Sea ice and productivity changes over the last glacial cycle in the Adélie Land region, East Antarctica, based on diatom assemblage variability

Diatoms can provide important paleoenvironmental information about seasonal sea ice extent, productivity, sea surface temperature and ocean circulation variability, yet there are relatively few studies analysing the last glacial cycle near the Antarctic continent. This study examines diatom assemblages over the last glacial cycle 15 from core TAN1302-44, from off Adélie Land, East Antarctica. Four distinct diatom assemblages were identified using principal components analyses. The PC 1 assemblage is associated with the interglacial, sedimentary facies, Facies 1, and comprises Thalassiosira lentiginosa, Actinocyclus actinochilus, Eucampia antarctica , Azpeitia tabularis and Asteromphalus hyalinus , suggesting that MIS 5e and Holocene interglacial time periods were characterised by seasonal sea ice environments with similar ocean temperature and 20 circulation. The PC 2 assemblage is associated with the glacial, Facies 2, and comprises Fragilariopsis obliquecostata, Asteromphalus parvulus, Rhizosolenia styliformis, Thalassiosira tumida, Chaetoceros dichaeta , and a Eucampia antarctica terminal/intercalary ratio. This indicates that, during the MIS 4-2 glacial there was an increase in the length of the sea ice season compared with the interglacial period, yet still no permanent sea ice cover. The PC 2 assemblage is also associated with the glaciation and deglacial facies. There is an initial 25 increase of PC 2 at the start of MIS 5d-a glaciation stage and then a gradual increase throughout late MIS 4-2, suggests that sea ice cover steadily increased reaching a maximum at the end of MIS 2. The PC 3 assemblage is associated with all four facies and comprises Actinocyclus ingens , Actinocyclus actinochilus , Thalassiosira oliverana and Fragilariopsis kerguelensis , suggesting that reworking of sediments and an influx of older sediments occurred throughout the last glacial cycle. Finally, the PC 4 assemblage is associated with the 30 deglacial, glaciation, and glacial facies and comprises Fragilariopsis kerguelensis , Thalassiothrix antarctica, Chaetoceros bulbosum and Eucampia antarctica, suggesting that during the last glaciation, the last two deglacials, and the early glacial, there was a period of enhanced upwelling of nutrient-rich, warmer water, which is inferred to reflect an increase in Circumpolar Deep Water. Interestingly, the diatom data suggest the onset of increased Circumpolar Deep Water during the last deglacial occurred after the rapid loss of a prolonged sea ice 35 season at the end of last glacial. Together, these results suggest changes in ocean circulation and sea ice season important factors during climate transitions. The results fill a gap in our understanding of the sea ice extent and ocean circulation changes proximal to East Antarctica over the last glacial cycle. the identification and quantification of groups of reconstruct a n of lighter species ’ valves bottom currents or variable 70 influx of terrigenous matter when reconstructing the past environment, is to consider all these processes. is reported to comprise oscillations in the sea ice season (Hartman et al. 2021). Here we use diatom assemblages to understand the changes in the duration of the sea ice season and in CDW upwelling in the Adélie region over the last glacial cycle, ~140 ka, including the deglacial 95 and glaciation transitions. at 240 cm, within the deglacial, glacial and interglacial 280 facies, respectively; Asteromphalus parvulus was found at 180-160 cm, 200 cm and at 240-230 cm, within the glacial and interglacial facies, respectively; and Asteromphalus hyalinus was found at 170-150 cm, at 240 cm and at 270 cm, within the glacial, interglacial and deglacial facies.