STABLE ISOTOPIC PALEOCLIMATE RECORD IN CALCAREOUS TUFA FROM MALANAPHY SPRINGS STATE PRESERVE, WINNESHIEK COUNTY, IOWA, USA

The stable isotope ratios of carbon (δ¹³C), hydrogen (δD), and oxygen (δ¹⁸O) in geologic archives are valuable indicators of climate and hydrological change. Specifically, δD and δ¹⁸O from precipitation can serve as indicators of hydroclimate because isotopic fractionation is dependent on the source signal and temperature. Tufa is a porous CaCO₃ deposit that precipitates from degassing bodies of water (often associated with karst deposits) and often aggregates near stream gradients or waterfalls. The δ¹³C and δ¹⁸O isotopic signature from tufa, in conjunction with radiocarbon analysis (or Uranium/Thorium dating), can be used to construct a climate/hydroclimate record for the geographic area in which they are found. As part of an NSF funded REU (Research Experience for Undergraduates) project, tufa mounds at Malanaphy Springs State Preserve were cored during the summer of 2012. The objectives of this study were to: 1) complete an initial analysis of the field location; 2) determine the viability of coring tufa at this site; and 3) construct a preliminary record of isotopic composition (δ¹³C and δ¹⁸O) within the sampled tufa deposits. Tufa and spring water samples were analyzed for isotopic ratios at Iowa State University. In the upper 900 mm of the tufa deposit δ¹³C had a mean value of ^-3.832 ± 0.272 ⁰⁄₀₀ and δ¹⁸O fluctuated around a mean of ^-5.847 ± 0.143 ⁰⁄₀₀. The δ¹³C and δ¹⁸O values from the upper platform core yielded co-registered isotopic shifts in the upper 100 mm, however this pattern was not evident throughout the entire record. The trend in δ¹⁸O over the upper 900 mm may represent a shift in the hydrological cycle, perhaps representing the last 1000 years (pending radiocarbon analysis), while the variability in δ¹³C likely reflects changes in the vegetation. Spring outlet temperatures remained nearly constant at 9.4 ± 0.025 °C, hence we suggest that the isotopic record derived from the tufa most likely represents changes in hydroclimate.