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The δ18O of carbonate minerals that formed at Earth’s surface is widely used to investigate paleoclimates and paleo-elevations. However, a multitude of hydrologic processes can affect δ18O values, including mixing, evaporation, distillation of parent waters, and carbonate growth temperatures. We combined traditional carbon and oxygen isotope analyses with clumped (Δ47) and triple oxygen isotopes (Δ′17O) analyses in oyster shells (Acutostrea idriaensis) of the Goler Formation in southern California (USA) to obtain insights into surface temperatures and δ18O values of meteoric waters during the early Eocene hothouse climate. The Δ47-derived temperatures ranged from 9 °C to 20 °C. We found a correlation between the δ18O of growth water (δ18Ogw) (calculated using Δ47 temperatures and δ18O of carbonate) and the δ13C values of shells. The Δ′17O values of shell growth waters (0.006‰–0.013‰ relative to Vienna standard mean ocean water–standard light Antarctic precipitation [VSMOW-SLAP]) calculated from Δ′17O of carbonate (–0.087‰ to –0.078‰ VSMOW-SLAP) were lower than typical meteoric waters. These isotopic compositions are consistent with oyster habitation in an estuary. We present a new triple oxygen isotope mixing model to estimate the δ18O value of freshwater supplying the estuary (δ18Ofw). The reconstructed δ18Ofw of –11.3‰ to –14.7‰ (VSMOW) is significantly lower than the δ18Ogw of –4.4‰ to –9.9‰ that would have been calculated using “only” Δ47 and δ18O values of carbonate. This δ18Ofw estimate supports paleogeographic reconstructions of a Paleogene river fed by high-elevation catchments of the paleo–southern Sierra Nevada. Our study highlights the potential for paired Δ47 and Δ′17O analyses to improve reconstructions of meteoric water δ18O, with implications for understanding ancient climates and elevations.
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The Evolution of Cordilleran Topography in the Americas: Records of Surface Uplift and the Onset of Orogenic Collapse, American Geophysical Union Fall Meeting, December 2019.
High elevation regions cover a relatively small portion of Earth’s surface, but have an out-sized influence on regional hydrology, continental sediment transport, and global climate. What factors control the lifespan of mountains, and what are the mechanics of the transition to orogenic collapse? The Cenozoic morphology and evolution of the Cordilleran orogen is widely studied and debated in both North and South America, particularly the timing and rates of uplift, the links to possible mantle delamination, and the controls on extensional collapse. At least part of this discrepancy is due to the inherent complexity of atmospheric circulation and moisture sourcing in continental interiors, such as in the modern Rocky Mountains and Altiplano, where the distribution of hydrogen and oxygen isotope ratios (δD/δ18O) of precipitation does not correlate with temperature. Here we reconstruct past elevations across the northern and southern Basin and Range of the western U.S. and the Central Andes of southern Peru, using hydrogen isotope ratios (δD) of paleo-precipitation preserved in volcanic glass from widespread air-fall ashes and ignimbrites. Data span from the paleo-Pacific shoreline across the Cordilleran orogen and are paired with new sanidine 40Ar/39Ar geochronology and detailed chemo- and litho-stratigraphy, providing a novel approach to interpreting changes in δD values over space and time. Paleogene data in the western U.S. show that Pacific-sourced moisture crossed a high elevation Cordilleran hinterland prior to reaching what are now the Rocky Mountains and Great Plains. Mixing with Gulf-sourced moisture likely took place over the central and southern Rocky Mountains, making applications of the modern lapse rate or a Rayleigh modeled lapse rate inaccurate as both use a single moisture source. Data from west of the Sevier foreland, dominated by Pacific-sourced moisture, however, reveal the locations of peak Paleogene elevations. Across the southern Peruvian Altiplano, we observe a similar pattern of paleo-air-mass mixing from relatively higher and lower elevation moisture sources.
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- Award ID(s):
- 1848563
- PAR ID:
- 10168791
- Date Published:
- Journal Name:
- American Geophysical Union Fall Meeting 2019
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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