Seafloor linear and curvilinear erosive features on polar and subpolar continental margins are generally interpreted as ploughmarks left by the keels of grounded icebergs. A large number of ploughmarks made by icebergs with keel depths of up to 500 m have been observed on the South Patagonian continental margin as far north as 44°50′ S. The ploughmarks show a variety of forms and are the most northerly such features reported in the South Atlantic Ocean (López-Martínez et al. 2011).
Description
Over 2500 individual linear and curvilinear seafloor landforms occur on the South Patagonian margin at modern water depths of 135–646 m (Fig. 1); the great majority are found at depths of 250–550 m. These landforms are predominantly orientated from south to north. Their individual lengths range over c. 0.3–32 km, with a mean of almost 2 km. Ploughmark widths are 73–685 m, with depths of 10–25 m and an average of about 15 m. In plan view, some cross-cutting relationships are also apparent (Fig. 1b–d). Individual ploughmarks on the South Patagonian margin show a variety of shapes and shape-associations when viewed in detail (Fig. 1). In some areas they consist of sets of parallel linear grooves (Fig. 1c). Some also display chevron shapes and sometimes have subrounded pits, often in connection with direction changes.
Giant ploughmarks on the South Patagonian margin. (a) Location of study area (red box; map from GEBCO_08). (b) Bathymetric image of ploughmarks on the outer shelf and uppermost continental slope. Acquisition system Kongsberg EM302. Frequency 30 kHz. Grid-cell size 25 m. (c, d) Sets of sub-parallel linear and chevron-shaped ploughmarks. (e) Ploughmarks and location of seismic profile. (f) Sub-bottom profile showing truncation of acoustically laminated sediments by an iceberg keel. VE×50. Acquisition system Kongsberg TOPAS PS18. Centre frequency 3.5 kHz. (g) Cross-profiles of parallel ploughmarks. VE×30 (x–x′) and×15 (y–y′).
Shallow acoustic stratigraphy shows that the features are erosional (Fig. 1f), cutting the underlying acoustically layered sediments of the margin. Some features also have berms along their length that rise slightly above the mean level of the surrounding seafloor. There is a thin veneer (<1 m) of acoustically homogeneous sediment draping the landforms in some areas.
Interpretation
The landforms described above are interpreted as iceberg ploughmarks, formed by the grounding of keels of icebergs that drifted northwards across the study area. They are similar in morphology and dimensions to both modern and relict iceberg ploughmarks observed off Antarctica and in Arctic waters (e.g. Dowdeswell et al. 1993; O'Brien et al. 1997; Metz et al. 2008). Considering the water depths in which these ploughmarks occur today, and the global sea-level lowering of about 120 m at the Last Glacial Maximum, we calculate a maximum iceberg keel depth to be over 500 m.
Parallel linear grooves are interpreted as ploughmarks produced by a single iceberg with a multi-keeled base. The largest of these landforms were produced by icebergs with three keels, leading to grooves totalling about 2 km wide (including inter-groove spacing) and up to 16 m deep in a water depth of 465 m. This implies a minimum width of 2 km for the iceberg base, suggesting that the icebergs were very large in width and therefore probably tabular. The chevron-shaped ploughmarks imply abrupt changes in drift direction of the parent icebergs that may be linked to grounding and ungrounding events (Fig. 1b–d). In most cases the new orientation is towards deeper waters, which is the most usual way for an iceberg to become ungrounded. Two of the deepest ploughmarks in the study area at 575 m water depth, which are chevron-shaped, show grounding pits about 10 m deep at their shallowest points (Fig. 1e, f). The preservation of the ploughmarks on the seafloor also indicates that very little or no sediment accumulation has occurred since their formation.
The ploughmarks probably formed during a time when cold currents allowed icebergs to drift northwards to reach the Patagonian margin, probably during the last glacial period about 20 000 years ago (López-Martínez et al. 2011). Given the inferred iceberg dimensions and the direction of the iceberg-transporting ocean current (which appear to have remained the same from the last glacial to the present interglacial), the icebergs that produced the ploughmarks are interpreted to be sourced from fast-flowing ice streams draining the Antarctic Peninsula (e.g. Canals et al. 2000; Ó Cofaigh et al. 2005; Graham et al. 2010). Iceberg provenance from the Patagonian Ice Cap or the subantarctic archipelagos is unlikely, given the bathymetry of the shelves in these areas and the limited extent of their ice cover during the Last Glacial Maximum.
After drifting NE in the Antarctic Circumpolar Current, the icebergs were transported northwards to the South Patagonian margin by the Falklands/Malvinas Current for a total distance of 2000–4000 km. The waters north of the Falkland Islands and along the South Patagonian margin were probably several degrees colder than today, which prevented rapid iceberg melting and deterioration (López-Martínez et al. 2011).
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
References
Giant ploughmarks on the South Patagonian continental margin produced by Antarctic icebergs
