USING PARTIAL DISSOLUTIONS TO ASSESS THE DEGREE OF ALTERATION IN SHELLS USED FOR HIGH PRECISION STRONTIUM ISOTOPE STRATIGRAPHY

Current attempts to model future sea-level rise rely, at least in part, on estimates of past sea-level acquired from carbonate sequences across the globe. The utility of this data depends upon whether the record of sea-level change seen in carbonate sequences can be measured with sufficient certainty and age resolution to allow correlation with other climate data. However, providing accurate age determinations of carbonate rocks can be difficult. One common approach is the use of Strontium Isotope Stratigraphy (SIS). A crucial part of SIS is appraisal of the preservation of the original ⁸⁷Sr/⁸⁶Sr ratio. This is because all carbonate material can suffer at least some degree of alteration, which can alter the ⁸⁷Sr/⁸⁶Sr value leading to inaccurate ages. Here we explore, both theoretically and experimentally, the potential for using partial dissolutions to assess the degree of alteration in materials used for SIS.

Well-preserved material will contain a single isotopic reservoir of Sr, whereas, altered material may contain two or more distinct reservoirs. In the case of altered material, the reservoirs will include both the original seawater Sr together with contaminating Sr of variable concentration, such that the analyzed ⁸⁷Sr/⁸⁶Sr ratios from this altered material will represent a mixture of any of the isotopic reservoirs incorporated during the dissolution undertaken for analysis. In poorly preserved material, the measured ⁸⁷Sr/⁸⁶Sr ratios of progressively more aggressive leaches will show variation as each dissolution will scavenge different amounts of the isotopically different pools of Sr; whereas, leaches from well preserved material should be exactly the same. In the case of altered material, the first leach will most likely dissolve the most loosely bound Sr and subsequent leaches may get closer to the original isotopic ratio.

Therefore, successive leaches from a sample may be analyzed and the pattern of the resulting ⁸⁷Sr/⁸⁶Sr values compared to identify contamination. We present preliminary results from tests of this partial dissolution approach using different biogenic material, including mollusks and sharks teeth, of various ages. The results suggest that partial dissolution can be used to discriminate between well-preserved and altered materials and can be used to assess the robustness of SIS data.