IMIS

Iron, manganese, and trace elements play an important role in the marine carbon cycle as they are limiting nutrients for marine primary productivity. Water column concentrations of these bio-essential elements are controlled by the balance between input and removal, with burial in marine sediments being the main sink. The efficiency of this burial sink is dependent on the redox state of the water column, with sediments underlying a sulphidic (euxinic) water column being the most efficient sinks for Fe, but also Mn and trace elements (Co, Cd, Ni, Mo, As, W, V, and U). Transient changes in ocean redox state can hence affect trace element burial, and correspondingly, the ocean’s trace element inventory, but the impact of transient oxygenation events on trace element cycling is currently not well understood.

Here, we investigate the impact of a natural oxygenation event on trace element release and burial in sediments of the Eastern Gotland Basin (EGB), a sub-basin of the Baltic Sea. After being anoxic (<0.5 µM O₂) for ∼10 years, the deep waters of the EGB experienced a natural oxygenation event (Major Baltic Inflow, MBI) in 2015. Following this oxygenation event, we deployed benthic chamber landers along a depth transect in the EGB in April 2016, 2017 and 2018. We complemented these in situ flux measurements with analyses of water column, solid phase and pore water chemistry. Overall, the event increased the benthic effluxes of dissolved trace elements, though particular responses were element-specific and were caused by different mechanisms. Enhanced fluxes of Cd and U were caused by oxidative remobilisation, while Ni showed little response to the inflow of oxygen. In contrast, enhanced release of Co, Mo, As, W, and V was caused by the enhanced transient input of Mn oxides into the sediment, whereas Fe oxides were of minor importance. Following the dissolution of the oxides in the sediment, Mn and W were nearly completely recycled back to the water column, while fractions of Fe, Co, Mo, As, and V were retained in the sediment. Our results suggest that transient oxygenation events in euxinic basins may decrease the water column inventory of certain trace elements (Fe, Co, Mo, As, and V), thus potentially affecting global marine primary productivity on longer timescales.