Saturday, 15 July 2006

Micromorphology and Stable Isotope Investigation of Lacustrine Sediments on the Southern High Plains of Texas and New Mexico.

Dusten Russell, Wayne Hudnall, and B.L. Allen. Texas Tech University, Department of Plant and Soil Science, Box 42122, Lubbock, TX 79409

We investigated the depositional processes of lacustrine sediments on the Southern High Plains, which existed during late Pliocene to early Pleistocene. Petrographic analyses and d18O and d13C isotope were used in this identification.

Isotopic values from soil carbonate have been used as identifiers of carbon whether derived from the atmosphere or from plants. Isotopes have also been used to identify climatic conditions and depositional processes for soils. Isotopic composition of lacustrine precipitates on the Southern High Plains is dependent upon the temperature, elevation, and condensation history of the air masses from the Gulf of Mexico, since this particular body of water is the primary source of moisture for this region (Soliz, 1996).

The stable isotopes measured were d18O and d13C. The d18O isotopes were analyzed to determine the climatic condition at the time of deposition of the lacustrine sediments. Oxygen isotopes provided information about paleoclimatic conditions through differences in atomic weights. During glaciated periods, d16O values are in higher concentrations than d18O within the glaciated ice. The d18O is concentrated in the atmosphere and deposited into lakes and rivers via precipitation. As climatic conditions changed from glaciated to non-glaciated periods, the ice melts releasing d16O into the atmosphere, reducing the concentrations of d18O in the lakes and rivers.

d13C isotope values were examined to establish plant fauna type, whether the plants present during the time of deposition were C3, C4, or CAM plants. C3 plants have d13C isotope values between -32 and -20 . C4 plants have a higher capability of CO2 uptake (Ehleringer et al., 1993) and exhibit d13C values ranging from -17 to -9 . The d13C values of CAM plants are between -25 and -11 . d13C isotopes can also be used to determine if the carbon present formed via lithogenic or pedogenic processes. Pedogenic carbonates have d13C values ranging from -10 to 0 , while lithogenic carbonates will have d13C values from -2 to +2 (Ryskov et al., 2000). Results of d13C values suggest that the lake was shallow, a couple of meters in depth, and carbonate deposition was due to evaporitic processes, which was emphasized by small grain size observed under a petrographic microscope. d13C values indicate pedogenic and lithogenic processes were operative within the geologic section. The results from the lake researched will further the general comprehension of the geological processes and development of the lithogenic and pedogenic processes of the Southern High Plains during the late Pliocene to early Pleistocene.

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