Isabel P. Montañez

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.

A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern Pangaea

Two hundred thirty million years ago, in what is now Argentina, dinosaurs could be found as the dominant carnivores or as small herbivores. Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with 40Ar/39Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.

High-Precision U-Pb Calibration of Carboniferous Glaciation and Climate History, Paganzo Group, NW Argentina

The duration and geographic extent of Carboniferous glacial events in southern Gondwana remain poorly constrained despite recent evidence for a more dynamic glacial history than previously considered. We report 10 high-precision (2! ± <0.1%) U-Pb ages for the Permian-Carboniferous Paganzo Group, NW Argentina, that redefi ne the chronostratigraphy of the late Paleozoic Paganzo and Rio Blanco Basins, and signifi cantly refi ne the timing of glacial events and climate shifts in the western region of southern Gondwana. Radiometric calibration of the Paganzo Group indicates three pulses of Carboniferous glaciation in the mid-Visean, the late Serpukhovian to earliest Bashkirian, and between the latest Bashkirian to early Moscovian. An abrupt shift in depositional style from high-sinuosity single-storied fl uvial deposits and clay-rich paleosols to low-sinuosity multi storied feldspathic fluvial deposits inter calated with eolianites and calcic paleosols is constrained to the latest Moscovian and earliest Kasimovian. These constraints indicate a relatively abrupt climate shift from humid-subhumid to nonseasonal semiarid regional climate conditions that occurred signifi cantly earlier than previously inferred (Early Permian). This period of high-latitude aridity was contemporaneous with a shift to dryland depositional environments and a major vegetation regime shift documented throughout the Pangean paleotropics in the Pennsylvanian.

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.

Late Paleozoic tropical climate response to Gondwanan deglaciation

Coupled climate-biome model simulations of the late Paleozoic were developed to determine the response of Pangean tropical climate to Gondwanan deglaciation. The model simulations predict substantial changes over equatorial Pangea including continental drying, a reversal of equatorial winds, warming, heavier δ 18 O values of meteoric precipitation, and the expansion of deserts and the contraction of forests. The magnitude of these tropical responses is sensi- tive to the extent of Gondwana continental ice and the deglacial rise in atmospheric pCO 2 , boundary conditions that are not well known for the late Paleozoic. Nonetheless the model predictions are consistent with climatic and environmental trends determined from terrestrial proxy data, implying that the deglaciation of Gondwana was a transformational climate event in tropical Pangea.

Shifts in late Paleozoic atmospheric circulation over western equatorial Pangea: Insights from pedogenic mineral δ18O compositions

The d 18 O values of pedogenic calcites, phyllosilicates, and iron (oxyhydr)oxides from fossil soils throughout the southwestern United States show systematic paleolatitudinal and temporal trends that indicate a significant change in soil moisture conditions and atmospheric circulation patterns over southwestern Euramerica throughout latest Pennsylvanian and Early Permian time. A progressive depletion of as much as 6‰ in mineral 18 O values with increasing distance northward (to 108) of the paleoequator indicates a weakening or disruption of zonal easterly flow over the study area by latest Pennsylvanian time. Furthermore, elevated d 18 O values suggest a proximal source of paleoprecipitation over the study area, perhaps due to initiation of reversed equatorial flow over tropical southwestern Euramerica. Coupling of the geographic and temporal trends defined by pedogenic mineral d 18 O values with those defined by climatically sensitive paleosol pedotypes in the study area, along with overall elevated mineral d 18 O values, provides some of the strongest evidence that Northern Hemisphere monsoonal circulation was well established over Pangea by Early Permian time.

Dynamic carboniferous climate change, Arrow Canyon, Nevada.

The Phanerozoic9s longest-lived and most widespread glaciation, the late Paleozoic ice age, is undergoing a resurgence in interest. Long-held models of the timing, duration, and magnitude of glaciation are being reevaluated due to emerging evidence from former high latitudes, evidence that the late Paleozoic ice age was punctuated by long-lived glacial minima or possibly ice-free times. The history of the late Paleozoic ice age is archived within the biostratigraphically well-constrained, carbonate-dominated succession of Arrow Canyon, Nevada, United States. In this paleo-tropical succession, the distribution of lithofacies, flooding surfaces, and subaerial exposure horizons and their stacking into meter-scale cycles record a detailed climate history. The onset of this phase of glaciation during the middle Mississippian was followed by a dynamic evolution of glacioeustasy through the late Mississippian to late Pennsylvanian. Moderate- to high-amplitude glacioeustasy was likely interrupted by an earliest Pennsylvanian short-lived glacial minimum, but otherwise appears to have persisted through the middle Pennsylvanian. Upper Pennsylvanian strata record low- to moderate-amplitude relative sea-level changes, suggesting a long-lived interval of diminished ice volume. This proposed glacial minimum is coincident with a notable minimum in glaciogenic sedimentation near the former southern pole, aridification across paleo-tropical Pangea, and significant floral and faunal turnover, suggesting a link between tropical environmental change and high-latitude glaciation. These conclusions, however, are at odds with those traditionally inferred from Euramerican cyclothems, i.e., persistent high-amplitude glacioeustasy driven by a single, large ice sheet. Rather, the Arrow Canyon archive of varying depositional facies and cycle stacking patterns records major changes in the magnitude of short-term glacioeustasy. This finding contributes to recent and growing near- and far-field evidence for a more dynamic glaciation history than previously inferred from the classic Euramerican cyclothems.

Climatically driven glacial-interglacial variations in C3 and C4 plant proportions on the Chinese Loess Plateau

High-resolution δ13C records of bulk organic matter (OM) from three loess-paleosol successions from the central Chinese Loess Plateau document systematic fluctuations in C3 and C4 plant abundance for certain intervals during the past 620 k.y. Measured δ13C values of OM in paleosols are less negative by 0.5‰–4‰, and total organic carbon (TOC) contents are higher by up to 0.5 wt% relative to corresponding values in loess deposits. The δ13COM and TOC decrease from peak values in paleosols progressively into overlying and underlying loess deposits. Integrated δ13COM, TOC, and magnetic susceptibility rec ords indicate an increase in C4 vegetation to ≥50% during peak paleosol formation and a decrease in C4 vegetation to ≤30% during loess deposition for three of the past glacial-interglacial cycles. These results indicate that C4 plant abundance declined during glacials in this mid-latitude region driven primarily by a shift to enhanced winter precipitation and lower temperatures, and further contribute to the rapidly growing body of evidence that low p CO2 during Pleistocene glacials was insufficient to trigger C4 plant expansion without coincident favorable climatic conditions.

Evolution of Cupido and Coahuila carbonate platforms, Early Cretaceous, northeastern Mexico

The Cupido and Coahuila platforms of northeastern Mexico are part of the extensive carbonate platform system that rimmed the ancestral Gulf of Mexico during Barremian to Albian time. Exposures of Cupido and Coahuila lithofacies in several mountain ranges spanning an ~80 000 km 2 area reveal information about platform morphology and composition, paleoenvironmental relations, and the chronology of platform evolution. New biostratigraphic data, integrated with carbon and strontium isotope stratigraphy, significantly improve chronostratigraphic relations across the region. These data substantially change previous age assignments of several formations and force a revision of the longstanding stratigraphy in the region. The revised stratigraphy and enhanced time control, combined with regional facies associations, allow the construction of cross sections, isopach maps, and timeslice paleogeographic maps that collectively document platform morphology and evolution. The orientation of the Cupido (BarremianAptian) shelf margin was controlled by the emergent Coahuila basement block to the northwest. The south-facing margin is a highenergy grainstone shoal, whereas the margin facing the ancestral Gulf of Mexico to the east is a discontinuous rudist-coral reef. A broad shelf lagoon developed in the lee of the Cupido margin, where as much as 660 m of cyclic peritidal deposits accumulated. During middle to late Aptian time, a major phase of flooding forced a retrograde backstep of the Cupido platform, shifting the locus of shallow-marine sedimentation northwestward toward the Coahuila block. This diachronous flooding event records both the demise of the Cupido shelf and the consequent initiation of the Coahuila ramp. The backstepped Coahuila ramp (AptianAlbian) consisted of a shallow shoal margin separating an interior evaporitic lagoon from a low-energy, muddy deep ramp. More than 500 m of cyclic carbonates and evaporites accumulated in the evaporitic lagoon during early to middle Albian time. Restriction of the platform interior dissipated by middle to late Albian time with the deposition of peloidal, miliolid-rich packstones and grainstones of the Aurora Formation. The Coahuila platform was drowned during latest Albian to early Cenomanian time, and the deep-water laminites of the Cuesta del Cura Formation were deposited. This study fills in a substantial gap in the Cretaceous paleogeography of the eastern Gulf of Mexico coast, improving regional correlations with adjacent hydrocarbon-rich platforms. The enhanced temporal relations and chronology of events recorded in the Cupido and Coahuila platforms significantly improve global correlations with coeval, economically important platforms worldwide, perhaps con

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.