Reconstructing hydrological changes in (sub)tropical South America over the last 36 kyrs: insights into the low-latitude expressions of high-latitude climate forcing
Final Report Abstract
The aim of this project was to investigate the interplay of insolation and oceanic forcing on the spatial and temporal precipitation distribution in South America. For this purpose, paleo-precipitation records have been generated on marine sediment cores from off the Orinoco, Amazon, and Paraíba do Sul Rivers. To reconstruct precipitation variability an multi-proxy approach has been taken, combining foraminiferal-based proxies of salinity and river discharge (Ba/Ca and δ18O of sea water) with sediment geochemical (e.g. K/Al), lipid biomarker (e.g. n-alkane and branched GDGT accumulation rates, BIT index) and sedimentological proxies (lightness). In addition, sea surface temperature (SST) records obtained on the same cores have been used to disentangle the influence of large-scales oceanographic changes on continental hydroclimate. Investigated were the past 36 kyr in high temporal resolution, providing insights into periods of highly dynamic changes in ocean circulation strength (e.g. Heinrich Stadials) occurring against different background levels of orbitally driven insolation. Our results demonstrate that the sensitivity of tropical South American paleoclimate towards high-latitude climate change varies strongly between climatologically different regions. In the northern South American realms which are under the influence of the Intertropical Convergence Zone (ITCZ) we found abrupt variations in line with high northern latitude climate variability, caused by latitudinal shifts of the ITCZ. These fluctuations are most pronounced if occurring against a background of high boreal summer insolation, which causes the ITCZ to occupy a northerly position. Variations in the core region of the South Atlantic Convergence Zone (SACZ) are on the other hand less pronounced, which speaks against the development of a “super SACZ” in eastern South America during intervals of sluggish ocean circulation (e.g. during Heinrich Stadial 1). However, we find that the SACZ might have been expanded further northward, fueled by anomalously high SST off the southern portions of Eastern South America. These high SSTs might have further increased moisture transport by trade winds during austral winter, which increased water availability over the otherwise relatively arid eastern South America. As a third crucial component of the (sub)tropical South American climate system, the South American Low Level Jet (SALLJ) appeared to have been severely weakened during periods of anomalous warming over south America, reinforcing notions of an anti-correlation between SACZ and SALLJ strength in modern observational data. Notably, to distinct influence of high southern latitude climatic changes on the (sub)tropical South American precipitation patterns were found. Lastly, a previously unrecognized interplay of continental hydroclimate and deep marine ecosystems has been found, as cold water corals growing on the slope off southeastern Brazil appear to grow during times of intensified precipitation on the continent. It is argued that this temporal match is due to the increased transport of nutrients by enhanced river run-off during intervals of intensified precipitation, fueling the growth of the heterotrophic cold water coral ecosystems.
Publications
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“Oceanic-forced multicentennial-scale precipitation variability in Eastern Brazil” (EGU General Assembly; Vienna, 2018)
A. Bahr et al.
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(2020): Monsoonal forcing controlled cold water coral growth off southeastern Brazil during the past 160 kyrs. Biogeosciences
Bahr, A., Doubrawa, M., Titschack, J., Austermann, G., Koutsodendris, A., Nürnberg, D., Albuquerque, A.L., Friedrich, O., Raddatz, J.
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(2021): Atlantic Subtropical Gyre in relation to Western Boundary Current dynamics during abrupt glacial climate fluctuations. Global and Planetary Change, 21, 103497
Nürnberg, D., Riff, T., Bahr, A., Karas, C., Meier, K., Lippold, J.
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(2021): Role of the tropical Atlantic for the interhemispheric heat transport during the last deglaciation. Paleoceanography and Paleoclimatology, 36, e2020PA004107
Meier, K., Bahr, A., Chiessi, C., Albuquerque, A.L., Friedrich, O.
