Comparative carbon cycle dynamics of the present and last interglacial

Simultaneous measurement of C2H6 and CH4 concentrations, and of the δ13C-CH4 isotope ratio is demonstrated using a cavity-enhanced absorption spectroscopy technique in the mid-IR region. The spectrometer is compact and has been designed for field operation. It relies on optical-feedback-assisted injection of 3.3 μm radiation from an interband cascade laser (ICL) into a V-shaped high-finesse optical cavity. A minimum absorption coefficient of 2.8×10-9 cm-1 is obtained in a single scan (0.1 s) over 0.7 cm-1. Precisions of 3 ppbv, 11 ppbv, and 0.08‰ for C2H6, CH4, and δ13C-CH4, respectively, are achieved after 400 s of integration time. Laboratory calibrations and tests of performance are reported here. They show the potential for the spectrometer to be embedded in a sensor probe for in situ measurements in ocean waters, which could have important applications for the understanding of the source and fate of hydrocarbons from the seabed and in the water column. decomposition of organic matter under anoxic conditions. The abiogenic processes include biomass burning and thermal breakdown of organic molecules at high temperature in deep reservoirs. CH4 is the main component of natural gas, but heavier hydrocarbons (HCs) can also be present in natural gas such as ethane (C2H6) and propane (C3H8), depending on the gas origin. In addition the carbon isotopic composition of methane (δ13C-CH4 hereafter) differs between biogenic and abiogenic sources. While biogenic gas has a high concentration of CH4 with respect to heavier HCs and a more negative δ13C-CH4 (typically from -60‰ to -90 ‰), thermogenic gas is characterized by a lower ratio of CH4=C2H6 and a less negative δ13C-CH4 signature (-50‰ to -40 ‰). The combination of these two measurements leads to a quite unambiguous identification of the origin of natural gas (Claypool and Kvenvolden, 1983). A significant part of Earth's HC reservoirs lies in marine environments, at variable depth below the seafloor. Questions arise about their origin and fate, and notably about their contribution through leakage into the ocean. Such leakage contributes to the carbon balance of the oceans, to acidification after oxidation in the water column, and possibly to the atmospheric concentration of these HCs if gas flaring from the seabed reaches the ocean surface. Therefore it is important to document the origin and fate of methane and ethane present below the seabed and dissolved in the water column, for process understanding and for future climate and ocean acidity projections. © Author(s) 2019.