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We document an apparent downward displacement of the Matuyama‐Brunhes magnetic reversal by ∼20 m at Scotia Sea International Ocean Discovery Program Site U1538 (Pirie Basin) by comparison with the well‐defined paleomagnetic record at nearby Site U1537 (Dove Basin). Detailed stratigraphic correlation between the two sites is possible due to similar lithologic variations. However, the two sites have distinctly different porewater geochemistry. Notably, Site U1538 indicates a greater demand for electron acceptors to oxidize organic carbon and Fe 2+ enrichment below the depth of SO 4 2− depletion. Magnetic parameters indicate enrichment of an authigenic magnetic mineral with strong remanence properties around the depth of SO 4 2− depletion (∼46 m at Site U1538) relative to magnetic parameters at correlative depths at Site U1537. Fe 2+ enrichment below the depth of SO 4 2− depletion is not predicted based on the energetically favorable order of electron acceptors for microbial respiration but is documented here and in other depositional settings. This indicates Fe 2+ production exceeds the production of H 2 S by SO 4 2− reduction, providing a geochemical environment that favors the production and preservation of ferrimagnetic remanence‐bearing iron sulfides over paramagnetic pyrite and, thus, a mechanism for deep chemical remanent magnetization acquisition at depths of tens of meters. The influence of authigenic ferrimagnetic iron sulfides on paleomagnetic signals can be difficult to demonstrate with magnetic properties alone; therefore, this finding has implications for evaluating the fidelity of magnetostratigraphic records with complementary geochemical data. Such situations should be considered in other depositional environments with similar porewater Fe 2+ accumulation below the SO 4 2− reduction depth. Plain Language Summary Sediment on the seafloor is an archive of Earth history, including past climate variations, ice sheet dynamics, and geomagnetic field variatio

The reaction-transport model determines changes in sediment biogeochemistry and benthic fluxes at the study site (Boknis Eck) following the artificial addition of calcite to the seafloor as a potential carbon dioxide removal strategy. The code is written in MATHEMATICA v12 software (https://www.wolfram.com/mathematica/). No input files are needed to run the model. The model was built in the framework of the RETAKE project funded by the German Federal Ministry of Education and Research, Grant No. 03F0895, Project RETAKE, DAM Mission “Marine carbon sinks in decarbonization pathways” (CDRmare).

The research cruise M182 (MOSES Eddy-III) concludes a trilogy of research cruises conducted in the frame of the Helmholtz Association's Modular Observation Solutions for Earth Systems (MOSES) infrastructure, aiming at a comprehensive multi- and interdisciplinary study of ocean eddies in the northern tropical Atlantic. As an integral part of the BMBF-funded project "Role of eddies in the carbon pump of eastern upwelling systems - Demonstration Case Canary Current System" (REEBUS), launched in 2019, this study focuses on investigating the multi- layered influence of eddies on the lateral transport of biogeochemical properties and the carbon pump within deep-sea ecosystems. Ocean eddies have a significant impact on the physical, biogeochemical and biological aspects of coastal upwelling regions. Given the profound socio- economic impact of coastal upwelling areas, REEBUS takes a multidisciplinary approach to investigate the influence of ocean eddies on these systems. This expedition is motivated by the objective of acquiring novel insights into the functional dynamics of eddies, specifically elucidating their role in the CO2 source and sink functions inherent to eastern boundary upwelling systems. Furthermore, our investigative efforts encompassed a thorough examination of the biological carbon pump within both the designated upwelling areas and their associated oligotrophic regions. The synthesis of data from all REEBUS cruises, coupled with the inclusion of data from prior expeditions (MOSES Eddy-I and II) within this geographical region, promises to yield a high scientific potential.

Sediment fluxes to the seafloor govern the fate of elements and compounds in the ocean and serve as a prerequisite for research on elemental cycling, benthic processes and sediment management strategies. To quantify these fluxes over seafloor areas, it is necessary to scale up sediment mass accumulation rates (MAR) obtained from multiple sample stations. Conventional methods for spatial upscaling involve averaging of data or spatial interpolation. However, these approaches may not be sufficiently precise to account for spatial variations of MAR, leading to poorly constrained regional sediment budgets. Here, we utilize a machine learning approach to scale up porosity and 210 Pb data from 145 and 65 stations, respectively, in the Skagerrak. The models predict the spatial distributions by considering several predictor variables that are assumed to control porosity and 210 Pb rain rates. The spatial distribution of MAR is based on the predicted porosity and existing sedimentation rate data. Our findings reveal highest MAR and 210 Pb rain rates to occur in two parallel belt structures that align with the general circulation pattern in the Skagerrak. While high 210 Pb rain rates occur in intermediate water depths, the belt of high MAR is situated closer to the coastlines due to lower porosities at shallow water depths. Based on the spatial distributions, we calculate a total MAR of 34.7 Mt yr -1 and a 210 Pb rain rate of 4.7 · 10 14 dpm yr -1 . By comparing atmospheric to total 210 Pb rain rates, we further estimate that 24% of the 210 Pb originates from the local atmospheric input, with the remaining 76% being transported laterally into the Skagerrak. The updated MAR in the Skagerrak is combined with literature data on other major sediment sources and sinks to present a tentative sediment budget for the North Sea, which reveals an imbalance with sediment outputs exceeding the inputs. Substantial uncertainties in the revised Skagerrak MAR and the literature data might clo

The greenhouse gas methane (CH4) contributed to a warm climate that maintained liquid water and sustained Earth’s habitability in the Precambrian despite the faint young sun. The viability of methanogenesis (ME) in ferruginous environments, however, is debated, as iron reduction can potentially outcompete ME as a pathway of organic carbon remineralization (OCR). Here, we document that ME is a dominant OCR process in Brownie Lake, Minnesota (midwestern United States), which is a ferruginous (iron-rich, sulfate-poor) and meromictic (stratified with permanent anoxic bottom waters) system. We report ME accounting for ≥90% and >9% ± 7% of the anaerobic OCR in the water column and sediments, respectively, and an overall particulate organic carbon loading to CH4 conversion efficiency of ≥18% ± 7% in the anoxic zone of Brownie Lake. Our results, along with previous reports from ferruginous systems, suggest that even under low primary productivity in Precambrian oceans, the efficient conversion of organic carbon would have enabled marine CH4 to play a major role in early Earth’s biogeochemical evolution.

Iodine cycling in the ocean is closely linked to productivity, organic carbon export, and oxygenation. However, iodine sources and sinks at the seafloor are poorly constrained, which limits the applicability of iodine as a biogeochemical tracer. We present pore water and solid phase iodine data for sediment cores from the Peruvian continental margin, which cover a range of bottom water oxygen concentrations, organic carbon rain rates and sedimentation rates. By applying a numerical reaction‐transport model, we evaluate how these parameters determine benthic iodine fluxes and sedimentary iodine‐to‐organic carbon ratios (I:C org ) in the paleo‐record. Iodine is delivered to the sediment with organic material and released into the pore water as iodide (I − ) during early diagenesis. Under anoxic conditions in the bottom water, most of the iodine delivered is recycled, which can explain the presence of excess dissolved iodine in near‐shore anoxic seawater. According to our model, the benthic I − efflux in anoxic areas is mainly determined by the organic carbon rain rate. Under oxic conditions, pore water dissolved I − is oxidized and precipitated at the sediment surface. Much of the precipitated iodine re‐dissolves during early diagenesis and only a fraction is buried. Particulate iodine burial efficiency and I:C org burial ratios do increase with bottom water oxygen. However, multiple combinations of bottom water oxygen, organic carbon rain rate and sedimentation rate can lead to identical I:C org , which limits the utility of I:C org as a quantitative oxygenation proxy. Our findings may help to better constrain the ocean's iodine mass balance, both today and in the geological past. Key Points The impact of early diagenesis on benthic iodine fluxes and iodine burial was quantitatively evaluated using a reaction‐transport model Dissolved iodine anomalies in the water column are indicative of benthic efflux from anoxic sediments with high organic carbon turnover Not only