Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

The presence of a complex bedform arrangement on the sea floor of the continental shelf in the western Amundsen Sea Embayment, West Antarctica, indicates a multi-temporal record of flow related to the activity of one or more ice streams in the past. Mapping and division of the bedforms into distinct landform assemblages reveals their time-transgressive history, which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo-ice velocity, as suggested previously. A main control on the bedform imprint is the geology of the shelf, which is divided broadly between rough bedrock on the inner shelf, and smooth, dipping sedimentary strata on the middle to outer shelf. Inner shelf bedform variability is well preserved, revealing information about local, complex basal ice conditions, meltwater flow, and ice dynamics over time. These details, which are not apparent at the scale of regional morphological studies, indicate that past ice streams flowed across the entire shelf at times, and often had onset zones that lay within the interior of the Antarctic Ice Sheet today. In contrast, highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow, and may be a truer representation of fast palaeo-ice flow in these locations. A revised model for ice streams on the shelf captures complicated multi-temporal bedform patterns associated with an Antarctic palaeo-ice stream for the first time, and confirms a strong substrate control on a major ice stream system that drained the West Antarctic Ice Sheet during the Late Quaternary.     

Reports from the Smithsonian’s Global Volcanism Network, January 2010

Information included in this summary is based on more detailed reports published in the Bulletin of the Global Volcanism Network, vol. 35, no. 1, January 2010 (on the Internet at http://www.volcano.si.edu/). Edited by scientists at the Smithsonian, this bulletin includes reports provided by a worldwide network of correspondents. The reports contain the names and contact information for all sources. Please note that these reports are preliminary and subject to change as events are studied in more detail. The Global Volcanism Program welcomes further reports of current volcanism, seismic unrest, monitoring data, and field observations.     

Steric Sea-Level Change and its Impact on the Gravity Field caused by Global Climate Change

It is sometimes assumed that steric sea-level variations do not produce a gravity signal as no net mass change, thus no change of ocean bottom pressure is associated with it. Analyzing the output of two CO₂ emission scenarios over a period of 2000 years in terms of steric sea-level changes, we try to quantify the gravitational effect of steric sea-level variations. The first scenario, computed with version 2.6 of the Earth System Climate Model developed at the University of Victoria, Canada (UVic ESCM), is implemented with a linear CO₂ increase of 1% of the initial concentration of 365 ppm and shows a globally averaged steric effect of 5.2 m after 2000 years. In the second scenario, computed with UVic ESCM version 2.7, the CO₂ concentration increases quasi-exponentially to a level of 3011 ppm and is hold fixed afterwards. The corresponding globally averaged steric effect in the first 2000 years is 2.3 m. We show, due to the (vertical) redistribution of ocean water masses (expansion or contraction), the steric effect results also in a small change in the Earth’s gravity field compared to usually larger changes associated with net mass changes. Maximum effects for computation points located on the initial ocean surface can be found in scenario 1, with the effect on gravitational attraction and potential ranging from 0.0 to −0.7·10⁻⁵ m s⁻² and −3·10⁻³ to 6·10⁻³ m² s⁻², respectively. As expected, the effect is not zero but negligible for practical applications.     

How bright is the [Si X] 1431 nm coronal emission line?

Airborne eclipse observations of the [Six] 1430.5 nm coronal emission line are reviewed, and new ground-based out-of-eclipse coronagraph observations obtained at NSO/Sacramento Peak are reported. We find that the [Six] 1430.5 nm coronal emission line brightness is less than 8 × 10⁻⁶ B⊙ in small active region corona which showed [Fexiii] 1074.7 nm emission (corrected for sky background) of about 20 × 10⁻⁶ B⊙. Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

Do Stable Isotope Data from Calcrete Record Late Pleistocene Monsoonal Climate Variation in the Thar Desert of India?

Late Pleistocene terrestrial climate records in India may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete. Petrography shows that calcrete nodules in Quaternary sediments of the Thar Desert in Rajasthan are pedogenic, with little evidence for postpedogenic alteration. The calcrete occurs in four laterally persistent and one nonpersistent eolian units, separated by colluvial gravel. Thermoluminescence and infrared- and green-light-stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units I–IV of ca. 70,000–60,000, ca. 60,000–55,000, ca. 55,000–43,000, and ca. 43,000–25,000 yr, respectively. The youngest eolian unit (V) is <10,000 yr old and contains no calcrete. Stable oxygen isotope compositions of calcretes in most of eolian unit I, in the upper part of eolian unit IV, and in the nonpersistent eolian unit, range between −4.6 and −2.1‰ PDB. These values, up to 4.4‰ greater than values from eolian units II and III, are interpreted as representing nonmonsoonal¹⁸O-enriched “normal continental” waters during climatic phases when the monsoon weakened or failed. Conversely, 25,000–60,000-yr-old calcretes (eolian units II and III) probably formed under monsoonal conditions. The two periods of weakened monsoon are consistent with other paleoclimatic data from India and may represent widespread aridity on the Indian subcontinent during isotope stages 2 and 4. The total variation in δ¹³C is 1.7‰ (0.0–1.7‰), and δ¹³C covaries positively and linearly with δ¹⁸O. δ¹³C values are highest when δ¹⁸O values indicate the most arid climatic conditions. This is best explained by expansion of C₄grasses at the expense of C₃plants at low latitudes during glacial periods when atmosphericpCO₂was lowered. C₄dominance was overridingly influenced by global change in atmosphericpCO₂despite the lowered summer rainfall.     

Solar radiation incident on Mars and the outer planets: Latitudinal,seasonal, and atmospheric effects

Calculations of the daily solar radiation incident at the tops of the atmospheres of Mars and the outer planets and its variability with latitude and season are presented in a series of figures and tables similar to those for Earth in The Smithsonian Meteorological Tables. The changes in the latitudinal and seasonal distributions of daily surface insolation during the great Martian dust storm of 1971 (when Martian atmospheric optical depth increased from about τ = 0.1 to 2.0 were significant and dramatically illustrate the effect of atmospheric aerosols on surface insolation; i.e., the mean annual daily insolation at the poles decreased by more than a factor of 100 as τ increased from 0.1 to 2.0.     

Magnetic susceptibility and ice-rafted debris in surface sediments of the Atlantic sector of the Southern Ocean

Spatial distribution of magnetic susceptibility and the gravel fraction in surface sediments in the Atlantic sector of the Southern Ocean were investigated to reconstruct source areas and recent transport pathways of magnetominerals and ice-rafted debris. Maxima of magnetic susceptibility were observed offshore from areas where mafic source rocks occur, e.g. Queen Maud Land and the northern Antarctic Peninsula. The glacigenic input of debris and subsequent redeposition of fine material by bottom and turbidity currents on the continental margins result in regional variations of the gravel and susceptibility values. In the deep sea, however, the mixing of ice-rafted debris and turbidites from distal source areas causes a homogenous distribution of the susceptibility signal. On submarine elevations such as Maud Rise and Astrid Ridge, dust input may be an additional source for magnetominerals.     

Recognition of contour-current influence in mixed contourite-turbidite sequences of the western Weddell Sea,Antarctica

Sedimentary processes and structures across the continental rise in the western Weddell Sea have been investigated using sediment acoustic and multichannel seismic data, integrated with multibeam depth sounding and core investigations. The results show that a network of channels with associated along-channel ridges covers the upper continental slope. The seismic profiles reveal that the channels initially developed as erosive turbidite channels with associated levees on their northern side due to Coriolis force. Later they were partly or fully infilled, probably as a result of decreasing turbidite activity. Now the larger ones exist as erosive turbidite channels of reduced size, whereas the smaller ones are non-erosive channels, their shape being maintained by contour current activity. Drift bodies only developed where slumps caused a distinctive break in slope inclination on the upper continental rise, which served to initiate the growth of a drift body fed by contour currents or by the combined action of turbidites and contourites. The history of sedimentation can be reconstructed tentatively by correlation of seismo-stratigraphic units with the stages of evolution of the drifts on the western side of the Antarctic Peninsula. Three stages can be distinguished in the western Weddell Sea after a pre-drift stage, which is delimited by an erosional unconformity at the top: (1) a growth stage, dominated by turbidites, with occasional occurrence of slumps during its initial phase; (2) during a maintenance stage turbiditiy-current intensity (and presumably sedimentation rate also) decreased, probably as a result of the ice masses retreating from the shelf edge, and sedimentation became increasingly dominated by contour current activity; and (3) a phase of sheeted-sequence formation. A southward decrease in sediment thickness shows that the Larsen Ice Shelf plays an important role in sediment delivery to the western Weddell Sea. This study shows that the western Weddell Sea has some characteristics in common with the southern as well as the northwestern Weddell Sea: contour currents off the Larsen Ice Shelf have been present for a long time, probably since the late Miocene, but during times of high sediment input from the shelves as a result of advancing ice masses a channel-levee system developed and dominated over the contour-current transport of sediment. At times of relatively low sediment input the contour-current transport dominated, leading to the formation of drift deposits on the upper continental rise. Seaward of areas without shelf ice masses the continental rise mainly shows a rough topography with small channels and underdeveloped levees. The results demonstrate that sediment supply is an important, maybe the controlling factor of drift development on the Antarctic continental rise.