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Sulfur isotope evidence for a Paleoarchean subseafloor biosphere, Barberton, South Africa //GEOLOGY  Volume: 40   Issue: 11   Pages: 1031-1034   DOINOV 2012
The Archean sub-seafloor has been proposed as an environment for the emergence of life, with septate clusters of titanite microtextures in pillow lava rims argued to be the earliest traces of microbial microboring. Here we use nanoscale secondary ion mass spectrometry (NanoSIMS) to test possible geochemical traces of life in ca. 3.45 Ga pillow lavas of the Barberton Greenstone Belt, South Africa. Sulfide inclusions in the titanite microtextures record strongly negative sulfur isotope fractionations of delta S-34(VCDT) -39.8 parts per thousand to -3.2 parts per thousand (VCDT-Vienna Canyon Diablo Troilite). These represent the largest range and most negative delta S-34 values so far reported from the Archean, and are consistent with an early biogenic origin for the sulfides. Extensive in situ elemental mapping did not find any organic linings associated with the microtextures, despite the high spatial resolution and sensitivity of the NanoSIMS. The absence of organic linings thus excludes a key line of evidence previously used to support the biogenicity of the microtextures. In contrast, in situ sulfur isotope analysis of basalt-hosted sulfides provides an alternative approach to investigating the existence and nature of an Archean subseafloor biosphere.

54. Shen, Jun; Algeo, Thomas J.; Hu, Qing; et al.

Negative C-isotope excursions at the Permian-Triassic boundary linked to volcanism //GEOLOGY  Volume: 40   Issue: 11   Pages: 963-966    NOV 2012
Two Permian-Triassic boundary (PTB) sections from south China provide insights regarding the origin of negative excursions in carbonate and organic carbon isotope records associated with the largest mass extinction in Earth history. Xiakou, a carbonate ramp section, exhibits delta C-13(carb) excursions of up to -2 parts per thousand, and Xinmin, a deep-shelf section, exhibits delta C-13(org) excursions of up to -6 parts per thousand. In both sections, these excursions are associated with volcanic ash layers, and excursion size scales with ash layer thickness. These relationships document the direct influence of volcanism on the Earth-surface carbon cycle during the PTB crisis. Previous studies of ash layers in south China PTB sections have invoked a source in regional subduction-zone volcanism in the eastern Tethys, but our analysis suggests that these ash layers may represent distal deposits from large-scale explosive eruptions of the Siberian Traps. If confirmed by further investigation, this hypothesis would have important implications both for kill mechanisms during the PTB crisis as well as for refinement of the eruption history of the Siberian Traps.

55. Scambelluri, Marco; Tonarini, Sonia

Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle //GEOLOGY  Volume: 40   Issue: 10   Pages: 907-910    OCT 2012
Serpentinites formed by alteration of oceanic and forearc mantle are major volatile and fluid-mobile element reservoirs for arc magmatism, though direct proof of their dominance in the subduction-zone volatile cycles has been elusive. Boron isotopes are established markers of fluid-mediated mass transfer during subduction. Altered oceanic crust and sediments have been shown to release in the subarc mantle B-11-depleted fluids, which cannot explain B-11 enrichment of many arcs. In contrast to these crustal reservoirs, we document high delta B-11 values retained in subduction-zone Alpine serpentinites. No B-11 fractionation occurs in these rocks with progressive burial: the released B-11-rich fluids uniquely explain the elevated delta B-11 of arc magmas. B, O-H, and Sr isotope systems indicate that serpentinization was driven by slab fluids that infiltrated the slab-mantle interface early in the subduction history.

56. Blaether, Clara L.; Henderson, Gideon M.; Jenkyns, Hugh C.Explaining the Phanerozoic Ca isotope history of seawater //GEOLOGY  Volume: 40   Issue: 9   Pages: 843-846    SEP 2012

A new geochemical budget for the modern marine carbonate sink helps to explain the major features of the Phanerozoic Ca isotope record. A large compilation of Ca isotope ratios for modern carbonates, incorporating more than 50 new measurements, represents the quantitatively important components of the system. With this data set, distinct Ca isotope ratios are identified for different types of marine carbonate, the balance of which has changed over time with shifts between calcite and aragonite seas and with the development of pelagic calcification during the Mesozoic. It is suggested that large-scale changes in the Ca isotope ratio of seawater, as exemplified by that in the Carboniferous, were no longer possible after Jurassic time because of the generation of a deep-sea calcite sink expressed by deposition of foraminiferal-coccolith ooze across the world ocean. This work demonstrates the close connection between isotopic cycling, carbonate sedimentation, seawater chemistry, and evolutionary trends.

57. Hinojosa, Jessica L.; Brown, Shaun T.; Chen, Jun; et al.

Evidence for end-Permian ocean acidification from calcium isotopes in biogenic apatite //GEOLOGY  Volume: 40   Issue: 8   Pages: 743-746  AUG 2012
End-Permian (ca. 252 Ma) carbon isotope, paleobiological, and sedimentary data suggest that changes in ocean carbonate chemistry were directly linked to the mass extinction of marine organisms. Calcium isotopes provide a geochemical means to constrain the nature of these changes. The delta Ca-44/40 of carbonate rocks from southern China exhibits a negative excursion across the end-Permian extinction horizon, consistent with either a negative shift in the delta Ca-44/40 of seawater or a change in the calcite/aragonite ratio of carbonate sediments at the time of deposition. To test between these possibilities, we measured the delta Ca-44/40 of hydroxyapatite conodont microfossils from the global stratotype section and point (GSSP) for the Permian-Triassic boundary at Meishan, China. The conodont delta Ca-44/40 record shows a negative excursion similar in stratigraphic position and magnitude to that previously observed in carbonate rocks. Parallel negative excursions in the delta Ca-44/40 of carbonate rocks and conodont microfossils cannot be accounted for by a change in carbonate mineralogy, but are consistent with a negative shift in the delta Ca-44/40 of seawater. Such a shift is best accounted for by an episode of ocean acidification, pointing toward strong similarities between the greatest catastrophe in the history of animal life and anticipated global change during the twenty-first century.

58. Arnold, G. L.; Lyons, T. W.; Gordon, G. W.; et al.Extreme change in sulfide concentrations in the Black Sea during the Little Ice Age reconstructed using molybdenum isotopes //GEOLOGY  Volume: 40   Issue: 7   Pages: 595-598    JUL 2012

The Black Sea is the largest and most studied anoxic basin in the modern world. Much of this research has focused on the redox structure of the water column, specifically on the driving forces behind variations in the position, stability, and structure of the oxic-anoxic interface (chemocline). However, none of these studies has been able to quantify the historical sulfide concentrations associated with the changes in chemocline depth. Using the isotopic composition of molybdenum in sediments as a proxy, we show for the first time that varying concentrations of dissolved sulfide can be fingerprinted in historical systems. Our molybdenum isotope data indicate that in the region of the Bosporus inlet, the chemocline rose more than 65 m, reaching concentrations over 100 mu M sulfide in the bottom water ca. 300 yr B.P. This historical shoaling of the chemocline and extreme change in bottom-water sulfide concentration exceeds the modern changes that have been observed directly and attributed to anthropogenic influences on the Black Sea chemistry/hydrology. The first cold interval of the Little Ice Age, when temperature and circulation changes occurred in the Black Sea basins, may have provided the natural trigger for this extreme rise in bottom-water sulfide concentrations.

59. Bottini, Cinzia; Cohen, Anthony S.; Erba, Elisabetta; et al.

Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a// GEOLOGY  Volume: 40   Issue: 7   Pages: 583-586    JUL 2012
The early Aptian Oceanic Anoxic Event (OAE 1a) resulted from an exceptional set of interactions between the geosphere, the biosphere, and the ocean-atmosphere system. We present new Re-Os data from two sites spanning OAE 1a in the Tethys and Pacific Oceans. The patterns of variation in the seawater Os-isotope composition from both sites are very similar, and together they constrain the timing and duration of continental weathering in relation to the large-scale volcanic activity of the Ontong Java Plateau. The dominant feature through the OAE is an interval of similar to 880 k.y. when the Os-isotope composition of the global ocean was exceptionally unradiogenic, implicating unambiguously the Ontong Java Plateau as the trigger and sustaining mechanism for OAE 1a. A relatively short-lived (similar to 100 k.y.) Os-isotope excursion to radiogenic compositions in the Tethyan record is clearly linked to an abrupt perturbation to the global carbon cycle, and is fully consistent with the Pacific record. These highly distinctive features of seawater Os in contemporaneous samples from three high-resolution sections, two of which were very remote from the Ontong Java Plateau, indicate that ocean mixing at that time was very efficient. The results suggest that OAE 1a was also related to rapid global warming and elevated rates of silicate weathering both on the continents and in the oceans.

60. Woodhead, Jon; Stern, Robert J.; Pearce, Julian; et al.

Hf-Nd isotope variation in Mariana Trough basalts: The importance of "ambient mantle" in the interpretation of subduction zone magmas //GEOLOGY  Volume: 40   Issue: 6   Pages: 539-542   JUN 2012
In the study of geochemical mass balances at subduction zones, the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter. Deconvolving the influence of ancient versus modern enrichments is particularly difficult, especially when considering elements that are highly mobile. Here we provide an alternative approach, using less mobile elements, and a filter to remove the effects of recent slab additions. We provide new Hf isotope data for 30 Mariana Trough (MT) backarc basin lavas. Once filtered, Hf and Nd isotope ratios are highly correlated, of Indian mid-oceanic ridge basalt character, and display variations similar to ocean ridges of comparable lengths. The isotopic variability observed in this "ambient mantle" provides a new paradigm for the interpretation of the varied volcanic products of the arc. Thus, shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted sediment melt with an isotopically enriched mantle source. In contrast, the large volcanoes of the Central Island province have a consistent offset in Nd isotope compositions from the MT array resulting from fluid addition. Existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale. We suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide.

61. Peucker-Ehrenbrink, Bernhard; Hanghoj, Karen; Atwood, Tracy; et al.Rhenium-osmium isotope systematics and platinum group element concentrations in oceanic crust //GEOLOGY  Volume: 40   Issue: 3   Pages: 199-202    MAR 2012

Knowledge of the Os-187/Os-188 ratio as well as the inventories of rhenium and platinum group elements (PGE) in oceanic crust allows quantification of the proportion of recycled oceanic crust in oceanic basalt sources. Our knowledge is limited by the availability of well-characterized sections of oceanic crust, specifically of the plutonic, lower portion that has not been drilled in situ to the Moho. Here we report new data for plutonic rocks that compose the bottom 4680 m of an ocean crust section from the Oman ophiolite. Major and trace element data as well as mineral analyses indicate that Oman gabbros are primitive cumulates from melts similar to typical mid-oceanic ridge basalt. The mean weighted composition of this section (Re: 427 pg/g; Os: 55 pg/g; Ir: 182 pg/g; Pd: 2846 pg/g; Pt: 4151 pg/g; initial (POs)-P-187/Os-188: 0.142) indicates significantly higher Os and lower Re concentrations than previously analyzed partial sections of ocean crust that lack cumulate lower crust [Deep Sea Drilling Project Ocean Drilling Program (DSDP-ODP) Hole 504B, ODP Hole 735B], emphasizing that the lower, cumulate oceanic crust dominates the Os budget of oceanic crust. Analyses of mineral grain size fractions indicate that rhenium, PGE, and lead are enriched in the sulfur-rich, fine fraction. This corroborates the notion that small accessory phases, and the melt migration processes affecting them, control these elements' budgets, distributions, and susceptibilities to alteration. The Re-Os-PGE inventories of a hypothetical 6.5-km-thick composite section that consists of 1825 m of DSDP Hole 504B like upper oceanic crust and 4680 m of Oman-like lower ocean crust (Re: 736 pg/g; Os: 45 pg/g; Ir: 133 pg/g; Pd: 2122 pg/g; Pt: 2072 pg/g; initial Os-187/Os-188: 0.146) provide a new comprehensive assessment of oceanic crust composition. Upon recycling and mixing with reasonable proportions of mantle peridotite, this composite requires at least 2 G.y. to develop sufficiently radiogenic Os-187/Os-188 to generate high mu (HIMU: mu = U-238/Pb-204) basalts.

62. Barker, Philip A.; Hurrell, Elizabeth R.; Leng, Melanie J.; et al.

Seasonality in equatorial climate over the past 25 k.y. revealed by oxygen isotope records from Mount Kilimanjaro //GEOLOGY  Volume: 39   Issue: 12   Pages: 1111-1114    DEC 2011
Multiproxy analysis of a well-dated 25 ka lake sediment sequence from Lake Challa, on the eastern flank of Mount Kilimanjaro (East Africa), reveals the climatic controls that govern both the lake's paleohydrology and the climate-proxy record contained in the mountain's receding ice cap. The oxygen isotope record extracted from diatom silica (delta O-18(diatom)) in Lake Challa sediments captured dry conditions during the last glacial period and a wet late-glacial transition to the Holocene interrupted by Younger Dryas drought. Further, it faithfully traced gradual weakening of the southeastern monsoon during the Holocene. Overall, delta O-18(diatom) matches the branched isoprenoid tetraether (BIT) index of rainfall-induced soil runoff, except during 25-22 ka and the past 5 k.y. when insolation forcing due to orbital precession enhanced the northeastern monsoon. This pattern arises because during these two periods, a weakened southeastern monsoon reduced the amount of rainfall during the long rainy season and enhanced the opposing effect of evaporation intensity and/or length of the austral winter dry season. Importantly, our lake-based reconstruction of moisture-balance seasonality in equatorial East Africa also helps us understand the oxygen isotope record contained in Mount Kilimanjaro ice. Negative correlation between ice core delta O-18 and Lake Challa delta O-18(diatom) implies that moisture balance is not the primary climate control on the long-term trend in ice core delta O-18.

63. Elrick, Maya; Rieboldt, Sarah; Saltzman, Matthew R.; et al.

Oxygen-isotope trends and seawater temperature changes across the Late Cambrian Steptoean positive carbon-isotope excursion (SPICE event) //GEOLOGY  Volume: 39   Issue: 10   Pages: 987-990    OCT 2011
The globally recognized Late Cambrian Steptoean positive C-isotope excursion (SPICE) is characterized by a 3 parts per thousand-5 parts per thousand positive delta C-13 shift spanning <4 m.y. Existing hypotheses suggest that the SPICE represents a widespread ocean anoxic event leading to enhanced burial/preservation of organic matter (C-org) and pyrite. We analyzed delta O-18 values of apatitic inarticulate brachiopods from three Upper Cambrian successions across Laurentia to evaluate paleotemperatures during the SPICE. delta O-18 values range from similar to 12.5 parts per thousand to 16.5 parts per thousand. Estimated seawater temperatures associated with the SPICE are unreasonably warm, suggesting that the brachiopod delta O-18 values were altered during early diagenesis. Despite this, all three localities show similar trends with respect to the SPICE delta C-13 curve, suggesting that the brachiopod apatite preserves a record of relative delta O-18 and temperature changes. The trends include relatively high delta O-18 values at the onset of the SPICE, decreasing and lowest values during the main event, and an increase in values at the end of the event. The higher delta O-18 values during the global extinction at the onset of the SPICE suggests seawater cooling and supports earlier hypotheses of upwelling of cool waters onto the shallow shelf. Decreasing and low delta O-18 values coincident with the rising limb of the SPICE support the hypothesis that seawater warming and associated reduced thermohaline circulation rates contributed to decreased dissolved O-2 concentrations, which enhanced the preservation/burial of C-org causing the positive delta C-13 shift.

64. Wolff, John A.; Ellis, Ben S.; Ramos, Frank C Strontium isotopes and magma dynamics: Insights from high-temperature rhyolites //GEOLOGY  Volume: 39   Issue: 10   Pages: 931-934    OCT 2011

Volcanic rocks often exhibit internal heterogeneity in radiogenic isotopes. Isotopic disequilibrium between coexisting phenocrysts and isotopic zoning within single crystals has been demonstrated in basalts, andesites, dacites, and rhyolites. High-temperature Snake River-type rhyolites appear to be an exception. Despite the occurrence of Snake River Plain rhyolites in a region of isotopically highly variable crust and mantle, and significant differences from rhyolite unit to rhyolite unit, little to no Sr isotopic zoning is found within their feldspar phenocrysts, and feldspars within a single unit define tightly grouped unimodal populations. High-temperature rhyolitic magmas possess a unique combination of temperature and melt viscosity. Although typically 200 degrees C hotter than common rhyolites, the temperature effect on viscosity is offset by lower water contents (<3.5 wt%), so melt viscosities are in the same range as common, water-rich, cool rhyolites (10(5)-10(6) Pa s). However, magmatic temperatures are in the same range as basaltic andesites and andesites; consequently, cation diffusion rates are orders of magnitude greater than common rhyolites. We hypothesize that this combination of characteristics promotes crystal isotopic homogeneity: viscosities are too high to permit crystal transfer between liquids of contrasting Sr-87/Sr-86 on time scales shorter than those required for diffusive homogenization of Sr within phenocrysts (500-10,000 yr). This is not true for most magma types, in which crystal transfer is rapid (<<100 yr) due to low melt viscosities (basalts and intermediate magmas), or Sr diffusion rates are so slow that equilibration times are longer than the lifetime of the system (e.g., cool, wet rhyolites: 10(5)-10(6) yr).

65. Wang, Xuan-Ce; Li, Zheng-Xiang; Li, Xian-Hua; et al.

Nonglacial origin for low-delta O-18 Neoproterozoic magmas in the South China Block: Evidence from new in-situ oxygen isotope analyses using SIMS //GEOLOGY  Volume: 39   Issue: 8   Pages: 735-738    AUG 2011
Low-delta O-18 signatures in supracrustal rocks have been used as geochemical proxies for cold paleoclimates, e.g., glaciations. Unusual low-delta O-18 values found in Neoproterozoic igneous rocks in parts of the South China Block have thus been genetically linked to Neoproterozoic glaciation events. However, we report here new oxygen isotope compositions from Neoproterozoic magmatic zircons in central southern China using in-situ techniques that argue against such an interpretation. Our results show that (1) low-delta O-18 magmatic zircons started to appear in the South China Block from ca. 870 Ma, coinciding with the tectonic switching from Sibao orogenesis to postorogenic extension, which occurred more than 150 m.y. prior to the first glaciation event. The most abundant low-delta O-18 magmatic zircons have ages of 800-700 Ma. (2) The 830-700 Ma magmatic zircons are characterized by their bimodal nature of oxygen isotope compositions, i.e., mantle-like delta O-18 values (+4.4 parts per thousand to +5.8 parts per thousand) and high-delta O-18 values (+9.3 parts per thousand to +10.8 parts per thousand). (3) A sharp temporal change in maximum zircon delta O-18 values in the South China Block coincided with the onset of continental rifting and the possible arrival of a plume head. (4) No negative delta O-18 zircons have been identified in this study, contrary to previous studies. These features strongly argue against a glaciation origin for low to negative delta O-18 values in Neoproterozoic magmatic zircons from southern China. We propose that two stages of high-temperature water-magma interaction during plume-driven magmatism and continental rifting best explain the low-delta O-18 magmas. The most important implication of this study is that formation of such low-delta O-18 magmatic zircons was not necessarily related to glacial events and should not be used as a geochemical proxy for a cold paleoclimate.

66. Griffith, Elizabeth M.; Paytan, Adina; Eisenhauer, Anton; et al Seawater calcium isotope ratios across the Eocene-Oligocene transition //GEOLOGY  Volume: 39   Issue: 7   Pages: 683-686    JUL 2011

During the Eocene-Oligocene transition (EOT, ca. 34 Ma), Earth's climate cooled significantly from a greenhouse to an icehouse climate, while the calcite (CaCO3) compensation depth (CCD) in the Pacific Ocean increased rapidly. Fluctuations in the CCD could result from various processes that create an imbalance between calcium (Ca) sources to, and sinks from, the ocean (e. g., weathering and CaCO3 deposition), with different effects on the isotopic composition of dissolved Ca in the oceans due to differences in the Ca isotopic composition of various inputs and outputs. We used Ca isotope ratios (delta Ca-44/40) of coeval pelagic marine barite and bulk carbonate to evaluate changes in the marine Ca cycle across the EOT. We show that the permanent deepening of the CCD was not accompanied by a pronounced change in seawater delta Ca-44/40, whereas time intervals in the Neogene with smaller carbonate depositional changes are characterized by seawater delta Ca-44/40 shifts. This suggests that the response of seawater delta Ca-44/40 to changes in weathering fluxes and to imbalances in the oceanic alkalinity budget depends on the chemical composition of seawater. A minor and transient fluctuation in the Ca isotope ratio of bulk carbonate may reflect a change in isotopic fractionation associated with CaCO3 precipitation from seawater due to a combination of factors, including changes in temperature and/or in the assemblages of calcifying organisms.

67. Ferry, John M.; Passey, Benjamin H.; Vasconcelos, Crisogono; et al.Formation of dolomite at 40-80 degrees C in the Latemar carbonate buildup, Dolomites, Italy, from clumped isotope thermometry //GEOLOGY  Volume: 39   Issue: 6   Pages: 571-574    JUN 2011

The temperature of formation of replacement dolomite and delta O-18(H2O) of dolomitizing fluid in the Latemar carbonate buildup, Dolomites, Italy, were estimated independently from carbonate clumped isotope thermometry. Dolomite formed at 42-72 +/- 9-11 degrees C (+/- 2 standard deviations, SD) from fluid with delta O-18(H2O) that averages -0.3% +/- 3.3% (Vienna standard mean ocean water; +/- 2 SD). The estimated temperature and delta O-18(H2O) are similar to those of modern diffuse flow fluids at mid-ocean ridges, the kind of fluid that has been proposed previously as the dolomitizing fluid in the Latemar buildup, based on the trace element compositions of dolomite. Calcite in limestone preserves original d18O, but records clumped isotope temperatures, 44-76 +/- 9-11 degrees C (+/- 2 SD), that are higher than those at which the limestone formed. Temperature recorded by calcite, but not delta O-18, was likely reset during dolomitization. Clumped isotope thermometry has great potential for application to studies of burial and diagenesis by retrieving independent estimates of temperature and delta O-18(H2O) with uncertainties as low as +/- 5 degrees C (+/- 2 standard errors, SE) and +/- 0.75% (+/- 2 SE), respectively, from a single stable isotope analysis of a carbonate mineral.

68. Whittaker, Thomas E.; Hendy, Chris H.; Hellstrom, John C.

Abrupt millennial-scale changes in intensity of Southern Hemisphere westerly winds during marine isotope stages 2-4 //GEOLOGY  Volume: 39   Issue: 5   Pages: 455-458  MAY 2011
The strength of mid-southern latitude westerly atmospheric circulation plays an important role in global climate. Due to a lack of long, continuous, high-resolution paleoclimate archives from mid-southern latitudes, it remains unclear what factors control changes in its intensity and how past changes affected the climates of landmasses in their path. Here we show growth rate and stable isotope (delta O-18, delta C-13) profiles from a South Island, New Zealand, stalagmite (HW3) that permit centennial-scale investigation of Southern Hemisphere westerly paleointensity between 73 and 11 ka. Correlation between HW3 growth rate and isotope profiles suggests sensitivity to changes in annual precipitation, a factor controlled by westerly intensity. Low growth rates and relatively enriched isotope ratios define long-term trends in HW3, supporting existing evidence that weaker westerlies predominated during the last glacial period. Abrupt millennial-scale events occur frequently, such that the HW3 record resembles Greenland ice core stable isotope profiles. Furthermore, nearly synchronous timing of nine prominent wet and cool intervals with Heinrich events supports studies showing that increased westerly intensity is closely linked to North Atlantic cooling. As well as Heinrich events, the HW3 profiles also show an Antarctic Cold Reversal-like event during deglaciation, advocating for a bipolar seesaw of global climate at that time.

69. Dhuime, Bruno; Hawkesworth, C. J.; Storey, C. D.; et al.

From sediments to their source rocks: Hf and Nd isotopes in recent river sediments //GEOLOGY  Volume: 39   Issue: 4   Pages: 407-410    APR 2011
Unraveling continental evolution from the sedimentary record requires an understanding of time-integrated erosion laws that link sediments to their source rocks, and the extent to which erosion laws vary in different erosion systems. Detrital zircons from the Frankland River (southwestern Australia) define a continental growth curve that is strikingly similar to the Nd in shales curve for the Australian continent. This suggests that the detrital zircon data can be used as a good proxy for the sedimentary record through time. The advantage is that the age distribution of the zircons allows the contributions from different source regions to be determined for any sediment sample. Using integrated Hf and U-Pb isotopes in detrital zircons, and Nd isotope ratios of bulk recent sediments along the Frankland River, the relative contributions of different source terrains have been determined and expressed through an erosion parameter K, which relates the proportions of the material from different source rocks in the sediments to the proportions of those source rocks present in the overall catchment of the sediments analyzed. The results suggest that values of K = 4-6 are representative of mature river systems that sample large source areas, and that these should be used to reevaluate models of the evolution of the continental crust that were generally limited by the assumption of K. For the Gondwana supercontinent, K values of 4-6 indicate that at least 50% of the present-day volume of the continental crust was generated by the end of the Archean.

70. Widerlund, Anders; Andersson, Per S.

Late Holocene freshening of the Baltic Sea derived from high-resolution strontium isotope analyses of mollusk shells //GEOLOGY  Volume: 39   Issue: 2   Pages: 187-190    FEB 2011
Strontium isotopic composition (Sr-87/Sr-86) data from subfossil C-14-dated mollusk shells in raised beach sediments are used as a paleosalinity proxy in the brackish Baltic Sea, the precision (+/- 5%) and accuracy (+/- 0.7%) of the method being judged from replicate analyses of modern shells. Paleosalinity data with an average time resolution of similar to 200 yr for the period 7130-2775 calibrated C-14 yr B. P. indicate maximum surface salinities of 10%-11%, 11%-12%, and 12%-13% for the Bothnian Bay, Bothnian Sea, and Baltic Proper (the three major Baltic subbasins). The relative salinity differences between the basins were small (<= 30%) compared to the as much as eightfold present-day relative salinity differences (Bothnian Bay 1%-3%; Bothnian Sea 4%-5%; Baltic Proper 6%-8%). Late Holocene freshening (ca. 3000 calibrated C-14 yr B. P. to present) is most pronounced in the northernmost subbasin, the Bothnian Bay, consistent with the absence of a permanent halocline, sequestering of phosphorus in well-oxygenated bottom sediments, and phosphorus limitation of primary production in the present-day Bothnian Bay. This study suggests that paleosalinity data may be crucial to improving our understanding of the possible effects of any future, climate-induced freshening of the Baltic Sea.

71. Crow, Ryan; Karlstrom, Karl; Asmerom, Yemane; et al.

Shrinking of the Colorado Plateau via lithospheric mantle erosion: Evidence from Nd and Sr isotopes and geochronology of Neogene basalts //GEOLOGY  Volume: 39   Issue: 1   Pages: 27-30    JAN 2011
Geochronologic data from the southern margins of the Colorado Plateau (western United States) show an inboard radial migration of Neogene basaltic magmatism. Nd and Sr isotopic data show that as basaltic volcanism migrates inboard it also becomes increasingly more asthenospheric. Strongly asthenospheric alkali basalt (epsilon(Nd) > 4) appeared on the western plateau margin ca. 5 Ma, on the southeastern margin at 7 Ma, and is lacking from the plateau's other margins. Tomographic data suggest that low-velocity mantle underlies almost all recent (younger than 1 Ma) basaltic volcanism in a ring around much of the Colorado Plateau at a depth of 80 km. The combined isotopic and tomographic data indicate that the low-velocity mantle is asthenosphere along the western and southeastern margins of the plateau, but modified lithosphere around the remaining margins. Temporal and spatial patterns suggest a process by which upwelling asthenosphere is progressively infiltrating and replacing lithospheric mantle, especially where Proterozoic boundaries exist. This model explains (1) the dramatic velocity contrast seen well inboard of the physiographic edge of the plateau, (2) the inboard sweep of Neogene magmatism, and (3) isotopic evidence that much (but not all) of the low-velocity mantle is asthenospheric. These data support models that ongoing uplift of the edges of the Colorado Plateau is driven by mantle processes.

72. Kennedy, C. B.; Gault, A. G.; Fortin, D.; et al.

Carbon isotope fractionation by circumneutral iron-oxidizing bacteria //GEOLOGY  Volume: 38   Issue: 12   Pages: 1087-1090    DEC 2010
Bacteriogenic iron oxides in natural environments are characterized by an abundance of ferrihydrite precipitates intermixed with bacterial structures that commonly resemble those produced by the lithoautotrophic microorganisms Gallionella ferruginea and Leptohtrix ochracea. These species have been inferred to play a causal role in the formation of bacteriogenic iron oxides, providing a pathway for the reduction of CO2 and the depletion of C-13 in the organic constituents of bacteriogenic iron oxides. In this study, stable carbon isotope fractionation was determined for bacteriogenic iron oxide samples collected from submarine hydrothermal vents (Axial Volcano, Juan de Fuca Ridge), subterranean (Aspo Hard Rock Laboratory, Sweden) and surficial (Chalk River, Canada) groundwater seeps, and cultures of G. ferruginea. Data were also collected from ferrihydrite samples lacking evidence of bacteria from Bounty Seamount in the vicinity of Pitcairn Island. The mean delta C-13 (parts per thousand) of ferrihydrite was determined to be -15.87 parts per thousand +/- 4.96 parts per thousand for the samples from Axial Volcano, -24.97 parts per thousand +/- 0.43% for Aspo, -27.80 parts per thousand +/- 0.85 parts per thousand for Chalk River, -29.3 parts per thousand +/- 0.2 parts per thousand for the microbial culture, and -8.43 parts per thousand +/- 1.89 parts per thousand for the samples from Pitcairn. Samples with the highest concentration of organic carbon also had the lightest delta C-13 in a logarithmic relationship. The consistency of carbon isotope values in relation to the presence of iron-oxidizing bacteria from natural and laboratory samples is interpreted as the ability of these microorganisms to fractionate carbon. The potential of this fractionation to serve as a biosignature holds promise when the resistance of carbon and bacteriogenic ferrihydrite to diagenesis is taken into consideration.

73. Kuroda, Junichiro; Hori, Rie S.; Suzuki, Katsuhiko; et al.Marine osmium isotope record across the Triassic-Jurassic boundary from a Pacific pelagic site //GEOLOGY  Volume: 38   Issue: 12   Pages: 1095-1098    DEC 2010

The Triassic-Jurassic (T-J) boundary ca. 200 Ma represents one of the major mass extinction events of the Phanerozoic; however, the cause of this event remains controversial because of a paucity of geological evidence. In this study we present an isotopic record of osmium extracted from a bedded chert succession across the T-J boundary in the Kurusu section of Japan, deposited within a Paleo-Pacific (Panthalassa) deep basin. The data show a gradual decrease in seawater Os-187/Os-188 values during the Rhaetian, followed by a sharp increase in the latest Rhaetian, and a subsequent stable phase across the T-J boundary. The decreasing trend of Os-187/Os-188 values during the Rhaetian indicates a gradual increase in the relative supply rate of unradiogenic Os from the mantle associated with emplacement of the Central Atlantic Magmatic Province. The subsequent shift toward radiogenic values reflects an increased supply of radiogenic Os due to enhanced continental weathering. This interval marks more negative isotopic values of organic carbon, the onset of radiolarian faunal turnover, and conodont extinctions, indicating that the rapid increase in continental weathering rate was closely linked to the perturbation of the carbon cycle and the T-J biotic crisis.

74. Finlay, Alexander J.; Selby, David; Osborne, Mark J.; et al.

Fault-charged mantle-fluid contamination of United Kingdom North Sea oils: Insights from Re-Os isotopes //GEOLOGY  Volume: 38   Issue: 11   Pages: 979-982    NOV 2010
United Kingdom North Sea oils sourced from the Kimmeridge Clay Formation contain unradiogenic (similar to 0.17 to similar to 0.48) and radiogenic (similar to 1.04 to similar to 3.34) Os-187/Os-188 values. The unradiogenic Os-187/Os-188 values are spatially associated with the main basin-bounding faults of the Viking Graben and East Shetland Basin. In contrast, the radiogenic Os-187/Os-188 values are associated with North Sea basins located farther away from the basin boundary faults. We suggest that crustal thinning and strain localization within the Viking Graben and East Shetland Basin are sufficient to have allowed basin-bounding faults to propagate to sufficient depth to act as conduits for mantle-derived fluids to interact with oil. This hypothesis is supported by previous geochemical data for North Sea oil fields. Furthermore, we demonstrate the use of Os isotopes as an important tracker of crustal-scale fluid dynamics and petroleum migration pathways in extensional basins.

75. Robinson, Stuart A.; Murphy, Daniel P.; Vance, Derek; et al.

Formation of "Southern Component Water" in the Late Cretaceous: Evidence from Nd-isotopes //GEOLOGY  Volume: 38   Issue: 10   Pages: 871-874    OCT 2010
Constraining deep-ocean circulation during past greenhouse climatic periods, such as the Cretaceous, is important for understanding meridional heat transfer processes, controls on ocean anoxia, and the relative roles of climate and tectonics in determining paleocirculation patterns. Ocean circulation models for the Late Cretaceous and early Paleogene suggest that significant deep-water production occurred in the Southern Ocean, but cannot constrain when this process commenced or what the temporal relationship was between opening tectonic gateways and Late Cretaceous climatic cooling. Nd-isotope data obtained from biogenic apatite (fish teeth and bones) are presented from lower bathyal and abyssal sites in the South Atlantic and Indian Oceans. During the mid-Cretaceous, relatively radiogenic Nd-isotope values suggest that deep-water circulation in these basins was sluggish with inputs likely dominated by seawater-particle exchange processes and, possibly, easily weathered volcanic terranes. In the Campanian-Maastrichtian the Nd-isotopic composition of proto-Indian and South Atlantic deep waters became less radiogenic, suggesting the onset of deep-water formation in the Southern Ocean (Southern Component Water, SCW), consistent with Paleogene reconstructions and ocean circulation models. A combination of Southern Hemisphere cooling and the opening of tectonic gateways during the Campanian likely drove the onset of SCW.

76. Kasemann, Simone A.; Prave, Anthony R.; Fallick, Anthony E.; et al.

Neoproterozoic ice ages, boron isotopes, and ocean acidification: Implications for a snowball Earth// GEOLOGY  Volume: 38   Issue: 9   Pages: 775-778    SEP 2010
The Neoproterozoic Earth underwent at least two severe glaciations, each extending to low paleomagnetic latitudes and punctuating warmer climates. The two widespread older and younger Cryogenian glacial deposits in Namibia are directly overlain by cap carbonates deposited under inferred periods of high atmospheric carbon dioxide concentrations. Oceanic uptake of carbon dioxide decreases ocean pH; here we present a record of Cryogenian interglacial ocean pH, based on boron ( B) isotopes in marine carbonates. Our data suggest a largely constant ocean pH and no critically elevated pCO(2) throughout the older postglacial and interglacial periods. In contrast, a marked ocean acidification event marks the younger deglaciation period and is compatible with elevated postglacial pCO(2) concentration. Our data are consistent with the presence of two panglacial climate states in the Cryogenian, but indicate that each had its own distinct environmental conditions.

77. Stap, Lucy; Lourens, Lucas J.; Thomas, Ellen; et al.High-resolution deep-sea carbon and oxygen isotope records of Eocene Thermal Maximum 2 and H2 //GEOLOGY  Volume: 38   Issue: 7   Pages: 607-610    JUL 2010

Eocene Thermal Maximum 2 (ETM2) and H2 were two short-lived global warming events that occurred similar to 2 m.y. after the Paleocene-Eocene thermal maximum (PETM, ca. 56 Ma). We have generated benthic foraminiferal stable carbon and oxygen isotope records of four sites along a depth transect on Walvis Ridge (similar to 3.5-1.5 km paleodepth, southeast Atlantic Ocean) and one site on Maud Rise (Weddell Sea) to constrain the pattern and magnitude of their carbon isotope excursions (CIEs) and deep-sea warming. At all sites, ETM2 is characterized by similar to 3 degrees C warming and a -1.4 parts per thousand CIE. The H2 event that occurred similar to 100 k.y. later is associated with similar to 2 degrees C warming and a -0.8 parts per thousand CIE. The magnitudes of the delta C-13 and delta O-18 excursions of both events are significantly smaller than those during the PETM, but their coherent relation indicates that the delta C-13 change of the exogenic carbon pool was similarly related to warming during these events, despite the much more gradual and transitioned onset of ETM2 and H2.

78. Martin, Victoria M.; Davidson, Jon; Morgan, Dan; et al.

Using the Sr isotope compositions of feldspars and glass to distinguish magma system components and dynamics //GEOLOGY  Volume: 38   Issue: 6   Pages: 539-542    JUN 2010
The individual components of magmatic rocks are being increasingly targeted to gain insight into volcanic systems. Here we demonstrate the use of Sr isotope microsampling to distinguish distinct populations of glass and plagioclase that represent the sources of components (liquids and crystals) within each erupted magma batch. We analyzed glasses and plagioclase feldspar crystal cores to characterize crystal cargoes and host liquids from pumice-fall units from the Minoan cycle, Santorini, Greece. Using this approach, we identify the magmatic components that interacted prior to eruption and formation of pumice clasts, allowing us to interpret the magma dynamics associated with each eruption. Varying degrees of complexity can be identified, from relatively simple cases where the crystals are in isotopic equilibrium with the glass, to more complex, such as the large caldera-forming Minoan eruption, which is characterized by at least two liquid components and three different crystal provenances.

79. Canfield, Donald E.; Farquhar, James; Zerkle, Aubrey L.

High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog //GEOLOGY  Volume: 38   Issue: 5   Pages: 415-418    MAY 2010
A detailed record of the early-Earth sulfur (S) cycle is chronicled by the S isotope values of sulfide and sulfate preserved in the rock record. Interpretation of this record rests on our understanding of sulfur cycling in modern systems, experiments, and the resulting isotopic signatures. Very large fractionations in delta S-34 of >= 70 parts per thousand are commonly measured between sulfide and sulfate in modern systems and in ancient sediments. Theoretical calculations suggest that sulfate-reducing prokaryotes are capable of producing such large fractionations during the reduction of sulfate to sulfide, although they have only been demonstrated to generate fractionations up to 48 parts per thousand. Here we report the first direct determination of 60 parts per thousand-70 parts per thousand fractionations by natural populations of sulfate reducers. These high fractionations occur under the relatively low-sulfate conditions (1.1-2 mM) of meromictic Lago di Cadagno in Switzerland. The major and minor isotopic composition of sulfide and sulfate in the lake water is consistent with sulfide produced by sulfate reduction, with little evidence for modification by further oxidative sulfur cycling. These observations help us to constrain the evolution of seawater sulfate concentrations.

80. Horikawa, Keiji; Asahara, Yoshihiro; Yamamoto, Koshi; et al.

Intermediate water formation in the Bering Sea during glacial periods: Evidence from neodymium isotope ratios //GEOLOGY  Volume: 38   Issue: 5   Pages: 435-438  MAY 2010
Changes in the flux and location of overturning circulation may have large effects on marine ecosystems and CO2 exchange between the ocean and atmosphere. However, unlike the Atlantic, little is known about ocean circulation and ventilation under glacial boundary conditions in the North Pacific, especially in regard to intermediate water circulation. Here we present new records of neodymium (Nd) isotopes (epsilon(Nd)) in Fe-Mn oxyhydroxides of Bering Sea sediments (884 m water depth). We found a systematic variation between radiogenic values (up to +0.8 epsilon(Nd)) during cold periods and relatively less radiogenic values (<-1 epsilon(Nd)) during warm periods. There are no water masses with such radiogenic Nd isotope signatures in the intermediate or deep North Pacific. Potential sources of radiogenic values in the subarctic North Pacific are limited to surface waters adjacent to the Aleutian Arc and Kamchatka Peninsula. Therefore, the radiogenic epsilon(Nd) values of Fe-Mn oxyhydroxides observed at the intermediate depth during glacial periods are best explained by subduction of the surface water to the intermediate depth (at least similar to 800 m) due to brine rejection. Our data strongly indicate that the northwestern Bering Sea (off northeastern Kamchatka) was a possible source region of glacial intermediate water in the Bering Sea and the subarctic North Pacific.

81. Montoya-Pino, Carolina; Weyer, Stefan; Anbar, Ariel D.; et al.

Global enhancement of ocean anoxia during Oceanic Anoxic Event 2: A quantitative approach using U isotopes //GEOLOGY  Volume: 38   Issue: 4   Pages: 315-318    APR 2010
During the Mesozoic greenhouse world, the oceans underwent several oceanic anoxic events (OAEs) characterized by intervals during which organic-rich black shales were deposited, indicating strong oxygen depletion in the marine realm. The Cenomanian-Turonian OAE2 (ca. 93 Ma) represents one of the most prominent events of the Cretaceous, with significant perturbations of the global carbon cycle. Although OAE2 likely reached a global scale, the spatial extent of seawater anoxia during this OAE is poorly constrained. Here we demonstrate that variations in the U-238/U-235 isotope ratio (delta U-238), a newly developed paleoredox proxy, can be used to quantify the extent of marine anoxia. For black shales from the mid-Cretaceous OAE2 we find a systematic shift toward lighter delta U-238 and lower U concentrations as compared to modern equivalent organic-rich sediments from the Black Sea. This shift translates to a global increase of oceanic anoxia during OAE2 by at least a factor of three as compared to the present day or to periods before and after OAE2. The constant offset in U concentrations and isotope compositions of black shales throughout OAE2 compared to modern Black Sea sediments indicates an enhancement of oceanic anoxic conditions already prior to the onset of OAE2.

82. Cai, Yanjun; Cheng, Hai; An, Zhisheng; et al.

Large variations of oxygen isotopes in precipitation over south-central Tibet during Marine Isotope Stage 5 //GEOLOGY  Volume: 38   Issue: 3   Pages: 243-246   DOIMAR 2010
Oxygen isotope (delta O-18) records of two stalagmites from Tianmen Cave, on the south-central Tibetan Plateau, grew during much of Marine Isotope Stage 5. The Tianmen record, the first cave record from the Tibetan Plateau, characterizes a precipitation delta O-18 history larger in amplitude but similar in structure to Asian Monsoon records from the adjacent regions, providing essential evidence that the Asian Monsoon system, including the East Asian and Indian Monsoon subsystems, responds largely to changes in Northern Hemisphere summer insolation. Extremely low delta O-18 values during Marine Isotope Stages 5a, 5c, and 5e suggest that precipitation, rather than temperature, was a major factor controlling delta O-18 in precipitation on orbital time scales in the south-central Tibetan Plateau. The Tianmen record may help in the interpretation of regional ice core delta O-18 records. The large range of orbital-scale shifts in meteoric delta O-18 (>9%) raises important considerations related to reconstructing the uplift history of the plateau.

83. Parnell, John; Boyce, Adrian; Thackrey, Scott; et al.

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