655 / 2024-09-18 20:30:44

Using dissolved oxygen isotope (δ18O) to quantify sedimentary oxygen consumption and assess oxygen depletion in marginal seas

hypoxia,oxygen isotope,sedimentary oxygen consumption,dissolved oxygen minimum

The oxygen isotope δ18O plays a unique role in revealing dissolved oxygen (DO) cycling in the water column. This is largely attributed to the second dimension defined by oxygen isotope, in addition the first dimension of oxygen concentration. The core advantage brought by δ18O is the coupling between concentration [O2] and isotope (δ18O), namely the negative deviation between [O2] and δ18O coupling. First, the apparent fractionation of [O2] and δ18O coupling reflect the contribution from sedimentary oxygen consumption (SOC), relative to water column respiration (WCR), which show contrasting endmember fractionation factors (~-3.5‰ vs. -25‰). Based on the field observation to the [O2] and δ18O, we quantified that the WCR and SOC contributions to the total AOU is not stationary, but instead varied along with time. Over hypoxia stage, WCR is the key contributor of AOU (over 70%), whereas during non-hypoxic stage, SOC is the dominant oxygen sink (over 70%). The mixing effect on the negative deviation is also at work. With the respiration-mixing model, we are now able to provide the WCR and SOC contribution to total AOU considering mixing effect and the bias due to mixing is further overviewed with various samples collected in the China marginal seas. In addition, with the quantification of mixing effect on [O2] and δ18O coupling, we reversely quantified the DO minimum over field investigation, which is ~20μM lower than the measured values on board in the ship. Though a verification with independent approach is needed, our applications proves that δ18O approach provide a neat option in hypoxia and marine DO cycling study.