Holocene and Pleistocene fringing reef growth and the role of accommodation space and exposure to waves and currents (Bora Bora,Society Islands,French Polynesia)

Holocene fringing reef development around Bora Bora is controlled by variations in accommodation space (as a function of sea‐level and antecedent topography) and exposure to waves and currents. Subsidence ranged from 0 to 0·11 m kyr^?1, and did not create significant accommodation space. A windward fringing reef started to grow 8·7 kyr bp , retrograded towards the coast over a Pleistocene fringing reef until ca 6·0 kyr bp , and then prograded towards the lagoon after sea‐level had reached its present level. The retrograding portion of the reef is dominated by corals, calcareous algae and microbialite frameworks; the prograding portion is largely detrital. The reef is up to 13·5 m thick and accreted vertically with an average rate of 3·12 m kyr^?1. Lateral growth amounts to 13·3 m kyr^?1. Reef corals are dominated by an inner Pocillopora assemblage and an outer Acropora assemblage. Both assemblages comprise thick crusts of coralline algae. Palaeobathymetry suggests deposition in 0 to 10 m depth. An underlying Pleistocene fringing reef formed during the sea‐level highstand of Marine Isotope Stage 5e, and is also characterized by the occurrence of corals, coralline algal crusts and microbialites. A previously investigated, leeward fringing reef started to form contemporaneously (8·78 kyr bp ), but is thicker (up to 20 m) and solely prograded throughout the Holocene. A shallow Pocillopora assemblage and a deeper water Montipora assemblage were identified, but detrital facies dominate. At the Holocene reef base, only basalt was recovered. The Holocene windward–leeward differences are a consequence of less accommodation space on the eastern island side that eventually led to a more complex reef architecture. As a result of higher rates of exposure and flushing, the reef framework on the windward island side is more abundant and experienced stronger cementation. In the Pleistocene, the environmental conditions on the leeward island side were presumably unfavourable for fringing reef growth.     

New estimates of potential impacts of sea level rise and coastal floods in Poland

Natural Hazards - Polish coastal zone is thought to be one of the most exposed to sea level rise in Europe. With mean sea levels expected to increase between 28 and 98 cm by the end of the...     

Durations of soil formation and soil development indices in a Holocene Mediterranean floodplain

Holocene and late Pleistocene alluvial sequences of the mid-Medjerda floodplain (Northern Tunisia) reveal three types of soils with gradual transitions: Fluvisols (Calcaric), Cambisols (Calcaric) and Calcic Luvisols (Chromic). Stratigraphic cross-correlations, palaeomagnetic secular variation, and ¹⁴C and IRSL dating enable detailed information about ages and durations of soil formation in the floodplain.Weakly developed Fluvisols (Calcaric) commonly reveal late Holocene ages with soil formation durations lasting between 100 and 300 years. Hence, Holocene soil formation is detectable in the exposures from durations of around 100 years onwards. Cambisols show predominantly mid-Holocene ages. The durations of soil formation are between 800 and 5000 years. Calcic Luvisols (Chromic) feature late Pleistocene ages, with durations of soil formation between 10,000 and 40,000 years.Profile Development Indices were computed from simple field parameters like soil structure, soil colour, horizon thickness and leaching features. The derived development indices show good correlation (R² = 0.804) with the calculated durations of soil formation. Field parameters are well suited for a quantitative development index of Holocene soil formation, even if very weak developed soils are predominant. The study shows that maturity stages of Holocene alluvial soils in a homogeneous Mediterranean environment are predominantly driven by soil formation duration.     

The mechanism of cation and oxygen isotope exchange in alkali feldspars under hydrothermal conditions

The mechanism of re-equilibration of albite in a hydrothermal fluid has been investigated experimentally using natural albite crystals in an aqueous KCl solution enriched in ¹⁸O at 600°C and 2 kbars pressure. The reaction is pseudomorphic and produces a rim of K-feldspar with a sharp interface on a nanoscale which moves into the parent albite with increasing reaction time. Transmission electron microscopy (TEM) diffraction contrast and X-ray powder diffraction (XRD) show that the K-feldspar has a very high defect concentration and a disordered Al, Si distribution, compared to the parent albite. Raman spectroscopy shows a frequency shift of the Si-O-Si bending vibration from ~476 cm⁻¹ in K-feldspar formed in normal ¹⁶O aqueous solution to ~457 cm⁻¹ in the K-feldspar formed in ¹⁸O-enriched solution, reflecting a mass-related frequency shift due to a high enrichment of ¹⁸O in the K-feldspar silicate framework. Raman mapping of the spatial distribution of the frequency shift, and hence ¹⁸O content, compared with major element distribution maps, show a 1:1 correspondence between the reaction rim formed by the replacement of albite by K-feldspar, and the oxygen isotope re-equilibration. The textural and chemical characteristics as well as the kinetics of the replacement of albite by K-feldspar are consistent with an interface-coupled dissolution-reprecipitation mechanism.     

Origin and post-glacial evolution of surface cracks: A case study from the area of the Last Glaciation,north-eastern Poland

The analysis of LiDAR-based digital elevation models revealed the existence of groups of longitudinal fractures in the ground in northern Poland at the limit of the ice sheet's extent during its last maximum. Our research on the closed elongated depressions (CEDs) of the Jedwabno test field (Szuć site, north-east Poland) focuses on explaining their origins and their post-glacial history. This region was covered by an ice sheet and glacitectonically active during the Vistulian, and at least some surface fractures are possible witnesses to this activity. Using geomorphological mapping, sedimentological and geophysical research, we assumed it was related that the origin of these features here is associated with groundwater migration at the end of the Vistulian glaciation or later when groundwater flow intensified due to a rapid climate warming that caused permafrost to melt. The thawing of permafrost caused to transition from continuous permafrost to discontinuous, which in turn created groundwater flow that was probably responsible for the development of the surface cracks (fractures). Radiocarbon, palaeobiological (pollen, Cladocera) and geochemical studies allowed for an estimation of the formation time of these unique surface cracks in the Older Dryas. Prevailing conditions were also reconstructed for the later dynamic changes of the end of the Late Vistulian glaciation and in the Late Holocene until the Subatlantic Period (Megalayan stage). The surface cracks with steep slopes, despite their small area, are extraordinary sedimentation traps that have, in a special way, retained an almost complete record of the environmental and climate changes of the Late Glacial. There are sedimentological gaps in the Holocene, especially after the Preboreal (old part of the Greenlandian Stage), caused by changes in water levels, aeolian processes and human activity.     

New insights into Southern Caucasian glacial–interglacial climate conditions inferred from Quaternary gastropod fauna

In the present study, we performed gastropod analyses on loess–palaeosol sequences from northeast Armenia (Southern Caucasia) covering at least three glacial–interglacial cycles. The elaborated ecostratigraphy shows significant patterns of species composition related to the succession of pedocomplexes and loess, respectively. Pedocomplexes included species that can be associated with high-grass to forest-steppe biomes, indicating increased humidity for these sections compared to the loess layers. In contrast, loess layers that relate to glacial periods are associated with gastropod species of semidesert environments with shrub- and shortgrass-steppes, indicating semiarid to arid conditions. Furthermore, the loess deposits do not show any evidence for cold-adapted gastropod species. Therefore, we suggest that average July temperatures in the study area were above 10 °C, even during periods of loess deposition. Consequently, we propose that the limiting factor for tree growth during glacial periods was aridity, rather than temperature. In addition, we observe environmental differences between the various glacial times, with our results indicating a trend towards steadily increasing aridity in Southern Caucasia across the Middle to Late Pleistocene.     

Late Pleistocene palaeolake in the interior of Oman: a potential key area for the dispersal of anatomically modern humans out‐of‐Africa?

Relict fluvial and lacustrine deposits in the interior of Oman near Saiwan consist of waterlain breccias with pebble imbrications and current ripples, covered by algal laminites containing calcified reeds and charcoal pieces. Geomorphological evidence suggests that the palaeolake covered a maximum surface of 1400 km² and had a maximum depth of about 25 m. Optically stimulated luminescence (OSL) and thermally transferred OSL dating indicate that this palaeolake existed sometime between 132 and 104 ka. The rich archaeological evidence in the area suggests substantial human occupation, possibly at the time when Palaeolake Saiwan existed. Interestingly, the lithic traditions of the archaeological material show no clear relation to coeval findings from neighbouring areas, putting a question mark on the origin of the culture found at Saiwan. This finding raises questions concerning the origin of the population inhabiting Saiwan, as the site lies along one possible route for the dispersal of anatomically modern humans out‐of‐Africa. Copyright © 2011 John Wiley & Sons, Ltd.     

Fluvial system response to external forcing and human impact – Late Pleistocene and Holocene fluvial dynamics of the lower Guadalete River in western Andalucía (Spain)

The 170 km long river course of the Guadalete River (western Andalucía) provides an excellent record of Late Pleistocene and Holocene fluvial sedimentation dynamics. Furthermore, its floodplain sediments are very well suited to describe geomorphic changes forced by climate fluctuations, sea‐level changes, tectonic influences and human activity. Multiproxy investigations were based on field mapping and the study of 18 profile sections, mainly including sedimentological characterisation, soil‐chemical analyses and radiocarbon dating of 34 samples. Findings were complemented by drillings and electrical resistivity tomography. The lowermost 50 km of the river section are divided into an upper and lower part (each 25 km long), based on different sediment preservation conditions. The boundary corresponds to the disappearance of the Late Pleistocene river terrace. Significant floodplain aggradation occurred at around 10 000 cal. years BP, while dynamics were strongly affected by sea‐level fluctuations until the early Holocene. Furthermore, sedimentation starting at 8000, 6100, 4600, 2200, 900 and 400 cal. years BP is related to enhanced fluvial dynamics due to environmental stress that presumably was triggered by climate fluctuations, that is, aridification. However, the strongest intensity of sedimentation at 400 cal. years BP points to climate anomalies in the course of the Little Ice Age. In contrast, several periods of stability associated with alluvial soil formation took place during the Bølling and Allerød interstadials, prior to 8000, 6100 and 5100, and after 4300 and 2000 cal. years BP. The anthropogenic signal in floodplain evolution is not clearly distinguishable from that of climate. However, human land use had the potential to amplify geomorphic processes, especially during periods of climate deteriorations that caused increasing stress on the environment.     

Meteorites from the United Arab Emirates: Description,weathering, and terrestrial ages

We report on the first meteorite search campaign in the United Arab Emirates (UAE). The geology and proximity of our search region suggest that it is the north‐western extension of the Oman meteorite fields. We found 26 ordinary chondrites, bringing the total number of official meteorites from the UAE to 28. The campaign was organized and conducted in close cooperation with the UAE government and the main masses of the meteorites remained in the country where they will become part of an exhibition. The bulk composition of five meteorite and three soil samples indicates an uptake of U, Mo, Sr, Ba, Li, and Pb from the soil into the meteorites during terrestrial weathering. Terrestrial ages determined from ¹⁴C decay of 21 meteorites range from recent falls to 24.4 ka, with two meteorites having >37 ka and approximately 39 ka, respectively. Weak correlations between weathering degree, meteorite bulk chemical composition, and terrestrial age suggest highly localized weathering conditions, possibly related to abundant occurrences of sabkhas in the search region.