11/11/2012
Weather and climate in the Northern Hemisphere is profoundly affected by the Arctic Oscillation, a quasi-periodic fluctuation in atmospheric pressure that occurs on interannual to interdecadal timescales. Reconstructions of the Arctic Oscillation over longer timescales have suggested additional centennial- to millennial-scale variations in the phase of the oscillation, but often with conflicting results. Here we assess patterns of sea-ice drift in the Arctic Ocean over the past 8,000 years by geochemically determining the source of ice-rafted iron grains in a sediment core off the coast of Alaska. We identify pulses of sediment carried by sea ice from the Kara Sea, which can reach the coast of Alaska only during a strongly positive Arctic Oscillation. On the basis of these observations, we construct a record of the Arctic Oscillation phase, and identify a 1,500-year periodicity similar to that found in Holocene records of ice-rafted debrisin the North Atlantic, distinct from a 1,000-year cycle that has been found in total solar irradiance. We conclude that the 1,500-year cycle in the Arctic Oscillation arises from either internal variability of the climate system or as an indirect response to low-latitude solar forcing.
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05/23/2006
The Laramide Black Hills uplift of southwest South Dakota exposes a Precambrian crystalline core of ∼2560–2600 Ma basement granitoids nonconformably overlain by two Paleoproterozoic intracratonic rift successions. In the northern Black Hills, a 1 km thick, layered sill (the Blue Draw metagabbro) that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift-basin sedimentation. Ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207Pb/206Pb upper-intercept age of 2480 ± 6 Ma (all age errors ±2σ), comparable to two-point 207Pb/206Pb errorchron ages obtained by Pb stepwise leaching of the same titanites. Nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 ± 16 to 2284 ± 20 Ma (i.e., differentially reset along U–Pb concordia), and hornblende from an associated metadiorite yields a partially reset date with oldest apparent-age increments ranging between 2076 ± 16 and 2010 ± 8 Ma. We interpret these data as indicating that an episode of gabbroic magmatism occurred at 2480 Ma, in response to earlier rifting of the eastern edge of the Wyoming craton. Layered mafic intrusions of similar thickness and identical age occur along a rifted belt in the southern Superior craton (Sudbury region, Ontario). Moreover, these mafic intrusions are spatially aligned using previous supercontinent restorations of the Wyoming and Superior cratons (Kenorland–Superia configurations). This new “piercing point” augments one previously inferred by spatial–temporal correlation of the Paleoproterozoic Huronian (southern Ontario) and Snowy Pass (southeastern Wyoming) supergroups. We propose that layered mafic intrusions extending from Nemo, South Dakota, to Sudbury, Ontario, delineate an axial rift zone along which Wyoming began to separate from Superior during initial fragmentation of the Neoarchean supercontinent at ≥2480 Ma.
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01/01/2002
Late Paleoproterozoic (19001600 Ma) tectonothermal activity on all borders of the Archean Wyoming Province has long been established by low-precision KAr and RbSr studies. However, recent tectonic models advanced for supercontinent aggregation require improved constraints on the timing of tectonothermal activity along major boundaries. On its northwestern boundary, the Great Falls tectonic zone separates the Archean Wyoming and Hearne provinces. Recently published U/Pb ages and geochemical data reveal the presence of a Paleoproterozoic (ca. 1860 Ma) marginal-arc magmatic complex along a portion of the Great Falls tectonic zone in central Montana. We present nine new 40Ar/39Ar mineral ages (on hornblende and biotite) from these same arc rocks, which indicate ca. 1800 Ma thermal activity (>500°C) and subsequent rapid cooling to below 300°C by 1775 Ma. This new data set constrains the timing of WyomingHearne collision to between 1860 and 1800 Ma and the timing of last significant tectonothermal activity of this portion of the Great Falls tectonic zone (18001775 Ma). We note that our data add to a growing geochronologic database indicating ca. 1800 Ma tectonothermal activity (via either initial suturing or continued tectonic activity) associated with Paleoproterozoic docking of the Wyoming Province with Laurentia.
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09/15/2009
We present an updated classification for the entire Crustacea Decapoda, listing all known families and genera organized by higher taxonomic groups and including estimates of the number of species in every genus. All taxonomic names are also linked to the verified literature in which they were described, the first compilation of its kind for the Decapoda. To arrive at this compilation, we began with the classification scheme provided by Martin & Davis (2001) for extant families, updated the higher classification and included the fossil taxa. The resultant framework was then populated with the currently valid genera and an estimate of species numbers within each genus. Our resulting classification, spanning both extant (living) and fossil taxa, is the first comprehensive estimate of taxonomic diversity within the entire Decapoda. The classification consists of 233 families of decapods containing 2,725 genera and an estimated 17,635 species (including both extant and fossil species). Of the families in our classification, 53 are exclusively fossil, 109 contain both fossil and extant species, and 71 are extant only. The current estimate for extant species is 14,756, whereas 2,979 species are known exclusively as fossils.
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06/20/2013
Background: The infraorder Anomura has long captivated the attention of evolutionary biologists due to its impressive morphological diversity and ecological adaptations. To date, 2500 extant species have been described but phylogenetic relationships at high taxonomic levels remain unresolved. Here, we reconstruct the evolutionary history-phylogeny, divergence times, character evolution and diversification-of this speciose clade. For this purpose, we sequenced two mitochondrial (16S and 12S) and three nuclear (H3, 18S and 28S) markers for 19 of the 20 extant families, using traditional Sanger and next-generation 454 sequencing methods. Molecular data were combined with 156 morphological characters in order to estimate the largest anomuran phylogeny to date. The anomuran fossil record allowed us to incorporate 31 fossils for divergence time analyses.
Results: Our best phylogenetic hypothesis (morphological + molecular data) supports most anomuran superfamilies and families as monophyletic. However, three families and eleven genera are recovered as para- and polyphyletic. Divergence time analysis dates the origin of Anomura to the Late Permian ~259 (224-296) MYA with many of the present day families radiating during the Jurassic and Early Cretaceous. Ancestral state reconstruction suggests that carcinization occurred independently 3 times within the group. The invasion of freshwater and terrestrial environments both occurred between the Late Cretaceous and Tertiary. Diversification analyses found the speciation rate to be low across Anomura, and we identify 2 major changes in the tempo of diversification; the most significant at the base of a clade that includes the squat-lobster family Chirostylidae.
Conclusions: Our findings are compared against current classifications and previous hypotheses of anomuran relationships. Many families and genera appear to be poly- or paraphyletic suggesting a need for further taxonomic revisions at these levels. A divergence time analysis provides key insights into the origins of major lineages and events and the timing of morphological (body form) and ecological (habitat) transitions. Living anomuran biodiversity is the product of 2 major changes in the tempo of diversification; our initial insights suggest that the acquisition of a crab-like form did not act as a key innovation.
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08/01/1997
This study develops an empirical crystal-chemical framework for systematizing the kinetics of Pb loss and fission-track annealing in U-bearing minerals. Ionic porosity, Z (the fraction of a mineral's unit-cell volume not occupied by ions) potentially accounts for kinetic behavior by monitoring mean metal-oxygen bond length/strength. Various tests of a general kinetics-porosity relationship are presented, based upon diverse mineral data including: (1) Pb diffusion parameters; (2) measured closure temperatures (TC) for fission-track annealing and (3) retentivities of both Pb and fission tracks, from apparent-age data. Every kinetic parameter (including TC and mineral age for both the U/Pb and fission-track systems) is inversely correlated with Z within the sub-assemblage: zircon (Z ≈ 29%), titanite (∼ 34%) and apatite (∼ 38%). Assuming a diffusional closure model, Pb isotopic transport phenomena are described by a TC-Zscale “calibrated” with field-based TC data for titanite (≥ 680 ± 20°C) and apatite (∼ 500°C). Extrapolation of this scale yields TC estimates for the following minerals: staurolite (TC ≥ 1060°C, Z ≈ 25%); garnet (≥ 1010°C, ∼ 26.5%); zircon (≥900°C); monazite, xenotime, and epidote (≥ 750°C, ∼ 32%); and Ca-clinopyroxene (≥ 670 ± 30°C, ∼ 34 ± 1%, depending on composition). These empirical results imply that a (U/)Pb/Pb date for staurolite or garnet records the time of mineral growth, not post-growth isotopic closure, as also concluded in recent field studies. Because Z systematizes fission-track annealing, this recrystallization process, like volume-diffusion, must also be rate-limited by the strength of chemical bonds. The extent to which other recrystallization processes are likewise rate-limited is important to U/Pb geochronology because they potentially compete with diffusion as mechanisms for Pb-isotopic resetting in nature.
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01/01/2012
Goniodromitidae is an extinct family of primarily Jurassic and Cretaceous crabs that are found mainly in Europe. Herein, we report upon a diversity hotspot for goniodromitids from the Koskobilo quarry in northern Spain exposing mid-Cretaceous (Albian/Cenomanian) coral reef limestones. Five species of goniodromitid are described and discussed: Distefania incerta, D. renefraaijei n. sp., Eodromites grandis, Goniodromites laevis, and Navarradromites pedroartali n. gen., n. sp. Furthermore,Distefania centrosa is herein synonymized with D. incerta. Eodromites grandis was previously known only from the Late Jurassic, resulting in a time gap of 45 Myr. Other species of decapod with long time ranges are known. Additionally, the Spanish representatives of Eodromites grandis are the only Cretaceous specimens known from this genus. Species of the generaDistefania, Eodromites, and Goniodromites were predominantly found in coral/sponge limestones from the Jurassic and Cretaceous of Europe and were important for the survival of the family into the Cretaceous and indirectly into the Paleocene.
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01/01/2009
Discovery of a single specimen of cirolanid isopod from the Late Cretaceous of Lebanon permits definition of a new species, Cirolana garassinoi. Preservation with the ventral surface exposed is unique among isopod fossils. The evidence of a thin, apparently transparent cuticle and three pairs of dermoliths suggests that the specimen died while in the pre-molt condition. The ability to sequester calcium and possibly other mineral salts in a marine isopod may indicate a preadaptation to terrestrial lifestyles where the process is common in extant forms.
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12/31/2007
The Prosopidaevon Meyer, 1860, sensu lato as currently construed is too broad to be considered as a single family. A new family, Tanidromitidae,and two new genera,Sabellidromites andTanidromites, are erected to embrace several Jurassic species of brachyurans from northern Europe, based upon their possession of a unique combination of orbital and dorsal carapace characters. These revisions have also resulted in 11 new combinations. The GoniodromitidaeBeurlen, 1932, and the new family are placed within the Homolodromioidea. Members of the Tanidromitidae seem to have preferred sponge-dominatedhabitats and environments in which laminated limestones were deposited. Unlike other Jurassic members of the Homolodromioidea,members of the Tanidromitidae thus far have not been recovered from coral reef environments. The Tanidromitidae ranges fromMiddle to Late Jurassic in age and is known from Germany, Poland, and Britain. The Goniodromitidae ranged from the Jurassic to Cretaceous throughout Europe.
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01/01/2013
Examination of a single specimen of glypheoid lobster from the collections of The Natural History Museum, London, warrants designation of a new species, Glyphea macromuscula. The specimen, collected from Lyme Regis, Dorset, UK, increases the number of Early Jurassic glypheids to eleven, and the number of species of Glyphea from the United Kingdom to six.
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