Modern Hydrothermal Systems and the Oxygen Isotopic Evolution of Seawater

The oxygen isotope record of cherts and carbonates shows rocks that are increasingly depleted in 18O going back in time. If these data are used as a proxy for paleotemperatures, this would imply surface water temperatures of approximately 70°C during the Archean (3.3 Ga), and elevated temperatures (>45°C) until as recently as 350 Ma; these high temperatures are hard to reconcile with the geologic record, in particular the onset of well-documented glacial episodes. The secular trend in the oxygen isotope record can easily be explained by a change in the oxygen isotopic composition of seawater. Such changes in composition could result from an increase in ridgecrest depth with time, in addition to a modest rise in sea level with time. This change in physical chemistry within a hydrothermal system would tend to accelerate high- temperature rock-water interaction, thus affecting the critical point and convection within the system. This ridgecrest hypothesis has recently been explored in an ideal, 1-D case, and has not been tested in multiple dimensions. New drill core data also provide an opportunity to study temperature and pressure conditions of modern ridges with increasing depth. I propose to test this ridgecrest hypothesis in multiple dimensions using a finite element heat and mass transfer model (FEHM) in which an axisymmetric ridge is assumed and the depth of water above the ridgecrest is varied. Results of this modeling will then be compared with trends in modern hydrothermal systems using new data from the Ocean Drilling Program (ODP) and the Iceland Deep Drilling Program (IDDP). In particular, two contrasting environments are to be used—data from deeply submerged ridgecrests of the deep ocean, and new data from Reykjanes Ridge, Iceland in which the ridge is subaerial and ridgecrest depths can be varied spatially.