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  • 1
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    PANGAEA
    In:  Supplement to: Benz, Verena; Esper, Oliver; Gersonde, Rainer; Lamy, Frank; Tiedemann, Ralf (2016): Last Glacial Maximum sea surface temperature and sea-ice extent in the Pacific sector of the Southern Ocean. Quaternary Science Reviews, 146, 216-237, https://doi.org/10.1016/j.quascirev.2016.06.006
    Publication Date: 2020-01-17
    Description: Sea surface temperatures and sea-ice extent are the most critical variables to evaluate the Southern Ocean paleoceanographic evolution in relation to the development of the global carbon cycle, atmospheric CO2 variability and ocean-atmosphere circulation. In contrast to the Atlantic and the Indian sectors, the Pacific sector of the Southern Ocean has been insufficiently investigated so far. To cover this gap of information we present diatom-based estimates of summer sea surface temperature (SSST) and winter sea-ice concentration (WSI) from 17 sites in the polar South Pacific to study the Last Glacial Maximum (LGM) at the EPILOG time slice (19,000-23,000 cal. years BP). Applied statistical methods are the Imbrie and Kipp Method (IKM) and the Modern Analog Technique (MAT) to estimate temperature and sea-ice concentration, respectively. Our data display a distinct LGM east-west differentiation in SSST and WSI with steeper latitudinal temperature gradients and a winter sea-ice edge located consistently north of the Pacific-Antarctic Ridge in the Ross sea sector. In the eastern sector of our study area, which is governed by the Amundsen Abyssal Plain, the estimates yield weaker latitudinal SSST gradients together with a variable extended winter sea-ice field. In this sector, sea-ice extent may have reached sporadically the area of the present Subantarctic Front at its maximum LGM expansion. This pattern points to topographic forcing as major controller of the frontal system location and sea-ice extent in the western Pacific sector whereas atmospheric conditions like the Southern Annular Mode and the ENSO affected the oceanographic conditions in the eastern Pacific sector. Although it is difficult to depict the location and the physical nature of frontal systems separating the glacial Southern Ocean water masses into different zones, we found a distinct temperature gradient in latitudes straddled by the modern Southern Subtropical Front. Considering that the glacial temperatures north of this zone are similar to the modern, we suggest that this represents the Glacial Southern Subtropical Front (GSSTF), which delimits the zone of strongest glacial SSST cooling (〉4K) to its North. The southern boundary of the zone of maximum cooling is close to the glacial 4°C isotherm. This isotherm, which is in the range of SSST at the modern Antarctic Polar Front (APF), represents a circum-Antarctic feature and marks the northern edge of the glacial Antarctic Circumpolar Current (ACC). We also assume that a glacial front was established at the northern average winter sea ice edge, comparable with the modern Southern Antarctic Circumpolar Current Front (SACCF). During the glacial, this front would be located in the area of the modern APF. The northward deflection of colder than modern surface waters along the South American continent leads to a significant cooling of the glacial Humboldt Current surface waters (4-8K), which affects the temperature regimes as far north as into tropical latitudes. The glacial reduction of ACC temperatures may also result in the significant cooling in the Atlantic and Indian Southern Ocean, thus may enhance thermal differentiation of the Southern Ocean and Antarctic continental cooling. Comparison with temperature and sea ice simulations for the last glacial based on numerical simulations show that the majority of modern models overestimate summer and winter sea ice cover and that there exists few models that reproduce our temperature data rather well.
    Type: Dataset
    Format: application/zip, 29 datasets
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  • 2
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    PANGAEA
    In:  Supplement to: Basak, Chandranath; Fröllje, Henning; Lamy, Frank; Gersonde, Rainer; Benz, Verena; Anderson, Robert F; Molina-Kescher, Mario; Pahnke, Katharina (2018): Breakup of last glacial deep stratification in the South Pacific. Science, 359(6378), 900-904, https://doi.org/10.1126/science.aao2473
    Publication Date: 2020-01-17
    Description: Stratification of the deep Southern Ocean during the Last Glacial Maximum is thought to have facilitated carbon storage and subsequent release during the deglaciation as stratification broke down, contributing to atmospheric CO2 rise. Here, we present neodymium isotope evidence from deep to abyssal waters in the South Pacific that confirms stratification of the deepwater column during the Last Glacial Maximum. The results indicate a glacial northward expansion of Ross Sea Bottom Water and a Southern Hemisphere climate trigger for the deglacial breakup of deep stratification. It highlights the important role of abyssal waters in sustaining a deep glacial carbon reservoir and Southern Hemisphere climate change as a prerequisite for the destabilization of the water column and hence the deglacial release of sequestered CO2 through upwelling.
    Type: Dataset
    Format: application/zip, 5 datasets
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  • 3
    Publication Date: 2020-01-17
    Description: The last transition from a full glacial to a full interglacial state is of special importance to investigate processes that control the Earth's climate evolution. Out of phase interhemispheric climate variability over the last deglaciation has been associated with orbital induced insolation changes as well as with the “bipolar seesaw”, hence related to changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The Southern Ocean (SO) as only water territory connecting the Pacific, Atlantic and Indian Ocean, plays a crucial role as southern limb of the AMOC in propagating signals within its basins and into the different world oceans. The Antarctic Circumpolar Current (ACC), steered by the strong Southern Westerly Winds (SWW), redistributes heat, salt and nutrients via wind-driven upwelling and thus has the high potential of regulating atmospheric CO2 concentration via the biological pump as well as surface and deep-water ventilation. Sea surface temperature and sea-ice extent are important surface water parameters related to the oceanic frontal and current systems as well as to water mass formation via brine release and bioproductivity changes. Despite numerous marine studies from the Pacific sector of the SO, the (sub)antarctic realm is still underrepresented in paleoceanographic research. This thesis examines the environmental changes of the last 30,000 years (30 kyr) in the Pacific sector of the SO using diatom-based transfer function estimates of summer sea surface temperature (SSST) and winter sea-ice (WSI) concentrations reconstructed from 17 selected sediment cores. Including available sea surface temperatures and sea-ice records from the Pacific sector the thesis objectives are primarily a basin and circum-Antarctic wide comprehension of last glacial, deglacial and Holocene climate variability with respect to forcing mechanisms, lead-lag conditions and ice-ocean-atmosphere-ocean feedbacks. The first manuscript deals with the reconstruction of temperature and sea-ice signals during the Last Glacial Maximum (LGM; 19-23 kyr before present, BP) in the Pacific sector of the SO using new diatom data from a total of 17 cores. Consistent with estimates from previous studies, the Pacific sector shows a distinct basin-wide cooling with a temperature decrease of ≥4 K in the present Subantarctic Zone. Most prominent is an E-W gradient concerning the cold-water expansion and the maximum extent of winter sea ice (WSI) that results from strong topographic forcing also steering the frontal system. Hence, the frontal system was characterized by colder SSSTs Atmospheric forcing mechanisms such as the SWW and the El Niño Southern Oscillation (ENSO) are proposed to amplify the E-W gradient and to have a high regional impact in the Pacific sector. Regarding the average latitudinal expansion of the cold-water realm and the WSI extent in the different SO sectors, a coherent and uniform circum-Antarctic picture of the LGM time slice arises. The second manuscript carefully examines the deglacial history of the Pacific sector of the SO, based in the same set of sediment cores. A major outcome is a decoupling of the eastern Pacific sector to its western counterpart and the other SO sectors, which has not been shown before. An early deglacial warming around 22 kyr BP observed in the eastern sector is in close agreement with a warming in the adjacent West Antarctic Ice Sheet (WAIS), and is most likely related to the rising Southern Hemisphere insolation. Hence, the synchronous CO2 rise recorded in East and West Antarctica might have been triggered by the shutdown of the AMOC, rapid sea-ice retreat due to intense warming and strong upwelling due to strengthened SWW. Over the course of the deglaciation, the Pacific records show the common “Antarctic timing” consisting of increasing temperatures until the Antarctic Holocene Optimum (AHO; ~12-9 kyr BP) only interrupted by the Antarctic Cold Reversal (ACR; ~14.5-12.5 kyr BP). A sole contribution of the WAIS to meltwater pulse 1 A (14,400 yr BP) can not be ascertained as less cooling occurred during the ACR than expected by model simulations. The Holocene climate in the SO is of special importance in deciphering small-scale changes induced by atmospheric forcings, which allos to infer possible present climate changes. The sediment cores, presented in the third manuscript, are relatively high resolved for SO sediments (8-34 cm/kyr) and were retrieved in the western Pacific's Antarctic Zone. The SSST and WSI estimates show a Mid-Holocene cooling which corroborates results from model simulations of freshwater shedding from the rapid WAIS retreat. This sea surface cooling, most likely originating in the Pacific sector is propagated via the “cold water route” into the other SO sectors. The variability of warm and cold periods during the Mid- and Late Holocene reveals a strong dependence to regional influencing factors such as the close vicinity to the sea ice edge as well as to the atmospheric shift of a SWW-to a ENSO governed climate state. In summary, this thesis provides for the first time SSST and winter sea-ice estimates in the Pacific sector of the SO on a wide spatial range and for time slices whose pale oceanographic history is crucial for the understanding of global climate change. The investigated environmental parameters point to the sensitivity of this SO sector, concerning the drainage of the WAIS and the impact of atmospheric changes, that has the high potential of triggering climate change.
    Type: Dataset
    Format: application/zip, 16 datasets
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  • 4
    Publication Date: 2020-01-17
    Type: Dataset
    Format: text/tab-separated-values, 5346 data points
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  • 5
    Publication Date: 2020-01-17
    Type: Dataset
    Format: text/tab-separated-values, 3645 data points
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  • 6
    Publication Date: 2020-01-17
    Type: Dataset
    Format: text/tab-separated-values, 1539 data points
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  • 7
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    PANGAEA
    In:  Supplement to: Studer, Anja S; Sigman, Daniel M; Martínez‐García, Alfredo; Benz, Verena; Winckler, Gisela; Kuhn, Gerhard; Esper, Oliver; Lamy, Frank; Jaccard, Samuel L; Wacker, Lukas; Oleynik, Sergey; Gersonde, Rainer; Haug, Gerald H (2015): Antarctic Zone nutrient conditions during the last two glacial cycles. Paleoceanography, 30(7), 845-862, https://doi.org/10.1002/2014PA002745
    Publication Date: 2020-01-17
    Description: In a sediment core from the Pacific sector of the Antarctic Zone (AZ) of the Southern Ocean, we report diatom-bound N isotope (d15Ndb) records for total recoverable diatoms and two distinct diatom assemblages (pennate and centric rich). These data indicate tight coupling between the degree of nitrate consumption and Antarctic climate across the last two glacial cycles, with d15Ndb (and thus the degree of nitrate consumption) increasing at each major Antarctic cooling event. Coupled with evidence from opal- and barium-based proxies for reduced export production during ice ages, the d15Ndb increases point to ice age reductions in the supply of deep ocean-sourced nitrate to the AZ surface. The two diatom assemblages and species abundance data indicate that the d15Ndb changes are not the result of changing species composition. The pennate and centric assemblage d15Ndb records indicate similar changes but with a significant decline in their difference during peak ice ages. A tentative seasonality-based interpretation of the centric-to-pennate d15Ndb difference suggests that late summer surface waters became nitrate free during the peak glacials.
    Type: Dataset
    Format: application/zip, 5 datasets
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  • 8
    Publication Date: 2020-01-17
    Type: Dataset
    Format: text/tab-separated-values, 951 data points
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  • 9
    Publication Date: 2020-01-17
    Type: Dataset
    Format: text/tab-separated-values, 78 data points
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  • 10
    Publication Date: 2020-01-17
    Type: Dataset
    Format: text/tab-separated-values, 72 data points
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