Influence of vegetation type and site-to-site variability on soil carbonate clumped isotope records, Andean piedmont of Central Argentina (32-34°S)

Abstract

The clumped isotope geothermometer estimates the formation temperature (T(Δ47)) of carbonates and has great potential to enhance the extraction of environmental data from pedogenic (soil) carbonate in the geologic record. However, the influence of vegetation type and site-specific conditions on carbonate formation processes and T(Δ47) records remains poorly understood. This study examines the potential for variability in T(Δ47) data between nearby, same elevation sites with different C3/C4 biomass. Pedogenic carbonates (undercoatings and nodules) were collected from five modern soil pits in the semi-arid eastern Andean piedmont of Argentina under a summer precipitation regime. Three pits were instrumented with temperature and moisture sensors to 1 m depth, and a fourth was instrumented with additional soil CO2 and atmospheric (temperature, relative humidity, insolation, and rainfall) sensors. T(Δ47) values (mean: 30±6°C (±1SE)) are invariant with depth and are statistically indistinguishable between the four instrumented sites, though a 10 °C difference between our T(Δ47) values and those of a nearby Peters et al. (2013, EPSL) study suggests the potential for significant site-to-site variability, likely due to local soil hydrology. The results of this study suggest that deeper (≥40 cm) T(Δ47) values are consistent with carbonate formation during the early part of soil drying immediately after large mid-summer rainstorms. Carbonate formation ≤ 40 cm depth may be biased to soil drying after small, frequent precipitation events occurring throughout the spring, summer, and fall months, averaging to shallow summer T(Δ47) values and resulting in a near-isothermal T(Δ47) profile.