A B S T R A C T

Figure 7.1 Variation of 87 Sr/ 86 Sr through Phanerozoic time. LOWESS fit to data sources in Table7.1.The 87 Sr/ 86 Sr value of Sr dissolved in the world's oceans has varied though time, which allows one to date and correlate sediments. This variation and its stratigraphic resolution is discussed and graphically displayed.

[1] Profiles of Mo/total organic carbon (TOC) through the Lower Toarcian black shales of the Cleveland Basin, Yorkshire, United Kingdom, and the Posidonia shale of Germany and Switzerland reveal water mass restriction during the interval from late tenuicostatum Zone times to early bifrons Zone times, times which include that of the putative Early Toarcian oceanic anoxic event. The degree of restriction is revealed by crossplots of Mo and TOC concentrations for the Cleveland Basin, which define two linear arrays with regression slopes (ppm/%) of 0.5 and 17. The slope of 0.5 applies to sediment from the upper semicelatum and exaratum Subzones. This value, which is one tenth of that for modern sediments from the Black Sea (Mo/TOC regression slope 4.5), reveals that water mass restriction during this interval was around 10 times more severe than in the modern Black Sea; the renewal frequency of the water mass was between 4 and 40 ka. The Mo/TOC regression slope of 17 applies to the overlying falciferum and commune subzones: the value shows that restriction in this interval was less severe and that the renewal frequency of the water mass was between 10 and 130 years. The more restricted of the two intervals has been termed the Early Toarcian oceanic anoxic event but is shown to be an event caused by basin restriction local to NW Europe. Crossplots of Re, Os, and Mo against TOC show similar trends of increasing element concentration with increase in TOC but with differing slopes. Together with modeling of 187 Os/ 188 Os and d 98 Mo, the element/TOC trends show that drawdown of Re, Os, and Mo was essentially complete during upper semicelatum and exaratum Subzone times (Mo/TOC regression slope of 0.5). Drawdown sensitized the restricted water mass to isotopic change forced by freshwater mixing so that continental inputs of Re, Os, and Mo, via a low-salinity surface layer, created isotopic excursions of up to 1.3% in d 98 Mo and up to 0.6% for 187 Os/ 188 Os. Restriction thereby compromises attempts to date Toarcian black shales, and possibly all black shales, using Re-Os chronology and introduces a confounding influence in the attempts to use d 98 Mo and initial 187 Os/ 188 Os for palaeo-oceanographic interpretation.

No abstract

We provide a best-fit curve to 1849 strontium isotope data for the period 0 to 206 Ma using the LOcally-WEighted regression Scatterplot Smoother (LOWESS) method. This is a robust, nonparametric modern regression technique. Since it does not yield an explicit mathematical equation relating 87 Sr/ 86 Sr to time, a look-up table to determine numeric age has been generated in steps of 1 ϫ 10 Ϫ6 in 87 Sr/ 86 Sr. The calibration uses the timescales of Shackleton and coworkers for 0-7 Ma; Cande and Kent for 7-72 Ma; Obradovich for 72-95 Ma and Gradstein and coworkers for Ͼ95 Ma. The look-up table includes 95% confidence intervals on the predictions of numeric age. When using this table, the uncertainty on the 87 Sr/ 86 Sr of the sample whose age is sought must be added to that inherent in the LOWESS regression. We show how to determine the uncertainty in 87 Sr/ 86 , i.e., how best to obtain the 95% confidence bounds on a single measurement of 87 Sr/ 86 for a sample, and on the mean 87 Sr/ 86 value for 2 or more replicate measurements of 87 Sr/ 86 in the sample; these confidence intervals reflect analytical-system reproducibility for routine samples (as opposed to that of standard control materials, e.g., NIST 987) and are necessary to establish the final upper and lower bounds on predicted numeric age.