T146 Soils and Long-Term Environmental Change II

Wednesday, 28 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

USING FACTORS OF SOIL FORMATION TO INVESTIGATE STABLE CARBON ISOTOPE DISEQUILIBRIUM IN LATE PLEISTOCENE (MIS 3) BURIED SOILS OF THE CENTRAL GREAT PLAINS, USA

Paper number: T146-4
Program: Technical Sessions
Time: 2:05 PM – 2:20 PM

Authors

Abstract

Buried soils serve as important archives of environmental change and landscape history. In particular, the stable carbon isotope (δ13C) compositions of preserved organic matter and pedogenic carbonates in buried soils have proved to be important proxies for reconstructing late Quaternary ecological and climatic change. In this study, we investigate a series of buried soils from the High Plains of western Kansas. Three cores were obtained that contain three superposed loess units: the Gilman Canyon Formation, Peoria Loess, and Bignell Loess. All cores contained buried soils that date to ca. 28-48 ka (Marine Isotope Stage [MIS] 3) and stratigraphically correlate to the Gilman Canyon Formation. These buried soils have Ak-Bk horizonation and pedogenic carbonates consists of small (<10 mm) nodules and filaments (Stage I-II). δ13C analyses were performed on bulk pedogenic carbonates (δ13Ccarb) and co-occurring soil organic matter (δ13CSOM) to reconstruct late-Quaternary bioclimatic change in the region.

Results indicate a unique decoupling between δ13Ccarb and δ13CSOM in the buried soils. Theoretically, δ13Ccarb values are enriched by 14-17‰ relative to δ13CSOM. However, Δ13C values (δ13Ccarb – SOM) are 2 to 7 per mil higher than expected equilibrium values in the Bk horizons. In contrast, Δ13C values are 1 to 5 per mil lower than expected equilibrium values in the Ak horizons. Considering the different soil-forming factors allows for the establishment of multiple working hypotheses regarding the influence of climate, vegetation, parent material, and time on the observed isotopic disequilibrium. In particular, we consider the effect of temperature, aridity, seasonality, and pCO2 levels during MIS 3 as well as potential changes in parent material sources, phases of carbonate growth, and diagenetic overprinting over time. A variety of techniques are utilized, including strontium isotope (87Sr/86Sr) and particle-size analyses together with petrographic and cathodoluminescence imaging of carbonate components. Given that pedogenic carbonates are widely used as paleoenvironmental proxies, our results highlight the necessity of screening pedogenic carbonates for isotopic equilibrium as well as understanding landscape histories when interpreting isotopic trends.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-355553

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