Coupled δ44/40Ca, δ88/86Sr, and 87Sr/86Sr geochemistry across the end-Permian mass extinction event

Calcium isotopes in deep time: Potential and limitations

\n Stratigraphic variability in the geochemistry of sedimentary rocks provides critical data for interpreting paleoenvironmental change throughout Earth history. However, the vast majority of pre-Jurassic geochemical records derive from shallow-water carbonate platforms that may not reflect global ocean chemistry. Here, we used calcium isotope ratios (δ44Ca) in conjunction with minor-element geochemistry (Sr/Ca) and field observations to explore the links among sea-level change, carbonate mineralogy, and marine diagenesis and the expression of a globally documented interval of elevated carbon isotope ratios (δ13C; Hirnantian isotopic carbon excursion [HICE]) associated with glaciation in Upper Ordovician shallow-water carbonate strata from Anticosti Island, Canada, and the Great Basin, Nevada and Utah, USA. The HICE on Anticosti is preserved in limestones with low δ44Ca and high Sr/Ca, consistent with aragonite as a major component of primary mineralogy. Great Basin strata are characterized by lateral gradients in δ44Ca and δ13C that reflect variations in the extent of early marine diagenesis across the platform. In deep-ramp settings, deposition during synglacial sea-level lowstand and subsequent postglacial flooding increased the preservation of an aragonitic signature with elevated δ13C produced in shallow-water environments. In contrast, on the mid- and inner ramp, extensive early marine diagenesis under seawater-buffered conditions muted the magnitude of the shift in δ13C. The processes documented here provide an alternative explanation for variability in a range of geochemical proxies preserved in shallow-water carbonates at other times in Earth history, and challenge the notion that these proxies necessarily record changes in the global ocean.

/pdf/sea-level-carbonate-mineralogy-and-early-diagenesis-w1a36ihx9q.pdf

Sea level, carbonate mineralogy, and early diagenesis controlled δ13C records in Upper Ordovician carbonates

Estuaries are dynamic mixing zones where river water interacts with seawater, resulting in large and complex geochemical changes. How two key factors, particle aggregation and pH, affect metal behaviour in estuaries is reviewed and integrated in this paper. Riverine particles are coated with organic matter and electrostatic repulsive forces restrict aggregation. In estuaries, increased concentrations of divalent cations reduce the repulsive forces between particles at low salinities, resulting in their rapid coagulation and removal of particulate-associated metals (e.g. Fe and Pb). However, truly dissolved metals may mix conservatively, and metals associated more with colloidal and dissolved organic material (e.g. Cu and Zn) can show variable behaviour. In many field studies and modelling of river inputs with different compositions, pH decreases slightly at low salinity. Geochemical model simulations of dissolved metal speciation indicated that Zn would be desorbed from iron oxide binding surfaces due to these pH and cation concentration changes, with Cu also showing less binding to dissolved organic matter (DOM). DOM, pH and particle surfaces can influence individual metal behaviour at various spatial and temporal scales. Further integrated field and laboratory research in estuaries where key geochemical processes affecting metal concentrations are measured and modelled is needed.

Particle aggregation, pH changes and metal behaviour during estuarine mixing: review and integration

Ca and Sr isotope constraints on the formation of the Marinoan cap dolostones

Calcium and strontium isotope systematics in the lagoon-estuarine environments of South Australia: Implications for water source mixing, carbonate fluxes and fish migration

Isaac Kell-Duivestein

Papers

Calcium and strontium isotope systematics in the lagoon-estuarine environments of South Australia: Implications for water source mixing, carbonate fluxes and fish migration