TWO CASE STUDIES OF HYDROLOGIC MODIFICATION OF AQUATIC RECORD OF CLIMATE PRESERVED IN LAKE SEDIMENTS

Hydrologic processes often modify the aquatic record of climate signal in terms of its magnitude and even its sign. We use stable isotope data in this study of two semi-permanent wetlands with different hydrology to illustrate these relations. The wetlands are located within 30 meters of one another and receive water in a similar way, but one (P1) loses nearly all of its water to evaporation whereas the other (P8) loses some water to surface and subsurface outflow in addition to evaporation. These two wetlands represent simple hydrologic settings within the same terrain responding to identical climate influences. The modern environment is characterized by offsets in stable carbon and oxygen isotopes of the water, likely due in part to different residence times. Stable isotope data from short sediment cores collected from the center of each wetland also reveal records reflecting the system hydrology rather than the shared climate. The stable isotope data from the youngest carbonate sediments is consistent with the δ¹³C DIC and δ¹⁸O water from the modern time series in the two wetlands. The stable isotope profiles from the two cores maintain a systematic offset downcore that at times is consistent with modern relations and in conflict at other times. In the youngest sediments, the two wetlands have diverging oxygen isotope values that if interpreted literally as climate would imply a simultaneous increase and decrease in moisture at the same location. Covariance of δ¹³C and δ¹⁸O data in the core from P1 and a general lack of covariance in P8 reflect the importance of evaporation in P1 over that in P8. The study of these two wetlands demonstrates the difficulties in reconciling the climate influence and the sediment record. Interpreting climate and climate variability depends on a thorough understanding of the modern internal lake processes.