Episodes of Early Pleistocene West Antarctic Ice Sheet Retreat Recorded by Iceberg Alley Sediments

by Selleckchem | Jun 16 2023

Ice loss in the Southern Hemisphere has been greatest over the past 30 years in West Antarctica. The high sensitivity of this region to climate change has motivated geologists to examine marine sedimentary records for evidence of past episodes of West Antarctic Ice Sheet (WAIS) instability. Sediments accumulating in the Scotia Sea are useful to examine for this purpose because they receive iceberg-rafted debris (IBRD) sourced from the Pacific- and Atlantic-facing sectors of West Antarctica. Here we report on the sedimentology and provenance of the oldest of three cm-scale coarse-grained layers recovered from this sea at International Ocean Discovery Program Site U1538. These layers are preserved in opal-rich sediments deposited ∼1.2 Ma during a relatively warm regional climate. Our microCT-based analysis of the layer's in-situ fabric confirms its ice-rafted origin. We further infer that it is the product of an intense but short-lived episode of IBRD deposition. Based on the petrography of its sand fraction and the Phanerozoic 40Ar/39Ar ages of hornblende and mica it contains, we conclude that the IBRD it contains was likely sourced from the Weddell Sea and/or Amundsen Sea embayment(s) of West Antarctica. We attribute the high concentrations of IBRD in these layers to "dirty" icebergs calved from the WAIS following its retreat inland from its modern grounding line. These layers also sit at the top of a ∼366-m thick Pliocene and early Pleistocene sequence that is much more dropstone-rich than its overlying sediments. We speculate this fact may reflect that WAIS mass-balance was highly dynamic during the ∼41-kyr (inter)glacial world.

Comments:

The passage describes a study conducted in the Scotia Sea to investigate the sedimentary records and provenance of ice-rafted debris (IBRD) found in the region. The researchers focused on a specific cm-scale coarse-grained layer that was preserved in opal-rich sediments deposited approximately 1.2 million years ago during a relatively warm regional climate.

The study utilized microCT-based analysis to confirm that the layer had an ice-rafted origin, meaning it was carried and deposited by icebergs. The researchers inferred that the layer resulted from a brief but intense episode of IBRD deposition. By examining the petrography of the sand fraction and the ages of certain minerals within the layer, such as hornblende and mica, the researchers concluded that the IBRD likely originated from the Weddell Sea and/or Amundsen Sea embayment(s) in West Antarctica.

The high concentrations of IBRD found in these layers were attributed to "dirty" icebergs that calved from the West Antarctic Ice Sheet (WAIS) after it had retreated from its modern grounding line. These layers were found at the top of a sequence of sediments spanning the Pliocene and early Pleistocene, which was significantly more enriched with dropstones (stones carried and deposited by icebergs) compared to the overlying sediments.

The presence of these dropstone-rich layers suggests that the mass balance of the WAIS was highly dynamic during the approximately 41,000-year (inter)glacial cycles of that time period. This implies that the WAIS experienced periods of retreat and advance, leading to the deposition of ice-rafted debris and subsequent sedimentation in the Scotia Sea.

Overall, this study provides valuable insights into past episodes of West Antarctic Ice Sheet instability and demonstrates the usefulness of sedimentary records in the Scotia Sea for understanding the region's response to climate change.