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GEOLOGY
Iceland, the Farallon Slab, and dynamic topography of the North Atlantic
Clinton Conrad, University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1063, U.S.A., et al. Pages 177-180.
After removing known sources of topography from seafloor depths of the North Atlantic Ocean, we find that the North American side of the North Atlantic is anomalously deep by approximately half a kilometer when compared with the European side. We demonstrate that this depressed seafloor results from slow ongoing motion within Earth's mantle, the rocky layer that lies between the crust and the core. Within this layer lies the Farallon plate, which dove beneath the west coast of North America more than 30 million years ago and is now ~1500 km beneath North America's east coast. Our numerical models for the current motion of the mantle show that the continued descent of this dense, ancient seafloor drives a circulation pattern within the mantle that draws the surface of Earth downward and explains the anomalously deep seafloor in the western North Atlantic. Anomalously shallow seafloor near Iceland can also be explained by mantle flow. In this case light rock beneath Iceland drives upward motion that feeds the volcanism on Iceland but also pushes the seafloor upward by ~1–2 km. Thus, flow within Earth's interior contributes significantly to the topography of the seafloor.
A space-based estimate of the volcanic heat flux into the atmosphere during 2001 and 2002
Robert Wright and Luke Flynn, Hawaii Institute of Geophysics and Planetology, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, USA. Pages 189-192.
Some of Earth's active volcanoes are persistently active, whereas others erupt only sporadically. Using infrared satellite data acquired by NASA's Moderate Resolution Imaging Spectroradiometer (MODIS), we have calculated the total amount of heat radiated into Earth's atmosphere by 45 volcanoes active during 2001 and 2002, in order to quantify the contribution that active volcanism makes to Earth's energy budget. During 2001 and 2002 these volcanoes radiated approximately 5 x 1016 J of energy into the atmosphere, some three orders of magnitude less than the amount of energy consumed by the USA for manufacturing, residential and transportation purposes, in 1999. From the geographic perspective of the authors, the amount of energy radiated by Kilauea volcano, Hawaii, during each of these years was almost equivalent to the amount of energy consumed by the state of Hawaii for residential purposes during 1999.
Iron isotope fractionation during microbial reduction of iron: The importance of adsorption
Gary Icopini, Los Alamos National Laboratory, Chemistry Division, Los Alamos, NM 87545, U.S.A., et al. Pages 205-208.
One proposed method for documenting the presence of living organisms in ancient rocks or in rocks on other planets involves the use of iron isotopes as a biomarker. We have investigated iron isotopic signatures associated with one of the oldest forms of bacterial respiration, the bacterial reduction of iron. We found that much of the iron fractionation observed during microbial reduction of goethite is due to adsorption of dissolved iron to the goethite. The results of this research imply that much more work is needed to characterize the basic reactions that lead to iron isotope fractionation in biogeochemical systems before such isotopic signatures can be used as a biomarker.
Latest Pleistocene alpine glacier advances in the Sawtooth Mountains, Idaho, USA: Reflections of mid-latitude moisture transport at the close of the last glaciation
Glenn Thackray, et al., Idaho State University, Department of Geology, Box 8072, Pocatello, ID 83209-8072, U.S.A. Pages 225-228.
Glaciers in the Sawtooth Mountains of central Idaho advanced and retreated in a complex manner at the end of the last glaciation. Periods of glacier growth have been dated to ca. 17,000 and 14,000 years ago, using carbon-14 techniques. The growth and shrinkage of the glaciers occurred several thousand years after the final growth and shrinkage of glaciers in many other areas, and indicates that the influx of precipitation at the end of the last ice age nourished the Sawtooth Mountain glaciers. The influences of precipitation on glaciers appear quite variable across the region and across the globe, and the patterns of those influences promise to reveal further details of climate change during the latter stages of the most recent ice age.
Plant DNA: A new substrate for carbon stable isotope analysis and a potential paleoenvironmental indicator
A. Jahren, Johns Hopkins University, Department of Earth and Planetary Sciences, Baltimore, MD 21218-2681, U.S.A., et al. Pages 241-244.
We selected 12 higher plant species that span the range of plant diversity and extracted DNA; we then analyzed this DNA for its carbon stable isotope composition and compared it to the carbon stable isotope composition of bulk plant material from the same organisms. The difference was that DNA was found to be enriched in 13C by a constant amount (= 1.39 per mil) relative to the bulk organisms. This means that DNA, in addition to being useful as a way to identify the organism that lived long ago and contributed its carbon to bulk fossil material, could also be used to give insight into the stable carbon isotope signature of the organism that lived. This is useful since plants are the oldest DNA-recognized fossils, ~400,000 years in age.
Drowning of the –150 m reef off Hawaii: A casualty of global meltwater pulse 1A?
Jody Webster, MBARI, RDS, 7700 Sandholdt Road, Moss Landing, CA 95039, U.S.A., et al. Pages 249-252.
Due to a combination of rapid subsidence and eustatic sea-level changes over the past 450 k.y., a series of submerged reefs are preserved around the flanks of Hawaii. New evidence from the shallowest reef, now at –150 m, indicates it drowned because of rapid sea-level rise associated with meltwater pulse 1A (MWP-1A). This dramatic paleoclimate event is still controversial and is thought to have been caused by catastrophic ice sheet collapse during the last deglaciation, causing ~20 m of sea-level rise in <500 yr. In early 2002, the Larsen B ice shelf in Antarctica collapsed. Given continued concerns about the impact of global warming on the ice sheets, gathering more data from the geological record about past events is critical. Submerged coral reefs are one such record. New radiometric ages constrain the age of the coral reef at Hawaii to 15.2–14.7 ka, indicating (1) that reef growth persisted for 4.3 k.y. following the end of the Last Glacial Maximum at 19 ka, and (2) that the drowning age was roughly synchronous with the onset of MWP-1A (14.7–14.2 ka). During this event, paleowater depths increased rapidly above a critical depth (30–40 m), drowning the shallow reef-building corals and causing a shift to deeper coralline algal growth.
Cosmogenic nuclides 10Be and 26Al imply limited Antarctic Ice Sheet thickening in the Shackleton Range for over 1 Ma
Michael Bentley, University of Durham, Department of Geography, Science Laboratories, Durham, County of Durham DH1 3LE, U.K., et al. Pages 265-268.
For several years a key debate in Antarctic geoscience has been the stability of the East Antarctic Ice Sheet during the past few million years. Up until now, evidence for former ice sheet stability has been largely confined to the Transantarctic Mountains, close to the Ross Sea sector of Antarctica. The results presented in this paper suggest that the evidence for a stable East Antarctic Ice Sheet may extend as far as the Shackleton Range, a remote mountain plateau on the opposite side of the continent. We collected rock samples from the top of the plateau and have used analysis of in situ cosmogenic isotopes to show how long the Shackleton Range has been free of overriding ice. The results show that parts of the plateau have not been overridden by the East Antarctic Ice Sheet for over 1 million years, and possibly not for as long as 3 million years. The results also imply that bedrock erosion rates in the Shackleton are extraordinarily low (10–35 cm per million years), which supports the concept of long-term climatic stability in East Antarctica. The East Antarctic Ice Sheet (EAIS) locks up the greatest volume of water on Earth and thus has a critical role in terms of possible global warming and rising sea levels. Therefore, understanding its former behavior during cold and warm periods is vital to be able to better predict its future response to a warmer climate.
Geophysical Investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October–November, 2002, M = 7.9 Denali earthquake
Michael Fisher, U.S. Geological Survey, Menlo Park, CA 94025, U.S.A., et al. Pages 269-272.
On 3 November 2002, a M = 7.9 earthquake ruptured along the Denali fault in south-central Alaska. A diverse suite of geophysical data as well as outcrop geology and earthquake seismology indicate that the Denali fault is nearly vertical and extends to depths as great as 25 km. Strong seismic reflections from below the Alaska Range reveal what are probably faults that developed ca. 100 Ma ago, but in the past 20 yr these faults have produced little seismicity. The most intriguing finding from geophysical data is that earthquake aftershocks occurred above a rock body, with low electrical resistivity (>10·Ω m), that lies at depths below ~10 km. Aftershocks of the Denali fault earthquake have mainly occurred shallower than 10 km. A high geothermal gradient may cause the shallow seismicity. Another possibility is that the low resistivity results from fluids, which could have played a role in locating the aftershock zone by reducing rock friction within the middle and lower crust.
GSA TODAY Science Article
CO2 as a primary driver of Phanerozoic climate
Dana L. Royer, Department of Geosciences and Institutes of the Environment, Pennsylvania State University, University Park, Pennsylvania 16802, USA, et al.
Carbon dioxide as a greenhouse gas in deep time: Carbon dioxide has long been accepted as a greenhouse gas capable of having a major impact on the climate of planet Earth. This basic tenet of climate science was challenged recently by researchers who suggested that variations in cosmic ray flux imposed a more direct control on climate. In order to test the controls of CO2, Dr. Dana Royer from Pennsylvania State University and colleagues from Yale University, University of California, Southern Methodist University, and University of Sheffield examined the geological record for variations in carbon dioxide over the past 500 million years of earth history and compared those with direct indications of ancient climate such as glacial deposits. Royer compiled more than 370 geological estimates of ancient CO2 levels using indicators such as the structure in fossil leaves, chemical composition of calcite in ancient soils, and chemical composition of fossils. The result is a reconstruction of atmospheric levels of CO2 over the last 500 m.y. To further complement the observed changes they developed a mathematical model that quantifies how CO2varies over time as part of the global carbon cycle. They also developed a novel approach that considers the effects of variations in pH of the oceans over time on estimates of sea surface temperatures, again derived from the composition of ancient limestones. The combined observational and modeling approaches show a close correlation between CO2 and temperature, and reinforce the conclusion that CO2 exerts a first-order control on the climate of Earth and suggests that variations in cosmic ray fluxes are of secondary importance.
To view the complete table of contents for the March issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.