Geochronology and paleoenvironment of pluvial Harper Lake,Mojave Desert,California, USA

Accurate reconstruction of the paleo-Mojave River and pluvial lake (Harper, Manix, Cronese, and Mojave) system of southern California is critical to understanding paleoclimate and the North American polar jet stream position over the last 500 ka. Previous studies inferred a polar jet stream south of 35°N at 18 ka and at ~ 40°N at 17–14 ka. Highstand sediments of Harper Lake, the upstream-most pluvial lake along the Mojave River, have yielded uncalibrated radiocarbon ages ranging from 24,000 to > 30,000 ¹⁴C yr BP. Based on geologic mapping, radiocarbon and optically stimulated luminescence dating, we infer a ~ 45–40 ka age for the Harper Lake highstand sediments. Combining the Harper Lake highstand with other Great Basin pluvial lake/spring and marine climate records, we infer that the North American polar jet stream was south of 35°N about 45–40 ka, but shifted to 40°N by ~ 35 ka. Ostracodes (Limnocythere ceriotuberosa) from Harper Lake highstand sediments are consistent with an alkaline lake environment that received seasonal inflow from the Mojave River, thus confirming the lake was fed by the Mojave River. The ~ 45–40 ka highstand at Harper Lake coincides with a shallowing interval at downstream Lake Manix.     

Experiments on scalar dispersion within a model plant canopy,part III: An elevated line source

An experiment is reported in which heat was released as a passive tracer from an elevated lateral line source within a model plant canopy, with h _s = 0.85 h _c (h _s and h _c being the source and canopy heights, respectively). A sensor assembly consisting of three coplanar hot wires and one cold wire was used to measure profiles of mean temperature % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaiikamaana% aabaGaeqiUdehaaiaacMcaaaa!390C!\[(\overline \theta )\], temperature variance (Σ_θ ²), vertical and streamwise turbulent heat fluxes, and third moments of wind and temperature fluctuations. Conclusions were:

1. Despite the very heterogeneous flow within the canopy, the observed dispersive heat flux (due to spatial correlation between time-averaged temperature and vertical velocity) was small. However, there is evidence from the plume centroid (which was lower than h _s at the source) of systematic recirculating motions within the canopy.
2. The ratio % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdm3aaS% baaSqaaiabeI7aXjaab2gacaqGHbGaaeiEaaqabaGccaGGVaWaa0aa% aeaacqaH4oqCaaWaaSbaaSqaaiaab2gacaqGHbGaaeiEaaqabaaaaa!41DF!\[\sigma _{\theta {\text{max}}} /\overline \theta _{{\text{max}}} \] (of maximum values on vertical profiles) decreased from 1 near the source to an asymptotic value of 0.4 far downstream, in good agreement with previous experimental and theoretical work for concentration fluctuations in the surface layer well above the canopy.
3. The eddy diffusivity for heat from the line source (K _HL ) increased, downstream of the source, to a nearly constant ‘far-field’ vertical profile. Within the canopy, the far-field K _HL was an order of magnitude larger than K _HP , the equivalent diffusivity for a plane source; well above the canopy, the two were equal. The time scale defined by (far-field K _HL )/(vertical velocity variance) was independent of height within the canopy.
4. Budgets for temperature variance, vertical heat flux and streamwise heat flux are remarkably similar to the equivalent budgets for an elevated line source in the surface layer well above the canopy, except in the lower part of the canopy in the far field, where vertical transport is much more important than in the surface layer.
5. A random flight simulation of the mean height and depth of the temperature plume was generally in good agreement with experiment. However, details of the temperature and streamwise turbulent heat flux profiles were not correct, suggesting that the model formulation needs to be improved.

     

LABORATORY OBSERVATIONS OF SEDIMENT ENTRAINMENT BY FREEZING SUPERCOOLED WATER

Debris in basal ice produced by glaciohydraulic supercooling is typically characterized by high proportions of silt. A prominent hypothesis for this silt‐dominance is that frazil ice growing in supercooled water preferentially traps silt from sediment‐laden water percolating through it. It has therefore been suggested that silt‐dominance may be diagnostic of glaciohydraulic supercooling. The aim of our work is to test this hypothesis that freezing sediment‐laden supercooled water necessarily produces ice dominated by silt. We do this by simulating two freezing processes under laboratory conditions: (1) percolation of sediment‐laden water through frazil ice; (2) turbulent supercooling and subsequent freezing of sediment‐laden water. In experiments repeated using different particle sizes (sand, silt and clay in individual experiments) both processes entrained sand most effectively and silt least effectively. In experiments using a sediment mixture dominated by medium to coarse silt, both processes produced ice facies dominated by particle sizes between fine sand and coarse silt. These results suggest that silt‐dominance should therefore not be expected for supercooled freeze‐on, and is not a reliable diagnostic signature for supercooling. The silt‐dominated character of basal ice types associated with supercooling may result from other controls such as a silt‐dominated sediment supply or subglacial water flow rates, rather than the freezing process.     

Electromagnetic research in the ocean

The results of electromagnetic researches at sea and in the oceans are considered. Those in the U.S.S.R. are given particular attention.     

Lateral continuity of basement seismic reflections in 15 Ma ultrafast-spreading crust at ODP Site 1256

The Ocean Drilling Program (ODP) initiated drilling at Site 1256D in the Guatemala Basin, about 1,000 km off the East Pacific Rise to penetrate plutonic rocks, anticipated to be relatively shallow in this region, formed at an ultra-fast spreading rate. IODP Expedition E312 successfully drilled into gabbros at ~1,150 m in basement. Multi-channel seismic traces show weak laterally coherent sub-basement reflections at borehole depths. Synthetic reflectivity seismograms were computed using a Ricker wavelet and impedance profiles from borehole sonic logs. These seismograms show significant sub-basement amplitude peaks. A zero-offset vertical seismic profile, shot on E312, was processed to investigate the authenticity of these reflections and their relationship to borehole geology. A dual scheme of the median filtering and F–K dip filtering was used. Tests with synthetic seismograms indicate the approach is effective at reasonable SNR levels. Downgoing energy is clearly identified but negligible upgoing energy is visible over random noise. These results indicate that lava flows and igneous contacts in upper ocean crust have significant topography on lateral scales less than the Fresnel Zone (~300 m) due to igneous and tectonic processes.     

INDUSTRY-GOVERNMENT-ACADEMIC COOPERATION: POSSIBLE BENEFITS FOR GEOGRAPHY

Some current developments outside of academe and their relationship to geography are presented; comments are based on experience from a three year National Science Foundation Faculty Fellowship. Suggestions for improving our educational efforts are discussed, including new roles for the AAG, changes in teaching methods, establishment of advisory boards, and closer ties with industry and government.     

Behavior of the cosmic ray density during the initial phase of the Forbush effect

Variations in the cosmic ray density during the initial phase of the Forbush effect during the first hours after the arrival of the interplanetary shock wave have been studied with the use of data on variations in the cosmic ray density with a rigidity of 10 GV obtained by the global survey method by the world network of neutron monitors in 1957?2012. It is found that behavior of this parameter after the arrival of the shock wave demonstrates high variability. A small (~1/5 of total number), though distinct, group of Forbush effects, in which the density of the cosmic ray increases (not decreases) after the arrival of the shock wave, is defined. As a whole, the initial variation in cosmic ray density is correlated with the Forbush effect magnitude and the strength of the associated geomagnetic disturbance.     

Sedimentological analysis of two drill cores through the crater moat‐filling breccia,Flynn Creek impact structure,Tennessee

Sedimentological (line‐logging) analysis of two drill cores, FC77‐3 and FC67‐3, situated, respectively, in the northwestern and southeastern quadrants of the Flynn Creek impact structure's crater‐moat area reveals that the ~27 m thick crater moat‐filling breccia consists of three subequal parts. These parts, which were deposited during early modification stage of this marine‐target impact structure, are distinguished on the basis of vertical trends in sorting, grain size, and counts of clasts per meter in comparison with other well‐known marine‐target impact structures, namely Lockne, Tvären, and Chesapeake Bay. The lower part is interpreted to represent mainly slump deposits, and the middle part is interpreted to represent a stage intermediate between slump and marine resurge, that is, a traction flow driven by overriding suspension flow. The upper part (size graded, and relatively well sorted and fine grained) is interpreted to represent marine resurge flow only. The upper part is capped by a relatively thin and relatively fine‐grained calcarenite to calcisiltite deposit.     

Sediment deposition in the flood plain of Stemple Creek Watershed, northern California

Over the past 150 years, major land use changes have occurred in the Stemple Creek Watershed in northern California that have caused erosion to move soils from the upland to the flood plain, stream channels, and the bay. The purpose of this study is to document the recent (1954 to present) sediment deposition patterns in the flood plain area adjacent to Stemple Creek using the ¹³⁷Cesium technique. Sediment deposition ranged from 0.26 to 1.84 cm year⁻¹ for the period from 1964 to 2002 with an average of 0.85±0.41 cm year⁻¹. Sediment deposition rates were higher for the 1954 to 1964 period with a range of 0.31–3.50 cm year⁻¹ and an average of 1.29±1.04 cm year⁻¹. These data indicate that sediment deposition in the flood plain has decreased since the middle 1950s, probably related to reduction in row crop agriculture and an increase in pasturelands. This study shows that the flood plains in the Stemple Creek Watershed are a significant sink for the soils being eroded from the upland area. Given the significance of the flood plain for trapping eroded materials before they reach the stream channels or the bay, efforts need to be made to manage these flood plain areas to insure that they do not change and become a source rather than a sink for eroded materials as improved management practices on the upland areas reduce sediment input to the flood plain.