A climate crisis in stone
Along the coast of the Basque Country, near Zumaia, a series of elegant, tilted rock layers known as a flysch sequence meets the sea. These stone pages record a dramatic event. One specific boundary, dividing the Paleocene and Eocene epochs around 56 million years ago, records a geologically brief period of intense global warming called the Paleocene-Eocene Thermal Maximum (PETM). This boundary is visible as a shift in the rock composition. The older Paleocene layers are full of carbonates from the shells of marine microorganisms, but these abruptly give way to a darker, clay-rich layer deposited during the PETM.
This change in geology signals a crisis in the oceans. The PETM was triggered by a massive release of thousands of gigatons of carbon into the atmosphere and ocean over several thousand years. The source of this carbon is still debated, with possibilities including massive volcanic activity, the sudden melting of frozen methane hydrates from the seafloor, or the burning of vast peat deposits. This injection of carbon dioxide caused severe ocean acidification. The increasingly corrosive water dissolved the calcium carbonate shells of tiny planktonic organisms called foraminifera before they could be preserved on the seafloor, leaving behind the dark clay layer visible today. The chemical signature of this event is a sharp negative excursion in the ratio of carbon-13 to carbon-12 isotopes, a marker found in rock layers of this age worldwide.