Eustatic and climatic controls on the development of the Wahiba Sand Sea, Sultanate of Oman

Aeolian sand sea accumulations can serve as valuable archives of climate change in continental environments. The Wahiba Sand Sea is situated at the northern margin of the area presently affected by Indian Summer Monsoon Circulation and it records environmental changes associated with this major climatic boundary over the last 160 000 years. The internal stratigraphy and evolution of the sand sea is investigated using a combination of outcrop, borehole, seismic and luminescence data. Proximity to the Indian Ocean means that the sand sea succession shows the influence of sea level changes on the sedimentary architecture and composition of the dune deposits. During the last two glacial periods, low global sea level was associated with a high input of bioclastic grains, reflecting the significance of subaerially exposed shelf areas as one of the main sources of aeolian sediment. The onset of aeolian sediment transport and deposition was related to the breakdown of stabilizing vegetation during arid periods that equate with sea level lowstands. The preservation of aeolian sediments by the formation of supersurfaces and associated palaeosoils took place during times of increased wetness and elevated groundwater tables. This interplay of constructive and destructive periods greatly influenced the sedimentary architecture. Oscillations of wet and dry periods between 160 000 and 130 000 years and 120 000–105 000 years ago are attributed to the evolution of a wet aeolian system. Younger periods of aeolian deposition around and after the last glacial maximum were characterized by dry aeolian conditions. No soil horizons developed during these times.     

Climate variability at the southern boundaries of the Namib (southwestern Africa) and Atacama (northern Chile) coastal deserts during the last 120,000 yr

In this study, we present grain-size distributions of the terrigenous fraction of two deep-sea sediment cores from the SE Atlantic (offshore Namibia) and from the SE Pacific (offshore northern Chile), which we ‘unmix’ into subpopulations and which are interpreted as coarse eolian dust, fine eolian dust, and fluvial mud. The downcore ratios of the proportions of eolian dust and fluvial mud subsequently represent paleocontinental aridity records of southwestern Africa and northern Chile for the last 120,000 yr. The two records show a relatively wet Last Glacial Maximum (LGM) compared to a relatively dry Holocene, but different orbital variability on longer time scales. Generally, the northern Chilean aridity record shows higher-frequency changes, which are closely related to precessional variation in solar insolation, compared to the southwestern African aridity record, which shows a remarkable resemblance to the global ice-volume record. We relate the changes in continental aridity in southwestern Africa and northern Chile to changes in the latitudinal position of the moisture-bearing Southern Westerlies, potentially driven by the sea-ice extent around Antarctica and overprinted by tropical forcing in the equatorial Pacific Ocean.     

Timescales determining the degree of kinetic isotope fractionation by evaporation and condensation

A first order characteristic of the relative abundance of the elements in solar system materials ranging in size from inclusions in primitive meteorites to planetary sized objects such as the Earth and the Moon is that they are very much like that of the Sun for the more refractory elements but systematically depleted to varying degrees in the more volatile elements. This is taken as evidence that evaporation and and/or condensation were important processes in determining the distinctive chemical properties of solar system materials. In some instances there is also isotopic evidence suggesting evaporation in that certain materials are found enriched in the heavy isotopes of their more volatile elements. Here model calculations are used to explore how the relative rates of various key processes determine the relationship between elemental and isotopic fractionation during partial evaporation and partial condensation. The natural measure of time for the systems considered here is the evaporation or condensation timescale defined as the time it would take under the prevailing conditions for evaporation or condensation to completely transfer the element of interest between the two phases of the system. The other timescales considered involve the rate of change of temperature, the rate at which gas is removed from further interaction with the condensed phase, and the rates of diffusion in the condensed and gas phases. The results show that a key determinant of whether or not elemental fractionations have associated isotopic effects is the ratio of the partial pressure of a volatile element (P_i) to its saturation vapor pressure (P_i,sat) over the condensed phase. Systems in which the rate of temperature change or of gas removal are slow compared to the evaporation or condensation timescale will be in the limit P_i ∼ P_i,sat and thus will have little or no isotopic fractionation because at the high temperatures considered here there is negligible equilibrium fractionation of isotopes. If on the other hand the temperature changes are relatively fast, then P_i ≠ P_i,sat and there will be both elemental and isotopic fractionation during partial evaporation or partial condensation. Rapid removal of evolved gas results in P_i ? P_i,sat which will produce isotopically heavy evaporation residues. Diffusion-limited regimes, where transports within a phase are not sufficiently fast to maintain chemical and or isotopic homogeneity, will typically produce less isotopic fractionation than had the phases remained well mixed. The model results are used to suggest a likely explanation for the heavy silicon and magnesium isotopic composition of Type B CAIs (as due to rapid partial melting and subsequent cooling at rates of a few °C per hour), for the uniformity of the potassium isotopic composition of chondrules despite large differences in potassium depletions (as due to volatilization of potassium by reheating in regions of large but variable chondrules per unit volume), and that the remarkable uniformity of the potassium isotopic composition of solar system materials is not a measure of the relative importance of evaporation and condensation but rather due to the solar nebula having evolved sufficiently slowly that materials did not significantly depart from chemical equilibrium.     

Late Holocene climatic changes in Tierra del Fuego based on multiproxy analyses of peat deposits

A ca. 1400-yr record from a raised bog in Isla Grande, Tierra del Fuego, Argentina, registers climate fluctuations, including a Medieval Warm Period, although evidence for the ‘Little Ice Age’ is less clear. Changes in temperature and/or precipitation were inferred from plant macrofossils, pollen, fungal spores, testate amebae, and peat humification. The chronology was established using a ¹⁴C wiggle-matching technique that provides improved age control for at least part of the record compared to other sites. These new data are presented and compared with other lines of evidence from the Southern and Northern Hemispheres. A period of low local water tables occurred in the bog between A.D. 960-1020, which may correspond to the Medieval Warm Period date range of A.D. 950-1045 generated from Northern Hemisphere tree-ring data. A period of cooler and/or wetter conditions was detected between ca. A.D. 1030 and 1100 and a later period of cooler/wetter conditions estimated at ca. cal A.D. 1800-1930, which may correspond to a cooling episode inferred from Law Dome, Antarctica.     

Late Quaternary climatic control on erosion and weathering in the eastern Tibetan Plateau and the Mekong Basin

High-resolution siliciclastic grain size and bulk mineralogy combined with clay mineralogy, rubidium, strontium, and neodymium isotopes of Core MD01-2393 collected off the Mekong River estuary in the southwestern South China Sea reveals a monsoon-controlled chemical weathering and physical erosion history during the last 190,000 yr in the eastern Tibetan Plateau and the Mekong Basin. The ranges of isotopic composition are limited throughout sedimentary records: ⁸⁷Sr/⁸⁶Sr = 0.7206–0.7240 and ε_Nd(0) = −11.1 to −12.1. These values match well to those of Mekong River sediments and they are considered to reflect this source region. Smectites/(illite + chlorite) and smectites/kaolinite ratios are used as indices of chemical weathering rates, whereas the bulk kaolinite/quartz ratio is used as an index of physical erosion rates in the eastern Tibetan Plateau and the Mekong Basin. Furthermore, the 2.5–6.5 μm/15–55 μm siliciclastic grain size population ratio represents the intensity of sediment discharge of the Mekong River and, in turn, the East Asian summer monsoon intensity. Strengthened chemical weathering corresponds to increased sediment discharge and weakened physical erosion during interglacial periods. In contrast, weakened chemical weathering associated with reduced sediment discharge and intensified physical erosion during glacial periods. Such strong glacial–interglacial correlations between chemical weathering/erosion and sediment discharge imply the monsoon-controlled weathering and erosion.     

The upper Cenomanian (Cretaceous) ammonite Neolobites vibrayeanus () in the Middle East: taxonomic and palaeoecologic remarks

In the Middle East and Africa, the Cenomanian ammonite genus Neolobites occurs, partly in great abundance, in shallow marine shelf settings. The genus includes several species but it has remained uncertain to what extent these species reflect biospecies, chronospecies or morphological variants. Based on material from Jordan and Egypt, the morphological variation of Neolobites vibrayeanus is described and discussed. From the several species of Neolobites described in the literature, only N. vibrayeanus, N. fourtaui and N. peroni appear to deserve specific separation. Sedimentological and lithological data, notably the common occurrence in strata that were possibly deposited above storm wave base, suggest that Neolobites was able to inhabit shallow marine settings. In these settings, it seems to form a distinct assemblage with the nautiloid Angulithes. Although N. vibrayeanus shows morphological variability between distinct populations, local N. vibrayeanus assemblages seem to develop morphologically stable populations that show only a little morphological overlap with those of other areas. This may be the reason for the earlier taxonomic splitting. The occurrence in habitats in which other stratigraphically significant ammonites are often missing may be the reason for the previous poor dating of Neolobites assemblages.     

Tracing the history of submarine hydrothermal inputs and the significance of hydrothermal hafnium for the seawater budget—a combined Pb-Hf-Nd isotope approach

Secular variations in the Pb isotopic composition of a mixed hydrogenous-hydrothermal ferromanganese crust from the Bauer Basin in the eastern Equatorial Pacific provide clear evidence for changes in hydrothermal contributions during the past 7 Myr. The nearby Galapagos Rise spreading center provided a strong hydrothermal flux prior to 6.5 Ma. After 6.5 Ma, the Pb became stepwise more radiogenic and more similar to Equatorial Pacific seawater, reflecting the westward shift of spreading to the presently active East Pacific Rise (EPR). A second, previously unrecognized enhanced hydrothermal period occurred between 4.4 and 2.9 Ma, which reflects either off-axis hydrothermal activity in the Bauer Basin or a late-stage pulse of hydrothermal Pb from the then active, but waning Galapagos Rise spreading center.Hafnium isotope time-series of the same mixed hydrogenous-hydrothermal crust show invariant values over the past 7 Myr. Hafnium isotope ratios, as well as Nd isotope ratios obtained for this crust, are identical to that of hydrogenous Equatorial Pacific deep water crusts and clearly indicate that hydrothermal Hf, similar to Nd, does not travel far from submarine vents. Therefore, we suggest that hydrothermal Hf fluxes do not contribute significantly to the global marine Hf budget.