Time-variable gravity signal in Greenland revealed by high-low satellite-to-satellite tracking

[1] In the event of a termination of the Gravity Recovery and Climate Experiment (GRACE) mission before the launch of GRACE Follow-On (due for launch in 2017), high-low satellite-to-satellite tracking (hl-SST) will be the only dedicated observing system with global coverage available to measure the time-variable gravity field (TVG) on a monthly or even shorter time scale. Until recently, hl-SST TVG observations were of poor quality and hardly improved the performance of Satellite Laser Ranging observations. To date, they have been of only very limited usefulness to geophysical or environmental investigations. In this paper, we apply a thorough reprocessing strategy and a dedicated Kalman filter to Challenging Minisatellite Payload (CHAMP) data to demonstrate that it is possible to derive the very long-wavelength TVG features down to spatial scales of approximately 2000 km at the annual frequency and for multi-year trends. The results are validated against GRACE data and surface height changes from long-term GPS ground stations in Greenland. We find that the quality of the CHAMP solutions is sufficient to derive long-term trends and annual amplitudes of mass change over Greenland. We conclude that hl-SST is a viable source of information for TVG and can serve to some extent to bridge a possible gap between the end-of-life of GRACE and the availability of GRACE Follow-On.

Earth gravity field recovery using GPS, GLONASS, and SLR satellites

The time variable Earth’s gravity field provides the information about mass transport within the system Earth, i.e., the relationship of mass transport between atmosphere, oceans, and land hydrology. We recover the low-degree parameters of the time variable gravity field using microwave observations from GPS and GLONASS satellites and from SLR data to five geodetic satellites, namely LAGEOS-1/2, Starlette, Stella, and AJISAI. GPS satellites are particularly sensitive to specific coefficients of the Earth's gravity field, because of the deep 2:1 orbital resonance with Earth rotation (two revolutions of the GPS satellites per sidereal day). The resonant coefficients cause, among other, a “secular” drift (actually periodic variations of very long periods) of the semi-major axes of up to 5.3 m/day of GPS satellites. We processed 10 years of GPS and GLONASS data using the standard orbit models from the Center of Orbit Determination in Europe (CODE) with a simultaneous estimation of the Earth gravity field coefficients and other parameters, e.g., satellite orbit parameters, station coordinates, Earth rotation parameters, troposphere delays, etc. The weekly GNSS gravity solutions up to degree and order 4/4 are compared to the weekly SLR gravity field solutions. The SLR-derived geopotential coefficients are compared to monthly GRACE and CHAMP results.

Counting scars on tree stems to assess rockfall hazards: A low effort approach, but how reliable?

Rockfall is a widespread and hazardous process in mountain environments, but data on past events are only rarely available. Growth-ring series from trees impacted by rockfall were successfully used in the past to overcome the lack of archival records. Dendrogeomorphic techniques have been demonstrated to allow very accurate dating and reconstruction of spatial and temporal rockfall activity, but the approach has been cited to be labor intensive and time consuming. In this study, we present a simplified method to quantify rockfall processes on forested slopes requiring less time and efforts. The approach is based on a counting of visible scars on the stem surface of Common beech (Fagus sylvatica L.). Data are presented from a site in the Inn valley (Austria), where rocks are frequently detached from an ~ 200-m-high, south-facing limestone cliff. We compare results obtained from (i) the “classical” analysis of growth disturbances in the tree-ring series of 33 Norway spruces (Picea abies (L.) Karst.) and (ii) data obtained with a scar count on the stem surface of 50 F. sylvatica trees. A total of 277 rockfall events since A.D. 1819 could be reconstructed from tree-ring records of P. abies, whereas 1140 scars were observed on the stem surface of F. sylvatica. Absolute numbers of rockfalls (and hence return intervals) vary significantly between the approaches, and the mean number of rockfalls observed on the stem surface of F. sylvatica exceeds that of P. abies by a factor of 2.7. On the other hand, both methods yield comparable data on the spatial distribution of relative rockfall activity. Differences may be explained by a great portion of masked scars in P. abies and the conservation of signs of impacts on the stem of F. sylvatica. Besides, data indicate that several scars on the bark of F. sylvatica may stem from the same impact and thus lead to an overestimation of rockfall activity.

Matrix pore water in low-permeable crystalline bedrock: An archive for the palaeohydrogeological evolution of the Olkiluoto Investigation Site

Matrix pore water in the connected inter- and intragranular pore space of low-permeable crystalline bedrock interacts with flowing fracture groundwater predominately by diffusion. Based on the slow exchange between the two water reservoirs, matrix pore water acts as an archive of past changes in fracture groundwater compositions and thus of the palaeohydrological history of a site. Matrix pore water of crystalline bedrock from the Olkiluoto investigation site (SW Finland) was characterised using the stable water isotopes (δ18O, δ2H), combined with the concentrations of dissolved chloride and bromide as natural tracers. The comparison of tracer concentrations in pore water and present-day fracture groundwater suggest for the pore water the presence of old, dilute meteoric water components that infiltrated into the fractures during various warm climate stages. These different meteoric components can be discerned based on the diffusion distance between the two reservoirs and be brought into context with the palaeohydrological evolution of the site.

Assessing Initial Conditions for Chloride Transport Across Low- permeability Argillaceous Rocks, Wellenberg, Switzerland

Information about fluid evolution and solute transport in a low-permeability metamorphic rock sequence has been obtained by comparing chloride concentrations and chlorine isotope ratios of pore water, groundwater, and fluid inclusions. The similarity of d37Cl values in fluid inclusions and groundwater suggests a closed-system evolution during the metamorphic overprint, and signatures established at this time appear to form the initial conditions for chloride transport after exhumation of the rock sequence.

Low Energy Neutral Atoms from the Heliosheath

In the heliosheath beyond the termination shock, low energy (<0.5 keV) neutral atoms are created by charge exchange with interstellar neutrals. Detecting these neutrals from Earth's orbit is difficult because their flux is reduced substantially by ionization losses as they propagate from about 100 to 1 AU and because there are a variety of other signals and backgrounds that compete with this weak signal. Observations from IBEX-Lo and -Hi from two opposing vantage points in Earth's orbit established a lower energy limit of about 0.1 keV on measurements of energetic neutral atoms (ENAs) from the heliosphere and the form of the energy spectrum from about 0.1 to 6 keV in two directions in the sky. Below 0.1 keV, the detailed ENA spectrum is not known, and IBEX provides only upper limits on the fluxes. However, using some assumptions and taking constraints on the spectrum into account, we find indications that the spectrum turns over at an energy between 0.1 and 0.2 keV.

GOCE precise orbit determination for the entire Mission – challenges in the final mission phase

The Gravity field and steady-state Ocean Circulation Explorer (GOCE), ESA’s first Earth Explorer core mission, was launched on March 17, 2009 into a sunsynchronous dusk-dawn orbit and eventually re-entered into the Earth’s atmosphere on November 11, 2013. A precise science orbit (PSO) product was provided by the GOCE High-level Processing Facility (HPF) from the GPS high-low Satellite-to-Satellite Tracking (hl-SST) data from the beginning until the very last days of the mission. We recapitulate the PSO procedure and refer to the results achieved until the official end of the GOCE mission on October 21, 2013, where independent validations with Satellite Laser ranging (SLR) measurements confirmed a high quality of the PSO product of about 2 cm 1-D RMS. We then focus on the period after the official end of the mission, where orbits could still be determined thanks to the continuously running GPS receivers delivering high quality data until a few hours before the re-entry into the Earth’s atmosphere. We address the challenges encountered for orbit determination during these last days and report on adaptions in the PSO procedure to also obtain good orbit results at the unprecedented low orbital altitudes below 224 km.

P-T-t estimation of syn-kinematic strain in low-grade quartz-feldspar bearing rocks using thermodynamic modeling and 40Ar/39Ar dating techniques: example of the Plan-de-Phasy shear zone unit (Briançonnais Zone, Western Alps)

Pressure–Temperature–time (P–T–t) estimates of the syn-kinematic strain at the peak-pressure conditions reached during shallow underthrusting of the Briançonnais Zone in the Alpine subduction zone was made by thermodynamic modelling and 40Ar/39Ar dating in the Plan-de-Phasy unit (SE of the Pelvoux Massif, Western Alps). The dated phengite minerals crystallized syn-kinematically in a shear zone indicating top-to-the-N motion. By combining X-ray mapping with multi-equilibrium calculations, we estimate the phengite crystallization conditions at 270 ± 50 °C and 8.1 ± 2 kbar at an age of 45.9 ± 1.1 Ma. Combining this P–T–t estimate with data from the literature allows us to constrain the timing and geometry of Alpine continental subduction. We propose that the Briançonnais units were scalped on top of the slab during ongoing continental subduction and exhumed continuously until collision.

Microscale mapping of alteration conditions and potential biosignatures in basaltic-ultramafic rocks on early Earth and beyond

Subseafloor environments preserved in Archean greenstone belts provide an analogue for investigating potential subsurface habitats on Mars. The c. 3.5-3.4 Ga pillow lava metabasalts of the mid-Archean Barberton greenstone belt, South Africa, have been argued to contain the earliest evidence for microbial subseafloor life. This includes candidate trace fossils in the form of titanite microtextures, and sulfur isotopic signatures of pyrite preserved in metabasaltic glass of the c. 3.472 Ga Hooggenoeg Formation. It has been contended that similar microtextures in altered martian basalts may represent potential extraterrestrial biosignatures of microbe-fluid-rock interaction. But despite numerous studies describing these putative early traces of life, a detailed metamorphic characterization of the microtextures and their host alteration conditions in the ancient pillow lava metabasites is lacking. Here, we present a new nondestructive technique with which to study the in situ metamorphic alteration conditions associated with potential biosignatures in mafic-ultramafic rocks of the Hooggenoeg Formation. Our approach combines quantitative microscale compositional mapping by electron microprobe with inverse thermodynamic modeling to derive low-temperature chlorite crystallization conditions. We found that the titanite microtextures formed under subgreenschist to greenschist facies conditions. Two chlorite temperature groups were identified in the maps surrounding the titanite microtextures and record peak metamorphic conditions at 315 ± 40°C (XFe3+(chlorite) = 25-34%) and lower-temperature chlorite veins/microdomains at T = 210 ± 40°C (lower XFe3+(chlorite) = 40-45%). These results provide the first metamorphic constraints in textural context on the Barberton titanite microtextures and thereby improve our understanding of the local preservation conditions of these potential biosignatures. We suggest that this approach may prove to be an important tool in future studies to assess the biogenicity of these earliest candidate traces of life on Earth. Furthermore, we propose that this mapping approach could also be used to investigate altered mafic-ultramafic extraterrestrial samples containing candidate biosignatures.

Experimental dissolution of molybdenum-sulphides at low oxygen concentrations: A first-order approximation of late Archean atmospheric conditions

The abundance of atmospheric oxygen and its evolution through Earth's history is a highly debated topic. The earliest change of the Mo concentration and isotope composition of marine sediments are interpreted to be linked to the onset of the accumulation of free O2 in Earth's atmosphere. The O2 concentration needed to dissolve significant amounts of Mo in water is not yet quantified, however. We present laboratory experiments on pulverized and surface-cleaned molybdenite (MoS2) and a hydrothermal breccia enriched in Mo-bearing sulphides using a glove box setup. Duration of an experiment was 14 days, and first signs of oxidation and subsequent dissolution of Mo compounds start to occur above an atmospheric oxygen concentration of 72 ± 20 ppmv (i.e., 2.6 to 4.6 × 10−4 present atmospheric level (PAL)). This experimentally determined value coincides with published model calculations supporting atmospheric O2 concentrations between 1 × 10−5 to 3 × 10−4 PAL prior to the Great Oxidation Event and sets an upper limit to the molecular oxygen needed to trigger Mo accumulation and Mo isotope variations recorded in sediments. In combination with the published Mo isotope composition of the rock record, this result implies an atmospheric oxygen concentration prior to 2.76 Ga of below 72 ± 20 ppmv.

Reconstruction of in-situ porosity and porewater compositions of low-permeability crystalline rocks: Magnitude of artefacts induced by drilling and sample recovery

Geological site characterisation programmes typically rely on drill cores for direct information on subsurface rocks. However, porosity, transport properties and porewater composition measured on drill cores can deviate from in-situ values due to two main artefacts caused by drilling and sample recovery: (1) mechanical disruption that increases porosity and (2) contamination of the porewater by drilling fluid. We investigated the effect and magnitude of these perturbations on large drill core samples (12–20 cm long, 5 cmdiameter) of high-grade, granitic gneisses obtained from 350 to 600 m depth in a borehole on Olkiluoto Island (SW Finland). The drilling fluid was traced with sodium–iodide. By combining out-diffusion experiments, gravimetry, UV-microscopy and iodide mass balance calculations, we successfully quantified the magnitudes of the artefacts: 2–6% increase in porosity relative to the bulk connected porosity and 0.9 to 8.9 vol.% contamination by drilling fluid. The spatial distribution of the drilling-induced perturbations was revealed by numerical simulations of 2D diffusion matched to the experimental data. This showed that the rims of the samples have a mechanically disrupted zone 0.04 to 0.22 cm wide, characterised by faster transport properties compared to the undisturbed centre (1.8 to 7.7 times higher pore diffusion coefficient). Chemical contamination was shown to affect an even wider zone in all samples, ranging from 0.15 to 0.60 cm, inwhich iodide enrichmentwas up to 180 mg/kgwater, compared to 0.5 mg/kgwater in the uncontaminated centre. For all samples in the present case study, it turned out that the magnitude of the artefacts caused by drilling and sample recovery is so small that no correction is required for their effects. Therefore, the standard laboratory measurements of porosity, transport properties and porewater composition can be taken as valid in-situ estimates. However, it is clear that the magnitudes strongly depend on site- and drilling-specific factors and therefore our results cannot be transferred simply to other locations. We recommend the approach presented in this study as a route to obtain reliable values in future drilling campaigns aimed at characterising in-situ bedrock properties.

MRD's approach: Ideally suited to support Future Earth research agenda in mountains

Both Future Earth and Mountain Research and Development (MRD) aim to support production and dissemination of knowledge for sustainable development. As shown in Future Earth’s Strategic Research Agenda 2014, the global research community has begun to acknowledge its societal role and the need for a new type of research in which scientists link disciplines and coproduce transformation knowledge with stakeholders. Future Earth has defined three research themes that conceptualize the issues to be dealt with at the same time as the way in which this should be done. In many ways, MRD’s policy has made the journal a forerunner of Future Earth’s stipulated “step-change in research”. Indeed, MRD’s section policies aim to support similar contents and ways of producing these forms of knowledge. MRD publishes “systems knowledge” in its MountainResearch section, “target knowledge” in its MountainAgenda section, and “transformation knowledge” in its MountainDevelopment section. Each of these sections has dedicated review criteria to assess and enhance the quality of the knowledge presented in the papers. In this poster, we provide examples from each of the three sections of what the knowledge types look like, how they are assessed, and how they contribute to the three Future Earth themes.

Variations of clinopyroxene/melt element partitioning during assimilation of olivine/peridotite by low-Mg diorite magma

The effects of crystal chemistry and melt composition on the control of clinopyroxene/melt element partitioning (D) during the assimilation of olivine/peridotite by felsic magma have been investigated in Mesozoic high-Mg diorites from North China. The assimilation resulted in significant increase of Mg, Cr and Ni and only slight (< 30%) decrease of incompatible elements of the magma, and the compositional variations have been mirrored by the normally and reversely zoned clinopyroxene microphenocrysts formed at the early stage of the magma evolution. The Mg# [100 × Mg / (Mg + Fe)] values of the reversely zoned clinopyroxenes increase from 65 to 75 in the core to 85–90 in the high-Mg midsection, and reduce back to 73–79 at the rim. Trace element profiles across all these clinopyroxene domains have been measured by LA-ICP-MS. The melt trace element composition has been constrained from bulk rock analyses of the fine-grained low- and high-Mg diorites. Clinopyroxene/melt partition coefficients for rare earth elements (REE) and Y in the high-Mg group zonings (Mg# > 73–79, DDy < 1.2) are positively correlated with tetrahedral IVAl and increase by a factor of 3–4 as tetrahedral IVAl increases from 0.01 to 0.1 per formula unit (pfu). These systematic variations are interpreted to be controlled by the clinopyroxene composition. In contrast, partition coefficients for low-Mg group zonings (Mg# < 75–79, DDy > 1.2) are elevated by up to an order of magnitude (for REE and Y) or more (for Zr and Hf) at similar IVAl, indicating dominant control of melt composition/structure. DZr and DHf show a larger sensitivity to the compositional change of crystal and melt than DREE. DTi values for the low- and high-Mg zonings show a uniform dependence on IVAl. DSr and DLi are insensitive to the compositional change of clinopyroxene and melt, resulting in Sr depletions in the clinopyroxene zonings with elevated REE without crystallization of plagioclase. Our observations show that crystal chemistry and melt composition/structure may alternatively control clinopyroxene/melt partitioning during the assimilation of peridotite by felsic magma, and may be useful for deciphering clinopyroxene compositions and related crust–mantle processes.

Impact of the arc length on GNSS analysis results

Homogeneously reprocessed combined GPS/GLONASS 1- and 3-day solutions from 1994 to 2013, generated by the Center for Orbit Determination in Europe (CODE) in the frame of the second reprocessing campaign REPRO-2 of the International GNSS Service, as well as GPS- and GLONASS-only 1- and 3-day solutions for the years 2009 to 2011 are analyzed to assess the impact of the arc length on the estimated Earth Orientation Parameters (EOP, namely polar motion and length of day), on the geocenter, and on the orbits. The conventional CODE 3-day solutions assume continuity of orbits, polar motion components, and of other parameters at the day boundaries. An experimental 3-day solution, which assumes continuity of the orbits, but independence from day to day for all other parameters, as well as a non-overlapping 3-day solution, is included into our analysis. The time series of EOPs, geocenter coordinates, and orbit misclosures, are analyzed. The long-arc solutions were found to be superior to the 1-day solutions: the RMS values of EOP and geocenter series are typically reduced between 10 and 40 %, except for the polar motion rates, where RMS reductions by factors of 2–3 with respect to the 1-day solutions are achieved for the overlapping and the non-overlapping 3-day solutions. In the low-frequency part of the spectrum, the reduction is even more important. The better performance of the orbits of 3-day solutions with respect to 1-day solutions is also confirmed by the validation with satellite laser ranging.