Sedimentology of the debris-flow-dominated Warm Spring Canyon alluvial fan, Death Valley, California

Facies analysis of widely distributed exposures of the 32·6 km² and 8·1-km-long Warm Spring Canyon fan, central Death Valley, shows that it has been built principally by debris-flow deposits. These deposits were derived from a mature Panamint Range catchment mostly underlain by Precambrian mudrock, quartzite and dolomite. Stacked, clast-rich and matrix-supported debris-flow lobes of slightly bouldery, muddy, pebble–cobble gravel in beds 20–150 cm thick dominate the fan from apex to toe, accounting for 75–98% of most exposures. Interstratified with the debris flows are less abundant (2–25% of cuts), thinner (5–30 cm) and more discontinuous beds of clast-supported and imbricated, pebble–cobble gravel deposited by overland flows and gully flows. This facies formed by the surficial fine-fraction water winnowing of the debris flows primarily during recessional flood stage of the debris-flow events. Two other facies associations make up a small part of the fan. The incised-channel tract consists of a 250-m-wide clast-supported ribbon of irregularly to thickly bedded, boulder, pebble, cobble gravel nested within debris-flow deposits. This channel fill is oriented generally perpendicular to the Panamint range front. It formed by extensive erosion and winnowing of debris flows deposited within the incised channel, into which all water discharge from the catchment is funnelled. The limited presence of this facies only straddling the present incised channel indicates that this channel overall has maintained a consistent position on the fan except for slight lateral shifts, some caused by strike-slip offset. Fault offset temporarily closed the upper incised channel, causing recessional debris-flow mud to be ponded behind the dam. The other local facies assemblage consists of subrounded to rounded, moderately sorted pebble gravel in low-angle cross-beds that slope both basinwards and fanwards. This gravel was deposited in beachface, backshore and shoreface barrier-spit environments that developed where Lake Manly impinged on the Warm Spring fan during late Pleistocene time. These deposits straddle headcuts into, and were derived from, erosion of the debris-flow deposits. Wave energy sorted finer sediment from the shore zone, concentrated coarser sediment and rounded the coarse to very coarse pebble fraction by selective reworking.     

西藏纳木错湖相沉积的铀系年代学研究

纳木错是西藏海拔最高的大湖,位于藏北内流区东南部.研究中应用铀系全溶样品的等时线技术所测定的纳木错西北岸3个剖面的8个富含碳酸盐湖相沉积物样品的年龄.其实验程序是:先按矿物的粒径将样品分成若干子样品,全溶子样品并分别测试它们的230Th/232Th和234U/232Th比值,然后通过年龄等时线来校正由碎屑物带来的初始钍的影响,从而避免了常用的稀酸淋滤法难于验证的同位素分馏问题.测试结果显示,在等时线上所有数据都有良好的线性关系,表明所获得的年龄数据是合理的.由此可以初步认为,纳木错拔湖47.5 m以上的高位湖相沉积, 形成于90.7±9.9~71.8±8.5 ka B P间的晚更新世早期;而在拔湖42.8 m、27.7 m、23 m、17 m和10.7 m的第六、五、四、三和二级阶地,则分别形成于53.7±4.2 ka B P、41.2±4.7 ka B P、35.2±3.0 ka B P、32.3±4.4 ka B P和28.2±2.8 ka B P左右的晚更新世中晚期.     

Sedimentary processes and facies of the waterlaid Anvil Spring Canyon alluvial fan, Death Valley, California

The Anvil Spring Canyon fan of the Panamint Range piedmont in central Death Valley was built entirely by water-flow processes, as revealed by an analysis of widespread 2- to 12-m-high stratigraphic cuts spanning the 9·7 km radial length of this 2·5–5·0° sloping fan. Two facies deposited from fan sheetfloods dominate the fan from apex to toe. The main one (60–95% of cuts) consists of sandy, granular, fine to medium pebble gravel that regularly and sharply alternates with cobbly coarse to very coarse pebble gravel in planar couplets 5–25 cm thick oriented parallel to the fan surface. The other facies (0–25% of cuts) comprises 10- to 60-cm-thick, wedge-planar and wedge-trough beds of pebbly sand and sandy pebble gravel in backsets sloping 3–28°. Both facies are interpreted as resulting from rare, sediment-charged flash floods from the catchment, and were deposited by supercritical standing waves of expanding sheetfloods on the fan. Standing waves were repeatedly initiated, enlarged, migrated, and then terminated either by gradually rejoining the flood or by more violent breakage and washout. The frequent autocyclic growth and destruction of standing waves during an individual sheetflood resulted in the deposition of multiple coarse and fine couplet and backset sequences 50–250 cm thick across the active depositional lobe of the fan. Erosional intensity during washout of the standing wave determined whether early-phase backset-bed deposits or washout-phase sheetflood couplet deposits were selectively preserved in a given cycle. Two minor facies are also found in the Anvil fan. Pebble–cobble gravel lags (0–20% of cuts) are present above erosional scours into the sheetflood couplet and backset deposits. They consist of coarse gravel concentrated through fine-fraction winnowing of the host sheetflood facies by sediment-deficient water flows. This reworking occurred during recessional flood stage or from non-catastrophic discharge during the long intervals between major flash floods. This facies is common at the surface, giving rise to a ‘braided-stream’ appearance. However, it is stratigraphically limited, present as thin, continuous to discontinuous beds or lenses that bound 50- to 250-cm-thick sheetflood sequences. The other minor facies of the Anvil fan consists of clast-supported and imbricated, thickly stratified, pebbly, cobbly, boulder gravel present in narrow, radially aligned ribbons nested within sheetflood deposits. This facies is interpreted as representing deposition in the incised channel of the fan, a subenvironment characterized by greater flow competence resulting from maintained depth from channel-wall confinement, and by more frequent water flows and winnowing events caused by its direct connection with the catchment feeder channel.     

Cause of dominance by sheetflood vs. debris-flow processes on two adjoining alluvial fans, Death Valley, California

Two large, adjoining alluvial fans of the Panamint Range piedmont, Death Valley, California, are composed of different facies assemblages deposited by contrasting sedimentary processes. The Anvil Spring fan was built solely by water-flow processes (incised-channel floods and sheetfloods), whereas the neighbouring Warm Spring fan has been constructed principally by debris flows. The boundary between these fans delineates a sharp provincial piedmont contact between sheetflood-dominated fans to the south and debris-flow-dominated fans to the north. Factors such as climate, catchment area, fan area, catchment relief, aspect, vegetation types and density, and neotectonic setting are essentially identical for these two fans. The key difference between them is that their catchments are underlain by dissimilar bedrock types, which weather to yield distinctive sediment suites. Weathering of the granite and andesite of the Anvil fan catchment produces significant volumes of medium to very coarse sand, granules, pebbles, cobbles and boulders, but minimal silt and clay. In contrast, the shale, quartzite and dolomite that dominate bedrock in the Warm Spring catchment weather to yield a wide suite of sedimentary particles spanning from clay to boulders. The abundance of mud, and the unsorted character of the yielded sediment, cause precipitation-induced slope failures in the Warm Spring catchment to transform readily into debris flows. This propensity is due to the low permeability of the colluvial sediment, which causes added water to become trapped quickly and pore pressure to rise rapidly, promoting transformations to debris flows. In contrast, the limited volume of sediment finer than medium sand yielded from the Anvil fan catchment causes the colluvium to have high permeability. This factor prevents the transformation of wet colluvium to a debris flow during hydrologically triggered slope failures, instead maintaining sediment transport as entrained bed load or suspended load in a water flow.     

西藏扎布耶湖晚更新世沉积物230 年代学研究

本文应用全溶样品的等时线模式,较成功地测定了藏北高原西南部扎布耶盐湖SZK01孔岩芯中含不等量碳酸盐粘土的230Th/238U年龄,建立了青藏高原腹地海拔4000 m以上湖泊120 ka以来连续的同位素年龄标尺,从而为扎布耶盐湖及其周缘地区古环境古气候演化研究奠定了时间坐标。根据230Th/238U年龄数据,SZK01孔的平均沉积速率约为68 cm/ka。然而不同层段的沉积速率差异较大,变化在20.8~128.8 cm/ka,其快慢变化反映了该湖从短命深湖、动荡浅湖、滨湖到盐湖的环境变化,与高分辨率SZK02孔的研究结果相一致。讨论了测年模式的适应性﹑样品的U封闭性和应用的优越性,提出开展湖区及周边各类型水体的U、Th同位素化学行为研究和检验沉积物中多源Th对定年的影响将是今后年代学研究的重要内容。     

Are the Benches at Mormon Point, Death Valley, California, USA, Scarps or Strandlines?

The benches and risers at Mormon Point, Death Valley, USA, have long been interpreted as strandlines cut by still-stands of pluvial lakes correlative with oxygen isotope stage (OIS) 5e/6 (120,000–186,000 yr B.P.) and OIS-2 (10,000–35,000 yr B.P.). This study presents geologic mapping and geomorphic analyses (Gilbert's criteria, longitudinal profiles), which indicate that only the highest bench at Mormon Point (90 m above mean sea level (msl)) is a lake strandline. The other prominent benches on the north-descending slope immediately below this strandline are interpreted as fault scarps offsetting a lacustrine abrasion platform. The faults offsetting the abrasion platform most likely join downward into and slip sympathetically with the Mormon Point turtleback fault, implying late Quaternary slip on this low-angle normal fault. Our geomorphic reinterpretation implies that the OIS-5e/6 lake receded rapidly enough not to cut strandlines and was 90 m deep. Consistent with independent core studies of the salt pan, no evidence of OIS-2 lake strandlines was found at Mormon Point, which indicates that the maximum elevation of the OIS-2 lake surface was −30 m msl. Thus, as measured by pluvial lake depth, the OIS-2 effective precipitation was significantly less than during OIS-5e/6, a finding that is more consistent with other studies in the region. The changed geomorphic context indicates that previous surface exposure dates on fault scarps and benches at Mormon Point are uninterpretable with respect to lake history.     

Using fossil spring deposits in the Death Valley region,USA to evaluate palaeoflowpaths

Spring deposits reveal the timing and environment of past groundwater discharge. Herein, however, the potential for fossil spring deposits to infer water sources and palaeoflowpaths through trace elements and stable and radiogenic isotopes is examined. Past discharge (70 to 285 ka) in the Tecopa Basin in the Death Valley region of southeastern California is represented by tufa deposits, including mounds, pools, cemented ledges and rare calcite feeder veins. δ¹⁸O values indicate that spring discharge was a mixture of far‐travelled (regional) water with a significant, and perhaps dominant contribution of local recharge on a nearby range front and alluvial pediment, rather than simply representing an elevated regional water table. δ¹³C values indicate regional water had a high TDS, whereas solute data imply low overall solute contents, consistent with dilution by a large component of local recharge. Radiogenic isotope data (U‐series, ⁸⁷Sr/⁸⁶Sr) for tufa indicate that siliciclastic rocks (a regional aquitard) interacted with discharging water. To access this aquitard, regional flow was probably partitioned into a permeable north–south damage zone of a north–south range‐bounding fault along the foot of the Resting Spring Range, which ultimately controlled the location of groundwater discharge. Existing models for modern discharge in the Tecopa Basin, by contrast, call upon westward interbasin flow in carbonate rocks from the Spring Mountains through the intervening (and nearly perpendicular) Nopah and Resting Spring Ranges. Understanding the controls on regional groundwater flow is critical in this and other arid regions where water is, by definition, a scarce resource. Thus, although it is a case study, this report highlights a fruitful approach to palaeohydrology that can be widely applied. Copyright © 2007 John Wiley & Sons, Ltd.     

Measuring alluvial fan sensitivity to past climate changes using a self‐similarity approach to grain‐size fining,Death Valley,California

The effects of climate change on eroding landscapes and the terrestrial sedimentary record are poorly understood. Using mountain catchment–alluvial fan systems as simple analogues for larger landscapes, a wide range of theoretical studies, numerical models and physical experiments have hypothesized that a change in precipitation rate could leave a characteristic signal in alluvial fan sediment flux, grain size and down‐system fining rate. However, this hypothesis remains largely untested in real landscapes. This study measures grain‐size fining rates from apex to toe on two alluvial fan systems in northern Death Valley, California, USA, which each have well‐exposed modern and ca 70 ka surfaces, and where the long‐term tectonic boundary conditions can be constrained. Between them, these surfaces capture a well‐constrained temporal gradient in climate. A grain‐size fining model is adapted, based on self‐similarity and selective deposition, for application to these alluvial fans. This model is then integrated with cosmogenic nuclide constraints on catchment erosion rates, and observed grain‐size fining data from two catchment‐fan systems, to estimate the change in sediment flux from canyon to alluvial fan that occurred between mid‐glacial and modern interglacial conditions. In a fan system with negligible sediment recycling, a ca 30% decrease in precipitation rate led to a 20% decrease in sediment flux and a clear increase in the down‐fan rate of fining, supporting existing landscape evolution models. Consequently, this study shows that small mountain catchments and their alluvial fan stratigraphy can be highly sensitive to orbital climate changes over <10⁵ year timescales. However, in the second fan system it is observed that this sensitivity is completely lost when sediment is remobilized and recycled over a time period longer than the duration of the climatic perturbation. These analyses offer a new approach to quantitatively reconstructing the effects of past climate changes on sedimentation, using simple grain‐size data measured in the field.     

The hydrogeochemistry of a heavily used aquifer in the Mexican wine-producing Guadalupe Valley, Baja California

The Guadalupe Valley aquifer is the only water source for one of the most important wine industries in Mexico, and also the main public water supply for the nearby city of Ensenada. This groundwater is monitored for major ion, N-NO₃, P-PO₄, Fe, As, Se, Mo, Cd, Cu, Pb, Zn and Sb concentrations, as well as TDS, pH, dissolved oxygen and temperature. High concentrations of N-NO₃ (26 mg l⁻¹), Se (70 μg l⁻¹), Mo (18 μg l⁻¹) and Cu (4.3 μg l⁻¹) suggest that groundwater is being polluted by the use of fertilizers only in the western section of the aquifer, known as El Porvenir graben. Unlike the sites located near the main recharge area to the East of the aquifer, the water in El Porvenir graben has low tritium concentrations (<1.9 TU), indicating a pre-modern age, and thus longer water residence time. No significant variations in water quality (generally <10%) were detected throughout 2001–2002 in the aquifer, suggesting that reduced rainfall and recharge during this dry period did not significantly affect water quality. However, the wells nearest to the main recharge area in the Eastern aquifer show a slight but constant increase in TDS with time, probably as a result of the high (∼200 L S⁻¹) uninterrupted extraction of water at this specific recharge site. Relatively high As concentrations for the aquifer (10.5 μg l⁻¹) are only found near the northern limit of the basin associated with a geological fault.     

长江上源布曲盆地晚更新世湖相沉积的发现及其古植被与古气候

在青海省西南部青藏公路东南侧的通天河支流——布曲北岸 (33°4 4′4 5 .2″N,92°11′2 8.1″E) ,发现一条拔河 0～ 18m的湖相沉积剖面。湖相沉积物的 U系测年结果为 88.9～ 5 6 .2 ka,确定其沉积时代为晚更新世早中期。结合孢粉分析结果 ,本文探讨了青海省西部地区通天河及布曲流域晚更新世早中期的古植被与古气候。初步研究表明 ,在该剖面的下部和上部沉积期间 ,当地生长松、桦、冷杉等乔木和蒿等草本植物 ,气候比较温暖湿润。剖面中部沉积期间 ,蒿、藜、禾本科和麻黄等草原植被明显增多 ,气候凉爽干燥。剖面沉积期间气候曾经历过温暖湿润和凉爽干燥的波动。植被与气候的变化具有从末次间冰期晚期经末次冰期早冰段到末次冰期间冰段中期的特征     

Chronology of latest Pleistocene lake‐level fluctuations in the pluvial Lake Chewaucan basin,Oregon, USA

New accelerator mass spectrometer radiocarbon ages from gastropods in shore deposits within the pluvial Lake Chewaucan basin, combined with stratigraphical and geomorphological evidence, identify an abrupt rise and fall of lake level at ca. 12 ¹⁴C ka. The lake‐level high is coeval with lake‐level lows in the well‐dated records of palaeolakes Bonneville and Lahontan, and with a period of relatively wet conditions in the more southerly Owens Lake basin. This spatial pattern of pluvial lake levels in the western USA at 12 ¹⁴C ka indicates a variable synoptic response to climate forcing at this time. Copyright © 2001 John Wiley & Sons, Ltd.     

Shoreline tufa and tufaglomerate from Pleistocene Lake Bonneville,Utah, USA: stable isotopic and mineralogical records of lake conditions,processes, and climate

Shoreline carbonate deposits of Pleistocene Lake Bonneville record the conditions and processes within the lake, including the evaporative balance as well as vertical and lateral chemical and isotopic gradients. Tufas (swash‐zone) and tufaglomerates (cemented, subaqueous colluvium or beachrock) on multiple, well‐developed shorelines near the Silver Island Range, Utah, also present an opportunity to examine physicochemical lake processes through time. Three shorelines are represented by carbonate deposits, including the 23–20 ka Stansbury stage, 15–14.5 ka Bonneville stage, and 14.5–14 ka Provo stage. Mean δ¹⁸O_VSMOW values of all three shorelines are statistically indistinguishable ( ～ 27 ± 1‰), when a few Bonneville samples of unusual composition are neglected. However, differences in primary carbonate mineralogy indicate that the correspondence is an artefact of the different fractionation factors between calcite or aragonite and water. Second, in order to sustain a much smaller, shallower lake during the colder Stansbury stage, the climate must have also been relatively dry. Third, δ¹⁸O values in tufa are higher than tufaglomerate by ～ 0.5‰, consistent with greater evaporative enrichment of lake water in the swash zone. Fourth, mean δ¹³C values for the Provo, Stansbury and Bonneville shorelines (4.4, 5.0 and 5.2‰, respectively) show that carbon species were dominated by atmospheric exchange, with the variations produced by differences in the oxidation of organic matter. Comparisons of shoreline carbonates with deep‐lake marls of the same approximate age indicate that shoreline carbonate was much higher in δ¹³C and δ¹⁸O values (both ～ 2.5‰) during Bonneville time, whereas isotopic differences were minor (both ～ 1‰) in Stansbury time. In particular, the Bonneville stage may have sustained large vertical or lateral isotopic gradients due to evaporative enrichment effects on δ¹⁸O values. In contrast, the lake during the much shallower Stansbury stage may have been well mixed. Differences in the primary mineralogy (Stansbury and Bonneville, aragonite > calcite; Provo, calcite > aragonite) reflect profound differences in lake chemistry in terms of open versus closed‐basin lakes. The establishment of a continuous outlet during Provo time probably reduced the Mg²⁺/Ca²⁺ ratio of lake water. Curiously, regardless of primary mineralogy, tufaglomerate cements are enriched in Na⁺ and Cl^? and depleted in Mg²⁺ relative to capping tufa of the same age. This probably reflects vital or kinetic effects in the swash zone (tufa). We suspect that ‘abiotic’ effects may have been important in the dark pore space of developing tufaglomerate, where the absence of light suppressed photosynthesis. Copyright © 2005 John Wiley & Sons, Ltd.     

西藏谷露热泉型铯矿床年代学及意义

谷露间歇喷泉位于西藏那曲地区那曲县谷露乡西北的桑曲西岸,海拔4700～4750m,现今仍在强烈活动中,其以硅华为主的泉华沉积形成了热泉型中型硅华铯矿床,属印度-亚洲陆陆碰撞效应的产物。本文根据野外第四纪地质与地貌的系统调查研究,将硅华分为南、北两区共5套沉积。由泉胶砾岩组成的第Ⅰ套硅华下伏于南区大硅华台地之下,并越过桑曲而延伸至河东,构成倒数第二次冰期(爬然冰期)冰水沉积的基座。第Ⅱ套硅华组成南区的大硅华台地。由泉胶砾岩组成的第Ⅲ套硅华下伏于北区硅华台地之下,也越过桑曲延伸至河东,构成末次冰期(拉曲冰期)冰水沉积的基座。第Ⅳ套硅华构成北区硅华台地,已被一东西向小沟分为南、北两个小片,均由数列小的硅华锥组成。第Ⅴ套硅华则为现今仍在活动和堆积的硅华锥。根据9个样品的U系法测年结果,并参考野外地貌与第四纪地质的研究结果,可将这5套硅华的铯矿床的形成,划分为如下5个阶段:第1阶段虽暂缺年龄数据,但可按地貌部位和地层层序,判断其堆积于倒数第三次冰期(宁中冰期)以后,很可能为0.5～0.4MaBP之间的大间冰期早中期;第2阶段为0.38～0.25MaBP,相当于大间冰期晚期至爬然冰期早期;第3阶段为0.22MaBP前后的爬然冰期早期;第4阶段为108.6～17.2kaBP的晚更新世的末次间冰期晚期至拉曲冰期;第5阶段为5.3kaBP的全新世中期以来。由老到新,各期泉华的SiO2含量呈现出明显的降低趋势,而铯含量则呈升高趋势,是随着矿石中SiO2有序度的降低而其他成分被保留在晶格中所致。     

Laminated sediments in the Bay of La Paz, Gulf of California: a depositional cycle regulated by pluvial flux

Abstract Core BAP96‐CP, sampled from the deepest part of the Bay of La Paz, Gulf of California, has been analysed sedimentologically taking into account regional climate and oceanography. Laminated sediments at the bottom of the bay are essentially not bioturbated by benthic fauna. A subanoxic condition (O₂ < 0·2 mL L^?1) inhibits the proliferation of benthic fauna. Within the bay, the relative abundances of terrigenous and biogenic inputs change periodically. The terrigenous input is greater than the biogenic input and apparently experiences larger fluctuations. The terrigenous input dominates in dark laminae, whereas the biogenic input mostly occurs in light laminae. Thus, it is assumed that, down the core, the alternation of dark and light laminae represents cycles in the extent of dilution of the biogenic input by terrigenous input. The terrigenous input into the Bay of La Paz is mostly regulated by pluvial runoff. Thus, its temporal fluctuation follows the periods shown by the regional pluvial regime, particularly the 11·2 year period. This is equal to the frequency of sunspot cycles.     

Geomorphological analysis of neotectonic deformation,northern Owens Valley,California

At the western edge of the Basin and Range Province, the Owens Valley is the site of active seismicity and deformation. Morphometric analyses of three geomorphological features are used to determine the patterns and rates of neotectonic deformation: (l) a network of Pleistocene channels cut on top of the Bishop Tuff; (2) uplifted terraces of the Owens River; and (3) alluvial fans of the White Mountain front.In the Owens Valley, the three analyses are consistent with the same solution: net eastward tilt of the Owens Valley block at a rate of between 3.5 and 6.1°/Ma. Given the dip on the basement determined from geophysical data and extrapolating the rate of tilt in the Owens Valley back in time, it is inferred that the break-up of the Sierra Nevada and the northern Owens Valley occurred in the Pliocene, between around 2 and 4 Ma ago. The pattern of deformation in the northern Owens Valley matches anticlinal flexure on the Coyote warp, near the front of the Sierra Nevada, and faulting across the Volcanic Tableland is consistent with flexural extension. It is proposed that the Coyote warp is an expression of the tectonic hinge between westward rotation of the Sierra Nevada and eastward rotation of the Owens Valley since the Pliocene.     

四川九寨沟地区金矿床特征与找矿远景

九寨沟地区金矿床主要赋存于三叠系、寒武一志留系、泥盆系浊积岩中,并受构造控制。矿石矿物以多金属硫化物为主,见有孔雀石、铜蓝和褐铁矿等次生矿物及各种氧化物类,主成矿元素金的资源量可达特大型以上,属于构造岩浆热液型金矿床。该区及外围均具有很大的找矿潜力。     

The Helium Isotopic Chemistry of Lake Bonney, Taylor Valley, Antarctica: Timing of Late Holocene Climate Change in Antarctica

To better understand the long-term climate history of Antarctica, we studied Lake Bonney in Taylor Valley, Southern Victoria Land (78°S). Helium isotope ratios and He, Ne, Ar and N₂ concentration data, obtained from hydrocasts in the East (ELB) and West (WLB) Lobes of Lake Bonney, provided important constraints on the lakes Holocene evolution. Based on very low concentrations of Ar and N₂ in the ELB bottom waters, ELB was free of ice until 200 ± 50 years ago. After which, low salinity water flowing over the sill from WLB to ELB, covered ELB and formed a perennial ice cover, inhibiting the exchange of gases with the atmosphere. In contrast to the ELB, the WLB retained an ice cover through the Holocene. The brine in the WLB bottom waters has meteoric N₂ and Ar gas concentrations indicating that it has not been significantly modified by atmospheric exchange or ice formation. The helium concentrations in the deep water of WLB are the highest measured in non-thermal surface water. By fitting a diffusional loss to the ³He/⁴He, helium, and Cl profiles, we calculate a time of 3000 years for the initiation of flow over the sill separating the East and West Lobes. To supply this flux of helium to the lake, a helium-rich sediment beneath the lake must be providing the helium by diffusion. If at any time during the last million years the ice cover left WLB, there would be insufficient helium available to provide the current flux to WLB. The variations in water levels in Lake Bonney can be related to climatic events that have been documented within the Southern Victoria Land region and indicate that the lakes respond significantly to regional and, perhaps, global climate forcing.     

Fish Remains from Homestead Cave and Lake Levels of the Past 13,000 Years in the Bonneville Basin

A late Quaternary ichthyofauna from Homestead Cave, Utah, provides a new source of information on lake history in the Bonneville basin. The fish, represented by 11 freshwater species, were accumulated between 11,200 and 1000 ¹⁴C yr B.P. by scavenging owls. The ⁸⁷Sr/⁸⁶Sr ratio of Lake Bonneville varied with its elevation; ⁸⁷Sr/⁸⁶Sr values of fish from the lowest stratum of the cave suggest they grew in a lake near the terminal Pleistocene Gilbert shoreline. In the lowest deposits, a decrease in fish size and an increase in species tolerant of higher salinities or temperatures suggest multiple die-offs associated with declining lake levels. An initial, catastrophic, post-Provo die-off occurred at 11,300–11,200 ¹⁴C yr B.P. and was followed by at least one rebound or recolonization of fish populations, but fish were gone from Lake Bonneville sometime before 10,400 ¹⁴C yr B.P. This evidence is inconsistent with previous inferences of a near desiccation of Lake Bonneville between 13,000 and 12,000 ¹⁴C yr B.P. Peaks in Gila atraria frequencies in the upper strata suggest the Great Salt Lake had highstands at 3400 and 1000 ¹⁴C yr B.P.     

Multi‐basin depositional framework for moisture‐balance reconstruction during the last 1300 years at Lake Bogoria,central Kenya Rift Valley

Multi‐proxy analysis of sediment cores from five key locations in hypersaline, alkaline Lake Bogoria (central Kenya Rift Valley) has allowed reconstruction of its history of depositional and hydrological change during the past 1300 years. Analyses including organic matter and carbonate content, granulometry, mineralogical composition, charcoal counting and high‐resolution scanning of magnetic susceptibility and elemental geochemistry resulted in a detailed sedimentological and compositional characterization of lacustrine deposits in the three lake basins and on the two sills separating them. These palaeolimnological data were supplemented with information on present‐day sedimentation conditions based on seasonal sampling of settling particles and on measurement of physicochemical profiles through the water column. A new age model based on ²¹⁰Pb, ¹³⁷Cs and ¹⁴C dating captures the sediment chronology of this hydrochemically complex and geothermally fed lake. An extensive set of chronological tie points between the equivalent high‐resolution proxy time series of the five sediment sequences allowed transfer of radiometric dates between the basins, enabling interbasin comparison of sedimentation dynamics through time. The resulting reconstruction demonstrates considerable moisture‐balance variability through time, reflecting regional hydroclimate dynamics over the past 1300 years. Between ca 690 and 950 AD , the central and southern basins of Lake Bogoria were reduced to shallow and separated brine pools. In the former, occasional near‐complete desiccation triggered massive trona precipitation. Between ca 950 and 1100 AD , slightly higher water levels allowed the build‐up of high pCO ₂ leading to precipitation of nahcolite still under strongly evaporative conditions. Lake Bogoria experienced a pronounced highstand between ca 1100 and 1350 AD , only to recede again afterwards. For a substantial part of the time between ca 1350 and 1800 AD , the northern basin was probably disconnected from the united central and southern basins. Throughout the last two centuries, lake level has been relatively high compared to the rest of the past millennium. Evidence for increased terrestrial sediment supply in recent decades, due to anthropogenic soil erosion in the wider Bogoria catchment, is a reason for concern about possible adverse impacts on the unique ecosystem of Lake Bogoria.