Search

The surface geology of the site of the Chicxulub impact crater in northwestern Yucatan, Mexico, has not been studied extensively since the discovery of the crater almost two decades ago. Strontium isotope ([sup.87]Sr/[sup.86]Sr) measurements in carbonate rock outcrops reveal near-uniform strontium signatures of 0.70905 inside the ring of cenotes (water-filled sinkholes), which represents the rim of the crater basin. Measured strontium isotope ratios were used to infer rock ages, employing the marine Sr isotope curve. We estimate the age of the exposed limestone within the Chicxulub crater basin to be late Miocene to early Pliocene, representing the age of the youngest sediment fill. Discovery of a large terrain of near-uniform strontium isotope ratios in northwestern Yucatan offers new geoarchaeological opportunities to track ancient Maya migration and determine sources of manufactured goods. Our results have implications for applying the Sr isotope method to Maya archaeological sites, such as Mayapan, the last Maya capital, and Chichen Itza.

During a 1977 cruise of the R/V Bannock, a core was taken from an area of extensive hydrothermal mineralization located on the slope of Eolo Seamount, Aeolian Arc (Tyrrhenian Sea). The Eolo hydrothermal sediments exhibit unusual Pb isotopic compositions that cannot be solely derived from the plausible natural sources in the area. Mixing considerations indicate that a significant part of the Pb in these sediments is anthropogenic in origin. The anthropogenic Pb was most probably scavenged from the Mediterranean seawater by sinking hydrothermal plume particles. Apparently the hydrothermal products entering the seawater column did not carry significant amounts of Pb extracted from the underlying rocks, so their Pb isotopic compositions were swamped by the seawater Pb. This indicates that Pb in low-temperature hydrothermal products is controlled by the seawater Pb, not by hydrothermal leaching from underlying rocks.

Lu-Hf depleted mantle model ([T.sub.DM]) ages, obtained by analysis of zircons previously dated by U-Pb methods, demonstrate that ca. 1.75 Ga bimodal Paleoproterozoic rocks in the Gunnison-Salida region of central Colorado, and by extension in much of the southwestern U.S., were formed by partial melting of preexisting crustal rocks, the ages of which mostly range from 1.83 to 1.87 Ga. Some calculated Lu-Hf [T.sub.Dm] ages greater than 2.0 Ga indicate that even older crust was probably involved, consistent with the limited presence of ca. 2.5 Ga xenocrystic zircons in some rocks. These results suggest that rocks related to the Trans-Hudson and Penokean orogens are cryptically present much farther to the south than previously believed. Coupled with the bimodality of the volcanic suite in central Colorado, and indeed in much of the southwestern U.S., these results indicate that, in contrast to current juvenile arc-accretion models, melting of older crust related to extensional tectonics played an important role in the genesis of many magmatic rocks between 1.6 and 1.8 Ga. Keywords: Laurentia, crustal evolution, Colorado, Paleoproterozoic, zircon, depleted mantle.

Byline: George D. Kamenov (1), Michael R. Perfit (1), Paul A. Mueller (1), Ian R. Jonasson (2) Abstract: The Tabar--Lihir--Tanga--Feni (TLTF) islands of Papua New Guinea mainly comprise high-K calc-alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction-related magmatism. Numerous sedimentary, mafic, and ultramafic xenoliths recovered from Tubaf seamount, located on the flank of Lihir Island, provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region. The sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with Pacific affinity. A majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the TLTF arc. Leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50% or more of elements such as Cu, Zn, Rb, U, Pb, and light REE (rare-earth elements), most likely introduced in the xenoliths via hydrous fluids released from a subducted slab. The leaching experiments demonstrated that the light REE enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit REE patterns that range from flat to light REE depleted. Sr--Nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87.sup.Sr/ [.sup.86]Sr with decreasing 143.sup.Nd/ [.sup.144]Nd ratios. The labile phases in the ultramafic xenoliths, represented by the leachates, show decoupling between Sr and Nd with distinctly more radiogenic 87.sup.Sr/ [.sup.86]Sr than the residues. Both leachates and residues exhibit very wide range in their Pb isotopic compositions, indicating the involvement of three components in the mantle wedge under the TLTF islands. Two of the components can be identified as Pacific Oceanic mantle and Pacific sediments. Some of the ultramafic samples and clinopyroxene separates, however, exhibit relatively low 206.sup.Pb/ [.sup.204]Pb at elevated 207.sup.Pb/ [.sup.204]Pb suggesting that the third component is either Indian Ocean-type mantle or Australian subcontinental lithospheric mantle. Geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized, it did not significantly contribute to the isotopic and incompatible trace element compositions of TLTF lavas. Compared to the mantle samples, the TLTF lavas have very restricted Pb isotopic compositions that lie within the Pacific MORB range, indicating that magma compositions were dominated by melts released from a stalled subducted slab with Pacific MORB affinity. Interaction of slab melts with depleted peridotitic component in the mantle wedge, followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area. Author Affiliation: (1) Department of Geological Sciences, University of Florida, Gainesville, FL, 32611, USA (2) Geological Survey of Canada, K1A OE8, Ottawa, ON, Canada Article History: Registration Date: 31/10/2007 Received Date: 26/03/2007 Accepted Date: 31/10/2007 Online Date: 22/11/2007 Article note: Communicated by T.L. Grove. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-007-0262-0) contains supplementary material, which is available to authorized users.

Numerous studies have shown that large quantities of Grenville-age detritus dominate Neoproterozoic to Cambrian arenites in southwest Laurentia (southwestern United States). U-Pb ages and Hf isotopic compositions of zircons and [sup.40]Ar/[sup.39]Ar ages of white mica from clastic sedimentary rocks of the Neoproterozoic Uinta Mountain Group also indicate significant Mesoproterozoic detritus mixed with a variably abundant Archean component. Zircons with ages representative of the Paleoproterozoic basement in the eastern Uinta Mountains or the younger Paleoproterozoic rocks of the adjacent Yavapai-Mazatzal terranes were not observed. A limited range of initial [[epsilon].sub.Hf] (~90% between -3 and +3) for Mesoproterozoic zircons suggests derivation from a source region (or regions) characterized by mixing between juvenile and reworked older crust during Grenville orogenesis. The enriched Grenville-age basement proposed to underlie much of southeastern North America may be this source based on similarities of Hf isotopic data from Mesoproterozoic zircons in Mississippi River sand and available paleocurrent data. If so, then disruption of this supply in the Cambrian may be related to Iapetan rifting and, perhaps, the separation of the Precordillera terrane from Laurentia. Keywords: sedimentation, Precambrian, zircon, argon, hafnium, tectonics, Uinta Mountains, Laurentia, Grenville.

Bahamian hutias (Geocapromys ingrahami) are the only endemic terrestrial mammal in The Bahamas and are currently classified as a vulnerable species. Drawing on zooarchaeological and new geochemical datasets, this study investigates human management of Bahamian hutias as cultural practice at indigenous Lucayan settlements in The Bahamas and the Turks & Caicos Islands. In order to determine how hutia diet and distribution together were influenced by Lucayan groups we conducted isotopic analysis on native hutia bone and tooth enamel recovered at the Major’s Landing site on Crooked Island in The Bahamas and introduced hutias from the Palmetto Junction site on Providenciales in the Turks & Caicos Islands. Results indicate that some hutias consumed 13C-enriched foods that were either provisioned or available for opportunistic consumption. Strontium isotope ratios for hutia tooth enamel show a narrow range consistent with local origin for all of the archaeological specimens. In contrast, analysis of strontium isotopes in modern Bahamian hutia teeth from animals relocated to Florida from The Bahamas demonstrates that these animals rapidly lost their Bahamian signature and adopted a Florida signature. Therefore, strontium should be used cautiously for determining hutia provenance, particularly for individuals that were translocated between islands. Overall, our findings suggest that ancient human presence did not always result in hutia vulnerability and that the impact to hutia populations was variable across pre-Columbian indigenous settlements. [ABSTRACT FROM AUTHOR]

Contemporary West Indian biodiversity has been shaped by two millennia of non-native species introductions. Understanding the dynamics of this process and its legacy across extended temporal and spatial scales requires accurate knowledge of introduction timing and the species involved. Richard Ligon’s 17th century account and celebrated map of early colonial Barbados records the translocation of several Old World species to the island in the post-contact era, including pigs (Sus scrofa) believed to have been released by passing sailors the century prior. Here we challenge this long-accepted historical narrative, presenting evidence that Ligon’s “pigs” were in fact peccaries, a New World continental mammal often confused with wild boars. We document the first recorded instance of non-native peccary (Tayassuidae) on Barbados based on a securely identified mandibular specimen from a historic archaeological context. Results of specimen 87Sr/86Sr and AMS radiocarbon assays, along with newly reported data from Sr isotope environmental analyses, indicate a local origin dating to AD 1645–1670/1780–1800. These data support the presence of living peccary on Barbados some time during the first 175 years of English settlement, which, based on review of historical and archaeological data, most likely arises from 16th century peccary introduction from the Guianas/Trinidad by the Spanish or Portuguese. We argue dimorphic representations of “pigs” on Ligon’s map reflect the co-occurrence of peccary and European domestic swine on historic Barbados. Our findings overturn conventional history and provide greater taxonomic and chronological resolution for Caribbean bioinvasion studies, helping to refine our understanding of potential ecological impacts. In addition, the new bioavailable 87Sr/86Sr data for Barbados reported here advance current efforts toward mapping the Caribbean Sr isoscape. [ABSTRACT FROM AUTHOR]

We examined the potential use of lead (Pb) isotopes to source archaeological materials from the Maya region of Mesoamerica. The main objectives were to determine if: 1) geologic terrains throughout the Maya area exhibit distinct lead isotope ratios (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb), and 2) a combination of lead and strontium ratios can enhance sourcing procedures in the Mesoamerica region. We analyzed 60 rock samples for lead isotope ratios and a representative subset of samples for lead, uranium, and thorium concentrations across the Maya region, including the Northern Lowlands of the Mexican Yucatan Peninsula, the Southern Lowlands of Guatemala and Belize, the Volcanic Highlands, the Belizean Maya Mountains, and the Metamorphic Province/Motagua Valley. Although there is some overlap within certain sub-regions, particularly the geologically diverse Metamorphic Province, lead isotopes can be used to distinguish between the Northern Lowlands, the Southern Lowlands, and the Volcanic Highlands. The distinct lead isotope ratios in the sub-regions are related to the geology of the Maya area, exhibiting a general trend in the lowlands of geologically younger rocks in the north to older rocks in the south, and Cenozoic volcanic rocks in the southern highlands. Combined with other sourcing techniques such as strontium (87Sr/86Sr) and oxygen (δ18O), a regional baseline for lead isotope ratios can contribute to the development of lead isoscapes in the Maya area, and may help to distinguish among geographic sub-regions at a finer scale than has been previously possible. These isotope baselines will provide archaeologists with an additional tool to track the origin and movement of ancient humans and artifacts across this important region. [ABSTRACT FROM AUTHOR]