Chlorine isotope data of chlorides challenge the pore fluid paradigm

Abstract

In order to examine the seawater--seafloor sediment interactions that influence the 9 chemical composition of seawater through time, we examined hundreds of pore fluid 10 geochemical analyses from 13 clay--rich sedimentary successions drilled by the ODP--11 IODP. Chemical trends such as monotonous increases in Ca 2+ , and decreases in Mg 2+ and 12 δ 18 O with depth are traditionally interpreted to result from water--rock interaction. In 13 this view, the release of Ca 2+ into fluids and the uptake of Mg 2+ and 18 O mainly results 14 from the formation of low--temperature clays in the sediment and within underlying 15 basalts. Chloride concentration profiles and isotopic compositions, however, suggest 16 that different processes may influence pore water geochemistry. The data examined 17 here show relatively constant chloride contents but with a systematic decrease in δ 37 Cl 18 of chlorides with depth from 0 permil (the seawater value) down to --8.5 permil. The 19 δ 37 Cl data are highly correlated with δ 18 O (with δ 18 O down to --5.7 permil). 20 further experimental work on both chlorine and oxygen isotopes is certainly needed, the 45 results of this study indicate that ion--filtration should be considered as a potential 46 mechanism for fractionating isotopic species in sediment pore waters, particularly for 47 oxygen isotope ratios whose variations are often commonly attributed to water--rock 48 exchange. 49 I) Introduction 50 Water--rock interactions and transport of solutes by water are means by which 51 chemical elements are redistributed at the surface of the Earth. Through these processes, 52 the Earth's surface and subsurface reservoirs, such as seawater, continental crust, 53 sediments, altered oceanic crust, were created, maintained and modified all along the 54 Earth's history. To decipher this history, geochemical tools were developed. They 55 include stable isotope geochemistry of traditional elements (O, H, C, N, S) and non--56 traditional elements (e.g., Cl, Li, B, Cu, U, Zn, Se). Although chlorine stable isotope 57 geochemistry started in the early times of the stable isotope geochemistry, it remains an 58 under--utilized tool. Yet, chlorine stable isotopes ( 37 Cl & 35 Cl) of dissolved chlorides are 59 proved to be powerful tools to describe transport mechanism of fluids. Physical 60 processes mainly fractionate them during fluid transports in geological media: diffusion 61 (

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

“…Whatever the process causing the decrease, the conservation of stable isotopes of chlorine requires that it is due to either the preferential removal of 37 Cl isotopes from the chlorides of the original pore fluid (seawater) or by the addition of 37 Cl-depleted chlorides. This latter possibility has been considered in previous works (Ransom et al, 1995;Spivack et al, 2002;Wei et al, 2008) does not operate for the many other sedimentary piles, not associated with subduction zones, that nevertheless systematically show the depletion in of their pore fluid chlorides (Agrinier et al, 2021). The ubiquity of the 37 Cl depletion of chloride in sediments confirms that the process fractionating the chlorine isotope of chlorides is local and specific to the type of the sediments.…”

“…Indeed, such ranges of chlorine isotope fractionation are observed in natural and experimental systems (Desaulnier et al;1986;Lavastre et al, 2005;Eggenkamp and Coleman, 2009;Sharp et al, 2010;Richard et al, 2011;Giunta et al, 2017a,b). For the ion filtration process of chlorides in clay-rich oceanic sediments undergoing compaction, α values in the range of 1.00025 to 1.008 are proposed (Agrinier et al, 2021). Larger chlorine isotope fractionations are produced when chlorine is oxidised to an oxydation number of +2 to +7.…”

“…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. 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).…”

“…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.…”

Unprecedently large 37Cl/35Cl equilibrium isotopic fractionation on nano-confinement of chloride anion

“…Chlorine isotopic fractionation is a very informative tool used in many life-controlling processes 21 , including environmental 22 – 25 , geochemical 26 – 30 and biochemical 31 – 33 studies. Therefore a detailed understanding of this phenomenon is an important scientific (but with practical consequences in the interpretation of experimental results 34 and analytical techniques 35 – 37 ) issue.…”