Millennial-scale variability in eastern South American climate and western South Atlantic circulation during the last 70,000 years

• Ano de defesa: 2020
• Autor(a) principal: Campos, Marília de Carvalho
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Heinrich Stadials; América do Sul; Atlantic meridional overturning circulation; Atlântico Sul; Circulação meridional do Atlântico; Late Quaternary; Oceanic ventilation; Precipitação; Precipitation; Quaternário tardio; Rio São Francisco; Salinidade da superfície do mar; São Francisco River; Sea surface salinity; Sea surface temperature; South America; South Atlantic; Temperatura da superfície do mar; Ventilação oceânica
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/100/100136/tde-19062020-163854/

During the last glacial and deglacial periods, the Earth experienced several abrupt millennial-scale climate change events, named Heinrich Stadials (HS) and Dansgaard-Oeschger events. The HS in particular are commonly attributed to reductions in the strength of the Atlantic meridional overturning circulation (AMOC). Given the marked AMOC influence over global climate and the possibility of the AMOC to reduce its strength in the future due to ongoing climate change, the study of HS became a topic of key importance. Here we investigate the effects of last glacial and deglacial HS to eastern (E) South American hydroclimate as well as western South Atlantic oceanography. To do so, we studied marine sediment core M125-95-3 collected from the western tropical South Atlantic mid-depth (10.94°S, 36.20°W, 1897 m water depth), near the mouth of the São Francisco River (i.e., off E South America), spanning the last ca. 70,000 years. We produced radiocarbon ages from planktonic foraminifera, X-ray fluorescence analyses from bulk sediment samples, stable oxygen and carbon isotopic analyses from planktonic and benthic foraminifera, and Mg/Ca analyses from planktonic foraminifera. We conclude that the last glacial and deglacial HS were marked by positive precipitation anomalies over the São Francisco River drainage basin, and that this was the southernmost drainage basin from the South American Atlantic seaboard that experienced substantial increases in precipitation. We propose a new mechanism for explaining tropical South America HS positive precipitation anomalies. This mechanism involves austral summer precipitation increases only over E South America while the rest of tropical South America experienced precipitation increases during the winter, challenging the widely held assumption of a strengthened monsoon during HS. During the same abrupt events, the mid-depth western tropical South Atlantic experienced decreases in d13C and increases in sulfur (unprecedentedly used as a proxy for abrupt millennial-scale changes in bottom water ventilation) that we attributed to an increased Northern Component Water (NCW) residence time and to the accumulation of respired carbon at mid-depths. We also suggest that the negative d13C excursions progressively increase along the NCW southwards pathway, reaching its maximum in the western tropical South Atlantic from where the signal dissipates/dilutes by mixing with Southern Component Water. Regarding the upper water column, the western tropical South Atlantic surface waters were warmer and saltier during HS. Data from the Agulhas Leakage region also recorded similar features, however, with larger positive excursions. We conclude that the heat and salt imported from the Indian Ocean during HS were only partially transferred to the western tropical South Atlantic. Thus, Indian Ocean salt that eventually reached the high latitudes of the North Atlantic helping on the recovering of the AMOC was most probably transported mainly within the thermocline. Finally, the data shown herein indicate that past events of weak AMOC profoundly affected South American hydroclimate and western South Atlantic oceanography