Arid Coastal Carbonates and the Phanerozoic Record of Carbonate Chemistry

B P Smith,E J Ramos,M D Cantine,R C Martindale,K D Bergmann,C Kerans

doi:10.1029/2021av000386

Abstract

AbstractOcean chemistry and carbonate sedimentation link Earth's climate, carbon cycle, and marine pH. The carbonate system in seawater is complex and there are large uncertainties in key parameters in deep time. Here, we link sedimentary textures formed in arid coastal environments and preserved in the rock record to past seawater carbonate chemistry. Prior to the mid‐Mesozoic, tepee structures and pisoids – features associated with peritidal environments – co‐vary with available shelf area during cycles of supercontinent formation and rifting. In contrast, tepees and pisoids are consistently scarce after the mid‐Mesozoic, which coincides with a radiation in pelagic calcifiers as well as the breakup of Pangea. Numerical models suggest that the global and temporal abundances of tepee structures and pisoids are correlated with secular shifts in seawater chemistry, and that trends likely reflect the underlying influence of tectonics and biotic innovation on marine alkalinity and the saturation states of carbonate minerals. As independent sedimentary proxies, tepees and pisoids serve as benchmarks for global carbon cycle models and provide a new proxy record of seawater chemistry that can help discern links among tectonics, biotic innovation, and seawater chemistry.

Highlights

IntroductionChemical, and biological products of Earth's carbon cycle
Carbonate rocks are physical, chemical, and biological products of Earth's carbon cycle
Numerical models suggest that the global and temporal abundances of tepee structures and pisoids are correlated with secular shifts in seawater chemistry, and that trends likely reflect the underlying influence of tectonics and biotic innovation on marine alkalinity and the saturation states of carbonate minerals

Summary

Introduction

Chemical, and biological products of Earth's carbon cycle. Carbonate burial regulates both global climate and marine pH by removing atmospheric CO2 through silicate weathering (Berner et al, 1983; Urey, 1952). Carbonate precipitation depends on the carbonate chemistry of seawater, a system with two degrees of freedom among six variables: pCO2 (bars), HCO3 (mmol/kgw), CO32 (mmol/ kgw), pH, total carbon (mmol/kgw), and alkalinity (mmol/kgw) (Zeebe & Wolf-Gladrow, 2001). The CO32 concentration is usually depicted in a related quantity, the saturation state Ω, for either calcite or aragonite:. Where γ is the activity of dissolved ions, brackets denote concentrations, and Ksp is a thermodynamic solubility constant of the mineral.

Results

Discussion

Conclusion