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MARINE ORGANIC CARBON AND RADIOCARBON—PRESENT AND FUTURE CHALLENGES

Published online by Cambridge University Press:  25 January 2022

Ellen R M Druffel*
Affiliation:
Department of Earth System Science, UC Irvine, Keck Carbon Cycle AMS, Irvine, CA, USA
Steven R Beaupré
Affiliation:
School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA
Hendrik Grotheer
Affiliation:
Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Marine Geochemistry, Bremerhaven, Germany
Christian B Lewis
Affiliation:
Department of Earth System Science, UC Irvine, Keck Carbon Cycle AMS, Irvine, CA, USA
Ann P McNichol
Affiliation:
Department of Geology and Geophysics, Wood Hole Oceanographic Institution, Woods Hole, MA, USA
Gesine Mollenhauer
Affiliation:
Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Marine Geochemistry, Bremerhaven, Germany
Brett D Walker
Affiliation:
Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, ON, Canada
*
*Corresponding author. Email: edruffel@uci.edu
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Abstract

We discuss present and developing techniques for studying radiocarbon in marine organic carbon (C). Bulk DOC (dissolved organic C) Δ14C measurements reveal information about the cycling time and sources of DOC in the ocean, yet they are time consuming and need to be streamlined. To further elucidate the cycling of DOC, various fractions have been separated from bulk DOC, through solid phase extraction of DOC, and ultrafiltration of high and low molecular weight DOC. Research using 14C of DOC and particulate organic C separated into organic fractions revealed that the acid insoluble fraction is similar in 14C signature to that of the lipid fraction. Plans for utilizing this methodology are described. Studies using compound specific radiocarbon analyses to study the origin of biomarkers in the marine environment are reviewed and plans for the future are outlined. Development of ramped pyrolysis oxidation methods are discussed and scientific questions addressed. A modified elemental analysis (EA) combustion reactor is described that allows high particulate organic C sample throughput by direct coupling with the MIniCArbonDAtingSystem.

Type
Research Article
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
© The Author(s), 2022. Published by Cambridge University Press for the Arizona Board of Regents on behalf of the University of Arizona

INTRODUCTION

Organic carbon (C) is reduced C whose molecular structure is formed on skeletons of C atoms. There are two pools of organic C in seawater, dissolved organic C (DOC) that passes a 0.2 to 1.0 micron filter, and particulate organic C (POC) that is retained on the filter (Figure 1). The pool size of DOC is 682 GtC (Hansell Reference Hansell2013), similar in size to the organic C present in the upper one meter of oceanic sediments (1000 GtC, Hedges et al. Reference Hedges2000); the POC pool is much smaller (30–40 GtC). Early studies demonstrated that bomb 14C was incorporated into subsurface and deep organisms (technically a portion of the POC pool), such as decapods and fish, along with significant amounts of pre-bomb C, on decadal timescales (Williams et al. Reference Williams, McGowan and Stuiver1970; Williams and Linick Reference Williams and Linick1975).

Figure 1 Size ranges of organic matter constituents in seawater and their relationships to the operationally defined pools of DOC (gray background) and POC (white background). (After Beaupré Reference Ausín, Magill, Haghipour, Fernández, Wacker, Hodell, Baumann and Eglinton2019.)

Organic C is primarily produced during photosynthetic reduction of dissolved inorganic C (DIC) by phototrophs in the surface ocean. The concentration of DOC in seawater is 2–4% that of DIC, which makes DOC more sensitive to changes in its sources and sinks than those for DIC. The processes controlling DIC in the ocean are CO2 exchange with the atmosphere, photosynthesis, respiration and production and dissolution of carbonate. The processes controlling DOC in seawater are more complex, including river input, microbial production of recalcitrant DOC (Jiao et al. Reference Jiao, Herndl, Hansell, Benner, Kattner, Wilhelm, Kirchman, Weinbauer, Chen and Azam2010), chemoautotrophy (Ingalls et al. Reference Ingalls, Shah, Hansman, Aluwihare, Santos, Druffel and Pearson2006; Hansman et al. Reference Hansman, Griffin, Watson, Druffel, Ingalls, Pearson and Aluwihare2009), hydrothermal processes (McCarthy et al. Reference McCarthy, Beaupré, Walker, Voparil, Guilderson and Druffel2011; Lang et al. Reference Lang, Butterfield, Lilley, Paul Johnson and Hedges2006; Shah Walter et al. Reference Shaw Walter, Jaekel, Osterholz, Fisher, Huber and Pearson2018; Estes et al. Reference Estes, Berti, Coffey, Hochella, Wozniak and Luther2019), enhanced refractory DOC utilization by priming (Bianchi et al. Reference Bianchi2011; Fang et al. Reference Fang, Lee, Lee, Hahm, Kim, Druffel and Hwang2020; Shen and Benner Reference Shen and Benner2018), gel formation (Verdugo et al. Reference Verdugo, Alldredge, Azam, Kirchmand, Passowe and Santschif2004) and dissolution/aggregation of POC (Smith et al. Reference Smith, Baldwin and Williams1992; Druffel et al. Reference Druffel, Griffin, Coppola and Walker2016).

The introduction of 14C analyses using AMS in the 1980s made it possible to study a broad range of organic C pools in the sea that were not possible with gas counting or liquid scintillation methods due to sample size restrictions. We discuss the present and developing advances for studying marine organic C using the mother of all isotopes, 14C.

Bulk DOC 14C

Methods for measuring bulk DOC Δ14C in seawater were developed in the 1960s that required collection of 400–500 L of water in Gerard barrels (Figure 2), UV oxidation in 60 L batches, and gas counting of 14C in the CO2 (Williams et al. Reference Williams, Oeschger and Kinney1969). Improvements to the method required UV oxidation of 5 L of seawater using AMS measurements of 14C (Williams and Druffel Reference Williams and Druffel1987). Beaupré et al. (Reference Beaupré, Druffel and Griffin2007) improved the method further by using UV oxidation of samples as small as 30 mL of seawater. Quantification of CO2 lost during breakthrough of CO2 from liquid nitrogen cooled traps during collection improved DOC concentrations reported with isotopic values (Walker et al. Reference Walker, Beaupré, Griffin and Druffel2019). However, one day is required to process one sample, standard, or blank. New methods are critically needed to increase processing of DOC samples for 14C measurement. Batch processing of bulk DOC in 100 mL reactors improves throughput of high [DOC] samples, such as freshwater and sediment pore water, to several per day (Xue et al. Reference Xue, Tiantian and Xuchen2015).

Figure 2 Peter M. Williams on the R/V Melville with a Gerard barrel, a 270 L steel vessel used to collect large volume seawater samples. The barrel is covered to protect it from particles coming mostly from the ship’s stack. Circa 1989. (Photo by Ellen Druffel.)

Radiocarbon in DIC has been used to measure the timescale of global deep ocean circulation. Deep water is produced in the northern North Atlantic, flows south to the Southern Ocean where it flows eastward, and then northward into the Indian and Pacific oceans, taking ∼1500 14C years to complete its journey (Broecker et al. Reference Broecker, Gerard, Ewing and Heezen1960; Bien et al. Reference Bien, Rakestraw and Suess1965; Stuiver et al. Reference Stuiver, Quay and Ostlund1983). This result agrees with pre-bomb bulk DOC Δ14C values that decrease from the deep North Atlantic (–456‰, 4900 14C years), to the South Atlantic (–475‰, 5200 14C years), the Southern Ocean (–500‰, 5600 14C years), and are lowest in the North Pacific (–560‰, 6600 14C years) (Druffel et al. Reference Druffel, Griffin, Coppola and Walker2016, Reference Druffel, Griffin, Garcia, Wang, McNichol, Key and Walker2019; Bercovici et al. Reference Bercovici, McNichol, Xu and Hansell2018; Druffel and Bauer Reference Druffel and Bauer2000), increasing in 14C age by 1700 years. This remarkable similarity between the deep cycling time of DOC and DIC suggests that DOC Δ14C is controlled primarily by circulation, indicating that deep DOC is refractory on millennial timescales (Hansell Reference Hansell2013). Yet the DOC 14C ages are much lower (∼3000 14C years) than those of DIC.

There are significant correlations between DOC and DIC Δ14C values for seawater taken from the same niskin bottle at numerous locations in the world oceans (Beaupré and Aluwihare Reference Beaupré and Aluwihare2010; Beaupré et al. Reference Beaupré, Walker and Druffel2020). Similar to DIC Δ14C results in the central Pacific (<1,000 m) that contained bomb 14C in the 1980s (Ostlund and Stuiver Reference Ostlund and Stuiver1980) and >1990s (Key et al. Reference Key, Kozyr, Sabine, Lee, Wanninkhof and Bullister2004; McNichol et al. Reference McNichol, Key and Guilderson2022), bomb 14C is seen in the DOC Δ14C values in the upper 1000 m in the Pacific Ocean (Figure 3). The highest surface values are found in the mid-ocean gyres around 20°N and 20°S (>–250‰), and lower values are near the equator and polar regions (–260‰ to –400‰) where deeper waters upwell to the surface (Bercovici et al. Reference Bercovici, McNichol, Xu and Hansell2018; Druffel and Bauer Reference Druffel and Bauer2000). Subantarctic Mode Water, produced in the southwest Pacific, flows between 200 and 400 m to the Equatorial Undercurrent in the equatorial Pacific, and has Δ14C values that range from –460 to –350‰ (Figure 3). Formed between 60° and 50°S, Antarctic Intermediate Water flows north at depths of ∼700–1200 m and has Δ14C values that range from –530 to –410‰ (Figure 3).

Figure 3 DOC Δ14C values (‰) of water samples (indicated by black dots) collected from seven stations along 150°W on the P16N cruise in 2015 (Druffel et al. Reference Druffel, Griffin, Garcia, Wang, McNichol, Key and Walker2019), one station on the P06 cruise (stn 130, 32.5°S 144.7°W) in 2010 (Druffel and Griffin Reference Druffel and Griffin2015) and one station from the Southern Ocean (SOce; 54.0°S 176.0°W) in 1995 (Druffel and Bauer Reference Druffel and Bauer2000) using Ocean Data View (Schlitzer Reference Schlitzer2015). (After Druffel et al. Reference Druffel, Griffin, Garcia, Wang, McNichol, Key and Walker2019.)

Questions that remain unanswered about marine DOC include the sources and sinks of DOC from hydrothermal systems, chemoautotrophy, sediment porewaters, interaction with POC, and photooxidation of DOC in the surface ocean and aerosols (Beaupré et al. Reference Beaupré, Walker and Druffel2020). 14C and molecular identification methods (e.g., NMR and FT-ICR-MS) can help to determine the importance of these mechanisms.

SPE-DOC, and Size-Fractionated DOC

Solid-phase extraction (SPE) is a method of separating DOC (SPE-DOC) from bulk DOC using hydrophobic, silica or polystyrene resins. Seawater is acidified to enhance the relative abundance of protonated carboxyl groups, decreasing solubility and increasing sorption onto the resin (Mopper et al. Reference Mopper, Stubbins, Ritchie, Bialk and Hatcher2007). XAD and C18 resins have been used to collect SPE-DOC and generally isolate humic substances from total DOC (Druffel et al. Reference Druffel, Williams, Bauer and Ertel1992; Green and Blough Reference Green and Blough1994; Hedges Reference Hedges1992; Kieber et al. Reference Kieber, Hydro and Seaton1997; Peuravuori and Pihlaja Reference Peuravuori and Pihlaja1997). C18 resins were especially effective in isolating chromophoric humic substances (Amador et al. Reference Amador, Milne, Moore and Zika1990). More recently, styrene divinylbenzene polymer sorbents, such as PPL are more efficient at extracting DOC and leach less C than C18 resins. NMR analysis shows that SPE-DOC isolated using PPL is more representative of bulk DOC than that isolated using C18 resins (Dittmar et al. Reference Dittmar, Koch, Hertkorn and Kattner2008). The ease of use, high recoveries, and ability to desalinate SPE-DOC samples have led to wide use of PPL resins in investigations of marine DOC. Subsequent methodological studies have compared PPL, C18, and other SPE resins in more detail, and show that DOC extraction can be increased up to 89% with the correct ratio of DOC/PPL (Li et al. Reference Li, Harir, Lucio, Kanawati, Smirnov, Flerus, Koch, Schmitt-Kopplin and Hertkorn2016). Lewis et al. (Reference Lewis, Walker and Druffel2020) demonstrated that the optical and isotopic characteristics of SPE-DOC using PPL cartridges are heterogeneous during elution with methanol, which may be useful in further separation of compounds based on their relative polarities.

PPL has been used to isotopically and chemically characterize SPE-DOC in the east South Atlantic Ocean, the Bermuda Atlantic Time Series, Station ALOHA in the North Pacific, the Weddell Sea, and the deep North Pacific Ocean (Broek et al. Reference Broek, Walker, Guilderson, Vaughn, Mason and McCarthy2020; Flerus et al. Reference Flerus, Lechtenfeld, Koch, McCallister, Schmitt-Kopplin, Benner, Kaiser and Kattner2012; Hertkorn et al. Reference Hertkorn, Harir, Koch, Michalke and Schmitt-Kopplin2013; Lechtenfeld et al. Reference Lechtenfeld, Kattner, Flerus, McCallister, Schmitt-Kopplin and Koch2014; Zigah et al. Reference Zigah, McNichol, Xu, Johnson, Santinelli, Karl and Repeta2017). SPE-DOC typically has lower Δ14C and δ13C values than those of total DOC (Broek et al. Reference Broek, Walker, Guilderson, Vaughn, Mason and McCarthy2020; Coppola and Druffel Reference Coppola and Druffel2016; Zigah et al. Reference Zigah, McNichol, Xu, Johnson, Santinelli, Karl and Repeta2017; Lewis et al. Reference Lewis, Walker and Druffel2021), is rich in aromatic C (Simjouw et al. Reference Simjouw, Minor and Mopper2005), and has elemental compositions indicative of thermogenic processes found throughout the water column (Dittmar and Koch Reference Dittmar and Koch2006; Hertkorn et al. Reference Hertkorn, Harir, Koch, Michalke and Schmitt-Kopplin2013). The presence of carboxyl-rich alicyclic molecules (CRAM), with low H/C and O/C ratios indicative of recalcitrance, is often found in SPE-DOC and tends to increase with depth in the water column (Broek et al. Reference Broek, Walker, Guilderson, Vaughn, Mason and McCarthy2020; Hertkorn et al. Reference Hertkorn, Harir, Koch, Michalke and Schmitt-Kopplin2013; Lechtenfeld et al. Reference Lechtenfeld, Hertkorn, Shen, Witt and Benner2015). Lechtenfeld (Reference Lechtenfeld, Hertkorn, Shen, Witt and Benner2015) found that bacterial SPE-DOC isolated in laboratory experiments is similar in composition with natural marine DOC and the composition of recalcitrant molecules in the ocean. The link between SPE-DOC recalcitrance, low Δ14C, and low δ13C may all be indicative of the influence of marine heterotrophic bacteria on DOC degradation, transformation, cycling, and the production of refractory DOC (Flerus et al. Reference Flerus, Lechtenfeld, Koch, McCallister, Schmitt-Kopplin, Benner, Kaiser and Kattner2012; Lechtenfeld et al. Reference Lechtenfeld, Hertkorn, Shen, Witt and Benner2015; Osterholz et al. Reference Osterholz, Niggemann, Giebel, Simon and Dittmar2015). Solid-phase extraction will likely continue to advance our knowledge of DOC molecular composition as techniques to isolate and further characterize DOC develop further.

For several decades, ultrafiltration (UF) has been used as a method to isolate DOC from abundant salts in seawater. The technique has shaped much of our understanding of DOC cycling and molecular composition, and samples material that can be isolated without the chemical bias inherent to all resin chromatography methods. Early UF studies have shown that, despite low DOC recovery (10–30%), ultrafiltered DOC (UDOC), also referred to as high molecular weight (HMW DOC), was generally representative of total DOC in terms of its bulk elemental (C:N) and isotopic (δ13C, δ15N) values (Benner et al. Reference Benner, Paculksy, McCarthy and Hedges1992, Reference Benner, Biddanda, Black and McCarthy1997; McCarthy et al. Reference McCarthy, Hedges and Benner1993). However, several studies have shown that UDOC differs from bulk DOC in molecular composition (Skoog and Benner Reference Skoog and Benner1997; Dittmar et al. Reference Dittmar, Fitznar and Kattner2001) and Δ14C values (Santschi et al. Reference Santschi, Guo, Baskaran, Trumbore, Southon, Bianchi, Honeyman and Cifuentes1995; Loh et al. Reference Loh, Bauer and Druffel2004; McNichol and Aluwihare Reference McNichol and Aluwihare2007). Further parameterization of UDOC Δ14C values as a function of UF experiment concentration factor (CF) reconciled >250‰ variability in reported surface and deep Pacific UDOC Δ14C values (Figure 4a; Walker et al. Reference Walker, Beaupre, Guilderson, Druffel and McCarthy2011). Further work suggested that changes in CF could be used for the targeted isolation of semi-labile or even bioavailable UDOC (Walker et al. Reference Walker2016a). Robust, size-age-composition relationships within both the DOC pool and total marine organic matter size continuum (Figure 4b,c,d) suggest a “precursor-product” relationship controlling the cycling (14C age) and chemical composition (N-content) of marine organic matter. As a result, DOC C:N content and 14C age can be generally predicted from molecular size (Walker et al. Reference Walker, Beaupre, Guilderson, McCarthy and Druffel2016b).

Figure 4 Size-Δ14C and elemental composition (C:N) relationships of DOC in the Pacific Ocean. (A) DOC Δ14C as a function of ultrafiltration concentration factor (CF) for the surface and deep Pacific ocean. Relationships between the C:N content of size-fractionated marine organic matter (B,D) and Δ14C (C,E) in the surface and deep Pacific. (Figures adapted from Walker et al. Reference Walker, Beaupre, Guilderson, Druffel and McCarthy2011 and Walker et al. Reference Walker, Beaupre, Guilderson, McCarthy and Druffel2016b.)

A recent advancement is the combination of UF and SPE techniques to view organic matter through various analytical windows simultaneously. By filtering out POC, then performing solid-phase extraction on UDOC permeate fractions, the comparison of up to three different sub-pools of DOC is possible (HMW-DOC, hydrophobic LMW-DOC, hydrophilic LMW DOC) (Broek et al. Reference Broek, Walker, Guilderson and McCarthy2017; Zigah et al. Reference Zigah, McNichol, Xu, Johnson, Santinelli, Karl and Repeta2017; Broek et al. Reference Broek, Walker, Guilderson, Vaughn, Mason and McCarthy2020). This method has provided new insights regarding the cycling of hydrothermal vent DOC in the open ocean, the possible addition of high molecular weight DOC to the deep ocean, and the selective utilization of some LMW-DOC during meridional overturning circulation (Broek et al. Reference Broek, Walker, Guilderson and McCarthy2017; Zigah et al. Reference Zigah, McNichol, Xu, Johnson, Santinelli, Karl and Repeta2017; Broek et al. Reference Broek, Walker, Guilderson, Vaughn, Mason and McCarthy2020).

Compound Classes and 14C

Marine organic matter exists as molecules, some of which are characterizable. The characterizable organic C is made up mostly of amino acids/proteins, sugars/carbohydrates, lipids, and nucleic acids. However, more than half of the organic matter in seawater and sediments is not characterizable at the molecular level (Hedges et al. Reference Hedges2000). Whereas 85% of surface plankton and POC in the central Pacific is identified molecularly (Wakeham et al. Reference Wakeham, Lee, Hedges, Hermes and Peterson1997), less than 25% of deep POC is so identified, due to extensive remineralization as POC falls through the water column. The Δ14C of sinking POC in the deep east North Pacific is lower than that of surface DIC that was photosynthesized to produce the organic C (Druffel et al. Reference Druffel, Bauer, Williams, Griffin and Wolgast1996). The surface sediment organic C has even lower Δ14C values (Wang et al. Reference Wang, Druffel and Lee1998). A correlation was found between Δ14C and aluminum content of sinking POC, demonstrating that organic C associated with lithogenic material from sediment resuspension causes the observed low Δ14C values (Hwang et al. Reference Hwang, Druffel and Eglinton2010).

To understand these trends, separation of POC into four compound classes (or organic fractions), total hydrolyzable amino acids (THAA), total hydrolyzable carbohydrates (THCO), lipids and acid insoluble fraction were used to study 14C (Wang et al. Reference Wang, Druffel and Lee1996, Reference Wang, Druffel and Lee1998; Hwang and Druffel Reference Hwang and Druffel2003). Dried POC, zooplankton, phytoplankton, detrital aggregates on surface sediment and sediment from Stn M (34°50ʼN, 123°0ʼW, 4100 m depth) were extracted with methylene chloride:methanol to extract lipids. Half of the residue was hydrolyzed with 6M HCl under N2 for THAA and the other half with 72% and 0.6M H2SO4. The hydrolyzates were eluted through ion exchange columns to separate the two fractions, and the organic C leftover from the THAA extraction was used for the acid insoluble fraction. All THAA and THCO fractions had higher Δ14C values than those for the lipid and acid insoluble fractions (Wang et al. Reference Wang, Druffel and Lee1996, Reference Wang, Druffel and Lee1998; Hwang and Druffel Reference Hwang and Druffel2003) (Figure 5). The proportion of acid insoluble fractions increased with depth in the water column, and was highest in sediment samples (49–69%) and lowest in zooplankton and phytoplankton (<10%) from the upper 100 m. These results implied that the acid insoluble fraction was composed of lipid-like macromolecules.

Figure 5 Δ14C of organic compound classes (THAA, open triangles; TCHO, open circles; lipids, solid triangles; acid insoluble fraction, solid circles). The range of DIC 14C values in surface waters at Stn M ranged from about 40 to 70‰. (After Hwang and Druffel Reference Hwang and Druffel2003.)

Compound classes have also been measured in HMW-DOC. Results similar to those obtained for POC and sedimentary organic C were obtained, where THAA and TCHO had higher Δ14C values than those for lipid and acid insoluble fractions (Loh et al. Reference Loh, Bauer and Druffel2004).

Dissolved black C (DBC) is incompletely combusted organic matter produced from fossil fuel and biomass burning. DBC was measured in HMW-DOC from 4 samples from the Pacific and Atlantic Oceans and found to have Δ14C values that were 470–860‰ lower than those for bulk DOC from the same water (Ziolkowski and Druffel Reference Ziolkowski and Druffel2010). DBC was also measured in SPE-DOC from 6 samples in the Atlantic, Pacific and Arctic Oceans and DBC Δ14C values were 0–560‰ lower than those of SPE-DOC (Coppola and Druffel Reference Coppola and Druffel2016). They concluded that there are two distinct pools of DBC, that in the surface ocean and that in the deep ocean. Coppola et al. (Reference Coppola, Ziolkowski, Masiello and Druffel2014) measured black C in sinking POC and sediments and found that the main transport mechanism of black C to sediments may be sorption of ancient DBC onto POC. Further studies of DBC in marine systems are needed to understand more fully the sources and sinks of DBC and its residence time in the ocean.

Compound Specific Radiocarbon Analyses (CSRA)

Organic matter in the ocean consists of a complex mixture of different compounds, both of terrigenous and marine origin. Greater understanding of C cycling processes in the marine realm can be obtained by isolating specific fractions of marine organic matter that can be ascribed to a known source (biomarkers), by investigating the 14C signature of individual organic compounds. Compound-specific 14C analyses (CSRA) of marine biomarkers have been used to elucidate processes affecting the fate of marine organic matter during transport (e.g., Ohkouchi et al. Reference Ohkouchi, Eglinton, Keigwin and Hayes2002; Mollenhauer et al. Reference Mollenhauer, Eglinton, Ohkouchi, Schneider, Müller, Grootes and Rullkötter2003, Reference Mollenhauer, Inthorn, Vogt, Zabel, Sinninghe Damsté and Eglinton2007; Ausín et al. Reference Ausín, Magill, Haghipour, Fernández, Wacker, Hodell, Baumann and Eglinton2019), the reactivity of different types of marine organic compounds (e.g., Shah et al. Reference Shah, Mollenhauer, Ohkouchi, Eglinton and Pearson2008; Kusch et al. Reference Kusch, Ogawa, Altabet, Butzin, Friedrich, Ohkouchi and Mollenhauer2010, Reference Kusch, Rethemeyer, Hopmans, Wacker and Mollenhauer2016), and C sources utilized for biosynthesis (Ingalls et al. Reference Ingalls, Shah, Hansman, Aluwihare, Santos, Druffel and Pearson2006; Hansmann et al. Reference Hansman, Griffin, Watson, Druffel, Ingalls, Pearson and Aluwihare2009). Improvement of stratigraphies for sediments that are otherwise difficult to date have also relied on CSRA of marine organic compounds (Ingalls et al. Reference Ingalls, Anderson and Pearson2004; Ohkouchi and Eglinton Reference Ohkouchi and Eglington2008). These studies have revealed the long-term residence of marine organic matter in continuous re-suspension loops (e.g., Mollenhauer et al. Reference Mollenhauer, Inthorn, Vogt, Zabel, Sinninghe Damsté and Eglinton2007, Reference Mollenhauer, McManus, Wagner, McCave and Eglinton2011), which has also been shown for terrigenous biomarkers (e.g., Bröder et al. Reference Bröder, Tesi, Andersson, Semiletov and Gustafsson2018; Wei et al. Reference Wei, Mollenhauer, Hefter, Kusch, Grotheer, Schefuß and Jia2021), its preservation in association with fine-grained lithogenic material resulting in long-distance lateral transport and the millennial scale duration of these transport processes (Ohkouchi et al. Reference Ohkouchi, Eglinton, Keigwin and Hayes2002; Mollenhauer et al. Reference Mollenhauer, Kienast, Lamy, Meggers, Schenider, Hayes and Eglinton2005, Reference Mollenhauer, Inthorn, Vogt, Zabel, Sinninghe Damsté and Eglinton2007). Chemoautotrophic metabolism has been shown to prevail among mesopelagic microbial communities (Ingalls et al. Reference Ingalls, Shah, Hansman, Aluwihare, Santos, Druffel and Pearson2006). Steroids and pigments and their derivatives, biomineral occluded compounds and as nucleic acids (Cherrier et al. Reference Cherrier, Bauer, Druffel, Coffin and Chanton1999) have been investigated. Recent developments use CSRA of amino acids to study their sources and cycling (Bour et al. Reference Bour, Walker, Broek and McCarthy2016; Blattmann et al. Reference Blattmann, Montluçon, Haghipour, Ishikawa and Eglinton2020).

All applications of CSRA on marine organic compounds require extraction and purification of sufficient mass of the compound from samples where they are typically present in trace amounts. As a consequence, most CSRA studies are conducted using sediments that have sufficient material available. In most cases, CSRA of compounds isolated from POC or DOC sampled from the water column is not possible, unless dedicated sampling campaigns that collect several thousands of liters of water are conducted (e.g., Ingalls et al. Reference Ingalls, Shah, Hansman, Aluwihare, Santos, Druffel and Pearson2006). An exception was sugars collected from HMW-DOC in surface waters of the North Pacific that had post-bomb Δ14C values (Repeta and Aluwihare Reference Repeta and Aluwihare2006).

Commonly used methods for extraction and purification targeting mostly lipid compounds have been summarized by Mollenhauer et al. (Reference Mollenhauer, Kusch, Eglinton and Pearson2019). Often, a sequence of wet-chemical and chromatographic techniques is applied, and typically samples of 100 µg C or less are obtained. The samples need to be combusted prior to 14C analysis and in many cases will be graphitized. AMS analyses of small samples are conducted using size-matched standards and blanks for normalization. For such small samples, accurate determination of the amount and Δ14C value of extraneous C added to the sample during the various steps of sample preparation is of critical importance (Santos et al. Reference Santos, Southon, Drenzek, Ziolkowski, Druffel, Xu, Zhang, Trumbore, Eglinton and Hughen2010). Recently, isotope dilution techniques are increasingly used to obtain reliable estimates of this extraneous C contribution and its 14C content, which can subsequently be used to correct CSRA results (e.g., Haghipour et al. Reference Haghipour, Ausin, Usman, Ishikawa, Wacker, Welte, Ueda and Eglinton2019; Sun et al. Reference Sun2020).

Taking advantage of the elemental-analyzer-accelerator-mass-spectrometer (EA-AMS) setup, recent methodological developments allow for direct injection of CO2 gas produced from combustion of purified compounds into a gas accepting ion source (Haghipour et al. Reference Haghipour, Ausin, Usman, Ishikawa, Wacker, Welte, Ueda and Eglinton2019; Mollenhauer et al. Reference Mollenhauer, Grotheer, Gentz, Bonk and Hefter2021). Purified compounds are transferred into small tin cups, dried and combusted in the EA (Elementar Elemental Analyzer) coupled to the AMS. Thereby, the graphitization step is omitted, and high-precision 14C data can be obtained for samples containing 50 µg C or less. For samples younger than approximately 5000 years, precise 14C analyses can be achieved for samples as small as 10 µg C (Haghipour et al. Reference Haghipour, Ausin, Usman, Ishikawa, Wacker, Welte, Ueda and Eglinton2019).

Ramped Pyrolysis Oxidation (RPO) and 14C

In the mid-2000s, attempts to understand the chronology of Antarctic sediments led John Hayes to develop the Ramped Pyrolysis Oxidation system (RPO) at the National Ocean Sciences AMS facility, a system that separates organic matter based on thermochemical properties. RPO 14C analysis involves the controlled step heating of samples containing organic C, with or without oxygen, typically at a heating rate of 5°C/minute (Figure 6). As heating proceeds, the most reactive organic components become volatile and are swept into an oxidizing reactor by a helium stream, allowing separation of CO2 from other pyrolysis/combustion components and collection of CO2 over discrete temperature intervals for analysis of both stable and radiocarbon isotopes. A major advantage is the ability to analyze all of the organic matter in a sample, not just an isolated fraction. In sediments, the thermal separation allows the isolation of fresh detrital matter from eroded bedrock. The initial goal of improving the chronologies of Antarctic sediments was realized (Rosenheim et al. Reference Rosenheim, Day, Domack, Schrum, Benthien and Hayes2008) and has been expanded upon (Rosenheim et al. Reference Rosenheim, Santoro, Gunter and Domack2013; Subt et al. Reference Subt, Fangman, Wellner and Rosenheim2016).

Figure 6 Schematic of the RPO reactor (left) and the results (right) from a POC sample collected from the Narayani River, Nepal (Hemingway Reference Hemingway, Galy, Gagnon, Grant, Rosengard, Soulet, Zigha and McNichol2017). The top half of the quartz reactor heats the sample while the gases are swept into a CO2 analyzer before passing through a cryogenic trap. The gray shaded region is the thermogram, the variation in ppm CO2 vs. temperature (units not shown), white circles show the δ13C values, and bars show the fraction modern values for each temperature interval (Hemingway Reference Hemingway, Galy, Gagnon, Grant, Rosengard, Soulet, Zigha and McNichol2017). The data show that the high temperature, more refractory, fractions have dramatically less 14C, i.e., are older than the lower temperature fractions.

Figure 7 Preliminary F14C POCsusp values of samples collected along a shelf to open ocean transect off Cape Blanc (NW Africa, ∼21°N) collected in November 2018 during research cruise MSM79 onboard the German R/V Maria S. Merian and processed according to the EA–AMS method described above (H. Grotheer unpublished data). Red dots indicate sampling depths. (The map was generated using Ocean Data View; Schlitzer Reference Schlitzer2015).

Building on this success, RPO has since been applied to a wide variety of biogeochemical questions in many environments including the oceans. It is being used to study the nature of the organic material transported to the oceans from rivers (Hemingway et al. Reference Hemingway, Rothman, Grant, Rosengard, Eglinton, Derry and Galy2019). Hemingway et al. (Reference Hemingway, Rothman, Rosengard and Galy2017) developed an inverse method to calculate organic C reactivity from RPO results. They used the method on RPO results from 62 sediment organic C, riverine POC, soil organic C, and DOC samples to indicate that mineral interactions are more important for the preservation of organic C than selective preservation (Hemingway et al. Reference Hemingway, Rothman, Grant, Rosengard, Eglinton, Derry and Galy2019). RPO has been applied to studying the fate of petroleum hydrocarbons, both natural and pollutant, in marine and coastal ecosystems (Pendergraft et al. Reference Pendergraft, Dincer, Sericano, Wade, Kolasinski and Rosenheim2013; Pendergraft and Rosenheim Reference Pendergraft and Rosenheim2014; Adikhari et al. Reference Adhikari, Maiti, Overton, Rosenheim and Marx2016; Rogers et al. Reference Rogers, Bosman, Lardie-Gaylord, McNichol, Rosenheim and Montoya2019). A time series study of the impact of the Deep Water Horizon oil spill on sediments from four sites demonstrated the range of impacts and the time evolution of the oil spill. RPO and C isotope analysis allowed the separation of the spilled petrocarbon from natural hydrocarbon seep C. Emerging work is using the technique to explore the nature of DOC, particularly the high molecular weight fraction.

EA-AMS for Suspended POC 14C

During oceanographic expeditions, suspended POC (POCsusp) for radiocarbon analysis is typically collected using in-situ water pumps with GF/F filters that are lowered to water depths where up to 1000 L are pumped (Griffith et al. Reference Griffith, McNichol, Xu, McLaughlin, Macdonald, Brown and Eglinton2012). Because water pumps are expensive, multiple casts per station are required to obtain full-depth water column 14C POCsusp profiles, a time consuming process that is also expensive.

Utilizing the EA-AMS setup can overcome in-situ water pump limitations, because the MICADAS (MIniCArbonDAtingSystem) allows routine, high precision analysis of small samples (> 100 µgC). For EA-AMS, seawater is processed onboard directly from CTD-ROS (Conductivity Temperature Depth-Rosette) niskin bottles, where 2–40 L of water are collected to obtain sufficient C on GF/F filters, depending on the POCsusp concentration. It is possible to sample up to 24 depths during one deployment and generate high resolution depth profiles of POCsusp 14C (Figure 7). POCsusp is concentrated on pre-combusted GF/F filters using a vacuum supported glass filtration device that allows the sampling of filtrate to obtain the corresponding DOC fraction. In the lab, the filters are acidified, packed into tin capsules, and combusted in the EA. The generated CO2 is measured in the MICADAS via direct injection into the gas ion source (Mollenhauer et al. Reference Mollenhauer, Grotheer, Gentz, Bonk and Hefter2021).

The conventional CuO oxidation reactor of the EA operating at 950°C is prone to cracking, due to halogen radicals reacting with CuO that attack the quartz reactor. Recently, a modified EA reactor has replaced the CuO with WO3 and changed the position of the Ag wool. The WO3 does not react with halogen radicals and Ag wool is positioned at low reaction temperatures further preventing unwanted side reactions. The reaction temperature is lowered to 850°C, and an Inconel liner and ash fingers are used to further protect the quartz reactor. This reactor setup withstands 150–200 samples, whereas a typical CuO reactor processing saline POCsusp filters cannot withstand more than 50 samples.

The disadvantage of this setup is that the C content of the blank increases significantly. The C content of a typical blank for a pre-combusted ½ 45 mm GF/F analyzed on a CuO reactor is ∼10 μg C, but for the modified WO3 reactor it is ∼20 μgC. The higher blank C originates from the WO3 and can be reduced by carefully inspecting the material and removing incompletely oxidized tungsten particles and foreign objects. Nonetheless, pre-combusted GF/F filters contain a significant amount of blank C and appropriate blank determination and subsequent correction is critical. The high blank C content has further implications for the minimum amount of seawater that needs to be filtered per sample in order to acquire an optimum blank C to POCsusp ratio.

In the first example of a high-resolution F14C (Fraction modern 14C) POCsusp profile (Figure 7, H. Grotheer unpublished data) was generated for a transect from the shelf to the open ocean off Cape Blanc (NW Africa, ∼21°N). The Cape Blanc region is located in the southern part of the Canary Current Eastern Boundary Upwelling system off northwest Africa. Here, the Cape Blanc upwelling filament transports cold, nutrient-rich, upwelled waters for hundreds of kilometers offshore towards the oligotrophic ocean (Van Camp et al. Reference Van Camp, Nykjaer, Mittelstaedt and Schlittenhardt1991; Gabric et al. Reference Gabric, Garcia, Van Camp, Nykjaer, Eifler and Schrimpf1993; Àlvarez-Salgado et. al. Reference Àlvarez-Salgado, Doval, Borges, Joint, Frankignoulle, Woodward and Figueiras2001) and is an important transport pathway for particles (Karakas et al. Reference Karakas, Nowald, Blaas, Marchesiello, Frickenhaus and Schlitzer2006).

Fresh POCsusp (F14C 1.0) produced by primary production on the shelf is transported along the continental slope within the bottom layer and travels offshore at ∼500 m water depth. Further offshore, underneath the photic zone, F14C POCsusp decreases to ∼0.8 at 500 m depth. Below 500 m, F14C values show a heterogeneous distribution with pre-aged POCsusp (F14C ∼0.6) separated by layers of POCsusp with intermediate F14C values, suggesting the presence of a refractory POCsusp pool, potentially originating from sediment resuspension. These preliminary data highlight the potential of the EA-AMS method to generate high-resolution, F14C POCsusp profiles that will advance our understanding of POC cycling, transport and sequestration processes in the ocean.

FUTURE WORK

Studies of marine organic C have benefitted from research conducted outside of the marine scientific community. Effort to continue this strong communication and collaboration is important to maintain and to nurture. New ways to unlock the secrets of the biogeochemistry of the marine system will benefit humanity as we chart a course through the Anthropocene.

References

REFERENCES

Adhikari, PL, Maiti, K, Overton, EB, Rosenheim, BE, Marx, BD. 2016. Distributions and accumulation rates of polycyclic aromatic hydrocarbons in the northern Gulf of Mexico sediments. Environmental Pollution 212:413423. doi: 10.1016/j.envpol.2016.01.064.CrossRefGoogle ScholarPubMed
Àlvarez-Salgado, XA, Doval, MD, Borges, AV, Joint, I, Frankignoulle, M, Woodward, M, Figueiras, FG. 2001. Off-shelf fluxes of labile materials by an upwelling filament in the NW Iberian upwelling system. Progress in Oceanography 51(2–4):321337. doi: 10.1016/S0079-6611(01)00073-8.CrossRefGoogle Scholar
Amador, J, Milne, PJ, Moore, CA, Zika, RG. 1990. Extraction of chromophoric humic substances from seawater. Marine Chemistry 29:117. doi: 10.1016/0304-4203(90)90002-T.CrossRefGoogle Scholar
Ausín, B, Magill, C, Haghipour, N, Fernández, A, Wacker, L, Hodell, D, Baumann, K-H, Eglinton, TI. 2019. (In)coherent multiproxy signals in marine sediments: Implications for high-resolution paleoclimate reconstruction. Earth and Planetary Science Letters 515:3846. doi: 10.1016/j.epsl.2019.03.003.CrossRefGoogle Scholar
Beaupré, SR, Aluwihare, L. 2010. Constraining the 2-component model of marine dissolved organic radiocarbon. Deep Sea Research Part II: Topical Studies in Oceanography 57(16):14941503. doi: 10.1016/j.dsr2.2010.02.017.CrossRefGoogle Scholar
Beaupré, SR, Walker, BD, Druffel, ERM. 2020. The two–component model coincidence: Evaluating the validity of marine dissolved organic radiocarbon as a stable–conservative tracer at Station M. Deep-Sea Research II 173:104737.CrossRefGoogle Scholar
Beaupré, SR, Druffel, ERM, Griffin, S. 2007. A low–blank photochemical extraction system for concentration and isotopic analyses of marine dissolved organic carbon. Limnology and Oceanography: Methods 5:174184.Google Scholar
Beaupré et al. 2019. Oceanic efflux of ancient marine dissolved organic carbon in primary marine aerosol. Science Advances 5:eaax6535.CrossRefGoogle Scholar
Benner, R, Biddanda, B, Black, B, McCarthy, M. 1997. Abundance, size distribution, and stable carbon and nitrogen isotopic compositions of marine organic matter isolated by tangential-flow ultrafiltration. Marine Chemistry 57:243263.CrossRefGoogle Scholar
Benner, R, Paculksy, J, McCarthy, M, Hedges, J, et al. 1992. Bulk chemical characteristics of dissolved organic matter in the ocean. Science 255:15611564.CrossRefGoogle ScholarPubMed
Bercovici, SK, McNichol, AP, Xu, L, Hansell, DA. 2018. Radiocarbon content of dissolved organic carbon in the South Indian Ocean. Geophysical Research Letters 45(2):872879. doi: 10.1002/2017gl076295.CrossRefGoogle Scholar
Blattmann, TM, Montluçon, DB, Haghipour, N, Ishikawa, NF, Eglinton, TI. 2020. Liquid chromatographic isolation of individual amino acids extracted from sediments for radiocarbon analysis. Front. Mar. Sci. 7:174. doi: 10.3389/fmars.2020.00174.CrossRefGoogle Scholar
Bianchi, TS. 2011. The role of terrestrially derived organic carbon in the coastal ocean: a changing paradigm and the priming effect. Proc. National Academy Sciences 108(49):1947319481.CrossRefGoogle ScholarPubMed
Bien, G, Rakestraw, NW, Suess, HE. 1965. Radiocarbon in the Pacific and Indian Oceans and its relation to deep water movements. Limnology and Oceanography 10:R25R36.CrossRefGoogle Scholar
Bour, AL, Walker, BD, Broek, TAB, McCarthy, MD. 2016. Radiocarbon analysis of individual amino acids: carbon blank quantification for a small-sample high-pressure liquid chromatography purification method Analytical Chemistry 88(7):35213528. doi: 10.1021/acs.analchem.5b03619.CrossRefGoogle ScholarPubMed
Bröder, L, Tesi, T, Andersson, A, Semiletov, I, Gustafsson, Ö. 2018. Bounding cross-shelf transport time and degradation in Siberian-Arctic land-ocean carbon transfer. Nature Communications 9(1):806. doi: 10.1038/s41467-018-03192-1.CrossRefGoogle ScholarPubMed
Broecker, WS, Gerard, R, Ewing, M, and Heezen, BC. 1960. Natural radiocarbon in the Atlantic Ocean: Jour. Geophys. Research 55:29032931.Google Scholar
Broek, TAB, Walker, BD, Guilderson, TP, McCarthy, MD. 2017. Coupled ultrafiltration and solid phase extraction approach for the targeted study of semi-labile high molecular weight and refractory low molecular weight dissolved organic matter. Marine Chemistry 194:146157.CrossRefGoogle Scholar
Broek, TA, Walker, BD, Guilderson, TP, Vaughn, JS, Mason, HE, McCarthy, MD. 2020. Low molecular weight dissolved organic carbon: aging, compositional changes, and selective utilization during global ocean circulation. Global Biogeochemical Cycles 34(6):120. doi: 10.1029/2020GB006547.CrossRefGoogle Scholar
Cherrier, J, Bauer, J, Druffel, ERM, Coffin, R, Chanton, J. 1999. Radiocarbon in marine bacteria: Evidence for the ages of assimilated carbon. Limnology and Oceanography 44(3):730736.CrossRefGoogle Scholar
Coppola, AI, Ziolkowski, LA, Masiello, CA, Druffel, ERM. 2014. Aged black carbon in marine sediments and sinking particles. Geophys. Res. Lett. 41. doi: 10.1002/2013GL059068.CrossRefGoogle Scholar
Coppola, AI, Druffel, ERM. 2016. Cycling of black carbon in the ocean. Geophysical Research Letters 43(9):44774482. doi: 10.1002/2016GL068574.CrossRefGoogle Scholar
Dittmar, T, Koch, BP. 2006. Thermogenic organic matter dissolved in the abyssal ocean. Marine Chemistry 102(3–4):208217. doi: 10.1016/j.marchem.2006.04.003.CrossRefGoogle Scholar
Dittmar, T, Fitznar, HP, Kattner, G. 2001. Origin and biogeochemical cycling of organic nitrogen in the eastern Arctic Ocean as evident from D- and L-amino acids. Geochim. Cosmochim. Acta 65:41034114.CrossRefGoogle Scholar
Dittmar, T, Koch, B, Hertkorn, N, Kattner, G. 2008. A simple and efficient method for the solid-phase extraction of dissolved organic matter (SPE-DOM) from seawater. Limnology and Oceanography: Methods 6(6):230235.Google Scholar
Druffel, ERM, Bauer, JE. 2000. Radiocarbon distributions in Southern Ocean dissolved and particulate organic matter. Geophysical Research Letters 27(10):14951498. doi: 10.1029/1999GL002398.CrossRefGoogle Scholar
Druffel, ERM, Griffin, S. 2015. Radiocarbon in dissolved organic carbon of the South Pacific Ocean, Geophy. Res. Lett. 42:40964101. doi: 10.1002/2015GL063764.CrossRefGoogle Scholar
Druffel, ERM, Bauer, JE, Williams, PM, Griffin, S, Wolgast, D. 1996. Seasonal variability of radiocarbon in particulate organic carbon in the Northeast Pacific Ocean. Journal of Geophysical Research 101:2054320552.CrossRefGoogle Scholar
Druffel, ERM, Griffin, S, Coppola, AI, Walker, BD. 2016. Radiocarbon in dissolved organic carbon of the Atlantic Ocean. Geophysical Research Letters 43. doi: 10.1002/2016GL068746.CrossRefGoogle Scholar
Druffel, ERM, Griffin, S, Garcia, NG, Wang, N, McNichol, AP, Key, R, Walker, BD. 2019. Dissolved organic radiocarbon in the central Pacific Ocean, Geophysical Research Letters 46. doi: 10.1029/2019GL083149.CrossRefGoogle Scholar
Druffel, ERM, Williams, PM, Bauer, JE, Ertel, JR. 1992. Cycling of dissolved and particulate organic matter in the open ocean. Journal of Geophysical Research 97(C10):15639. doi: 10.1029/92JC01511 CrossRefGoogle Scholar
Estes, ER, Berti, D, Coffey, NR, Hochella, MF, Wozniak, AS, Luther, GW. 2019. Abiotic synthesis of graphite in hydrothermal vents. Nature Communications 10. doi: 10.1038/s41467-019-13216-z.CrossRefGoogle Scholar
Fang, L, Lee, SH, Lee, S-A, Hahm, D, Kim, G, Druffel, ERM, Hwang, J. 2020. Removal of refractory dissolved organic carbon in the Amundsen Sea, Antarctica. Scientific Reports. doi: 10.1038/s41598-020-57870-6.CrossRefGoogle Scholar
Flerus, R, Lechtenfeld, OJ, Koch, BP, McCallister, SL, Schmitt-Kopplin, P, Benner, R, Kaiser, K, Kattner, G. 2012. A molecular perspective on the ageing of marine dissolved organic matter. Biogeosciences 9(6):19351955. doi: 10.5194/bg-9-1935-2012 CrossRefGoogle Scholar
Gabric, AJ, Garcia, L, Van Camp, L, Nykjaer, L, Eifler, W, Schrimpf, W. 1993. Offshore export of shelf production in the Cape Blanc (Mauritania) giant filament as derived from coastal zone color scanner imagery. Journal of Geophysical Research 98:46974712. doi: 10.1029/92jc01714.CrossRefGoogle Scholar
Green, SA, Blough, NV. 1994. Optical absorption and fluorescence properties of chromophoric dissolved organic matter in natural waters. Limnology and Oceanography 39(8):19031916. doi: 10.4319/lo.1994.39.8.1903.CrossRefGoogle Scholar
Griffith, DR, McNichol, AP, Xu, L, McLaughlin, FA, Macdonald, RW, Brown, KA, Eglinton, TI. 2012. Carbon dynamics in the western Arctic Ocean: insights from full-depth carbon isotope profiles of DIC, DOC, and POC. Biogeosciences 9. doi: 10.5194/bg-9-1217-2012.CrossRefGoogle Scholar
Haghipour, N, Ausin, B, Usman, MO, Ishikawa, N, Wacker, L, Welte, C, Ueda, K, Eglinton, TI. 2019. Compound-Specific Radiocarbon Analysis by Elemental Analyzer–Accelerator Mass Spectrometry: precision and limitations. Analytical Chemistry 91(3):20422049. doi: 10.1021/acs.analchem.8b04491.CrossRefGoogle ScholarPubMed
Hansell, DA. 2013. Recalcitrant dissolved organic carbon fractions. Annual Reviews of Marine Science 5:421448.CrossRefGoogle ScholarPubMed
Hansman, RL, Griffin, S, Watson, JT, Druffel, ERM, Ingalls, AE, Pearson, A, Aluwihare, LI. 2009. The radiocarbon signature of microorganisms in the mesopelagic ocean. Proceedings of the National Academy of Sciences 106:65136518. doi: 10.1073/Pnas.0810871106.CrossRefGoogle ScholarPubMed
Hedges, JI. 1992. A comparison of dissolved humic substances from seawater with Amazon River counterparts by 13C-NMR spectroscopy. Geochemica et Cosmochimica Acta 56(4):17531757.CrossRefGoogle Scholar
Hedges, JI, et al. 2000. The molecularly-uncharacterized component of nonliving organic matter in natural environments. Organic Geochemistry 31:945958.CrossRefGoogle Scholar
Hemingway, JD, Galy, VV, Gagnon, AR, Grant, KE, Rosengard, SZ, Soulet, G, Zigha, PK, McNichol, AP. 2017. Assessing the blank carbon contribution, isotope mass balance and kinetic isotope fractionation of the Ramped Pyrolysis/Oxidation Instrument at NOSAMS. Radiocarbon 59:179193. doi: 10.1017/RDC.2017.3.CrossRefGoogle Scholar
Hemingway, JD, Rothman, DH, Rosengard, SZ, Galy, VV. 2017. Technical note: An inverse method to relate organic carbon reactivity to isotope composition from serial oxidation. Biogeosciences 14(22):50995114. doi: 10.5194/bg-14-5099-2017.CrossRefGoogle Scholar
Hemingway, JD, Rothman, DH, Grant, KE, Rosengard, SZ, Eglinton, TI, Derry, LA, Galy, VV. 2019. Mineral protection regulates long-term global preservation of natural organic carbon. Nature 570(7760):228231. doi: 10.1038/s41586-019-1280-6.CrossRefGoogle ScholarPubMed
Hertkorn, N, Harir, M, Koch, BP, Michalke, B, Schmitt-Kopplin, P. 2013. High-field NMR spectroscopy and FTICR mass spectrometry: Powerful discovery tools for the molecular level characterization of marine dissolved organic matter. Biogeosciences 10(3):15831624. doi: 10.5194/bg-10-1583-2013.CrossRefGoogle Scholar
Hwang, J, Druffel, ERM. 2003. Lipid–like material as the source of the uncharacterized organic carbon in the ocean? Science 299:881884.CrossRefGoogle ScholarPubMed
Hwang, J, Druffel, ERM, Eglinton, TI. 2010. Widespread influence of resuspended sediments on oceanic particulate organic carbon: Insights from radiocarbon and aluminum contents in sinking particles, Global Biogeochemical Cycles 24:GB4016. doi: 10.1029/2010GB003802.CrossRefGoogle Scholar
Ingalls, A, Anderson, RF, Pearson, A. 2004. Radiocarbon dating of diatom-bound organic compounds. Marine Chemistry 92:91105.CrossRefGoogle Scholar
Ingalls, AE, Shah, SR, Hansman, RL, Aluwihare, LI, Santos, GM, Druffel, ERM, Pearson, A. 2006. Quantifying archeal community autotrophy in the mesopelagic ocean using natural radiocarbon. Proceedings of the National Academy of Sciences 103(17):64426447. doi: 10.1073/pnas.0510157103.CrossRefGoogle Scholar
Jiao, N, Herndl, GJ, Hansell, DA, Benner, R, Kattner, G, Wilhelm, SW, Kirchman, DL, Weinbauer, TL, Chen, F, Azam, F. 2010. Microbial production of recalcitrant dissolved organic matter: long-term carbon storage in the global ocean. Nature Reviews Microbiology 8:593599. doi: 10.1038/nrmicro2386.CrossRefGoogle ScholarPubMed
Karakas, G, Nowald, N, Blaas, M, Marchesiello, P, Frickenhaus, S, Schlitzer, R. 2006. High-resolution modeling of sediment erosion and particle transport across the northwest African shelf. Journal of Geophysical Research 111:C06025. doi: 10.1029/2005JC003296.CrossRefGoogle Scholar
Key, RM, Kozyr, A, Sabine, CL, Lee, K, Wanninkhof, R, Bullister, JL, et al. 2004. A global ocean carbon climatology: Results from Global Data Analysis Project (GLODAP). Global Biogeochemical Cycles 18:GB4031. doi: 10.1029/2004GB002247.CrossRefGoogle Scholar
Kieber, RJ, Hydro, LH, Seaton, PJ. 1997. Photooxidation of triglycerides and fatty acids in seawater: Implication toward the formation of marine humic substances. Limnology and Oceanography 42(6):14541462. doi: 10.4319/lo.1997.42.6.1454.CrossRefGoogle Scholar
Kusch, S, Kashiyaman, Ogawa, AN, Altabet, M, Butzin, M, Friedrich, J, Ohkouchi, N, Mollenhauer, G. 2010. Implications for chloro- and pheopigment synthesis and preservation from combined compound-specific d13C, d15N, and Δ14C analysis. Biogeosciences 7(12):41054118. doi: 10.5194/bg-7-4105-2010.CrossRefGoogle Scholar
Kusch, S, Rethemeyer, J, Hopmans, EC, Wacker, L, Mollenhauer, G. 2016. Factors influencing 14C concentrations of algal and archaeal lipids and their associated sea surface temperature proxies in the Black Sea. Geochimica et Cosmochimica Acta 188:3557. doi: 10.1016/j.gca.2016.05.025.CrossRefGoogle Scholar
Lang, SQ, Butterfield, DA, Lilley, MD, Paul Johnson, H, Hedges, JI. 2006. Dissolved organic carbon in ridge-axis and ridge-flank hydrothermal systems. Geochimica et Cosmochimica Acta 70:38303842. doi: 10.1016/j.gca.2006.04.031.CrossRefGoogle Scholar
Lechtenfeld, OJ, Hertkorn, N, Shen, Y, Witt, M, Benner, R. 2015. Marine sequestration of carbon in bacterial metabolites. Nature Communications 6(1):8. doi: 10.1038/ncomms7711.CrossRefGoogle ScholarPubMed
Lechtenfeld, OJ, Kattner, G, Flerus, R, McCallister, SL, Schmitt-Kopplin, P, Koch, BP. 2014. Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean. Geochimica et Cosmochimica Acta 126:321337. doi: 10.1016/j.gca.2013.11.009.CrossRefGoogle Scholar
Lewis, CB, Walker, BD, Druffel, ERM. 2020. Isotopic and optical heterogeneity of solid phase extracted marine dissolved organic carbon. Marine Chemistry 219:103752. doi: 10.1016/j.marchem.2020.103752 CrossRefGoogle Scholar
Lewis, CB, Walker, BD, Druffel, ERM. 2021. Constraining the cycling of global solid-phase extracted dissolved organic carbon using radiocarbon measurements, Geophysical Research Letters. doi: 10.1002/(ISSN)1944-8007.CrossRefGoogle Scholar
Li, Y, Harir, M, Lucio, M, Kanawati, B, Smirnov, K, Flerus, R, Koch, BP, Schmitt-Kopplin, P, Hertkorn, N. 2016. Proposed guidelines for solid phase extraction of Suwannee River dissolved organic matter. Analytical Chemistry 88(13):66806688. doi: 10.1021/acs.analchem.5b04501.CrossRefGoogle ScholarPubMed
Li, Y, Harir, M, Uhl, J, Kanawati, B, Lucio, M, Smirnov, KS, Koch, BP, Schmitt-Kopplin, P, Hertkorn, N. 2017. How representative are dissolved organic matter (DOM) extracts? A comprehensive study of sorbent selectivity for DOM isolation. Water Research 116:316323. doi: 10.1016/j.watres.2017.03.038.CrossRefGoogle ScholarPubMed
Loh, AN, Bauer, JE, Druffel, ERM. 2004. Variable aging and storage of dissolved organic components in the open ocean. Nature 430(7002):877881.CrossRefGoogle Scholar
McCarthy, M, Hedges, J, Benner, R. 1993. The chemical composition of dissolved organic matter in seawater. Chemical Geology 107(3–4):503507.CrossRefGoogle Scholar
McCarthy, MD, Beaupré, SR, Walker, BD, Voparil, I, Guilderson, TP, Druffel, ERM. 2011. Chemosynthetic origin of 14C-depleted dissolved organic matter in a ridge-flank hydrothermal system. Nature Geoscience 4:3236. doi: 10.1038/ngeo1015.CrossRefGoogle Scholar
McNichol, AP, Aluwihare, LI. 2007. The power of radiocarbon in biogeochemical studies of the marine carbon cycle: insights from studies of dissolved and particulate organic carbon (DOC and POC). Chem. Rev. 107:443466.CrossRefGoogle Scholar
McNichol, AP, Key, RM, Guilderson, TO. 2022. Global ocean radiocarbon programs. Radiocarbon. In press (this issue).CrossRefGoogle Scholar
Mollenhauer, G, Eglinton, TI, Ohkouchi, N, Schneider, RR, Müller, PJ, Grootes, PM, Rullkötter, J. 2003. Asynchronous alkenone and foraminifera records from the Benguela Upwelling System. Geochimica et Cosmochimica Acta 67(12):21572171.CrossRefGoogle Scholar
Mollenhauer, G, Grotheer, H, Gentz, T, Bonk, E, Hefter, J. 2021. Standard operation procedures and performance of the MICADAS radiocarbon laboratory at Alfred Wegener Institute (AWI), Germany. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 496:4551. doi: 10.1016/j.nimb.2021.03.016.CrossRefGoogle Scholar
Mollenhauer, G, Inthorn, M, Vogt, T, Zabel, M, Sinninghe Damsté, JS, Eglinton, TI. 2007. Aging of marine organic matter during cross-shelf lateral transport in the Benguela upwelling system revealed by compound-specific radiocarbon dating, Geochemistry, Geophysics, Geosystems G3 8(9):Q09004. doi: 10.1029/2007GC001603.Google Scholar
Mollenhauer, G, Kienast, M, Lamy, F, Meggers, H, Schenider, RR, Hayes, JM, Eglinton, TI. 2005. An evaluation of 14C age relationships between co-occurring foraminifera, alkenones, and total organic carbon in continental margin sediments. Paleoceanography 20:PA1016. doi: 10.1029/2004PA001103.CrossRefGoogle Scholar
Mollenhauer, G, Kusch, S, Eglinton, TI, Pearson, A. 2019. Compound-specific radiocarbon measurements. In: Kirk CJ, Bokuniewicz JH, Yager LP, editors. Encyclopedia of ocean sciences. 3rd edition. Vol. 1:235–244. Elsevier. ISBN: 978-0-12-813081-0.CrossRefGoogle Scholar
Mollenhauer, G, McManus, JF, Wagner, T, McCave, IN, Eglinton, TI. 2011. Radiocarbon and 230Th data reveal rapid redistribution and temporal changes in sediment focussing at a North Atlantic drift. Earth and Planetary Science Letters 301:373382. doi: 10.1016/j.epsl.2010.11.022.CrossRefGoogle Scholar
Mopper, K, Stubbins, A, Ritchie, JD, Bialk, HM, Hatcher, PG. 2007. Advanced instrumental approaches for characterization of marine dissolved organic matter: extraction techniques, mass spectrometry, and nuclear magnetic resonance spectroscopy. Chemical Reviews 107(2):419442. doi: 10.1021/cr050359b CrossRefGoogle ScholarPubMed
Ohkouchi, N, Eglinton, TI. 2006. Radiocarbon constraint on relict organic carbon contributions to Ross Sea sediments, Geochemistry, Geophysics, Geosystems G3 7(4):Q04012. doi: 10.1029/2005GC001097.Google Scholar
Ohkouchi, N, Eglington, TI. 2008. Compound-specific radiocarbon dating of Ross Sea sediments: a prospect for constructing chronologies in high-latitude oceanic sediments. Quaternary Geochronology 3(3):235243.CrossRefGoogle Scholar
Ohkouchi, N, Eglinton, TI, Keigwin, LD, Hayes, JM. 2002. Spatial and temporal offsets between proxy records in a sediment drift. Science 298:12241227.CrossRefGoogle Scholar
Osterholz, H, Niggemann, J, Giebel, HA, Simon, M, Dittmar, T. 2015. Inefficient microbial production of refractory dissolved organic matter in the ocean. Nature Communications 6:7422. doi: 10.1038/ncomms8422.CrossRefGoogle ScholarPubMed
Ostlund, G, Stuiver, M. 1980. GEOSECS Pacific radiocarbon. Radiocarbon 22(1):2553.CrossRefGoogle Scholar
Pendergraft, MA, Dincer, Z, Sericano, JL, Wade, TL, Kolasinski, J, Rosenheim, BE. 2013. Linking ramped pyrolysis isotope data to oil content through PAH analysis. Environmental Research Letters 8(4). doi: 10.1088/1748-9326/8/4/044038.CrossRefGoogle Scholar
Pendergraft, MA, Rosenheim, BE. 2014. Varying relative degradation rates of oil in different forms and environments revealed by ramped pyrolysis. Environmental Science, Technology 48(18):1096610974. doi: 10.1021/es501354c.CrossRefGoogle ScholarPubMed
Peuravuori, J, Pihlaja, K. 1997. Molecular size distribution and spectroscopic properties of aquatic humic substances. Analytica Chimica Acta 337(2):133149.CrossRefGoogle Scholar
Repeta, DJ, Aluwihare, LI. 2006. Radiocarbon analysis of neutral sugars in high-molecular-weight dissolved organic carbon: implications for organic carbon cycling. Limnology and Oceanography 51(1):10451053.CrossRefGoogle Scholar
Rogers, KL, Bosman, SH, Lardie-Gaylord, M, McNichol, A, Rosenheim, BE, Montoya, JP, et al. 2019. Petrocarbon evolution: ramped pyrolysis/oxidation and isotopic studies of contaminated oil sediments from the Deepwater Horizon oil spill in the Gulf of Mexico. PLoS ONE 14(2):e0212433. doi: 10.1371/journal.pone.0212433.CrossRefGoogle ScholarPubMed
Rosenheim, BE, Day, MB, Domack, E, Schrum, H, Benthien, A, Hayes, JM. 2008. Antarctic sediment chronology by programmed-temperature pyrolysis: Methodology and data treatment, Geochem. Geophys. Geosyst. 9(4):Q04005 doi: 10.1029/2007GC001816.CrossRefGoogle Scholar
Rosenheim, BE, Santoro, JA, Gunter, M, Domack, EW. 2013. Improving Antarctic sediment 14C dating using ramped pyrolysis: an example from the Hugo Island Trough. Radiocarbon 55(1):115126.CrossRefGoogle Scholar
Santos, GM, Southon, JS, Drenzek, NJ, Ziolkowski, L, Druffel, ERM, Xu, X, Zhang, D, Trumbore, SE, Eglinton, TI, Hughen, KA. 2010. Blank assessment for ultra–small radiocarbon samples: chemical extraction and separation vs. AMS. Radiocarbon 52(3):13221335.CrossRefGoogle Scholar
Santschi, PH, Guo, LD, Baskaran, M, Trumbore, S, Southon, J, Bianchi, TS, Honeyman, B, Cifuentes, L. 1995. Isotopic evidence for the contemporary origin of high-molecular-weight organic-matter in oceanic environments. Geochim. Cosmochim. Acta 59:625631.CrossRefGoogle Scholar
Schlitzer, R. 2015. Ocean data view. http://odv.awi.de Google Scholar
Shah, SR, Mollenhauer, G, Ohkouchi, N, Eglinton, TI, Pearson, A. 2008. Origins of archaeal tetraether lipids in sediments: Insights from radiocarbon analysis. Geochimica et Cosmochimica Acta 72(18):45774594.CrossRefGoogle Scholar
Shaw Walter, SR, Jaekel, U, Osterholz, H, Fisher, AT, Huber, JA, Pearson, A, et al. 2018. Microbial decomposition of marine dissolved organic matter in cool oceanic crust. Nature Geoscience 11. doi: 10.1038/s41561-018-0109-5.CrossRefGoogle Scholar
Shen, Y, Benner, R. 2018. Mixing it up in the ocean carbon cycle and the removal of refractory dissolved organic carbon. Scientific Reports. doi: 10.1038/s41598-018-20857-5.CrossRefGoogle Scholar
Simjouw, J-P, Minor, EC, Mopper, K. 2005. Isolation and characterization of estuarine dissolved organic matter: comparison of ultrafiltration and C18 solid-phase extraction techniques. Marine Chemistry 96(3–4):219235. doi: 10.1016/j.marchem.2005.01.003.CrossRefGoogle Scholar
Skoog, A, Benner, R. 1997. Aldoses in various size fractions of marine organic matter: Implications for carbon cycling. Limnology And Oceanography 42:18031813.CrossRefGoogle Scholar
Smith, KLJ, Baldwin, RJ, Williams, PM. 1992. Reconciling particulate organic carbon flux and sediment community oxygen consumption in the deep North Pacific. Nature 359:313315. doi: 10.1038/359313a0 CrossRefGoogle Scholar
Stuiver, M, Quay, PD, Ostlund, HG. 1983. Abyssal water 14C distribution and the age of the world oceans. Science 219(4586):849851. doi: 10.1126/science.219.4586.849.CrossRefGoogle Scholar
Subt, C, Fangman, KA, Wellner, JS, Rosenheim, BE. 2016. Sediment chronology in Antarctic deglacial sediments: reconciling organic carbon 14C ages to carbonate 14C ages using ramped PyrOx. The Holocene 26(2):265273. doi: 10.1177/0959683615608688.CrossRefGoogle Scholar
Sun, S, et al. 2020. 14C blank assessment in small-scale compound-specific radiocarbon analysis of lipid biomarkers and lignin phenols. Radiocarbon 62(1):207218. doi: 10.1017/RDC.2019.108.CrossRefGoogle Scholar
Van Camp, L, Nykjaer, L, Mittelstaedt, E, Schlittenhardt, P. 1991. Upwelling and boundary circulation off northwest Africa as depicted by infrared and visible satellite observations. Prog. Oceanogr. 26:357402.CrossRefGoogle Scholar
Verdugo, P, Alldredge, AL, Azam, F, Kirchmand, DL, Passowe, U, Santschif, PH. 2004. The oceanic gel phase: a bridge in the DOM–POM continuum. Marine Chemistry 92:6785. doi: 10.1016/j.marchem.2004.06.017 CrossRefGoogle Scholar
Walker, B, Beaupre, S, Guilderson, T, Druffel, ERM, McCarthy, M. 2011. Large-volume ultrafiltration for the study of radiocarbon signatures and size vs. age relationships in marine dissolved organic matter. Geochimica et Cosmochimica Acta 75:51875202.CrossRefGoogle Scholar
Walker, BD, et al. 2016a. Linked changes in marine dissolved organic carbon molecular size and radiocarbon age. Geophysical Research Letters 43:10,385–10,393.CrossRefGoogle Scholar
Walker, BD, Beaupre, SR, Guilderson, TP, McCarthy, MD, Druffel, ERM. 2016b. Pacific carbon cycling constrained by organic matter size, age and composition relationships. Nature Geoscience 9:888891.CrossRefGoogle Scholar
Walker, BD, Beaupré, SR, Griffin, S, Druffel, ERM. 2019. UV photochemical oxidation and extraction of marine dissolved organic carbon at UC Irvine: status, surprises, and methodological recommendations. Radiocarbon 61:16031618. doi: 10.1017/RDC.2019.9.CrossRefGoogle Scholar
Wakeham, SG, Lee, C, Hedges, JI, Hermes, PJ, Peterson, ML. 1997. Fate of major biochemicals in water column particles and sediments of the central Equatorial Pacific Ocean. Geochimica et Cosmochimica Acta 61:53635369.CrossRefGoogle Scholar
Wang, X-C, Druffel, ERM, Lee, C. 1996. Evidence for sorption of old carbon to particulate material in the deep sea from radiocarbon in organic compound classes. Geophysical Research Letters 23:35833586.CrossRefGoogle Scholar
Wang, X-C, Druffel, ERM, Lee, C. 1998. Radiocarbon studies of organic compound classes in plankton and sediment of the NE Pacific Ocean. Geochim Cosmochim Acta 62:13651378.CrossRefGoogle Scholar
Wei, B, Mollenhauer, G, Hefter, J, Kusch, S, Grotheer, H, Schefuß, E, Jia, G. 2021. The nature, timescale, and efficiency of riverine export of terrestrial organic carbon in the (sub)tropics: insights at the molecular level from the Pearl River and adjacent coastal sea. Earth and Planetary Science Letters 565:116934. doi: 10.1016/j.epsl.2021.116934.CrossRefGoogle Scholar
Williams, PM, Druffel, ERM. 1987. Radiocarbon in dissolved organic carbon in the central North Pacific Ocean. Nature 330:246248.CrossRefGoogle Scholar
Williams, PM, Oeschger, H, Kinney, P. 1969. Natural radiocarbon activity of the dissolved organic carbon in the Northeast Pacific. Nature 224:256258.CrossRefGoogle Scholar
Williams, PM, McGowan, JA, Stuiver, M. 1970. Bomb carbon-14 in deep sea organisms. Nature 227(5256):375376.CrossRefGoogle Scholar
Williams, PM, Linick, TW. 1975. Cycling of organic carbon in the ocean: Use of naturally occurring radiocarbon as a long and short term tracer. IAEA, Vienna. Symposium on isotope ratios as pollutant source and behavior indicators. Vienna, Austria, 18 Nov 1974. IAEA-SM--191/26.Google Scholar
Van Camp, L, Nykjaer, L, Mittelstaedt, E, Schlittenhardt, P. 1991. Upwelling and boundary circulation off Northwest Africa as depicted by infrared and visible satellite observations. Progress in Oceanography 26:357402. doi: 10.1016/0079-6611(91)90012-B.CrossRefGoogle Scholar
Xue, Y, Tiantian, GE, Xuchen, W. 2015. An effective method of UV-oxidation of dissolved organic carbon in natural waters for radiocarbon analysis by accelerator mass spectrometry. J. Ocean Univ. China. doi: 10.1007/s11802-015-2935-z.CrossRefGoogle Scholar
Zigah, PK, McNichol, AP, Xu, L, Johnson, C, Santinelli, C, Karl, DM, Repeta, DJ. 2017. Allochthonous sources and dynamic cycling of ocean dissolved organic carbon revealed by carbon isotopes. Geophysical Research Letters 44:24072415. doi: 10.1002/2016GL071348.CrossRefGoogle Scholar
Ziolkowski, LA, Druffel, ERM. 2010. Aged black carbon identified in marine dissolved organic carbon, Geophysical Research Letters 37:L16601. doi: 10.1029/2010GL043963.CrossRefGoogle Scholar
Figure 0

Figure 1 Size ranges of organic matter constituents in seawater and their relationships to the operationally defined pools of DOC (gray background) and POC (white background). (After Beaupré 2019.)

Figure 1

Figure 2 Peter M. Williams on the R/V Melville with a Gerard barrel, a 270 L steel vessel used to collect large volume seawater samples. The barrel is covered to protect it from particles coming mostly from the ship’s stack. Circa 1989. (Photo by Ellen Druffel.)

Figure 2

Figure 3 DOC Δ14C values (‰) of water samples (indicated by black dots) collected from seven stations along 150°W on the P16N cruise in 2015 (Druffel et al. 2019), one station on the P06 cruise (stn 130, 32.5°S 144.7°W) in 2010 (Druffel and Griffin 2015) and one station from the Southern Ocean (SOce; 54.0°S 176.0°W) in 1995 (Druffel and Bauer 2000) using Ocean Data View (Schlitzer 2015). (After Druffel et al. 2019.)

Figure 3

Figure 4 Size-Δ14C and elemental composition (C:N) relationships of DOC in the Pacific Ocean. (A) DOC Δ14C as a function of ultrafiltration concentration factor (CF) for the surface and deep Pacific ocean. Relationships between the C:N content of size-fractionated marine organic matter (B,D) and Δ14C (C,E) in the surface and deep Pacific. (Figures adapted from Walker et al. 2011 and Walker et al. 2016b.)

Figure 4

Figure 5 Δ14C of organic compound classes (THAA, open triangles; TCHO, open circles; lipids, solid triangles; acid insoluble fraction, solid circles). The range of DIC 14C values in surface waters at Stn M ranged from about 40 to 70‰. (After Hwang and Druffel 2003.)

Figure 5

Figure 6 Schematic of the RPO reactor (left) and the results (right) from a POC sample collected from the Narayani River, Nepal (Hemingway 2017). The top half of the quartz reactor heats the sample while the gases are swept into a CO2 analyzer before passing through a cryogenic trap. The gray shaded region is the thermogram, the variation in ppm CO2 vs. temperature (units not shown), white circles show the δ13C values, and bars show the fraction modern values for each temperature interval (Hemingway 2017). The data show that the high temperature, more refractory, fractions have dramatically less 14C, i.e., are older than the lower temperature fractions.

Figure 6

Figure 7 Preliminary F14C POCsusp values of samples collected along a shelf to open ocean transect off Cape Blanc (NW Africa, ∼21°N) collected in November 2018 during research cruise MSM79 onboard the German R/V Maria S. Merian and processed according to the EA–AMS method described above (H. Grotheer unpublished data). Red dots indicate sampling depths. (The map was generated using Ocean Data View; Schlitzer 2015).