Endogenic Carbonate Research Articles

Modern and late Holocene dolomite formation: Manito Lake, Saskatchewan, Canada

Major advances have occurred in our understanding of modern dolomite formation and penecontemporaneous dolomitization over the past several decades. Manito Lake, located in west-central Saskatchewan, Canada, is a large (65km2), deep (zmax: 22m) perennial saline (~45ppt TDS) lake in which modern and late Holocene dolomite coexists with other endogenic and authigenic carbonate precipitates, including aragonite, monohydrocalcite, calcite, and Mg-calcite. Like many other lacustrine dolomites, Manito Lake dolomite is microcrystalline (less than 1μm to 5μm), Ca-rich and poor to moderately ordered. It occurs as relatively pure hardgrounds and as a component of nearshore microbialites. It also forms isopachous cements in consolidated siliciclastic shoreline sediments. Manito Lake dolomite is most likely forming by mainly biomediated precipitation at or near the sediment–water interface (i) in pore spaces of coarse siliciclastic sediments (i.e., beachrock), (ii) as fine laminae associated with microbialites, and (iii) as a major component of mudstone hardgrounds and pavements.

Impacts climatiques et anthropiques sur le fonctionnement hydrogéochimique du Lac Ifrah (Moyen Atlas marocain)

Résumé Un suivi hydrogéochimique saisonnier du Lac Ifrah (Maroc) a été réalisé au cours des années 2003–2006 dans le but de caractériser son fonctionnement hydrogéochimique vis-à-vis des impacts climatiques et anthropiques. Le Lac Ifrah a beaucoup régressé au cours de la dernière décennie. Les eaux sont relativement alcalines, oxygénées, assez turbides et sursaturées en alcalino-terreux. La charge particulaire est moyennement élevée. Le faciès hydrogéochimique est généralement de type bicarbonaté chloro-sodique magnésien. Le lac a connu un déficit hydrique important sous l'effet de la récurrence des sécheresses et de la forte pression anthropique au cours des trois dernières décennies. Ceci s'est traduit par l'élévation de la salinité des eaux impliquant une augmentation de la conductivité électrique et des teneurs en alcalins (Na+ et K+) et en chlorures. Ce déficit est évalué également par la diminution des teneurs en alcalino-terreux (Ca2+ et Mg2+) à la suite de leur précipitation sous forme de carbonates endogéniques. Les indices de saturation vis-à-vis de la calcite, de l'aragonite et de la dolomite sont toujours supérieurs à zéro. L'augmentation des éléments nutritifs (phosphore et nitrates) et la dégradation de l'oxygénation des eaux reflètent l'accélération de l'eutrophisation du lac qui est reflétée aussi par l'augmentation de la charge particulaire et la turbidité des eaux. Editeur Z.W. Kundzewicz Citation Etebaai, I., Damnati, B., Raddad, H., Benhardouz, H., Benhardouz, O., Miche, H. et Taieb, M., 2012. Impacts climatique et anthropique sur le fonctionnement hydrogéochimique du Lac Ifrah (Moyen Atlas marocain). Hydrological Sciences Journal, 57 (3), 547–561.

A high-resolution record of climate variability and landscape response from Kettle Lake, northern Great Plains, North America

A decadal-scale multiproxy record of minerals, pollen, and charcoal from Kettle Lake, North Dakota provides a high-resolution record of climate and vegetation change spanning the entire Holocene from the northern Great Plains (NGP) in North America. The chronology is established by over 50 AMS radiocarbon dates. This record exhibits millennial-scale trends evident in other lower-resolution studies, but with much more detail on short-term climate variability and on the rapidity and timing of major climatic shifts. As a proxy for precipitation, we utilize the rate of endogenic carbonate sedimentation, which depends on groundwater inflow, which in turn depends on precipitation. Independent cluster analyses of mineral and pollen data reveal major Holocene mode shifts at 10.73 ka (ka = cal yr BP), 9.25 ka, and 4.44 ka. The early Holocene, 11.7–9.25 ka, was generally wet, with perhaps a trend to higher evaporation associated with warming temperatures. A switch from calcite to aragonite deposition associated with a severe, but brief drought occurred at 10.73 ka. From 10.73 ka to 9.25 ka, climate was generally humid but punctuated at 100–300 yr intervals by brief droughts, including the most severe drought of the entire Holocene at 9.25 ka. This event was coeval with the 9.3–9.2 ka event in the Greenland ice cores and observed at a number of sites worldwide. In contrast, the prominent 8.2 ka event in Greenland is not remarkable at Kettle Lake. The prominence of the 9.25 event locally in the NGP may be due to a major drawdown and northward retreat of Lake Agassiz at this time, reducing its mesoclimatic effect on the NGP and thrusting the region into an insolation controlled regime. The mid-Holocene, 9.25–4.44 ka, was characterized by great variability in moisture on a multi-decadal scale, with severe droughts alternating with more humid periods. The high abundance of the weedy but drought intolerant Ambrosia generally during the mid-Holocene and specifically during the multi-decadal drought periods is seemingly paradoxical, but can be explained by high interannual variability of moisture overlaid on the multi-decadal variability. The late Holocene, 4.44 ka–present, was also characterized by multi-decadal variability in moisture, but was generally wetter than the mid-Holocene and the magnitude of variability was less. The trends in wet-dry mineral, pollen, and charcoal proxies were similar to the mid-Holocene, but late Holocene mineral-pollen assemblages are distinct from mid-Holocene. The shift to wetter climate in the late Holocene was more gradual than the abrupt shift to arid conditions 9.25 ka, which may explain the asymmetric retreat and readvance of forest along the eastern margin of the NGP. Precipitation variations in the NGP have been linked with Pacific and Atlantic sea-surface temperatures, and mid-Holocene drought in the NGP has been linked with sustained La Niña-like conditions in the Pacific. These linkages may explain the decadal- and millennial-scale variations seen in the NGP, but cause of the prominent century-scale variations remains elusive.

Carbonate and organic matter sedimentation and isotopic signatures in Lake Chungará, Chilean Altiplano, during the last 12.3 kyr

Sediments in lakes in the Andean volcanic setting are often made up of diatomaceous ooze together with volcaniclastics and small amounts of carbonates. Despite their scarcity, carbonates along with organic matter provide significant paleoenvironmental information about lake systems. This study focuses on the carbonates in Lake Chungará, their morphologies, distribution and origin deduced from the isotopic markers. These markers reflected changes in the water and the biomass between the onset of the Holocene and around 9.6 cal kyr BP. These changes are marked by general increases in TOC, TN, and TN-δ 15N AIR, and by fluctuating values of TOC-δ 13C VPDB in its sediments and are probably related to major shifts in the lake surface/volume associated with rises in lake level. An increase in salinity around 10 cal kyr BP is thought to be linked to a short dry period, giving rise to the onset of carbonate production. The mid-Holocene arid period between 7.3 and 3.5 cal ka BP, with a maximum of aridity around 6.0 cal kyr BP, was deduced from δ 18O VPDB values in the endogenic carbonates. These results match the reconstructions in Lake Titicaca based on benthic diatoms and paleoshore levels. Offshore sediments mainly consist of a diatomaceous ooze, laminated in the lower half (Unit 1), and banded-massive with tephra layers in the upper half of the sequence (Unit 2). TOC-δ 13C VPDB and the C/N ratio confirm that phytoplankton was the main source of organic matter in these sediments. Shallower sediments (units 3 to 5) developed in platform and littoral settings, providing evidence of subaqueous macrophytes and, to a lesser extent, land plants. Carbonate content ranges between 0.1 and 6 wt.% in offshore settings (30 to 40 m water depth) and reaches the maximum values in the lower part of Unit 2. Carbonate minerals (low magnesium calcite and minor amounts of high magnesium calcite and aragonite) are scarce and are arranged in mm-thick layers, commonly forming cm-thick levels or bioclasts. Carbonate layers are made up of euhedral-to-subhedral spindle-shaped calcite crystals and, to a lesser extent, aragonite needles, all in the μm range. Aragonite spheroids coexist in littoral sediments with other carbonate shapes and charophyte remains, where carbonate reaches locally up to 20 wt.%. CO 2 photosynthetic depletions related to seasonal phytoplankton blooms were responsible for the high frequency deposition of mm-thick carbonate layers. The average values for δ 13C VPDB in lake water, plankton and sediments of Lake Chungará (as organic matter or as carbonate) are around 15‰ higher than commonly reported values in other lakes. This 13C enrichment is attributed to carbon assimilation from a DIC affected by methanogenesis, in which HCO 3 − is the dominant species. The δ 13C VPDB and δ 18O VPDB variations and their covariation in endogenic carbonates suggest that lake water volume and lake level increased along the Holocene.

The age of carbonatites and mafic rocks (SHRIMP II and Rb-Sr dating) from the Oshurkovo apatite-bearing pluton ( western Transbaikalia)

The paper presents geochronological data on silicate and endogenic carbonate rocks from the Oshurkovo apatite-bearing pluton, namely, the U-Pb and Rb-Sr isochron ages of gabbros and carbonatites. The U-Pb age of the carbonatites from this pluton (126.55 ± 0.85 Ma) is close to that of the other carbonatite occurrences in southwestern Transbaikalia. Also, its similarity to the age of the gabbro (125.4 ± 2 Ma), which makes up the bulk of the pluton, suggests that these rocks are comagmatic.

Kinetics of the Exchange Reactions between Gd(DTPA)2−, Gd(BOPTA)2−, and Gd(DTPA-BMA) Complexes, Used As MRI Contrast Agents, and the Triethylenetetraamine-Hexaacetate Ligand

The kinetics of ligand exchange reactions occurring between the Gd(DTPA), Gd(BOPTA), and Gd(DTPA-BMA) complexes, used as contrast agents in MRI, and the ligand TTHA, have been studied in the pH range 6.5-11.0 by measuring the water proton relaxation rates at 25 °C in 0.15 M NaCl. The rates of the reactions are directly proportional to the concentration of TTHA, indicating that the reactions take place with the direct attack of the H(i)TTHA((6-i)-) (i = 0, 1, 2 and 3) species on the Gd(3+) complexes, through the formation of ternary intermediates. The rates of the exchange reactions of the neutral Gd(DTPA-BMA) increase when the pH is increased from 6.5 to 9, because the less protonated H(i)TTHA((6-i)-) species can more efficiently attack the Gd(3+) complex. The rates of the exchange reactions of [Gd(DTPA)](2-) and [Gd(BOPTA)](2-) also increase from pH 8.5 to 11, but from 6.5 to 8.5 an unexpected decrease was observed in the reaction rates. The decrease has been interpreted by assuming the validity of general acid catalysis. The protons from the H(i)TTHA((6-i)-) species (i = 2 and 3) can be transferred to the coordinated DTPA or BOPTA in the ternary intermediates when the dissociation of the Gd(3+) complexes occurs faster. The kinetic inertness of Gd(DTPA), Gd(BOPTA), and Gd(DTPA-BMA) differs very considerably; the rates of the ligand exchange reactions of Gd(DTPA-BMA), thus the rates of its dissociation, are 2 to 3 orders of magnitude higher than those of Gd(DTPA) and Gd(BOPTA). The rates of the ligand exchange reactions increase with increasing concentration of the endogenous citrate, phosphate, or carbonate ions at a pH of 7.4, but the effect of citrate and phosphate is negligible at their physiological concentrations. The increase in the reaction rates at the physiological concentration of the carbonate ion is significant (20-60%), and the effect is the largest for the Gd(DTPA-BMA) complex.

Lake carbonate-δ 18O records from the Yukon Territory, Canada: Little Ice Age moisture variability and patterns

A 1000-yr history of climate change in the central Yukon Territory, Canada, is inferred from sediment composition and isotope geochemistry from small, groundwater fed, Seven Mile Lake. Recent observations of lake-water δ 18O, lake level, river discharge, and climate variations, suggest that changes in regional effective moisture (precipitation minus evaporation) are reflected by the lake’s hydrologic balance. The observations indicate that the lake is currently 18O-enriched by summer evaporation and that during years of increased precipitation, when groundwater inflow rates to the lake increase, lake-water δ 18O values decrease. Past lake-water δ 18O values are inferred from oxygen isotope ratios of fine-grained sedimentary endogenic carbonate. Variations in carbonate δ 18O, supplemented by those in carbonate and organic δ 13C, C/N ratios, and organic carbon, carbonate and biogenic silica accumulation rates, document changes in effective moisture at decadal time scales during the early Little Ice Age period to present. Results indicate that between ∼AD 1000 and 1600, effective moisture was higher than today. A shift to more arid climate conditions occurred after ∼AD 1650. The 19th and 20th centuries have been the driest of the past millennium. Temporal variations correspond with inferred shifts in summer evaporation from Marcella Lake δ 18O, a similarly small, stratified, alkaline lake located ∼250 km to the southwest, suggesting that the combined reconstructions accurately document the regional paleoclimate of the east-central interior. Comparison with regional glacial activity suggests differing regional moisture patterns during early and late Little Ice Age advances.

Acquisition of Ca2+ and HCO3 −/CO3 2− for shell formation in embryos of the common pond snail Lymnaea stagnalis

Embryos of the freshwater common pond snail Lymnaea stagnalis develop to hatch within 10 days under control conditions (22°C, Miami-Dade tap water) and this development is impaired by removal of ambient calcium. In contrast, embryos did not exhibit dependence upon an ambient HCO3 −/CO3 2− source, developing and hatching in HCO3 −/CO3 2−-free water at rates comparable to controls. Post-metamorphic, shell-laying embryos exhibited a significant saturation-type calcium uptake as a function of increasing ambient calcium concentration. However, changes in ambient bicarbonate concentration did not influence calcium or apparent titratable alkalinity uptake. There was a distinct shift from no significant flux in pre-metamorphic embryos to net uptake of calcium in post-metamorphic stages as indicated by an increased uptake from the micro-environment surrounding the egg mass and increased net uptake in 24-h, whole egg mass flux measurements. Furthermore, HCO3 −/CO3 2− acquisition as measured by titratable alkalinity flux is at least partially attributable to an endogenous carbonate source that is associated with acid extrusion. Thus, calcium requirements for embryonic shell formation are met via uptake but HCO3 −/CO3 2−, which is also necessary for shell formation is acquired in part from endogenous sources with no detectable correlation to ambient HCO3 −/CO3 2− availability.

Carbonate microbialites and hardgrounds from Manito Lake, an alkaline, hypersaline lake in the northern Great Plains of Canada

Manito Lake is a large, perennial, Na–SO 4 dominated saline to hypersaline lake located in the northern Great Plains of western Canada. Significant water level decrease over the past several decades has led to reduction in volume and surface area, as well as an increase in salinity. The salinity has increased from 10 ppt to about 50 ppt TDS. This decrease in water level has exposed large areas of nearshore microbialites. These organogenic structures range in size from several cm to over a meter and often form large bioherms several meters high. They have various external morphologies, vary in mineralogical composition, and show a variety of internal fabrics from finely laminated to massive. In addition to microbiolities and bioherms, the littoral zone of Manito Lake contains a variety of carbonate hardgrounds, pavements, and cemented clastic sediments. Dolomite and aragonite are the most common minerals found in these shoreline structures, however, calcite after ikaite, monohydrocalcite, magnesian calcite, and hydromagnesite are also present. The dolomite is nonstoichiometric and calcium-rich; the magnesian calcite has about 17 mol% MgCO 3. AMS radiocarbon dating of paired organic matter and endogenic carbonate material confirms little or no reservoir affect. Although there is abundant evidence for modern carbonate mineral precipitation and microbialite formation, most of the larger microbialites formed between about 2300 and 1000 cal BP, whereas the hardgrounds, cements, and laminated crusts formed about 1000–500 cal BP.

Geochemical processes in a Mediterranean Lake: a high-resolution study of the last 4,000 years in Zoñar Lake, southern Spain

High-resolution geochemical analysis of a 6-m-long sediment core from Zonar Lake, southern Spain, provides a detailed characterization of major changes in lake and watershed processes during the last 4,000 years. Geochemical variables were used as paleolimnological indicators and complement Zonar Lakes’s paleoenvironmental reconstruction based on sedimentological and biological proxies, which define periods of increasing allochthonous input to the lake and periods of dominant autochthonous sedimentation. Chemical ratios identify periods of endogenic carbonate formation (higher Ca/Al, Sr/Al and Ba/Al ratios), evaporite precipitation (higher S/Al, Sr/Al ratios), and anoxic conditions (higher Mo/Al, U/Th ratios and Eu anomaly). Higher productivity is marked by elevated organic carbon content and carbonate precipitation (Mg/Ca). Hydrological reconstruction for Zonar Lake was based on sedimentological, mineralogical and biological proxies, and shows that lower lake levels are characterized by Sr-rich sediments (a brackish lake with aragonite) and S-rich sediments (a saline lake with gypsum), while higher lake levels are characterized by sediments enriched in elements associated with alumino-silicates (Al, K, Ti, Fe, trace and rare earth elements), reflecting fresher conditions. Geochemical indicators also mark periods of higher detrital input to the lake related to human activity in the watershed: (1) during the Iberian Roman Humid Period (650 BC–AD 300), around the onset of the Little Ice Age (AD 1400), during the relatively drier Post-Roman and Middle Ages (AD 800–1400), and over the last 50 years, due to mechanized farming practices. Heavy metal enrichment in the sediments (Cu and Ni) suggests intensification of human activities during the Iberian Roman Period, and the use of fertilizers during the last 50 years.

Lateglacial and Holocene hydroclimate inferred from a groundwater flow-through lake, Northern Rocky Mountains, USA

Climate-driven variations in lake-groundwater exchange are recorded by sediments in groundwater-dominated lakes. A groundwater flow-through lake in west-central Montana (USA) registers latest Pleistocene and Holocene hydroclimatic variation in fluid and solute balance, as controlled by rates and timing of groundwater recharge. Early Holocene warming occurred under conditions of relative aridity and low groundwater throughflow, punctuated by a c. 450-yr episode of lake dilution centered on 11 000 cal. yr BP. Maximum evaporative concentration of lake waters, registered in both δ18O values and mineralogy of endogenic carbonates, coincided with the early-Holocene peak in insolation seasonality at about 9750 cal. yr BP. Subsequently, progressively decreasing lake residence time drove a sustained long-term decline in salinity while having a very subdued effect on mean δ18O values. We explain this decoupling by (1) limits placed on oxygen isotope sensitivity by groundwater throughflow, and (2) a shift toward greater summer rain contribution to lake inflow after mid-Holocene time. Superimposed multidecadal- and century-scale variation in lake—groundwater exchange generated high-frequency but low-amplitude isotopic oscillations throughout the record. High rates of groundwater throughflow maintaining low lake salinity similar to that observed today were established around 1400 cal. yr BP. We infer reduced regional stream baseflow, decrease in permanent wetlands (relative increase in ephemeral wetlands) and enhanced lake and wetland salinity prior to this time, relative to the late Holocene.

Lake Hovsgol basin as a new study site for long continental paleoclimate records in continental interior Asia: General context and current status

Lacustrine sedimentary strata in the Lake Hovsgol rift basin, NW Mongolia, preserved an important regional archive with new powerful proxies for climate change in continental interior Asia. Two scientific drilling campaigns of the Hovsgol Drilling Project (HDP) recovered the most complete and best resolved mid-late Pleistocene paleoclimate archive in Mongolia. With the basal age of ca. 1 Ma, Lake Hovsgol drill core section complements those obtained previously in the neighboring Lake Baikal, and contains regional paleoclimate proxy records on a timescale comparable to that of marine sedimentary records and the longest Antarctic ice core record. Works in this volume discuss changes in lake chemistry, biota and environment, closely linked with drill core lithology and depositional trends apparent from sediment facies. The current collection of works illustrates the potential of proxy signals in the Hovsgol record and highlights new important constraints on their interpretation. Because of a nearly continuous record of endogenic carbonate accumulation in Lake Hovsgol, a wealth of paleoclimate proxy data is preserved in carbonate mineralogy, elemental geochemistry and stable isotope composition; such a unique long record was not previously available in this part of the world. The repetitive pattern of changes in the lake's water budget in response to the climatic cycles of the Pleistocene is established and constrained by absolute dating and the robust paleomagnetic event/reversal timescale. The new proxy records from the HDP sediment sections are both relevant and timely contributions to the discussion of the nature of paleoclimate changes in continental interior Asia during prior glacial and interglacial intervals of the Pleistocene. These records help establish the early timing of the post-glacial changes in atmospheric circulation and effective moisture budget in the region and resolve the previously proposed scenarios of extreme precipitation gradients during glacial periods. The key primary data presented in this volume make a compelling case for further in-depth studies of the available records and for future deeper drilling into the early Pleistocene and the Pliocene strata in this basin.

Stable isotope records of Late Quaternary climate and hydrology from Mediterranean lakes: the ISOMED synthesis

Lake isotope records can be used to assess the spatial coherency of Late Quaternary climate change across the circum-Mediterranean region. We place modern and palaeo-data within a simple conceptual lake response model to show that the isotope hydrology of most Mediterranean lakes has been influenced strongly by water balance, even in those systems that are chemically dilute (i.e. freshwater). δ 18O data on biogenic and endogenic carbonates from 24 lake basins are used to reconstruct multi-millennial-scale trends since the LGM. While it is difficult to make direct comparisons between lake records in terms of single climatic parameters, coherent regional isotopic trends can be identified. During glacial times Mediterranean lakes deposited carbonates isotopically heavier in δ 18O compared to the Holocene, partly due to source area effects. Isotopic enrichment was most marked during intervals corresponding to the H1 and Younger Dryas events, confirming that Late Pleistocene cold stages in the North Atlantic region were marked by aridity around much of the Mediterranean. Almost all Mediterranean lake records shifted to more depleted isotopic values during the Last Glacial–Interglacial Transition (LGIT). This shift is the reverse of the trend which characterised the same transition in lakes from northern and central Europe, and suggests that temperature changes were not an important direct driver of Mediterranean lake isotopic records over glacial–interglacial timescales. In the early Holocene, many lakes in the eastern part of the region were more depleted isotopically than in recent millennia. This corresponds with marine sapropel formation, both chronologically and geographically, and implies that increases in local rainfall contributed significantly to the creation of a freshwater lid and anoxia in the East Mediterranean Sea. In contrast, no such pattern is currently apparent from lake isotope records from the West Mediterranean, suggesting a possible NW–SE contrast in climate history during the Holocene.

A ratiometric optical imaging probe for intracellular pH based on modulation of europium emission

A set of three pH-responsive ratiometric Eu(III)complexes has been synthesised incorporating a coordinated azathioxanthone sensitiser and a pH dependent alkylsulfonamide moiety. Emission properties, anion binding affinities, pH response curves and protein binding constants were studied in detail in aqueous media, and solutions containing various concentrations of interfering anions and protein were also examined. The complex, [EuL3] exhibited some interference from protein and endogenous anions, e.g. lactate and hydrogen carbonate, but possessed a protonation constant of 7.2 in human serum solution. A suitable calibration curve was obtained and was used to determine the local pH using a 680/589 nm intensity ratio vs. pH plot. Confocal fluorescence microscopy images revealed fast uptake of the complex and a well distributed localisation within the cell; fast egress also occurred. Ribosomal localisation, with a high concentration within the protein-dense nucleoli was observed, in a similar manner to structurally related complexes bearing the same coordinated sensitising moiety. An IC(50) value of 67 (+/-20) microM was estimated using an MTT assay. Selected emission band ratio versus pH plots allow pH measurement in the range 6 to 8, enabling intracellular pH to be measured by microscopy. A value of 7.4 was estimated for NIH 3T3 cells in the protein rich regions of the nucleolus and ribosomes.

Geochemical evidence for hydroclimatic variability over the last 2460 years from Crevice Lake in Yellowstone National Park, USA

A 2460-year-long hydroclimatic record for Crevice Lake, Yellowstone National Park, Montana was constructed from the δ 18O values of endogenic carbonates. The δ 18O record is compared to the Palmer Hydrologic Drought Index (PHDI) and Pacific Decadal Oscillation (PDO) indices, as well as inferred discharge of the Yellowstone River. During the last century, high δ 18O values coincide with drought conditions and the warm phase of the PDO index. Low δ 18O values coincide with wet years and a negative PDO index. Comparison of tree-ring inferred discharge of the Yellowstone River with the δ 18O record over the last 300 years indicates that periods of high discharge (i.e., wet winters with significant snow pack) correspond with low δ 18O values. Extrapolating this relationship we infer wet winters and high river discharge for the periods of 1090–1030, 970–870, 670–620, and 500–430 cal years BP. The wet intervals at 670 and 500 cal BP are synchronous with similar events in Banff, Canada and Walker Lake, Nevada. The wet intervals at 970 and 670 cal BP overlap with wet intervals at Walker Lake and major drought events identified in the western Great Basin. These results suggest that the northern border of Yellowstone National Park straddles the boundary between Northern Rocky Mountains and Great Basin climate regimes.

A 14 kyr record of the tropical Andes: The Lago Chungará sequence (18°S, northern Chilean Altiplano)

High-resolution geochemical analyses obtained using an X-ray fluorescence (XRF) Core Scanner, as well as mineralogical data from the Lago Chungará sedimentary sequence in the northern Andean Chilean Altiplano (18°S), provided a detailed reconstruction of the lacustrine sedimentary evolution during the last 14,000 cal. yr BP. The high-resolution analyses attained in this study allowed to distinguish abrupt periods, identify the complex structures of the early and mid-Holocene arid intervals and to compare their timing with Titicaca lake and Sajama ice records. Three main components in the lake sediments have been identified: (a) biogenic component, mainly from diatoms (b) volcanics (ash layers) from the nearby Parinacota Volcano and (c) endogenic carbonates. The correlation between volcanic input in Lago Chungará and the total particles deposited in the Nevado Sajama ice core suggests the Parinacota Volcano as the common source. The geochemical record of Lago Chungará indicates an increase in siliceous productivity during the early Holocene, lagging behind the rise in temperatures inferred from the Nevado Sajama ice core. The regional mid-Holocene aridity crisis can be characterized as a number of short events with calcite and aragonite precipitation in the offshore lake zones.

Late Holocene moisture balance variability in the southwest Yukon Territory, Canada

Analyses of sediment cores from Marcella Lake, a small, hydrologically closed lake in the semi-arid southwest Yukon, provides effective moisture information for the last ∼4500 years at century-scale resolution. Water chemistry and oxygen isotope analyses from lakes and precipitation in the region indicate that Marcella Lake is currently enriched in 18O by summer evaporation. Past lake water values are inferred from oxygen isotope analyses of sedimentary endogenic carbonate in the form of algal Charophyte stem encrustations. A record of the δ 18O composition of mean annual precipitation at Jellybean Lake, a nearby evaporation-insensitive system, provides data of simultaneous δ 18O variations related to decade-to-century scale shifts in Aleutian Low intensity/position. The difference between the two isotope records, Δδ, represents 18O-enrichment in Marcella Lake water caused by summer effective moisture conditions. Results indicate increased effective moisture between ∼3000 and 1200 cal BP and two marked shifts toward increased aridity at ∼1200 and between 300 and 200 cal BP. These prominent late Holocene changes in effective moisture occurred simultaneously with changes in Aleutian Low circulation patterns over the Gulf of Alaska indicated by Jellybean Lake. The reconstructed climate patterns are consistent with the topographically controlled climatic heterogeneity observed in the coastal mountains and interior valleys of the region today.

A continuous 250,000 yr record of oxygen and carbon isotopes in ostracode and bulk-sediment carbonate from Bear Lake, Utah-Idaho

Oxygen and carbon isotopes from a continuous, 120-m-long, carbonate-rich core from Bear Lake, Utah-Idaho, document dramatic fluctuations in the hydrologic budget of the lake over the last 250,000 yr. Isotopic analyses of bulk sediment samples capture millennial-scale variability. Ostracode calcite was analyzed from 78 levels, mainly from the upper half of the core where valves are better preserved, to compare the isotopic value of purely endogenic carbonate with the bulk sediment, which comprises both endogenic and detrital components. The long core exhibits three relatively brief intervals with abundant endogenic aragonite (50±10%) and enriched δ 18O and δ 13C. These intervals are interpreted as warm/dry periods when the lake retracted into a topographically closed basin. We correlate these intervals with the interglacial periods of marine oxygen-isotope stages 1, 5e, and 7a, consistent with the presently available geochronological control. During most of the time represented by the core, the lake was fresher than the modern lake, as evidenced by depleted δ 18O and δ 13C in bulk-sediment carbonate.

Factors controlling δ13C values of sedimentary carbon in hypertrophic Baldeggersee, Switzerland, and implications for interpreting isotope excursions in lake sedimentary records

We use stable carbon isotope values (δ13C) in sedimented organic matter and carbonate as proxy indicators of productivity changes in a highly eutrophic to hypertrophic lake. Stable isotope data from a seasonally sampled sediment core recovered from Baldeggersee, Switzerland were compared with direct and inferred total phosphorus (P) concentrations. Carbon isotope values of sedimentary organic matter (δ13Corg) in both seasonal varves and over the past 100 yr are not linearly related to total P concentrations, nor do they mirror observed trends in carbonate isotope values (δ13CCaCO3). Baldeggersee δ13Corg values are influenced by both variations in the relative inputs of eukaryotic biomass, which becomes enriched in 13C with increasing primary productivity, and the contribution of microbial biomass produced in the expanding anoxic bottom waters, which is typically very depleted in 13C. We also examined the fractionation within the organic matter‐CO2–CaCO3 system, calculated as εtotal organic carbon(TOC) = 103{[(δ13CCaCO3 + 1,000)/(δ13Corg + 1,000)] − 1}. Thresholds within the measured εTOC values represent Baldeggersee CaCO3 trophic status more accurately than δ13Corg or δ13CCaCO3 values alone. In alkaline lakes with endogenic carbonate precipitates, εTOC values can facilitate accurate interpretation of values and trends in organic carbon isotope data, and can help to better assess trophic history and lake remediation efforts in lake systems that have been heavily affected by cultural eutrophication.

04/00118 Producing hydrocarbons and non-hydrocarbon containing materials from a hydrocarbon containing formation including oil shale: Vinegar, H. J. et al. PCT Int. Appl. WO 03 35,801 (Cl. C10G) 1 May 2003, US Appl. PV 374,970

The northern Great Plains of western Canada, a vast region stretching from the Precambrian Shield east of Winnipeg, Manitoba, westward for some 1600 km to the foothills of the Rocky Mountains, contains literally millions of lakes and wetlands. Although often characterized as a saline, Na-SO4 system, in fact the wide range of water chemistries exhibited by the lakes results in an unusually large diversity of sediment composition. Despite a long history of limnogeological study, it is only recently that the spectrum of carbonate minerals and sedimentological processes in these lakes has been realized. About 30 species of carbonate minerals have been reported from the modern and Holocene sediment of about 50 basins in the region. The ubiquity of detrital calcite and dolomite is a legacy of the carbonate bedrock and carbonate-rich glacial sediments. Elevated salinities of the lakes, together with high alkalinities, productivity, and pH values, act in concert to create thermodynamically saturated or supersaturated conditions with respect to many carbonate minerals. The most common non-detrital components are Mg-calcite, aragonite and non-stoichiometric dolomite. Many of the basins whose brines have very high Mg/Ca ratios also contain hydromagnesite, magnesite, and nesquehonite. Although not common, sodium carbonates, including trona, natron and nahcolite, also occur in some of the hypersaline lakes.Because of their great range of formative conditions, carbonates have been the workhorse for much of the physical and geochemical paleolimnology in the Canadian Great Plains. However, the often-difficult task of distinguishing endogenic lacustrine carbonates from allogenic and authigenic minerals has limited the use of carbonate stratigraphy in the region. Despite this problem, the carbonates have been useful in deciphering (i) past changes in hydrology and drainage basin characteristics, (ii) lake level and water column stratification fluctuations, and (iii) water chemistry and salinity variations.