Stable Isotope Proxies for Polar Paleoprecipitation in Ancient Greenhouse Worlds

Polar regions are sensitive to changing global heat budgets, and global warming is strongly expressed near the poles. There are uncertainties about responses of hydrologic cycle to episodes of global warming, and about roles of the hydrologic cycle in amplifying polar warming. Stable isotope proxies from authigenic minerals and fossil materials from ancient terrestrial ecosystems of the mid-Cretaceous and Early Eocene have been used to interpret records of the δ¹⁸O and δD of precipitation during ancient greenhouse worlds. From one example, a transect of mid-Cretaceous pedogenic carbonate δ¹⁸O in the Americas spans from the Colombian paleoequator to the North Slope Alaskan paleo-Arctic. This Cretaceous latitudinal mineral δ¹⁸O gradient is isotopically lighter and declines northward more steeply than that of the modern; ¹⁸O mass-balance models of these data argue for higher Cretaceous precipitation rates in temperate-polar latitudes than today. They also argue for increased latent heat fluxes (3.6x -- 10.6x at 50º N) to the poles; a mechanism for amplified polar warming. Models of Cretaceous paleoclimates by GCMs with water isotope modules do not simulate the steeper empirical Cretaceous mineral ¹⁸O gradient or mass balance-modelled higher precipitation rates; orographic influences of the Brooks Range on Alaskan δ¹⁸O data have been cited as a possible confounding effect masking true δ¹⁸O values of polar paleoprecipitation. From another example, long-chain n-alkanes (n-C_27/29) recovered from Early Eocene sediments recording the PETM event (IODP Expedition 302) in the Arctic Ocean have characterized the δD of Arctic paleoprecipitation. The δD values of n-C_27/29 alkanes are dramatically higher in the PETM interval than in bounding strata, arguing for decreased rainout during poleward moisture transport, but in higher moisture and latent heat delivery to the North Pole during this event. Discrepant results call for coordination of multiple proxies, and we are pursuing this objective for the Cretaceous North Slope example.