Molecular Composition Evolution of Dissolved Organic Matter With Water Depth in Prydz Bay of East Antarctic: Carbon Export Implications.

This study analyzes the molecular composition of dissolved organic matter (DOM) in Prydz Bay by Fourier Transform Ion Cyclotron Resonance mass spectrometry to probe the carbon sequestration capacity in the continental shelf system. Concentrations of particulate organic carbon (POC), particulate nitrogen and dissolved organic carbon (DOC) with water depth show that POC could be mainly decomposed into DOC and/or microbially degraded. Highly labile DOC is further degraded and remineralized by microorganisms within the upper 200 m, as evidenced by a downward enrichment of 13CPOC and increases in the average molecular weight, oxygen atom number (O) and double bond equivalents of DOM molecules, indicating that biodegradation is the main driver for particulate organic matter and DOM evolution with water depth. Semi‐quantitative calculation demonstrates that ∼83% of POC was transformed to DOC as well as dissolved inorganic carbon (DIC), and ∼30% of DOC further to DIC via microbial degradation within the upper 200 m in summer, resulting in a relatively low total organic carbon content in sediments of Prydz Bay. The newly transformed DIC and residue DOC can be preserved in the deep layer due to the formation of well stratified and stable water body in summer of Prydz Bay, ultimately entering the regional circulation system instead of being released back into the atmosphere. This could be one of the most important processes determining the atmosphere CO2 uptake in the continental shelf system of Southern Ocean. Plain Language Summary: The continental shelf system in the Southern Ocean, especially with the development of polynyas, plays a critical role in the global carbon cycle and disproportionally accounts for almost half of the anthropogenic CO2 uptake by the biological pump, with ensuing deep‐sea sequestration by the form of particulate and dissolved organic carbon, and even CO2 due to microbial degradation of primary productivity. However, complex interactions between physical, chemical and biological processes significantly affect the fate of organic carbon through the water column, resulting in an uncertainty of vertical carbon export. In this study, we conduct an investigation on the dissolved organic matter sources and evolution with water depth in summer of Prydz Bay, a typical polynya‐developed continental shelf system in the East Antarctica. The results clearly show that biodegradation is the main factor controlling organic carbon mineralization and mainly occurs within the upper 200 m of water column, with a transformation into dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). Integrated with regional circulation system and hydrological characteristics, our study further indicates that newly formed DIC and DOC can be preserved in the deep layer of Prydz Bay, then into the regional circulation system rather back to the atmosphere. Key Points: Novel evidence at molecular level shows the vital role of biodegradation during carbon sink in Prydz BayThe conversion of POC into DOC/DIC mainly occurs within the upper 200 m of water columnHighlighting regional circulation's contribution to carbon transport and sequestration in the Southern Ocean [ABSTRACT FROM AUTHOR]