Groundwater Sr, Cl, and SO4 chemistry in the western and southern Yucatan Peninsula: insight into formation of Edzna Valley, Mexico

Strontium isotope chemistry, presented here, combined with previously obtained groundwater ion chemistry elucidates the stratigraphy of buried sedimentary rocks of the southern Yucatan Peninsula, Mexico. Whereas in groundwater of the northern Peninsula, ion chemistry is dominated by contact with a saline intrusion, the groundwater of southern Campeche and southern Quintana Roo has high SO4/Cl and Sr/Cl indicating extensive contact with beds of dissolving gypsum (and celestite) in the subsurface. The newlyobtained strontium isotope data presented here strengthen a conclusion of previous research, namely that, in marked contrast with the north, there is no saline intrusion in the southern Peninsula. Sr/Sr measurements confirm that much of the evaporite that is contributing ions to southern Peninsula groundwater has a KtEocene seawater isotope signature. A key conclusion from combined study of Sr, Sr isotopes, Cl, and SO4 is that a large geomorphic feature in western Campeche, the Valley of Edzna polje, was once occupied by a post-Eocene seawater incursion. with calcite and dolomite of the aquifer (Perry et al. 2002). Cl and SO4 typically behave conservatively except where redox processes are active within the aquifer. Thus, the ratio of SO4/Cl in seawater (100xSO4/Cl equivalent of about 10.3) indicates exchange between an upper lens of fresh groundwater and a deeper intrusion of modified seawater. Values significantly higher indicate dissolution of gypsum from evaporite. Low values, which are unusual, indicate sulfate reduction (Perry et al. 2002, 2009 & in press). Sr in groundwater has proven to be a reliable indicator of contact of the groundwater with an evaporite in this area (Perry et al. 2002). Apparently, celestite (SrSO4) is a ubiquitous minor mineral in evaporites of the region. Thus, a Sr/Cl ratio higher than the seawater ratio is an additional indicator of the presence of evaporite in the aquifer, and this criterion is useful even for deep groundwater of the saline intrusion. Strontium isotopes provide another useful tracer. Unless subsequently altered, Sr/Sr values of minerals precipitated from seawater record the Sr isotopic composition of the water from which they crystallized. Therefore, the changes in seawater Sr/Sr over time (McArthur et al. 2001) can be used to date marine chemical sedimentary rocks including evaporites and thus used as a stratigraphic tool. Figure 1. Geologic map of the Yucatan Peninsula (after Perry et al. 2009 & in press). Apparent Sr/Sr "ages" of groundwater reported in that paper are as shown in the legend. Location of water samples of this study are shown as open circles.