HOLOCENE CLIMATIC AND HUMAN INFLUENCES ON LAKES OF THE YUCATAN PENINSULA, MEXICO - AN INTERDISCIPLINARY, PALEOLIMNOLOGICAL APPROACH

We used palaeolimnological techniques to examine effects of Holocene c limate change and human influence on lakes of the Yucatan Peninsula, M exico. The three study lakes are located along a west-east transect th at represents a gradient of increasing modern precipitation and densit y of former Maya settlement. At Lake Coba, an 880-cm sediment core yie lds a complete record of lacustrine sedimentation that began when the lake first filled similar to 8000 BP as groundwater level rose in resp onse to rising sea level and increased precipitation. Diatom, ostracod , and delta(18)O evidence indicate that Lake Coba was initially shallo w and saline. Coba, presently in the region of greatest rainfall, show ed more episodic water-level changes than Lake San Jose Chulchaca or L ake Sayaucil. High lake level and fresh water were evident at 440 cm ( similar to 2600 BP), followed by a decline in water levels and an incr ease in total ionic salinity to the present time. In a 613-cm core fro m Sayaucil, in the intermediate precipitation zone, total salinity was high between 600 and 400 cm (similar to 3050 and 2000 BP), followed b y consistently higher water levels. Salinity was high in the lower por tion of a 110-cm San Jose Chulchaca core (beginning similar to 1860 BP ), followed by a gradual and consistent freshening of water to the pre sent time. Trophic state changes and human influence on lakes were eva luated using diatom, delta(18)O, total P, sedimentary organic matter, and preliminary pollen data. Maximal human disturbance at Lake Coba, a densely settled Maya urban site, occurred during a deep-water event a t 440 cm, followed by a decline in human influence and trophic state t o the present time. Trophic state and linear sedimentation rates in Sa yaucil increased significantly above 400 cm (after similar to 2000 BP) , probably associated with initial Maya settlement near Xtojil and sub sequent small-scale farming. Limnological disturbance may have precede d the period of maximal human occupation because initial land clearanc e and consequent soil erosion probably affected water quality substant ially. San Jose Chulchaca lacks archaeological evidence of human occup ation in the drainage, and shows gradual changes in trophic state not caused by human disturbance. With the exception of a C-14 date on wood from the base of the Coba core, C-14 dates and chronologies may be ar tificially old as a consequence of hard-water-lake error. Trophic stat e changes in the study lakes were generally consistent with known patt erns of human settlement and population change. Late-Holocene water-le vel fluctuations were most pronounced in Lake Coba in eastern Yucatan, where modern rainfall is currently greatest, but lake level is genera lly lower than during much of the past. Lake level was relatively cons tant in Sayaucil in the central peninsula, whereas lake level in San J ose Chulchaca in the arid western portion of the peninsula increased g radually over time.