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Atlantic Meridional Overturning Circulation (AMOC) plays a major role in the climate system by modulating the depth and rate of oceanic heat storage. Some climate simulations suggest that reduced AMOC decreases bottom water ventilation and that the heat absorbed by the ocean starts to mix downwards, warming Atlantic intermediate waters. This has been corroborated for the western North Atlantic by benthic foraminifera geochemical records from periods of reduced AMOC during the last deglaciation. However, the deep‐water response remains poorly constrained, and the lack of direct paleotemperature reconstructions limits our understanding about the effects of reduced circulation on ocean heat uptake. We present a new reconstruction of bottom water temperatures from core GeoB9508‐5 (2,384 m water depth, 15°29.90°N/17°56.88°W) off the northwestern African Margin. Our paleotemperature record, based on Uvigerina spp. Mg/Ca, shows two episodes of intense transient deep water warming in times of decreasing overturning circulation, followed by long periods of heat uptake stagnation. First, during AMOC slowdown in the Heinrich stadial 1, when paleotemperatures of ∼2°C persisted for ∼5.4 Kyr coincident with the weakest stage of AMOC; and second in the Younger Dryas, when bottom water temperatures >4°C lasted ∼2.5 Kyr during a less intense AMOC decline. This suggests a stagnation of deep‐water heat uptake in the deep NE Atlantic possibly linked to a reduced downward advection of heat during times of a reduced AMOC, supporting the hypothesis that AMOC strength sets the depth of oceanic heat storage in the North Atlantic. Plain Language Summary: Anthropogenic activity affects earth‐atmosphere energy balance enhancing climate change in the last decades. The ocean plays a key role in this balance, by taking up to 90% of the excess heat from the atmosphere and redistributing it globally though the Atlantic Meridional Overturning Circulation (AMOC). For the 21st century, a possible 34%–45% AMOC reduction has been hypothesized, raising concern on its effects on ocean heat uptake and climate change. To contribute to the understanding of these possible effects, we reconstructed bottom water temperatures changes for eastern North Atlantic deep waters over the last 46,000 years, including two periods with a significantly reduced AMOC: (a) Heinrich Stadial 1 (18,200–14,900 years ago) and (b) the Younger Dryas (12,800–11,700 years ago). Our results suggest that with a weak AMOC, Atlantic intermediate waters warm as ventilation decreases, at the same time this heat stops being transferred to the deep eastern North Atlantic for thousands of years. The implications of these processes for global warming still need to be investigated. Key Points: Intense North Atlantic Deep‐Water (NADW) warming at the beginning of Heinrich Stadial 1 (HS1) and the Younger Dryas (YD)With a weak Atlantic Meridional Overturning Circulation during HS1 and the YD deep water temperature remained stable in poorly ventilated bottom watersWeaker HS1 and YD NADW formation, may reduce heat dissipation into the deep Atlantic [ABSTRACT FROM AUTHOR]

Particulate pyrogenic carbon (PyC) transported by rivers and aerosols, and deposited in marine sediments, is an important part of the carbon cycle. The chemical composition of PyC is temperature dependent and levoglucosan is a source‐specific burning marker used to trace low‐temperature PyC. Levoglucosan associated to particulate material has been shown to be preserved during riverine transport and marine deposition in high‐ and mid‐latitudes, but it is yet unknown if this is also the case for (sub)tropical areas, where 90% of global PyC is produced. Here, we investigate transport and deposition of levoglucosan in suspended and riverbed sediments from the Amazon River system and adjacent marine deposition areas. We show that the Amazon River exports negligible amounts of levoglucosan and that concentrations in sediments from the main Amazon tributaries are not related to long‐term mean catchment‐wide fire activity. Levoglucosan concentrations in marine sediments offshore the Amazon Estuary are positively correlated to total organic content regardless of terrestrial or marine origin, supporting the notion that association of suspended or dissolved PyC to biogenic particles is critical in the preservation of PyC. We estimate that 0.5–10 × 106 g yr−1 of levoglucosan is exported by the Amazon River. This represents only 0.5–10 ppm of the total exported PyC and thereby an insignificant fraction, indicating that riverine derived levoglucosan and low‐temperature PyC in the tropics are almost completely degraded before deposition. Hence, we suggest caution in using levoglucosan as tracer for past fire activity in tropical settings near rivers. Plain Language Summary: During plant organic matter burning, most of the carbon is emitted to the atmosphere as CO2, but a fraction is retained as pyrogenic biomass. The chemical composition of pyrogenic biomass depends on fire temperature and allows to differentiate between high and low‐temperature pyrogenic biomass. Here, we analyzed if low‐temperature pyrogenic biomass is preserved during transport in the Amazon River and deposited in western tropical Atlantic sediments. We found that only negligible amounts of low‐temperature pyrogenic biomass reach the Atlantic through riverine transport. While most pyrogenic carbon (PyC) originates in the tropics, our study suggests that only an insignificant fraction of low‐temperature PyC is permanently stored in marine sediments, where it would be removed from the short‐term carbon cycle. Key Points: Only negligible amounts of the source‐specific low‐temperature biomass burning tracer levoglucosan are exported by the Amazon River systemMarine sediment levoglucosan yields are controlled by organic carbon content regardless of marine or terrestrial source of organic matterDust and river derived levoglucosan escape burial in the tropical Atlantic, despite the dominant tropical source of pyrogenic carbon [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The tropical rainbelt impacts food and water security for 1 billion people. Knowing its dynamics is of utmost importance. The suggestion of a southward migration of the tropical rainbelt through the Holocene (last 11,700 years) has influenced paleoclimatology for two decades. However, most of the available evidence supporting this suggestion comes from tropical Northern Hemisphere precipitation reconstructions like northernmost South America. They systematically show a decrease in precipitation through the Holocene. In the tropical Southern Hemisphere like northeastern Brazil, at the opposite side of the tropical rainbelt, precipitation reconstructions are, however, rare. We reconstructed mid‐ to late Holocene (last 5,200 years) changes in precipitation over northeastern Brazil (tropical Southern Hemisphere), where modern precipitation is associated with the southern border of the tropical rainbelt. We analyzed three independent indicators of changes in precipitation recorded in marine sediments collected off northeastern Brazil. All indicators suggest a decrease in precipitation over northeastern Brazil from the mid‐ to the late Holocene. Together with climate model simulations, our results indicate a latitudinal contraction of the tropical rainbelt. A Holocene contraction of the rainbelt, in contrast to a southward migration, reconciles apparently conflicting precipitation reconstructions and provides valuable insights into the dynamics of the tropical rainbelt. Key Points: Precipitation over northern northeastern Brazil decreased from the mid‐ to the late HoloceneThe meridional migration range of the Intertropical Convergence Zone contracted from the mid‐ to the late HoloceneTogether with the intensification of the regional Walker circulation, our results reconcile previously conflicting records [ABSTRACT FROM AUTHOR]

Depth transects of benthic foraminiferal oxygen isotopes from the Atlantic Ocean show that glacial‐interglacial changes are larger at deep (> ~2,000 m) than at intermediate water levels. Our model results suggest that the smaller changes in the upper 1,000 m of the water column are a result of the glacial sea level lowering of about 120 m, leading to warmer temperatures of around 1 °C and hence a smaller glacial‐interglacial stable oxygen isotope difference. In contrast, a shoaling of the water mass boundary to ~2,000‐m water depth between the northern and southern source waters is caused by the expansion of a cold (close to the freezing point) southern source water in the abyssal ocean, increasing the oxygen isotope values of benthic foraminifera from the Last Glacial Maximum in the deep Atlantic. These two effects explain the different amplitudes of glacial‐interglacial stable oxygen isotope differences in the upper and deeper water column of the Atlantic Ocean. Key Points: Benthic δ18O glacial‐interglacial changes off north West Africa and in the west Atlantic Ocean were investigated with an ocean modelModel results suggest smaller glacial‐interglacial δ18O differences around the thermocline layer due to glacial sea level loweringThe expansion of cold southern source water during the LGM led to larger glacial‐interglacial δ18O differences in the deep Atlantic Ocean [ABSTRACT FROM AUTHOR]

Surface ocean circulation in the western equatorial Atlantic is mainly wind driven and plays a major role for the transport of warm waters to the North Atlantic. Past changes in the strength and direction of the trade winds are well documented, but the response of the western equatorial Atlantic circulation and water column structure to these changes is unclear. Here we used the difference between the stable isotopic oxygen composition of two species of planktonic foraminifera (Globigerinoides ruber white and Neogloboquadrina dutertrei) from two sediment cores collected off northeastern Brazil to investigate millennial‐ and orbital‐scale changes in upper ocean stratification since the Last Interglacial. Our records indicate enhanced upper ocean stratification during several Heinrich stadials, partly due to a shoaling of the thermocline, which was linked to a decrease in the strength of southeast trades winds. In addition, we show that a decrease in wind zonality induced by increases in Northern Hemisphere low‐latitude summer insolation causes a shoaling of the thermocline in the western equatorial Atlantic. These ocean‐atmosphere changes contributed to a reduction in the cross‐equatorial transport of warm waters, particularly during Heinrich stadials and Marine Isotope Stage 4. Key Points: Changes in western equatorial Atlantic thermocline depth were recorded on millennial and orbital time scalesShoaling of the thermocline during Heinrich stadials was associated with weak southeast trade windsShoaling of the thermocline during periods of low‐latitude boreal summer insolation maxima occurred due to decreased wind zonality [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

During Termination 1, millennial-scale weakening events of the Atlantic meridional overturning circulation (AMOC) supposedly produced major changes in sea surface temperatures (SSTs) of the western South Atlantic, and in mean air temperatures (MATs) over southeastern South America. It has been suggested, for instance, that the Brazil Current (BC) would strengthen (weaken) and the North Brazil Current (NBC) would weaken (strengthen) during slowdown (speed-up) events of the AMOC. This antiphase pattern was claimed to be a necessary response to the decreased North Atlantic heat piracy during periods of weak AMOC. However, the thermal evolution of the western South Atlantic and the adjacent continent is so far largely unknown. Here we address this issue, presenting high-temporal-resolution SST and MAT records from the BC and southeastern South America, respectively. We identify a warming in the western South Atlantic during Heinrich Stadial 1 (HS1), which is followed first by a drop and then by increasing temperatures during the Bølling-Allerød, in phase with an existing SST record from the NBC. Additionally, a similar SST evolution is shown by a southernmost eastern South Atlantic record, suggesting a South Atlantic-wide pattern in SST evolution during most of Termination 1. Over southeastern South America, our MAT record shows a twostep increase during Termination 1, synchronous with atmospheric CO2 rise (i.e., during the second half of HS1 and during the Younger Dryas), and lagging abrupt SST changes by several thousand years. This delay corroborates the notion that the long duration of HS1 was fundamental in driving the Earth out of the last glacial. [ABSTRACT FROM AUTHOR]

During Termination 1, millennial-scale weakening events of the Atlantic meridional overturning circulation (AMOC) supposedly produced major changes in sea surface temperatures (SST) of the western South Atlantic, and in mean air temperatures (MAT) over southeastern South America. It was suggested, for instance, that the Brazil Current (BC) would strengthen (weaken) and the North Brazil Current (NBC) would weaken (strengthen) during slowdown (speed-up) events of the AMOC. This anti-phase pattern was claimed to be a necessary response to the decreased North Atlantic heat piracy during periods of weak AMOC. However, the thermal evolution of the western South Atlantic and the adjacent continent is largely unknown and a compelling record of the BC-NBC anti-phase behavior remains elusive. Here we address this issue, presenting high temporal resolution SST and MAT records from the BC and southeastern South America, respectively. We identify a warming in the western South Atlantic during Heinrich Stadial 1 (HS1), which is followed first by a drop and then by increasing temperatures during the Bølling--Allerød, in-phase with an existing NBC record. Additionally, a similar SST evolution is shown by a southernmost eastern South Atlantic record, suggesting a South Atlantic-wide pattern in SST evolution during most of Termination 1. Over southeastern South America, our MAT record shows a two-step increase during Termination 1, synchronous with atmospheric CO2 rise (i.e., during the second half of HS1 and during the Younger Dryas), and lagging abrupt SST changes by several thousand years. This delay corroborates the notion that the long duration of HS1 was fundamental to drive the Earth out of the last glacial. [ABSTRACT FROM AUTHOR]

We investigate changes in the delivery and oceanic transport of Amazon sediments related to terrestrial climate variations over the last 250 ka. We present high-resolution geochemical records from four marine sediment cores located between 5 and 12°N along the northern South American margin. The Amazon River is the sole source of terrigenous material for sites at 5 and 9° N, while the core at 12° N receives a mixture of Amazon and Orinoco detrital particles. Using an endmember unmixing model, we estimated the relative proportions of Amazon Andean material ("%-Andes", at 5 and 9°N) and of Amazon material ("%-Amazon", at 12°N) within the terrigenous fraction. The %-Andes and %-Amazon records exhibit significant precessional variations over the last 250 ka that are more pronounced during interglacials in comparison to glacial times. High %-Andes values observed during periods of high austral summer insolation reflect the increased delivery of suspended sediments by Andean tributaries and enhanced Amazonian precipitation, in agreement with western Amazonian speleothem records. However, low %-Amazon values obtained at 12°N during the same periods seem to contradict the increased delivery of Amazon sediments. We propose that reorganisations in surface ocean currents modulate the northwestward transport of Amazon material. In agreement with published records, the seasonal North Brazil Current retroflection is intensified (or prolonged in duration) during cold substages of the last 250 ka (which correspond to intervals of high DJF or low JJA insolation) and deflects eastward the Amazon sediment and freshwater plume. [ABSTRACT FROM AUTHOR]

This study presents linear trends of coastal upwelling intensity in the later part of the 20th century (1960-2001) employing various indices of upwelling, derived from meridional wind stress and sea surface temperature. The analysis was conducted in the four major coastal upwelling regions in the world, which are off North-West Africa, Lüderitz, California and Peru respectively. The trends in meridional wind stress showed a steady increase of intensity from 1960-2001, which was also reflected in the SST index calculated for the same time period. The steady cooling observed in the instrumental records of SST off California substantiated this observation further. Correlation analysis showed that basin-scale oscillations like the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) could not be directly linked to the observed increase of upwelling intensity off NW Africa and California respectively. The relationship of the North Atlantic Oscillation (NAO) with coastal upwelling off NW Africa turned out to be ambiguous due to a negative correlation between the NAO index and the meridional wind stress and a lack of correlation with the SST index. Our results give additional support to the hypothesis that the coastal upwelling intensity increases globally because of raising greenhouse gas concentrations in the atmosphere and an associated increase of the land-sea pressure gradient and meridional wind stress. [ABSTRACT FROM AUTHOR]

Abstract: A global foraminiferal model was used to determine the seasonal imprint of planktonic foraminifera on the sedimentary record. The model provides monthly concentrations of five planktonic foraminiferal species used in paleoceanographic reconstructions including N. pachyderma (sin. and dex.), G. bulloides, G. ruber (white) and G. sacculifer. The temperature imprint in foraminiferal shells varies according to the season of calcification, and the sedimentary records retain this seasonal imprint. Proxy records for a species will therefore be weighted towards the values during the season of maximum production for that species. Our model prediction reveals that, in general, at high latitudes, close to the geographical limit of occurrence of each species, the signal is biased towards summer conditions. In contrast, at lower latitudes the signal is biased towards winter or annual mean conditions. Temperatures derived from G. ruber (white) and G. sacculifer are most suitable for estimating annual mean SST in tropical waters, between 20° N/S, while G. ruber (white) when collected at mid-latitudes, near to 40° latitude, reflects mainly summer conditions. We carried out sensitivity experiments to study the response of planktonic foraminiferal seasonality to changes in temperature. We forced the model decreasing the temperature globally by 2 °C and 6 °C. In most of the regions, due to the cooling, the season of maximum production shifted to a warmer season. Thus, the annual species population recorded little change in the temperature. In tropical waters, where temperature seasonality is low, foraminiferal population recorded the entire temperature variation. These findings highlight the importance of considering changes in seasonality through time, specially for temperature reconstructions based on monospecific samples, such as those derived from planktonic foraminiferal isotopic and trace element composition. [Copyright &y& Elsevier]

We present a new planktonic foraminifera model developed for the global ocean mixed-layer. The main purpose of the model is to explore the response of planktonic foraminifera to different boundary conditions in the geological past, and to quantify the seasonal bias in foraminiferabased paleoceanographic proxy records. This model is forced with hydrographic data and with biological information taken from an ecosystem model to predict monthly concentrations of the most common planktonic foraminifera species used in paleoceanography: N. pachyderma (sinistral and dextral varieties), G. bulloides, G. ruber (white variety) and G. sacculifer. The sensitivity of each species with respect to temperature (optimal temperature and range of tolerance) is derived from previous sediment-trap studies. Overall, the spatial distribution patterns of most of the species are in agreement with core-top data. N. pachyderma (sin.) is limited to polar regions, N. pachyderma (dex.) and G. bulloides are the most common species in high productivity zones, while G. ruber and G. sacculifer are more abundant in tropical and subtropical oligotrophic waters. For N. pachyderma (sin) and N. pachyderma (dex.), the season of maximum production coincides with that observed in sediment-trap records. Model and sediment-trap data for G. ruber and G. sacculifer show, in general, lower concentrations and less seasonal variability at all sites. A sensitivity experiment suggest that, within the temperature-tolerance range of a species, food availability may be the main parameter controlling its abundance. [ABSTRACT FROM AUTHOR]

Seasonal changes in the flux of planktonic foraminifera have to be understood to interpret corresponding proxy-based reconstructions. To study the seasonal cycle of planktonic foraminifera species we developed a numerical model of species concentration (PLAFOM). This model is forced with a global hydrographic dataset (e.g. temperature, mixed layer depth) and with biological information taken from an ecosystem model (e.g. "food type", zooplankton abundance) to predict monthly concentrations of the most common planktonic foraminifera species used for proxies: N. pachyderma (sinistral and dextral varieties), G. bulloides, G. ruber (white variety) and G. sacculifer. The sensitivity of each species with respect to temperature (optimal temperature and range of tolerance) is derived from sediment-trap studies. Overall, the spatial distribution patterns of most of the species are comparable to core-top data. N. pachyderma (sin.) is limited to polar regions, N. pachyderma (dex.) and G. bulloides are the most common species in high productivity zones like upwelling areas, while G. ruber and G. sacculifer are more abundant in tropical and subtropical oligotrophic waters. Modeled seasonal variation match well with sediment-trap records in most of the locations for N. pachyderma (sin), N. pachyderma (dex.) and G. bulloides. G. ruber and G. sacculifer show, in general, lower concentrations and less seasonal variability in all sites. The lower variability is reflected in the model output, but the small scale variations are not reproduced by the model in several locations. Due to the fact that the model is forced by climatological data, it can not capture interannual variations. The sensitivity experiments we carried out show that, inside the temperature tolerance range, food availability is the main parameter which controls the abundance of some species. The here presented model represents a powerful tool to explore the response of planktonic foraminifera to different boundary conditions, and to quantify the seasonal bias in foraminifera-based proxy records. [ABSTRACT FROM AUTHOR]

The article discusses a study on predicting planktic foraminiferal fluxes on a global scale. It highlights the use of empirical model to predict the fluxes. The materials and methodologies used in the study are outlined. It also provides insights on the statistical analysis of foraminiferal species fluxes.

Understanding and quantifying the seasonal and spatial distribution of planktic foraminiferal fluxes reflected in sedimentary assemblages is key to interpret foraminifera-based proxies in paleoceanography. Towards this goal we present an empirical model to predict foraminiferal fluxes on a global scale. A compilation of planktic foraminiferal flux and export production data from globally distributed sediment traps together with environmental data of sea-surface temperature and mixed-layer depth from online databases is used to calibrate the model that calculates monthly foraminiferal fluxes for the 18 most common species. The calibrated model is then forced with a global data set of hydrographic and productivity data to predict monthly foraminiferal fluxes worldwide. The predictive skills of the model are assessed by comparing the model output with planktic foraminiferal assemblages from globally distributed surface sediments as well as with measured foraminiferal fluxes of sediment traps not included in the calibration data set. Many general distribution patterns of foraminiferal species recognized from the model output compare favorably with observations from coretops or sediment traps, even though the model still produces problematic results in some places. Among others, meridional gradients in species richness and diversity, increased relative abundances of Neogloboquadrina pachyderma (dex.) in upwelling areas, and peak abundances of Globigerinella siphonifera in oligotrophic subtropical gyres show good agreement between model and coretops. Absolute foraminiferal fluxes are significantly underestimated in most cases, while seasonal variations can be reproduced for some species. Interannual differences in foraminiferal fluxes are not reflected by the model which might partly be due to a lack of actual environmental data for the calibration and model experiments. The limited predictive skills of the model suggest that additional parameters should be considered. Export production should probably be replaced by depth-dependent primary production data reflecting the true food availability for planktic foraminifera. Results might also be improved by adding a dynamic component to the model and linking it to an ecosystem model. [ABSTRACT FROM AUTHOR]

The long-term orbital-scale sea surface temperature (SST) variability in the tropics is thought to be mainly driven by greenhouse gases (GHG) forcing. However, few studies have investigated the drivers of such variability in the tropical Atlantic. Given the evidence of orbital-scale changes in Atlantic Meridional Overturning Circulation (AMOC) strength, one can hypothesize that AMOC variability also modulates the long-term tropical Atlantic SST through the bipolar seesaw mechanism. According to this mechanism, under weak [strong] AMOC conditions, the Southern Hemisphere is expected to warm up [cool down], while the Northern Hemisphere cools down [warms up]. Here, we investigate the long-term SST variability of the western tropical South Atlantic (WTSA), i.e., along the main pathway of the upper AMOC branch towards the equator, using a new 300 thousand years (kyr)-long Mg/Ca-based SST record. Our SST record shows glacial-interglacial variability superimposed by four remarkable long-term warm events during the three recorded glacial periods. These glacial warm events occurred between ca. 280–260, 160–143, 75–60, and 40–24 ka before present. Our results support the notion that atmospheric GHG plays a leading role in modulating the glacial-interglacial SST variability in the WTSA. However, it does not explain the occurrence of glacial warm events. Our study supports that the glacial warm events were caused by an orbital-scale bipolar seesaw mechanism operating in the Atlantic due to changes in the AMOC strength. These warm events may have been amplified by annual mean insolation driven by obliquity. Finally, we suggest that the long-term bipolar seesaw warmed the western tropical (South) Atlantic during the MIS 5/4 transition when the Earth's climate was cooling off. • The western tropical South Atlantic SST was reconstructed for the last 300 kyr. • Greenhouse gases control the glacial-interglacial SST variability. • Four long-term warm events were identified during glacial conditions. • The warm events were caused by an orbital-scale bipolar seesaw in the Atlantic. [ABSTRACT FROM AUTHOR]

The Amazon region hosts the world's largest watershed spanning from high elevation Andean terrains to lowland cratonic shield areas in tropical South America. This study explores variations in optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) signals in suspended silt and riverbed sands retrieved from major Amazon rivers. These rivers drain Pre-Cambrian to Cenozoic source rocks in areas with contrasting denudation rates. In contrast to the previous studies, we do not observe an increase in the OSL sensitivity of quartz with transport distance; for example, Tapajós and Xingu Rivers show more sensitive quartz than Solimões and Madeira Rivers, even though the latter have a significantly larger catchment area and longer sediment transport distance. Interestingly, high sensitivity quartz is observed in rivers draining relatively stable Central Brazil and Guiana shield areas (denudation rate ξ = 0.04 mm yr − 1 ), while low sensitivity quartz occurs in less stable Andean terrains ( ξ = 0.24 mm yr − 1 ). An apparent linear correlation between quartz OSL sensitivity and denudation rate suggests that OSL sensitivity may be used as a proxy for erosion rates in the Amazon basin. Furthermore, luminescence sensitivity measured in sand or silt arises from the same mineral components (quartz and feldspar) and clearly discriminates between Andean and shield sediments, avoiding the grain size bias in provenance analysis. These results have implications for using luminescence sensitivity as a proxy for Andean and shield contributions in the stratigraphic record, providing a new tool to reconstruct past drainage configurations within the Amazon basin. [ABSTRACT FROM AUTHOR]

We present a multi-year record of shell fluxes and δ 18 O of six planktonic foraminifera species ( Globigerinoides ruber pink, Globigerinoides ruber white, Trilobatus sacculifer , Orbulina universa , Neogloboquadrina dutertrei , Globorotalia menardii ) from sediment traps located in the southwestern Atlantic. Among the six species, only the fluxes of G. ruber white and N. dutertrei exhibit a significant seasonal component, with G. ruber white showing a single flux peak in austral summer, and N. dutertrei exhibiting two flux peaks in spring and autumn. To estimate calcification depths of the studied species, we then compare their measured δ 18 O to vertical δ 18 O profiles predicted for each collection time from in-situ temperature profiles and climatological salinity profiles. For the majority of the cases, the measured δ 18 O could be accounted for by in-situ calcification, assuming species-specific temperature-δ 18 O calibrations. The resulting estimates of the calcification depth imply that each species exhibits a characteristic typical mean calcification depth. The estimated calcification depths for N. dutertrei (mode 60–70 m) and G. menardii (mode 70–80 m) appear to track the depth of the thermocline in the region, whereas the calcification depths of the remaining four species correspond to conditions in the mixed layer. Among the apparent mixed-layer calcifiers, G. ruber pink and white appeared to calcify consistently shallower (mode 30–40 m) while T. sacculifer calcified deeper (mode 50–60 m). Because of the low flux seasonality, the observed oxygen isotope offsets among the species are similar to the flux-weighted mean annual δ 18 O offsets, indicating that isotopic offsets among the species in sediment samples are mainly due to different calcification depths. Since the habitat offsets among the species are consistent across seasons, δ 18 O in sedimentary shells can be used to track conditions in different parts of the water column and the difference in the oxygen-isotope composition between surface species (best represented by G. ruber pink) and thermocline species (best represented by N. dutertrei ) can be used as a proxy for stratification in the southwestern Atlantic. [ABSTRACT FROM AUTHOR]

The supply of nutrients to the low-latitude thermocline is largely controlled by intermediate-depth waters formed at the surface in the high southern latitudes. Silicic acid is an essential macronutrient for diatoms, which are responsible for a significant portion of marine carbon export production. Changes in ocean circulation, such as those observed during the last deglaciation, would influence the nutrient composition of the thermocline and, therefore, the relative abundance of diatoms in the low latitudes. Here we present the first record of the silicic acid content of the Atlantic over the last glacial cycle. Our results show that at intermediate depths of the South Atlantic, the silicic acid concentration was the same at the Last Glacial Maximum (LGM) as it is today, overprinted by high silicic acid pulses that coincided with abrupt changes in ocean and atmospheric circulation during Heinrich Stadials and the Younger Dryas. We suggest these pulses were caused by changes in intermediate water formation resulting from shifts in the subpolar hydrological cycle, with fundamental implications for the nutrient supply to the Atlantic. [ABSTRACT FROM AUTHOR]

Stable carbon isotopic (δ13C) minimum events have been widely described in marine archives recording the properties of thermocline and intermediate waters during glacial terminations. However, the mechanisms associated with these events remain ambiguous. Here we present three high temporal resolution deep-dwelling planktonic foraminifera δ13C records from the main thermocline and one benthic δ13C record from the modern core of Antarctic Intermediate Water (AAIW). Our sediment cores are distributed along the western South Atlantic from the equator to the subtropics, with the longest record spanning the last ∼300 kyr. The results show that δ13C minimum events were pervasive features of the last three glacial terminations and Marine Isotope Stage 4/3 transition in the western South Atlantic. Two distinct mechanisms were responsible for the δ13C minima at the thermocline and intermediate depths of the Atlantic, respectively. We suggest that the δ13C minimum events at the thermocline were mostly driven by the thermodynamic ocean-atmosphere isotopic equilibration, which is supported by calculated δ13C of dissolved inorganic carbon in the subtropical western South Atlantic as well as by previously published model simulations. On the other hand, intermediate depths δ13C minimum events in the tropics were likely caused by the slowdown of the Atlantic meridional overturning circulation and the associated accumulation of isotopically light carbon at mid and intermediate depths of the Atlantic Ocean. • We investigated δ13C minimum events in the western South Atlantic. • δ13C minima occurred in the upper ocean over the last three glacial terminations. • Thermodynamic equilibration drives δ13C minima at the thermocline. • AMOC slowdown drives δ13C minima at intermediate depth of the tropical Atlantic. [ABSTRACT FROM AUTHOR]

• The western tropical Atlantic stratification was reconstructed for the last 300 ka. • Stratification in the upper western tropical Atlantic is precession-paced. • ITCZ system and South Atlantic Subtropical Gyre dynamics govern stratification. The upper ocean circulation in the western tropical Atlantic (WTA) is responsible for the northward cross-equatorial heat transport as part of the Atlantic Meridional Overturning Circulation (AMOC). This cross-equatorial transport is influenced by the thermocline circulation and stratification. Although seasonal thermocline stratification in the WTA is precession-driven, the existence of an orbital pacemaker of changes in the entire WTA upper ocean stratification, which comprises the main thermocline, remains elusive. Here, we present a 300 ka-long record of the WTA upper ocean stratification and main thermocline temperature based on oxygen isotopes (δ 18 O) and Mg/Ca of planktonic foraminifera. Our Δ δ 18 O record between Globigerinoides ruber and Globorotalia truncatulinoides , representing upper ocean stratification, shows a robust precession pacing, where strong stratification was linked to high summer insolation in the Northern Hemisphere (precession minima). Mg/Ca-based temperatures support that stratification is dominated by changes in thermocline temperature. We present a new mechanism to explain changes in WTA stratification, where during the Northern Hemisphere summer insolation maxima, the Intertropical Convergence Zone shifts northward, developing a negative wind stress curl anomaly in the tropical Atlantic. This, in turn, pulls the main thermocline up and pushes the South Atlantic Subtropical Gyre southwards, increasing the stratification to the north of the gyre. This mechanism is supported by experiments performed with the Community Earth System Model (CESM1.2). Finally, we hypothesize that the precession-driven WTA stratification may affect the cross-equatorial flow into the North Atlantic. [ABSTRACT FROM AUTHOR]