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Sterol and fatty acid biomarkers and isotopic composition (δ13C and δ15N) of bulk organic matter (OM) were quantified in a sediment core to characterize the accumulation of autochthonous OM in an area on the continental shelf adjacent to Rio de Janeiro State. In the sediment surface (0-1 cm) the concentration of total sterols and fatty acids was at least one order of magnitude higher than that measured deeper down in the core and was dominated by labile and planktonic-derived biomarker compounds. These results suggest, as is confirmed by multivariate statistical analysis, the occurrence of an event of enhanced primary production in the water column and efficient export of particles to the bottom. Similar conditions have been observed at Cabo Frio, located 150 km to the north of our study site, during an upwelling event, suggesting that such events may exert a regional influence on primary production on the south-eastern Brazilian continental shelf. Beyond the signatures from this event, the presence of biomarker compounds from vascular plants suggests the additional influence of an outflow from Guanabara Bay at the study site. These results point to the need for further investigation of the relative influence of physical forcings and continental inputs on the biogeochemical processes on the section of the continental shelf considered in the present study.
Marcadores moleculares na classe de lipídios (esterois, ácidos graxos e hidrocarbonetos) e a composição isotópica (δ13C e δ15N) da matéria orgânica bruta foram quantificados em amostras de um testemunho de sedimento para caracterizar o histórico recente de sedimentação da matéria orgânica na plataforma continental adjacente à Baía de Guanabara, no Estado do Rio de Janeiro. Na superfície do sedimento (0-1 cm), a concentração total de esterois e ácidos graxos foi cerca de uma ordem de grandeza maior do que observado nas camadas mais profundas do sedimento, com predominância de lipídios derivados da produção planctônica. Esses resultados sugerem, como confirmado através de estatística multi-variada, a ocorrência de um evento de elevada produção primária na coluna d'água e uma exportação rápida e eficiente das partículas para o sedimento. Condições similares são observadas na região de ressurgência de Cabo Frio, localizada cerca de 150 km ao norte da área de estudo. Portanto, nossos resultados sugerem que tais eventos têm uma influência regional sobre a produção primária na margem continental sudeste do Brasil. Por outro lado, a presença de lipídios derivados de plantas vasculares de origem continental ressalta a necessidade de investigar com maior profundidade a influência relativa entre forçantes físicas e o aporte continental sobre processos biogeoquímicos na porção da plataforma continental considerada no presente trabalho

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Abstract. The number of sedimentary records collected along the Brazilian continental margin has increased significantly in recent years, but relatively few are located in shallow waters and register paleoceanographic processes in the outer shelf–middle slope prior to 10–15 ka. For instance, the northward flow up to 23–24∘ S of cold and fresh shelf waters sourced from the Subantarctic region is an important feature of current hydrodynamics in the subtropical western South Atlantic Ocean, and yet limited information is available for the long-term changes of this system. Herein, we considered a suite of organic and inorganic proxies – alkenones-derived sea surface temperature (SST), δD-alkenones, δ18O of planktonic foraminifera, and ice-volume free seawater δ18OIVF−SW – in sediment from two cores (RJ-1501 and RJ-1502) collected off the Rio de Janeiro Shelf (SE Brazilian continental shelf) to shed light on SST patterns and relative salinity variations since the end of the last glacial cycle in the region and the implications of these processes over a broader spatial scale. The data indicate that, despite the proximity (∼40 km apart) of both cores, apparently contradictory climatic evolution occurred at the two sites, with the shallower (deeper) core RJ-1501 (RJ-1502) showing consistently cold (warm) and fresh (salt) conditions toward the Last Glacial Maximum (LGM) and last deglaciation. This can be reconciled by considering that the RJ-1501 core registered a signal from mid- to high latitudes on the upper slope off Rio de Janeiro represented by the influence of the cold and fresh waters composed of Subantarctic Shelf Water and La Plata Plume Water transported northward by the Brazilian Coastal Current (BCC). The data from core RJ-1502 and previous information for deep-cores from the same region support this interpretation. In addition, alkenone-derived SST and δ18OIVF−SW suggest a steep thermal and density gradient formed between the BCC and Brazil Current (BC) during the last climate transition which, in turn, may have generated perturbations in the air–sea heat flux with consequences for the regional climate of SE South America. In a scenario of future weakening of the Atlantic Meridional Overturning Circulation, the reconstructed gradient may become a prominent feature of the region.

Alkenones or long-chain unsaturated ketones (C36-C39) are a category of lipids produced exclusively by a class of marine phytoplankton (haptophyte algae) present in the surface of the oceans. These compounds are biomarkers (molecular proxies) traditionally used in paleoceanographic reconstructions through the proportions between their most abundant compounds (C37 alkenones) or the determination of their specific isotopic composition, which is dependent on suitable isolation of the lipid compounds. Samples from two sedimentary cores collected from the continental slope of southeastern Brazil were used to apply a methodology of alkenone isolation. The isolation steps, beginning with organic extraction of the sediment, contemplate the formation of urea adduct and silica gel column chromatography impregnated with silver ions. The results of each alkenone isolation phase were monitored by gas chromatography coupled with a flame-ionization detector (GC-FID) and the isolated fractions were injected into a mass spectrometer with accelerators (AMS) for radiometric analysis. The purification methodology was efficient in adequately isolating the alkenones for isotopic analysis, with low content of interfering compounds (< 2 %) and high global recovery (89 %). Losses of less than 11 % during the process occurred particularly in the Ag +-SiO2 step (7 %). Throughout the isolation steps, a preferential loss of C37 ketones over C38 and of C37:3 over C37:2 was observed. This removal is confirmed by the lack of significant correlation (p < 0.05) between the fraction prior to isolation, and the isolated fraction of both the ΣC37/ΣC38 and U 37 K´ (C37:2/C37:2 + C37:3) values, which indicates that this paleotemperature index should not be evaluated based on the isolated fraction. The final masses of isolated alkenones (12.2-55.2 μg) acquired in the purification process allowed for the determination of this compound specific radiocarbon, an important tool that can be used to evaluate paleoenvironmental processes.

Abstract. The number of sedimentary records collected along the Brazilian margin has grown significantly in recent years. However, few are useful in elucidating the paleoclimatic evolution of relatively shallow waters (