Palynology vs. model simulation: oceanographic reconstruction of incomplete data from the Cretaceous Greenland Norwegian Seaway

摘要

Dinoflagellate cyst-based analyses of the upper Albian to upper Maastrichtian successions in the Greenland-Norwegian Seaway (GNS), cores 6711/4-U-1 and 6707/10-1, show significant changes in assemblage composition. Palaeoenvirorunental reconstructions from the early Late Cretaceous are in general agreement with actual reconstructions indicating the highest eustatic sea-level and wannest sea surface temperatures near the Cenomanian-Turonian transition, with sea-level regression and cooling towards the end of the Cretaceous. The study of an incomplete Maastrichtian interval indicates short warming episodes that were superimposed on the global long-term cooling trend during the Late Cretaceous. The presence of Maastrichtian dinoflagellate cysts indicative of oceanic conditions reflects coeval deepening of the GNS. The region was also influenced by pre-rifting processes, resulting in palaeogeographical reconfigurations. Comparable late Campanian dinoflagellate cyst assemblages from the Western Interior and Greenland-Norwegian seaways suggest a marine connection between these regions despite an enhanced global marine regression during the Late Cretaceous. To better understand our palynological signal we use numerical climate model simulations examining the local current regime for the latest Cretaceous (winter and summer seasons) assuming an atmospheric carbon dioxide level of 4x pre-industrial (1120 ppm) and applying various palaeogeographical reconfigurations: 1) Western Interior Seaway (WIS) and Hudson Seaway (HUD) closed; 2) WIS opened and HUD closed; 3) WIS and HUD opened. The results demonstrate that GNS currents are dominated by warmer water-masses from the southwest during times when the GNS and WIS are connected. In addition, HUD surface currents show a tendency to drive WIS water eastward, i.e. towards GNS that most likely facilitated biogeographic species exchange. These findings support the palynological data suggesting that the WIS and GNS could have been connected through the HUD at least during the late Campanian. In this case, the seaway must have played a significant role in the exchange of water-masses and heat transport. Further restrictions between the Arctic Basin and the global ocean influenced by lowering sea- and salinity-levels could consequently be an important factor contributing to the latest Mesozoic global cooling.