The rate and consequences of Sr diagenesis in deep-sea carbonates

Abstract

We have used a diffusion-advection-reaction model for Sr exchange during deep-sea carbonate diagenesis to determine the rate and consequences of Sr exchange between porewaters and bulk carbonate at six DSDP sites (590B, 575, 593, 516, 305 and 289) where the required Sr concentration and isotopic composition data for the interstitial waters are available from the literature. The model can reproduce both the Sr 2+ and 87Sr/ 86Sr profiles in the porewaters at each of the sites by adjusting the rate of Sr exchange due to solution-reprecipitation as a function of sediment age. We find that the rate of solution-reprecipitation at all of the sites is highest for the youngest sediment (1–10%/m.y.) and decreases by about an order of magnitude over the first 10 m.y. of sediment age. The effect of this exchange on the properties of the bulk carbonate is to reduce the Sr content by 5–25%, depending primarily on the sedimentation rate, and shift the 87Sr/ 86Sr by as much as 0.00004 from the value the sediment had when first deposited. Isotopic shifts of this magnitude are large compared to present analytical uncertainties and therefore should be corrected for when using bulk carbonate 87Sr/ 86Sr for stratigraphic correlation. The average Sr loss in the carbonate sections studied is about 15%, from which we conclude that the Sr flux from carbonate sediments to the overlying ocean is of the order of 15% of the riverine Sr flux. We also show that the isotopic composition of the diagenetic Sr flux is at all times quite close to the contemporaneous seawater value and therefore this flux plays a very minor role in the overall Sr budget that determines the Sr isotopic composition of seawater. The range of Sr losses from bulk carbonate due to diagenesis that we calculate is too small to account for the typical decrease in Sr content seen in pelagic carbonates as a function of age (about 70% over the first 100 m.y.). We conclude that this decrease is not the result of diagenesis and must involve some changing property of the ocean itself.