Plant growth chamber design for subambient pCO2 and δ13 C studies.

RATIONALE: Subambient pCO2 has persisted across the major Phanerozoic ice ages, including the entire late Cenozoic (ca 30 Ma to present). Stable isotope analysis of plant-derived organic matter is used to infer changes in pCO2 and climate in the geologic past, but a growth chamber that can precisely control environmental conditions, including pCO2 and δ13 C value of CO2 (δ13 CCO2 ) at subambient pCO2 , is lacking. METHODS: We designed and built five identical chambers specifically for plant growth under stable subambient pCO2 (ca 100 to 400 ppm) and δ13 CCO2 conditions. We tested the pCO2 and δ13 CCO2 stability of the chambers both with and without plants, across two 12-hour daytime experiments and two extended 9-day experiments. We also compared the temperature and relative humidity conditions among the chambers. RESULTS: The average δ13 CCO2 value within the five chambers ranged from -18.76 to -19.10‰; the standard deviation never exceeded 0.14‰ across any experiment. This represents better δ13 CCO2 stability than that achieved by all previous chamber designs, including superambient pCO2 chambers. Every pCO2 measurement (n = 1225) was within 5% of mean chamber values. The temperature and relative humidity conditions differed by no more than 0.4°C and 1.6%, respectively, across all chambers within each growth experiment. CONCLUSIONS: This growth chamber design extends the range of pCO2 conditions for which plants can be grown for δ13 C analysis of their tissues at subambient levels. This new capability allows for careful isolation of environmental effects on plant 13 C discrimination across the entire range of pCO2 experienced by terrestrial land plants.

Reconciliation of marine and terrestrial carbon isotope excursions based on changing atmospheric CO2 levels.

Negative carbon isotope excursions measured in marine and terrestrial substrates indicate large-scale changes in the global carbon cycle, yet terrestrial substrates characteristically record a larger-amplitude carbon isotope excursion than marine substrates for a single event. Here we reconcile this difference by accounting for the fundamental increase in carbon isotope fractionation by land plants in response to increasing atmospheric CO2 concentration (pCO2). We show that for any change in pCO2 concentration (ΔpCO2), terrestrial and marine records can be used together to reconstruct background and maximum pCO2 levels across the carbon isotope excursion. When applied to the carbon isotope excursion at the Palaeocene-Eocene boundary, we calculate pCO2=674-1,034 p.p.m.v. during the Late Palaeocene and 1,384-3,342 p.p.m.v. during the height of the carbon isotope excursion across all sources postulated for the carbon release. This analysis demonstrates the need to account for changing pCO2 concentration when analysing large-scale changes in the carbon isotope composition of terrestrial substrates.