EXPLORING THE ROLE OF THE HYDROLOGIC CYCLE IN ARCTIC AMPLIFICATION DURING THE GREENHOUSE WORLDS OF THE CRETACEOUS AND PALEOGENE

A characteristic of warm periods in Earth History like those of the Cretaceous-Paleogene is development of extreme polar warmth, to the extent that existence of polar cryospheres is in doubt. Paleoclimatologic researchers working on Cretaceous-Paleogene deposits in the Arctic have studied the stable isotopes of paleoprecipitation, using a variety of pedogenic mineral, vertebrate and higher plant fossil, and organic molecular proxies from the sedimentary record to consider the possible role of the hydrologic cycle in sustaining ancient polar warmth. A major Earth System Science question concerns applicability of a modern empirical relationship between mean annual surface air temperature and weighted mean annual δ¹⁸O of precipitation first described by Dansgaard (1964), a relationship that associates higher water δ¹⁸O values with higher air temperatures, and lower water δ¹⁸O values with lower air temperatures. Ufnar et al. (2002 in Palaeo-3 188:51-71) noted that despite a warmer Albian (Early Cretaceous) Arctic, Albian pedogenic siderite δ¹⁸O values from North Slope Alaska are lower, rather than higher than those expected from the modern, and much lower than predicted for the Albian by the Dansgaard relationship. Ufnar et al. (2004 in Geology 32:1049-1052) interpreted the data as resulting from increased rainout effects of an intensified Cretaceous hydrologic cycle, with stable isotope mass balance modeling to quantify increased latent heat flux as a possible mechanism for sustaining polar warmth. Earth System Model (ESM) simulations by Poulsen (2007 in Geology 35:199-202) did not match the Cretaceous siderite δ¹⁸O data, but rather predicted higher δ¹⁸O precipitation values than modern, positing complicating factors such as orographic and continental effects as possibly confounding the published data. The results of Poulsen et al. (2007) are compatible with the findings of other investigators of the stable isotope paleohydrology of Paleogene deposits in the Arctic, and other ESM studies of the Paleogene Arctic. Were published Cretaceous δ¹⁸O proxy data from Alaska taken from locales that fortuitously produced anomalous results? Our recent studies strengthen earlier conclusions about isotopically lighter paleoprecipitation in the Cretaceous Arctic, and highlight a genuine paleoclimatic enigma.