Achim D. Herrmann

Global-ocean redox variation during the middle-late Permian through Early Triassic based on uranium isotope and Th/U trends of marine carbonates

Uranium isotopes (238U/235U) in carbonates, a proxy for global-ocean redox conditions owing to their redox sensitivity and long residence time in seawater, exhibit substantial variability in the Daxiakou section of south China from the upper-middle Permian through the mid-lower Triassic (∼9 m.y.). Middle and late Permian ocean redox conditions were similar to that of the modern ocean and were characterized by improving oxygenation in the ∼2 m.y. prior to the latest Permian mass extinction (LPME), countering earlier interpretations of sustained or gradually expanding anoxia during this interval. The LPME coincided with an abrupt negative shift of >0.5‰ in δ238U that signifies a rapid expansion of oceanic anoxia. Intensely anoxic conditions persisted for at least ∼700 k.y. (Griesbachian), lessening somewhat during the Dienerian. Th/U concentration ratios vary inversely with δ238U during the Early Triassic, with higher ratios reflecting reduced U concentrations in global seawater as a consequence of large-scale removal to anoxic facies. Modeling suggests that 70%–100% of marine U was removed to anoxic sinks during the Early Triassic, resulting in seawater U concentrations of <5% that of the modern ocean. Rapid intensification of anoxia concurrent with the LPME implies that ocean redox changes played an important role in the largest mass extinction event in Earth history.

ASSESSING THE PALEOENVIRONMENTAL SIGNIFICANCE OF MIDDLE–LATE PENNSYLVANIAN CONODONT APATITE δ18O VALUES IN THE ILLINOIS BASIN

ABSTRACT Conodont apatite &dgr;18OV-SMOW values from Middle though Upper Pennsylvanian (Desmoinesian–Missourian) laminated, marine black shale units within cyclic deposits of intercalated terrestrial and marine strata (cyclothems) from the Illinois Basin (United States) were measured in order to evaluate their utility as a proxy for changes in the oxygen isotopic composition of the epicontinental Late Pennsylvanian Midcontinent Sea (LPMS). The average &dgr;18OV-SMOW values of well-preserved monogeneric (Idiognathodus) separates of conodont apatite from 12 lithologic units representing nine cyclothems range from 17.0‰ to 20.1‰ and average 19.0‰ ± 0.4‰ (1&sgr;). Within the limits of analytical uncertainty of stable isotope measurements, the stratigraphic distribution of conodont apatite &dgr;18O values is nontrending; particularly, there is no significant shift in &dgr;18O values across the Desmoinesian–Missourian boundary, a period that has been interpreted to preserve a shift toward a warmer climate, increased seasonality, and shorter periods of wet conditions in the terrestrial record. Conodont apatite &dgr;18O values from stratigraphically equivalent black shale members across the Illinois Basin vary up to 2.6‰, which is nearly equivalent to the observed stratigraphic range of conodont apatite &dgr;18O values, and suggests differences in local (basin-scale) seawater &dgr;18O values affected the conodont apatite &dgr;18O values. Within analytical uncertainty, conodont apatite &dgr;18O values from the Illinois Basin and Midcontinent Basin (United States) are indistinguishable, suggesting a component of overarching broader regional to global controls on seawater &dgr;18O values. Nevertheless, if the large variability observed in stratigraphically equivalent black shale members in the Illinois Basin is attributed to regional factors, these results indicate caution should be used when attempting to interpret temporal shifts from single aliquots of conodonts in epicontinental settings.

Multiple episodes of extensive marine anoxia linked to global warming and continental weathering following the latest Permian mass extinction

Multiple episodes of extensive oceanic anoxia delayed the marine ecosystem recovery from the latest Permian mass extinction. Explaining the ~5-million-year delay in marine biotic recovery following the latest Permian mass extinction, the largest biotic crisis of the Phanerozoic, is a fundamental challenge for both geological and biological sciences. Ocean redox perturbations may have played a critical role in this delayed recovery. However, the lack of quantitative constraints on the details of Early Triassic oceanic anoxia (for example, time, duration, and extent) leaves the links between oceanic conditions and the delayed biotic recovery ambiguous. We report high-resolution U-isotope (δ238U) data from carbonates of the uppermost Permian to lowermost Middle Triassic Zal section (Iran) to characterize the timing and global extent of ocean redox variation during the Early Triassic. Our δ238U record reveals multiple negative shifts during the Early Triassic. Isotope mass-balance modeling suggests that the global area of anoxic seafloor expanded substantially in the Early Triassic, peaking during the latest Permian to mid-Griesbachian, the late Griesbachian to mid-Dienerian, the Smithian-Spathian transition, and the Early/Middle Triassic transition. Comparisons of the U-, C-, and Sr-isotope records with a modeled seawater PO43− concentration curve for the Early Triassic suggest that elevated marine productivity and enhanced oceanic stratification were likely the immediate causes of expanded oceanic anoxia. The patterns of redox variation documented by the U-isotope record show a good first-order correspondence to peaks in ammonoid extinctions during the Early Triassic. Our results indicate that multiple oscillations in oceanic anoxia modulated the recovery of marine ecosystems following the latest Permian mass extinction.

The relationship of conodont biofacies to spatially variable water mass properties in the Late Pennsylvanian Midcontinent Sea

Molybdenum and uranium enrichment factors and nitrogen isotopes suggest that an interplay of open ocean upwelling and riverine runoff led to distinct spatial and secular variations in water mass properties within the epicontinental Late Pennsylvanian Midcontinent Sea of North America. In particular, the intensity of continental runoff influenced the flux of bulk organic matter to the sediment. Benthic anoxia appears to have been controlled by the vertical density gradient in the water column associated with continental runoff combined with the advection of basinal water. Anoxic conditions were stronger in proximal (i.e., more shoreward) areas of the Midcontinent Shelf, indicating that anoxia did not develop primarily due to upwelling of nutrient-rich waters along the southern shelf margin, as previously suggested. Changes in water mass redox conditions not only drove authigenic enrichment of redox-sensitive trace elements across the basin but also had a strong effect on the spatial distribution of various conodont taxa. Our analysis suggests that the widely accepted depth-stratification model for the distribution of conodonts is incomplete. Conodont biofacies distributions seem to have been controlled by physicochemical properties of the water mass (e.g., salinity, temperature, nutrients, turbidity, and/or dissolved oxygen levels) that may correspond less directly to water depth. The proximity to terrestrial freshwater influx and the strength of anoxia/euxinia in the subpycnoclinal water mass played significant roles in the spatial and temporal distributions of conodont taxa.

An ancient estuarine-circulation nutrient trap: The Late Pennsylvanian Midcontinent Sea of North America

The Late Pennsylvanian Midcontinent Sea (LPMS) of North America, which existed during glacioeustatic highstands of the late Paleozoic ice age, was an immense (>1 × 106 km2) cratonic interior sea exhibiting large-scale estuarine circulation, with a low-salinity surface plume overlying a high-salinity, anoxic, deep water mass. As in river estuaries, these conditions resulted in trapping and recycling of nutrients and organic-reactive elements (e.g., trace metals such as Mo, U, and Zn) in the subpycnoclinal water mass, leading to total organic carbon (TOC) to 40% and peak trace metal enrichment factors (EFs) of 540 (Mo), 73 (U), and 75 (Zn) in the black shale facies of the Hushpuckney Shale. Trace metal EFs increase gradually from distal (Oklahoma) to proximal (Iowa) areas of the Midcontinent Shelf before decreasing abruptly in the Illinois Basin, located to the east of the Mississippi River Arch (MRA). Owing to similar productivity (TOC) and redox (FeT/Al) proxy values on both sides of this arch, the large differences in trace metal EFs are interpreted to reflect divergent deep-water chemistries; specifically, much larger aqueous trace metal inventories on the Midcontinent Shelf than in the Illinois Basin. This condition implies that (1) deep waters of the Midcontinent Shelf and Illinois Basin were physically separated by the MRA, demonstrating its existence as a positive bathymetric feature during the Late Pennsylvanian, and (2) the saltwater wedge to the west of the MRA functioned as a nutrient trap in which organic-reactive trace metals were strongly concentrated through water-column recycling.