Peir K. Pufahl

An authigenic origin for Precambrian greenalite: Implications for iron formation and the chemistry of ancient seawater

Persistent anoxia and the lack of a skeletal silica sink through the Precambrian would have promoted a variety of reactions between iron and dissolved silica through much of Earth’s early history. However, although both iron and silica have each left clear fingerprints in the Precambrian record, evidence for their interaction, and the attendant biogeochemical consequences, is cryptic. Here, experimental evidence is presented showing that Fe 2+ and SiO 2 (aq) in anoxic seawater–derived solutions promote rapid nucleation of a hydrous Fe(II)-silicate gel at 25 °C. By merging experimental data with crystallographic constraints, we observe that structural rearrangement and dehydration produce Fe-rich serpentine nanoparticles within the gel, which eventally aggregate to form the mineral greenalite. This nonclassical crystal growth pathway is consistent with the crystal structure of greenalite and with its syndepositional origin in iron formation. A mechanistic underpinning for greenalite precipitation also permits new constraints on the chemistry of ferruginous Precambrian waters. For example, greenalite may have nucleated from waters with a pH as high as 7.7–8.3, implicating alkalinity as a key trigger in coupling and decoupling Fe and Si during the anoxic deposition of several late Archean and Paleoproterozoic iron formations. The common, though not exclusive, association of greenalite with deeper-water iron formation facies (i.e., below the fair-weather wave base) suggests that the upwelling of silica-rich alkaline water masses played an important role in driving precipitation. More broadly, our results prompt a reconsideration of the inorganic reactions that determine the upper limits on water-column Fe 2+ concentrations in nonsulfidic seawater. The primary precipitation of greenalite and/or siderite would set a ceiling for dissolved Fe 2+ that is sensitive to pH, and higher than previously estimated. These results indicate that a better understanding of greenalite distributions in chemical and siliciclastic sediments will help to disentangle the coevolution of redox and acid-base chemistries through the Precambrian.

Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan

An ejecta layer produced by the Sudbury impact event ca. 1850 Ma occurs within the Baraga Group of northern Michigan and provides an excellent record of impact-related depositional processes. This newly discovered, ∼2–4-m-thick horizon accumulated in a peritidal environment during a minor sea-level lowstand that punctuated a period of marine transgression. Common ejecta clasts include shock-metamorphosed quartz grains, splash-form melt spherules and tektites, accretionary lapilli, and glassy shards, suggesting sedimentation near the terminus of the continuous ejecta blanket. Sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact-generated tsunami wave train. Observed platinum group element anomalies (Ir, Rh, and Ru) within the Sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite. Documenting and interpreting the detailed characteristics of the Sudbury ejecta horizon in Michigan have yielded a fingerprint to identify this chronostratigraphic marker in other Paleoproterozoic basins. For the first time a foundation exists to assess the consequences of the Sudbury impact on Precambrian ocean chemistry and early life.

USING MULTIPLE ENVIRONMENTAL PROXIES TO DETERMINE DEGREE OF MARINE INFLUENCE AND PALEOGEOGRAPHICAL POSITION OF THE JOGGINS FOSSIL CLIFFS, UNESCO WORLD HERITAGE SITE

Abstract The Joggins Fossil Cliffs site was selected as a UNESCO World Heritage site because of its unparalleled preservation of Pennsylvanian terrestrial organisms in their environmental context. Despite an abundance of research over the past 150 years, significant questions remain regarding the Joggins paleoenvironment, including the degree of marine influence and whether the gymnospermous Cordaites trees may represent the earliest mangroves. Sedimentologic and paleontologic data from interbedded limestone beds indicate open marine conditions in the oldest part of the section, with a waning marine influence up section. Limestone beds, which occur primarily at the base of cycles interbedded with coal and floodplain deposits, are 15–100 cm thick and contain ostracodes, bivalves, and echinoderm fragments. Independent lines of evidence to support a diminishing marine influence in fluvial and coastal deposits with time include: (1) the presence of punctate brachiopods, echinoderm fragments, and ostracodes infilled with framboidal pyrite in older limestone beds; (2) antithetic abundances between ostracodes and freshwater bivalves; and (3) an overall coarsening upward in the sequence. These results indicate that Joggins was, at least in the oldest portion of the formation, closer to the open ocean than previously surmised.

A Marine Incursion In the Lower Pennsylvanian Tynemouth Creek Formation, Canada: Implications For Paleogeography, Stratigraphy and Paleoecology

Abstract We document the occurrence of a marine bed, and its associated biota, in the Lower Pennsylvanian (Langsettian) Tynemouth Creek Formation of New Brunswick, and discuss its implications for paleogeography, stratigraphy, and paleoecology. This is only the second marine interval found in the entire Pennsylvanian fill of the Maritimes Basin of Canada, the other being recently found in the broadly same-age Joggins Formation of Nova Scotia. Evidence for the new marine transgression comprises an echinoderm-rich limestone that infills irregularities on a vertic paleosol surface within the distal facies of a syntectonic fluvial megafan formed under a seasonally dry tropical climate. Gray, platy ostracod-rich shales and wave-rippled sandstone beds that directly overlie the marine limestone contain trace fossils characteristic of the Mermia Ichnofacies, upright woody trees, and adpressed megafloras. This association represents bay-fills fringed by freshwater coastal forests dominated by pteridosperms, cordaites, and other enigmatic plants traditionally attributed to dryland/upland habitats. The fossil site demonstrates that marine transgressions extended farther into the interior of Pangea than has previously been documented, and may allow correlation of the Tynemouth Creek and Joggins Formations with broadly coeval European successions near the level of the Gastrioceras subcrenatum and G. listeri marine bands. It also helps explain the close similarity of faunas between the Maritimes Basin and other paleotropical basins, if transgressions facilitated migration of marine taxa into the continental interior.

Riverine mixing and fluvial iron formation: A new type of Precambrian biochemical sediment

Precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early Earth. The appearance of large, economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean-atmosphere system near the end of the Archean. Exhalative iron formation contains a record of hydrothermal vent chemistry through time. We introduce here fluvial iron formation, a new type of Fe-rich microbial-biochemical sediment that formed by mixing river discharge and seawater in coastal environments. The Paleoproterozoic Chiall Formation (ca. 1.8 Ga), Earaheedy Basin, Western Australia, contains laminated and granular hematitic iron formation in delta channel deposits. Where mixing occurred in adjacent peritidal settings, laminated iron formation and hematitic oncoids formed. Because fluvial iron formations precipitated at the interface between terrestrial and marine realms, the locus of known Fe precipitation processes is shifted landward into paleoestuarine settings and reflects Fe derived from both terrestrial weathering and coastal upwelling, providing a new window into ocean-atmosphere evolution.