Interstitial Water Chemistry and Diagenesis of Biogenic Sediments from the Eastern Equatorial Pacific, Deep Sea Drilling Project Leg 85

Abstract

The concentration changes in pore waters of dissolved calcium, magnesium, sulfate, strontium, and silica and of alkalinity are controlled by diagenetic reactions occurring within the biogenic sediments of DSDP Sites 572, 573, and 574. Downcore increases in dissolved Sr 2+ indicate recrystallization of calcite, and increases in dissolved SiO 2 reflect dissolution of amorphous silica. Minor gradients in dissolved Ca 2+ and Mg 2+ suggest little if any influence from reactions involving volcanic sediments or basalt. Differences in interstitial water profiles showing the downhole trends of these chemical species mark variations in carbonate and silica diagenesis, sediment compositions, and sedimentation rate histories among the sites.The location and extent of carbonate diagenesis in these sediments are determined from Sr/Ca distributions between the interstitial waters and the bulk carbonate samples. Pore water strontium increases in the upper 100 to 250 m of sediment are assumed to reflect diffusion from underlying zones where calcite recrystallization has occurred. On the basis of calculations of dissolved strontium production and comparisons between observed and calculated "equilibrium" Sr/ Ca ratios of the solids, approximately 30 to 50% of the carbonate has recrystallized in these deeper intervals. These estimates agree with the observed amounts of chalk at these sites. Variations in Sr/Ca ratios of these carbonates reflect differences in calcareous microfossil content, in diagenetic history, and, possibly, in changes in seawater Sr/Ca with time.Samples of porcelanite recovered below 300 m at Site 572 suggest formation at temperatures 20 to 30° C greater than ones estimated assuming oceanic geothermal gradients from sedimentary sections similar to those recovered on Leg 85. The higher temperatures may partially account for higher Sr/Ca ratios determined for recrystallized carbonates from this site.

Interstitial Water and Bulk Calcite Chemistry, Leg 130, and Calcite Recrystallization

“…4B), as are relatively constant values from 40 to 10 Ma. Absolute Sr/Ca ratios in this study are higher especially in the youngest sediments, but this may be a result of the higher Sr/Ca ratios in nannofossil calcite and thus bulk calcite relative to foraminifer tests (Baker et al, 1982;Stout, 1985). Although age-controlled recrystallization may play a part, the controls on these bulk calcite Sr/Ca records may be dominantly seawater chemistry.…”

“…Coupled with the debate about the total extent of calcite recrystallization are different interpretations about recrystallization rate variations with depth or age (cf. Richter and DePaolo, 1987, 1988Stout, 1985;Gieskes et al, 1986). It is not known if recrystallization is a one-step process as most studies have assumed, or whether recrystallized calcite itself can further recrystallize (Apitz, 1991).…”

“…It is not known if recrystallization is a one-step process as most studies have assumed, or whether recrystallized calcite itself can further recrystallize (Apitz, 1991). The importance of interstitial water chemical variation through time, especially calcium concentration gradients, in determining the composition of recrystallized calcite has been recognized (Stout, 1985;Delaney, 1989), but quantitative estimates of this effect require making assumptions about past variations that cannot necessarily be verified. The role of minor sedimentary phases in controlling interstitial water chemistry and thus the environment for calcite recrystallization is not well known (e.g., Baker and Bloomer, 1986).…”

A study of celestine equilibrium in marine sediments using the entire ODP/IODP porewater data base

“…Indeed, the Sr elemental mineral/solution partition coefficients for biogenic aragonite and calcite are higher than those for inorganic calcite (Banner, 1995;Hampt and Delaney, 1997, and references therein). The comparison between Sr/Ca ratios in interstitial waters and in solid carbonates has thus led to extensive studies of the extent of carbonate recrystallization in marine sediments and their influence on the recorded values of 87 Sr/ 86 Sr and d 18 O (e.g., Baker et al, 1982;Gieskes, 1983;Stout, 1985;Elderfield, 1986;Gieskes et al, 1986;Richter and DePaolo, 1987;Richter and DePaolo, 1988;Delaney, 1989;Richter, 1993Richter, , 1996Richter and Liang, 1993;Andreasen and Delaney, 2000;Rudnicki et al, 2001;Ando et al, 2006;Fantle and DePaolo, 2006). Several numerical models have been used to calculate the rate of recrystallization of Ca-carbonates, the Sr distribution coefficient for calcite and the Sr isotopic exchange rate between marine sediments and interstitial waters (e.g., Baker et al, 1982;Richter and DePaolo, 1987;Richter and DePaolo, 1988;Delaney, 1989;Richter, 1993Richter, , 1996Richter and Liang, 1993;Rudnicki et al, 2001;Ando et al, 2006;Fantle and DePaolo, 2006).…”

Pore-Water Chemistry of the Sulu and Celebes Seas: Extensive Diagenetic Reactions at Sites 767 and 768

“…Such increases have been observed at other DSDP and ODP sites and have been attributed to recrystallization of biogenic carbonates to inorganic calcite, with the consequent release of strontium to the pore fluids (Baker et al, 1982). As the dissolved strontium and sulfate concentrations are below celestite saturation, the diffusive flux of strontium from the sediment and the degree of carbonate recrystallization at Sites 767 and 768 may be estimated (Baker, 1986;Stout, 1985;Swart and Guzikowski, 1988). In the method outlined by Baker (1986), the strontium flux out of or into the sediments is summed over the time interval represented by two successive pore-water samples.…”