Architecture and preservation in the fluvial to marine transition zone of a mixed-process humid-tropical delta: Middle Miocene Lambir Formation,Baram Delta Province,north-west Borneo

The interaction of river and marine processes in the fluvial to marine transition zone fundamentally impacts delta plain morphology and sedimentary dynamics. This study aims to improve existing models of the facies distribution, stratigraphic architecture and preservation in the fluvial to marine transition zone of mixed-process deltas, using a comprehensive sedimentological and stratigraphic dataset from the Middle Miocene Lambir Formation, Baram Delta Province, north-west Borneo. Eleven facies associations are identified and interpreted to preserve the interaction of fluvial and marine processes in a mixed-energy delta, where fluvial, wave and tidal processes display spatially and temporally variable interactions. Stratigraphic successions in axial areas associated with active distributary channels are sandstone-rich, comprising fluvial-dominated and wave-dominated units. Successions in lateral areas, which lack active distributary channels, are mudstone-rich, comprising fluvial-dominated, tide-dominated and wave-dominated units, including mangrove swamps. Widespread mudstone preservation in axial and lateral areas suggests well-developed turbidity maximum zones, a consequence of high suspended-sediment concentrations resulting from tropical weathering of a mudstone-rich hinterland. Within the fluvial to marine transition zone of distributary channels, interpreted proximal–distal sedimentological and stratigraphic trends suggest: (i) a proximal fluvial-dominated, tide-influenced subzone; (ii) a distal fluvial-dominated to wave-dominated subzone; and (iii) a conspicuously absent tide-dominated subzone. Lateral areas preserve a more diverse spectrum of facies and stratigraphic elements reflecting combined storm, tidal and subordinate river processes. During coupled storm and river floods, fluvial processes dominated the fluvial to marine transition zone along major and minor distributary channels and channel mouths, causing significant overprinting of preceding interflood deposits. Despite interpreted fluvial–tidal channel units and mangrove influence implying tidal processes, there is a paucity of unequivocal tidal indicators (for example, cyclical heterolithic layering). This suggests that process preservation in the fluvial to marine transition zone preserved in the Lambir Formation primarily records episodic (flashy) river discharge, river flood and storm overprinting of tidal processes, and possible backwater dynamics.     

A review of the chondrite–achondrite transition,and a metamorphic facies series for equilibrated primitive stony meteorites

Here, the petrological features of numerous primitive achondrites and highly equilibrated chondrites are evaluated to review and expand upon our knowledge of the chondrite–achondrite transition, and primitive achondrites in general. A thermodynamic model for the initial silicate melting temperature and progressive melting for nearly the entire known range of oxidation states is provided, which can be expressed as T_m = 0.035Fa²?3.51Fa + 1109 (in °C, where Fa is the proportion of fayalite in olivine). This model is then used to frame a discussion of textural and mineralogical evolution of stony meteorites with increasing temperature. We suggest that the metamorphic petrology of these meteorites should be based on diffusive equilibration among the silicate minerals, and as such, the chondrite–achondrite transition should be defined by the initial point of silicate melting, not by metal–troilite melting. Evidence of silicate melting is preserved by a distinctive texture of interconnected interstitial plagioclase ± pyroxene networks among rounded olivine and/or pyroxene (depending on ?O₂), which pseudomorph the former silicate melt network. Indirectly, the presence of exsolution lamellae in augite in slowly cooled achondrites also implies that silicate melting occurred because of the high temperatures required, and because silicate melt enhances diffusion. A metamorphic facies series is defined: the Plagioclase Facies is equivalent to petrologic types 5 and 6, the Sub‐calcic Augite Facies is bounded at lower temperatures by the initiation of silicate melting and at higher temperatures by the appearance of pigeonite, which marks the transition to the Pigeonite Facies.     

Bottom water circulation in Cascadia Basin

A combination of beta spiral and minimum length inverse methods, along with a compilation of historical and recent high-resolution CTD data, are used to produce a quantitative estimate of the subthermocline circulation in Cascadia Basin. Flow in the North Pacific Deep Water, from 900-1900 m, is characterized by a basin-scale anticyclonic gyre. Below 2000 m, two water masses are present within the basin interior, distinguished by different potential temperature-salinity lines. These water masses, referred to as Cascadia Basin Bottom Water (CBBW) and Cascadia Basin Deep Water (CBDW), are separated by a transition zone at about 2400 m depth. Below the depth where it freely communicates with the broader North Pacific, Cascadia Basin is renewed by northward flow through deep gaps in the Blanco Fracture Zone that feeds the lower limb of a vertical circulation cell within the CBBW. Lower CBBW gradually warms and returns to the south at lighter density. Isopycnal layer renewal times, based on combined lateral and diapycnal advective fluxes, increase upwards from the bottom. The densest layer, existing in the southeast quadrant of the basin below 2850 m, has an advective flushing time of 0.6 years. The total volume flushing time for the entire CBBW is 2.4 years, corresponding to an average water parcel residence time of 4.7 years. Geothermal heating at the Cascadia Basin seafloor produces a characteristic bottom-intensified temperature anomaly and plays an important role in the conversion of cold bottom water to lighter density within the CBBW. Although covering only about 0.05% of the global seafloor, the combined effects of bottom heat flux and diapycnal mixing within Cascadia Basin provide about 2-3% of the total required global input to the upward branch of the global thermohaline circulation.     

Stormwater plume detection by MODIS imagery in the southern California coastal ocean

Stormwater plumes in the southern California coastal ocean were detected by MODIS-Aqua satellite imagery and compared to ship-based data on surface salinity and fecal indicator bacterial (FIB) counts collected during the Bight'03 Regional Water Quality Program surveys in February–March of 2004 and 2005. MODIS imagery was processed using a combined near-infrared/shortwave-infrared (NIR-SWIR) atmospheric correction method, which substantially improved normalized water-leaving radiation (nLw) optical spectra in coastal waters with high turbidity. Plumes were detected using a minimum-distance supervised classification method based on nLw spectra averaged within the training areas, defined as circular zones of 1.5–5.0-km radii around field stations with a surface salinity of S < 32.0 (“plume”) and S > 33.0 (“ocean”). The plume optical signatures (i.e., the nLw differences between “plume” and “ocean”) were most evident during the first 2 days after the rainstorms. To assess the accuracy of plume detection, stations were classified into “plume” and “ocean” using two criteria: (1) “plume” included the stations with salinity below a certain threshold estimated from the maximum accuracy of plume detection; and (2) FIB counts in “plume” exceeded the California State Water Board standards. The salinity threshold between “plume” and “ocean” was estimated as 32.2. The total accuracy of plume detection in terms of surface salinity was not high (68% on average), seemingly because of imperfect correlation between plume salinity and ocean color. The accuracy of plume detection in terms of FIB exceedances was even lower (64% on average), resulting from low correlation between ocean color and bacterial contamination. Nevertheless, satellite imagery was shown to be a useful tool for the estimation of the extent of potentially polluted plumes, which was hardly achievable by direct sampling methods (in particular, because the grids of ship-based stations covered only small parts of the plumes detected via synoptic MODIS imagery). In most southern California coastal areas, the zones of bacterial contamination were much smaller than the areas of turbid plumes; an exception was the plume of the Tijuana River, where the zone of bacterial contamination was comparable with the zone of plume detected by ocean color.     

Evolution of boundary‐layer aerosol particles due to in‐cloud chemical reactions during the 2nd Lagrangian experiment of ACE‐2

The second Aerosol Characterisation Experiment (ACE‐2) was aimed at investigating the physical, chemical and radiative properties of aerosol and their evolution in the North Atlantic region. In the 2nd "Lagrangian" experiment, an air mass was tracked over a 30‐h period during conditions of extensive stratocumulus cover. Boundary‐layer measurements of the aerosol size distribution obtained with a passive cavity aerosol spectrometer probe (PCASP) during the experiment show a gradual growth in size of particles in the 0.1–0.2 μm diameter mode. Simultaneously, SO₂ concentrations were found to decrease sharply from 800 to 20 ppt. The fraction of sulphate in aerosol ionic mass increased from 0.68±0.07 to 0.82±0.09 for small particles (diameter below 1.7 μm) and from 0.21±0.04 to 0.34±0.03 for large particles (diameter above 1.7 μm). The measurements were compared with a multicyclic parcel model of gas phase diffusion into cloud droplets and aqueous phase chemical reactions. The model was able to broadly reproduce the observed transformation in the aerosol spectra and the timescale for the transformation of SO₂ to sulphate aerosol. The modelled SO₂ concentration in the boundary layer fell to below half its initial value over a 6.5‐h time period due to a combination of the entrainment of cleaner tropospheric air and cloud chemical reactions. NH₃ and HCl gas were also found to play an important rôle in cloud processing in the model.     

A method for estimating the sub-wavelength sway of a sonar towfish

The essentially random motion of the towfish in a side scan sonar system limits the fidelity of the resultant image. Although corrective techniques have been developed to deal with the gross departures from a set of regular amplitude versus range samples along a straight locus as well as the gross rotational errors, sub-wavelength displacement errors are mostly ignored. This paper suggests a technique to estimate the appropriate displacement errors from the statistics of the measured data when there are few or no strong scatterers on an otherwise featureless sea floor. As it happens, the proposed technique is somewhat similar to an existing optical technique but is new to underwater sonar. We show in a series of simulated trials that it is possible to estimate the resultant sub-wavelength horizontal departure from a straight locus under some (hopefully) realistic operating conditions     

Tectonic and eustatic control on the distribution of black-shale source beds in the Wufeng and Longmaxi formations (Ordovician-Silurian),South China

It is known that high-quality, black-shale source rocks occur in the uppermost Ordovician Wufeng Formation and in the lowermost Silurian Longmaxi Formation in South China. Hence, it is important to understand their lithostratigraphy and the controls on their deposition. A review of lithostratigraphic criteria for subdividing the two adjacent formations provides new regional correlations between the formations and related stratigraphic successions and facies. Both the black shales and the related, overlying flysch deposits at the Ordovician-Silurian transition in South China appear to have migrated northwestward in time and space, reflecting probable flexural control in a foreland basin that developed in response to subduction-type orogeny southeast of the Yangtze block. The black shales also contain K-bentonites from explosive, felsic-intermediate volcanism, the distribution of which also supports orogeny to the southeast. Finally, the analysis of sequence stratigraphy, which shows that the initiation of transgressive system tracts (TST) and condensed section (CS) in the related third-order sequences coincided with the two black-shale horizons respectively, indicates that the main controlling factors for the deposition of the Ordovician-Silurian black shales in South China are (1) northwestwardly migrating, foreland-basin subsidence caused by deformational loading related to episodic accretion of the Cathaysia block to the Yangtze block during this period, and (2) the anoxic, sediment-starved water column caused by rapid rise of the sea-level during the two successive phases of third-order global sea-level rise near the Ordovician-Silurian transition in South China. In future exploration for hydrocarbon source rocks in the area, it is important to consider likely flexural and eustatic causes for subsiding, deep, anoxic seas in recognizing other source rock intervals, and our understanding of the Wufeng and Longmaxi formations may serve as models for future source rock exploration. __________ Translated from Earth Science—Journal of China University of Geosciences, 2007, 32(6): 818–827 [译自: 地球科学—中国地质大学学报]     

Worm holes and their space-time continuum: Spatial and temporal variability of macroinfaunal annelids in a Northern New England salt marsh

Coastal systems serve many human uses and as a result are susceptible to anthropogenic activities such as nutrient loading and overfishing. In soft sediments, infauna frequently serve as key indicators of such activities. To use infauna effectively as bioindicators, it is important to understand how infaunal abundances and community patterns vary naturally within ecosystems. We examined the spatial and temporal dynamics of infaunal annelids in four tidal creeks of the Plum Island Estuary, Massachusetts, USA, from June to October 2003, sampling along a tidal inundation gradient that crossed five distinct habitats from creek bottoms to the vegetated high marsh platform. Annelids comprised 97% of the total number of macroinfauna. Highest densities were found in creek wall habitats (33,418–65,535 individuals m⁻²), and lowest densities (2,421–10,668 individuals m⁻²) were found inSpartina patens habitats. Five numerically abundant species comprised 87% of the annelid assemblage and three species,Manayunkia aestuarina (Polychaeta),Paranais litoralis (Oligochaeta), andCernosvitoviella immota (Oligochaeta), were broadly distributed across the marsh landscape.Streblospio benedicti (Polychaeta) andFabricia sabella (Polychaeta) were abundant only in mudflat and creek wall habitats, respectively.P. litoralis experienced a summer decline in all habitats, whereasM. aestuarina abundance increased 4–5 fold, in October relative to June in creek wall and tall-formSpartina alterniflora habitats. Hierarchical spatial, analysis revealed that >90% of the variability in annelid abundances was found at the mesospatial scale (<50 m). Variation among the four creeks, (>1 km) was relatively small.