Leibniz Institute for Baltic Sea Research, Germany

Interstitial water and sediment samples of Integrated Ocean Drilling Program (IODP) Expedition 313 (New Jersey Shallow Shelf) were analyzed for chemical composition and stable isotope ratios. The analyses indicate a previously unknown complex geometry of the underlying fresh-water lens with alternating fresh-water-salt-water intervals divided by sharp boundaries in the upper part of the cores. Three fluid sources were identified: (1) meteoric fresh water, (2) marine seawater, and (3) brine. The pore-fluid stable isotope values define a mixing line with end members that have delta (super 18) O and delta (super 2) H values of -7.0 per mil and -41 per mil for fresh water, and -0.8 per mil and -6 per mil for salt water, respectively. This is similar to the modern mean value of New Jersey precipitation and today's New Jersey shelf water. For fresh water, this either indicates modern meteoric recharge via aquifers that crop out on mainland New Jersey or emplacement at a time with climatic and hydrologic conditions similar to modern. An origin from Pleistocene glacial meltwaters with depleted isotope values is not confirmed by stable isotope data of this study. Salt water also represents modern isotope values suggesting an infiltration along permeable, coarse-grained sandy units. The lower core parts are characterized by mixing with brine fluids that originate from evaporites in the deep underground.Stable carbon isotope analyses of gas and fluids prove the existence of methane formation from degradation of marine organic matter and CO (sub 2) reduction in the lower core parts below approximately 350 m below seafloor. Methane concentrations above 10000 ppm and delta (super 13) C (sub methane) values of approximately -80 per mil were measured. Methane formation is also indicated by authigenic carbonates with low delta (super 13) C (sub carbonate) values. Although not reaching the surface at present conditions, the venting out of variable fluxes of methane from passive continental margins due to sea-level fluctuations is significant for the long-term carbon cycle. Authigenic carbonates indicate the precipitation from pore fluids with marine oxygen stable isotope ratios at low temperatures. The geochemical data and interpretations presented in this study supply the missing link between existing onshore and offshore data and may provide the basis for an integrated approach to construct a geochemical transect across the New Jersey shallow shelf.