Reliable Age Determinations Research Articles

Re-Os geochronology and geochemical evolution of late Cambrian to Middle Ordovician Alum and Tøyen shales, Sweden

The limited number of accurate and precise radiometric ages through the ∼100 Myr span of the Cambrian and Ordovician impedes reliable age determinations for stage boundaries in these periods. Here, we fill significant gaps in the early Paleozoic chronostratigraphy by providing precise Re-Os time-pins. Sample selection is linked to a firm biostratigraphic framework built on the appearance and distribution of trilobites, graptolites, and conodonts. A Furongian (upper Cambrian) Alum Shale section (Andrarum-3 drill core, Scania, Sweden) at the onset of the Steptoean Positive Carbon Isotopic Excursion (SPICE) yields highly non-isochronous Re-Os isotopic data from a section with wildly fluctuating δ13Corg; however, selected data from a narrow sediment band with steady carbon isotope stratigraphy provides an imprecise Re-Os age of 497 ± 28 Ma (2σ; Model 3; n = 3), with an initial 187Os/188Os ratio (Osi) of 0.74 ± 0.05. Organic-rich Alum Shale (Tomten-1 drill core, Västergötland, Sweden) from ∼120 cm below the Cambrian-Ordovician boundary yields a Model 1 age of 488.6 ± 5.1 Ma (2σ; MSWD = 1.5; n = 25) and an Osi of 0.82 ± 0.04 for Stage 10, uppermost Cambrian. Biostratigraphic data indicate the dated Alum Shale is from an interval slightly below the Top Of Cambrian Excursion (TOCE) and slightly above the First Appearance Datum (FAD) of the agnostoid Lotagnostus americanus. Organic-rich Tøyen Shale (Lerhamn drill core, Scania, Sweden) yields a precise Model 1 Re-Os age of 469.7 ± 1.4 Ma (2σ; MSWD = 1.0; n = 10) and Osi of 0.802 ± 0.002 for the maximum age of the Floian–Dapingian stage boundary (Lower–Middle Ordovician boundary). The Os isotopic composition of seawater from the latest Ediacaran through the Cambrian to Early-Middle Ordovician hovers around 0.8 but falls to 0.54 by early Silurian. This significant decrease in seawater 187Os/188Os is consistent with reduced chemical weathering and cooler seawater temperatures through the Middle–Late Ordovician. Overall, Redox Sensitive Element (RSE; Re, Os, Mo, U) abundances correlate positively with Total Organic Carbon (TOC), suggesting efficient removal of these elements from an anoxic water column by organic matter. However, these relationships break down for high TOC (>10%) shales depositing under euxinic conditions. The RSE-TOC relationship breakdown supports enhanced metal drawdown from the water column with local pyrite accumulation. Geochemical data suggest the deposition of Alum and Tøyen shales under hydrographically restricted settings with increased primary productivity along the Baltica's margin during the latest Cambrian to Early-Middle Ordovician.

Age and Petrogenesis of Paleoproterozoic Diorites from Dikes of the Baikal Uplift of the Siberian Craton

Abstract ––We carried out a detailed geological, geochronological, geochemical, and isotope study of diorites from a dike located in the central part of the Baikal uplift of the Siberian craton. The geochemical and isotope data obtained for diorites of the studied intrusion were compared with coeval mafic and intermediate igneous rocks of the southern part of the Siberian craton. The U–Pb (ID-TIMS) baddeleyite age of 1862 ± 7 Ma has been estimated for diorite from a dike located in the area of the Onguren Village. The obtained data are the first reliable age determination for the Paleoproterozoic mafic and intermediate igneous rocks of the Baikal uplift, which are part of the South Siberian postcollisional magmatic belt. The dike is of NE strike. The dike rocks correspond in chemical composition to diorites and are highly differentiated varieties (mg# = 36.5–37.4). There are no significant variations in the composition of diorites in the marginal and central parts of the dike. The rocks are characterized by low contents of TiO2, P2O5, and Nb, high contents of Th, Zr, Ba, and LREE, and low negative values of εNd(T) (–5.9 to –6.2). We assume that the diorites formed from the enriched subcontinental lithospheric mantle. The obtained geochemical and isotope data show similar negative values of εNd(T) from –4.3 to –11.6 for most of Paleoproterozoic mafic and intermediate igneous rocks of the South Siberian postcollisional magmatic belt within the Aldan Shield, the Baikal uplift, and the Irkut block of the Sharyzhalgai uplift. These rocks correspond in geochemical features to rocks resulted from the melting of subduction-modified lithospheric mantle sources in the postcollisional extension setting at the final Paleoproterozoic stage of formation of the Siberian craton.

The Ages of Galactic Bulge Stars with Realistic Uncertainties

Using modern isochrones with customized physics and carefully considered statistical techniques, we recompute the age distribution for a sample of 91 microlensed dwarfs in the Galactic bulge presented by Bensby et al. and do not produce an age distribution consistent with their results. In particular, our analysis finds that only 15 of 91 stars have ages younger than 7 Gyr, compared to their finding of 42 young stars in the same sample. While we do not find a constituency of very young stars, our results do suggest the presence of an ∼8 Gyr population at the highest metallicities, thus contributing to the long-standing debate about the age–metallicity distribution of the Galactic bulge. We supplement this with attempts at independent age determinations from two sources of photometry, BDBS and Gaia, but find that the imprecision of photometric measurements prevents reliable age and age uncertainty determinations. Lastly, we present age uncertainties derived using a first-order consideration of global modeling uncertainties in addition to standard observational uncertainties. The theoretical uncertainties are based on the known variance of free parameters in the 1D stellar evolution models used to generate isochrones, and when included, result in age uncertainties of 2–5 Gyr for this spectroscopically well-constrained sample. These error bars, which are roughly twice as large as typical literature values, constitute realistic lower limits on the true age uncertainties.

Fish and Sclerochronology Research in the Mediterranean: Challenges and Opportunities for Reconstructing Environmental Changes

Over the past two decades, the field of sclerochronology has been rapidly developing, with scientists devoting significant efforts to studying the physical and chemical variations in hard tissues of aquatic organisms. Most of this research has been limited to certain taxa and geographic areas. Although growth increments in fish otoliths are used for sclerochronology purposes, relatively little has been done in the Mediterranean Sea. According to the literature, the chemical composition of otoliths from Mediterranean fish species has primarily been used for analysing migration patterns, habitat use and population structure of commercially important fish species. To the best of our knowledge, there are no studies on fish growth chronology construction conducted in the Mediterranean Sea. In order to identify the opportunities for sclerochronology research on fish from the Mediterranean, we used FishBase to identify potential candidate species with a sufficiently long-life span and clearly defined growth increments for growth chronology construction and otolith chemistry research. We also present the challenges and limitations for sclerochronology research, including: (i) very few fish species in the Mediterranean Sea have a longevity of several decades, (ii) issues associated with reliable age determination for certain long-lived fish species, (iii) a general lack of understanding and effort to constructed and manage otolith collections, and (iv) limitations imposed by the availability of funding, expertise and instrumentation. Despite these challenges, fish sclerochronology research has strong potential in the Mediterranean and adjacent seas. Recent studies in the Adriatic Sea have resulted in the construction of bivalve chronologies and the geochemical analysis of shells, providing important time-series data for comparative analysis and a multispecies approach. Furthermore, studies conducted in other parts of the world have demonstrated great potential for the use of fish otoliths in monitoring environmental variability and the effects of pollutants and disturbance.

Surprisingly young age for the mamenchisaurid sauropods in South China

Mamenchisauridae, a sauropod group named after the largest known proto-Asian sauropod Mamenchisaurus, is generally believed to range from the Early to the Late Jurassic in age. Fossil remains of Mamenchisaurus are primarily found in the Upper Shaximiao Formation and less frequently in the overlying Suining Formation in the Sichuan Basin. An accurate absolute age for Mamenchisaurus is critical for understanding the evolution and radiation of the mamenchisaurids. However, reliable age determinations for the Mamenchisaurus-bearing formations are lacking. In this study, we provide robust detrital zircon U/Pb ages (mean ages of 114.4 ± 1.1 Ma; Aptian) for rocks from the classic fossil outcrops of the Suining Formation in the Sichuan Basin. Our data indicate an extremely long chronological gap between M. anyuensis (114.4 ± 1.1 Ma) and the earlier Mamenchisaurus (Late Jurassic). This indicates mamenchisaurids may have lived at least 30 million years longer than previously thought.

New Substellar Discoveries from Kepler and K2: Is There a Brown Dwarf Desert?

Abstract We present the discoveries of a brown dwarf (BD) and a low-mass star from the Kepler and K2 missions. The newly discovered BD is EPIC 212036875b and the low-mass star is KOI-607b. EPIC 212036875b has a mass of M b = 52.3 ± 1.9 M J, a radius of R b = 0.874 ± 0.017 R J, and orbits its host star in P = 5.169885 ± 0.000027 days. Its host star is a late F-type star with M ⋆ = 1.288 ± 0.065 M ⊙, R ⋆ = 1.498 ± 0.025 R ⊙, and T eff = 6238 ±60 K. KOI-607b has a mass of M b = 95.1 ± 3.4 M J, a radius of R b = 1.089 ± 0.089 R J, and an orbital period of P = 5.89399148 ± 0.00000060 days. The primary star in the KOI-607 system is a G dwarf with M ⋆ =0.993 ± 0.052 M ⊙, R ⋆ = 0.915 ± 0.031 R ⊙, and T eff = 5418 ± 87 K. We also revisit a BD, CWW 89Ab, that was previously published by Nowak et al. (under the designation EPIC 219388192b). CWW 89Ab is one of two known transiting BDs associated with a star cluster, which illustrates the need for more BDs with accurate masses, radii, and reliable age determinations to test theoretical models. We find that the newly discovered BD, EPIC 212036875b, falls in the middle of the so-called BD desert, indicating that EPIC 212036875b is either a particularly rare object, or the BD desert may not be so dry after all.

Age of the dacite of Sunset Amphitheater, a voluminous Pleistocene tephra from Mount Rainier (USA), and implications for Cascade glacial stratigraphy

The dacite of Sunset Amphitheater, Mount Rainier (USA), illustrates the difficulties in establishing accurate ages of Pleistocene tephra eruptions. Nearly uniform whole-rock, glass, and mineral compositions, texture, and phenocryst assemblage establish that certain conspicuous dissected pumice exposures scattered from Mount Rainier to southern Puget Sound are products of the same Pleistocene Plinian eruption. Deposit thicknesses and pumice sizes support an eruption on the order of low Volcanic Explosivity Index (VEI) 5, atypically explosive for dominantly lava-producing Mount Rainier. Statistically permissible 40Ar/39Ar plateau ages of plagioclase phenocryst separates are 138 ± 20 ka and 101 ± 11 ka (2σ). A previously published result of 206 ± 11 ka is herein shown to result from a sample selection error. Zircon from the pumice yields a U-Th crystallization age of 147 ± 8 ka if the isochron is required to pass through the tephra U-Th isotopic composition. In contrast, pooled (U-Th)/He measurements on the zircon yield an age of 85 ± 6 ka (2σ), which accords with well-behaved 40Ar/39Ar ages of stratigraphically associated lavas high on Mount Rainier, and is the best estimate of the pumice's true eruption age. Inclusions of undegassed melt (glass) in the plagioclase separates are proposed as biasing apparent 40Ar/39Ar plateau ages to old values through coupling of undegassed magmatic excess Ar with radiogenic Ar that accumulated post-eruptively from relatively K-rich glass. U-Th ages record zircon growth prior to eruption, consistent with a possible complex history of advanced solidification followed by remobilization. The ca. 85 ka eruption age confirms that bracketing glacial tills on the flanks of Mount Rainier were products of the Penultimate Glaciation (MIS 6) and Last Glacial Maximum (MIS 2). This eruption age also provides an important time marker for glacial and other sedimentary deposits in southern Puget Sound lowland that, excepting the Vashon Drift (MIS 2), generally lack reliable age determinations.

A 2207 Ma radiating mafic dyke swarm from eastern Dharwar craton, Southern India: Drift history through Paleoproterozoic

We report new TE-TIMS precise Pb-Pb baddeleyite ages together with consistent paleomagnetic results on two Paleoproterozoic mafic dyke swarms, intruding Archean basement rocks in the northern and southern region of Cuddapah basin, Eastern Dharwar Craton. The distinct E-W to NW-SE striking dykes, which gives a weighted mean age of 2206.8 ± 2.1 Ma, confirms the presence of a radiating mafic dyke swarm with a fanning angle of 50°, convergence towards a focal point in the north-western part of Dharwar Craton. The North-South trending parallel dyke swarm yields an emplacement age of 2252.4 ± 1.2 Ma, making this the second oldest well dated dyke swarm of the Dharwar craton after the ∼2367 Ma event.Paleomagnetic results from the ∼2207 Ma radiating dykes have revealed a stable reverse polarity remanence directions with a mean direction of D = 26°, I = 65° (α95 = 6°) and corresponding paleopole situated at 57°N and 113°E (dp = 8°, dm = 10°). Reliable baddeleyite age determinations at paleomagnetic sampling sites, positive baked contact test and a secular variation correlation test proves that the reporting remanence directions are primary. We have also reviewed the published paleomagnetic results from similar N-S dykes, which are yielding a normal polarity with a paleopole position at 16°S and 299°E (dp = 13°, dm = 15°). We consider this result as a key paleopole for the ∼2252 Ma dyke swarm.We have attempted to reconstruct an updated APWP for Dharwar craton utilizing six well constrained key paleopoles from 2.37 Ga to 1.89 Ga periods. The path suggests that the Dharwar craton has relatively rapid drift rate (28.77 cm/yr) from 46° to 53° paleolatitude along with a clockwise rotation (30°) during the short time span of ∼9 Ma (2216 Ma–2207 Ma), when a magnetic polarity reversal occurred. However, we speculate that major tectonic activity or plate tectonic processes were responsible for rapid drift of Dharwar craton at that time. The identification of coeval mafic magmatic episodes (∼2.2 Ga) from several other Archean cratons over the globe firmly supports the popular hypothesis of supercraton breakup at ∼2.2 Ga time.

Effects of thermal annealing and chemical abrasion on ca. 3.5 Ga metamict zircon and evidence for natural reverse discordance: Insights for U[sbnd]Pb LA-ICP-MS dating

We present a microstructural and UPb systematics study comparing pristine, thermally annealed (TA) and chemically abraded (CA) ~3500Ma zircon from a quartz-dioritic gneiss, with the aim to improve pre-analytical workflows for more accurate and precise LA-ICP-MS UPb dating of ancient zircon. Four zircon domains are identified: i) low- to medium-U concentric-oscillatory zoned cores, ii-iii) two porous high-U outer alteration domains, and iv) low-U narrow inward growing recrystallization rims. Raman spectroscopy on the pristine zircon reveals a positive correlation between increasing structural damage and U content. The porous high-U outer domains show a drastic increase in non-formula Ca above estimated amorphous fractions of ~0.8, which we ascribe to hydrothermal alteration of high-damage zircon characterized by percolating networks of amorphous areas. Upon treatment (TA, CA), hyper-spectral CL and Raman spectroscopy suggest structural recovery of point defects, but not full repair of high-damage amorphous areas. Calculated Raman radiation damage ages suggest that natural annealing affected all domains at ~500Ma, consistent with negligible laser ablation matrix effects comparing pristine and treated cores.We show that reverse discordance in pristine and TA cores is not an analytical artifact. High-U alteration domains are normal discordant and were partially reset at ca. 3410Ma. Upon CA, UPb discordance and scatter are reduced in the cores, yielding intercepts of 3503±14Ma and 560±550Ma (MSWD=1.0). The lower intercept matches the timing of Pb-loss recorded in the alteration domains and the Raman radiation damage age. We argue that short distance Pb redistribution within the cores, which led to reverse discordance, was controlled by UPb systematics recording both the crystallization age and the timing of Pb redistribution. Some excess Pb was redistributed from the partially re-set alteration domains into the cores, leading to mixed UPb systematics and further reverse discordance. Removal of the latter excess Pb upon CA suggests that radiogenic Pb from the alteration domains accumulated within distinct damage sites accessible for CA leaching.We conclude that standardization of pre-analytical treatment is not recommended; we propose to investigate the structural state of unknown specimen by means of Raman spectroscopy, in combination with commonly applied CL imaging. During LA-ICP-MS analyses, co-measurement of non-formula and selected trace elements is deemed highly valuable in detecting ablations for use in reliable UPb age determination.

Dating of Cambrian-Ordovician boundary strata in northernmost Vietnam and methodological aspects of evolutionary biostratigraphic inference

Unrepeatability of evolution and the correspondence of the fossil record to ancestor-descendant successions of species are the unavoidable, although usually hidden, assumptions in any reliable age determination based on fossils. We expose these assumptions while dating early Paleozoic carbonate rock deposits in the Lung Cu section at the Vietnamese/Chinese border. The best-preserved and most abundant fossils in this section are shumardiid trilobites. The succession of shumardiid species, based on data from elsewhere, provides an evolutionary reference standard. The shumardiid record is not sufficiently complete to verify hypotheses of ancestor-descendant relationships but enables estimation of the "degree of evolutionary advancement" of the Vietnamese species. This suggests an age close to the Cambrian/Ordovician boundary. Although considered non-scientific by cladists, such inferences are testable. Support for a late Furongian or early Tremadocian age is provided by the occurrence of Cordylodus conodonts in strata above the trilobite-bearing bed. The conodont evolution has a good fossil record interpreted in population terms in the Baltic region and Australia, including the lineage represented in Vietnam. Age determination based on such evolutionary reasoning is reliable but of a relatively low resolution, because the rate of morphological evolution is generally low. Generally, more precise dating is offered by distribution of fossils controlled by ecological factors, which are repeatable and mostly diachronous over large geographic distances, but they may have happened relatively rapidly. The appearance of the Iapetognathus-Chosonodina-bearing conodont assemblage in the Lung Cu area, as suggested by its occurrences elsewhere, was probably due to abrupt faunal migration into the region.

The role of cryptotephra in refining the chronology of Late Pleistocene human evolution and cultural change in North Africa

Sites in North Africa hold key information for dating the presence of Homo sapiens and the distribution of Middle Stone Age (MSA), Middle Palaeolithic (MP) and Later Stone Age (LSA) cultural activity in the Late Pleistocene. Here we present new and review recently published tephrochronological evidence for five cave sites in North Africa with long MSA/MP and LSA cultural sequences. Four tephra horizons have been identified at the Haua Fteah (Cyrenaica, Libya). They include cryptotephra evidence for the Campanian Ignimbrite (CI) eruption dating to ∼39 ka that allows correlation with other Palaeolithic sequences in the eastern Mediterranean and as far north as Russia. Cryptotephra have also been recorded from the Moroccan sites of Taforalt, Rhafas and Dar es-Soltane 1. At Taforalt the geochemical composition suggests a provenance in the Azores, while examples from Sodmein (Egypt) appear to derive from central Anatolia and another unknown source. In these latter examples chemical compositional data from relevant proximal volcanic centres is currently lacking so the identification of tephra in layers of known age and cultural association provides the first reliable age determinations for distal volcanic events and their geographical extent. The future potential for tephrochronological research in North Africa is also discussed.

Retreat of the West Antarctic Ice Sheet from the western Amundsen Sea shelf at a pre- or early LGM stage

Recent palaeoglaciological studies on the West Antarctic shelf have mainly focused on the wide embayments of the Ross and Amundsen seas in order to reconstruct the extent and subsequent retreat of the West Antarctic Ice Sheet (WAIS) since the Last Glacial Maximum (LGM). However, the narrower shelf sectors between these two major embayments have remained largely unstudied in previous geological investigations despite them covering extensive areas of the West Antarctic shelf. Here, we present the first systematic marine geological and geophysical survey of a shelf sector offshore from the Hobbs Coast. It is dominated by a large grounding zone wedge (GZW), which fills the base of a palaeo-ice stream trough on the inner shelf and marks a phase of stabilization of the grounding line during general WAIS retreat following the last maximum ice-sheet extent in this particular area (referred to as the Local Last Glacial Maximum, ‘LLGM’). Reliable age determination on calcareous microfossils from the infill of a subglacial meltwater channel eroded into the GZW reveals that grounded ice had retreated landward of the GZW before ∼20.88 cal. ka BP, with deglaciation of the innermost shelf occurring prior to ∼12.97 cal. ka BP. Geophysical sub-bottom information from the inner-, mid- and outer shelf indicates grounded ice extended to the shelf edge prior to the formation of the GZW. Assuming the wedge was deposited during deglaciation, we infer the timing of maximum grounded ice extent occurred before ∼20.88 cal. ka BP. This could suggest that the WAIS retreat from the outer shelf was already underway during or even prior to the global LGM (∼23–19 cal. ka BP). Our new findings give insights into the regional deglacial behaviour of this understudied part of the West Antarctic shelf and at the same time support early deglaciation ages recently presented for adjacent drainage sectors of the WAIS. If correct, these findings contrast with the hypothesis that initial deglaciation of Antarctic Ice Sheets occurred synchronously at ∼19 cal. ka BP.

MOLECULAR GAS IN YOUNG DEBRIS DISKS

Gas-rich primordial disks and tenuous gas-poor debris disks are usually considered as two distinct evolutionary phases of the circumstellar matter. Interestingly, the debris disk around the young main-sequence star 49 Ceti possesses a substantial amount of molecular gas, and possibly represents the missing link between the two phases. Motivated to understand the evolution of the gas component in circumstellar disks via finding more 49 Ceti-like systems, we carried out a CO J=3-2 survey with Atacama Pathfinder EXperiment, targeting 20 infrared-luminous debris disks. These systems fill the gap between primordial and old tenuous debris disks in terms of fractional luminosity. Here we report on the discovery of a second 49 Ceti-like disk around the 30 Myr old A3-type star HD21997, a member of the Columba Association. This system was also detected in the CO(2-1) transition, and the reliable age determination makes it an even clearer example of an old gas-bearing disk than 49 Ceti. While the fractional luminosities of HD21997 and 49 Ceti are not particularly high, these objects seem to harbor the most extended disks within our sample. The double-peaked profiles of HD21997 were reproduced by a Keplerian disk model combined with the LIME radiative transfer code. Based on their similarities, 49 Ceti and HD21997 may be the first representatives of a so far undefined new class of relatively old (>~8 Myr), gaseous dust disks. From our results, neither primordial origin nor steady secondary production from icy planetesimals can unequivocally explain the presence of CO gas in the disk of HD21997.

Age determination and reproductive traits of killer whales entrapped in ice off Aidomari, Hokkaido, Japan

On 7 February 2005 a group of 9 killer whales (Orcinus orca) were trapped in drifting sea ice and died at Aidomari, Hokkaido, Japan. We carried out age determination based on tooth growth layers and examined the reproductive organs of these whales. Growth layer groups (GLGs) in the dentine and cementum were readable, even in the old specimens, and complementary to each other in decalcified and stained thin sections of lower teeth. Reliable age determination of killer whales is feasible, and GLGs are accumulated annually. The longitudinal growth of the teeth continued until about 20 years of age, which is much longer than for the corresponding age in other delphinids. Counts of corpora lutea and albicantia increased linearly with age from 3 to 7 in whales 13–34 years old, but the oldest female (59 years old) had only 8 corpora albicantia, which could indicate a decline in the ovulation rate in old females. Photographs of the original group trapped in the ice confirmed that at least 2 whales escaped or died and did not strand with the others. Therefore, this group was composed originally of a mature male, 1 possibly postreproductive female, 5 reproductively active females, 3 calves, and 2 or 3 unidentified individuals.

Implications of Prebasic and a Previously Undescribed Prealternate Molt For Aging Rusty Blackbirds

Abstract. Aging birds often relies on differences in plumage between immatures and adults, and understanding these patterns can improve our ability to discern demographic patterns within populations. We investigated patterns of prebasic molt of the Rusty Blackbird (Euphagus carolinus) in fall at Whitehorse, Yukon Territory, and developed a new technique for aging based on characteristics of the head plumage acquired during prebasic molt. Furthermore, we investigated the possibility of a prealternate molt among wintering Rusty Blackbirds on the basis of captures in Mississippi and museum specimens from across the species' winter range. Finally, we examined how a prealternate molt might affect the aging of winter birds by plumage. Rusty Blackbirds completed their prebasic molt by the end of September, and immature birds had a more prominent eye ring and a paler chin than adults, allowing a reliable age determination. Previously, the Rusty Blackbird was thought to attain its breeding plumage through feather ...

Holocene Radiocarbon-Dated Sites in Northeastern Siberia: Issues of Temporal Frequency, Reservoir Age, and Human-Nature Interaction

The existing corpus of data on radiocarbon dates for Holocene sites in Northeastern Siberia was used as proxy to reconstruct the chronology of human occupation of the region. The problem of reservoir age correction in the Bering Sea region complicated this task and this issue needs to be solved in order to obtain more reliable age determinations for coastal archaeological sites. Using a chronology built after excluding the questionable dates from the database, the major patterns of human population dynamics and their possible correlation with climatic fluctuations were examined. No direct relationship appears to exist between these two processes. Additional archaeological and paleo environmental work needs to be carried out in this region of the North.

Age determination enhanced by embryonic foot bud and foot plate measurements in relation to Carnegie stages, and the influence of maternal cigarette smoking

Reliable age determination of first-trimester human embryos and fetuses is an important parameter for clinical use and basic science. Age determination by ultrasound or morphometric parameters of embryos 4-6 weeks post conception (p.c.) have been questioned, and more accurate methods are required. Data on whether and how maternal smoking and alcohol consumption influence embryonic and fetal foot growth is also lacking. Embryonic tissue from 102 first-trimester legal abortions (aged 35-69 days p.c.) were collected. All women answered a questionnaire concerning smoking and drinking habits, and delivered a urine sample for cotinine analysis. Embryonic age was evaluated by vaginal ultrasound measurements and by post-termination foot length and compared with the Carnegie stages. Foot bud and foot plate were defined and measured as foot length in embryos aged 35-47 days p.c. (range 0.8-2.1 mm). In embryos and fetuses aged 41-69 days p.c., heel-toe length was measured (range 2.5-7.5 mm). We found a significant linear correlation between foot length and age. Morphology of the feet was compared visually with the Carnegie collection, and we found that the mean ages of the two collections correlated well. Foot length was independent of gender, Environmental Tobacco Smoke, maternal smoking and alcohol consumption. Foot length correlated linearly to embryonic and foetal age, and was unaffected by gender, ETS, maternal smoking and alcohol consumption.

Problems with radiocarbon dating the Middle to Upper Palaeolithic transition in Italy

Radiocarbon dating of material from Late Pleistocene archaeological sites is challenging. Small amounts of modern 14C-labelled contamination will significantly affect the reliability of dates from the period, producing erroneous results. Recent developments in sample pre-treatment chemistry have shown that problems in reliable age determination during this period are surmountable. In this paper we provide an example of one such case, from the Middle to Upper Palaeolithic transitional site of the Grotta di Fumane, in northern Italy. We AMS dated two fractions of the same charcoal samples derived from a series of superimposed Mousterian, Aurignacian and Gravettian levels excavated at the site. One fraction was treated using the routine acid–base–acid (ABA) method, the other with the more rigorous acid–base-oxidation/stepped combustion (ABOx–SC) method. The latter method produced consistently older, and almost certainly more reliable, results. The eruption of the known-age Campanian Ignimbrite from the Phlegrean Fields near present-day Naples at 39.3 ka yr BP seals Ulluzzian and Proto Aurignacian levels in the south of Italy. Equivalent cultural levels are present at Fumane and the results obtained with the ABOx–SC methods are consistent with the ages inferred for sites in the south of Italy based on the presence of the Campanian Ignimbrite. New results from a sample found beneath the Campanian Ignimbrite at the Russian site of Kostenki, obtained using both the ABA and ABOx–SC, methods are also presented. They support the conclusion reached at Fumane by demonstrating that, in many cases, the ABOX–SC treatment effectively removes contamination where the ABA treatment does not. The results of the work offer a sobering examination of the problems inherent in the current radiocarbon database relating to the period, and highlight the dangers of an uncritical use of the corpus of 14C results obtained over the last few decades. Based on our results, we predict that more than 70% of the 53 previously available determinations from Fumane are erroneously young. A way forward is suggested, using these improved chemical preparation methods, applying analytical methods to characterise the material dated, and testing existing site chronologies to establish which previous determinations are liable to be inaccurate.

Fetal Age Estimation Using Orbital Measurements: 3D CT‐Scan Study Including the Effects of Trisomy 21

This study evaluates a new method of fetal age estimation based on orbital measurement and including the potential trisomy 21 of the fetus. Six orbital and one facial CT-scan measurements were taken on 71 fetuses ranging from 14 to 41 weeks of gestational age. Forty-eight fetuses were "normal" and 23 fetuses presented the Down syndrome (trisomy 21). The anatomy/imagery correspondence was evaluated comparing our results to those obtained by direct bone measurements on the same fetuses and revealed no significant differences between the two kinds of measurements. Moreover, a multiple linear stepwise regression was realized to estimate fetal age and showed that the "trisomy 21 parameter" is not conserved in the final determination model. Therefore, we conclude that the good radio-anatomical correspondence offers an interesting alternative to direct bone measurement (necessitating dissections) and allows a reliable fetal age determination, whatever is the trisomy 21 condition of the fetus.

Gonad development and spawning in one and two year old mussels (Mytilus edulis) from Western Norway

Sexual maturation and spawning was followed in one and two year old rope cultured mussels (Mytilus edulis) from April to December. Development of gonad and storage tissue was followed using descriptive categories from histology, and development of total soft tissue was followed using meat yield and condition index.Both age groups were mature both during spring and autumn and had similar patterns of spawning and maturation. April and May were characterized by scattered spawning activity and accumulation of storage reserves at the same time, resulting in relative constant condition indices during this period. This was followed by ripening of the gonads and an increase in condition index that culminated in a spawning in late June. Parts of the population then entered a resting phase, while a part of the population underwent new maturation towards an autumn spawning in September. In December all the mussels had started the winter maturation.Condition index and meat yield were higher throughout the season in the one year old mussels. This was probably due to the sum of several factors, including differences in specific feeding rates and biomass density, rather than spawning.The present study is the first to compare the reproductive patterns over time of one and two year old mussels with reliable age determination, and provide information that there are no obvious differences in neither timing of gonad development nor spawning patterns between the age-classes.