Hypoxia-driven variations in iron and manganese shuttling in the Baltic Sea over the past 8 kyr

The Baltic Sea has experienced three major intervals of bottom water hypoxia following the intrusion of seawater circa 8 kyr ago. These intervals occurred during the Holocene Thermal Maximum (HTM), Medieval Climate Anomaly (MCA), and during recent decades. Here we show that sequestration of both Fe and Mn in Baltic Sea sediments generally increases with water depth, and we attribute this to shelf-to-basin transfer (“shuttling”) of Fe and Mn. Burial of Mn in slope and basin sediments was enhanced following the lake-brackish/marine transition at the beginning of the hypoxic interval during the HTM. During hypoxic intervals, shelf-to-basin transfer of Fe was generally enhanced but that of Mn was reduced. However, intensification of hypoxia within hypoxic intervals led to decreased burial of both Mn and Fe in deep basin sediments. This implies a nonlinearity in shelf Fe release upon expanding hypoxia with initial enhanced Fe release relative to oxic conditions followed by increased retention in shelf sediments, likely in the form of iron sulfide minerals. For Mn, extended hypoxia leads to more limited sequestration as Mn carbonate in deep basin sediments, presumably because of more rapid reduction of Mn oxides formed after inflows and subsequent escape of dissolved Mn to the overlying water. Our Fe records suggest that modern Baltic Sea hypoxia is more widespread than in the past. Furthermore, hypoxia-driven variations in shelf-to-basin transfer of Fe may have impacted the dynamics of P and sulfide in the Baltic Sea thus providing potential feedbacks on the further development of hypoxia

Coupled dynamics of CH4-S-FeP in Black Sea sediments

Surface sediments in the deep basin of the Black Sea are underlain by extensive deposits of iron (Fe) oxide-rich lake sediments that were deposited prior to the inflow of marine Mediterranean Sea waters ca. 9000 years ago. The ongoing downward diffusion of marine sulfate into the methane (CH4)-bearing lake sediments has led to a multitude of diagenetic reactions in the sulfate-methane transition zone (SMTZ). While the cycles of sulfur (S), CH4 and Fe in the SMTZ have been extensively studied, relatively little is known about their impact on sedimentary phosphorus (P) and the biogeochemical processes occuring below the SMTZ. In this study, we combine detailed geochemical analyses with multicomponent diagenetic modeling to demonstrate that sulfate-mediated anaerobic oxidation of CH4 substantially enhances the downward sulfidization of the lake deposits. This drives the release of Fe oxide bound P to the pore water and subsequent formation of authigenic Fe(II)-P minerals below the sulfidization front. We further show that downward migrating sulfide becomes partly re-oxidized to sulfate by reaction with oxidized Fe minerals, fueling a cryptic S cycle with slow rates of sulfate reduction in the deep limnic deposits. However, our results reveal that cryptic S cycling is unlikely to explain the observed release of dissolved Fe2+ below the SMTZ. Instead, we suggest that CH4 oxidation coupled to the reduction of Fe oxides may provide a possible mechanism for the apparent Fe oxide reduction at depth in the sediment. The coupled CH4-S-Fe-P dynamics described here may strongly overprint burial records of Fe, S and P in depositional marine systems subject to changes in organic matter loading or water column salinity. Such diagenetic alterations should not be interpreted as primary sedimentary signals