EVALUATING NORTHERN HIGH-LATITUDE PALEOCLIMATE MODEL RESULTS USING PALEOBOTANICAL EVIDENCE FROM THE MIDDLE CRETACEOUS

摘要

Climate plays a significant role in determining the style of depositional processes at different latitudes, which in turn influences the location of hydrocarbon systems. Results of climate modelling may therefore provide important information for predicting the presence or absence of suitable hydrocarbon plays. The critical step is to validate the model results against proxy data where they are available, to determine whether the models provide realistic results. Paleoclimate proxy data are most often derived from moreaccessible low to mid latitude regions and are biased towards warm climate states. However, General Circulation Models (GCMs) have traditionally been biased to colder temperatures, in particular at high latitudes, struggling to maintain the high latitude regions warm enough to sustain forests that were present during greenhouse periods, such as the mid-Cretaceous (~110-90 Ma), without exaggerated warming of the equatorial regions. To improve this approach the HadCM3L coupled atmosphere-ocean GCM, a state-of-the-artmodel for the long simulations required to reach an equilibrium climate, has been run for each Stage of the Cretaceous using new paleogeographic basemaps. Here, we compare the results for the Aptian (118.5 Ma) and Albian (105.8 Ma) with paleoclimate proxy data from the high northern latitudes in order to determine if the model produces viable results for this region. Paleoclimate analysis of fossil wood from conifer forests from Svalbard ofAptian-Albian age suggests that they grew in moist cool upland areas adjacent to warmer temperate lowland regions, probably with rivers and/or swamps present. Studies of conifers from the Canadian Arctic islands indicate that they grew under slightly cooler conditions than on Svalbard, similar to northern Canada today. The HadCM3L GCM results for Svalbard show that the dominant biome was evergreen taiga/montane forest with lowland temperate vegetation present during the Albian Stage possibly with an element of deciduous taiga/montane forest in the Aptian (both cold boreal forest with short hotsummers according to the Köppen-Geiger classification). The modelled Mean Annual Temperature (MAT) was ~-3.7oC at the sample sites with summer temperatures rising to a mean of ~18oC during the Albian. Mean Annual Precipitation (MAP) was ~571 mm. In the Canadian Arctic the model results indicate that the biomes were more mixed than on Svalbard. The Aptian biome was dominantly deciduous taiga/montane forest with temperate vegetation in low laying areas. The Albian landscape was dominated by evergreen taiga/montane forest with some elements of deciduous taiga. Both Stages wereclassified as cold boreal forest with short hot summers under the Köppen-Geigerclassification scheme. MAT was modelled to be ~-6.5oC at the sample sites with summer temperatures reaching a mean of ~13oC and MAP was ~406 mm. These results suggest that the HadCM3L GCM, coupled with updated paleogeographic maps, can produce a good match to the climate proxy data in these difficult-to-model high latitude areas.