Continental Shield Research Articles

In situ 10Be modeling and terrain analysis constrain subglacial quarrying and abrasion rates at Sermeq Kujalleq (Jakobshavn Isbræ), Greenland

Abstract. Glacial erosion creates diagnostic landscapes and vast amounts of sediment. However, knowledge about the rate at which glaciers erode and sculpt bedrock and the proportion of quarried (plucked) versus abraded material is limited. To address this, we quantify subglacial erosion rates and constrain the ratio of quarrying to abrasion during a recent, ∼ 200-year long overriding of a bedrock surface fronting, Sermeq Kujalleq (Jakobshavn Isbræ), Greenland, by combining 10Be analyses, a digital terrain model, and field observations. Cosmogenic 10Be measurements along a 1.2 m tall quarried bedrock step reveal a triangular wedge of quarried rock. Using individual 10Be measurements from abraded surfaces across the study area, we derive an average abrasion rate of 0.13 ± 0.08 mm yr−1. By applying this analysis across a ∼ 1.33 km2 study area, we estimate that the Greenland Ice Sheet quarried 378 ± 45 m3 and abraded 322 ± 204 m3 of material at this site. These values result in an average total erosion rate of 0.26 ± 0.16 mm yr−1, with abrasion and quarrying contributing in roughly equal proportions within uncertainty. Additional cosmogenic 10Be analysis and surface texture mapping indicate that many lee steps are relicts from the prior glaciation and were not re-quarried during the recent overriding event. These new observations of glacier erosion in a recently exposed landscape provide one of the first direct measurements of quarrying rates and indicate that quarrying accounts for roughly half of the total glacial erosion in representative continental shield lithologies.

The extreme yet transient nature of glacial erosion

Ice can sculpt extraordinary landscapes, yet the efficacy of, and controls governing, glacial erosion on geological timescales remain poorly understood and contended, particularly across Polar continental shields. Here, we assimilate geophysical data with modelling of the Eurasian Ice Sheet — the third largest Quaternary ice mass that spanned 49°N to 82°N — to decipher its erosional footprint during the entire last ~100 ka glacial cycle. Our results demonstrate extreme spatial and temporal heterogeneity in subglacial erosion, with rates ranging from 0 to 5 mm a−1 and a net volume equating to ~130,000 km3 of bedrock excavated to depths of ~190 m. A hierarchy of environmental controls ostensibly underpins this complex signature: lithology, topography and climate, though it is basal thermodynamics that ultimately regulates erosion, which can be variously protective, pervasive, or, highly selective. Our analysis highlights the remarkable yet fickle nature of glacial erosion — critically modulated by transient ice-sheet dynamics — with its capacity to impart a profound but piecemeal geological legacy across mid- and high latitudes.

Relations between earthquake distributions, geological history, tectonics and rheology on the continents.

This paper is concerned with the distribution of earthquakes, particularly their depths, with the temperature of the material in which they occur, and with the significance of both for the rheology and deformation of the continental lithosphere. Earthquakes on faults are generated by the sudden release of elastic energy that accumulates during slow plate motions. The nonlinear high-temperature creep that localizes such energy accumulation is, in principle, well understood and can be described by rheological models. But the same is not true of seismogenic brittle failure, the main focus of this paper, and severely limits the insights that can be obtained by simulations derived from geodynamical modelling of lithosphere deformation. Through advances in seismic tomography, we can now make increasingly detailed maps of lithosphere thickness on the continents. The lateral variations are dramatic, with some places up to 300 km thick, and clearly relate to the geological history of the continents as well as their present-day deformation. Where the lithosphere thickness is about 120 km or less, continental earthquakes are generally confined to upper crustal material that is colder than about 350°C. Within thick lithosphere, and especially on its edges, the entire crust may be seismogenic, with earthquakes sometimes extending into the uppermost mantle if the Moho is colder than 600°C, but the continental mantle is generally aseismic. Earthquakes in the continental lower crust at 400-600°C require the crust to be anhydrous and so are a useful guide or proxy to both composition and strength. These patterns and correlations have important implications for the geological evolution of the continents. They can be seen to have influenced features as diverse as the location of post-collisional rifting; cratonic basin formation; the location, origin and timing of granulite-facies metamorphism; and the formation, longevity and strength of cratons. In addition, they have important consequences for earthquake hazard assessment in the slowly deforming edges and interiors of continental shields or platforms, where the large seismogenic thickness can host very large earthquakes. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.

MAGLAB: A computing platform connecting geophysical signatures to melting processes in Earth's mantle

Decompression melting of the upper mantle produces magmas and volcanism at the Earth's surface. Experimental petrology demonstrates that the presence of CO2 and H2O enhances peridotite melting anywhere within the upper mantle down to approximately 200–300 km depth. The presence of mantle melts with compositions ranging from carbonate-rich to silicate-rich unavoidably affects the geophysical signals retrieved from Earth's mantle. Geochemical investigations of erupted intraplate magmas along with geophysical surveys allow for constraining the nature and volume of primary melts, and a sound formalism is required to integrate these diverse datasets into a realistic model for the upper mantle including melting processes. Here, we introduce MAGLAB, a model developed to calculate the composition and volume fraction of melts in the upper mantle, together with the corresponding electrical conductivity of partially molten mantle peridotites at realistic pressure-temperature conditions and volatile contents. We use MAGLAB to show how the compositions of intraplate magmas relate to variations in lithosphere thickness. Progressive partial melting of a homogeneous peridotitic mantle source can in theory create the diversity of compositions observed among the spectrum of intraplate magma types, with kimberlite melts beneath thick continental shields, alkaline magmas such as melilitite, nephelinite and basanite beneath thinner continents and relatively old plus thick oceanic lithospheres, and ‘regular’ basalts beneath the youngest and thinnest oceanic lithospheres as well as beneath significantly thinned continental lithospheres. MAGLAB calculations support recent experimental findings about the role of H2O in the upper mantle on producing primary kimberlitic melts in addition to CO2. We demonstrate the robustness of MAGLAB calculations by reproducing the compositions of erupted melts as well as associated mantle electrical conductivities beneath the Society hotspot in the Pacific Ocean. A comparison of our simulations with magnetotelluric surveys at various oceanic settings shows that the heterogeneities in electrical conductivity of Earth's upper mantle are related to variations in volatile content via the presence of small (generally <<1 wt%) and heterogeneously distributed fractions of CO2-H2O-bearing melts.

Elevated Equilibrium Geotherm in Stable Continental Shield: Evidence From Integrated Structural, Hydrological, and Electromagnetic Studies on Nonvolcanic Hot Springs in the Eastern Ghats Belt, India

AbstractNonvolcanic hot springs occur within Neoproterozoic granulites of the tectonically stable Eastern Ghats Belt in the Indian shield. Penetrative granulite facies planar structural fabrics in the northern Eastern Ghats Belt are reoriented within an E‐W striking, northerly dipping ductile shear zone that is subsequently dissected by WNW‐ESE trending, subvertical pseudotachylite‐bearing faults and fractures. Tube and dug wells in the area yield both thermal (~60 °C) and nonthermal (~28 °C) water. Chemical analyses reveal that the thermal water is richer in Na+, K+, and Cl−, with lower bicarbonate content. Stable isotope analyses (δ2H and δ18O) of the thermal and nonthermal waters both plot on the Global Meteoric Water Line; however, tritium and 14C ages indicate that the nonthermal water is relatively modern, while the thermal waters are older. This suggests that both waters come from distinct reservoirs. Very low frequency electromagnetic studies show that water circulates through the WNW‐ESE trending fracture systems but also exists in isolated pockets within the crystalline country rock. Heat production studies reveal that some host rocks have exceptionally high heat‐producing element concentrations, primarily as thorium within the minerals monazite and thorite. Meteoric waters entrapped for longer times near these heat‐producing element‐rich pockets undergo radiogenic heating, and are shielded from the nonthermal groundwater circulating within the fracture system. The high heat‐producing element distribution in the crust resulting from Neoproterozoic geological events has, thus, elevated the present‐day equilibrium geotherm in the Eastern Ghats Belt, forming sources for shallow‐level, nonvolcanic hot springs within a tectonically inactive terrane.

S-wave velocity structure of the upper mantle beneath the Arctic region from Rayleigh wave dispersion data

A new model of the upper mantle structure of the Arctic region based on a representative dataset of Rayleigh wave group velocity dispersion curves from 10 to 250 s period is presented. A surface wave tomography procedure is used to image the group velocity distributions. Local dispersion curves are constructed and inverted to vertical S-wave velocity sections up to the depth of 500 km. Results are presented in two forms: as horizontal slices of S-wave velocity variations relative to the average velocities at several mantle depths and as 2D velocity sections along three profiles crossing different tectonic units. The model shows lateral and vertical velocity variations over the whole depth range within the study area. The most prominent velocity heterogeneities are localized in the uppermost mantle up to the 200 km depth. In this depth range, the velocity anomalies are strongly correlated with the main tectonic structures. The calculated S-wave velocity distribution demonstrates that the upper mantle of the continental shields is characterized by high S-wave velocities and contains regional velocity anomalies associated with small-scale blocks. Low mantle velocities are attributed to the areas with high level of tectonic and seismic activity. Significant difference in deep velocity structure of the marginal seas and deep-water oceanic basins of various ages is revealed. The obtained model suggests some insights into the geodynamical evolution of the lithosphere of the Arctic region.

Oceanic-like axial crustal high in the central Red Sea

The Red Sea is an important example of a rifted continental shield proceeding to seafloor spreading. However, whether the crust in the central Red Sea is continental or oceanic has been controversial. Contributing to this debate, we assess the basement geometry using seismic reflection and potential field data. We find that the basement topography from seismically derived structure corrected for evaporite and other sediment loading has an axial high with a width of 70–100 km and a height of 0.8–1.6 km. Basement axial highs are commonly found at mid-ocean ridges affected by hotspots, where enhanced mantle melting results in thickened crust. We therefore interpret this axial high as oceanic-like, potentially produced by recently enhanced melting associated with the broader Afar mantle anomaly. We also find the Bouguer gravity anomalies are strongly correlated with basement reflection depths. The apparent density contrast necessary to explain the Bouguer anomaly varies from 220 kg m−3 to 580 kg m−3 with no trend with latitude. These values are too small to be caused primarily by the density contrast between evaporites and mantle across a crust of uniform thickness and density structure, further supporting a thickened crustal origin for the axial high. Complicating interpretation, only a normal to modestly thickened axial crust is predicted from fractionation-corrected sodium contents (Na8.0), and the basement reflection is rugged, more typical of ultra-slow spreading ridges that are not close to hotspots. We try to reconcile these observations with recent results from seismic tomography, which show modest mantle S-wave velocity anomalies under this part of the Red Sea.

Crustal heat production and estimate of terrestrial heat flow in central East Antarctica, with implications for thermal input to the East Antarctic ice sheet

Abstract. Terrestrial heat flow is a critical first-order factor governing the thermal condition and, therefore, mechanical stability of Antarctic ice sheets, yet heat flow across Antarctica is poorly known. Previous estimates of terrestrial heat flow in East Antarctica come from inversion of seismic and magnetic geophysical data, by modeling temperature profiles in ice boreholes, and by calculation from heat production values reported for exposed bedrock. Although accurate estimates of surface heat flow are important as an input parameter for ice-sheet growth and stability models, there are no direct measurements of terrestrial heat flow in East Antarctica coupled to either subglacial sediment or bedrock. As has been done with bedrock exposed along coastal margins and in rare inland outcrops, valuable estimates of heat flow in central East Antarctica can be extrapolated from heat production determined by the geochemical composition of glacial rock clasts eroded from the continental interior. In this study, U, Th, and K concentrations in a suite of Proterozoic (1.2–2.0 Ga) granitoids sourced within the Byrd and Nimrod glacial drainages of central East Antarctica indicate average upper crustal heat production (Ho) of about 2.6 ± 1.9 µW m−3. Assuming typical mantle and lower crustal heat flux for stable continental shields, and a length scale for the distribution of heat production in the upper crust, the heat production values determined for individual samples yield estimates of surface heat flow (qo) ranging from 33 to 84 mW m−2 and an average of 48.0 ± 13.6 mW m−2. Estimates of heat production obtained for this suite of glacially sourced granitoids therefore indicate that the interior of the East Antarctic ice sheet is underlain in part by Proterozoic continental lithosphere with an average surface heat flow, providing constraints on both geodynamic history and ice-sheet stability. The ages and geothermal characteristics of the granites indicate that crust in central East Antarctica resembles that in the Proterozoic Arunta and Tennant Creek inliers of Australia but is dissimilar to other areas like the Central Australian Heat Flow Province that are characterized by anomalously high heat flow. Age variation within the sample suite indicates that central East Antarctic lithosphere is heterogeneous, yet the average heat production and heat flow of four age subgroups cluster around the group mean, indicating minor variation in the thermal contribution to the overlying ice sheet from upper crustal heat production. Despite these minor differences, ice-sheet models may favor a geologically realistic input of crustal heat flow represented by the distribution of ages and geothermal characteristics found in these glacial clasts.

Upper mantle anisotropy inferred from shear wave splitting beneath the Eastern Indian Shield region

We estimate the shear wave splitting parameters vis-à-vis the thicknesses of the continental lithosphere beneath the two permanent seismic broadband stations located at Dhanbad (DHN) and Bokaro (BOKR) in the Eastern Indian Shield region. Broadband seismic data of 146 and 131 teleseismic earthquake events recorded at DHN and BOKR stations during 2007–2014 were analyzed for the present measurements. The study is carried out using rotation-correlation and transverse component minimization methods. We retain our “Good”, “Fair” and “Null” measurements, and estimate the splitting parameters using 13 “Good” results for DHN and 10 “Good” results for BOKR stations. The average splitting parameters (ϕ, δt) for DHN and BOKR stations are found to be 50.76°±5.46° and 0.82 ± 0.2 s and 56.30°±5.07° and 0.95 ± 0.17 s, and the estimated average thicknesses of the anisotropic layers beneath these two stations are ∼ 94 and ∼109 km, respectively. The measured deviation of azimuth of the fast axis direction (ϕ) from the absolute motion of the Indian plate ranges from ∼8° to 14°. The measured deviation of azimuth of the fast axis direction (ϕ) from the absolute motion of the Indian plate ranges from ∼8° to 14°. The eastward deviation of the fast axis azimuths from absolute plate motion direction is interpreted to be caused by induced outflow from the asthenosphere. Further, the delay time found in the present analysis is close to the global average for continental shield areas, and also coherent with other studies for Indian shield regions. The five “Null” results and the lower delay time of ∼0.5–0.6 s might be indicating multilayer anisotropy existing in the mantle lithosphere beneath the study area.

Geodynamics of kimberlites on a cooling Earth: Clues to plate tectonic evolution and deep volatile cycles

Kimberlite magmatism has occurred in cratonic regions on every continent. The global age distribution suggests that this form of mantle melting has been more prominent after 1.2 Ga, and notably between 250–50 Ma, than during early Earth history before 2 Ga (i.e., the Paleoproterozoic and Archean). Although preservation bias has been discussed as a possible reason for the skewed kimberlite age distribution, new treatment of an updated global database suggests that the apparent secular evolution of kimberlite and related CO2-rich ultramafic magmatism is genuine and probably coupled to lowering temperatures of Earth's upper mantle through time.Incipient melting near the CO2- and H2O-bearing peridotite solidus at >200 km depth (1100–1400 °C) is the petrologically most feasible process that can produce high-MgO carbonated silicate melts with enriched trace element concentrations akin to kimberlites. These conditions occur within the convecting asthenospheric mantle directly beneath thick continental lithosphere. In this transient upper mantle source region, variable CHO volatile mixtures control melting of peridotite in the absence of heat anomalies so that low-degree carbonated silicate melts may be permanently present at ambient mantle temperatures below 1400 °C. However, extraction of low-volume melts to Earth's surface requires tectonic triggers. Abrupt changes in the speed and direction of plate motions, such as typified by the dynamics of supercontinent cycles, can be effective in the creation of lithospheric pathways aiding kimberlite magma ascent.Provided that CO2- and H2O-fluxed deep cratonic keels, which formed parts of larger drifting tectonic plates, existed by 3 Ga or even before, kimberlite volcanism could have been frequent during the Archean. However, we argue that frequent kimberlite magmatism had to await establishment of an incipient melting regime beneath the maturing continents, which only became significant after secular mantle cooling to below 1400 °C during post-Archean times, probably sometime shortly after 2 Ga. At around this time kimberlites replace komatiites as the hallmark mantle-derived magmatic feature of continental shields worldwide.The remarkable Mesozoic–Cenozoic ‘kimberlite bloom’ between 250–50 Ma may represent the ideal circumstance under which the relatively cool and volatile-fluxed cratonic roots of the Pangea supercontinent underwent significant tectonic disturbance. This created more than 60% of world's known kimberlites in a combination of redox- and decompression-related low-degree partial melting. Less than 2% of world's known kimberlites formed after 50 Ma, and the tectonic settings of rare ‘young’ kimberlites from eastern Africa and western North America demonstrate that far-field stresses on cratonic lithosphere enforced by either continental rifting or cold subduction play a crucial role in enabling kimberlite magma transfer to Earth's surface.

Seismic hazard assessment of Koyna region, Peninsular India: using geospatial approach

BackgroundEarthquake-prone regions from the stable continental regions (like Peninsular India) warrant total seismic hazard estimation from possible sources — reservoir induced or tectonic. The Koyna region falls in the so-called stable continental region, though previously considered as aseismic because of its location in the Peninsular India, which was argued to be the continental Shield region; however, it is seismically active ever since the December 10, 1967, strong earthquake (M6.5) which killed around 200 people and damaged properties. The event was initially attributed to reservoir-induced seismicity. The present study aims at mapping the spatial distribution of hazard intensity in terms of peak ground acceleration and damage potential expressed as Modified Mercalli Intensity (MMI) levels owing to the scenario earthquakes (M4.0 to M6.9) from the linear sources and background seismicity in the Koyna region. For the seismic source model, geospatial integration approach is used. To delineate the source boundaries we perform spatial analysis of seismicity attributes and assigning weights based on the geological information. In addition, spatial buffering of thematic layers of faults and lineaments is applied to truncate the spatial uncertainties. Thematic attribute layers are integrated over the SRTM-DEM-derived geomorphic features and spatial seismicity pattern to define the possible linear seismic source zones. We employ ArcGIS® to perform geospatial analysis and to integrate thematic spatial information finally to prepare the seismic hazard and damage potential zones for the Koyna region.ResultsWe present the probabilistically estimated seismic hazard and damage potential estimates for the Koyna region. Spatial mapping of the hazard parameters is performed on Geographic Information System (GIS) platform. Along with the historical and instrumental earthquake data, we incorporate updated and relocated seismic events from circa 1618 to 2017 covering ~ 400 years period, for analyzing the mean annual rates of seismicity and earthquake recurrence intervals. The anticipated recurrence intervals of earthquakes ~ M5.5 is around 100 ± 10 years. The estimated peak ground acceleration (PGA) for 10% probability being exceeded in 50 years (i.e. 500 years of return period) is greater than 21% g is anticipated for the area located (~30 km radius) between the Koyna and Warna Reservoirs.ConclusionsThe hazard estimates for the 500 years return period earthquake scenarios (M4.0 to M6.9) are based on the bedrock level estimation. Estimated PGA-values are grouped into three categories that are relatively defined as high, moderate, and low hazard zones, respectively. Slightly damaging intensity (MMI-VII level) expected in 50 years has ~ 40% probability and PGA greater than 21% g for the area which is located between Koyna and Warna Reservoirs, which is identified as high hazard zone.

Plant\u2013soil interactions maintain biodiversity and functions of tropical forest ecosystems

Tropical forests are characterized by high biodiversity and aboveground biomass growing on strongly weathered soils. However, the distribution of plant species and soils are highly variable even within a tropical region. This paper reviews existing and novel knowledge on soil genesis, plant and microbial physiology, and biogeochemistry. Typically, forests in Southeast Asia are dominated by dipterocarps growing on acidic Ultisols from relatively young parent material. In the Neotropics and Africa, forests contain abundant legume trees growing on Oxisols developed in the older parent materials on stable continental shields. In Southeast Asia, the removal of base cations from the surface soil due to leaching and uptake by dipterocarp trees result in intensive acidification and accumulation of exchangeable Al3+, which is toxic to most plants. Nutrient mining by ectomycorrhizal fungi and efficient allocation within tree organs can supply phosphorus (P) for reproduction (e.g., mast fruiting) even on P-limited soils. In the Neotropics and Africa, nitrogen (N) fixation by legume trees can ameliorate N or P limitation but excess N can promote acidification through nitrification. Biological weathering [e.g., plant silicon (Si) cycling] and leaching can lead to loss of Si from soil. The resulting accumulation of Al and Fe oxides in Oxisols that can reduce P solubility through sorption and lead to limitation of P relative to N. Thus, geographical variation in geology and plant species drives patterns of soil weathering and niche differentiation at the global scale in tropical forests.

Transferability of microsatellite loci to Vellozia plicata (Velloziaceae), a widespread species on Brazilian inselbergs

Inselbergs are isolated, mainly granitic and gneissic rock outcrops, forming old landscape elements on crystalline continental shields on all continents. Many Velloziaceae which occur specifically as mat-formers on these rock outcrops are well adapted to harsh environmental conditions and do not occur in the surrounding matrix. Vellozia plicata Mart. is a widespread desiccation-tolerant mat-forming species on inselbergs in southeastern Brazil, with naturally fragmented populations. Here, we tested the transferability of 53 microsatellite loci previously developed for two Vellozia species (V. gigantea N. L. Menezes & Mello-Silva and V. squamata Pohl) in two populations of V. plicata. We succeeded in the amplification of 11 loci, and six of them were polymorphic. The number of alleles per locus ranged from 2 to 17, the expected and observed heterozygosities ranged from 0.000 to 0.875 and from 0.000 to 0.895, respectively. Our results show that cross-amplification works within the genus Vellozia, but at a lower level when compared to other monocot families on inselbergs, such as Bromeliaceae and Orchidaceae. This work might help in further studies on population genetics of V. plicata, in order to guide conservation actions in the future and also to promote further investigations about speciation processes of inselberg specialists.

Sources and mobility of carbonate melts beneath cratons, with implications for deep carbon cycling, metasomatism and rift initiation

Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr–Nd–Hf–Pb–Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ΔεHf coupled with >+5‰δ7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05–3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (−3.6 to −2.4‰δ13C for carbonatites versus −5.7 to −3.6‰δ13C for kimberlites) require open-system fractionation at magmatic temperatures.Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of ‘pure’ carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16–33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated ‘cratonic carbonatites’ have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiation – as exemplified by the Mesozoic–Cenozoic breakup of the North Atlantic craton.

Abiotic and biotic controls on methane formation down to 2.5 km depth within the Precambrian Fennoscandian Shield

Despite a geological history characterised by high temperature and pressure processes and organic carbon deprived crystalline bedrock, large amounts of hydrocarbons are found in deep groundwaters within Precambrian continental shields. In many sites, methane comprises more that 80% of the dissolved gas phase reaching concentrations of tens of mmoll−1. In this study, we used isotopic methods to study the carbon isotope systematics and sources of crustal methane within the Fennoscandian Shield. The main study sites were the Outokumpu Deep Drill Hole and the Pyhäsalmi mine in Finland, both of which allow groundwater sampling down to 2.5km depth and have been previously studied for their groundwater chemistry and microbiology. We show that the differences in the amount and isotopic composition of methane are related to the availability of carbon sources as well as processes behind the incorporation of hydrogen and carbon via abiotic and biotic pathways into hydrocarbon molecules. Supported by previously reported occurrences and isotopic data of deep groundwater methane in lithologically different locations in Finland and Sweden, we show that methane formation is controlled by microbial methanogenesis and abiotic reactions, as well as lithology with the metasedimentary environments being the most favourable for methane occurrence. Rather than a thermogenic relic, crustal methane within the Fennoscandian Shield is more likely the result of low temperature formation from ancient organic compounds or their inorganic intermediates such as graphite. Such crustal gases are characterised by the lack of major amounts of C2+ hydrocarbons and 13C-rich methane. Further, microbiological and isotopic geochemical evidence suggest that microbial methane is more common at depths shallower than 1.5km.

Very long-term stability of passive margin escarpment constrained by40Ar/39Ar dating of K-Mn oxides

The post-rift denudation history of high-elevation divergent continental margins is central to deciphering source-to-sink systems across such margins and to unraveling their topographic evolution. We performed 40Ar-39Ar dating of supergene K-Mn oxides from supergene manganese ore deposits in order to constrain the age of in situ formed laterites on both the lowland and highland sides of the Western Ghats Escarpment of Peninsular India. Documentation of laterites as old as 53 Ma on the highland and 47 Ma at the foot of the escarpment shows that the escarpment stabilized before 47 Ma (possibly 60 Ma). The topographic setting of the dated weathering mantles also constrains denudation of the lowland and the highland since ca. 50 Ma to maximums of 5 m/m.y. and 15 m/m.y., respectively. Our results challenge denudation rates derived from apatite fission track thermochronology by documenting the topography and relief longevity of the southwest Indian margin and its hinterland. The results preclude large-magnitude uplift of Peninsular India after the Eocene, and more generally argue against late Neogene uplift and topographic rejuvenation of continental shields.

Sugar Loaf Landin south-eastern Brazil: a centre of diversity for mat-forming bromeliads on inselbergs

Inselbergs sensu lato are isolated, mainly granitic and gneissic, rock outcrops, forming old landscape elements on crystalline continental shields on all continents. Mats consisting mainly of monocots occur on open rocky slopes, forming a conspicuous habitat on these outcrops. Brazilian lowland inselbergs consist of species-rich mats with large numbers of endemics and Bromeliaceae standing out as characteristic elements in these communities. For the first time, we provide a checklist of mat-forming bromeliads on lowland inselbergs in Brazil and investigate whether the regional species pool influences the species diversity of inselberg mats. Furthermore, through modelling analysis, we looked for the potential distribution of bromeliads endemic to inselbergs, expecting that they would show a narrower niche width than non-endemic species. Our data indicate the existence of a particular phytogeographical region in the Atlantic Forest in south-eastern Brazil, comprising inselbergs harbouring a highly diverse flora of Bromeliaceae, which we call Sugar Loaf Land. In addition, our projections reveal smaller predicted areas for bromeliads endemic to inselbergs compared with non-endemic species, the former apparently being mainly controlled by climatic factors. In addition to climate and regional species pools, the island-like character of inselbergs seems to play an important role in the speciation of rupicolous Bromeliaceae.

Erosion patterns and mantle sources of topographic change across the southern African Plateau derived from the shallow and deep records of kimberlites

AbstractFlow in the sublithospheric mantle is increasingly invoked as a mechanism to explain both modern and past surface topography, but the importance of this phenomenon and its influence at different localities are debated. Southern Africa is an elevated continental shield proposed to represent dynamically supported topography. However, this region is also characterized by a complex lithospheric architecture variably affected by Cretaceous heating, thinning, and metasomatic alteration. We used apatite (U‐Th)/He thermochronometry on 15 Cretaceous kimberlites from an ∼600 km long transect across the Kaapvaal Craton, combined with information from xenoliths in these pipes, to determine the plateau interior erosion history. The goal was to determine the relationships with lithospheric modification patterns and thereby better isolate the sublithospheric contribution to elevation. The results document a wave of erosion from west to east across the craton from ∼120 to &lt;60 Ma, initially focused along paleorivers and then retreating as a scarp across the landscape. This spatially variable erosion event was associated with limited modification of the Archean cratonic lithospheric mantle as recorded by mantle xenoliths and xenocrysts, implying that dynamic buoyancy sources may be required to explain the elevations. In contrast, off‐craton to the southwest, a more pronounced regional erosion phase at ∼110–90 Ma was coincident with significant modification of the Proterozoic lithospheric mantle. This relationship suggests that lithospheric processes were more important contributors to erosion and topographic change off‐craton than on‐craton. Together, these results suggest that lithospheric architecture can have an important control on the surface expression of mantle dynamics.

Comment on "A Unified Seismic Catalog for the Iranian Plateau (1900-2011)" by Mohammad P. Shahvar, Mehdi Zare, and Silvia Castellaro

A recent paper in the March/April 2013 issue of Seismological Research Letters , “A Unified Seismic Catalog for the Iranian Plateau (1900–2011)” by Mohammad P. Shahvar, Mehdi Zare, and Silvia Castellaro (henceforth referred to as Shahvar et al. , 2013) provides a unified homogeneous earthquake catalog, for the period 1900–2011, particularly for seismic‐hazard assessment studies in Iran. They subdivide the territory of the Iranian plateau into two domains only according to seismicity to derive empirical relationships to convert the original magnitudes ( M s, m b, M L, and M N) to a uniform scale ( M w), applying standard least squares (SR) and inverted least squares (ISR) regression methods and orthogonal regression method (OR). Some contradictions in Shahvar et al. (2013) encouraged us to declare a few points. The distinction between seismotectonic zones is based on all available geologic, tectonic, geophysical, and seismological data. In other words, a seismotectonic zone is considered an area that under present‐day geodynamic regimes has an internally consistent tectonic setting and unified seismicity pattern (Ye et al. , 1993, 1995). Shahvar et al. (2013) claim that the Iranian plateau can de facto be divided into two domains only because seismicity in the Alborz and Central Iran zones show statistically undistinguishable features. In contrast, Karimiparidari et al. (2013) distinguish six seismotectonic zones based on Mirzaei et al. (1998) and Tavakoli (1996). It is noteworthy that the second author of the two concurrent inconsistent papers is in common. Based on Nowroozi (1971) and Bird et al. (1975), it is stated by Shahvar et al. (2013) that the continental Arabian shield subducts beneath the Zagros belt. However, local seismograph networks have found no reliable depths in the Zagros deeper than ∼20 km, even in places where deeper earthquakes were reported to have occurred (e.g., Niazi et al. , 1978; Tatar et al. , …

Antarcticaʼs hypsometry and crustal thickness: Implications for the origin of anomalous topography in East Antarctica

The hypsometry of Antarctica revealed by BEDMAP2 data is characterised by deglaciated modal elevations of ∼−450 m and ∼650 m for West and East Antarctica, respectively, and an East Antarctic plateau that is topographically anomalous by ∼400–600 m with respect to global continental modal elevation estimates. Superimposed on the East Antarctic plateau are the Gamburtsev Subglacial Mountains, the Dronning Maud Land Mountains and the Vostok Highlands with modal elevations ∼400 m in excess of the East Antarctic mode. To ascertain whether East Antarcticaʼs anomalous topography can be attributed to Airy-type crustal compensation, a continental-scale crustal thickness model was derived from the inversion of GOCO03S satellite gravity data constrained by seismic crustal thickness estimates. The average crustal thickness of East Antarctica is ∼40 km (for West Antarctica ∼24 km), a value typical of continental shields, and while crustal thicknesses of >50 km locally beneath the Gamburtsev Subglacial Mountains and Dronning Maud Land can account for their differential modal elevation above the plateau, crustal thicknesses elsewhere across East Antarctica offer no suggestion of crustal-level continental-scale support for the broader plateau. Enderby Land, for example, resides on the plateau and is characterised by a modal elevation of ∼750 m and crust ∼40 km thick, whereas off the plateau in East Antarctica, the Aurora and Wilkes Subglacial Basins have modal elevations of ∼−50 m and ∼50 m, respectively, yet similarly thick crust. The lack of crustal support for the elevated broader East Antarctic plateau, coupled with seismic images showing fast upper mantle velocities beneath the plateau, suggest a mid-to-lower mantle source for East Antarcticaʼs anomalous topography.