Low content of highly reactive iron in sediments from Prydz Bay and the adjacent Southern Ocean: Controlling factors and implications for sedimentary organic carbon preservation

Examining iron (Fe) speciation in marine sediments is critical to understand Fe and carbon biogeochemical cycling in polar regions. In this study, we investigated the speciation of Fe in sediments from Prydz Bay and the adjacent Southern Ocean, and examined the factors controlling Fe speciation and its relationship with total organic carbon (TOC). Our results reveal that unreactive silicate Fe (Fe-U) is the dominant pool of total Fe (Fe-T), followed by poorly reactive sheet silicate Fe (Fe-PRS), reducible crystalline Fe oxides (Fe-ox2), easily reducible amorphous/poorly crystalline Fe oxides (Fe-ox1), and magnetite (Fe-mag), with carbonate-associated ferrous Fe (Fe-carb) being the smallest pool. The highly reactive Fe (Fe-HR)/Fe-T ratios (0.13 +/- 0.06) in our study area are among the lowest end-member globally, primarily due to weak bedrock weathering and slow glacier melting. The Fe-ox1/Fe-T ratios are similar to those in continental shelf and marginal seas containing highly weathered materials, while the Fe-ox2/Fe-T ratios are significantly lower. This result implicates that low temperature inhibits the aging of iceberg melting-sourced Fe-ox1 potentially, and accordingly the regulation of weathering on the Fe-HR/Fe-T ratio is mainly reflected in Fe-ox2/Fe-T ratio. There are no significant correlations between TOC and Fe-HR, Fe-carb, Fe-ox1 or Fe-ox2 in the research region. Four distinct patterns of TOC/Fe-HR ratio can be discerned by summarizing the global data set: (a) high TOC/Fe-HR ratios (> 2.5) are likely the result of high marine primary productivity and low chemically weathered source materials; (b) low TOC/Fe-HR ratios (< 0.6) are caused by high rates of Fe-HR inputs and OC remineralization; (c) mid-range TOC/Fe-HR ratios (0.6 - 2.5) typical of most river particulates and marginal sea sediments indicate the same Fe-HR and OC sources and/or interactions between each other; (d) both low TOC and Fe-HR content is the result of low marine primary productivity and weak chemical weathering. Our findings provide new insights into the relationship between Fe-HR and TOC in polar sediments.