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GEOLOGY
Geologic constraints on the chaotic diffusion of the solar system
Heiko Pälike, Southampton Oceanography Centre, School of Ocean and Earth Science, University of Southampton, Southampton, UK; et al. Pages 929-932.
This paper shows that Earth's climate system of the past, as recorded in deep-sea sediments, is sensitive enough to encode information about fundamental astronomical parameters over millions of years--an exciting new find. Palike et al. use the fidelity of the Earth system "recorder" to extract information about the history of orbital variations for the past ca. 30 million years. In particular, the solar system's orbital variations behave in a chaotic fashion, thus making it increasingly difficult to calculate past orbital variations with astronomical models alone: the precision of astronomical calculations, adjusted to recent observations, decreases by a factor of 10 every 10 million years into the past. Palike's new constraints are thus of great interest to earth scientists as well as astronomers: astronomical calculations are used as a template for the astronomical calibration of the geological time scale and show that a new improved calculation of Earth's orbit fits geological data much better than previous ones. More importantly, Palike's results show why astronomers and physicists should look again at geological records to obtain precise constraints on their models, with the possibility to gain new and challenging constraints on models of the orbital evolution of the solar system. It is of particular interest to forecast the prolongation of this work to longer periods of time (40–60 million years ago and beyond), which could provide some ultimate constraints for the dynamical models of the solar system evolution.
Bacterial sulfate reduction limits natural arsenic contamination in groundwater
Matthew F. Kirk and Craig M. Bethke (corresponding author), University of Illinois, Geology, Urbana, IL; et al. Page 953-956.
Researchers at the University of Illinois have discovered that a common, subsurface class of bacteria controls whether or not high levels of arsenic occur in groundwater. The researchers studied a large aquifer in central Illinois, where water from some wells contains high levels of arsenic, even though water from nearby wells may be devoid of the substance. Where sulfate-reducing bacteria are active, the sulfide produced reacts to strip arsenic from solution. In the absence of sulfate reaction, methanogenesis is the dominant type of metabolism, and arsenic accumulates to high levels.
Tropical response to the 8200 yr B.P. cold event?: Speleothem isotopes indicate a weakened early Holocene monsoon in Costa Rica
Matthew S. Lachniet, University of Nevada, Geological Science, 4505 Marlyland Parkway, Las Vegas, NV 89154, USA; et al. Pages 957-960.
When one thinks of Costa Rica, a small Central American nation, the image that comes to mind is of a tropical paradise far removed from the bitter cold winters of the high latitudes. Costa Rica is a country replete with verdant vegetation and abundant waterfalls, where parrots and toucans fly among summer-like rain showers and clouds billow high into the sky. However, new evidence recovered from a Costa Rican cave shows that the tropical climate of past millennia was abruptly affected by catastrophic changes thousands of miles away in the Canadian tundra. As the northern hemisphere ice sheets began their slow retreat at the end of the last Ice Age, large lakes accumulated at their southern margin. One such lake, Glacial Lake Agassiz, drained rapidly when new outlets to Hudson Bay were opened by ice retreat and released a massive pulse of freshwater to the North Atlantic Ocean. Like the plot in a recent Hollywood blockbuster movie, this freshwater pulse weakened the ocean circulation that transports warm tropical water to higher latitudes and provoked climate perturbations as far south as the Caribbean Sea. The monsoon rains of Costa Rica were not unaffected and rainfall amounts decreased for approximately 400 years following this ocean circulation disruption. This conclusion is based on chemical measurements of oxygen isotopes preserved in a stalagmite--a growth of calcite--from a Costa Rican cave that was dated by Uranium-series methods to have grown between 8800 and 4900 yr BP.
Codependent histories of the San Andreas and San Jacinto fault zones from inversion of fault displacement rates
Richard A. Bennett, University of Arizona, Department of Geosciences, Tucson, AZ 85721, USA; et al. Pages 961-964.
The present-day partitioning of strain between the San Jacinto and southernmost San Andreas fault zones is of particular interest since there are very different rates of seismic strain release on the two fault systems--the San Jacinto fault zone has ruptured in several M > 6 earthquakes in the last century, whereas the southernmost San Andreas fault has produced very few earthquakes. Accurate estimates of present day strain accumulation rates could help to clarify whether the relative absence of historical seismicity on the San Andreas fault is an indication that this fault posseses a lower seismic threat to southern California, or alternatively suggests that it ruptures primarily during infrequent large to great earthquakes. We investigated the displacement histories of the two fault zones using geologic fault slip data spanning time periods ranging from 5 Ma to 5000 years. We found that apparent discrepancies between long- and short-term average displacement rates can be reconciled using a model for time variable displacement rate for both faults. In this model, the displacement rate on the southernmost San Andreas fault decelerated from ~35 mm/yr at 1.5 Ma to as low as 9 + 4 mm/yr by 90 ka. Over this same time period, the rate of displacement on the San Jacinto fault zone accelerated from zero prior to 1.5 Ma to a rate of 26 + 4 mm/yr. The data also imply that the San Andreas fault displacement rate accelerated since ~ 90 ka, from ~ 9 mm/yr to a modern rate of 27 + 4 mm/yr, whereas the San Jacinto fault zone decelerated from 26 + 4 mm/yr to a modern rate of 8 + 4 mm/yr. The time scale of these changes is significantly longer than the earthquake cycle, but shorter than time-scales characteristic of lithospheric-scale dynamics. The emergence of the San Jacinto fault zone around 1.5 Ma coincides with the development of a major restraining bend in the San Andreas fault zone. These observations suggest that the formation of new sub-parallel faults (e.g., San Jacinto) could be driven by conditions that inhibit displacement on pre-existing faults (e.g., San Andreas). Such interactions between these faults could thus have important implications for geodynamics, bearing on issues such as the relative strengths of upper mantle, and lower and upper crustal layers, and the degree to which the deformation field within the upper crust is coupled to that within the upper mantle. Moreover, the discrepancy between present-day rate estimates (faster San Andreas) and seismic strain release patterns (more active San Jacinto) would appear to suggest that the historical earthquake record is not a good proxy for present-day loading rate. If so, the absence of historical seismicity on the San Andreas fault would suggest that this fault zone ruptures primarily in large to great earthquakes.
Thermal-hydrologic mechanism for rainfall-triggered collapse of lava domes
D. Elsworth, Pennsylvania State University, Department of Energy and Geo-Environmental Engineering, University Park, PA 16802, USA; et al. Pages 969-972.
Hot lava domes that cap active stratovolcanoes are shown to be suceptible to collapse triggered by intense rainfall. Collapse may occur both in periods of active dome growth and volcanic repose, evolve into energetic failures involving as much as 90% of the dome, or >100 × 106 m3 of dome lava, and may result in lethal pyroclastic flows that reach many kilometers from the source. This work develops a new thermal-hydrologic mechanism that can cause such failures: pressure buildup within fissures due to effusive gas trapped by a rain-saturated dome carapace results in increased destabilizing forces and the loss of mass strength. This ultimately results in failure of the dome. Mechanistic models are consistent with field observations and provide a quantification of threshold rainfall intensities and durations required to trigger failure.
Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis
Andrea Marzoli, Università di Padova, Mineralogia e Petrologia, Padova, Italy; et al. Pages 973-976.
New multidisciplinary data show that the largest known continental volcanic province, the CAMP, was synchronous, some 200 million years ago with the Triassic-Jurassic boundary mass extinction, one of the most severe climatic and biotic catastrophes. Widespread basaltic lava flows erupted in a short time first in Morocco and then in North America and triggered, through emission of volcanic gases, a global warming of Earth's climate and thus extinction of numerous continental and marine species.
Effect of ambient Mg/Ca ratio on Mg fractionation in calcareous marine invertebrates: A record of the oceanic Mg/Ca ratio over the Phanerozoic
Justin B. Ries, Johns Hopkins University, Morton K. Blaustein Department of Earth and Planetary Sciences, Baltimore, MD 21218, USA. Pages 981-984.
The concentrations of magnesium (Mg) and calcium (Ca) in the oceans are believed to have been controlled over geologic time by a reaction between seawater brine and magma along midocean ridges, which is driven by the rate of ocean crust production. Four high-Mg calcareous organisms (echinoids, crabs, shrimps, and calcareous serpulid worms) decreased their skeletal Mg when grown in artificial ancient seawaters of reduced Mg/Ca ratios, and actually changed to low-Mg calcite skeletons in the seawater of lowest Mg/Ca (~1 molar ratio), corresponding to ~100 mya. This suggests the skeletal chemistry of these organisms has varied significantly over geologic time. Algorithms relating skeletal Mg/Ca to ambient Mg/Ca and temperature were developed for the four organisms. A Phanerozoic history of oceanic Mg/Ca, predicted from the echinoid algorithm using previously published skeletal Mg/Ca ratios of fossil echinoderms and paleotemperatures, is consistent with the predictions of other models (fluid inclusions, history of calcite-aragonite seas, brine-magma-river water flux model). The history of oceanic Mg/Ca could potentially be reconstructed from unaltered fossils of the other organisms evaluated, as well.
Stratigraphic paleoecology: Bathymetric signatures and sequence overprint of mollusk associations from upper Quaternary sequences of the Po Plain, Italy
Daniele Scarponi, University of Bologna, Earth Sciences, Bologna 40126, Italy and Michal Kowalewski, Virginia Polytechnic Institute, Department of Geological Sciences, Blacksburg, VA 24061-0420, USA. Pages 989-992.
Since at least Galileo's times, fossils have been used to gain insights into the nature of ancient environments. In particular, paleontologists and geologists have argued in recent years that fossil shellfish can provide an important recorder of water depth of prehistoric oceans. Now, a new study of the relatively recent geological past (the last 125 thousand years) shows that ancient mollusk shells can indeed provide outstanding depth gauging devices. Paleontologists from University of Bologna and Virginia Tech used shells of fossil mollusks extracted from marine sedimentary rocks to estimate the water depth at which these rocks were originally deposited. By applying multivariate numerical techniques to their fossil data and testing the resulting patterns against the depth distribution of the present-day shellfish, the researchers were able to estimate the water depth of ancient seas to the nearest 3 meters (~9 feet). The fact that fossil shellfish can provide so accurate and precise bathymetric (depth gauging) tool is an exciting discovery: it offers geoscientists a precise strategy for enhancing our understanding of sedimentary rocks, which represent the primary source of information about the history of life and past environments and climates of Earth.
Canterbury Drifts at Ocean Drilling Program Site 1119, New Zealand: Climatic modulation of southwest Pacific intermediate water flows since 3.9 Ma
R.M. Carter, James Cook University, Marine Geophysical Laboratory, Townsville, Qld. 4811, Australia; et al. Pages 1005-1008.
The New Zealand Southern Alps run longitudinally across Southern Hemisphere midlatitudes, at 42–47 °S. The Alps, situated athwart both the oceanic Subtropical Front and the northern edge of the zonal westerly wind belt, are therefore located in a location of great climatic sensitivity. The Alps rise to 3754 m (Mount Cook) and today host a number of small mountain glaciers. However, at the peak of the previous glaciations, the Alps were the site of an expanded, midlatitude, maritime ice cap. Ocean Drilling Program drill site 1119 is situated on the upper continental slope, just east of the Southern Alps, and in such a position that it lies in the direct path of riverine outwash plumes from the melting alpine snows. The 500-m core from the site extends back to almost 4 million years ago, in the middle Pliocene. Measurements of natural gamma radiation levels down the hole, which respond to the mud content of the sediment, reveal a high resolution (1–2 kyr) pattern of alternating glacial and interglacial periods. Back to 2.5 Ma, the climatic pattern matches closely with oceanic oxygen isotope curves, and back to 0.73 Ma with Antarctic ice cap temperature as revealed by isotope records from the Vostok and Dome C ice cores. The Site 1119 climate record matches the known Antarctic temperature record so closely that its earlier parts, back to 3.91 Ma, can be used as a surrogate measure for Antarctic air temperature. The presence of several sharply alternating warm and cold periods in the Late Pliocene (2.7–3.6 Ma) supports the dynamicist interpretation of a fluctuating Antarctic ice cap at that time. Site 1119 is globally unique in that it contains high resolution proxies for both atmospheric (gamma ray curve) and oceanic (stable isotopes, grainsize) climate change. The drillsite is located on sediment drifts deposited from intermediate depth waters and also contains a physical record of the fluctuating flow strengths of Subantarctic Mode Water, Antarctic Intermediate Water, and ancillary nearby ocean fronts.
GSA TODAY Science Article
Geology, geomorphology, and the restoration ecology of salmon
David R. Montgomery, Quaternary Research Center and Department of Earth & Space Sciences, University of Washington Seattle, Washington 98195-1310, USA.
Rock and Roe: The decline of many species of wild salmon in the Pacific Northwest is well known. Less well known is the role of geologic factors in the evolution and diversification of salmon and their role in potential rescue of salmon from extinction. In the feature article in the November issue of GSA Today, David Montgomery, a geomorphologist at the University of Washington, details the range of geologic conditions that have impacted and controlled the rise and fall of salmon. At long time scales (millions of years) the rise of the mountains along the Pacific coast of North America led to diversification as salmon adapted to new environments created by the changing topography. At the human time scale, studies of the geomorphology of river channels has shown that salmon are heavily affected by human activities that influence channel characteristics. The most critical of these activities are those that influence the presence and abundance of logjams, which are essential microhabitat for salmon that spawn and raise their young in rivers. Montgomery shows how activities such as channel dredging to improve river navigation and the practices of forest clear cutting have led to dramatic losses in logjams along many of the rivers traditionally used by salmon. The integration of geology and the discipline of river geomorphology is critical to developing effective strategies for restoring salmon habitat and populations. Montgomery is the recent author of King of Fish: The Thousand Year Run of Salmon, published by Westview Press.
To view the complete table of contents for the November issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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