Sea-floor spreading rate changes in the South Atlantic

A computer correlation technique was used to deduce the spreading history of the Mid-Atlantic Ridge from 5 magnetic profiles between 28°S and 43°S. In general, several possible histories are indicated for each profile involving changes of spreading rate and faulting, some of which are easily overlooked by the visual method. The only spreading history that was consistent will all the profiles required spreading at approximately 2.2 cm yr^-1 from 11 m.y.b.p. to approximately 5.5 m.y.b.p., followed by a decrease in rate to 1.7 cm yr^-1 relative to the Vine (1966) magnetic reversal model based on the South Pacific. Comparison of the data with other reported spreading rate discontinuities suggests that the South Pacific may be reponsible for the reported spreading rate changes.     

Light regulation of benthic sulfate reduction rates mediated by seagrass (Thalassia testudinum) metabolism

The effects of light reduction on community metabolism and sediment sulfate reduction rates (SRR) were assessed experimentally in a shallow (<2.0 m) seagrass (Thalassia testudinum) meadow along Florida's north-central Gulf coast. Nine experimental plots (1.5 m×1.5 m) were shaded differentially to achieve a 0–90% gradient in light reduction within the seagrass meadow. Gross primary production and net community production (NCP), estimated with in situ benthic chamber incubations, decreased with increasing light reduction. The compensation irradiance for community metabolism, i.e., the shading level at which NCP shifted from net autotrophic to net heterotrophic, was determined to be 52.5% of the incoming irradiance at canopy height in the seagrass bed (308.7 μE m⁻² s⁻¹ PAR at noon). Sediment SRR, determined with the use of a³⁵S−SO₄ ²⁻ radiotracer technique, increased quickly (within 5 d) and markedly with increased shade, i.e., simulated light reduction. SRR increased 50-fold when shading exceeded the light compensation point for the seagrass community, rendering the community net heterotrophic. Five days after restoring ambient light conditions, SRR had decreased sharply for all shading treatments. The observed decrease in NCP, coincident with the increase in the SRR with light reduction, suggests that light reduction has an indirect influence on sediment SRR mediated through its effect on seagrass metabolism.     

AVIATR—Aerial Vehicle for In-situ and Airborne Titan Reconnaissance

We describe a mission concept for a stand-alone Titan airplane mission: Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR). With independent delivery and direct-to-Earth communications, AVIATR could contribute to Titan science either alone or as part of a sustained Titan Exploration Program. As a focused mission, AVIATR as we have envisioned it would concentrate on the science that an airplane can do best: exploration of Titan??s global diversity. We focus on surface geology/hydrology and lower-atmospheric structure and dynamics. With a carefully chosen set of seven instruments??2 near-IR cameras, 1 near-IR spectrometer, a RADAR altimeter, an atmospheric structure suite, a haze sensor, and a raindrop detector??AVIATR could accomplish a significant subset of the scientific objectives of the aerial element of flagship studies. The AVIATR spacecraft stack is composed of a Space Vehicle (SV) for cruise, an Entry Vehicle (EV) for entry and descent, and the Air Vehicle (AV) to fly in Titan??s atmosphere. Using an Earth-Jupiter gravity assist trajectory delivers the spacecraft to Titan in 7.5 years, after which the AVIATR AV would operate for a 1-Earth-year nominal mission. We propose a novel ??gravity battery?? climb-then-glide strategy to store energy for optimal use during telecommunications sessions. We would optimize our science by using the flexibility of the airplane platform, generating context data and stereo pairs by flying and banking the AV instead of using gimbaled cameras. AVIATR would climb up to 14?km altitude and descend down to 3.5?km altitude once per Earth day, allowing for repeated atmospheric structure and wind measurements all over the globe. An initial Team-X run at JPL priced the AVIATR mission at FY10 $715M based on the rules stipulated in the recent Discovery announcement of opportunity. Hence we find that a standalone Titan airplane mission can achieve important science building on Cassini??s discoveries and can likely do so within a New Frontiers budget.     

Standard and Non-Standard Solar Models

We summarize the physical input and assumptions commonly adopted in modern standard solar models that also produce good agreement with solar oscillation frequencies. We discuss two motivations for considering non-standard models: the solar neutrino problem and surface lithium abundance problem. We begin to explore the potential for mixed core models to solve the neutrino problem, and compare the structure, neutrino flux, and oscillation frequency predictions for several models in which the inner 25% of the radius is homogenized, taking into account the effects of non-local equilibrium abundances of ³He. The results for the neutrino flux and helioseismic predictions are far from satisfactory, but such models have the potential to reduce the predicted ⁷Be/⁸B neutrino flux ratio, and further studies are warranted. Finally, we discuss how much the neutrino problem can be alleviated in the framework of the standard solar model by using reaction rates, abundances and neutrino capture cross-sections at the limits of their uncertainties, while still satisfying the constraints of helioseismology.     

Secular and orbital variability of Cir X-1 observed in optical spectra

We have observed variations in the optical emission lines from the X-ray binary Circinus X-1. These variations may be attributed both to orbital variations and to long term secular changes in line strength. We have detected double-peaked H α emission lines on two occasions, providing the first direct evidence for an accretion disc in the system. The separation of the peaks was different on the two occasions, suggesting that the disc might have a different size. The equivalent width of the emission lines dropped by more than a factor of three between 1999 and 2000; this continues the trend seen in earlier data, so that the H α equivalent width has now declined by a factor of 20 since 1976. The emission lines do not appear to show signature of orbital motion, except for the data taken near phase 0, which show a significant velocity shift.
We have observed an absorption component to the He  i lines on one occasion. We suggest that, unlike the P Cygni profiles seen in X-ray spectra, this absorption does not arise in the accelerating zone of a radiatively driven wind. Instead, the absorption arises in material previously ejected from the system. It was only seen on this one occasion because the strength of the emission line had dropped dramatically.     

VLA Measurements of Faraday Rotation through Coronal Mass Ejections

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the Sun, which play an important role in space weather. Faraday rotation is the rotation of the plane of polarization that results when a linearly polarized signal passes through a magnetized plasma such as a CME. Faraday rotation is proportional to the path integral through the plasma of the electron density and the line-of-sight component of the magnetic field. Faraday-rotation observations of a source near the Sun can provide information on the plasma structure of a CME shortly after launch. We report on simultaneous white-light and radio observations made of three CMEs in August 2012. We made sensitive Very Large Array (VLA) full-polarization observations using 1?–?2 GHz frequencies of a constellation of radio sources through the solar corona at heliocentric distances that ranged from 6?–?\(15~\mathrm{R}_{\odot}\). Two sources (0842+1835 and 0900+1832) were occulted by a single CME, and one source (0843+1547) was occulted by two CMEs. In addition to our radioastronomical observations, which represent one of the first active hunts for CME Faraday rotation since Bird et al. (Solar Phys., 98, 341, 1985) and the first active hunt using the VLA, we obtained white-light coronagraph images from the Large Angle and Spectrometric Coronagraph (LASCO) C3 instrument to determine the Thomson-scattering brightness [\(\mathrm{B}_{\mathrm{T}}\)], providing a means to independently estimate the plasma density and determine its contribution to the observed Faraday rotation. A constant-density force-free flux rope embedded in the background corona was used to model the effects of the CMEs on \(\mathrm{B}_{\mathrm{T}}\) and Faraday rotation. The plasma densities (\(6\,\mbox{--}\,22\times10^{3}~\mbox{cm}^{-3}\)) and axial magnetic-field strengths (2?–?12 mG) inferred from our models are consistent with the modeling work of Liu et al. (Astrophys. J., 665, 1439, 2007) and Jensen and Russell (Geophys. Res. Lett., 35, L02103, 2008), as well as previous CME Faraday-rotation observations by Bird et al. (1985).     

Dissipation of Titan's north polar cloud at northern spring equinox

Saturn's Moon Titan has a thick atmosphere with a meteorological cycle. We report on the evolution of the giant cloud system covering its north pole using observations acquired by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft. A radiative transfer model in spherical geometry shows that the clouds are found at an altitude between 30 and 65 km. We also show that the polar cloud system vanished progressively as Titan approached equinox in August 2009, revealing at optical wavelengths the underlying sea known as Kraken Mare. This decrease of activity suggests that the north-polar downwelling has begun to shut off. Such a scenario is compared with the Titan global circulation model of Rannou et al. (2006), which predicts a decrease of cloud coverage in northern latitudes at the same period of time.     

Phase relationship between sea level and abrupt climate change

Direct traces of past sea levels are based on the elevation of old coral reefs at times of sea level highstands. However, these measurements are discontinuous and cannot be easily correlated with climate records from ice cores. In this study we show a new approach to recognizing the imprint of sea level changes in continuous sediment records taken from the continental slope at locations that were continuously submerged, even during periods of sea level lowstand. By using a sediment core precisely synchronized with Greenland ice cores, we were able to recognize major floods of the Mediterranean continental shelf over the past 270 kyr. During the last glacial period five flooding events were observed at the onset of the warmest Greenland interstadials. Consistent correspondence between warm climate episodes and eustatic sea level rises shows that these global flooding events were generated by pronounced melting of the Northern Hemisphere ice sheets, due to rapid intensification of Atlantic Meridional Overturning Circulation.The method described in this study opens a new perspective for inter-hemispheric synchronization of marine climate records if applied in other continental margins from the Southern Hemisphere or the equatorial regions.     

Predicting contaminant fate and transport in sediment caps: Mathematical modelling approaches

Sediment capping is a remedial option for managing contaminated sediments that involves the artificial placement of a layer of material over a contaminated area. Sorbent materials such as activated C and coke can be used to amend sand caps to improve cap performance. In this study, analytical and numerical modelling approaches were compared for predicting contaminant fate and transport in sediment caps using several diffusion-controlled and advection-dominated contaminant transport scenarios. An analytical tool was used to predict cap performance at steady-state. These results were compared with the results from the numerical CoReTranS model in which the effective diffusivity and degradation rates were modelled as discontinuous functions at a prescribed bioturbation depth. The numerical approach was also applied to modelling a sorptive cap. It was shown that, while the analytical approach can be used to predict steady-state contaminant transport, the numerical approach is needed to evaluate multiple sediment layers with different transport and sorption characteristics and to examine the transient performance between the time that the single layer transient is applicable (i.e., before penetration of the cap containment layer) and until steady-state in the upper layer. For the 30 cm thick sand cap that was considered in this study, the predicted time to reach steady-state conditions for a diffusion-controlled scenario is 1 ka. For an advection-dominated transport, the time to reach steady-state conditions is reduced to 100 a. The activated C-amended sand cap was more effective in isolating the contaminant within the sorbent layer for a sustained period of time (∼100 a). Results from both modelling approaches showed that capping can effectively reduce contaminant flux to the overlying water with critical variables being cap thickness, groundwater velocity, and sediment sorptivity.