Ocean alkalinity enhancement gets its ecosystem test: five mesocosm runs, 8,000 litres at a time

The chemistry has never been the difficult part. Crushed rock dissolves, alkalinity rises, seawater takes up more CO2; Steve Archer of Bigelow Laboratory calls it an antacid for the ocean, and says it copies the weathering that has held ocean pH steady over geologic time, except that emissions now outrun weathering. In theory, he says, it is tractable ^[11]. The biology is where trials earn their keep, because phytoplankton sit at the base of the marine food web and drive much of the ocean's biogeochemistry, and how they respond to a sudden alkalinity dose has been the open question ^[15].

An 8,000-litre enclosure is eight cubic metres of seawater ^[1][16]. That is enough to hold a plankton community rather than a sample of one, and to dose it at a concentration you actually know. It is also the source of the main limit on the results: the mesocosm is deliberately isolated from the surrounding water ^[14], so it reports what a community does at a fixed dilution, not where the added alkalinity travels afterwards.

The site list is the more interesting design decision. Five campaigns since 2021, after five years of this work, is roughly one a year across four locations in three countries ^[17]: two in Gran Canaria, one in Bergen, one at Kiel on the Baltic and one at Heligoland in the North Sea ^[7], with GEOMAR in Kiel, Las Palmas and Malaga as partners ^[8]. Those water bodies are not versions of each other. Archer's group describes the range as open subtropics, cold North Atlantic and the heavily diluted semi-enclosed seas of northern Europe, each dosed with different quantities and forms of mineral ^[13]. A benign result in subtropical water would say little about a Baltic spring bloom.

At Kiel the comparison was quicklime against brucite, tested both for how fast each dissolves and for what it does ^[9]. Dissolution speed is the operational variable, since it sets how concentrated the chemistry change is in the patch of water the plankton are sitting in. Both minerals are cement feedstocks already produced at industrial volume ^[9], so supply chains are not the novelty here. Volume is: Archer's summary of what OAE would need is that it takes a heck of a lot of rock ^[12].

Set that against where the market already is. Only a handful of small ocean-based OAE tests have been run ^[10], and Archer's objection to the commercial pace is blunt: companies are proposing to do this without sufficient information on what is going to happen ^[6]. The ocean absorbs about a quarter of annual CO2 emissions and its surface has become 30% more acidic on average since the Industrial Revolution, hard enough on corals, coccolithophores and young oysters already ^[2][3]. That is the argument for wanting OAE and the argument for measuring the biology before selling the tonnes.