THE RATE AND CONSEQUENCES OF SR DIAGENESIS IN DEEP-SEA CARBONATES

We have used a diffusion-advection-reaction model for Sr exchange duri ng deep-sea carbonate diagenesis to determine the rate and consequence s of Sr exchange between porewaters and bulk carbonate at six DSDP sit es (590B, 575, 593, 516, 305 and 289) where the required Sr concentrat ion and isotopic composition data for the interstitial waters are avai lable from the literature. The model can reproduce both the Sr2+ and S r-87/Sr-86 profiles in the porewaters at each of the sites by adjustin g the rate of Sr exchange due to solution-reprecipitation as a functio n of sediment age. We find that the rate of solution-reprecipitation a t all of the sites is highest for the youngest sediment (1-10%/m.y.) a nd 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 bul k carbonate is to reduce the Sr content by 5-25%, depending primarily on the sedimentation rate, and shift the Sr-87/Sr-86 by as much as 0.0 0004 from the value the sediment had when first deposited. Isotopic sh ifts of this magnitude are large compared to present analytical uncert ainties and therefore should be corrected for when using bulk carbonat e Sr-87/Sr-86 for stratigraphic correlation. The average Sr loss in th e 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 t he contemporaneous seawater value and therefore this flux plays a very minor role in the overall Sr budget that determines the Sr isotopic c omposition of seawater. The range of Sr losses from bulk carbonate due to diagenesis that we calculate is too small to account for the typic al 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 decreas e is not the result of diagenesis and must involve some changing prope rty of the ocean itself.