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 (f(mw_humic) and f(cdw_humic)) were compared to those using oxygen isotope (delta O-18) (f(mw)) and optimum multiparameter analysis (OMP) (f(cdw_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 f(cdw_ humic) (0.5-1.0) was narrower than that of f(cdw_ OMP) (0.1-1.0), indicating that the f(cdw_ 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 x C1-1.14 for transect 1 and f(cdw_ OMP) = 126.04 x C1-1.41 for transect 2). The CDW fraction calculated using the empirical equations (f(cdw_ humic_empirical)) was in good agreement with the f(cdw_OMP). We also found a significant positive relationship between f(mw) and f(mw_humic), indicating that a reasonable method can be applied with a high percentage of explained variance and that f(mw) can be largely explained by f(mw_humic). Our results show that the humic-like component can be a useful tracer for identifying CDW and glacial meltwater in the Amundsen Sea.