Date of Award

2018

Document Type

Honors Thesis (Open Access)

Department

Colby College. Geology Dept.

Advisor(s)

Robert A. Gastaldo

Second Advisor

Walter A. Sullivan

Third Advisor

Bess G. Koffman

Abstract

The Karoo Basin, South Africa, contains a reportedly continuous stratigraphic record spanning the terrestrial vertebrate extinction event equated with the Permian-Triassic boundary (PTB) in the marine record. The current hypothesis links this major loss in biodiversity and vertebrate turnover to a global change in climate towards aridification. Rapid climate change is interpreted to be reflected in continental rocks by a changeover from (1) greenish (Permian) to reddish (Triassic) mudrock, (2) a transition in the river architectures from meandering to braided regimes, and (3) wetland to calcic-bearing paleosols. Here, we present geochemical results on a 1.3-meter calcic paleoVertisol interval found ~34 meters below the vertebrate-defined PTB as described by previous workers. This interval differs from contemporaneous paleosols because of the presence of abundant stage II pedogenic carbonate nodules. Previous workers hypothesized that such calcic paleosols formed during the Early Triassic after the extinction event(s). As such, their presence below the biozone boundary is surprising.

The physical properties of the paleosol interval are described both in the field and with thin sections, and geochemical trends are identified through the profile. Thin sections of the siltstone, sampled at 10-cm intervals, were described to identify primary structures, fabrics, and diagenetic features. The geochemical proxies used in this study include: molecular weathering ratios and mass balance techniques. ICP-MS elemental data on bulk-rock composition were acquired at ALS Global Laboratory. Four calcium-carbonate nodules, taken from different stratigraphic positions in the paleosol profile, were analyzed for δ13C and δ18O at Southern Methodist University.

The calcic paleoVertisol section—one of several identified below the vertebrate-defined boundary—contains at least two stacked-soil horizons. Geochemical data indicate probable pedogenic overprinting of each soil horizon, which collectively exhibit a possible drying trend. Mean annual precipitation (MAP) estimates transition from 890.09 mm/yr at the base of the interval to 779.88 mm/yr at its top, supporting the interpretation of a seasonal climate. Mean annual temperature (MAT) estimates remain constant at 8° C. Calcium-carbonate cemented nodules analyzed from the paleoVertisol interval have δ13C values ranging from -5.44 ‰ to -8.62 ‰ and were almost certainly precipitated under well-drained conditions. The data suggest that the onset of change in paleoclimate began much earlier in the Late Permian than previously hypothesized in the Karoo Basin.

Keywords

Karoo Basin, Late Permian, Vertisol, paleosol, paleoclimate, calcic paleoVertisol

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