The onset of the Messinian salinity crisis in the deep Eastern Mediterranean basin

Astronomical tuning of the Messinian pre‐salt succession in the Levant Basin allows for the first time the reconstruction of a detailed chronology of the Messinian salinity crisis (MSC) events in deep setting and their correlation with marginal records that supports the CIESM ( 2008 ) 3‐stage model. Our main conclusions are (1) MSC events were synchronous across marginal and deep basins, (2) MSC onset in deep basins occurred at 5.97 Ma, (3) only foraminifera‐barren, evaporite‐free shales accumulated in deep settings between 5.97 and 5.60 Ma, (4) deep evaporites (anhydrite and halite) deposition started later, at 5.60 Ma and (5) new and published ⁸⁷Sr/⁸⁶Sr data indicate that during all stages, evaporites precipitated from the same water body in all the Mediterranean sub‐basins. The wide synchrony of events and ⁸⁷Sr/⁸⁶Sr homogeneity implies inter‐sub‐basin connection during the whole MSC and is not compatible with large sea‐level fall and desiccation of the Mediterranean.     

A Lagrangian Cloud Model for the Study of Marine Fog

A large-eddy simulation model is coupled with a Lagrangian cloud model to study marine fog. In this model, aerosols and droplets are treated from a Lagrangian frame of reference, in contrast to the traditional bulk and bin microphysical models. Droplet growth via condensation is governed by Köhler theory and environmental conditions local to the droplet. Coupling to the vapour and temperature fields of the flow ensures mass, momentum, and energy conservation between the air and droplet phases. Based on the recent C-FOG field campaign, a simulation is performed which highlights the benefits and potential of this type of model. By initializing the simulation with the measured aerosol size distribution and making assumptions about the chemical composition of the multiple peaks, the simulations provide a clear explanation for the observed bimodal droplet distribution during C-FOG: high supersaturation levels cause condensational growth of nearly all coarse-mode aerosols (presumed to be composed of marine salt), as well as a large number of accumulation model aerosols (presumed to be of continental origin with a lower hygroscopicity). The largest peak in the resulting droplet distribution is created from coarse-mode aerosols with high hygroscopicity, while the secondary peak is only possible due to the limited impact of the largest peak on saturation levels inside the fog. Thus, for the simulated levels of supersaturation, it is the limited number of coarse-mode aerosols which is responsible for the emergence of a larger second peak.

     

Determination of Selenium Concentrations in NIST SRM 610, 612, 614 and Geological Glass Reference Materials Using the Electron Probe, LA-ICP-MS and SHRIMP II

A combination of EMPA, sensitive high resolution ion microprobe (SHRIMP II) and/or LA-ICP-MS techniques was used to measure the concentration of selenium (Se) in NIST SRM 610, 612, 614 and a range of reference materials. Our new compiled value for the concentration of Se in NIST SRM 610 is 112 ± 2 μg g⁻¹. The concentration of Se in NIST SRM 612, using NIST SRM 610 for calibration, determined using LA-ICP-MS (confirmed using SHRIMP II) was 15.2 ± 0.2 μg g⁻¹. The concentration of Se in NIST SRM 614, using LA-ICP-MS was 0.394 ± 0.012 μg g⁻¹. LA-ICP-MS determination of Se in synthetic geological glasses BCR-2G, BIR-1G, TB-1G and the MPI-DING glasses showed a range in concentrations from 0.062 to 0.168 μg g⁻¹. Selenium in the natural glass, VG2, was 0.204 ± 0.028 μg g⁻¹.     

Thermokarst lakes and ponds on Mars in the very recent (late Amazonian) past

The history of water is fundamental to understanding the geological evolution of Mars and to questions concerning the possible development of life on the Red Planet. Today, Mars is cold and dry; its regolith is permanently frozen and except under highly localised and transient conditions, liquid water is unstable at the surface. Intriguingly, we have identified geological features that could be markers of very late-Amazonian “wet” or ice-rich periglacial processes in Utopia and western Elysium Planitiae: 1. rimless, flat-floored and lobate, sometimes scalloped, depressions that are suggestive of terrestrial alases (evaporated/drained thermokarst lakes); 2. small-sized polygonal patterned-ground (perhaps formed by thermal-contraction cracking and possibly underlain by ice wedges); and, 3. circular/near-circular raised-rim depressions (consistent in morphology and scale with pingo-scars) that are nested in rimless depressions. In terrestrial cold-climate, non-glacial environments, landscape assemblages of this type occur only in the presence of ice-rich permafrost.Commenting upon the origin of the putative periglacial features on Mars, most workers have suggested that sublimation and not evaporation has been the dominant process. By contrast, we propose that two key characteristics of the rimless depressions – inner terraces and orthogonally-oriented polygons – are markers of stable, ponded water and its slow loss by evaporation or drainage. If the raised-rim landforms are pingo scars, then this also points to boundary conditions that are supportive of stable liquid water.With regard to the relative age of the features described above, previous work identified some lobate depressions superposed on crater-rim gullies in the region (Soare et al., 2007). Gullies could be amongst the youngest geological features on Mars; superposed depressions point to an origin that is more youthful than the gullies. In turn, as some raised-rim landforms are superposed on rimless depressions, this is indicative of an origin that is even more recent than that of the depressions. Together with the geological evidence showing that the rimless depressions could have been formed by ponded water, the stratigraphy of the putative periglacial-landscape in this region suggests that the very late Amazonian period could have been warmer and wetter than had been thought hitherto.     

Fractionation of silicon isotopes during biogenic silica dissolution

Silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization, primarily by diatoms, in surface waters of the ocean. Interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean, as driven by biological fractionation during the production of biogenic silica (bSiO₂, or opal) by diatoms. However, other factors which may influence the silicon isotope composition of diatom opal, particularly post-formational aging and maturation processes, remain largely uninvestigated. Here, we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bSiO₂ suspended in seawater under closed conditions. This fractionation acts counter to that occurring during bSiO₂ production and at about half its absolute magnitude, with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor ε_DSi–BSi of −0.55‰, corresponding to a fractionation factor α_30/28 of 0.99945. The enrichment factor did not vary with source material, indicating the lack of a significant species effect, or with temperature from 3 to 20 °C. Thus, the dissolution of bSiO₂ produces dissolved silicon with a δ³⁰Si value that is 0.55‰ more negative than its parent bSiO₂, an effect that must be accounted for when interpreting oceanic δ³⁰Si distributions. The δ³⁰Si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed, implying a measurable (±0.1‰) change in the relative δ³⁰Si of opal samples whenever the difference in preservation efficiency between them is >20%. This effect could account for 10–30% of the difference in diatom δ³⁰Si values observed between glacial and interglacial conditions. It is unlikely, however, that the inferred maximum possible change in δb³⁰SiO₂ of +0.55‰ would be manifested in situ, as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage.     

A diatom record of environmental change in Fryxell Basin,Taylor Valley,Antarctica, late Pleistocene to present

We present a diatom record from a sediment core taken in Lake Fryxell, Taylor Valley, Antarctica. Six zones were defined using diatom assemblage changes that indicate varying limnological conditions. The early lake stage, ca. 35,000 cal years BP, is characterized by Mayamea atomus f. permitis, a species rarely reported in modern Antarctic Dry Valley environments. An extended period from ca. 35,000 to 19,000 cal years BP is characterized by low diatom abundance, with dominant taxa Luticola spp., Muelleria spp., and Diadesmis contenta. The modern assemblage was established ca. 13,000 cal years BP, after two relatively brief transitional stages. One key species for this recent period, Navicula lineola var. perlepida, is absent in surface sediments and the modern environment, indicating an environmental change within the last several centuries. The diatom assemblage is compared to modern diatom communities in Dry Valley streams, which provide the most complete information on diatom distributions in this region. Although precise environmental interpretation of the core is hampered by limited knowledge of environmental constraints on many of the diatom taxa present in the lake core, the data provide important new insights into the history of Glacial Lake Washburn.     

Holocene floristic diversity and richness in northeast Norway revealed by sedimentary ancient DNA (sedaDNA) and pollen

We present a Holocene record of floristic diversity and environmental change for the central Varanger Peninsula, Finnmark, based on ancient DNA extracted from the sediments of a small lake (sedaDNA). The record covers the period c. 10 700 to 3300 cal. a BP and is complemented by pollen data. Measures of species richness, sample evenness and beta diversity were calculated based on sedaDNA sampling intervals and 1000‐year time windows. We identified 101 vascular plant and 17 bryophyte taxa, a high proportion (86%) of which are still growing within the region today. The high species richness (>60 taxa) observed in the Early Holocene, including representatives from all important plant functional groups, shows that modern shrub‐tundra communities, and much of their species complement, were in place as early as c. 10 700 cal. a BP. We infer that postglacial colonization of the area occurred prior to the full Holocene, during the Pleistocene‐Holocene transition, Younger Dryas stadial or earlier. Abundant DNA of the extra‐limital aquatic plant Callitriche hermaphroditica suggests it expanded its range northward between c. 10 200 and 9600 cal. a BP, when summers were warmer than present. High values of Pinus DNA occur throughout the record, but we cannot say with certainty if they represent prior local presence; however, pollen influx values >500 grains cm^?2 a^?1 between c. 8000 and 7300 cal. a BP strongly suggest the presence of pine woodland during this period. As the site lies beyond the modern tree limit of pine, it is likely that this expansion also reflects a response to warmer Early Holocene summers.     

Numerical implementation and sensitivity analysis of a wave energy converter in a time-dependent mild-slope equation model

Several Wave Energy Converters (abbreviated as WECs) have intensively been studied and developed during the last decade and currently small farms of WECs are getting installed. WECs in a farm are partly absorbing, partly redistributing the incident wave power. Consequently, the power absorption of each individual WEC in a farm is affected by its neighbouring WECs. The knowledge of the wave climate around the WEC is needed to predict its performance in the farm. In this paper a technique is developed to implement a single and multiple WECs based on the overtopping principle in a time-dependent mild-slope equation model. So far, the mild-slope equations have been widely used to study wave transformations around coastal and offshore structures, such as breakwaters, piles of windmills and offshore platforms. First the limitations of the WEC implementation are discussed through a sensitivity analysis. Next the developed approach is applied to study the wave height reduction behind a single WEC and a farm. The wake behind an isolated WEC is investigated for uni- and multidirectional waves; it is observed that an increase of the directional spread leads to a faster wave redistribution behind the WEC. Further the wake in the lee of multiple WECs is calculated for two different farm lay-outs, i.e. an aligned grid and a staggered grid, by adapting the performance of each WEC to its incident wave power. The evolved technique is a fast tool to find the optimal lay-out of WECs in a farm and to study the possible influence on surrounding activities in the sea.