Using Carbon Isotope Equilibrium to Screen Pedogenic Carbonate Oxygen Isotopes: Implications for Paleoaltimetry and Paleotectonic Studies
Using Carbon Isotope Equilibrium to Screen Pedogenic Carbonate Oxygen Isotopes: Implications for Paleoaltimetry and Paleotectonic Studies
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利用碳同位素平衡筛选成土碳酸盐氧同位素：对古海拔测量和古构造研究的意义
作者：
N. Sheldon
成土碳酸盐岩的稳定同位素组成（δ 13 Ccarb、δ 18 Ocarb）在古环境和古测高研究中有着广泛的应用。同时，在单一古土壤的垂直地层剖面和水平剖面中，δ 18 Ocarb值的显著变化远远超过了可合理归因于海拔变化的变化。本文提出了一种新的筛选方法，即利用碳酸盐包裹体或剖面有机质的δ 13 Corg组成，确定哪些成土碳酸盐δ 18 Ocarb组成反映了同位素平衡与环境条件的关系，其中Δ 13 C = δ 13 Ccarb- δ 13 Corg。基于51个现代土壤的季风、大陆和地中海水分状况，Δ 13 C = +15.6 ± 1.1‰（1σ），这与碳同位素平衡时通过菲克扩散形成的碳酸盐的理论预测非常吻合。从地质记录中的不平衡和平衡的情况下的例子进行了检查，它表明，以前的δ 18 Ocarb记录用于推断新生代隆起在蒙大拿州西南部不提供任何约束的古海拔，因为>90%的成壤碳酸盐同位素组成的平衡。未来古测高研究的指导方针包括收集垂直地层剖面和横向剖面，每个层位至少有三个结核，从岩石学角度筛选结核的成岩作用，收集至少一个独立的古气候或古植被代用指标，并利用对每种古土壤测量的Δ 13 C筛选δ 18 Ocarb值。
Stable isotope compositions of pedogenic carbonates (δ13Ccarb, δ18Ocarb) are widely used in paleoenvironmental and paleoaltimetry studies. At the same time, both in vertical stratigraphic sections and in horizontal transects of single paleosols, significant variability in δ18Ocarb values is observed well in excess of what could reasonably be attributed to elevation changes. Herein, a new screening tool is proposed to establish which pedogenic carbonate δ18Ocarb compositions reflect formation in isotopic equilibrium with environmental conditions through the use of the co-occurring δ13Corg composition of carbonate-occluded or in profile organic matter, where Δ13C = δ13Ccarb – δ13Corg. Based upon 51 modern soils from monsoonal, continental, and Mediterranean moisture regimes, Δ13C = +15.6 ± 1.1‰ (1σ), which closely matches theoretical predictions for carbonates formed at carbon isotope equilibrium through Fickian diffusion. Examples from both disequilibrium and equilibrium cases in the geologic record are examined, and it is shown that previous δ18Ocarb records used to infer Cenozoic uplift in southwestern Montana do not provide any constraint on paleoelevation because >90% of the pedogenic carbonate isotopic compositions are out of equilibrium. Guidelines for future paleoaltimetry studies include collection of both vertical stratigraphic sections and lateral transects, of at least three nodules per horizon, petrographic screening of nodules for diagenesis, collection of at least one independent proxy for paleoclimate or paleovegetation, and screening δ18Ocarb values using Δ13C measured for each paleosol.