Iceberg fragments recovered from the sea floor near Swift Glacier, Antarctica, contained sufficient sediment to sink the ice. Sediment concentrations in the samples would have caused them to settle at 0.13 to 0.35 m/s through the water column. Impact with the sea floor would significantly turbate soft sediments. Unlike sediment dumped from icebergs, the stratigraphy of the frozen sediments created by glacial processes may be preserved in the marine sedimentary record after melting of the ice. Negatively buoyant berg fragments may be common in polar regions, and when driven by currents may scour the sea floor up and down slopes unlike floating ice.
Des fragments d’icebergs recueillis sur le fond océanique, près du glacier de Swift, en Antarctique, contenaient suffisamment de sédiments pour couler à une vitesse de 0,13 à 0,35 m/s. La collision de tels fragments avec le plancher marin entraînerait un brassage important des sédiments mous. Au contraire de celle de sédiments délestés par les icebergs, la stratigraphie de ces sédiments gelés résultant de processus glaciaires peut être préservée au sein des dépôts marins après la fonte des fragments de glace dans lesquels ils sont emprisonnés. Ces fragments, dont la densité est supérieure à celle de l’eau, pourraient être communs dans les régions polaires et causer, sous l’action des courants, un labourage ascendant et descendant des pentes des fonds marins, contrairement aux glaces flottantes.
Corps de l’article
Rafting of terrestrial sediment by sea ice and icebergs has long been documented as an important glacimarine process in arctic (Bischof, 2000) and antarctic waters (Anderson et al., 1980). Sediment is actively incorporated in ice by basal freezing of sea ice to sediment in the nearshore zone (Gilbert, 1983), by frazil and anchor ice scavenging from the water column and sea floor (Reimnitz et al., 1990; Smedsrud, 2002), or into glacier ice by basal processes, especially associated with freezing (Iverson, 1993). Passive loading onto sea ice occurs associated with tidal (Gilbert, 1990), colluvial, fluvial (Reimnitz and Bruder, 1972) and aeolian (Gilbert, 1983) processes, and onto glacier ice principally by colluvial processes on the lateral valley sides (Small, 1987).
Although it is known that the loads of these sediments may be large, there is very little evidence whether loads sufficient to increase the bulk density of the ice and sediment to greater than that of sea water occur commonly (cf. Goldschmidt, 1994), and of the significance of this in the character of the glacimarine sedimentary record.
The concentration, C, of sediment necessary to make the ice neutrally buoyant at the surface is
where ρ is the density of seawater (w), ice (i) and sediment (s), respectively. As little as 170 to 200 g/L of sediment in the ice may be sufficient to render the ice negatively buoyant in seawater (Gilbert, 1990). Sediment incorporated into the ice either at the time of formation or subsequently, by melting and refreezing, will be more effective than sediment passively loaded on the ice surface, because the latter is likely to be washed from the ice as it submerges, allowing the ice to re-float. Variable concentration through the ice offers a means to raft frozen-in sediment to sea and subsequently allow a portion to sink due to breaking off of relatively dirtier ice, or by melting. This is especially so if the ice is oriented with the area of higher concentration submerged in cold sea water while the cleaner, less dense region is exposed to warmer air and solar radiation.
Antarctic Sediment-Laiden Ice
Grab samples from the sea floor in an embayment in front of Swift Glacier, James Ross Island, western Weddell Sea (Fig. 1) collected during a U.S. Antarctic Programme cruise on RVIP N.B. Palmer in January 2002 provide the first documentation of iceberg fragments submerged by their sediment content. This is a region that has experienced rapid climate warming (Vaughan et al., 2001) and associated disintegration of the Larsen A Ice Shelf, principally in January 1995 (Scambos et al., 2001), and the Larsen B Ice Shelf in February 2002. Bergs from the Swift Glacier were heavily laiden as a result of the development of large medial moraines (Fig. 1) associated with rapid erosion of the weakly indurated and heavily weathered upper Cretaceous sedimentary and igneous volcanic rock of James Ross Island.
The sea water in the vicinity of Swift Glacier consisted of four layers on January 3, 2002 (mid summer): (1) from 0-15 m depth, temperature increased from -0.58 to -0.39 °C, salinity decreased from 33.25 to 33.67 ‰, and density assessed from the UNESCO International Equation of State for Seawater (IES80) increased from 1026.72 to 1027.06 kg/m3; (2) by 60 m depth temperature decreased to -0.75 °C, salinity increased to 33.865 ‰ and density increased to 1027.25 kg/m3; (3) by 120 m depth temperature decreased to -1.69 °C, salinity increased to 34.43 ‰, and density increased to 1027.77 kg/m3: and (4) by 185 m (the sea floor), temperature decreased slightly to -1.83 °C, salinity increased slightly to 34.47 ‰ and density increased slightly to 1027.84 kg/m3.
In grab sample 42 (Fig. 1), collected in 23 m of water, were two fragments of sediment-laiden ice. One (Fig. 2a) contained a concentration of banded sediment of 396 g/L (57 % gravel, 23 % sand, and 20 % silt and clay size by weight), generating a negative buoyancy in sea water of about 134 g/L. The other (Fig. 2b) contained a single large fragment (672 g) of vesicular basalt for a concentration of sediment of 795 g/L, and a negative buoyancy in sea water of about 381 g/L. In grab sample 41 a fragment of sediment-laiden ice had a density of 1 470 g/m3. Compare these values with concentrations of up to 57 g/L in arctic sea ice (Nürnberg et al., 1994) and up to 68 g/L in Antarctic icebergs (Anderson et al, 1980).
According to the nomograph presented by Deitrich (1982), these fragments would have fallen through sea water described above at about 0.13, 0.27 and 0.35 m/s, respectively, assuming they were then as they were when recovered. Thus, they may have struck the sea floor with sufficient force to re-suspend a cloud of the highly under-consolidated, rapidly deposited, fine-grained sediment that dominates the sea floor in this region, and they may have become partially buried in this material.
Rafting over large areas of polar oceans and delivery to the seafloor of frozen sediment may be more widespread than previously understood. The implications for glacimarine sedimentation are significant. Sediment is delivered to the sea floor as it was incorporated into the ice, significantly preserving sedimentary structures such as foliation, lamination and layering. It may be encapsulated in fine hemipelagic sediment during impact or as deposited subsequently but before melting occurs, thus preserving the sedimentary structures (Gilbert, 1990). Because these fragments are slightly to moderately negatively buoyant, some may not be implanted in the sediment, especially on relatively hard sea floors. Thus, they may be easily moved across the sea floor by currents, leaving furrows that go up and down slopes (Josenhans and Woodworth-Lynas, 1988) unlike scours created by floating ice which only occur at and just less than depths corresponding to the draft of the ice keel. The samples we recovered were very small (limited to several decimetres by the size of the grab sampler) but it is likely that much larger, negatively buoyant bergs and berg fragments occur in the vicinity of calving glaciers.
Funding was provided by the Natural Sciences and Engineering Research Council of Canada and by the U.S. National Science Foundation, Office of Polar Programmes (OPP-0003060). The diligence and skill of captain, officers, and crew of RVIP N.B. Palmer made the work possible. The first author thanks Foreign Affairs Canada and the University of Copenhagen, Institute of Geography, for the opportunities provided by a research chair during which this paper was written. We also thank Michel Parent who reviewed the manuscript.
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