ALBERT

Description: The temporal succession of changes in Amazonian hydroclimate during Heinrich Stadial 1 (HS1) (ca. 18-14.7 cal ka BP) is currently poorly resolved. Here we present HS1 records based on isotope, inorganic and organic geochemistry from a marine sediment core influenced by the Amazon River discharge. Our records offer a detailed reconstruction of the changes in Amazonian hydroclimate during HS1, integrated over the basin. We reconstructed surface water hydrography using stable oxygen isotopes (d18O) and Mg/Ca-derived paleotemperatures from the planktonic foraminifera Globigerinoides ruber, as well as salinity changes based on stable hydrogen isotope (dD) of palmitic acid. We also analyzed branched and isoprenoid tetraether concentrations, and compared them to existing bulk sediment ln(Fe/Ca) data and vegetation reconstruction based on stable carbon isotopes from n-alkanes, in order to understand the relationship between continental precipitation, vegetation and sediment production. Our results indicate a two-phased HS1 (HS1a and HS1b). During HS1a (18-16.9 cal ka BP), a first sudden increase of sea surface temperatures (SST) in the western equatorial Atlantic correlated with the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and the associated southern hemisphere warming phase of the bipolar seesaw. This phase was also characterized by an increased delivery of terrestrial material. During HS1b (16.9-14.8 cal ka BP), a decrease in terrestrial input was, however, associated with a marked decline of seawater d18O and palmitic acid dD. Both isotopic proxies independently indicate a drop in sea surface salinity (SSS). A number of records under the influence of the North Brazil Current, in contrast, indicate increases in SST and SSS resulting from a weakened AMOC during HS1. Our records thus suggest that the expected increase in SSS due to the AMOC slowdown was overridden by a two-phased positive precipitation anomaly in Amazonian hydroclimate.

Description: Seafloor methane release can significantly affect the global carbon cycle and climate. Appreciable quantities of methane are stored in continental margin sediments as shallow gas and hydrate deposits, and changes in pressure, temperature and/or bottom-currents can liberate significant amounts of this greenhouse gas. Understanding the spatial and temporal dynamics of marine methane deposits and their relationships to environmental change are critical for assessing past and future carbon cycle and climate change. Here we present foraminiferal stable carbon isotope and sediment mineralogy records suggesting for the first time that seafloor methane release occurred along the southern Brazilian margin during the last glacial period (40–20 cal ka BP). Our results show that shallow gas deposits on the southern Brazilian margin responded to glacial−interglacial paleoceanographic changes releasing methane due to the synergy of sea level lowstand, warmer bottom waters and vigorous bottom currents during the last glacial period. High sea level during the Holocene resulted in an upslope shift of the Brazil Current, cooling the bottom waters and reducing bottom current strength, reducing methane emissions from the southern Brazilian margin.

Description: The modern state of the Atlantic meridional overturning circulation promotes a northerly maximum of tropical rainfall associated with the Intertropical Convergence Zone (ITCZ). For continental regions, abrupt millennial-scale meridional shifts of this rainbelt are well documented, but the behavior of its oceanic counterpart is unclear due the lack of a robust proxy and high temporal resolution records. Here we show that the Atlantic ITCZ leaves a distinct signature in planktonic foraminifera assemblages. We applied this proxy to investigate the history of the Atlantic ITCZ for the last 30,000 years based on two high temporal resolution records from the western Atlantic Ocean. Our reconstruction indicates that the shallowest mixed layer associated with the Atlantic ITCZ unambiguously shifted meridionally in response to changes in the strength of the Atlantic meridional overturning with a southward displacement during Heinrich Stadials 2-1 and the Younger Dryas. We conclude that the Atlantic ITCZ was located at ca. 1°S (ca. 5° to the south of its modern annual mean position) during Heinrich Stadial 1. This supports a previous hypothesis, which postulates a southern hemisphere position of the oceanic ITCZ during climatic states with substantially reduced or absent cross-equatorial oceanic meridional heat transport.

Description: Changes in heat transport associated with fluctuations in the strength of the Atlantic meridional overturning circulation (AMOC) are widely considered to affect the position of the Intertropical Convergence Zone (ITCZ), but the temporal immediacy of this teleconnection has to date not been resolved. Based on a high-resolution marine sediment sequence over the last deglaciation, we provide evidence for a synchronous and near-linear link between changes in the Atlantic interhemispheric sea surface temperature difference and continental precipitation over northeast Brazil. The tight coupling between AMOC strength, sea surface temperature difference, and precipitation changes over northeast Brazil unambiguously points to a rapid and proportional adjustment of the ITCZ location to past changes in the Atlantic meridional heat transport.