ALBERT

Description: Methane clathrate hydrate (structure I) is found to be very strong, based on laboratory triaxial deformation experiments we have carried out on samples of synthetic, high‐purity, polycrystalline material. Samples were deformed in compressional creep tests (i.e., constant applied stress, σ), at conditions of confining pressure P = 50 and 100 MPa, strain rate 4.5 × 10−8 ≤ equation image ≤ 4.3 × 10−4 s−1, temperature 260 ≤ T ≤ 287 K, and internal methane pressure 10 ≤ PCH4 ≤ 15 MPa. At steady state, typically reached in a few percent strain, methane hydrate exhibited strength that was far higher than expected on the basis of published work. In terms of the standard high‐temperature creep law, equation image = Aσne−(E*+PV*)/RT the rheology is described by the constants A = 108.55 MPa−n s−1, n = 2.2, E* = 90,000 J mol−1, and V* = 19 cm3 mol−1. For comparison, at temperatures just below the ice point, methane hydrate at a given strain rate is over 20 times stronger than ice, and the contrast increases at lower temperatures. The possible occurrence of syntectonic dissociation of methane hydrate to methane plus free water in these experiments suggests that the high strength measured here may be only a lower bound. On Earth, high strength in hydrate‐bearing formations implies higher energy release upon decomposition and subsequent failure. In the outer solar system, if Titan has a 100‐km‐thick near‐surface layer of high‐strength, low‐thermal conductivity methane hydrate as has been suggested, its interior is likely to be considerably warmer than previously expected.

Description: Most Central American volcanoes occur in an impressive volcanic front that trends parallel to the strike of the subducting Cocos Plate. The volcanic front is a chain, made of right-stepping, linear segments, 100 to 300 Km in length. Volcanoes cluster into centers, whose spacing is random but averages about 27 Km. These closely spaced, easily accessible volcanic centers allow mapping of geochemical variations along the volcanic front. Abundant back-arc volcanoes in southeast Guatemala and central Honduras allow two cross-arc transects. Several element and isotope ratios (e.g. BalLa, Uffh, B/La, IOBe/9Be, 87Sr/86Sr) that are thought to signal subducted marine sediments or altered MORB consistently define a chevron pattern along the arc, with its maximum in Nicaragua. BalLa, a particularly sensitive signal, is 130 at the maximum in Nicaragua but decreases out on the limbs to 40 in Guatemala and 20 in Costa Rica, which is just above the nominal mantle value of 15. This high amplitude regional variation, roughly symmetrical about Nicaragua, contrasts with the near constancy, or small gradient, in several plate tectonic parameters such as convergence rate, age of the subducting Cocos Plate, and thickness and type of subducted sediment. The large geochemical changes over relatively short distances make Central America an important margin for seeking the tectonic causes of geochemical variations; the regional variation has both a high amplitude and structure, including flat areas and gradients. The geochemical database continues to improve and is already adequate to compare to tectonic models with length scales of 100 Km or longer.

Description: The southern central Chilean margin at the site of the largest historically recorded earthquake in the Valdivia region, in 1960 (Mw = 9.5), is part of the 5000-km-long active subduction system whose geodynamic evolution is controversially debated and poorly understood. Covering the area between 36° and 40°S, the oceanic crust is segmented by prominent fracture zones. The offshore forearc and its onshore continuation show a complex image with segments of varying geophysical character, and several fault systems active during the past 24 m.y. In autumn 2001, the project SPOC was organized to study the Subduction Processes Off Chile, with a focus on the seismogenic coupling zone and the forearc. The acquired seismic data crossing the Chilean subduction system were gathered in a combined offshore-onshore survey and provide new insights into the lithospheric structure and evolution of active margins with insignificant frontal accretion.

Description: [1] Very rare, halogen-rich andesite melt inclusions (HRA) in bytownitic plagioclase phenocrysts (An89–90) from tephra fallout of the Izu arc volcanic front (Izu VF) provide new insights into the processes of fluid release from slab trenchward to the volcanic front in a cool subduction zone. These HRA are markedly enriched in Cl, F and Li - by factors of up to 8 (Cl, F) and 1.5 (Li) - but indistinguishable with respect to the fluid-mobile large-ion lithophile elements (LILE; K, Sr, Rb, Cs, Ba, Pb, U), rare earths (REE) or high field strength elements (HFSE) from the low-K tholeiitic magmas of the Izu VF. We suggest that the chemical signature of the HRA reflects the presence of a fluid in the mantle source that originated from the serpentinized mantle peridotite above the metacrust. This “wedge serpentinite” presumably formed by fluid infiltration beneath the forearc and was subsequently down-dragged with the slab to arc front depths. The combined evidence from the Izu VF (∼110 km above slab) and the outer forearc serpentinite seamounts (∼25 to 30 km above slab) suggests that the slab flux of B and Cl is highest beneath the forearc, and decreases with increasing slab depths. In contrast, the slab flux of Li is minor beneath the forearc, but increases with depth. Fluorine may behave similarly to Li, whereas the fluid-mobile LILE appear to be largely retained in the slab trenchward from the Izu VF. Consequently, the chemical signatures of both Izu trench sediments and basaltic rocks appear preserved until arc front depths.

Description: The convergent Sunda margin off Indonesia displays all geological features characteristic of an accretion-dominated subduction zone. A combined interpretation of prestack depth-migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high-resolution bathymetric data and for the first time allows the exact mapping of backstop regimes. Initially, the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin. Increasing material strength of the outer high due to lithification formed a dynamic backstop, which controls accretion today. An out-of-sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area. The existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc, which is observed in regimes of frontal as well as of oblique subduction. Mass balance calculations, which account for porosity changes and metamorphism, indicate a subduction history dominated by accretionary processes since the late Eocene. Accretion is associated with the low values of basal friction inferred for the Sunda margin. Structural investigations of conjugate fault planes indicate a very weak basal detachment. Effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments, whereas overpressuring within the frontal accretionary prism is negligible.

Description: Simplified representations of spatially inhomogeneous (three-dimensional (3-D)) clouds in radiative transfer models provide systematic errors when calculating solar broadband radiative fluxes. An example is the neglect of horizontal photon transports as it is the case for the independent column approximation (ICA). The present work tries to quantify and interpret these errors on the basis of a large set of 3-D mixed phase cloud scenarios with 3-D varying extinction coefficients, scattering phase functions, and single-scattering albedos. The cloud cases result from a mesoscale atmospheric circulation model with detailed cloud microphysics. Domain-averaged cloud radiative fluxes are calculated by means of a Monte Carlo radiative transfer model. Depending on cloud type and solar zenith angle (SZA) the differences between 3-D and ICA results range from +20 W m−2 to −30 W m−2 for the upward reflected fluxes and from +10 W m−2 to −7 W m−2 for the absorbed fluxes. The mean (averaged over all cloud realizations) errors of the ICA-based upward fluxes vary between 5 W m−2 overestimation at 15°SZA and 6 W m−2 underestimation at 75°SZA. The ICA underestimates the absorbed flux by ∼1–2 W m−2 for most SZA except for 75°. It is found that neglecting the horizontal variability of the absorption and scattering properties of the cloud hydrometeors leads to a general underestimation of solar broadband absorption by as much as 15 W m−2 with average values between 4 W m−2 at small SZA and 1 W m−2 at large SZA.

Description: Between 1996 and 1998, a concerted effort was made to study the deep open ocean convection in the Labrador Sea. Both in situ observations and numerical models were employed with close collaboration between the researchers in the fields of physical oceanography, boundary layer meteorology, and climate. A multitude of different methods were used to observe the state of ocean and atmosphere and determine the exchange between them over the experiment's period. The Labrador Sea Deep Convection Experiment data collection aims to assemble the observational data sets in order to facilitate the exchange and collaboration between the various projects and new projects for an overall synthesis. A common file format and a browsable inventory have been used so as to simplify the access to the data.

Description: Current hydrographic data can provide snapshots but no continuous timeseries of the meridional overturning circulation (MOC). Using output from two eddy-permitting numerical ocean models we test the feasibility of a monitoring system for the MOC in the North Atlantic. The results suggest that a relatively simple arrangement, using moorings placed across a longitude-depth section and the zonal wind stress, is able to capture most of the MOC strength and vertical structure as a function of time. Being closely related to the transport of energy to the North Atlantic, measuring the MOC would open the prospect of having continuous information about a key element of northern hemisphere climate.