THE ULTRA-WARM ARCTIC CA. 90 MILLION YEARS AGO: TESTING LINKAGES WITH VOLCANIC CO2 OUTGASSING

The Arctic region preserves a unique geologic history of past climate that might provide insight into modern conditions and the potential for future change. We focus on an excursion within the Cretaceous greenhouse world to ultra-warm conditions, ca. 90 million years ago. Evidence for both the high temperature and anomalous CO₂ outgassing is found in the geology of the High Arctic. The former comes in the form of a spectacular assemblage of vertebrate fossils, including large-bodied crocodile-like champsosaurs, turtles and fish. The latter is inferred by the presence of continental flood basalts and the Alpha-Mendeleev Oceanic Ridge; these features may form one of Earth's most voluminous large igneous provinces. The fossil vertebrates are from a key Arctic site on Axel Heiberg Island at 79° N. The fossil champsosaurs suggest a minimum mean annual temperature of 14 °C; paleomagnetic data indicate that the site was at Arctic paleolatitudes (approximately 71° N). Fish fossils record an expansion of ranges toward the poles. Turtles are abundant and diversity is unusually high compared with other high latitude vertebrate assemblages, consistent with warm temperatures. The fossil champsosaurs define a population dominated by juveniles, incompatible with freezing. Aurorachelys gaffneyi, a large macrobaenid fossil turtle with affinities to Asia, suggests the possibility of dispersal over a warm, brackish Arctic Ocean. Isotopic analyses of bone apatite using a multiple taxon approach can be used to obtain quantitative estimates of paleotemperature. The availability of multiple species of fossil fish and turtles, and bones from champsosaurs affords the possibility of consistency tests. Scales from the fossil gar Lepisosteus yield an average δ¹⁸O of 7.0‰ relative to Vienna Standard Mean Ocean Water. The fossil vertebrates overlie continental flood basalts of the Strand Fiord Formation, which may be the on-land expression of magmatism that formed the Alpha-Mendeleev Ridge. CO₂ outgassing related to this volcanism, together with outgassing at several other sites of coeval magmatism, may have been responsible for the interval of extreme climatic warmth. Such a linkage is supported by new radiometric age data from volcanic rocks of Hansen Point (High Canadian Arctic).