561 research outputs found
Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization on atmospheric pCO2 and seawater composition over the last 130 000 years: a model study
We have developed and employed an Earth system
model to explore the forcings of atmospheric pCO2 change
and the chemical and isotopic evolution of seawater over the
last glacial cycle. Concentrations of dissolved phosphorus
(DP), reactive nitrogen, molecular oxygen, dissolved inorganic
carbon (DIC), total alkalinity (TA), 13C-DIC, and 14CDIC
were calculated for 24 ocean boxes. The bi-directional
water fluxes between these model boxes were derived from a
3-D circulation field of the modern ocean (Opa 8.2, NEMO)
and tuned such that tracer distributions calculated by the box
model were consistent with observational data from the modern
ocean. To model the last 130 kyr, we employed records
of past changes in sea-level, ocean circulation, and dust deposition.
According to the model, about half of the glacial
pCO2 drawdown may be attributed to marine regressions.
The glacial sea-level low-stands implied steepened ocean
margins, a reduced burial of particulate organic carbon, phosphorus,
and neritic carbonate at the margin seafloor, a decline
in benthic denitrification, and enhanced weathering of
emerged shelf sediments. In turn, low-stands led to a distinct
rise in the standing stocks of DIC, TA, and nutrients in the
global ocean, promoted the glacial sequestration of atmospheric
CO2 in the ocean, and added 13C- and 14C-depleted
DIC to the ocean as recorded in benthic foraminifera signals.
The other half of the glacial drop in pCO2 was linked
to inferred shoaling of Atlantic meridional overturning circulation
and more efficient utilization of nutrients in the Southern
Ocean. The diminished ventilation of deep water in the
glacial Atlantic and Southern Ocean led to significant 14C
depletions with respect to the atmosphere. According to our
model, the deglacial rapid and stepwise rise in atmospheric
pCO2 was induced by upwelling both in the Southern Ocean
and subarctic North Pacific and promoted by a drop in nutrient
utilization in the Southern Ocean. The deglacial sea-level
rise led to a gradual decline in nutrient, DIC, and TA stocks,
a slow change due to the large size and extended residence
times of dissolved chemical species in the ocean. Thus, the
rapid deglacial rise in pCO2 can be explained by fast changes
in ocean dynamics and nutrient utilization whereas the gradual
pCO2 rise over the Holocene may be linked to the slow
drop in nutrient and TA stocks that continued to promote an
ongoing CO2 transfer from the ocean into the atmosphere
MRI-validation of SEP monitoring for ischemic events during microsurgical clipping of intracranial aneurysms
OBJECTIVE:
During surgical clipping of intracranial aneurysms, reduction in SEP amplitude is thought to indicate cortical ischemia and subsequent neurological deficits. Since the sensitivity of SEP is questioned, we investigated SEP with respect to post-operative ischemia.
METHODS:
In 36 patients with 51 intracranial aneurysms, clinical evaluation and diffusion-weighted MRI (DWI) was performed before and within 24h after surgery. During surgery, time of temporary occlusion was recorded. MRI images were reviewed for signs of ischemia.
RESULTS:
For 43 clip applications (84%), we observed neither pathologic SEP events nor ischemia in MRI. In two cases where reduction lasted >10 min after clip release, SEP events correlated with ischemia in the MRI. Only one of the ischemic patients was symptomatic and developed a transient hemiparesis.
CONCLUSIONS:
While pathologic SEP events correlated with visible ischemia in MRI only in two cases with late SEP recovery, ischemia in MRI may have been transient or may not have reached detection threshold in the other cases, in agreement with the absence of permanent neurological deficits.
SIGNIFICANCE:
In complex aneurysm cases, where prolonged temporary occlusion is expected, SEP should be used to detect ischemia at a reversible stage to improve the safety of aneurysm clipping.
Copyright © 2011 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved
Mid-Pliocene shifts in ocean overturning circulation and the onset of Quaternary-style climates
A major tipping point of Earth's history occurred during the mid-Pliocene: the onset of major Northern-Hemisphere Glaciation (NHG) and of pronounced, Quaternary-style cycles of glacial-to-interglacial climates, that contrast with more uniform climates over most of the preceding Cenozoic and continue until today (Zachos et al., 2001). The severe deterioration of climate occurred in three steps between 3.2 Ma (warm MIS K3) and 2.7 Ma (glacial MIS G6/4) (Lisiecki and Raymo, 2005). Various models (sensu Driscoll and Haug, 1998) and paleoceanographic records (intercalibrated using orbital age control) suggest clear linkages between the onset of NHG and the three steps in the final closure of the Central American Seaways (CAS), deduced from rising salinity differences between Caribbean and the East Pacific. Each closing event led to an enhanced North Atlantic meridional overturning circulation and this strengthened the poleward transport of salt and heat (warmings of +2–3°C) (Bartoli et al., 2005). Also, the closing resulted in a slight rise in the poleward atmospheric moisture transport to northwestern Eurasia (Lunt et al., 2007), which probably led to an enhanced precipitation and fluvial run-off, lower sea surface salinity (SSS), and an increased sea-ice cover in the Arctic Ocean, hence promoting albedo and the build-up of continental ice sheets. Most important, new evidence shows that the closing of the CAS led to greater steric height of the North Pacific and thus doubled the low-saline Arctic Throughflow from the Bering Strait to the East Greenland Current (EGC). Accordingly, Labrador Sea IODP Site 1307 displays an abrupt but irreversible EGC cooling of 6°C and freshening by ~2 psu from 3.25/3.16–3.00 Ma, right after the first but still reversible attempt of closing the CAS
First-principles molecular-dynamics simulations of a hydrous silica melt: Structural properties and hydrogen diffusion mechanism
We use {\it ab initio} molecular dynamics simulations to study a sample of
liquid silica containing 3.84 wt.% HO.We find that, for temperatures of
3000 K and 3500 K,water is almost exclusively dissolved as hydroxyl groups, the
silica network is partially broken and static and dynamical properties of the
silica network change considerably upon the addition of water.Water molecules
or free O-H groups occur only at the highest temperature but are not stable and
disintegrate rapidly.Structural properties of this system are compared to those
of pure silica and sodium tetrasilicate melts at equivalent temperatures. These
comparisons confirm the picture of a partially broken tetrahedral network in
the hydrous liquid and suggest that the structure of the matrix is as much
changed by the addition of water than it is by the addition of the same amount
(in mole %) of sodium oxide. On larger length scales, correlations are
qualitatively similar but seem to be more pronounced in the hydrous silica
liquid. Finally, we study the diffusion mechanisms of the hydrogen atoms in the
melt. It turns out that HOSi triclusters and SiO dangling bonds play a
decisive role as intermediate states for the hydrogen diffusion.Comment: 25 pages, 18 figures. submitte
Overview of Glacial Atlantic Ocean Mapping (GLAMAP 2000)
GLAMAP 2000 presents new reconstructions of the Atlantic's sea surface temperatures (SST) at the Last Glacial Maximum (LGM), defined at both 21,500–18,000 years B.P. (“Last Isotope Maximum”) and 23,000–19,000 years B.P. (maximum glacial sea level low stand and orbital minimum of solar insolation; EPILOG working group; see Mix et al. [2001]). These reconstructions use 275 sediment cores between the North Pole and 60°S with carefully defined chronostratigraphies. Four categories of core quality are distinguished. More than 100 core sections provide a glacial record with subcentennial- to multicentennial-scale resolution. SST estimates are based on a new set of almost 1000 reference samples of modern planktic foraminifera and on improved transfer-function techniques to deduce SST from census counts of microfossils, including radiolarians and diatoms. New proxies also serve to deduce sea ice boundaries. The GLAMAP 2000 SST patterns differ significantly in crucial regions from the CLIMAP [1981] reconstruction and thus are important in providing updated boundary conditions to initiate and validate computational models for climate prediction
Proof of the thermodynamical stability of the E' center in SiO2
The E' center is a paradigmatic radiation-induced defect in SiO2 whose
peculiar EPR and hyperfine activity has been known since over 40 years. This
center has been traditionally identified with a distorted, positively-charged
oxygen vacancy V_O+. However, no direct proof of the stability of this defect
has ever been provided, so that its identification is still strongly
incomplete. Here we prove directly that distorted V_O+ is metastable and that
it satisfies the key requirements for its identification as E', such as thermal
and optical response, and activation-deactivation mechanisms.Comment: RevTeX 4 pages, 2 figure
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