CHAPHOLO: Paleolimnological Evaluation of Lake Chapala (Western Mexico) During the Past 10,000 Years (CONACYT CB2011, Grant 168685, In Progress). PHASE I: Drilling Campaign.

Abstract:

Lake Chapala being the Mexican largest shallow (ca. 5 m) lake, of tectonic origin, is located in western Mexico. Based on magnetic profiles carried on by Alatorrre et al. (2007), the depocenter area, having an extension of ~ 10 km (E-W) x ~ 5 km (N-S), was delimited. The drilling campaign was carried out in April 2014, recovering a total of 51.50 m of sediment with a Usinger drilling machine (Ø 80 mm and 55 mm) and a Uwitec platform. To achieve efficient stratigraphic control at the site CHD (103° 02'W, 20° 15' N), 51.50 m of sediment were cored at three equidistant holes spread ca. 4 m: CHD-A (4.20 m), CHD-B (27.15 m) and CHD (20.15 m). The stratigraphic depth reached was 27.15 m. After recovery, the undisturbed sediment was extruded from iron tubes into PVC tubes and split longitudinally resulting one working half for laboratory work and the other as archive half. In total we obtained 103 "half tubes", that were protected with plastic film wrap and stored in plastic bags and keeping refrigerated at 4° C. The sediment collected is loose and its grain size is silty-clay, with the exception of volcanic material. The preliminary megascopic description of the sediment in CHD-A core based on color, presence of laminations, volcanic ash and black particles is as follows:

COLOR. (0 m-4.15 m) gray-brown; (4.15 m -22.15 m) dark gray; (22.15 m -27.15 m) gray-brown.

LAMINATION. (0m-11.15m) Incipient to defined; (11.15m-22.15m) Absent; (22.15m-23.15m) Incipient; (23.15m-27.15m) Absent.

VOLCANIC ASH. (0m- 27.15m) Few horizons mm in thickness. From 26.52 m - 26.66 m a thin layer of 14 cm was observed.

BLACK PARTICLES. (0m-27.15m) Presence at different concentration of black particles (cf. charcoal)

DIATOM & POLLEN. (0m-10m) Central diatom (Surirella sp and Cyclotella sp); Pinus sp.

The identification of different concentrations of charcoal is the opportunity to reconstruct the Holocene landscape dynamics in the watershed of Lake Chapala, regarding the fire history through anthracological studies. At the bottom (27.15 m) the conventional radiocarbon age is 9,740 BP. Thus a preliminary linear sedimetation rate is 2 mm yr^-1. We wish thank UDG and UNAM authorities for their support to the development of this first phase and Drs. Socorro Lozano and Margarita Caballero for their support for the Contract in Comodato.