A seasonally ''dry'' interglacial climate in eastern England during the early Middle Pleistocene: palaeopedological and stable isotopic evidence from Pakefield, UK

The pedology and the isotope geochemistry of the early Middle Pleistocene interglacial 'rootlet bed' at Pakefield, eastern England, record the climate when this unit accumulated. Horizons of nodular pedogenic calcrete indicate the existence of a strongly seasonal precipitation regime that produced a pronounced net soil moisture deficit during which secondary carbonate was precipitated. The isotopic composition of these nodules confirms the seasonality of annual precipitation, as this indicates that intense soil moisture evaporation was a major mechanism in secondary carbonate formation. The climatic interpretation is enhanced by the palaeoecology of the unit that indicates average summer temperatures significantly warmer than those of the present. The climate of this period was, therefore, different from that of the present day in terms of both temperature and the seasonality of precipitation. Although it has previously been shown that certain interglacial periods in northwest Europe were warmer than the Holocene, this is the first study to show that the precipitation regime of one such episode may also have been significantly different.     

Coastal dune conservation on an Irish commonage: community-based management or tragedy of the commons?

In Ireland 'commonage' refers to lands jointly owned by several individuals who have grazing rights. Commonage can provide the low-intensity grazing regime regarded as optimal for habitat conservation, and it is also unlikely to suffer the negative impacts of building development or coastal engineering. Today, however, the traditional control systems of coastal commonage are generally moribund, leading to habitat degradation. The only viable future management model is likely to be one based on local community control. Community management would have the legitimacy to counter the negative perceptions of external authority that generate environmental degradation.     

Lunar highland meteorite Dhofar 026 and Apollo sample 15418: Two strongly shocked,partially melted,granulitic breccias

Abstract— Studies of lunar meteorite Dhofar 026, and comparison to Apollo sample 15418, indicate that Dhofar 026 is a strongly shocked granulitic breccia (or a fragmental breccia consisting almost entirely of granulitic breccia clasts) that experienced considerable post‐shock heating, probably as a result of diffusion of heat into the rock from an external, hotter source. The shock converted plagioclase to maskelynite, indicating that the shock pressure was between 30 and 45 GPa. The post‐shock heating raised the rock's temperature to about 1200 °C; as a result, the maskelynite devitrified, and extensive partial melting took place. The melting was concentrated in pyroxene‐rich areas; all pyroxene melted. As the rock cooled, the partial melts crystallized with fine‐grained, subophitic‐poikilitic textures. Sample 15418 is a strongly shocked granulitic breccia that had a similar history, but evidence for this history is better preserved than in Dhofar 026. The fact that Dhofar 026 was previously interpreted as an impact melt breccia underscores the importance of detailed petrographic study in interpretation of lunar rocks that have complex textures. The name “impact melt” has, in past studies, been applied only to rocks in which the melt fraction formed by shock‐induced total fusion. Recently, however, this name has also been applied to rocks containing melt formed by heating of the rocks by conductive heat transfer, assuming that impact is the ultimate source of the heat. We urge that the name “impact melt” be restricted to rocks in which the bulk of the melt formed by shock‐induced fusion to avoid confusion engendered by applying the same name to rocks melted by different processes.     

Influence of snow upon rates of granodiorite weathering, Colorado Front Range, USA

Measurements of phenocryst relief in 1-m² quadrats placed randomly on southwest-facing granodiorite outcrops exposed by late Pleistocene deglaciation of a high-altitude Front Range col suggest that weathering is strongly influenced by snow. Depending upon local differences in snow-cover duration (inferred from differences in lichen cover), rates of bedrock-surface lowering vary from 0.0001 to 0.0013 mm/year. A quadratic polynomial best describes the relationship. Weathering is least effective beneath late-lying snowbanks that protect the rock from freeze-thaw and wetting-drying cycles for most of the year, and in winter snow-free sites where dryness and the binding effects of crustose lichens discourage granular disintegration. Weathering is most effective in areas of thin to moderate winter snow cover where meltout occurs early, moisture is available at critical seasons, and episodic lichen snowkill ensures that bare rock is exposed. If granite-weathering data are to be used for dating, measurements should be restricted to microenvironments with comparable snow-accumulation histories.     

The glaciation of northeastern Kansas

The traditional pre-Illinoian glacial stratigraphy in the central United States has been abandoned, which leaves a void in the Pleistocene stratigraphy of Kansas. The Independence Formation is proposed as a tithostratigraphic unit in northeastern Kansas. The Independence Formation consists of all diamictons and stratified sediments interbedded with diamictons. The Independence Formation vanes greatly in thickness and texture, but it always contains erratics derived from the Canadian Shield. At least two glacial advances took place in northeastern Kansas, first by the Minnesota lobe coming from the northeast, and later by the Dakota lobe from the northwest. During the first advance, glacial Lake Atchison was dammed in a now-buried valley. Overflow from this lake cut spillways that form parts of the Blue, Kansas, and Missouri valleys. During the later ice advance, the Kansas valley was blocked, and temporary lakes formed, from which catastrophic flooding took place. The age of the Independence Formation is interpreted as 0.7 to 0.6 million years BP. It is considered equivalent in age and stratigraphic position to type A2 and A3 tills of Nebraska and western Iowa and to the McCredie Formation of northern Missouri. This age range corresponds to oxygen-isotope stages W16.     

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette     

Parameterization of the Melting Regime of the Shallow Upper Mantle and the Effects of Variable Lithospheric Stretching on Mantle Modal Stratification and Trace-Element Concentrations in Magmas

Parameterization of melting phenomena in the upper mantle hasprimarily focused on two basic themes, namely the physical andchemical processes that govern partial melting. Parameterizationof physical processes mainly refers to establishing relationshipsbetween parameters such as the temperature, pressure, matrixand melt flow geometry, lithospheric stretching, and volumeof magma. By contrast, parameterization of chemical processeslargely implies unravelling the relationships between type anddegree of melting, and source and melt composition. Few attemptshave been made, however, to interrelate the two processes. Thepresent work is an effort to provide a link between physicaland chemical parameters associated with mantle melting and toallow in-depth modelling of partial melting processes in upwellingasthenosphere in a rigorous yet simplified manner. Several correlationsamong the most important physical parameters (e.g., equilibrationand extrusion temperature and pressure of magma, melt fractionand thickness, stretching factor, etc.) are explored. On thisbasis, a model for the compositional stratification of the lithosphereis proposed, and its bearing on the nature of intra-oceanicarc magmatism is emphasized. Trends of melting residues in termsof modal olivine and clinopyroxene are calculated for a widerange of possible potential temperatures that may be appliedto xenolith or abyssal peridotite suites to constrain furthertheir original depth of upwelling. Dry solidus equations fordepleted peridotite compositions are also derived that may beused to infer the effects of volatiles on the melting of refractorysupra-subduction zone mantle. The sensitivity of certain elementsto temperature variations during melting in a column of ascendingmantle is highlighted using Ni as an example, and the dangersof using single-value distribution coefficients to predict concentrationsof transition metals in magmas are emphasized. MORB-normalizedmulti-element profiles calculated using a variety of sources,mantle potential temperatures, and stretching factors are presented,and the differences between instantaneous and pooled melts arediscussed. A technique to calculate mineral proportions duringtransformation of garnet lherzolite to spinel lherzolite, togetherwith estimates of the modal composition of fertile spinel andgarnet lherzolite are included. Selected trace-element abundancesin various sources [bulk silicate Earth, depleted MORB (mid-oceanridge basalt) mantle, N-MORB) and distribution coefficientsfor common rock-forming minerals are also tabulated.     

Modern carbonate and terrigenous clastic sediments on a cool water, high energy, mid-latitude shelf: Lacepede, southern Australia

The wide Lacepede Shelf and narrow Bonney Shelf are contiguous parts of the south-eastern passive continental margin of Australia. The shelves are open, generally deeper than 40 m, covered by waters cooler than 18°C and swept by oceanic swells that move sediments to depths of 140 m. The Lacepede Shelf is proximal to the ‘delta’of the River Murray and the Coorong Lagoon. Shelf and upper slope sediments are a variable mixture of Holocene and late Pleistocene quartzose terrigenous clastic and bryozoa-dominated carbonate particles. Bryozoa grow in abundance to depths of 250 m and are conspicuous to depths of 350 m. They can be grouped into four depth-related assemblages. Coralline algae, the only calcareous phototrophs, are important sediment producers to depths of 70 m. Active benthic carbonate sediment production occurs to depths of 350 m, but carbonate sediment accumulation is reduced on the open shelf by continuous high energy conditions. The shelf is separated into five zones. The strandline is typified by accretionary sequences of steep shoreface, beach and dune carbonate/siliciclastic sediments. Similar shoreline facies of relict bivalve/limestone cobble ridges are stranded on the open shelf. The shallow shelf, c.40–70 m deep, is a wide, extremely flat plain with only subtle local relief. It is a mosaic of grainy, quartzose, palimpsest facies which reflect the complex interaction of modern bioclastic sediment production (dominated by bryozoa and molluscs), numerous highstands of sea level over the last 80 000 years, modern mixing of sediments from relatively recent highstands and local introduction of quartz-rich sediments during lowstands. The middle shelf, c.70–140 m deep, is a gentle incline with subtle relief where Holocene carbonates veneer seaward-dipping bedrock clinoforms and local lowstand beach complexes. Carbonates are mostly modern, uniform, clean, coarse grained sands dominated by a diverse suite of robust to delicate bryozoa particles produced primarily in situ but swept into subaqueous dunes. The deep shelf edge, c. 140–250 m deep, is a site of diverse and active bryozoa growth. Resulting accumulations are characteristically muddy and distinguished by large numbers of delicate, branching bryozoa. The upper slope, between 250 and 350 m depth, contains the deepest platform-related sediments, which are very muddy and contain a low diversity suite of delicate, branching cyclostome bryozoa. This study provides fundamental environmental information critical for the interpretation of Cenozoic cool water carbonates and the region is a good model for older mixed carbonate-terrigenous clastic successions which were deposited on unrimmed shelves.     

Origin of a cool-water, Oligo-Miocene deep shelf limestone, Eucla Platform, southern Australia

The Abrakurrie Limestone is an areally extensive, bryozoan-rich unit within the Eucla Platform, a Tertiary carbonate shelf which caps the central part of the southern Australian continental margin. The onshore portion, the topic of this study, has been exposed since middle Miocene time and lies beneath the Nullarbor Plain. The rocks are fine-sand- to granule-sized calcarenites, composed of bryozoans, bivalves, benthic foraminifera and echinoids with lesser numbers of brachiopods, solitary corals and serpulids. They conspicuously lack significant numbers of planktonic foraminifera and coralline algae. Most bryozoan remains are from delicate branching cyclostomes although delicate branching, robust branching, foliose, fenestrate, multilaminar encrusting and free-living cheilostomes are variably abundant in specific units. The poorly lithified sequence is punctuated by well-cemented layers with erosional tops, which are interpreted as hardgrounds. The limestone is interpreted as a cool-water, deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the Eucla Platform. A model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation. The shelf is envisaged as partitioned by the depth of the zone of wave abrasion. Sediments were produced throughout, but accumulated only below this depth. When the seafloor was above this depth it was an environment of cementation and erosion. The vertical sequence correlates in a general way with the global sea-level model for the mid-Cenozoic. While accumulation rates for southern Australian carbonates are similar to rates of cool-water carbonate deposition elsewhere (c. 2.5 cm kyr^-1), the rate of Abrakurrie accumulation is much less, suggesting that significant time periods are represented by the hardgrounds.