USING PALEOCLIMATE DATA TO ESTIMATE PALEOLATITUDE OF CORDILLERAN TERRANES

Paleoclimate observations were the first evidence used by Alfred Wegener and Wladimir Köppen to demonstrate large paleolatitudinal shifts of the continents. Our ability to measure paleotemperature has greatly improved over the last several decades, especially with the introduction of the clumped isotope method. We review the use of paleotemperature as a proxy for paleolatitude. Mean annual temperature (MAT) provides the best kind of measurement because it has low variance from year to year. The distribution of MAT with latitude is well defined by the energy-balance method of North et al., 1981, which gives T = T_m – ΔT (sin²θ –1/3), where θ is latitude, T is the MAT at that latitude, T_m is the average global temperature, and ΔT is the equator-to-pole difference in temperature. This equation is used to estimate the average meridional temperature profile for a stable reference continent with a known apparent polar wander path. PaleoMAT measurements are determined over a large range of paleolatitude in the reference continent (at sites close to paleosealevel) and those data are used to estimate the parameters, T_m and ΔT, for the meridional temperature equation. Next, paleoMAT measurements are determined for one or more sites in a suspect terrane, and the meridional temperature equation is used to determine the paleolatitude of the terrane. As an example, Miller et al., 2006 used this approach to estimate the paleolatitude of a 110 Ma floral site in the Methow Valley of Washington State. The shapes of modern leaves are known to be well correlated to MAT. Paleofloral sites at near shore sites in eastern North America were used to estimate an Albian-Cenomanian meridonial temperature equation for stable North America (T_m = 23.7 C, ΔT = 18.1 C). The suspect Methow site gave a MAT = 23.4 C and a paleolatitude of 38.4 N, indicating ~2200 km of northward offset after formation. We will report on new clumped isotope data from ammonites from the Late Cretaceous Nanaimo Group in western Washington and southwest British Columbia. Five localities show a steady cooling from 31 to 14 C over the time span 85 to 74 Ma, also consistent with large northward offset after 85 Ma.