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Abstract. In 1977–1978 and 1978–1979, the Ross Ice Shelf Project (RISP) recovered sediments from beneath the largest ice shelf in Antarctica at Site J-9 (∼82° S, 168° W), ∼450 km from open marine waters at the calving front of the Ross Ice Shelf and 890 km from the South Pole, one of the southernmost sites for marine sediment recovery in Antarctica. One important finding was the discovery of an active macrofauna, including crustaceans and fish, sustained below the ice shelf far from open waters. The sediment has a thin, unconsolidated upper unit (up to 20 cm thick) and a texturally similar but compacted lower unit (>1 m thick) containing reworked early, middle, and late Miocene diatom and calcareous benthic foraminiferal assemblages. A probable post-Last Glacial Maximum (LGM) disconformity separates the upper unit containing a dominantly agglutinated foraminiferal assemblage, from the lower unit consisting mostly of reworked Miocene calcareous benthic species, including Trifarina fluens, Elphidium magellanicum, Globocassidulina subglobosa, Gyroidina sp., and Nonionella spp. The presence of the polar planktic foraminiferal species Neogloboquadrina pachyderma and the endemic Antarcticella antarctica supports the late Miocene diatom age for the matrix of the lower unit. The microfossil assemblages indicate periods of ice sheet collapse and open-water conditions south of Site J-9 during warm intervals of the early, middle, and late Miocene, including the Miocene Climatic Optimum (∼17–14.7 Ma), demonstrating the dynamic nature of the West Antarctic Ice Sheet (WAIS) and Ross Ice Shelf during the Neogene. The foraminiferal assemblage of the upper unit is unique to the Ross Sea and suggests the influence of a sub-ice-shelf water mass proximal to the retreating post-LGM grounding zone. This unique assemblage is strongly dominated by the bathyal, cold-water agglutinated genus Cyclammina. 

Abstract. With the onset of anthropogenic climate change, it is vital that we understand climate sensitivity and rates of change during periods of warming in the Earth's past to properly inform climate forecasts. To best inform modeling of ongoing and future changes, environmental conditions during past periods of extreme warmth are ideally developed from multiproxy approaches, including the development of novel proxies where traditional approaches fail. This study builds on a proposed sea surface temperature (SST) proxy for the high-latitude Southern Ocean, based on the morphometrics of the ubiquitous Antarctic diatom Fragilariopsis kerguelensis. This species has been shown to display two distinct morphotypes; a low-rectangularity morphotype is interpreted to be more common in warmer waters while a high-rectangularity morphotype is more common in cooler waters. The proportion of the low-rectangularity morphotype (pLR) has been correlated to SST and summer SST (SSST). Here, we examine this proxy by reconstructing SST using sediment samples from the modern seafloor surface in the Amundsen Sea and the Sabrina Coast to test how well two published calibrations of this relationship (Kloster et al., 2018; Glemser et al., 2019) reconstruct SST and SSST in the modern ocean. In the Amundsen Sea surface sediments, we calculate derived SST −1.6 to −1.2 °C and derived SSST 0.6 to 0.7 °C. In the Sabrina Coast surface sediments, we calculate derived SST −0.3 to 0.5 °C and derived SSST 1.4 to 2.5 °C. We discuss the differing population dynamics of F. kerguelensis in our surface samples between the Amundsen Sea and Sabrina Coast because the Amundsen Sea specimens display a lower pLR than Sabrina Coast specimens, although they exist in warmer waters and should display a higher pLR. We also use the two published calibrations to preliminarily reconstruct SST and SSST in the Amundsen Sea over the last interglacial, Marine Isotope Stage 5 (MIS-5). We calculate SSTs that are slightly cooler or within the range of the modern Amundsen Sea for the duration of the last interglacial; we calculate summer SSTs ∼ 1 °C warmer than the modern Amundsen Sea. This suggests MIS-5 SSTs were at most marginally warmer than the modern Amundsen Sea. 

Abstract. Iceberg influence on diatom productivity has been observed for the present and suggested for the past, but direct seeding of the Southern Ocean during times of ice sheet collapse has never been directly demonstrated. Here we demonstrate enhanced diatom production and accumulation in the Amundsen Sea during a Mid-Pliocene interglacial that precisely coincides with pulses of ice-rafted debris (IRD) accumulation, and we infer a causal relation. International Ocean Discovery Program (IODP) Expedition 379 obtained continuous sediment records from the Amundsen Sea continental rise to document West Antarctic Ice Sheet (WAIS) history in an area currently experiencing the largest ice loss in Antarctica. Scanning electron microscopy (SEM) imagery of Mid-Pliocene interglacial sediments of Marine Isotope Stage (MIS) (GI-17, ∼ 3.9 Ma) documents distinct intervals of IRD-rich diatomite, whereas the overall diatom abundance and concentration of bloom species is relatively low in the absence of visible IRD. Sand- and granule-sized IRD grains are documented fully encased within diatomite laminae, with some displaying soft-sediment micro-deformation formed by grains falling into soft diatom ooze. IRD-rich diatomite layers are often characterized by nearly monospecific assemblages of the pelagic diatom Thalassiothrix antarctica, indicating very high primary productivity as IRD grains fell. Diatom-filled fecal pellets with clusters of barite grains are also documented within some of these laminae, further indicating direct mass sinking of diatom mats. Melting icebergs release soluble nutrients along with IRD; thus the coincidence of IRD and bloom species in Amundsen Sea sediments provides compelling evidence that iceberg discharge and melting directly initiates enhanced diatom productivity in the Southern Ocean. These results may contribute to interpreting past WAIS history by providing another proxy for potential collapse events. Furthermore, we suggest that ice sheet collapse may more broadly enhance Southern Ocean diatom production, which in itself can contribute to increased carbon export, potentially attenuating or countering the warming that may have triggered the collapse.