High-Temporal-Resolution In Situ Sensor for Oceanic CO2 Isotope Measurement Enabling Multidimensional Isotope Tracing Analysis (R13C, R18O, and R17O) via Laser Absorption Spectroscopy.

Abstract

Combined in situ analysis of oceanic CO2 concentrations and diverse C and O isotope characteristics can offer a unique perspective with multiple isotopic tracing dimensions for identifying marine biogeochemical processes. Applying this strategy in marine environments is urgently required, yet it faces inherent challenges in terms of existing analytical methods and instruments, e.g., a lack of in situ sensors, limited detectable isotope variety, and low-temporal-resolution data. Here, we report an underwater in situ dissolved CO2 isotope sensor based on mid-infrared tunable diode laser absorption spectroscopy (MIR-TDLAS) and membrane extraction technology. Through the proposed targeted strategies, the sensor is capable of providing high-temporal-resolution in situ measurement of all monosubstituted isotopes of dissolved CO2 (16O13C16O, 18O12C16O, and 17O12C16O) at marine background concentrations. The sensor is demonstrated to provide comparable precision to that of isotope ratio mass spectrometry. At 400 ppmv, the precision for R13C, R18O, and R17O could achieve 0.084, 0.042, and 0.013‰, respectively, for a 1 s integration time. By enabling a high-frequency in situ analysis in fixed-point time-series field deployment, a 17O anomaly with strong regularity is observed, which is not obvious in 18O and 13C, and therefore, the superiority of the proposed multidimensional in situ isotope tracing strategy is demonstrated. The developed sensor has great potential to open up new prospects for advancing marine carbon research.