An isotopic perspective on the correlation of surface ocean carbon dynamics and sea ice melting in Prydz Bay (Antarctica) during austral summer

Abstract The stable carbon isotope composition of particulate organic carbon (δ 13 C POC ) and naturally occurring long-lived radionuclide 226 Ra ( T 1/2 =1600 a) were applied to study the variations of upper ocean ( 13 C POC values ranged from −27.4‰ to −19.0‰ and generally decreased from inner bay (south of 67°S) toward the Antarctic Divergence. Surface water 226 Ra activity concentration ranged from 0.92 to 2.09 Bq/m 3 (average 1.65±0.32 Bq/m 3 , n =20) and increased toward the Antarctic Divergence, probably reflecting the influence of 226 Ra-depleted meltwater and upwelled 226 Ra-replete deep water. The fraction of meltwater, f i , was estimated from 226 Ra activity concentration and salinity using a three-component (along with Antarctic Summer Surface Water, and Prydz Bay Deep Water) mixing model. Although the fraction of meltwater is relatively minor (1.6–11.9%, average 4.1±2.7%, n =20) for the surface waters (sampled at ~6 m), a positive correlation between surface δ 13 C POC and f i (δ 13 C POC = 0.94× f i −28.44, n =20, r 2 =0.66, p 13 C POC values in the inner Prydz Bay compared to the open oceanic waters. This is the first time for a relationship between δ 13 C POC and meltwater fraction to be reported in polar oceans to our knowledge. We propose that sea ice melting may have affected surface ocean δ 13 C POC by enhancing water column stability and providing a more favorable light environment for phytoplankton photosynthesis, resulting in drawdown of seawater CO 2 availability, likely reducing the magnitude of isotope fractionation during biological carbon fixation. Our results highlight the linkage of ice melting and δ 13 C POC , providing insights into understanding the carbon cycling in the highly productive Antarctic waters.