William T. Holser

The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation

Abstract Three hundred new samples of marine evaporite sulfate, of world-wide distribution, were analyzed for δ34S, and 60 of these also for δ18O in the sulfate ion. Detailed δ34S age curves for Tertiary—Cretaceous, Permian—Pennsylvanian, Devonian, Cambrian and Proterozoic times document large variations in δ34S. A summary curve for δ18O also shows definite variations, some at different times than δ34S, and always smaller. The measured δ34S and δ18O correspond to variations in these isotopes in sulfate of the world ocean surface. The variations of δ18O are controlled by input and output fluxes of sulfur in the ocean, three of which are the same that control δ34S: deposition and erosion of sulfate, and deposition of sulfide. Erosion of sulfide differs in its effect on the S and O systems. δ18O in the sulfate does not seem to be measurably affected by equilibration with either seawater or with subsurface waters after crystallization. In principle, the simultaneous application of both δ34S and δ18O age curves should help reduce the number of assumptions in calculations of the cycles of sulfur and oxygen through geological time, and a new model involving symmetrical fluxes is introduced here to take advantage of the oxygen data. However, all previously published models as well as this one lead to anomalies, such as unreasonable calcium or oxygen depletions in the ocean—atmosphere system. In addition, most models are incapable of reproducing the sharp rises of the δ34S curve in the late Proterozoic, the Devonian and the Triassic which would be the result of unreasonably fast net sulfide deposition. This fast depletion could result from an ocean that has not always been mixed (as previously assumed in all model calculations).

Paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite

Abstract Rare earth element concentrations in biogenic apatite of conodonts, fish debris and inarticulate brachiopods were determined in over 200 samples from Cambrian to modern sediments. Tests for experimental bias caused by the chemicals used to separate fossils from the rock matrix and for interlaboratory, interexperiment or interspecies related variations clearly show that no resolvable fractionation of REE occurs. Incorporation of REE in biogenic apatite of Recent fish debris occurs near the sediment-water interface soon after deposition and reflects characteristics of seawater. The original REE signature apparently survives subsequent burial and diagenesis. Cerium variations in fish debris from modern environments are controlled by redox potential. Ce is fractionated by co-precipitation with metallic oxides under oxidizing conditions. This fractionation produces a negative Ce anomaly (Ce anom ) in seawater that is reflected in the sedimented fish debris. Conversely, no fractionation of Ce occurs under reducing conditions, resulting in Ce concentrations that are normal to enriched in anoxic seawater and fish debris deposited under local or basinwide anoxic conditions. Extrapolation of observations of the redox control of Ce anom in modern oceans to fossil apatite indicates that anoxic conditions were prevalent in lower Paleozoic and Lower Triassic oceans, whereas in the upper Paleozoic, the world ocean was generally oxidizing.

Search for evidence of impact at the Permian-Triassic boundary in Antarctica and Australia

Life on Earth was almost destroyed some 250 m.y. ago in the most profound of all known mass extinction events. We investigated the possible role of impact by an extraterrestrial bolide through chemical and mineralogical characterization of boundary breccias, search for shocked quartz, and analysis for iridium in Permian-Triassic boundary sections at Graphite Peak and Mount Crean, Antarctica, and Wybung Head, Australia. Thin claystone breccias at the isotopically and paleobotanically defined boundary at all three locations are interpreted as redeposited soil rather than impact ejecta. The breccias at all three locations also yielded shocked quartz, but it is an order of magnitude less abundant (0.2 vol%) and smaller (only as much as 176 micrometers m diameter) than shocked quartz at some Cretaceous-Tertiary boundary sites. Faint iridium “anomalies” were detected (up to 134 pgṁg −1 ). These values are an order of magnitude less than iridium anomalies at some Cretaceous-Tertiary boundary sites. Furthermore, peak iridium values are as much as 1 m below the isotopically and paleobotanically defined boundary. The idea that impact caused the extinctions thus remains to be demonstrated convincingly.

Isotope geochemistry of oxygen in the sedimentary sulfate cycle

Abstract A re-evaluation of the isotopic geochemistry of oxygen involved in the exogenic sulfur cycle leads to a model in which δ 18OSO4 of seawater is determined by a dynamic balance of four fluxes: inputs by erosion of evaporite sulfate and by oxidative erosion of sulfide, and outputs of sulfate into evaporites and of carbon dioxide into seawater by bacterial reduction of sulfate to sulfide. The sulfate deposition-erosion loop is closed, but the sulfide loop is open, connected through the fixed reservoir of 18OH2O of seawater. The level of δ 18OSO4 is apparently not appreciably affected by either equilibration with 18OH2O, or by Lloyds proposed fast reduction-oxidation cycle on the sea floor. The possible effect of equilibration during a subsea hydrothermal circulation is unclear from available data. Calculations of hypothetical equilibration of 18 O 16 O SO 4 with 18 O 16 O H 2 O , either from the dehydration of gypsum or from formation waters, give δ 18OSO4 values much higher than any observed in old evaporites. Consequently, these processes were probably not significant in altering the δ 18OSO4 of seawater that is recorded in evaporites.

Isotope shifts in the Late Permian of the Delaware Basin, Texas, precisely timed by varved sediments

Closely spaced samples (285 in number) of varved sediments from the Upper Permian in Delaware Basin, Texas, have been analyzed for δ13Ccarb, δ13Corg, δ18Ocarb, Corg, Ccarb, and calcite/dolomite. δ13C records a dramatic rise from −2.8 to +5.7‰ in only 4400 years, detected in three sections across the basin, extrapolating smoothly through a 600-year interruption by a local (west side of the basin) fresh-water inflow evidenced by low δ18O. This continuity and low Corg within the basin, both indicate that the excess net deposition of Corg, necessary to generate the rise in δ13C, took place in the ocean external to the Delaware Basin. Correlation with similar records from the Zechstein Basin suggest that the event was world-wide, although this poses obvious difficulties for the carbon cycle. The rate of rise of δ13C, and its sustained high level, must imply conversions of oxidized carbon to reduced carbon that are very large depending on which reservoirs were involved.

Marine and Nonmarine Salts of Western Interior, United States

Bromide concentrations were compared among several Upper Permian and Jurassic salt rocks from various basins in the Western Interior. The Opeche salt (Guadalupian) from the Williston basin showed low concentrations of bromide, suggesting a strong nonmarine influence at the time of deposition. A Guadalupian salt from the Alliance basin showed values that fell in the normal marine range. Ochoan salts (Salado and Rustler Formations) from the Delaware basin, also showed normal marine values. The Jurassic Dunham Salt from the Williston basin contained no measurable bromide, suggesting that this is a second-cycle salt.