New correlation of stable carbon isotopes with changing late-Holocene fluvial environments in the Trinity River basin of Texas, USA

Abstract

A late-Holocene alluvial sequence in north-central Texas has a 1 m thick buried cumulic soil with an A-C profile called the West Fork paleosol. It formed 2300 to 1000 yr BP and is a local equivalent of the Copan paleosol that is present throughout the southern US Great Plains. Stable carbon isotopes indicate that the paleosol and underlying gray clay formed under vegetation dominated by C4 species (mean δ13C: −18.3 ± 0.3‰). Diverse paleoenvironmental studies indicate that the period of paleosol formation was cool and wet and that alluvial water-tables were high, resulting in broad wet meadows across alluvial valleys, characterized by communities of grasses. Present-day wet meadows and bottomlands with Mollisols with A-C profiles along streams in the Great Plains are dominated by C4 tallgrass species and may serve as analogues to wet-meadow environments during the late Holocene. A shift in climate to warm-dry conditions about 1000 yr BP was accompanied by deep channel cutting, low alluvial water-tables, and colonization of abandoned floodplains by trees and other C3 species, as indicated by a change in carbon isotopes to lower values (mean δ13C: −20.8 ± 0.5‰) and correlating with the ‘Medieval Warm Period’. Other stable carbon isotope studies from late-Holocene alluvium in Texas have been mistakenly interpreted as evidence for paleoenvironmental conditions opposite to those presented in this investigation. We conclude that interpretations of stable carbon isotopes from alluvium based on broad patterns of upland C4 grasses and climate can be in error, especially in cases where wet-meadow deposits and soils are present.