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

Abstract

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.

Post-depositional formation of vivianite-type minerals alters sediment phosphorus records

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Holocene Refreshening and Reoxygenation of a Bothnian Sea Estuary Led to Enhanced Phosphorus Burial

“…The variations in bottom water oxygen may have stimulated the transformation of Mn oxides to more stable Mn carbonates in these sediments, in particular at site M0062 where Mn content is >100 μmol/g ( Fig. 3; Huckriede and Meischner 1996;Lenz et al 2015). The high C org content in these Moenriched sediments is likely the combined effect of enhanced preservation of organic matter in sediments under anoxic conditions (e.g., De Hartnett et al 1998;Tsandev et al 2012), and an increased flux of organic matter from the productive water column to the seafloor.…”

Mn∕Ca intra- and inter-test variability in the benthic foraminifer <i>Ammonia tepida</i>

“…Sedimentary records of manganese may reflect a variety of past environmental conditions, including bottom water redox state, continental runoff, surface water productivity and bottom water current dynamics (Reichart et al, 1997;van der Weijden et al, 2006;Lenz et al, 2015). The use of sedimentary manganese as a proxy is complicated because of its complex biogeochemical dynamics, including the remobilization of once precipitated manganese oxides, and subsequent diagenetic overprinting (Schenau et al, 2002).…”