Abstract: Isotope geochemistry has established relationships between formation environment (e.g. temperature) and “clumped” isotopes of carbonates. This work indicates that by measuring the “clumping” of 18O and 13C, carbonates can be used as proxies for paleoenvironmental conditions. Pedogenic carbonates are potentially long-term integrators ( >1000 y) of environmental processes and conditions. However, soils are complex, dynamic systems that are not optimal for carbonate isotope preservation. Little experimental work has examined soil processes that may alter carbonate clumped isotope signatures, though theoretically some are likely important. For example, forest fires have been shown to increase soil temperatures to > 250 °C—hot enough to rearrange the carbon and oxygen isotopes in the carbonate mineral. If this occurs, the carbonates may not reflect their formation environments as previously predicted, though they may indicate historical forest fire burn temperatures. We simulated forest fires to test their influence on carbonate isotopes signatures experimentally. We hypothesized that in dry soils, heating from a simulated forest fire alters carbonate isotopic signatures of carbonates found close to the soil surface (experiencing >250 °C) but not at depth (experiencing < 250 °C).
To test this hypothesis, we used an experimental setup: a 120-liter metal container filled with air-dried sand. Carbonate samples with known isotopic signatures, and temperature probes were placed at depths of 2 cm, 10 cm, 20 cm, and 40 cm—relating to peak temperatures experienced of >500 °C, 130 °C, 47.5 °C, 26 °C, respectively. A wood fire was burned at the surface for 5.5 hr, with new wood added for the first 1.5 hr. We then excavated the carbonate samples, and analyzed their O, C, and clumped isotopic signatures. Prior to heating, the clumped isotope derived temperature = 17 °C, 18O = -12.612 ‰, and 13C = -6.166 ‰. After heating, only the shallowest sample (2 cm, T >500 °C) was altered (T47 = 58.6 ± 7.6 °C; 18O = -4.2886 ± 0.5136 ‰; 13C = -6.1667 ± 0.0153 ‰). All other samples were within 1se of accepted values.
Given the alteration in the T47 and 18O, there is likely isotopic exchange of the O with any residual moisture in the soil upon heating. These data indicate that forest fires have the potential to alter carbonates in shallow soil horizons. However, the alteration did not change the carbonate temperature to the temperature of the soil during the fire.
