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Ice rafting: An indication of glaciation?

Published online by Cambridge University Press:  20 January 2017

Erk Reimnitz  and
Edward W. Kempema
Erk Reimnitz
Affiliation:
Branch of Pacific Marine Geology, U.S. Geological Survey, Menlo Park California 94025. U.S.A
Edward W. Kempema
Affiliation:
Branch of Pacific Marine Geology, U.S. Geological Survey, Menlo Park California 94025. U.S.A
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Abstract

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Type
Correspondence
Information
Journal of Glaciology , Volume 34 , Issue 117 , 1988 , pp. 254 - 255
Copyright
Copyright © International Glaciological Society 1988

The Editor, Journal of Glaciology

SIR,

the Cenozoic climatic and oceanographic history of the Arctic Ocean basin and surrounding coastal areas depends largely on our ability to interpret the sedimentary record that accumulated on its floor. This ability relies on the adequacy of models for sediment supply used in the interpretation of sediment cores. There is general agreement that much of the sediment was supplied by ice rafting, and that periods of more or less sea-ice cover alternated with periods of drifting icebergs (both from glaciers and from ice shelves), but how these cycles relate to timing of glaciations on surrounding continents is under debate (Reference Herman and HopkinsHerman and Hopkins, 1980; Reference Hopkins and HermanHopkins and Herman, 1981; Reference SpjeldnaesSpjeldnaes, 1981; Reference Clark, Hanson and MolniaClark and Hanson, 1983; Reference Minicucci, Clark and MolniaMinicucci and Clark, 1983). As Reference SpjeldnaesSpjeldnaes (1981), we question the models of sediment supply and the use of two particular criteria to recognize the contribution by icebergs as opposed to that by sea ice: coarse-sediment texture and presence of shallow-water micro-faunas. Until recently, nobody had looked into sea ice for its sediment texture and content, and the contribution by sea ice therefore has been misjudged.

There seems little doubt among most workers that drop stones represent drifting icebergs. Some sedimentologists (Reference Clark, Hanson and MolniaClark and Hanson, 1983; Reference Minicucci, Clark and MolniaMinicucci and Clark, 1983), however, believe that more subtle textural criteria give clues of the two different ice types and transport mechanisms under question here, and seem convinced that sand in ice-rafted deposits records drifting icebergs. If sand-size particles include shallow-water foraminifers, these are thought to have been “gouged up” by glacial ice and rafted to the deep sea (Reference Herman and HopkinsHerman and Hopkins, 1980). Some even go so far as to differentiate the mud fraction of cores into four types, where dominance of silt reflects icebergs and dominance of clay a sea-ice cover (Reference Clark, Hanson and MolniaClark and Hanson, 1983; Reference Minicucci, Clark and MolniaMinicucci and Clark, 1983). Sand/silt/clay ratios extracted from their published histograms are plotted on a ternary diagram in Figure 1.

Recent studies in the Beaufort Sea indicate that turbid (sediment-bearing) sea ice can transport considerable quantities of sediment indistinguishable by texture and micro-fossils from that presumed to record glacial climate in the deep basin. The mechanism of sediment entrainment leading to turbid ice is very different from any of the classic entrainment mechanisms (e.g. Reference KindleKindle, 1924), which in fact are of little importance for Beaufort Sea ice rafting (Reference Reimnitz, Barnes, Reed and SaterReimnitz and Barnes, 1974). Turbid ice forms from rising frazil-ice crystals originally suspended in supercooled water during freeze-up storms (Reference Barnes, Reimnitz and FoxBarnes and others, 1982). Frazil ice scavenges particulate matter and plankton from the water column and directly from the sea floor (Reference Osterkamp, Gosink, Barnes, Schell and ReimnitzOsterkamp and Gosink, 1984; Reference Reimnitz and KempemaReimnitz and Kempema, 1987; Reference Reimnitz, Kempema and BarnesReimnitz and others, 1987; Kempema and others, unpublished). The resulting ice canopy contains finely disseminated sediment in those upper parts that formed from frazil slush after the storm subsided (Fig. 2). Turbid ice generally includes small patches of coarser material, such as granules, pebbles, mollusk shells, plant material, kelp, sticks, etc., raised to the surface by masses of anchor ice (ice that accretes on a substrate submerged in either quiet or turbulent supercooled water and that remains attached to the substrate) (Reference Reimnitz, Kempema and BarnesReimnitz and others, 1987). Lastly, this ice also carries neritic ostracodes and foraminifers (Reference BriggsBriggs, 1983, unpublished). Sand/silt/clay ratios for sediment samples, both from newly formed frazil ice at freeze-up and from the surface of turbid ice as concentrated by summer melting (Kempema and others, unpublished), are plotted in Figure 1. The sand fraction of two composite anchor-ice samples collected at the time of freeze-up plot at the very top of Figure 1. Clasts of cobble size do occur in turbid ice but representative samples of the very coarse components are not available. This floating seasonal ice canopy, although having never physically contacted the sea floor as massive ice, may carry over 1000 m3/km2 of sediment, or 16 times more than the annual sediment supply by rivers feeding the same area (Reference Reimnitz and KempemaReimnitz and Kempema, 1987). In some winters, the turbid ice canopy extends as a contiguous band from shore to the 20 or 30 m isobath, and in patches across the entire shelf (Reference Barnes, Reimnitz and FoxBarnes and others, 1982; Reference Reimnitz and KempemaReimnitz and Kempema, 1987; Kempema and others, unpublished).

Fig. 1. Sand/silt/clay ratios attributed by Reference Clark, Hanson and MolniaClark and Hanson (1983) and Reference Minicucci, Clark and MolniaMinicucci and Clark (1983) to four dominant modes of deposition under the influence of ice, and those actually measured in floating sea ice (Kempema and others, unpublished). The two coarse samples at the very top become mixed with finer fractions in the drifting/settling process.

Fig. 2. Block of seasonal fast ice. including turbid ice formed from sediment-bearing frazil during a few days at freeze-up. (Photograph by G. Meltveldt, University of Alaska.).

The texture of mud carried by turbid sea ice today is indistinguishable from the mud texture of deep Arctic Ocean cores attributed to glacial conditions (Fig. 1). The patchy coarse components entrained into sea ice by anchor ice become homogenized in the common settling process after release from drifting floes into deep water. Therefore, not even a sandy texture and drop stones can be used as sole criteria for glacial conditions. Also, drifting sea ice imparts striations on rocks in the coastal zone (Reference DionneDionne, 1985), on boulders embedded in firm mud at 10 m water depth (Reimnitz and others, unpublished), may shatter them (Reference Josenhans, Barrie, Woodworth–Lynas and ParrottJosenhans and others, 1985), and possibly produce fracture faces on quartz grains (Reference Hodel, Reimnitz and BarnesHodel and others, 1988). Most damaging to the theory that certain surface features and shapes of drop stones are unique to glacial conditions is the fact that pebbles lifted by anchor ice today, and then are rafted by sea ice, include Flaxman lithologies (Reference Hopkins and HermanHopkins and Herman, 1981) with “glacial striations”. Lastly, icebergs impacting the bottom produce gouges but are highly unlikely to entrain shallow-water organisms (Kempema and others, unpublished). However, frazil ice scavenging suspended matter from supercooled water is an effective mechanism to enrich sea ice with planktonic foraminifers and diatoms, as found in the Antarctic (Reference Garrison, Ackley and BuckGarrison and others, 1983; Reference Dieckmann, Rohardt, Helmer and KipfstuhlDieckmann and others, 1986; Reference Spindler and DieckmannSpindler and Dieckmann, 1986; Reference SullivanSullivan and others, 1986), and confirmed by our unpublished laboratory experiments. Similar mechanisms lift shallow shelf micro-organisms off the bottom and incorporate them into the sea ice (Reference BriggsBriggs, 1983, unpublished; Reference Reimnitz, Kempema and BarnesReimnitz and others, 1987; Kempema and others, unpublished). The occurrence of such micro-fossils, along with plant debris and mollusks, in cores to over 1000 m water depths on the continental slope off northern Alaska (personal communication from E. Brouwers, 1983) suggest modern sea-ice transport of shelf sediments to the deep ocean. Ice-rafted components, such as drop stones, sand, and shallow-water benthic organisms in sediment cores of the Arctic Basin therefore are not necessarily an indication of glaciation on the surrounding continents. They are incorporated, moved, and released more effectively by short-lived and more quickly recycled sea ice.

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Figure 0

Fig. 1. Sand/silt/clay ratios attributed by Clark and Hanson (1983) and Minicucci and Clark (1983) to four dominant modes of deposition under the influence of ice, and those actually measured in floating sea ice (Kempema and others, unpublished). The two coarse samples at the very top become mixed with finer fractions in the drifting/settling process.

Figure 1

Fig. 2. Block of seasonal fast ice. including turbid ice formed from sediment-bearing frazil during a few days at freeze-up. (Photograph by G. Meltveldt, University of Alaska.).