Combining soil water balance and clumped isotopes to understand the nature and timing of pedogenic carbonate formation

Abstract

Pedogenic carbonate is an important archive for paleoclimate, paleoecology, and paleoelevation studies. However, it can form under seasonal environmental conditions that differ significantly from the mean growing season environment or mean annual conditions, potentially complicating its use for proxy reconstructions. The observed seasonal temperature is typically, but not always, biased high relative to mean annual air temperature (MAT). To evaluate the annual timing of pedogenic carbonate formation, ten different soils were sampled across the western United States. Sites were selected to span a variety of precipitation regimes and soil orders. Precipitation regimes ranged from arid sites (mean annual precipitation (MAP) < 20 cm) that receive the majority of precipitation during the winter to wetter sites (MAP > 50 cm) dominated by summer precipitation. Pedogenic carbonate formation temperatures derived from clumped isotope measurements ranged between 6 and 22 °C, with most samples falling at or below MAT. Clumped isotope temperatures were compared to monthly precipitation normals and modeled monthly values of evapotranspiration and soil water content. Results show that carbonate formation temperatures agree with the annual timing of soil water depletion,

Seasonal Bias in Soil Carbonate Formation and Its Implications for Interpreting High‐Resolution Paleoarchives: Evidence From Southern Utah

“…In addition, studies must consider the effects of multiple formation times on bulk soil carbonate isotope composition (Burgener et al, 2016). Studies regularly, and not necessarily incorrectly, assume that soil carbonate only forms at one time of the year (Gallagher & Sheldon, 2016;Oerter & Amundson, 2016), but our data also suggest that the potential for mixed signals from multiple events throughout the year exists. Although…”

“…Other workers have also inferred the overarching importance of soil moisture in soil carbonate formation. Variable T(Δ 47 ) in soil carbonates of the western United States across a range of precipitation regimes and soil types were interpreted as the result of differences in the timing of soil moisture depletion (Gallagher & Sheldon, 2016), but support for the interpreted mechanism was limited by a lack of in situ monitoring. Others have suggested carbonate formation during the wet season as the soil dries after rain events (Hough et al, 2014;Snell et al, 2013), during the driest part of the year (Breecker et al, 2009b), or with significant variability due to elevation-driven climate differences (Oerter & Amundson, 2016).…”

Interpreting the Difference in Magnitudes of PETM Carbon Isotope Excursions in Paleosol Carbonate and Organic Matter: Oxidation of Methane in Soils Versus Elevated Soil Respiration Rates

“…A sustained 4–5 times increase in respiration rates is conceivably easier if a shift in seasonal timing of soil carbonate formation occurred during the PETM. Soil carbonate, while generally thought to form during hot, dry periods (Breecker et al, ), can be pushed to other seasons depending on the seasonal timing of rainfall (Peters et al, ; Gallagher & Sheldon, ). There is evidence to support a shift in regional hydrology during the PETM (Bowen et al, ).…”

Warm Terrestrial Subtropics During the Paleocene and Eocene: Carbonate Clumped Isotope (Δ47) Evidence From the Tornillo Basin, Texas (USA)

“…Modern summer precipitation in western Texas comes primarily from the Gulf of Mexico through a monsoon‐like system and is more enriched in 18 O than winter precipitation that originates primarily from frontal systems forced by cool air masses that have traveled overland from the Pacific (Licht et al, ; Nativ & Riggio, ; Vachon et al, ; Vera et al, ) (Figure S7). Recent work shows that soils yield carbonates with calculated δ 18 O w that resembles the δ 18 O of rainfall during the months in which the carbonates grew (Gallagher & Sheldon, ; Hough et al, ). Thus, assuming that these moisture source patterns were similar to modern throughout the Paleogene (as Fricke et al, , predict for the Cretaceous North America), an increase in δ 18 O of rainfall in the Eocene could also be explained by preferential carbonate accumulation during summer rain events from moisture derived from the proto‐Gulf of Mexico.…”