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The vegetation history and climate in southeastern Brazil, as well as the oceanic dynamics of the tropical Atlantic Ocean offshore, were reconstructed for the last 7.4 cal ka BP. This reconstruction was based on pollen, fern spores and dinoflagellate cysts identified in a marine core (CF10–04B). It was possible to verify the presence of an ombrophilous forest from 7.4 cal ka BP. Near the base of the PI zone low concentrations of pollen and fern spores are recorded, along with low frequencies of forest taxa and fern spores and an increase in pollen types of open vegetation, suggesting less humid climatic conditions than currently observed in the coastal regions of Southeastern Brazil. The assemblages of dinoflagellate cysts suggest a neritic zone with warm upper column temperature, high salinity and oligotrophic environment (evidenced by the presence of O. centrocarpum, Spiniferites spp. and L. machaerophorum). An increase in Tropical Waters (TW) and the approximation of the Brazilian Current (BC) in the middle shelf of southeastern Brazil in the mid-Holocene probably influenced the low accumulation of cysts, pollen grains, fern spores, and their associations. It is still possible to verify that at approximately 4.6 cal ka BP, an increase in the rainfall in the southeastern Brazilian region was probably the fundamental factor for the expansion of vegetation, mainly based on the considerable increases in hygrophytic and aquatic plants. The marked increase in the accumulation of dinoflagellate cysts, with the dominance of autotroph taxa (mainly O. centrocarpum, followed by Spiniferites spp), evidences the warmer waters of the BC and more intense surface upwelling of the South Atlantic Central Water (SACW).

This study presents new quantitative data on benthic foraminifera from three bathymetric transects of the Brazil (11-14°S, 420-1900 m) and Campos (22°S, 430-2000 m) basins. The quantity and quality of organic matter flux as well as substrate properties and hydrodynamic conditions at the sediment-water interface are identified as key parameters controlling assemblage distribution. Based on the total (stained and unstained) fauna, a distinct biogeographic divide between a Globocassidulina subglobosa/crassa assemblage in the Campos Basin and a rosalinid/bolivinid assemblage in the Brazil Basin occurs across the bifurcation of the South Atlantic Central Water into its southward subtropical and northward tropical branches. In the Campos Basin, coarser sediments, increased bottom current activity, and variable nutrient supply favor an assemblage of Globocassidulina subglobosa/crassa, Nuttalides umbonifer and Alabaminella weddellensis. Occurrences of cold-water coral mounds in 870 m provide an ecological niche favoring species such as Alabaminella weddellensis which benefit from trapped nutrients. The Brazil Basin is characterized by increased abundances of Rosalina and Bolivina, while Globocassidulina subglobosa/crassa is comparatively less frequent. Assemblages with G. subglobosa/crassa, Rosalina spp., Bolivina variabilis and Bolivina subreticulata are favored by a relatively high nutrient input at 14°S. Further north, assemblages with Bolivina subreticulata, Bolivina variabilis, Epistominella exigua, G. subglobosa/crassa are located beneath the velocity core of the North Brazil Undercurrent (NBUC), coinciding with more clayey sediments rich in TOC. Occurrences of delicate branching forms such as Saccorhiza ramosa indicate a more stable setting, distal to the main current. Rose Bengal stained (living) specimens are scarce in all three regions, as is typical for deep-sea foraminiferal faunas. Their patterns of species distribution largely reflect those observed for the total fauna.

International audience; 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.

Mercury (Hg) is a toxic globally spread pollutant that has been found at increasing concentrations in the South Atlantic Ocean. The present work provides the first insight into the total mercury (HgT, unfiltered waters) content in the water of the Brazilian Exclusive Economic Zone (BEEZ), within a 24°S to 20°S. Water samples were collected from surface to 3400 m depth along transects, and analyzed with atomic fluorescence. The mean HgT concentration for the Tropical Water mass (TW) was 6.3 ± 1.4 pM (n = 16), for the South Atlantic Central Water (SACW), 5.9 ± 0.7 pM (n = 8), for the Antarctic Intermediate Water (AAIW), 5.0 ± 0.6 pM (n = 2), for the Upper Circumpolar Deep Water (UCDW), 6.5 pM (n = 1), and for the North Atlantic Deep Water (NADW), 5.7 ± 0.9 pM (n = 12). HgT concentrations were highest throughout the BEEZ in comparison with other parts of the Atlantic Ocean, farther from the coast.

Cold-water corals (CWC), dominantly Desmophyllum pertusum (previously Lophelia pertusa), and their mounds have been in the focus of marine research during the last two decades; however, little is known about the mound-forming capacity of other CWC species. Here, we present new 230Th/U age constraints of the relatively rarely studied framework-building CWC Solenosmilia variabilis from a mound structure off the Brazilian margin combined with computed tomography (CT) acquisition. Our results show that S. variabilis can also contribute to mound formation, but reveal coral-free intervals of hemipelagic sediment deposits, which is in contrast to most of the previously studied CWC mound structures. We demonstrate that S. variabilis only occurs in short episodes of

Expansions of coastal upwelling spots along the Brazilian coast were previously reported for Marine Isotope Stage (MIS) 5, but open questions remain regarding the climatic mechanisms and the periodicity of such changes. Based on two marine sediment cores, we provide evidence for multiple intensifications of the upwelling regime off the Southeast Brazilian margin (SBM) during several interglacials and highlight the major role of eccentricity as the responsible forcing. In addition, we show a two-step change in the upwelling regime across the Mid-Brunhes Event (MBE) and an increase in the amplitude of upwelling variability after this climatic transition. Our findings point to substantial modifications of the upwelling regions during several glacial-interglacial transitions that probably altered the regional marine productivity regime and the carbon budget.

Subtropical ocean gyres play a key role in modulating the global climate system redistributing energy between low and high latitudes. A poleward displacement of the subtropical gyres has been observed over the last decades, but the lack of long-term monitoring data hinders an in-depth understanding of their dynamics. Paleoceanographic records offer the opportunity to identify meridional changes in the subtropical gyres and investigate their consequences to the climate system. Here we use the abundance of planktonic foraminiferal species Globorotalia truncatulinodes from a sediment core collected at the northernmost boundary of the South Atlantic Subtropical Gyre (SASG) together with a previously published record of the same species from the southernmost boundary of the SASG to reconstruct meridional fluctuations of the SASG over last ca. 70 kyr. Our findings indicate southward displacements of the SASG during Heinrich Stadials (HS) 6-4 and HS1, and a contraction of the SASG during HS3 and HS2. During HS6-4 and HS1, the SASG southward displacements likely boosted the transfer of heat to the Southern Ocean, ultimately strengthening deep-water upwelling and CO2 release to the atmosphere. We hypothesize that the ongoing SASG poleward displacement may further increase oceanic CO2 release.

During the last deglaciation abrupt millennial‐scale perturbations of the Atlantic Meridional Overturning Circulation massively altered the interhemispheric heat distribution affecting, for example, continental ice volume and hydroclimate. If and how the related cross‐equatorial heat transport was controlled by the interplay between the southward‐flowing Brazil Current (BC) and northward‐flowing North Brazil Current (NBC) remains controversial. To assess the role of tropical heat transport during the last deglaciation, we obtained a high‐resolution foraminiferal Mg/Ca‐based sea surface temperature (SST) record from the BC domain at 21.5°S. The data reveal a yet undocumented warming of at least 4.6°C of the BC during Heinrich Stadial 1 at ∼16 ka indicating massive oceanic heat accumulation in the tropical South Atlantic. Simultaneously, a strongly diminished NBC prevented the release of this excess heat into the northern tropics. The observed magnitude of heat accumulation substantially exceeds numerical model simulations, stressing the need to further scrutinize atmospheric and oceanic heat transport during extreme climatic events., Plain Language Summary: The Atlantic overturning circulation underwent abrupt millennial‐scale perturbations. Such phases of sluggish oceanic circulation resulted in a substantial reduction of northward heat transport. As a consequence, substantial cooling occurred in the Northern Hemisphere and warming occurred in the Southern Hemisphere with severe effects on tropical precipitation. The distribution of heat within the western tropical Atlantic is accomplished by the southward‐flowing BC and the northward‐flowing NBC. By reconstructing SSTs for the interval between 20,000 and 10,000 yr before present, we assess the role of both currents in the interhemispheric heat transport during weak Atlantic overturning. We found that a sluggish overturning circulation resulted in anomalous southward heat transport by the BC in concert with a weak NBC, which lead to a yet undocumented warming of at least 4.6°C in the western tropical South Atlantic. This warming significantly exceeds reconstructions based on numerical simulations. This points to the need to further improve our understanding of changes in the cross‐equatorial oceanic and atmospheric heat transport in response to rapid changes in ocean circulation, in particular as a significant weakening of the Atlantic overturning circulation is predicted in the wake of anthropogenic climate change., Key Points: Brazil Current heat transport coupled to changes in strength of the Atlantic Meridional Overturning Circulation during the last deglacial. Antiphased heat transport by the Brazil and North Brazil Currents during Heinrich Event 1. Warming of western tropical South Atlantic sea surface based on foraminiferal Mg/Ca exceeds numerical model results for Heinrich Event 1., Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659, Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) http://dx.doi.org/10.13039/501100001807, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) http://dx.doi.org/10.13039/501100002322, MCTI, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) http://dx.doi.org/10.13039/501100003593, Focus Program of the Goethe University Frankfurt, Alexander von Humboldt‐Stiftung http://dx.doi.org/10.13039/100005156

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.

Summary The ¹⁴C-age offset between the organic matter (OM) and carbonates buried in marine sediments of the Cabo Frio upwelling system during the past 6.5 kyr was investigated. The ¹⁴C-ages of the OM were 787 ± 166 years older than those of foraminifera, which might be indicative of a long-term storage and re-processing of the OM on the mid-shelf sediments caused by local upwelling-related hydrodynamic processes. A gradual increase up to the maximum ¹⁴C-offset from 5.8 to 4.0 cal kyr BP may be a response to the intensification of the Brazil Current activity over the shelf, extending the sediment translocation and, consequently, the OC aging. Also, an intensification of the South American monsoon during this period might had favored the input of pre-aged terrigenous OM. Between 4.0 and 2.0 cal kyr BP the ¹⁴C-offset decreases, possibly due to a subsequent weakening in the bottom currents. After 1.8 cal kyr BP, a slightly increase in the ¹⁴C-offset could be caused by an upwelling enhancement. These results reinforce the existence of a temporal decoupling between the organic proxies commonly used in paleoceanographic studies and the correspondent age models applied, which highlights the need to interpret proxy-based information with caution, especially in upwelling regions.

Cold-water corals (CWC) constitute important deep-water ecosystems that are increasingly under environmental pressure due to ocean acidification and global warming. The sensitivity of these deep-water ecosystems to environmental change is demonstrated by abundant paleo-records drilled through CWC mounds that reveal a characteristic alteration between rapid formation and dormant or erosive phases. Previous studies have identified several parameters such as food supply, oxygenation, and carbon saturation state of bottom water as central for driving or inhibiting CWC growth, yet there is still a large uncertainty about the relative importance of the different environmental parameters. To advance this debate we have performed a multi-proxy study on a sediment core retrieved from the 25 m high Bowie Mound, located in 866 m water depth on the continental slope off south-eastern Brazil, a structure built up mainly by the CWC Solenosmilia variabilis. Our results indicate a multi-factorial control on CWC growth and mound formation at Bowie Mound during the past ~160 kyrs, which reveals distinct formation pulses during glacial high northern latitude cold events (Heinrich Stadials, HS) largely associated with anomalous continental wet periods. The ensuing enhanced run-off elevated the terrigenous nutrient and organic matter supply to the continental margin, and might have boosted marine productivity. The dispersal of food particles towards the CWC colonies during HS was facilitated by the highly dynamic hydraulic conditions along the continental slope that prevailed throughout glacial periods. These conditions caused the emplacement of a pronounced nepheloid layer above Bowie Mound aiding the concentration and along-slope dispersal of organic matter. Our study thus demonstrates a yet unrecognized impact of continental climate variability on a highly vulnerable deep-marine ecosystem.

The last glacial was an interval characterized by a sequence of abrupt millennial-scale events well documented mainly from the Greenland and Antarctica ice-cores. Although the triggers are not fully understood, most of the works agree that they occurred in consonance with oscillations in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Paleoceanographic reconstructions have shown that cold millennial-scale stadials were accompanied by high temperatures in the subsurface to intermediate waters of the Atlantic Ocean that may have acted in both the basal melting of ice-sheets and in the rapid atmospheric warming during the onset of warm interstadials. Assuming that recent transient models indicated an accentuated response of the subsurface western South Atlantic to the millennial-scale disturbances, here we present a paleoceanographic reconstruction in this area based on the deep-dwelling planktic foraminifer Globorotalia inflata. Our high-resolution oxygen isotope (d18O) presents a sequence of millennial-scale variability that strongly resembles the structure of the Greenland Dansgaard-Oeschger cycles, mainly during Marine Isotope Stage (MIS) 5. On the other hand, during MIS 3, this millennial-scale feature is absent or weakly represented. Cross-spectral analyzes indicate a meaningful north-to-south forcing over the western South Atlantic subsurface during early-glacial. Mg/Ca-derived temperature and ice-volume free seawater d18O (d18OIVF-SW) executed for the MIS 5 interval demonstrated that the subsurface western South Atlantic was warmer and saltier (colder and fresher) during early glacial stadial (interstadials). We hypothesized that a wide reorganization of the northward heat transport throughout the last glacial occurred, in which regions so far south as 24 ºS worked as prominent heat reservoirs in periods of weakened AMOC during MIS 5 but not necessarily during MIS 3. Our data suggest that future impacts over the AMOC along the Brazilian margin will likely be recognized in the subsurface layers of the western South Atlantic.

Hydrological extremes related to the South American Summer Monsoon (SASM) are expected to become more frequent in the near future and might have devastating socioeconomic consequences for the densely populated region of eastern Brazil. Given the complexity in SASM behaviour in space and time, a dense coverage of monsoonal precipitation records, particular those spanning multiple glacial-interglacial cycles, are urgently needed to constrain this high spatial-temporal variability. This information is necessary to reduce the uncertainty associated with projections of SASM precipitation in response to rising anthropogenic greenhouse gas (GHG) emissions. Here we use elemental ratios from X-ray fluorescence scanning of two sediment cores retrieved off the eastern Brazil margin to reconstruct long-term rainfall changes in the hinterland. Our findings from core M125-55-7 (offshore the Doce River, 20°S) reveal that during the past ~320 kyr, precession-paced insolation forcing is the primary pacemaker of variations in SASM precipitation over the Doce basin. We also determined an anomalous interval of weak monsoonal response to insolation forcing during Marine Isotope Stage 6, which we attribute to enhanced wintertime precipitation due to exceptionally strong southeast trade winds created by a steep South Atlantic latitudinal temperature gradient. Moreover, our results suggest that albeit predominantly driven by insolation forcing, the intensity of SASM rainfall responds negatively to GHG forcing, most likely through indirect feedbacks. We propose that GHG forcing directly influences the magnitude of both the inter- and intrahemispheric latitudinal temperature gradients, which in turn modify the strength of atmospheric circulation and precipitation in the tropics. Thus, we suggest that SASM rainfall intensity over tropical eastern Brazil will likely be suppressed by rising CO2 emissions in the future. Our preliminary analysis of core M125-73-3 (off the Contas River; 12°S) reveals regional differences in monsoonal precipitation between the more northerly Contas basin and the more southerly Doce basin. Most notably, unlike the insolation-paced continental rainfall variability recorded at site M125-55-7, SASM rainfall intensity over the Contas basin appears to be more sensitive to glacial-interglacial scale pacing over the past ~800 kyr. Taken together, our records reveal both the high spatial variability in SASM precipitation over eastern Brazil and the dominant influence of orbital forcing on monsoonal rainfall intensity.

Marine sediments from the Western and Eastern South Atlantic continental margins are used to reconstruct mercury (Hg) accumulation over the last glacial/interglacial cycle. Sediment core GL-1248, c...

The last glacial period (116–11.7 ka BP) was an interval characterized by a sequence of abrupt millennial-scale events well documented in Greenland and Antarctica ice-cores. Throughout this period, Greenland cold stadials were accompanied by warm conditions in the thermocline to intermediate waters of the Atlantic Ocean that may have played a role in both the basal melting of ice shelves and the rapid atmospheric warming during the onset of warm interstadials. Climate model simulations indicated an accentuated response of the subtropical western South Atlantic thermocline to the disturbances in the Atlantic circulation. Such works encourage investigations upon thermocline/deep-dwelling planktic foraminifera in this region; however, a study with this aim was not performed. Here we present a paleoceanographic reconstruction from the subtropical western South Atlantic based on the thermocline planktic foraminifera Globorotalia inflata. Our high-resolution δ18O record for the last glacial period presents a millennial-scale variability that strongly resembles the structure of the Greenland Dansgaard-Oeschger cycles during Marine Isotope Stage (MIS) 5. During MIS 3, this millennial-scale variability is absent or considerably dampened. Mg/Ca-derived temperature and seawater δ18O corrected for ice-volume for the MIS 5 interval demonstrate that the region was warmer and saltier (colder and fresher) during early-glacial stadials (interstadials). Our data suggest a reorganization of the northward heat transport throughout the last glacial, in which regions as far south as 24 °S acted as heat reservoirs in periods of weakened Atlantic Meridional Overturning Circulation during MIS 5 but not necessarily (or only marginally) during MIS 3.

Slowdowns of the Atlantic meridional overturning circulation during Heinrich stadials (HS) caused reductions in cross-equatorial heat transport, southward shifts of the Intertropical Convergence Zone and intensification of precipitation over eastern tropical South America. While these changes are well described, the associated spatial sea surface temperature (SST) patterns are still unclear. Here, we analyze proxy data to assess changes in ocean-atmosphere interactions during HS over the western South Atlantic. Our SST proxy records show contrasting patterns between the tropical (warming) and the subtropical (no change) western South Atlantic during HS. We propose that the distinct SST behavior in the subtropics was associated with the cloud cover effect of the South Atlantic Convergence Zone over the upper ocean.

The western tropical Atlantic (WTA) supplies warm and saline waters to the upper-limb of the Atlantic Meridional Overturning Circulation (AMOC) and may store excess heat and salinity during periods of AMOC slowdown. Since previous sea surface temperature (SST) reconstructions from the WTA typically focus on the Last Glacial Maximum and the last deglaciation, additional long-term records spanning multiple glacial-interglacial transitions are needed in order to elucidate the drivers of long-term WTA SST variability. We performed Mg/Ca analyses on the surface-dwelling planktic foraminifera Globigerinoides ruber (pink) from a sediment core raised from the southern WTA to reconstruct SST changes over the past 322 kyr. We evaluate the relative importance of atmospheric pCO2, AMOC strength and trade-wind intensity in driving the thermal evolution of the WTA across three glacial-interglacial cycles. Our results indicate a lack of pronounced glacial-interglacial variability in the SST record, prompting us to exclude atmospheric pCO2 as a direct driver of SST variations in the southern WTA. Similarly, we conclude that variations in AMOC strength also likely did not have a strong influence on long-term WTA SST, based on the low and relatively stable interhemispheric SST gradient over the past 322 kyr. Our results reveal high-amplitude variability in zonal SST gradients within the (sub)tropical South Atlantic and similarities between the long-term patterns of the intrahemispheric meridional SST gradient and our SST record. Based on these findings, we propose that changes in the intrahemispheric meridional SST gradient modulate southeast trade wind intensity, which in turn drives variations in zonal SST gradients and southern WTA SSTs.

Benthic foraminiferal assemblages, Shannon diversity, BFAR index, and oxygen and carbon stable isotopes from Cibicides kullenbergi and Uvigerina peregrina were analyzed in three sedimentary cores dating from the last two centuries from an oligotrophic area influenced by Cabo Frio Upwelling System events in the southwestern Atlantic. The geochemistry and ecology of the benthic foraminifera were compared to organic carbon flux and planktonic foraminiferal records in order to understand the impact of phytodetritus produced by upwelling events and disturbances on the benthic community on a multidecadal time-scale. Values of δ13C for C. kullenbergi were lower (0.26 ± 0.10‰) compared to δ13CDIC, suggesting the presence of phytodetritus in the sediment water where C. kullenbergi calcifies its test. Changes in the benthic foraminiferal assemblage suggest that distinct levels of disturbance differentially influence community-level diversity. Colonization and dominance by opportunistic species resulted in reduced diversity after 1960 CE, with low diversity being observed in areas under the influence of seasonal and intermittent phytodetritus inputs, and higher diversity in a less disturbed area. Certain species can compensate for the negative impact of opportunistic species in terms of disturbance across the community structure, supporting the spatial insurance hypothesis. This community seems to be far from a stable diversity, given the negative interactions between species over the past two centuries, with the r-selective species G. subglobosa and A. weddellensis limiting B. marginata growth during periods of high food availability.

In order to investigate the chemical and magnetic characteristics of sediments of the western boundary upwelling system of Southwest Atlantic we analyzed magnetic susceptibility, grain size distribution, total organic carbon, heavy mineral abundance, Fe associated with Mossbauer spectra, and Fe and Mn of pore water to evaluate the deposition patterns of sediments. Four box-cores were collected along a cross-shelf transect. Brazil Current and coastal plume exert a primary control at the inner and outer shelf cores, which exhibited similar depositional patterns characterized by a high abundance of heavy minerals (mean 0.21% and 0.08%, respectively) and very fine sand, whereas middle shelf cores presented low abundances of heavy minerals (mean 0.03%) and medium silt. The inner shelf was dominated by sub-angular grains, while in middle and outer shelf cores well-rounded grains were found. The increasing Fe3+:Fe2+ ratio from the inner to the outer shelf reflects farther distance to the sediment source. The outer shelf presented well-rounded minerals, indicating abrasive processes as a result of transport by the Brazil Current from the source areas. In the middle shelf, cold-water intrusion of the South Atlantic Central Water contributes to the primary productivity, resulting in higher deposition of fine sediment and organic carbon accumulation. The high input of organic carbon and the decreased grain size are indicative of changes in the hydrodynamics and primary productivity fueled by the western boundary upwelling system, which promotes loss of magnetization due to the induction of diagenesis of iron oxide minerals.

Paleoceanographic reconstructions in upwelling regions can provide relevant information about changes in primary productivity, ocean–atmosphere interactions and the carbon budget. Here, we assessed new data on planktonic foraminifera from a sediment core located near to coastal upwelling zones along the Brazilian coast. Our new data was combined with previous records to reveal the state of upwelling systems along the western South Atlantic margin throughout the last two deglacial and interglacial periods sensu lato . Despite the contemporary oligotrophic scenario of the Santos Basin, a remarkably high relative abundance of Globigerina bulloides and low temperatures at a depth of 100 m indicated upwelling conditions similar to current shelf upwelling zones from 130 to 90 kyr BP. Comparing these results with previous studies, we argue that Brazilian shelf upwelling zones expanded offshore between 20 and 28°S. We develop two conceptual scenarios to characterize the system: (1) during Marine Isotopic Stage (MIS 5), the system expanded along the continental margin between 20 and 28°S following the eccentricity maximum; and (2) after 20 kyr BP, the system retracted to current continental shelf zones. We propose a new mechanism whereby variation of the Earth's eccentricity, which drives seasonality, is the main factor controlling expansion or retraction of the Brazilian upwelling system. Absence of such conditions in more recent periods supports our model and indicates that current upwelling zones are the remnants of a larger upwelling system. However, more studies are required to better define the latitudinal boundaries of the Brazilian upwelling system in the past and its possible influences over the regional carbon budget.

We present a multi-year record of shell fluxes and δ18O 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 δ18O to vertical δ18O profiles predicted for each collection time from in-situ temperature profiles and climatological salinity profiles. For the majority of the cases, the measured δ18O could be accounted for by in-situ calcification, assuming species-specific temperature-δ18O 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 δ18O 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, δ18O 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.

The accumulation of trace elements in sediments from highly productive continental margins may depend on the affinity of these elements for organic matter and their degrees of further incorporation into pyrite (FeS2). We tested the hypothesis that the relative contributions of these geochemical phases play a substantial role as trace element (As, Cd, Cr, Cu, Mn, Ni, and Zn) sinks in the highly bioturbated sediments from the tropical upwelling system off Cabo Frio, southeastern Brazil. Four sediment cores sampled across the Cabo Frio continental shelf were submitted to a sequential extraction procedure performed to separate three different operationally defined fractions, i.e., the geochemical phases soluble in 1 M HCl (considered as the “reactive” fraction), concentrated H2SO4 (considered as the organic matter-bound phase), and concentrated HNO3 (considered as the pyrite-bound phase). The trace metal incorporation into pyrite was assessed by estimating the degree of trace metal pyritization (DTMP), while the pyrite sulfur stable isotope signatures (δ 34SPyr) were used as proxies for sulfur redox cycling intensity. Relative contributions of trace element fixation by organic matter and pyrite were positively correlated for Mn, Cr, and Ni on one hand, and negative correlated for Cu, on the other hand. The positive correlations imply in synergistic roles of these geochemical phases in determining the trace elements sedimentary sinks, while the negative relationship found for Cu reflects differences in the predominant retention mechanisms along with sediment burial. The δ34SPyr signatures were negatively correlated with DTMP values of As, Cd, and Mn, suggesting a diminishing effect of the sulfur redox cycling on trace elements pyritization. These δ 34SPyr signatures were not correlated with DTMP values of Cr, Cu, and Ni, which were dominantly associated with the high organic matter contents found in this upwelling system. The role of pelagic organic matter scavenging of metals and later fueling of benthic microbial sulfate reduction and pyrite accumulation were evidenced as highly variable across the Cabo Frio shelf sediments. Differences in the organic matter accumulation in response to upwelling-enhanced primary productivity and in the intensity of bioturbation-driven sulfur redox cycling help to explain the spatial variability in the biogeochemical processes affecting the sedimentary trace metal sinks.

Rapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies., Design Type(s)modeling and simulation objective • data collection and processing objective • source-based data analysis objectiveMeasurement Type(s)age-depth modelTechnology Type(s)computational modeling techniqueFactor Type(s)geographic location • depthSample Characteristic(s)Atlantic Ocean • marine sediment Machine-accessible metadata file describing the reported data (ISA-Tab format)

Stable isotopes have been widely used in the literature both to discuss current ocean circulation processes, as well as to reconstitute paleoceanographic parameters. The distribution of oxygen and deuterium stable isotopes in seawater (δ18Osw and δDsw) at the Western Tropical South Atlantic border was investigated to better understand the main fractionation processes of these isotopes and establish a regional salinity and δ18Osw relation to improve the paleoceanographic knowledge in the region. This study was conducted during a quasi-synoptic oceanographic cruise in which 98 discrete seawater samples were collected in the core of the main water masses for stable isotope analysis. A strong correlation between δ18Osw and δD was found, which made it possible to extrapolate the results for δ18Osw to δD. Although it was not possible to distinguish the water masses based only on their isotopic signatures, the water masses had a strong salinity and δ18Osw relation, and compared with previous studies, a seasonal pattern was observed. Paleosalinity differences of up to 0.2 psu between Summer and Winter are reported. Considering the limitations of the current techniques to seasonally separate the samples for the paleoceanographic studies, an intermediate Mixing Line for the Tropical South Atlantic (SSS = 1.942* δ18Osw + 34.56) was proposed to reduce the estimated errors associated with these seasonal variations.

Intensification of the Agulhas Leakage (AL) during glacial terminations has long been proposed as a necessary mechanism for reverting the Atlantic Meridional Overturning Circulation (AMOC) to its i...

Strong floristic affinities indicate past ecosystem rearrangements that integrated northeast Brazil with the distant montane range of the Brazilian highlands, sustaining past plant migrations possibly due to climatic patterns. This palynological study of the marine core GL-1248 (0°55.2′S, 43°24.1′W) spans the past ~130-k years to reconstruct the montane rainforest dynamics with the predominant provenance of Parnaiba Hydrographic Basin in northeastern Brazil. In this context, the reduced continental temperatures of the last glacial enabled the migration of ontane rainforest taxa into lowland areas. In these circumstances, the pollen cluster of montane rainforest taxa increased in relative abundance during the early Marine Isotopic Stage 3. The evidence suggests that a secondary pathway evolved, instead of the widely debated coastal route, enabling the biological exchange via the expansion of montane forests throughout the central Brazilian highlands. Two montane pollen types with distinct provenance and ecological features highlighted the ocean-atmosphere dynamics. First, the increase in the relative abundance of Podocarpus correlated with periods of low moisture levels and reduced weathering in the Parnaiba Basin and led to questioning the influence of southeast trade winds as one of the leading carriers of this pollen type. Second, the evidence of Alnus pollen grains, characteristic of Andean vegetation, and low-temperature regions suggest that the Amazon river plume reached the study area. In this sense, the northeast trade winds restrained the North Brazilian Current and guided the southeastward transport of the Amazon river sediments. The presence of Alnus pollen grains emphasizes periods of a weakened North Brazilian Current. Our findings have significant climatic repercussions since heat transport to the northern hemisphere was possibly impaired at the time when Alnus occurred at our core site.

Neodymium (Nd) and strontium (Sr) isotopic signatures in marine sediments reflect the geology of the associated drainage basin from which those sediments are supplied. These signatures are significantly affected by relief, weathering, transport, and deposition of sedimentary contributions from the respective sources. This work makes available new data of Sr Nd isotope ratios and Rare Earth Element (REE) concentrations, which provides a record of the sedimentary provenance of marine sediment supplied to the continental slope off the coast of northeastern Brazil since the last 130 thousand years. Our results indicate the main source of sediments to this site was the Parnaiba sedimentary basin. Furthermore, the fluctuations in Fe/K, Al/Si ratios, and Sr values are explained by geochemical processes operating within the region, which likely reflects changes in the hydrological regime and relative sea-level, and intensity of weathering in continental rocks in glacial-interglacial timescales. We suggest that low values of 87Sr/86Sr ratios during glacial are a result of dry conditions. Also, minor fluctuations within the last glacial period may be a consequence of displacements of the Intertropical Convergence Zone (ITCZ). Such displacements cause changes in rainfall and consequently variations in chemical weathering which affect Sr isotopic fractionation. During past humid intervals, the low Nd (non-radiogenic) signal observed could be explained as a process of penetration of rainwater into the deepest non-radiogenic sediment and subsequent replacement of the isotopic signal in the environment in glacial-interglacial timescales.

Scientific projections point to the increase of greenhouse gas levels in the atmosphere will turn the oceans warmer and, consequently, the upper layer of the thermohaline circulation will get thicker, restricting the productivity areas. Oligotrophic conditions will tend to prevail, and expanding the range of proxy data in oligotrophic oceans, across different climate scenarios, has the potential to improve predictions of climate models about how primary productivity will respond. Still, the variability of past primary production in the equatorial Atlantic remains elusive. We used organic-walled dinoflagellate cysts (dinocysts) assemblages to reconstruct changes in the sea surface hydrography and productivity in the western equatorial Atlantic over the last ~130 kyr. Four dinocysts assemblages were defined: open ocean, river outflow, neritic and nutricline assemblages. Multiple proxies were used to support the environmental changes evidenced by the observed variations in the dinocyst assemblages, such as the X-ray fluorescence (XRF) Ti/Ca ratio, and planktonic foraminifera abundances. We observed glacial-interglacial as well as the millennial-scale changes in the dinocyst assemblages. A clear glacial-interglacial pattern was marked by more autotrophs dinocysts over the interglacials and the prevalence of heterotrophs dinocysts during the glacial period. We pointed out the interchange of autotrophs assemblages (nutricline and open ocean) over the interglacials intervals, denoting divergences in nutrient availability. During the last glacial period, precipitation events occurred in northeastern Brazil due to southward displacements of the Intertropical Convergence Zone (ITCZ). The increase of precipitation boosted the freshwater input to the adjacent ocean, and the dinocyst assemblages oscillated in response to such freshwater/salinity changes. Over the glacial period, dinocysts with different nutritional requirements (autotrophs and heterotrophs) varied following the ITCZ displacements. Thus, in this study, we showed the influence of ocean-atmospheric processes on the ocean and investigated the productivity in an oligotrophic region over distinct past climatic backgrounds.

The reproductive behavior of planktonic foraminifera is an important variable for the interpretation of paleoproxies based on their shells and for the understanding of the role of these organisms in oceanic carbonate flux. Observations from plankton tows have initially provided evidence for the existence of reproductive cycles synchronized with lunar phases in several species. However, subsequent observations from sediment traps yielded inconclusive results. Here we report shell flux data of four key species of planktonic foraminifera (Trilobatus sacculifer, Globigerinoides ruber, Orbulina universa and Neogloboquadrina dutertrei) from multiple deployments of a high-resolution (3-7 days) sediment trap in the southwestern Atlantic. Despite the potential bias related to lateral advection at the shallow deployment depths of the traps, the unusually high sampling resolution makes it possible to better constrain the short-term (lunar) dynamics of shell flux than most previous studies. Using periodic regression on the high-resolution series, we detected for all species evidence for a single flux maximum during one lunar cycle, occurring approximately 4-6 days after the full moon. In this series, 44-52 % of the shell flux in the deep (100 m) trap occurred during the last quarter. Different flux behavior between the shallow (50 m) and the deep (100 m) traps co-located on the same mooring revealed evidence for migration to deeper levels prior to reproduction in T. sacculifer. Although a monthly peak in shell flux was observed in the 3-day resolution deployment, its signature disappeared when all deployments were analyzed together. This analysis still reveals an elevated flux during the last quarter of the lunar cycle, but it seems that the period of the reproductive cycle is not fixed in time. Combined with aliasing at the sampling resolution of 5-7 days, this variable timing overwhelms the strictly periodic component of the shell flux series. We conclude that planktonic foraminifera shell flux and thus the carbonate export to the seafloor is affected by periodicity in the lunar band, but that reproduction does not seem to occur at exactly the same day of the lunar cycle in each month.

Acoustic remote sensing is a highly effective tool for exploring the seafloor of both deep and shallow marine settings. Indeed, the acoustic response depends on several physicochemical factors such as sediment grain size, bulk density, water content, and mineralogy. The objective of the present study is to assess the suitability of seismic peak amplitude as a predictor of total organic carbon (TOC) content in shallow marine sediments, based on data collected in the Cabo Frio mud belt in an upwelling zone off southeastern Brazil. These comprise records of P-wave velocity (V P) along 680 km of high-resolution single-channel seismic surveys, combined with analyses of grain size, wet bulk density, absolute water content and TOC content for four piston-cores. TOC contents of sediments from 13 box-cores served to validate the methodology. The results show well-defined positive correlations between TOC content and mean grain size (phi scale) as well as absolute water content, and negative correlations with V P, wet bulk density, and acoustic impedance. These relationships yield a regression equation by which TOC content can be satisfactorily predicted on the basis of acoustic impedance for this region: y = − 4.84 ln(x) + 40.04. Indeed, the derived TOC contents differ by only 5% from those determined by geochemical analysis. After appropriate calibration, acoustic impedance can thus be conveniently used as a predictor of large-scale spatial distributions of organic carbon enrichment in marine sediments. This not only contributes to optimizing scientific project objectives, but also enhances the cost-effectiveness of marine surveys by greatly reducing the ship time commonly required for grid sampling.

During glacial/interglacial cycles, changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) modified the intermediate and deep-water mass proportions and high latitude productivity in the Atlantic Ocean. These factors influence the distribution and geochemical partitioning of trace metals in the ocean. Mercury is a redox and productivity-sensitive trace metal, making it a potential proxy of paleoenvironmental changes. Therefore, this work examines the effect of Atlantic Ocean circulation changes during the last two glacial/interglacial cycles on the biogeochemistry of Hg. For this, a high-resolution record of the total Hg concentration was determined in core GL-1090 collected from the Southwestern Subtropical Atlantic that represents the last 185 thousand years. During the reported glacial/interglacial cycles, Hg showed a distinct trend throughout Marine Isotope Stages with higher concentrations during periods of enhanced penetration of northern component water into the southwestern Atlantic. This is supported by the similarity of mercury variability with benthic foraminifera δ13C, suggesting a strong influence of deep ocean circulation on the availability and accumulation of this metal in deep-sea sediments. Mercury geochemistry and particle scavenging were correlated with organic matter (OM) input at the core site. We also noted that mercury responded to redox variation in sediment after Termination II, which can be explained by the increase in deep ocean ventilation due to AMOC strengthening. This hypothesis was confirmed by the antiphase behavior of Hg and Total Organic Carbon when compared with Mn/Al ratios and CaCO3. Our work, therefore, allows for a better understanding of the processes leading to long-term mercury removal to sediments.

During the Younger Dryas (YD), paleoceanographic proxies indicate a weakening of the Atlantic Meridional Overturning Circulation (AMOC) resulting in a widespread surface cooling of the North Atlantic and a southward displacement of the Intertropical Convergence Zone (ITCZ). In the western tropical Atlantic, competing ocean and atmospheric processes may result in contrasting surface hydrography scenarios north and south of the equator during the YD. Based on new and compiled data, we provide an up-to-date model-data comparison of changes in western tropical Atlantic surface hydrography during the YD. We show that both transient model simulations and proxy results indicate warming of sea surface temperatures (SST) in the western tropical South Atlantic, but disagree for the SST in the western tropical North Atlantic. Proxies also reveal a complex spatial pattern in surface salinity in the western tropical Atlantic, while a broad negative anomaly is observed in the model output. By comparing planktonic foraminiferal Ba/Ca records from different records in the western tropical Atlantic we constrain ITCZ shifts from the YD to the early Holocene. Based on the Ba/Ca records, the ITCZ reached its southernmost position between 13 and 11.8 ka and started to move northward reaching its northernmost position between 10.8 and 9.7 ka.

International audience; 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.

After glacial terminations, large amounts of heat and salt were transferred from low to high latitudes, which is a crucial phenomenon for the reestablishment of the Atlantic Meridional Overturning Circulation (AMOC). However, how different glacial terminations evolved in the (sub)tropics is still poorly documented. Here we use foraminifera oxygen (δ18O) and carbon (δ13C) stable isotopes to show that the North Atlantic heat piracy, following the AMOC resumption at the early Last Interglacial, affected the thermocline δ18O levels of the subtropical western South Atlantic. Because of the cooling imposed by this process, glacial δ18O persisted in the thermocline for ~7 kyr after the onset of the Last Interglacial, dampening the effect of sea level rise usually imprinted on foraminifera δ18O during terminations. Faunal composition and δ13C also suggest the existence of a colder and thicker South Atlantic Central Water coeval with the AMOC recovery. This process apparently did not occur during the last deglaciation.

Paleoceanographic reconstructions from the Brazil Current are scarce and lack the required temporal resolution to appropriately represent its variability during key periods of the last glacial–interglacial cycles. Here, we present the first high-temporal resolution multiproxy reconstruction of the Brazil Current at 24 °S covering the last 185 ka. During the last and penultimate glacial periods, our Mg/Ca-derived sea surface temperature (SST) record shows a strong cooling at ca. 47 and ca. 156 ka, respectively, that is followed by a warming trend from late-Marine Isotope Stage (MIS) 3 to MIS 1 and from late-MIS 6 to MIS5e, respectively. Importantly, the Brazil Current warmed uninterruptedly towards Termination I (II) after the low SST at ca. 47 and ca. 156 ka, with no SST minima during the Last Glacial Maximum or penultimate glacial maximum. The reason for the strong cooling and the warming trend during late-MIS 3 and late-MIS 6 could reside in the favorable obliquity configuration. However, this mechanism is not sufficient to sustain the warming observed for the rest of the last and penultimate glacial periods. We propose that the change in the Atlantic meridional overturning circulation (AMOC), as described in the literature, from a “warm” to a “cold mode” for MIS 2 and MIS 6 is responsible for the accumulation of warm waters in the subtropical western South Atlantic, preventing SST minima during the last and penultimate glacial maxima in the region. Change in benthic δ 13 C corroborates that a fundamental modification in the AMOC mode might have triggered the heat accumulation. Our data also show a sudden increase in SST and surface salinity during the last glacial descent (MIS 4), indicating that the western portion of the subtropical gyres may have acted as a heat and salt reservoir, while higher latitude climates transited to a glacial background. Our findings imply that the AMOC “cold mode” induces heat storage in the subtropical western South Atlantic and, because of that, the last two regional SST minima occurred out-of-phase with the glacial maxima of higher latitudes.

Modern sea surface temperatures (SSTs) were estimated from UK′37 values of ten core-top sediment samples collected under the Cabo Frio upwelling system and validated by comparison with satellite-derived SSTs. UK′37-SSTs were found to reflect annual SST means in the offshore area and seasonal means (winter/spring) in the inshore and central areas. The latter areas are mainly influenced by upwelling processes whereas the offshore area is under the influence of the thermal front of the Brazil Current (BC). SST variability during the last century was then reconstructed from three short sediment cores. The record showed a general cooling trend in the inshore and central areas, suggesting intensification of the upwelling process. The trends at the offshore area reflected oscillations of the BC thermal front, with piling up of warmer BC waters closer to the coast since ca. 1950.

The North Brazil Current in the western equatorial Atlantic is the main route for poleward heat transfer and therefore is a key component of the climate system. To understand the parameters that influence this region, we present a paleoceanographic reconstruction of the last 40 kyr based on study of sediment core MC 10/3. This reconstruction is based on: i) the oxygen isotope composition of the planktonic foraminifer Globigerinoides ruber (white); ii) census counts of the planktonic foraminifera assemblage to estimate sea surface temperature (SST) via the Modern Analogue Technique (SST MAT ) and to deduce characteristics of the water column; and iii) values of δ 18 O ivc-sw (a proxy for sea surface salinity (SSS)). The oxygen isotope composition of G. ruber showed a strong shift of greater than 1‰ after 21 cal kyr BP. Such a change can be attributed mainly to a salinity reduction of two units, as well as a slight SST increase (~ 1 °C), between 21 and 17.5 kyr BP. This change may be related to a southward displacement of the Intertropical Convergence Zone (ITCZ) and its belt of convective activity, which would have increased precipitation over the region and diminished surface salinity. The structure of the planktonic foraminiferal assemblage also indicates such conditions. The productive/thermocline-dwelling Globigerinita glutinata showed higher abundance prior to 21 cal kyr BP. Other productive/deep-dwellers, i.e., Neogloboquadrina dutertrei , Globorotalia truncatulinoides (left- and right-coiling) and Globorotalia inflata , were slightly more abundant prior to 21 cal kyr BP and became less frequent with the development of low-salinity surface water that created more stratified and oligotrophic conditions in the water column. However, the warm/oligotrophic surface-dwellers ( G. ruber , Globigerinoides sacculifer and Globigerinella siphonifera ) became more abundant after 21 cal kyr BP. The species Globorotalia menardii was nearly absent during the Last Glacial Maximum (LGM), and its growth in abundance, mainly after 17.5 cal kyr BP, may indicate the resumption of the Agulhas leakage, which would have reached the study area via the South Equatorial Current (SEC). The return of high salinity values synchronous with the G. menardii increase also denotes the influence of Agulhas leakage into the western equatorial Atlantic.

Texto completo: acesso restrito. p.127–136 Submitted by Edileide Reis (leyde-landy@hotmail.com) on 2015-03-24T11:47:19Z No. of bitstreams: 1 A.Y.V. Cardozo.pdf: 1407391 bytes, checksum: feec92b74d278949937b1df641ee425f (MD5) Approved for entry into archive by Alda Lima da Silva (sivalda@ufba.br) on 2015-03-27T20:17:23Z (GMT) No. of bitstreams: 1 A.Y.V. Cardozo.pdf: 1407391 bytes, checksum: feec92b74d278949937b1df641ee425f (MD5) Made available in DSpace on 2015-03-27T20:17:23Z (GMT). No. of bitstreams: 1 A.Y.V. Cardozo.pdf: 1407391 bytes, checksum: feec92b74d278949937b1df641ee425f (MD5) Previous issue date: 2013 Aquatic environments in the high-Andean may be quite vulnerable to climatic conditions and hydrological processes (precipitation/evaporation) that have the potential to alter water levels and chemistry. This study reconstructs the paleohydrological history of Lake Tota, Colombia, to provide a record of environmental changes in the eastern flank of the Colombian Andes during the late Holocene. A 54 cm core was collected at the margin of the lake at a depth of 1.5 m, and the paleohydrological reconstructions were based on changes in diatom assemblages, granulometry, and bulk organic geochemical parameters. The core was subdivided into 5 biozones by assessing the significance of the variance among the CONISS clusters. Biozone T5 (base, 4000 calibrated years BP) was dominated by the planktonic diatom Discostella stelligera and was deposited in a period of high-water level. Sedimentation during this period was rich in carbon due to phytoplanktonic production; the granulometry data suggests that the lake was under high to moderate hydrological dynamic conditions, indicating active deposition at the river delta front. Biozone T4, beginning at 3600 cal. yrs BP, was dominated by three species D. stelligera, Cocconeis placentula var. lineata and Fragilaria sensu lato, and this association could indicate that the lake was under the influence of large fluctuations in water level. This interpretation is supported by granulometry and geochemical data, particularly by the low levels of total organic carbon (TOC). In biozone T3, corresponding to 3000 cal. yrs BP, we found a lowering of the water level, suggested by low levels of TOC. Poor granulometric sorting and higher carbon/nitrogen (C/N) ratios suggest a macrophyte influence on the sedimentation, avoiding direct deltaic influence. In this biozone the benthic diatom F. sensu lato group was dominant. In biozone T2, starting 2400 cal. yrs BP, the proxies indicate that the lake experienced increasing water levels and that stronger circulation patterns occurred in the water column than in the preceding periods, as inferred by the presence of Aulacoseira granulata a ticoplanktonic species. The sediment in this biozone is well sorted and finer, indicating a greater depth. Biozone T1 at the top of the core and since 2000 cal. yrs BP to the present, evidence for a recent reduction in water level of Lake Tota can be deduced by the presence of Achnanthidium minutissimum var. minutissimum.

article i nfo Stable isotopic composition (δ 18 Oa ndδD of water, δ 13 CDIC) of the water column in the open ocean is related to the origin of water masses. Due to the recent increase of paleoceanographic studies on continental shelves, it is alsoimportant tounderstand theirdistribution and variabilityinthosesystems. Toexamine theinfluence of con- tinental shelves internal processes on isotopic composition of water masses, we present data of stable isotopes and phosphate content from a western boundary upwelling system located on the Southeastern Brazilian coast and compare them with offshore observations. High mixing of the main water masses (SSW, TW and SACW) was observed in the majority of the samples collected during different seasons in 2011 and 2012. A mixing trian- gle approach was used to separate the water masses contribution and characterize their isotopic composition. In addition, an isotopic three end-member model was established, proposing it as a paleoceanographic tool to re- construct relative contribution of these water masses in sediment records. Variations of δ 18 O values are linked to oceanographic dynamics, mixing, continental runoff and upwelling processes on the shelf. Differently the δ 13 CDIC variations in the middle and inner parts of the shelf are related to the productivity of the upwelling sys- tem. Seasonal variability of the δ 13 CDIC values may be also related to changes in the upwelling intensity.

Physical and biogeochemical processes in continental shelves act synergistically in both transporting and transforming suspended material, and ocean dynamics control the dispersion of particles by the coastal zone and their subsequent mixing and dilution within the shelf area constrained by oceanic boundary currents, followed by their gradual settling in a complex sedimentary scenario. One of these regions is the Cabo Frio Upwelling System located in a significantly productive area of Southeastern Brazil, under the control of the nutrient-poor western boundary Brazil Current but also with a wind-driven coastal upwelling zone, inducing cold-water intrusions of South Atlantic Central Water on the shelf. To understand these synergic interactions among physical and biogeochemical processes in the Cabo Frio shelf, a series of four experiments with a total of 98 discrete samples using sediment traps was performed from November 2010 to March 2012, located on the 145 m isobath on the edge of the continental shelf. The results showed that lateral transport might be relevant in some cases, especially in deep layers, although no clear seasonal cycle was detected. Two main physical-geochemical coupling scenarios were identified: singular downwelling events that can enhance particles fluxes and are potentially related to the Brazil Current oscillations; and events of significant fluxes related to the intrusion of the 18°C isotherm in the euphotic zone. The particulate matter settling in the Cabo Frio shelf area seems to belong to multiple marine and terrestrial sources, in which both Paraiba do Sul River and Guanabara Bay could be potential land-sources, although the particulate material might subject intense transformation (diagenesis) during its trajectory to the shelf edge. Processos físicos e biogeoquímicos em plataformas continentais agem sinergicamente, tanto transportando, quanto transformando o material em suspensão, enquanto a dinâmica do oceano controla a dispersão destas partículas pela zona costeira e sua posterior mistura na plataforma continental. O sistema de ressurgência de Cabo Frio, localizado na região Sudeste do Brasil, se caracteriza por ser um região de alta produtividade sob a influência da Corrente do Brasil, uma corrente de contorno oeste oligotrófica, mas também com uma zona de ressurgência costeira induzida pelo vento, o que produz as intrusões das águas frias e ricas em nutrientes da Água Central do Atlântico Sul sob a plataforma. Visando entender as interações sinergéticas entre os processos físicos e biogeoquímicos neste sistema, uma série de quatro experimentos com armadilhas de sedimentos e outros sensores físicos foram fundeados na isóbata de 145 metros, no período entre novembro de 2010 e março de 2012. Um total de 98 amostras discretas foram analisadas. Os resultados mostraram que o transporte lateral pode ser importante em alguns casos, especialmente nas camadas mais profundas. Embora não tenha sido detectado nenhum ciclo sazonal claro, dois cenários de acoplamento físicogeoquímicos foram identificados: eventos de subsidência que produzem um aumento nos fluxos de partículas e são potencialmente relacionados às oscilações da Corrente do Brasil; e eventos de fluxos significativos relacionados com a intrusão da isoterma de 18°C na zona eufótica. As partículas em suspensão na plataforma continental de Cabo Frio representam uma mistura de fontes marinhas e continentais. Tanto o Rio Paraíba do Sul quanto a Baía de Guanabara podem ser potenciais fontes continentais, embora o material particulado possa ainda estar sujeito a intensa transformação (diagênese) durante sua trajetória para a borda da plataforma continental.

The Cabo Frio region in the state of Rio de Janeiro, southeast coast of Brazil, is characterized by a local coastal upwelling system and converging littoral sediment transport systems that are deflected offshore at Cabo Frio, as a consequence of which a thick cross-shelf sediment deposit has developed over time. To investigate the evolution of this muddy deposit, geophysical, sedimentological and geochemical data from four sediment cores (3.8–4.1 m in length) recovered in water depths between 88 and 141 m were analyzed. The high-resolution seismic data show variable sediment thicknesses ranging from 1 to 20 m, comprising two sedimentary units separated by a high-impedance layer at a depth of about 10 m below the seafloor at the coring sites. According to the available age datings, the upper sedimentary unit is late Pleistocene to Holocene in age, whereas the lower unit (not dated) must, by implication, be entirely Pleistocene in age. The boomer-seismic reflection signal can be divided into three echo-types, namely transparent (inner shelf), stratified (middle shelf) and reflective (outer shelf), each type seemingly related to the local sediment composition. The upper 4 m of the upper sedimentary unit is dominated by silty sediment on the middle shelf, and by upward-fining sediments (silty sand to sandy silt) on the inner and outer shelf. The downcore trends of P-wave velocity, gamma-ray density and acoustic impedance are largely similar, but generally reversed to those of water and organic carbon contents. Total organic carbon contents increase with decreasing mean grain size, periodic fluctuations suggesting temporal changes in the regional hydrodynamics and primary productivity fuelled by the local upwelling system. The reconstruction of sedimentation rates in the course of the Holocene is based on 35 AMS age datings of organic material recovered from variable downcore depths. These range from a maximum of 13.3 cm/decade near the base of the inner shelf core (7.73–7.70 ka BP) to generally very low values (

Six sediment cores were obtained from the Cabo Frio shelf region of coastal Brazil to quantify the accumulation of organic carbon in a highly productive upwelling region. The sampled locations, 10–60 km offshore at ~100 m water depth, were investigated for excess 210Pb (210Pbex) as well as 239+240Pu fallout activities to determine sedimentary dynamics. The 210Pbex and 239+240Pu dating models show that the sediment accumulation rates varied substantially throughout this complex hydrodynamic system (0.8–5.5 mm yr−1). Excess 210Pb and 239+240Pu fluxes indicate lateral transport, with varying intensity along the continental shelf. The stations with the greatest 210Pbex and 239+240Pu sediment inventories are also the sites with the highest carbon accumulation rates (CAR). The total organic carbon (TOC) and total nitrogen (TN) contents, along with the δ13C results, indicate that the organic matter deposited in this region is mainly of marine origin. The results of this work suggest that lateral transport, with varying intensity along the shelf, contribute to the large quantities of marine plankton buried at specific depositional settings in the Cabo Frio upwelling region (~1–8 mol of OC cm−2 yr−1).

Continental and marine paleoclimate archives from northwestern and northeastern South America recorded positive precipitation anomalies during Heinrich Stadials (HS). These anomalies have been classically attributed to enhanced austral summer (monsoon) precipitation. However, the lack of marine paleoclimate records off eastern South America as well as inconsistencies between southeastern South American continental and marine records hamper a comprehensive understanding of the mechanism responsible for (sub-) tropical South American hydroclimate response to HS. Here we investigate piston core M125-95-3 collected off eastern South America (10.94°S) and simulate South American HS conditions with a high-resolution version of an atmosphere-ocean general circulation model. Further, meridional changes in precipitation over (sub-) tropical South America were assessed with a thorough compilation of previously available marine paleorecords. Our ln(Ti/Ca) and ln(Fe/K) data show increases during HS6-Younger Dryas. It is the first core off eastern South America and the southernmost from the Atlantic continental margin of South America that unequivocally records HS-related positive precipitation anomalies. Based on our new data, model results and the compilation of available marine records, we propose a new mechanism for the positive precipitation anomalies over tropical South America during HS. The new mechanism involves austral summer precipitation increases only over eastern South America while the rest of tropical South America experienced precipitation increases during the winter, challenging the widely held assumption of a strengthened monsoon. South American precipitation changes were triggered by dynamic and thermodynamic processes including a stronger moisture supply from the equatorial North Atlantic (tropical South Atlantic) in austral winter (summer).

The sediment core GL-1248, collected from the continental slope off northeastern Brazil, was used to reconstruct mercury (Hg) variations in NE South American continental margin and understand its variability in response to paleoclimate changes over the past 128 ka. Mercury concentrations in GL-1248 ranged between 14.95 and 69.43 ng/g, showing a glacial-interglacial pattern with higher (lower) concentrations in the glacial period (interglacial period). Parallel trends of Hg and XRF-Fe plots suggest that following atmospheric Hg deposition onto the continent, Hg is incorporated with Fe compounds before transportation and eventual immobilization at the NE Brazil continental slope. Peaks of Hg and Fe/Ca ratio peaks occurred concurrently during certain Heinrich Stadials, indicating that Hg is transported from the continent alongside fluvial sediments during periods of increased precipitation and erosion in NE Brazil continent. Mercury concentrations varied with periodicities of 56 ka and 900 yr suggesting glacial-interglacial changes and millennial-scale variability, respectively. Total Hg and total organic carbon are poorly correlated, although the latter likely influenced Hg sequestration into marine sediments during millennial-scale events between 60 ka and 30 ka. Altogether, our results suggest that the atmosphere is the primary source of Hg to GL-1248 and glacial-interglacial climate variations were the major determinant of atmospheric Hg deposition. Furthermore, increased precipitation during millennial-scale events played a secondary role by enhancing Hg transport to the continental slope of NE Brazil.

We performed δ18OC (Globigerinoides ruber, 250–300 μm white) and SST (modern analogue technique) paleoceanographic reconstructions for the western tropical South Atlantic Ocean at the northeast Brazilian margin to assess millennial- to centennial-scale climatic shifts that may be propagated by the thermohaline circulation. The results show a progressive SST increase (~ 1 °C) over the Holocene, with a prominent shift occurring during the mid-Holocene that may be linked to changes in insolation distribution. Furthermore, spectral and coherency analyses reveal several centennial- to millennial-scale modes of variability that are similar in both proxy records (4.1–3.8 kyr, 1.5–1.0 kyr, ~ 700 yr, 570–560 yr, ~ 390 yr, ~ 350 yr and ~ 330 yr). Such variability could be the result of (1) solar-induced atmospheric changes at northern high latitudes (possibly propagated southward by the meridional overturning circulation and hence inducing shifts in tropical δ18Oc and sea surface temperature), and/or (2) salinity anomalies propagated from the Southern Atlantic Ocean and transmitted to the study site through the Agulhas Leakage. These climate oscillations may have had substantive effects on the Holocene climate system, especially over the tropical western South Atlantic Ocean and the South American continent. Our data show that the western tropical South Atlantic may respond to multi-centennial to millennial oscillations that are possibly triggered by external (solar) and internal (northern and southern high latitudes) climate forcing. Further investigations are necessary to illuminate the role of the western tropical South Atlantic in inter-hemispheric heat transfer.