CHARACTERIZING THE GEOLOGY OF A VERNAL POOL: A CASE STUDY FROM FOUR MILE CREEK, OHIO

Vernal pools are seasonal wetlands, typically underlain by low-permeability sediments, that generally form during the spring due to enhanced precipitation. Vernal pools provide important ecosystem services such as water filtration, carbon sequestration, and are habitat for diverse species. However, in the Midwestern U.S., there has been a significant loss of vernal pools and other wetland environments. Significant restoration efforts of streams and wetlands are underway but are limited by a poor understanding of riparian wetlands, including vernal pools, prior to European settlement. To map the spatial distribution, determine the controls on formation, and assess temporal stability of pre-European settlement riparian wetlands in a host of environments, it is necessary to identify the characteristics of such deposits. We investigated the sedimentology and geochemistry of a small (~3000 cm x 3000 cm) vernal pool in the floodplain of Four Mile Creek, Ohio. At the base of a 150 cm sediment profile is a fluvial cobble conglomerate (150-140 cm), followed by ~20 cm of light brownish gray (2.5Y 6/2) clayey silt. From a depth of 120 cm to 60 cm the sediments are a very dark greyish brown (2.5Y 3/2) and have an organic carbon concentration of ~2%. The uppermost 60 cm of the profile, above a historic tile drain, is predominately silt and are grayish brown (10YR 5/2). AMS radiocarbon analyses of charcoal at depths of 120 cm and 57 cm yield ¹⁴C ages of 1,720+/-20 and 295+/-20 ¹⁴C years BP. The sediments are non-calcareous, except for the lower 20 cm. Elemental analysis of the sediments with a portable XRF identified large fluctuations in redox sensitive elements such as manganese, with concentration as low as 255 pm at 135 cm depth to 1,365 at 105 cm depth. Pollen analysis from samples at depths of 120 cm and 50 cm showed low concentrations of pollen grains (<1,000 grains per gram), indicating significant loss of pollen grains due to wetting and drying. In general, geologists look for green mudstones, marls, and high organic-carbon concentrations in association with wetland pollen assemblages and micro-fauna such as ostracods to identify wetland deposits. It is important for geologists to learn to recognize seasonal wetlands such as vernal pools fed by precipitation, and not groundwater, to characterize the full range of wetland habitats.