Relict sea-floor ploughmarks record deep-keeled Antarctic icebergs to 45°S on the Argentine margin
Highlights
► Icebergs up to 500 m in submarine keel depth drifted and grounded at 45°S. ► Ploughmarks 32 km long, 685 m wide and 20 m deep on the Argentine continental slope. ► Most northerly morphological evidence of giant icebergs in the southern Atlantic. ► Provenance from fast-flowing ice streams of Western Antarctica. ► General currents pattern retained between Last Glacial Maximum and modern conditions.
Introduction
Icebergs are produced where glacier ice and ice sheets, sometimes including attendant ice shelves, reach the sea. The highest flux of modern icebergs is from fast-flowing ice streams and outlet glaciers draining huge interior basins of the Antarctic and Greenland ice sheets (e.g. Bentley, 1987, Rignot and Kanagaratnam, 2006), and from collapse of major ice shelves (e.g. Silva et al., 2006). Evidence of iceberg occurrence in the marine-geological record is an important indicator of the former presence of extensive ice sheets that reached to sea level (e.g. Dowdeswell et al., 2007). Ploughmarks form when the submarine keels of icebergs ground on the sea floor (Woodworth-Lynas et al., 1991). The resistance of the soft, sedimentary bed is often insufficient to halt drifting icebergs, and iceberg keels cut through the sea floor to form ploughmarks that are often linear to curvilinear and sometimes exhibit abrupt changes in direction (Dowdeswell et al., 2007, Rebesco et al., 2010). The continental shelves surrounding Antarctica and Greenland are heavily disturbed by icebergs to a depth of about 400 to 500 m, with evidence of well-preserved streamlined subglacial landforms indicative of past ice flow found mainly at greater shelf depths because few icebergs have keels deeper than this (e.g. Canals et al., 2000, Anderson et al., 2002, Ó Cofaigh et al., 2002, Evans et al., 2006). However, Dowdeswell and Bamber (2007) suggested that Antarctic icebergs disturb the sea floor to water depths of 500 to 600 m, and in the Amundsen Sea sector there are scours to depths of 720 m (Larter et al., 2009). In the Northern Hemisphere, ploughmarks have been observed to almost 1000 m offshore West Greenland (Kuijpers et al., 2007) and to between 650 and 700 m on the Canadian margin (Piper and Pereira, 1992).
Today, few icebergs are found in the relatively warm waters north of the Antarctic Polar Front at about 60°S, and have been observed only rarely north of the Falkland Islands (e.g. Long et al., 2002, Silva et al., 2006). Here we present sea-floor morphological evidence to demonstrate, for the first time, that icebergs up to about 500 m in submarine keel-depth drifted and grounded as far north as 45°S on the Argentine continental slope of eastern South America (Fig. 1).
Marine-geophysical data were acquired from the continental shelf and upper slope off Argentina during cruises of the Spanish research vessel Miguel Oliver in 2007 and 2008 between approximately 44°40′S to 47°50′S and 59°W to 60°30′W (Fig. 1). The area is part of the terraced slope that developed in the middle Miocene between Rio de la Plata and the Falkland Islands (Hernández-Molina et al., 2009). Navigation data were acquired using differential GPS and a Seapath inertial navigation system. A Kongsberg-Simrad EM 302 multibeam swath-bathymetry system was used to acquire about 34,000 km2 of sea-floor bathymetric and morphological data, which were subsequently gridded at a horizontal resolution of 25 m and a vertical accuracy of about 0.4 to 1.5 m depending on the depth. Shallow sea-bed stratigraphy was obtained from a TOpographic PArametric Sonar (TOPAS) PS18 sub-bottom profiler operating at a centre frequency of 3.5 kHz.
Section snippets
Location, morphology and orientation of iceberg ploughmarks
The study area comprises a relatively flat continental shelf (< 0.5°), with a well-defined shelf edge at about 150 m in water depth (Fig. 2A). To the east, the uppermost slope is relatively steep at between about 2° and 5°, with lower gradients of < 1° beyond this. The sediments on the slope are part of a contourite depositional system built up since the Eocene (Hernández-Molina et al., 2009). The slope is dissected by seven large, branching and steep-sided (> 10°) canyon systems up to about 500 m
Timing of iceberg occurrence
There are no dates on sediment cores from the study area. Our many TOPAS profiles, with a resolution of 0.3 m and recorded simultaneously with multibean data, demonstrate that there has been very little or no sedimentation since the formation of the ploughmarks. The landforms are also well-preserved and clearly defined on sea-floor imagery (Fig. 2, Fig. 3). We infer, therefore, that the ploughmarks were produced the last time relatively cold currents allowed icebergs to drift northwards to the
Sources of icebergs
The most likely source for icebergs that ploughed the sea floor east of South America to 45°S was the full-glacial ice sheet that reached the shelf edge along the Amundsen–Bellingshausen Sea and western Antarctic Peninsula margin at the LGM (Fig. 1). This is because of both the eastward flow direction of the Antarctic Circumpolar Current (ACC), together with the northward-flowing Falklands/Malvinas Current (Fig. 1), and the presence of major ice streams draining the ice sheet (e.g. Canals et
Iceberg drift tracks and ocean currents
Once icebergs are calved, bathymetry and ocean currents are important controls on their drift tracks, and wind also plays a role in iceberg dispersal. The drift tracks of icebergs, derived from ice streams of the full-glacial West Antarctic and Antarctic Peninsula ice sheets (Fig. 1), to the southern Argentine margin are also constrained severely by bathymetry. Water depths south of the Falkland Islands and across the passage between the Falkland Islands and southernmost Argentina reach less
Conclusions
Imaging of sea-floor ploughmarks has demonstrated, for the first time, that deep-keeled icebergs drifted and grounded as far north as 45°S offshore of Argentina at the LGM (Fig. 1). The bergs were probably tabular in form and up to more than 500 m thick. These mega-bergs were most likely derived from fast-flowing ice streams draining the WAIS and Antarctic Peninsula. They drifted several thousand kilometres in ocean currents similar in pattern to those of today, but probably several degrees
Acknowledgements
This project was funded by the Spanish General Secretariat of the Sea and the Spanish Oceanographic Institute (IEO). Spanish Antarctic research is funded by the National R&D Plan. We thank the officers, crew and operational personnel of the RV Miguel Oliver. The authors thank two anonymous referees and the Editor, D.J.W. Piper, for helpful comments that have contributed to improving this paper.
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