ALBERT

Description: The alkenone unsaturation index (Uk'37) as proxy for sea surface temperature (SST) is an important tool in paleoclimatology for reconstructing past ocean temperature variability. Typically, Uk'37 recorded in marine surface sediments shows a linear correlation with modern mean annual SST. However, in high-latitude oceanic regions, such as the subpolar Pacific, Uk'37-based SSTs do overestimate the mean annual temperature by up to 6 °C, potentially leading to obscured paleoclimatic information drawn from stratigraphic Uk'37-records. The reason for this “warm bias” is still not well understood. Here, we present a compilation of 97 sediment surface samples from Multicores collected in the Bering Sea, the Okhotsk Sea and the North Pacific to evaluate the alkenone-temperature proxy against observational data from the North Pacific. Sediment surface samples were analysed for alkenones and the derived Uk'37-indices converted to water temperatures using different calibration equations established in the literature. Uk'37-based SSTs were then compared to instrumental SST data, as well as modern alkenone flux data from sediment traps in the North Pacific. Our results confirm that most Uk'37-based SSTs from the subpolar Pacific are 2–6 °C too warm compared to instrumental mean annual SSTs for calibrations applied. However, with an uncertainty at the level of ±1.5 °C or less reconstructed SSTs fit quite well to modern autumn temperatures north of the Subarctic Front (SAF), when maximum export flux of alkenones to the seafloor is indicated by sediment trap data. South of the SAF, reconstructed SSTs largely mimic the modern mean annual SST signal with an uncertainty of ±1.5 °C or less, which is likely due to the attenuation of seasonality and longer growth season of coccolithophorids according to sediment trap data. Our study further demonstrates that Uk'37, when seasonality in alkenone production and export are known and considered, is able to provide reasonable estimates of SSTs in modern high-latitude ocean settings. We conduct a case study using available alkenone time-series derived from a sediment core collected from the south-western Okhotsk Sea to better understand the potential effect of seasonality in alkenone production on stratigraphic Uk'37-record in the subpolar Pacific. The case study from the Okhotsk Sea indicates that even a small shift in seasonality may lead to strongly biased SSTs with broader regional implications for paleoclimate reconstructions in high-latitude ocean settings.

Description: As an integral part of the Earth's climate system, the Kuroshio Current (KC) plays a crucial role in shaping the regional oceanography and climate in the Northern Hemisphere. However, how the KC dynamics have varied over glacial-interglacial cycles is still under debate. The dynamic transfer and accumulation of submarine hydrothermal source materials by deep-reaching KC offer us a unique opportunity to examine the variations in dynamics of the KC. Here, we used novel proxies of sedimentary mercury (Hg) and antimony (Sb) in core MD01-2404 retrieved from the middle Okinawa Trough (OT) to reconstruct the evolution of the KC hydrodynamics over the last 92,000 years. We infer the enrichments of sedimentary Hg and Sb to signify hydrothermal input, which is delivered laterally to the study site by deep circulation in association with the KC, thus indicating the dynamics of KC. Overall, both the sedimentary Hg and Sb in core MD01-2404 indicate a persistent influence on the KC dynamics within the OT over the last glacial-interglacial cycles. Furthermore, our Hg and Sb proxies suggest a significantly weakened influence during the last deglaciation and last glacial period while a strengthened influence during the Holocene and late Marine Isotope Stage 5. Our studies imply that the orbital-scale dynamics of KC are controlled by tropical atmosphere-ocean interactions induced by sea surface temperature changes and regulated by the extratropical climate conditions.

Description: Strength of Southern Ocean upwelling controls the exchange of carbon dioxide (CO2) between deep ocean reservoirs and atmosphere, as well as the communication of dissolved silicon with the euphotic zone of the Southern Ocean. The silicon supply could limit diatom opal productivity in the high-latitudes of Southern Ocean and the subsequent burial of biogenic opal in underlying sediments. Here we report a record of biogenic opal export off the Prydz Bay south of the polar front of the Southern Ocean, indicating strengthened upwelling during the past five glacial terminations. In all five terminations (Isingle bondV), opal peaks occur in line with Northern Hemisphere summer insolation intensity as well as the existing IRDs, indicating that freshwater injection associated with retreat of the Northern Hemisphere ice sheets could be the cause of enhanced upwelling in the Southern Ocean during terminations. This could in turn promote CO2 outgassing, finally accelerating the completion of the terminations. In addition, the enhanced upwelling could export the Si-rich deep water to low latitudes via Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW), potentially leading to deglacial opal peaks in subtropical North Atlantic.