ALBERT

Description: Long‐term addition of N and P in a boreal rich fen caused changes in species composition following three different pathways. Addition of NP: reduced species and functional richness. Highly competitive grasses out‐competed other species, especially bryophytes. Addition of P: large species turnover, but no reduction in species or functional richness; bryophyte diversity increased. Addition of N: only smaller changes without clear dominants. Abstract Questions How does long‐term increase in nutrient availability affect species composition, species diversity and functional composition in boreal rich fens, and how does this differ from short‐term effects? What are the possible mechanisms behind the observed changes, and how does nutrient limitation influence species diversity in these communities? Location Sølendet Nature Reserve, central Norway. Methods A full‐factorial field experiment. Plots in two localities received one of following treatments (n = 3): no nutrient addition (control), N, P, K, NP, NK, PK and NPK addition. Cover of plant species was recorded before treatment, and after two and 15 years of treatment. Results Two years of nutrient addition caused small changes in species composition, but addition of NP led to large increase in abundance of species with high ability to exploit the added nutrients—a direct result of the elimination of nutrient limitation in the communities. After 15 years of nutrient addition there were significant changes following three different pathways, one for each of N, P and NP addition. The addition of NP led to large community shifts, considerable species turnover and reduced species and functional richness, mainly caused by increase in cover of highly competitive and tussock‐forming grasses like Deschampsia cespitosa, Festuca ovina and Molinia caerulea, out‐competing other species, especially bryophytes. Addition of N led to smaller changes in species turnover, and without clear dominant species. Addition of P led to considerable species turnover, but no reduction in species or functional richness, and the bryophyte diversity increased. This is explained by the bryophytes’ association with N‐fixing cyanobacteria, suggesting less N limitation and a greater ability to utilize the added P when vascular plants suffer from N shortage. In addition, bryophytes are more sensitive to low P availability, due to larger P requirements compared to vascular plants. There was no effect of K addition. Conclusions Both N and P limitation is essential for the maintenance of high species diversity in boreal rich fens, and P limitation controls bryophyte diversity. From a management perspective, N and P limitation is vital in the conservation of boreal rich fens or when a functional fen system is reestablished through restoration measures.

Description: ABSTRACT 36Cl production in the atmosphere is modulated by the magnetic field intensity of both the Sun and the Earth. The record of 36Cl concentration along with that of 10Be in ice cores may therefore provide information as to their variability. To better understand the 36Cl signal in glaciological archives, we measured its concentration in Talos Dome snow samples (mean accumulation rate of 8 g.cm‐2.yr‐1 water equivalent) spanning the 1955 to 1980 C.E. period with a resolution of one sample every 3 years, and in Vostok snow samples (mean accumulation rate of 1.96 g.cm‐2.yr‐1 water equivalent) spanning the 1949 to 2007 C.E. period with a six‐month resolution that had never before been obtained. Marine nuclear bomb tests in the late 1950s produced anthropogenic 36Cl which was injected into the stratosphere and spread around the globe. In the late 1950s this anthropogenic pulse led to an increase of 36Cl concentration at Talos Dome that was more than 100 times higher than the pre‐ and post‐ bomb values. It is noteworthy that the atmosphere of Vostok remains polluted by anthropogenic 36Cl today. This pollution results from gaseous H36Cl mobility at low accumulation sites and implies re‐emission of 36Cl from the snowpack that is not observed at Talos Dome. The 36Cl/Cl‐ ratio may be used to discriminate the stratospheric anthropogenic 36Cl source from the tropospheric natural 36Cl source, which allows us to discuss the immobile vs. mobile 36Cl in the Vostok snowpack.

Description: A high-resolution sedimentological and geochemical study of a high-altitude proglacial lake (Lake Blanc, Aiguilles Rouges, 2352 m a.s.l.) revealed 195 turbidites, 190 of which are related to flood events over the last 1400 years. We used the coarsest sediment fraction of each turbidite as a proxy for the intensity of each flood event. Because most flood events at this locality are triggered by localized summer convective precipitation events, the reconstructed sedimentary record reveals changes in the frequency and intensity of such events over the last millennium. Comparisons with other temperature, palaeohydrological and glacier reconstructions in the region suggest that the most intense events occurred during the warmest periods, i.e. during the Medieval Climate Anomaly (AD 800–1300) and the current period of global warming. On a multi-decadal time scale, almost all the flood frequency peaks seem to correspond to warmer periods, whereas multi-centennial variations in flood frequency appear to follow the regional precipitation pattern. Consequently, this new Alpine flood record provides further evidence of a link between climate warming and an increase in the frequency and intensity of flooding on a multi-decadal time scale, whereas the centennial variability in flood frequencies is related to regional precipitation patterns. Copyright © 2013 John Wiley & Sons, Ltd.

Description: The Eurasian inland propagation of temperature anomalies during glacial millennial-scale climate variability is poorly understood but this knowledge is crucial to understanding hemisphere-wide atmospheric teleconnection patterns and climate mechanisms. Based on biomarkers and geochemical paleothermometers, a pronounced continental temperature variability between 64,000 and 20,000 years ago, coinciding with the Greenland Dansgaard-Oeschger cycles, was determined in a well-dated sediment record from the formerly enclosed Black Sea. Cooling during Heinrich events was not stronger than during other stadials in the Black Sea. This is corroborated by modeling results showing that regular Dansgaard-Oeschger cycles penetrated deeper into the Eurasian continent than Heinrich events. The pattern of coastal ice-rafted detritus suggests a strong dependence on the climate background state, with significantly milder winters during periods of reduced Eurasian ice sheets and an intensified meridional atmospheric circulation.

Description: This study aims at providing robust temperature reconstructions for a key oceanographic setting in the North Atlantic and at understanding the nature of the temperature signal recorded by the two biomarkers U k’ 37 and TEX 86 , considering season and depth of production. To do so, high-resolution signals of U k’ 37 and TEX 86 are determined for the last 70 ky for core MD95-2042, located off Portugal. Signals of U k’ 37 and TEX 86 present a tight correlation, demonstrating a dominant temperature effect. U k’ 37 signals correspond well to the annual mean sea surface temperature (SST), whereas TEX H 86 -derived temperatures are 5.6 °C higher, which is unrealistically warm for this area. Unsuitable TEX 86 global linear calibrations on the Iberian Margin may suggest a possible occurrence of archaeal communities with specific temperature response. To assess the impact of different season or depth of production of the biomarkers on the recorded temperature in the sediment, modeled temperature proxies (Tproxies) are introduced in a regional oceanic model (ROMS) and tested for different seasons (Annual/Summer/Winter) and depths (Surface and 0-200 m) of production for three climate modes (Present Day (PD), Last Glacial (LGM) and Heinrich Stadials (HS)). Similar temperature amplitudes between climate modes are found at MD95-2042 core site for observations, for both biomarkers and for modeled Annual Surface Production Tproxy: 5.5-7 °C for ΔT(PD-LGM), and 3-4 °C for ΔT(LGM-HS). Therefore, we propose a new TEX H 86 regional calibration to reconstruct present and past annual mean SSTs on the Iberian Margin.

Description: Climatic models simulate abrupt oscillations that are associated, in the North Atlantic, with Dansgaard-Oeschger and Heinrich events. However, the geographic extension of temperature anomalies is largely uncontrolled due to the scarcity of quantitative records of sufficient time resolution on the European continent. Here, we propose, based on a recently developed temperature proxy (TEX86), a reconstruction of millennial-scale temperature variations in a Black Sea sediment archive for the last 40,000 years. Prior to any paleoclimatological interpretations the effects of potential bias, such as seasonality and depth of maximum export production on temperature reconstructions, are considered for the Black Sea. Based on previous work, a tentative method for temperature corrections, taking into account varying terrigenous inputs, is further proposed. Reconstructed temperatures for Black Sea core MD042790 were remarkably stable during the last glacial. However, significant shifts toward lower temperatures of 2°C occurred during Heinrich events 2 and 3. The deglaciation displayed a temperature increase of 10°C consistent with neighboring European reconstructions. A Younger Dryas cooling of approximately 5–6°C was clearly expressed in the reconstruction. In notable contrast to observations from nearby archives, Heinrich events imprinted our glacial temperature record consistent with a strong reorganization of oceanic circulation and a large spreading of the temperature anomaly from the North Atlantic toward the southeast. Furthermore, in contrast to high-latitude records, our Black Sea record lacks the signatures of Dansgaard-Oeschger interstadials, suggesting a decreasing temperature gradient away from the North Atlantic.

Description: During the last glacial period, the Oxygen Minimum Zone (OMZ) within the northeastern Pacific Ocean strengthened and weakened on a millennial time scale, demonstrating a tight linkage with northern high latitude climate, although the precise mechanisms responsible remain unknown. Core MD02–2508, retrieved off Baja California, was analyzed for major and trace elements (Br, Ca, Ti, Fe, Mn, and Sr) using a XRF scanner and redox-sensitive trace elements (Cu, Ni, Cd, As, V, Cr, Mo, and U) using the ICP-MS. The trace element content, the Fe/Ti ratio, and Br-based organic carbon exhibit higher values during the Holocene and during warm Dansgaard-Oeschger events than during the Last Glacial Maximum (LGM), stadials, and Heinrich (H) events. A principal component analysis of the element/Al ratio indicated that the following two main factors controlled the chemical composition of the sediments: (1) export production, as represented by organic carbon, that was lower during cold periods; and (2) regional intermediate water oxygenation, as represented by U and Mo variability, that was not supported by a change in export production. The latter suggests that intermediate water oxygenation improved during H events, but slightly deteriorated during late Marine Isotope Stage (MIS) 3 and MIS 2. A local biogeochemical effect, forced by atmospheric processes, impacted the LGM and H events in the same manner. Whereas regional intermediate oceanic circulation varied in an opposite manner during the LGM and H events, possibly as a result of the global reorganization of intermediate water circulation during the LGM.

Description: Abstract We developed a fully automated magnetic field scanner dedicated to uniaxial magnetic field measurements to determine remanent magnetization intensities and their variations in sedimentary U‐channels. A fluxgate magnetometer located as close as possible to the sedimentary section is used to perform uniaxial measurements of magnetic fields generated by the isothermal remanent magnetization of the sediment. This artificial magnetization, which is known to be a powerful proxy in environmental magnetism, is produced perpendicular to the U‐channel long axis, and parallel to the fluxgate axis, using a Halbach cylinder prior to the measurement. The present magnetic scanner offers a maximal spatial resolution of 5.8 mm for point sources. A spatial resolution of 14 mm is obtained for U channel samples. The magnetic scanner presents a reliable magnetic field range over about 3 orders of magnitude allowing measurement of magnetizations that saturate the Superconducting Rock Magnetometer in its classical configuration. The estimation of remanent magnetization intensities along the U‐channel is based on a modeling approach that uses successive uniformly magnetized prisms. In lacustrine laminated sections, comparison between modeling results based on prisms of a constant thickness, on prisms determined from sedimentary facies and on prisms determined from XRF (X‐Ray Fluorescence) data helps to understand the detrital vs. diagenetic history of the sedimentary succession.

Description: Abstract More than 2000 analysis of beryllium‐10 (10Be) and sulfate concentrations were performed at a nominal sub‐annual resolution on an ice core covering the last millennium as well as on records from three sites in Antarctica (Dome C, South Pole and Vostok) to better understand the increase in 10Be deposition during stratospheric volcanic eruptions. A significant increase in 10Be concentration is observed in 14 of the 26 volcanic events studied. The slope and intercept of the linear regression between 10Be and sulfate concentrations provide different and complementary information. Slope is an indicator of the efficiency of the draining of 10Be atoms by volcanic aerosols depending on the amount of SO2 released and the altitude it reaches in the stratosphere. Intercept gives an image of the 10Be production in the stratospheric reservoir, ultimately depending on solar modulation. The Samalas event (1257 A.D.) stands out from the others as the biggest eruption of the last millennium with the lowest positive slope of all the events. We hypothetize that the persistence of volcanic aerosols in the stratosphere after the Samalas eruption have drained the stratospheric 10Be reservoir for a decade, meaning that solar reconstructions based on 10Be should be considered with caution during this period. The slope of the linear regression between 10Be and sulfate concentrations can also be used to correct the 10Be snow/ice signal of the volcanic disturbance.