132 research outputs found
Climate Variability in Europe and Africa: a PAGES-PEP III Time Stream II Synthesis
The PEP III Europe-Africa transect extends from the arctic fringes of NW Eurasia to South Africa. It encompasses the presently
temperate sector of mid-latitude Europe, the Mediterranean region, the arid and semi-arid lands of the Sahara, Sahel and the
Arabian Peninsula, and the inter-tropical belt of Africa. The palaeoenvironmental evidence available from these regions, which
has been summarised in earlier chapters of this volume and which collectively spans the last 250,000 years, clearly bears
the stamp of long-term global climate forcing induced by variations in solar insolation. External forcing is ultimately the
reason why the Eurasian continental ice sheets waxed and waned repeatedly during the late Quaternary, and why the southerly
limit of permafrost migrated southwards across mid-latitude Europe, periodically becoming degraded during warmer episodes.
At the same time, pronounced fluctuations in atmospheric and soil moisture have affected the Mediterranean, desert and Sahel
regions, while there is abundant evidence from every sector of the PEP III transect for marked migrations of the principal
vegetation belts, as well as for other major environmental changes, that are also considered to reflect long-term climate
forcing. It is only in the last decade or so, however, that the full complexity of the history of climate changes during the
last interglacial-glacial cycle, and their environmental impacts in continental Europe and Africa, have begun to be recognised.
The discovery of evidence for the abrupt Dansgaard-Oeschger (D-O) and Heinrich (H) climatic oscillations in Greenland ice-core
(Johnsen et al. 1992) and North Atlantic (Bond et al. 1993) records, have prompted a re-examination of the continental record.
This, together with a number of technical improvements in field and laboratory equipment, greater access to sites in remote
and difficult terrain, diversification in the range of available palaeoecological and geochronological tools, and closer inter-disciplinary
collaboration, have led to a more penetrating examination of the field evidence, which has progressed the science considerably.
We can now see that the stratigraphical record is much more complex than appreciated hitherto, and more detailed and refined
models of past climatic and environmental models are beginning to emerge. There is, for example, a growing body of evidence
which suggests that D-O and H events had significant impacts on the environment of Europe and Africa, as well as on the Mediterranean
Sea
Impact of monsoons, temperature, and CO 2 on the rainfall and ecosystems of Mt. Kenya during the Common Era
Late-Holocene climate dynamics recorded in the peat bogs of Tierra del Fuego, South America
The ombrotrophic peat bogs of Tierra del Fuego are located within the southern westerly wind belt (SWWB), which dominates climate variability in this region. We have reconstructed late-Holocene water-table depths from three peat bogs and aimed to relate these records to shifts in regional climate. Water-table depths were quantified by the analysis of testate amoeba assemblages, and a regional transfer function was used to infer past water-table depths. During the last 2000 years, testate amoeba assemblages have been relatively stable, with a dominance of Difflugia pulex and Difflugia pristis type, and an increase in Assulina muscorum and other Euglyphida at the top of each section. Multivariate analyses show that water-table depth remained the main environmental variable explaining assemblages along the TiA12 core, but reconstructions were not significant for the two other cores. In line with the low variability in assemblages, water tables were relatively stable during the last 2000 years. Slightly wetter conditions were found between ~1400 and 900 cal. BP and a pronounced recent dry shift was reconstructed in all of the three peat profiles. Considering the regional climatic context, this recent shift may have been forced by a decrease in precipitation and warmer conditions linked to an increase in the importance of the SWWB. Nevertheless, we cannot exclude the influence of higher UV-B radiation resulting from the local degradation of the ozone layer since the late 1970s, which may have had an additional effect on the relative presence of A. muscorum in the southern Patagonian region
The Holocene isotopic record of aquatic cellulose from Lake Äntu Sinijärv, Estonia: Influence of changing climate and organic-matter sources
An alternative approach to transfer functions? Testing the performance of a functional trait-based model for testate amoebae
publisher: Elsevier articletitle: An alternative approach to transfer functions? Testing the performance of a functional trait-based model for testate amoebae journaltitle: Palaeogeography, Palaeoclimatology, Palaeoecology articlelink: http://dx.doi.org/10.1016/j.palaeo.2016.12.005 content_type: article copyright: © 2016 The Authors. Published by Elsevier B.V
Orbital forcing of glacial/interglacial variations in chemical weathering and silicon cycling within the upper White Nile basin, East Africa: Stable-isotope and biomarker evidence from Lakes Victoria and Edward
On Quaternary time scales, the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae (notably diatoms, for which Si is an essential nutrient). Large tropical river systems dominate the export of Si from the continents to the oceans. Here, we investigate variations in Si cycling in the upper White Nile basin over the last 15 ka, using sediment cores from Lakes Victoria and Edward. Coupled measurements of stable O and Si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and Si cycling, while the abundances of lipid biomarkers characteristic of terrestrial/emergent higher plants, submerged/floating aquatic macrophytes and freshwater algae document past ecosystem changes. During the late-glacial to mid-Holocene, 15–5.5 ka BP, orbital forcing greatly enhanced monsoon rainfall, forest cover and chemical weathering. Riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower Si-isotope values. Since 5.5 ka BP, increasingly dry climates and more open vegetation, reinforced by the spread of agricultural cropland over the last 3–4 ka, have reduced dissolved silica inputs into the lakes. Centennial-to millennial-scale dry episodes are also evident in the isotopic records and merit further investigation
OGLE-2009-BLG-092/MOA-2009-BLG-137: A Dramatic Repeating Event With the Second Perturbation Predicted by Real-Time Analysis
We report the result of the analysis of a dramatic repeating gravitational
microlensing event OGLE-2009-BLG-092/MOA-2009-BLG-137, for which the light
curve is characterized by two distinct peaks with perturbations near both
peaks. We find that the event is produced by the passage of the source
trajectory over the central perturbation regions associated with the individual
components of a wide-separation binary. The event is special in the sense that
the second perturbation, occurring days after the first, was
predicted by the real-time analysis conducted after the first peak,
demonstrating that real-time modeling can be routinely done for binary and
planetary events. With the data obtained from follow-up observations covering
the second peak, we are able to uniquely determine the physical parameters of
the lens system. We find that the event occurred on a bulge clump giant and it
was produced by a binary lens composed of a K and M-type main-sequence stars.
The estimated masses of the binary components are
and , respectively, and they are separated in
projection by . The measured distance to the
lens is . We also detect the orbital motion
of the lens system.Comment: 18 pages, 5 figures, 1 tabl
The Extreme Microlensing Event OGLE-2007-BLG-224: Terrestrial Parallax Observation of a Thick-Disk Brown Dwarf
Parallax is the most fundamental technique to measure distances to
astronomical objects. Although terrestrial parallax was pioneered over 2000
years ago by Hipparchus (ca. 140 BCE) to measure the distance to the Moon, the
baseline of the Earth is so small that terrestrial parallax can generally only
be applied to objects in the Solar System. However, there exists a class of
extreme gravitational microlensing events in which the effects of terrestrial
parallax can be readily detected and so permit the measurement of the distance,
mass, and transverse velocity of the lens. Here we report observations of the
first such extreme microlensing event OGLE-2007-BLG-224, from which we infer
that the lens is a brown dwarf of mass M=0.056 +- 0.004 Msun, with a distance
of 525 +- 40 pc and a transverse velocity of 113 +- 21 km/s. The velocity
places the lens in the thick disk, making this the lowest-mass thick-disk brown
dwarf detected so far. Follow-up observations may allow one to observe the
light from the brown dwarf itself, thus serving as an important constraint for
evolutionary models of these objects and potentially opening a new window on
sub-stellar objects. The low a priori probability of detecting a thick-disk
brown dwarf in this event, when combined with additional evidence from other
observations, suggests that old substellar objects may be more common than
previously assumed.Comment: ApJ Letters, in press, 15 pages including 2 figure
Binary microlensing event OGLE-2009-BLG-020 gives a verifiable mass, distance and orbit predictions
We present the first example of binary microlensing for which the parameter
measurements can be verified (or contradicted) by future Doppler observations.
This test is made possible by a confluence of two relatively unusual
circumstances. First, the binary lens is bright enough (I=15.6) to permit
Doppler measurements. Second, we measure not only the usual 7 binary-lens
parameters, but also the 'microlens parallax' (which yields the binary mass)
and two components of the instantaneous orbital velocity. Thus we measure,
effectively, 6 'Kepler+1' parameters (two instantaneous positions, two
instantaneous velocities, the binary total mass, and the mass ratio). Since
Doppler observations of the brighter binary component determine 5 Kepler
parameters (period, velocity amplitude, eccentricity, phase, and position of
periapsis), while the same spectroscopy yields the mass of the primary, the
combined Doppler + microlensing observations would be overconstrained by 6 + (5
+ 1) - (7 + 1) = 4 degrees of freedom. This makes possible an extremely strong
test of the microlensing solution. We also introduce a uniform microlensing
notation for single and binary lenses, we define conventions, summarize all
known microlensing degeneracies and extend a set of parameters to describe full
Keplerian motion of the binary lenses.Comment: 51 pages, 8 figures, 2 appendices. Submitted to ApJ. Fortran codes
for Appendix B are attached to this astro-ph submission and are also
available at http://www.astronomy.ohio-state.edu/~jskowron/OGLE-2009-BLG-020
MOA-2009-BLG-387Lb: A massive planet orbiting an M dwarf
We report the discovery of a planet with a high planet-to-star mass ratio in
the microlensing event MOA-2009-BLG-387, which exhibited pronounced deviations
over a 12-day interval, one of the longest for any planetary event. The host is
an M dwarf, with a mass in the range 0.07 M_sun < M_host < 0.49M_sun at 90%
confidence. The planet-star mass ratio q = 0.0132 +- 0.003 has been measured
extremely well, so at the best-estimated host mass, the planet mass is m_p =
2.6 Jupiter masses for the median host mass, M = 0.19 M_sun. The host mass is
determined from two "higher order" microlensing parameters. One of these, the
angular Einstein radius \theta_E = 0.31 +- 0.03 mas, is very well measured, but
the other (the microlens parallax \pi_E, which is due to the Earth's orbital
motion) is highly degenate with the orbital motion of the planet. We
statistically resolve the degeneracy between Earth and planet orbital effects
by imposing priors from a Galactic model that specifies the positions and
velocities of lenses and sources and a Kepler model of orbits. The 90%
confidence intervals for the distance, semi-major axis, and period of the
planet are 3.5 kpc < D_L < 7.9 kpc, 1.1 AU < a < 2.7AU, and 3.8 yr < P < 7.6
yr, respectively.Comment: 20 pages including 8 figures. A&A 529 102 (2011
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