Fluorescence characteristics and dynamics of dissolved organic matter (DOM) in the Amundsen Sea, Antarctica

Abstract

The Amundsen Sea in West Antarctica has undergone a rapid ice loss through basal melting due to the warm Circumpolar Deep Water (CDW) intrusion in the bottom layer. This warm and salty CDW intrusion to the continental shelf drives the basal melting of glaciers. Glacial melt water can supply bioavailable iron to the euphotic zone, which can affect the primary production, resulting in the changes in the dynamics of dissolved organic matter (DOM) and carbon cycle in the Amundsen Sea. Thus, it is important to understand the dynamics and quality of DOM and its sources in the Amundsen Sea. Based on the hydrographic observations conducted in the Amundsen Sea from 6 January to 16 February 2020, the relatively high values of dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) absorption coefficient at 350 nm (a350) were observed in the WGIS. Additionally, the low value of spectral slope coefficient (S275-295) and the high value of specific UV absorbance at 254 nm (SUVA254) indicated that the high molecular weight of DOM in the WGIS. On the other hand, the values of a350 and SUVA254 decreased with increasing S275-295 value, indicating that the low molecular weight of DOM was predominant in the DIS. By the fluorescence excitation-emission matrices (EEMs), which analyzed by parallel factor (PARAFAC) modeling, a humic-like component (C1, Exmax/Emmax = 250/427) and a protein-like component (C2, Exmax/Emmax = 275/339) were identified. The vertical distribution of C1, a tracer for CDW, suggested that the DIS was more directly influenced by the supply of DOM from sediment origin and glacier meltwater to the euphotic zone due to the intrusion of warm CDW. The supply of DOM by the CDW to the euphotic zone in the DIS can expose DOM to UV radiation, which enhances CDOM photo-degradation in the upper layer. In contrast, the CDW intrusion to the basal glacier in the WGIS was prevented by the Antarctic slope front (ASF), which forms isopycnal gradients in bottom depth. As a result, the distinct regional differences of optical properties between DIS and WGIS occurred. The FDOMH* values, calculated by a significant positive relationship between C1 and apparent oxygen utilization (AOU) (r2=0.45, p < 0.05 in the WGIS, and r2=0.47, p < 0.05 in the DIS), showed positive values, revealing that C1 was derived from sediments in both the WGIS and DIS. In addition, the relationships between C2, nutrient, and particulate organic carbon (POC) suggested that C2 was mainly originated from marine primary production and sea ice or microbial activity origin in both the WGIS and DIS. The results of this study indicate that the differences in the characteristics of DOM between the WGIS and DIS are likely caused by the difference of the CDW intrusion pathway rather than the differences of DOM sources. Therefore, this study suggests that the changes in the CDW intrusion pathway by glacial basal melting and/or global warming may result in the change in DOM quantity and quality in West Antarctica.