USING TRIPLE OXYGEN AND CLUMPED ISOTOPES IN SOILS TO UNDERSTAND THE HYDROSPHERE IN CRITICAL ZONES (Invited Presentation)

Understanding the flux of water in the hydrosphere is an important aspect of understanding the Critical Zone (CZ). New isotopic methods to understand evaporation in soils using pedogenic carbonates is now possible using clumped isotopes (Δ₄₇) and triple oxygen isotopes (Δ¹⁷O) in conjunction with δ¹⁸O and δ¹³C. Samples from 11 soils from a transect across the Serengeti Ecosystem in Tanzania were collected to look at isotopic and hydrologic variability across this CZ. Carbonate morphologies are not consistent across the landscape and range from coatings to nodules to petrocalcic horizons and soil orders include Inceptisols, Mollisols, and Ultisols. Water samples from precipitation, lakes, rivers, and soils were also collected for δ¹⁸O, δD, and Δ¹⁷O to better understand the isotopic variability across the landscape. For soil waters and other waters, δ¹⁸O ranges from -3.8 to 5.5‰ and -5.0 to 10.4‰, δD ranges from -58.2 to 13.1‰ and -26.6 to 68.9‰, and Δ¹⁷O ranges from -11 to 62 per meg and -25 to 24 per meg. This falls within the range of known values for δ¹⁸O and δD in East Africa and are the first Δ¹⁷O values from Kenya and Tanzania. In addition, 10 soil temperature loggers were buried between Feb. 2018 to Jan. 2019 to understand the variability in soil temperature across the transect. Mean soil temperatures range from 2 to 4°C higher than mean annual air temperatures of 19 to 23°C. Δ₄₇ temperatures range from 14 to 29°C (mean 24°C), which more closely matches the maximum soil temperatures recorded in the by the loggers (24 to 30°C), but are still significantly lower than Δ₄₇ temperature recorded in other parts of Africa. Pedogenic carbonate isotopic values range from -2.7 to 1.8‰ for δ¹³C and -4.5 to 4.3‰ for δ¹⁸O. Ongoing work on Δ¹⁷O of soil carbonates indicates that this method will be a useful proxy for evaporation in Deep Time CZs (DTCZs). This transect has a range of precipitation from 499 to 846 mm yr^-1 and an Aridity Index ranging from 0.33 to 0.55 (semi-arid to dry sub-humid). There is a positive increase of ~40 per meg in Δ¹⁷O (SE ±15) between the semi-arid and dry sub-humid environments. This preliminary data suggests that with additional analysis of Δ¹⁷O in pedogenic carbonates from both wetter and drier environments Δ¹⁷O will be a valuable addition for understanding the hydrologic balance of modern soils and reconstructing DTCZs.