Complexity of diatom response to Lateglacial and Holocene climate and environmental change in ancient, deep and oligotrophic Lake Ohrid (Macedonia and Albania)

© Author(s) 2016. Lake Ohrid (Macedonia and Albania) is a rare example of a deep, ancient Mediterranean lake and is a key site for palaeoclimate research in the northeastern Mediterranean region. This study conducts the analysis of diatoms as a proxy for Lateglacial and Holocene climate and environmental change in Lake Ohrid at a higher resolution than in previous studies. While Lake Ohrid has the potential to be sensitive to water temperature change, the data demonstrate a highly complex diatom response, probably comprising a direct response to temperature-induced lake productivity in some phases and an indirect response to temperaturerelated lake stratification or mixing and epilimnetic nutrient availability in others. The data also demonstrate the possible influence of physical limnological (e.g. the influence of wind stress on stratification or mixing) and chemical processes (e.g. the influence of catchment dynamics on nutrient input) in mediating the complex response of diatoms. During the Lateglacial (ca. 12 300-11 800 cal yr BP), the low-diversity dominance of hypolimnetic Cyclotella fottii indicates low lake productivity, linked to low water temperature. Although the subsequent slight increase in small, epilimnetic C. minuscula during the earliest Holocene (ca. 11 800-10 600 cal yr BP) suggests climate warming and enhanced stratification, diatom concentration remains as low as during the Lateglacial, suggesting that water temperature increase was muted across this major transition. The early Holocene (ca. 10 600-8200 cal yr BP) is characterised by a sustained increase in epilimnetic taxa, with mesotrophic C. ocellata indicating high water-temperature-induced productivity between ca. 10 600-10 200 cal yr BP and between ca. 9500-8200 cal yr BP and with C. minuscula in response to low nutrient availability in the epilimnion between ca. 10 200-9500 cal yr BP. During the middle Holocene (ca. 8200-2600 cal yr BP), when sedimentological and geochemical proxies provide evidence for maximum Holocene water temperature, anomalously low C. ocellata abundance is probably a response to epilimnetic nutrient limitation, almost mimicking the Lateglacial flora apart from the occurrence of mesotrophic Stephanodiscus transylvanicus in the hypolimnion. During the late Holocene (ca. 2600 cal yr BP-present), high abundance and fluctuating composition of epilimnetic taxa are probably a response more to enhanced anthropogenic nutrient input, particularly nitrogen enrichment, than to climate. Overall, the data indicate that previous assumptions concerning the linearity of diatom response in this deep, ancient lake are invalid, and multi-proxy analysis is essential to improve understanding of palaeolimnological dynamics in future research on the long, Quaternary sequence

To assess and be assessed: Upper secondary school students’ narratives of credibility judgements

Purpose – The purpose of this paper is to explore how students construct narratives of themselves as information seekers in a school context where their descriptions of their information activities are assessed and graded. Design/methodology/approach – Blog posts on credibility judgements written by 28 students at a Swedish upper secondary school were analysed through a bottom-up coding process based in the sociocultural concept of narratives of selves. Findings – Two tensions in the students’ accounts are identified. The first tension is that between the description of the individual, independent student and the description of the good group member. The second tension is between describing oneself as an independent information seeker and at the same time as someone who seeks information in ways that are sanctioned within the school setting. Research limitations/implications – The study focuses on a specific social practice and on situated activities, but also illustrates some aspects of information activities that pertain to educational contexts in general. It explores how social norms related to credibility judgements are expressed and negotiated in discursive interaction. Practical implications – The study highlights that when information activities become objects of assessment, careful consideration of what aspects are meant to be assessed is necessary. Originality/value – The study is based on the idea of information activities as socially and discursively shaped, and it illustrates some of the consequences when information activities become objects of teaching, learning, and grading

Information Research

Introduction. This study explored how various meanings are attributed to the term facts in Swedish schools and how this may shape conditions for students' learning. The understanding of information activities as social, communicative, and discursive, which motivates the study, is informed by a sociocultural perspective of learning and information interaction. Method. The study re-analyses empirical data from four previous research projects, where material was collected through various qualitative methods, mainly interviews, observations, and document analysis. The material involved 14 classes from year 2 to year 12.Analysis. The data were analysed thematically. In the material, 565 occurrences of facts are identified and categorised. Results. The analysis generated three themes. Firstly, facts were associated with specific genres or modalities. Secondly, facts were seen as distinguishable, external, and tangible. Thirdly, facts came across as having strong connections to neutrality and they were viewed as evidence. Conclusions. The analysis showed variation in how the study participants talked about facts. Despite the dominant associations, each theme displayed more complex meanings of the term, which indicates that fact cannot be equated to how the term has been used as an analytical term in previous research. The frequent use of the term facts can be understood as a result of the strong focus on students seeking and using certain types of information for writing school reports. The results show how important it is that information researchers adopt an open and explorative approach to the meaning of the language used in school activities that they study

Clustering of Intermediate Luminosity X-ray selected AGN at z~3

We present the first clustering results of X-ray selected AGN at z~3. Using Chandra X-ray imaging and UVR optical colors from MUSYC photometry in the ECDF-S field, we selected a sample of 58 z~3 AGN candidates. From the optical data we also selected 1385 LBG at 2.8<z< 3.8 with R<25.5. We performed auto-correlation and cross-correlation analyses, and here we present results for the clustering amplitudes and dark matter halo masses of each sample. For the LBG we find a correlation length of r_0,LBG = 6.7 +/- 0.5 Mpc, implying a bias value of 3.5 +/- 0.3 and dark matter (DM) halo masses of log(Mmin/Msun) = 11.8 +/- 0.1. The AGN-LBG cross-correlation yields r_0,AGN-LBG = 8.7 +/- 1.9 Mpc, implying for AGN at 2.8<z<3.8 a bias value of 5.5 +/- 2.0 and DM halo masses of log(Mmin/Msun) = 12.6 +0.5/-0.8. Evolution of dark matter halos in the Lambda CDM cosmology implies that today these z~3 AGN are found in high mass galaxies with a typical luminosity of 7+4/-2 L*.Comment: Accepted for publication in ApJ Letters. 4 pages, 4 figures (1 in color

Assessment of remote cavitation detection methods with flow visualization in a full scale francis turbine

This paper describes the experimental investigations carried out in the Francis turbine at Svorka power plant operated by Statkraft in Norway. The unit, with a head of 260 m, can deliver a maximum output load of 25 MW. The rated flow rate is 11 m3/s and the machine rotates at 600 rpm. The turbine runner shows cavitation pitting on the suction side of the blades but some blades present more erosion than others. Moreover, preliminary studies based on remote monitoring of vibrations and acoustic emissions in this particular unit have predicted risk of erosion at high loads and the presence of a draft tube swirl affecting the cavity dynamics. In order to assess the sensitivity of these methods and the validity of the predictions, several acrylic-glass windows have been installed on the draft tube wall to visualize the runner outlet flow during operation. A high speed camera has been used to record the flow field during the tests with rates up to 5000 frames per second. A cavitation detection system has been installed comprising three high-frequency uniaxial integrated electronics piezoelectric (IEPE)-type accelerometers and an acoustic emission sensor, mounted in the turbine guide bearing pedestal and a guide vane arm. In particular, a series of measurements at different operation conditions have been carried out to correlate the simultaneous camera observations with the acceleration and acoustic emission overall levels in high frequency bands. The preliminary analysis of the camera records permits to certify the existence of erosive blade cavitation with the closure region close to the eroded areas at high loads. It can be seen that cavitation appears only in some blades and that it presents different cavity sizes for the same operation condition. As the load increases towards maximum powers, both the number of blades with cavitation and the size of the cavities grow. Moreover, the overall vibration levels also rise as expected.Postprint (published version

Mediterranean climate since the Middle Pleistocene: a 640 ka stable isotope record from Lake Ohrid (Albania/Macedonia)

Lake Ohrid (Macedonia/Albania) is an ancient lake with a unique biodiversity and a site of global significance for investigating the influence of climate, geological and tectonic events on the generation of endemic populations. Here, we present oxygen (δ18O) and carbon (δ13C) isotope data on carbonate from the upper ca. 248 m of sediment cores recovered as part of the Scientific Collaboration on Past Speciation Conditions in Lake Ohrid (SCOPSCO) project, covering the past 640 ka. Previous studies on short cores from the lake (up to 15 m, < 140 ka) have indicated the Total Inorganic Carbon (TIC) content of sediments to be highly sensitive to climate change over the last glacial–interglacial cycle, comprising abundant endogenic calcite through interglacials and being almost absent in glacials, apart from discrete bands of early diagenetic authigenic siderite. Isotope measurements on endogenic calcite (δ18Oc and δ13Cc) reveal variations both between and within interglacials that suggest the lake has been subject to hydroclimate fluctuations on orbital and millennial timescales. We also measured isotopes on authigenic siderite (δ18Os and δ13Cs) and, with the δ18OCc and δ18Os, reconstruct δ18O of lakewater (δ18Olw) through the 640 ka. Overall, glacials have lower δ18Olw when compared to interglacials, most likely due to cooler summer temperatures, a higher proportion of winter precipitation (snowfall), and a reduced inflow from adjacent Lake Prespa. The isotope stratigraphy suggests Lake Ohrid experienced a period of general stability through Marine Isotope Stage (MIS) 15 to MIS 13, highlighting MIS 14 as a particularly warm glacial, and was isotopically freshest during MIS 9. After MIS 9, the variability between glacial and interglacial δ18Olw is enhanced and the lake became increasingly evaporated through to present day with MIS 5 having the highest average δ18Olw. Our results provide new evidence for long-term climate change in the northern Mediterranean region, which will form the basis to better understand the influence of major environmental events on biological evolution within the lake

Mediterranean climate since the Middle Pleistocene: A 640 ka stable isotope record from Lake Ohrid (Albania/Macedonia)

Lake Ohrid (Macedonia/Albania) is an ancient lake with a unique biodiversity and a site of global significance for investigating the influence of climate, geological and tectonic events on the generation of endemic populations. Here, we present oxygen (δ18O) and carbon (δ13C) isotope data on carbonate from the upper ca. 248 m of sediment cores recovered as part of the Scientific Collaboration on Past Speciation Conditions in Lake Ohrid (SCOPSCO) project, covering the past 640 ka. Previous studies on short cores from the lake (up to 15 m, < 140 ka) have indicated the Total Inorganic Carbon (TIC) content of sediments to be highly sensitive to climate change over the last glacial-interglacial cycle, comprising abundant endogenic calcite through interglacials and being almost absent in glacials, apart from discrete bands of early diagenetic authigenic siderite. Isotope measurements on endogenic calcite(δ18Oc and δ13Cc) reveal variations both between and within interglacials that suggest the lake has been subject to hydroclimate fluctuations on orbital and millennial timescales. We also measured isotopes on authigenic siderite (δ18Os and δ13Cs) and, with the δ18OCc and δ18Os, reconstruct δ18O of lakewater (δ18Olw) through the 640 ka. Overall, glacials have lower δ18Olw when compared to interglacials, most likely due to cooler summer temperatures, a higher proportion of winter precipitation (snowfall), and a reduced inflow from adjacent Lake Prespa. The isotope stratigraphy suggests Lake Ohrid experienced a period of general stability through Marine Isotope Stage (MIS) 15 to MIS 13, highlighting MIS 14 as a particularly warm glacial, and was isotopically freshest during MIS 9. After MIS 9, the variability between glacial and interglacial δ18Olw is enhanced and the lake became increasingly evaporated through to present day with MIS 5 having the highest average δ18Olw. Our results provide new evidence for long-term climate change in the northern Mediterranean region, which will form the basis to better understand the influence of major environmental events on biological evolution within the lake

The Very Large Telescope Lyman-Break Galaxy Redshift Survey – IV. Gas and galaxies at z ∼ 3 in observations and simulations

We use a combination of observations and simulation to study the relationship between star-forming galaxies and the intergalactic medium at z ≈ 3. The observed star-forming galaxy sample is based on spectroscopic redshift data taken from a combination of Very Large Telescope (VLT) Lyman-break galaxy (LBG) Redshift Survey (VLRS) data and Keck Low-Resolution Imaging Spectrometer (LRIS) observations in fields centred on bright background quasi-stellar objects (QSOs), whilst the simulation data is taken from the Galaxies–Intergalactic Medium Interaction Calculation (GIMIC). In the simulation, we find that the dominant peculiar velocities are in the form of large-scale coherent motions of gas and galaxies. Gravitational infall of galaxies towards one another is also seen, consistent with expectations from linear theory. At smaller scales, the root-mean-square (RMS) peculiar velocities in the simulation overpredict the difference between the simulated real- and z-space galaxy correlation functions. Peculiar velocity pairs with separations smaller than 1 h−1 Mpc have a smaller dispersion and explain the z-space correlation function better. The Lyα auto- and cross-correlation functions in the GIMIC simulation appear to show infall smaller than implied by the expected βLyα ≈ 1.3 (McDonald et al.). There is a possibility that the reduced infall may be due to the galaxy-wide outflows implemented in the simulation. The main challenge in comparing these simulated results with the observed Keck + VLRS correlation functions comes from the presence of velocity errors for the observed LBGs, which dominate at ≲ 1 h− 1 Mpc scales. When these are taken into account, the observed LBG correlation functions are well matched by the high amplitude of clustering, shown by higher mass (M* > 109 M⊙) galaxies in the simulation. The simulated cross-correlation function shows similar neutral gas densities around galaxies to those seen in the observations. The simulated and observed Lyα z-space autocorrelation functions again agree better with each other than with the βLyα ≈ 1.3 infall model. Our overall conclusion is that, at least in the simulation, gas and galaxy peculiar velocities are generally towards the low end of expectation. Finally, little direct evidence is seen in either simulation or observations for high transmission near galaxies due to feedback, in agreement with previous results

Environmental control on the occurrence of high-coercivity magnetic minerals and formation of iron sulfides in a 640 ka sediment sequence from Lake Ohrid (Balkans)

The bulk magnetic mineral record from Lake Ohrid, spanning the past 637 kyr, reflects large-scale shifts in hydrological conditions, and, superimposed, a strong signal of environmental conditions on glacial–interglacial and millennial timescales. A shift in the formation of early diagenetic ferrimagnetic iron sulfides to siderites is observed around 320 ka. This change is probably associated with variable availability of sulfide in the pore water. We propose that sulfate concentrations were significantly higher before  ∼  320 ka, due to either a higher sulfate flux or lower dilution of lake sulfate due to a smaller water volume. Diagenetic iron minerals appear more abundant during glacials, which are generally characterized by higher Fe / Ca ratios in the sediments. While in the lower part of the core the ferrimagnetic sulfide signal overprints the primary detrital magnetic signal, the upper part of the core is dominated by variable proportions of high- to low-coercivity iron oxides. Glacial sediments are characterized by high concentration of high-coercivity magnetic minerals (hematite, goethite), which relate to enhanced erosion of soils that had formed during preceding interglacials. Superimposed on the glacial–interglacial behavior are millennial-scale oscillations in the magnetic mineral composition that parallel variations in summer insolation. Like the processes on glacial–interglacial timescales, low summer insolation and a retreat in vegetation resulted in enhanced erosion of soil material. Our study highlights that rock-magnetic studies, in concert with geochemical and sedimentological investigations, provide a multi-level contribution to environmental reconstructions, since the magnetic properties can mirror both environmental conditions on land and intra-lake processes