Oxygen isotope composition of annually banded modern and mid-Holocene travertine and evidence of paleomonsoon floods, Grand Canyon, Arizona, USA

Holocene and modern travertine formed in spring-fed Havasu Creek of the Grand Canyon, Arizona, was studied to determine the factors governing its oxygen-isotope composition. Analysis of substrate-grown travertine indicates that calculated calcite-formation temperatures compare favorably with measured water temperatures, and include silt-rich laminae deposited by monsoon-driven floods. Ancient spring-pool travertine is dated by U-series at 7380 ± 110 yr and consists of 14 travertine-silt couplets of probable annual deposition. One hundred eighty high-resolution δ¹⁸O analyses of this mid-Holocene sample average −11.0‰ PDB. The average value for modern travertine is 0.5‰ lower, perhaps because mid-Holocene temperature was higher or there was proportionally greater summer recharge. δ¹⁸O cyclicity in the mid-Holocene travertine has average amplitude of 1.9 ± 0.5‰ PDB, slightly less than the inferred modern-day annual temperature range of Havasu Creek. The annual temperature range might have been reduced during the 14-yr interval compared to present, although other non-temperature factors could account for the muted annual variation. Silt-rich laminae within isotopically lower calcite in the modern and mid-Holocene travertine verifies the seasonal resolution of both samples, and suggests that similar temperature-precipitation conditions, as well as monsoon-generated summer floods, prevailed in the mid-Holocene as they do throughout the Grand Canyon region today.     

Live (stained) benthic foraminifera in the Whittard Canyon, Celtic margin (NE Atlantic)

Living (Rose Bengal stained) benthic foraminifera were investigated at 18 deep-sea stations sampled in the Whittard Canyon area (NE Atlantic). The stations were positioned along 4 bathymetric transects ranging from 300 to 3000 m depth: two along the main canyon axes (Western and Eastern branches) and two along adjacent open slopes (Western and Eastern slopes). The aim of this study was to assess changes of foraminiferal standing stock, composition and microhabitat in relation to the physico-chemical conditions prevailing at and below the sediment-water interface in various canyon and open-slope environments. Minimal oxygen penetration depths and maximal diffusive oxygen uptakes were recorded at upper canyon stations, suggesting a high mineralisation rate. This is confirmed by the high phytopigment concentrations measured in the sediment of the upper canyon axes. Foraminiferal abundance was positively correlated with diffusive oxygen uptake and phytopigment concentration in the sediment. This suggests a control of organic matter fluxes on the foraminiferal communities. Foraminiferal abundance was generally higher along the canyon axis compared to open-slope sites at comparable water depths. The species composition varied with water depth along all four transects, but was also different between canyon branches and adjacent slopes. The silty/sandy intercalations at many of the deeper canyon stations may have been rapidly deposited by fairly recent gravity flows. At station 51WB (3002 m), the faunal characteristics (strong dominance, shallow infaunal microhabitats) suggest that the foraminiferal community is in an early state of ecosystem colonisation after these recent sedimentation events, which would have supplied the important amounts of phytopigments.     

Block-supported cataclastic flow within the upper crust

Recent work from portions of the Sevier fold-thrust belt that have deformed primarily within the elastico-frictional regime, demonstrates that cataclastic flow can be subdivided into two types: matrix- and block supported. The two types may operate simultaneously within the same deforming material. However, their activity can vary spatially, temporally and across scales. Although block-supported cataclastic flow is a critical process in upper crustal deformation, it continues to be largely ignored and/or misunderstood, primarily because established concepts and definitions for cataclastic flow are chiefly based on matrix-supported cataclastic flow. Here, block-supported cataclastic flow is examined to better understand cataclastic flow in general and to explore its relationship with matrix-supported cataclastic flow.     

Soil seed bank longevity of the exotic annual grass Bromus rubens in the Mojave Desert,USA

Although arid environments are often considered among the least invaded terrestrial biomes, the impacts of exotic plant species can be severe and long lasting. Bromus rubens (red brome) is an exotic annual grass species in the Mojave Desert known to outcompete native plant species, alter habitat, and promote accumulation of fuel that contributes to increasing fire frequency and severity. We assessed longevity of the exotic B. rubens seeds in the soil by burying seeds at four depths (0, 2, 5, and 10 cm) and recovering seeds 6, 12, 18, and 24 months after burial. Seed viability was reduced with greater burial depth and greater time since burial. A relatively small proportion of seeds retained viability for two years, suggesting that while the B. rubens seed bank can be large, it is relatively short-lived. Although B. rubens apparently relies more on the annual production, dispersal, and germination of seeds than on a long-lived seed bank for its annual recruitment, the numerous seeds produced by individual plants indicate that even a small proportion of seeds remaining viable for more than a year can aid recruitment from the seed bank and is an important factor in understanding population dynamics.     

A JÖKULHLAUP FROM A LAURENTIAN CAPTURED ICE SHELF TO THE GULF OF MEXICO COULD HAVE CAUSED THE BØLLING WARMING

Since the rapid rate of global warming at the onset of the Bølling interstadial became evident, its cause has been under debate. It coincides closely in time with a strong global transgression called meltwater pulse 1a. One attempt at solution says that a meltwater pulse of Antarctic origin could cause an increase in North Atlantic Deep Water formation, and thus give rise to the Bølling interstadial. However, others have disputed that Antarctic meltwater would have that effect, and furthermore, the start of the Bølling interstadial is not even associated with an increase in North Atlantic Deep Water. A controversial hypothesis says that some Laurentian meltwater came from a jökulhlaup (sub-glacial outburst flood), but no study has yet shown unequivocally that sufficient amounts of water could be stored under the ice. Furthermore, according to all available data a melt-water pulse from the Laurentian ice would give rise to strong cooling, not warming. Nevertheless, meg-afloods appear instrumental in accumulating the Mississippi Fan, created entirely during the Quaternary period, and dramatic climate changes are characteristic of this period. This paper presents a hypothetical chain of events, building on the published literature and simple calculations, to investigate whether the order of magnitude is reasonable. The hypothesis is that a jökulhlaup from a Laurentian captured ice shelf flowed out through the Mississippi, boosted the Gulf Stream, reinvigorated the North Atlantic circulation, and as a result triggered the Bølling warm phase.     

Origin of Taiwan Canyon and its effects on deepwater sediment

The continental slope of the Taiwan Shoal, which has cultivated numerous submarine canyons, is located in a passive continental margin environment. However, the trend of the Taiwan Canyon, with its 45° intersection angle, is obviously different from that of the erosion valley downward along the continental slope. A distinct break is present in the lower segment of the Taiwan Canyon, which then extends from west to east parallel to the continental slope until finally joining the Manila Trench. By utilizing multiple-beam water depth data, high-resolution seismic data, and sediment cores, this study describes the topographic characteristics of the Taiwan Canyon and provides a preliminary discussion on the origin of the Taiwan Canyon and its effect on deepwater sediment. The terrain, landform, and sediment of the Taiwan Canyon exhibit segmentation characteristics. The upper segment is characterized primarily by erosion, downward cutting with a V shape, and wide development of sliding, slumping, and other gravity flow types. The middle segment is characterized mostly by U-shaped erosion-sedimentation transition and development of an inner levee. The lower segment is characterized primarily by sedimentation and development of a sediment wave. The bottom current has a significant reworking effect on the interior sediments of the canyon and forms reworked sands. The formation and evolution of the Taiwan Canyon is closely related to sediment supply, gravity sliding(slumping), faulting activities, and submarine impaling. Given the sufficient terrigenous clastic supply, the sediments along the continental shelf edge continuously proceed seaward; gliding and slumping in the front edge provide driving forces for the formation of the canyon. Faulting activities result in stratum crushing, and the gravity flow takes priority in eroding the relatively fragile stratum. Thus, the direction of the extension of the canyon crosses the surrounding erosion valley obliquely. Seamounts are formed through submarine impaling. Owing to seamount blocking, the lower segment of the canyon is turned toward the east–west direction. Large amounts of sediments overflow at the turning, forming sediment waves.     

Potential for gas hydrate formation at the northwest New Zealand shelf margin - New insights from seismic reflection data and petroleum systems modelling

In this study we provide evidence for methane hydrates in the Taranaki Basin, occurring a considerable distance from New Zealand's convergent margins, where they are well documented. We describe and reconstruct a unique example of gas migration and leakage at the edge of the continental shelf, linking shallow gas hydrate occurrence to a deeper petroleum system. The Taranaki Basin is a well investigated petroleum province with numerous fields producing oil and gas. Industry standard seismic reflection data show amplitude anomalies that are here interpreted as discontinuous BSRs, locally mimicking the channelized sea-floor and pinching out up-slope. Strong reverse polarity anomalies indicate the presence of gas pockets and gas-charged sediments. PetroMod™ petroleum systems modelling predicts that the gas is sourced from elevated microbial gas generation in the thick slope sediment succession with additional migration of thermogenic gas from buried Cretaceous petroleum source rocks. Cretaceous–Paleogene extensional faults underneath the present-day slope are interpreted to provide pathways for focussed gas migration and leakage, which may explain two dry petroleum wells drilled at the Taranaki shelf margin. PetroMod™ modelling predicts concentrated gas hydrate formation on the Taranaki continental slope consistent with the anomalies observed in the seismic data. We propose that a semi-continuous hydrate layer is present in the down-dip wall of incised canyons. Canyon incision is interpreted to cause the base of gas hydrate stability to bulge downward and thereby trap gas migrating up-slope in permeable beds due to the permeability decrease caused by hydrate formation in the pore space. Elsewhere, hydrate occurrence is likely patchy and may be controlled by focussed leakage of thermogenic gas. The proposed presence of hydrates in slope sediments in Taranaki Basin likely affects the stability of the Taranaki shelf margin. While hydrate presence can be a drilling hazard for oil and gas exploration, the proposed presence of gas hydrates opens up a new frontier for exploration of hydrates as an energy source.     

ages of the recent two episodes of glacially dammed lakes along the upstream of the yarlung zangbo gorge

There are many episodes of multiple-level lacustrine terraces along the entrance of the Yarlung Zangbo Great Canyon. Besides, very thick fluvio-lacustrine sediments are buried beneath the cover of the riverbed. Optically stimulated luminescence and radiocarbon dating provide an approximate timeline of upper valley deposits and reveal at least two glacially dammed lake events (Ⅰ and Ⅱ) which have deposition ages of 7～9ka (Ⅰ) and 20～30ka(Ⅱ), respectively. The recent two episodes of glacially dammed lakes produced two steps of lacustrine terraces (T₁, T₂) correspondingly, which are of elevations 2906～2 956m and 3100～3 060m. The formation of paleo-dammed lakes reflects that the Zelunglung Glacier in the west slope of Mt. Namche Barwa had progressively advanced to block the Yarlung Tsangpo River during the early Holocene and the Last Glacial Maximum. The glacially dammed lake I has a relatively smaller extent. Its lacustrine sediments are distributed mainly from Datuoka to Mirui with maximum thickness about 5～8m. Its end is roughly at the south of Milin County. The glacially dammed lake Ⅱ occupies a large area with the end roughly nearby Lang County. Its sediments are exposed from Datuoka to Wolong with maximum thickness about 100m. After the later fluvial erosion, the lacustrine sediments of this lake formed 1～3 levels of secondary terraces.     

A year in Barkley Canyon: A time-series observatory study of mid-slope benthos and habitat dynamics using the NEPTUNE Canada network

Understanding long-term trends in species abundance and distribution represents an important challenge for future research in the deep sea, particularly as management of human impacts becomes a more important concern. However, until natural higher frequency variability is better understood, it will be difficult to interpret any long-term trends that may be apparent in data sets. We present here the results of the first year of observations at the NEPTUNE Canada cabled observatory site in Barkley Canyon, off the coast of Vancouver Island, in the northeast Pacific Ocean. Presence/absence and abundance data for 28 faunal groups were extracted from daily video records from an observatory camera. Concurrent CTD and current meter data were collected from co-located instruments. Water mass properties, currents and faunal community composition exhibited notable seasonal trends. Distinct seasonal faunal groupings were observed, together with summer and winter trends in temperature, salinity and current patterns. Variations in abundance of decapod crustaceans and fishes were responsible for most differences between faunal groups. We suggest that faunal composition may have been responding to seasonal variations in food availability, together with direct and indirect physical influences on predator and prey abundance.