The White Cliffs of Dover rise several hundred feet above the surf, along a southern stretch of the English Channel. They are beautiful and abrupt, as if a giant cleaver chopped open the hillside to expose the chalky layer below. The cliffs, tragically, seem to invite death: The height of Beachy Head at Dover makes it a notorious suicide peak. The structures themselves are also something of a mystery — exactly how the headlands formed, 100 million years ago, left scientists puzzled.
What has long been known is that the cliffs’ chalk is made up of pulverized shells. Not any old seashell, though, will do — the mineral dust once belonged to a single-celled plant known as a coccolithophore. Such organisms are special among marine plants, as they sheath themselves in plates of a calcium-based crust. (You might know this crusty secretion better by the name of limestone.)
The question remained how these algae — each little more than one ten-thousandth of an inch in diameter — could bloom in the massive numbers required to form what would be known as the White Cliffs. In typical conditions, competition with other microorganisms keep these algae populations in check. For clues, a few researchers looked far to the south.
The Great Calcite Belt is a band of coccolithophores that exists today in the Antarctic Ocean. From above, the belt appears as a circle of reflective water that rings the Antarctic continent. It was first described by William Balch, an oceanographer at the Bigelow Laboratory for Ocean Sciences in Maine, and his co-authors in 2011.
In a study published recently in Global Biogeochemical Cycles, Balch and his colleagues examined the conditions that allow these plants to flourish so much so that the water is a shimmery blue-green. The scientists took samples on two research cruises, in early 2011 near South Africa and 2012 near Australia. They tracked the coccolithophore species and their abundance, as well as the concentrations of the plant’s microscopic competitors known as diatoms.
The marine biologists discovered that three nutrients — nitrate, iron and silicate — tip the scales in favor of the coccolithophores or the diatoms. Both diatoms and coccolithophores fare well when iron concentrations are high in the water. Diatoms ensconce themselves in a glasslike skeleton built from silicate. And where there’s plenty of iron and silicate, the diatoms outcompete coccolithophores.
Conversely, where nitrate was high but iron was relatively low, coccolithophores have the advantage. Such conditions are found, in particular, along the Patagonian Shelf, near the westernmost edge of the Indian Ocean and the Subantarctic front.
Important, too, was the role of coccolithophores in sucking carbon dioxide from the environment and turning it into biological compounds. “The Great Calcite Belt is significant because this gigantic area of the ocean is full of these organisms that are fixing carbon,” Balch said in a news release.
Will the White Cliffs of the Patagonian Shelf eventually rise from the sea near Argentina? Perhaps. The scientists pointed to a paper published in 2015 that indicates chalky oozes are accruing on the southern seabed. To form new chalk layers, however, the microorganisms’ discarded plates still need to build up over thousands of millennia.
“While we don’t have the great cliffs of the Southern Ocean,” Balch said, “there is solid evidence that the calcite is making it to the seafloor.”