Diagenetic evaluation of a Pennsylvanian carbonate succession (Bird Spring Formation, Arrow Canyon, Nevada, U.S.A.) — 1: Brachiopod and whole rock comparison

Abstract

Diagenetic evaluation of the Pennsylvanian Bird Spring Formation, Arrow Canyon, Nevada (U.S.A.) involved the detailed petrographic and geochemical examination of its brachiopod and whole rock components. δ13C, δ18O and 87Sr/86Sr of whole rock (WR) statistically (p=0.0001) do not agree with isotope values of coeval brachiopod low-Mg calcite (LMC). More importantly, a detailed investigation of the best preserved, fine-grained whole rock material demonstrates that it does not universally preserve its carbon isotope values, in contrast with hypothetical predictions. Furthermore, lumped trace chemistry or stable isotope tests do not unequivocally identify brachiopod or whole rock (fine- to coarse-grained) samples that carry an ‘original’ seawater signal. Instead, diagenetic evaluation, involving as many screening tests as possible, on a horizon-by-horizon level to compensate for spatial and temporal variations, is necessary to identify the sample(s) that carries an original seawater signature. Many brachiopods from the Bird Spring Formation are blue- to non-luminescent and carry microstructures and trace element contents similar to those of their modern tropical counterparts. Concurrence of chemical results between well preserved brachiopods and whole rock (if originally low-Mg calcite) is achieved in 22% of coeval samples for ∆13CbLMC-WR (with natural limits of ±0.25‰), in 20% of samples for ∆18ObLMC-WR (with natural limits of ±0.25‰), and in 9% of samples for ∆87SrbLMC-WR (with global limits of ±0.000025). After an adjustment for mineralogical fractionation, if whole rock is considered originally aragonite, the concurrence of ∆13CbLMC-WR drops to 0%, and for ∆18ObLMC-WR it drops to 6%. Furthermore, comparison of stable isotopes of our well-preserved brachiopods with those of preserved whole rock from the literature, suggests that they are significantly different (p=0.0001), with results of the latter dataset being more negative by about −0.77 (−2.57‰ with mineralogical adjustment) for δ13C (PDB) and −1.31 (−2.11‰) for δ18O (PDB). Thus, our integrated screening and comparative tests indicate that: (1) an overwhelming number of the whole rock samples from the Bird Spring Formation carry a diagenetic instead of a primary signal for both carbon and oxygen isotopes, and (2) many of the biogenic low-Mg calcite brachiopods in this succession carry an original chemical signal suitable for the reconstruction of high-resolution seawater isotope curves representative of the Pennsylvanian North American epeiric sea, and potentially of the global oceans.