Little is known about anthropogenic carbon dioxide (CO2) concentrations and distributions within marginal sea systems that are subject to complex physical and biogeochemical variability. Cruises were conducted from 2009-2011, 2014, and 2016 as part of the CHOICE-C I and CHOICE-C II in the northern South China Sea basin. These high-spatial-resolution measurements were compared with one another and with prior investigations to assess the spatial, seasonal, and interannual variability of anthropogenic CO2 based on both back-calculation and transient-time distribution approaches. From examining summer 2009, summer 2014, spring 2011, and spring 2016 datasets, the average increases in the surface water were ~21.6-26.5 μmol kg-1 over a 5-year period at the same stations (n=10-13). A comparison on the approaches for the summer 2014 dataset showed that anthropogenic CO2 has penetrated into the deep water, ranging between 6.11 ± 4.88 μmol kg-1 (n=102) and 6.41± 0.79 μmol kg-1 (n=16) between 2000-3800m. Based on same station comparisons (n=4 for spring, n=5-10 for summer), the aragonite saturation state (ΩAr.) horizon in summer 2009 (ΩAr. = 0.98 ± 0.5, n=10) was at ~870 ± 116m, whereas the ΩAr. horizon (ΩAr. = 0.98 ± 0.5, n=10) in summer 2014 (ΩAr. = 1 ± 0.6, n=10) was ~586 ± 119m. These findings were associated with an increase of ~7.0-19.3 μmol kg-1, n=5 in anthropogenic carbon. The annual ΩAr. horizon decrease in the summer based on these two datasets is 57m yr-1. This differed from the ΩAr. horizon temporal evolution during spring, where anthropogenic CO2 stagnated at ~800m at 12.32 μmol kg-1 ± 4.03 during spring 2011 (ΩAr. = 0.95 ± 0.5, n=4) to 12.23 μmol kg-1 ± 9.53 during spring 2016 (ΩAr. = 0.96 ± 0.6, n=4). Yet, the average anthropogenic CO2 during spring increased by ~8.56 μmol kg-1 at 300m and ~6.67 μmol kg-1 at 2000m (n=3). This unique inconsistency in temporal variations in anthropogenic carbon concentrations highlights the need for spatially resolved data in these coastal systems, particularly in regions subjected to anthropogenic influences with overprints in both seasonal and spatial changes. As a whole, the 32-39% increase in anthropogenic carbon in the surface waters within 5 years that is affecting the entirety of the water column is alarming. Continued basin-wide inter-annual and seasonal investigations are needed to determine the rate of ΩAr. decline as this has deleterious consequences on the marine ecosystem.