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GEOLOGY
Glendonites in Neoproterozoic low-latitude, interglacial, sedimentary rocks, northwest Canada: Insights into the Cryogenian ocean and Precambrian cold-water carbonates
Noel P. James, Queen's University, Geological Sciences, Kingston, ON K7L 3N6, Canada; et al. Pages 9-12.
The Cryogenian (ca. 720–630 million years ago) was characterized by the most intense glaciations Earth has ever experienced, often called the "snowball Earth" glaciations. The nature of climate between these extensive glaciations remains uncertain, with various workers suggesting either icehouse temperatures or a rapid return to normal or even abnormally high temperatures following each glaciation. The uppermost Twitya and Keele formations of northwestern Canada provide a critical test of this, as they were deposited in equatorial latitudes at a time midway between the Sturtian and Marinoan glaciations. Pseudomorphs of ikaite, a carbonate that is stable only at near-freezing seawater temperatures, have been discovered at three levels in this interglacial succesion. While the encompassing sedimentary rocks have all the characteristics of warm-water, continental margin oceanography, the ikaite crystals could only have formed from near-freezing seawater. These findings imply that the shallow world ocean at tropical latitudes between glacial epochs was likely very cold, much colder than previously supposed. Such an ocean would easily have cooled to yield widespread sea ice and, through positive feedback, growth of the low-latitude glaciers hypothesized by the snowball Earth theory. The discovery further implies that the components of carbonate rocks generally utilized to confirm warm-water conditions also formed in cool-waters in Precambrian time when the world ocean was more highly saturated with calcium carbonate prior to the evolution of shelly invertebrates.
Distinguishing base-level change and climate signals in a Cretaceous alluvial sequence
T. White, Pennsylvania State University, University Park, Earth and Environmental Systems Institute, University Park, PA 16802-6813, USA; et al. Pages 13-16.
Paleosols, or ancient soils, often contain records of ancient climatic conditions that existed when the soils formed. However, the task of reading the record from paleosols often involves first understanding other factors involved in paleosol formation. In North America, peculiar middle Cretaceous paleosols formed under globally warm greenhouse conditions are broadly distributed. The paleosols formed along a seaway that existed in central North America at that time--paleosols on either side of the seaway basin display similarities that suggest variations in sea level may have contributed to their formation. We interpret geochemical data and microscopical observations from the paleosols as recording an overall waning of marine influence on the paleosols during their formation. We focus on horizons that precipitated from fresh water and find that during relative sea-level highstands, precipitation was derived from the seaway, whereas during sea-level lowstands, hemispheric-scale atmospheric moisture transport from the tropics led to lower precipitation rates. We conclude that the middle Cretaceous greenhouse atmosphere was subjected to long-term amplification of the global hydrologic cycle, an observation that may have important implications to our not-too-distant future.
Permeability of fine-grained rocks: New evidence from chalks
Anthony J. Mallon, University of Durham, Department of Earth Sciences, Durham, Co. Durham DH1 3LE, UK; et al. Pages 21-24.
The main aquifers in which water and oil/gas are stored naturally underground include sandstones, fractured rocks and limestones, including chalks. Other rocks, such as claystones, salt, and other fine-grained rocks are the seals or caprocks trapping oil/gas beneath them and/or preventing water from flowing freely underground. Deeply buried chalks are shown in this paper to be effective barriers to flow and where thick enough can hold back high pressures which are a hazard when drilling boreholes for water or oil/gas.
Transition from arc to oceanic magmatism at the Kamchatka-Aleutian junction
Maxim Portnyagin, IFM- GEOMAR, Kiel 24148, Germany; et al. Pages 25-28.
The paper presents the first data on the composition of rocks from the most northern young volcanoes in the Kamchatka Peninsula. Geochemistry is used to trace changes in magma generation associated with the broken Pacific Plate and to map upwelling and southward flow of the fertile mantle beneath Kamchatka. The mantle rejuvenation explains recent volcanism in northern Kamchatka and the exceptional productivity of volcanoes in central Kamchatka, the most active volcanoes on Earth's continental margins.
Mineral isochrons and isotopic fingerprinting: Pitfalls and promises
Jon Davidson, University of Durham, Department of Earth Sciences, Durham, Co. Durham DH1 3LE, UK; et al. Pages 29-32.
One of the most widely-used methods for dating rocks is determining the isotopic compositions of the minerals which make up the rocks. Comparing these isotopic compositions, and the parent-daughter ratios of the minerals, we can determine the age at which the rock solidified from magma. This approach relies on a fundamental assumption that the minerals all crystallize with the same isotopic composition to start with, and differences in isotopic ratios in minerals with different parent-daughter ratios then develop with time. Recent work has shown that this fundamental assumption may be commonly flawed. Many of the minerals found in igneous rocks are not crystallized simply from a single magma. Instead, they grow at different times and in different places, commonly from different magmas, and are brought together by mixing of magmas prior to or during eruption or emplacement. This contribution is, on the one hand, a cautionary tale pointing out this realization (which may well go unnoticed in old rocks) and on the other an expression of opportunity, in that the different isotopic ratios among minerals at the time when the rocks form give valuable insights into the processes of magma evolution
Atmospheric CO2 fluctuations during the last millennium reconstructed by stomatal frequency analysis of Tsuga heterophylla needles
Lenny Kouwenberg, Utrecht University, Paleoecology, Utrecht 3584, CD, Netherlands; et al. Pages 33-36.
Investigation of the coupling between atmospheric CO2 levels and climate change under natural conditions in the historical past is crucial in the forecasting of the impact of anthropogenically produced CO2 in the coupled climate-carbon cycle system. Therefore, the inverse relation between atmospheric CO2 concentrations and stomatal numbers on plant leaves was applied to reconstruct atmospheric CO2 mixing ratios over the period A.D. 800–2000 from fossil conifer needles from Washington, United States. In contrast to previous ice-core studies, the stomatal record shows century scale fluctuations in atmospheric CO2, between 10 and 50 ppmv, occurring before the reconstructed anthropogenic CO2 rise over the last century. These atmospheric CO2 variations may, among other sources, partly originate from contemporaneous changes in sea surface temperature in the Atlantic Ocean. This study furthermore suggests that century scale CO2 changes could have played a role as a forcing factor in climate over the last millennium.
Constriction during exhumation: Evidence from eclogite microstructures
Walter Kurz, Graz University of Technology, Institute of Applied Geosciences, Rechbauerstrasse 12, Austria. Pages 37-40.
During the orogenesis of the Alps, large volumina of both oceanic and continental crust were subducted. Most of these materials have been assimilated in the mantle. However, some of these subducted complexes have been exhumed back to Earth's surface from depths as much as 100 km and are now exposed in the Alps as high-pressure metamorphic rocks. During the past 20 years, many models for the exhumation of high-pressure rocks have been proposed, which may be applied to the boundary conditions in the Alps. The model proposed here is based on microstructural investigations on high-pressure rocks and assumes that buoyancy forces that are related to differences in rock-density result in the exhumation of these rocks. This requires that high-pressure rocks, which actually have high densities, are enclosed within rather light, acidic crustal rocks, which again are surrounded by heavy rocks of Earth's mantle, causing their exhumation. In this model the transition from subduction to exhumation is by the break-off of a subducted slab.
Neoproterozoic S isotopes, the evolution of microbial S species, and the burial efficiency of sulfide as sedimentary pyrite Matthew T. Hurtgen, Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA 02138, USA; et al. Pages 41-44.
In 1996, Canfield and Teske proposed that oxygen concentrations in Earth's atmosphere rose to greater than 5-18% of present levels sometime in the Neoprotorozoic (1000-544 Ma). Evidence for this oxidation event came in the form of sulfur isotopes and more specifically, in an increase in the sulfur isotope difference between oceanic sulfate and contemporaneous sedimentary pyrite (?34Ssulfate – pyrite or just ?34S for short) [Delta 34 Sulfur]. During bacterial sulfate reduction, bacteria preferentially separate the lighter sulfur isotope - that is 32S versus 34S - during the production of sulfide and pyrite. The maximum extent of this depletion during bacterial sulfate reduction alone is 46 ‰ [per mil.]. In order to get ?34S values > 46 ‰, the oxidative part of the sulfur cycle must be operating. This occurs when sulfide is oxidized to intermediate sulfur species as opposed to sulfate (the fully oxidized form).
Intermediate sulfur species can under go an additional process termed disproportionation under which bacteria convert intermediate sulfur species into both sulfide and sulfate. During this process, bacteria again separate the lighter 32S in the production of sulfide (by 7-20 ‰) thereby driving ?34S values to > 46 ‰. Importantly, Canfield and Teske (1996) argued that a nonphotosynthetic sulfide-oxidizing (NPSO) bacteria were necessary to produce these intermediate sulfur species and thereby facilitate disproportionate reactions and ?34S values > 46 ‰. Canfield and Teske (1996) argued that ?34S first exceeded 46 ‰ sometime between 1000 and 600 Ma and therefore, NPSO bacteria must have evolved during this time. However, the sulfur isotope composition of Neoproterozoic seawater sulfate was poorly constrained, and therefore ?34S was not precisely known. In this paper, we reconstructed the sulfur isotope composition of seawater sulfate during this time using carbonate-associated sulfate and found that ?34S values in fact did not exceed 46 ‰ from 780 to 570 Ma and that the apparent increase in ?34Spyrite variability during this time manifests an ocean with low sulfate concentrations and rapidly fluctuating ?34Ssulfate - not necessarily the radiation of a NPSO bacteria. Furthermore, we argue that disproportionation reactions have likely occurred at least since the Early Proterozoic (with or without the existence of NPSO bacteria), and ?34S values remained relatively low as a consequence of efficient pyrite burial in an ocean with few oxidants and a low sulfate concentration.
Patchy deposits of Cenozoic pelagic sediments in the central Pacific
Neil C. Mitchell, Cardiff University, School of Earth, Ocean, and Planetary Sciences, Cardiff, Wales CF10 3YE, UK; and Mitchell W. Lyle, Boise State University, Department of Geosciences, Center for Geophysical Investigation of the Shallow Subsurface, Boise, ID 83725, USA. Pages 49-52.
Along the equator in the central Pacific Ocean, pelagic organisms grow unusually strongly and abundantly. The accumulation of their skeletons over millions of years has created a giant mound of sediment across the Pacific, in places more than 600 m thick, which forms the largest such deposit on Earth. Mitchell and Lyle have used techniques traditionally used by the oil business to detect layering within these sediments, which reveals that the deposits are surprisingly patchy in detail. They hypothesize that the patchiness could represent how corrosion of the pelagic skeletons varies across the region because of different chemistry of the deep Pacific Ocean.
Frozen dynamics of migrating bedforms
Douglas J. Jerolmack and David Mohrig, Massachusetts Institute of Technology, Earth, Atmospheric, and Planetary Sciences, Cambridge, MA 02139, USA. Pages 57-60.
Sand ripples in rivers and oceans can leave patterns preserved in rocks. These patterns may be used to infer the size of the ripples that made them, which relates to the speed of flowing water over the ripples. These relationships are complex and have eluded mathematical description until now. The authors mathematically reproduce the dynamic behaviour of ripples and explore the sedimentary patterns generated by these modeled ripples. Results should allow geologists to better interpret ripple marks in the field, which is essential for understanding the history of flowing water on planetary surfaces.
Sensitivity of the Australian Monsoon to insolation and vegetation: Implications for human impact on continental moisture balance
Gifford Miller, University of Colorado, INSTAAR, and Department of Geological Sciences, Boulder, CO 80309-0450, USA; et al. Pages 65-68.
The planetary monsoon explains much of the variability in the annual exchange of energy and moisture between the oceans and land. This paper evaluates changes in the Australian Summer Monsoon, a component of the Asian Monsoon system. Monsoonal rainfall is essential to human sustainability in the past, present, and future, and is a critical driver of landscape ecology and biogeography. Miller et al. show that on hundred-thousand-year time scales the strength of the Australian Monsoon is determined by ice-age cycles of temperature change over central Asia, rather than conditions over Australian itself, as found in a classic monsoon setting, illustrating how the Northern Hemisphere can influence Southern Hemisphere climate. An intriguing element of the Australian Monsoon record is the relative failure of monsoon moisture to intensify in the early Holocene (10,000 years ago), despite reinvigoration of the rest of the planetary monsoon system. They show that this enigma can be explained by altered environmental conditions in Australia.
Using a General Circulation Model, they test the sensitivity of the Australian Monsoon to vegetation and soil properties, and find that if northern Australia is mantled by dense vegetation, monsoon rains deliver twice as much moisture to the interior as they do if it is a desert. Miller et al. point out that now-extinct large marsupial herbivores that roamed across much of northern Australia until their extinction 50,000 years ago suggest more abundant vegetation than occurs there at present. They suggest that a changed burning regime, initiated by the earliest human colonizers, may have transformed the vegetation of semi-arid Australia from a drought-tolerant, relatively dense ecosystem to the modern desert scrub, and in so doing led to not only the extinction of the dependent fauna, but also to the long-term desertification of the continent. At a time when landscape modification by human activity is preceding rapidly, this offers a sobering lesson.
Evidence for solar forcing of sea-surface temperature on the North Icelandic Shelf during the late Holocene
Hui Jiang, East China Normal University, Laboratory of Geographic Information Science, Shanghai 200062, China; et al. Pages 73-76.
Diatom proxies from the modern position of the oceanographic Polar Front north of Iceland record variability in sea-surface temperatures during the past two millennia. Comparison of changes in sea-surface temperatures with variations in the atmospheric circulation above Greenland, North American Atlantic coastal sea-surface temperatures, and mean temperature anomalies for the Northern Hemisphere suggest synchronous North Atlantic–wide fluctuations, which would seem to imply a common forcing factor. A positive and significant correlation between sea-surface temperature record from the North Icelandic Shelf and reconstructed solar irradiance, together with modeling results, supports the hypothesis that solar forcing is an important constituent of natural climate variability in the northern North Atlantic region.
Questioning the evidence for Earth's earliest life--Akilia revisited
Aivo Lepland, Geological Survey of Norway, Geochemistry, Trondheim 7491, Norway; et al. Pages 77-79.
Metamorphic rocks on Akilia Island off the coast of southern West Greenland, claimed to be the oldest (ca. 3.85 Ga) known remains of sediments on Earth, are the focus of interest. Mass spectrometrically determined inclusions of isotopically fractionated carbon in apatite crystals in a sample from these rocks have been interpreted to represent the earliest known traces of life. Recent optical and electronic analyses of a large number of apatite crystals from the original and adjacent samples have, however, failed to reveal any carbon inclusions, thus challenging the original interpretation. It is not clear what objects were analyzed to lead to the widely publicized conclusion about the oldest traces of life in the Akilia rocks, but our data indicate that the claim for early life cannot be founded on an occurrence of graphite inclusions in apatite.
GSA TODAY Science Article
Urban Lead Poisoning and Medical Geology: An Unfinished Story
Gabriel M. Filippelli et al., Indiana University-Purdue University Indianapolis, Dept. of Geology, Indianapolis, Indiana 46202-5132, USA.
Lead, widely recognized as toxic to humans, with particularly negative impacts on the brain development of children, was phased out of paint and gasoline over two decades ago, yet it remains a threat in some U.S. cities. In this month's issue of GSA Today, geologist Gabe Filippelli and coworkers report on the distribution of lead in soils in Indianapolis. They found that lead reaches high levels adjacent to arterial roadways (interstates, for example), indicating that it is residue from the long term use of leaded gasoline. By combining soil lead maps with public health records showing the occurrences of lead toxicity in children, Filippelli et al. have found that many of the cases of lead toxicity occur in children in lower incomes housing areas adjacent to such roadways and nearer to the city center. Filippelli et al. suggest that soil lead is mobilized as airborne dust and ingested by children, particularly during the summer months. The research of Filippelli et al. should serve as a warning that the collocation of arterial roadways and housing may be hazardous to children's health and may be an unappreciated long term hazard of many urban centers.
To view the complete table of contents for the January issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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