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The Nansen Legacy paleo cruise was carried out from September 26 to October 20, 2018 with RV “Kronprins Haakon”. The cruise took place in the northern Barents Sea and the Nansen Basin, and it went through the sea ice to 83.3 N. The overriding objective of the cruise was to reconstruct the natural variability and range of sea ice cover and Atlantic Water through flow in the Barents Sea on longer time scales. During the cruise four ocean moorings were deployed in northwest Barents Sea, where one ARGO float was also deployed. Twelve “paleo stations” were identified using multibeam and sub bottom profilers. At these stations, short and long sediment cores were obtained. This cruise report gives an overview of methods used and samples taken.

Sediment deposition along continental margins and especially close to the outlets of major river systems is highly dynamic and influenced by changing environmental conditions, such as sea-level variations and the shifting of ocean currents. The upper slope of the Mozambique margin (SE Africa) receives its sediments from the Zambezi River and is the largest river-fed deposition center along the Eastern African Margin. Global sea level rise during the last glacial-Holocene transition led to a re-routing of the Zambezi River sediment plume. This caused order-of-magnitude changes in sedimentation rates along the shelf break of the Mozambique margin. The variable sediment input as well as changing organic matter load and quality resulted in non-steady state early diagenesis leading to changes in formation and upward flow of methane. This is reflected in temporally and spatially variable formation conditions of authigenic minerals (such as pyrite), especially at the sulfate-methane transition zone (SMTZ) where upward-diffusing methane is anaerobically oxidized by sulfate. Pyrite accumulations in sediment cores can be used to define the past positioning of SMTZs. The isotopic composition of sulfur in pyrite can provide information about the geochemical and environmental factors (e.g., availability of methane, sulfate, reactive iron) controlling the formation of these authigenic minerals during different times of sediment deposition.We present geochemical data from sediment cores acquired in 2015 during the PAMELA-MOZ4 campaign onboard R/V Pourquoi Pas? offshore Mozambique. A reactive transport model is used to simulate the evolution of early diagenetic conditions over the time of sediment deposition (i.e., the last 27,000 years). By reproducing the currently observed mineral accumulations, the temporal development of methane generation and upward flux, and the past positioning of the SMTZ, can be reconstructed. With this, we are able to put a time constraint on past events of authigenic mineral accumulation and reveal their response to sedimentation rate changes caused by sea-level rise. We further discuss isotope signatures of small-scale diagenetic processes at the Mozambique margin.This research was co-funded by TOTAL and IFREMER as part of the PAMELA scientific project.

Marine anaerobic oxidation of ammonium (anammox) plays a central role in the nitrogen cycle of modern Oxygen Deficient Zones (ODZs). The newly developed bacteriohopanetetrol stereoisomer (BHT-x) biomarker for anammox, which is largely unaffected by early diagenesis, allows for the reconstruction of the presence and dynamics of past ODZs from the sedimentary record of continental margins. In this study, we investigate the development and dynamics of the ODZ in the Gulf of Alaska (GOA) between 60 and 15 cal ka BP using records of redox sensitive trace metals (TM) and the BHT-x anammox biomarker from IODP Site U1419 (~700 m water depth). The biomarker record indicates that the ODZ in the GOA was in concert with global climate fluctuations in the late Pleistocene. Anammox was more pronounced during warmer periods and diminished during cooler periods, as indicated by correlation with the δ18O signal obtained by the North Greenland Ice core Project (NGRIP). Trace metal enrichments, however, do not match the trend in BHT-x. Systematic metal enrichments in intervals where biomarkers point to more intense water column deoxygenation are not observed. We suggest that this proxy discrepancy was caused by environmental factors, other than water column redox conditions, with opposing effects on the TM and biomarker records. Two of the most widely used redox indicators, Mo and U, are not significantly enriched throughout the sediment record at Site U1419. SiteU1419 experienced some of the highest sedimentation rates (100–1000 cm ka −1) ever reported for late Pleistocene continental margin sediments, leading to a continuous and rapid upward migration of the sediment-water interface. We suggest that despite water column and seafloor oxygen depletion, significant sedimentary enrichments of these redox sensitive trace metals were prevented by a limited time for their diffusion across the sediment-water interface and subsequent enrichment as authigenic phases. Thus, depositional conditions were ideal for biomarker preservation but prevented significant authigenic trace metal accumulations. Similar discrepancies between organic and inorganic redox proxies could exist in other high sedimentation rate environments, potentially putting constraints on paleo-redox interpretations in such settings if they are based on trace metal enrichments alone.

Sediment samples from the Gulf of Alaska (GOA, IODP Expedition 341, Site U1417) have been analyzed to understand present and past diagenetic processes that overprint the primary sediment composition. No Sulfate-Methane Transition Zone (SMTZ) was observed at the zone of sulfate depletion, but a >200 m thick sulfate- and methane-free sediment interval occurred between the depth of sulfate depletion (~200 m) and the onset of methanogenesis (~440 m). We suggest that this apparent gap in biogeochemical processing of organic matter is caused by anaerobic oxidation of methane fueled by sulfate which is released during dissolution of barites at the upper boundary of the methane rich layer. Beneath the methanogenic zone, at ~650 m depth, pore-water sulfate concentrations increase again, indicating sulfate supply from greater depth feeding into a deep, inverse SMTZ. A likely explanation for the availability of sulfate in the deep sub-seafloor at U1417 is the existence of a deep aquifer related to plate bending fractures, which actively transports sulfate-rich water to, and potentially along, the interface between sediments and oceanic crust. Such inverse diagenetic zonations have been previously observed in marine sediments, but have not yet been linked to subduction-related plate bending. With the discovery of a deep inverse SMTZ in an intra-oceanic plate setting and the blocking of upward methane diffusion by sulfate released from authigenic barite dissolution, Site U1417 provides new insights into sub-seafloor pore-fluid and gas dynamics, and their implications for global element cycling and the deep biosphere.