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We have performed the first known semicontinuous monitoring of particulate organic carbon (POC) fluxes and dissolved Si concentrations delivered to the ocean during a typhoon. Sampling of the Choshui River in Taiwan during Typhoon Mindulle in 2004 revealed a POC flux of 5.00 × 105 t associated with a sediment flux of 61 Mt during a 96 h period. The linkage of high amounts of POC with sediment concentrations capable of generating a hyperpycnal plume upon reaching the ocean provides the first known evidence for the rapid delivery and burial of POC from the terrestrial system. These fluxes, when combined with storm-derived CO2 consumption of 1.65 × 108 mol from silicate weathering, elucidate the important role of these tropical cyclone events on small mountainous rivers as a global sink of CO2. [ABSTRACT FROM AUTHOR]

The process by which continental crust has formed is not well understood, though such crust mostly forms at convergent plate margins today. It is thus imperative to study modern intra-oceanic arcs, such as those common in the western Pacific Ocean. New seismic studies along the representative Izu-Bonin intra-oceanic arc provide unique along-strike images of arc crust and uppermost mantle to complement earlier, cross-arc lithospheric profiles. These reveal two scales (1000–10 km scale) of variations, one at the scale of the Izu versus Bonin (thick versus thin) arc crust, the other at the intervolcano (∼50 km) scale. These images show that: (1) the bulk composition of the Izu-Bonin arc crust is more mafic than typical continental crust, (2) the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes, (3) the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments, and (4) a process to return lower crustal components to the mantle, such as delamination, is required for an arc crust to evolve into continental crust. Continued thickening of the Izu-Bonin crust, accompanied by delamination of lowermost crust, can yield velocity structure of typical continental crust. [ABSTRACT FROM AUTHOR]

The 231Pa-235U disequilibria provide greater ability to constrain the rate of mantle melting in convergent margin settings than the more often analyzed 238U-230Th-226Ra systems, which are strongly affected by fluid-addition processes from the subducting slab. Here we present new 231Pa-235U data for 12 samples from the Kick'em Jenny (KEJ) submarine volcano in the Southern Lesser Antilles to define the melting rate at a subduction zone with one of the lowest convergence rates. The KEJ samples have the highest average (231Pa)/(235U) yet measured in global arcs, consistent with other studies of the Southern Lesser Antilles lavas. These results reinforce the previously noted negative correlation between average (231Pa)/(235U) and convergence rate in all arc settings globally. We develop a model to explain this negative correlation and to better constrain melting rates at convergent margins. Assuming that the corner flow velocity is coupled to and equal to the subducting slab velocity, the melting rate, directly reflecting the flux of water added, becomes a linear function of subduction rate. This physical model is then coupled to three different melting models previously developed for calculating U-series disequilibria (reactive porous flow, dynamic, and flux melting). All three models reproduce the globally observed negative correlation between subduction rate and 231Pa excess. Although the style of melting cannot be easily discriminated, the good correspondence between models and observation provides an example of how geochemical and geophysical models can be linked to provide a self-consistent model of melt generation in convergent margin settings. [ABSTRACT FROM AUTHOR]

Using triaxial numerical experiments, we investigated the evolution of the state of stress and that of the bulk instantaneous and finite strain during ongoing convergence and subsequent progressive tectonic unloading of a warm and buoyant continental lithosphere. Various unloading histories of the driving tectonic force were considered. As the tectonic force progressively declines, the instantaneous strain evolves from plane strain to horizontal constriction in a direction perpendicular to that of convergence, and finally to horizontal flattening. During the progressive unloading of the tectonic force driving convergence, bulk constrictional strain accommodates the release of accumulated gravitational stress. The decline of the triaxial strain rates to low values reduces the potential for the orogen-parallel linear fabric to be erased by horizontal flattening. This is confirmed by the finite strain ellipsoid that evolves toward plane strain with a long axis parallel to the orogen. In the ca. 2.5 Ga Gawler and Terre Adélie cratons, we have identified a well-preserved and widespread horizontal linear fabric. As suggested by our numerical experiments, we associate the development of this linear fabric with the waning stages of late Archean convergence. [ABSTRACT FROM AUTHOR]

Seismic refraction and reflection data were acquired in 2000 and 2003 to study the morphology and sedimentary fill of the remote El'gygytgyn crater (Chukotka, northeastern Siberia; diameter 18 km). These data allow a first insight into the deeper structure of this unique impact crater. Wide-angle data from sonobuoys reveal a five-layer model: a water layer, two lacustrine sedimentary units that fill a bowl-shaped apparent crater morphology consisting of an upper layer of fallback breccia with P-wave velocities of ∼3000 m/s, and a lower layer of brecciated bedrock (velocities >3600 m/s). The lowermost layer shows a distinct anticline structure that, by analogy with other terrestrial and lunar craters of similar size, can be interpreted as a central ring structure. The El'gygytgyn crater exhibits a well-expressed morphology that is typical of craters formed in crystalline target rocks. [ABSTRACT FROM AUTHOR]

Magnesium/calcium (Mg/Ca) ratios of foraminiferal shells from a sediment core from the northern South China Sea, a semi-enclosed basin in the western tropical Pacific, document variations in sea-surface temperature (SST) during the past 145 k.y. Glacial SSTs were 4°C colder than interglacial SSTs. During the last deglaciation, most of the warming was accomplished in a single abrupt step after continental ice-sheet decay had already begun, but warming and ice-sheet demise were nearly synchronous during the penultimate deglaciation. Abrupt SST changes of the past 15 k.y. were apparently synchronous with events in East Asian monsoon rainfall, suggesting that variations in monsoon winds and their influence on surface circulation of the western Pacific exerted a strong control on northern South China Sea SSTs. We suggest that this link persisted for the previous 130 k.y., during which time orbital-scale 2-3 °C SST changes and several small (≤2 °C) abrupt SST events occurred in the northern South China Sea. The similar timing of northern South China Sea SST, on a benthic δ18O time scale, to a well-dated speleothem record from eastern China suggests that the demise of ice sheets associated with the penultimate deglaciation did not precede Northern Hemisphere summer insolation increase. Our re suits suggest that surface waters had higher δ18O values during times of strong summer monsoon than during times of weak monsoon, likely reflecting a redistribution of 18O. depleted rainfall from land during times of strong summer monsoons, to the western Pacific during times of weaker summer monsoons. [ABSTRACT FROM AUTHOR]

Ice thickness in West Antarctica at the Last Glacial Maximum (LGM) is poorly known, yet is key information for understanding ice streams and interpreting ice cores. Although trim lines, moraine limits, and exposure-age dating provide geologic constraints on ice thickness near the Transantarctic Mountains and in Marie Byrd Land, lack of exposed bedrock hampers traditional geologic methods in a central, ∼2 × 106 km² region. Here we infer ice-sheet thickness changes in the central Ross Sea Embayment by using a transient ice-flow model to find combinations of accumulation-rate and ice-sheet thickness histories that match the depth-age relationship and the measured layer-thickness pattern in the Siple Dome ice core. After we reject unreasonable accumulation-rate histories, the remaining history pales indicate thinning of 200–400 m since the LGM. Our estimate is lower than previous reconstructions that were constrained by geologic evidence from the Transantarctic Mountains and by marine data from the Ross Sea floor, which indicate that a grounded ice sheet extended to the continental shelf margin during the last glacial period. Low surface elevations in the central Ross Sea Embayment during the LGM do not preclude thicker ice along its boundaries. However, if this grounded ice sheet came over 1000 km from interior West Antarctica, as is usually assumed, then it had very low surface slope, requiring a very slippery bed. Alternatively, the grounded Ross Sea ice might have flowed from the Transantarctic Mountains and western Marie Byrd Land. [ABSTRACT FROM AUTHOR]

Hypoxia on the Louisiana continental shelf is tied to nutrient loading and freshwater stratification from the Mississippi River. Variations in the relative abundance of low-oxygen-tolerant benthic foraminifers in four sediment cores from the Louisiana shelf provide a proxy record of low-oxygen events. Core chronologies are obtained using 210pb dating techniques. The foraminiferal data are consistent with previous studies indicating that the intensity of hypoxic events (oxygen <2 rag/L) has increased over the past 50 yr owing to the higher nutrient loading associated with the use of commercial fertilizer, and also reveal several low-oxygen events between A.D. 1817 and 1910, prior to the widespread use of fertilizer. The pre-1910 low-oxygen events are associated with high Mississippi River discharge rates, indicating that these low-oxygen episodes are related to natural variations in river drainage that enhance transport of nutrients and freshwater to the continental shelf. Our data show that the low-oxygen events of the past few decades were more extreme than any that occurred in the previous ∼180 yr, and support the interpretation that the increased use of fertilizer has amplified an otherwise naturally occurring process. [ABSTRACT FROM AUTHOR]

In contrast to Eocene Himalayan foreland basin sedimentary deposits in India and Pakistan, coeval sedimentary rocks of the Bhainskati Formation in Nepal contain scant petrographic evidence of orogenic input. Such data have been used as evidence to promote models of diachroneity of India-Asia collision. In this paper we document orogenic input into the Eocene foreland basin rocks of Nepal, from fission-truck analyses of detrital zircons. Our data provide evidence that significantly reduces the possible duration of any diachroneity of collision, and brings the interpretation of the sedimentary record into better agreement with ages of early thrusting and metamorphism in the orogen. We also use our detrital fission-track data to bolster previous age determinations of the overlying Dumri Formation, confirming the basin-wide occurrence of a major unconformity. Comparison of our data set with that from coeval along-strike rocks in India suggests that there is no evidence of diachroneity in early stages of the orogen's exhumation. [ABSTRACT FROM AUTHOR]

The Eclogite zone (Tauern Window, Eastern Alps, Austria) represents one of only a few examples of high-pressure units providing both the prograde and the retrograde metamorphic evolution. The eclogites are associated with rocks originating from continental and transitional crust that also have been affected by high-pressure metamorphism. Eclogites that formed along the prograde path indicate metamorphic conditions of 17-20 kbar (1700-2000 MPa) at 550-580 °C; the peak assemblages formed at 21-25 kbar (2100-2500 MPa) at 600-620 °C. Omphacite microstructures, in particular shape fabrics and crystallographic preferred orientations, indicate that the final phases of the prograde evolution were characterized by flattening strain. The evolution at the pressure peak and along the exhumation path shows a constrictional strain geometry. This change is interpreted to have been controlled by the force balance between slab pull (related to an oceanic slab) and the buoyancy of subducted adjacent continental crustal material, including the eclogites. At a certain lithospheric level, where the subducted continental rocks were entirely surrounded by high-density lower-crustal and upper-mantle material, the buoyancy forces exceeded the slab-pull forces. This force change resulted in the buoyancy-driven extrusion of continental material between two lithospheric plates. The part of the subducted slab that remained in buoyant equilibrium was therefore affected by constriction. Moreover, this process may have resulted in breakoff of the subducted oceanic slab. If so, the cessation of slab pull resulted in accelerated buoyancy-driven extrusion of the continental material. Extrusion of these high-pressure sheets would have been accompanied by constriction subparallel to the subduction channel as well. [ABSTRACT FROM AUTHOR]

Correlation of continental shelf-slope stratigraphy in Santa Monica Bay (southern California) with Ocean Drilling Program records for nearby slope-basin sites has illuminated the timing and scale of terrigenous sediment dispersal on margin since the Last Glacial Maximum (LGM). Marine flooding surfaces preserved in a transgressive sequence on the Santa Monica Shelf provide a key link between base-level elevation and sediment transport across shelf. Sediment-accumulation rates at slope-basin sites were maximal ca. 15-10 ka, well after the LGM, decreased during the 12-9 ka transition from fluvial-estuarine to fully marine conditions on the shelf, and decelerated throughout the Holocene to 30%-75% of their values at the LGM. The deceleration is interpreted to manifest a landward shift in the margin depocenter with the onset of transgressive sedimentation beginning when sea level surmounted the shelf edge ca. 13 ka, as predicted by sequence-stratigraphic models. However, the records make clear that factors other than base level modulated slope-basin accumulation rates during the deglaciation. [ABSTRACT FROM AUTHOR]

The sediments of Santa Barbara Basin, a valuable source of information about the history of the California Current, show cyclic deposition in three properties here investigated: varve thickness, total organic carbon (TOC) accumulation, and fish-scale abundance. We analyzed the record since A.D. 1350 for varves, A.D. 1120 for TOC, and A.D. 1200 for fish scales. Prominent periods are 169 yr, 93 yr, 51.4 yr, 17.7 yr (varve thickness), 124 yr, 56 yr, 37.4 yr (TOC flux), and 103 yr, 128 yr (fish-scale abundance). A surprising number of the varve and TOC cycles can be interpreted as deriving from the common tidal cycles, i.e., the 4.425 yr perigee period (describing the changing distance between Earth and Moon) and the 18.6 yr lunar nodal period (which pertains to the lineup of Earth, Moon, and Sun). We suggest that the record of the basin is partly a result of pulsed sedimentation aligned with tides. These findings support the hypothesis that a considerable portion of sediment destined for deposition in the center of the basin goes through a phase of shallow storage on the shelf where it can be remobilized through storms and tides. Mechanisms calling on tidally pulsed productivity or tidally influenced climate change are not excluded, but are considered less attractive than remobilization. Fish-scale cycles may reflect the influence of solar forcing. [ABSTRACT FROM AUTHOR]