Droughts in the desert: Medieval Warm Period associated with coarse sediment layers in the Gulf of Aqaba-Eilat,Red Sea

Prolonged droughts can cause environmental stress, even in ecosystems that are already adapted to dry conditions. In these areas life greatly depends on rare, sporadic rainfall events that can produce flash floods. Today, urban expansion, population growth and development are occurring worldwide, including along hyperarid coastlines, an ecological zone on the extreme of habitability. In such places, the absence of the already limited precipitation can lead to long-lasting damage, and recognizing drought conditions in the past is useful for planning. Recognizing droughts, however, is challenging because they are not known as depositional events, and therefore may be under-recognized in the sedimentological record. Floods in these hyperarid deserts carry eroded sediments and deposit them in either terrestrial or marine terminal basins. In the hyperarid (ca 30 mm rain per year) desert surrounding the northern Gulf of Aqaba-Eilat some flash floods reach the sea and deposit alluvial sediments (mostly silt and clay) on the shelf. Following a recent 17 year drought period (1995 to 2012) a coarser surface layer was recognized offshore the outlet of an ephemeral river, despite the lack of incoming flood material. This coarsened layer resulted from a prolonged period wherein mixing and winnowing of fine particles at the surface was not interrupted by recurrent flash floods. In the bottom of two sediment cores collected from the same shelf, a similarly coarsened layer was observed. The terminal level of this deep coarse layer was dated to ca 660 yr bp , corresponding with the end of the Medieval Warm Period (ca 1350 ad ). These findings suggest that the medieval anomaly resulted in a prolonged drought period in this already arid region. These results present a new interpretation of grain-size distribution records that provides a means to reconstruct drought histories in hyperarid regions globally.     

Isotopic constraints on crustal architecture and Permo‐Triassic tectonics in New Guinea: Possible links with eastern Australia

New U–Pb zircon ages and Sr–Nd isotopic data for Triassic igneous and metamorphic rocks from northern New Guinea help constrain models of the evolution of Australia's northern and eastern margin. These data provide further evidence for an Early to Late Triassic volcanic arc in northern New Guinea, interpreted to have been part of a continuous magmatic belt along the Gondwana margin, through South America, Antarctica, New Zealand, the New England Fold Belt, New Guinea and into southeast Asia. The Early to Late Triassic volcanic arc in northern New Guinea intrudes high‐grade metamorphic rocks probably resulting from Late Permian to Early Triassic (ca 260–240 Ma) orogenesis, as recorded in the New England Fold Belt. Late Triassic magmatism in New Guinea (ca 220 Ma) is related to coeval extension and rifting as a precursor to Jurassic breakup of the Gondwana margin. In general, mantle‐like Sr–Nd isotopic compositions of mafic Palaeozoic to Tertiary granitoids appear to rule out the presence of a North Australian‐type Proterozoic basement under the New Guinea Mobile Belt. Parts of northern New Guinea may have a continental or transitional basement whereas adjacent areas are underlain by oceanic crust. It is proposed that the post‐breakup margin comprised promontories of extended Proterozoic‐Palaeozoic continental crust separated by embayments of oceanic crust, analogous to Australia's North West Shelf. Inferred movement to the south of an accretionary prism through the Triassic is consistent with subduction to the south‐southwest beneath northeast Australia generating arc‐related magmatism in New Guinea and the New England Fold Belt.     

Retrieval of chlorophyll and nitrogen in Norway spruce (Picea abies L. Karst.) using imaging spectroscopy

The research evaluated the information content of spectral reflectance (laboratory and airborne data) for the estimation of needle chlorophyll (C_AB) and nitrogen (C_N) concentration in Norway spruce (Picea abies L. Karst.) needles. To identify reliable predictive models different types of spectral transformations were systematically compared regarding the accuracy of prediction. The results of the cross-validated analysis showed that C_AB can be well estimated from laboratory and canopy reflectance data. The best predictive model to estimate C_AB was achieved from laboratory spectra using continuum-removal transformed data (R²cv = 0.83 and a relative RMSEcv of 8.1%, n = 78) and from hyperspectral HyMap data using band-depth normalised spectra (R²cv = 0.90, relative RMSEcv = 2.8%, n = 13). Concerning the nitrogen concentration, we observed somewhat weaker relations, with however still acceptable accuracies (at canopy level: R²cv = 0.57, relative RMSEcv = 4.6%). The wavebands selected in the regression models to estimate C_AB were typically located in the red edge region and near the green reflectance peak. For C_N, additional wavebands related to a known protein absorption feature at 2350 nm were selected. The portion of selected wavebands attributable to known absorption features strongly depends on the type of spectral transformation applied. A method called “water removal” (WR) produced for canopy spectra the largest percentage of wavebands directly or indirectly related to known absorption features. The derived chlorophyll and nitrogen maps may support the detection and the monitoring of environmental stressors and are also important inputs to many bio-geochemical process models.     

Partial melting and recrystallization of Archeaan komatiites by residual heat from rapidly accumulated flows

Spinifex-textured komatiites at Honeymoon Well, Western Australia, show evidence of partial melting and recrystallization of original igneous textures. Their textures and mineral compositions differ markedly from those typical of komatiites. Spinifex olivine plates are bent and broken, while interstitial space between spinifex and cumulus olivine is occupied by polygonal aggregates of clinopyroxene, orthopyroxene, minor olivine and plagioclase. Similar granular pyroxene-plagioclase aggregates occur as diffuse veins cutting spinifex zones and cumulate zones of the flows and, in places, form the matrix to a breccia containing corroded fragments of spinifex rock. Thermometry based on the two pyroxene assemblages yields temperatures of 1055° to 1141° C, just below the low-pressure komatiite solidus. Mineral compositions are different from those of typical komatiites: clinopyroxenes are Al-poor and Cr-rich, olivines are unusually iron-rich and depleted in Cr and Ca, and the low-Ca pyroxene is bronzite rather than the more typical pigeonite. We interpret these observations as the results of thermal metamorphism, partial remelting and subsequent slow crystallization of originally normal spinifex-textured komatiite flows. The rocks in question occupy a 40–70 m interval sandwiched between two olivine-rich units: an underlying 90 m-thick olivine adcumulate layer, forming part of the cumulate zone of a basal 160 m-thick flow, and an overlying 1 km-thick extrusive body composed mostly of olivine mesocumulate and adcumulate and capped in turn by spinifex-textured flows. Thermal modelling shows that a sinusoidal temperature profile of cool flow tops and hot flow centres would exist within this sequence shortly after eruption. Conductive thermal relaxation of this profile could reheat spinifex zones to the extent of inducing partial melting and textural reconstitution. Such reheating is largely dependent on the time interval between the emplacement of successive flows. Calculations suggest that at Honeymoon Well the emplacement interval must have been of the order of 10 years or less. Textural reconstitution may have contributed to the development of the thick orthocumulate sequences characteristic of komatiites in the Agnew-Wiluna belt. Present address: Geochemex Australia, P.O. Box 281, West Perth, 6005, Western Australia     

Abundances in Magellanic Clouds Young Clusters:

Detailed abundances are measured in cool (K type) supergiants in LMC young populous clusters (NGC 2004, NGC 2100 and NGC 1818), and compared to those, recently observed in samples of field supergiants in both Magellanic Clouds. This completes the work already done onthe SMC cluster NGC 330 (Hill et al., 1997b).The analysis of the field supergiants showed no evidence of any abundance dispersion among the stars, compatible with a well mixed gas in thesegalaxies. However, previous determinations of the metallicity ofNGC 330 and NGC 1818 gave values well below the respective field values. This difference is seeked, using a homogeneous method for the field andcluster samples, and the abundance ratios of various elements are examined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spatial patterns of primary production on the shelf, slope and basin of the Western Arctic in 2002

In the spring and summer of 2002 primary production in the Chukchi Sea was measured, using ¹⁴C uptake experiments. Our cruise track encompassed the shelf and continental slope area of the Chukchi and Beaufort Seas progressing into deep water over the Canada Basin. The study area experienced upwards of 90% ice cover during the spring, with ice retreating into the basin during the summer. Production in the spring was light-limited due to ice cover, with average euphotic zone production rates of <0.3 g C m⁻² d⁻¹. Values of 8 g C m⁻² d⁻¹ were observed in association with surface bloom conditions during the initial ice breakup. Considerable nutrient reduction in the surface waters took place between the spring and summer cruise, and although not observed, this was attributed to a spring bloom. Decreased ice cover and increased clarity of surface waters in the summer allowed greater light penetration. The highest rates of production during the second cruise were found at 25–30 m, coincident with the top of the nutricline. Daily euphotic zone productivity in the summer averaged 0.78 g C m⁻² d⁻¹ on the shelf and 0.32 g C m⁻² d⁻¹ on the edge of the Canada basin. These data provide an estimated annual production of 90 g C m⁻² yr⁻¹ in the study area.     

Origin of pockmarks and chimney structures on the flanks of the Storegga Slide,offshore Norway

Seafloor pockmarks and subsurface chimney structures are common on the Norwegian continental margin north of the Storegga Slide scar. Such features are generally inferred to be associated with fluid expulsion, and imply overpressures in the subsurface. Six long gravity and piston cores taken from the interior of three pockmarks were compared with four other cores taken from the same area but outside the pockmarks, in order to elucidate the origins and stratigraphy of these features and their possible association with the Storegga Slide event. Sulfate gradients in cores from within pockmarks are less steep than those in cores from outside the pockmarks, which indicates that the flux of methane to the seafloor is presently smaller within the pockmarks than in the adjacent undisturbed sediments. This suggests that these subsurface chimneys are not fluid flow conduits lined with gas hydrate. Methane-derived authigenic carbonates and Bathymodiolus shells obtained from a pockmark at >6.3 m below the seafloor indicate that methane was previously available to support a chemosynthetic community within the pockmark. AMS ¹⁴C measurements of planktonic Foraminifera overlying and interlayered with the shell-bearing sediment indicate that methane was present on the seafloor within the pockmark prior to 14 ka ¹⁴C years b.p., i.e., well before the last major Storegga Slide event (7.2 ka ¹⁴C years b.p., or 8.2 ka calendar years b.p.). These observations provide evidence that overpressured fluids existed within the continental margin sediments off Norway during the last major advance of Pleistocene glaciation.     

Through the impact glass: Insight into the evolution of melt at the Mistastin Lake impact structure

By analyzing impact glass, the evolution of the impact melt at the Mistastin Lake impact structure was investigated. Impact glass clasts are present in a range of impactites, including polymict breccias and clast‐rich impact melt rock, and from a variety of settings within the crater. From the glass clasts analyzed, three petrographic subtypes of impact glass were identified based on their clast content, prevalence of schlieren, color, texture, and habit. Several alteration phases were also observed replacing glass and infilling vesicles; however, textural observations and quantified compositional data allowed for the identification of pristine impact glass. Although the various types of glasses show significant overlap in their major oxide composition, several subtle variations in the major oxide chemistry of the glass were observed. To investigate this variation, a least‐squares mixing model was implemented utilizing the composition of the glass and the known target rock chemistry to model the initial melt composition. Additionally, image analysis of the glass clasts was used to investigate whether the compositional variations correlated to textural difference in the lithologies. We propose that the textural and compositional dichotomy observed is a product of the evolution, assimilation, and emplacement of the glass. The dichotomy is reflective of the melt either being ballistically emplaced (group 2 glasses: occurring in melt‐poor polymict breccias at lowermost stratigraphic position outside the transient crater) or the result of late‐stage melt flows (group 1 glasses, occurring in melt‐bearing polymict breccias and impact melt rocks at higher stratigraphic positions outside the transient crater).     

Petrography and geochemistry of lunar meteorites Dhofar 1673, 1983, and 1984

The Dhofar 1673, Dhofar 1983, and Dhofar 1984 meteorites are three lunar regolith breccias classified based on their petrography, mineralogy, oxygen isotopes, and bulk chemistry. All three meteorites are dominated by feldspathic lithic clasts; however, impact melt rock clasts and spherules are also found in each meteorite. The bulk chemistry of these samples is similar to other feldspathic highland meteorites with the Al₂O₃ content only slightly lower than average. Within the lithic clasts, the Mg # of mafic phases versus the anorthite content of feldspars is similar to other highland meteorites and is found to plot intermediate of the ferroan‐anorthositic suite and magnesian suite. The samples lack any KREEPy signature and have only minor indications of a mare basalt component, suggesting that the source region of all three meteorites would have been distal from the Procellarum KREEP Terrane and could have possibly been the Feldspathic Highland Terrane. All three meteorites were found within 500 m of each other in the Dhofar region of Oman. This, together with their similar petrography, stable isotope chemistry, and geochemistry indicates the possibility of a pairing.