Upper mantle anisotropy of southeast Arabia passive margin [Gulf of Aden northern conjugate margin], Oman

In this study, we used data recorded by two consecutive passive broadband deployments on the Gulf of Aden northern margin, Dhofar region, Sultanate of Oman. The objective of these deployments is to map the young eastern Gulf of Aden passive continental margin crust and upper mantle structure and rheology. In this study, we use shear-wave splitting analysis to map lateral variations of upper mantle anisotropy beneath the study area. In this study, we found splitting magnitudes to vary between 0.33 and 1.0 s delay times, averaging about 0.6 s for a total of 17 stations from both deployment periods. Results show distinct abrupt lateral anisotropy variation along the study area. Three anisotropy zones are identified: a western zone dominated by NW–SE anisotropy orientations, an eastern zone dominated with NE–SW anisotropy orientations, and central zone with mixed anisotropy orientations similar to the east and west zones. We interpret these shorter wavelength anisotropy zones to possibly represent fossil lithospheric mantle anisotropy. We postulate that the central anisotropy zone may be representing a Proterozoic suture zone that separates two terranes to the east and west of it. The anisotropy zones west and east were being used indicative of different terranes with different upper mantle anisotropy signatures.     

Isotope variations in a Sierra Nevada snowpack and their relation to meltwater

Isotopic variations in melting snow are poorly understood. We made weekly measurements at the Central Sierra Snow Laboratory, California, of snow temperature, density, water equivalent and liquid water volume to examine how physical changes within the snowpack govern meltwater δ¹⁸O. Snowpack samples were extracted at 0.1 m intervals from ground level to the top of the snowpack profile between December 1991 and April 1992. Approximately 800 mm of precipitation fell during the study period with δ¹⁸O values between −21.35 and −4.25‰. Corresponding snowpack δ¹⁸O ranged from −22.25 to −6.25‰. The coefficient of variation of δ¹⁸O in snowpack levels decreased from −0.37 to −0.07 from winter to spring, indicating isotopic snowpack homogenization. Meltwater δ¹⁸O ranged from −15.30 to −8.05‰, with variations of up to 2.95‰ observed within a single snowmelt episode, highlighting the need for frequent sampling. Early snowmelt originated in the lower snowpack with higher δ¹⁸O through ground heat flux and rainfall. After the snowpack became isothermal, infiltrating snowmelt displaced the higher δ¹⁸O liquid in the lower snowpack through a piston flow process. Fractionation analysis using a two-component mixing model on the isothermal snowpack indicated that δ¹⁸O in the initial and final half of major snowmelt was 1.30‰ lower and 1.45‰ higher, respectively, than the value from simple mixing. Mean snowpack δ¹⁸O on individual profiling days showed a steady increase from −15.15 to −12.05‰ due to removal of lower δ¹⁸O snowmelt and addition of higher δ¹⁸O rainfall. Results suggest that direct sampling of snowmelt and snow cores should be undertaken to quantify tracer input compositions adequately. The snowmelt sequence also suggests that regimes of early lower δ¹⁸O and later higher δ¹⁸O melt may be modeled and used in catchment tracing studies.     

Estimating the underwater light field from remote sensing of ocean color

We present a new approach that incorporates two models to estimate the underwater light field from remote sensing of ocean color. The first employs a series of analytical, semi-analytical, and empirical algorithms to retrieve the spectrum of inherent optical properties (IOPs), including the absorption and the backscatter coefficients, from the spectrum of remote sensing reflectance. The second model computes the profile of photosynthetically available radiation E _0,PAR (z) for a vertically homogeneous water column using the information of the retrieved IOPs and the ambient optical environment. This computation is based on an improved look-up table technology that possesses high accuracy, comparable with the full solution of the radiative transfer equation, and meets the computational requirement of remote sensing application. This new approach was validated by in situ measurements and an extensive model-to-model comparison with a wide range of IOPs. We successfully mapped the compensation depth by applying this new approach to process the SeaWiFS imagery. This research suggests that E _0,PAR (z) can be obtained routinely from ocean-color data and may have significant implications for the estimation of global heat and carbon budget.     

Fluctuation of transports and Sea level in the western boundary region of the tropical Pacific Ocean

Large fluctuation in transport of the Equatorial Countercurrent flowing eastward from the western boundary is not the direct result of fluctuation in transport of the North Equatorial Current, but rather relates to fluctuation of sea level anomaly in the Philippine Sea.     

Carbonate cement fabrics displayed: A traverse across the margin of the Bahamas Platform near Lee Stocking Island in the Exuma Cays

Consolidated to friable carbonate rocks found in the Lee Stocking Island area in the Exuma Cays include: (1) reef rock, (2) channel stromatolites, (3) shallow-water hardgrounds, (4) beachrock rimming the islands and (5) Pleistocene bedrock.

The most common cement fabrics observed are: aragonitic fibers, which include acicular fan-druse and square-tipped coarse fibers cementing beachrock and stromatolites; and an isopachous needle-fiber rim cementing hardgrounds and stromatolites.

Less common are high-Mg calcite bladed textures of the reef rock and stromatolites. Two types of blades are present: the more common stubby variety of either high-Mg or low-Mg calcite, and an elongated variety of high-Mg calcite which was found in only three beachrock samples.

Aragonitic micrite envelopes usually surround grains in beachrock, hardgrounds and stromatolites, but only in association with fibrous cement. An aragonitic crust cements the surfaces of lime mud beds of the tidal channel, while a high-Mg calcite cryptocrystalline cement occurs in all the rock types. Calcified algal filaments of high-Mg calcite, from the abundant green and blue-green algae in the area, are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock. A low-Mg calcite equant spar cements the Pleistocene samples and is associated with meteoric diagenesis and cementation of the Pleistocene surface.

Cement precipitation coincides with the path of the cool oceanic water from Exuma Sound as it warms and loses CO₂ and moves up onto the bank near Lee Stocking with the incoming tide. Cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward, fibrous cements become predominant. As suggested by their crystal size and location on the shelf margin, we think that the reef rock cryptocrystalline material are the fastest forming of the cements, where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming. The rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate. On the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds, but these may form at a slower rate than that of the margin, though this rate is still quite high. Cementation gradients occur from the tidal channel to the intertidal zones of: (1) west Norman's Pond Cay, where cement fabric suggests a reduced calcium carbonate availability, and (2) west Lee Stocking Island, where a change in mineralogy suggests a change in water chemistry.

Thus, a sequence of cement fabrics and mineralogies can be traced. Micritic textures occur in a more seaward position; fine, fibrous aragonite fibers in a more lagoonal and levee position; and coarser aragonite fibers and Mg-calcite cements in the intertidal and supratidal position. This sequence is thought to track the evolution of the water mass.     

Depth gradients of benthic standing stock and diversity on the continental margin at a high-latitude ice-free site (off Spitsbergen, 79°N)

High-latitude seas are mostly covered by multi-year ice, which impacts processes of primary production and sedimentation of organic matter. Because of the warming effect of West Spitsbergen Current (WSC), the waters off West Spitsbergen have only winter ice cover. That is uncommon for such a high latitude and enables to separate effects of multiyear-ice cover from the latitudinal patterns. Macrofauna was sampled off Kongsfjord (79°N) along the depth gradient from 300 to 3000 m. The density, biomass and diversity at shallow sites situated in a canyon were very variable. Biomass was negatively correlated with depth (R=-0.86, p<0.001), and ranged from 61 g ww m⁻² (212 m) to 1 g ww m⁻² (2025 m). The biomasses were much higher than in the multiyear-ice covered High Arctic at similar depths, while resembling those from temperate and tropical localities. Species richness (expressed by number of species per sample and species–area accumulation curves) decreased with depth. There was no clear depth-related pattern in diversity measures: Hurbert rarefaction, Shannon–Wiener or Pielou. The classic increase of species richness and diversity with depth was not observed. Species richness and diversity of deep-sea macrofauna were much lower in our study than in comparable studies of temperate North Atlantic localities. That is related to geographic isolation of Greenland–Icelandic–Norwegian (GIN) seas from the Atlantic pool of species.