Robert Michael Deconto

Alternative global Cretaceous paleogeography

Plate tectonic reconstructions for the Cretaceous have assumed that the major continental blocks—Eurasia, Greenland, North America, South America, Africa, India, Australia, and Antarctica—had separated from one another by the end of the Early Cretaceous, and that deep ocean passages connected the Pacific, Tethyan, Atlantic, and Indian Ocean basins. North America, Eurasia, and Africa were crossed by shallow meridional seaways. This classic view of Cretaceous paleogeography may be incorrect. The revised view of the Early Cretaceous is one of three large continental blocks— North America–Eurasia, South America–Antarctica-India-Madagascar-Australia; and Africa—with large contiguous land areas surrounded by shallow epicontinental seas. There was a large open Pacific basin, a wide eastern Tethys, and a circum- African Seaway extending from the western Tethys (“Mediterranean”) region through the North and South Atlantic into the juvenile Indian Ocean between Madagascar-India and Africa. During the Early Cretaceous the deep passage from the Central Atlantic to the Pacific was blocked by blocks of northern Central America and by the Caribbean plate. There were no deep-water passages to the Arctic. Until the Late Cretaceous the Atlantic-Indian Ocean complex was a long, narrow, sinuous ocean basin extending off the Tethys and around Africa. Deep passages connecting the western Tethys with the Central Atlantic, the Central Atlantic with the Pacific, and the South Atlantic with the developing Indian Ocean appeared in the Late Cretaceous. There were many island land areas surrounded by shallow epicontinental seas at high sea-level stands.

Late Cretaceous climate and vegetation interactions: Cold continental interior paradox

The Campanian age of the Late Cretaceous was warm, with no evidence for permanent or seasonal sea ice at high latitudes. Sea level was high, creating extensive epicontinental and shallow shelf seas. Very low meridional thermal gradients existed in the oceans and on land. Campanian (80 Ma) climate and vegetation have been simulated using GENESIS (Global ENvironmental and Ecological Simulation of Interactive Systems) Version 2.0 and EVE (Equilibrium Vegetation Ecology model), developed by the Climate Change Research section of the Climate and Global Dynamics division at NCAR (National Center for Atmospheric Research). GENESIS is a comprehensive Earth system model, requiring high resolution (2^circ by 2^circ) solid earth boundary condition data as input for paleoclimate simulations. Boundary condition data define certain prescribed global fields such as the distribution of land-sea-ice, topography, orographic roughness, and soil texture, as well as atmospheric chemistry, the solar constant, and orbital parameters that define the latitudinal distribution of solar insolation. A comprehensive, high resolution paleogeography has been reconstructed for the Campanian. The paleogeography, based on a new global plate tectonic model, provides the framework for the solid earth boundary conditions used in the paleoclimate simulation. Because terrestrial ecosystems influence global climate by affecting the exchange of energy, water and momentum between the land surface and the atmosphere, the distribution of global vegetation should be included in pre-Quaternary paleoclimate simulations. However, reconstructing global vegetation distributions from the fossil record is difficult. EVE predicts the equilibrium state of plant community structure as a function of climate and fundamental ecological principles. The model has been modified to reproduce a vegetation distribution based on life forms that existed in the Late Cretaceous. EVE has been applied as a fully interactive component of the Campanian simulation. 1500 ppm CO_2 and a QFACTOR of 4 were sufficient to maintain forest over Antarctica and high northern latitudes. The QFACTOR is the multiplicative of the oceanic heat diffusion coefficient in the slab-mixed layer ocean component of GENESIS. The simulated Campanian oceanic heat transport has maximum values of about 1.7 times 10^{15} W at 25 ^circ north and 2.6 times 10^{15} W at 25^ circ south, similar to present day observed values. Late Cretaceous forests played an important role in the maintenance of low meridional thermal gradients, polar warmth, and equable continental interiors. The Campanian high to polar latitude forests decreased surface albedo (especially in late winter-early spring, prior to snow melt), and increased net radiation and fluxes of sensible and latent heat. This warmed the high latitude troposphere and increased atmospheric moisture. The warmer atmospheric temperatures reduced winter cooling of the high latitude sea surface and aided the advection of warm, moist air from the oceans into the continental interiors.

Mass-Balanced Reconstruction of Overburden

The mass-balanced paleogeographic reconstructions technique determines the thickness of the rock eroded from the source areas to supply sediment to the sedimentary basins. The thickness of this eroded overburden and the time of its erosion are valuable inputs for calculations of the thermal history of the rocks and the maturation of hydrocarbons.