Rodrigo Azevedo Nascimento

Subtropical gyres contribute significantly to climate regulation by constituting the main pathways for energy redistribution between low and high latitudes. The South Atlantic Subtropical Gyre (SASG) operates in a region that is critical to the Atlantic energy balance. Its northern boundary, defined by the southern branch of the South Equatorial Current (sSEC), constitutes an important interhemispheric connection for heat and salt exchange. The sSEC bifurcates in the western tropical South Atlantic, giving rise to the Brazil Current, which transports warm and saline tropical waters southward, and the North Brazil Current, which transports heat and salt northwestward. In addition to acting as a linkage between both Atlantic subtropical gyres, the North Brazil Current constitutes an essential part of the upper branch of the Atlantic Meridional Overturning Circulation. Recently, observational data have recorded a reduction in the intensity of heat and salt transport toward the North Atlantic, along with a southward displacement of the SASG. These phenomena are likely influenced by the progressive weakening of the Atlantic Meridional Overturning Circulation, detected since the late 20th century and projected to continue in the coming decades. The lack of long-term oceanic records with adequate spatial coverage for the South Atlantic basin prevents a more complete understanding of the trends and impacts associated with SASG displacements. Here we investigate meridional changes in the position of the northern boundary of the SASG during Termination V and Marine Isotope Stage 11, through a multiproxy approach to reconstruct upper-ocean water-column stratification from a sediment core in the western tropical South Atlantic. To this end, relative abundance counts of the planktonic foraminifer species Globorotalia truncatulinoides (dextral and sinistral) and stable oxygen isotope (δ¹⁸O) analyses of G. truncatulinoides (dextral) and Globigerinoides ruber albus have been conducted. Due to the deeper apparent calcification depth of G. truncatulinoides, the difference in the δ¹⁸O signal of both species (Δδ¹⁸Otrunca-ruber) functions as an indicator of thermocline depth. The strong association of G. truncatulinoides with regions of deep thermocline allows the establishment of a relationship between variations in species abundance and changes in the stratification of the upper ocean. Since deep thermocline conditions can be interpreted as a signature of the presence of both Atlantic subtropical gyres, the proxies employed allow tracking meridional shifts in the SASG. Preliminary results are promising and suggest that the northern boundary of the SASG varied meridionally on millennial and orbital timescales. Mg/Ca ratio analyses will be performed on both species to reconstruct surface and subsurface temperatures, as well as to discriminate the individual roles of temperature and salinity in upper-ocean stratification.

The Amazon basin is one of the most influential hydroclimatic systems on the planet, modulating the water cycle and energy balance of the tropical regions. The long-term stability of the Amazon rainforest is closely linked to regional hydroclimate, as shifts in rainfall amount and seasonality can drive substantial ecological transformations across the basin. Understanding how the Amazon system has naturally responded to past oceanic and atmospheric forcings is crucial, and paleoclimate records provide information to investigate the mechanisms governing Amazonian hydroclimate variability through time. Yet, paleoclimate archives that allow us to explore its variability beyond the last 50 ka are limited. The objective of this study is to characterize orbital- and millennial-scale hydroclimatic changes within the Amazon basin over the last 220,000 years through high-resolution X-ray fluorescence (XRF) analysis on marine sediment cores recovered from the northern margin of French Guyana during the AMARYLLIS–AMAGAS II cruise, specifically at stations S6 and S7 where a composite record was produced for each station by combining cores MD23-3652Q/53 and MD23-3655Q/56, respectively.XRF results of S6 cores, which have an age model, allowed us to associate geochemical changes with Marine Isotopic Stages (MIS 1–7) and Heinrich Stadials of the last 60 ka. High values of Fe/Ca and Al/K log-ratios are observed during Heinrich Stadials (HS1–H6), indicating increased input of terrigenous vs. biogenic material, consistent with enhanced fluvial discharge, and an enhanced contribution of chemically weathered material from the Amazon basin. Elevated ln(Fe/K) and ln(Al/K) ratios specifically suggest a stronger contribution from lowland, highly leached soils and enhanced precipitation-driven weathering within the basin, rather than changes in sediment provenance. These patterns suggest globally wetter conditions over the Amazon Basin during HS, in agreement with the documented southward shift of the Intertropical Convergence Zone (ITCZ) and strengthening of the South American monsoon. During interglacial periods such as MIS 5e and MIS 1, higher sea levels likely reduced the continental influence on sedimentation at the core sites, enhancing the relative contribution of marine carbonates. This is reflected by lower ln(Fe/Ca) and ln(Fe/K) ratios, together with higher ln(Sr/Ca) values, which indicate a decline in terrigenous input and a stronger oceanic influence. During glacial stages (MIS 6, 4 and 2), the combination of high ln(Fe/Ca) and an increased ln(Al/K), denotes intensified fluvial supply and stronger chemical weathering under humid conditions, despite lowered sea level.S7 cores, although lacking an age model, allow for a qualitative comparison due to their geographic proximity to S6. The general trends in Fe/Ca and Al/K log-ratios are consistent with those of S6, suggesting that S7 cores record the same regional signal of Amazonian fluvial variability, modulated by the tropical hydroclimatic regime. These preliminary results demonstrate that XRF records from S6 and S7 cores constitute an exceptional archive for evaluating the interaction between the Amazonian hydroclimatic system and North Atlantic forcings, indicating that during Heinrich Stadials, a southward migration of the ITCZ and intensified tropical rainfall enhanced Amazonian river discharge and continental runoff.

The scientifically and politically agreed-upon benchmark for limiting global warming in the coming decades, as stated in the Paris Agreement, was set to “well below 2 °C above pre-industrial levels”. The interglacial period known as Marine Isotope Stage (MIS) 11, which occurred ca. 400 thousand years ago, is thought to have reached temperatures up to ~2 °C warmer than pre-industrial conditions, making it an excellent case study for investigating the behaviour of Earth’s climate under warmer-than-pre-industrial conditions.The South Atlantic is particularly important for Earth’s climate, as it represents a major heat reservoir and plays a crucial role in heat transport between the hemispheres. To better understand the behaviour of the South Atlantic under a ~2 °C warmer-than-pre-industrial climate, we are generating and compiling paleoceanographic records from the eastern and western margins of the basin spanning MIS 11 and its preceding deglaciation (Termination V). The outcomes of this research have the potential to greatly improve our understanding of South Atlantic dynamics under warmer-than-pre-industrial climates, thereby helping to constrain plausible future climate scenarios.

The modern rainfall regime over semiarid northeastern Brazil (NEB) is primarily controlled by the seasonal migration of the Intertropical Convergence Zone (ITCZ), with the rainy season occurring during March-April, when the ITCZ reaches its southernmost position. It is well accepted that reductions in cross-equatorial northward heat transport mediated by the Atlantic Meridional Overturning Circulation (AMOC) during abrupt cold phases of Dansgaard-Oeschger (DO) cycles, namely Greenland stadials (GS) and Heinrich stadials (HS), triggered southward migrations of the ITCZ. These migrations led to enhanced precipitation over NEB, a signal that is more clearly captured in paleoclimate records during HS.Here, we present a reconstruction of millennial-scale Atlantic ITCZ dynamics based on the longest continuous paleoprecipitation records available for NEB, spanning the last 160 thousand years (kyr) at a temporal resolution of ca. 30 years. In addition, we use numerical climate model outputs to investigate the mechanisms underlying this millennial-scale variability. The hydroclimate records are derived from a composite of iron-to-calcium (Fe/Ca) and iron-to-potassium (Fe/K) log-ratios measured in bulk sediments from marine sediment cores MD23-3670Q and MD23-3671 (1365 m water depth; 1°34.7′ S, 43°1.4′ W), retrieved offshore NEB during the AMARYLLIS-AMAGAS II cruise in 2023. High ln(Fe/Ca) and ln(Fe/K) values reflect increases in continental precipitation, which enhance chemical weathering, erosion, and terrigenous discharge to the adjacent continental margin. Our records reveal enhanced continental precipitation during cold phases (i.e., GS and HS) of the 25 DO cycles identified in the NGRIP ice core, reinforcing the strong teleconnection between tropical hydroclimate variability and high-latitude climate changes.The records further indicate consistently higher continental precipitation over NEB during HS than during GS. We show that terrigenous input (i.e., continental precipitation) is inversely related to AMOC strength (r = 0.78, p < 0.05) and to mid- to high-latitude North Atlantic sea surface temperatures (SSTs) (r = 0.9, p < 0.05). Particularly, HS are systematically associated with the highest ln(Fe/Ca) values, the weakest AMOC conditions, and the lowest North Atlantic SSTs.Numerical simulations performed with the Institut Pierre Simon Laplace climate model version 4 show a gradual increase in annual NEB rainfall as AMOC intensity is progressively reduced. This enhanced rainfall results from a gradual (i) lengthening of the rainy season over NEB and (ii) increase in mean monthly precipitation during the rainy season. The lengthening of the rainy season is driven by both a southward shift in the annual mean ITCZ position and an expansion of its southward seasonal migration range. Meanwhile, we propose that the increase in mean monthly precipitation is related to warmer SSTs in the tropical South Atlantic, which can enhance deep atmospheric convection and act as a direct moisture source for the adjacent continent. Together, these findings suggest that enhanced rainfall over NEB during North Atlantic cold events is not solely driven by a southward migration of the ITCZ, thereby advancing our understanding of tropical atmospheric dynamics during episodes of AMOC slowdown.

Previous studies have linked increased precipitation over northeastern Brazil to millennial-scale climate events, particularly Heinrich Stadials (HS), which are associated with increased freshwater input into the subpolar North Atlantic and weakening of the Atlantic Meridional Overturning Circulation (AMOC). During these intervals, reduced northward heat transport promotes a southward displacement of the Intertropical Convergence Zone, leading to enhanced precipitation over northeastern Brazil. While the atmospheric response to AMOC variability during HS is relatively well documented, the variability of ocean circulation at intermediate depths, especially in the western equatorial Atlantic (WEA), remains poorly constrained.Here, we reconstruct intermediate depth circulation and northeastern Brazil climate over the last glacial period (i.e., the last 35 ka) using marine sediment core MD23-3670Q (1ºS 43ºW; 1,357 mbsl) from the WEA. Stable carbon isotopes (δ13C) were measured in epibenthic (i.e., Cibicidoides pachyderma, C. lobatulus, C. incrassatus) and endobenthic (i.e., Uvigerina peregrina, Globobulimina affinis) foraminiferal species at a minimum resolution of 4 cm as a proxy for ventilation and carbon cycle. X-ray fluorescence (XRF) scanning performed every 1 cm provided proxies for redox conditions (i.e., ln(Mn/Ti)) and continental input (i.e., ln(Ti/Ca)).Negative δ13C excursions in epibenthic foraminifera during the Younger Dryas and HS 1, 2, and 3 suggest the accumulation of respired carbon at intermediate depths in the WEA. This interpretation is supported by the low input of terrestrial and marine organic matter to the bottom of the ocean, inferred from the small δ13C gradient between C. pachyderma and U. peregrina. In addition, neodymium isotope records from nearby core indicate only minor changes in intermediate water mass provenance throughout the last glacial period, suggesting the persistent predominance of southern sourced waters at our site. Negative C. pachyderma δ13C excursions, together with reduced ln(Mn/Ti) values during HS, indicate decreased oxygen penetration in the sediments due to a combination of reduced intermediate depth ventilation and increased sedimentation rates. A reduced δ13C gradient between C. pachyderma and G. affinis further suggests a shallower redox boundary during HS, corroborating the reduced oxygen penetration into the bottom sediments. The close correspondence between our ventilation proxies and millennial-scale variations in ln(Ti/Ca) provides evidence for ocean-atmosphere coupling between reduced intermediate water ventilation in the WEA and enhanced precipitation over northeastern Brazil, driven by changes in the AMOC strength over the last 35 ka.

The climate system experienced several periodic oscillations over the last ca. 800 ka known as glacial-interglacial (G-IG) cycles. Disruptions of the global carbon cycle were evident on this time scale, promoting fluctuations in the atmospheric CO2 concentration leading to global climate variability. In the more recent interglacials, both Antarctic temperatures and atmospheric CO2 concentrations are significantly higher than in the previous “lukewarm interglacials” (ca. 800 – 430 ka) before the Mid-Brunhes Transition (MBT). Changes in the Atlantic Meridional Overturning Circulation (AMOC) and deepwater formation rate around Antarctica have been invoked to explain a 30 ppm increase in the atmospheric CO2 ­during post-MBT interglacial periods. Deepwater variability is tightly coupled to the ventilation of CO2 in the Southern Ocean by atmospheric and oceanic connections, contributing to carbon storage in the deep ocean and the atmospheric CO2. Here, we present a new 770 ka benthic foraminifera δ13C record from sediment core GL-854 retrieved from the western South Atlantic (WSA) at 2200 m water depth. We compare our record with published δ13C data from the eastern margin to investigate the zonal gradient variability of the North Atlantic Deep Water (NADW) in the deep South Atlantic basin. WSA δ13C variability and absolute values strongly mimic the North Atlantic mid-depth record at the NADW formation region. This similarity is interpreted as NADW preferentially carrying a modified signal through the deep western boundary current towards the WSA (rather than towards the eastern margin) after the MBT. The δ13C gradient based on the difference between benthic foraminifera C. wuellerstorfi from both margins (Δδ13Cw-e) gradually increases after a transitional period between ca. 400 ka to 300 ka towards the Holocene. We suggest that the mechanism behind this long-term increasing trend on the Δδ13Cw-e record post-MBT is the result of enhanced production of North Component Water due to Agulhas Leakage intensification driven by reduced sea-ice extent after the MBT. Furthermore, reduced sea-ice extent decreases the Antarctic Bottom Water density and formation in the Southern Ocean, contributing to the deepening of the AMOC during post-MBT interglacial periods. Our interpretation proposes a framework connecting sea-ice and ocean-atmosphere dynamics to deepwater geometry within the South Atlantic basin, which ultimately contributed to the climate change observed across the MBT.