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Organic matter is an important source of information on the transport and consolidation processes of sediments. In this study, the isotopes of carbon and nitrogen (δ 13 C and δ 15 N), total organic carbon (TOC), total nitrogen, carbon/nitrogen (C/N) ratio, and 13 C-NMR were utilized to understand the origin and behavior of organic material in the Abrolhos region. It were analyzed nine sediment cores taken from a mangrove, a channel between the mainland and the coral reefs. The average value of the C/N ratio in the mangrove was 18, which characterizes purely terrigenous areas. For the reefs, the average value of the C/N ratio was 8,which is characteristic of marine and coastal regions. For the sediment cores taken from the channel, the average value of the C/N ratio was 10, a typical value of areas under the influence of mangroves. The mean values of δ 13 C were -26.9‰ for the mangrove, -20.7‰ for the channel region, and -18.2‰ for the reefs. This variation is associated with the main source of organic matter, which in the mangrove is derived from vascular plants (mainly C3 metabolism) and for the reefs is derived from phytoplankton. The 13 C-NMR results corroborate the isotopic and elemental analyses. The analyses of these cores indicate that the anthropogenic influence on the coast did not significantly alter the composition of the material that has been deposited in about the last 80 years in the region of study.

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 (δ 18 O sw and δD sw) 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 δ 18 O sw 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 δ 18 O sw and δD was found, which made it possible to extrapolate the results for δ 18 O sw 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 δ 18 O sw 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* δ 18 O sw + 34.56) was proposed to reduce the estimated errors associated with these seasonal variations.

Quantitative information about the variation in natural isotopic abundances in water is of great importance in a variety of fields. Due to the wide range of applications and types of samples, it is necessary that isotopic analyses have precision, accuracy and reproducibility. The present study compares the techniques of cavity ring-down spectroscopy (CRDS) and isotope-ratio mass spectrometry (IRMS) for the determination of the isotopic ratios of 2 H/ 1 H and 18 O/ 16 O in water in the two secondary standards, denoted PB3 and PB4, and in a certified material, GISP, Greenland Ice Sheet Precipitation, used as a quality tester of such measurements. The traditional method for measuring isotopic ratios is IRMS. Because of the nature of the molecule, the samples are not introduced directly into the mass spectrometer. Instead, the water is chemically converted to CO 2 and H 2 . The other technique, CRDS, is a system of laser absorption that has great potential for the detection of atomic and molecular species with high sensitivity by measuring the light absorption ratio as a function of time, confined within an optical cavity of high finesse. In this technique, the water sample is converted into steam without undergoing conversion processes. Parametric (test T) and nonparametric (Wilcoxon) statistical tests were performed to compare the results obtained in the system, and CRDS and IRMS are from the same population. The values of the isotopic abundances of the two secondary standards [PB3, δD = − 1.9 ± 0.4 (‰) and δ 18 O = − 2.19 ± 0.24 (‰) and PB4, δ 2 H = − 71.4 ± 0.4 (‰) and δ 18 O = − 10.08 ± 0.19 (‰)] were determined with accuracy. For the certified standard GISP, values of δ 2 H = − 189.3 ± 0.5 (‰) and δ 18 O = − 24.69 ± 0.20 (‰) were obtained. Both techniques have factors that interfere with the accuracy of the measurements and require corrections. Comparing the results revealed that there was a greater accuracy for measurements with CRDS and greater precision for IRMS. However, the results are within the tolerance range of 0.2‰ for δ 18 O and 2.0‰ for δ 2 H in isotope hydrology.

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