TEMPORAL VARIATIONS IN SEDIMENT FLUX TO THE WESTERN GULF OF MEXICO OVER MULTIPLE GLACIAL-INTERGLACIAL CYCLES

Temporal variations in sediment flux are important indicators of landscape response to changes in climate, tectonics, and threshold mechanisms. Establishing prehistoric long-term sediment flux rates from volumetric records is typically problematic due to difficulties in accounting for material absent from the surveyed deposit. Terrestrial cosmogenic nuclides (TCN) are being used to measure sediment flux from two non-glaciated and tectonically quiescent river systems (Colorado and Trinity Rivers) draining into the western Gulf of Mexico. This study will provide insight into the magnitude of sediment flux variability over the past 5 million years and quantify the response of non-glaciated catchments to glacial-interglacial climate changes.

Deposits already chronostratigraphically associated with specific glacial or interglacial intervals in the past 200 ka are the main focus of the climate-response portion of this study; however, older deposits are also being analyzed. Preliminary results have been acquired for one TCN depth profile in the Pleistocene Lissie (ca. 620 ka) surface along the Colorado River. A partially shielded depth profile consisting of five regularly-spaced samples of quartz grains (355-500 μm) was constructed from unmixed fluvial sediment excavated from a gravel pit. Cosmogenic ¹⁰Be concentrations were measured with 2σ precisions of ~5% and were used to determine an average inherited concentration of 1.6 ± 0.1 x 10⁵ atoms g^-1. Monte Carlo simulation of TCN concentration vs. depth allowing parallel variability in erosion rate, exposure age, bulk density, and inheritance indicate an insensitivity of inheritance to these other parameters. This inherited concentration corresponds to a basin-wide average erosion rate of ~0.04 ± 0.02 mm a^-1.