The Amazon River is at present winding through lush tropical forests, thick with green leaves and vines. Meanwhile in the mid-Atlantic coastal plain of the US, leaves are falling from the trees and grass is turning brown, ready for the first dusting of snow. At first glance, it would seem that the two systems have little in common. Yet, 55.5 million years ago, the mid-Atlantic shelf may have looked far more like Amazonia than any modern environment in North America.

Credit: © ISTOCKPHOTO/BRASIL2

During the Palaeocene–Eocene Thermal Maximum — a period of abrupt warming that began about 55.5 million years ago and lasted for around 100,000 years — temperatures rose by 5 to 9 °C. This warming came on top of temperatures that were already higher than today. This climate event is marked by a thick (2- to 15-m) layer of clay that was deposited in the Salisbury embayment, which stretches along the North American coast from Virginia to New Jersey. To the naked eye, the clay layer can be an attractive red colour. But the more exciting characteristics, at least for this line of research, are found by looking more closely: the clay is filled with an unusually high concentration of tiny single-domain magnetite grains. Based on an analysis of the magnetite grains, and their organic carbon-rich matrix, Robert Kopp of Princeton University and colleagues concluded that these grains were deposited on a hot and humid river-dominated shelf, not unlike a present-day tropical setting (Paleoceanography doi:10.1029/2009PA001783; in the press).

The grains in question were formed primarily by bacteria that secrete nanometre-size magnetite crystals to orient themselves in the water column. Kopp and colleagues concluded that to trigger a bloom of these bacteria, and preserve their remains, the suboxic zone of the seafloor sediments must have expanded from pre-Palaeocene–Eocene Thermal Maximum levels. Furthermore, in part owing to altered cycling of iron in the expanded suboxic zone, more reactive iron must have been available to the magnetotactic bacteria.

These conditions are surprisingly similar to those found on the modern Amazon shelf: soil and rock weathering in the hot and humid conditions brings abundant iron to the sediments, and high organic-matter deposition promotes up to 2 m of suboxic conditions in the underlying sediments. The Amazon-like conditions in the mid-Atlantic could have been supported by ancient precursors to either the Susquehanna or Potomac rivers, as both were in existence during the Palaeocene and Eocene epochs.

The temperature drop at the end of the Palaeocene–Eocene Thermal Maximum marked the end of the deposition of magnetofossil-rich clays, and the termination of the Amazon-like conditions.