THE SOLUTE ECOTONE, A KEY TO PAST HYDROLOGY

An ecotone, which typically develops along one or more environmental gradients, is a biologic gradient where species change from one community to another. Accordingly, distinctive ecotones among aquatic species, such as mollusks and ostracodes, are produced by changes in solute composition. The solute composition (or dominant ions) of natural waters includes bicarbonate-carbonate, sulfate, and chloride anions, and sodium, calcium, and magnesium cations. The sum of ion concentration is commonly expressed as total dissolved solids (TDS) in mg per liter. Three major solute types exist that are of biologic importance and the solute ecotone between them generally occurs between 1,500 and 3,000 mg/L TDS, although it may form at lower TDS values.

Solute composition is commonly determined by hydrologic setting and process, local geology, and climate variation. Because the distribution of many mollusks is linked to solute composition along the ecotone, a change in waters within the solute ecotone can affect the occurrence of these taxa. Therefore, the presence of aquatic mollusks in sediment cores may be used to help interpret paleohydrology and paleoclimate by mapping modern hydrochemical fields to estimate changes in calcium, (bi)carbonate and TDS over time. Hence, change in aquatic molluscan assemblages in sediment cores may be used to describe past changes in inflow or outflow and hydrologic characteristics.