WETLAND HYDROLOGY AND OSTRACODE SHELL δ18O VARIABILITY

Water and ostracodes in 2 semi-permanent wetlands, P1 and P8, in Cottonwood Lakes, ND, were monitored for 3 years to study the relation between isotopes from ostracode shells and hydrology. Ostracode shells in sediment cores were then analyzed to see if variation found in modern shells existed in the past. These two wetlands are supported largely by rain and snowmelt, but groundwater (GW) contributes to part of their hydrologic budgets. Both reside in glacial till with low GW flow, however a small sand lens allows more GW transmission in P8. P1 loses most of its water by evaporation, whereas P8 loses water to GW outflow and evaporation, and overland flow during deluge. Accordingly, the δ¹⁸O of P1 water is higher than P8. P8, unlike P1, has a thick fringe of cattails and sedges, creating isolated pockets of water. P8 receives GW from prairie and a pasture, thus containing several isotopically distinct local habitats for ostracodes. The δ¹⁸O of P1 and δ¹⁸O of individual sample sites in P8 increased by 4~6‰ from May to September, following snowmelt influx. Ostracode shell production commonly occurs during the warm water seasons, so these are most relevant to understanding what the δ¹⁸O shell values reflect.

δ¹⁸O of calcite shells of adult Cypridopsis vidua, an ostracode common in P1 and P8, showed wide variation. C. vidua is thought to require T>15°C to grow and reproduce. When T is above 15°C, C. vidua may go through a complete life cycle in just weeks. Each generation thus records the δ¹⁸O of coeval water via T-dependent isotope fractionation during the shell calcification following the final molt, so C. vidua from P1 showed δ¹⁸O variation similar to that of water, with an expected isotopic offset. C. vidua from P8 also showed a similar range for each sample site but an aggregate range of 8~10‰.

C. vidua shells in 1-cm thick sediment slices from P1 and P8 cores also showed a 4~6‰ range with offsets between two cores consistent with modern population. Thus, ostracode shells in the cores record the seasonal variability of these wetlands over the time span represented by 1-cm thick layers (several years to a decade). δ¹⁸O of C. vidua in these cores then records the system's hydrologic variability, which in turn is one step removed from the local climate variability; the climate signal is modified when it is transmitted through particulars of site hydrology.