The clumped isotope geothermometer in soil and paleosol carbonate

• Published in: Geochimica et cosmochimica acta Vol. 105; pp. 92 - 107
• Main Authors: Quade, J., Eiler, J., Daëron, M., Achyuthan, H.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15-03-2013; Elsevier
• Subjects: air temperature; buried soils; carbon; carbonates; climate; Continental interfaces, environment; dewatering; equations; heat; isotopes; Ocean, Atmosphere; oxygen; rain; Sciences of the Universe; shade; soil depth; solar radiation; summer; surface temperature; Himalayan region; Pakistan; China; Arizona; India; Nevada
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2012.11.031

We studied both modern soils and buried paleosols in order to understand the relationship of temperature (T°C(47)) estimated from clumped isotope compositions (Δ47) 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(r2=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(r2=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 (⩽5km) 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 2.5–4km 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 >25cm, 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 (<3–4km) paleosol carbonates. Despite diagenetic resetting of Δ47 values, δ18O and δ13C 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.