A DCO-sponsored team led by Martin Van Kranendonk (University of New South Wales), with geochemist John Valley (University of Wisconsin), economic geologist Franco Pirajno (Geological Survey of Western Australia), and mineralogist Robert Hazen (Carnegie Institution), conducted field studies in the remote North Pole Dome area of the Pilbara Complex, Western Australia, from 17-27 June 2014. They were accompanied by several graduate students, as well as Emmy Award winning producer/director Doug Hamilton and his film crew from the celebrated NOVA TV series (WGBH, Boston).
The North Pole Dome area is a roughly circular ring of hills, approximately 12 kilometers in diameter, surrounding a relatively flat depression that represents a “caldera”—the collapsed center of a 3.5 billion-year-old volcano. Following the collapse, the caldera gradually filled with layers of sediments, some of which contain pristine microbial fossil mounds called “stromatolites,” as well as other features that point to a dynamic shallow water environment. The North Pole Dome is a small feature within the ancient Pilbara Complex, which represents a remarkable raft of virtually unaltered crust from Earth’s Archean Eon.
Van Kranendonk and colleagues focused on three DCO-related studies during the 11-day effort. First, they systematically collected samples of the oldest known unaltered primary carbonate formations, including biogenic carbonate stromatolites and abiogenic layers and cavity-filling masses of chemically precipitated carbonates. These specimens have been added to a growing collection of carbonates from around the world, spanning the past 3.5 billion years. Systematic studies of the trace and minor elements in these specimens are revealing changing near-surface environments, and associated evolution of the carbon cycle through deep time.
A second focus of the North Pole Dome field studies was systematic collection of black, carbon-bearing chert (a form of silica deposited as veins by warm to hot hydrothermal fluids). Black chert from this region is famous for holding possible fossil microbes preserved as microscopic black spheres and filaments. Several dozen specimens will be analyzed for total organic carbon content, as well as isotopic characteristics that might be used to distinguish biogenic versus abiogenic sources of carbon. Once these specimens are surveyed, subsequent analyses will attempt to characterize the detailed nature of the organic carbon residues that produce the distinctive black color.
Finally, in an effort to understand Earth’s varied near-surface environments at the time of life’s origins, the field team collected sandstones from a distinctive sediment horizon of the 3.5 billion-year-old formations. Similar, somewhat younger, rocks in nearby regions of Western Australia have yielded a treasure trove of ancient grains of zircon, a sturdy mineral that survives billions of years as sediment grains in much younger rocks. Individual zircon grains can be dated—some as old as 4.4 billion years provide the oldest known surviving fragments of our planet. Extending this exciting work, the DCO team will search for other distinctive “heavy detrital minerals,” each of which might reveal clues about Earth’s earliest crustal formation and evolution.
Submitted by Robert M. Hazen, 4 July 2014