Peak discharge of a Pleistocene lava-dam outburst flood in Grand Canyon, Arizona, USA

The failure of a lava dam 165,000 yr ago produced the largest known flood on the Colorado River in Grand Canyon. The Hyaloclastite Dam was up to 366 m high, and geochemical evidence linked this structure to outburst-flood deposits that occurred for 32 km downstream. Using the Hyaloclastite outburst-flood deposits as paleostage indicators, we used dam-failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph. Failure of the Hyaloclastite Dam released a maximum 11 × 10⁹ m³ of water in 31 h. Peak discharges, estimated from uncertainty in channel geometry, dam height, and hydraulic characteristics, ranged from 2.3 to 5.3 × 10⁵ m³ s⁻¹ for the Hyaloclastite outburst flood. This discharge is an order of magnitude greater than the largest known discharge on the Colorado River (1.4 × 10⁴ m³ s⁻¹) and the largest peak discharge resulting from failure of a constructed dam in the USA (6.5 × 10⁴ m³ s⁻¹). Moreover, the Hyaloclastite outburst flood is the oldest documented Quaternary flood and one of the largest to have occurred in the continental USA. The peak discharge for this flood ranks in the top 30 floods (>10⁵ m³ s⁻¹) known worldwide and in the top ten largest floods in North America.     

50,000 years of vegetation and climate history on the Colorado Plateau, Utah and Arizona, USA

Sixty packrat middens were collected in Canyonlands and Grand Canyon National Parks, and these series include sites north of areas that produced previous detailed series from the Colorado Plateau. The exceptionally long time series obtained from each of three sites (> 48,000 ¹⁴C yr BP to present) include some of the oldest middens yet discovered. Most middens contain a typical late-Wisconsinan glaciation mixture of mesic and xeric taxa, evidence that plant species responded to climate change by range adjustments of elevational distribution based on individual criteria. Differences in elevational range from today for trees and shrubs ranged from no apparent change to as much as 1200 m difference. The oldest middens from Canyonlands NP, however, differ in containing strictly xeric assemblages, including middens incorporating needles of Arizona single-leaf pinyon, far north of its current distribution. Similar-aged middens from the eastern end of Grand Canyon NP contain plants more typical of glacial climates, but also contain fossils of one-seed juniper near its current northern limit in Arizona. Holocene middens reveal the development of modern vegetation assemblages on the Colorado Plateau, recording departures of mesic taxa from low elevation sites, and the arrival of modern dominant components much later.     

Paleomagnetism of Middle Proterozoic mafic intrusions and Upper Proterozoic (Nankoweap) red beds from the Lower Grand Canyon Supergroup, Arizona

Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, DM=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, DM=7.0°). A new ⁴⁰Ar/³⁹Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred 200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence.     

Oxygen isotope records of goethite from ferricrete deposits indicate regionally varying Holocene climate change in the Rocky Mountain region, U.S.A.

Oxygen isotopes of goethite from ferricrete deposits were measured from both northern and southern Rocky Mountain localities to assess geographic variability in regional Holocene paleoclimate change. A ∼3.7‰ increase in oxygen isotope values of ¹⁴C-dated goethites in the northern Rocky Mountains suggests a regional-scale relative increase in amounts of isotopically heavy summer precipitation since the early Holocene. In contrast, oxygen isotope values from the southern Rocky Mountains increase abruptly ∼2.1‰ at ∼6200 ¹⁴C yr B.P., then decrease ∼2.4‰ between ∼2000 ¹⁴C yr B.P. and the present. We interpret this period of relatively high δ¹⁸O values as evidence for a middle Holocene warm period combined with a relatively strong summer monsoon. These variable climate records suggest that the Rocky Mountains of the western United States have had a spatially heterogeneous Holocene climate history.     

Evidence for high-frequency late Glacial to mid-Holocene (16,800 to 5500 cal yr B.P.) climate variability from oxygen isotope values of Lough Inchiquin, Ireland

A 7.6-m core recovered from Lough Inchiquin, western Ireland provides evidence for rapid and long-term climate change from the Late Glacial period to the Mid-Holocene. We determined percentage of carbonate, total organic matter, mineralogy, and δ¹⁸O_calcite values to provide the first high-resolution record of climate variability for this period in Ireland. Following deglaciation, rapid climate amelioration preceded large increases in GISP2 δ¹⁸O_ice values by ∼2300 yr. The Oldest Dryas (15,100 to 14,500 cal yr B.P.) Late Glacial event is documented in this record as a decrease in δ¹⁸O_calcite values. Brief warming at ∼12,700 cal yr B.P. was followed by characteristic Younger Dryas cold and dry climate conditions. A rapid increase in δ¹⁸O_calcite values at ∼10,500 cal yr B.P. marked the onset of Boreal warming in western Ireland. The 8200 cal yr B.P. event is represented by a brief cooling in our record. Prior to general warming, a larger and previously undescribed climate anomaly between 7300 and 6700 cal yr B.P. is characterized by low δ¹⁸O_calcite values with high-frequency variability.     

Thickness and stable isotopic characteristics of modern seasonal climate-controlled sub-annual travertine laminas in a travertine-depositing stream at Baishuitai, SW China: implications for paleoclimate reconstruction

A continuous high-resolution (monthly) record of stable isotopes (δ¹³C and δ¹⁸O) in a well-laminated freshwater travertine deposited at Baishuitai, SW China from May 1998 to November 2001 was presented. The travertine exhibits clear annual bands with coupled brown/white color laminations. Throughout field investigation, it was found that the thin (1.5–2.2 mm), brown porous lamina was formed in the monsoonal rainy season from April to September, whereas the thick (5–8 mm), dense white lamina was formed in the dry season from October to March. The comparisons of lamina thickness and stable isotope signals in the travertine with the meteorological records allow us to constrain the relevant geochemical processes in the travertine formation under different climate conditions and to relate climate variables to their physicochemical proxies in the travertine record. Sympathetic variations in lamina thickness, δ¹³C and δ¹⁸O along the sampled profile reflect changes in hydrogeochemistry, showing that thin lamina and low δ¹³C and δ¹⁸O values occur in warm and rainy seasons. The decreased amount of calcite precipitation and low δ¹³C values during the warm and rainy seasons is caused by dilution of overland flow after rainfall. The low δ¹⁸O values are believed to be related to the rainfall amount effect in subtropical monsoonal regions. This process is thought to be markedly subdued whenever the amount of rainfall is lower than a given threshold. Accordingly, distinct minima in lamina thickness, δ¹³C and δ¹⁸O are interpreted to reflect events with above-average rainfall, possibly heavy floods, and vice versa. This study demonstrates the potential of freshwater travertine to provide valuable information on seasonal or even monthly rainfall variations.     

Oxygen, carbon, and strontium isotope geochemistry of diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav Massif, Kazakhstan

Diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav massif, Kazakhstan, were strongly altered by fluids flowing through fractures and infiltrating along grain boundaries during exhumation. Alteration includes retrogradation of high-grade silicate assemblages by hydrous minerals, replacement of diamond by graphite and of dolomite by calcite. Diamond-bearing carbonate rocks are among the most intensely altered isotopically with δ¹⁸O_VSMOW values as low as +9‰, δ¹³C_VPDB=−9‰, and ⁸⁷Sr/⁸⁶Sr as high as 0.8050. Evidence of isotopic equilibration between coexisting dolomite and high-Mg calcite during ultrahigh-pressure metamorphism (UHPM) is preserved only rarely in samples isolated from infiltrating fluids by distance from fractures. Isotopic heterogeneity and isotopic disequilibrium are widespread on a hand-specimen scale. Because of this lack of homogeneity, bulk analyses cannot provide definitive measurements of ¹³C/¹²C fractionation between coexisting diamond and carbonate. Our study adequately documents alteration on a scale commensurate with observed vein structures. But, testing the hypothesis of metamorphic origin of microdiamonds has not fully succeeded because our analytical spatial resolution, limited to 0.5 mm, is not small enough to measure individual dolomite inclusions or individual diamond crystals.     

Early Holocene climate signals from stable isotope composition of ice wedges in the Chara Basin,northern Transbaikalia,Russia

Stable isotope composition of syngenetic and epigenetic ice wedges, radiocarbon age, and pollen spectra of the surrounding deposits were studied during long term investigations at the "Belyi Klyuch" site on the first(6-8 m height) terrace of the Chara River(720 m.a.s.l.) in northern Transbaikalia to assess climatic conditions during ice-wedge formation. It was revealed that Holocene ice wedges had been formed from 10 to 7.5 ka 14 C BP. The isotope composition(δ~(18)O, δ~2 H) of relict ice wedges is the lightest and amounts-23‰ and-185‰, correspondingly. The isotopic compositions of ice lenses from sandy loam above ice wedges are-15.7‰, and-133‰; of small ice wedge in peat and sand are-15.3‰ and-117.9‰, accordingly.Interpretation of the ice wedge isotope composition has yielded that mean winter temperatures during cold stages of Holocene optimum were lower than today, during warm stages they were close to modern ones. During the coldest stages of Holocene optimum the total annual freezing index varied from-5100 to-5700 ℃ degree days, i.e. 300-600 ℃ degree days colder than during extremely severe modern winters. The total annual thawing index varied from 1300 to 1800 ℃ degree days, which was slightly higher than modern ones.     

High-resolution evidence from southern China of an early Holocene optimum and a mid-Holocene dry event during the past 18,000 years

Computer models suggest that the Holocene Optimum for East Asian summer monsoon precipitation occurred at different times in different regions of China. Previous studies indicate that this time-transgressive Holocene Optimum should have been experienced about 3000 yr ago in southern China. In this study we describe a section which allows us to test this timing directly. We have closely examined high-resolution eutrophic peat/mud sequences covering the past 18,000 cal yr at Dahu, Jiangxi, on the southern boundary of the mid subtropical zone in China. Late Pleistocene successions in the Dahu record indicate cooler and much wetter conditions relative to synchronous events in north-central China. Our results indicate that the Holocene Optimum occurred between ca. 10,000 and 6000 cal yr ago in southern China, consistent with the global pattern. Conditions were relatively dry and cold from 6000 to 4000 cal yr ago. Our data also support the conclusion that the last deglaciation to early Holocene in the south was much wetter, resulting in the formation of dense broad-leaved forests, which could have acted to moderate land temperature ∼10,000 to 6000 cal yr ago, yielding a stable early-Holocene climate. After 6000 cal yr, forest reduction led to unstable land temperatures, and possibly to a northerly shift of the subtropical high-pressure system. Whatever the mechanism, these changes resulted in decreased precipitation between 6000 and 4000 cal yr B.P. in southern China.     

Oxygen isotope composition and geothermometry of granulite to greenschist facies metamorphic rocks: a study from the Neoproterozoic collision-related nappe system, south of São Francisco Craton, SE Brazil

Oxygen isotope studies were carried out across units of a Neoproterozoic nappe system, south of São Francisco Craton. A temperature decrease toward the base of the system is found, consistent with a previously recognized inverted metamorphic pattern. The tectonic contact of the basal unit and the reworked southern São Francisco craton show a steep temperature gradient, suggesting that low temperature thrusting acted as the dominant tectonic process. The contrasts between the δ¹⁸O values of the Três Pontas-Varginha and Carmo da Cachoeira nappes and the differences among the samples and minerals are consistent with the preservation of sedimentary isotopic composition during metamorphism. The small differences in the δ¹⁸O values between the undeformed and the deformed calc-silicate samples (1.6‰) suggest that the δ¹⁸O value of mylonitization fluids was close to that which equilibrated with the metamorphic assemblage. The distinct δ¹⁸O values of metapelitic and calc-silicate samples and the great temperature difference from one type to the other indicate that no large-scale fluid interaction processes occurred during metamorphism. Oxygen isotopic estimations of both Três Pontas-Varginha undeformed rocks and Carmo da Cachoeira unaltered equivalents indicate δ¹⁸O values of up to 18‰. Comparison between these values and those from the ‘basement’ orthogneisses (8.3–8.5‰) indicates the latter are not sources for the metapelites.     

Oxygen and carbon isotope composition of Gordon Group carbonates (Ordovician), Florentine Valley,Tasmania, Australia

The Gordon Group carbonates consist of biota of the Chlorozoan assemblage, diverse non‐skeletal grains and abundant micrite and dolomite, similar to those of modern warm water carbonates. Cathodoluminescence studies indicate marine, meteoric and some burial cements. Dolomites replacing burrows, mudcracks and micrite formed during early diagenesis.

δ¹⁸O values (‐5 to ‐7%ō PDB) of the non‐luminescent fauna and marine cement are lighter than those of modern counterparts but are similar to those existing within low latitudes during the Ordovician because of the light δ¹⁸O values of Ordovician seawater (‐3 to ‐5%o SMOW). The δ¹⁸O difference (2%o) between marine and meteoric calcite indicates that Ordovician meteoric water was similar to that in modern subtropics. Values of δ¹³C relative to δ¹⁸O indicate that during the Early Ordovician there were higher atmospheric CO₂ levels than at present but during the Middle and Late Ordovician they became comparable with the present because of a change from ‘Greenhouse’ to glacial conditions. δ¹⁸O values of Late Ordovician seawater were heavier than in the Middle Ordovician mainly because of glaciation.

Dolomitization took place in marine to mixed‐marine waters while the original calcium carbonate was undergoing marine to meteoric diagenesis.     

Paleoenvironment of the Folsom archaeological site, New Mexico, USA, approximately 10,500 C yr B.P. as inferred from the stable isotope composition of fossil land snail shells

Well-preserved aragonitic land snail shells (Vallonia) from late Pleistocene Eolian sediment in the Folsom archaeological site in New Mexico exhibit an overall decrease of δ¹⁸O_PDB from maximum values of +2.7‰ (more positive than modern) to younger samples with lower average values of about −3.6‰ (within the modern range). The age of the samples (approximately 10,500 ¹⁴C yr B.P.) suggests that the decrease in δ¹⁸O may manifest climatic changes associated with the Younger Dryas. Some combination of increased relative humidity and cooler temperatures with decreased δ¹⁸O of precipitation during the times of snail activity can explain the decrease in shell δ¹⁸O. A well-known Paleoindian bison kill occurred at the Folsom site during this inferred environmental transition.Average δ¹³C values of the aragonite shells of the fossil Vallonia range from −7.3 to −6.0‰ among different archaeological levels and are not as negative as modern values. This suggests that the proportion of C₄ vegetation at the Folsom site approximately 10,500 ¹⁴C yr B.P. was greater than at present; a result which is consistent with other evidence for higher proportions of C₄ plants in the region at that time.     

A geochemical view into continental palaeotemperatures of the end-Permian using oxygen and hydrogen isotope composition of secondary silica in chert rubble breccia: Kaibab Formation,Grand Canyon (USA)

The upper carbonate member of the Kaibab Formation in northern Arizona (USA) was subaerially exposed during the end Permian and contains fractured and zoned chert rubble lag deposits typical of karst topography. The karst chert rubble has secondary (authigenic) silica precipitates suitable for estimating continental weathering temperatures during the end Permian karst event. New oxygen and hydrogen isotope ratios of secondary silica precipitates in the residual rubble breccia: (1) yield continental palaeotemperature estimates between 17 and 22 °C; and, (2) indicate that meteoric water played a role in the crystallization history of the secondary silica. The continental palaeotemperatures presented herein are broadly consistent with a global mean temperature estimate of 18.2 °C for the latest Permian derived from published climate system models. Few data sets are presently available that allow even approximate quantitative estimates of regional continental palaeotemperatures. These data provide a basis for better understanding the end Permian palaeoclimate at a seasonally-tropical latitude along the western shoreline of Pangaea.     

Lipid biomarkers and carbon-isotopes of modern travertine deposits (Yellowstone National Park, USA): Implications for biogeochemical dynamics in hot-spring systems

Lipid biomarkers and ¹³C fractionation patterns were used to understand the dynamics of carbon cycling during microbial metabolisms in different environments of travertine precipitation (called facies) at Spring AT-1 on Angel Terrace in the Mammoth Hot Springs complex of Yellowstone National Park, USA. Microbial mats that encrust travertine deposits were collected for analyses of lipid biomarkers and carbon isotopes along the continuous drainage outflow system of Spring AT-1. The spring water exhibits a continuous temperature drop from 71°C in the vent at top to 24°C in the distal slope at bottom. Phospholipid fatty acids (PLFA) and glycolipid fatty acids (GLFA) exhibit distinctly different compositions in each of the facies, which are consistent with partitioning of the bacterial 16S rRNA gene sequences in the Spring AT-1 travertine facies (Fouke et al., 2003).The δ¹³C composition of total biomass within the microbial mats decreases from −16.1‰ in the vent to −23.5‰ in the distal slope. However, lower values occur in the pond (−26.0‰) and the proximal slope (−28.0‰) between the vent and the distal slope. Isotopic compositions of PLFA and GLFA have variations similar to those of total biomass. The average δ¹³C values of PLFA are −12.4 ± 5.2‰ (n = 10 individual fatty acids, same below) in the vent, −33.0 ± 3.1‰ (n = 11) in the pond, −33.7 ± 3.8‰ (n = 16) in the proximal slope, and −22.4 ± 3.4‰ (n = 10) in the distal slope; the average δ¹³C values of GLFA are −19.6 ± 3.0‰ (n = 3) in the vent, −30.4 ± 4.7‰ (n = 8) in the pond, −36.9 ± 2.8‰ (n = 12) in the proximal slope, and −27.9 ± 3.1‰ (n = 13) in the distal slope. In particular, fatty acids in the vent are enriched in ¹³C relative to the total biomass, which is consistent with the notion that the biosynthetic pathways of the extant microbial community in the vent may be dominated by Aquificales using the reversed tricarboxylic acid cycle. Fractionations between fatty acids and total biomass in the pond, the proximal slope and the distal slope suggest the involvement of other biosynthetic pathways for CO₂ fixation by extant microbial populations. The results indicate that lipid biomarkers provide valuable information on the changing diversity and activity of microbial communities in different depositional environments. Carbon-isotope fractionations, on the other hand, can provide insight into the operating biosynthetic pathways associated with different organisms in the changing environment. This integrated approach may serve as a powerful tool for identifying functional metabolism within a community and identify shifts in microbial community structure in modern hot-spring systems.     

Mineralogy,petrography, and oxygen and aluminum-magnesium isotope systematics of grossite-bearing refractory inclusions

Grossite (CaAl₄O₇) is one of the one of the first minerals predicted to condense from a gas of solar composition, and therefore could have recorded isotopic compositions of reservoirs during the earliest stages of the Solar System evolution. Grossite-bearing Ca,Al-rich inclusions (CAIs) are a relatively rare type of refractory inclusions in most carbonaceous chondrite groups, except CHs, where they are dominant. We report new and summarize the existing data on the mineralogy, petrography, oxygen and aluminum-magnesium isotope systematics of grossite-bearing CAIs from the CR, CH, CB, CM, CO, and CV carbonaceous chondrites. Grossite-bearing CAIs from unmetamorphosed (petrologic type 2―3.0) carbonaceous chondrites preserved evidence for heterogeneous distribution of ²⁶Al in the protoplanetary disk. The inferred initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] in grossite-bearing CAIs is generally bimodal, ˜0 and ˜5×10⁻⁵; the intermediate values are rare. CH and CB chondrites are the only groups where vast majority of grossite-bearing CAIs lacks resolvable excess of radiogenic ²⁶Mg. Grossite-bearing CAIs with approximately the canonical (²⁶Al/²⁷Al)₀ of ˜5×10⁻⁵ are dominant in other chondrite groups. Most grossite-bearing CAIs in type 2–3.0 carbonaceous chondrites have uniform solar-like O-isotope compositions (Δ¹⁷O ˜ ‒24±2‰). Grossite-bearing CAIs surrounded by Wark-Lovering rims in CH chondrites are also isotopically uniform, but show a large range of Δ¹⁷O, from ˜ ‒40‰ to ˜ ‒5‰, suggesting an early generation of gaseous reservoirs with different oxygen-isotope compositions in the protoplanetary disk. Igneous grossite-bearing CAIs surrounded by igneous rims of ±melilite, Al-diopside, and Ca-rich forsterite, found only in CB and CH chondrites, have uniform ¹⁶O-depleted compositions (Δ¹⁷O ˜ ‒14‰ to ‒5‰). These CAIs appear to have experienced complete melting and incomplete O-isotope exchange with a ¹⁶O-poor (Δ¹⁷O ˜ ‒2‰) gas in the CB impact plume generated about 5 Ma after CV CAIs. Grossite-bearing CAIs in metamorphosed (petrologic type >3.0) CO and CV chondrites have heterogeneous Δ¹⁷O resulted from mineralogically-controlled isotope exchange with a ¹⁶O-poor (Δ¹⁷O ˜ ‒2 to 0‰) aqueous fluid on the CO and CV parent asteroids 3–5 Ma after CV CAIs. This exchange affected grossite, krotite, melilite, and perovskite; corundum, hibonite, spinel, diopside, forsterite, and enstatite preserved their initial O-isotope compositions. The internal ²⁶Al-²⁶Mg isochrons in grossite-bearing CAIs from weakly-metamorphosed CO and CV chondrites were not disturbed during this oxygen-isotope exchange.HCCJr is grateful to Klaus Keil for all his sound profession counsel and collegial friendship over the years. He has always been willing to talk and has the generous nature of listening and sharing his thoughts freely and constructively. Professor Klaus Keil has been a mentor to and played a key role in the careers of three of the authors of this paper (ANK, KN, and GRH). He has also influenced the careers of the other authors and most of the people who have worked on meteorites over the past 50+ years. We therefore dedicate this paper to Professor Keil and present it in this Special Issue of Geochemistry.     

Mineralogy,petrology, and oxygen isotopic composition of Northwest Africa 12379, metal-rich chondrite with affinity to ordinary chondrites

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa_18.1–28.3, average Fa_24.9±3.2, PMD = 12.8; Cr₂O₃ = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs_3.2–18.7Wo_0.2–4.5; average Fs_14.7±3.7Wo_1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ¹⁷O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa₂₇) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ¹⁷O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ¹⁸O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.     

Petrotectonic evolution of the high- to ultrahigh-pressure Maksyutov Complex, Karayanova area, south Ural Mountains: structural and oxygen isotope constraints

The Maksyutov Complex consists of three fault-bounded lithologic units: a quartzofeldspathic gneiss containing mafic eclogite boudins (Unit #1); a metasedimentary blueschist-facies (Yumaguzinskaya) unit; and a meta-ophiolitic mélange (Unit #2). The geologic history of the high- to ultrahigh-pressure (HP–UHP) assembly of the Maksyutov Complex is complicated by several stages of prolonged retrograde metamorphism and deformation. The Sakmara River exposes all three units near the former village of Karayanova. A structural/petrologic cross-section through the area yields new quantitative data for the complex and, regionally, for the south Urals. Analysis of the Karayanova area has identified the major structures. Regional folding within the complex is parallel to the dominant foliation trending northeast–southwest. Stereonet data show that, during exhumation, this large-scale folding was refolded about axes trending southeast. Unit #1 and the Yumaguzinskaya are tectonically and petrologically distinct units juxtaposed by west-vergent thrusting and recrystallization within the same subduction zone. A shear zone developed later between Unit #2 and the Unit #1+Yumaguzinskaya tectonic package accompanying exhumation. Field relations and petrofabric demonstrate that blueschist-facies recrystallization overprinted an earlier eclogite-facies metamorphism. Thermobarometric measurements yield P–T values of 594–637°C, 1.5–1.7 GPa for eclogite, but these conditions may reflect annealing during the early-stage exhumation at 375 Ma. Cuboid graphite aggregates testify to precursor conditions for Unit #1 within the diamond stability field, if such textures are correctly interpreted. Measured ¹⁸O/¹⁶O partitioning between pairs of coexisting phases yield three main recrystallization temperature ranges: (1) 678±83°C, attending Unit #1 eclogite-facies metamorphism; (2) 453±17°C, during transitional blueschist/greenschist-facies metamorphism for the amalgamated Unit #1+Yumaguzinskaya+Unit #2 assembly; and (3) 250±68°C, reflecting late-stage hydrothermal alteration and exhumation. Oxygen isotope data for Units #1 and #2 indicate that garnet, blue amphibole, and pyroxene crystallized in isotopic equilibrium, validating previous thermobarometric calculations for a Unit #1 retrograde metamorphic event. Variations in δ¹⁸O values for phengites suggest the possibility of late metamorphic fluid infiltration. Retrograde recrystallization at high pressure in the presence of fluids and a calculated slow exhumation rate for the Maksyutov Complex account for the fact that inferred UHP coesite and diamond were completely back-reacted during decompression.     

Stable carbon isotope composition of terrestrial leaves: inter‐ and intraspecies variability,cellulose and whole‐leaf tissue difference,and potential for climate reconstruction

Stable carbon isotope analysis of terrestrial plant leaves preserved in Quaternary lake sediments has the potential to provide high‐resolution reconstructions of past climatic conditions. Preferably, δ¹³C measurements should be made on a single leaf component, e.g. cellulose, but this approach is often precluded by limited leaf availability. Previous work suggests that reliable palaeoclimatic information also may be derived from δ¹³C measurements on whole‐leaf tissue, given a similar degree of leaf decomposition between samples. Leaf δ¹³C data for 12 Scandinavian species of dwarf‐shrubs, shrubs and trees, and a comparison of δ¹³C data on recent and late Holocene Salix herbacea leaves, revealed that the δ¹³C signal registered by holocellulose is largely reflected by measurements on whole‐leaf tissue. Holocellulose was found to be consistently enriched in ¹³C, although this δ¹³C offset was smaller for subfossil leaves. This supports the use of δ¹³C measurements on whole‐leaf tissue for climate reconstruction, at least for leaves preserved in soft, late Holocene sediments with minimal diagenetic effects. Leaf carbon and nitrogen data on fresh leaves of the same 12 Scandinavian species, and corresponding data on late Holocene Salix herbacea leaves, suggest that the leaf C:N ratio is a suitable indicator of the degree of leaf decomposition. Copyright © 2003 John Wiley & Sons, Ltd.     

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.