Mark E. Patzkowsky

Phanerozoic trends in the global diversity of marine invertebrates.

It has previously been thought that there was a steep Cretaceous and Cenozoic radiation of marine invertebrates. This pattern can be replicated with a new data set of fossil occurrences representing 3.5 million specimens, but only when older analytical protocols are used. Moreover, analyses that employ sampling standardization and more robust counting methods show a modest rise in diversity with no clear trend after the mid-Cretaceous. Globally, locally, and at both high and low latitudes, diversity was less than twice as high in the Neogene as in the mid-Paleozoic. The ratio of global to local richness has changed little, and a latitudinal diversity gradient was present in the early Paleozoic.

Effects of sampling standardization on estimates of Phanerozoic marine diversification.

Global diversity curves reflect more than just the number of taxa that have existed through time: they also mirror variation in the nature of the fossil record and the way the record is reported. These sampling effects are best quantified by assembling and analyzing large numbers of locality-specific biotic inventories. Here, we introduce a new database of this kind for the Phanerozoic fossil record of marine invertebrates. We apply four substantially distinct analytical methods that estimate taxonomic diversity by quantifying and correcting for variation through time in the number and nature of inventories. Variation introduced by the use of two dramatically different counting protocols also is explored. We present sampling-standardized diversity estimates for two long intervals that sum to 300 Myr (Middle Ordovician-Carboniferous; Late Jurassic-Paleogene). Our new curves differ considerably from traditional, synoptic curves. For example, some of them imply unexpectedly low late Cretaceous and early Tertiary diversity levels. However, such factors as the current emphasis in the database on North America and Europe still obscure our view of the global history of marine biodiversity. These limitations will be addressed as the database and methods are refined.

A weathering hypothesis for glaciation at high atmospheric pCO2 during the Late Ordovician

New paired carbonate and organic-carbon isotope analyses from Nevada, USA, together with a consideration of the effects of mountain-building and ice-sheet coverage of the continents on atmospheric pCO2, lead to a new hypothesis for the cause of the Late Ordovician glaciation. We suggest that the Taconic orogeny, which commenced in the late-middle Ordovician, caused a long-term decline in atmospheric pCO2 through increased weatherability of silicate rocks. Ice-sheet growth was triggered when pCO2 decreased to a threshold of ∼10× present atmospheric level and proceeded by positive ice-albedo feedback. In the midst of glaciation, atmospheric pCO2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets. At first, this enhanced greenhouse effect was overcompensated for by ice-albedo effects. Ultimately, however, atmospheric pCO2 reached a level which overwhelmed the cooling effects of ice albedo, and the glaciation ended. The isotope results can be interpreted to indicate that atmospheric pCO2 rose during the glaciation, consistent with other proxy information, although alternative interpretations are possible. The large, positive carbonate isotope excursion observed in Late Ordovician rocks around the world is explained as the expected response to increased carbonate-platform weathering during glacioeustatic sea-level lowstand, rather than as a response to increased organic-carbon burial.

GRADIENT ECOLOGY OF A BIOTIC INVASION: BIOFACIES OF THE TYPE CINCINNATIAN SERIES (UPPER ORDOVICIAN), CINCINNATI, OHIO REGION, USA

Abstract Recent studies have emphasized that faunal change is typically brief and most commonly occurs at sequence boundaries and major flooding surfaces. The Upper Ordovician of the Cincinnati, Ohio, region records a major biotic invasion in the Richmondian Stage, which offers an opportunity to test these generalizations and to understand how episodes of faunal change are reflected in the structure of ecological gradients. The early Cincinnatian (C1–early C4 depositional sequences) displays two relatively stable faunal gradients, with the primary gradient reflecting onshore-offshore setting and the secondary gradient reflecting substrate consistency. During the mid-C4 sequence, dominant taxa of the shallow subtidal are extirpated, while deep subtidal taxa expand into those habitats, leading to a loss of cross-shelf faunal differentiation. This faunal breakdown is accompanied into the mid-C5 by a series of ecological epiboles, indicating an ongoing flux in ecological associations. The onshore-offshore gradient is reestablished in the C5, albeit with new associations dominated by or containing immigrant taxa. Recognition of this gradient is hindered by widespread increased abundance of bryozoans and by the delayed appearance of at least seven common genera of brachiopods and corals. The Richmondian Invasion plays out over multiple sequences and is not confined to a brief interval at the beginning of a sequence. These faunal changes do not coincide with sequence boundaries or major flooding surfaces and therefore cannot be sequence stratigraphic artifacts, nor can they represent a geologically instantaneous faunal response to sea-level change.

Biofacies replacement in a sequence stratigraphic framework; Middle and Upper Ordovician of the Nashville Dome, Tennessee, USA

Quantitative analysis of fossil assemblages from ten depositional sequences identified the spatial and temporal distribution of shallow and deep subtidal biofacies in Middle and Upper Ordovician strata of the Nashville Dome. Articulate brachiopods and bryozoans dominate most biofacies, although corals, sponges, gastropods, and bivalves are also important taxa that dominate biofacies locally. Temporal patterns in both lithofacies and biofacies indicate a close link between environmental shifts and faunal changes that took place over a period of approximately 13 million years. The primary cause of the environmental and biotic changes was Taconic orogenesis, which increased the flux of fine-grained siliciclastics and nutrients to the epeiric seaway and affected circulation patterns by changing basin topography. Biofacies replacement was accomplished not only by taxonomic turnover, but also by wholesale changes in the dominance structure of local assemblages. Changes in dominance also occurred at the guild level, with dominance by reclining epifaunal suspension feeders (RESF) shifting to pedunculate epifaunal suspension feeders and back to RESF in concert with environmental changes. Although all major macrofaunal groups involved in biofacies replacement were affected by the changes in paleoceanographic conditions, local extinction and immigration was not synchronous among all genera, but occurred at different times for different genera. This suggests that these Middle and Upper Ordovician biofacies are composed of communities that were not tightly integrated in their structure, but rather were fluid with species and genera moving in and out of communities in response to local changes in environmental conditions.

Biotic response to a Middle Ordovician paleoceanographic event in eastern North America

Global paleontological events such as mass extinctions have been the focus of attention in recent years, yet regional biotic events account for a greater proportion of extinction in the fossil record and should receive greater attention if we are to fully understand extinction processes over the Phanerozoic. We use a sequence stratigraphic framework to document a major extinction and migration episode that occurred in the Middle and Late Ordovician of North America. This event is characterized in eastern North America by coordinated changes in faunal assemblages, carbonate lithologies, siliciclastic influx, and phosphatization. We ascribe these changes to a regional paleoceanographic event brought on by thrust-driven flexural subsidence and sea-level fluctuations that lowered water temperature and increased turbidity and nutrient input.

From Cyclothems to Sequences: The Record of Eustasy and Climate on an Icehouse Epeiric Platform (Pennsylvanian-Permian, North American Midcontinent)

The stratigraphic succession of the North American Midcontinent contains a high-resolution (< 100,000 year) record of sea level and climate change spanning the Pennsylvanian-Permian boundary. Outcrop-based sequence stratigraphic analysis on the upper Wabaunsee, Admire, and lower Council Grove groups in Kansas and Nebraska reveals a hierarchy of fifty-one fifth-order meter-scale cycles grouped into five fourth-order composite sequences. Fifth-order cycles are bounded by subaerial unconformities in nearshore settings and correlative conformities in offshore settings, and are therefore very thin depositional sequences. Lowstand systems tracts, observed only in nearshore deposits, are expressed as sandy, incised-valley fills in these meter-scale cycles. Transgressive systems tracts, which are dominated by carbonate deposition, include coastal evaporites indicating arid climatic conditions. Highstand systems tracts, which are dominated by siliciclastic deposition, include thin but persistent coals in muddy coastal successions indicating humid climatic conditions. Paleosols at sequence boundaries evolve from vertic to calcic, indicating climate change from relatively humid to relatively arid conditions during subaerial exposure. This indicates that relatively arid climate coincided with eustatic lows while relatively humid climate coincided with eustatic highs on this low-latitude platform. Composite sequences are bounded by subaerial, angular unconformities and display a transgressive-regressive stacking pattern of meter-scale cycles. The high-resolution stratigraphic record of the Midcontinent helps constrain the rate and magnitude of environmental change in ancient icehouse conditions and can serve as a baseline of environmental change for comparison with other coeval successions.

Distal orogenic effects on peripheral bulge sedimentation; Middle and Upper Ordovician of the Nashville Dome

ABSTRACT A major switch in depositional style in the Ordovician carbonates of the Nashville Dome corresponds closely with the onset of the late Middle Ordovician Taconic orogeny. This time marks a shift from tropical-type to temperate-type carbonates, the initiation of widespread major phosphate deposition, the introduction of large amounts of terrigenous silt and clay, the occurrence of widespread seismically induced soft-sediment deformation, and a change from a low-energy flat-topped carbonate shelf to a high-energy doubly dipping carbonate ramp. Soft-sediment deformation and the introduction of siliciclastics are direct effects of the Taconic orogeny; the switch from tropical-type to temperate-type carbonates, the initiation of phosphate deposition, and the switch in carbonate ramp are lar ely oceanographic effects triggered by the orogeny. In particular, phosphate deposition and the switch to temperate-type limestones appears to have been driven by upwelling along the eastern side of the Nashville Dome within the newly deepened Taconic foreland basin. A fourfold decrease in the rate of relative sea-level rise occurred on the Nashville Dome nearly 3 m.y. following the onset of thrusting and foreland basin initiation. Subsidence rates were constant before and after this decrease, and no evidence of a change in subsidence rates is seen to coincide with the onset of thrusting. The slowing of subsidence may reflect viscoelastic uplift of the Nashville Dome, but the abrupt change from one constant subsidence rate to another is not predicted by existing foreland basin models.

Patterns of turnover in Middle and Upper Ordovician brachiopods of the eastern United States: a test of coordinated stasis

A compilation of species occurrences in a chronostratigraphic framework of depositional sequences from a 250,000 km2 area in the eastern United States is used to test for coordinated stasis in Middle and Upper Ordovician articulate brachiopods. Two rapid pulses of turnover in brachio- pod species separate three periods of relatively lower turnover (ecologic-evolutionary (EE) sub- units) that range from 3 to 9 m.y. in duration. Turnover within these EE subunits is characterized by high levels of percent species origination (ca. 60%) and percent species extinction (ca. 80%) and low levels of percent species persistence (<10%), all of which fall outside the range reported for coordinated stasis. Turnover between EE subunits is characterized by low levels of percent species holdover and percent species carryover (ca. 10% or less) and is consistent with coordinated stasis, although turnover pulses are driven largely by pulses in either extinction or origination, and not by pulses in both, as reported for coordinated stasis. Taken together, although these data display a marked bimodality in turnover, high levels of turnover within EE subunits is inconsistent with a pattern of coordinated stasis. Turnover rates within these EE subunits are much higher than pre- vious global estimates for Cambro-Ordovician brachiopods or Phanerozoic marine species and in- dicate that local extirpation and migration play a significant role in regional biodiversity dynamics. Despite the high level of turnover observed within these EE subunits, some level of ecologic sta- bility occurs because abundant genera persist throughout entire EE subunits. Ordovician species in this study behaved relatively independently of other taxa and were not tightly integrated as suggested by the broadly overlapping taxon abundance curves, the shifting habitat preference of some taxa, the piecemeal turnover between EE subunits, and the continuous creation of new species associations due to background levels of turnover within EE subunits. Turnover within EE subunits was associated with relatively stable or only mildly fluctuating environments. Rapid turnover be- tween EE subunits was caused by extreme perturbations to the regional or possibly global ocean- climate system.

Measuring Recurrence of Marine Biotic Gradients: A Case Study from the Pennsylvanian-Permian Midcontinent

Abstract The aim of this study was to determine whether biotic associations of Pennsylvanian-Permian brachiopods and bivalves from the northern Midcontinent differ in their degree of recurrence through time. The study interval includes 2.5 Myr that can be divided into 5 full and 2 partial composite depositional sequences separated by subaerial unconformities. These stratigraphic packages represent replicate natural experiments in establishing the benthic marine ecosystem of the basin. Based on cluster and ordination analyses, two discrete biofacies can be recognized—one dominated by brachiopods and the other by bivalves. Within each of these, environmental gradients can be recognized. The brachiopod gradient is interpreted to reflect the degree of water-column oxygenation, whereas the bivalve gradient is interpreted to reflect the transition from restricted to open-marine conditions. Comparison of measured recurrence with randomized data indicates that the ecological segregation of the two biofacies is maintained to a significant degree through the succession of depositional sequences in the study interval. In contrast, the gradients within each biofacies, although recognizable, are not maintained rigidly from sequence to sequence. There is also no significant difference in gradient recurrence between the two biofacies. These results imply that there is no need to call upon strong interspecific interactions to maintain the structure of these paleocommunities through time.