Modern rainfall over northeastern (NE) South America is highly sensitive to and strongly controlled by the position of the Intertropical Convergence Zone (ITCZ), that is, the rising limb of the Hadley circulation (meridionally oriented overturning circulation cell) (Hastenrath, 2012;Poveda et al., 2006;Schneider et al., 2014). Today, semi-arid northern NE Brazil receives ca. 50% of its total annual precipitation during March-April, when the ITCZ and the associated equatorial precipitation reach its seasonal southernmost position over NE South America (Figure 1). A suggested mid-to late Holocene southward migration of the Abstract Modern precipitation over northeastern (NE) South America is strongly controlled by the seasonal meridional migration of the Intertropical Convergence Zone (ITCZ). Ample evidence from the Northern Hemisphere suggests a mid-to late Holocene southward migration of the ITCZ. Such a shift would be expected to increase precipitation over semi-arid northern NE Brazil (Southern Hemisphere). However, the most robust precipitation record from northern NE Brazil shows a drying trend throughout the Holocene. Here, we address this issue presenting a high-temporal resolution reconstruction of precipitation over northern NE Brazil based on data from a marine sediment core, together with analyses of mid-and late Holocene simulations performed with the fully coupled climate model FGOALS-s2. Both, our data and the climate model simulations show a decrease in precipitation over northern NE Brazil from the mid-to the late Holocene. The model outputs further indicate a latitudinal contraction of the seasonal migration range of the ITCZ that, together with an intensification of the regional Walker circulation, were responsible for the mid-to late Holocene changes in precipitation over NE South America. Our results reconcile apparently conflicting precipitation records and climate mechanisms used to explain changes in precipitation over NE South America.
Abstract. This study explores the mechanisms behind the high glacial productivity in the southern Brazilian margin (SBM) during the last 70 kyr using planktonic foraminifera assemblage and subsurface temperature information derived using the modern analogue technique. We show that enhanced glacial productivity was driven by the synergy of two mechanisms operating in different seasons: (i) enhanced productivity in the upwelling region during short austral summer events; and (ii) the persistent presence of the Plata Plume Water (PPW) due to prolonged austral winter conditions. We suggest that the upwelling systems in the southern Brazilian margin were more productive during the last glacial period due to the enhanced Si supply for diatom production by high-Si thermocline waters preformed in the Southern Ocean. We hypothesize that orbital forcing did not have a major influence on changes in upwelling during the last glacial period. However, the more frequent northward intrusions of the Plata Plume Water were modulated by austral winter insolation at 60∘ S via changes in the strength of alongshore southwesterly winds. After the Last Glacial Maximum, the reduced Si content of thermocline waters decreased upwelling productivity, while lower austral winter insolation decreased the influence of the Plata Plume Water over the southern Brazilian margin, reducing regional productivity.
Negative excursions in the stable carbon isotopic composition (δ13C) at Atlantic intermediate to mid‐depths are common features of millennial‐scale events named Heinrich Stadials. The mechanisms behind these excursions are not yet fully understood, but most hypotheses agree on the central role played by the weakening of the Atlantic meridional overturning circulation. Marine records registering millennial‐scale negative δ13C excursions in the Atlantic are mostly restricted to the Heinrich Stadials of the last deglacial, while the Heinrich Stadials of the last glacial are poorly studied. Here, we constrain changes in bottom water ventilation in the western tropical South Atlantic mid‐depth during Heinrich Stadials of the last glacial and deglacial by investigating marine core M125‐95‐3. The concurrent decreases in benthic foraminifera δ13C and increases in bulk sediment sulfur indicate an increased Northern Component Water (NCW) residence time in the western tropical South Atlantic mid‐depth during Heinrich Stadials. Furthermore, a coherent meridional pattern emerges from the comparison of our new data to previously published mid‐depth records from the western South Atlantic. While our record shows the largest negative δ13C excursions during almost all Heinrich Stadials, the western equatorialAtlantic showed medium and the subtropical South Atlantic showed the smallest negative excursions. This meridional pattern supports the notion that during Heinrich Stadials, a reduction in the NCW δ13C source signal together with the accumulation of respired carbon at NCW depths drove the negative δ13C excursions. We suggest that the negative δ13C excursions progressively increase along the NCW southwards pathway until the signal dissipates/dilutes by mixing with Southern Component Water.
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