Investigating the major carbon input to cave-air CO2 and speleothem calcite by using the respiratory quotient

Abstract:

Speleothems (cave mineral deposits) are used to reconstruct changes in rainfall, moisture sources, atmospheric temperatures, and vegetation. Soil respiration is generally considered to be one of the major sources of cave-air CO₂, and by extension a major source of carbon in speleothem calcite. However, the δ¹³C values from speleothem calcite are difficult to interpret. The purpose of this study is to investigate the major source of carbon in cave-air CO₂ using a novel tracer, and thereby increase the accuracy of δ¹³C from speleothem calcite as a paleoenvironmental proxy. Potential sources of CO₂ in cave-air include (1) soil respiration (primarily from roots and microbes), (2) animal respiration, (3) in-cave decomposition of organic matter, (4) deep magmatic or metamorphic sources, and (5) atmospheric air. Of these potential sources, soil respiration and atmospheric air are currently considered to be most significant in most caves. We use the respiratory quotient (RQ, which is the number of moles of CO₂ produced per mole of O₂ consumed, defined here in relation to atmospheric air) to compare cave air and overlying soil gas at two localities in central Texas: Natural Bridge Caverns and Inner Space Cavern. Soil gas samples (RQ = 1.32) follow a trend expected for respiration followed by diffusion whereas cave air samples (RQ = 0.97) follow a trend expected for respiration without subsequent diffusion. We suggest that root and rhizomicrobial respiration below the soil in the epikarst fracture network, where gas transport is dominated by advection rather than diffusion, contributes significantly to cave-air CO₂. This is important because ¹²CO₂ preferentially diffuses out of soils, elevating the d¹³C values of residual soil CO₂, whereas no carbon isotope fractionation occurs during advection. Our interpretation of RQ values suggests that the d¹³C value of cave-air CO₂ is not influenced by diffusive loss of CO₂. In order to further investigate soil and cave carbon sources and transport, we plan to collect gas samples from fractures in the epikarst by drilling wells into the epikarst. In addition, we plan to incubate roots and root-free soil in order to characterize the RQ and d¹³C values associated with these two types of respiration.