ALBERT

Description: To investigate channelization of a reactive melt in mantle rocks, we imposed a gradient in fluid pressure across a partially molten rock composed of olivine and clinopyroxene, sandwiched between a source of alkali basalt melt and a sink of porous alumina. We performed experiments at a confining pressure of 300 MPa and pore pressures of 0.1–300 MPa, resulting in fluid pressure gradients of 0–88 MPa/mm at temperatures of 1200–1250 °C. When the gradient in fluid pressure is zero, only a planar reaction layer composed of olivine + melt develops, in agreement with previous experiments. However, if the gradient in fluid pressure is greater than zero, in addition to the planar reaction layer, finger-like melt-rich channels that contain olivine + melt develop and propagate into the rock, significantly past the interface between the melt reservoir and the partially molten rock. Channelization of the melt results in a significant increase in permeability and hence in the flux of melt through the partially molten rock.

Description: Discerning the timing and pattern of late Quaternary glacier variability in the tropical Andes is important for our understanding of global climate change. Terrestrial cosmogenic nuclide (TCN) ages (48) on moraines and radiocarbon-dated clastic sediment records from a moraine-dammed lake at Nevado Huaguruncho, Peru, document the waxing and waning of alpine glaciers in the Eastern Cordillera during the past ~15 k.y. The integrated moraine and lake records indicate that ice advanced at 14.1 ± 0.4 ka, during the first half of the Antarctic Cold Reversal, and began retreating by 13.7 ± 0.4 ka. Ice retreated and paraglacial sedimentation declined until ca. 12 ka, when proxy indicators of glacigenic sediment increased sharply, heralding an ice advance that culminated in multiple moraine positions from 11.6 ± 0.2 ka to 10.3 ± 0.2 ka. Proxy indicators of glacigenic sediment input suggest oscillating ice extents from ca. 10 to 4 ka, and somewhat more extensive ice cover from 4 to 2 ka, followed by ice retreat. The lack of TCN ages from these intervals suggests that glaciers were less extensive than during the late Holocene. A final Holocene advance occurred during the Little Ice Age (LIA, ca. 0.4 to 0.2 ka) under colder and wetter conditions as documented in regional proxy archives. The pattern of glacier variability at Huaguruncho during the Late Glacial and Holocene is similar to the pattern of tropical Atlantic sea-surface temperatures, and provides evidence that prior to the LIA, ice extent in the eastern tropical Andes was decoupled from temperatures in the high-latitude North Atlantic.

Description: Alpine-style glaciation was rare in the Arctic during the last glaciation because ice sheets occupied most of the glaciated high latitudes. Due to the tight coupling of alpine-glacier fluctuations with climate, the geomorphic evidence of such fluctuations in the Brooks Range, Alaska (USA), presents a unique opportunity to study past climate changes in this portion of the Arctic. We use cosmogenic 10 Be exposure dating to directly date Last Glacial Maximum (LGM) terminal moraines and deglaciation in the central Brooks Range. 10 Be ages from moraine boulders indicate that the LGM culminated at ca. 21 ka and was followed by substantial retreat upvalley prior to a second moraine-building episode culminating at ca. 17 ka. Subsequent rapid deglaciation occurred between ca. 16 ka and 15 ka, when glaciers receded to within their Neoglacial limits. Initial deglaciation after the LGM was likely caused by ice sheet–induced atmospheric circulation changes and increasing insolation. Brooks Range glaciers largely disappeared during Heinrich Stadial 1, prior to significant warming in the North Atlantic region during the Bølling-Allerød, but coincident with global CO 2 rise. Glacier fluctuations during the late-glacial period, if any, were restricted to within their Neoglacial extents. This new chronology suggests that ice sheet–modulated atmospheric circulation and global CO 2 dominate glacial climate forcings in Arctic Alaska.

Description: The study of abrupt changes in global climate requires high-resolution records for which the connection to the climate system is well understood. Because lake systems are by their nature unique, ground truthing of geochemical measurements against directly observable physical evidence is required. The Mono Lake basin exposes multiple outcrops of lake sediments deposited during the last glacial period, providing the opportunity to reconstruct lake-level changes through stratigraphy-based interpretation of high-resolution records. Here we present a record of bulk-sediment carbonate derived from overlapping sections in three outcrops around the Mono Lake basin. We interpret this record as a reflection of lake-level variation, based on well-exposed stratigraphy and sedimentary facies changes. The co-variation of lake level with Sr isotopes measured in ostracodes is interpreted to reflect increased proportion of water supplied from the eastern basin during wet times. This high carbonate-high lake-level relationship is the opposite of the high carbonate-low lake-level relationship inferred in nearby Owens Lake, a difference attributable to extreme differences in basin geometry affecting the frequency of spilling conditions and resultant lake chemistry.

Description: The West Tahoe–Dollar Point fault (WTDPF) extends along the western margin of the Lake Tahoe Basin (northern Sierra Nevada, western United States) and is characterized as its most hazardous fault. Fallen Leaf Lake, Cascade Lake, and Emerald Bay are three subbasins of the Lake Tahoe Basin, located south of Lake Tahoe, and provide an opportunity to image primary earthquake deformation along the WTDPF and associated landslide deposits. Here we present results from high-resolution seismic Chirp (compressed high intensity radar pulse) surveys in Fallen Leaf Lake and Cascade Lake, multibeam bathymetry coverage of Fallen Leaf Lake, onshore Lidar (light detection and ranging) data for the southern Lake Tahoe Basin, and radiocarbon dates from piston cores in Fallen Leaf Lake and Emerald Bay. Slide deposits imaged beneath Fallen Leaf Lake appear to be synchronous with slides in Lake Tahoe, Emerald Bay, and Cascade Lake. The temporal correlation of slides between multiple basins suggests triggering by earthquakes on the WTDPF system. If this correlation is correct, we postulate a recurrence interval of ~3–4 k.y. for large earthquakes on the Fallen Leaf Lake segment of the WTDPF, and the time since the most recent event (~4.5 k.y. ago) exceeds this recurrence time. In addition, Chirp data beneath Cascade Lake image strands of the WTDPF offsetting the lake floor as much as ~7.5 m. The Cascade Lake data combined with onshore Lidar allow us to map the WTDPF continuously between Fallen Leaf Lake and Cascade Lake. This improved mapping of the WTDPF reveals the fault geometry and architecture south of Lake Tahoe and improves the geohazard assessment of the region.

Description: Glaciers on the world’s highest tropical mountains are among the most sensitive components of the cryosphere, yet the climatic controls that influence their fluctuations are not fully understood. Here we present the first 10 Be ages of glacial moraines in Africa and use these to assess the climatic conditions that influenced past tropical glacial extents. We applied 10 Be surface exposure dating to determine the ages of quartz-rich boulders atop moraines in the Rwenzori Mountains (~1°N, 30°E), located on the border of Uganda and the Democratic Republic of Congo. The 10 Be ages document expanded glaciers ca. 23.4 and 20.1 ka, indicating that glaciers in equatorial East Africa advanced during the global Last Glacial Maximum (ca. 26–19.5 ka). A comparison of these moraine ages with regional paleoclimate records indicates that Rwenzori glaciers expanded contemporaneously with dry and cold conditions. Recession from the moraines occurred after ca. 20.1 ka, similar in timing to a rise in air temperature documented in East African lake records. Our results suggest that, on millennial time scales, past fluctuations of Rwenzori glaciers were strongly influenced by air temperature.

Description: Travertine mounds form at the mouth of springs where CO 2 degassing drives carbonate precipitation from water flowing from depth. Building of such mounds commonly involves the successive "stratigraphic" deposition of carbonate layers that precipitate from waters rising from depth along vertical to horizontal open fissures that are episodically sealed by radiating crystals. Much more intriguing structures can also be observed, such as widespread horizontal white veins of carbonate with vertical aragonite fibers, parallel or oblique to the "stratigraphic" travertines, which extend laterally over distances of several tens of meters and could represent up to 50% of the total volume of the travertine mound. Using highly precise U-Th dating, the growth direction of these horizontal veins is shown to be from top to bottom, and this fact clearly indicates that they developed within the mound over a period of ~1000 yr for the vein analyzed. A vein growth mechanism is proposed that is able to uplift the rock above the vein thanks to the force of crystallization. The possible development of such structures in other places and the consequences of reverse growth direction when interpreting travertine data are discussed.

Description: We conducted compressional, tensile, and torsional creep experiments on fine-grained forsterite plus Ca-bearing pyroxene aggregates. A distinct microstructure with aggregation of the same phase in the direction of compression was formed in our samples after all the experiments. The stress–strain rate relationship, grain-size dependent flow strength, and the achievement of large tensile strain all indicate that samples underwent creep due to grain boundary sliding (GBS). As a result of GBS, grain-switching events allow dispersed phases to contact grains of the same phase and orient in the direction of compression. We identify similar aggregated microstructures in previously reported micrographs of polymineralic granite-origin ultramylonites. Mineral phase mixing through GBS, which helps to retain fine grain size in rocks due to grain boundary pinning, has been speculated to occur during formation of mylonites. However, our results contradict this hypothesis because mineral aggregation through GBS promotes demixing rather than mixing of the mineral phases. GBS processes alone will not promote a transformation of well-developed monomineralic bands to polymineralic bands during mylonitization.