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

We investigate the chlorine isotope disequilibrium between chlorides in pore fluids and chlorides in seawater and infer its consequences for chlorine isotope exchange between ocean sediment pore fluids and seawater. We illustrate our methodology with pore fluids from two IODP drilled to depths of approximate to 1000 m (U1456 and U1457) in the Indus River fan dominated by clay-detritus sediments of the western Himalayas. At these two sites, the concentrations of chloride and sodium ions do not show significant changes with depth and remain very close to those of seawater. As a function of depth, however, chlorides show a progressive decrease in Cl-37 (downto -2.5 and -1.4 parts per thousand respectively), while Ca2+ increases and Mg2+ decreases, as commonly observed in clay-rich oceanic sediments. The rate of the delta Cl-37 decrease is correlated to the lithology and sedimentation rate. Examining the chlorine budget on these two sites, we conclude that: (1) Most of the chlorine (>= 96% of the total Cl) is contained in the pore fluids as chlorides, while the chlorine stored in the other sinks (minerals or organochlorine) is generally very minor (representing <4% of the total Cl). (2) These other sinks are too small to sequester the lost Cl-37-enriched chlorine, which could compensate for the decreased delta Cl-37 of the chlorides observed in the pore fluids (assuming that the pore fluids originally had chlorides with the chlorine isotopic composition of seawater, delta Cl-37 = 0 parts per thousand). (3) This lost Cl-37-enriched chlorine has been released from the sediment into the overlying seawater, with fluxes of about 20 mol of chloride per square metre per kiloyear.In order to evaluate the consequence of this unbalanced on a global scale, we extend this chlorine budget analysis to 22 other oceanic sites drilled by IODP in a variety of tectonic environments. It is estimated that worldwide oceanic sediment pore fluids contain 1 x 10(20) mol of chloride with mean delta Cl-37 at -2.3 +/- 0.1 parts per thousand and mean chlorinity at 0.290 +/- 0.055 mol Cl- per kg of sediment. Most of the calculated values of the lost Cl-37-enriched chlorine flux are between 3.5 and 45.6 mol of chloride per square metre per kiloyear (mean value of 18.3 mol Cl.m(-2).kyr(-1)), corresponding to a typical chloride exchange time between sediments and seawater of between 5.5 and 71 Myr with a mean value of 14 Myr. Importantly, in oceanic sediments, a relationship between the delta Cl-37 of the chlorides in their pore fluids and the nature of the detrital minerals (smectite, illite, kaolinite, carbonate, quartz, & mldr;) may exists. This relationship should be modulated by the detrital input from the continents to the oceanic sediments and should therefore be a consequence of the climatic conditions on the continents (mainly rain, temperature). The delta Cl-37 of the seawater should vary according to these detrital inputs. The expected range of variation would be between -0.3 and +0.3 parts per thousand. This is consistent with the range of delta Cl-37 in seawater predicted by the study of evaporites deposited since 2 Ga (Eggenkamp et al., 2019).