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

Abstract

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 = 87 Sr/ 86 Sr) of seawater (sw) during the Phanerozoic utilizing as a constraint the

Effect of paleoseawater composition on hydrothermal exchange in midocean ridges

“…Possibly counterbalancing this effect is the amount of global hydrothermal fluid flux, which some have inferred was higher than modern in the Cretaceous (53, 54) as a consequence of globally higher seafloor generation rates. Our results therefore suggest that either low-temperature exchange of Sr between basalt and seawater (including weathering of ocean-island basalts) was responsible for a large fraction of the overall basalt-seawater exchange (43,49,55), or conversely, that there was an exceptionally small continental weathering flux of radiogenic Sr during the Cretaceous. Supporting the prior interpretation are records of low-temperature hydrothermal crust alteration (49), which suggest that higher water temperatures increased the magnitude of low-temperature basalt-seawater exchange.…”

“…Although we have not attempted to create a global model for seawater [Ca] and [Sr] through time, differences in lowtemperature fluxes and retrograde outputs from altered oceanic crust (e.g., refs. 43,49), along with potential differences in reaction rates caused by paleoseawater variability, would be important to consider [a dual-porosity model is presented in the SI Appendix, section I to explore the effects of reaction rate differences on hydrothermal exchange (SI Appendix, section I and Fig. S5)].…”

Inclusion of a suite of weathering tracers in the cGENIE Earth System Model – muffin release v.0.9.10

“…There is no evidence for a generic dependence of δ 88/86 Sr fractionation during biogenic carbonate formation on environmental conditions, although temperature and growth rate effects were observed in some calcifying species (Fietzke and Eisenhauer, 2006;Böhm et al, 2012;Stevenson et al, 2014;Vollstaedt et al, 2014). Kristall et al (2017) input from diagenesis 3-5 0.7035-0.7084 0.27 Kristall et al (2017) dissolved riverine input 20.2-47 0.7111-0.7136 0.32 Allègre et al (2010) Peucker-Ehrenbrink et al 2010Pearce et al 2015; Kristall et al (2017) particulate riverine input 5.2 <0.7136 uncertain Allègre et al (2010) Peucker-Ehrenbrink et al 2010Kristall et al 2017groundwater discharge 7.1-16.6 0.7089 0.354 ± 0.028 Basu et al (2001); Beck et al (2013) dust flux and rainwater uncertain 0.7075 -0.7191 0.05 -0.31 Pearce et al (2015) pelagic Sr burial in carbonates 12.5 -174 seawater 0.20 Krabbenhöft et al (2010) Stevenson et al (2014) Kristall et al (2017) neritic Sr burial in carbonates 19 seawater 0.21 Krabbenhöft et al (2010) Sr burial in sea floor alteration uncertain seawater seawater Menzies and Seyfried Jr (1979) Kristall et al (2017)…”

“…Continental weathering is the main source for marine Sr, Os, Li and Ca today and changes in the weathered lithology or weathering intensity are invoked as major drivers of the evolution of trace metal concentrations and their isotopic compositions in seawater over time (e.g. Misra and Froelich, 2012;Kristall et al, 2017). ROKGEM, the weathering module of cGENIE, provides a framework for calculating climate-and CO 2 -dependent additions of Ca, Mg and alkalinity from carbonate weathering (following Berner, 1994) and silicate weathering (following Brady, 1991)…”

“…Mass balances and box models have been most commonly used to interpret geological records of trace metal variations (e.g. Tejada et al, 2009;Misra and Froelich, 2012;Kristall et al, 2017;Them et al, 2017). They however rely on assumed fluxes and residence times based on today's ocean which might not always be applicable throughout the geological record.…”