Showing papers by "Elisabeth Michel published in 2020"

Southern Ocean upwelling, Earth's obliquity, and glacial-interglacial atmospheric CO2 change

Abstract: Previous studies have suggested that during the late Pleistocene ice ages, surface-deep exchange was somehow weakened in the Southern Ocean's Antarctic Zone, which reduced the leakage of deeply sequestered carbon dioxide and thus contributed to the lower atmospheric carbon dioxide levels of the ice ages. Here, high-resolution diatom-bound nitrogen isotope measurements from the Indian sector of the Antarctic Zone reveal three modes of change in Southern Westerly Wind-driven upwelling, each affecting atmospheric carbon dioxide. Two modes, related to global climate and the bipolar seesaw, have been proposed previously. The third mode-which arises from the meridional temperature gradient as affected by Earth's obliquity (axial tilt)-can explain the lag of atmospheric carbon dioxide behind climate during glacial inception and deglaciation. This obliquity-induced lag, in turn, makes carbon dioxide a delayed climate amplifier in the late Pleistocene glacial cycles.

Benthic foraminifera as tracers of brine production in the Storfjorden “sea ice factory”

Abstract: . The rapid response of benthic foraminifera to environmental factors (e.g. organic matter quality and quantity, salinity, pH) and their high fossilisation potential make them promising bio-indicators for the intensity and recurrence of brine formation in Arctic seas. Such an approach, however, requires a thorough knowledge of their modern ecology in such extreme settings. To this aim, seven stations along a north–south transect across the Storfjorden (Svalbard archipelago) have been sampled using an interface multicorer. This fjord is an area of intense sea ice formation characterised by the production of brine-enriched shelf waters (BSW) as a result of a recurrent latent-heat polynya. Living (rose bengal-stained) foraminiferal assemblages were analysed together with geochemical and sedimentological parameters in the top 5 cm of the sediment. Three major biozones were distinguished. (i) The “inner fjord” zone, dominated by typical glacier proximal calcareous species, which opportunistically respond to fresh organic matter inputs. (ii) The “deep basins and sill” zone, characterised by glacier distal agglutinated fauna; these are either dominant because of the mostly refractory nature of organic matter and/or the brine persistence that hampers the growth of calcareous species and/or causes their dissolution. (iii) The “outer fjord” zone, characterised by typical North Atlantic species due to the intrusion of the North Atlantic water in the Storfjordrenna. The stressful conditions present in the deep basins and sill (i.e. acidic waters and low food quality) result in a high agglutinated ∕ calcareous ratio ( A∕C ). This supports the potential use of the A∕C ratio as a proxy for brine persistence and overflow in Storfjorden.

The nature of deep overturning and reconfigurations of the silicon cycle across the last deglaciation.

Abstract: Changes in ocean circulation and the biological carbon pump have been implicated as the drivers behind the rise in atmospheric CO2 across the last deglaciation; however, the processes involved remain uncertain. Previous records have hinted at a partitioning of deep ocean ventilation across the two major intervals of atmospheric CO2 rise, but the consequences of differential ventilation on the Si cycle has not been explored. Here we present three new records of silicon isotopes in diatoms and sponges from the Southern Ocean that together show increased Si supply from deep mixing during the deglaciation with a maximum during the Younger Dryas (YD). We suggest Antarctic sea ice and Atlantic overturning conditions favoured abyssal ocean ventilation at the YD and marked an interval of Si cycle reorganisation. By regulating the strength of the biological pump, the glacial-interglacial shift in the Si cycle may present an important control on Pleistocene CO2 concentrations.

New pH evidence for changes in intermediate South East Pacific carbon storage during the last deglaciation

Abstract:

The leading hypotheses proposed to explain the rise in atmospheric CO2 during the last glacial to interglacial transition proposes enhanced carbon transfer from the intermediate and deep oceans to the atmosphere via the intensification of southern ocean upwelling. To test this scenario, we generated a high resolution record of boron isotopes (d¹¹B) and B/Ca (proxies for pH and carbonate ion concentration, respectively) measured on shells of the benthic foraminifera C. wuellestorfi from a marine sedimentary core located at intermediate depth (1536m) on the Chilean margin. Our records confirm the link between changes in ocean circulation and variations in the carbonate chemistry at this site. The data also reveal the increase of intermediate water pH at the very late LGM, before the beginning of the deglaciation and the rise in atmospheric pCO₂. To account for this observation, we suggest the existence of an early release of carbon from the intermediate ocean to the atmosphere in response to sea ice retreat occurring at the same time. The lack of any clear increase in atmospheric CO2 suggests that this release of intermediate ocean carbon was compensated by enhanced biological pumping.