Sedimentological and geophysical investigation of sediment cores from the Amundsen Sea

Ice loss from the marine-based, potentially unstable West Antarctic Ice Sheet (WAIS) contributes to current sea-level rise and may raise sea level by up to 3.3 to 5 meters in the future. Over the past few decades, glaciers draining the WAIS into the Amundsen Sea Embayment (ASE) have shown accelerated ice flow, rapid thinning and grounding-line retreat. However, the long-term context of this ice-sheet retreat is poorly constrained, limiting our ability to accurately predict future WAIS behaviour. Here we present a new chronology for WAIS retreat from the inner continental shelf of the eastern ASE based on radiocarbon dates from three marine sediment cores. The ages document a retreat of the grounding line to within ~93 km of its modern position before 11.7±0.7 kyr BP (thousand years before present). This early deglaciation is consistent with ages for grounding-line retreat from the western ASE. Our new data demonstrate that, other than in the Ross Sea, WAIS retreat in the ASE has not continued progressively since the Last Glacial Maximum. Furthermore, our results suggest that the grounding-line position in the ASE was predominantly stable throughout the Holocene, and that any episodes of fast retreat similar to that observed today must have been short-lived. Alternatively, today's rapid retreat was unprecedented during the Holocene. Therefore, the current ice loss must originate in recent changes in regional climate, ocean circulation or ice-sheet dynamics. Incorporation of these results into models is essential to produce robust predictions of future ice-sheet change and its contribution to sea-level rise.

(Table 2) Radiocarbon ages on Arctic box and multicores

Paleo-sea-ice history in the Arctic Ocean was reconstructed using the sea-ice dwelling ostracode Acetabulastoma arcticum from late Quaternary sediments from the Mendeleyev, Lomonosov, and Gakkel Ridges, the Morris Jesup Rise and the Yermak Plateau. Results suggest intermittently high levels of perennial sea ice in the central Arctic Ocean during Marine Isotope Stage (MIS) 3 (25-45 ka), minimal sea ice during the last deglacial (16-11 ka) and early Holocene thermal maximum (11-5 ka) and increasing sea ice during the mid-to-late Holocene (5-0 ka). Sediment core records from the Iceland and Rockall Plateaus show that perennial sea ice existed in these regions only during glacial intervals MIS 2, 4, and 6. These results show that sea ice exhibits complex temporal and spatial variability during different climatic regimes and that the development of modern perennial sea ice may be a relatively recent phenomenon.

Abyssal community analysis from replicate box cores in the central North Pacific

A 0.25 m**2 United States Naval Electronics Laboratory box corer was used to take replicate samples from an oligotrophic bottom under the North Pacific Central Water Mass (~28°N, 155°W). The bottom is a red clay with manganese nodules at a depth of 5500-5800 m. Macrofaunal density ranges from 84 to 160 individuals per m**2 and is therefore much the same as in Northwest Atlantic Gyre waters. Of the macrofaunal taxa, polychaetes dominate (55 %), followed by tanaids (18 %), bivalves (7 %), and isopods (6 %). Meiofaunal taxa were only partially retained by the 297 µm screen used in washing. Even then, they are 1.5-3.9 times as abundant as the macrofaunal taxa, with nematodes being numerically dominant by far. Foraminifera seem to comprise an important portion of the community, but could not be assessed accurately because of the inability to discriminate living and dead tests. Remains of what are probably xenophyophoridans are also very important, but offer the same problem. Faunal diversity is extremely high, with deposit feeders comprising the overwhelming majority. Most species are rare, being encountered only once. The distributions of only three species show any significant deviation from randomness. The polychaete fauna from box cores collected from 90 miles to the north was not significantly different from that of the principal study locality. Concordance appeared at several taxonomic levels, from species through macrofaunal/meiofaunal relationships. As a result, the variation in total animal abundance shows aggregation among cores. We discuss Sokolova's concept of a deep-sea oligotrophic zone dominated by suspension feeders, and reconcile it with our present findings. The high diversity of the fauna combined with the low food level contradict theories that relate diversity directly with productivity.