Response of Pediastrum in German floodplain lakes to Late Glacial climate changes

Subfossil remains of the Pediastrum-algae group (Fam. Hydrodictyaceae) are frequently found in lake sediments, but very few studies have used them as palaeoclimate and palaeolimnological indicators. This study explored the species composition of Pediastrum assemblages in shallow floodplain lakes in northern Germany during two transitional periods from cold to temperate climate conditions (GS-2/GI-1 and GI-1/Holocene). We identified Pediastrum taxa to subspecies level and used multivariate statistics (constrained clustering, principal component analysis and redundancy analysis) to show that shifts in taxonomic composition reflected the strong Late Glacial climate oscillations. The Pediastrum assemblages indicate that climate amelioration already begun shortly before the main GS-2/GI-1 summer-temperature transition. In contrast to previous studies that identified trophic state as the main driver of change in Pediastrum species composition, we identified climate shifts and related factors as the major drivers of community change. Water depth and trophic state probably acted as secondary factors that were responsible for differences in Pediastrum response between the first and second investigated climate transitions. During cold periods, Pediastrum algae composition was controlled mainly by environmental variables, whereas during warm periods, Pediastrum assemblages may have been influenced to a greater extent by intra- and intergeneric competition. This study contributes to knowledge about ecological niches of Pediastrum species. Size-measurements on Pediastrum coenobia reveal that one response of Pediastrum algae to climate shifts probably was size change. Coenobia of Pseudopediastrum boryanum var. boryanum were significantly smaller during cold climate periods than during temperate periods. The shifts in Pediastrum species composition and coenobium size suggest this algae group has great potential for enabling palaeoecological and palaeoclimate inferences.     

Long-wave radiative analysis of cloudy scattering atmospheres using a net exchange formulation

The Net Exchange Formulation (NEF) is an alternative to the usual radiative transfer equation. It was proposed in 1967 by Green [Q. J. R. Meteorol. Soc. 93 (1967) 371] for atmospheric sciences and by Hottel [H.C. Hottel, A.F. Sarofim. Radiative Transfer McGraw Hill, New York, 1967] for engineering sciences. Until now, the NEF has been used only in a very few cases for atmospheric studies. Recently we have developed a long-wave radiative code based on this formulation for a GCM of the Mars planet. Here, we will present results for the Earth atmosphere, obtained with a Monte Carlo Method based on the NEF. In this method, fluxes are not addressed any more. The basic variables are the net exchange rates (NER) between each pair of atmospheric layer (i, j), i.e. the radiative power emitted by i and absorbed by j minus the radiative power emitted by j and absorbed by i. The graphical representation of the NER matrix highlights the radiative exchanges that dominate the radiative budget of the atmosphere and allows one to have a very good insight of the radiative exchanges. Results will be presented for clear sky atmospheres with Mid-Latitude Summer and Sub-Arctic Winter temperature profiles, and for the same atmospheres with three different types of clouds. The effect of scattering on long-wave radiative exchanges will also be analysed.     

Coarse clast ridge sequences as suitable archives for past storm events? Case study on the Houtman Abrolhos,Western Australia

Prehistoric storm records are relatively scarce in most parts of the world. This article presents storm records derived from coral rubble‐based geological archives of the Houtman Abrolhos Archipelago located off the west coast of Australia, where the southernmost coral reefs of the Indian Ocean are found. Winter storm swell from the circum‐Antarctic ‘Brave Westerlies’, as well as tropical cyclone waves, have left numerous ridge systems on dozens of islands of the archipelago, all composed of coral rubble from adjacent reefs. At three islands, seven ridge systems were dated by three different methods: U‐series (68 dates), radiocarbon (64 dates), electron spin resonance (7 dates); 139 radiometric dates span the last 5500 years of the Holocene. In contrast to the geomorphological interpretation, the age sequences show ‘inversions’, hiatuses and different ages for the same ridge, all pointing to complicated ridge formation processes. Time gaps, some exceeding 1000 years, are interpreted as phases of erosion and not as phases without storm activity. Copyright © 2012 John Wiley & Sons, Ltd.     

Tidal gravity variations revisited at Vostok Station, Antarctica

In 1969, prior to the discovery of the subglacial Lake Vostok, an Askania Gs-11 gravimeter was operated at Vostok Station (78.466°S, 106.832°E; 3478 m asl) to observe tidal gravity variations. To gain a better understanding of the lake's tidal dynamics, we reanalyzed these data using a Bayesian Tidal Analysis Program Grouping method (BAYTAP-G and -L programs). The obtained phase leads for the semidiurnal waves M2 (6.6 ± 2.1°) and S2 (10.1 ± 4.2°) are more pronounced than those of the diurnal waves, among which the largest phase lead (for K1) was 5.0 ± 0.5°. The obtained δ factor for M2 was 0.890 ± 0.032, significantly less than the theoretical value of 1.16. For three global ocean tide models (NAO99b, FES2004, and TPXO6.2), the estimated load tides on waves Q1, O1, P1, K1, M2, and S2 range from 0.1–0.2 μGal (Q1 and S2) to 0.6–0.7 μGal (K1). The difference in amplitude among the three models is less than 0.14 μGal (M2), and the difference in phase is generally less than 10°. In calculating the residual tide vectors using the ocean models, the TPXO6.2 model generally gave the smallest residual amplitudes. Our result for the K1 wave was anomalously large (1.36 ± 0.25 μGal), while that for the M2 wave was sufficiently small (0.37 ± 0.17 μGal). The associated uncertainty is half that reported in previous studies. It is interesting that the residual K1 tide is approximately 90° phase-leaded, while the M2 tide is approximately 180° phase-leaded (delayed). Importantly, a similar reanalysis of data collected at Asuka Station (71.5°S, 24.1°E) gave residual tides within 0.2–0.3 μGal for all major diurnal and semidiurnal waves, including the K1 wave. Therefore, the anomalous K1 residual tide observed at Vostok Station must be linked to the existence of the subglacial lake and the nature of solid–ice–water dynamics in the region.     

Traces of historical tropical cyclones and tsunamis in the Ashburton Delta (north‐west Australia)

Although the north‐western coast of Western Australia is highly vulnerable to tropical cyclones and tsunamis, little is known about the geological imprint of historic and prehistoric extreme wave events in this particular area. Despite a number of site‐specific difficulties such as post‐depositional changes and the preservation potential of event deposits, both tropical cyclones and tsunamis may be inferred from the geomorphology and the stratigraphy of beach ridge sequences, washover fans and coastal lagoons or marshes. A further challenge is the differentiation between tsunami and storm deposits in the geological record, particularly where modern deposits and/or historical reports on the event are not available. This study presents a high‐resolution sedimentary record of washover events from the Ashburton River delta (Western Australia) spanning approximately the last 150 years. A detailed characterization of event deposits is provided, and a robust chronostratigraphy for the investigated washover sequence is established based on multi‐proxy sediment analyses and optically stimulated luminescence dating. Combining sedimentological, geochemical and high‐resolution optically stimulated luminescence data, event layers are assigned to known historical events and tropical cyclone deposits are separated from tsunami deposits. For the first time, the 1883 Krakatoa and 1977 Sumba tsunamis are inferred from sedimentary records of the north‐western part of Western Australia. It is demonstrated that optically stimulated luminescence applied in coastal sedimentary archives with favourable luminescence characteristics can provide accurate chronostratigraphies even on a decadal timescale. The results contribute to the data pool of tropical cyclone and tsunami deposits in Holocene stratigraphies; however, they also demonstrate how short‐lived sediment archives may be in dynamic sedimentary environments.     

The dependence of organohalogen compound concentrations on herring age and size in the Bothnian Sea, northern Baltic

The concentrations and composition profiles of polychlorinated dibenzo-p-dioxins and dibenzofurans, polychlorinated biphenyls, polybrominated diphenyl ethers and polychlorinated naphthalenes were determined in herring individuals collected from the commercial catches of the Bothnian Sea, northern Baltic. The median age of herring was 5.0 years and in the muscle the median toxic equivalent concentration of PCDD/Fs was 5.6 WHO(PCDD/F)-TEQ pg/g fresh weight (fw) and that of PCBs 2.7 WHO(PCB)-TEQ pg/g fw. The median sum concentration of PBDEs was 1.4 ng/g fw and that of PCNs 0.1 ng/g fw. Differences in age-dependent accumulation between the organohalogen groups and individual congeners were major. In the Bothnian Sea the content of organohalogen compounds in herring is obviously elevated due to the availability and large proportion of Mysis crustaceans in their diet. More intensive fishing could reduce the concentrations of organohalogens, including the abundant, accumulative and toxic 2,3,4,7,8-PeCDF congener in the Baltic herring catch.     

Insights into rock avalanche emplacement processes from detailed morpho‐lithological studies of the Tschirgant deposit (Tyrol,Austria)

Large, rapid rockslope failures generate deposits with complex morphologies due to a number of causal and influencing factors. To investigate these, we conducted a detailed case study at the carbonate Tschirgant deposit (Tyrol, Austria). It preserved evidence of simultaneous rock sliding (very large, coherent hummocks) and rock avalanche spreading (smaller, more scattered hummocks and ridges). Motion indicators, such as longitudinal ridges furthermore pinpoint the transition between linear sliding and radial spreading. The lithological distribution in the Tschirgant deposit shows that it retained source stratigraphy despite being split into two accumulation lobes by a high bedrock ridge. Furthermore, lithology had a very strong control on the final deposit morphology in that the different lithologic units form individual deposit surfaces. River erosion has created fortuitous outcrops that reveal the basal rock avalanche contact. The underlying valley‐fill sediments (substrates) have been intricately involved in shaping the rock avalanche morphology and, where entrained, highlight internal rock avalanche deformation features. This study shows that intrinsic dynamic properties of granular media (e.g. tendency for longitudinal alignments), emplacement mode, lithology (and source predisposition), runout path topography, and substrates form the quintet of causal factors that shape rock avalanche morphology. Copyright © 2015 John Wiley & Sons, Ltd.     

Optically stimulated luminescence dating of sublittoral and intertidal sediments from Dubai, UAE: Radioactive disequilibria in the uranium decay series

Shallow marine and intertidal shell rich sediments from the low coastal plain of Dubai were dated by optically stimulated luminescence. Combined high-resolution gamma spectrometry and ICP–MS analyses revealed significant disequilibria in the ²³⁸U decay series. Time dependent dose rates were modelled for two scenarios including a closed system and an open system with linear U and Ra uptake. OSL ages that were calculated using the linear uptake model are in good agreement with archaeological evidence and radiocarbon datings on shells and wood. The presented results illustrate that luminescence-dating techniques are suitable to set up a chronostratigraphic frame for calcareous coastal environments if an appropriate model is applied to consider the influence of radioactive disequilibria and time dependent dose rates. A comprehensive radionuclide analysis of the sediments is thought necessary to perform geochemically supported dose rate simulations.