Transitions of redox state and nutrient status in the Southern Ocean since the last glacial: Evidence from speciation analyses of C, Fe, and P in sediments at the Conrad Rise

The Southern Ocean, a high-nutrient and low-chlorophyll (HNLC) region, has played an important role in regulating global climate system. The Southern Ocean became suboxic during the last glacial period ( 40 to 19 kyr ago) and changed to oxic toward Holocene. In order to elucidate changes of the seawater for its the redox state and corresponding nutrient status, we performed C, Fe and P speciation analyses of the marine sediments (COR-1bPC) recovered in 2010 at the Conrad Rise in the Southern Ocean (KH10-7 cruise). Thirty-seven samples were quantified for five P-bearing species (Pabs, PFe, Pauth, Pdet & Porg) by modified SEDEX method of Ruttenberg (1992) and four Fe-bearing species (FeHCl, Fecarb, Feox & Femag) by the method of Poulton et al. (2005). The abundance and stable isotope compositions of organic carbon were measured using EA-irMS at Kochi University. Higher abundance of Corg, Fe and P were observed in the last glacial period when compared to interglacial period. In the deglaciation period, abundance of Corg decreased but that of all P-bearing phases, FeHCl, and Feox abruptly increased to the maximum values. Fepy was only detected in the last glacial. Abundance of FeHCl was highly variable among other species. A major sink of P in the analyzed samples is found to be Pauth throughout the profile (Avg. = 0.0142 wt.%). The d13Corg values in last glacial period (Avg. = -23.63 ‰) were isotopically lighter than those in the interglacial period (Avg. = -21.73 ‰). These results suggest that primary productivity was higher in last glacial by increased supply of nutrients (Fe and P) to surface ocean, most likely due to expansion of sea ice supported by occurrence of IRD. Suppression by sea ice expansion of atmosphere-ocean interactions would have allowed CO2 to be stored in deep ocean. Retreat of the sea ice and recovery of upwelling would have increased supply of P and Fe to the surface ocean and of dissolved oxygen to the deep ocean, leading to enhanced decomposition of organic matter. Concurrently, stored CO2 in deep ocean would have been released into the atmosphere. We conclude that Fe exerts the key control for differential storage of atmospheric CO2 in the Southern Ocean between glacial and interglacial periods.