Coupling of equatorial Atlantic surface stratification to glacial shifts in the tropical rainbelt

Abstract

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.

Millennial‐ to Orbital‐Scale Responses of Western Equatorial Atlantic Thermocline Depth to Changes in the Trade Wind System Since the Last Interglacial

“…Hence, the increase in δ 18 O of N. dutertrei during HS1 and the YD can be explained by a decrease in subsurface temperature, linked to a shoaling of the mixed layer and thermocline depth. In addition, the changes recorded by their R NG/g ratio (proxy for mixed layer depth, Portilho‐Ramos et al, ) are in agreement with our Δδ 18 O dut‐rub changes for the LGM, HS1, and the YD. A dissimilarity between both proxies can only be noticed during the EH (Figure S7b).…”

“…Our Δδ 18 O dut‐rub record indicates an increase in upper ocean stratification in the western equatorial Atlantic (partly due to a shoaling of the thermocline; see section ) during HS, particularly during HS5a, HS5, and HS1 (Figures d and h). Our Δδ 18 O dut‐rub record from GeoB16202‐2 agrees with the R N/Gg tracer from core GeoB16206‐1 (defined as % Neogloboquadrina / (% Neogloboquadrina + % G. glutinata , Portilho‐Ramos et al, ) (Figures d and S7), which further supports our interpretation of enhanced upper ocean stratification. Portilho‐Ramos et al () interpret the change in R N/Gg during HS1 as a shallow mixed layer depth, which would allow the presence of thermocline waters closer to the surface.…”

“…In order to explain our N. dutertrei δ 18 O record, temperature and δ 18 O sw reconstructions at subsurface would be needed, but this type of data is still scarce for this area. To our knowledge, Portilho‐Ramos et al () is the only study that presents a subsurface temperature reconstruction for the last 30 ka in this region, using the Modern Analog Technique applied to planktonic foraminifera assemblages. Their data from core GeoB16206‐1 (Figure b) indicates a subsurface cooling during HS2, HS1, and the YD, which were associated with a shoaling of the mixed layer (Figure S7a).…”

“…However, these previous works in the western equatorial Atlantic could not explore millennial‐scale changes of the thermocline depth due to the low resolution of their records. Recently, a high‐resolution record of planktonic foraminifera assemblages covering the last 30 ka, indicated that a shallow mixed layer depth occurred in the western equatorial Atlantic during Heinrich stadials (HS) 2 and 1, as well as the Younger Dryas (YD; Portilho‐Ramos et al, ). The authors proposed that the increased stratification was linked to the excess of freshwater flux below the Intertropical Convergence Zone (ITCZ) that hinders wind‐driven vertical mixing.…”

“…The dashed white rectangle marks the area enlarged in panel b. (b) Besides cores GL‐1248 and GeoB16202‐2, the other cores in the western equatorial Atlantic are GeoB2204‐2 (Wolff et al, ), GeoB1523‐1 (Kinkel et al, ), CDH‐86 (Nace et al, ), and GeoB16206‐1 (Portilho‐Ramos et al, ). Black solid lines with arrows indicate surface currents and white dotted lines the seasonal range of the Intertropical Convergence Zone (ITCZ).…”