Lipid biomarkers in Holocene and glacial sediments from ancient Lake Ohrid (Macedonia, Albania)

Abstract. Organic matter preserved in Lake Ohrid sediments originates from aquatic and terrestrial sources. Its variable composition reflects climate-controlled changes in the lake basin’s hydrology and related organic matter export, i.e. changes in primary productivity, terrestrial plant matter input and soil erosion. Here, we present first results from lipid biomarker investigations of Lake Ohrid sediments from two near-shore settings: site Lz1120 near the southern shore, with low-lying lands nearby and probably influenced by river discharge, and site Co1202 which is close to the steep eastern slopes. Variable proportions of terrestrial n-alkanoic acids and n-alkanols as well as compositional changes of !- hydroxy acids document differences in soil organic matter supply between the sites and during different climate stages (glacial, Holocene, 8.2 ka cooling event). Changes in the vegetation cover are suggested by changes in the dominant chain length of terrestrial n-alkanols. Effective microbial degradation of labile organic matter and in situ contribution of organic matter derived from the microbes themselves are both evident in the sediments. We found evidence for anoxic conditions within the photic zone by detecting epicholestanol and tetrahymanol from sulphur-oxidising phototrophic bacteria and bacterivorous ciliates and for the influence of a settled human community from the occurrence of coprostanol, a biomarker for human and animal faeces (pigs, sheep, goats), in an early Holocene sample. This study illustrates the potential of lipid biomarkers for future environmental reconstructions using one of Europe’s oldest continental climate archives, Lake Ohrid.

Sediment core fossils in ancient Lake Ohrid: testing for faunal change since the Last Interglacial

Ancient Lake Ohrid is probably of early Pleistocene or Pliocene origin and amongst the few lakes in the world harbouring an outstanding degree of endemic biodiversity. Although there is a long history of evolutionary research in Lake Ohrid, particularly on molluscs, a mollusc fossil record has been missing up to date. For the first time, gastropod and bivalve fossils are reported from the basal, calcareous part of a 2.6 m long sediment succession (core Co1200) from the north-eastern part of Lake Ohrid. Electron spin resonance (ESR) dating of mollusc shells from the same stratigraphic level yielded an age of 130 ± 28 ka. Lithofacies III sediments, i.e. a stratigraphic subdivision comprising the basal succession of core Co1200 between 181.5–263 cm, appeared solid, greyish-white, and consisted almost entirely of silt-sized endogenic calcite (CaCO3>70%) and intact and broken mollusc shells. Here we compare the faunal composition of the thanatocoenosis with recent mollusc associations in Lake Ohrid. A total of 13 mollusc species (9 gastropod and 4 bivalve species) could be identified within Lithofacies III sediments. The value of sediment core fossils for reconstructing palaeoenvironmental settings was evaluated and the agreement between sediment and palaeontological proxies was tested. The study also aims at investigating major faunal changes since the Last Interglacial and searching for signs of extinction events. The combined findings of the ecological study and the sediment characteristics suggest deposition in a shallow water environment during the Last Interglacial. The fossil fauna exclusively included species also found in the present fauna, i.e. no extinction events are evident for this site since the Last Interglacial. The thanatocoenosis showed the highest similarity with recent Intermediate Layer (5–25 m water depth) mollusc assemblages. The demonstrated existence of a mollusc fossil record in Lake Ohrid sediment cores also has great significance for future deep drilling projects. It can be hoped that a more far reaching mollusc fossil record will then be obtained, enabling insight into the early evolutionary history of Lake Ohrid.

Spatial variability of recent sedimentation in Lake Ohrid (Albania/Macedonia)

Abstract. Lake Ohrid is likely of Pliocene age and thus commonly referred to as the oldest existing lake in Europe. In this study spatial variability of recent sediment composition is assessed using >50 basin wide distributed surface sediment samples. Analysis of biogeochemical bulk parameters, selected metals, pigment concentrations as well as grain size distributions revealed a significant spatial heterogeneity in surface sediment composition. It implies that sedimentation in Lake Ohrid is controlled by an interaction of multiple natural and anthropogenic factors and processes. Major factors controlling surface sediment composition are related to differences in geological catchment characteristics, anthropogenic land use, and a counterclockwise rotating surface water current. In some instances processes controlling sediment composition also seem to impact distribution patterns of biodiversity, which suggests a common interaction of processes responsible for both patterns.

Stratigraphic analysis of lake level fluctuations in Lake Ohrid: an integration of high resolution hydro-acoustic data and sediment cores

Abstract. Ancient Lake Ohrid is a steep-sided, oligotrophic, karst lake that was tectonically formed most likely within the Pliocene and often referred to as a hotspot of endemic biodiversity. This study aims on tracing significant lake level fluctuations at Lake Ohrid using high-resolution acoustic data in combination with lithological, geochemical, and chronological information from two sediment cores recovered from sub-aquatic terrace levels at ca. 32 and 60m water depth. According to our data, significant lake level fluctuations with prominent lowstands of ca. 60 and 35m below the present water level occurred during Marine Isotope Stage (MIS) 6 and MIS 5, respectively. The effect of these lowstands on biodiversity in most coastal parts of the lake is negligible, due to only small changes in lake surface area, coastline, and habitat. In contrast, biodiversity in shallower areas was more severely affected due to disconnection of today sublacustrine springs from the main water body. Multichannel seismic data from deeper parts of the lake clearly image several clinoform structures stacked on top of each other. These stacked clinoforms indicate significantly lower lake levels prior to MIS 6 and a stepwise rise of water level with intermittent stillstands since its existence as water-filled body, which might have caused enhanced expansion of endemic species within Lake Ohrid.

Comparison of iron isotope variations in modern and Ordovician siliceous Fe oxyhydroxide deposits

Formation pathways of ancient siliceous iron formations and related Fe isotopic fractionation are still not completely understood. Investigating these processes, however, is difficult as good modern analogues to ancient iron formations are scarce. Modern siliceous Fe oxyhydroxide deposits are found at marine hydrothermal vent sites, where they precipitate from diffuse, low temperature fluids along faults and fissures on the seafloor. These deposits exhibit textural and chemical features that are similar to some Phanerozoic iron formations, raising the question as to whether the latter could have precipitated from diffuse hydrothermal fluids rather than from hydrothermal plumes. In this study, we present the first data on modern Fe oxyhydroxide deposits from the Jan Mayen hydrothermal vent fields, Norwegian-Greenland Sea. The samples we investigated exhibited very low δ56Fe values between -2.09‰ and -0.66‰. Due to various degrees of partial oxidation, the Fe oxyhydroxides are with one exception either indistinguishable from low-temperature hydrothermal fluids from which they precipitated (-1.84‰ and -1.53‰ in δ56Fe) or are enriched in the heavy Fe isotopes. In addition, we investigated Fe isotope variations in Ordovician jasper beds from the Løkken ophiolite complex, Norway, which have been interpreted to represent diagenetic products of siliceous ferrihydrite precursors that precipitated in a hydrothermal plume, in order to compare different formation pathways of Fe oxyhydroxide deposits. Iron isotopes in the jasper samples have higher δ56Fe values (-0.38‰ to +0.89‰) relative to modern, high-temperature hydrothermal vent fluids (ca. -0.40‰ on average), supporting the fallout model. However, formation of the Ordovician jaspers by diffuse venting cannot be excluded, due to lithological differences of the subsurface of the two investigated vent systems. Our study shows that reliable interpretation of Fe isotope variations in modern and ancient marine Fe oxyhydroxide deposits depends on comprehensive knowledge of the geological context. Furthermore, we demonstrate that very negative δ56Fe values in such samples might not be the result of microbial dissimilatory iron reduction, but could be caused instead by inorganic reactions.

Pelagic molybdenum concentration anomalies and the impact of sediment resuspension on the molybdenum budget in two tidal systems of the North Sea

The seasonal dynamics of molybdenum (Mo) were studied in the water column of two tidal basins of the German Wadden Sea (Sylt-Rømø and Spiekeroog) between 2007 and 2011. In contrast to its conservative behaviour in the open ocean, both, losses of more than 50% of the usual concentration level of Mo in seawater and enrichments up to 20% were observed repeatedly in the water column of the study areas. During early summer, Mo removal by adsorption on algae-derived organic matter (e.g. after Phaeocystis blooms) is postulated to be a possible mechanism. Mo bound to organic aggregates is likely transferred to the surface sediment where microbial decomposition enriches Mo in the pore water. First δ98/95Mo data of the study area disclose residual Mo in the open water column being isotopically heavier than MOMo (Mean Ocean Molybdenum) during a negative Mo concentration anomaly, whereas suspended particulate matter shows distinctly lighter values. Based on field observations a Mo isotope enrichment factor of ε = −0.3‰ has been determined which was used to argue against sorption on metal oxide surfaces. It is suggested here that isotope fractionation is caused by biological activity and association to organic matter. Pelagic Mo concentration anomalies exceeding the theoretical salinity-based concentration level, on the other hand, cannot be explained by replenishment via North Sea waters alone and require a supply of excess Mo. Laboratory experiments with natural anoxic tidal flat sediments and modelled sediment displacement during storm events suggest fast and effective Mo release during the resuspension of anoxic sediments in oxic seawater as an important process for a recycling of sedimentary sulphide bound Mo into the water column.

Denudation rates of small transient catchments controlled by former glaciation: The Hörnli nunatak in the northeastern Swiss Alpine Foreland

Extensive glaciers repeatedly occupied the northern Alpine Foreland during the Pleistocene and left a strongly glacially overprinted low slope landscape. Only few islands appeared as nunataks standing above the surface of the large piedmont glacier lobes. These nunatak areas kept their original shape, manifested in steep catchments with mean slopes up to 33 . Even though not glaciated, these catchments where significantly affected by base-level changes occurring as a consequence of phases of glacier advances and retreats. Both domains, the glacially eroded and non-eroded, are therefore prone to different mechanisms and time-scales of fluvial and colluvial re-adjustment. In this study we investigate these effects by exploring the spatial distribution and magnitude of denudation in the Hörnli region of the eastern Swiss Alpine Foreland in the present Interglacial. The area represents both domains in a relatively small area with largely uniform tectonic, lithologic and climatic conditions. The differences in Holocene andscape evolution are investigated using topographic analyses and catchment-averaged denudation rates derived from 10Be concentrations in fluvial quartz sand. We find that in formerly non-glaciated, fluvially dominated catchments close hillslope-channel coupling prevails and that these catchments yield high average denudation rates of 350 mm/ka. Glacially overprinted catchments yielded catchment-wide denudation rates an order of magnitude lower. These low denudation rates are hypothesized to be the consequence of both (i) a dominance of slow hillslope processes and (ii) admixture of high concentration, pre-LGM glacial sediment. This suggests that a) a careful field investigation must accompany the denudation rate studies and b) that the concept of area-weighted cosmogenic nuclide denudation rates must be considered in light of the predominant catchment processes.

In situ dehydration behavior of zeolite-like pentagonite: A single-crystal X-ray study

The structural modifications upon heating of pentagonite, Ca(VO)(Si4O10)·4H2O (space group Ccm21, a=10.3708(2), b=14.0643(2), c=8.97810(10) Å, V=1309.53(3) Å3) were investigated by in situ temperature dependent single-crystal X-ray structure refinements. Diffraction data of a sample from Poona district (India) have been measured in steps of 25 up to 250 °C and in steps of 50 °C between 250 and 400 °C. Pentagonite has a porous framework structure made up by layers of silicate tetrahedra connected by V4+O5 square pyramids. Ca and H2O molecules are extraframework occupants. Room temperature diffraction data allowed refinement of H positions. The hydrogen-bond system links the extraframework occupants to the silicate layers and also interconnects the H2O molecules located inside the channels. Ca is seven-fold coordinated forming four bonds to O of the tetrahedral framework and three bonds to extraframework H2O. The H2O molecule at O9 showing a high displacement parameter is not bonded to Ca. The dehydration in pentagonite proceeds in three steps. At 100 °C the H2O molecule at O8 was released while O9 moved towards Ca. As a consequence the displacement parameter of H2O at O9 halved compared to that at room temperature. The unit-cell volume decreased to 1287.33(3) Å3 leading to a formula with 3H2O per formula unit (pfu). Ca remained seven-fold coordinated. At 175 °C Ca(VO)(Si4O10)·3H2O transformed into a new phase with 1H2O molecule pfu characterized by doubling of the c axis and the monoclinic space group Pn. Severe bending of specific TOT angles led to contraction of the porous three-dimensional framework. In addition, H2O at O9 was expelled while H2O at O7 approached a position in the center of the channel. The normalized volume decreased to 1069.44(9) Å3. The Ca coordination reduced from seven- to six-fold. At 225 °C a new anhydrous phase with space group Pna21 but without doubling of c had formed. Release of H2O at O7 caused additional contraction of TOT angles and volume reduction (V=1036.31(9) Å3). Ca adopted five-fold coordination. During heating excursion up to 400 °C this anhydrous phase remained preserved. Between room temperature and 225 °C the unit-cell volume decreased by 21% due to dehydration. The dehydration steps compare well with the thermo-gravimetric data reported in the literature.

High-temperature induced dehydration, phase transition and exsolution in amicite: A single-crystal X-ray study

Temperature dependent single-crystal X-ray data were collected on amicite K4Na4(Al8Si8O32)·11H2O from Kola Peninsula (Russia) in steps of 25 °C from room temperature to 175 °C and of 50 °C up to 425 °C. At room temperature amicite has space group I2 with a = 10.2112(1), b = 10.4154(1), c = 9.8802(1) Å, β = 88.458(1)°, V = 1050.416(18) Å3. Its crystal structure is based on a Si–Al ordered tetrahedral framework of the GIS type with two systems of eight-membered channels running along the a and c axes. Extraframework K and Na cations are ordered at two fully occupied sites. Above 75 °C amicite was found to partly dehydrate into two separate but coherently intergrown phases, both of space group I2/a, one K-rich ∼K8(Al8Si8O32) ·4H2O (at 75 °C: a = 10.038(2), b = 9.6805(19), c = 9.843(2) Å, β = 89.93(3)°, V = 956.5(3) Å3) and the other Na-rich ∼Na8(Al8Si8O32)·2H2O (at 75 °C: a = 9.759(2), b = 8.9078(18), c = 9.5270(19) Å, β = 89.98(3)°, V = 828.2(3) Å3). Upon further heating above 75 °C the Na- and K-phases lost remaining H2O with only minor influence on the framework structure and became anhydrous at 175 °C and 375 °C, respectively. The two anhydrous phases persisted up to 425 °C. Backscattered electron images of a heated crystal displayed lamellar intergrowth of the K- and Na-rich phases. Exposed to ambient humid conditions K- and Na-rich phases rehydrated and conjoined to the original one phase I2 structure.

In situ dehydration behavior of veszelyite (Cu,Zn)2Zn(PO4)(OH)3·2H2O: A single-crystal X-ray study

The rare mixed copper-zinc phosphate mineral veszelyite (Cu,Zn)2Zn(PO4)(OH)3·2H2O space group P21/c, a = 7.5096(2), b = 10.2281(2), c = 9.8258(2) Å, β = 103.3040(10)°, V = 734.45(3) Å3 was investigated by in situ temperature-dependent single-crystal X-ray structure refinements. The atomic arrangement of veszelyite consists of an alternation of octahedral and tetrahedral sheets. The Jahn-Teller distorted CuO6 octahedra form sheets with eight-membered rings. The tetrahedral sheet composed of PO4 and ZnO3(OH) tetrahedra shows strong topological similarities to that of cavansite, gismondine, and kipushite.Diffraction data of a sample from Zdravo Vrelo, near Kreševo (Bosnia and Herzegovina) have been measured in steps of 25 up to 225 °C. Hydrogen positions and the hydrogen-bond system were determined experimentally from the structure refinements of data collected up to 125 °C. At 200 °C, the hydrogen-bonding scheme was inferred from bond-valence calculations and donor-acceptor distances. The hydrogen-bond system connects the tetrahedral sheet to the octahedral sheet and also braces the Cu sheet.At 150 °C, the H2O molecule at H2O2 was released and the Cu coordination (Cu1 and Cu2) decreased from originally six- to fivefold. Cu1 has a square planar coordination by four OH groups and an elongate distance to O3, whereas Cu2 has the Jahn-Teller characteristic elongate bond to H2O1. The unit-cell volume decreased 7% from originally 734.45(3) to 686.4(4) Å3 leading to a formula with 1 H2O pfu. The new phase observed above 150 °C is characterized by an increase of the c axis and a shortening of the b axis. The bending of T-O-T angles causes an increasing elliptical shape of the eight-membered rings in the tetrahedral and octahedral sheets. Moreover a rearrangement of the hydrogen-bond system was observed.At 225 °C, the structure degrades to an X-ray amorphous residual due to release of the last H2O molecule at H2O1. The stronger Jahn-Teller distortion of Cu1 relative to Cu2 suggests that Cu1 is fully occupied by Cu, whereas Cu2 bears significant Zn. H2O1 is the fifth ligand of Cu2. Zn at Cu2 is not favorable to adopt planar fourfold coordination. Thus, if the last water molecule is expelled the structure is destabilized.This study contributes to understanding the dehydration mechanism and thermal stability of supergene minerals characterized by Jahn-Teller distorted octahedra with mixed Cu, Zn occupancy.