Timing of Pedogenic Carbonate Formation in Fine‐Grained Soils: Decoupled T(Δ₄₇) and δ¹⁸O_w Seasonal Bias

Abstract Historically, clumped isotope thermometry (T(∆ 47 )) of soil carbonates has been interpreted to represent a warm‐season soil temperature based dominantly on coarse‐grained soils. Additionally, T(∆ 47 ) allows the calculation of the oxygen isotope composition of soil water (δ 18 O w ) in the past using the temperature‐dependent fractionation factor between soil water and pedogenic carbonate, but previous work has not measured δ 18 O w values with which to compare to these archives. Here, we present clumped isotope thermometry of modern soil carbonates from three soils in Colorado and Nebraska, USA, that have a fine‐to‐medium grain size, contain clay, and are representative of many carbonate‐bearing paleosols preserved in the rock record. At two of the three sites, Briggsdale, CO and Seibert, CO, T(∆ 47 ) overlaps with mean annual soil temperature (MAST), and the calculated δ 18 O w overlaps within uncertainty with measured δ 18 O w at carbonate bearing depths. At the third site, in Oglala National Grassland, NE, mean T(∆ 47 ) is 8–11°C warmer than MAST, and the calculated δ 18 O w has a significantly higher isotope value than any observations of δ 18 O w . At all three sites, even in the fall season, δ 18 O w values at carbonate bearing depths overlap with spring rainfall δ 18 O w , and there is little to no evaporative enrichment of δ 2 H w and δ 18 O w values. These data challenge long‐held assumptions that all pedogenic carbonate records a warm‐season bias, and that δ 18 O w at carbonate‐bearing depths is affected by evaporative enrichment.