Phanerozoic Research Articles

Why Geology Matters: Decoding the Past, Anticipating the Future

Review: Why Geology Matters: Decoding the Past, Anticipating the Future By Doug Macdougall Reviewed by Byron Anderson DeKalb, Illinois, USA Macdougall, Doug. Why Geology Matters: Decoding the Past, Anticipating the Future. Berkeley, CA: University of California Press, 2011. xv, 285 pp. ISBN: 9780520266421,US$29.95, cloth. Printed on 100 percent post consumer waste, recycled, de-inked fiber. E-book available. Why Geology Matters pursues two goals: to better understand how Earth works and to provide clues to the future of the planet. Macdougall, Professor Emeritus of Earth Sciences at Scripps Institution of Oceanography, University of California San Diego, hopes “that this book will, in some small way, stimulate a deeper interest in the field among its readers, and perhaps help improve this situation” (p. xiii). Geology is defined as “an intellectual pursuit that encompasses studies of the atmosphere, the oceans, the solid earth, and even other planets of the solar system” (p. 249). It is interdisciplinary incorporating mathematics, physics, chemistry, engineering, and biology, and has spun off a number of related studies, for example, astrogeology and biogeochemistry. Interspersed chapters explore Earth’s history with specific events and processes, for example, the Chicxulub Crater in the Yucatan Peninsula. Geoscience studies natural processes, for example, volcanic dust, tsunamis, and earthquakes, and addresses areas, such as sustainable management of water, mineral, and energy resources. The book emphasizes the importance of greenhouse gases, particularly carbon dioxide, as a primary agent driving widespread environmental change. The geological timescale is introduced covering epochs, periods, eras, and eons all the way back to the Hadean eon which started at Earth’s formation about 4.6 billion years ago and ended roughly 3.8 billion years ago, though most of the book deals with the Phanerozoic eon, approximately 540 million years ago to the present. Geoscientists look for clues about Earth’s past by examining evidence, for example, the presence or absence of different types of fossils which shed light on the record of changing environments. This evidence is then combined with inferences from theory. The “Wandering Plates” chapter does an excellent job in explaining why most natural phenomena are influenced by plate tectonics. Study of these plates has led to a better understanding of volcanoes, earthquakes, and other phenomena that affect living things. While analytical tools for decoding the past have become very sophisticated, for example, x- rays and three dimensional images, geoscientists still find it necessary to use of terms, such as, “may have” and “perhaps” particularly in determining certain conditions that occurred billions of years ago. From the beginning of Earth’s formation through the Proterozoic eon, many details will never be known. Macdougall drives home the importance of geology, an understudied subject, and concludes somewhat ominously that “to a considerable extent, the immediate future of our planet is now in our own hands” (p. 248). The book is for students of geology and educated laypersons. It could be used as an introductory textbook, and is recommended for all libraries with a geology collection. The text is nicely supplemented with over thirty illustrations, a bibliography and future reading section, and a subject index. Byron Anderson, , retired from Northern Illinois University Libraries, DeKalb, IL 60115, USA. Electronic Green Journal, Issue 32, Fall 2011, ISSN:1076-7975

The subsurface record of the Late Palaeozoic glaciation in the Chaco Basin, Bolivia

Abstract The Late Palaeozoic glaciation was the longest of the Phanerozoic era. It is recorded in numerous Gondwanan basins, with some, such as the Chaco Basin, having a high petroleum potential. In this basin, the quality of the available seismic, well and outcrop data permits us to characterize the Late Palaeozoic glacial record. Palaeovalleys that are c . 500 m deep and c . 7 km wide have been analysed here. Focusing on the glaciogenic Carboniferous deposits, seismic data with well ties and outcrop analogues provide new sedimentological insights. The palaeovalley infill is imaged as a chaotic seismic facies overlain by an aggrading–prograding prism, interpreted as tillites covered by a fluvio-deltaic system. Tillites form both under the ice and during rapid ice recession, whereas fluvio-deltaic systems can only originate from a stable ice margin and last until the ice sheets withdraw inland. These two depositional modes are repeated several times, generating a progressive burial of the Carboniferous palaeovalleys. This succession of erosions and fills records major glacial stages, including a series of glacial and interglacial phases from the Late Devonian to the Early Permian. Depicting the Late Palaeozoic glacial history of the Chaco Basin seems crucial for the localization of potential good reservoirs.

Climate Stability and Policy

Starting in the 1980s and culminating in the Kyoto accords of 1997, followed by the awarding of the 2007 Nobel Peace Prize to Al Gore and the United Nation’s International Panel on Climate Change (IPCC), international attention has been focused on the dangers of global warming owing to anthropogenic carbon dioxide emissions. In this essay, however, I will argue that humanity faces a much greater danger from the glaciation associated with the coming of the next Ice Age, and that the carbon dioxide increases that we have seen during the past two hundred years are not sufficient to avert such glaciation and its associated disruptions to the biosphere and civilization as we know it. Such conflicting considerations have obvious implications for the formulation of public policy regarding human attempts to manage climate change. During most of the Phanerozoic eon, which began about a half-billion years ago, there were few glacial intervals until the late Pliocene 2.75 million years ago. Beginning at that time, the Earth’s climate entered a period of instability with the onset of cyclical ice ages. At first these had a 41,000 year cycle, but about 1 million years ago the period lengthened to 100,000 years, which has continued to the present [1]. Over this period of instability the climate has been extraordinarily sensitive to small forcings,* whether due to Milankovitch cycles, solar variations, aerosols, or albedo variations driven by cosmic rays. The current interglacial has lasted for some ten thousand years—about the duration of past interglacials—and serious policy considerations arise as it nears its likely end. It is extremely unlikely that the current rise in carbon dioxide concentration—some 30% since 1750, and projected further increase over the next few decades—will significantly postpone the next glaciation. Figure 1 shows the representation from Wikipedia [2] of temperature over the Phanerozoic derived from the available proxy data.

Widespread iron-rich conditions in the mid-Proterozoic ocean

The chemical composition of the ocean changed markedly with the oxidation of the Earth's surface, and this process has profoundly influenced the evolutionary and ecological history of life. The early Earth was characterized by a reducing ocean-atmosphere system, whereas the Phanerozoic eon (less than 542 million years ago) is known for a stable and oxygenated biosphere conducive to the radiation of animals. The redox characteristics of surface environments during Earth's middle age (1.8-1 billion years ago) are less well known, but it is generally assumed that the mid-Proterozoic was home to a globally sulphidic (euxinic) deep ocean. Here we present iron data from a suite of mid-Proterozoic marine mudstones. Contrary to the popular model, our results indicate that ferruginous (anoxic and Fe(2+)-rich) conditions were both spatially and temporally extensive across diverse palaeogeographic settings in the mid-Proterozoic ocean, inviting new models for the temporal distribution of iron formations and the availability of bioessential trace elements during a critical window for eukaryotic evolution.

Evolution of fixed-channel alluvial plains in response to Carboniferous vegetation

The establishment of terrestrial plants during the Palaeozoic era was one of the most significant changes to the Earth system during the Phanerozoic eon. The continuing evolution and expansion of land plants irrevocably altered the alluvial landscape: the broad, unconfined and unconsolidated blankets of coarse sediment found at the start of the Cambrian period were replaced by a diverse array of braided and meandering channel styles and stable floodplains by the end of the Devonian period. Here we show that the first appearance of a sedimentary facies suite attributed to low-energy, organic-rich river systems with multiple channels and stable alluvial islands, known as anabranching or anastomosing rivers, occurred during the Carboniferous period. Our field studies and literature review demonstrate that the appearance of these rivers coincides with the continuing evolution of tree-like plants. We suggest that increased floodplain stability and new triggers for channel avulsion were provided by the increase in complexity and diversity of root assemblages, density of floodplain forests and production of woody debris associated with expansion of arborescence. We conclude that the expansion of tree habitats led to the crossing of a threshold in vegetative control of floodplain and river morphology during the Carboniferous.

Conditions required for oceanic anoxia/euxinia: Constraints from a one-dimensional ocean biogeochemical cycle model

Widespread black shale depositional intervals termed oceanic anoxic events (OAEs) occurred repeatedly during the Phanerozoic Eon. Here we developed a new vertical one-dimensional ocean biogeochemical cycle model that involves several chemical reactions in an oxic–anoxic–sulfidic water column. To explore the theoretical constraints for global oceanic anoxia/euxinia quantitatively and systematically, we conducted sensitivity analyses of the proposed causal mechanisms, including elevated rates of riverine phosphorus (P) input, ocean stagnation, and lowered oxygen solubility due to climate warming. We gave special attention to the vertical chemical structure of the ocean and also to the characteristic behaviors of the marine P cycle under anoxic conditions, because the relationship between the depth of anoxia and the benthic phosphorus flux could be important for the occurrence of oceanic anoxia/euxinia. Steady-state simulations indicated that (1) a decrease in ocean stagnation or oxygen solubility is not enough by itself to achieve widespread anoxia with the present reactive P river input rate, and (2) shallow water anoxia followed by massive P liberation from surface sediments can lead to widespread eutrophication and anoxia/euxinia. We conclude that elevated riverine flux of reactive P is the most important factor for triggering global anoxic events via a positive feedback loop among ocean anoxia, phosphorus regeneration, and surface biological productivity.

Geochemical evidence for widespread euxinia in the Later Cambrian ocean

Widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long-term inhibitor of evolutionary radiation on early Earth. In recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans, possibly influencing evolutionary events during this time. Physical evidence of widespread anoxia in Cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored. Here we present sulphur isotope records from six globally distributed stratigraphic sections of later Cambrian marine rocks (about 499 million years old). We find a positive sulphur isotope excursion in phase with the Steptoean Positive Carbon Isotope Excursion (SPICE), a large and rapid excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation. Numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions. Independently, molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia. These results identify the SPICE interval as the best characterized ocean anoxic event in the pre-Mesozoic ocean and an extreme example of oxygen deficiency in the later Cambrian ocean. Thus, a redox structure similar to those in Proterozoic oceans may have persisted or returned in the oceans of the early Phanerozoic eon. Indeed, the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in Cambrian oceans.

Experimental measurement of growth patterns on fossil corals: Secular variation in ancient Earth-Sun distances

In recent years, much attention has been given to the increase in the Earth-Sun distance, with the modern rate reported as 5–15 m/cy on the basis of astronomical measurements. However, traditional methods cannot measure the ancient leaving rates, so a myriad of research attempting to provide explanations were met with unmatched magnitudes. In this paper we consider that the growth patterns on fossils could reflect the ancient Earth-Sun relationships. Through mechanical analysis of both the Earth-Sun and Earth-Moon systems, these patterns confirmed an increase in the Earth-Sun distance. With a large number of well-preserved specimens and new technology available, both the modern and ancient leaving rates could be measured with high precision, and it was found that the Earth has been leaving the Sun over the past 0.53 billion years. The Earth’s semi-major axis was 146 million kilometers at the beginning of the Phanerozoic Eon, equating to 97.6% of its current value. Measured modern leaving rates are 5–14 m/cy, whereas the ancient rates were much higher. Experimental results indicate a special expansion with an average expansion coefficient of 0.57H 0 and deceleration in the form of Hubble drag. On the basis of experimental results, the Earth’s semi-major axis could be represented by a simple formula that matches fossil measurements.

Stepwise and large-magnitude negative shift in δ13Ccarb preceded the main marine mass extinction of the Permian–Triassic crisis interval

Large perturbations to the global carbon cycle occurred during the Permian–Triassic boundary mass extinction, the largest extinction event of the Phanerozoic Eon (542Ma to present). Controversy concerning the pattern and mechanism of variations in the marine carbonate carbon isotope record of the Permian–Triassic crisis interval (PTCI) and their relationship to the marine mass extinction has not been resolved to date. Herein, high-resolution carbonate carbon isotope profiles (δ13Ccarb), accompanied by lithofacies, were generated for four sections with microbialite (Taiping, Zuodeng, Cili, and Chongyang) in South China to better constrain patterns and controls on δ13Ccarb variation in the PTCI and to test hypotheses about the temporal relationship between perturbations to the global carbon cycle and the marine mass extinction event. All four study sections exhibit a stepwise negative shift in δ13Ccarb during the Late Permian–Early Triassic, with the shift preceding the end-Permian crisis being larger (>3‰) than that following it (1–2‰). The pre-crisis shifts in δ13Ccarb are widely correlatable and, hence, represent perturbations to the global carbon cycle. The comparatively smaller shifts following the crisis demonstrate that the marine mass extinction event itself had at most limited influence on the global carbon cycle, and that both Late Permian δ13Ccarb shifts and the mass extinction must be attributed to some other cause. Their origin cannot be uniquely determined from C-isotopic data alone but appears to be most compatible with a mechanism based on episodic volcanism in combination with collapse of terrestrial ecosystems and soil erosion.

The evolution of the marine phosphate reservoir

Phosphorus is a biolimiting nutrient that has an important role in regulating the burial of organic matter and the redox state of the ocean-atmosphere system. The ratio of phosphorus to iron in iron-oxide-rich sedimentary rocks can be used to track dissolved phosphate concentrations if the dissolved silica concentration of sea water is estimated. Here we present iron and phosphorus concentration ratios from distal hydrothermal sediments and iron formations through time to study the evolution of the marine phosphate reservoir. The data suggest that phosphate concentrations have been relatively constant over the Phanerozoic eon, the past 542 million years (Myr) of Earth's history. In contrast, phosphate concentrations seem to have been elevated in Precambrian oceans. Specifically, there is a peak in phosphorus-to-iron ratios in Neoproterozoic iron formations dating from ∼750 to ∼635 Myr ago, indicating unusually high dissolved phosphate concentrations in the aftermath of widespread, low-latitude 'snowball Earth' glaciations. An enhanced postglacial phosphate flux would have caused high rates of primary productivity and organic carbon burial and a transition to more oxidizing conditions in the ocean and atmosphere. The snowball Earth glaciations and Neoproterozoic oxidation are both suggested as triggers for the evolution and radiation of metazoans. We propose that these two factors are intimately linked; a glacially induced nutrient surplus could have led to an increase in atmospheric oxygen, paving the way for the rise of metazoan life.

Hyperoxia reduces costs of digestion in snakes: potential bioenergetic consequences of the paleoatmosphere

Hyperoxia reduces costs of digestion in snakes: potential bioenergetic consequences of the paleoatmosphere Marshall D McCueDepartment of Biological Sciences, St Mary’s University, One Camino Santa Maria, San Antonio, TX, USAAbstract: Oxygen levels in the Earth’s atmosphere fluctuated dramatically during the Phanerozoic eon, and may have reached concentrations that were 60% higher than current levels. These atmospheric changes have engendered much speculation about the relationship between O2 availability and major evolutionary events among animals (eg, insect gigantism, flapping flight, endothermy). With the exception of studies of exercising humans, a limited number of investigations have examined the potential effects of hyperoxic conditions on physiologic performance variables in animals. Given that the energetic costs of maintenance and digestion can account for the majority of the total energy budget of snakes, this study was conducted to determine the effect of simulated paleoatmospheric oxygen concentration on the metabolic rates of postabsorptive and postprandial snakes. A series of three repeated-measures trials conducted on Western diamondback rattlesnakes under oxygen concentrations ranging from 21% to 50% revealed that hyperoxia did not affect resting metabolic rates, but that 35% O2 was sufficient to reduce specific dynamic action by an average of 11%. Significant increases in respiratory exchange ratios were also observed in postprandial snakes. Given the degree to which extant snakes are energy-limited, such an energetic benefit may have had significant implications for the feeding behaviors and life histories of ancient snakes as they radiated from their lizard ancestors.Keywords: oxygen limitation, pitvipers, postprandial calorigenesis, rattlesnakes, respiratory quotient, specific dynamic action

Constraints on early Cambrian carbon cycling from the duration of the Nemakit-Daldynian–Tommotian boundary δ13C shift, Morocco

The Nemakit-Daldynian–Tommotian (ND-T) boundary marks the first appearance of metazoan reefs and calcite biomineralizers and is associated with the largest δ 13 C shift during the Phanerozoic Eon. Biological transitions in Earth history are often accompanied by excursions in the carbon isotopic composition (δ 13 C) of the ocean, where δ 13 C variability is interpreted to reflect changes in the global carbon cycle. The duration and thus rate of these δ 13 C anomalies are rarely known, making it difficult to constrain their possible causes and their relationship, if any, to biologic transitions. We report sedimentological and δ 13 C data from a new 2.5-km-thick section that spans the early Cambrian evolutionary “explosion” in the Moroccan Anti-Atlas Mountains. Three new zircon 206 Pb- 238 U ages from tuffs within the stratigraphy constrain the timing of the ND-T boundary to 524.84 ± 0.09 Ma. Two of the tuffs exactly bracket the ND-T transition and constrain the duration of the −8‰ δ 13 C shift to 506 ± 126 k.y. With a simple box model, we explore a range of geochemical processes that could account for such a rapid ND-T δ 13 C shift, and conclude that metamorphic and/or volcanic fluxes of carbon may have been sustained at levels 4–16 times higher than today for millions of years.

Contrasting Structural Features at Different Boundary Areas of the North China Craton and Its Tectonic Implications

The destruction of the North China Craton(NCC) during the Phanerozoic is highly spatially uneven.The inherent heterogeneity of the cratonic lithosphere and the tectonics of interactions among the sourrounding blocks may be responsible for the distinctly different tectonic processes and deformation patterns in the different parts of the NCC.The tectonic diversity certainly would have left significant imprints on the deep structure beneath different geological domains of the craton.A detailed knowledge of the deep structure of the NCC therefore can provide important constrains on the dynamic process and mechanism of the Phanerozoic craton destruction.In this paper,we present the deep structural images of the different boundary zones of the NCC.These images were constructed from the data recently collected by the dense broadband seismic arrays of the North China Interior Structure Project(NCISP).Considering the contrasting features of the crustal and lithospheric structure as well as the upper mantle deformation pattern between these boundary zones,we discuss the spatial heterogeneity and possible tectonic processes of the craton destruction.Our results show that as a mechanically weak zone along the eastern margin of the NCC,the Tanlu Fault Zone has the thinnest lithosphere(~60 km) and obviously attenuated crust(35 km) at the present time,indicating that this fault zone may have been the most significantly affected during the whole-sale lithospheric destruction and thinning in the eastern NCC.On the other hand,both the Yanshan belt and the northern Ordos block around the northeastern and northwestern boundaries of the NCC,respectively,are characterized by the coexistence of both thinned and thick lithosphere and sharp changes in the crustal and upper mantle structure,suggesting an uneven lithopsheric remobilization and thinning in these regions.The sharp Moho and fine-scale variations in both the lithospheric structure and upper mantle deformation pattern beneath the Yanshan belt probably resulted from the joint influence of the multiple phases of compressional deformation in the Mesozoic and intense regional extension from Late Mesozoic to Cenozoic.Striking structural contrasts are even more pronounced between the stable interior of the Ordos block and the surrounding Cenozoic rift areas near the northwestern boundry of the NCC.This as well as the localized lithospheric thinning observed in the rift areas may be attributed to the pre-existing structural heterogeneity and repeated thermo-tectonic reactivation during the long term evolution of the craton.

Effect of nutrient availability on marine origination rates throughout the Phanerozoic eon

The diversity of marine life has varied throughout the past 500 million years. Statistical analyses suggest that fluctuations in the availability of marine nutrients has been one important regulator of rates of origination during this time.

High-resolution carbon isotopic records from the Ordovician of South China: Links to climatic cooling and the Great Ordovician Biodiversification Event (GOBE)

The Great Ordovician Biodiversification Event (GOBE) represented the largest increase in diversity in the marine biosphere during the Phanerozoic Eon, yet its causes and consequences remain poorly understood. Patterns of isotopic variation in high-resolution δ 13C carb and δ 13C org records from a well-exposed section at Honghuayuan in South China may provide important insights regarding the GOBE. The Honghuayuan isotopic profiles, which can be correlated with C-isotopic records from contemporaneous sections globally, reveal large perturbations to the global carbon cycle during the Ordovician. A + 8‰ increase in δ 13C org values in the Floian implies a large, albeit transient increase in the burial rate of organic matter during the mid-Early Ordovician that may have contributed to climatic cooling and played an important role in triggering the GOBE. A + 4‰ increase in δ 13C carb and high-frequency variation in δ 13C org in the Darriwilian to Sandbian suggest a second episode of elevated organic carbon burial rates accompanied by substantial instability in the global carbon cycle during the late Middle and early Late Ordovician. This pattern may mark the onset of climate changes culminating in the end-Ordovician Hirnantian glaciation and mass extinction event that terminated the GOBE.

Influences of alkalinity and pCO2 on CaCO3 nucleation from estimated Cretaceous composition seawater representative of “calcite seas”

Research Article| February 01, 2010 Influences of alkalinity and pCO2 on CaCO3 nucleation from estimated Cretaceous composition seawater representative of “calcite seas” Janie Lee; Janie Lee * Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA *E-mail: jlee@ocean.tamu.edu. Search for other works by this author on: GSW Google Scholar John W. Morse John W. Morse Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Janie Lee * Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA John W. Morse Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA *E-mail: jlee@ocean.tamu.edu. Publisher: Geological Society of America Received: 17 Jul 2009 Revision Received: 20 Aug 2009 Accepted: 21 Aug 2009 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2010 Geological Society of America Geology (2010) 38 (2): 115–118. https://doi.org/10.1130/G30537.1 Article history Received: 17 Jul 2009 Revision Received: 20 Aug 2009 Accepted: 21 Aug 2009 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Janie Lee, John W. Morse; Influences of alkalinity and pCO2 on CaCO3 nucleation from estimated Cretaceous composition seawater representative of “calcite seas”. Geology 2010;; 38 (2): 115–118. doi: https://doi.org/10.1130/G30537.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Experiments were conducted on CaCO3 nucleation in seawater of estimated Cretaceous Period “calcite sea” (i.e., low-magnesium calcite being the primary inorganic marine calcium carbonate precipitate) composition to determine the influences of alkalinity and pCO2 on calcite versus aragonite formation. The pH was continually monitored throughout the experiments and it was used, in combination with the initial alkalinity, to calculate the pCO2 and saturation state of aragonite and calcite at the time of nucleation. The morphology and mineralogy of the precipitates were determined using scanning electron microscopy and X-ray diffraction analyses. It was observed that the initial alkalinity affected greatly the nucleation pCO2 and the CaCO3 polymorph that is precipitated. In seawater with Mg2+/Ca2+ = 1.2, ∼10 mM alkalinity, and a pCO2 <2500 μatm, calcite is the initial polymorph that nucleates. Aragonite nucleates when the pCO2 is >2500 μatm. Seawater with Mg2+/Ca2+ = 1.2 and a wide range of initial alkalinities (5–50 mM) produces variable results. Seawater with Mg2+/Ca2+ = 1.7 produces only aragonite at lower alkalinities (<∼11 mM), but calcite is formed when the alkalinity is >∼18 mM. These results demonstrate the need to also consider the effects of alkalinity and pCO2 in the critical Mg2+/Ca2+ ratio region of ∼1–2 for calcite seas versus “aragonite seas” (in which the primary inorganic carbonate precipitates are aragonite and high-magnesium calcite) at various times throughout the Phanerozoic Eon and to use reasonable estimated values of alkalinity and pCO2 in experimental studies. The results of this study indicate that it may have been possible for CaCO3 to commonly nucleate directly from Cretaceous seawater due to elevated calcium and alkalinity concentrations, even though atmospheric pCO2 was higher. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Early Cambrian ocean anoxia in South China

The cause of the most marked changes in the evolution of life, which define the first-order stratigraphic boundary between the Precambrian and the Phanerozoic eon, remains enigmatic and a highly topical subject of debate. A global ocean anoxic event, triggered by large-scale hydrogen sulphide (H(2)S) release to surface waters, has been suggested by Wille et al., on the basis of two data sets from South China and Oman, to explain the fundamental biological changes across the Precambrian/Cambrian (PC/C) boundary. Here we report a new precise SHRIMP U-Pb zircon age of 532.3 +/- 0.7 million years (Myr) ago (Fig. 1) for a volcanic ash bed in the critical unit that reflects the ocean anoxic event, the lowermost black shale sequence of the Niutitang Formation in the Guizhou Province, South China. This age is significantly younger than the precise PC/C boundary age of 542.0 +/- 0.3 Myr ago, approximately 10 Myr younger than the extinction of the Ediacaran fauna, and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the PC/C boundary.

Quartz crystal size distribution of the Ordovician Millbrig K-bentonite

Crystal size distribution (CSD) analysis has been applied to quartz crystals of the Ordovician Millbrig K-bentonite, which represents one of the largest known fallout ash deposits in the Phanerozoic Era, to establish crystal growth histories and conditions in the magma chamber prior to eruption. Specific CSDs were examined for crystal growth conditions. All Millbrig samples exhibit lognormal concave-down shapes in agreement with previously reported CSDs on large silicic systems but in contrast to more mafic systems characterized by linear CSDs. Concave-down CSDs are possibly an intrinsic property of phenocryst growth in large volume silicic magmas as documented in the literature [Bindeman, I.N., 2003. Crystal sizes in evolving silicic magma chambers. Geology, 31: 367–370.]. Quartz crystals of the Millbrig must have been produced by surface-controlled, size-dependent growth with a minor Ostwald ripening component, which could have easily changed to supply-controlled growth due to a limited supply of nutrients, as the crystals grew larger. CSDs were also used to examine the possible origin of multiple ash beds in the Millbrig. These ash beds appear to be a product of a series of separate eruptions that represent separate magma layers or batches, each with slightly different crystal growth conditions.

CO2‐forced evolution of plant gas exchange capacity and water‐use efficiency over the Phanerozoic

The capacity of plants to fix carbon is ultimately constrained by two core plant attributes: photosynthetic biochemistry and the conductance to CO(2) diffusion from the atmosphere to sites of carboxylation in chloroplasts, predominantly stomatal conductance. Analysis of fossilized plant remains shows that stomatal density (number per unit area, D) and size (length by width, S) have fluctuated widely over the Phanerozoic Eon, indicating changes in maximum stomatal conductance. Parallel changes are likely to have taken place in leaf photosynthetic biochemistry, of which maximal rubisco carboxylation rate, V(cmax) is a central element. We used measurements of S and D from fossilized plant remains spanning the last 400 Myr (most of the Phanerozoic), together with leaf gas exchange data and modeled Phanerozoic trends in atmospheric CO(2) concentration, [CO(2)](a), to calibrate a [CO(2)](a)-driven model of the long-term environmental influences on S, D and V(cmax). We show that over the Phanerozoic large changes in [CO(2)](a) forced S, D and V(cmax) to co-vary so as to reduce the impact of the change in [CO(2)](a) on leaf CO(2) assimilation for minimal energetic cost and reduced nitrogen requirements. Underlying this is a general negative correlation between S and D, and a positive correlation between water-use efficiency and [CO(2)](a). Furthermore, the calculated steady rise in stomatal conductance over the Phanerozoic is consistent with independent evidence for the evolution of plant hydraulic capacity, implying coordinated and sustained increase in gas exchange capacity and hydraulic capacity parallel long-term increases in land plant diversity.

Seeing changes in a changing sea

The ratio of magnesium to calcium in sea water is thought to have influenced the skeletal mineralogy of certain marine calcifiers throughout the Phanerozoic eon. A fresh look at old data suggests that mass extinctions may have also played a role.