| 1. |
- Kekli, A, Meili M, Aldahan A, Possnert G, Buraglio N, et al.
(författare)
-
High levels of 129I in rivers of south
- 2001
-
Ingår i: Proceedings of the 8th Nordic Seminar on Radioecology., 25-28 Feb. ; , s. 194-
-
Konferensbidrag (refereegranskat)
|
|
| 2. |
- Abbuehl, Luca M., et al.
(författare)
-
El Niño forcing on 10Be-based surface denudation rates in the northwestern Peruvian Andes?
- 2010
-
Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 123:3-4, s. 257-268
-
Tidskriftsartikel (refereegranskat)abstract
- High magnitude precipitation events provide large contributions to landscape formation and surface denudation in arid environments. Here, we quantify the precipitation-dependent geomorphic processes within the Rio Piura drainage basin located on the Western Escarpment of the northern Peruvian Andes at 5 degrees S latitude. In this region, monsoonal easterly winds bring precipitation to the >3000 m asl high headwaters, from where the annual amount of precipitation decreases downstream toward the Pacific coast. Denudation rates are highest in the knickzones near the headwaters, similar to 200-300 mm ky(-1), and sediment discharge is limited by the transport capacity of the channel network. Every few years, this situation is perturbed by westerly, wind-driven heavy precipitation during El Nino events and results in supply-limited sediment discharge as indicated by bedrock channels. The detailed analysis of the stream-long profiles of two river basins within the Rio Piura catchment reveals a distinct knickzone in the transition zone between the easterly and westerly climatic influences, suggesting an En Nino forcing on the longitudinal channel profiles over at least Holocene timescales. Measured trunk stream catchment-wide denudation rates are up to ca. 300 mm ky(-1) and decrease successively downstream along the river profiles. Denudation rates of tributary rivers are ca. 200 mm ky(-1) near the plateau and show a stronger downstream decreasing trend than trunk stream rates. This suggests that the landscape is in a transient stage of local relief growth, which is driven by fluvial incision. This corroborates the results of paleoclimate studies that point towards higher El Nino frequencies during the past ca. 3000 years, leading to higher runoff and more erosion in the trunk channel compared to the hillslopes and thus growth of local relief. Downstream increases in channel gradient spatially coincide with the reaches of highest precipitation rates during El Nino events, we therefore interpret that Holocene landscape evolution has largely been controlled by climate. The ky-timescale of the Be-10 data together with the transience of the landscape implies that El Nino events in northwestern Peru have occurred since at least the Holocene, and that adjustment to channel incision is still taking place.
|
|
| 3. |
- Abbühl, Luca, et al.
(författare)
-
Erosion rates and mechanisms of knickzone retreat inferred from Be-10 measured across strong climate gradients on the northern and central Andes Western Escarpment
- 2011
-
Ingår i: Earth Surface Processes and Landforms. - : John Wiley & Sons. - 0197-9337 .- 1096-9837. ; 36, s. 1464-1473
-
Tidskriftsartikel (refereegranskat)abstract
- escarpment edge, deep gorges and distinct knickzones in river profiles characterize the landscape on the Western Escarpment of the Andes between ~5°S and ~18°S (northern Peru to northern Chile). The Western Escarpment straddles strong north-south and east-west precipitation gradients, which we exploit in order to determine how climate affects denudation rates in three river basins spanning an otherwise relatively uniform geologic and geomorphologic setting. The Western Escarpment reflects an ongoing transient response to major tectonic perturbation in the Late Miocene when surface uplift created a steep ramp separating drainage systems above the growing escarpment from those below. Upstream of this ramp, and now forming the uplifted Meseta/Altiplano (~3000 m a.s.l.), streams remain graded to the Late Miocene base level constituted by a series of Tertiary volcanic-volcanoclastic rocks, Streams below the ramp have responded to the Late Miocene surface uplift by incising deeply into fractured Mesozoic rocks via a series of steep, headward retreating knickzones that grade to the present-day base level defined by the Pacific Ocean. We find that the Tertiary units function as cap-rocks, which aid in the parallel retreat of the sharp escarpment edge and upper knickzone tips. Upstream of these knickzones, on the Meseta/Altiplano, 10Be-derived catchment denudation rates of the Rio Piura (5°S), Rio Pisco (13°S) and Rio Lluta (18°S) average ~10 mm ky–1 irrespective of precipitation rates; whereas, downstream of the escarpment edge, denudation rates range from 10 mm ky-1 to 250 mm ky-1 and correlate positively with precipitation rates, but show no significant correlation with hillslope angles or channel steepness. These relationships are explained by the presence of the cap-rock and climate-driven fluvial incision that steepens hillslopes to near-threshold conditions. In addition, valley width and the extent of dissection both appear to increase with increasing precipitation to the north, consistent with climate-forcing of landscape morphology in the deeply incised terrain beneath the escarpment edge.Since escarpment retreat and the precipitation pattern were established at least in the Miocene, we speculate that the present-day distribution of morphology and denudation rates has probably remained largely unchanged during the past several millions of years as the knickzones have propagated headward into the plateau.
|
|
| 4. |
- Adolphi, Florian, et al.
(författare)
-
Persistent link between solar activity and Greenland climate during the Last Glacial Maximum
- 2014
-
Ingår i: Nature Geoscience. - 1752-0908 .- 1752-0894. ; 7:9, s. 662-666
-
Tidskriftsartikel (refereegranskat)abstract
- Changes in solar activity have previously been proposed to cause decadal- to millennial-scale fluctuations in both the modern and Holocene climates(1). Direct observational records of solar activity, such as sunspot numbers, exist for only the past few hundred years, so solar variability for earlier periods is typically reconstructed from measurements of cosmogenic radionuclides such as Be-10 and C-14 from ice cores and tree rings(2,3). Here we present a high-resolution Be-10 record from the ice core collected from central Greenland by the Greenland Ice Core Project (GRIP). The record spans from 22,500 to 10,000 years ago, and is based on new and compiled data(4-6). Using C-14 records(7,8) to control for climate-related influences on Be-10 deposition, we reconstruct centennial changes in solar activity. We find that during the Last Glacial Maximum, solar minima correlate with more negative delta O-18 values of ice and are accompanied by increased snow accumulation and sea-salt input over central Greenland. We suggest that solar minima could have induced changes in the stratosphere that favour the development of high-pressure blocking systems located to the south of Greenland, as has been found in observations and model simulations for recent climate(9,10). We conclude that the mechanism behind solar forcing of regional climate change may have been similar under both modern and Last Glacial Maximum climate conditions.
|
|
| 5. |
- Aldahan, Ala, et al.
(författare)
-
129I isotope in an ice core from Greenland
- 2000
-
Ingår i: 6th International conference on Acid Rain, 10-16 December, Tsukuba, Japan. ; , s. 122-
-
Konferensbidrag (refereegranskat)
|
|
| 6. |
- Aldahan, Ala Adin, et al.
(författare)
-
10Be in rhodochrosite nodules from Neogene sediments along the Galapagos Ridge, equatorial Pacific
- 2010
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 268:7-8, s. 1253-1256
-
Tidskriftsartikel (refereegranskat)abstract
- Microcrystalline, calcian rhodochrosite occurs as nodules around burrows in late Neogene pelagic sediments from the Galapagos Ridge in the Guatemala Basin, eastern equatorial Pacific (DSDP Leg 68; Site 503). Be-10 isotope revealed that the rhodochrosite nodules have formed under growth conditions much faster than those reported for Fe-Mn nodules. The overall REE patterns of the nodules and host pelagic sediments indicate element derivation mainly from marine pore water. However, variations in the shale normalised Eu values suggest influx of hydrothermal fluids into mounds area at Galapagos, which is also evidenced by the similar minor and major element contents in the nodules and host sediments.
|
|
| 7. |
- Aldahan, Ala, et al.
(författare)
-
Anthropogenic I-129 in the Baltic Sea
- 2005
-
Ingår i: 10th International conference on accelerator mass spectrometry, Berkeley. ; , s. 33-
-
Konferensbidrag (refereegranskat)
|
|
| 8. |
- Aldahan, Ala, et al.
(författare)
-
Anthropogenic I-129 in the Baltic Sea
- 2007
-
Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 259:1, s. 491-495
-
Tidskriftsartikel (refereegranskat)abstract
- We report here data on the distribution of I-129 in water of the Baltic Sea and Skagerrak-Kattegat basins obtained through sampling campaigns during the years 2000 and 2001. I-129 in the water of the Skagerrak-Kattegat shows persistently high concentrations in both the surface and deep parts. A decreasing trend in surface water I-129 concentration is observed away from the Kattegat basin and into the Baltic Sea. Inventory calculations indicate that at least 95%of the isotope supply to the Baltic Sea is from the North Sea marine water via Skagerrak-Kattegat and only minor part is from riverine inflow and the Chernobyl accident. As the discharges from the nuclear reprocessing facilities were not decreased during the last decade, it is expected that more I-129 will accumulate in the Baltic Sea and related basins in the future. Consequently, systematic sampling is needed to reevaluate the concentration levels not only in the water, but also in the sediments and biota of the region.
|
|
| 9. |
- Aldahan, Ala, et al.
(författare)
-
Atmospheric impact on beryllium isotopes as solar activity proxy
- 2008
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:L21812
-
Tidskriftsartikel (refereegranskat)abstract
- Reconstructing solar activity variability beyond the time scale of actual measurements provides invaluable data for modeling of past and future climate change. The 10 Be isotope has been a primary proxy archive of past solar activity and cosmic ray intensity, particularly for the last millennium. There is, however, a lack of direct high-resolution atmospheric time series on 10 Be that enable estimating atmospheric modulation on the production signal. Here we report quasi-weekly data on 10 Be and 7 Be isotopes covering the periods 1983-2000 and 1975-2006 respectively, that show, for the first time, coherent variations reflecting both atmospheric and production effects. Our data indicate intrusion of stratosphere/upper troposphere air masses that can modulate the isotopes production signal, and may induce relative peaks in the natural 10 Be archives (i.e., ice and sediment). The atmospheric impact on the Be-isotopes can disturb the production signals and consequently the estimate of past solar activity magnitude. Citation: Aldahan, A., J. Hedfors, G. Possnert, A. Kulan, A.-M. Berggren, and C. Soderstrom (2008), Atmospheric impact on beryllium isotopes as solar activity proxy, Geophys. Res. Lett., 35, L21812, doi: 10.1029/2008GL035189.
|
|
| 10. |
|
|
