Radiocarbon age offsets, ontogenetic effects, and potential old carbon contributions from soil organic matter for pre-bomb and modern detritivorous gastropods from central Texas, USA

Abstract

Carbon isotopes of terrestrial gastropod yield insights on age and paleoenvironments for many depositional settings, but questions remain on the carbon source for gastropod carbonate and the climatic significance. This study documented for four terrestrial gastropod species ( Rabdotus dealbatus, A. alternata, V. indentata, and Rumina decollata) from central Texas 14 C age offsets between shell‑carbonate and corresponding atmosphere values that lived on carbonate- and kerogen-rich Cretaceous bedrock in the 20th and 21st centuries. Also, the ontogenetic variations in 14 C content were assessed to better understand carbon cycling between gastropods and the associated reservoirs. The carbon sources were partitioned based on 13 C isotopic values to evaluate the possible causes for age anomalies using initially a three-component model that includes contributions from atmosphere, vegetation, and limestone pools, with a new added fourth component, for soil organic carbon. The average ( n = 4) of the outer and inner shell samples yielded the respective 14 C ages of 550 ± 325 and 1520 ± 525 yr B.P. The apparent linearized 14 C distance-age anomaly for an individual gastropod whorl is 4–26 mm/yr calculate from shell 14 C ages on apex and aperture material. The smallest gastropod dated, a single V. indentata test (3.9 mg) yielded the oldest 14 C age of 2340 yr B.P. The four-component model for the oldest 14 C ages indicates that 50–85% of shell carbon may be from ingestion, with a smaller pool from respiration. Up to 30% of the organic carbon in gastropod shell may be derived from soil organic carbon. Terrestrial gastropods that yield anomalously old 14 C ages of >500 years may be detritivores and live in soils with sources of labile old carbon either inherited from the bedrock and/or the older pool of soil organic matter common in well-developed calcium-rich soils. • Modern and prebomb terrestrial gastropods from Texas gave 14 C ages 300–2400 yr old. • Gastropod shells show ontogenetic effect with older ages at apex by 100–1700 yr. • Identify new old carbon source for snails from kerogen-rich soil organic matter (SOM) • Develop new four component model (air, rock, SOM and plants) to explain old 14 C ages • Up to 30% of carbon in gastropod shell may be derived from SOM.