Comment on egusphere-2022-1009

Abstract

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 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 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 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 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 season at the end of last glacial. Together, these results suggest changes in ocean circulation and sea ice season were 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.