Deltas in an estuary: clumped and triple oxygen isotope analyses reveal isotopically depleted headwaters in the early Eocene of Southern CA

Abstract:

The δ¹⁸O of carbonate minerals that form in the hydrosphere is widely used to investigate hydrologic processes in ancient high CO_{2 ­}worlds. However, a multitude of hydrologic processes can affect δ¹⁸O values (mixing, evaporation, and distillation of parent waters; carbonate growth temperatures). We combine traditional carbon and oxygen isotopes with clumped (∆₄₇) and triple oxygen isotopes (∆'¹⁷O) in oyster shells (Acutostrea idriaensis) from the early Eocene to provide insights into the δ¹⁸O of meteoric waters during hothouse climates. The oyster shells were collected from the Goler Formation of California, USA, near the transition from the terrestrial to marine environments. In addition to evidence from the geologic setting itself, we find isotopic evidence to suggest that the oysters lived in an estuary. δ¹⁸O of growth water (δ¹⁸O_gw) is calculated using ∆₄₇ temperatures and δ¹⁸O of carbonate and ranges from -4.4 to -9.0 ‰ (SMOW). δ¹³C values range from -0.5 to -6.0 ‰ (VPDB). A correlation exists between δ¹³C and δ¹⁸O_gw (r² of a linear model is 0.95); a mix of low δ¹³C and δ¹⁸O fresh water with high δ¹³C and δ¹⁸O ocean water can explain this isotopic covariation. ∆'¹⁷O of growth waters (calculated from ∆'¹⁷O of carbonate and ∆₄₇ temperatures (–0.004 to +0.006 ‰ (SMOW-SLAP)) is consistent with a mix of fresh and ocean waters in δ¹⁸O - ∆'¹⁷O space, which we consider further evidence for an estuarine environment. We use these mixing relationships (δ¹³C - δ¹⁸O and δ¹⁸O - ∆'¹⁷O) to estimate the δ¹⁸O of the freshwater feeding the estuary. We find that δ¹⁸O of freshwater is -15 ‰ or lower – an estimate that is >5 ‰ lower than what would have inferred using “only” clumped and traditional oxygen isotopes (-9 ‰). This estimate of δ¹⁸O of fresh water is lower than the δ¹⁸O of modern snowmelt springs in the Sierra Nevada. Isotopically depleted freshwater supports paleogeographic reconstructions of a Paleogene river that flowed southward along the moderately high-elevation eastern paleo-Sierra Nevada before turning west to the Pacific. This result highlights the potential for clumped and triple oxygen isotopes to enhance reconstructions of δ¹⁸O of meteoric waters, which are critical for making comparisons with modeled predictions and will ultimately improve understanding hydroclimate in high CO₂ worlds.