Erosion of carbonate-bearing sedimentary rocks may close the alkalinity budget of the Baltic Sea and support atmospheric CO2 uptake in coastal seas

Abstract

High alkalinity values observed in coastal seas promote the uptake of CO2 from the atmosphere. However, the alkalinity budget of coastal areas and marginal seas is poorly understood, even though some of the recently observed alkalinity enhancement can be ascribed to riverine fluxes and anaerobic processes in shelf sediments. Here, we investigate the alkalinity budget of the Baltic Sea to identify previously unrecognized alkalinity sources. We quantify the generation of alkalinity and dissolved calcium (Ca) in this marginal sea applying simple mass balance calculations. Using this approach, we identify alkalinity and Ca sources of approximately 324 Gmol yr-1 and 122 Gmol yr-1, respectively, that cannot be ascribed to the riverine input. The magnitude of the Ca source suggests that a major fraction of the excess alkalinity (244 Gmol yr-1) is induced by the dissolution of calcium carbonate (CaCO3). A review of available field data shows that carbonate-bearing rocks at the coast and the seabed of the Baltic Sea are rapidly eroded and may provide sufficient CaCO3 to close the Ca budget. Hence, dissolution of eroded CaCO3 is the most likely source for the Ca enrichment observed in Baltic Sea water. This hypothesis is supported by mass accumulation rates of sediments derived from radioisotope data that are evaluated to derive a basin-wide rate of mud to muddy sand accumulation at the bottom of the Baltic Sea. The resulting value (139 Tg yr-1) exceeds current estimates of riverine particle fluxes into the Baltic Sea by more than one order of magnitude and confirms that rates of till erosion are sufficiently high to account for the Ca and most of the alkalinity excess in Baltic Sea water. Finally, we show that deliberate addition of CaCO3 to sediments deposited in the Baltic Sea could neutralize significant amounts of CO2 and help to achieve net-zero greenhouse gas emissions in the Baltic region.

Disentangling artificial and natural benthic weathering in organic rich Baltic Sea sediments

“…Dissolved Ca could be used to quantify dissolution rates of carbonates (calcite, aragonite) added to the seabed. Data obtained in this study indicate that this may be possible but that the interpretation of benthic Ca flux data is complicated by the dissolution of calcite delivered by coastal and seabed erosion in the study area (Wallmann et al, 2022). Similar problems may arise in other coastal and shelf settings where carbonate minerals produced by erosion and calcareous plankton are deposited and dissolved at the seabed.…”

“…However, the shallow sedimentary depocenters in the Baltic Sea display strong biogeochemical and physical seasonality, which leads to a highly complex chemical system governed by variable oxygen availability, bottom water temperatures, microbial activities, and organic matter degradation rates (Dale et al, 2011;Melzner et al, 2013;Perner et al, 2022). Moreover, the large benthic background fluxes induced by natural CaCO 3 input via continental erosion and diatom dissolution (Gasiunaite et al, 2005;Borawska et al, 2022;Wallmann et al, 2022) constitute further challenges for disentangling natural and enhanced weathering, as they might overprint potential fluxes induced by enhanced benthic weathering.…”

Benthic-pelagic coupling and isotopic fractionation of barium in Kiel Bight, SW Baltic Sea

“…Coarse grained sediment particles are retained near-shore whereas the fine-grained fraction is transported over longer distances and accumulates in deeper channels and basins (Seibold et al, 1971). The glacial till also contains up to 30% of calcium carbonate (Averes et al, 2021), which is mostly dissolved due to undersaturation in the bottom water and pore waters of surface sediments (Wallmann et al, 2022).…”

Guide to Best Practices in Ocean Alkalinity Enhancement Research

“…In areas where there are strong bottom currents, fine-grained minerals can be eroded and transported. These minerals are then ultimately delivered to regional depocenters in deeper basins in shallow coastal seas such as the Baltic Sea (e.g., Wallmann et al, 2022) or to the continental slopes on open margins (e.g., Anderson et al, 1994). Once deposited in deep waters, they are removed from the shallow regions, where the benthic-pelagic water mass exchange is rapid.…”