David L. Kidder

Causes and consequences of extreme Permo-Triassic warming to globally equable climate and relation to the Permo-Triassic extinction and recovery

Permian waning of the low-latitude Alleghenian/Variscan/Hercynian orogenesis led to a long collisional orogeny gap that cut down the availability of chemically weatherable fresh silicate rock resulting in a high-CO2 atmosphere and global warming. The correspondingly reduced delivery of nutrients to the biosphere caused further increases in CO2 and warming. Melting of polar ice curtailed sinking of O2- and nutrient-rich cold brines while pole-to-equator thermal gradients weakened. Wind shear and associated wind-driven upwelling lessened, further diminishing productivity and carbon burial. As the Earth warmed, dry climates expanded to mid-latitudes, causing latitudinal expansion of the Ferrel circulation cell at the expense of the polar cell. Increased coastal evaporation generated O2- and nutrient-deficient warm saline bottom water (WSBW) and delivered it to a weakly circulating deep ocean. Warm, deep currents delivered ever more heat to high latitudes until polar sinking of cold water was replaced by upwelling WSBW. With the loss of polar sinking, the ocean was rapidly filled with WSBW that became increasingly anoxic and finally euxinic by the end of the Permian. Rapid incursion of WSBW could have produced ∼20 m of thermal expansion of the oceans, generating the well-documented marine transgression that flooded embayments in dry, hot Pangaean mid-latitudes. The flooding further increased WSBW production and anoxia, and brought that anoxic water onto the shelves. Release of CO2 from the Siberian traps and methane from clathrates below the warming ocean bottom sharply enhanced the already strong greenhouse. Increasingly frequent and powerful cyclonic storms mined upwelling high-latitude heat and released it to the atmosphere. That heat, trapped by overlying clouds of its own making, suggests complete breakdown of the dry polar cell. Resulting rapid and intense polar warming caused or contributed to extinction of the remaining latest Permian coal forests that could not migrate any farther poleward because of light limitations. Loss of water stored by the forests led to aquifer drainage, adding another ∼5 m to the transgression. Non-peat-forming vegetation survived at the newly moist poles. Climate feedback from the coal-forest extinction further intensified warmth, contributing to delayed biotic recovery that generally did not begin until mid-Triassic, but appears to have resumed first at high latitudes late in the Early Triassic. Current quantitative models fail to generate high-latitude warmth and so do not produce the chain of events we outline in this paper. Future quantitative modeling addressing factors such as polar cloudiness, increased poleward heat transport by deep water and its upwelling by cyclonic storms, and sustainable mid-latitude sinking of warm brines to promote anoxia, warming, and thermal expansion of deep water may more closely simulate conditions indicated by geological and paleontological data.

Secular Distribution of Biogenic Silica through the Phanerozoic: Comparison of Silica‐Replaced Fossils and Bedded Cherts at the Series Level

The modern marine silica cycle is dominated by silica‐secreting phytoplankton, principally diatoms, but this cycle has evolved considerably during the Phanerozoic. We analyzed the temporal distribution of silica‐replaced fossils and bedded chert to determine the influence of factors such as extinctions and climate change on siliceous facies. Trends in silica replacement of fossils match faunal radiations during the Ordovician and Siluro‐Devonian, with peaks in silica replacement in the Late Ordovician and Middle Devonian corresponding to peaks in the abundance and diversity of siliceous sponges. Sharp drops in the abundance of silica‐replaced fossils and/or bedded cherts coincide with four of the five major mass extinctions. No discernible decrease marks the extinction at the end of the Triassic. We expected peaks in bedded chert deposition during glacial episodes because enhanced ocean circulation should favor more and stronger upwelling. Orogenic episodes, which may trigger continental glaciation, may also increase silica supply and further enhance siliceous deposition during these intervals. The data we have sampled provide mixed results. The Late Ordovician and Late Devonian glaciations do not correspond to peaks in bedded chert abundance, although the increase in bedded cherts relative to silica‐replaced fossils during the Carboniferous may reflect a climatic influence. The well‐known Middle Miocene circum‐Pacific chert event does correspond with a glacial interval. Lows in silica deposition should mark intervals when the ocean was stratified and/or ocean circulation was sluggish. Data from the mid‐Cretaceous and Early Triassic appear consistent with this expectation. Following the shift to a diatom‐dominated silica cycle in the Late Jurassic and Cretaceous, patterns of chert abundance through time became more volatile and more responsive to external influences on marine silica burial such as icehouse and greenhouse effects. Prior to the diatom radiation, biogenic silica burial was probably more equitably divided between radiolarians and siliceous sponges. This more cosmopolitan control of silica burial may have dampened the effects of climatic factors on silica accumulation, though better resolved data are needed to test this possibility.

Elemental mobility in phosphatic shales during concretion growth and implications for provenance analysis

Shale-normalized rare-earth element (REE) patterns record variable redox conditions in coprolitic phosphorite concretions and their host shales from the Pennsylvanian-age cyclothems in the U.S. midwestern states of Oklahoma, Kansas, Nebraska, Iowa, and Ohio as well as the Mississippian-age Fayetteville Shale of Arkansas. Shale-normalized enrichments in middle REE (MREE) developed as the phosphatic concretions lithified under reducing conditions in pore waters of organic-rich muds. Depletions in MREE (Sm–Ho) in host shales correspond to the MREE enrichments in some nearby concretions. Rare and weak negative cerium depletions in both reworked and unreworked phosphorite concretions suggest exposure to oxygenated waters. Positive europium anomalies in thick phosphorite coatings on coprolitic phosphorite concretion cores suggest extreme reduction during lithification of the phosphorite coating. Flat REE patterns in both host shales and phosphorite concretions probably reflect detrital influence. Phosphate diagenesis can affect the trace element chemistry of black shale enough to alter provenance signals. Overall enrichment in REE in phosphorite relative to host shale suggests movement of REE as does the MREE-enriched phosphorite near the MREE-depleted host shale. Diagenetically mobile REE can affect La–Th–Sc relationships used in provenance analysis. The extent of the diagenetic modification of La–Th–Sc distribution is related to the amount of P2O5 present. Typical La–Th–Sc distribution in host shale results when P2O5 values are below 0.5%. High La abundance substantially skews La–Th–Sc distribution when P2O5 is above 5.0%, but when P2O5 is between 0.5% and 5.0% the extent of the effect is unclear. Oxides of aluminum and other major elements may be useful in recording detrital influence and proximity to the shoreline in phosphorite concretions. Most of the concretions nearest to shoreline sources of detrital input have both flat REE patterns and elevated values of Al2O3/(Al2O3+Fe2O3).

Silica-replaced fossils through the Phanerozoic

A systematic survey of 1863 papers on macrobenthic assemblages reveals that an average of 21% of published Paleozoic papers concern silicified fossils, but that average drops to just 4% for post-Paleozoic papers. During the Paleozoic, silicified fossil occurrences do not significantly correlate with the amount of shelf chert, outcrop area, time, duration of geologic intervals, or carbonate rock volume. This substantial drop in numbers of silicified fossils coincides temporally with increased importance of aragonite faunas following the end-Permian extinctions. However, qualitative measurements of secular changes in abundance and diversity of siliceous sponges are consistent not only with the post-Paleozoic decline in fossil silicification, but also with fluctuations in the amount of silicified fossils throughout the Paleozoic. The facies distribution of silicified fossils in the Permian of West Texas also suggests that the distribution of silicified fossils may reflect the occurrence of siliceous sponges. The decline in silicified fossils after the Permian may be related to a concomitant rise in offshore bedded chert deposition and movement of the locus of biogenic silica formation from nearshore to offshore regions beginning in the Triassic, rather than with the expansion of diatoms in the Cretaceous.

First-order coupling of paleogeography and CO2, with global surface temperature and its latitudinal contrast

The authors propose that for any geography, halving the amount of emergent land area will elevate CO{sub 2} levels enough to raise land surface temperature 10C and vice versa. They have evaluated this relation by specifying latitude and level of emergence for six end-member continental configurations. They show that a world with polar continents (capworld) will be warmest, whereas a world dominated by tropical ones (ringworld) will be coldest - a result superficially counterintuitive to established climate dogma. A meridional configuration (sliceworld) will have intermediate temperatures. The model is consistent with modern, Pleistocene maximum-emergence and mid-Cretaceous minimum-emergence climates. It also predicts a cool global climate for the half-emergent mid-Cambrian ringworld and a very warm, equable climate for the half-emergent mid-Silurian capworld. Furthermore, the relations among latitude, land area, temperature, and CO{sub 2} levels predict that a Late Proterozoic, equator-straddling land mass could have been glaciated. A strong point of the model is that it yields realistic results with no knowledge of paleolongitude, sea-floor-generation rates, or orogeny (or, by implication, degassing and erosion rates), non of which is obtainable for pre-Mesozoic paleogeographies.

Impact of Grassland Radiation on the Nonmarine Silica Cycle and Miocene Diatomite

Abstract The Early Miocene rise of the grass-dominated ecosystem is a plausible trigger for a sharp Miocene increase in accumulation of nonmarine diatomaceous sediment as well as diversification of nonmarine diatoms. This grassland radiation introduced a biogeochemical mechanism for enhancing widespread and sustained mobilization of usable silica and other nutrients. Volcanism was probably responsible for episodic nonmarine diatomaceous sediments from the advent of the oldest known nonmarine diatoms in the Late Cretaceous through the Oligocene. Although prolific Miocene volcanism was undoubtedly still important in the development of many diatomites, feedback from grassland colonization of volcanic soils may explain why diatomaceous sedimentation surged in the Miocene following a more sparse pre-Miocene record. The initial rise of the grass-dominated ecosystem, increased nonmarine diatomite accumulation, and Early Miocene evolutionary radiations of nonmarine diatom taxa are at least approximately coeval. Although the earliest known grass is Paleocene, multiple lines of evidence, including mollic-epipedon paleosols, fossil occurrences of hypsodontic ungulate grazers, and fossil phytoliths, suggest that the grass-dominated ecosystem did not expand significantly until Early Miocene. The grassland radiation apparently was delayed until Middle Miocene in parts of Eurasia, Africa, and Australia. If that delay is real, the diatom/diatomite record in those regions should coincide with it. The onset of increased Miocene diatomite accumulation is as yet imprecisely dated, but coincidence with the rise of the grass-dominated ecosystem is predicted herein. Early Miocene diversifications of Actinocyclus and Thalassiosira diatoms are consistent temporally with grassland expansion where it is Early Miocene. Subsequent adjustments in the silica cycle also may be attributed to grasslands. Nonmarine diatom radiations in the Late Miocene and Pliocene coincide with sharp regressions that may have released nutrients and soluble phytolith opal stored in Miocene soils and paleosols as well as dissolved silica in soil pore waters. Regressional erosive pulses of phytoliths provide a new explanation for low Ge/ Si ratios in marine diatoms during Pleistocene glacial intervals. Nonmarine diatoms from regressive intervals should record lower Ge/Si ratios than before and after those regressions because of phytolith contributions with low Ge/Si ratios. Late Miocene radiations of C4 and moist tall-grass ecosystems may have mobilized even more silica than the short, dry-climate Early Miocene grasses. Abundance of diatomite may have fluctuated in concert with changes in degree of volcanism, even after grassland expansion, but at substantially higher levels than before this new terrestrial ecosystem arose.

Rare-Earth Element Variation in Phosphate Nodules from Midcontinent Pennsylvanian Cyclothems

ABSTRACT The rare-earth element (REE) geochemistry of phosphate nodules from eastern Kansas and northeastern Oklahoma is dominated by patterns that are generally flat or are enriched in middle REE (MREE). Flat patterns are typical of phosphate nodules preserved in thick shales and in nodules from shales deposited nearest to detrital sources. The flat patterns are probably derived from terrigenous constituents in the host shale. MREE enrichment is evident in phosphate found in relatively thin shales and in distal shales. We suggest that the MREE-enriched pattern reflects the contribution of MREE-enriched fecal phosphate. The initial MREE enrichment mechanism may have been analogous to that in which some modern algae preferentially extract MREE from water of marine composition. The MREE-enric ed signature may be preserved only in phosphate nodules that formed where terrigenous input was so low that it did not mask the characteristic fecal pattern. Rare Ce depletion patterns reflect a primary seawater REE source that has not been obscured by fecal or detrital components.

Petrology and Origin of Phosphate Nodules from the Midcontinent Pennsylvanian Epicontinental Sea

ABSTRACT Midcontinent Pennsylvanian phosphate nodules (concretions), that occur in widespread black and gray shales, have not been largely affected by reworking processes that commonly enrich other phosphate deposits with respect to P. A sequence of events consisting of 1) upwelling, 2) development of a prolific pelagic biota, 3) decay of dead pelagic organisms and fecal material that released P to the interstitial waters of the black muds, and 4) cementation by apatite, pyrite, quartz, and calcite is proposed to have formed the phosphate nodules prior to compaction of the enclosing shales. Phosphate nodules are best developed at the top (and sometimes bottom) of black-shale units, indicating that nodular growth was enhanced near the boundary between anoxic and disoxic conditions. The conditions under which these phosphates formed are generally similar to those cited for Holocene pericontinental phosphorites. This study provides an example of an epicontinental phosphate deposit that fits Cook and McElhinnys (1979) spatial and temporal model for distribution of pericontinental phosphorites.

A human-induced hothouse climate?

Hothouse climate has been approached or achieved more than a dozen times in Phanerozoic history. Geologically rapid onset of hothouses in 10–10 yr occurs as HEATT (haline euxinic acidic thermal transgression) episodes, which generally persist for less than 1 million years. Greenhouse climate preconditions conducive to hothouse development allowed large igneous provinces (LIPs), combined with positive feedback amplifiers, to force the Earth to the hothouse climate state. The two most significant Cenozoic LIPs (Columbia River Basalts and much larger Early Oligocene Ethiopian Highlands) failed to trigger a hothouse climate from icehouse preconditions, suggesting that such preconditions can limit the impact of CO 2 emissions at the levels and rates of those LIPs. Human burning of fossil fuels can release as much CO 2 in centuries as do LIPs over 10–10 yr or longer. Although burning fossil fuels to exhaustion over the next several centuries may not suffice to trigger hothouse conditions, such combustion will probably stimulate enough polar ice melting to tip Earth into a greenhouse climate. Long atmospheric CO 2 residence times will maintain that state for tens of thousands of years.

Syntectonic sedimentation in the Proterozoic upper Belt Supergroup, northwestern Montana

Deepening environments in the Proterozoic Libby Formation record a tectonically induced style of sedimentation distinctly different from that of older Belt rocks. Facies associations and sedimentary structures indicate that deposition in the lower Libby Formation occurred above fair-weather wave base. Thick, widespread hummocky cross-stratified quartzite in the upper Libby Formation lacks the association of shallow-water features present in the lower Libby Formation, suggesting that upper Libby deposition occurred below fair-weather wave base and above storm wave base. Independent evidence for tectonism during deposition of the Belt Supergroup exists but is plagued by poor age control. The angular unconformity that occurs between Libby-equivalent rocks and the overlying Windermere Supergroup indicates tectonic activity between deposition of the Belt-Purcell and Windermere Supergroups. The interpreted subsidence or rise in basin water level combined with newly uplifted source areas as recorded in the upper Libby Formation and Garnet Range Formation could have been an early manifestation of movements that produced this unconformity.