The clumped isotope geothermometer in soil and paleosol carbonate

Abstract

We studied both modern soils and buried paleosols in order to understand the relationship of temperature (T°C(47)) estimated from clumped isotope compositions (Δ_(47)) of soil carbonates to actual surface and burial temperatures. Carbonates from modern soils with differing rainfall seasonality were sampled from Arizona, Nevada, Tibet, Pakistan, and India. T°C(47) obtained from these soils shows that soil carbonate forms in the warmest months of the year, in the late morning to afternoon, and probably in response to intense soil dewatering. T°C(47) obtained from modern soil carbonate ranges from 10.8 to 39.5 °C. On average, T°C(47) exceeds mean annual temperature by 10–15 °C due to summertime bias in soil carbonate formation, and to summertime ground heating by incident solar radiation. Secondary controls on T°C(47) are soil depth and shading. Site mean annual air temperature (MAAT) across a broad range (0–30 °C) of site temperatures is highly correlated with T°C(47) from soils, following the equation: MAAT(°C) = 1.20(T°C(47)_0)- 21.72 (r^2 = 0.92) where T°C(47)_0 is the effective air temperature at the site estimated from T°C(47). The effective air temperature represents the air temperature required to account for the T°C(47) at each site, after consideration of variations in T°C(47) with soil depth and ground heating. The highly correlated relationship in this equation should now permit mean annual temperature in the past to be reconstructed from T°C(47) in paleosol carbonate, assuming one is studying paleosols that formed in environments generally similar in seasonality and ground cover to our calibration sites. T°C(47)_0 decreases systematically with elevation gain in the Himalaya, following the equation: elevation (m) = -229(T°C(47)_0)+ 9300 (r^2 = 0.95) Assuming that temperature varied similarly with elevation in the past, this equation can be used to reconstruct paleoelevation from clumped isotope analysis of ancient soil carbonates. We also measured T°C(47) from long sequences of deeply buried (⩽5 km) paleosol carbonate in the Himalayan foreland in order to evaluate potential diagenetic resetting of clumped isotope composition. We found that paleosol carbonate faithfully records plausible soil T°C(47) down to 2.5–4 km burial depth, or ∼90–125 °C. Deeper than this and above this temperature, T°C(47) in paleosol carbonate is reset to temperatures >40 °C. We observe ∼40 °C as the upper limit for T°C(47) in modern soils from soil depths >25 cm, and therefore that T°C(47) >40 °C obtained from ancient soil carbonate indicates substantially warmer climate regimes compared to the present, or non-primary temperatures produced by resetting during diagenesis. If representative, this limits the use of T°C(47) to reconstruct ancient surface temperature to modestly buried (<3–4 km) paleosol carbonates. Despite diagenetic resetting of Δ_(47) values, δ^(18)O and δ^(13)C values of the same deeply buried paleosol carbonate appear unaltered. We conclude that solid-state reordering or recrystallization of clumping of carbon and oxygen isotopes can occur in the absence of open-system exchange of paleosol carbonate with significant quantities of water or other phases.

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