Holocene and Pleistocene pluvial periods in Yemen,southern Arabia

Arabia is an important potential pathway for the dispersal of Homo sapiens (“out of Africa”). Yet, because of its arid to hyper-arid climate humans could only migrate across southern Arabia during pluvial periods when environmental conditions were favorable. However, knowledge on the timing of Arabian pluvial periods prior to the Holocene is mainly based on a single and possibly incomplete speleothem record from Hoti Cave in Northern Oman. Additional terrestrial records from the Arabian Peninsula are needed to confirm the Hoti Cave record. Here we present a new speleothem record from Mukalla Cave in southern Yemen. The Mukalla Cave and Hoti Cave records clearly reveal that speleothems growth occurred solely during peak interglacial periods, corresponding to Marine Isotope Stages (MIS) 1 (early to mid-Holocene), 5.1, 5.3, 5.5 (Eemian), 7.1, 7.5 and 9. Of these humid periods, highest precipitation occurred during MIS 5.5 and lowest during early to middle Holocene.     

Occurrence of an exceptional carbonate dissolution episode during early glacial isotope stage 6 in the Southeastern Atlantic

Concentration and mass accumulation rate profiles from Southeastern Atlantic sediment cores located off Namibia show that an exceptional episode in benthic carbonate dissolution occurred during early glacial isotope stage 6 (substages 6.6 and 6.5) between about 186 000 and 170 000 yr BP. Although this episode is restricted to or is more pronounced in this region than in other areas of the Atlantic Ocean, its exceptional character with respect to older and younger climatic episodes at the same site cannot be fully explained by local factors alone, but requires a combination of local and global influences. The onset of the carbonate dissolution episode is related to a more efficient transfer of organic matter from surface eutrophic areas to the lower and is due to low sea level, while its termination relates to a change in either global ocean alkalinity or bottom water circulation. An evaluation of the magnitude of this local carbonate dissolution episode suggests that its contribution to a global alkalinity change may have been significant. Carbonate dissolution was probably amplified by stronger upwelling activity of the Benguela System linked to an exceptional northern excursion of the boreal summer ITCZ during early glacial isotope stage 6. This low latitude global linkage may explain how this carbonate dissolution event as well as other ‘anomalies’ observed for early stage 6, like an important Dole effect minimum or a ‘cold’ Mediterranean sapropel, are related.     

Geology and geochemistry of jasperoids from the Gold Bar district, Nevada

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J₀, J₁, J₂, and J₃, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J₀ formed during an early event. Petrographic observations, geochemistry, and δ¹⁸O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J₁, J₂, and J₃ jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J₀ jasperoids caused them to have higher δ¹⁸O values than J_1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ¹⁸O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ¹⁸O values are associated with the largest deposits. The 28‰ range of jasperoid δ¹⁸O values is best explained by mixing between an ¹⁸O-enriched fluid and an ¹⁸O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ¹⁸O composition of jasperoids indicates that gold was introduced by the ¹⁸O-enriched fluid. The lowest calculated δ¹⁸O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the ¹⁸O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the ¹⁸O-enriched ore fluid is not constrained. The δ³⁴S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H₂S and sulfate were derived from sedimentary sources. Likewise, the δ¹³C and δ¹⁸O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO₂ was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.     

Landslide inventory validation and susceptibility mapping in the Gerecse Hills,Hungary

Landslides pose a threat to property both in the populated and cultivated areas of the Gerecse Hills (Hungary). The currently available landslide inventory database holds the records from many sites in the area, but the database is out-of-date. Here we address the problem of revising the National Landslides Cadastre landslide inventory database by creating a landslide suscept- ibility map with a multivariate model based on likelihood ratio functions. The model is applied to the TanDEM-X DEM (0.4″ res.), the current landslide inventory of the area, and data acquired from geological maps. By comparing the distributions of four variables in the landslide and non-landslide area with grid computation methods, the model yields landslide susceptibility estimates for the study area. The estimations show to what extent a certain area is similar to the sample areas, therefore, its likelihood to be affected by landslides in the future. The accuracy of the model predictions was checked in the field and compared to the results of our previous study using the SRTM-1 DEM for a similar analysis. The model gave accurate estimates when certain correction measures were applied to the input datasets. The limitations of the model, the input datasets, and the suggested correction measures are also discussed.     

Oscillations of sea level rise along the Atlantic coast of North America north of Cape Hatteras

It is shown in the short comment that the sea levels are oscillating about a longer-term trend and that the sea level rise (SLR) computed with time windows of 20, 30 or 60?years also oscillates, with the amplitude of these latter oscillations reducing as the time window increases. The use of only two values of the SLR distribution is misleading to infer conclusions about the accelerating behaviour. In particular, the comparison of the 30-year SLR 1950?C1979 with the 30-year SLR 1980?C2009 for the tide gauges along the Atlantic coast of North America north of Cape Hatteras to infer an accelerating behaviour is particularly wrong because the 30-year time window is a too short interval to appreciate the longer-term sea level trend cleared of the multi-decadal oscillations, and the two values from the SLR distribution are computed, respectively, at the times of a valley and a peak for the 60-year Atlantic Ocean multi-decadal oscillation. By using a 60-year time window or all the data since opening when more than 60?years of recording are available and by analysing the SLR time history, the only conclusion that can be inferred from the analysis of the tide gauges along the North American Atlantic coast is that the sea levels are oscillating without too much of a positive acceleration along their longer-term trend.     

The importance of subsurface nepheloid layers in transport and delivery of sediments to the eastern Cariaco Basin,Venezuela

Optical transmissometer measurements were coupled with particulate organic matter (POM) observations to understand suspended sediment composition and distribution in the eastern Cariaco Basin during the rainy seasons of September 2003 and 2006. Our results suggest that nepheloid layers originating at the mouth of small mountainous rivers discharging into the eastern Basin are a major delivery mechanism of terrigenous sediments to the Basin interior. Intermediate nepheloid layers (INL) were observed near the shelf break (～100 m) and appear to effectively transport terrigenous material laterally from the shelf to deep waters, thereby providing a plausible supply mechanism of the terrestrial material observed in sediment traps. These findings highlight the importance of small, local rivers in the Cariaco Basin as sources of terrestrial material. In contrast, these nepheloid layers contained only limited POM. When this information is combined with published sediment trap POM data, it suggests that nepheloid layers may not be a primary mechanism for delivering terrigenous POM to the deeper waters of the basin during the rainy season. Rather, BNL may redistribute marine-derived POM from shallow waters to the Basin's interior by providing ballast materials, particularly during episodic events driven by wind and precipitation. Though we have determined that nepheloid layers play an important role in the seaward transport of particulate material in the Cariaco Basin, their composition and temporal variability have not been fully characterized. This is critical to understand lateral particle transport, since nepheloid layers constitute a significant source of sediment to the deep Cariaco Basin.     

A paleoclimate model of Northern Hemisphere ice sheets

A model for predicting the growth and decay of ice sheets based on the astronomical theory of climate change is presented. The purpose of the study in part is to isolate the role of the ice-sheet physics and earth response under varying ice load by simplifying to the extreme the role of the hydrosphere-atmosphere. Ice sheet physics and the response of the lithosphere-asthenosphere under the ice load are modeled explicitly. Insolation anomalies (taken at a fixed latitude) directly force latitudinal displacement of the snow line. Accumulation rate a, and ablation rate a′ evaluated at mean sea level are specificed as external constants; a,a′ decrease linearly with ice sheet elevation. Rough tuning of the model to the general shape of the ice-volume record of the last two major glacials determines the external constants. Model predictions of the ages of several events in the last major glaciation compare well with the radiological ages. The six glacial terminatios (I–VI) over the last 600,000 yr are identified and the predicted ages compare reasonably well with the δ¹⁸O record for two deep-sea cores. A direct comparison of model power spectra of ice volume as a function of period with spectra of the δ¹⁸O record shows apparent underprediction of power near 100,000 yr. When a quantitative but heuristic method for taking into account the “red noise” spectrum evident in the geological records is used, a much more favorable comparison is possible. The model prediction lends support to the hypothesis that the nonlinearity of the ice-sheet physics is responsible for the 100,000-yr periodicity in the geological record of the late Pleistocene.     

Note on the generalized thermal theory for gravity currents in the deceleration phase

The generalized thermal theory for gravitational convection, produced from instantaneous buoyancy sources on sloping boundaries, developed in Dai and Garcia (2010) is examined in this note. An assumption implicitly made therein, that detrained fluid carries no momentum, was inappropriate and the solution was not physical in special cases. The generalized thermal theory is now improved by considering the momentum carried away by detrained mixed fluid. An asymptotic velocity–distance relation for gravity currents further downslope in the deceleration phase is provided and agreement with reported experimental data is found.