High-Temporal-Resolution In Situ Sensor for Oceanic CO Isotope Measurement Enabling Multidimensional Isotope Tracing Analysis (RC, RO, and RO) via Laser Absorption Spectroscopy.

Combined in situ analysis of oceanic CO 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 CO 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 CO (OCO, OCO, and OCO) 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 RC, RO, and RO 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 O anomaly with strong regularity is observed, which is not obvious in O and C, 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.