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7 July 2014
GSA Release No. 14-45
Contact:
Kea Giles
Managing Editor,
GSA Communications
+1-303-357-1057
Denali tracksite
Figure 3 from Fiorillo et al. A–C: Size ranges of tracks found at Denali National Park, Alaska, tracksite.
D: Adult hadrosaurid track with skin impressions.
Scale bar for C1 is 5 cm.

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Denali Duck-Billed Dino Tracks

New GEOLOGY articles published online on 30 June 2014

Boulder, Colo., USA – A trio of paleontologists has discovered a remarkable new tracksite in Alaska’s Denali National Park filled with duck-billed dinosaur footprints -- technically referred to as hadrosaurs -- that demonstrates they not only lived in multi-generational herds but thrived in the ancient high-latitude, polar ecosystem. The paper provides new insight into the herd structure and paleobiology of northern polar dinosaurs in an arctic greenhouse world.

The article, "Herd structure in Late Cretaceous polar dinosaurs: A remarkable new dinosaur tracksite, Denali National Park, Alaska, USA," was written for Geology by lead author Anthony R. Fiorillo, curator of earth sciences at the Perot Museum of Nature and Science, and co-authors Stephen Hasiotis of the University of Kansas and Yoshitsugu Kobayashi of the Hokkaido University Museum.

"Denali is one of the best dinosaur footprint localities in the world. What we found that last day was incredible -- so many tracks, so big and well preserved," said Fiorillo. "Many had skin impressions, so we could see what the bottom of their feet looked like. There were many invertebrate traces -- imprints of bugs, worms, larvae and more -- which were important because they showed an ecosystem existed during the warm parts of the years."

**FEATURED ARTICLE**
Herd structure in Late Cretaceous polar dinosaurs: A remarkable new dinosaur tracksite, Denali National Park, Alaska, USA
Anthony R. Fiorillo et al., Perot Museum of Nature and Science, 2201 North Field Street, Dallas, Texas 75201, USA. Published online 30 June 2014; http://dx.doi.org/10.1130/G35740.1

Other GEOLOGY articles (see below) cover such topics as
1. The beneficial effect of Saharan dust in the Bahamas;
2. Geochemical fingerprinting of 25 trans-Atlantic dust events from North Africa to Barbados;
3. Study of shark teeth from Banks Island, NWT, Canada, shows that in the Eocene Arctic Ocean, sharks were abundant even in lower-saline conditions; and
4. Sample analysis from the largest volcanic eruption of the 20th century: the 1912 Novarupta eruption, Alaska.

GEOLOGY articles published online ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary articles by contacting Kea Giles at the address above.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, .

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The fertilization of the Bahamas by Saharan dust: A trigger for carbonate precipitation?
P.K. Swart et al., Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149, USA. Published online 30 June 2014; http://dx.doi.org/10.1130/G35744.1.

It has long been known the dust from Africa causes hazy sunsets and dirty cars during certain periods of the year in the south-eastern United States. However, this same dust has a beneficial effect on the shallow waters of the Bahamas. Here it provides iron to photosynthesizing bacteria which in turn fix N2 from the atmosphere and provides nitrogen to all of the biological communities. Evidence for this is present in the ratio of 15N/14N of the sedimentary organic material which is virtually indistinguishable from atmospheric values. The photosynthetic activity of the cyanobacteria in turn induces the precipitation of calcium carbonate in the shallow waters producing large amount of carbonate mud which is swept into deeper waters where it accumulates building and expanding the platform towards the west. It might be speculated that periodic increases in the supplies of dust through time have in turn been responsible for increased deposition of carbonate sediments and enlargement of the Bahamas.

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Geochemical fingerprinting of trans-Atlantic African dust based on radiogenic Sr-Nd-Hf isotopes and rare earth element anomalies
Ali Pourmand et al., Neptune Isotope Laboratory, Dept. of Marine Geosciences, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA. Published online 30 June 2014; http://dx/doi.org/10.1130/G35624.1

Mineral dust is an important component of Earth’s climate system and biogeochemical cycles on a global scale. In order to understand the relationship between climate processes in the source areas and the properties of aerosols at distant receptor sites, we must be able to identify the source provenance of dust. We studied the geochemical composition of 25 trans-Atlantic dust events from North Africa to the Caribbean island of Barbados between 2003 and 2011 to investigate the possibility of tracking aerosols to their source origin. We find systematic differences between the geochemical composition of individual dust events, and the first evidence for seasonal shifts in dust source activity and transport between the months of May-September compared with November-April over the decade of study. These results indicate that coherent geochemical source signatures of trans-Atlantic dust can be preserved even after transport across thousands of kilometers. In the absence of geochemical data in aerosols over potential source areas in North Africa, however, it is difficult to make a direct comparison between aerosols over Barbados and the source origins. Nevertheless, our data suggests that many aerosol samples from Barbados are lined to sources in Mali and sub-Saharan regions.

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Evidence from shark teeth for a brackish Arctic Ocean in the Eocene greenhouse
Sora L. Kim et al., Current Address: Dept. of Geophysical Sciences, University of Chicago , 5734 S. Ellis Avenue, Chicago, Illinois 60637, USA (also Dept. of Geology & Geophysics, University of Wyoming, 1000 E. University Drive #3006, Laramie, Wyoming 82071, USA). Published online 30 June 2014; http://dx.doi.org/10.1130/G35675.1.

Temperatures are rising in the Arctic at twice the global rate, affecting the sensitive hydrological cycle with increased freshwater inputs to the ocean. A deep-time analogue often used to understand and predict global warming impacts is the early to middle Eocene greenhouse (about 38 to 53 million years ago). Although the terrestrial Arctic during this interval is well documented, marine records are rare and largely based on one central Arctic Ocean site (Lomonosov Ridge). Kim et al. estimate a mean paleosalinity of ~13 PSU for coastal waters of the western Arctic Ocean by analyzing the oxygen isotope composition of Eocene shark teeth from Banks Island, NWT, Canada, and incorporated these values into a salinity model. This paleo-salinity is substantially lower than others for the Eocene central Arctic Ocean (21 to 25 PSU) and today’s Arctic Ocean (32 to 35 PSU), but similar to Louisiana's Lake Pontchartrain. This is the first salinity estimate for the western Arctic Ocean during Eocene time and, when considered with prior results from the central Arctic Ocean, suggests a reduced surface salinity but larger salinity gradient (~10 PSU) across the Arctic Ocean during the Eocene greenhouse. From an ecologic perspective, while today’s lamniform sharks (e.g., sand tigers, thresher, and great white sharks) are largely intolerant of low salinity, Eocene species were abundant in the brackish Arctic Ocean, suggesting their past environmental tolerances were much greater than today.

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Explosive to effusive transition during the largest volcanic eruption of the 20th century (Novarupta 1912, Alaska)
Chinh T. Nguyen et al., Dept. of Earth Science, Rice University, Houston, Texas 77005, USA. Published online 30 June 2014; http://dx.doi.org/10.1130/G35593.1

Silicic volcanic eruptions commonly show abrupt shifts between violently explosive episodes and a gently effusive lava dome. It has been a subject of debate whether the gas loss through inter-connected permeable networks (outgassing) controls these transitions. Sample analysis from the A.D. 1912 Novarupta eruption, Alaska, shows that the degree of vesicle inter-connectivity (measured as the ratio of connected to total vesicle volume fraction) decreases with phenocryst content and increases with eruption intensity. Permeabilities of explosive samples show a weak dependence on vesicle volume fraction. Dome samples are not significantly different in permeability, but are of lower vesicle volume fraction. Quantitative analysis indicates that outgassing alone was insufficient to affect the transition to effusive activity. Rather, the change from explosive to effusive activity was probably a consequence of high versus low magma ascent rates.

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Ammonite extinction and nautilid survival at the end of the Cretaceous
Neil H. Landman et al., Division of Paleontology (Invertebrates), American Museum of Natural History, New York, New York 10024, USA. Published online 30 June 2014; http://dx.doi.org/10.1130/G35776.1

One of the puzzles about the end-Cretaceous extinctions is why some organisms disappeared and others survived. A notable example is the differential extinction of ammonites and survival of nautilids, the two groups of co-occurring, externally shelled cephalopods at the end of the Cretaceous. To investigate the role of geographic distribution in explaining this outcome, we compiled a database of all the occurrences of ammonites and the nautilid genus Eutrephoceras in the last 0.5 m.y. of the Maastrichtian. We also included recently published data on ammonite genera that appear to have briefly survived into the Paleocene. Using two metrics to evaluate the geographic range of each genus (first, a convex hull encompassing all of the occurrences of each genus, and second, the maximum distance between occurrences for each genus), we documented that most ammonite genera at the end of the Maastrichtian were restricted in their geographic distribution, possibly making them more vulnerable to extinction. The geographic distribution of those genera that may have briefly survived into the Paleocene is significantly greater than that of non-surviving genera, implying that more broadly distributed genera were more resistant to extinction. This pattern is further emphasized by the broad distribution of Eutrephoceras, which matches that of the most widely distributed ammonites at the end of the Maastrichtian. However, even the most widely distributed ammonites eventually succumbed to extinction, whereas Eutrephoceras survived. Evidently, a broad geographic distribution may have initially protected some ammonites against extinction, but it did not guarantee their survival.

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Persistent intermediate water warming during cold stadials in the southeastern Nordic seas during the past 65 k.y.
Mohamed M. Ezat et al., Centre for Arctic Gas Hydrate, Environment and Climate (CAGE), Uit, The Arctic University of Norway, NO-9037 Tromsø, Norway, and Dept. of Geology, Faculty of Science, Beni-Suef University, 62111 Beni-Suef, Egypt. Published online 30 June 2014; http://dx.doi.org/10.1130/G35579.1.

Greenland ice core records reveal millennial-scale climate variability during the last glacial (~100,000 to 10,000 years before present), known as Dansgaard-Oeschger (DO) events. A typical DO cycle (~1500 years) is characterized by an abrupt atmospheric warming of 8 to 16 degrees C from stadial (cold) to interstadial (warm) conditions followed by a gradual cooling and eventually a sudden cooling back to stadial conditions. In the modern Nordic seas, conversion of inflowing warm Atlantic surface water to deep cold water (<0 degrees C) through convection is closely linked with climate. A bottom water temperature (BWT) record based on benthic foraminiferal Mg/Ca from the southeastern Nordic seas shows a pronounced BWT increase by 2 to 5 degrees C during stadials compared to the interstadials and the Holocene. The study suggests that the warm Atlantic water never ceased to flow into the Nordic seas during the past 65,000 years with inflow at the surface during the Holocene and warm interstadials switching to subsurface and intermediate inflow during cold stadials. The BWT record shows that vertical shifts in the position of the warm Atlantic Water played a key role in the abrupt surface warming during interstadial phases.

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Effects of temperature, sulfur, and oxygen fugacity on the composition of sphalerite from submarine hydrothermal vents
Manuel Keith et al., GeoZentrum Nordbayern, Universität Erlangen-Nürnberg, D-91054 Erlangen, Germany. Published online 30 June 2014; http://dx.doi.org/10.1130/G35655.1.

Experimental studies have shown that temperature, pressure, sulfur fugacity (fS2), and oxygen fugacity (fO2) influence the Fe content of sphalerite. We present compositional in situ data on sphalerite from submarine volcanic-hosted massive sulfide (VHMS) ores of hydrothermal vents from different plate tectonic settings and with variable host-rock compositions. Sphalerite from sediment-hosted vents has systematically higher S contents and Fe/Zn ratios than those of the sediment-starved vents, reflecting an influence of fS2 and fO2 on Fe partitioning between fluid and sphalerite. The Fe/Zn ratios of sphalerite from sediment-starved vent systems apparently increase systematically with the fluid temperatures of the corresponding vents. We conclude that the composition of sphalerite can be used to (1) distinguish between sediment-hosted and sediment-starved hydrothermal processes, and (2) estimate minimum fluid temperatures of sphalerite precipitation from inactive sediment-starved hydrothermal vent sites and fossil VHMS deposits.

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Pliocene orographic barrier uplift in the southern Central Andes
Heiko Pingel et al., Institut für Erd- und Umweltwissenschaften, Universität Potsdam, 14476 Potsdam, Germany. Published online 30 June 2014; http://dx.doi.org/10.1130/G35538.1

Many of the world’s largest mountain belts feature extensive high-elevation/low-relief sectors. Today and in Earth’s distant past, such orogenic plateaus have formed efficient obstacles to atmospheric flows influencing a variety of environmental aspects. Sedimentary deposits exposed in intermontane basins along the Puna Plateau in NW Argentina furnish ideal archives to assess the relations between plateau building and environmental change. This is possible because, here, sediments often contain datable volcanic ash layers that help constrain the timing of deformation or sedimentary deposition and erosion. Importantly, volcanic ashes also contain microscopic glass shards with the potential to incorporate surface water present during deposition. Since isotopic compositions of surface water are a reliable indicator for altitude and rainfall amount under present-day conditions, their variation through time may provide valuable information on past topographic changes during mountain building. In this study, Pingel et al. conclude that observed changes in the stable hydrogen isotope signal of volcanic glass in intermontane basin sediments are related to the development of dry leeward conditions following mountain-range uplift. This forced extreme east-west rainfall and erosion gradients and helped create humid conditions on the eastern flanks of the mountains and a harsh desert environment within the mountain interior.

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Calcium isotopes in evaporites record variations in Phanerozoic seawater SO4 and Ca
Clara L. Blättler and John A. Higgins, Dept. of Geosciences, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA. Published online 30 June 2014; http://dx.doi.org/10.1130/G35721.1.

This work establishes a new geological archive for constraining the composition of ancient seawater. Evaporite formations that are derived from seawater, and which include the minerals gypsum, anhydrite, and halite, contain isotopic evidence of the relative concentrations of sulfate and calcium ions. Under high sulfate conditions, such as in the modern and the Permian periods, gypsum and anhydrite minerals record a large range in calcium isotope ratios. Under low sulfate and high calcium conditions, such as in the Cretaceous and Silurian periods, these same minerals have a very restricted calcium isotope range. These data represent a new, independent source of information on the changing composition of seawater over the Phanerozoic.

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The magnetic signature of ultramafic-hosted hydrothermal sites
Florent Szitkar et al. (Jérôme Dyment corresponding), Institut de Physique du Globe de Paris, CNRS UMR 7154, Sorbonne Paris Cité, Université Paris Diderot, 75005 Paris, France. Published online 30 June 2014; http://dx.doi.org/10.1130/G35729.1.

Deep-sea hydrothermal activity has attracted a strong public interest as it involves vigorous physical and chemical exchanges between the solid and liquid Earth envelopes and hosts spectacular oases of life in the otherwise desert abysses. Due to lower magma supply, slow spreading centers such as the Mid-Atlantic Ridge present a variety of outcropping rocks, including mantle rocks (or ultramafics), and therefore various types of hydrothermal sites. Based on the deep-sea submersible magnetic and bathymetric data and rock samples of three cruises, Szitkar et al. present the discovery of positive magnetic anomalies and a strong magnetization associated with the large ultramafic-hosted hydrothermal sites. The main magnetic mineral is magnetite within the subsurface part of the sites (the stockwork). This observation reflects the abundance of iron at these sites : the stockwork represents a major chemical reactor where iron and other metals precipitate, forming massive sulfide deposits of interest for mineral exploration. The clear magnetic signature revealed by this study can easily be traced with appropriate underwater vehicles and instruments. It may be of great use to detect and characterize inactive ultramafic-hosted hydrothermal sites, which count among the most economically valuable deep-sea mineral deposits.

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Deep plate serpentinization triggers skinning of subducting slabs
Katharina Vogt, Dept. of Earth Sciences, Utrecht University, Budapestlaan 4, 3584CD Utrecht, Netherlands; and Taras Gerya, Institute of Geophysics, ETH Zürich, Sonneggstrasse 5, 8092 Zurich, Switzerland. Published online 30 June 2014; http://dx.doi.org/10.1130/G35565.1.

Most of the present day oceans floor is currently being consumed in subduction zones, but fragments of oceanic crust have also been found on land. Emplacement of such fossil oceanic crust onto continental crust is a fundamental problem, because oceanic crust is significantly denser than upper continental crust. Based on numerical experiments we suggest that oceanic crust may decouple along a mechanically weak serpentinized horizon, break and be incorporated into accretionary wedges or emplaced on land. Such mechanically weak horizons (i.e., serpentinized mantle) may be the outcome of extensive hydration in the outer rise region of subduction zones.

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Orogeny forced terrestrial climate variation during the late Eocene-early Oligocene in Europe
László Kocsis et al., Institute of Earth Sciences, University of Lausanne, UNIL-Geopolis, 1015 Lausanne, Switzerland. Published online 30 June 2014; http://dx/doi.org/10.1130/G35673.1.

Large stable isotope dataset (delta-18O-PO4 and delta-13C) is presented here from European fossil mammal teeth across the Eocene-Oligocene (approx. 30 to 40 million years ago), the period of the most recent greenhouse-icehouse climatic shift. While the oxygen isotopic composition reflects the drinking waters of the mammals, the carbon isotopic composition can be linked to the paleoflora. Interpretation from the combined results show regionalized continental climate in Europe and separation between warm, semi-arid and more humid regions is evident. In such spatial heterogeneity the proximity of warm seas and the emergence of mountains played key roles. The data prove that the Alps already acted as climatic barrier during latest Eocene and earliest Oligocene (approx. 33 to 35 million years ago). After 31 million years ago, large drop in delta-18O-PO4 values are explained by sudden altitude change that influenced the meteoric water isotopic composition and hence the drinking water of the mammals. The estimated paleo-altitude change is 1200 m and the uplift could have taken place along the Alpine-Dinaridic orogenic system.


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