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Serpentine alteration is recognized as an important process for element cycling, however, related silicon fluxes are unknown. Pore fluids from serpentinite seamounts sampled in the Mariana forearc region during IODP Expedition 366 were investigated for their Si, B, and Sr isotope signatures (δ30Si, δ11B, and 87Sr/86Sr, respectively) to study serpentinization in the mantle wedge and shallow serpentine alteration to authigenic clays by seawater. While serpentinization in the mantle wedge caused no significant Si isotope fractionation, implying closed system conditions, serpentine alteration by seawater led to the formation of authigenic phyllosilicates, causing the highest natural fluid δ30Si values measured to date (up to +5.2 ± 0.2‰). Here we show that seafloor alteration of serpentinites is a source of Si to the ocean with extremely high fluid δ30Si values, which can explain anomalies in the marine Si budget like in the Cascadia Basin and which has to be considered in future investigations of the global marine Si cycle.

Anthropogenic impacts are perturbing the global nitrogen cycle via warming effects and pollutant sources such as chemical fertilizers and burning of fossil fuels. Understanding controls on past nitrogen inventories might improve predictions for future global biogeochemical cycling. Here we show the quantitative reconstruction of deglacial bottom water nitrate concentrations from intermediate depths of the Peruvian upwelling region, using foraminiferal pore density. Deglacial nitrate concentrations correlate strongly with downcore δ13C, consistent with modern water column observations in the intermediate Pacific, facilitating the use of δ13C records as a paleo-nitrate-proxy at intermediate depths and suggesting that the carbon and nitrogen cycles were closely coupled throughout the last deglaciation in the Peruvian upwelling region. Combining the pore density and intermediate Pacific δ13C records shows an elevated nitrate inventory of >10% during the Last Glacial Maximum relative to the Holocene, consistent with a δ13C-based and δ15N-based 3D ocean biogeochemical model and previous box modeling studies., Understanding controls on past nitrogen budgets can improve predictions for future global biogeochemical cycling. Here, using foraminiferal pore density and δ13C, the authors present a quantitative record of deglacial nitrate from the intermediate Pacific and infer close coupling between carbon and nitrogen cycles.

During R/V Meteor cruise 141/1, pore fluids of near surface sediments were investigated to find indications for hydrothermal activity in the Terceira Rift (TR), a hyperslow spreading center in the Central North Atlantic Ocean. To date, submarine hydrothermal fluid venting in the TR has only been reported for the D. João de Castro seamount, which presently seems to be inactive. Pore fluids sampled close to a volcanic cone at 2,800-m water depth show an anomalous composition with Mg, SO4, and total alkalinity concentrations significantly higher than seawater and a nearby reference core. The most straightforward way of interpreting these deviations is the dissolution of the hydrothermally formed mineral caminite (MgSO4 0.25 Mg (OH)2 0.2H2O). This interpretation is corroborated by a thorough investigation of fluid isotope systems (δ26Mg, δ30Si, δ34S, δ44/42Ca, and 87Sr/86Sr). Caminite is known from mineral assemblages with anhydrite and forms in hydrothermal recharge zones only under specific conditions such as high fluid temperatures and in altered oceanic crust, which are conditions generally met at the TR. We hypothesize that caminite was formed during hydrothermal activity and is now dissolving during the waning state of the hydrothermal system, so that caminite mineralization is shifted out of its stability zone. Ongoing fluid circulation through the basement is transporting the geochemical signal via slow advection toward the seafloor.

The Permian-Triassic mass extinction represents the most severe environmental crisis in Earth’s history, which dictated the course for evolution of life until today. Volcanism from Siberian traps played a significant role involving a substantial input of relatively light carbon into the atmosphere leading to a combination of global warming by ~6°C, sporadic anoxia or euxinia, and ocean acidification. However, its detailed manifestation and environmental impact is yet to be fully understood. This lack of knowledge also extends to a better quantification of emitted and sequestered carbon budgets (cf. Gutjahr et al., 2017).

High-latitude cold-water coral (CWC) reefs are particularly vulnerable due to enhanced CO2 uptake in these regions. For both modern and fossil Lophelia pertusa samples, retrieved along the Norwegian margin from Lopphavet (70.6°N) over Sula to Oslofjord (59°N), we have analysed the boron isotope systematics (δ11B) as a potential seawater pH proxy [1]. This case study investigated potential heterogeneities within single specimens and recent regional to local variations Analyses were undertaken using MC-ICP-MS based bulk (solution) analyses and continuous laser ablation (LA) profiles, as well as secondary ion mass spectrometry (SIMS) spot measurementsin combination with fluorescence microscopy. The latter provided a fast pre-screening routine for phase-specific subsampling. Bulk analyses of around 5 mg milled from the massive thecal walls from samples of the main transect sites resulted in a recent average δ11B value of 26. 7‰(±0.3, 2SD). In contrast, an adjacent subsample dominated by centres of calcification (COC) gave 22.65‰, hence an offset of about-3 ‰, with an approximately 40% lower B/Ca ratio. LA profiles [2] showed significant δ11B single sample heterogeneities of up to 10 ‰. Supporting preliminary SIMS results yielded similar ranges, and indicate multiple COC zones as the source of low δ11B values. This suggests stronger isotope fractionation occurred with lower B concentrations in the COC compared to the theca during mineralisation.

IODP Expedition 307 made it for the first time possible to investigate the entire body of a cold-water coral carbonate mound. Here we provide new insights into the long-term history of Challenger Mound on the European continental margin off Ireland. This study is based on age determinations (230Th/U, 87Sr/86Sr) and geochemical signals (Mg/Li and Ba/Ca) measured in the scleractinian cold-water coral Lophelia pertusa from IODP Site 1317 in the Porcupine Seabight. The paleoceanographic reconstructions reveal that coral growth in the Porcupine Seabight was restricted to specific oceanographic conditions such as enhanced export of primary production and Bottom-Water Temperatures (BWT) between ~8 and 10 °C, related to the water mass stratification of the Mediterranean Outflow Water (MOW) and Eastern North Atlantic Water (ENAW). The geochemical signals from the coral skeletons can be explained by the close interaction between cold-water coral growth, sea-surface productivity and the surrounding water masses – the boundary layer between MOW and ENAW. Enhanced sea-surface productivity and the build-up of a stable water mass stratification between ENAW and MOW caused enhanced nutrient supply at intermediate water depths and facilitated a steady mound growth between ~3.0 and 2.1 Ma. With the decrease in sea-surface productivity and related reduced export productivity the food supply was insufficient for rapid coral mound growth between ~1.7 and 1 Ma. During the late Pleistocene (over the last ~0.5 Myr) mound growth was restricted to interglacial periods. During glacials the water mass boundary between ENAW/MOW probably was below the mound summit and hence food supply was not sufficient for corals to grow.

The increasing pCO2 in seawater is a serious threat for marine calcifiers and alters the biogeochemistry of the ocean. Therefore, the reconstruction of past-seawater properties and their impact on marine ecosystems is an important way to investigate the underlying mechanisms and to better constrain the effects of possible changes in the future ocean. Cold-water coral (CWC) ecosystems are biodiversity hotspots. Living close to aragonite undersaturation, these corals serve as living laboratories as well as archives to reconstruct the boundary conditions of their calcification under the carbonate system of the ocean.We investigated the reef-building CWC Lophelia pertusa as a recorder of intermediate ocean seawater pH. This species-specific field calibration is based on a unique sample set of live in situ collected L. pertusa and corresponding seawater samples. These data demonstrate that uranium speciation and skeletal incorporation for azooxanthellate scleractinian CWCs is pH dependent and can be reconstructed with an uncertainty of ±0.15. Our Lophelia U / Ca–pH calibration appears to be controlled by the high pH values and thus highlighting the need for future coral and seawater sampling to refine this relationship. However, this study recommends L. pertusa as a new archive for the reconstruction of intermediate water mass pH and hence may help to constrain tipping points for ecosystem dynamics and evolutionary characteristics in a changing ocean.

The Gulf of Cadiz is located at the plate boundary between the Eurasian and African plates and is characterized by widespread mud volcanism all along the continental margins. It is commonly believed that this results from sediment dewatering enhanced by the compressional forces within the accretionary prism. Recently, 3D seismic data and geochemical sediment cores indicated that in addition to compressional tectonics, halokinesis controls the location and dynamics of the Mercator mud volcano (MMV). The MMV is located in the El Arraiche mud volcano field offshore Morocco at a water depth of 350 m and was one of the main target areas of recent HERMES cruises with the RV MERIAN and RV JAMES COOK. During both cruises fluid flow and gas ebullition were observed. The acquired seismic and geochemical pore water data are best explained by the existence of a shallow rising salt diapir possibly playing a major role as driving force for the mud volcanism. In a number of cores collected at the center of the MMV, both, gypsum and halite crystals of several cm-size have been found. A detailed geochemical and mineralogical analysis of the minerals suggests that these crystals ascended as clasts within the mud matrix. These evaporates are likely of Tri- assic origin and might be connected to the salt province 500 km further south on the Moroccan Margin, thus suggesting, that the Moroccan salt province extends this far to the north. Further to the halokinesis, the geochemical analysis of the pore fluids and mineralogical measurements on the sediments provide information about diagenetic processes at greater depth and the origin of the fluids and mud matrix. Regarding the fluids, the enrichment in Li and B indicates a deep fluid source from mineral dewatering reactions at elevated temperatures (>150 ̊C).