ALBERT

Description: Shipboard bathymetry and gravity data from 30 crossings of 6 great Pacific fracture zones (FZs), the Mendocino, Murray, Molokai, Clarion, Clipperton, and Udintsev, are compared with the predictions of a model in which FZs are locked beyond the ridge-transform intersection, such that no vertical motion occurs on the fault in response to differential thermal subsidence. At least some sections of all of these FZs, except the Molokai, are consistent with this model and sustain shear stresses as high as 20 MPa. However, none of the FZs is locked along its entire length, as inferred from observed shear stresses dropping below 75% of the value necessary to maintain a locked fault. There is some suggestion that the unlocking may be related to excess volcanism.

Description: Seismically derived depth estimates to the top of the oceanic crust beneath the Hawaiian Islands indicate that the curvature of the deflected lithosphère is much larger than commonly believed. The conservative and model-independent curvature estimates exceed 10−7 m−1 and are comparable in magnitude to curvatures at trenches and outer rise systems. The depth estimates are used to constrain both two-dimensional (2-D) and three-dimensional (3-D) flexural models. The curvature constraints require a 2-D variable elastic thickness that decreases from 35 km in areas away from the volcanic load to 25 km directly beneath the load. In an attempt to understand the nature of the yielding beneath the Hawaiian Islands we introduce two new 3-D models. The first model combines a realistic yield strength based rheology with a new technique for 3-D flexure calculations in which the elastic plate thickness is curvature-dependent. The new variable rigidity model predicts an undeformed (mechanical) plate thickness of 44 km, decreasing to 33 km beneath the big island of Hawaii. The best-fitting mechanical thickness corresponds approximately to the depth to the 600 °C isotherm in 90-m.y.-old lithosphere. The second model uses a broken plate, but here the crack is oriented along the weak Molokai fracture zone rather than along the island chain trend. This unconventional flexure model can explain the observed asymmetry in the depth data across the fracture zone without requiring the excessively large elastic thickness of more conventional broken plate models. Both the proposed models imply that modeling with constant thickness plates may underestimate the true mechanical plate thickness by being unduly influenced by the weak zone beneath the seamounts.

Description: Drift is a prominent parameter characterizing the Arctic sea ice cover that has a deep impact on the climate system. Hence it is a key issue to both the remote sensing as well as the modeling community, to provide reliable sea ice drift fields. This study focuses on the comparison of sea ice drift results from different sea ice‐ocean coupled models and the validation with observational data in the period 1979–2001. The models all take part in the Arctic Ocean Model Intercomparison Project (AOMIP) and the observations are mainly based on satellite imagery. According to speed distributions, one class of models has a mode at drift speeds around 3 cm s −1 and a short tail toward higher speeds. Another class shows a more even frequency distribution with large probability of drift speeds of 10 to 20 cm s −1 . Observations clearly agree better with the first class of model results. Reasons for these differences are manifold and lie in discrepancies of wind stress forcing as well as sea ice model characteristics and sea ice‐ocean coupling. Moreover, we investigated the drift patterns of anticyclonic and cyclonic wind‐driven regimes. The models are capable of producing realistic drift pattern variability. The winter of 1994/1995 stands out because of its maximum in Fram Strait ice export. Although export estimates of some models agree with observations, the corresponding inner Arctic drift pattern is not reproduced. The reason for this is found in the wind‐forcing as well as in differences in ocean velocities.

Description: Structure and growth of the Izu‐Bonin‐Mariana arc crust: 1. Seismic constraint on crust and mantle structure of the Mariana arc–back‐arc system Narumi Takahashi Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Shuichi Kodaira Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Yoshiyuki Tatsumi Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Yoshiyuki Kaneda Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Kiyoshi Suyehiro Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan A high‐resolution seismic velocity model is presented for the crust and upper mantle of the Mariana arc–back‐arc system (MABS) based on active source seismic profiling. The major characteristics are (1) slow mantle velocity of 〈8 km s −1 in the uppermost mantle, especially, and deep reflectors under the Mariana arc (MA) and the West Mariana Ridge (WMR), (2) a deep reflector in the upper mantle beneath the relative thick crust of the Mariana Trough (MT) axis, (3) distribution of lower‐velocity lower crusts (6.7–6.9 km s −1 ) beneath the volcanic front and adjacent to the MT, and (4) high‐velocity lower crust (7.2–7.4 km s −1 ) beneath the boundary regions between the MA and MT, and between the WMR and the Parece Vela Basin (PVB), adding to structural characteristics of crust and upper mantle beneath the MABS. Of the characteristics described above, characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the Moho, considering that the WMR is extinct arc currently. On the other hand, characteristic 2 suggests that the origin of deep reflectors beneath the MT axis might be lower velocity materials due to the diffractive signals with strong amplitudes, characteristic 3 suggests that the lower‐velocity lower crust advanced crustal growth and characteristic 4 suggests that the high‐velocity lower crust beneath arc–back‐arc transition zone is composed of mafic/ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite.

Description: On the Pacific margin off central Costa Rica, an anomalous lens-shaped zone is located between the overriding plate and the subducting oceanic lithosphere approximately 25 km landward of the deformation front. This feature was previously recognized in reflection seismic data when it was termed 'megalens'. Its origin and seismic velocity structure, however, could not unambiguously be derived from earlier studies. Therefore during RV SONNE cruise SO163, seismic wide-angle data were acquired in 2002 using closely spaced ocean bottom hydrophones and seismometers along two parallel strike and two parallel dip lines above the 'megalens', intersecting on the middle slope. The P-wave velocities and structure of the subducting oceanic Cocos Plate and overriding Caribbean Plate were determined by modelling the wide-angle seismic data in combination with the analysis of coincident reflection seismic data and the use of synthetic seismograms. The margin wedge is defined by high seismic velocities (4.3-6.1 km s(-1)) identified within a wedge-shaped body covered by a slope sediment drape. It is divided into two layers with different velocity gradients. The lower margin wedge is clearly constrained by decreasing velocities trenchward and terminates beneath the middle slope at the location of the 'megalens'. Seismic velocities of the 'megalens' are lower (3.8-4.3 km s(-1)) relative to the margin wedge. We propose that the 'megalens' represents hybrid material composed of subducted sediment and eroded fragments from the base of the upper plate. Upward-migrating overpressured fluids weaken the base of the margin wedge through hydrofracturing, thus causing material transfer from the upper plate to the lower plate. Results from amplitude modelling support that the 'megalens' observed off central Costa Rica is bound by a low-velocity zone documenting fluid drainage from the plate boundary to the upper plate.

Description: Lilliput was discovered in 2005 as the southernmost known hydrothermal field along the Mid-Atlantic Ridge. It is exceptional in that it lacks high-temperature venting probably because of a thickened crust. The absence of thermophilic and hyperthermophilic prokaryotes in emissions supports the argument against the presence of a hot subsurface at Lilliput, as is typically suggested for diffuse emissions from areas of high-temperature venting. The high phylogenetic diversity and novelty of bacteria observed could be because of the low-temperature influence, the distinct location of the hydrothermal field or the Bathymodiolus assemblages covering the sites of discharge. The low-temperature fluids at the Lilliput are characterized by lowered pH and slightly elevated hydrogen (16 nM) and methane (∼2.6 μM) contents compared with ambient seawater. No typical hydrogen and methane oxidizing prokaryotes were detected. The higher diversity of reverse tricarboxylic acid genes and the form II RubisCO genes of the Calvin Benson-Bassham (CBB) cycle compared with the form I RubisCO genes of the CBB cycle suggests that the chemoautotrophic community is better adapted to low oxygen concentrations. Thiomicrospira spp. and Epsilonproteobacteria dominated the autotrophic community. Sulfide is the most abundant inorganic energy source (0.5 mM). Diverse bacteria were associated with sulfur cycling, including Gamma-, Delta- and Epsilonproteobacteria, with the latter being the most abundant bacteria according to fluorescence in situ hybridization. With members of various Candidate Divisions constituting for 25% of clone library sequences we suggest that their role in vent ecosystems might be more important than previously assumed and propose potential mechanisms they might be involved in at the Lilliput hydrothermal field.

Description: Changes in permeability and porosity during shortening deformation of Carrara marble and hot-pressed calcite aggregates were measured under high pressure at room temperature using argon as pore fluid. At effective pressures of 30 and 50 MPa, the permeability of Carrara marble increased by up to 2 orders of magnitude with less than 2% strain during which the connected porosity increased by only 0.005. The permeability increased more slowly with further strain up to 18%, during which the connected porosity increased by a further 0.05 to 0.06. At effective pressures of 100 MPa to 200 MPa, these effects were much less marked. In hot-pressed calcite aggregates, deformed at an effective pressure of 50 MPa, the permeability increased by about 2 orders of magnitude after about 12% strain and an increase in connected porosity of about 0.03. Microstructural studies indicate that, in the coarse-grained Carrara marble specimens, both transgranular and grain boundary cracks are present after room temperature deformation. For a given strain, the average length and the linear density of transgranular cracks decrease with increasing effective pressure. In fine-grained, hot-pressed calcite aggregates, dilatancy is mainly due to opening of grain boundary cracks. The very marked increase in permeability with small strain at low effective pressure can be correlated with the proliferation of connected microcracks of relatively large apertures, deduced on the basis of theoretical models.

Description: This is the first record of intersexuality to have been found within the Cephalopoda. Seven out of 16 sexually mature Ancistrocheirus lesueurii males from southern African waters had nidamental glands in the mantle cavity in addition to a normally developed male reproductive system (spermatophoric complex, testis and hectocotylus). The frequency of occurrence suggests that feminization in male A. lesueurii is not a rare phenomenon in the south-eastern Atlantic. Normal A. lesueurii shows sexual size dimorphism, with females growing larger than males. The intersexual males formed a distinct size group intermediate between normal males and females, and their testis mass and spermatophore length were significantly larger than in normal males. The absence of oocytes and oviducts in intersexual males indicates that feminization represents pseudohermaphroditism. Intersexuality does not seem to affect male functionality and is apparently advantageous in that larger body size is accompanied by larger testis and spermatophores.