Early to mid-Miocene Ross Sea oceanographic and climate evolution

Abstract

Global climate has been rapidly changing due to on-going anthropogenic CO2 input to the atmosphere, causing the Antarctic Ice Sheet (AIS) to retreat and subsequently cause global sea level rise. AIS melting is heavily dependent on ocean-ice interactions. To be able to more accurately predict the future behaviour of the AIS - with respect to the ocean-driven melting - studying the interactions between AIS and ocean during past intervals of rapid climate warming can provide insights into mechanisms and feedbacks. One interval in the past when climate rapidly warmed is the Miocene Climatic Optimum (18 to 16 Ma), when the Southern Ocean was several degrees warmer in comparison to present sea surface temperatures. This research hence is a palaeoceanographic reconstruction of the Ross Sea in the early to middle Miocene and is performed using samples made into microscopic slides from the Ross Sea at Site U1521, recently retrieved during IODP. Dinoflagellate cyst assemblages suggest a highly dynamic marine-based Ross Sea ice sheet before and throughout the MCO and a clear warming trend towards the MCO. The studied interval starts with the reworking period indicating a period of increased glacial activity. Before the onset of the MCO the Ross Sea is mostly dominated by gonyaulacoid dinocysts suggesting a low SST and a locally ice-free sea to allow light to penetrate and stimulate photosynthesis. The onset of the MCO is indicated by the chert layer with dominance of heterotrophic dinocysts, indicating upwelling of nutrient rich water and ice sheet retreat with a higher Ross Sea SST. A small cooling trend is visible after the onset with fluctuating sea surface temperatures, indicated by autotrophic gonyaulacoid dominance, until the MCO where temperatures rise and heterotrophic dinocysts become abundant again.