THE HYDROLOGICAL CYCLE ON A GREENHOUSE EARTH AND ITS IMPLICATIONS – DIFFERENT FROM TODAY

Numeric modeling of the climate system of the Late Cretaceous suggests that the relation between surface runoff and groundwater during warm geologic (“greenhouse”) times was very different from today. About 2/3 of the total river discharge entering the sea today is from surface runoff and about 1/3 is supplied by ground water. For the Late Cretaceous, GCM- simulations show a relation opposite to that of present, with subsurface runoff dominating over surface runoff both globally and regionally. The simulations indicate that globally the groundwater runoff was about 6 times greater than the surface runoff. These differences in the amount of subsurface runoff should have severe consequences for a variety of geological and paleoceanographic factors. E. g. subsurface geological conduits play a critical role in ground water and nutrient supply to the margins of continents and islands. Recent studies have shown that the 226Ra signal provides a sensitive tracer of ground water input to the coastal ocean. Many other elements and ions are expected to have similar geochemistry, and should desorb into brackish ground water. Important constituents include Ba, Cs, phosphate, ammonia, and cadmium. Other elements may be enriched in brackish ground waters due to variable redox conditions. These include Mn, Fe, U, Cr, and V, as well as species adsorbed on Mn and Fe oxides. Concentrations of these elements in ground water may be orders of magnitude higher than in river water. The ground water flow may largely bypass the estuary filter and deliver the elements directly to the ocean. If ground water input is as high as the 226Ra data require, a re-evaluation of “paleo-terrestrial” fluxes to the oceans is required. Furthermore, ground water inputs will vary with sea level. During sea level rise, salt water intrusion into coastal aquifers will release desorbable components. Transition from glacial to interglacial conditions may be accompanied by fresh inputs of phosphate and barium to the ocean. Such inputs have not been considered for these important paleoceanographic tracers.