Paleoclimate reconstruction of the Chukchi Plateau in Arctic Ocean over the last climatic cycles based on trace element ratio of ostracods

Abstract

Arctic sea ice plays a crucial role in regulating Earth’s climate system through its albedo effect, control on freshwater influx and nutrient supply to oceans. Sea ice retreat has become more vigorous in Arctic Ocean from the mid-1990s, however, its driving mechanism is not fully understood. The halocline separates Arctic sea ice and a warm water mass, the Atlantic Layer, which is originated from North Atlantic Ocean. The halocline acts as a buffer layer to inhibit heat flux from the warm Atlantic Layer to Arctic sea ice, and thus, slow down sea ice decline. Research suggested that the fluctuation of halocline thickness in the Arctic could be a potential controlling factor on sea ice development. This study aims to evaluate the variability of halocline’s depth and its impact on sea ice cover in the last climatic cycles. The first part of this study will discuss the chemical composition in ostracod shells and test potential limitation in paleoceanography studies while the second part will use Mg/Ca ratio of ostracods to reconstruct bottom water temperature of the Chukchi Plateau and examine its implication on the fluctuation of water mass stratification. This study revealed element distribution of adult and juvenile ostracod valves between two genera, Krithe and Polycope. We discovered homogeneous Ca distribution and Mg inner and outer bands for both Krithe and Polycope valves. Adult and juvenile (A-1 and A-2) Krithe valves showed similar Mg/Ca ratios, suggesting future use of juvenile Krithe shells to reconstruct bottom water temperature. We confirmed that shell cleaning was necessary before trace element analysis of ostracods while diagenetic effect from the formation of dolomite was trivial on Krithe shells. We postulated a possible relationship between Polycope Mg/Ca ratio and bottom water temperature, extending future research to examine the potential use of Polycope Mg/Ca ratio in reconstructing paleo-water temperature. To unveil the fluctuation of halocline thickness in Arctic Ocean, we measured Mg/Ca ratio of ostracod shells from multiple sediment cores retrieved at different depths (434 m–741 m) in the Chukchi Plateau of western Arctic Ocean. We measured bottom water temperature from coretop ostracod samples from 434 m, 580 m and 741 m water depths as –0.27 °C, 2.36 °C and 0.46 °C respectively. The water depths from 434 m to 580 m might be occupied by the Atlantic Layer while deeper 741 m water depth was dominant by the Arctic Intermediate Water during the Holocene. In MIS 3, bottom water temperature fluctuated between 0 °C and around 1.5 °C from water depths of 434 m and 580 m, suggesting the presence of the Atlantic Layer up to 434 m. Therefore, we refined the upper boundary of the Atlantic Layer in MIS 3, which might indicate a thinner and shallower halocline in MIS3 than results from previous research. Arctic sea ice decline in last climatic cycle might possibly be explained by the shift of halocline to a shallower depth.