<p>The Earth system has entered a new geological epoch, the Anthropocene. The oceans’ capacity to regulate atmospheric carbon dioxide (CO 2 ) at various<br />timescales is amongst the most crucial players to maintain climate on Earth in a habitable range. The biogeochemical property exerting this regulatory mechanism is alkalinity, the oceans’ CO 2 and pH buffer capacity. The proposed project will investigate how the oceans’ alkalinity is impacted firstly by human measures, required by the Paris agreement (COP 21) to mitigate climate change via bioenergy production and its downstream effects on shallow oceans, and secondly by climate change, in particular by increased weathering in the Arctic because of ice retreat.</p>

<p><strong>Investigation of different anaerobic respiratory pathways and their impacts on the release ratio of DIC/alkalinity at selected North Sea regions</strong></p><p>Tentative authors: Nele Treblin<sup>1,2</sup>, Michael E. Böttcher<sup>3</sup>, Tristan Zimmermann<sup>1</sup>, Daniel Pröfrock<sup>1</sup>, Mona Norbisrath<sup>1</sup>, Bryce van Dam<sup>1</sup>, Helmuth Thomas<sup>1</sup></p><p><sup>1</sup>Institute for Coastal Research, Helmholtz Center Geesthacht</p><p><sup>2</sup>Alfred-Wegener-Institute Helmholtz Center for Polar and Marine Research</p><p><sup>3</sup>Leibniz Institute for Baltic Sea Research Warnemünde</p><p> </p><p>Coastal sediments play a crucial role in carbon metabolism, which decreases with increasing distance from the shoreline. The North Sea, a NW European shelf sea, represents a relatively shallow, well-ventilated (on annual timescales) system, connected to the Baltic Sea and the North Atlantic. Especially the southern part of the North Sea receives a large amount of organic matter (OM), both from riverine input and internal North Sea sources. After the depletion of oxygen due to aerobic OM respiration, anaerobic metabolic activities become dominant in the sediment. In the absence of oxygen, electron acceptors, such as NO<sub>3</sub><sup>-</sup>, Fe<sup>3+</sup>, Mn<sup>4+</sup> and SO<sub>4</sub><sup>2-</sup>, facilitate not only the release of respired CO<sub>2</sub>, but also of alkalinity, furthermore enhanced by potential dissolution of sedimentary carbonates. Therefore, under these conditions, benthic-pelagic coupling may impact on the potential to absorb CO<sub>2</sub> from the atmosphere.</p><p>To investigate the described processes, porewater and sediment samples, collected from six different stations in the German Bight (North Sea) during the RV Heincke cruise HE541 in September 2019, have been analyzed for their vertical concentration profiles of nutrients, various trace metals, sulfur, DIC and alkalinity.</p><p>Benthic oxic and anoxic zones have been identified based on the vertical concentration gradients. Furthermore, alkalinity and DIC are set in relation to anaerobic metabolic activities. Finally, active reworking and ventilation becomes pivotal in areas such as the North Sea. Thus, the influence of bioturbation on anaerobic respiration is also considered.</p>