ISLAND-WIDE ISOTOPIC ANALYSIS OF METEORIC AND HYDROTHERMAL WATERS IN DOMINICA, LESSER ANTILLES

The island of Dominica (750 km²) is dominated by nine Pleistocene volcanic centers, has one of the highest mean annual precipitation rates (up to 10,000 mm yr^-1) and one of the highest stream densities (~365 streams) in the Lesser Antilles arc, and contains more than 30 hydrothermally active regions. In June 2014, an island-wide hydrothermal and meteoric water study was conducted for 71 bodies of water in Dominica. The main object of the study was to establish a baseline for field, geochemical, and isotopic compositions. Future deviations from this baseline could be indicative of volcanic unrest in the magmatic system. In June 2015 and June 2016, all locations were re-sampled. Between 2014 and 2015, pH levels increased in acidity by an average of 0.45, whereas all elemental, anion, and cation data did not systematically vary. In 2014, δ¹³C_DIC values at samples sites ranged from -21.6‰ to +2.0‰, and in 2015, values ranged from ­­­­­­-17.3‰ to +3.8‰ including both magmatic and biogenic water sources. Enriched magmatic carbon isotope values are localized to known hydrothermally active regions, suggestive of magmatic carbon sources and degassing. Carbon isotopes are an average of 2.1‰ more enriched island-wide in 2015 compared to 2014. In 2014, oxygen isotope compositions for all samples ranged from -3.3‰ to +7.0‰, and in 2015, oxygen isotopes ranged from -3.3‰ to +8.1‰. In 2015, oxygen isotopes became heavier by 0.05‰ indicating no significant enrichment or depletion of δ¹⁸O values between the two years. Several of the samples in 2014 and 2015 record δ¹⁸O values outside of the range of -3.3‰ to -2.0‰ that most closely corresponds to the GMWL, suggesting increased evaporation at those sites. In 2014, deuterium isotopes ranged from -15.0‰ to +12.2‰, while in 2015, values ranged from -6.5‰ to +13.6‰, with an average δD increase of 2.1‰ island-wide in 2015. Although geographically isolated enriched δ¹³C_DIC values could be indicative of magmatic carbon sources, the island-wide enrichment of carbon and deuterium isotopes, in addition to increased water acidity, suggest elevated evaporation island-wide in response to decreased annual precipitation (2870 mm yr^-1 in 2013, 1910 mm yr^-1 in 2014, and 1680 mm yr^-1 in 2015) as recorded at the east coast international airport.