Subglacial Lake Discharge Affects the Age and Concentration Distribution of Dissolved Organic Carbon in Ross Sea Surface Waters

Abstract

The Ross Sea continental shelf plays a key role in the global carbon cycle. However, the sources and cycling of dissolved organic carbon (DOC) in the region are poorly understood. Here, we determined the radiocarbon ( ¹⁴ C) and stable carbon isotope ( ¹³ C) contents of DOC in addition to those of dissolved inorganic carbon (DIC) in the Little America Basin, eastern Ross Sea. Carbon isotope values of DIC indicate rapid vertical mixing across the continental shelf, along with high surface primary productivity near the ice shelf. Despite the supply of freshly produced DOC from high primary productivity, the ¹⁴ C age of the DOC throughout the water column was unexpectedly old (5,100–7,400 years), with surface DOC being older than that in deeper waters, in contrast to the typical oceanic distribution. A Keeling plot of the relationship between DOC ¹⁴ C contents and the inverse of the DOC concentrations exhibits a positive linear correlation, which is the opposite trend to that of the global ocean. This demonstrates the presence of DOC with a ¹⁴ C age of >10,000 years in the surface waters. Although the exact source of this aged DOC remains uncertain, we suggest that DOC from subglacial lake discharge is the likely source.