Big discovery for biogenic magnetite

Abstract

One of the most significant characteristics of the Anthropocene (the present age of geologic time) is the rate at which humans are perturbing the global carbon cycle. The potency of carbon dioxide and methane as greenhouse gases and their effects on Earth's temperature balance is well established (1), and the myriad of climate and ecological changes and feedbacks in response to this abrupt warming is the focus of much ongoing research (1, 2). The geologic record is one of our greatest assets in understanding the short- and long-term environmental responses to extreme fluctuations in the carbon cycle, and the Paleocene–Eocene Thermal Maximum (PETM), which occurred ≈55 million years ago, is an ideal analogue for the Anthropocene. The PETM is marked by an abrupt negative carbon isotope excursion that indicates a massive injection of light carbon into the oceans and atmosphere over a period of a few thousand years. This perturbation to the carbon cycle resulted in supergreenhouse conditions that persisted for as long as 180,000 years. Mean annual temperatures and deep and surface ocean temperatures at all latitudes rose by 5–8°C (3). Terrestrial plants and mammals diversified and radiated, and new marine microorganisms evolved and flourished while others disappeared forever. In this issue of PNAS, Schumann et al. (4) report evidence for new microorganisms that appeared and disappeared with the PETM, signaling another specific ecological response to the biogeochemical changes associated with this extreme warming event.