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.