||December 20, 2002
GSA Release No. 02-57
Boulder, Colo.-The Geological Society of America's January issue of GEOLOGY contains several potentially newsworthy items. Topics of interest include: mineralization of embryonic material and early animal evolution; discovery of Ediacaran fossils suggesting that complex animals appeared soon after "snowball" glacier meltdown; significant earthquake and tsunami hazards in coastal Portugal, Spain, and Morocco; and first documentation of a link between increased Indian monsoon seasonality and initiation of Northern Hemisphere glaciation.
Highlights are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns for copies of articles and for additional information or other assistance.
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- Experimental mineralization of invertebrate eggs and the preservation of Neoproterozoic embryos
Derek Martin, Department of Earth Sciences, Bristol University, Bristol BS8 1RJ, UK. Pages 39-42.
- The enigma of early animal evolution is the apparent discrepancy between the first appearance of abundant, unambiguous, fossilized animals in the Cambrian Period (543 million years ago) and "molecular clock" estimates of up to 1500 million years ago for the divergence of animals. Possible reasons for the paucity of animals in the Precambrian fossil record include the lack of hard parts for fossilization and the possibility that Precambrian animals were small in size and thus easily degraded. However, recently, fossilized eggs and embryos have been discovered in Precambrian deposits, which may help to shed light on early animal evolution. Here we show that mineralization of soft tissue, in this case invertebrate eggs, is possible under laboratory conditions without the presence of hard parts. Mineralization is very rapid, being produced within three weeks under conditions of anoxia and lowered pH, and is bacterially mediated. Preservation is in a style similar to that observed in fossils and reinforces the opinion that the fossils are eggs and embryos of animals, rather than algal cells, which the fossil material was originally described as. This experimental approach will provide a basis for more extensive investigations of the processes and conditions that lead to the mineralization of embryonic material and, subsequently, should help us assess the significance of fossil material for analyzing the early evolution of animals.
- Mapping active faults offshore Portugal (36°N-38°N): Implications for seismic hazard assessment along the southwest Iberian Margin
Eulàlia Gràcia, Centre Mediterrani d'Investigacions Marines i Ambientals, Unidad de Tecnología Marina (CSIC), 08003 Barcelona, Spain, et al. Pages 83-86.
- Submarine mapping reveals the surficial expression of several fault structures <100 km offshore Portugal (SW Iberian Margin), at the boundary between the European and African plates. High-resolution seismic data show folding and reverse faulting of the Quaternary units associated with submarine-landslide deposits, suggesting present-day tectonic activity. The location and dimension of these newly identified structures agree with the modeled source suggested for the 1755 Lisbon earthquake and tsunami, possibly the most destructive event in western Europe during historical times. These fault escarpments and deformed seafloor sediments associated with a cluster of shallow seismicity suggest that these thrusts are active and may pose a significant earthquake and tsunami hazard to the coasts of Portugal, Spain, and Morocco.
- Life after snowball: The oldest complex Ediacaran fossils
Guy M. Narbonne and James G. Gehling, Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada. Pages 27-30.
- The oldest known animal fossils on Earth are the Ediacara biota, an assemblage of soft-bodied animals preserved as impressions on sandstone beds. Prior to our work, the oldest examples of these fossils were from the Mistaken Point Formation (565 million years old) of southeastern Newfoundland. Our continuing search for Ediacara fossils below this level has now extended their range nearly 2 km downwards to the upper part of the Drook Formation. These fossils are large (up to nearly 2 m long) fronds that are probably about 570-580 million years old. They represent the oldest known evidence of megascopic life after the meltdown of the "snowball Earth" glaciers, the most severe ice age Earth has ever known. The unexpectedably large size and complexity of these fronds implies that complex animals appeared very soon after the meltdown of the "snowball" glaciers.
- Anthropogenically induced changes in sediment and biogenic silica fluxes in Chesapeake Bay
Steven M. Colman and John F. Bratton, U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, Massachusetts 02543, USA. Pages 71-74.
- As part of an effort to estimate the character and magnitude of anthropogenic disturbance of Chesapeake Bay, we measured the accumulation of total sediment and biogenic silica in long, continuous, well-dated sediment cores. The data show that biogenic silica flux to sediments, an index of diatom productivity in the bay, as well as its variability, were relatively low before European settlement of the Chesapeake Bay watershed. In the succeeding 300-400 years, the flux of biogenic silica increased by a factor of four to five. Biogenic silica fluxes still appear to be increasing, despite recent nutrient-reduction efforts. The increase in diatom-produced biogenic silica was matched by a similar increase in total sediment flux. This history suggests the magnitude of anthropogenic disturbance of the estuary and indicates that significant changes had occurred long before the twentieth century.
- Looking for clues to paleoceanographic imprints: A diagnosis of the Gulf of Cadiz contourite depositional systems
Javier Hernández-Molina, Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, Spain, et al. Pages 19-22.
- In our work, we present a very new morphosedimentary map of the Gulf of Cadiz, where is shown the Contourite Depositional System on the middle slope. This map was constructed using a broad database collected since 1989, obtained during several cruises and projects supported by the Spanish Research Council. In addition, new data of swath bathymetric, multi-channel, and high-resolution seismic profiles were collected during a recent cruise of the Spanish oceanographic vessel BIO HESPERIDES. These data represent around 12 years of work of our research group, which have enabled us to produce a new regional morphosedimentary map of the slope, which was completed in the SE sector with an important data set provided by the U.S. Naval Research Laboratory. Also, the combination of techniques used in this contribution provided a good insight into the nature of the depositional bottom processes active in this sea, which will contribute to a better understanding of the Contourite Depositional System's characterization and evolution. The Gulf of Cadiz Contourite Depositional System could be considered a detached combined drift fan produced by the Mediterranean outflow water, which has an important impact on global paleoceanography.
- Initiation of Northern Hemisphere glaciation and strengthening of the northeast Indian monsoon: Ocean Drilling Program Site 758, eastern equatorial Indian Ocean
Anil K. Gupta, Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 302, W.B., India, and Ellen Thomas, Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459-0139, USA, and Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109, USA. Pages 47-50.
- Meteorologists are debating possible future trends in the Indian monsoon system; its failure this year caused widespread droughts as well as floods in Asia. Global warming and the El Nino Southern Oscillation have been mentioned as possible causes of this failure of the Southwest (summer) monsoon. Longer time-scale oscillations in the Southwest (SW) monsoon have been linked to changes in the North Atlantic circulation, but such long-distance connections have not been thoroughly documented. On the other hand, it was recently argued that intensification of the SW monsoon during the past four centuries was caused by heating of the Indian-Eurasian landmass (Science, 297, 596-599, 2002). Many scientists argue that even longer-term changes in the Indian monsoon system are caused by such tectonic processes as the Tibetan-Himalayan uplift, and the closing of the Indonesian Seaway, but the evidence is not conclusive. In this study, we use records over the past 5.5 million years from Ocean Drilling Program Site 758, eastern equatorial Indian Ocean. We obtained information on bottom-dwelling faunas of unicellular, eukaryotic organisms (benthic foraminifera), which can be used as indicators for changes in oceanic productivity and its seasonality. We combined this information with oxygen isotopic data, which are indicative of changes in the size of the polar ice sheets. We thus provide the first documentation of a link between increased seasonality of the Indian monsoons (increasing strength of the NE monsoon and decreasing strength of the SW monsoon) and increased glaciation of the Northern Hemisphere during the Plio-Pleistocene, and document that the changes in seasonality had a major effect on the biota in the eastern equatorial Indian Ocean.
- Synchroneity of meltwater pulse 1a and the Bølling warming: New evidence from the South China Sea
M. Kienast, Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada, et al. Pages 67-70.
- Following the last glaciation, sea level rose extremely rapidly (13.5-24 m in <290-500 yr) during meltwater pulse (MWP) 1a. Together with the Bølling warming (an abrupt rise in Northern Hemisphere air temperature of at least 5 °C within a few decades during the last deglaciation), MWP 1a arguably represents the most dramatic event in Earth's climate history during the past 25,000 years, and determining its precise age is of utmost importance for a mechanistic understanding of the oceanographic, glaciological, and climatic changes that occurred during the last glacial-interglacial transition. Nevertheless, the absolute timing of MWP 1a and its phase relation with the Bølling warming are still a matter of considerable debate. Here we present a downcore record of terrigenous organic matter supply to the northern South China Sea (SCS), which indicates a rapid drop in the supply of terrigenous organic matter to the open SCS during the last deglaciation. This abrupt decrease is paralleled by an equally rapid increase in sea-surface temperatures, corresponding with the Bølling warming at 14.7 ka. The sudden drop in terrigenous organic matter delivery to this marginal basin is interpreted to reflect a short-term response of local rivers to rapid sea-level rise, strongly implying that the Bølling warming and the onset of MWP 1a are synchronous. This phase relation contrasts with the widely cited onset of this MWP 1a at ca. 14 ka, and implies that previous studies postulating a weakening of deep-water formation in the North Atlantic due to massive meltwater discharge during MWP 1a need to be reevaluated.
- Widespread, prolonged late Middle to Late Ordovician upwelling in North America: A proxy record of glaciation?
Michael C. Pope and Jessica B. Steffen, Department of Geology, Washington State University, Pullman, Washington 99164-2812, USA. Pages 63-66.
- The Late Ordovician glaciation is an enigmatic event in Earth's history because it occurred during a time of greatly enhanced atmospheric CO2. We present data indicating that during the late Middle and Late Ordovician chert, carbonate, and phosphate were deposited along the southern margin of North America. These sediments, along with similar sediments around the periphery or North America and other mid-latitude continents indicate abundant oceanic upwelling during this period. We interpret the upwelling to be a result of vigorous ocean circulation caused by the onset, and waxing and waning of glaciers on Gondwana during the late Middle and Late Ordovician. Thus, widespread, long-lived upwelling events may indirectly record the duration of glacial events.
- Petrologic indicators of preeruption magma dynamics
Julia E. Hammer, Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA, and Malcolm J. Rutherford, Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. Pages 79-82.
- Physical, petrologic, and atmospheric observations of the 1991 Pinatubo, Philippines, eruption are unmatched in variety and scientific impact by those of any other large explosive volcanic eruption. This study reevaluates assessments of magma storage and movements in the crust prior to eruption using mineralogical pressure indicators. Sudden changes in the pressure recorded in the erupted materials may result from magma ascent or introduction of volatiles such as CO2. Either of these scenarios may be related to an injection of mafic magma thought to have triggered the explosive eruptions of this volcano.
- Magma transport and coupling between deformation and magmatism in the continental lithosphere
Keith A. Klepeis, Department of Geology, University of Vermont, Burlington, Vermont 05405-0122, USA, firstname.lastname@example.org; et al.
- The mechanisms by which magma is generated and transported through continental crust and how these processes affect the chemical and mechanical evolution of the lithosphere are some of the least understood issues of continental dynamics. This paper reports on the evolution of an unusually well-exposed early Mesozoic arc that is now located in western New Zealand. The deeply eroded character of this arc lead to the following conclusions about magmatism and deformation at 10-50 km paleodepths. (1) The mafic-intermediate composition of the lower crust and the mineral reactions controlling melt production strongly influenced pathways of melt transfer and controlled the mechanical behavior of the lithosphere during orogenesis. (2) Evolving lithospheric strength profiles during magmatism and convergence produced transient periods of vertical coupling and decoupling of crustal layers. (3) Late orogenic extension was driven by plate interactions rather than by gravitational forces and a weak lower crust. This study also shows that the approach of using parallel field, laboratory, and experimental studies is important for understanding the feedbacks that developed among magmatism, metamorphism and deformation as continental lithosphere evolves during cycles of orogenesis.
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Review the complete table of contents for this issue of GEOLOGY.