What is the Geochemical Record of the Mixing-Zone?

When carbonate sediments are exposed to meteoric fluids during changes in sea level, the they become exposed to a number well described hydrological regimes, which include the vadose zone (a zone in which the pore space is... [ view full abstract ]

When carbonate sediments are exposed to meteoric fluids during changes in sea level, the they become exposed to a number well described hydrological regimes, which include the vadose zone (a zone in which the pore space is mainly filled with air), the fresh-water phreatic zone (the zone below the water table where the pore space is saturated with freshwater), the mixing-zone (a region in which the freshwater mixes with seawater), and the marine phreatic zone (the pore water is filled with marine derived fluids). Within these zones the original sediments are dissolved and then reprecipitated. The fluids and processes which take place within these zones impart characteristic C and O isotopic values which allows these zones to be recognized in ancient rocks. These changes have become a standard tool for defining diagenetic zone within ancient rocks. In particular the mixing-zone is supposed to be characterized by a covarying trend between C and O isotopic values in the diagenetic carbonates. However, in the case of the Bahamas there is a significant problem with the dimension of the diagenetic zones associated with the model, namely that during the last glacial period, sea level is estimated to have fallen by at approximately 120 m. Assuming a hydrological head similar to what is observed in South Florida of about 2 m, then the Ghyben-Herzberg relationship for a fresh-water lens predicts a freshwater water phreatic zone of about 80 m. This would place the top of the mixing zone at a depth of 200 m, and completely encompass the mixing-zone as defined by the covariation of C and O isotopic values. In fact the region of co-variation between the C and O isotopic values would correspond to the freshwater phreatic zone and the mixing-water zone would be located in a region of the core in which the C and O isotopic values were both relatively positive and which showed a relatively poor correlation between the two isotopes. I will present evidence that the covarying trends of C and O isotopic values do not represent the mixing-zone at all, but rather variable amounts of freshwater diagenesis associated with the interface between the phreatic and vadose zones.