Paleohydrology of the upper Laurentian Great Lakes from the late glacial to early Holocene

Between 10,500 and 9000 cal yr BP, δ¹⁸O values of benthic ostracodes within glaciolacustrine varves from Lake Superior range from − 18 to − 22‰ PDB. In contrast, coeval ostracode and bivalve records from the Lake Huron and Lake Michigan basins are characterized by extreme δ¹⁸O variations, ranging from values that reflect a source that is primarily glacial ( − 20‰ PDB) to much higher values characteristic of a regional meteoric source ( − 5‰ PDB). Re-evaluated age models for the Huron and Michigan records yield a more consistent δ¹⁸O stratigraphy. The striking feature of these records is a sharp drop in δ¹⁸O values between 9400 and 9000 cal yr BP. In the Huron basin, this low δ¹⁸O excursion was ascribed to the late Stanley lowstand, and in the Lake Michigan basin to Lake Agassiz flooding. Catastrophic flooding from Lake Agassiz is likely, but a second possibility is that the low δ¹⁸O excursion records the switching of overflow from the Lake Superior basin from an undocumented northern outlet back into the Great Lakes basin. Quantifying freshwater fluxes for this system remains difficult because the benthic ostracodes in the glaciolacustrine varves of Lake Superior and Lake Agassiz may not record the average δ¹⁸O value of surface water.     

Geochemical relation between gas hydrates and water venting in the seismic blanking zone on the northern Cascadia continental margin offshore Vancouver Island, Canada

The isotopic composition (δD and δ¹⁸O) and chloride concentration (Cl⁻) of pore waters from the northern Cascadia continental margin offshore Vancouver Island were measured to characterize the relations between the water flow regime and the distribution, formation and dissociation of gas hydrates. The δD values of pore waters in gas hydrate-bearing sediments are slightly higher ( 1‰) than those of seawater as the result of gas hydrate dissociation during core recovery and handling. Within the seismic blanking zone, the δD values were slightly lower (− 1‰) than values measured from sites outside the blanking area (0‰). We attribute these differences to 1) distillation of D-rich water during hydrate formation in the center of the blanking zone and 2) limited migration of pore water between inside and outside of the blanking zone due to different fluid fluxes. In contrast, the δ¹⁸O values and Cl⁻ concentrations do not show significant spatial variation due to decreased isotopic fractionation of oxygen and small fraction of chloride relative to hydrogen isotope during gas hydrate formation. The δD value of pore water, therefore, appears to be a sensitive indicator of gas hydrate occurrence. We estimate that gas hydrate occupied at least 2.0 to 6.3% of sediment pore space using δD distribution in this area.     

Pleistocene paleoclimate of the arid region of Israel as recorded in calcite deposits along regional transverse faults and in veins

The δ¹⁸O and δ¹³C values of the calcites associated with E-W and NE-SW transverse faults in the Negev, Israel, indicate that calcite was deposited from meteoric water. A regional change in the δ¹⁸O and δ¹³C values was observed. The ¹⁸O content in the calcite increases, from the southwestern (δ¹⁸O = −17.8‰) to the northeastern (δ¹⁸O = −2.9‰) part of the region. The δ¹³C values show the opposite trend of the ¹³C content decrease: from +2‰ in the south to −10‰ in the northeast. These trends had to reflect changes in regional paleoclimate, suggesting a change in the isotopic composition of the solution from which the calcite was deposited in different periods. The variations in the δ¹⁸O values reflect shifts in the δ¹⁸O values of precipitation and are associated with a change in the source of moist air masses which came from the equatorial Atlantic in the early Pleistocene and from the Mediterranean during a later period. Variations in δ¹³C values reflect changes from humid to arid conditions. Two modes of calcite deposition are suggested: (1) precipitation of calcite minerals in the unsaturated zone following the dissolution in the soil or (2) calcite deposition that occurred as CO₂ was lost during emergence of paleogroundwater from Lower Cretaceous and Jurassic aquifers.     

Diagenetic mineralization in Pennsylvanian coals from Indiana, USA: C/C and O/O implications for cleat origin and coalbed methane generation

Cleats and fractures in southwestern Indiana coal seams are often filled with authigenic kaolinite and/or calcite. Carbon- and oxygen-stable isotope ratios of kaolinite, calcite, and coalbed CO₂ were evaluated in combination with measured values and published estimates of δ¹⁸O of coalbed paleowaters that had been present at the time of mineralization. δ¹⁸O_mineral and δ¹⁸O_water values jointly constrain the paleotemperature of mineralization. The isotopic evidence and the thermal and tectonic history of this part of the Illinois Basin led to the conclusion that maximum burial and heat-sterilization of coal seams approximately 272 Ma ago was followed by advective heat redistribution and concurrent precipitation of kaolinite in cleats at a burial depth of < 1600 m at  78 ± 5 °C. Post-Paleozoic uplift, the development of a second generation of cleats, and subsequent precipitation of calcite occurred at shallower burial depth between  500 to  1300 m at a lower temperature of 43 ± 6 °C. The available paleowater in coalbeds was likely ocean water and/or tropical meteoric water with a δ¹⁸O_water  − 1.25‰ versus VSMOW. Inoculation of coalbeds with methanogenic CO₂-reducing microbes occurred at an even later time, because modern microbially influenced ¹³C-enriched coalbed CO₂ (i.e., the isotopically fractionated residue of microbial CO₂ reduction) is out of isotopic equilibrium with ¹³C-depleted calcite in cleats.     

Hydrologic and climatic implications of stable isotope and minor element analyses of authigenic calcite silts and gastropod shells from a mid-Pleistocene pluvial lake, Western Desert, Egypt

Authigenic calcite silts at Wadi Midauwara in Kharga Oasis, Egypt, indicate the prolonged presence of surface water during the Marine Isotope Stage 5e pluvial phase recognized across North Africa. Exposed over an area of  4.25 km², these silts record the ponding of water derived from springs along the Libyan Plateau escarpment and from surface drainage. The δ¹⁸O values of these lacustrine carbonates (− 11.3‰ to − 8.0‰ PDB), are too high to reflect equilibrium precipitation with Nubian aquifer water or water of an exclusively Atlantic origin. Mg/Ca and Sr/Ca of the silts have a modest negative covariance with silt δ¹⁸O values, suggesting that the water may have experienced the shortest residence time in local aquifers when the water δ¹⁸O values were highest. Furthermore, intra-shell δ¹⁸O, Sr/Ca, and Ba/Ca analyses of the freshwater gastropod Melanoides tuberculata are consistent with a perennially fresh water source, suggesting that strong evaporative effects expected in a monsoonal climate did not occur, or that dry season spring flow was of sufficient magnitude to mute the effects of evaporation. The input of a second, isotopically heavier water source to aquifers, possibly Indian Ocean monsoonal rain, could explain the observed trends in δ¹⁸O and minor element ratios.     

12,000-Year, preliminary results of the stable nitrogen and carbon isotope record from the Empakai Crater lake sediments, Northern Tanzania

The stable isotope compositions of organic carbon and nitrogen, the contents of organic carbon and nitrogen and C/N ratios for two cores recovered from the Empakai Crater at water depths of 11 and 20 m are used to document climatic changes in northern Tanzania. Eight ¹⁴C AMS dates determined on total organic matter (OM) indicate that the sedimentation rate in this lake is about 30 cm/ka for the late Pleistocene to early Holocene period. There are differences in the δ¹³C values of organic carbon between the two cores, which may be a result of differences in location from the present shoreline and of different water depths. In the deeper-water core the δ¹³C values show a general downcore decrease to the base of the core with a sharp change to lower values of about 4‰ at a depth of 100 cm (8.7 ka). The general trend of downcore decrease in ¹³C values can be attributed either to a systematic decrease in the relative proportion of C₄ type of OM, owing to an increase in precipitation and change in vegetation cover from grassland to forest, or to utilization of isotopically enriched carbon during photosynthesis. The δ¹⁵N values show a general downcore increase with again a sharp change of about 5‰ to lower values at about 8.7 ka. A sharp change of about 5‰ and 4‰ to more depleted values at a depth of 100 cm of both ¹⁵N and ¹³C, respectively, suggests either hiatus or abrupt change in climatic condition from wetter conditions to drier conditions. There is enhanced preservation of OM in the lake as depicted by high mean values of organic carbon and nitrogen at both sites.     

Micro-scale tracing of Fe and Si isotope signatures in banded iron formation using femtosecond laser ablation

We have detected micrometre-scale differences in Fe and Si stable isotope ratios between coexisting minerals and between layers of banded iron formation (BIF) using an UV femtosecond laser ablation system connected to a MC-ICP-MS. In the magnetite–carbonate–chert BIF from the Archean Old Wanderer Formation in the Shurugwi Greenstone Belt (Zimbabwe), magnetite shows neither intra- nor inter-layer trends giving overall uniform δ⁵⁶Fe values of 0.9‰, but exhibits intra-crystal zonation. Bulk iron carbonates are also relatively uniform at near-zero values, however, their individual δ⁵⁶Fe value is highly composition-dependent: both siderite and ankerite and mixtures between both are present, and δ⁵⁶Fe end member values are 0.4‰ for siderite and −0.7‰ for ankerite. The data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by Fe(III)-reducing bacteria; or more likely an abiotic reaction of organic carbon and Fe(III) during low-grade metamorphism. Si isotope composition of the Old Wanderer BIF also shows significant variations with δ³⁰Si values that range between −1.0‰ and −2.6‰ for bulk layers. These isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal-rich waters. Interestingly, Fe and Si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures. Moreover, the changes of Fe and Si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal-rich water which govern the rates of Fe and Si precipitation and therefore also the development of layering. During periods of low hydrothermal activity, precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate–chert layers. During periods of intensive hydrothermal activity, precipitation rates of ferric oxyhydroxide were high, and subsequent diagenesis triggered only partial reduction, forming magnetite–carbonate–chert layers. We are confident that our micro-analytical technique is able to detect both the solute flux history into the sedimentary BIF precursor, and the BIF’s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors.     

Oxygen and carbon isotope analysis of the Mooreville Chalk and late Santonian-early Campanian sea level and sea surface temperature changes, northeastern Gulf of Mexico, U.S.A.

Oxygen and carbon isotope profiles for strata of the Mooreville Chalk (upper Santonian-lower Campanian) of the eastern Gulf Coastal Plain, U.S.A., show correlations with published curves for these isotopes. The δ¹⁸O curve exhibits a strong similarity to the Exmouth Plateau δ¹⁸O curve from ODP drilling sites offshore northwestern Australia, and the δ¹³C curve can be correlated with the δ¹³C curve from the English Chalk Trunch section.A thermal maximum probably occurred in the northeastern Gulf of Mexico during the latest Santonian (83.8 Ma), as evidenced by the minimum δ¹⁸O values in the lower Mooreville beds. A δ¹³C positive excursion occurs at the same stratigraphic level, which has been recognized as the “Santonian/Campanian boundary event” worldwide. After this event, ocean surface water temperature decreased throughout the early Campanian. This carbon isotope excursion is followed by a plateau in δ¹³C values with a peak value occurring in a condensed section (80 Ma), which has been correlated to a downlap or maximum flooding surface on seismic data from offshore Alabama. The section characterized by increasing δ¹³C values corresponds to a marine transgression. The interval characterized by decreasing δ¹³C values corresponds to regression and progradation. The maximum flooding event occurred 0.8 Ma later than the thermal maximum event.The Mooreville chalk/marl cycles are most likely a product of fluctuations in siliciclastic sediment influx into the northeastern Gulf of Mexico modulated by the precession band of the Earth's orbital cycles. Higher carbon isotope values occur in the marl beds indicating that these beds were formed in a more anoxic/dysoxic environment characterized by higher clay, silt input and higher organic carbon accumulation.     

Controls on the isotopic composition of surface water and precipitation in the Northern Andes, Colombian Eastern Cordillera

Empirical datasets provide the constraints on the variability and causes of variability in stable isotope compositions (δD or δ¹⁸O) of surface water and precipitation that are essential not only for models of modern and past climate but also for investigations of paleoelevation. This study presents stable isotope data for 76 samples from four elevation transects and three IAEA GNIP stations in the Eastern Cordillera of Colombia and the northern Andean foreland. These data are largely consistent with theories of stable isotope variability developed based on a global dataset. On a monthly basis, the precipitation-amount effect exerts the dominant control on δD_p and δ¹⁸O_p values at the IAEA GNIP stations. At the Bogotá station (2547 m), the δD_p and δ¹⁸O_p values vary seasonally, with isotopic minima correlating with maxima in precipitation-amount. Although surface water samples from Eastern Cordilleran streams and rivers fall on the Global Meteoric Water Line, samples from three of four lakes (2842–3459 m) have evaporatively elevated δD_sw and δ¹⁸O_sw values. The IAEA GNIP station data averaged over multiple years, combined with stream and river water data, define vertical lapse rates of −1.8‰ km⁻¹ for Δδ¹⁸O and −14.6‰ km⁻¹ for ΔδD, and are a close fit to a common thermodynamically based Rayleigh distillation model. Elevation uncertainties for these relationships are also evaluated. Comparison of this Colombian dataset with the elevation uncertainties generated by the thermodynamically based model shows that the model underestimates uncertainty at high Δδ¹⁸O and ΔδD values while overestimating it for low Δδ¹⁸O and ΔδD values. This study presents an independent, empirical assessment of stable isotope-based elevation uncertainties for the northern Andes based on a dataset of sufficient size to ensure statistical integrity. These vertical lapse rates and associated uncertainties form the basis for stable isotope paleoelevation studies in the northern Andes.     

Sources and transport of carbon and nitrogen in the River Sava watershed, a major tributary of the River Danube

Carbon and nitrogen dynamics were examined throughout the River Sava watershed, a major tributary of the River Danube, in 2005 and 2006. The River Sava exported 2.1 × 10¹¹ mol C/yr as dissolved inorganic carbon (DIC), and emitted 2.5 × 10¹⁰ mol C/yr as CO₂ to the atmosphere. Stable carbon isotope ratios indicate that up to 42% of DIC originated from carbonate weathering and 23% from degradation of organic matter. Loads of dissolved and particulate organic carbon increased with discharge and export rates were calculated to be 2.1 × 10¹⁰ mol C/yr and up to 4.1 × 10⁹ mol C/yr, respectively. Isotopic compositions (δ¹³C and δ¹⁵N) and C/N ratios indicated that soil organic matter was the dominant source of particulate organic matter for 59% of the samples. Eighteen percent of the samples were dominated by plankton, 12% by periodic inputs of fresh terrestrial plant detritus with C/N > 15, and about 11% of the samples were dominated by the contribution of aquatic vascular plants. Nitrate inputs were controlled by land use in the River Sava watershed. δ¹⁵N_NO3 values <6‰ were found in predominantly forested watersheds, while values >6‰ typically represented watersheds with a higher percentage of agricultural and/or urban land use. Elevated δ¹⁵N_NO3 values (up to +25.5‰) at some sites were probably due to the combined effects of low-flow and inputs from sewage and/or animal waste.     

Cryogenic carbonates in cave environments: A review

Cryogenic cave carbonate (CCC) represents a specific type of speleothem. Its precipitation proceeds at the freezing point and is triggered by freezing-induced concentration of solutes. Compared to classical speleothems (stalagmites, flowstones), CCC occurs as accumulations of loose uncemented aggregates. The grain sizes range from less than 1 μm to over 1 cm in diameter. Karst groundwater chemistry and its freezing rate upon entering the cave are responsible for highly variable grain morphology. Rapid freezing of water results in the formation of CCC powders with grain size typically below 50 μm. Slow freezing of water in caves (usually in systems where the CO₂ escape is partly restricted; e.g., ice covered water pools) results in the formation of large mineral grains, with sizes from less than 1 mm to about 20 mm. The range of carbon and oxygen stable isotope compositions of CCC is larger than for a typical carbonate speleothem. Rapid freezing of water accompanied by a quick kinetic CO₂ degassing results in large ranges of δ¹³C of the CCC powders (between –10‰ and +18‰ PDB). Slow freezing of water, with a restricted CO₂ escape results in gradual increase of δ¹³C values (from −9‰ to +6‰ PDB; data ranges in individual caves are usually much more restricted), accompanied by a δ¹⁸O decrease of the precipitated carbonate (overall range from −10‰ to −24‰ PDB). These unusual trends of the carbonate δ¹⁸O evolution reflect incorporation of the heavier ¹⁸O isotope into the formed ice. New isotope data on CCC from three Romanian ice caves allow better understanding of the carbon and oxygen isotope fingerprint in carbonates precipitated from freezing of bulk water. CCCs are proposed as a new genetic group of speleothems.     

Fractionation of silicon isotopes during biogenic silica dissolution

Silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization, primarily by diatoms, in surface waters of the ocean. Interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean, as driven by biological fractionation during the production of biogenic silica (bSiO₂, or opal) by diatoms. However, other factors which may influence the silicon isotope composition of diatom opal, particularly post-formational aging and maturation processes, remain largely uninvestigated. Here, we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bSiO₂ suspended in seawater under closed conditions. This fractionation acts counter to that occurring during bSiO₂ production and at about half its absolute magnitude, with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor ε_DSi–BSi of −0.55‰, corresponding to a fractionation factor α_30/28 of 0.99945. The enrichment factor did not vary with source material, indicating the lack of a significant species effect, or with temperature from 3 to 20 °C. Thus, the dissolution of bSiO₂ produces dissolved silicon with a δ³⁰Si value that is 0.55‰ more negative than its parent bSiO₂, an effect that must be accounted for when interpreting oceanic δ³⁰Si distributions. The δ³⁰Si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed, implying a measurable (±0.1‰) change in the relative δ³⁰Si of opal samples whenever the difference in preservation efficiency between them is >20%. This effect could account for 10–30% of the difference in diatom δ³⁰Si values observed between glacial and interglacial conditions. It is unlikely, however, that the inferred maximum possible change in δb³⁰SiO₂ of +0.55‰ would be manifested in situ, as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage.     

Carbonate stable isotope signals in the 1-Ma sedimentary record of the HDP-04 drill core from Lake Hovsgol, NW Mongolia

Oxygen and carbon stable isotope ratios in carbonates from the HDP-04 drill core from Lake Hovsgol, NW Mongolia, show an overall covariant relationship suggesting that for the most of the past 1 Ma Hovsgol remained a closed-basin lake. Carbonate δ¹⁸O ratio is responsive to regional climate change: a ca. +1.5‰ basinwide δ¹⁸O shift has occurred with the onset of Bølling–Allerød warming (sensu lato), followed by a ca. 0.8‰ depletion during the Younger Dryas. The post-glacial δ¹⁸O shift of the same magnitude is recorded in bulk carbonates, shells of two ostracod species and in wet-sieved fine fraction <63 μm. Associated with the lake-level rise and correlative with the post-glacial warming in the northern hemisphere, the observed δ¹⁸O shift is nevertheless positive. This argues against changes in local temperature and hydrology as key driving mechanisms. Most likely, Lake Hovsgol δ¹⁸O reflects a climate-driven shift in the composition of regional precipitation. Tied into a distinct lithologic succession, the radiocarbon-dated late glacial δ¹⁸O shift apparently represents a ‘template’ of the lake's response to glacial–interglacial transitions: a similar pattern of parallel changes in lithology and carbonate stable isotope composition is observed in at least 10 more intervals in the 1-Ma record, including the MIS 20/MIS19 transition at the Brunhes/Matuyama paleomagnetic reversal boundary. The comparison of carbon stable isotope ratios of untreated and in vacuo roasted bulk sediment with those of detrital carbonates suggests that clastic input of carbonates by lake tributaries does not affect the geochemistry of bulk carbonates in the HDP-04 section. The profiles of bulk carbonate δ¹⁸O and δ¹³C in the Pleistocene section of the HDP-04 drill core suggest at ca. 15.4 ka, at ca. 100 m below today's level, Lake Hovsgol still stood relatively high as compared with prior extended periods of time during late Matuyama and early Brunhes. Isotopically heavy δ¹⁸O and δ¹³C ratios during the mid–late Brunhes, particularly, in carbonate crusts and oolites, are suggestive of past episodes of dramatic evaporative ¹⁸O-enrichment of lake waters. Despite the expectation of muted amplitudes of temperature- and precipitation-related isotope signals, the sedimentary record from the sensitive ‘water gauge’ basin of Lake Hovsgol has high potential for providing important constraints on past hydrologic evolution of continental interior Asia during the Pleistocene.     

Low-density CO2-rich fluid inclusions from charnockites of southwestern Madurai Granulite Block, southern India; implications on graphite mineralization

Characterization of fluid inclusions in graphite-bearing charnockites from the southwestern part of the Madurai Granulite Block in southern India reveals a probable relation with the formation and break down of graphite during the high-grade metamorphism. The first-generation monophase pure CO₂ inclusions, the composition of which is confirmed by laser Raman spectroscopy, recorded moderate density (0.77–0.87 g/cc) corresponding to low tapping pressure (around 2 kb) than that of the peak granulite-facies metamorphism. The precipitation of graphite, as inferred from graphite inclusions and δ¹³C values of the graphite from the outcrops, is interpreted as the cause of this lowering of fluid density. An intermediate generation of pseudosecondary inclusions resulted from the re-equilibration or modification of the first-generation fluids and the CO₂ formed is interpreted to be the oxidation product from graphite. The youngest generation of fluids which caused widespread retrogression of the granulites is a low-temperature (350 °C) high-saline (32.4–52.0 wt% NaCl equivalent) brine. Carbon isotope data on the graphite from the charnockites show δ¹³C values ranging from −11.3 to −19.9‰, suggesting a possibility of mixing of carbon sources, relating to earlier biogenic and later CO₂ fluid influx. Combining the information gathered from petrologic, fluid inclusion and carbon stable isotope data, we model the fluid evolution in the massive charnockites of the southwestern Madurai Granulite Block.     

Developing oxygen isotope proxies from archaeological sources for the study of Late Holocene human–climate interactions in coastal southwest Florida

Oxygen isotopes (δ¹⁸O) derived from archaeological Mercenaria campechiensis shells and Ariopsis felis otoliths potentially provide low-latitude paleoclimate data for studying Late Holocene human–climate interactions in coastal southwest Florida. Specimens analyzed come from the Pineland site complex. Deposits record abrupt and subtle environmental changes appearing to have been climate-related and to have impacted the sedentary human residents. One archaeological shell-otolith set dates to 2nd/3rd century A.D. within the Roman Optimum (RO) climatic episode. A second set dates to 13th/14th century A.D. within the Little Ice Age (LIA). A modern shell-otolith set was analyzed for comparison. δ¹⁸O_ARAGONITE of modern and LIA shells suggest similar seasonal conditions. RO shell is 1‰ more positive during summer, suggesting higher estuarine salinity than in modern and LIA times. Modern and LIA otoliths also have similar δ¹⁸O_ARAGONITE. Estimated Winter temperatures are within measured instrument records. Summer temperatures are overestimated reflecting Summer migration into less-saline water. Estimated Summer temperatures for RO otolith are similar to today's, suggesting elevated estuarine salinity and diminished rainy season, consistent with similar aged zooarchaeological assemblages. Comparisons of two taxa aid in interpreting archaeological δ¹⁸O data; however, early results are mixed with expected profiles for RO specimens and unexpected profiles for LIA specimens.     

A 37-Meter Record of Paleoclimatological Events from Stable Isotope Data on Continental Molluscs in Valle di Castiglione, Near Rome, Italy

New stable isotope analyses on molluscan shells from a long core drilled in the crater lake of Valle di Castiglione, near Rome, extended the investigated portion of the core to 37 m. The succession of δ¹⁸O‰ values in the core interval 37–2.3 m ranges from −2.8 to +6.9‰ with only six samples below 0‰ _(PDB). These results point to arid climatic phases coupled with the high measured δ¹⁸O values of the biogenic carbonate. In contrast, depleted ¹⁸O samples correspond to wet climatic periods, in agreement with a strong evaporative control on the lake water isotopic composition. The ¹³C content of the shells shows sharp changes controlled by the dissolved inorganic carbon isotope budget. Isotopic data suggest that the whole body of water behaved as a closed system, thus resembling lacustrine systems located in arid and semiarid regions where hydrological control dominates the geochemical parameters.     

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.     

Holocene hydrological reconstructions from stable isotopes and paleolimnology, Cordillera Real, Bolivia

Multiproxy analyses of sediment cores from Lago Taypi Chaka Kkota (LTCK) Cordillera Real, Bolivia, provide a record of drier conditions following late Pleistocene deglaciation culminating in pronounced aridity between 6.2 and 2.3 ka B.P. Today LTCK is a glacial-fed lake that is relatively insensitive to changes in P–E because it is largely buffered from dry season draw-down through the year-round supply of glacial meltwater. This was not the case during the middle to late Holocene when glaciers were absent from the watershed. Lake-water δ¹⁸O values inferred from δ¹⁸O analysis of sediment cellulose range from −12.9 to −5.3‰ and average −8.7‰ between 6.2 and 2.3 ka B.P. Modern lake-water δ¹⁸O from LTCK averages −14.8‰ which is compatible with the δ¹⁸O_lw value of −14.3‰ for the surface sediment cellulose. Analyses of δ¹⁸O from modern surface waters in 23 lakes that span the range from glacial-fed to closed basin vary from −16.6 to −2.5‰. This approximates the magnitude of the down-core shift in δ¹⁸O_lw values in LTCK during the middle to late Holocene from −12.9 to −5.3‰. Strong paleohydrologic change during the middle Holocene is also evident in diatom assemblages that consist of shallow-water, non-glacial periphytic taxa and bulk organic δ¹³C and δ¹⁵N that show increases likely resulting from degradation of lacustrine organic matter periodically exposed to subaerial conditions. Neoglaciation began after 2.3 ka B.P. as indicated by changes in the composition of the sediments, lower δ¹⁸O values, and a return to diatom assemblages characteristic of the glacial sediments that formed during the Late Pleistocene. Collectively, these data indicate that the past 2.3 ka B.P. have been the wettest interval during the Holocene. Millennial-scale shifts in the paleohydrologic record of LTCK during the early to middle Holocene conform to other regional paleoclimatic time-series, including Lake Titicaca and Nevado Sajama, and may be driven by insolation and resultant changes in atmospheric circulation and moisture supply. In contrast, an apparent 1200-year lag in the onset of wetter conditions at LTCK (2.3 ka B.P.) compared to Lake Titicaca (3.5 ka B.P.) provides evidence for variable sub-regional hydrologic response to climate change during the middle to late Holocene.     

Middle Pleistocene climate and habitat change at Zhoukoudian, China, from the carbon and oxygen isotopic record from herbivore tooth enamel

The Pleistocene deposits at Zhoukoudian, often referred to as the “Peking Man” site, contain dental remains from a diverse group of herbivores, including Equus sanmeniensis, Cervus elaphus, Cervus nippon, Megaloceros pachyosteus, Sus lydekkeri, and Dicerorhinus choukoutienensis. The carbon and oxygen isotopic compositions of structural carbonate within the enamel of these teeth are used to reconstruct the paleodiet and paleoenvironment of the mammals. The δ¹³C values of enamel from Zhoukoudian range from −2.3‰ to −13.0‰, indicating that these mammals consumed between 25% and 100% C₃ plants. The presence of significant amounts of C₄ plants in the diets of some herbivore species indicates that at the onset of the Middle Pleistocene local habitats included mixed C₃/C₄ vegetation. By approximately 470,000 yr ago, C₃ plants dominated the diets of herbivores studied, suggesting that the abundance of C₄ flora had decreased in the area. For all deer analyzed in this study, the values of δ¹³C and δ¹⁸O decrease substantially from about 720,000 to 470,000 yr ago. This trend may be due to a strengthening of the winter monsoon during the Middle Pleistocene.     

Carbon and Oxygen Isotope Geochemistry of the Amba Dongar Carbonatite Complex, Gujarat, India

A carbon and oxygen isotope survey based on 42 samples from the Amba Dongar carbonatite complex of Gujarat, India, indicates that the magmatic differentiation series sövite → alvikite → ankeritic carbonatite is beset with a distinct isotope trend characterized by a moderate rise in ¹³C coupled with a sizeable increase in ¹⁸O. From an average of −4.6 ± 0.4 ‰ [PDB] for the least differentiated (coarse) sövite member, δ¹³C values slowly increase in the alvikite (−3.7 ± 0.6 ‰) and ankeritic fractions (−3.0 ± 1.1 ‰), whereas δ¹⁸O rises from 10.3 ± 1.7 ‰ [SMOW] to 17.5 ± 5.8 ‰ over the same sequence, reaching extremes between 20 and 28 ‰ in the latest generation of ankeritic carbonatite. While an apparent correlation between δ¹³C and δ¹⁸O over the δ¹⁸O range of 7–13 ‰ conforms with similar findings from other carbonatite complexes and probably reflects a Rayleigh fractionation process, the observed upsurge of ¹⁸O notably in the ankeritic member is demonstrably related to a late phase of low-temperature hydrothermal activity involving large-scale participation of ¹⁸O-depleted groundwaters. As a whole, the Amba Dongar carbonatite province displays the characteristic ¹³C/¹²C label of deep-seated (primordial) carbon, reflecting the carbon isotope composition of the subcontinental upper mantle below the Narmada Rift Zone of the Indian subcontinent.