Coupled changes in intermediate water ventilation and northeastern Brazil precipitation during the last glacial period

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.