September 2025 | Oceanography
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deep ocean for long-term carbon removal. Ocean nutrient
fertilization enhances marine productivity by adding
micronutrients essential for the growth of phytoplank-
ton, thus stimulating primary production, whereby CO2
is taken up from the surface water and stored in organic
biomass that sinks out of the surface ocean. Surface pro-
ductivity can also be stimulated through artificial upwell-
ing, whereby deep, nutrient-rich waters are brought to the
surface and utilized there by phytoplankton. Ocean alka-
linity enhancement (OAE) involves the addition of alka-
linity, either by adding crushed alkaline minerals or aque-
ous hydroxides, or by using electrochemical methods to
generate base, which can then be added to seawater. This
addition of alkalinity enhances the CO2 uptake potential
of the ocean. CO2 is then stored within the ocean as part of
the dissolved bicarbonate (HCO3
–) pool for a longer time
scale. Direct ocean carbon capture and storage (DOCCS)
uses technological processes to extract CO2 directly from
the ocean, usually through electrochemical methods that
create a pH swing and force all the dissolved inorganic
carbon (DIC) into CO2. The extracted CO2 can either be
stored in sub-sea geological formations or used in indus-
trial processes (following known carbon capture utiliza-
tion and storage pathways, or CCUS). Low-CO2 water is
then returned to the ocean where net carbon removal is
only achieved after air-sea CO2 (re)equilibration.
Research on mCDR can, and should, be informed by
other adjacent, relevant fields, such as the work on OA,
where there is a wealth of knowledge about the marine car-
bonate system and ecosystem responses to it. Together the
OA and carbon communities have developed experimen-
tal and field-based methodologies, created best practices
guides (Riebesell et al., 2011; Currie et al., 2024), and built
carbon measurement observing capacity through a num-
ber of networks. One such network is the Global Ocean
Acidification Observing Network (GOA-ON). GOA-ON
was established in 2012 as a result of increased awareness
and concern about OA from scientists who recognized the
need for coordinated efforts to understand this global issue
(Newton et al., 2015). Today, GOA-ON has >1,000 members
from >115 countries and territories that are organized into
11 regional hubs (Figure 1). GOA-ON members also par-
ticipate in other carbon-related international initiatives,
including the Global Ocean Observing System (GOOS),
the Scientific Committee on Oceanic Research (SCOR),
the Commonwealth Blue Charter, the International Ocean
Carbon Coordination Project (IOCCP), the Surface Ocean
CO2 Atlas (SOCAT), and the Global Ocean Data Analysis
Project (GLODAP). This interaction is actively maintained
through partnerships, joint initiatives, and GOA-ON sec-
retariat members, the latter of which are financially sup-
ported by three partner organizations, the US National
Ocean acidification is occurring because of (1) the rapid uptake of CO2
from the atmosphere, which is causing a shift in marine chemistry:
CO2 reacts with seawater to become (2) part of the marine carbonate
system, increasing the pool of dissolved inorganic carbon (DIC), and
(3) causing an increase in bicarbonate ions (HCO3
–)and hydrogen ions
(H+) (a decrease in pH—Equation 1) and (4) a decrease in carbonate
ions (CO3
2–).
pH = –log[H+]
(1)
Total alkalinity (TA) is the sum of all the bases in seawater that are
able to buffer hydrogen ions. TA does not change as a result of CO2
addition but rather absorbs the additional DIC by shifting the equilib-
rium between CO2, HCO3
–, and CO3
2–. Over geological time frames,
mineral weathering of carbonate sediments (e.g., CaCO3), but also
silicates, plays an important role in buffering these shifts in chemistry
by (5) adding alkalinity to the ocean.
The saturation state (Ω) of minerals, such as CaCO3, is important for
determining the stability of the mineral in a solution (Equation 2). As
Ω increases, seawater becomes increasingly saturated (Ω > 1) and
minerals are more likely to precipitate; as Ω decreases and eventually
becomes undersaturated (Ω < 1), minerals are more likely to dissolve.
Ω =
[CO3
2–] [Ca2+]
Ksp
,
(2)
where Ksp is the solubility product constant.
Calcification and dissolution can also (6) alter the DIC and TA by the
following reactions (Equation 3):
2HCO3
– + Ca2+ ↔ CaCO3 + CO2 + H2O
(3)
BOX 1. CARBONATE CHEMISTRY AND
OCEAN ACIDIFICATION