U-series and oxygen isotope chronology of the mid-Pleistocene Lake Amora (Dead Sea basin)

This study establishes for the first time the chronology and limnological history of Lake Amora (Dead Sea basin, Israel), whose deposits (the Amora Formation) comprise one of the longest exposed lacustrine records of the Pleistocene time. The Amora Formation consists of sequences of laminated primary aragonite and silty-detritus, Ca-sulfate minerals, halite and clastic units. This sedimentary sequence was uplifted and tilted by the rising Sedom salt diapir, exposing ∼320 m of sediments on the eastern flanks of Mt. Sedom (the Arubotaim Cave (AC) section).The chronology of the AC section is based on U-disequilibrium dating (²³⁰Th-²³⁴U and ²³⁴U-²³⁸U ages) combined with floating δ¹⁸O stratigraphy and paleomagnetic constraints. The determination of the ²³⁰Th-²³⁴U ages required significant corrections to account for detrital Th and U. These corrections were performed on individual samples and on suites of samples from several stratigraphic horizons. The most reliable corrected ages were used to construct an age-elevation model that was further tuned to the oxygen isotope record of east Mediterranean foraminifers (based on the long-term similarity between the sea and lake oxygen isotope archives).The combined U-series-δ¹⁸O age-elevation model indicates that the (exposed) Amora sequence was deposited between ∼740 and 70 ka, covering seven glacial-interglacial cycles (Marine Isotope Stages (MIS) 18 to 5).Taking the last glacial Lake Lisan and the Holocene Dead Sea lacustrine systems as analogs of the depositional-limnological environment of Lake Amora, the latter oscillated between wet (glacial) and more arid (interglacial) conditions, represented by sequences of primary evaporites (aragonite and gypsum that require enhanced supply of freshwater to the lakes) and clastic sediments, respectively. The lake evolved from a stage of rapid shifts between high and low-stand conditions during ∼740 to 550 ka to a sabkha-like environment that existed (at the AC site) between 550 and 420 ka. This stage was terminated by a dry spell represented by massive halite deposition at 420 ka (MIS12-11). During MIS10-6 the lake fluctuated between lower and higher stands reaching its highest stand conditions at the late glacial MIS6, after which a significant lake level decline corresponds to the transition to the last interglacial (MIS5) low-stand lake, represented by the uppermost part of the Formation.δ¹⁸O values in the primary aragonite range between 6.0 and −1.3‰, shifting cyclically between glacial and interglacial intervals. The lowest δ¹⁸O values are observed during interglacial stages and may reflect short and intense humid episodes that intermittently interrupted the overall arid conditions. These humid episodes, expressed also by enhanced deposition of travertines and speleothems, seem to characterize the Negev Desert, and in contrast to the overall dominance of the Atlantic-Mediterranean system of rain patterns in the Dead Sea basin, some humid episodes during interglacials may be traced to southern sources.     

Cretaceous seamounts along the continent-ocean transition of the Iberian margin: U-Pb ages and Pb-Sr-Hf isotopes

To elucidate the age and origin of seamounts in the eastern North Atlantic, 54 titanite and 10 zircon fractions were dated by the U-Pb chronometer, and initial Pb, Sr, and Hf isotope ratios were measured in feldspars and zircon, respectively. Rocks analyzed are essentially trachy-andesites and trachytes dredged during the “Tore Madeira” cruise of the Atalante in 2001. The ages reveal different pulses of alkaline magmatism occurring at 104.4 ± 1.4 (2σ) Ma and 102.8 ± 0.7 Ma on the Sponge Bob seamount, at 96.3 ± 1.0 Ma on Ashton seamount, at 92.3 ± 3.8 Ma on the Gago Coutinho seamount, at 89.3 ± 2.3 Ma and 86.5 ± 3.4 Ma on the Jo Sister volcanic complex, and at 88.3 ± 3.3 Ma, 88.2 ± 3.9, and 80.5 ± 0.9 Ma on the Tore locality. No space-time correlation is observed for alkaline volcanism in the northern section of the Tore-Madeira Rise, which occurred 20-30 m.y. after opening of the eastern North Atlantic. Initial isotope signatures are: 19.139-19.620 for ²⁰⁶Pb/²⁰⁴Pb, 15.544-15.828 for ²⁰⁷Pb/²⁰⁴Pb, 38.750-39.936 for ²⁰⁸Pb/²⁰⁴Pb, 0.70231-0.70340 for ⁸⁷Sr/⁸⁶Sr, and +6.9 to +12.9 for initial epsilon Hf. These signatures are different from Atlantic MORB, the Madeira Archipelago and the Azores, but they lie in the field of worldwide OIB. The Cretaceous seamounts therefore seem to be generated by melts from a OIB-type source that interact with continental lithospheric mantle lying formerly beneath Iberia and presently within the ocean-continent transition zone. Inheritance in zircon and high ²⁰⁷Pb of initial Pb substantiate the presence of very minor amounts of continental material in the lithospheric mantle. A long-lived thermal anomaly is the most plausible explanation for alkaline magmatism since 104 Ma and it could well be that the same anomaly is still the driving force for tertiary and quaternary alkaline magmatism in the eastern North Atlantic region. This hypothesis is agreement with the plate-tectonic position of the region since Cretaceous time, including an about 30° anti-clockwise rotation of Iberia.     

Ar-Ar ages of martian nakhlites

We report ³⁹Ar-⁴⁰Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of ³⁹Ar recoil and variable amounts of trapped ⁴⁰Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2σ errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial ⁴⁰Ar on weathered grain surfaces that was degassed along with the first several percent of ³⁹Ar. A total K-⁴⁰Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2σ). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of ∼12 Ma is also similar to CRE ages for other nakhlites.     

ESR,U-series and paleomagnetic dating of Gigantopithecus fauna from Chuifeng Cave,Guangxi, southern China

Several Gigantopithecus faunas associated with taxonomically undetermined hominoid fossils and/or stone artifacts are known from southern China. These faunas are particularly important for the study of the evolution of humans and other mammals in Asia. However, the geochronology of the Gigantopithecus faunas remains uncertain. In order to solve this problem, a program of geochronological studies of Gigantopithecus faunas in Guangxi Province was recently initiated. Chuifeng Cave is the first studied site, which yielded 92 Gigantopithecus blacki teeth associated with numerous other mammalian fossils. We carried out combined ESR/U-series dating of fossil teeth and sediment paleomagnetic studies. Our ESR results suggest that the lower layers at this cave can be dated to 1.92 ± 0.14 Ma and the upper layers can be dated to older than 1.38 ± 0.17 Ma. Correlation of the recognized magnetozones to the geomagnetic polarity timescale was achieved by combining magnetostratigraphic, biostratigraphic and ESR data. The combined chronologies establish an Olduvai subchron (1.945–1.778 Ma) for the lowermost Chuifeng Cave sediments. We also analyzed the enamel δ¹³C values of the Gigantopithecus faunas. Our results show that southern China was dominated by C₃ plants during the early Pleistocene and that the Gigantopithecus faunas lived in a woodland-forest ecosystem.     

Sulfur and carbon isotope records from 1700 to 800 Ma carbonates of the Jixian section, northern China: Implications for secular isotope variations in Proterozoic seawater and relationships to global supercontinental events

This study is a comprehensive, stable isotope survey of the marine carbonate-dominated, upper Paleo- to lower Neoproterozoic stratigraphy of Jixian County, China. Carbonate-associated sulfate (CAS) was extracted and measured for δ³⁴S_CAS using the same samples analyzed for δ¹³C_carbonate. This integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of Proterozoic seawater. We specifically sampled marine carbonate intervals from the lower section of the Chuanlinggou Formation, Changcheng Group (ca. 1700 Ma) to the top of the Jingeryu Formation, Qingbaikou Group (ca. 800 Ma). δ¹³C_carbonate values are mostly negative in the upper Paleoproterozoic Changcheng Group, with an ascending trend from −3‰ to 0‰. We observed variation of approximately 0 ± 1‰ in the Mesoproterozoic Jixian Group, and positive values of +2 ± 2‰ characterize the lower Neoproterozoic Qingbaikou Group. Stratigraphic variations in δ³⁴S_CAS are more remarkable in their ranges and magnitudes, including conspicuously high values exceeding +30‰ in the three intervals at ca. 1700 Ma, 1300-1100 Ma, and 1000-900 Ma. In the Changcheng Group, δ³⁴S_CAS values are typically higher than +25‰, with only a few values of less than +15‰. In contrast, most of the data spanning from the Mesoproterozoic Tieling Formation of the Jixian Group to the lower Neoproterozoic Jingeryu Formation of the Qingbaikou Group are highly variable between +10‰ and +25‰, with some values exceeding +25‰.In the late Paleoproterozoic (1700-1600 Ma), a >10‰ decrease in δ³⁴S_CAS and ∼3‰ increase in δ¹³C_carbonate are coincident with, and likely related to, the breakup of Columbia, a supercontinent that predated Rodinia. Carbon and sulfur isotope data from the Mesoproterozoic, when global tectonic activity was comparatively weaker, fall mostly in the ranges of +15 ± 10‰ and 0 ± 1‰, respectively, but fluctuations of >20‰ for δ³⁴S_CAS and >3‰ for the δ¹³C_carbonate at ca. 1450-1400 Ma may reflect subduction and large-scale magmatic activity in island arcs marking the end of Columbia breakup. From the late Mesoproterozoic (ca. 1300-1100 Ma) to the early Neoproterozoic (ca. 800 Ma), the δ¹³C and δ³⁴S of seawater increased gradually with increasing variability. Most impressive areδ³⁴S_CAS values that exceed +30‰ in two intervals at ca. 1300-1100 Ma and ca. 1000-900 Ma, which may reflect the assembly and early breakup of Rodinia. Although gaps in the record remain, and studies of even higher resolution are warranted, our results suggest that changes in paleoceanographic conditions linked to global tectonics strongly influenced the biogeochemical cycles of C and S. Furthermore, periods of the Proterozoic previously noted for their isotopic invariability show clear isotopic expressions of this tectonic activity.     

Ar-Ar and I-Xe ages and thermal histories of three unusual metal-rich meteorites

Portales Valley, Sombrerete, and Northwest Africa (NWA) 176 are three unrelated meteorites, which consist of silicate mixed with substantial amounts of metal and which likely formed at elevated temperatures as a consequence of early impacts on their parent bodies. Measured ³⁹Ar-⁴⁰Ar ages of these meteorites are 4477 ± 11 Ma and 4458 ± 16 Ma (two samples of Portales Valley), 4541 ± 12 Ma, and 4524 ± 13 Ma, respectively (Ma = million years; all one-sigma errors). The Ar-Ar data for Portales Valley show no evidence of later open system behavior suggested by some other chronometers. Measured ¹²⁹I-¹²⁹Xe ages of these three meteorites are 4559.9 ± 0.5 Ma, 4561.9 ± 1.0 Ma, and ∼4544 Ma, respectively (relative to Shallowater = 4562.3 ± 0.4 Ma). From stepwise temperature release data, we determined the diffusion characteristics for Ar and Xe in our samples and calculated approximate closure temperatures for the K-Ar and I-Xe chronometers. Adopting results and interpretations about these meteorites from some previous workers, we evaluated all these data against various thermal cooling models. We conclude that Portales Valley formed 4560 Ma ago, cooled quickly to below the I-Xe closure temperature, then cooled deep within the parent body at a rate of ∼4 °C/Ma through K-Ar closure. We conclude that Sombrerete formed 4562 Ma ago and cooled relatively quickly. NWA 176 likely formed and cooled quickly ∼4544 Ma ago, or later than formation times of most meteorite parent bodies. For all three meteorites, the Ar-Ar ages are in better agreement with I-Xe ages and preferred thermal models if we increase these Ar-Ar ages by ∼20 Ma. Such age corrections would be consistent with probable errors in ⁴⁰K decay parameters in current use, as suggested by others. The role of impact heating and possible disruption and partial reassembly of meteorite parent bodies to form some meteorites likely was an important process in the early solar system.     

Re-Os and Mo isotope systematics of black shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, northern Australia

Joint application of the Mo isotope paleoredox proxy and Re-Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re-Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, northern Australia. New Re-Os ages of 1361 ± 21 Ma (2σ, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2σ, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105-119 ppm) and degree of pyritization [DOP] values (0.90-0.92) implying quantitative conversion of molybdate (MoO₄²⁻) to thiomolybdate (MoS₄²⁻) in overlying bottom waters. The average δ^97/95Mo (0.72 ± 0.10‰, 2σ, n = 6) of these shales is consistent with previous data, but represents a significantly more precise determination for global seawater δ^97/95Mo at 1.4 Ga. This value is lighter than present-day seawater by ∼0.85‰ and reflects expanded strongly euxinic deep ocean conditions ([H₂S]_aq > 11 μM) relative to oxic, suboxic, and weakly/intermittently euxinic ([H₂S]_aq < 11 μM) marine deposition in the 1.4 Ga oceans. Mass-balance modelling suggests Mo removal into strongly euxinic and oxic sediments may have comprised 30-70% and less than 15%, respectively, of the oceanic Mo sink at 1.4 Ga as opposed to 5% and 35% today, respectively.The Re-Os radioisotope system in organic-rich shales serves as a test for post-depositional alteration that could affect the usefulness of paleoredox tracers such as Mo stable isotopes. Re-Os isotope data for the Wollogorang Formation black shales are scattered and yield a highly imprecise date of 1359 ± 150 Ma (2σ, n = 21, MSWD = 85, Model 3). This age is younger than U-Pb zircon ages from interbedded tuffs that constrain the age of deposition at ca. 1730 Ma. In conjunction with previous petrological, geochemical, and paleomagnetic data, the Re-Os isotope data suggest hydrothermal fluid flow through the Wollogorang Formation, possibly associated with formation of the ca. 1640 Ma McArthur River Pb-Zn-Ag sedimentary exhalative deposit, resulted in post-depositional mobilization of Re and Os. Based on the degree of deviation of the Re-Os data from a 1730 Ma reference line, open-system behavior of Re and Os was greatest near the base of the black shale unit. Wollogorang Formation black shales are enriched in Mo (41-58 ppm), but are characterized by variable δ^97/95Mo (0.3-0.8‰) and DOP (0.57-0.92). The lightest δ^97/95Mo values occur near the base of the black shale unit. Based on the Re-Os systematics, hydrothermal fluids have probably overprinted the authigenic δ^97/95Mo signature in those shales. However, the heaviest δ^97/95Mo values in the Wollogorang Formation come from stratigraphically higher shales, and are similar to those observed for the Velkerri Formation, and thus may reflect seawater δ^97/95Mo at 1.73 Ga.     

Holocene rockfalls in the southern Negev Desert,Israel and their relation to Dead Sea fault earthquakes

Rockfall ages in tectonically active regions provide information regarding frequency and magnitude of earthquakes. In the hyper-arid environment of the Dead Sea fault (DSF), southern Israel, rockfalls are most probably triggered by earthquakes. We dated rockfalls along the western margin of the DSF using terrestrial cosmogenic nuclides (TCN). At each rockfall site, samples were collected from simultaneously exposed conjugate boulders and cliff surfaces. Such conjugate samples initially had identical pre-fall (“inherited”) TCN concentrations. After boulder detachment, these surfaces were dosed by different production rates due to differences in post-fall shielding and geometry. However, in our study area, pre-rockfall inheritance and post-rockfall production rates of TCN cannot be evaluated. Therefore, we developed a numerical approach and demonstrated a way to overcome the above-mentioned problems. This approach can be applied in other settings where rockfalls cannot be dated by simple exposure dating. Results suggest rockfall ages between 3.6 ± 0.8 and 4.7 ± 0.7 ka. OSL ages of sediment accumulated behind the boulders range between 0.6 ± 0.1 and 3.4 ± 1.4 ka and support the TCN results. Our ages agree with dated earthquakes determined in paleoseismic studies along the entire length of the DSF and support the observation of intensive earthquake activity around 4–5 ka.     

Zircon U-Pb and Lu-Hf isotopic systematics of the Daping plutonic rocks: Implications for the Neoproterozoic tectonic evolution of the northeastern margin of the Indochina block, Southwest China

The Daping Neoproterozoic plutonic rocks at the northeastern margin of Indochina block in southwest China provide an ideal opportunity for studying the tectonic setting and relationship between the Indochina and Yangtze Blocks. LA-ICP-MS U-Pb dating on the zircon cores and rims of a hornblende-gabbro yield ²⁰⁶Pb/²³⁸U weighted means ages of 873 ± 9.1 Ma and 769 ± 7 Ma, respectively, and that for cores, mantles and rims of a granodiorite yield ²⁰⁶Pb/²³⁸U weighted means ages of 981-987 Ma, 829 ± 10 Ma and 761 ± 11 Ma, respectively. The zircon cores and mantles are interpreted as inherited from their source region. The zircon rims are magmatic, their ages represent the emplacement timing. The zircon cores and rims from the hornblende-gabbro have ε_Hf^(t) values ranging from − 5.0 to − 5.8 and + 0.6 to + 6.4. Corresponding single-stage model ages range from 1626 to 1662 Ma and 1094 to 1311 Ma, respectively. For the granodiorite, the inherited mantles (including cores) show two groups: (1) ε_Hf^(t) values of + 3.3 to + 12.3 with single-stage Hf model ages of 897 to 1235 Ma; and (2) ε_Hf^(t) values of − 1.9 to − 7.8 with single stage model ages of 1470-1667 Ma. The zircon rims are characterized by positive ε_Hf^(t) values (+ 5.4 to + 8.2) with single-stage model ages ranging from 977 to 1108 Ma. Whole-rock geochemical data for the hornblende-gabbro, such as enrichment of LILE and LREE, negative anomaly of Nb and Ta, and high Mg^# (52.1-65.4), suggest magma generation in a subduction-related setting. An island-arc affinity is strongly supported by the features of high-alumina basalt and abundant hornblende in a large hornblende-gabbro sill. The granodiorites are characterized by high Sr contents and Sr/Y ratios, strong enrichment of LILE and LREE, and negative anomaly of Nb, Ta, P and Ti, comparable with the features of subduction-related plutonic rocks. These data show that the hornblende-gabbro was generated by the partial melting of a metasomatized mantle wedge peridotite with contribution from aqueous fluids derived from a subducted slab. The granodiorite magma is a product of the mixing of mafic magma produced by partial melting of a slab-fluid-enriched metasomatized mantle wedge peridotite and felsic magma formed by the partial melting of crustal materials. The emplacement ages and geochemical features of subduction-related Daping plutonic rocks are the same as those reported from the western margin of the Yangtze block, suggesting the presence of an oceanic crust in between, with subduction to either side generating island-arc magmatism in the Neoproterozoic.     

U-Pb ages of angrites

Precise U-Pb ages, determined with double spike (²⁰²Pb-²⁰⁵Pb) thermal ionization m1ass spectrometry, are reported for angrites Angra dos Reis (AdoR), Lewis Cliff 86010 (LEW), and D’Orbigny. Nineteen of 23 acid-washed pyroxene fractions from these meteorites and whole rock fractions from D’Orbigny contain between 0.5 and 1.3 pg of total common Pb, indistinguishable from analytical blank. Measured ²⁰⁶Pb/²⁰⁴Pb ratios in these fractions are between 6300 and 14,100 for AdoR, 1160-4500 for LEW, and 608-8500 for D’Orbigny. Blank-corrected ²⁰⁶Pb/²⁰⁴Pb ratios for all three meteorites vary from 2160 to over 100,000. These fractions yielded precise and reproducible ²⁰⁷Pb^∗/²⁰⁶Pb^∗ dates with the average values of 4557.65 ± 0.13 Ma for AdoR, 4558.55 ± 0.15 Ma for LEW, and 4564.42 ± 0.12 Ma for D’Orbigny. Pb-Pb isochrons including data with slightly elevated common Pb, and U-Pb upper concordia intercepts, yield similar dates. The implications of these new Pb-isotopic ages of angrites are threefold. First, they demonstrate that AdoR and LEW are not coeval, and the group of “slowly cooled” angrites is therefore genetically diverse. Second, the new age of LEW suggests an upward revision of ⁵³Mn-⁵³Cr “absolute” ages by 0.7 Ma. Third, a precise age of D’Orbigny allows consistent linking of the ⁵³Mn-⁵³Cr and ²⁶Al-²⁶Mg extinct nuclide chronometers to the absolute lime scale.     

Precise U–Pb baddeleyite ages of mafic dykes and intrusions in southern West Greenland and implications for a possible reconstruction with the Superior craton

The Archaean block of southern Greenland constitutes the core of the North Atlantic craton (NAC) and is host to a large number of Precambrian mafic intrusions and dyke swarms, many of which are regionally extensive but poorly dated. For southern West Greenland, we present a U–Pb zircon age of 2990 ± 13 Ma for the Amikoq mafic–ultramafic layered intrusion (Fiskefjord area) and four baddeleyite U–Pb ages of Precambrian dolerite dykes. Specifically, a dyke located SE of Ameralik Fjord is dated at 2499 ± 2 Ma, similar to a previously reported ⁴⁰Ar/³⁹Ar age of a dyke in the Kangâmiut area. For these and related intrusions of ca. 2.5 Ga age in southern West Greenland, we propose the name Kilaarsarfik dykes. Three WNW-trending dykes of the MD3 swarm yield ages of 2050 ± 2 Ma, 2041 ± 3 Ma and 2029 ± 3 Ma. A similar U–Pb baddeleyite age of 2045 ± 2 Ma is also presented for a SE-trending dolerite (Iglusuataliksuak dyke) in the Nain Province, the rifted western block of the NAC in Labrador. We speculate that the MD3 dykes and age-equivalent NNE-trending Kangâmiut dykes of southern West Greenland, together with the Iglusuataliksuak dyke (after closure of the Labrador Sea) represent components of a single, areally extensive, radiating swarm that signaled the arrival of a mantle plume centred on what is presently the western margin of the North Atlantic craton. Comparison of the magmatic ‘barcodes’ from the Nain and Greenland portions of the North Atlantic craton with the established record from the north-eastern Superior craton shows matches at 2500 Ma, 2214 Ma, 2050–2030 Ma and 1960–1950 Ma. We use these new age constraints, together with orientations of the dyke swarms, to offer a preliminary reconstruction of the North Atlantic craton near the north-eastern margin of the Superior craton during the latest Archaean and early Palaeoproterozoic, possibly with the Core Zone craton of eastern Canada intervening.     

Asymmetry and basin migration in the dead sea rift

The Dead Sea depression sensu stricto, forms the deepest continental part of the Dead Sea rift, a transfer which separates the Levanthine and Arabian plates. It is occupied by three distinct sedimentary bodies, deposited in basins whose depocenters are displaced northward with time. They are: the continental red beds of the Hazeva Formation (Miocene), the Bira-Lido-Gesher marls and the exceptionally thick rocksalt of the Sedom Formation (Pliocene—Early Pleistocene), and the successive Amora, Lisan and Dead Sea evaporites and clastics (Early Pleistocene—Recent). Lengthwise and crosswise asymmetries of these sedimentary basins and their respective depocenters are due to: leftlateral shear combined with anticlockwise rotation of the Arabian (eastern) plate; steeper faulting of the crustal eastern margin than of the western sedimentary margin, and modification of depositional pattern by twice filling up of basins, by Hazeva red beds during Late Miocene pause of shear and by Sedom rocksalt during Pliocene marine ingression.     

Mn-Cr systematics of carbonates in CM chondrites: Implications for the timing and duration of aqueous alteration

CM chondrites contain carbonates and other secondary minerals such as phyllosilicates, sulfides, sulfates, oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid. We report in situ Mn-Cr isotope measurements in the highly aqueously altered CM2.1 chondrites QUE 93005 and ALH 83100 using secondary ion mass spectrometry (Cameca ims-1270 ion microprobe). The ⁵³Cr excesses are correlated with the ⁵³Mn/⁵⁵Mn ratio and result from the in situ decay of ⁵³Mn, a short-lived radioisotope with a half-life of 3.7 Ma. If we assume that carbonate grains in samples QUE 93005 and ALH 83100 are cogenetic, then the excesses define initial ⁵³Mn/⁵⁵Mn ratios ((⁵³Mn/⁵⁵Mn)₀) of (4.1 ± 1.2) × 10⁻⁶ and (5.1 ± 1.7) × 10⁻⁶, respectively. These values are comparable to those in carbonates from other CM chondrites as reported in the literature. Initial ⁵³Mn/⁵⁵Mn ratios for calculated model isochrones for individual carbonate grains range from (3.8 ± 1.4) × 10⁻⁶ to (4.8 ± 2.1) × 10⁻⁶ for QUE 93005 and from (3.1 ± 1.6) × 10⁻⁶ to (1.3 ± 0.5) × 10⁻⁵ for ALH 83100. A possible interpretation for the ranges in (⁵³Mn/⁵⁵Mn)₀ could be that alteration in individual CM chondrites was episodic and occurred over an extended period of time. However, isochrones based on the entire set of carbonate grains in each of the CM chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in CM chondrites of different subtypes and that alteration on the CM parent asteroid started contemporaneously with or shortly after CAI formation and lasted at least 4 Ma.     

The geochemical behavior and isotopic composition of Hg in a mid-Pleistocene western Mediterranean sapropel

The concentrations of mercury (Hg) and other trace metals (Ni, Cu, Zn, Mo, Ba, Re, U) and the Hg isotopic composition were examined across a dramatic redox and productivity transition in a mid-Pleistocene Mediterranean Sea sapropel sequence. Characteristic trace metal enrichment in organic-rich layers was observed, with organic-rich sapropel layers ranging in Hg concentration from 314 to 488 ng/g (avg = 385), with an average enrichment in Hg by a factor of 5.9 compared to organic-poor background sediments, which range from 39 to 94 ng/g Hg (avg = 66). Comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers Hg to the seafloor. Near complete scavenging of Hg from the water column renders the sapropel Hg isotopic composition representative of mid-Pleistocene Mediterranean seawater. Sapropels have an average δ²⁰²Hg value of −0.91‰ ± 0.15‰ (n = 5, 1 SD) and Δ¹⁹⁹Hg value of 0.11‰ ± 0.03‰ (n = 5, 1 SD). Background sediments have an average δ²⁰²Hg of −0.76‰ ± 0.16‰ (n = 5, 1 SD) and Δ¹⁹⁹Hg of 0.05‰ ± 0.01‰ (n = 5, 1 SD), which is indistinguishable from the sapropel values. We suggest that the sapropel isotopic composition is most representative of the mid-Pleistocene Tyrrhenian Sea.     

Sequestration mechanisms and anthropogenic inputs of rhenium in sediments from Eastern Canada lakes

The concentrations of Re, as well as those of several other geochemical variables, were measured in dated sediment cores and in porewater samples from four lacustrine basins in Eastern Canada: one, perennially oxic, located 40 km from Québec City and three, seasonally anoxic, located within 25 km of non-ferrous metal smelters. The drainage basins of these lakes are uninhabited and have not been affected by human activity or wildfires. All of the depth profiles of dissolved Re indicate: higher Re concentrations in the water overlying the sediment than in the porewater; diffusion of Re across the sediment-water interface; a progressive decrease in porewater Re concentrations to reach minimum values of ∼0.5 pM within a 10-cm sediment depth interval. Modeling of these Re porewater profiles with a one-dimensional transport-reaction equation indicates that Re is removed from porewater within this depth interval. Based on thermodynamic predictions of Re speciation and of saturation states and on comparison of these predictions with sulfide porewater profiles, we infer that Re is removed from porewater by precipitation of rheniite (ReS_2(s)). The rate constant for the formation of ReS_2(s) in sediments is estimated from the modeling exercise to be 0.51 ± 0.64 × 10⁻²¹ mol cm⁻³ s⁻¹. Accumulation of sedimentary Re shows a strong authigenic component, as in anoxic marine sediments. Sharp increases in solid-phase Re during the last century are attributed to atmospheric deposition of anthropogenic Re deriving from coal burning and nearby smelter emissions.     

Carbon and oxygen stable isotope compositions of late Pleistocene mammal teeth from dolines of Ajoie (Northwestern Switzerland)

Fossils of megaherbivores from eight late Pleistocene ¹⁴C- and OSL-dated doline infillings of Ajoie (NW Switzerland) were discovered along the Transjurane highway in the Swiss Jura. Carbon and oxygen analyses of enamel were performed on forty-six teeth of large mammals (Equus germanicus, Mammuthus primigenius, Coelodonta antiquitatis, and Bison priscus), coming from one doline in Boncourt (~ 80 ka, marine oxygen isotope stage MIS5a) and seven in Courtedoux (51–27 ka, late MIS3), in order to reconstruct the paleoclimatic and paleoenvironmental conditions of the region. Similar enamel δ¹³C values for both periods, ranging from − 14.5 to − 9.2‰, indicate that the megaherbivores lived in a C₃ plant-dominated environment. Enamel δ¹⁸O_PO4 values range from 10.9 to 16.3‰ with a mean of 13.5 ± 1.0‰ (n = 46). Mean air temperatures (MATs) were inferred using species-specific δ¹⁸O_PO4–δ¹⁸O_H2O-calibrations for modern mammals and a present-day precipitation δ¹⁸O_H2O-MAT relation for Switzerland. Similar average MATs of 6.6 ± 3.6°C for the deposits dated to ~ 80 ka and 6.5 ± 3.3°C for those dated to the interval 51–27 ka were estimated. This suggests that these mammals in the Ajoie area lived in mild periods of the late Pleistocene with MATs only about 2.5°C lower than modern-day temperatures.     

Trace element systematics and Sm-Nd and Lu-Hf ages of Larkman Nunatak 06319: Closed-system fractional crystallization of an enriched shergottite magma

Combined ¹⁴⁷Sm-¹⁴³Nd and ¹⁷⁶Lu-¹⁷⁶Hf chronology of the martian meteorite Larkman Nunatak (LAR) 06319 indicates an igneous crystallization age of 193 ± 20 Ma (2σ weighted mean). The individual ¹⁴⁷Sm-¹⁴³Nd and ¹⁷⁶Lu-¹⁷⁶Hf internal isochron ages are 183 ± 12 Ma and 197 ± 29 Ma, respectively, and are concordant with two previously determined ¹⁴⁷Sm-¹⁴³Nd and ⁸⁷Rb-⁸⁷Sr internal isochron ages of 190 ± 26 Ma and 207 ± 14 Ma, respectively (Shih et al., 2009). With respect to the ¹⁴⁷Sm-¹⁴³Nd isotope systematics, maskelynite lies above the isochron defined by primary igneous phases and is therefore not in isotopic equilibrium with the other phases in the rock. Non-isochronous maskelynite is interpreted to result from shock-induced reaction between plagioclase and partial melts of pyroxene and phosphate during transformation to maskelynite, which resulted in it having unsupported ¹⁴³Nd relative to its measured ¹⁴⁷Sm/¹⁴⁴Nd ratio. The rare earth element (REE) and high field strength element (HFSE) compositions of major constituent minerals can be modeled as the result of progressive crystallization of a single magma with no addition of secondary components. The concordant ages, combined with igneous textures, mineralogy, and trace element systematics indicate that the weighted average of the radiometric ages records the true crystallization age of this rock. The young igneous age for LAR 06319 and other shergottites are in conflict with models that advocate for circa 4.1 Ga crystallization ages of shergottites from Pb isotope compositions, however, they are consistent with updated crater counting statistics indicating that young volcanic activity on Mars is more widespread than previously realized (Neukum et al., 2010).     

Ancient relative and absolute ages for a basaltic meteorite: Implications for timescales of planetesimal accretion and differentiation

Asuka 881394 is a unique basaltic meteorite that originated in the crust of a differentiated planetesimal in the early Solar System. We present high precision Pb, Mg, and Cr isotopic compositions of bulk samples and mineral separates from this achondrite. A ²⁰⁷Pb-²⁰⁶Pb internal isochron obtained from the radiogenic pyroxene and whole-rock fractions of Asuka 881394 yields an absolute age of 4566.5 ± 0.2 Ma, which we consider to be the best estimate for the crystallization age of this basaltic achondrite. The ²⁶Al-²⁶Mg systematics show some evidence of disturbance, but 5 of the 6 analyzed whole-rock and mineral fractions define an isochron corresponding to a ²⁷Al/²⁶Al ratio of (1.28 ± 0.07) × 10⁻⁶. Comparison with the ²⁶Al-²⁶Mg and Pb-Pb systematics in the D’Orbigny achondrite translates to a ²⁶Al-²⁶Mg age of 4565.4 ± 0.2 Ma for Asuka 881394. The ⁵³Mn-⁵³Cr systematics in whole-rock, silicate and chromite fractions correspond to a ⁵³Mn/⁵⁵Mn ratio of (3.85 ± 0.23) × 10⁻⁶. Compared to the most precise ⁵³Mn-⁵³Cr and Pb-Pb systematics available for the D’Orbigny angrite, this translates to a ⁵³Mn-⁵³Cr age of 4565.3 ± 0.4 Ma; similarly, a comparison with the NWA 4801 angrite yields a ⁵³Mn-⁵³Cr age of 4565.5 ± 0.4 Ma, in agreement with the age obtained relative to D’Orbigny. While the ²⁶Al-²⁶Mg and ⁵³Mn-⁵³Cr ages appear to be concordant in Asuka 881394, these ages are ∼1 Ma younger than its ²⁰⁷Pb-²⁰⁶Pb age. This discordance might have been caused by one or more of several reasons, including differences in the closure temperatures for Pb versus Cr and Mg diffusion in their host minerals combined with slow cooling of the parent body as well as differential resetting of isotopic systems by a process other than volume diffusion, e.g., shock metamorphism. The ancient age of Asuka 881394 suggests that basaltic volcanism on its parent planetesimal occurred within ∼3 Ma of the formation of earliest solids in the Solar System, essentially contemporaneously with chondrule formation. This requires that the Asuka 881394 parent body was fully accreted within ∼500,000 yrs of Solar System formation.     

Effects of aridity and vegetation on plant-wax δD in modern lake sediments

We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (ε_a) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of ²H and ¹⁸O associated with evaporation of lake waters. The most negative ε_a values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum ε_a values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0-2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ¹⁸O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau.     

Paleozoic ages and excess Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from Ar/Ar dating by stepwise crushing

The ⁴⁰Ar/³⁹Ar stepwise crushing technique is applied for the first time to date garnet from ultra-high-pressure metamorphic (UHPM) eclogites. Three garnet samples from the Bixiling eclogites analyzed by ⁴⁰Ar/³⁹Ar stepwise crushing yield regular, predictable age spectra, and a clear separation between excess ⁴⁰Ar and concordant plateau and isochron ages. All three age spectra begin with high apparent ages followed by step by step decreasing ages, and finally age plateaux with apparent ages in the range from 427 ± 20 to 444 ± 10 Ma. The data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 ± 34 to 459 ± 58 Ma, corresponding to initial ⁴⁰Ar/³⁶Ar ratios from 292.1 ± 4.5 to 294.5 ± 6.7, statistically indistinguishable from the modern air. The high initial ages are interpreted to derive from secondary fluid inclusions containing excess ⁴⁰Ar, whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess ⁴⁰Ar. The plateau ages are interpreted to approximate the time of garnet growth during initial UHPM metamorphism. Phengite analyzed by laser stepwise heating yielded a complicated two-saddle age spectrum with a scattered isochron corresponding to age of 463 ± 116 Ma and initial ⁴⁰Ar/³⁶Ar ratio of 1843 ± 1740 indicative of the presence of extraneous ⁴⁰Ar within phengite. These concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that Dabie UHPM eclogites were first formed in the early Paleozoic, during the same event that caused the Qinling-Northern Qaidam Basin-Altyn Tagh eclogites.