Hydrogen Isotope Composition of Pore Waters and Interlayer Water in Sediments from the Central Western Pacific, Leg 129

Abstract

Hydrogen isotope compositions have been measured on pore waters from sediments of Leg 129 sites in the Pigafetta and East Mariana basins (central western Pacific). Total water (pore + sorbed waters) contents and their δD have been analyzed for three samples that contain smectite but no zeolite so that sorbed water can be attributed to interlayer water. The H budget for pore and total waters implies that interlayer water is 20‰ to 30‰ depleted in D compared to pore water. Because the interlayer/total water molar ratio (0.25 to 0.5) in smectitic sediments is very high, interlayer water represents an important reservoir of D-depleted water in sediments.δD depth profiles for pore water at Sites 800 and 801 show breaks related to chert and radiolarite layers and are relatively vertical below. Above these chert units, pore waters are similar to modern seawater t>ut below, they are between -lO‰ and -5.5‰. These values could represent little modified pre-Miocene seawater values, which were D-depleted because of the absence of polar caps, and were preserved from diffusive exchange with modern seawater by the relatively impermeable overlying chert layers. At Site 802, δD values of the pore waters show a decrease in the Miocene tuffs from O‰ values at the top to -8‰ at 250 mbsf. Below, δD values are relatively uniform at about -8‰. Miocene tuffs are undergoing low water/rock alteration. A positive covariation of δD and Cl content of pore water in the tuffs suggests that the increase of δD values could result from secondary smectite formation. Low diffusive exchange coupled with D enrichment due to alteration of preglacial waters could explain the observed profile.

Stable isotopic composition of upper oceanic crust formed at a fast spreading ridge, ODP Site 801

“…The slightly heavier δD values for the Site 801 composites may in part result from higher temperatures of alteration at Site 801 compared to the other sites. Another factor for the Site 801 composites is the presence of sediments, which at nearby Site 800 have relatively high δD values, −48.9 to −53.6‰ [ France‐Lanord and Sheppard , 1992]. Like in the basalts, the main water carrier in the sediments is smectite, and the slightly heavier δD of sediments is related to retention of some interlayer water that has estimated δD = −27‰ to −44‰ [ France‐Lanord and Sheppard , 1992].…”

“…Another factor for the Site 801 composites is the presence of sediments, which at nearby Site 800 have relatively high δD values, −48.9 to −53.6‰ [ France‐Lanord and Sheppard , 1992]. Like in the basalts, the main water carrier in the sediments is smectite, and the slightly heavier δD of sediments is related to retention of some interlayer water that has estimated δD = −27‰ to −44‰ [ France‐Lanord and Sheppard , 1992]. Decreasing the amount of sediment in the Site 801 composites would have the effect of decreasing the δD slightly, although the difference between the SUPER composite (−87‰) and the deepest composite (−90.4‰), which contains no sediment, may not be significant.…”

Stable isotope composition of CO2 outgassed from cores of argillites: a simple method to constrain δ18O of porewater and δ13C of dissolved carbon in mudrocks

“…We are aware of a growing body of evidence supporting isotopic fractionation of porewater in clay-rich rocks in relation with partitioning in different compartments, including interlayer, bound and free water (France-Lanord and Sheppard, 1992;Hsieh and Savin, 1998;Sacchi et al, 2000 and references there-in). Our results do not provide any insight into this issue and, at present, it is assumed that outgassed CO 2 reflects isotopic equilibrium with bulk porewater.…”

Stable Isotope Evolution of Volcanic Ash Layers During Diagenesis of the Miocene Monterey Formation, California

“…Extrapolation of the fractionation trend of Capuano (1992) to the estimated temperature of the glass to smectite reaction of 40 ~ predicts a greater hydrogen isotope fractionation of -45%0 which would correspond to formation water 8D values of between -41%o and -24%0. A greater hydrogen isotope fractionation of -45%0 at low temperatures is supported by smectite hydroxyl-porewater fractionations of -43 to -46%0 measured by France-Lanord and Sheppard (1992). Studies of marine pore waters indicate a decrease in ~D values of 5 to 15%0 with depth that is interpreted to represent the lower ~D values of seawater prior to major ice buildup since the middle Miocene (Friedman and Hardcastle 1988;France-Lanord and Sheppard 1992).…”

“…A greater hydrogen isotope fractionation of -45%0 at low temperatures is supported by smectite hydroxyl-porewater fractionations of -43 to -46%0 measured by France-Lanord and Sheppard (1992). Studies of marine pore waters indicate a decrease in ~D values of 5 to 15%0 with depth that is interpreted to represent the lower ~D values of seawater prior to major ice buildup since the middle Miocene (Friedman and Hardcastle 1988;France-Lanord and Sheppard 1992). Assuming a late Miocene seawater 8D value of -5%0 still indicates that the ~D values of the smectites are significantly lower (-36 to -19%o) than predicted for equilibrium exchange.…”