Abstract
The deep basins of the Baltic Sea, including the Gotland and Landsort Deeps, are well known for the exceptional occurrence of sedimentary Mn carbonate. Although the details of the mechanisms of Mn carbonate formation are still under debate, a close relationship with episodic major Baltic inflows (MBIs) is generally assumed, at least for the Gotland Basin. However, the few studies on Mn cycling during MBIs suffer from a limited temporal resolution. Here we report on Mn dynamics in the water column and sediments of the Gotland Deep following an MBI that entered the Baltic Sea in December 2014. Water column profiles of dissolved Mn were obtained at a monthly to bi-monthly resolution between February 2015 and March 2017 and revealed an impact of the MBI on the Gotland Deep bottom waters beginning in March 2015. Water column profiles and budget estimates provided evidence for remarkable losses of dissolved Mn associated with the enhanced deposition of Mn oxide particles, as documented in sediment trap samples and surface sediments. In July 2015, subsequent to the nearly full oxygenation of the water column, clear signals of the re-establishment of bottom water anoxia appeared, interrupted by a second inflow pulse around February 2016. However, dissolved Mn concentrations of up to 40 µM in the bottom waters in June 2016 again indicated a pronounced reduction of Mn oxide and the escape of dissolved Mn back into the open water column. The absence of substantial amounts of Mn carbonate in the surface sediments at the end of the observation period suggested that the duration of bottom water oxygenation plays an important role in the formation of this mineral. Data from both an instrumental time series and a dated sediment core from the Gotland Deep supported this conclusion. Enhanced Mn carbonate formation occurred especially between the 1960s and mid-1970s, when several MBIs caused a long-lasting oxygenation of the water column. By contrast, Mn carbonate layers were much less pronounced or even missing after single MBIs in 1993, 2003, and 2014, each of which provided a comparatively short-term supply of O2 to the deeper water column.
Highlights
The transition metal Mn is an essential micro-nutrient for every life-form and plays a central role in photosynthesis (Davidson and Marchant, 1987; Hansel, 2017)
In November 2011, the sulfide concentration in the bottom waters increased to a maximum of 148 μM, while Mndiss concentrations within the deeper water column remained at a similar level of ∼10 μM
After a basin-wide extrapolation of Mndiss concentrations from the water column profiles using the corresponding hypsography-based water volumes (Figure 8), the Mndiss inventories (∼1.5 × 109 mol) in the euxinic water bodies were identical during the 2 years, which argues against a sustained effect of the weak pre-major Baltic inflows (MBIs) oxygenation, at least for Mn
Summary
The transition metal Mn is an essential micro-nutrient for every life-form and plays a central role in photosynthesis (Davidson and Marchant, 1987; Hansel, 2017). The sensitivity of Mn to changing redox promotes the formation of massive Mn deposits in modern and ancient times; including e.g., ferromanganese crusts and nodules as well as Mn ore deposits comprising oxide, carbonate, and silicate phases (Hlawatsch et al, 2002; Nyame et al, 2003; Johnson et al, 2016). In addition to such exceptional deposits, whose mechanisms of formation are still not fully understood, sedimentary Mn signatures are used as proxy for past redox reconstructions (Pruysers et al, 1993; März et al, 2011)
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