PALEOCLIMATIC SIGNIFICANCE OF STABLE-ISOTOPE AND TRACE ELEMENT VARIATIONS IN SPRING-DERIVED TUFA DEPOSITS FROM BELIZE, CROATIA, AND EGYPT

Spring-derived tufa deposits, widespread in the Quaternary sedimentary record, are demonstrated archives of paleoclimatic information. However, the nature of the transfer functions relating tufa chemistry to paleoenvironment may vary with both time, space, and, confoundingly, climate. In order to examine geochemical variation in tufas across a range of climatic conditions, we analyzed the stable isotope (δ¹⁸O and δ¹³C), minor and trace element (Mg/Ca, Sr/Ca, Ba/Ca) geochemistry of modern tufas from the Mopan River, Belize (tropical), the Krka River, Croatia (temperate), and Pleistocene tufas from Kharga Oasis, Egypt (arid to semi-arid). Oxygen and carbon isotope ratio variability was significantly different between the three field sites, though Belizean and Croatian tufas show significantly less variation (~1.5‰ in oxygen and ~1‰ in carbon) than those from Egypt (~5.7‰ in oxygen and ~5.4‰ in carbon). Similarly, minor element concentrations demonstrate the least variation in Belizean tufas, while the largest ranges are seen in the Egyptian tufas.

Significant correlations exist between these geochemical parameters but the relationships are not consistent across all three field sites. Tufas from Belize demonstrate a negative covariance between Mg/Ca and δ¹⁸O, while tufas from Croatia and Egypt demonstrate no significant relationship between these two factors. Therefore, only the Belizean tufas show potential for Mg thermometry. Belizean tufas also exhibit a negative covariance between δ¹⁸O and δ¹³C, which is likely tied to along-flow processes. Conversely, the Croatian and Egyptian tufas demonstrate a positive covariance, probably due to degassing and, at least in the Egyptian samples, evaporation. The high variation in Egyptian tufa δ¹⁸O also suggests evaporative effects played a significant role in controlling O-isotope variation. All sites display a positive covariance between Mg/Ca, Sr/Ca, and Ba/Ca, though the slopes of the relationships are significantly different between localities. This relationship may derive from aquifer processes and thus is probably tied qualitatively to paleorainfall. By utilizing tufa samples from a wide range of climate conditions, we can identify the non-climatic sources of variation in these geochemical parameters.