53 research outputs found
Changes of the Atlantic meridional overturning circulation of the past 30ka recorded in a depth transect at the Blake Outer Ridge
Oceans and climate are a tightly coupled system interacting with each other in various ways such as storage of
carbon dioxide in the deep ocean. Within the global conveyor belt the Atlantic Meridional Overturning Circulation (AMOC) holds a key function, transporting warm salty surface waters from the tropical to the northern Atlantic where deep water formation takes place. Following the continental rise of North America this newly formed deep water propagates southward as Western Boundary Undercurrent (WBUC) ventilating the deep Atlantic. In the past (e.g. the last glacial cycle) strength and geometry of the AMOC have changed significantly. This study aims to provide a better understanding of the temporal and spatial (also depth depended) evolution of the AMOC in the western Atlantic sector since the last glacial (âŒ30 ka). We have investigated four sediment cores of the Blake Outer Ridge (30°N, 74°W; ODP 1059 to 1062) in a depth transect from 3000 to 4700 m water depth in the main flow path of the WBUC. We measured four down-core profiles of neodymium (ΔNd) and 231Pa/230Th isotopes for the reconstruction of water mass provenance and circulation strength of the last âŒ30 ka. In contrast to published Nd isotope and 231Pa/230Th records from the Blake Ridge area our records are of unprecedented resolution, resolving climate key features of the North Atlantic region: Heinrich Stadials (HS) 1 and 2, the Last Glacial Maximum (LGM), the BĂžlling-AllerĂžd and Younger Dryas (YD). Radiogenic Nd isotope signatures during the LGM reveal AABW to be the prevalent water mass in the deep western North Atlantic. The trend to more unradiogenic signatures during the deglaciation point to an increased formation of NADW which was again replaced by AABW during YD. The Holocene shows the most unradiogenic signatures and therefore established NADW. The circulation strength-proxy 231Pa/230Th indicates reduced LGM deep circulation, a pronounced slowdown during HS1 and a strong and deep circulation during the Holocene. Compared to isotopic records from the Bermuda Rise (ODP 1063) we found depth depended geometry changes of the WBUC which have occurred through the last glacial. Here, we focus on how deep northern sourced water has reached during phases of reduced circulation (indicated by increased 231Pa/230Th ratios) and the timing of this southward progradation of lower NADW
Millennial-scale deep ocean ventilation and sea-surface variability during the last four glacial cycles : a new assessment for the Northern Hemisphere ice-sheet growth
AGU Fall Meeting, S. Francisco (USA), 19-14 December 2007, Suppl., Abstract PP44B-0
Seismotectonic aspects of the Ms 7.3 1948 October 5 AĆgabat (Ashgabat) earthquake, TĂŒrkmenistan: right-lateral rupture across multiple fault segments, and continuing urban hazard
The Ms 7.3 1948 AĆgabat earthquake was one of the most devastating earthquakes of the 20th century, yet little is known about its location, style and causative fault. In this study, we bring together new seismic and geomorphic observations with previously published descriptions of surface rupture and damage distributions to determine the likely source of the earthquake. We determine the epicentre and focal mechanism of this earthquake from digitized historical seismograms and the relocation of regional seismicity to show that the earthquake most likely nucleated close to the city of AĆgabat. The earthquake ruptured a right-lateral strike-slip fault to the southeast of the city, which has a clear long-term expression in the landscape, and also likely reactivated a subparallel concealed thrust along the Gyaursdag anticline east of the city. The earthquake potentially also ruptured a right-lateral segment northwest of AĆgabat, which does not have an identifiable expression in the landscape. Using high-resolution satellite imagery and digital elevation models we investigate the geomorphology of active faulting around AĆgabat and adjacent parts of the Köpetdag (Kopeh Dagh) mountain range front, showing that there are significant strike-slip and oblique strike-slip segments adjacent to the city that apparently did not rupture in 1948, and yet show clear geomorphic expression and potential right-lateral displacement of Parthian-era (âŒ2000 yr) and post-Sassanian era (âŒ1500 yr) archaeological remains. Luminescence dating of displaced fluvial terraces west of AĆgabat yields a vertical displacement rate of 0.6 mmâyrâ1, though the strike-slip rate remains undetermined
The Status of Dosage Compensation in the Multiple X Chromosomes of the Platypus
Dosage compensation has been thought to be a ubiquitous property of sex chromosomes that are represented differently in males and females. The expression of most X-borne genes is equalized between XX females and XY males in therian mammals (marsupials and âplacentalsâ) by inactivating one X chromosome in female somatic cells. However, compensation seems not to be strictly required to equalize the expression of most Z-borne genes between ZZ male and ZW female birds. Whether dosage compensation operates in the third mammal lineage, the egg-laying monotremes, is of considerable interest, since the platypus has a complex sex chromosome system in which five X and five Y chromosomes share considerable genetic homology with the chicken ZW sex chromosome pair, but not with therian XY chromosomes. The assignment of genes to four platypus X chromosomes allowed us to examine X dosage compensation in this unique species. Quantitative PCR showed a range of compensation, but SNP analysis of several X-borne genes showed that both alleles are transcribed in a heterozygous female. Transcription of 14 BACs representing 19 X-borne genes was examined by RNA-FISH in female and male fibroblasts. An autosomal control gene was expressed from both alleles in nearly all nuclei, and four pseudoautosomal BACs were usually expressed from both alleles in male as well as female nuclei, showing that their Y loci are active. However, nine X-specific BACs were usually transcribed from only one allele. This suggests that while some genes on the platypus X are not dosage compensated, other genes do show some form of compensation via stochastic transcriptional inhibition, perhaps representing an ancestral system that evolved to be more tightly controlled in placental mammals such as human and mouse
Medicinal plants â prophylactic and therapeutic options for gastrointestinal and respiratory diseases in calves and piglets? A systematic review
Iron oxidation in sediment cores (Site 1062) during six months of storage in the Ocean Drilling Program archive
Changes in bulk sediment Fe(II)/Fe(III) ratio and in the distribution
of iron among different minerals as a result of Ocean Drilling Program
archive storage in the Bremen Core Repository were investigated using
Mössbauer spectroscopy.
Massive Fe(II) to Fe(III) oxidation, which involved between 24% and
45% of the initial Fe(II), occurred within only 6 months of refrigerated
storage. Prior to archive storage, >95% of the Fe(II) in the sediment
samples under investigation was structural iron in silicate minerals.
Hence, virtually the entire oxidation process took place within silicate
mineral lattices, and the sediment mineral assemblage was not changed
in this case. Nevertheless, the observed oxidation of the comparatively
shielded silicate lattice Fe(II) suggests that Fe(II) bound in authigenic
carbonates, phosphates, or sulfidesâsuch as that found in many marine
sedimentsâwould likely be oxidized at least as fast. Those minerals,
however, would be replaced by Fe(III)-bearing oxides and
oxyhydroxides, which implies a change of sediment composition, and
thus, of various sediment properties, including the magnetic signal,
within a few months of storage.
Furthermore, changes in the silicate lattice Fe(II)/Fe(III) ratio during
storage, such as those reported here, also signify loss of information.
This is because oxidation of the structural Fe(II) upon contact with atmospheric
oxygen may occur only inasmuch as the inverse Fe(III)âFe(II) redox transition has taken place in the seabed. Therefore, the reversible
shift, if it were measured under controlled reoxidation in the
laboratory, may suggest the chemical stress that was suffered by the
iron oxide minerals at the ocean bottom. Concerning Site 1062, this
process might help to judge both the authenticity of magnetic field excursion
records and the lithostratigraphic value of red lutites at given
sediment depths.
Although the nature and extent of information loss or alteration during
storage depend on sediment type, the reported observations emphasize
the need for special sample protection with respect to
properties that might be affected
Refraktometrische Bestimmung der verdauenden Kraft verschiedener PepsinprĂ€parate gegenĂŒber tierischem Fibrin
Paleoceanographic significance of sediment color on western North Atlantic Drifts: II. Late PliocenePleistocene sedimentation
Computer-Assistierte Pedikelinstrumentierung mit registrierungsfreier Navigation in 3D C-Arm Bildern
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