Variability of carbonate diagenesis in equatorial Pacific sediments deduced from radiogenic and stable Sr isotopes

The recrystallisation (dissolution-precipitation) of carbonate sediments has been successfully modelled to explain profiles of pore water Sr concentration and radiogenic Sr isotope composition at different locations of the global ocean. However, there have been few systematic studies trying to better understand the relative importance of factors influencing the variability of carbonate recrystallisation. Here we present results from a multi-component study of recrystallisation in sediments from the Integrated Ocean Drilling Program (IODP) Expedition 320/321 Pacific Equatorial Age Transect (PEAT), where sediments of similar initial composition have been subjected to different diagenetic histories. The PEAT sites investigated exhibit variable pore water Sr concentrations gradients with the largest gradients in the youngest sites. Radiogenic Sr isotopes suggest recrystallisation was relative rapid, consistent with modelling of other sediment columns, as the Sr-87/Sr-86 ratios are indistinguishable (within 2 sigma uncertainties) from contemporaneous seawater Sr-87/Sr-86 ratios. Bulk carbonate leachates and associated pore waters of Site U1336 have lower Sr-87/Sr-86 ratios than contemporaneous seawater in sediments older than 20.2 Ma most likely resulting from the upward diffusion of Sr from older recrystallised carbonates. It seems that recrystallisation at Site U1336 may still be on-going at depths below 102.5 rmcd (revised metres composite depth) suggesting a late phase of recrystallisation. Furthermore, the lower Sr/Ca ratios of bulk carbonates of Site U1336 compared to the other PEAT sites suggest more extensive diagenetic alteration as less Sr is incorporated into secondary calcite. Compared to the other PEAT sites, U1336 has an inferred greater thermal gradient and a higher carbonate content. The enhanced thermal gradient seems to have made these sediments more reactive and enhanced recrystallisation. In this study we investigate stable Sr isotopes from carbonate-rich deep sea sediments for the first time. Pore water delta Sr-88/86 increases with depth (from 0.428 parts per thousand to values reaching up to 0.700 parts per thousand) at Site U1336 documenting an isotope fractionation process during recrystallisation. Secondary calcite preferentially incorporates the lighter Sr isotope (86 Sr) leaving pore waters isotopically heavy. The delta(87)/Sr-86 values of the carbonates themselves show more uniform values with no detectable change with depth. Carbonates have a much higher Sr content and total Sr inventory than the pore waters meaning pore waters are much more sensitive to fractionation processes than the carbonates. The delta Sr-88/86 Sr results indicate that pore water stable Sr isotopes have the potential to indicate the recrystallisation of carbonate sediments. (C) 2014 Elsevier Ltd. All rights reserved.