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GEOLOGY
New Early Cambrian bilaterian embryos and larvae from China
Michael Steiner, TU Berlin, Berlin 13355, Germany, et al. Pages 833-836.
This article describes an exceptionally preserved association of bilaterian embryos and larvae from the Early Cambrian (ca. 535 Ma years) of the Yangtze Paraplatform (South China). The three-dimensional preserved embryos are phosphatized and even show cell structures. The investigated larvae are the oldest known undoubted examples described so far. The reconstruction of the developmental cycle of the genus Pseudooides indicates an assignment to the protostomes, most probably within an ancestral arthropod lineage.
Implications of lunar orbital periodicity from the Chaibasa tidal rhythmite (India) of late Paleoproterozoic age
Rajat Mazumder, Yokohama National University, Geological Institute,Yokohama, Kanagawa 2408501, Japan. Pages 841-844.
The pace of tidal evolution for the past ~450 million years leads geophysicists to speculate an Earth/Moon collision occurred sometime between 1.5–2.0 billion years ago resulting in the total melting of Earth's mantle. This report examines lunar orbital periodicities from late Paleoproterozoic (2.1–1.6 billion years ago) marine Chaibasa Formation, India. The article shows that a normal semidiurnal (two tides per day) tidal system similar to present day was active during Chaibasa sedimentation and no such collision took place during that time. Mazumder's data, in combination with other published tidal rhythmite data, indicate long-term stability of the lunar orbit since the Paleoproterozoic.
Orbital tuning and correlation of 1.7 m.y. of continuous carbon storage in an early Miocene peatland
David J. Large, University of Nottingham, School of Chemical, Environmental, and Mining Engineering, Nottingham, Nottinghamshire NG7 2RD, UK, et al. Pages 873-876.
Peatland is the main natural source of atmospheric methane and a sink for carbon dioxide. Whether peatland will help mitigate or enhance concentrations of these greenhouse gases during global warming is uncertain. New evidence of 1.7 million years of continuous peat accumulation demonstrates that short-term climate change, which controls carbon storage in modern peatland, does not affect carbon accumulation in peatland over periods exceeding 100,000 yrs. Simultaneous changes in peatland and the marine records may point to a link between peatland methane emissions and the marine carbon reservoir.
The shape of Mesozoic dinosaur richness
David E. Fastovsky, University of Rhode Island, Department of Geosciences, Kingston, RI 02881, USA, et al. Pages 877-880.
Did Mesozoic dinosaurs appear abruptly and increase in diversity or did they maintain more or less constant diversity throughout their 160 million years on Earth? Did they die out suddenly or did they decline in number and diversity over the final 15 million years of the Mesozoic Era? This research addresses aspects of these questions using a newly available global database of Mesozoic dinosaur genera, which is the largest and most up-to-date of its type known. Our research reaffirms that the number of dinosaur genera increased strikingly and steadily throughout the Mesozoic, a radiation that we believe can be linked to the evolution of new innovations. These allowed the exploration of new ecospace and appear to have fueled the increases in dinosaur diversity. All indicators are that the dinosaur clade was robust at the point of its Mesozoic extinction. Minor fluctuations in the last stages of the Mesozoic, heretofore commonly attributed to a decline in diversity during the last 15 million years of the Cretaceous, cannot be viewed as such in the context of overall Mesozoic dinosaur diversity. The data show that the supposed gradual dinosaur "extinction" of the last ~10 million years of the Cretaceous is in fact a much less profound drop in diversity than earlier fluctuations in the data. Therefore, our view is that all of these fluctuations represent imperfections in the data rather than real extinctions--a viewpoint that is reinforced by the robust global pattern of increasing generic diversity. Recorded here for the first time, the number of Cretaceous Asian dinosaur fossils is approaching that of North America. This contrasts with the dating of Mesozoic dinosaurs, in which the most refined dates are typically found in North America. Refined dating of dinosaurs that are not from North America, rather than the collection of yet more dinosaurs, would provide the best insights into the tempo and mode of dinosaur evolution.
A continuous 150,000 yr monsoon record from Lake Eyre, Australia: Insolation-forcing implications and unexpected Holocene failure
John W. Magee, Australian National University, Earth and Marine Sciences, Canberra ACT 0200, Australia; et al. Pages 885-888.
Lake Eyre is a very large (10,000 sq km) ephemerally flooded playa in the most arid part of Central Australia fed by runoff from summer monsoon rainfall. In the past it has been both wetter (enlarged perennial lake) and drier (playa basin deflation) than today and the lake-level record through time is a proxy for the strength of the Australian monsoon. This paper presents a continuous lake-level curve for Lake Eyre over the past 150,000 yrs dated by multiple methods, which demonstrates accord with the major Quaternary ice-age cycles with wet events during warm interglacials and dry events during cold glacials. The Australian monsoon is at the periphery of the Asian monsoon, a major component of the planetary climate system. Debate exists as to whether variations in the intensity of Australian summer monsoon rainfall, both in the modern system and through Quaternary climate cycles, are controlled by variations of Northern or Southern Hemisphere insolation. Abrupt transitions in the lake-level record from dry phases to wet phases at 125,000 and 12,000 yrs ago coincide with Northern Hemisphere winter-insolation minima rather than Southern Hemisphere summer-insolation maxima indicating that outflow from the Northern Hemisphere Siberian high-pressure cell is the major control. Regardless of the hemispheric forcing, the low intensity of the early Holocene Australian monsoon--by comparison with the last interglacial and especially the intervening high-level lacustrine events when all forcing elements were modest--is an enigma that can only be explained by a change in boundary conditions within Australia. We believe that a plausible boundary-condition change that could affect the efficiency of landward transfer of monsoon moisture is the large-scale alteration of the biota by human burning, across the northern half of Australia since the arrival of humans on the continent about 50–55,000 yrs ago.
Igneous dikes on Mars revealed by MOLA topography
Richard A. Schultz, University of Nevada--Reno, Department of Geological Sciences, Mackay School of Earth Sciences and Engineering, Reno, NV 89557-0138, USA; et al. Pages 889-892.
The origin of Martian grabens has remained controversial for decades because their morphology in spacecraft images could be equally well explained by either a pair of normal faults, caused by extensional tectonism, or by a shallow igneous dike. Knowledge of which of these alternatives operates on Mars, and in what localities, would focus discussions of Martian tectonic, magmatic, and hydrologic evolution onto a less conjectural framework. Although the imaged morphology of grabens may be nonunique, the topography is not, with definitive differences predicted for grabens formed from faults or dikes. Using high-resolution topography, we have identified the characteristic topographic signature of a dike that was intruded at shallow depth below the Martian surface, supporting inferences of geologically recent, near-surface magma transport in part of the Tharsis volcanotectonic complex.
U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania
Clive R. Calver, Geological Survey of Tasmania, Mineral Resources Tasmania, Rosny Park, Tasmania 7018, Australia; et al. Pages 893-896.
Two or more ice ages during the late Neoproterozoic era appear to have been more severe than at any other time in Earth's history. The Marinoan glacial deposits of South Australia record one of these ice ages, and correlatives of these deposits have previously been identified on several other continents. Correlation has generally relied on chemostratigraphic and lithostratigraphic evidence rather than radiometric (isotopic) dating, as suitable rocktypes for the latter have been generally lacking. In spite of the lack of numerical dates, the confidence of the geological community in a synchronous, globally distributed Marinoan ice age was recently formalized in the selection of the top of the Marinoan glacials in South Australia as the Global Stratotype Section and Point ('golden spike') for a new, terminal Proterozoic (Ediacaran) system. Global synchroniety of late Neoproterozoic ice ages is also an essential (but largely untested) element of the "snowball Earth" hypothesis. Recently, presumed correlatives of Marinoan glacial deposits in China and Namibia have been dated isotopically at 663–599 Ma and ~636 Ma, respectively. Our paper reports two U-Pb dates (582 ± 4 Ma and 575 ± 3 Ma) that closely bracket Marinoan glacial correlatives in Tasmania, and which together strongly suggest a significantly younger age (~580 Ma) for the Marinoan glacial deposits of Australia. The base of the Ediacaran System is therefore significantly younger than suggested by recent work, and correlation of late Neoproterozoic glacial units between continents needs to be reappraised.
Preservation of Early Cambrian animals of the Chengjiang biota
Sarah E. Gabbott, University of Leicester, Geology, Leicester, Leicestershire LE1 7RH, UK; et al. Pages 901-904.
In the Yunnan Province, China, the Chengjiang sediments have contained within them unique and exquisitely preserved fossils. Although these animal remains are over 500 million years old nearly every detail of their anatomy can be studied, from the spiny proboscis of ancient worms to the hairs on the legs of primitive arthropods. These animals lived in the Cambrian sea and record what life was like just after the Cambrian "evolutionary explosion" - a crucial time in the history of life on Earth. Until now the processes that acted to preserve these animals have been poorly understood. This paper shows that a common mineral, pyrite (often known as fool's gold) rapidly precipitated onto the rapidly decaying carcasses of the Chengjiang animals and faithfully captured their morphology. Even more remarkable is that the type of soft tissue may have influenced the shape of the mineralizing pyrite crystals. Raspberry shaped pyrites, termed framboids, replaced easily decayed animals and tissues, whereas, perfect octahedral and cubic shapes reflect animals and tissues that were tougher and so decayed more slowly.
A volcanic source for fixed nitrogen in the early Earth's atmosphere
Tamsin A. Mather, Cambridge University, Earth Sciences, Downing Street, Cambridge
Cambridgeshire CB2 3EQ, UK; et al. Pages 905-908.
All life on Earth needs nitrogen to survive. Although nitrogen makes up the majority of the air we breathe our bodies are unable to absorb it in this form. Before we can access this huge reservoir of atmospheric nitrogen it must first be fixed into forms (such as ammonia and nitrogen oxides) that can be absorbed by the plants that we, or animals lower down the food chain, eat. This is why we put nitrogen-containing fertilizers on soils to increase their fertility. Some bacteria and fungi have evolved the ability to fix nitrogen and these biological processes along with mankind's activities (such as the burning of fossil fuels) are the major sources of fixed nitrogen in present-day ecosystems. But the question remains as to where the fixed nitrogen came from that enabled life to evolve in the first place (about three billion years ago). Previously, lightning and asteroid impacts have been suggested as the major fixed nitrogen sources for the early Earth's atmosphere. However, new work presented here suggests that the high temperatures associated with volcanic activity might also have played a role. Researchers observed elevated levels of fixed nitrogen in the plume from a lava lake at Masaya volcano in Nicaragua. The heat from the volcano allows the nitrogen and oxygen in the atmosphere to react together to form fixed nitrogen. From their measurements the researchers were able to estimate the rate of nitrogen fixation that volcanoes might produce in the early Earth's atmosphere and to show that this process may have been as important as lightning and asteroid impacts.
100 years in the dark: Extreme longevity of Eocene bivalves from Antarctica
Devin P. Buick and Linda C. Ivany, Syracuse University, Earth Sciences, Syracuse, NY 13244, USA. Pages 921-924.
Why do organisms have the lifespans they do, and why do some species live fantastically long lives in comparison to their close relatives? A number of polar bivalves today exhibit extreme longevity, living upwards of a century or more, while their warmer-water cousins are lucky to make it to 10 years. This phenomenon is often attributed to the cold temperatures of the water in which the clams live, but on Earth today it is impossible to separate this factor from other variables characteristic of polar environments. Buick and Ivany examined fossil bivalves from the Eocene of Antarctica (~45 million years ago), a time when polar latitudes were relatively warm, in order to investigate the relationship between temperature and lifespan in high latitude organisms. High resolution sampling and oxygen isotope analysis of material collected across a portion of two shells reveal seasonal temperature variations during the life of the bivalve, confirm that the Eocene water temperature was warm, and show that dark growth bands in the shell correspond to summertime warm temperatures. The annual nature of these bands then allowed them to count bands and arrive at age estimates for individuals, revealing lifespans on the order of 120 years or more. These polar clams from Earth's distant past also lived exceedingly long lives, despite living in warm water. The authors suggest that extreme longevity in this case results from slow metabolism associated with the limited light and hence food (phytoplankton) availability that characterizes polar settings. Oxygen and carbon isotope analyses also surprisingly reveal that these clams stopped growing every year during the Austral summer, when sunlight would have stimulated phytoplankton production and presumably shell growth. The researchers suggest that summer growth stoppages reflect the dedication of metabolic energy to spawning at a time when larval survival is more likely due to the presence of food. The small chances for successful reproduction in a generally food-stressed setting may also make long life adaptive, in that there are more opportunities for seasonal spawning. The limited and seasonal availability of light and hence food in polar regions may contribute to the longevity of high latitude bivalves today as well.
GSA TODAY Science Article
Dilational fault slip and pit chain formation on Mars
David A. Ferrill, CNWRA, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238, USA, et al.
Ever since the earliest telescope observations in the 1600s by the likes of Galileo, Huygens, and Cassini, people have wondered about the origin of surface features on the planet Mars. New high-resolution images from satellites have shown the Martian surface in unprecedented detail, and scientists have relied on comparisons of geologic features with those on Earth to understand the processes affecting the Red Planet. In a recent paper in GSA TODAY, David Ferrill and colleagues at the Southwest Research Institute and University of Texas at San Antonio report on the origin of unusual linear chains of pits that are common on Mars. By using a combination of laboratory simulations, mathematical modeling to take into consideration the decreased gravity field on Mars, and by comparison with modern examples in Iceland, the authors show that these features are formed by active faulting. As the faults move, unconsolidated loose material above the fault collapses into the fault zone, forming distinctive conical depressions aligned along the trace of the fault. The pristine morphology of the pits is taken as an indication that the pits formed recently and thus, like Earth, Mars may have active faults.
To view the complete table of contents for the October issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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