ALBERT

Description: [1]  We investigate the effect of self-attraction and loading (SAL) induced by the projected accumulation of sea water on shallow continental shelf areas. [2]  Using output from a climate model we compute 21st century changes in regional steric sea surface height and find that steric changes are largest over the deep ocean and relatively small on the shallow continental shelves. The resulting redistribution of sea water towards the shelf areas leads to mass accumulation on the shelves and therefore to increased gravitational attraction as well as increased loading on the sea floor. [3]  We find that, depending on the scenario and region, SAL effects may result in an additional sea level rise of 1–3 cm on the world's continental shelf areas by the end of the 21st century. These estimates are at most 15% of the combined changes in sea surface height induced by redistribution of water masses and steric expansion.

Description: [1]  We quantify gravity changes after great earthquakes present within the 10-year-long time-series of monthly GRACE gravity fields. Using spherical harmonic normal-mode formulation, the respective source parameters of moment tensor and double-couple were estimated. For the 2004 Sumatra-Andaman earthquake, the gravity data indicate a composite moment of 1.2 × 10 23  N-m with a dip of 10°, in agreement with the estimate obtained at ultra-long seismic periods. For the 2010 Maule earthquake, the GRACE solutions range 2.0–2.7 × 10 22  N-m for dips of 12–24°, and centroid depths within the lower crust. For the 2011 Tohoku-Oki earthquake, the estimated scalar moments range 4.1–6.1 × 10 22  N-m, with dips of 9–19°, and centroid depths within the lower crust. For the 2012 Indian Ocean strike-slip earthquakes, the gravity data delineate a composite moment of 1.9 × 10 22  N-m regardless of centroid depth, comparing favorably with the total moment of the main ruptures and aftershocks. The smallest event we successfully analyzed with GRACE was the 2007 Bengkulu earthquake with M 0  ~ 5.0 × 10 21  N-m. We found that the gravity data constrain the focal mechanism with the centroid only within the upper and lower crustal layers for thrust events. Deeper sources ( i . e ., in the upper mantle) could not reproduce the gravity observation as the larger rigidity and bulk modulus at mantle depths inhibit the interior from changing its volume, thus reducing the negative gravity component. Focal mechanisms and seismic moments obtained in this study represent the behavior of the sources on temporal and spatial scales exceeding the seismic and geodetic spectrum.

Description: The only vertical land movement signal routinely corrected for when estimating absolute sea-level change from tide gauge data is that due to glacial isostatic adjustment (GIA). We compare modeled GIA uplift (ICE-5G + VM2) with vertical land movement at ∼300 GPS stations located near to a global set of tide gauges, and find regionally coherent differences of commonly ±0.5–2 mm/yr. Reference frame differences and signal due to present-day mass trends cannot reconcile these differences. We examine sensitivity to the GIA Earth model by fitting to a subset of the GPS velocities and find substantial regional sensitivity, but no single Earth model is able to reduce the disagreement in all regions. We suggest errors in ice history and neglected lateral Earth structure dominate model-data differences, and urge caution in the use of modeled GIA uplift alone when interpreting regional- and global- scale absolute (geocentric) sea level from tide gauge data.

Description: Bedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often over-estimated, particularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted root-mean-squares of observation-model differences: 4.9–5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high.

Description: We investigate numerically apparent multi-fractal behavior of samples from synthetically generated processes subordinated to truncated fractional Brownian motion (tfBm) on finite domains. We are motivated by the recognition that many earth and environmental (including hydrologic) variables appear to be self-affine (monofractal) or multifractal with Gaussian or heavy-tailed distributions. The literature considers self-affine and multifractal types of scaling to be fundamentally different, the first arising from additive and the second from multiplicative random fields or processes. It has been demonstrated theoretically (Neuman, 2010a, 2011) that square or absolute increments of samples from Gaussian/Lévy processes subordinated to tfBm exhibit apparent/spurious multifractality at intermediate ranges of separation lags, with breakdown in power-law scaling at small and large lags as is commonly exhibited by real data. A preliminary numerical demonstration of apparent multifractality by Neuman (2010b) was limited to Gaussian fields having nearest neighbor autocorrelations and led to rather noisy results. Here we improve upon Neuman's numerical analysis by adopting a much simpler but more complete and accurate generation scheme proposed by Neuman (2011). This allows us to investigate with greater fidelity apparent multifractal behaviors of samples taken from a broader range of processes including Gaussian with and without symmetric Lévy and log-normal (as well as potentially other) subordinators. Our results shed new light on the nature of apparent multifractality which has wide implications vis-a-vis the scaling of many hydrologic as well as other earth and environmental variables. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Variations in the Earth's water cycle are commonly quantified by their effect on global mean sea-level. However, the interaction between passive adjustment of the ocean to changes in gravitational attraction due to mass redistribution, the related deformation of the solid Earth and disturbances in the Earth's rotation vector will yield a distribution that is more complicated than a uniform rise or fall of the ocean's surface. In this study, we present the first estimates of seasonal changes in passive sea-level (which we define as the height difference between the sea surface at rest and ocean floor, excluding steric and dynamical effects) based on direct observations of surface mass redistribution, made by the Gravity Recovery and Climate Experiment (GRACE) between 2003 and 2010. We show that this “selfgravitation-effect” causes seasonal variations of the sea-level of up to 1 cm – comparable to the amplitude of the long-period tides – and that inclusion in numerical ocean models results in a better agreement between observed and modelled ocean bottom pressure variations, particularly in coastal zones.

Description: We analyze theoretically the ability of model quality (sometimes termed information or discrimination) criteria such as the negative log likelihood NLL, Bayesian criteria BIC and KIC and information theoretic criteria AIC, AICc, and HIC to estimate (1) the parameter vector $\boldrtheta$ of the variogram of hydraulic log conductivity (Y = ln K), and (2) statistical parameters $\sigma _{hE}^2$ and $\sigma _{YE}^2$ proportional to head and log conductivity measurement error variances, respectively, in the context of geostatistical groundwater flow inversion. Our analysis extends the work of Hernandez et al. (2003, 2006) and Riva et al. (2009), who developed nonlinear stochastic inverse algorithms that allow conditioning estimates of steady state and transient hydraulic heads, fluxes and their associated uncertainty on information about conductivity and head data collected in a randomly heterogeneous confined aquifer. Their algorithms are based on recursive numerical approximations of exact nonlocal conditional equations describing the mean and (co)variance of groundwater flow. Log conductivity is parameterized geostatistically based on measured values at discrete locations and unknown values at discrete “pilot points.” Optionally, the maximum likelihood function on which the inverse estimation of Y at pilot points is based may include a regularization term reflecting prior information about Y. The relative weight $\lambda = \sigma _{hE}^2/\sigma _{YE}^2$ assigned to this term and its components $\sigma _{hE}^2$ and $\sigma _{YE}^2$, as well as $\boldrtheta$ are evaluated separately from other model parameters to avoid bias and instability. This evaluation is done on the basis of criteria such as NLL, KIC, BIC, HIC, AIC, and AICc. We demonstrate theoretically that, whereas all these six criteria make it possible to estimate $\sigma _{hE}^2$, KIC alone allows one to estimate validly $\boldrtheta$ and $\sigma _{YE}^2$ (and thus $\lambda$). We illustrate this discriminatory power of KIC numerically by using a differential evolution genetic search algorithm to minimize it in the context of a two-dimensional steady state groundwater flow problem. We find that whereas $\sigma _{hE}^2,\sigma _{YE}^2$, and the integral scale of Y can be estimated on the basis of a zero-order mean flow equation, the sill of the Y-variogram is estimated more accurately by a second-order approximation of flow. This notwithstanding, KIC prefers the simpler zero-order moment over the more complex second-order version.

Description: Question: Does community assembly on roadsides differ between two contrasting habitats along a stress–productivity gradient? Is establishment success determined by the regional pool, environmental filters or historical events? Location: Highway roadcuts and embankments in central Spain (40°29′N, 03°34′W). Methods: Species composition was recorded annually in 45 plots distributed on steep slopes with newly exposed surfaces (roadcuts) or newly built topsoiled substrates (embankments), for 4 years following hydroseeding with standard or native seed mixture. Frequency of appearance and local colonization and extinction rates of individual species were clustered in establishment success groups. We examined the correspondence between descriptive plant traits and species performance on both roadslope types. Results: Roadslope species richness showed a sustained increase over time, although at consistently lower levels and rates on the more productive embankments. Sixty per cent of the colonization events involved species from the surrounding vegetation. Hydroseeded species persisted through time, but did not modify community composition or dynamics. A higher establishment success rate was found in wind-pollinated species with large seeds and in exotic species. Those species growing on embankments showed an equal or higher establishment success rate on roadcuts and, conversely, species from roadcuts exhibited an equal or lower success rate on embankments. Conclusions: At the initial stages of community assemblage on unproductive newly created areas, species richness was shaped by the regional species pool. Communities on less harsh topsoiled embankments were subjected, however, to the filtering effect of competitive exclusion. In a reclamation context, efforts to increase site productivity may have detrimental consequences for in situ conservation of local diversity.

Description: Approaches are needed to better predict spatial variation in riverine Hg concentrations across heterogeneous landscapes that include mountains, wetlands, and open waters. We applied multivariate linear regression to determine the landscape factors and chemical variables that best account for the spatial variation of total Hg (THg) and methyl Hg (MeHg) concentrations in 27 sub-basins across the 493 km2 upper Hudson River basin in the Adirondack Mountains of New York. THg concentrations varied by sixfold, and those of MeHg by 40-fold in synoptic samples collected at low-to-moderate flow, during spring and summer of 2006 and 2008. Bivariate linear regression relations of THg and MeHg concentrations with either percent wetland area or DOC concentrations were significant but could account for only about 1/3 of the variation in these Hg forms in summer. In contrast, multivariate linear regression relations that included metrics of (1) hydrogeomorphology, (2) riparian/wetland area, and (3) open water, explained about 66% to 〉90% of spatial variation in each Hg form in spring and summer samples. These metrics reflect the influence of basin morphometry and riparian soils on Hg source and transport, and the role of open water as a Hg sink. Multivariate models based solely on these landscape metrics generally accounted for as much or more of the variation in Hg concentrations than models based on chemical and physical metrics, and show great promise for identifying waters with expected high Hg concentrations in the Adirondack region and similar glaciated riverine ecosystems.