RIVERS, GROUNDWATER AND SEDIMENT TRANSPORT ON A GREENHOUSE EARTH – NEW IDEAS FROM THE LATE CRETACEOUS

Numeric modeling of the climate system at the Cenomanian/Turonian boundary (Late Cretaceous) suggests that the relation between surface runoff and groundwater during warm geologic (“greenhouse”) times was very different from today. Today the global average of subsurface runoff (ground water, ~13,320 km3) is about 30% of surface runoff (~43,790 km3). About 2/3 of the total river discharge entering the sea today is from surface runoff, much of it originating as snow melt, and about 1/3 is supplied by ground water. GENESIS (v.2.0) simulations of the paleoclimate of the Cenomanian/Turonian, show a relation opposite to that of present, with subsurface runoff dominating over surface runoff both globally and regionally. In warm geologic times surface runoff is generated mostly by intense convective storms when the rainfall rate exceeds the rate of infiltration into the soil; snow melt is relatively unimportant. The simulations suggest that globally the groundwater runoff, most of which would feed into rivers, was about 6 times greater than the surface runoff. These differences have significant implications for weathering and river transport of sedimentary materials. The major difference in weathering in greenhouse versus icehouse times appears to be enhancement of chemical weathering in the former and mechanical weathering in the latter. Enhanced chemical weathering tends to reduce silicate minerals to clay, whereas mechanical weathering tends to result in coarser detrital grains. Steadier flow in rivers during greenhouse times would be able to transport suspended load throughout the year, but more variable flow during icehouse times means that there would be more deposition and re-erosion in drainage basins.