Strong impact of ion filtration on the isotopic composition of chlorine in young clay-rich oceanic sediment pore fluids

Chloride exchanges between oceanic sediments and seawater: Constraints from chlorine isotopes

“…In terms of chloride geochemistry, the pore fluids exhibit the usual chloride geochemical features recorded in the vast majority of oceanic clay-rich sedimentary sites (Agrinier et al, 2019 and. They are:…”

“…Using the Cl filtration isotope mass balance model (Closed sedimentary pile : CSP), described in appendix B of Agrinier et al (2019), the Sites U1456 and U1457 pore fluids can be fitted with small  37 Cl/ 35 Cllost-porefluid ranging between 1.00025 and 1.0025 (Figure 6). Note that at the top of the sediment column, this model has a boundary condition to set chlorinity and  37 Cl to seawater values.…”

“…Note that at the top of the sediment column, this model has a boundary condition to set chlorinity and  37 Cl to seawater values. This condition is imposed to represent the rapid exchange of chlorides across the sediment-seawater interface (Agrinier et al, 2019). It erases the accumulation of 37 Clenriched chlorides, which are expelled from the bottom of the column where 37 Cl-depleted chlorides remain.…”

“…To explain these depletions in 37 Cl in pore fluids, suggestions have been made, ranging from external causes: the upward advection of 37 Cl-depleted fluid into the sediments from greater depths (Ransom et al, 1995;Spivack et al, 2002;Hesse et al, 2006;Wei et al, 2008), to internal cause: the generation of 37 Cl-depletion by ion filtration of chlorides when pore fluids are transported through clays during the compaction of the sediments (Godon et al, 2004a;Bonifacie et al, 2007;Agrinier et al, 2019Agrinier et al, & 2021, see an ilustration of the ion filtration process in Figure 1 of Agrinier et al 2021). Based on the fact that these 37 Cl depletions have been ubiquitously observed in a large variety of investigated tectonic contexts (proto-oceanic basins, oceanic plateaus, continental margins, ridge flanks, prisms of subduction zones, fluvial fans) Agrinier et al, (2021) concluded that these 37 Cl depletions were inherited from an internal process.…”

“…Based on the fact that these 37 Cl depletions have been ubiquitously observed in a large variety of investigated tectonic contexts (proto-oceanic basins, oceanic plateaus, continental margins, ridge flanks, prisms of subduction zones, fluvial fans) Agrinier et al, (2021) concluded that these 37 Cl depletions were inherited from an internal process. Agrinier et al (2019Agrinier et al ( & 2021 calculated the fractionation of chlorine isotopes of chlorides of pore fluids which could be caused by the ion filtration due to the repulsion of chloride by the negatively charged surface of clays (Phillips and Bentley, 1987). We suggested that the mineralogical nature of the sediment would be the primary factor controlling the importance of the chlorine isotope fractionation in pore fluid chlorides.…”