Modeling the paleo-seawater radiogenic strontium isotope record: A case study of the Late Jurassic-Early Cretaceous

While uniformitarianism underlies many aspects of the Earth sciences, modeling geochemical cycles throughout Earth history requires critically assessing the reliability and constancy of projecting modern fluxes and their isotope ratios into deep geologic time. In this study, we develop an iterative technique for modeling the radiogenic strontium isotope composition (R-87 = Sr-87/Sr-86) of seawater (sw) during the Phanerozoic utilizing as a constraint the composite record of McArthur et al. (2012) (McArthur, J. M., Howarth, R. J., & Shields, G. A. (2012). Strontium isotope stratigraphy. In F. M. Gradstein, J. G. Ogg, M. Schmitz, & G. Ogg (Eds.), The Geologic Time Scale 2012 (pp. 127-144): Elsevier). The model yields estimates for the Sr isotope composition of the riverine, submarine groundwater discharge, and diagenetic input fluxes. We use the Late Jurassic-Early Cretaceous segment of the marine Sr isotope record as a case study to demonstrate the efficacy of the new method. To begin, Sr-87/Sr-86 ratios are calculated for a combined riverine and submarine groundwater discharge flux (tw = total weathering) using a prescribed time-variable 41257,, determined from the LOWESS FIT 5 of McArthur et al. (2012). The calculated R-87,R-t, values are checked against the range of 1287,, for the time interval of interest, as well as older carbonates available for weathering, to determine the reasonableness of the estimates. From these estimates, different prescribed R-87.tw values are tested to identify the best match with target seawater 87Sr/86Sr ratios and the range of 1287,5,,v. For model runs focusing on long time intervals (>10 Myr), a single prescribed R-87,R-tw value does not produce a reasonable match to the Sr isotope record. Testing the frequency of variation in R-870, indicates that during the Early Cretaceous, R87,Lw varied at least every 5 Myr, if not more frequently. The minimum frequency at which 1287.t, changes likely varied throughout the Phanerozoic, as it depends on the variability of d/dt R-97,R-sw and residence time of Sr. Model results also demonstrate the significance of estimating variation in the total hydro thermal (high and low temperature) input flux of Sr along with seawater Sr concentrations in the geologic past, which are key for properly estimating R-57,R-tw. (C) 2017 Elsevier B.V. All rights reserved.