GEOCHEMICAL ANALYSIS OF LAKE SEDIMENTS AS A PROXY FOR WATERSHED URBANIZATION, WHITE ROCK LAKE, DALLAS, TX

White Rock Lake (WRL) is a shallow (~3 m average depth) 4.4 km² human-made reservoir that was constructed in 1912 by damming White Rock Creek (WRC) in North-Central Texas, U.S.A. Over the past century WRL, and the upper WRC watershed, have undergone change from a rural agrarian to urban landscape. A 2.2 meter core collected in 2003, as well as several short (30 cm) cores collected in 2011, from WRL provide a nearly continuous, and age-calibrated, record of sedimentation over the past century and preserve progressive temporal trends in C/N and ¹³C/¹²C ratios of organic matter that appear to be concomitant with historical records of urbanization and its associated changes in land-use practices across the upper WRC watershed.

Sedimentary organic matter δ¹³C values in the WRL cores range from -24.6 to -15.8‰, and exhibit a progressive trend toward more negative values in younger sediments that begins in the late 1940s. Sedimentary organic matter C/N ratios in the WRL cores range from 6.8 to 27.2 and exhibit a trend toward lower values that begins in the late 1940s. In addition, weight percent organic carbon increases from ~2.2% in older sediments to ~4.5% in the youngest sediments. This decrease in weight percent organic carbon at greater depths may reflect that recently deposited material has not undergone microbial alteration, or alternatively, it may represent larger contributions of organic carbon to the lake system in more recent times.

All of the aforementioned shifts in the geochemical record of lake sediment organic matter record begin with the onset of urbanization across the watershed. Major changes in the stable carbon isotope values and C/N ratios of organic matter preserved in the White Rock Lake sediments begin near the same stratigraphic interval. The shift in both proxies suggests that the lake sediments preserve an increasing proportion of autochthonous organic carbon with decreasing stratigraphic depth, and provides evidence of a shift from C4-to C3-dominated vegetation across the landscape.