THE LATE PLEISTOCENE-HOLOCENE LIMIT IN THE BASINS OF ZACAPU AND CUITZEO NORTH MICHOACÁN BASED IN THE DIATOM RECORD

Paleolimnological records of the Zacapu basin and the northern margin of lake Cuitzeo were studied using diatoms, TIC-TOC, X ray difractometry and magnetic susceptibility as proxies, with the objective of defining their evolution based in eight trenches.

The base of the Cuitzeo basin was dated at 23,380 a. The northern zone represented by Buenavista displayed more sandy facies, while more clayey silty facies were observed to the south

The base of the Zacapu basin was dated to 39,000 a, the diatom record being well preserved. Five phases of lake evolution were evidenced. The first phase displays low lake levels with high turbidity, conditions characterized by Aulacoseira granulata var. angustissima. During the second phase the lake reached its maximum water level, being dominated by Stephanodiscus niagarae followed by S. minutulus and S. excentricus, and by ticoplanktonic forms of Fragilaria tenera and Ulnaria ulna. A highly organic marsh with a trend for turbid waters was established during the third phase, in which Eunotia implicata and Hantzchia amphioxys are characteristic. The fourth phase shows a reestablishment of the lake with the ticoplanktonic species Fragilaria tenera and Ulnaria ulna. The last phase indicates a lake having a low water level, characterized by Gomphonema spp., Anomonoeis sphaerophora and Hantzchia amphioxys, which is associated to draining and anthropogenic activities.

Values of TIC-TOC for both basins agree with results from other proxies, with higher TIC during dry intervals or lower lake level and maximum TOC values corresponding to high productivity in deeper lakes.

During the late Quaternary, lake Cuitzeo extended further north than its present location and commonly presented marshy facies in sectors subject to high evaporation, while Zacapu remained to be a stable water body showing fluctuations towards the late Holocene with increased contributions from volcanic events. Both lakes were drained during Colonial and post-Colonial times.