Neil J. Tabor

CO 2 as a primary driver of Phanerozoic climate

Royer et al. (2004) introduce a seawater pH correction to the Phanerozoic temperature reconstruction based on δO variations in marine fossils. Although this correction is a novel idea and it is likely to have played some role in offsetting the δO record, we show that (a) The correction cannot be as large as claimed by Royer et al. (b) Irrespective of the size of the correction, a CO2 signature cannot possibly be seen in the data. (c) Even though the CO2 signature cannot be seen, the pH correction implies only a somewhat higher global temperature sensitivity than that in Shaviv and Veizer (2003), a sensitivity that is consistent with a “black body Earth”, but only marginally with the lower limit of the IPCC range.

Middle Miocene paleoaltimetry of southern Tibet: Implications for the role of mantle thickening and delamination in the Himalayan orogen

The stable isotope composition of pedogenic and early diagenetic carbonates from the Oiyug Basin of southern Tibet allows model estimates of the paleoaltimetry of the Tibetan Plateau for the middle Miocene. Pedogenic calcium carbonate nodules have average d 18 Occ values of 219.6‰, whereas nodular lacustrine dolomites range in composition from 27.6‰ to 25.5‰. The most negative of the carbonate isotope values can be used to constrain the oxygen isotope composition of paleoprecipitation, from which model estimates of paleoaltimetry can be made. Model results indicate that the southern Tibetan Plateau achieved elevations of ;5200 11370/2605 m by at least 15 Ma. Our results are identical within uncertainty to previous workers’ paleoelevation estimates based on Oiyug Basin fossil floral physiognomy. This is the first time that two paleoaltimeters have been directly compared and are in accord. Collectively, these data strongly support tectonic models in which thickening of mantle lithosphere beneath the domain of crustal thickening and subsequent detachment of the mantle lithosphere plays an indiscernible role in the elevation history of this part of the Himalaya-Tibet orogenic system.

Climate and vegetational regime shifts in the late Paleozoic ice age earth

The late Paleozoic earth experienced alternation between glacial and non-glacial climates at multiple temporal scales, accompanied by atmospheric CO2 fluctuations and global warming intervals, often attended by significant vegetational changes in equatorial latitudes of Pangaea. We assess the nature of climate-vegetation interaction during two time intervals: middle-late Pennsylvanian transition and Pennsylvanian-Permian transition, each marked by tropical warming and drying. In case study 1, there is a catastrophic intra-biomic reorganization of dominance and diversity in wetland, evergreen vegetation growing under humid climates. This represents a threshold-type change, possibly a regime shift to an alternative stable state. Case study 2 is an inter-biome dominance change in western and central Pangaea from humid wetland and seasonally dry to semi-arid vegetation. Shifts between these vegetation types had been occurring in Euramerican portions of the equatorial region throughout the late middle and late Pennsylvanian, the drier vegetation reaching persistent dominance by Early Permian. The oscillatory transition between humid and seasonally dry vegetation appears to demonstrate a threshold-like behavior but probably not repeated transitions between alternative stable states. Rather, changes in dominance in lowland equatorial regions were driven by long-term, repetitive climatic oscillations, occurring with increasing intensity, within overall shift to seasonal dryness through time. In neither case study are there clear biotic or abiotic warning signs of looming changes in vegetational composition or geographic distribution, nor is it clear that there are specific, absolute values or rates of environmental change in temperature, rainfall distribution and amount, or atmospheric composition, approach to which might indicate proximity to a terrestrial biotic-change threshold.

Hot summers in the Bighorn Basin during the early Paleogene

During the early Paleogene, climate in continental interiors is thought to have been warmer and more equable than today, but estimates of seasonal temperature variations during this period are limited. Global and regional climate models of the Paleogene predict cooler temperatures for continental interiors than are implied by proxy data and predict a seasonal range of temperature that is similar to today. Here, we present a record of summer temperatures derived from carbonate clumped isotope thermometry of paleosol carbonates from Paleogene deposits in the Bighorn Basin, Wyoming (United States). Our summer temperature estimates are ∼18 °C greater than mean annual temperature estimated from analysis of fossil leaves. When coupled, these two records yield a seasonal range of temperature similar to that in the region today, with winter temperatures that are near freezing. These data are consistent with our high-resolution climate model output for the Early Eocene in the Bighorn Basin. We suggest that temperatures in continental interiors during the early Paleogene greenhouse were warmer in all seasons, but not more equable than today. If generally true, this removes one of the long-standing paradoxes in our understanding of terrestrial climate dynamics under greenhouse conditions.

Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO2 concentrations

Abstract Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO2 components—atmospheric CO2 and CO2 from in situ oxidation of organic matter. The δ13C values measured for the Fe(CO3)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (∼283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (∼270 Ma BP) paleosol (SAP). These goethites contain the most 13C-rich Fe(CO3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO2 components. δ13C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric P co 2 value of about 1 × PAL, but the scatter in the measured δ13C values of organic matter permits a calculated maximum P co 2 of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO3)OH in MCGoeth and SAP correspond to soil CO2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO2 concentrations are similar to modern soils in warm, wet environments. The average δ13C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ13C value for all profiles of −5.4‰. Thus, the value of Δ13C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric P co 2 and organic matter δ13C values were the same for both paleosol types. Furthermore, the atmospheric P co 2 calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric P co 2 (permitted by one standard deviation of the organic matter δ13C value) of ∼5 × PAL. If, however, measured average δ13C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of 13C-depleted, labile organic compounds, calculated Permian atmospheric P co 2 using these 13C-enriched organic values would underestimate the actual atmospheric P co 2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO2. The Fe(CO3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in 13C relative to Fe(CO3)OH in goethites from soils in which there are mixtures of two isotopic CO2 components. Field-relationships and the δ13C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO2 components at the time of goethite crystallization. The three components were probably atmospheric CO2, CO2 from in situ oxidation of organic matter and CO2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO2 components, as recorded by Fe(CO3)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric P co 2.

Mineralogical and geochemical evolution of a basalt-hosted fossil soil (Late Triassic, Ischigualasto Formation, northwest Argentina): Potential for paleoenvironmental reconstruction

Reconstruction of paleoclimatic conditions in the Ischigualasto basin, northwestern Argentina, has been constrained by fi eld studies coupled with mineralogic, whole-rock, and fi ne-fraction chemical and stable isotope analysis of a Triassic (Carnian) basalt-hosted fossil soil. Field evidence, such as wedgeshaped aggregate structure and slickensides, indicate this was likely a paleo-Vertisol. Whole-rock analysis defi nes down-profi le trends in clay mineralogy and chemical composition that are consistent with modern soils developed upon basalt parent material. X-ray diffraction analysis indicates that the basaltic parent material is dominated by plagioclase with trace amounts of weathered 2:1 phyllosilicate. Overlying weathered horizons show a progressive loss of plagioclase and an increase in phyllosilicates with minor amounts of kandite clays and detrital quartz. X-ray diffraction analysis of the <2 µm fraction shows that the weathered layers are dominated by dioctahedral smectite (montmorillonite) with a minor fraction of kaolinite in the upper layers of the profi le. There is a progressive loss of basic cations in conjunction with an increase in concentration, on a wt% basis, of conservative elements from the basalt upward through the weathering profi le. The combined data indicate that this soil likely formed on a stable landscape in a cool and humid climate. In addition, the presence of quartz in the paleosol profi le suggests an eolian contribution of sediment during pedogenesis. Despite these apparent morphologic and bulk chemical trends indicative of a pedogenic origin, none of the authigenic minerals formed in isotopic equilibrium. However, based on measured oxygen and hydrogen isotope compositions, these minerals apparently formed from meteoric waters with a narrow range of δ 18 O and δD compositions at different temperatures. If this is correct, then amygdaloidal calcites formed at ~60‐100 °C, followed by precipitation of montmorillonites at 49‐57 °C during late-stage hydrothermal alteration. Finally, goethite formed at low temperatures of 6 ± 3 °C in a pedogenic environment. This complex history of hydrothermal alteration and pedogenic overprinting brings to light the need for cautious interpretation of bulk chemical trends in paleosols as a means for paleoclimate reconstruction. Comparison of the calculated Triassic oxygen isotopic compositions of meteoric water and soil temperature with modern environments suggests that this soil formed in a seasonal, humid, and cool climate.

Paleoenvironmental reconstruction from chemical and isotopic compositions of Permo-Pennsylvanian pedogenic minerals

Mineralogical and chemical analysis of Late Pennsylvanian and Early Permian paleosols from the eastern shelf of the Midland basin, north-central Texas, USA, are used to test hypothesized climate change in Late Paleozoic western equatorial Pangea, previously defined independently on the bases of sedimentologic and paleontologic proxies and climate models. The <0.2-μm size phyllosilicate fraction in the studied paleosols exhibits down-profile trends in mineralogy and chemical composition that are consistent with modern weathering profiles suggesting a dominantly pedogenic origin. A stratigraphic trend from kaolinite-dominated profiles in Upper Pennsylvanian paleosols to profiles dominated by smectite and hydroxy-interlayered 2:1 phyllosilicates in Lower Permian paleosols indicates a relatively rapid decrease in soil weathering and leaching in the latest Pennsylvanian followed by a more gradual decrease in leaching throughout the Early Permian. The chemical composition (cation ratios and exchange capacity) of these phyllosilicates further corroborates this shift toward less intensive leaching, presumably in response to climate change from humid to progressively more arid conditions. The phyllosilicates in the <0.2-μm size fraction and contemporaneous pedogenic calcites from the Permo-Pennsylvanian paleosols exhibit a long-term stratigraphic increase in their δ18O values of as much as ∼3.2‰ and ∼5.2‰, respectively. This long-term trend is consistent with a transition throughout the latest Pennsylvanian through Early Permian toward progressively more evaporatively enriched soil waters. Superimposed on the long-term trend is an apparent rapid enrichment (1.5 to 2‰) in phyllosilicate δ18O values immediately above the Pennsylvanian–Permian boundary. Observed oxygen isotope fractionation between the phyllosilicates and calcites within individual paleosols indicate isotopic disequilibrium between mineral pairs. This is attributed to a minor detrital component in the pedogenic clay-dominated phyllosilicate fraction coupled with the effects of seasonality of mineral formation. Inferred δ18O compositions of Late Paleozoic meteoric water (−2‰ to +4‰) are compatible with less intensive soil leaching under conditions of increasing aridity, possibly coupled with a shift in local precipitation from a continental source to a marine source.

New dicynodonts (Therapsida, Anomodontia) and updated tetrapod stratigraphy of the Permian Ruhuhu Formation (Songea Group, Ruhuhu Basin) of southern Tanzania

ABSTRACT Permian tetrapod fossils were discovered in the Tanzanian Ruhuhu Formation in 1963, but they have received far less attention than the tetrapods of the overlying Usili (formerly Kawinga) Formation. Here, we describe two dicynodonts collected in the Ruhuhu Formation in 2008. Abajudon kaayai, gen. et sp. nov., is represented by a partial skull and mandible and is characterized by autapomorphic upper teeth that are triangular in cross-section, have procurved tips, and bear a deep groove on the mesial surface. Although it shows similarities to taxa such as Endothiodon and Chelydontops, the exact relationships between A. kaayai and other dicynodonts are uncertain. The second specimen also consists of a partial skull and mandible. We refer it to cf. Endothiodontia based on the medial placement of the long maxillary tooth rows, the presence of depressions on the palatine pad, a long posterior dentary sulcus, and similarities of the mandibular dentition. Tetrapods occur in three fossiliferous horizons in the Ruhuhu Formation. The likely Middle Permian lower horizon includes dinocephalians, temnospondyls, and the dicynodonts described here. The middle horizon includes a new, tusked species of Endothiodon and at least one other dicynodont. The upper horizon appears to sample an assemblage similar to that of the Usili Formation and therefore may be of Late Permian age. The discovery of Middle Permian fossils in Tanzania and Zambia provides the opportunity to test whether southern Gondwana was characterized by a cosmopolitan tetrapod fauna for an extended period of time before the biogeographic restructuring caused by the end-Permian mass extinction.

Latest Permian paleosols from Wapadsberg Pass, South Africa: Implications for Changhsingian climate

Terrestrial settings preceding the endPermian crisis are reported to trend toward increasingly dry and arid conditions, resulting in landscape change and a shift in fl architectures and regimes. Much of the latest Permian (Changhsingian) stratigraphic record in the Karoo Basin, South Africa, consists of paleosols, which record the physical conditions across time and space. Preboundary sequences at Wapadsberg Pass, Eastern Cape Province, provide insight into the climate regime that infl uenced paleosol formation at that time. A high-resolution sedimentological and geochemical study of two, stacked aggradational paleosols, in conjunction with stable isotope geochemical characterization of paleosol carbonate-cemented concretions over a 90 m section at this locality, demonstrates that these landscapes were predominantly wetland terrains without a demonstrable trend in increasing drying up to the Permian-Triassic boundary, as defi ned by vertebrates in the area. Two paleosols examined 70 m below the Permian-Triassic boundary are identifi ed as Protosols, and the former soil-air interface of each is marked by an autochthonous forestfl oor litter in which canopy leaves of Glossopteris and groundcover plants of Trizygia are preserved. Molecular weathering ratios (e.g., base loss, clayeyness, chemical index of alteration minus potassium [CIA–K], etc.) determined from these horizons are indicative of immature soil development under watersaturated conditions. Assuming that paleosol-matrix concentrations of trace elements are indicative of Permian soil-solution chemistries, high concentrations of Ni, Cu, Ba, and Cr may have been growth-stress factors that may account for the small glossopterid leaf size in the megafl oras, in contrast to current models that implicate stress in response to climate change. Stable isotope δ 18 O and δ 13 C values are presented for micritic and microspar (<20 μm) calcite cements from carbonate nodules collected at 15 horizons through a 90 m stratigraphic interval up to, and including, the Permian-Triassic boundary. These isotopic ratios exhibit dissimilar trends. No clear trend exists in δ 18 O (Peedee belemnite [PDB] values range from –14.7‰ to –21.8‰). In contrast, a trend exists in δ 13 C values, where carbonate cements almost certainly precipitated under well-drained conditions in an interval that is 60 m below the PermianTriassic boundary (–5.3‰), while δ 13 C values as low as –16.9‰, indicative of water-logged conditions, begin 90 m below and continue up to the Permian-Triassic boundary. Hence, no evidence is found for a preboundary trend toward increasing aridity at this locality. The fi rst estimates of the latest Permian atmospheric pCO2 from paleosols, based on coexisting calcite and organic matter δ 13 C values from paleosols that developed under well-drained conditions, provide a range of values from 900 to 1900, and 500 to 1300 ppmV, respectively, which are signifi cantly lower than the latest Early Permian, when terrestrial biome replacement is documented to have occurred.

Juxtaposed Permian and Pleistocene isotopic archives: Surficial environments recorded in calcite and goethite from the Wichita Mountains, Oklahoma

A paleokarst fi ll deposit from the Wichita Mountains, south-central Oklahoma, United States, consists primarily of sparry calcite, Fe-sulfi des, and goethite. Previous cement-stratigraphic studies and paleontological fi nds suggest that calcite mineralization was initiated during Permian time, whereas goethite and other oxides apparently formed from oxidation of preexisting Fe-sulfi des during Pleistocene time. Therefore, these deposits have the potential to offer insight into surfi cial hydrology and paleoenvironment in an upland setting from two time periods at a single site. δC PDB and δO SMOW measurements of 17 samples from growth bands in a single karst-fi ll calcite crystal range from –10.7‰ to –6.6‰ (mean = –8.6‰) and 27.1‰ to 28.3‰ (mean = 28‰), respectively. Large oscillations in the δC values through the growth series may originate from seasonal changes in the magnitude of biological productivity during Permian time. These δC oscillations contrast with the relative stability of the δO values, which are more positive than would be expected for isotopic equilibrium with local modern waters. The δO values of the calcite may refl ect the δO values of ambient meteoric groundwaters in the Permian that were isotopically similar to waters in modern, seasonally dry, low-latitude coastal regions. Goethites are not in equilibrium with modern waters or coexisting calcites in the fi ssure-fi ll deposit as determined from δO and δD values of the goethites. Furthermore, the combined δO and δD values of the goethites are indicative of formation from meteoric waters at a temperature of ~9 °C ± 3 °C. This inferred temperature is 7 °C ± 3 °C cooler than local modern mean annual temperature and corresponds well with independent studies that propose temperatures ~6 °C cooler in this region during Pleistocene time. The mole fraction and δC values of the Fe(CO 3 )OH component in solid solution in the goethite sample are 0.0103 and –10.1‰, respectively. In combination, these values suggest that goethite formed in an environment characterized by mixing of three isotopically distinct CO 2 components: (1) oxidized biological carbon, (2) atmospheric CO 2 , and (3) CO 2 from dissolution of carbonate in the karst system. Oxidized biological carbon may have originated either from fl ora characterized by C3 or mixed C3:C4 photosynthesizers. Mass balance calculations between these three CO 2 end members correspond to an inferred soil CO 2 concentration [CO 2 contributed from (1) and (2) above] ranging from ~8,000 ppmV to ~16,000 ppmV for a local ecosystem dominated by C 3 fl ora. This inferred range of soil CO 2 concentrations is typical of grasslands characterized by relatively high biological productivity. If C4 fl ora were a signifi cant 56 N.J. Tabor and C.J. Yapp source of oxidizing carbon, the higher calculated ambient CO 2 concentration at the time of goethite crystallization in the cave (~20,000 ppmV) might be interpreted to correspond to an unusually productive C4 soil present at a time of generally cooler and drier conditions across the southern Great Plains of North America.