49-9 Climatic and Topographic Implications of Coastal Stable Isotope Records: Examples from the Oregon Cascades and Texas

Session: Exploring Feedbacks Between Tectonics and Climate on Lithospheric Evolution Using Multidisciplinary Approaches

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Stable isotope paleoaltimetry yields valuable insights into the geodynamic drivers of surface uplift and the paleoclimatic, sedimentologic, and paleobiologic effects of topographic change. Where high topography forces moist air masses to rise, cool, and condense, the stable isotope ratios (δD/δ18O) of the resulting precipitation progressively decrease as elevation increases. Paleoclimate-calibrated modeling applied to proxy records, such as the thermodynamic rainout model pioneered by David Rowley, can provide estimates of the magnitude of elevation change in the past. Coastal regions with one moisture source are ideal locations to apply rainout models, yet uncertainty remains in deciphering the possible effects of global climate change from orogenesis. Using δD values of paleoprecipitation preserved within hydrated volcanic glass, we reconstruct the early Cenozoic stable isotope records of two near coastal regions across major climatic and tectonic transitions to gain a better understanding of the signatures of each change.

Oligocene global temperature estimates are substantially colder than Eocene temperatures, attributed to rapid global cooling at the Eocene-Oligocene transition (EOT). New δD values for Texas paleocoastal precipitation, however, show minimal differences across this transition. High-resolution late Eocene to early Oligocene glass samples represent the first precipitation events from paleo-Gulf-of-Mexico moisture and have consistently high δD values. Glass δD values document a small decrease in initial rainout δD values across the EOT, but δD values before 34 and after 33 Ma are similar. Regional EOT data show that the competing effects of global cooling on initial coastal precipitation δD values were minimal in comparison to changes due to rainout across the continent.

In contrast to purely climate-driven changes in paleocoastal precipitation, we also present stable isotope data from the Oregon Cascades to better understand the timing and mechanisms of surface uplift and rainshadow formation. New glass δD values reflect increasing orographic rainout from the Pacific over the Oregon Cascades during the Oligocene and early Miocene. From ca. 33-17 Ma, δD values of samples collected east of the range decrease by 30‰. Interpreted using a rainout model, these data indicate arc elevations increased by 1.1 ± 0.4 km during this interval, coeval with geochemical evidence for 12 km of crustal thickening. This surface uplift was synchronous with leeward drying, highlighting the role of paleoelevation changes in controlling regional hydroclimate.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10335

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