Following are highlights from the June issue of Geology and a summary of the science article from the June issue of GSA Today. These articles are embargoed until June 1. We ask that you discuss articles of interest with the authors before publishing stories on their work, and that reference be made to Geology in stories published. Please contact Anika Burkard for additional information and assistance.
Geology
Dinosaur abundance was not declining in a "3 m gap" at the top of Hell Creek Formation, Montana and North Dakota
Peter M. Sheehan et al., p. 523
The mode of dinosaur extinction has long been contentious. One camp argues that dinosaurs went extinct suddenly (as would have occurred if an asteroid killed them), while the other argues that the dinosaurs went extinct gradually (as would have occurred if a non-catastrophic event were responsible for their demise). A key piece of evidence is the so-called "3 meter gap," a supposedly fossil-free 3 meter interval just below the Cretaceous-Tertiary boundary preserved in eastern Montana and western North Dakota. This study addresses the abundance of dinosaur fossils below and even within that 3 meter gap. The authors suggest that the gap does not really exist and that it is simply an illusion due to the rarity of dinosaur fossils. The uppermost 3 meters before the boundary is no more or less devoid of fossils than any other 3 meter interval. For this reason, the authors conclude that there is no evidence that dinosaurs went extinct gradually before the boundary. Rather, the extinction was catastrophic and the data are fully concordant with an asteroid impact as the cause of the dinosaur extinction.
Overlapping volcanoes: The origin of Hilo Ridge, Hawaii
Robin T. Holcomb et al., p. 547
University of Washington scientists have found evidence that the volcanic Island of Hawaii grew differently than was previously thought. A prominent submarine ridge that extends eastward from the island, just north of the city of Hilo, was not built by lofty Mauna Kea volcano but instead by older Kohala volcano, which was later overlapped and partly buried by Mauna Kea. Similar overlaps are suspected among several other Hawaiian volcanoes. If this is correct, the history and future behavior of those volcanoes may be quite different than is generally supposed. For example, giant landslides from the island may be more frequent, and eruptions from active Kilauea volcano may become much more frequent and more voluminous in future centuries.
Lithium isotope evidence for light element decoupling in the Panama subarc mantle
Paul B. Tomascak et al., p. 507
What happens to pieces of the oceanic crust in subduction zones, places where plates collide and are recycled into Earth's mantle? A provocative theory suggests that in some areas the oceanic crust is hot enough to melt, and the products of this melting form peculiar volcanic lavas called adakites. The authors use a new geochemical tool, lithium isotopes, to test if the subducted ocean crust beneath Panama has melted. Similarity between the lithium isotope signature of adakite lavas (<3 million years old) and of modern ocean crust suggest that the crust subducting under Panama is first 'sweated' off, then melted.
Dimethylsulfide production variations over the past 200 k.y. in the equatorial Atlantic: A first estimate
Anders S. Henriksson et al., p. 499
Using a core taken from the equatorial south Atlantic Ocean, the authors have studied the production of dimethylsulfide (DMS), a compound released by phytoplankton, over the past 200,000 years. Because DMS aerosols serve as sites around which clouds can develop, the production of DMS provides a direct link between climate and marine phytoplankton productivity. The study demonstrates that high DMS production is correlated with glaciated periods. The authors conclude that equatorial regions help to cool the earth during glaciated intervals in Earth history by being sites of high productivity and thus, sources of cloud-generating DMS.
Sclerosponges as a new potential recorder of environmental changes: Lead in Ceratoporella nicholsoni
Claire E. Lazareth et al., p. 515
The authors test the potential of using the massive basal skeleton of sclerosponges as a new paleoenvironmental recorder. Lead concentrations on a 223-yr-long profile through the calcareous skeleton of a Caribbean Ceratoporella nicholsoni were analyzed using the high-resolution laser-ablation microprobe technique. The lead trend in the specimen displays a general increase from 1760 to 1984, with a major threefold increase after 1930. This lead profile is analogous to results acquired from ice or coral cores and clearly highlights the potential of sclerosponges as a new proxy of environmental changes under tropical latitudes, for time series extending over several centuries.
Can slab melting be caused by flat subduction?
Marc-André Gutscher et al., p. 535
Current models of plate subduction (where oceanic crust descends into Earth's mantle at deep sea trenches, as in the circum-Pacific "ring of fire" region) and formation of volcanic chains parallel to the trench are based on a steep angle of descent. Magma generation is considered to occur at depths of roughly 100 km as fluids (mostly water) are released from the lower plate and migrate into the upper mantle "wedge." Here partial melting occurs and the magma ascends upward to form a "volcanic arc." Direct melting of the subducting oceanic crust, once favored in the early 1980s to explain arc magmatism, is now no longer considered to be possible, except in the rare cases of subduction of very hot, young oceanic crust (< 5 million years old). The magmas thus generated have a particular geochemical signature (typically a high silica content and a depletion in heavy rare earth elements) and are called "adakites" after their first discovery on Adak Island in the Aleutians. Paradoxically, of the 20 or so occurrences of adakitic magmas around the world, only 5 can be explained by the subduction of very young oceanic crust. The rest are associated with the subduction of older (10 - 45 million year old) crust, which according to published numerical models should not reach sufficiently high temperatures at 100 km depth to allow melting. An unusual type of subduction, "flat subduction" (where the downgoing plate flattens its angle of descent and "underslides" the upper, typically continental, plate sub-horizontally for several hundred kilometers), can produce a special set of temperature and pressure conditions allowing the direct melting of hydrous (water bearing) minerals in the downgoing oceanic plate. Ten "flat slab" zones have been identified worldwide. Eight of these are associated with modern or young (< 3 million year old) adakites. The thermal structure of "flat subduction" has never been modeled numerically and remains an important open question with a direct bearing on the generation of great interplate subduction earthquakes. The importance of the thermal structure of subduction zones and its impact on volcanism and earthquakes was briefly discussed in a "New and Views" article in the journal Nature (vol. 403, p. 31-34, 6 January, 2000) by Stephen Kirby of the USGS.
Alternative origin of aliphatic polymer in kerogen
B.A. Stankiewicz et al., p. 559
The authors discuss new insights into the origin of kerogen. Kerogen is insoluble organic matter, commonly composed of mostly aliphatic hydrocarbons, that occurs in ancient sedimentary rocks as a result of the accumulation and transformation of biomass into geologically long-lived material. Kerogen has been identified as a largely untapped hydrocarbon source. Here, the authors conclude that arthropod exoskeletons can make a significant contribution to kerogen deposits. Because arthropods are perhaps the most significant non-microscopic contributor to the world's biomass, and because they presumably have been since the Devonian (for the past 370 or more million years), there is reason to suppose that they have been major contributors to the world's kerogen reserves for a long time. This work gives new insights into kerogen formation, and suggests potential sources of kerogen that have previously been untapped.
GSA Today
Anatomy of the North Anatolian Fault Zone in the Marmara Sea, Western Turkey: Extensional Basins Above a Continental Transform
Ali E. Aksu, Tom J. Calon, and Richard N. Hiscott, p. 3 - 7.
The August 17, 1999, Izmit earthquake (M 7.4) caused more than 30,000 deaths and billions of dollars in damage (see GSA Today, January 2000 issue). It was followed by the November 12, 1999, Duzce earthquake (M 7.1). These were the 11th and 12th in a series of major quakes in the past 100 years that represent breaks along a 1000-km-long segment of the plate boundary. The breaks have systematically progressed westward, toward the Marmara region, home of the city of Istanbul and 25% of Turkey's population.
This study provides a new synthesis of the plate margin in the Marmara Sea, directly west of the last ruptures, and helps refine our understanding of potential future seismicity in the region.
Aksu et al. note that the seismically active North Anatolian transform fault dives under the Marmara Sea and changes character significantly. The paper documents a tulip-shaped fault system, where the stem of the tulip, 5 km below the surface, is the seaward continuation of the plate boundary transform. This fault branches upward to define the margins of a series of extensional basins, with mainly southwest-oriented long axes, that serve to accommodate both transform motion and crustal extension. The points of rupture that produce earthquakes are restricted to depths of more than 5 km along the trace of the single buried transform fault. At shallower depths in the tulip-shaped fault zone, deformation appears to proceed by aseismic creep.
The paper documents a complex segmentation of the crust and an interplay between transform-parallel strike slip and extension in the tulip structure. It is not clear how slip on the buried transform propagates through the tulip structure to the seabed, where it has induced dramatic subsidence that locally exceeds 1200 m. Likewise, implications for seismicity are not clear. But this study is an example of the type of detailed subsurface information that is needed to evaluate how a future westward progression of earthquake activity might affect the region. It also provides a look at an unusual type of microplate margin and gives further insight into the complex nature of intracontinental deformation along transform boundaries.
*Full text Geology articles and the science article from GSA Today are available on the first of each month on the GSA Web site, http://www.geosociety.org/pubs/journals.htm .
Journal
Geology