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Dimensions of Radiocarbon Variability within Sedimentary Organic Matter

Published online by Cambridge University Press:  30 May 2018

Rui Bao*
Geological Institute, ETH Zurich, Zurich, Switzerland National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA Present address: Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
Ann P McNichol
National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA
Cameron P McIntyre
Geological Institute, ETH Zurich, Zurich, Switzerland Laboratory for Ion Beam Physics, ETH Zurich, Zurich, Switzerland Scottish Universities Environmental Research Centre, Glasgow, United Kindom
Li Xu
National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA
Timothy I Eglinton
Geological Institute, ETH Zurich, Zurich, Switzerland
*Corresponding author. Email:


Organic carbon (OC) radiocarbon (14C) signatures in marine surface sediments are highly variable and the causes of this heterogeneity remain ambiguous. Here, we present results from a detailed 14C-based investigation of an Arabian Sea sediment, including measurements on organic matter (OM) in bulk sediment, specific grain size fractions, and OC decomposition products from ramped-pyrolysis-oxidation (RPO). Our results show that 14C ages of OM increase with increasing grain size, suggesting that grain size is an important factor controlling the 14C heterogeneity in marine sediments. Analysis of RPO decomposition products from different grain size fractions reveals an overall increase in age of corresponding thermal fractions from finer to coarser fractions. We suggest that hydrodynamic properties of sediment grains exert the important control on the 14C age distribution of OM among grain size fractions. We propose a conceptual model to account for this dimensionality in 14C variability that invokes two predominant modes of OM preservation within different grain size fractions of Arabian Sea sediment: finer (<63 µm) fractions are influenced by OM-mineral grain aggregation processes, giving rise to relatively uniform 14C ages, whereas OM preserved in coarser (>63 µm) fractions includes materials encapsulated within microfossils and/or entrained fossil (14C-depleted) OC hosted in detrital mineral grains. Our findings highlight the value of RPO for assessment of 14C age variability in sedimentary OC, and for assessing mechanisms of OM preservation in aquatic sediments.

Research Article
© 2018 by the Arizona Board of Regents on behalf of the University of Arizona 

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