The mid-Cretaceous water bearer: isotope mass balance quantification of the Albian hydrologic cycle
A latitudinal gradient in meteoric N 18 O compositions compiled from paleosol sphaerosiderites throughout the Cretaceous Western Interior Basin (KWIB) (34^75‡N paleolatitude) exhibits a steeper, more depleted trend than modern (predicted) values (3.0x [34‡N latitude] to 9.7x [75‡N] lighter). Furthermore, the sphaerosiderite meteoric N 18 O latitudinal gradient is significantly steeper and more depleted (5.8x [34‡N] to 13.8x [75‡N] lighter) than a predicted gradient for the warm mid-Cretaceous using modern empirical temperature^N 18 O precipitation relationships. We have suggested that the steeper and more depleted (relative to the modern theoretical gradient) meteoric sphaerosiderite N 18 O latitudinal gradient resulted from increased air mass rainout effects in coastal areas of the KWIB during the mid-Cretaceous. The sphaerosiderite isotopic data have been used to constrain a mass balance model of the hydrologic cycle in the northern hemisphere and to quantify precipitation rates of the equable ‘greenhouse’ Albian Stage in the KWIB. The mass balance model tracks the evolving isotopic composition of an air mass and its precipitation, and is driven by latitudinal temperature gradients. Our simulations indicate that significant increases in Albian precipitation (34^52%) and evaporation fluxes (76^96%) are required to reproduce the difference between modern and Albian meteoric siderite N 18 O latitudinal gradients. Calculations of precipitation rates from model outputs suggest mid^high latitude precipitation rates greatly exceeded modern rates (156^220% greater in mid latitudes [2600^3300 mm/yr], 99% greater at high latitudes [550 mm/yr]). The calculated precipitation rates are significantly different from the precipitation rates predicted by some recent general circulation models (GCMs) for the warm Cretaceous, particularly in the mid to high latitudes. Our mass balance model by no means replaces GCMs. However, it is a simple and effective means of obtaining quantitative data regarding the mid-Cretaceous hydrologic cycle in the KWIB. Our goal is to encourage the incorporation of isotopic tracers into GCM simulations of the midCretaceous, and to show how our empirical data and mass balance model estimates help constrain the boundary conditions.