Diamond precipitation and mantle metasomatism – evidence from the trace element chemistry of silicate inclusions in diamonds from Akwatia, Ghana

Trace element concentrations in the four principal peridotitic silicate phases (garnet, olivine, orthopyroxene, clinopyroxene) included in diamonds from Akwatia (Birim Field, Ghana) were determined using SIMS. Incompatible trace elements are hosted in garnet and clinopyroxene except for Sr which is equally distributed between orthopyroxene and garnet in harzburgitic paragenesis diamonds. The separation between lherzolitic and harzburgitic inclusion parageneses, which is commonly made using compositional fields for garnets in a CaO versus Cr₂O₃ diagram, is also apparent from the Ti and Sr contents in both olivine and garnet. Titanium is much higher in the lherzolitic and Sr in the harzburgitic inclusions. Chondrite normalised REE patterns of lherzolitic garnets are enriched (10–20 times chondrite) in HREE (La_N/Yb_N = 0.02–0.06) while harzburgitic garnets have sinusoidal REE_N patterns, with the highest concentrations for Ce and Nd (2–8 times chondritic) and a minimum at Ho (0.2–0.7 times chondritic). Clinopyroxene inclusions show negative slopes with La enrichment 10–100 times chondritic and low Lu (0.1–1 times chondritic). Both a lherzolitic and a harzburgitic garnet with very high knorringite contents (14 and 21 wt% Cr₂O₃ respectively) could be readily distinguished from other garnets of their parageneses by much higher levels of LREE enrichment. The REE patterns for calculated melt compositions from lherzolitic garnet inclusions fall into the compositional field for kimberlitic-lamproitic and carbonatitic melts. Much more strongly fractionated REE patterns calculated from harzburgitic garnets, and low concentrations in Ti, Y, Zr, and Hf, differ significantly from known alkaline and carbonatitic melts and require a different agent. Equilibration temperatures for harzburgitic inclusions are generally below the C-H-O solidus of their paragenesis, those of lherzolitic inclusions are above. Crystallisation of harzburgitic diamonds from CO₂-bearing melts or fluids may thus be excluded. Diamond inclusion chemistry and mineralogy also is inconsistent with known examples of metasomatism by H₂O-rich melts. We therefore favour diamond precipitation by oxidation of CH₄-rich fluids with highly fractionated trace element patterns which are possibly due to “chromatographic” fractionation processes. Received: 27 January 1996 / Accepted: 5 May 1997     

Sub-weekly to interannual variability of a high-energy shoreline

Sixty-one Global Positioning System (GPS), sub-aerial beach surveys were completed at 7 km long Ocean Beach, San Francisco, CA (USA), between April 2004 and March 2009. The five-year time series contains over 1 million beach elevation measurements and documents detailed changes in beach morphology over a variety of spatial, temporal, and physical forcing scales. Results show that seasonal processes dominate at Ocean Beach, with the seasonal increase and decrease in wave height being the primary driver of shoreline change. Storm events, while capable of causing large short-term changes in the shoreline, did not singularly account for a large percentage of the overall observed change. Empirical orthogonal function (EOF) analysis shows that the first two modes account for approximately three-quarters of the variance in the data set and are represented by the seasonal onshore/offshore movement of sediment (60%) and the multi-year trend of shoreline rotation (14%). The longer-term trend of shoreline rotation appears to be related to larger-scale bathymetric change. An EOF-based decomposition technique is developed that is capable of estimating the shoreline position to within one standard deviation of the range of shoreline positions observed at most locations along the beach. The foundation of the model is the observed relationship between the temporal amplitudes of the first EOF mode and seasonally-averaged offshore wave height as well as the linear trend of shoreline rotation. This technique, while not truly predictive because of the requirement of real-time wave data, is useful because it can predict shoreline position to within reasonable confidence given the absence of field data once the model is developed at a particular site.     

Transport and retention in an upwelling region: The role of across-shelf structure

The paradox of upwelling is the relationship between strong wind forcing, nutrient enrichment, and shelf productivity. Here we investigate how across-shelf structure in velocity and hydrography plays a role in the retention (inshore) and export (offshore) of particles such as nutrients, plankton and larvae. We examine the spatial structure of the coastal currents during wind-driven upwelling and relaxation on the northern Californian Shelf. The field work was conducted as part of the Wind Events and Shelf Transport (WEST) project, a 5-year NSF/CoOP-funded study of the role of wind-driven transport in shelf productivity off Bodega Bay (northern California) from 2000 to 2003. We combine shipboard velocity profiles (ADCP) and water properties from hydrographic surveys during the upwelling season to examine the mean across-shelf structure of the hydrography and velocity fields during three contrasting upwelling seasons, and throughout the upwelling-relaxation cycle. We also present results from two winter seasons that serve as contrast to the upwelling seasons.During all three upwelling seasons clear spatial structure is evident in velocity and hydrography across the shelf, exemplified by current reversals inshore and the presence of a persistent upwelling jet at the shelf break. This jet feature changes in structure and distance from the coast under different wind forcing regimes. The jet also changes from the north of our region, where it is a single narrow jet, adjacent to the coast, and to the south of our region, where it broadens and at times two jets become evident. We present observations of the California Under Current, which was observed at the outer edge of our domain during all three upwelling seasons. The observed across-shelf structure could aid both in the retention of plankton inshore during periods of upwelling followed by relaxation and in the export of plankton offshore in the upwelling jet.     

Vertical migration and mortality of benthos in dredged material—part I: Mollusca

Benthic invertebrates have many characteristics which make them prime candidates for burial studies in dredged material. A major concern in dredging and disposal projects is the effect of burial on the survival of benthic invertebrates. The purpose of the research reported in this paper was to determine the ability of estuarine benthos—in particular three species of molluscs (Mercenaria mercenaria, Nucula proxima and Ilyanassa obsoleta)—to migrate vertically in natural and exotic sediments and to determine the survival of benthos when exposed to particular amounts of simulated dredged material.Mortalities generally increased with increased sediment depth, with increased burial time and with overlying sediments whose particle size distribution differed from that of the species native sediment. Temperature affected mortalities and vertical migration. It was concluded that vertical migration is a viable process which can significantly affect rehabilitation of a dredged disposal area. Under certain conditions, vertical migration should be considered, together with larval settling and immigration from outside impacted areas, as a mechanism of recruiting a dredgedump site.     

High inside corners at ridge-transform intersections

A large topographic high commonly occurs near the intersection of a rifted spreading center and a transform fault. The high occurs at the inside of the 90° bend in the plate boundary, and is called the high inside corner, while the area across the spreading center, the outside corner, is often anomalously low. To better understand the origin of this topographic asymmetry, we examine topographic maps of 53 ridge-transform intersections. We conclude the following: (1) High inside corners occur at 41 out of 42 ridge-transform intersections at slow spreading ridges, and thus should be considered characteristic and persistent features of rifted slow spreading ridges. They are conspicuously absent at fast spreading ridges or at spreading centers that lack a rift valley. (2) High inside corners occur wherever an axial rift valley is present, and an approximate 1:1 correlation exists between the relief of the rift valley and the magnitude of the asymmetry. (3) Large high inside corners occur at both long and short transform offsets. (4) High inside corners at long offsets decay off-axis faster than predicted by the square root of age cooling model, precluding a thermalisostatic origin, but consistent with dynamic or flexural uplift models.These observations support the existing hypothesis that the asymmetry is due to the contrast in lithospheric coupling that occurs in the active transform versus the inactive fracture zone. Active faulting in the transform breaks the lithosphere along a high angle fault, permitting vertical movement of the inside corner block, whereas the inactive fracture zone forms a weld that couples the outside corner to the adjacent block, preventing it from rising. Large asymmetry at very short transform offsets appears to be caused by the added effect of a second uplift mechanism. Young lithosphere in the rift valley couples to the older plate, and when it leaves the rift valley it lifts the older plate with it. At very short offsets, this coupled uplift acts upon the high inside corner; at long offsets, it may upwarp the older plate or its expression may be muted.     

Polyester mooring for the Mad Dog spar—design issues and other considerations

The Mad dog project will use a polyester mooring system on the drilling and production truss spar. This will be the first use of a permanent polyester mooring system on a Floating Production System (FPS) outside of Brazil and the first time polyester has been used on a spar. As such, there were many challenges, which include:

• Regulatory approval.

• Designing a mooring system which is dominated by current loadings.

• Largest polyester rope break load ever required.

• Rope design qualification and testing.

• Quality control and assurance.

• Inspection, Maintenance, Repair and Retirement (IMRR) of such a mooring system.

This paper will focus on the design issues, rope design, manufacturing, qualification and testing, and the IMRR plan that was developed to provide the assurance that the polyester mooring system could be safely operated over the 20-year field life.     

The Loess/Paleosol record and the nature of the younger Dryas climate in central China

The use of latest Pleistocene-Holocene paleosols in defining Chinese climatic sequences is plagued by poor chronological controls caused primarily by the use of radiocarbon dates derived from bulk soil carbon. Dating of a post-glacial aeolian/paleosol sequence in the Pigeon Mountain basin of north-central China, using culturally deposited charcoal, support a wide array of other data suggesting the Younger Dryas was a period of cooler dryer conditions marked by wide-spread aeolian deposition. Periods of soil formation and higher lake levels bracket this climatic event. Climatic variability immediately before, during and immediately after the Younger Dryas interval is associated with rapid technological elaboration and innovation in the production and use of chipped stone tools, and perhaps, ground stone. © 1998 John Wiley & Sons, Inc.     

Origin of post‐Minoan caves and volcaniclastic cave fill,Thera (Santorini), Greece

The Aegean island of Thera (Santorini) was covered by tephra from its cataclysmic Late Bronze Age (ca. 3600 yr B.P.) eruption. Vertical exposures of the eruptive sequence show secondary, nonvolcanic, circular (in cross section) features composed of stratified sediment. Many are inaccessible from the floors of modern quarries and appear to be caves filled with younger sediment, but show no connection to the land surface. A filled cave was found in the wall of a modern gully outside the modern quarries, and a filled cave was found in a terrace scarp, well above the modern gully. Natural (and probably rapid) incision by gullies into the thick tephra deposit left many locations with lateral access to tephra. Inhabitants from post‐Minoan to recent times excavated tephra for materials and buildings, and caves were subsequently filled by sporadic (possibly seasonal) flood events that deposited sediment. These gullies may have provided access for modern tephra removal that isolated the filled caves high on the modern quarry walls. © 2003 Wiley Periodicals, Inc.