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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).

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.

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

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.

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.

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.

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 South American monsoon system (SAMS) plays a fundamental role in the precipitation regime of the most populous and economically important regions in Brazil. The South Atlantic Convergence Zone (SACZ) is a main component of the SAMS, characterizing its active phase, and is often associated with intense rainfall events: Strong and persistent episodes cause severe floods and landslides, while weak and sparse episodes are associated with droughts. The variability of the convergence zone caused great natural disasters in Southeast Brazil, associated with extreme precipitation conditions: 3562 landslides killed 947 people in Rio de Janeiro state in 2011, while a shortage of water in Sao Paulo affected around 20 million people between 2014 and 2015. In the present study, we build SACZ configuration series for the period between January 2000 and June 2014 and use them as indicators for the SAMS to quantify its influence on several atmospheric variables. Based on a principal component analysis, we present indices that identify the configuration of the SACZ in a local scale. The indices reached strong accuracy rates, especially for identifying days of extreme rainfall events associated with the SAMS and may, thus, serve as decision-making tools to help prepare for their impacts. Furthermore, the indices are composed by common variables simulated by numerical weather and climate models, other than precipitation, which is often a not very reliable output. The applied methodology is easily reproducible and different variables may be used to compose indices for different regions—an advantage of this local-scale approach.

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.

The Campos Basin (100,000 km2) is located on the continental shelf of southeastern Brazil. Despite the significant oil and gas industrial activities underway in the Campos Basin, scarce information is available regarding the hydrocarbon contents and microbial communities in the deep-sea sediments. To gain new insights on these aspects, we first obtained deep-sea sediment samples with different degrees of oil exposure. We obtained samples from a seabed fissure (N = 28), surroundings (250 m to 500 m from the fissure; N = 24), and a control area (N = 4). We used shotgun metagenomics to characterize the taxonomic and metabolic diversity and analyzed biogeochemical parameters (metal and oil concentration) of all samples. The high levels of unresolved complex mixture of hydrocarbons in the fissure indicate a potentially recent petrogenic contribution in these sediments. The fissure area was found to have a higher abundance of hydrocarbonoclastic bacterial genera and hydrocarbon degradation genes. These bacteria may be used as biosensors of sediment contamination. The effects of oil contamination, mainly around the fissure, are less clear at 250 m and 500 m, suggesting that the surroundings may not have been heavily affected by the oil leakage. Our study demonstrates that metagenomics can disclose biosensors for environmental monitoring.

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.

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.

International audience; Minerals and elements are important proxies that can provide information about variations in the delivery and deposition of coastal ocean sediments associated with past climate changes. In this study, postglacial changes in the accumulation of sediments on the upper shelf of southeastern Brazil are linked to the evolution of regional paleoceanographic and continental paleoclimatic conditions during the last 14.4 kyr CAL BP. Mineralogical and major and trace element analyses of a 14C-dated sediment core identify three main lithostratigraphic units of this core that reveal a succession of changes in sediment delivery and accumulation as postglacial sea level rose and Holocene climate on land evolved. The depositional setting has transitioned in response to the large sea level rise prior to 9.9 kyr CAL BP from being a shallow water high energy environment to a deeper water one. This location has since then been a lower energy environment that has persisted into modern times. Due to the high marine productivity associated with the Cabo Frio upwelling system in an oxygenated water column, the sediments have shown a complex and dynamic redox condition, making variations in redox-sensitive trace elements indicative of changes in primary production and organic matter diagenesis at this location. After sea level stabilized ~ 9 kyr CAL BP, variations in fluxes of Al, Fe, Ca, and minerals were small, indicating that climate driven changes had only secondary controls on delivery of detrital sediment components to the Cabo Frio shelf. Modern coastal upwelling conditions and the onset of mid-shelf organic-rich sediment deposition were established after 9.9 CAL kyr BP.

International audience; Dissolved silicate (DSi), NH4+, NO3− and PO43− susceptibility to be exchanged between sediment pore waters and overlying waters was evaluated in Jurujuba Sound (JS station) and Coroa Grande Sound (CGS station), southeastern Brazil. Sedimentary elemental (C, N and P) and isotopic (δ13C and δ15N) compositions evidenced stronger anthropogenic fertilization in JS station. Net NO3− influxes from overlying waters occurred, which was two orders of magnitude higher under the more fertilized condition. This condition resulted in 6–13-times higher net effluxes of NH4+, DSi and PO43− to overlying waters. Vertical alternation between production and consumption processes in pore waters contributed for a more limited regeneration in CGS station. This was associated with diagenetic responses to sedimentary grain size variability in deeper layers and biological disturbance in upper layers. Nearly continuous production of NH4+, DSi and PO43− in pore waters implied in intensified susceptibility to remobilization under the eutrophic condition of JS station.

The Brazil Current (BC) is a relevant feature in the Atlantic Meridional Overturning Circulation (AMOC). Its behavior during slowdown or intense AMOC remains poorly known because of the lack of paleoceanographic records, especially for the Holocene. Here, we investigate changes in a western boundary upwelling system (Cabo Frio, off Southeastern Brazil) which are driven by variations in the BC and NE winds during the last 9 kyr. To assess the variability of the BC, we used δ18O, Mg/Ca, and assemblages of planktonic foraminifera. Our results indicate five oceanographic phases during the last 9 kyr. During Phase I (from 9.0 to 7.0 cal kyr BP), the BC diverged offshore from the modern upwelling area because of the low sea level, increasing the influence of shelf waters and coastal upwelling plumes on foraminifera assemblages. Phase II (7.0–5.0 kyr BP) was marked by the approach of the internal front of the BC with low intensity and episodes of strong productivity that were linked primarily to the upwelling of the South Atlantic Central Water (SACW) and/or Subpolar Shelf Waters (SPSWs) (cold). Phase III (5.0–3.5 kyr BP) was a transition, marking a large oceanographic and climatic change from the weakening of the AMOC. The internal front of the BC became warm and subsurface SACW upwelling was stronger. In Phase IV (3.5–2.5 kyr BP), the BC acquired its modern dynamics, but weak NE winds weakened the SACW’s contribution to upwelling events. Finally, in Phase V (last 2.5 kyr BP), the NE winds reintensified, promoting frequent episodes of upwelling and intrusion by SPSWs during the Medieval Climate Anomaly.

Resume Cette etude porte sur un sondage preleve dans la lagune de Brejo do Espinho, sur le littoral de Cabo Frio (Bresil), soumis a l'influence aride d'un upwelling local, genere par les alizes de nord-est issus de l'Atlantique sud et particulierement renforces lors des evenements El Nino. L'etude des diatomees appuyee par des analyses sedimentaires et isotopiques met en evidence des phases arides peu frequentes avant 4000 ans, une forte variabilite climatique entre 3600 et 2900 ans et une intensification de l'aridite depuis 2400 ans. Pour citer cet article : B. Laslandes et al., C. R. Geoscience 338 (2006).

The distribution of planktonic foraminifera in box-core tops under the influence of a western boundary upwelling system along the southeastern Brazilian continental margin was examined to evaluate the similarity percentage (SIMPER) and to create a biofacies model for paleoenvironmental applications. Species associated with warm and oligotrophic water were distributed in the Rio de Janeiro sector of the Campos Basin, while productive water species were most abundant in the Rio de Janeiro sector of the Santos Basin, and cold-water species were most abundant in the Cabo Frio Upwelling System (CFUS). Four major biofacies are associated with the oceanographic setting of the CFUS: one from the Campos Basin (A — Brazil Current front), one from the Santos Basin (B — mixture of coastal and oceanic waters), and two associated with the Cabo Frio High in the northern biofacies (C) and southern biofacies (D). The distribution of biofacies C and D was associated with temperature differences. Biofacies C represents a mixture of upwelling and tropical waters, while biofacies D represents a mixture of cold, nutrient-rich and Santos Basin waters. These biofacies were also defined in core CF10-01B, in which six main paleoceanographic phases were characterized during the last 9 cal kyr, predominantly showing alternating dominance between biofacies C and D. From 9.0 to 5.0 cal kyr before the present (BP), the system was dominated by biofacies C. Between 4.0 and 3.5 cal kyr BP, biofacies D was dominant. Between 3.5 and 2.5 cal kyr BP, a strong influence of coastal waters and weakened upwelling activity were indicated by the dominance of biofacies B + C + D. The last 2.5 cal kyr was dominated by biofacies D. The SST in this core was reconstructed using the Modern Analog Technique (MAT), which revealed 0.35 °C of variability, indicating no SST changes during the Holocene. The weak sensitivity of the MAT was due to the interplay among different oceanographic features.

Key Points: Use of sedimentary nitrogen isotopes is examined; On average, sediment 15N/14N increases approx. 2 per mil during early burial; Isotopic alteration scales with water depth Abstract: Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15N/14N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle.

International audience; Northeast Brazil represents a key area in terms of Holocene environmental changes in South America due to its distinct semi-arid climate at near equatorial latitudes as well as to a wetter Early and Middle Holocene paleoclimate in comparison with other South America regions, which also experienced a reduced monsoon at that time. We studied the lacustrine sediments of Lake Boqueirão, which is located at 5°S latitude on the Brazilian Atlantic coast; it is directly influenced by displacement of the Intertropical Convergence Zone (ITCZ). A short (1 m) core "Boqc0701" was collected at a 7.5 m water depth during a seismic survey. Organic matter (OM) geochemical [Rock Eval Total Organic Carbon (TOC), Hydrogen Index (HI), Oxygen Index (OI)] and petrographical studies allowed us to distinguish five sedimentary intervals during the last 3 kyrs. Phase E (3000 to 2050 cal yr BP) presents the largest TOC, HI and OI fluctuations in the entire core. The most probable cause of these variations is a great instability in the lake level during this period. Phase D (2050 to 1830 cal yr BP) is marked by an increase in sedimentation rate and a higher contribution from a well-preserved OM with an algal origin; this was interpreted as a higher and more stable lake level. During phase C (1830 to 1550 cal yrs BP), low HI and high OI indicate the input of more oxidized, degraded and detrital OM, reflecting a lower lake level. In phase B (1550 to 1470 cal yr BP), the OM quality and quantity (HI and TOC) indicate an algal contribution. During phase A (1470 to 570 cal yr BP), the high stability of the proxies may be due to a deep environment where OM fluctuations are strongly buffered. These changes in lake level can be compared with other tropical South America high resolution records. A correspondence seems to exist between phases of low ENSO, cool North Atlantic Ocean and Boqueirão wet phases. We did not encounter this same pattern during the last 1050 years, during which the 1050-570 cal yrs BP interval was generally wet (although probable fluctuations are buffered in our proxies) while the North Atlantic was warm (Mann et al., 2009). A prolonged dryness occurred in Lake Boqueirão from 570 cal yrs BP until recent decades. This phase corresponds to the Little Ice Age characterized by a cooler North Atlantic Ocean (Mann et al., 2009). These data indicate that the teleconnection pattern between the tropical Atlantic, ENSO and Northeast Brazil rainfalls has changed in the past.

Lipid fractions extracted from sediment layers deposited in a small Brazilian lake during the last 20,000 yrs were investigated by gas chromatography-mass pectrometry. Considerable differences in the distribution and the amount of triterpene derivatives in the aliphatic as well as in the aromatic fractions were observed all along the series. Although no precise identification of these compounds was undertaken, our interpretation of mass spectral data allowed us discriminating between des-A-triterpenes, mono- and triaromatic derivatives of pentacyclic triterpenes and a series of compounds tentatively identified as diaromatic derivatives of tetracyclic triterpenes. The largest compound diversity was found in the lowest levels of the series dated back to the end of the Last Glacial Maximum when good preservation of terrestrial plant debris was ensured by rapid burial in a semi-arid climate. Then, except for a period corresponding to the Younger Dryas, only des-A-lupane was detected in significant amounts. The high predominance of des-A-lupane is interpreted as resulting from the development of a belt of Eleocharis sp. (spike-rush) that filtered most of organic inputs from the catchment and was the site of des-A-lupane production and exportation towards the lake centre. During the Younger Dryas, a strong influx of des-Alupane is attributed to the destruction of the spike-rush belt consecutive to the lowering of the lake level under drier conditions. The temporary destruction of this barrier allowed other triterpene derivatives to reach the lake. The distinct dynamics of des-A and aromatic triterpenes derivatives under variable medium conditions led us to hypothesise that these two families of compounds derive from distinct pools that contributed differently to the sediment depending on environmental and climatic conditions. Des-A-lupane was most probably produced in sub-aquatic conditions within the belt of spike-rush. Reversely, aromatic derivatives of triterpenes could have resulted from the degradation of their biological precursors within reducing micro-environments in the catchment.

International audience; We here report a reconstruction of hydrological balance variations in Northern Brazil for the last 20 ka deduced from the ΔD values of aquatic and land plant molecules extracted from the sediment infill of Lake Caçó. Our reconstructed precipitation, lake water isotope ratio and evaporation-evapotranspiration isotope effect allow us to obtain an estimate of moisture balance, and, to a lesser extent, precipitation amount and seasonality changes. During the end of the Last Glacial Maximum (LGM, between ca. 20 and 17.3 ka), high ΔD values and smaller fractionation of leaf waxes indicate an arid to semi-arid climate with a long lasting dry season. An abrupt change towards much wetter conditions occurred within ca. 500 years from 17.3 to 16.8 ka, as shown by a 50 ‰ decrease in D/H ratios and a marked increase in H isotopic fractionation of leaf waxes. This abrupt isotopic change coincides with a major transformation from savanna-dominated vegetation to humid rain forest around the lake, based on pollen data. Comparisons with other paleo-precipitation records from South American sites indicate that late glacial humid conditions were controlled by intensification of the ITCZ and/or a southward shift of its mean position across our study site. Our isotope data show only a small rise in aridity during Younger Dryas event (13 to 11.5 ka). Although the Holocene was not screened in details, D/H ratios of terrestrial and aquatic compounds show near constant offsets, suggesting stable and relatively humid climate conditions during this period.

The lipid extracts of sediments collected from the Quaternary filling of a tropical lacustrine series (Lagoa do Caco, Brazil) were investigated using gas chromatography-mass spectrometry (GC-MS). Various pentacyclic triterpene 3-methyl ethers (PTMEs) were present in the neutral fraction. Comparison of retention times and mass spectra with available standards allowed us to confirm the presence of olean-12-en-3beta-ol ME (beta-amyrin ME), olean-18-en-3beta-ol ME (miliacin), taraxer- 14-en-3beta-ol ME (crusgallin), fern-9(l I)-en-3beta-ol ME (arundoin) and arbor-9(11)-en-3beta-ol ME (cylindrin). The following other compounds could also be tentatively identified from their GC-MS characteristics: urs-12-en-3beta-ol ME, bauer-7-en-3beta-ol ME and fern-8-en-3beta-ol ME. Other compounds such as possible 3alpha isomers of the PTMEs as well as di- or tri-unsaturated counterparts might be PTME diagenetic derivatives. According to previous chemotaxonomic studies, all these compounds most probably originate from Gramineae that used to colonize the savanna of Northern Brazil at the time of deposition. (C) 2004 Elsevier Ltd. All rights reserved.

From west to east, the Fluminense coastline, composed of two series of lagoons, presents an aridity gradient induced by a local upwelling that is controlled by trade winds coming from the south Atlantic. The physico-chemical conditions of the lagoons are controlled by the duration of the dry season. They are characterized by hypersaline conditions with organic-carbonate sedimentation in the upwelling-influenced area, and by hyposaline conditions with organic-detrital sedimentation out of the upwelling-influenced area. On a core collected in the Brejo do Espinho hypersaline lagoon, diatom analysis supported by sedimentological data revealed a varying high water level from 4500 to 2200 14C yr BP induced by successive freshwater inputs in to the lagoon. Two major dry events occurred during this period, probably related to relative sea-level fluctuations. At 2200 14C yr BP, a drastic change occurred, as indicated by a carbonate nodule, carbonate-dominated sediments and polyto hypersaline diatoms, all suggesting arid conditions. The Brejo do Espinho record suggests that before 2200 14C yr BP, the central part of the Brazilian coast was mostly subjected to south to southeasterly winds and weaker trade winds, whereas after 2200 14C yr BP, trade winds dominated, implying drier conditions at the easternmost part of the Fluminense coastline. These results would also suggest a late Holocene increase in El Ninlo activity (after 2200 14C yr BP), consistent with several El Nifio records from western South America.

An unusual molecular fossil (onocerane I) has been detected in Quaternary lacustrine sediments (Lagoa do Caco, NE Brazil) for the first time. This component was initially thought to attest to the former presence of ferns or club mosses. From consideration of possible plant precursors of onocerane-related molecules recorded in the literature and comparison with palynological results and palaeoclimatic data, we here provide evidence that club mosses and ferns cannot account as sources of onocerane I in this setting. Onocerane I is abundant in the lipid extracts of sediments deposited during the two driest periods recorded in Northern Brazil (Last Glacial Maximum and Younger Dryas). This component is therefore suspected to be diagnostic of the development of plants adapted to dry or semi-arid conditions in this region.

International audience; Sediment samples collected along a 6-m core, drilled in the deepest part of the Lagoa do Caçó (NE Brazil), have been investigated in order to determine source(s) and degradation conditions of the organic matter (OM) with special emphasis on paleoenvironmental implications. Bulk organic geochemistry (Rock-Eval pyrolysis, C/N determination, δ13C and δ15N measurement) and petrography combined with sedimentological evidence and radiocarbon dates allowed to identify four major intervals documenting major environmental changes that occurred during the last 20,000 years. The first interval, dating back to the end of the Last Glacial Maximum (LGM), contains well-preserved OM derived from higher plants. This material was most probably produced in an ephemeral palustrine system and rapidly buried by sands. This level is thought to have been deposited under relatively arid climate conditions associated with strong but episodic rainfalls. Between 19,240 and 17,250 Cal years BP, the climate appears to have been more humid and seasonality more pronounced as suggested by the presence of a permanent lake. After a drastic environmental change dating back to 17,250 Cal years BP, the sediment became truly lacustrine with restricted mineral input and highly degraded higher plant-derived organic matter. After that, a stepwise improvement in the preservation of OM occurred, as revealed by several pronounced shifts in the Rock-Eval TpS2 signal. These changes could document abrupt climatically driven changes during the Late Glacial. Finally, around 5610 Cal years BP, environmental conditions, approaching those prevailing today were established. Minor climatic changes during the Holocene were probably buffered by a high water table which might explain the lack of paleoenvironmental fluctuations.