On the role of the Southern Ocean in the glacial-interglacial cycles of the past 460,000 years: Changes in wind-driven upwelling and ocean front position revealed by reconstructed surface ocean nutrient conditions and temperatures

The Southern Ocean, where the vertical circulation drives exchange between the voluminous ocean interior and the atmosphere, is proposed to play an important role in the Pleistocene glacial-interglacial cycles by redistributing heat and carbon between ocean and atmosphere. However, the mechanisms behind the Southern Ocean’s impact on global climate remain debated.This dissertation reconstructs changes in the Southern Ocean over the past 460,000 years by measuring the nitrogen isotopic (15N-to-14N) ratio of trace organic matter preserved in diatom microfossils, which reflects surface nutrient conditions, and the abundances of archaeal membrane lipids isolated from sediments, which reflect upper ocean temperature. First, data from the Indian sector of the Antarctic Zone (AZ), the more polar domain of the Southern Ocean, suggest that the upwelling driven by the Southern Westerly Winds responds to three modes of change: global mean climate, the northern-to-southern hemisphere temperature difference, and Earth’s axial tilt. The third mode can explain the lag of CO2 behind climate during glacial inceptions and deglaciations. Second, data from the Pacific sector of the AZ extending back 460,000 years show that the nutrient conditions of Antarctic surface waters were continuously correlated with the volume of land ice sheets, possibly through the effect of ice sheet size on Southern Ocean surface water conditions or the regional winds. Third, temperature reconstructions at different latitudes are used to reconstruct the latitudinal displacements of the Antarctic Polar Front (APF) in the last glacial-interglacial cycle, and the data show in general a more southward(northward) APF in warmer(colder) southern hemisphere climates, with a more southward APF than currently during the penultimate interglacial and at the end of the last deglaciation. Fourth, after correction for APF migration, the nutrient conditions at different latitudes of the Southern Ocean as reconstructed with the N isotopes of diatoms and deep-sea corals yield a consistent history for the increase in Southern Ocean upwelling at the end of the last ice age. These findings strengthen the case for the Southern Ocean’s vertical circulation as a cause of glacial-interglacial changes in atmospheric carbon dioxide, supporting the hypothesis of a central role for Southern Westerly Wind-driven upwelling

Status of the Antarctic Ocean "surface isolation" hypothesis for glacial/interglacial carbon dioxide change

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Foraminifera-bound nitrogen isotopes during the last 160 ka in the Mediterranean Sea

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Consistent Changes in Nitrate Consumption in Two Sectors of the Antarctic Zone Through the Last Glacial Cycle

We present new diatom-bound N isotope (δ15Ndb) records and complementary data from two sediment cores in the Indian sector of the Southern Ocean, with age models that benefit from correlation of TEX86 to Antarctic ice-core temperature. An unprecedented degree of consistency is observed between these two δ15Ndb records and with a previously published δ15Ndb record from the Pacific sector of the Southern Ocean. Even modest δ15Ndb excursions within the progression toward the LGM (e.g. one centered on MIS 5a at ~85 ka) are found to apply in both the Indian and Pacific sectors of the Antarctic. That such similar changes in δ15Ndb are measured on nearly opposite sides of the Antarctic continent argues that they represent the overall history of δ15Ndb in the open Antarctic Zone. In general, across the last glacial cycle, δ15Ndb is found to be strongly correlated with Antarctic climate and atmospheric CO2 ,with higher δ15Ndb (and thus the inference of more complete nitrate consumption) during Antarctic cold intervals. The low opal and biogenic barium export previously documented for the glacial Antarctic Zone is observed in the Indian sector cores as well. Thus, the data comport with the interpretation of reduced nitrate supply during glacial periods. In one Indian sector core with appropriate sedimentation rate, sampling density and age constraints, minima in δ15Ndb are observed during Marine Isotope Stage (MIS) 3 that appear to reflect the Antarctic Isotope Maxima (warm) events, indicating that the same processes producing glacial/interglacial changes in nitrate supply also operate at smaller amplitude and on shorter timescales against the glacial background state. We suggest that this and other findings from the data are best explained by a westerly wind-driven mechanism for the glacial/interglacial changes in nitrate supply. Finally, there are characteristic δ15Ndb histories shared by all records during the Holocene and previous interglacial MIS 5e, which have implications for the climate/CO2 relationships that have been reconstructed for these interglacials.info:eu-repo/semantics/publishe

Laboratory Assessment of the Impact of Chemical Oxidation, Mineral Dissolution, and Heating on the Nitrogen Isotopic Composition of Fossil‐Bound Organic Matter

Fossil‐bound organic material holds great potential for the reconstruction of past changes in nitrogen (N) cycling. Here, with a series of laboratory experiments, we assess the potential effect of oxidative degradation, fossil dissolution, and thermal alteration on the fossil‐bound N isotopic composition of different fossil types, including deep and shallow water scleractinian corals, foraminifera, diatoms and tooth enamel. Our experiments show that exposure to different oxidizing reagents does not significantly affect the N isotopic composition or N content of any of the fossil types analyzed, demonstrating that organic matter is well protected from changes in the surrounding environment by the mineral matrix. In addition, we show that partial dissolution (of up to 70%–90%) of fossil aragonite, calcite, opal, or enamel matrixes has a negligible effect on the N isotopic composition and N content of the fossils. These results suggest that the isotopic composition of fossil‐bound organic material is relatively uniform, and also that N exposed during dissolution is lost without significant isotopic discrimination. Finally, our heating experiments show negligible changes in the N isotopic composition and N content of all fossil types at 100°C. At 200°C and hotter, any N loss and associated nitrogen isotope changes appear to be directly linked to the sensitivity of the mineral matrix to thermal stress, which depends on the biomineral type. These results suggest that, so long as high temperature does not compromise the mineral structure, the biomineral matrix acts as a closed system with respect to N, and the N isotopic composition of the fossil remains unchanged.Plain Language Summary: The ratio of the heavy and light isotopes of nitrogen (15N and 14N) in the organic material contained within the mineral structure of fossils can be used to reconstruct past changes in biological and chemical processes. With a series of laboratory experiments, we evaluate the potential effects of chemical conditions, fossil dissolution, and heating on the nitrogen isotopic composition (15N/14N ratio) of corals, foraminifera, diatoms and tooth enamel. Our results indicate that these processes do not have a significant effect on the 15N/14N of fossils, suggesting that the mineral matrix provides a barrier that isolates a fossil's organic nitrogen from the surrounding environment, preventing alteration of its 15N/14N. In addition, we show that if part of the fossil‐bound organic nitrogen is exposed by dissolution or heating, it is lost without affecting the 15N/14N of the organic material that remains in the mineral. These findings imply that the original 15N/14N ratio incorporated by the organism is preserved in the geologic record. Therefore, measurements of the nitrogen isotopes on fossils can provide faithful biological, ecological, and environmental information about the past.Key Points: Fossil‐bound organic matter is well protected by the mineral matrix from chemical changes in the surrounding environment. Partial dissolution of fossil calcite, aragonite, opal, and enamel has a negligible effect on their N isotopic composition and N content. During heating, fossil N content and isotopic composition remains unchanged if the structure of the inorganic matrix is not compromised.Max Planck SocietyDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659US National Science FoundationPaul Crutzen Nobel Prize Fellowshiphttps://doi.org/10.5281/zenodo.688468

The Southern Ocean during the ice ages: A review of the Antarctic surface isolation hypothesis, with comparison to the North Pacific

The Southern Ocean is widely recognized as a potential cause of the lower atmospheric concentration of CO2 during ice ages, but the mechanism is debated. Focusing on the Southern Ocean surface, we review biogeochemical paleoproxy data and carbon cycle concepts that together favor the view that both the Antarctic and Subantarctic Zones (AZ and SAZ) of the Southern Ocean played roles in lowering ice age CO2 levels. In the SAZ, the data indicate dust-driven iron fertilization of phytoplankton growth during peak ice age conditions. In the ice age AZ, the area-normalized exchange of water between the surface and subsurface appears to have been reduced, a state that we summarize as “isolation” of the AZ surface. Under most scenarios, this change would have stemmed the leak of biologically stored CO2 that occurs in the AZ today. SAZ iron fertilization during the last ice age fits with our understanding of ocean processes as gleaned from modern field studies and experiments; indeed, this hypothesis was proposed prior to evidentiary support. In contrast, AZ surface isolation is neither intuitive nor spontaneously generated in climate model simulations of the last ice age. In a more prospective component of this review, the suggested causes for AZ surface isolation are considered in light of the subarctic North Pacific (SNP), where the paleoproxies of productivity and nutrient consumption indicate similar upper ocean biogeochemical changes over glacial cycles, although with different timings at deglaciation. Among the proposed initiators of glacial AZ surface isolation, a single mechanism is sought that can explain the changes in both the AZ and the SNP. The analysis favors a weakening and/or equatorward shift in the upwelling associated with the westerly winds, occurring in both hemispheres. This view is controversial, especially for the SNP, where there is evidence of enhanced upper water column ventilation during the last ice age. We offer an interpretation that may explain key aspects of the AZ and SNP observations. In both regions, with a weakening in westerly wind-driven upwelling, nutrients may have been “mined out” of the upper water column, possibly accompanied by a poleward “slumping” of isopycnals. In the AZ, this would have encouraged declines in both the nutrient content and the formation rate of new deep water, each of which would have contributed to the lowering of atmospheric CO2. Through several effects, the reduction in AZ upwelling may have invigorated the upwelling of deep water into the low latitude pycnocline, roughly maintaining the pycnocline’s supply of water and nutrients so as to (1) support the high productivity of the glacial SAZ and (2) balance the removal of water from the pycnocline by the formation of Glacial North Atlantic Intermediate Water. The proposed return route from the deep ocean to the surface resembles that of Broecker’s (1991) “global ocean conveyor,” but applying to the ice age as opposed to the modern ocean.ISSN:0277-379

Deglacial timing of biogeochemical changes in the Antarctic and Polar Frontal Zones: Evidence for a northward shift in wind-driven upwelling during the glacial interval

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The Southern Ocean during the ice ages: A slumped pycnocline from reduced wind-driven upwelling?

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The Southern Ocean during the ice ages: A slumped pycnocline from reduced wind-driven upwelling?

The Southern Ocean is recognized as a potential cause of the lower atmospheric concentration of CO2 during ice ages, but the mechanism is debated. In the ice age Antarctic Zone, biogeochemical paleoproxy data suggest a reduction in the exchange of nutrients (and thus water and carbon) between the surface and the deep ocean. We report simple calculations with those data indicating that the decline in the supply of nutrients during peak glacials was extreme, >50% of the interglacial rate. Weaker wind-driven upwelling is a prime candidate for such a large decline, and new, complementary aspects of this mechanism are identified here. First, reduced upwelling would have resulted in a “slumping” of the pycnocline into the AZ. Second, it would have allowed diapycnal mixing to “mine” nutrients out of the upper water column, possibly causing an even greater slumping of the vertical nutrient gradient (or “nutricline”). These mechanisms would have reduced shallow subsurface nutrient concentrations, decreasing wintertime resupply of nutrients to the surface mixed layer, beyond the reduction in upwelling alone. They would have complemented two changes previously proposed to accompany a decline in upwelling: (1) halocline strengthening and (2) reduced isopycnal mixing in the deep ocean. Together, the above changes would have encouraged declines in the nutrient content and/or the formation rate of new deep water in the AZ, enhancing CO2 storage in the deep ocean