Precipitation rates and atmospheric heat transport during the Cenomanian greenhouse warming in North America: Estimates from a stable isotope mass-balance model

David F. Ufnar, University of Southern Mississippi


Stable isotope mass-balance modeling results of meteoric 6180 values from the Cenomanian Stage of the Cretaceous Western Interior Basin (KWIB) suggest that precipitation and evaporation fluxes were greater than that of the present and significantly different from simulations of Albian KWIB paleohydrology. Sphaerosiderite meteoric 6180 values have been compiled from the Lower Tuscaloosa Formation of southwestern Mississippi (25 degrees N paleolatitude), The Dakota Formation Rose Creek Pit, Fairbury Nebraska (35 degrees N) and the Dunvegan Formation of eastern British Columbia (55 degrees N paleolatitude). These paleosol siderite delta(18)O values define a paleolatitudinal gradient ranging from -4.2 parts per thousand VPDB at 25 degrees N to -12.5 parts per thousand VPDB at 55 degrees N. This trend is significantly steeper and more depleted than a modern theoretical siderite gradient (25 degrees N: -1.7 parts per thousand; 65 degrees N; -5.6 parts per thousand VPDB), and a Holocene meteoric calcite trend (27 degrees N: -3.6 parts per thousand: 67 degrees N; -7.4 parts per thousand VPDB). The Cenomanian gradient is also comparatively steeper than the Albian trend determined for the KWIB in the mid- to high latitudes. The steep latitudinal trend in meteoric delta(18)O values may be the result of increased precipitation and evaporation fluxes (amount effects) under a more vigorous greenhouse-world hydrologic cycle. A stable-isotope mass-balance model has been used to generate estimates of precipitation and evaporation fluxes and precipitation rates. Estimates of Cenomanian precipitation rates based upon the mass-balance modeling of the KWIB range from 1400 mm/yr at 25 degrees N paleolatitude to 3600 mm/yr at 45 degrees N paleolatitude. The precipitation-evaporation (P-E) flux values were used to delineate zones of moisture surplus and moisture deficit. Comparisons between Cenomanian P-E and modern theoretical siderite, and Holocene calcite latitudinal trends shows an amplification of low-latitude moisture deficits between 5-25 degrees N paleolatitude and moisture surpluses between 40-60 degrees N paleolatitude. The low-latitude moisture deficits correlate with a mean annual average heat loss of 48 W/m(2) at 10 degrees N paleolatitude (present, 8 W/m(2) at 15 degrees N). The increased precipitation flux and moisture surplus in the mid-latitudes corresponds to a mean average annual heat gain of 180 W/m(2) at 50 degrees N paleolatitude (present, 17 W/m(2) at 50 degrees N). The Cenomanian lowlatitude Moisture deficit is similar to that of the Albian, however the mid-latitude (40-60 degrees N) precipitation flux values and precipitation rates are significantly higher (Albian: 2200 mm/yr at 45 degrees N; Cenomanian: 3600 mm/yr at 45 degrees N). Furthermore, the heat transferred to the atmosphere via latent heat of condensation was approximately 10.6x that of the present at 50 degrees N. The intensified hydrologic cycle of the mid-Cretaceous greenhouse warming may have played a significant role in the poleward transfer of heat and more equable global conditions. Paleoclimatological reconstructions from multiple time periods during the mid-Cretaceous wit[ aid in a better understanding of the dynamics of the hydrologic cycle and latent heat flux during greenhouse world conditions. Published by Elsevier B.V.