Microfossils reveal Southern Ocean changes over geologic time
The Antarctic Southern Ocean is key to mitigating the impacts of climate change as it absorbs as much as 75 % of excess heat created by humans, and 40 % of human-generated carbon dioxide. Yet, the Southern Ocean’s warmer currents are accelerating the destabilisation of the Antarctic ice sheet. Meanwhile, ocean current changes further offshore are stoking more extreme weather and climate events in the southern hemisphere. Yet, how oceanographic changes close to the ice sheet relate to those further offshore, remains poorly understood. The OceaNice project, which was funded by the European Research Council, developed and applied tools to reconstruct Southern Ocean conditions during warm periods in the geologic past. “Reconstructing how the Southern Ocean, the Antarctic ice sheet and the polar climate changed during geologic episodes of warming and cooling, helps us understand the system as a whole,” says Peter Bijl, the project coordinator. If the investigation periods have atmospheric carbon dioxide levels similar to those projected for this century, the results should improve modelling for future Southern Ocean (and beyond) climate change.
Reconstructing past surface ocean conditions
The team used a group of fossil plankton known as dinoflagellate cysts, as proxies for past sea ice, temperature and upwelling (surfacing of deep, nutrient-rich waters) conditions. Characterising the present-day affinities of modern dinoflagellate cyst species, (preferred temperature, nutrient conditions and sea ice conditions, for instance), lets researchers infer the past conditions that must have existed for fossilised sediment core versions to live. Fifty years of worldwide ocean drilling has yielded the necessary sediment cores for analysis of fossilised dinoflagellate cysts from many Southern Ocean locations. “Comparing the species composition on the modern Southern Ocean floor with the conditions in the overlying water, reveals the preferences of each species. We applied this knowledge to fossilised remains in the sediment cores, which contain a stack of fossil sea floors dating back to past warm climates. The fossil dinoflagellate cysts within these told us what ocean conditions must have been prevalent during those warm climates,” explains Bijl from Utrecht University, the project host. The focus was on glacial–interglacial transitions, alongside the long-term climate cooling during the past 20 million years. Additionally, the molecular remains of a group of archaea also preserved in fossil sea floor sediments, was used as a supplementary tool to reconstruct past ocean temperature. These organisms make their membrane-spanning molecules differently depending on the prevailing temperature. “These organic proxies provide an absolute palaeotemperature of the past – an actual number. With OceaNice we demonstrated that dinoflagellate cysts can also act as palaeothermometers,” adds Bijl. Another key result was a better understanding of how Southern Ocean conditions actually changed, alongside that of the Antarctic ice sheet, over the past 20 million years: the latitudinal position of the ocean fronts, the development of the latitudinal temperature gradient and ocean condition variability over glacial–interglacial cycles.
Boosting the accuracy of climate change projections
OceaNice’s contribution to a systems understanding of the interplay between climate change, ocean change and ice sheet change, ultimately improves the accuracy of emissions-based climate change projections. The team is currently further developing the dinoflagellate cyst-based proxy through a follow-up project working on the West Antarctic ice sheet. “We will reconstruct ice–ocean interactions during the most severe deglaciation events, to understand the consequences of rapid freshwater dumps on Southern Ocean oceanography,” says Bijl. “Next we will explore the Southern Ocean efficiency as a carbon sink.”
Keywords
OceaNice, Antarctic, ice sheet, plankton, fossil, climate change, Southern Ocean
