Carbon dioxide (CO2) emitted by human activities not only brings about a serious greenhouse effect but also accelerates global climate change. This has resulted in extreme climate hazards that can obstruct human development in the near future. Hence, there is an urgent need to achieve carbon neutrality by increasing negative emissions. The ocean plays a vital role in absorbing and sequestering CO2. Current research on marine carbon storage and sink enhancement mainly focuses on biological carbon sequestration using carbon sinks (macroalgae, shellfish, and fisheries). However, seawater inorganic carbon accounts for more than 95 % of the total carbon in marine carbon storage. Increasing total alkalinity at a constant dissolved inorganic carbon shifts the balance of existing seawater carbonate system and prompts a greater absorption of atmospheric CO2, thereby increasing the ocean\'s \"carbon sink\". This review explores two main mechanisms (i.e., enhanced weathering and ocean alkalinization) and materials (e.g., silicate rocks, metal oxides, and metal hydroxides) that regulate marine chemical carbon sink (MCCS). This work also compares MCCS with other terrestrial and marine carbon sinks and discusses the implementation of MCCS, including the following aspects: chemical reaction rate, cost, and possible ecological and environmental impacts.

The stable carbon isotope composition (δ13C) in coral skeletons can be used to reconstruct the evolution of the dissolved inorganic carbon (DIC) in surface seawater, and its long-term declining trend during the past 200 years (~1800-2000) reflects the effect of anthropogenic Suess effect on carbonate chemistry in surface oceans. The global atmospheric CO2 concentration still has been increasing since 2000, and the Suess effect is intensifying. Considering the coral\'s ability of resilience and acclimatization to external environmental stressors, the response of coral δ13C to Suess effect may change and needs to be re-evaluated. In this study, ten long coral δ13C time series synthesized from different oceans were used to re-evaluate the response of coral carbonate chemistry to Suess effect under the changing environments. These δ13C time series showed a long-term declining trend since 1960s, but the declining rates slowed in eight time series since around 2000s. Considering that the declining rates of the DIC-δ13C in surface seawater from the Hawaii Ocean Time-series Station and Bermuda Atlantic Time-series Station has not changed since 2000 compared with those during 1960-1999, the change in the coral δ13C trends at eight of ten locations may indicate that the response of coral δ13C to the anthropogenic Suess effect has changed since around 2000s. This change may have resulted from coral acclimatization to external environmental stressors. To adapt to acidifying oceans, coral may have the ability to regulate the source of DIC in extracellular calcifying fluid and/or the utilization way of DIC, therefore the response of coral δ13C to anthropogenic Suess effect will change accordingly.

Factors controlling ocean acidification and its temporal variations were studied over the 1995-2011 period at the Dyfamed site at 10 m depth, in the North Mediterranean Sea. The results indicated a mean annual decrease of 0.003 ± 0.001 pH units on the seawater scale. The seasonal variability was characterized by a pH decrease during springtime and a strong pH increase in late fall. Anthropogenic CO2 (CANT) absorption by the ocean was the key driver of seawater acidification in this region, accounting for about 70% of the observed drop in pH, followed by water temperature (about 30%). The total inorganic carbon (CT) data showed a CT increase of 30.0 ± 1.0 μmol kg(-1) per decade. This decadal increase is mainly due to the CANT penetration (43.2 μmol kg(-1) per decade) in surface waters, which is mitigated for by relatively small opposing changes in CT due to physical and biological processes.

Artificial upwelling is considered a promising way to reduce the accumulation of anthropogenic carbon dioxide in the atmosphere. This practice could transport nutrient-rich deep water to the euphotic zone, enhance phytoplankton growth and consequently increase organic carbon exportation to the deep ocean via the biological pump. However, only a few studies quantitatively assess changes in oceanic CO2 uptake resulting from artificial upwelling. This article uses a simulation to examine the effect of hypothetical artificial upwelling-induced variations of CO2 fugacity in seawater (fCO2) using observed carbon and nutrient data from 14 stations, ranging from 21 to 43°N, in the West Philippine Sea (WPS), the East China Sea (ECS) and the Sea of Japan. Calculations are based on two basic assumptions: First, a near-field mixing of a nutrient-rich deep-ocean water plume in a stratified ocean environment is assumed to form given the presence of an artificial upwelling devise with appropriate technical parameters. Second, it is assumed that photosynthesis of marine phytoplankton could deplete all available nutrients following the stoichiometry of the modified Redfield ratio C/H/O/N/S/P=103.1/181.7/93.4/11.7/2.1/1. Results suggest artificial upwelling has significant effects on regional changes in sea-air differences (ΔfCO2sea-air) and the carbon sequestration potential (ΔfCO2mixed-amb). Large variations of ΔfCO2sea-air and ΔfCO2mixed-amb are shown to be associated with different regions, seasons and technical parameters of the artificial upwelling device. With proper design, it is possible to reverse the contribution of artificial upwelling from a strong CO2 source to sink. Thus, artificial upwelling has the potential to succeed as a geoengineering technique to sequester anthropogenic CO2, with appropriate technical parameters in the right region and season.