DID GROUNDWATER STRATIFICATION DURING DROUGHT PROVIDE POTABLE GROUNDWATER IN THE NORTHERN YUCATAN PENINSULA DURING THE MAYA TERMINAL AND POST-CLASSIC?

Paleoclimate data from closed basin lakes on Mexico's Yucatan peninsula shows drought occurring in the Maya Lowlands: the drought coincides with the Terminal Classic Maya Collapse and has been ascribed to the southern migration of the ITCZ. However, drought as the trigger for the collapse has been controversial since Mayan cities in the Northern Lowlands (vs Southern), which should have been affected first by drought, continued to persist into the Terminal and Post-Classic periods. Here, we present microfossil (foraminifera and thecamoebian) data from aquatic cave sediment cores (n=5; Ox Bel Ha and Aktun Ha cave systems near Tulum) from the Northern Lowlands that may explain the observed pattern of population decline. Results from the cave sediments span the last 3500 yrs and show changes in the salinity of the meteoric lens with fresher conditions commencing in the Terminal Classic and continuing into the Post-Classic period where drought is the most pronounced. During drought periods, the density stratification of the meteoric and marine water masses ensures that potable groundwater remains available in the shallow subsurface, and may in fact have decreased in salinity. While drought caused a decreased thickness of the meteoric water, it also resulted in reduced flow and less turbulent mixing with the underlying marine water within the flooded cave conduits and may account for our observed freshening of the near surface meteoric water. In contrast, during wet periods, increased flow caused increased turbulent mixing with underlying marine water thus increasing its salinity and decreasing its potability. This may have varied in effect from area to area, and emphasizes that regional patterns of water resources may have allowed some population centers to survive the Classic Maya Collapse, particularly in the Northern Lowlands.