Carbonate and silicate cementation of siliciclastic sediments of the New Jersey shelf (IODP Expedition 313): relation with organic matter diagenesis and submarine groundwater discharge

Abstract

International audienceThe New Jersey continental shelf extends 150 km off the shoreline. During IODP Expedition 313, siliciclastic deposits of late Eocene to late Pleistocene age were drilled down to 631, 669 and 755 m below seafloor at sites 27A, 28A and 29A respectively in very shallow waters (33.5 to 36 m depth). Pore water salinities display multilayered brackish-salty-brine units 10 to 170 m thick, where low-salinity water is preferentially stored in fine-grained sediments. The sharp boundaries of these buried aquifers are often marked by cemented layers a few centimetres thick. The mineralogy and scanning electron microscope observations of these layers show two phases of cementation by authigenic minerals: (1) the early carbonate cement is frequently associated with pyrite, and (2) the late silicate cement infills the residual porosity. The isotopic compositions of the carbonate cements vary widely: −2.4 < δ¹⁸O ‰ VPDB < +2.8; −15.1 < δ¹³C ‰ VPDB < +15.6. The δ¹⁸O values indicate that the carbonate cements precipitated with pore waters comprising variable mixtures of seawater and ¹⁸O-depleted fresh water originating from submarine groundwater discharge. The δ¹³C values of the carbonate cements are related to organic matter diagenesis, providing ¹³C-depleted dissolved inorganic carbon during bacterial sulphate reduction and anaerobic oxidation of methane, and ¹³C-rich dissolved inorganic carbon 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 silicate cements. The estimated range of temperature (18±4 °C) during carbonate precipitation is consistent with carbonate cementation at moderate burial depths; however, silicate cementation occurred later during diagenesis at deeper burial depths

Data report: Expedition 313 elemental data from X-ray fluorescence scanning and measurements on core samples

“…F3C). There are also intervals of diagenetic cement within these intervals (Pierre et al, 2017; Fig. F3).…”

The Aquitaine Shelf Edge (Bay of Biscay): A Primary Outlet for Microbial Methane Release

“…In light of our study and recent discoveries in particular along the U.S. Atlantic and Cascadia margins, continental shelf edges around the world may be seen as primary outlets for microbial methane releases, independently of the prevalent tectonic regime and the presence of gas hydrates (Johnson et al, 2015; Prouty et al, 2016). Microbial methane circulation might play a predominant role in the shaping and evolution of the shelf edge in interactions with offshore groundwater discharge (Hong et al, 2019; Pierre et al, 2017; Pierre, Demange, et al, 2017) and possibly but not necessary with gas hydrate dissociation (Johnson et al, 2019; Skarke et al, 2014).…”

Seismic Reflection Methods in Offshore Groundwater Research

“…However, beneath the inner shelf fresh water is stored in both fine-grained and coarse-grained sediments, whereas beneath the middle shelf, freshened groundwater is preferentially stored in deeper fine grained and low-permeability deposits (clay and silt dominated) and salty waters recovered in coarse-grained deposits (unconsolidated sands). Low permeability barriers such as fine-grained maximum flooding/transgression sediments and cemented diagenetic layers potentially play a role in preserving the emplaced fresh water [17,104,105,107]. Conversely, the faults interpreted in the deeper parts of the seismic sections and based on the results from the resistivity modelling, appear to connect the shallow freshened water aquifers with upwelling fluids from dissolved deep evaporites, facilitating their salinization.…”

“…The relatively thick clay and silt layers in well USGS 6001 exhibited higher salinity levels (30-70% seawater) and displayed a geometry consistent with ongoing vertical diffusion. The permeability measured on Nantucket Island in low-salinity aquifers range from 10-780 mD and it allows flow of water through sediments [98,107]. The exact extent of the low-salinity groundwater offshore Massachusetts isis unknown, as no offshore drilling has confirmed the predicted models.…”