Disentangling global geochemical signals from local diagenetic alteration in shallow-water marine carbonates during OAE1a

Shallow-water marine carbonates are widely used to reconstruct short-lived (∼ million years) perturbations to global geochemical cycles (e.g., carbon, calcium, strontium, and lithium). However, local environmental phenomena like meteoric and marine diagenetic alteration or changes in carbonate facies often decouple carbonate geochemistry from open-ocean seawater chemistry. To accurately reconstruct global geochemical cycles using shallow-water carbonates, it is therefor crucial to separate signals of local environmental phenomena from genuine changes in open-ocean seawater chemistry. The well-documented shallow-water carbonate succession from Ocean Drilling Program Site 866A (Resolution Guyot, Mid-Pacific Mountains) offers an opportunity to disentangle global and local signals recorded in shallow-water carbonates during the Early Cretaceous oceanic anoxic event 1a (OAE1a), a brief, globally correlative perturbation to the carbon cycle. Recent studies at this site have documented a δ¹³C excursion, a modest decline in ⁸⁷Sr/⁸⁶Sr, and large stratigraphic variability in δ⁷Li and δ⁴⁴Ca values, all of which have been interpreted as reflecting temporal changes in the chemical composition of open-ocean seawater associated with OAE1a. However, other work at this site demonstrates clear evidence of meteoric and marine diagenesis as well as changes in carbonate facies across the OAE1a interval. Here, we aim to disentagle signals that reflect global geochemical cycling from those that reflect local environmental phenomena by employing a suite of carbonate-bound geochemical proxies (δ⁷Li, δ¹³C, δ¹⁸O_carb, δ²⁶Mg, δ⁴⁴Ca, Δ₄₇, [Mg], [Ca], [Sr], [Li]) and a numerical model of carbonate diagenesis. We show that while changes in ⁸⁷Sr/⁸⁶Sr ratios at this site reflect changes in primary seawater composition, stratigraphic variability in other geochemical systems (e.g., δ⁷Li, δ²⁶Mg and δ⁴⁴Ca values and trace element concentrations) arises primarily from early marine and meteoric diagenesis, with some contamination of Li from co-occurring clays. With much of the chemostratigraphic variability preserved today conceivably owing its origin to carbonate diagenesis and in recognition of the residence times for these elements in seawater (>1 million years) being longer than the short-lived OAE1a event, we make a case that our model-derived “snapshot” estimate for the calcium, magnesium, and lithium isotope composition of the marine diagenetic fluid reflects a stable open-ocean Early Cretaceous seawater signature. Although a global δ¹³C signal may still be present, our findings show how the stratigraphic record of δ¹³C values across the OAE1a at this site is a complex function of marine and meteoric diagenesis and changes in carbonate facies.