A Seafloor Turned Time Capsule
The rolling hills of North Canterbury hold a globally significant climate record within a rock unit called the Waipara Greensand. This formation, exposed by the Waipara River, was once sediment on a shallow sea floor between 62 and 58 million years ago. Its distinct greenish color comes from high concentrations of glauconite, an iron- and potassium-rich silicate mineral that forms in marine environments with slow rates of deposition. In the Waipara area, the formation reaches a thickness of about 80 meters.
This layer of ancient seabed sediment records an important moment in Earth's history. Its upper boundary coincides with the transition from the Paleocene to the Eocene epoch, an interval marked by extreme global warming. Scientists study drill cores and exposed cliffs here to understand the Paleocene-Eocene Thermal Maximum (PETM). During this event, a massive release of carbon into the atmosphere and oceans caused global temperatures to rise by around 5°C. The rock layers at Waipara provide a high-resolution account of this ancient episode of rapid climate change, paralleling modern environmental shifts.
Reading the Climate Record
The story of the PETM is written in the chemistry of the rocks and the microscopic fossils they contain. Geologists analyze the ratio of carbon isotopes—specifically carbon-12 and carbon-13—preserved in the marine sediments. The PETM is identified globally by a sharp negative shift in this ratio, known as a Carbon Isotope Excursion (CIE). This signature indicates a massive injection of carbon-12 into the environment, likely from sources like volcanic activity or the melting of methane hydrates.
The greensand also holds the fossilized shells of foraminifera, single-celled marine organisms. Different species of foraminifera thrive at different ocean temperatures. By tracking which species are present in each layer, scientists can reconstruct past sea-surface temperatures. The layers corresponding to the PETM show a dramatic turnover, with warm-water species abruptly replacing cooler-water ones. These fossil records, combined with geochemical data from sites like Waipara, allow for a detailed reconstruction of the environmental cascade that occurred during the warming event.
The area also has many fossils for larger, more charismatic fossils from the Paleocene. The Waipara Greensand has yielded the remains of some of the world's oldest known penguins, including giant species like Crossvallia waiparensis, which stood up to 1.6 meters tall. Fossils of at least 16 types of sharks and bony-toothed birds (Pelagornithids) have also been found. These discoveries show that the ancient seas off the coast of Zealandia—the submerged continent New Zealand sits upon—were a center for seabird evolution just after the extinction of the dinosaurs.