LATE HOLOCENE (3500 yBP) SALINITY CHANGES AND THEIR CLIMATIC IMPLICATIONS AS RECORDED IN AN ANCHIALINE CAVE SYSTEM, OX BEL HA, YUCATAN, MEXICO

Abstract

<p>Foraminifera and thecamoebians have been used extensively in coastal reconstructions and intertidal zonations. Recent work has shown that their distributions in anchialine cave systems are effective at characterizing modern hydrologic conditions and reconstructing climatic fluctuations over the mid-late Holocene. This thesis focuses on the lower extent (within 2 km of the Caribbean coast) of the Ox Bel Ha cave system in Tulum, Quintana Roo, Mexico.</p> <p>Surface sediment samples and sediment push cores were collected along a shore-normal transect using SCUBA techniques. The samples were analyzed for foraminifera and thecamoebian populations to establish: (1) Their modern distributions in different locales in the cave system including open water cenotes, main cave passages, and secondary cave passages; and (2) Their transitions through the late Holocene (3500 yBP) in response to climatic fluctuations, particularly precipitation. Hydrochemical parameters (specific conductivity, luminescent dissolved oxygen, pH, and temperature) of the modern water column were obtained to complement the microfossil assemblages in the surface samples. The sediment push cores were age-constrained using radiocarbon techniques and organic matter geochemistry (δ¹³ C, δ¹⁵N, TOC, TN, C/N) was used to determine the dominant source of sediment input.</p> <p>Salinity is the main controlling factor on the presence of foraminifera and thecamoebians; however the sedimentation patterns in the cave system had a significant role in sample biasing. In the open water and main cave conduit, sedimentation rates were higher (~0.22mm yr^-1) and the microfossil assemblage showed high abundances of Ammonia tepida and Elphidium gunteri, with a minor component being comprised of brackish thecamoebians Centropyxis aculeata and C. constricta. In the deeper areas of the cave where sedimentation rates were considerably lower the samples contained a more heterogeneous mix of A. tepida, E. gunteri, C. aculeata, and C. constricta. This biasing is important to consider when interpreting the core data.</p> <p>The core records showed four distinct phases of climatic changes which are closely tied to fluctuations in precipitation over the late Holocene. Periods of increased precipitation resulted in higher turbulent mixing at the interface between the superior fresh meteoric water and the basal saline marine water (halocline), resulting in more brackish conditions in the groundwater overall. Alternatively, lower precipitation resulted in a freshening of the groundwater due to the decreased turbulent mixing at the halocline. In addition, periods with high fluctuations in precipitation caused a heterogeneous mix of microfossils as a result of poor time averaging due to the low sedimentation rates. This inverse relationship has important implications in understanding the different periods of the ancient Maya culture, particularly the transition between the Classic and PostClassic Periods.</p>