Analyzing the 1pctCO2 and 1pctCO2-CDR experiments, we identify a significant early turnover in ocean carbon sink capacity prior to year 140 and a dramatic alteration in the carbon budget after achieving carbon neutrality. Employing a Taylor first-order expansion, our attribution analysis reveals that the early downturn is primarily governed by dissolved inorganic carbon (DIC) concentration changes. Conversely, the post-neutrality perturbation is jointly controlled by DIC, sea surface temperature (SST), and alkalinity (ALK). These mechanistic attributions are robustly corroborated through spatial mapping of DIC and SST anomalies. Our findings highlight a fundamental transition from chemically-dominated to thermo-chemically co-regulated ocean carbon dynamics, with critical implications for marine carbon uptake stability under carbon dioxide removal (CDR) scenarios.