Florian Scholz

Nutrient enrichment and warming cause widespread costal deoxygenation, leading to hypoxia or even anoxia in bottom waters. The benthic-water-column-feedbacks that develop under persistent seasonal oxygen depletion and amplify hypoxia remain poorly constrained. A 15-month study of a seasonally hypoxic to anoxic costal system in the southwestern Baltic Sea combined water-column monitoring, sediment chemistry, metabolic rates and microbial analyses with historic data to reveal pronounced seasonal shifts in sediment processes. During winter oxygenation, organic matter degradation, fermentation, aerobic respiration, nitrate- and iron-reduction and microbial competition increased. Re-ventilation following anoxia did not reset sediment biogeochemistry, but initiated intensified oxygen consumption, microbial biosynthetic activity and anaerobic carbon re-cycling. Reactive ironoxyhydroxides became exhausted, the sulfidic zone shoaled and sulfate reduction and pyrite formation rates increased through fall. These processes reinforced benthic feedbacks, progressively reducing the sediment’s capacity to retain sulfide over years. This weakening of sedimentary resilience exacerbates sulfide accumulation in eutrophic costal environments with growing chances of sulfidic deep-water upwelling.

We investigate late Holocene changes in the northern Benguela Upwelling System using a multi‐proxy approach on shallow shelf deposits from the Namibian mud belt. Three sediment cores characterized by organic carbon‐rich, anoxic, and partly gas‐bearing sediments were retrieved along a sampling transect at 23°S in August 2019. A sub‐bottom profile recorded in a sampling campaign in February 2022 enabled stratigraphic correlation of the cores. As a novelty in this area to date, variations in terrigenous input patterns are reconstructed in one gravity core using major and trace element data, including rare earth elements, in a statistical provenance modeling approach. Results indicate a progressive decline in input of Orange River‐derived material over the past 3800 years, attributed to a weakening of the Benguela Coastal Current (BCC). This reduction in northward directed transport energy is reflected in increasing lamination frequency observed in Computed Tomography‐scan imagery and declining Zr/Al ratios toward the present. Sulfidic events observed along the Namibian coastline are interpreted as a direct consequence of the BCC weakening, since decreasing depositional energy facilitated organic matter deposition and degradation, the formation of a shallow sulfate–methane transition zone (SMTZ) and the accumulation of free sulfide and methane gas in the central mud belt area beginning around 1450 years BP. The establishment of sulfidic redox conditions in gas‐bearing sediments is recorded in sedimentary Mo/U ratios exceeding modern seawater Mo/U. This study highlights the strong coupling of oceanographic changes, sedimentation processes, and biogeochemical conditions in the Benguela Upwelling System.

Nickel isotopes (δ ⁶⁰ Ni) are an emerging tool for understanding ocean biogeochemical evolution, given improved modern Ni budget constraints. Here, we provide constraints on past ocean Ni isotope compositions from organic-rich sediments from Tarfaya Basin core SN°4. The data constrain Late Cretaceous ocean δ ⁶⁰ Ni to ∼+0.8 to +1.0 per mil (‰), lower than the modern value of +1.34‰. Mass balance modeling suggests two drivers of this difference: changes in riverine input isotope composition during the Cenozoic and a more reducing Late Cretaceous ocean. Because Ni and Mo isotopes respond differently to changes in ocean inputs and redox-sensitive sinks, coupled Ni-Mo models help to deconvolve the relative importance of the two processes. The results show that the areal fraction of the euxinic sink was higher than today, reaching ∼2% during OAE2 (Oceanic Anoxic Event 2), and that the importance of the deep ocean oxic sink relative to the euxinic sink increased during the Cenozoic, potentially driven by changes in the biological pump and ocean circulation.

Formation and burial of pyrite (iron sulfide) in marine sediments exert a fundamental control on atmospheric oxygenation and seawater buffering over geological timescales. However, little is known about how the formation and delivery of its precursor, reactive iron (Fe) oxide minerals, have evolved throughout Earth history. Secular variability in reactive Fe (including Fe oxides and pyrite) preserved in marine sediments is commonly attributed solely to redox changes. Here, we develop an approach to distinguish redox-driven influences from other controls on sedimentary reactive Fe, including the intensity of continental silicate weathering and the transfer of terrigenous particles to the ocean. We apply this framework to a compilation of reactive Fe data spanning 1,200 My of Earth history. Our results reveal persistently low proportions of reactive Fe from the Mesoproterozoic through the Cambrian, followed by a pronounced mid- to late Paleozoic rise and a subsequent decline in the late Cenozoic. This temporal pattern is inconsistent with a purely redox-driven control. Comparison with independent proxies for continental weathering and land-ocean sediment and solute fluxes suggests a strong coupling between reactive Fe burial, oxidative silicate weathering, and tectonically driven erosion. Notably, the mid- to late Paleozoic rise in reactive Fe coincided with the proliferation of land plants and increasing atmospheric oxygen. We propose a positive feedback where Earth-surface Fe cycling was both amplified by atmospheric oxygenation and contributed to it through its influence on pyrite burial in marine sediments.

Molybdenum isotopes (δ ⁹⁸ Mo) have been used for more than two decades to investigate past‐ocean redox conditions due to the distinct behavior of this element in oxic and anoxic environments. The oxic sink is generally accepted to have a near‐constant δ ⁹⁸ Mo value of −0.7‰, a value derived primarily from the δ ⁹⁸ Mo signatures of Fe–Mn crusts and nodules. However, little information is available on the downcore evolution of δ ⁹⁸ Mo in oxic pelagic sediments, which constitute the largest fraction of the oxic Mo sink. Here, we present δ ⁹⁸ Mo authigenic data for two deep‐sea sites with oxygenated pore waters covering 90 Myr of sedimentation, one from the North Pacific Gyre and another from the South Pacific Gyre. In both records, sedimentary δ ⁹⁸ Mo _auth values systematically increase downcore from −0.5‰ at the sediment surface to a maximum of ∼+0.3‰. This shift cannot be explained by changes in the ocean's redox state but is likely related to the diagenetic conversion of Mn‐oxides into different mineral phases. Our findings indicate that the δ ⁹⁸ Mo signal generated by adsorption at the sediment‐water interface is not preserved during burial, suggesting that the canonical −0.7‰ value of the oxic sink requires re‐evaluation. In the lower section of the South Pacific Site, we also identify two positive excursions reaching δ ⁹⁸ Mo _auth values as high as +1.7‰. This portion of the site was influenced by near‐field hydrothermal processes, and we propose that these large excursions reflect Mo adsorption onto Fe (oxyhydr)oxides.

Due to upwelling of nutrient‐rich waters, the Peruvian Margin is one of the most productive regions of the global ocean, hosting one of the most pronounced and shallowest oxygen minimum zones. This combination of high productivity and low oxygen makes the Peruvian margin a critical site for reconstructing past environmental changes. In this study, we investigated the evolution of redox conditions, productivity, and sedimentary processes at ODP Site 680 since the Pliocene using multiple geochemical proxies, including iron speciation, redox‐sensitive element contents, and δ ⁹⁸ Mo _xs , δ ¹⁵ N _bulk , and δ ¹³ C _org . Our findings reveal that over the past 15 marine isotope stages, the mechanism of molybdenum accumulation in the sediments varied. During interglacial periods, elevated Mo, high δ ¹⁵ N _bulk , and positive correlations with reactive Fe suggest that particulate delivery was the main mode of Mo accumulation, potentially driven by Fe (oxyhydr)oxide precipitation in the water column coupled to denitrification. Glacial periods lack these correlations, and Mo accumulation appears to have been primarily controlled by diffusion from bottom waters. We observe a coupling between proxies for surface productivity and the redox state of both bottom waters and the overlying water column during interglacial intervals, indicating that during these periods, productivity controlled redox conditions. During the warm and humid Pliocene, comparably low U and Mo concentrations were accompanied by elevated ratios of reactive iron to total iron. We propose that elevated Fe _HR /Fe _T values were produced by enhanced delivery of reactive iron from a highly weathered terrigenous sediment source. However, an additional redox control on Fe speciation remains possible.

The ocean absorbs about 25% of anthropogenic carbon dioxide emissions, with this uptake regulated by acid-neutralizing anions collectively termed alkalinity. Most seawater alkalinity originates from the weathering of aluminosilicate and carbonate minerals on land, whose dissolved products are transported to the ocean by rivers, a slow process that causes carbon dioxide removal to lag behind emissions. Here we present geochemical evidence showing that fine-grained glacial sediments mobilized by coastal erosion undergo rapid seafloor weathering. While aluminosilicate weathering is largely balanced by secondary clay formation (reverse weathering), carbonate dissolution yields a significant net release of alkalinity to coastal waters. Because more than two-thirds of the global coastline was formerly glaciated, ongoing deglaciation and erosion may enhance alkalinity fluxes, providing a previously unrecognized shortcut in the global carbon cycle. As this enhanced flux is ultimately driven by climate warming, it may act as a negative feedback that helps moderate future increases in atmospheric carbon dioxide.