Carbonate and Silica Cementation of the Siliciclastic Sediments of the New Jersey Shelf (IODP Expedition 313) : Relation with Organic Matter Diagenesis and Submarine Groundwater Discharge

The New Jersey continental shelf extends up to 150 km away from the shoreline. During IODP Expedition 313 the siliciclastic deposits of late Eocene to late Pleistocene age were drilled down to 631 mbsf, 669 mbsf and 700 mbsf at the three sites 27A, 28A, 29A respectively, in very shallow water depth (33.5 to 36 m). High alkalinities and methane concentrations in pore waters point to microbial anaerobic oxidation of methane and methanogenesis during organic matter diagenesis. Pore water salinities display multilayered fresh-salty-brine units 10 to 170 m thick, where fresh water is preferentially stored in fine-grained sediments (van Geldern et al 2013 ; Lofi et al 2013). The sharp boundaries of these buried aquifers are often marked by hardly cemented layers a few centimeters thick. The mineralogy and SEM observations of these layers show two phases of cementation by authigenic minerals (1) the early carbonate cement is made of Fe-dolomite, siderite and occasionally calcite, frequently associated with pyrite (2) the late silica cement fills in the residual porosity. The oxygen and carbon isotopic compositions of the carbonate cements vary in wide ranges : -2.4 < δ¹⁸O‰ VPDB < +2.8 ; -15.1< δ¹³C ‰ VPDB <+15.6. The relatively low δ¹⁸O values indicate that carbonate cements precipitated with pore waters more or less depleted in ¹⁸O of the buried aquifers (van Geldern et al 2013). The δ¹³C values of carbonate cements are related to organic matter diagenesis providing ¹³C-depleted DIC during bacterial sulphate reduction and ¹³C-rich DIC during methanogenesis. The diagenetic cementation processes included chemical weathering of reactive silicate minerals by the CO₂-rich pore waters issued from organic matter diagenesis that released bicarbonate, cations and dissolved silica, which were further precipitated as carbonate and silica cements.

Lofi et al, 2013, Geosphere ; doi:10.1130/GES00855.1

Van Geldern et al, 2013, Geosphere ; doi:10.1130/GES00859.1