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‘Calcium-carbonate-producing organisms are the finger on the pulse of ocean climate’, says NIOZ oceanographer and endowed professor at VU Amsterdam, Geert-Jan Brummer. ‘Therefore, the tiny plants and animals that produce calcium carbonate are also key to understanding the past, present and future of the climate. Plankton that produces calcium carbonate thrives better or worse depending on the temperature, amount of sunlight, mixing of water during storms, and CO2 levels. It also means there are seasons in the blooming of calcium-carbonate-producing phytoplankton. In summer, a warm layer of water develops at the top of the ocean that does not mix easily with deeper, colder water. In that warm, isolated layer, nutrients are rapidly consumed as a result of which plankton growth is inhibited. In winter, there is little sunlight, and the water is too cold for algal blooms. This means that calcium-carbonate-producing plankton usually blooms in spring and autumn and that these blooming periods shift with the changing climate.’

Seasons in the archive

‘The presence or absence of plankton cannot be understood without considering those seasons, which are still recognisable when dead plankton sinks to the ocean floor. In samples from the ocean floor, we can still identify the seasons from the past from the remains of species that thrive at specific temperatures. We, therefore, investigate plankton in both the water column and ocean floor. We measure sinking plankton using fully automatic measurement stations, “sediment traps”, which we have anchored to the ocean floor using long cables.’

‘Strangely enough, we see that the “mountain tops” on the ocean floor are somewhat similar to the mountains on land: the “snow line” is shifting under the influence of the changing climate. For example, on top of the mid-Atlantic Ridge, there is a white layer of sunken calcium carbonate skeletons from dead plankton. The deeper you go, the less calcium carbonate there is on the slopes, because it has dissolved due to the shifting chemical balance in the deep ocean. However, the undersea snow line is moving ever higher due to the increasing CO2 concentrations as more CO2 ends up in the ocean water from the atmosphere. Studying the archives of the ocean floor subsequently teaches us what this can mean for sea life and ultimately life on land.’