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

Increased EEG power and slowed dominant frequency in patients with neurogenic pain

To study the mechanisms of chronic neurogenic pain, we compared the power spectra of the resting EEG of patients (n = 15, 38-75 years, median 64 years, 6 women) and healthy controls (n = 15, 41-71 years, median 60 years, 8 women). On an average, the patient group exhibited higher spectral power over the frequency range of 2-25 Hz, and the dominant peak was shifted towards lower frequencies. Maximal differences appeared in the 7-9 Hz band in all electrodes. Frontal electrodes contributed most to this difference in the 13-15 Hz band. Bicoherence analysis suggests an enhanced coupling between theta (4-9 Hz) and beta (12-25 Hz) frequencies in patients. The subgroup of six patients free from centrally acting medication showed higher spectral power in the 2-18 Hz frequency range. On an individual basis, the combination of peak height and peak frequency discriminated between patient and control groups: discriminant analysis classified 87% of all subjects correctly. After a therapeutic lesion in the thalamus (central lateral thalamotomy, CLT) we carried out follow-up for a subgroup of seven patients. Median pain relief was 70 and 95% after 3 and 12 months, respectively. The average EEG power of all seven patients gradually decreased in the theta band and approached normal values only after 12 months. The excess theta EEG power in patients and its decrease after thalamic surgery suggests that both EEG and neurogenic pain are determined by tightly coupled thalamocortical loops. The small therapeutic CLT lesion is thought to initiate a progressive normalization in the affected thalamocortical system, which is reflected in both decrease of EEG power and pain relief

Increased EEG power and slowed dominant frequency in patients with neurogenic pain

To study the mechanisms of chronic neurogenic pain, we compared the power spectra of the resting EEG of patients (n = 15, 38-75 years, median 64 years, 6 women) and healthy controls (n = 15, 41-71 years, median 60 years, 8 women). On an average, the patient group exhibited higher spectral power over the frequency range of 2-25 Hz, and the dominant peak was shifted towards lower frequencies. Maximal differences appeared in the 7-9 Hz band in all electrodes. Frontal electrodes contributed most to this difference in the 13-15 Hz band. Bicoherence analysis suggests an enhanced coupling between theta (4-9 Hz) and beta (12-25 Hz) frequencies in patients. The subgroup of six patients free from centrally acting medication showed higher spectral power in the 2-18 Hz frequency range. On an individual basis, the combination of peak height and peak frequency discriminated between patient and control groups: discriminant analysis classified 87% of all subjects correctly. After a therapeutic lesion in the thalamus (central lateral thalamotomy, CLT) we carried out follow-up for a subgroup of seven patients. Median pain relief was 70 and 95% after 3 and 12 months, respectively. The average EEG power of all seven patients gradually decreased in the theta band and approached normal values only after 12 months. The excess theta EEG power in patients and its decrease after thalamic surgery suggests that both EEG and neurogenic pain are determined by tightly coupled thalamocortical loops. The small therapeutic CLT lesion is thought to initiate a progressive normalization in the affected thalamocortical system, which is reflected in both decrease of EEG power and pain relie

Ab-initio study of oxygen vacancies in alpha-quartz

Extrinsic levels, formation energies, and relaxation geometries are calculated ab initio for oxygen vacancies in alpha-quartz SiO2. The vacancy is found to be thermodynamically stable in the charge states Q=+3, Q=0, Q=--2, and Q=-3. The charged states are stabilized by large and asymmetric distortions near the vacancy site. Concurrently, Franck-Condon shifts for absorption and recombination related to these states are found to be strongly asymmetric. In undoped quartz, the ground state of the vacancy is the neutral charge state, while for moderate p-type and n-type doping, the +3 and -3 states are favored, respectively, over a wide Fermi level window. Optical transitions related to the vacancy are predicted at around 3 eV and 6.5 eV (absorption) and 2.5 to 3.0 eV (emission), depending on the charge state of the ground state.Comment: 6 figures included, but only Fig.1 actually change

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

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.% H$_2$O.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$_2$ triclusters and SiO dangling bonds play a decisive role as intermediate states for the hydrogen diffusion.Comment: 25 pages, 18 figures. submitte

Motor-evoked potentials (MEP) during brainstem surgery to preserve corticospinal function

Background: Brainstem surgery bears a risk of damage to the corticospinal tract (CST). Motor-evoked potentials (MEPs) are used intraoperatively to monitor CST function in order to detect CST damage at a reversible stage and thus impede permanent neurological deficits. While the method of MEP is generally accepted, warning criteria in the context of brainstem surgery still have to be agreed on. Method: We analyzed 104 consecutive patients who underwent microsurgical resection of lesions affecting the brainstem. Motor grade was documented prior to surgery, early postoperatively and at discharge. A baseline MEP stimulation intensity threshold was defined and intraoperative testing aimed to keep MEP response amplitude constant. MEPs were considered deteriorated and the surgical team was notified whenever the threshold was elevated by ≥20mA or MEP response fell under 50%. Findings: On the first postoperative day, 18 patients experienced new paresis that resolved by discharge in 11. MEPs deteriorated in 39 patients, and 16 of these showed new postoperative paresis, indicating a 41% risk of new paresis. In the remaining 2/18 patients, intraoperative MEPs were stable, although new paresis appeared postoperatively. In one of these patients, intraoperative hemorrhage caused postoperative swelling, and the new motor deficit persisted until discharge. Of all 104 patients, 7 deteriorated in motor grade at discharge, 92 remained unchanged, and 5 patients have improved. Conclusions: Adjustment of surgical strategy contributed to good motor outcome in 33/39 patients. MEP monitoring may help significantly to prevent motor deficits during demanding neurosurgical procedures on the brainste