Salinity and stable oxygen isotope relationship in the Southwestern Atlantic: constraints to paleoclimate reconstructions

Abstract: 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.

摘要：稳定同位素在学术研究中应用广泛，既可用于探讨现代海洋环流过程，也可用于重建古海洋学参数。本研究针对南大西洋西部热带边缘海域的海水氧稳定同位素与氘稳定同位素（δ¹⁸O_sw和δD_sw）分布特征展开调查，旨在明确两类同位素的主要分馏机制，并建立区域盐度与δ¹⁸O_sw的相关关系，以深化该区域的古海洋学研究认知。本研究依托一次准同步海洋考察航次开展，在主要水团核心区域采集了98个离散海水样本，用于稳定同位素分析。研究发现δ¹⁸O_sw与δD之间存在显著相关性，据此可将δ¹⁸O_sw的分析结果外推至δD。尽管仅通过同位素特征无法区分各水团，但各水团的盐度与δ¹⁸O_sw均呈现极强的相关关系；与既往研究对比后，本研究观测到了显著的季节变化模式。研究记录到夏季与冬季的古盐度差值可达0.2 psu（实用盐度单位，Practical Salinity Unit）。考虑到当前古海洋学研究中难以按季节维度分离样本的技术局限，本研究提出了南大西洋热带海域的中间混合线公式：SSS = 1.942 × δ¹⁸O_sw + 34.56，以降低上述季节差异带来的估算误差。