Boulder, Colo., USA – New Geology postings extend the understanding of seawater chemistry by ~300 million years; determine erosion rates for exposure of today's southern Rocky Mountains; reveal new evidence for meltwater pulses; call mudstones "so complicated as to almost defy understanding"; describe a White Nile megalake; examine the oldest rocks on Earth; postulate that biomarkers heat up during earthquakes; investigate chemical denudation; describe plate tectonic influences on animal evolution; and analyze sulfides in abyssal peridotites.
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Analyses of fluid inclusions in Neoproterozoic marine halite provide oldest measurement of seawater chemistry
Natalie Spear et al., Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19014, USA; nahilln@sas.upenn.edu. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G34913.1.
The chemistry of the ocean in the mid-Neoproterozoic, approximately 830 million years ago, was very different than the chemistry of the modern ocean. Estimates by Natalie Spear and colleagues suggest that mid-Neoproterozoic marine sulfate concentrations were almost 90% lower than modern values. The major-ion composition (Na+, K+, Ca2+, Mg2+, Cl-, SO42-, and HCO3-) of seawater has varied considerably during Phanerozoic time (the past 541 million years). This is the first direct measurement of the composition of mid-Neoproterozoic seawater, making it the oldest measurement of seawater chemistry. This research extends present-day knowledge of seawater chemistry by approximately 300 million years.
Basins and bedrock: Spatial variation in 10Be erosion rates and increasing relief in the southern Rocky Mountains, USA
David P. Dethier et al., Department of Geosciences, Williams College, Williamstown, Massachusetts 01267, USA; ddethier@williams.edu. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G34922.1.
David Dethier and colleagues show that long-term erosion rates can be influenced by rock strength. In the southern Rocky Mountain landscape, weaker sedimentary rocks erode more rapidly than strong rocks such as granite, helping to produce sharp topographic contrasts. On a global basis, slope is the primary control of long-term erosion rates. To estimate erosion rates at time scales of thousands to tens of thousands of years, Dethier and colleagues used the concentration of 10Be (a cosmogenic isotope) in alluvial sediment in 60 small Colorado, Nebraska, and Wyoming watersheds. Their results demonstrate that at a constant slope, sedimentary rocks in Cenozoic basins (e.g., North Park, Colorado) are eroding at about 75 mm per thousand years, two to four times faster than the granitic rocks of the adjacent Rocky Mountains. Concentrations of 10Be and 26Al in near-surface granitic bedrock imply long-term (more than 105 years) erosion rates of 10 to 30 mm per thousand years, similar to shorter-term granitic values. The spatial distribution of erosion rates and geologic evidence imply that topographic relief in the southern Rocky Mountains increased in the past 40 million years; erosion exposed the modern landscape over the past five to 10 million years.
Geomorphic and stratigraphic signals of postglacial meltwater pulses on continental shelves
Andrew N. Green et al., Geological Sciences, University of KwaZulu-Natal, Private Bag X54001, Durban, South Africa; greena1@ukzn.ac.za. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G35052.1.
Andrew Green and colleagues provide new geomorphological and stratigraphic evidence for abrupt rises in sea level, termed Melt Water Pulses (MWPs), from the South African continental shelf. Two MWPs are documented based on the regional occurrence of remarkably well-preserved submerged shorelines that were drowned in place during these rapid rises in sea level. These are situated at 60 and 100 m depths and considered to correspond with MWP 1A and 1B respectively. Green and colleagues hypothesize that setting aside local differences in topography and sediment supply, areas of similar eustatic and tectonic regimes will have similar features preserved in their stratigraphic and geomorphic records.
Parasequence types in shelfal mudstone strata—Quantitative observations of lithofacies and stacking patterns, and conceptual link to modern depositional regimes
Kevin M. Bohacs et al., ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77252, USA; Kevin.M.Bohacs@exxonmobil.com. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G35089.1.
Mudstones, commonly thought to be homogenous and boring, turn out to be quite complex. Recent work on shale-gas and tight-liquid development, along with research on climate records and modern continental shelves, have revealed mudstones to be so complicated as to almost defy understanding. Rather than recording settling from suspension under persistently low-energy and low-oxygen waters, mudstones reflect the complex interaction of biological production, decay, dissolution of fossils, influx of sediments from land, and alteration during burial. Work presented here by Kevin Bohacs and colleagues reveals ordered patterns in this complexity that can be differentiated quantitatively and used to make predictions of rock properties. It appears that, on marine continental shelves, myriad processes converge to deposit relatively few associations with commonly recurring rock properties at the meter to tens of meter scale. These associations can be related quantitatively to ancient depositional environments with characteristic sets of processes. Bohacs and colleagues then used these patterns of sedimentary features to infer corresponding modern regimes with similar sets of commonly recurring depositional processes and from that, develop appropriate templates for their map patterns. These results can be used to make more robust interpretations of the rock record and to map the distribution of natural resources.
A White Nile megalake during the last interglacial period
Timothy T. Barrows et al., Department of Geography, College of Life and Environmental Science, University of Exeter, Exeter, Devon EX4 4RJ, UK; t.barrows@exeter.ac.uk. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G35238.1.
The eastern Sahara Desert of Africa is one of the most climatically sensitive areas on Earth, varying from lake-studded savannah woodland to hyperarid desert over the course of a glacial-interglacial cycle. In currently semiarid Sudan there is widespread evidence that a very large freshwater lake once filled the White Nile River valley. This paper by Timothy Barrows and colleagues presents the first quantitative estimate for the dimensions of the lake and a direct age for the emplacement of its shoreline. Using a dating approach based on exposure to cosmic rays, they estimate an exposure age of 109 thousand years (plus or minus eight thousand) for this megalake, indicating that it probably formed during the last interglacial period. This age is supported by optically stimulated luminescence dating of Blue Nile paleochannels associated with the lake. Using a high-resolution digital elevation model, Barrows and colleagues estimate that the lake was more than 45,000 square kilometers in area, making it comparable to the largest freshwater lakes on Earth today. They attribute the lake's existence to seasonal flood pulses as a result of local damming of the White Nile by a more southern position of the Blue Nile and greatly increased precipitation associated with an enhanced monsoon.
Heading down early on? Start of subduction on Earth
Simon Turner et al., Department of Earth and Planetary Sciences, Macquarie University, Sydney, Australia; simon.turner@mq.edu.au. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G34886.1.
This study by Simon Turner and colleagues compares the rock succession and geochemistry from a modern example of plate subduction (where one tectonic plate descends beneath another beneath a line of volcanoes) with this from the oldest rocks on Earth. The favorable comparison is used to suggest that modern-day plate tectonics have operated throughout Earth history. On the modern Earth, these are places containing abundant life and so a similar setting may have provided a place for development of earliest life.
Biomarkers heat up during earthquakes: New evidence of seismic slip in the rock record
Heather M. Savage et al., Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA; hsavage@ldeo.columbia.edu. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G34901.1.
While earthquakes must occur along faults, it is very difficult to determine what structural features in fault zones are directly related to earthquake slip. Identification of past earthquakes on exhumed faults would allow relation of structural features to earthquake processes. Frictional melt, called pseudotachylyte, has been the only way to detect past earthquakes on faults that have been exhumed to the surface. In this study, Heather Savage and colleagues show that organic compounds present in sedimentary rocks are altered along faults that were unambiguously heated during earthquakes, as evidenced by the presence of pseudotachylyte melt in the faults. They also observe a gradational heating signature away from the fault that is not observable with pseudotachylyte but is expected from thermal diffusion. These results show for the first time that the difference in organic thermal maturity between on and off-fault rocks is a robust earthquake indicator.
How accurate are rivers as gauges of chemical denudation of the Earth surface?
Julien Bouchez, GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Telegrafenberg, 14473 Potsdam, Germany, bouchez@gfz-potsdam.de; and Jérôme Gaillardet Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris-Cité, Institut Universitaire de France, 1 rue Jussieu,75238 Paris cedex 05, France. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G34934.1.
Rocks are continuously chemically weathered through a reaction with compounds contained in the atmosphere and in rain. As a result, rocks are dissolved, the remaining solid form soils, and continents are undergoing a slow "chemical denudation." To measure the rate of chemical denudation of Earth's surface, geochemists have used the chemical composition of the dissolved load of rivers (calcium, magnesium, potassium, sodium, silica), which transport the products of continental rock weathering to the ocean. Julien Bouchez and colleagues show how, during some weathering reactions, oxygen and hydrogen can also be released from rocks. For other types of reactions, oxygen and hydrogen can be incorporated into soils, partially compensating for chemical denudation. Therefore, oxygen and hydrogen atoms are involved in most chemical weathering reactions and thus influence chemical denudation rates. Presently, the way geochemists measure river chemical composition does not account for this fact.
Plate tectonic influences on Neoproterozoic-early Paleozoic climate and animal evolution
N. Ryan McKenzie et al., Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA; rmckenzie@jsg.utexas.edu. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G34962.1.
The initial diversification of animals paralleled some of the most dramatic episodes of climate and environmental change in Earth history. N. Ryan McKenzie and colleagues compile global Neoproterozoic-early Paleozoic detrital zircon age data to track spatiotemporal variations in continental arc systems to explore the influence of tectonic outgassing of CO2 on these climatic shifts. These data indicate that global continental arc systems were spatially reduced at the onset of the Cryogenian glacial interval, widespread during the Cambrian greenhouse, and reduced during Ordovician cooling. The Cambrian greenhouse was coincident with ecologically stressed conditions, whereas Ordovician global cooling was accompanied by a major biodiversification event. Thus, variation in the continental arc CO2 flux likely played a critical role in major climatic fluctuations, which profoundly influenced early animal evolution.
In-situ Pb isotopic analysis of sulfides in abyssal peridotites: New insights into heterogeneity and evolution of the oceanic upper mantle
J. Blusztajn et al., Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA; jblusztajn@whoi.edu. Published online ahead of print on 6 Jan. 2014, http://dx.doi.org/10.1130/G34966.1.
Abyssal peridotites and mid-ocean ridge basalts (MORBs) represent complementary residue-liquid products of melting and melt migration in the oceanic mantle. Because MORBs are mixtures of melts coming from different mantle depths, their isotopic signature do not directly describe the isotopic composition of the mantle source, but instead describes the average composition of different parts of the mantle. In contrast, abyssal peridotites, the residues of fractional melting and melt-rock reaction, should shed more light on distribution of isotopic heterogeneities. In-situ analysis of lead isotopes in sulfide grains from abyssal peridotites reveal that they preserve a record of mantle compositions not seen in whole rock basalts from the same area. The results presented here by J. Blusztajn and colleagues confirm that the source of MORBs, as represented by abyssal peridotites, is very heterogeneous. In situ lead isotopic data in sulfides provides a powerful tool for detecting the distribution, composition and length-scale of mantle heterogeneities.
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Geology