Investigating pedogenic carbonate formation by measuring the stable isotope composition of water in Vertisols
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The oxygen isotope compositions of pedogenic carbonates in paleosols are used to reconstruct paleoelevations, paleoatmospheric circulation, paleotemperatures, and paleoprecipitation. The oxygen isotope compositions of pedogenic carbonates are controlled by temperature and the oxygen isotope composition of soil water, which predominately originates from precipitation. In most calcic soils studied, pedogenic carbonates record the oxygen isotope composition of summer precipitation and/or mean annual precipitation subjected to evaporation. However, due to the complex hydrological properties of Vertiols, which are abundant in the rock record, the isotopic composition of soil water could potentially vary and could influence the isotopic composition of pedogenic carbonate. Furthermore, it is well established that soils contain multiple pools of water with different stable isotope compositions but little work has been done to investigate which pools are recorded by pedogenic carbonates. Therefore, the isotopic composition of soil water in modern Vertisols was monitored and compared with the oxygen isotope composition of pedogenic carbonate in the same soils to investigate if the oxygen isotope composition of pedogenic carbonates in Vertiols record mobile or immobile water. The isotope composition of soil water was determined in four ways: 1) measurement of isotope composition of water collected by vacuum distillation of soil samples collected by auger, 2) calculation from measured oxygen isotope compositions of soil CO₂, 3) calculation from measured oxygen isotope compositions of pedogenic carbonate, and 4) measured isotope compositions of water collected under tension in a soil solution sampler. The oxygen isotope compositions of water in equilibrium with CO₂ and water from the solution sampler were indistinguishable at 140cm and were interpreted as mobile water in macropores. The vacuum distilled water (which includes water from a mixture of macropores and micropores) always had lower δ ¹⁸O values than the macropore water and the other sampling methods, implying the presence of water with low δ ¹⁸O values. These oxygen isotope compositions of soil water pools were compared with δ ¹⁸O values of local precipitation (GNIP data from nearby Waco, TX). Below ~100cm, total soil water δ ¹⁸O values converge to -6.3 ± 0.7 %₀ (1σ, n=20), which is isotopically lighter than the δ ¹⁸O of mean annual precipitation (MAP) of Waco, Texas (-3.8 ± 2.7 %₀, 1σ, n=96). This could result from recharge of isotopically light September precipitation (SEPT); (-5.9 ± 2.4 %₀, 1σ, n=8)) replenishing the soil after dry periods and/or the contribution of winter precipitation (WP) (-5.5 ± 2.4 %₀, 1σ, n=25). The δ ¹⁸O values of soil water in equilibrium with soil CO₂ ((-4.1 ± 0.8%₀) are isotopically similar to or heavier than the isotopic composition of MAP. The δ ¹⁸O values of soil water in equilibrium with pedogenic carbonate (-2.7 ± 0.9%₀) are also isotopically similar to the isotopic composition of summer precipitation (SP, including June, July, and August) (-2.0 ± 2.9 %₀, 1σ, n=8). This suggests that, despite the more complex hydrology of Vertisols compared with other soils orders, the δ ¹⁸O values of pedogenic carbonates formed in central Texas Vertisols record SP and/or mean annual precipitation that has been subjected to evaporation, just as they do in other soils. If this holds true for Vertisols formed in other climates, then this facilitates the comparison among δ ¹⁸O values of paleosol carbonates from various soil orders, which is common practice in vertical successions of paleosols. Furthermore, the observation that the σ ¹⁸O values of water in equilibrium with pedogenic carbonate are more similar to the σ ¹⁸O values of macropore than micropore water suggests that pedogenic carbonates in central Texas Vertisols may form in macropores. Formation in macropores is more consistent with CO₂ degassing and/or evaporation, rather than root water uptake, as a proximal driver of calcite precipitation.