Paleoenvironmental analysis using thecamoebians and foraminifera in Mexican anchialine caves: a focus on Aktun Ha (Carwash) Mexico

Abstract

<p>Thecamoebians (testate amoebae) and foraminifera have never been systematically investigated in a phreatic cave system. Foraminifera (marine to brackish water protists) and thecamoebians (brackish to freshwater protists) are cosmopolitan and have been studied in virtually all environments on Earth. Although foraminifera-bearing sediment has been observed from vadose cave environments in the last 20 years, there is no mention of their ecology - or mere existence - in flooded cave passages in any micropaleontology or microbiology textbooks. Only one published manuscript briefly mentions foraminifera in preliminary test samples from phreatic caves in Bermuda. The unprecedented ability of these organisms to colonize any environment on Earth led to the hypothesis that they are also exploiting global phreatic caves.</p> <p>The karst landscape of the Yucatan Peninsula, Mexico, is an ideal setting for the formation of anastomosing, anchialine, phreatic cave systems. The stratified aquifer contains passages in both the superior freshwater lens (thecamoebian habitat} and marine water (foraminifera habitat) intruding below, creating an ideal location to test the hypothesis. Several important contributions have resulted from investigating surface samples and cores in this region. (1) Using thirty-three surface samples from three different cenotes, thecamoebians and foraminifera were found living in the sinkhole (cenote) environment, which are the physical entrance into the caves. An ecological boundary of -3.5 ppt was determined for thecamoebians, with Centropyxis constricta "aerophilia" determined as the most euryhaline thecamoebian taxa. (2) Foraminifera and thecamoebians have been colonizing Carwash Cave since the Sangamon Interglacial (MIS 5e), based on seventy-five surface samples collected throughout the system. The recovered taxa have responded to changes in cave environment similarly to other coastal systems, thereby indicating their potential as paleoenvironmental proxies in phreatic caves. (3) In conjunction with stable isotopes (δ¹³C_org and δ¹⁵N_org), thecamoebians and foraminifera were successful recorders of mid-late Holocene paleoenvironmental evolution in Carwash Cave. An important salinity transition in the freshwater lens (from >2 ppt to 1.5 ppt) of the aquifer at ~2760 Cal yrs BP is coincident with changes in the local Maya populations, indicating a possible environmental linkage between ancient freshwater resources and Maya cultural genesis. This thesis demonstrates unequivocally that thecamoebians and foraminifera exist in phreatic cave systems and hold wider potential as paleoenvironmental proxies in other global phreatic caves.</p>