Figure 7. The correlation between the normalized AT(AT35) and normalized CT(CT35) following Friis et al. 2003. The colors represent oxygen concentrations in µmol kg-1. The best fit lines (solid black lines), equation and r-squared for the hypoxic regions with oxygen concentrations ≤ 20.00 µmol kg-1 is also included. The inserted arrow-oriented diagram represents the direction of different processes involved in a carbonate system.
In order to investigate to what extent the processes of photosynthesis/respiration of organic matter and CaCO3formation/dissolution influence the nBUS carbonate system, the relationship between “AT – CT” and apparent oxygen utilization (AOU; the difference between the equilibrated oxygen concentration and oxygen concentrations derived from in-situ data) was also investigated. The “AT – CT” is conservative to ocean mixing and is used to unravel the mechanisms that affect the carbonate system (e.g. Xue and Cai, 2020). We found a significant negative correlation between ” AT – CT ” and AOU in subsurface water (depth ≤ 20 m), with a slope of -0.80 (Figure 8), in line with the previous study in the Gulf of Mexico with the calculated slope of -0.93 (Xue and Cai, 2020). These outcomes provide further confirmation that biological processes of photosynthesis and aerobic respiration play a crucial role in regulating TA and CT in the nBUS subsurface waters. In deeper waters (depth > 100 m), however, the slope decreases to -0.62, aligning with the confirmed slope of -0.70 in the deep waters of the global ocean (e.g., Xue and Cai, 2020). This decrease was attributed to a contribution from CaCO3 dissolution in deep waters, wherein the ”AT – CT ” ratio relative to AOU increases, providing additional evidence of CaCO3dissolution in the nBUS (Xue and Cai, 2020). Furthermore, we can also attribute this decrease in slope in deep waters to the occurrence of organic matter degradation mediated by anaerobic processes that produce significant amounts of AT and CT, such as sulfate reduction and denitrification.