ALBERT

Description: High-pressure and high-temperature Raman spectra of synthetic FeCO3 siderite and Mg0.76Fe0.24CO3 magnesite were measured across the spin transition up to nearly 60 GPa and 700 K. In pure siderite the spin transition is sharp and observed between 44 and 46 GPa, with no discernible temperature dependence up to 700 K. The spin transition in Fe-bearing magnesite (“ferromagnesite”) is also sharp and takes place between 45 and 47 GPa at ambient temperature, whereas the transition pressure range broadens significantly at about 600 K (45–52 GPa). Our results suggest that the onset pressure of the spin transition in the siderite–magnesite solid solution series is independent of temperature and composition up to 700 K, whereas the broadening of the spin transition range at higher temperature is driven by the Mg content of the sample. Finally, comparison of the (Mg,Fe)CO3 and the (Mg,Fe)O systems indicates that the onset pressure of the spin transition is temperature-independent in both cases, which is rationalized in terms of the FeO6 octahedral compression.

Description: The high-pressure phase transition of strontianite (SrCO3) was investigated at ambient temperature by means of powder and single-crystal X-ray diffraction. The samples were compressed in a diamond anvil cell to a maximum pressure of 49 GPa. Structure refinements confirm the existence of SrCO3 in the low pressure aragonite-type phase Pmcn (62) up to about 26 GPa. Above this pressure, SrCO3 transforms into a high-pressure phase with post-aragonite crystal structure Pmmn (59). Fitting the volume extracted from the compression data to the third-order Birch–Murnaghan equation of state for the low-pressure phase of SrCO3 yields K0=62.7(6) GPa and K′0=3.2(1), and for the high-pressure phase this yields K0=103(10) GPa and K′0=2.3(6). The unit cell parameters change non-uniformly, with the c axis being 4 times more compressible than the a and b axes. Our results unequivocally show the existence of a Pmmn structure in SrCO3 above 26 GPa and provide important structural parameters for this phase.

Description: A temporally highly resolved reconstruction of sea-ice conditions in eastern Fram Strait, using the sea-ice proxy IP25, sheds new light on potential feedback mechanisms between sea-ice variability and ocean circulation changes during rapid deglacial climate shifts. While a post-LGM sea-ice maximum probably played an important role for the timing of Heinrich Event 1, distinct sea-ice discharge events seem to be intrinsically tied to perturbations in the oceanic overturning circulation. The herein presented sea-ice record is the hitherto only continuous documentation of sea-ice changes in the subpolar North Atlantic that covers the transition from the last glacial into the Holocene. These data strengthen the need for more studies of high-resolution sediment cores to better assess the short-term palaeoenvironmental development and the feedback mechanisms between sea-ice variability and oceanic/ atmospheric circulation fluctuations during this crucial time of climate change.

Description: Eine auf dem Meereisproxy IP25 basierende, zeitlich hochauflösende Rekonstruktion der Meereisbedingungen in der östlichen Framstraße wirft ein neues Licht auf mögliche Wechselwirkungen zwischen Meereisschwankungen und Veränderungen der Ozeanzirkulation während schneller deglazialer Klimaänderungen. Während ein post-LGM Meereismaximum wahrscheinlich eine wichtige Rolle für den Beginn von Heinrich- Ereignis 1 gespielt hat, scheinen Intervalle eines verstärkten Meereisexports untrennbar mit der Schwächung der ozeanischen Umwälzzirkulation verbunden zu sein. Der hier präsentierte Datensatz ist die bisher einzige durchgehende Dokumentation der Meereisveränderungen im subpolaren Nordatlantik während des Übergangs vom letzten Glazial in das Holozän. Diese Daten belegen den Bedarf an weiteren Studien an ähnlich zeitlich hochaufgelösten Sedimentkernabfolgen, um die kurzzeitige Entwicklung der Paläoumweltbedingungen und die Wechselwirkungen zwischen Meereisveränderungen und der ozeanisch/atmosphärischen Zirkulation während dieser Zeit häufiger Klimaschwankungen besser abschätzen zu können.

Description: IfE_GOCE05s is a GOCE-only global gravity field model, which was developed at the Institut für Erdmessung (IfE), Leibniz Universität Hannover, Germany. The observations with a time span from 1 November 2009 to 20 October 2013 are used for the model recovery. The GOCE precise kinematic orbit with 1-s sampling rate is processed for the gravity field up to degree/order 150, while the three main diagonal gravity gradients are down-sampled to 2 s and used to recover the model up to degree/order 250. With two additional Kaula’s regularizations, the combined model “IfE_GOCE05s” is derived, with a maximum degree of 250. To develop IfE_GOCE05s, the following GOCE data (01.11.2009 - 20.10.2013) was used: * Orbits: SST_PKI_2, SST_IAQ_2; * Gradients: EGG_GGT_2, EGG_IAQ_2. None any priori gravity field information was used. Processing procedures: Gravity from orbits (SST): * Acceleration approach was applied to the kinematic orbit data; * PKI data was at 1 s sampling rate; * Model was derived up to d/o (degree/order) 150; * VCM (Variance-Covariance Matrix) was derived arc-wisely from the post-fit residuals. Gravity from gradients (SGG): * Gradients Vxx, Vyy and Vzz in the GRF (Gradiometer Reference Frame) were used; * Gradients were down-sampled to 2 s; * Model was derived up to d/o 250; * VCM was estimated arc-wisely from the post-fit residuals. Regularization: * A strong Kaula-regularization was applied to constrain the (near-)zonal coefficients that are degraded by the polar gap problem; * A slight Kaula-regularization was applied to improve the signal-to-noise ratio of the coefficients between d/o 201 and 250; * The regularization parameters were empirically determined. Combined solution: * The normal equations for SST and SGG were summed wih proper weighting factors; * Weighting factors for SST and SGG were determined from variance component estimation; * A direct inversion was applied on the final normal equation.

Description: The mission option OPTIMA (OPTical Interferometry for global MAss change detection from space) is one out of a variety of mission concepts which are investigated in the Future Gravity Missions project in detail. OPTIMA is based on experiences and observation concepts of GRACE and GOCE, that are combined and improved by innovative laser system components. More precisely two spacecraft in GRACE-like constellation for determining the long wavelengths of the gravity field are complemented by a gradiometer for the precise determination of medium and short wavelengths. Novelty here is that both the distance measurement between the satellites and the observation of the gradiometer test masses is based on optical techniques. The satellites will carry GNSS antennas that are mounted on the zenith side for spacecraft positioning and on the nadir side for GNSS reflectometry. GNSS reflectometry provides the opportunity to obtain a further functional of the gravity field at the same position and time besides gradients and range rates. Considering the noise behavior of the new instrumentation, the benefit of the OPTIMA concept in terms of RMS error per degree is compared to an »older« GRACE mission scenario. OPTIMA will allow monthly gravity field determination with much increased accuracy over all spherical harmonic degrees up to l = 250.

Description: Improving and homogenizing time and space reference systems on Earth and, more specifically, realizing the Terrestrial Reference Frame (TRF) with an accuracy of 1 mm and a long-term stability of 0.1 mm/year are relevant for many scientific and societal endeavors. The knowledge of the TRF is fundamental for Earth and navigation sciences. For instance, quantifying sea level change strongly depends on an accurate determination of the geocenter motion but also of the positions of continental and island reference stations, such as those located at tide gauges, as well as the ground stations of tracking networks. Also, numerous applications in geophysics require absolute millimeter precision from the reference frame, as for example monitoring tectonic motion or crustal deformation, contributing to a better understanding of natural hazards. The TRF accuracy to be achieved represents the consensus of various authorities, including the International Association of Geodesy (IAG), which has enunciated geodesy requirements for Earth sciences. Moreover, the United Nations Resolution 69/266 states that the full societal benefits in developing satellite missions for positioning and Remote Sensing of the Earth are realized only if they are referenced to a common global geodetic reference frame at the national, regional and global levels. Today we are still far from these ambitious accuracy and stability goals for the realization of the TRF. However, a combination and co-location of all four space geodetic techniques on one satellite platform can significantly contribute to achieving these goals. This is the purpose of the GENESIS mission, a component of the FutureNAV program of the European Space Agency. The GENESIS platform will be a dynamic space geodetic observatory carrying all the geodetic instruments referenced to one another through carefully calibrated space ties. The co-location of the techniques in space will solve the inconsistencies and biases between the different geodetic techniques in order to reach the TRF accuracy and stability goals endorsed by the various international authorities and the scientific community. The purpose of this paper is to review the state-of-the-art and explain the benefits of the GENESIS mission in Earth sciences, navigation sciences and metrology. This paper has been written and supported by a large community of scientists from many countries and working in several different fields of science, ranging from geophysics and geodesy to time and frequency metrology, navigation and positioning. As it is explained throughout this paper, there is a very high scientific consensus that the GENESIS mission would deliver exemplary science and societal benefits across a multidisciplinary range of Navigation and Earth sciences applications, constituting a global infrastructure that is internationally agreed to be strongly desirable.

Description: The first ESA Earth Explorer Core Mission GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) entered the operational measurement phase in September 2009. Before gravity field processing, the quality of the GOCE gradients in the measurement bandwidth (5-100 mHz), MBW, has to be assessed. Here, two procedures have been developed in Hanover, the mutual comparison and analysis of observed gradients in satellite track crossovers and the application of terrestrial gravity data which are upward continued and transformed into reference gradients for the GOCE gradiometer measurements. First the gravity gradients are filtered, where the longer wavelength signals below the MBW are replaced by global geopotential model (GPM) information. The filtered time series is used as input signal for both validation methods.

Description: Our joint high-pressure and high-temperature mid-infrared absorbance and Raman investigations on a synthetic strontianite sample led to the construction of the respective pressure–temperature P–T phase diagram (maximum P = 38 GPa, maximum T = 800 K). Our results allowed for the determination of the P–T slope for the onset pressure of the aragonite Pmcn → post-aragonite Pmmn structural transition reported in the literature, which is found to be negative and equal to ΔP/ΔT = − 0.012 GPa/K. The determined SrCO3 P–T phase diagram appears consistent with the respective P–T phase diagrams of relevant aragonite-type carbonates. In addition, we provide the first ever measured far-infrared absorbance spectra under pressure for the family of aragonite-type carbonates. Our complementary first-principle calculations were able to reproduce the infrared spectra of both Pmcn and Pmmn phases of strontianite.

Description: Using optical absorption and Raman spectroscopic measurements, in conjunction with the first-principles calculations, a pressure-induced high-spin (HS)-to-low-spin (LS) state electronic transition of Fe2+ (M2-octahedral site) was resolved around 76–80 GPa in a natural triphylite–lithiophilite sample with chemical composition M1LiM2Fe2+0.708Mn0.292PO4 (theoretical composition M1LiM2Fe2+0.5Mn0.5PO4). The optical absorption spectra at ambient conditions consist of a broad doublet band with two constituents ν1 (~ 9330 cm−1) and ν2 (~ 7110 cm−1), resulting from the electronic spin-allowed transition 5T2g → 5Eg of octahedral HSM2Fe2+. Both ν1 and ν2 bands shift non-linearly with pressure to higher energies up to ~ 55 GPa. In the optical absorption spectrum measured at ~ 81 GPa, the aforementioned HS-related bands disappear, whereas a new broadband with an intensity maximum close to 16,360 cm−1 appears, superimposed on the tail of the high-energy ligand-to-metal O2− → Fe2+ charge-transfer absorption edge. We assign this new band to the electronic spin-allowed dd-transition 1A1g → 1T1g of LS Fe2+ in octahedral coordination. The high-pressure Raman spectra evidence the Fe2+ HS-to-LS transition mainly from the abrupt shift of the P–O symmetric stretching modes to lower frequencies at ~ 76 GPa, the highest pressure achieved in the Raman spectroscopic experiments. Calculations indicated that the presence of M2Mn2+ simply shifts the isostructural HS-to-LS transition to higher pressures compared to the triphylite M2Fe2+ end-member, in qualitative agreement with our experimental observations.

Description: As changes in gravity are directly related to mass variability, satellite missions observing the Earth’s time varying gravity field are a unique tool for observing mass transport processes in the Earth system, such as the water cycle, rapid changes in the cryosphere, oceans, and solid Earth processes, on a global scale. The observation of Earth’s gravity field was successfully performed by the GRACE and GOCE satellite missions, and will be continued by the GRACE Follow-On mission. A comprehensive team of European scientists proposed the next-generation gravity field mission MOBILE in response to the European Space Agency (ESA) call for a Core Mission in the frame of Earth Explorer 10 (EE10). MOBILE is based on the innovative observational concept of a high-low tracking formation with micrometer ranging accuracy, complemented by new instrument concepts. Since a high-low tracking mission primarily observes the radial component of gravity-induced orbit perturbations, the error structure is close to isotropic. This geometry significantly reduces artefacts of previous along-track ranging low-low formations (GRACE, GRACE-Follow-On) such as the typical striping patterns. The minimum configuration consists of at least two medium-Earth orbiters (MEOs) at 10000 km altitude or higher, and one low-Earth orbiter (LEO) at 350-400 km. The main instrument is a laser-based distance or distance change measurement system, which is placed at the LEO. The MEOs are equipped either with passive reflectors or transponders. In a numerical closed-loop simulation, it was demonstrated that this minimum configuration is in agreement with the threshold science requirements of 5 mm equivalent water height (EWH) accuracy at 400 km wavelength, and 10 cm EWH at 200 km. MOBILE provides promising potential future perspectives by linking the concept to existing space infrastructure such as Galileo next-generation, as future element of the Copernicus/Sentinel programme, and holds the potential of miniaturization even up to swarm configurations. As such MOBILE can be considered as a precursor and role model for a sustained mass transport observing system from space.