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Historic droughts document the strong spatio-temporal variability of the South American Monsoon System, which currently provides more than two thirds of the rainfall in tropical South America. The drivers of this variability have remained not well understood due to the lack of continuous, highresolution paleorecords, especially from the more arid regions of tropical South America. Here we present a novel record of moisture availability across eastern South America for the past ~5,000 years from a sediment core retrieved off eastern Brazil. We document distinct decadal- to millennial-scale spatial shifts of major atmospheric convection centers that caused increasingly pronounced droughts in eastern South America over the past ~2,000 years. These fluctuations were triggered by climate anomalies in the high northern latitudes and propagated into equatorial latitudes via fluctuations in North Atlantic Overturning Circulation strength. As global warming is expected to decrease oceanic overturning due to enhanced meltwater input into the North Atlantic while at the same time reducing precipitation over eastern South America, an increasing risk for long-lasting droughts can be expected for this region, posing severe socioeconomic challenges. [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]

Fire causes dramatic energy and matter exchanges between biosphere and atmosphere on a regional to global scale. Predicting fires, however, is hindered by the complex interplay of fire, climate, and vegetation. Paleo‐fire records provide critical information beyond instrumental records that cover only the past few decades and may be used to assess the role of fire in large‐scale and long‐term environmental changes. Here we present a 22,000‐year multiproxy record of fire regime from a sediment core retrieved offshore South Java, Indonesia. We use microcharcoal in combination with two molecular markers of burning, levoglucosan and polycyclic aromatic hydrocarbons, to reconstruct fire occurrence as well as fire intensity in the past. We show that fire occurrence and intensity were high during the Last Glacial Maximum (LGM; around 21,000 years ago) and low during the Heinrich Stadial 1 and the early Holocene. Both fire regime and vegetation in tropical regions with high annual rainfall were primarily controlled by rainfall seasonality. However, fire additionally stabilized the savannah (rainforest)‐dominated ecosystem during the LGM (early Holocene) but caused transitions between the two vegetation types during the deglaciation and the late Holocene. Key Points: Microcharcoal reflects regional fire occurrence, while the ratio between two novel molecular markers reflects past fire intensityRainfall seasonality controlled both fire regime and vegetationFires of different frequencies and intensities were capable of either stabilizing or destabilizing savannah/rainforest vegetation [ABSTRACT FROM AUTHOR]

Late Holocene hydroclimate variations have been extensively recognized in Amazonia, but the effects of such changes on riverine discharge within the Amazon lowlands are still poorly understood. We investigated a sediment core covering circa 4,000 to 300 cal yr BP collected in the lower valley of the Xingu River (Xingu Ria) in an area under the influence of the Amazon River. Our results indicate a decrease in precipitation in the Amazon lowlands throughout the studied period and reduced input of coarser and potassium‐rich Amazon River sediments to the confluence from about 2,600 to 1,400 cal yr BP. We suggest that lower temperatures in the extratropical Southern Hemisphere weakened the South American Summer Monsoon and led to a decrease in the water discharge of the Amazon River during this period. Plain Language Summary: The variability of the Amazon River water discharge over the last millennia is still poorly understood. Sediments deposited in the lower valley of the Xingu River, a major eastern tributary of the Amazon River, allowed for the identification of a decrease in rainfall over the Amazon lowlands throughout the last 4,000 years. Changes in the relative input of Amazon and Xingu sediments in an area affected by both rivers indicate that the water discharge of the Amazon River decreased from about 2,600 to 1,400 years ago. We suggest that anomalously low temperatures in the Southern Hemisphere led to drier conditions in Amazonia during this period. Key Points: Sediments in fluvial rias reveal hydrological changes in Amazonia during the last millenniaCompound‐specific plant wax isotope records suggest a decrease in rainfall in Amazon lowlands over the last 4,000 yrA shift in the extratropical temperature gradient from 2,600 to 1,400 cal yr BP potentially led to lower Amazon River water discharge [ABSTRACT FROM AUTHOR]

The stable hydrogen isotope composition of lipid biomarkers, such as alkenones, is a promising new tool for the improvement of paleosalinity reconstructions. Laboratory studies confirmed the correlation between lipid biomarker δD composition (δDLipid), water δD composition (...) and salinity. Yet, there is limited insight into the applicability of this proxy in oceanic environments. To fill this gap, we test the use of the δD composition of alkenones (...) and palmitic acid (δDPA) as salinity proxies using samples of surface suspended material along the distinct salinity gradient induced by the Amazon Plume. Our results indicate a positive correlation between salinity and ..., while the relationship between ... and δDLipid is more complex: δDPA correlates strongly with ... (r² = 0.81) and shows a salinity dependent isotopic fractionation factor. δ only correlates with δ in samples with alkenone concentrations > 10ngL-1 (r² = 0.51). These findings are mirrored by alkenone based temperature reconstructions, which are inaccurate for samples with alkenone concentrations < 10ngL-1. Deviations in ... and temperature are likely to be caused by limited haptophyte algae growth due to low salinity and light limitation imposed by the Amazon Plume. Our study confirms the applicability of δDLipid as a salinity proxy in oceanic environments. But it raises a note of caution concerning regions where low alkenone production can be expected due to very low salinity conditions. To circumvent these limitations, we suggest the complementary use of ... and δDPA. [ABSTRACT FROM AUTHOR]

Abstract: High abundances of mangrove pollen have been associated with transgressive cycles on tropical margins, but the detailed relations between systems tracts and the taphonomy of the pollen are unclear. We report here the occurrence and high abundance of Rhizophora pollen, in association with taraxerol, a Rhizophora-sourced biomarker, from a high-resolution Congo fan core covering the last deglaciation. An age model based on 14C dates enables the temporal changes in taraxerol content and the percentage frequencies and flux (pollen grains (pg) cm−2 (103 yr)−1) of mangrove pollen to be compared quantitatively with the lateral rate of transgression across the flooding surface (derived from glacio-hydro-isostatic model output and the bathymetry of the margin). Rhizophora pollen concentrations and taraxerol content of the sediment are very strongly positively correlated with the lateral rate of transgression and indicate, independently of any sequence stratigraphic context, that mangrove pollen spikes are associated with the transgressive systems tract rather than the highstand systems tract or maximum flooding surface. Lower-resolution longer-term records from this margin indicate an association between taraxerol concentrations and transgressive rather than regressive phases. The flux of these materials to the Congo fan is interpreted as a function of the erosion of flooded mangrove swamp on the shelf and, less importantly, changing extent of mangrove habitat, during sea-level rise. Congo River palaeoflood events also result in reworking of mangrove pollen and supply to the fan, but this mechanism is subdominant. Rhizophora pollen has been underestimated in many palynological studies undertaken on cores from the African margin because of inappropriate sieve mesh size used during laboratory preparation. [Copyright &y& Elsevier]