Mo isotope composition in Mo-rich high- and low-T hydrothermal systems from the Swiss Alps

We analyzed the molybdenum (Mo) isotope compositions (IC) of 59 samples from two molybdenite mineralizations (Alpjahorn and Grimsel) and from a Mo-rich hydrothermal breccia (Grimsel) from the Aar Massif, Switzerland. The formation temperature of the Late Paleozoic Mo mineralizations (300-600 °C) is much higher than that of the Pliocene breccia (100-160 °C). The Mo IC of the molybdenites varies over 1.35‰. Even in a single hand specimen it spans 0.45‰, indicating that fractionation processes during molybdenite precipitation can vary on a cm scale. The Mo IC of most molybdenites analyzed here are significantly heavier than that of the host rock (δ^98/95Mo = (0.05 ± 0.1)‰) and show a bimodal distribution centered around δ^98/95Mo ≈ 1.1‰ and 0.2‰. This result rules out single stage Rayleigh fractionation as the relevant formation mechanism and instead, redox variations are suggested to be a main factor controlling the Mo IC of the studied high-temperature Mo deposits. The range of the Mo IC in one single deposit, the Alpjahorn, overlaps with the variation range of almost all other published values for Mo IC in Mo deposits. Compared to the molybdenites, the breccia shows an even wider variation of 3.0‰ (δ^98/95Mo between −1.6‰ and +1.4‰). In contrast to the high-T molybdenite deposits, here the Mo was transported via oxidized surface waters into the breccia system, where it was reduced and precipitated. This indicates that oxidation and reduction of Mo complexes may lead to highly variable Mo IC in hydrothermal systems.     

HP–UHP metamorphism as an indicator of slab dip variations in the Alpine arc

HP/UHP and LT metamorphic units that commonly occur in the inner parts of mountain belts result from the subduction of continental and oceanic material, most often exhumed prior to continental collision. The prograde pressure–temperature history of HP–UHP rocks strongly depends on the convergence rate and on the subduction zone geometry. The maximum pressure recorded provides a proxy for the depth of shearing off and stacking of HP metamorphic nappes. A 2-D thermal model of continental subduction at lithospheric scale is used to compute the length and pressure peak of detached HP metamorphic units as a function of the slab dip angle and the convergence rate. Model results are applied to the metamorphic nappe pile of the inner Alps. A mean convergence rate of 1 cm/year during the subduction of the Briançonnais terrane is indicated by the paleogeographic reconstructions between 46 and 38 Ma. On this basis, the available petrological data and lengths of metamorphic units are used to compute the variations of the slab dip angle. The slab dip angle is shown to increase, from the northeast to the southwest, along the Alpine arc with estimated values of 20° for Suretta, 30–45° for Monte Rosa and Gran Paradiso, and 60° for Dora Maira. From Eocene to Oligocene times, the increase in slab dip angle is controlled by changes of buoyancy, due to the spatial configuration of the Valaisan trough and the incoming of crustal material within the subduction zone.     

Molluskan species richness and endemism on New Caledonian seamounts: Are they enhanced compared to adjacent slopes?

Seamounts were often considered as ‘hotspots of diversity’ and ‘centers of endemism’, but recently this opinion has been challenged. After 25 years of exploration and the work of numerous taxonomists, the Norfolk Ridge (Southwest Pacific) is probably one of the best-studied seamount chains worldwide. However, even in this intensively explored area, the richness and the geographic patterns of diversity are still poorly characterized. Among the benthic organisms, the post-mortem remains of mollusks can supplement live records to comprehensively document geographical distributions. Moreover, the accretionary growth of mollusk shells informs us about the life span of the pelagic larva. To compare diversity and level of endemism between the Norfolk Ridge seamounts and the continental slopes of New Caledonia we used species occurrence data drawn from (i) the taxonomic literature on mollusks and (ii) a raw dataset of mainly undescribed deep-sea species of the hyperdiverse Turridae. Patterns of endemism and species richness were analyzed through quantitative indices of endemism and species richness estimator metrics. To date, 403 gastropods and bivalves species have been recorded on the Norfolk Ridge seamounts. Of these, at least 38 species (∼10%) are potentially endemic to the seamounts and nearly all of 38 species have protoconchs indicating lecithotrophic larval development. Overall, our results suggest that estimates of species richness and endemism, when sampling effort is taken into account, were not significantly different between slopes and seamounts. By including in our analyses 347 undescribed morphospecies from the Norfolk Ridge, our results also demonstrate the influence of taxonomic bias on our estimates of species richness and endemism.     

Holocene mass‐wasting events in Lago Fagnano,Tierra del Fuego (54°S): implications for paleoseismicity of the Magallanes‐Fagnano transform fault

High‐resolution seismic imaging and coring in Lago Fagnano, located along a plate boundary in Tierra del Fuego, have revealed a dated sequence of Holocene mass‐wasting events. These structures are interpreted as sediment mobilizations resulting from loading of the slope‐adjacent lake floor during mass‐flow deposition. More than 19 mass‐flow deposits have been identified, combining results from 800 km of gridded seismic profiles used to site sediment cores. Successions of up to 6‐m thick mass‐flow deposits, pond atop the basin floor and spread eastward and westward following the main axis of the eastern sub‐basin of Lago Fagnano. We developed an age model, on the basis of information from previous studies and from new AMS‐¹⁴C ages on cored sediments, which allows us to establish a well‐constrained chronologic mass‐wasting event‐catalogue covering the last ～12 000 years. Simultaneously triggered, basin‐wide lateral slope failure and the formation of multiple debris flow and postulated megaturbidite deposits are interpreted as the fingerprint of paleo‐seismic activity along the Magallanes‐Fagnano transform fault that runs along the entire lake basin. The slope failures and megaturbidites are interpreted as recording large earthquakes occurring along the transform fault since the early Holocene. The results from this study provide new data about the frequency and possible magnitude of Holocene earthquakes in Tierra del Fuego, which can be applied in the context of seismic hazard assessment in southernmost Patagonia.     

Late Mesozoic compressional to extensional tectonics in the Yiwulüshan massif,NE China and its bearing on the evolution of the Yinshan–Yanshan orogenic belt: Part I: Structural analyses and geochronological constraints

With a cratonic nucleus, the North China Craton (NCC) experienced a complex tectonic evolution with multiphase compressional and extensional events during Mesozoic times. Along the northern part of the NCC, the Yinshan–Yanshan fold and thrust belt was a typical intraplate orogen. Jurassic and Cretaceous continental sedimentation, magmatism, widespread intraplate characterize the Yinshan–Yanshan orogenic belt. The geodynamic significance of these tectonic events is still in dispute. In the western part of the Liaoning province, the Yiwulüshan massif crops out at the eastern end of the Yinshan–Yanshan orogenic belt. The Yiwulüshan massif presents an elliptical domal shape with a NE–SW striking long axis. The structural evolution of this massif brings new insights for the understanding of the Mesozoic plutonic–tectonic history of the NCC. A multidisciplinary study involving structural geology, geochronology, Anisotropy of Magnetic Susceptibility (AMS) and gravity modeling have been carried out. The presentation of the new results splits into two parts. Part I (this paper) deals with field and laboratory structural observations, and presents the main geochronological results. The AMS, gravity modeling data will be provided in a companion paper (Part II). The early compressional deformation (D₁) corresponds to a Late Jurassic to Early Cretaceous southward thrusting. The subsequent deformation is related to the Early Cretaceous exhumation of the Yiwulüshan massif. A detailed structural analysis allows us to distinguish several deformation events (D₂, D₃, and D₄). The Cretaceous extensional structures, such as syntectonic plutons bounded by ductile normal faults, metamorphic core complexes, and half-graben basins are recognized in many places in East Asia. These new data from the Yiwulüshan massif constitute a link between Transbaikalia, Mongolia, North China and South China, indicating that NW–SE extensional Mesozoic tectonics occurred throughout the entire region.     

Ecosystem Responses of the Subtropical Kaneohe Bay,Hawaii, to Climate Change: A Nitrogen Cycle Modeling Approach

The global coastal zone is characterized by high biological productivity and serves as an important channel through which materials are transferred from land to the open ocean, yet little is known how it will be affected by climate change. Here, we use Kaneohe Bay, Hawaii, a semi-enclosed subtropical embayment partially surrounded by a mountainous watershed and fed by river runoff as an example to explore the potential impact of climate change on the pelagic and benthic cycling of nitrogen. We employ a nine-compartment nitrogen cycle biogeochemical box model and perturb it with a set of four idealized climate scenarios. We find that hydrological changes play a dominant role in determining the ecosystem structure, while temperature changes are more important for the trophic state and stability of the ecosystem. The ecosystem stability against storm events does not significantly change under any scenario. The system remains autotrophic in the future; however, it becomes significantly less autotrophic under drier climate, while it turns slightly more autotrophic under wetter climate. These findings may have implications for other high island watershed and coastal ecosystems in the tropics and subtropics.     

A Top-Cut Model for Deposits with Heavy-Tailed Grade Distribution

In some ore deposits, the grade distribution is heavy-tailed and high values make the inference of first- and second-order statistics nonrobust. In the case of gold data, high values are usually cut and the block estimation is performed using truncated grades. With this method, a part of the deposit is omitted, resulting in a potential bias on resources figures. Ad-hoc corrections can be applied on the final figures to take into account the tail of the grade distribution, but no theoretical guidelines are available. A geostatistical model has been developed to handle high values based on the assumption that for high grade zones, the only tangible information is the geometry. The grade variable z can be split into three parts: the truncated grade ( $\operatorname{Min} (z, z_{\mathrm{e}})$ where z _e is the top-cut grade), a weighted indicator above top-cut grade (1{z≥z _e}), and a residual. Within this framework, the residual is poorly structured, and in most cases is a pure nugget-effect. Moreover, it has no spatial correlation with the truncated grade and the indicator above top-cut grade. This decomposition makes the variographic study more robust because variables (indicator and truncated grade) do not keep high grade values. The underlying hypotheses can be tested on experimental indicator variograms and the top-cut grade can be justified. Finally, the estimation is based on a truncated grade and indicator cokriging. The model is applied to blast holes from a gold deposit and on a simulated example. Both cases illustrate the benefits of keeping the high values in the estimation process via an appropriate mathematical model.     

Paleolimnological evidence of the response of the central Canadian treeline zone to radiative forcing and hemispheric patterns of temperature change over the past 2000 years

Instrumental climate records from the central Canadian treeline zone display a pattern of variation similar to general Northern Hemisphere temperature trends. To examine whether this general correspondence extends back beyond the instrumental record, we obtained a sediment core from Lake S41, a small lake in the Northwest Territories of Canada at 63°43.11′ N, 109°19.07′ W. A radiocarbon-based chronology was developed for the core. The sediments were analyzed for organic-matter content by loss-on-ignition (LOI), biogenic-silica content (BSi), and chironomid community composition to reconstruct July air temperature and summer water temperature. The paleolimnological records were compared with records of atmospheric CO₂ concentration, solar variability, and hemispheric temperature variations over the past 2000 years. The results of the analyses suggest that widely-documented long-term variations in Northern Hemisphere temperature associated with radiative forcing, namely the cooling following the medieval period during the Little Ice Age (LIA), and twentieth century warming, are represented in the central Canadian treeline zone. There is also evidence of a brief episode of warming during the eighteenth century. As evidenced by LOI and BSi, the twentieth century warming is typified by increased lake productivity relative to the LIA. Depending upon the measure, the increased productivity of the twentieth century nearly equals or exceeds that of any other period in the past 2000 years. In contrast, the rate of chironomid head capsule accumulation decreased and remained low during the twentieth century. Although the chironomid-inferred temperature reconstructions indicate cooling during the LIA, they present no evidence of greatly increased temperatures during the twentieth century. Warming during the twentieth century might have enhanced lake stratification, and the response of the chironomid fauna to warming was attenuated by decreased oxygen and lower temperatures in the hypolimnion of the more stratification-prone lake.

Wave train model for knickpoint migration

Rivers respond to a drop in their base level by incising the topography. The upstream propagation of an incision, as usually depicted by a knickpoint migration, is thought to depend on several parameters such as the drainage area, lithology, and the amplitude of the base level drop. We first investigate the case of the Messinian Salinity Crisis that was characterized by the extreme base level fall (1500 m) of the Mediterranean Sea at the end of the Miocene. The response of drainage areas of three orders of magnitude (10³ to 10⁶ km²) highlights the dominant role of the drainage area (with a square root relationship) in controlling the knickpoint migration after a base level fall. A compilation of mean rates of knickpoint propagation for time durations ranging from 10² to 10⁷ years displays a similar relationship indicating that successive wave trains of knickpoint can migrate in a river: first, wave trains linked to the release of the alluvial cover and then, wave trains related to the bedrock incision, which correspond to the real time response of rivers. Wave trains with very low retreat rates (long lived knickpoints > 1 My) rather correspond to the response time of regional landscape.