Wayne D. Goodfellow

Iridium anomaly in the upper devonian of the canning basin, Western australia.

A moderate iridium anomaly, about 20 times the local background, has been found in Upper Devonian rocks in the Canning Basin. It occurs at or near the Frasnian-Famennian boundary, which is known to be associated with a major massextinction event of global extent. The anomaly occurs in an extremely condensed limestone sequence laid down under quiet deepwater conditions. Its occurrence suggests a causal link with some form of meteoroid impact. Moreover, carbon isotope data indicate that a large reduction in biomass could have occurred at this level. However, the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium Frutexites; iridium, platinum, iron, manganese, cobalt, arsenic, antimony, and cerium are preferentially concentrated in filaments of this organism, with concentrations ranging from two to five times that of the matrix. It is possible that Frutexites extracted these elements directly from seawater, without the need for their derivation from an extraterrestrial source.

Ocean stagnation and ventilation defined by δ34S secular trends in pyrite and barite, Selwyn Basin, Yukon

Within the epicratonic Selwyn Basin, at least three cycles can be recognized for Paleozoic time when the water column alternated from open and ventilated to closed and stratified conditions. These cycles are recorded by δ34S values in pyrite that exceed those for coeval seawater during periods of stagnation and decrease markedly during periods of greater circulation. The marked increase in δ34S values for pyrite formed in stratified seas is controlled by the high percentage of sulfate bacterially reduced to sulfide, coupled with the removal of isotopically lighter sulfur from a closed system during pyrite sedimentation. The δ34S curve for barite during this time has a shape similar to the mean evaporite curve except that it is displaced positively, particularly in Frasnian time. This pronounced increase in δ34S values for barite of Frasnian age coincides with the mixing of isotopically heavier sulfur, accumulated earlier in a stratified water column, with surface waters during a ventilation event.

Anoxic stratified oceans as a source of sulphur in sediment-hosted stratiform ZnPb deposits (Selwyn Basin, Yukon, Canada)

Abstract Three periods of stagnation, viz. Middle Ordovician to Early Silurian, Early Devonian, and Late Devonian, have been recognized in the Selwyn Basin by extremely positive δ 34 S-values in pyrite. These δ 34 S-values in pyrite reflect almost complete bacterial sulphate reduction of sulphate to sulphide within a restricted anoxic layer of a stratified water column. W.C. Shanks and coworkers have reported a fourth anoxic event in the basin during the Early Cambrian. Each period of basin stagnation coincides with times when sedimentary Zn Pb-sulphide and barite deposits formed, while intervening periods correspond with the formation of stratiform barite deposits. The stratigraphic distribution of δ 34 S-values for pyrite are similar across the basin, whether near to or remote from mineralization, indicating a common seawater source of S. The generally parallel tracking of isotopic curves for sphalerite and galena with pyrite indicates that this sulphide also came from the bacterial reduction of sulphate within a stratified water column. This interpretation is supported by data on hydrothermal fluids that produced Zn Pb mineralization, which are near-neutral pH and range in temperature between 100° and 350°C. In such solutions, it is thermodynamically difficult to transport appreciable sulphide in the presence of Pb and Zn. Since these conditions apply to mineralizing fluids of many sedimentary-exhalative Zn Pb deposits, it follows that they too were likely S-poor, requiring an external seawater source of sulphide. This source is most effectively supplied by the sulphidic bottom layers of a stratified water column. This explains the correlation between anoxic events and Zn Pb sedimentary deposits, both within the Selwyn Basin and world-wide during the Phanerozoic. The latter observation also implies that the anoxic events within the Selwyn Basin were not controlled locally, but instead reflect periods of worldwide ocean stagnation.

CARBON AND SULFUR ISOTOPE ANOMALIES ACROSS THE FRASNIAN-FAMENNIAN EXTINCTION BOUNDARY, ALBERTA, CANADA

Stable isotopes of carbonate and organic carbon from two Upper Devonian sections in Alberta, Canada, indicate a “heavy carbon event” across the Frasnian-Famennian (F-F) boundary and a negative δ 13 C shift at the boundary. At Cinquefoil Mountain, δ 13 C values in carbonate are anomalously high (3.5‰ to 4.5‰ PDB [Peedee belemnite]) within an interval (10 m below to 23 m above the F-F boundary) representing about 1 m.y. (two standard conodont zones), compared to background values of ∼1‰ above and below this interval. Superimposed on this positive excursion is a marked δ 13 C decrease of about 1‰ at precisely the F-F boundary. At Medicine Lake, a similar δ 13 C excursion in organic carbon, from a background of −29‰ to a high of −26.5‰, is recorded across the F-F boundary. The δ 34 S data for iron sulfide in these sections show unusually high values (as much as 15‰ to 20‰ CDT [Canon Diablo troilite]) in the F-F boundary interval. These data, together with those from other areas, indicate a period of major global changes (e.g., greenhouse and marine anoxia) that took place well before and after the sudden biomass killing event at the F-F boundary. Existing evidence suggests that the F-F mass extinction was probably caused by a large extraterrestrial impact on Earth at a time when the global ecosystem was already severely stressed.

Massive sulfides in a sedimented rift valley, northern Juan de Fuca Ridge

A number of mounds each several hundred meters across and up to sixty meters high have been observed with SeaMARC II acoustic imagery and Seabeam bathymetry in the sediment-filled axial valley at the northern end of the Juan de Fuca Ridge. The mounds are located a few kilometers west of the eastern valley-bounding normal fault scarp where the local sediment fill is approximately 300 m thick. All of the mounds are believed to be of hydrothermal origin, and one is associated with anomalously high heat flow in excess of 1 W m−2. A piston core collected from that mound comprises coarse clastic sulfide units interbedded with sulfidic muds. Hydrothermal minerals present in the 2.3 m section include pyrrhotite, pyrite, marcasite, sphalerite, chalcopyrite, iss (intermediate solid solution in the CuFeZnS system), chalcopyrrhotite, galena, talc, barite, and amorphous silica. Mineral fabrics of the clasts indicate that the material was precipitated at or near the sea floor by mixing of hot hydrothermal fluids with cold seawater. Low concentrations of Zn, Cu, Cd, and Ag relative to those found in unsedimented ridge hydrothermal deposits, and the presence of pyrrhotite as an early phase mineral indicates that the vent fluids have been modified by reaction with sediments beneath the mound. Rapid sedimentation in a rift valley is clearly conducive to the formation of large hydrothermal mineral deposits like those believed to be present within and beneath these mounds. The relatively impermeable sediment cover insulates the crust, inhibits groundwater recharge, promotes long-lived discharge at a restricted number of sites, provides a substrate for the efficient subsurface precipitation of minerals, and through continued sedimentation, protects surficial deposits from the corrosive effects of seawater. No reliable estimate of the bulk composition of the mounds can be made with existing data, but their size is comparable to major hydrothermal mineral deposits found on land; ancient settings in which many land deposits formed are in many ways similar to the one in which the features described here are currently forming.

Sulfur-isotope anomaly associated with the Frasnian-Famennian extinction, Medicine Lake, Alberta, Canada

A sharp positive δ34S anomaly occurs in a 0–5 cm siltstone that contains syndepositional pyrite and was deposited below wave base as part of a basinal off-reef succession at Medicine Lake, Alberta. The bed rests on bioturbated, oxygenated dolomitic siltstones that are positively correlated with biostromal banks containing the last Frasnian shelly benthos over a wide region. Above, laminated anoxic argillaceous lime-mudstones grade upward into beds containing Famennian conodonts and brachiopods. Geochemical and sedimentological data suggest flooding of the craton by basin-derived anoxic water at the extinction horizon as a result of a sudden deep basin or ocean turnover. Comparable positive δ34S values are found in sedimentary pyrite in Frasnian black shales from a passive margin basin in the Yukon to the north; values decrease in the Famennian.

Facies and geochemical evidence bearing on the end‐triassic disappearance of the alpine reef ecosystem

A carbon and oxygen isotope profile is presented across the Triassic‐Jurassic boundary at the classic locality of Kendelbach, Austria. In conjunction with facies data it lends no support to the claim that the spectacular disappearance of the reef ecosystem at the end of the Triassic was due to a sharp fall of seawater temperature. A model relating mass extinction to sea‐level change is preferred.

Hydrothermal petroleum at the seafloor and organic matter alteration in sediments of Middle Valley, Northern Juan de Fuca Ridge

Abstract Sediments from high heat flow areas of Middle Valley, Northern Juan de Fuca Ridge, contain traces of bitumens with insignificant amounts of thermogenic components and mainly lipid material which has experienced accelerated diagenesis. These sediments have been exposed to mild hydrothermal conditions (50–120°C), based on their bitumen compositions. The hydrothermal bitumen in a barite alteration products. The aliphatic fraction represents the mildly transformed and immature matter and the major polynuclear aromatic hydrocarbons are the high temperature constituents (>300°C). The 14C age of this bitumen is about 29 000 aBP, reflecting rapid organic carbon cycling.

Ru/Ir ratios at the Cretaceous-Tertiary boundary: Implications for PGE source and fractionation within the ejecta cloud

Abstract Ruthenium and Ir are the least mobile platinum group elements (PGEs) within the Cretaceous-Tertiary (K-T) boundary clay. The Ru/Ir ratio is therefore the most useful PGE interelement parameter for distinguishing terrestrial and extraterrestrial contributions to the boundary clay. The Ru/Ir ratio of European marine K-T sections (1.67 ± 0.38) is statistically different from that of the North American continental sections (0.76 ± 0.26). It is unlikely that this difference is due to secondary PGE remobilization, PGE input to the boundary clay during multiple impacts, or volcanic emissions. The global difference in Ru/Ir ratios in the boundary clay may therefore be primary. The positive correlation between distance from the Chicxulub impact structure, Yucatan, Mexico, and Ru/Ir ratio and the more than 1000°C difference in the condensation temperatures of these elements lead us to propose that fractionation of Ru from Ir during condensation from the ejecta cloud may have occurred, resulting in the global difference in Ru/Ir ratios.

The Frasnian-Famennian extinction event in a stable cratonic shelf setting: Trout River, Northwest Territories, Canada

Abstract The Frasnian-Famennian (F-F) extinction event occurs in platformal carbonates along the Trout River, N.W.T. Paleogeographically the site was about 150 km east of the western cratonic margin. Faunal evidence, based primarily on corals, and sedimentation rates of passive cratonic margins suggest that the late Frasnian Redknife and Kakisa Formations include both the Upper rhenana and the linguiformis conodont zones. The high biodiversity of the benthic fauna during the late Frasnian decreases sharply at the F-F boundary and recovers only gradually during the early Famennian. This trend is also expressed by the profile of δ13C values. Microfacies analysis has identified two minor hiatuses at the F-F boundary. The lower break is represented by an unconformity at the upper contact of the latest Frasnian Kakisa Formation and is associated with surficial karstification and brecciation of the underlying biostromal carbonates. Anomalous trace element concentrations at the unconformity are interpreted as possible fallout material related to a bolide impact. This hiatus probably corresponds to the Lower triangularis Zone, because no diagnostic conodonts of that time interval are present. The lower and upper hiatuses are separated by the deposition of a wackestone-sandstone facies with an impoverished fauna of sponges, calcareous algae and some foraminifera, preserved only in neptunian dykes in the upper Kakisa and as fragments in the overlying Famennian Trout River Formation. The depauperate fauna suggests hostile conditions and/or a habitat ravaged by a catastrophic event—common characteristics after a global extinction event. Conodont evidence suggests that this brief depositional interval corresponds to the Middle triangularis Zone. The upper hiatus probably straddles the contact of the Middle and Upper triangularis zones. Evidence for this break are angular fragments of the wackestone-sandstone facies in the basal Trout River Formation. Conodonts from the basal siliciclastics of the Trout River Formation assign the lower part of this unit to the Upper triangularis Zone. In Alberta, 900 km to the south, similar platformal carbonates show a longer hiatus (the entire triangularis Zone) at the F-F contact, whereas slope sediments indicate continuous sedimentation with a sharp drop in biodiversity and a temporary proliferation of primitive organisms at the F-F boundary.