Tracing Circumpolar Deep Water and glacial meltwater using humic-like fluorescent dissolved organic matter in the Amundsen Sea, Antarctica

Abstract The Amundsen Sea is the most rapidly melting part of the West Antarctic Ice Sheet, due to increased heat transport by Circumpolar Deep Water (CDW). Tracing CDW and resulting glacial meltwater is important since glacial meltwater may change the water mass properties, leading to the change of the biogeochemical cycles. In this study, in order to investigate the potential for using the humic-like component of fluorescent dissolved organic matter (FDOM) as a tracer for CDW and glacial meltwater in the Amundsen Sea, a hydrographic survey was conducted during the austral summer of 2018 aboard the Korean icebreaker IBR/V Araon. The meteoric water and CDW fractions calculated using the humic-like component (fmw_humic and fcdw_humic) were compared to those using oxygen isotope (δ18O) (fmw) and optimum multiparameter analysis (OMP) (fcdw_OMP), respectively. The fluorescence intensity of the humic-like component varied from 0.007 to 0.021, with higher values in the deeper layer and lower intensities in the surface waters. The range of fcdw_humic (0.5–1.0) was narrower than that of fcdw_OMP (0.1–1.0), indicating that the fcdw_humic values were overestimated due to the remained humic-like C1. To minimize the effect of the remained humic-like C1 on the calculation of CDW fraction, we used newly derived empirical equations (i.e., fcdw_OMP = 105.17 × C1–1.14 for transect 1 and fcdw_OMP = 126.04 × C1–1.41 for transect 2). The CDW fraction calculated using the empirical equations (fcdw_humic_empirical) was in good agreement with the fcdw_OMP. We also found a significant positive relationship between fmw and fmw_humic, indicating that a reasonable method can be applied with a high percentage of explained variance and that fmw can be largely explained by fmw_humic. Our results show that the humic-like component can be a useful tracer for identifying CDW and glacial meltwater in the Amundsen Sea.