Stable carbon and oxygen isotope compositions of Eocene benthic foraminifera from ODP Site 207-1258

'Hyperthermals' are intervals of rapid, pronounced global warming known from six episodes within the Palaeocene and Eocene epochs (~65-34 million years (Myr) ago) (Zachos et al., 2005, doi:10.1126/science.1109004; 2008, doi:10.1038/nature06588; Roehl et al., 2007, doi:10.1029/2007GC001784; Thomas et al., 2000; Cramer et al., 2003, doi:10.1029/2003PA000909; Lourens et al., 2005, doi:10.1038/nature03814; Petrizzo, 2005, doi:10.2973/odp.proc.sr.198.102.2005; Sexton et al., 2006, doi:10.1029/2005PA001253; Westerhold et al., 2007, doi:10.1029/2006PA001322; Edgar et al., 2007, doi:10.1038/nature06053; Nicolo et al., 2007, doi:10.1130/G23648A.1; Quillévéré et al., 2008, doi:10.1016/j.epsl.2007.10.040; Stap et al., 2010, doi:10.1130/G30777.1). The most extreme hyperthermal was the 170 thousand year (kyr) interval (Roehl et al., 2007) of 5-7 °C global warming (Zachos et al., 2008) during the Palaeocene-Eocene Thermal Maximum (PETM, 56 Myr ago). The PETM is widely attributed to massive release of greenhouse gases from buried sedimentary carbon reservoirs (Zachos et al., 2005; 2008; Lourenbs et al., 2005; Nicolo et al., 2007; Dickens et al., 1995, doi:10.1029/95PA02087; Dickens, 2000; 2003, doi:10.1016/S0012-821X(03)00325-X; Panchuk et al., 2008, doi:10.1130/G24474A.1) and other, comparatively modest, hyperthermals have also been linked to the release of sedimentary carbon (Zachos et al., 2008, Lourens et al., 2005; Nicolo et al., 2007; Dickens, 2003; Panchuk et al., 2003). Here we show, using new 2.4-Myr-long Eocene deep ocean records, that the comparatively modest hyperthermals are much more numerous than previously documented, paced by the eccentricity of Earth's orbit and have shorter durations (~40 kyr) and more rapid recovery phases than the PETM. These findings point to the operation of fundamentally different forcing and feedback mechanisms than for the PETM, involving redistribution of carbon among Earth's readily exchangeable surface reservoirs rather than carbon exhumation from, and subsequent burial back into, the sedimentary reservoir. Specifically, we interpret our records to indicate repeated, large-scale releases of dissolved organic carbon (at least 1,600 gigatonnes) from the ocean by ventilation (strengthened oxidation) of the ocean interior. The rapid recovery of the carbon cycle following each Eocene hyperthermal strongly suggests that carbon was resequestered by the ocean, rather than the much slower process of silicate rock weathering proposed for the PETM (Zachos et al., 2005; 2003). Our findings suggest that these pronounced climate warming events were driven not by repeated releases of carbon from buried sedimentary sources (Zachos et al., 2008, Lourens et al., 2005; Nicolo et al., 2007; Dickens, 2003; Panchuk et al., 2003) but, rather, by patterns of surficial carbon redistribution familiar from younger intervals of Earth history.

Triassic redbeds in the Malaguide Complex (Betic Cordillera — Spain): petrography, geochemistry and geodynamic implications

Sandstone petrography and mudstone mineralogy and geochemistry of Triassic mudstones and sandstones from continental redbeds of the Malaguide Complex (Betic Cordillera, southern Spain) provide useful information on provenance, palaeoclimate and geodynamics during the early stages of the Pangea break-up, and on their diagenetic evolution. The sandstones are quartzarenites to sub-litharenites, with minor lithic fragments and rare feldspars. The mudstone samples show a PAAS like elemental distribution. The samples likely record recycling processes from their metasedimentary basement rocks that significantly affected the weathering indices, and monitors cumulative effects, including a first cycle of weathering at the source rocks. Sandstone composition and chemical–mineralogical features of mudstones record a provenance derived from continental block and recycled orogen that were weathered under warm and episodically wet climate. Source areas were located towards the east of the present-day Malaguide outcrops, and were formed by fairly silicic rock types, made up mainly of Palaezoic metasedimentary rocks, similar to those of the Paleozoic underlying series, with subordinate contributions from magmatic–metamorphic sources, and a rare supply from mafic metavolcanic rocks. Clay-mineral distribution of mudstones is dominated by illite and illite/smectite mixed-layer that result from differences in provenance, weathering, and burial/temperature history. Illite crystallinity values, illitization of kaolinite, occurrence of typical authigenic minerals and apatite fission-track studies, coupled with a subsidence analysis of the whole Malaguide succession suggest burial depths of at least 4–6 km with temperatures of 140–160 °C, typical of the burial diagenetic stage, and confirm the Middle Miocene exhumation of the Betic Internal Domain tectonic stack topped by the Malaguide Complex.

New data on the Vrancea Nappe (Moldavidian Basin, Outer Carpathian Domain, Romania): paleogeographic and geodynamic reconstructions

A study has been performed on the Cretaceous to Early Miocene succession of the Vrancea Nappe (Outer Carpathians, Romania), based on field reconstruction of the stratigraphic record, mineralogical-petrographic and geochemical analyses. Extra-basinal clastic supply and intra-basinal autochthonous deposits have been differentiated, appearing laterally inter-fingered and/or interbedded. The main clastic petrofacies consist of calcarenites, sub-litharenites, quartzarenites, sub-arkoses, and polygenic conglomerates derived from extra-basinal margins. An alternate internal and external provenance of the different supplies is the result of the paleogeographic re-organization of the basin/margins system due to tectonic activation and exhumation of rising areas. The intra-basinal deposits consist of black shales and siliceous sediments (silexites and cherty beds), evidencing major environmental changes in the Moldavidian Basin. Organic-matter-rich black shales were deposited during anoxic episodes related to sediment starvation and high nutrient influx due to paleogeographic isolation of the basin caused by plate drifting. The black shales display relatively high contents in sub-mature to mature, Type II lipidic organic matter (good oil and gas-prone source rocks) constituting a potentially active petroleum system. The intra-basinal siliceous sediments are related to oxic pelagic or hemipelagic environments under tectonic quiescence conditions although its increase in the Oligocene part of the succession can be correlated with volcanic supplies. The integration of all the data in the “progressive reorientation of convergence direction” Carpathian model, and their consideration in the framework of a foreland basin, led to propose some constrains on the paleogeographic-geodynamic evolutionary model of the Moldavidian Basin from the Late Cretaceous to the Burdigalian.

Magnetic anisotropy reveals Neogene tectonic overprint in highly strained carbonate mylonites from the Morcles nappe, Switzerland

The anisotropy of magnetic susceptibility (AMS) has been measured with low- and high-field methods, in deformed carbonate rocks along the Morcles nappe shear zone (Helvetic Alps). High-field measurements at room temperature and 77 K enable the separation of the ferrimagnetic, paramagnetic and diamagnetic anisotropy. The ferrimagnetic sub-fabric is generally insignificant in these rocks, contributing less than 10% to the total AMS. AMS results for both the separated diamagnetic and paramagnetic subfabrics are consistent with the regional shear movement in the late-stage formation of the Helvetic nappes, as seen in the Morcles nappe, whose inverted limb indicate shear displacement towards the northwest. The diamagnetic anisotropy correlates well quantitatively with the calculated magnetic anisotropy based on the calcite texture. There is a gradational change in the degree of anisotropy related to the strain gradient along the shear zone. A more complex magnetic fabric, resulting from partial overprinting due to displacement along the Simplon–Rhône fault, is evident at one site near the root zone of the nappe. Partial overprinting of the magnetic fabric appears to have taken place in two locations farther up the shear zone as well. This late phase deformation is associated with recent exhumation of the Mont Blanc and Belledonne external massifs and orogen parallel extension, and is reflected by the AMS. Rocks with bulk susceptibility ∼0 SI, and simple mineral compositions are ideal for low temperature high-field torque, as this method helps to enhance the paramagnetic susceptibility and anisotropy, which may otherwise be masked by the mixed magnetic contributions of the composite magnetic fabric.

An insight into the breakup of Gondwana: Identifying events through low-temperature thermochronology from the basement rocks of Madagascar

Fission track analysis was applied to the Precambrian suites of Madagascar in order to identify the lower-temperature cooling histories and their relationships to the Phanerozoic events that affected the island. Apatite ages range from 431 to 68 Ma, and zircon ages range from 452 to 238 Ma. Thermochronologically, the island can be divided into a southern, central, and northern region each with a subdivision on an east-west basis. The southern region is sharply separated from the central region by strongly contrasting apparent apatite ages over the northwest-southeast striking Ranotsara Shear Zone (RSZ). The change in apparent ages over the RSZ is indicative of later reactivation along younger brittle faults. The central region has the oldest ages of the island and has a diffuse contact to the third region northward. Along the entire western margin of the Precambrian basement initial Paleozoic exhumation was followed by heating (burial by sediments) during Jurassic and Cretaceous times. A decrease in ages along the eastern margin from 119 to 68 Ma coincides with the predicted positions of the Marion hot spot after effects of erosion are considered. On the other hand, these ages may represent progressive opening of the margin in a southward direction together with associated denudation of the rift shoulder. The eastern part of the central region has remained very stable since at least Devonian times, undergoing only long-term very slow exhumation at rates of 1–5 m/Myr.

Petrophysical investigations on sediment core CRP-2A from the Ross Sea, Antarctica

A suite of petropysical measurements - velocity versus pressure, bulk density, porosity, matrix density, and magnetic susceptibility -was undertaken on 63 core plugs from CRP-2A. These data are used to calibrate neutron, resistivity, and magnetic susceptibility well logs. Agreement between core-plug magnetic susceptibility measurements and both well-log and whole-core data is excellent. Comparison of core-plug bulk densities with continious well-log density records shows very good agreement. Core-plug measurements of matrix density permit conversion of the well-log and whole-core density records to porosity. Sands and muds exhibit similar downhole compaction patterns, and both patterns are consistent with 250 ± 150 m of exhumation. Pervasive cementation, particularly in the lower half of the core, has affected many CRP-2A petrophysical parameters: (1) fractional porosities are reduced by about 0.05 - 0.10 in the lower part of the hole; (2) velocity and porosity rebound are much smaller than is usually observed for unconsolidated sediments with burial depths similar to CRP-2A; (3) velocities are unusually insensitive to pressure, suggesting that any exhumation-induced microcracks have been scaled subsequently; (4) the velocity/porosity relationship lacks the characteristic signature of exhumation-induced microcracks; (5) the velocity/porosity relationship changes with depth, indicating downhole increase in consolidation; (6) Vp/Vs ratios of the highest-porosity sediments are unusually low, implying enhancement of framework stiffness.

Timing and chemistry of fluid-flow events in the Lawn Hill Platform, Northern Australia

Mudrocks and carbonates of the Isa superbasin in the Lawn Hill platform in northern Australia host major base metal sulfide mineralization, including the giant strata-bound Century Zn-Pb deposit. Mineral paragenesis, stable isotope, and K-Ar dating studies demonstrate that long-lived structures such as the Termite Range fault acted as hot fluid conduits several times during the Paleoproterozoic and Mesoproterozoic in response to major tectonic events. Illite and chlorite crystallinity studies suggest the southern part of the platform has experienced higher temperatures (up to 300 degrees C) than similar stratigraphic horizons in the north. The irregular downhole variation of illite crystallinity values provides further information oil the thermal regime in the basin and shows that clay formation was controlled not only by temperature increase with depth but also by high water/rock ratios along relatively permeable zones. K-Ar dating of illite, in combination with other data, may indicate three major thermal events in the central and northern Lawn Hill platform Lit 1500, 1440 to 1400, and 1250 to 1150 Ma. This study did not detect the earlier Century base metal mineralizing event at 1575 Ma. 1500 Ma ages are recorded only in the south and correspond to the age of the Late Isan orogeny and deposition of the Lower Roper superbasin. They may reflect exhumation of a provenance region. The 1440 to 1300 Ma ages are related to fault reactivation and a thermal pulse at similar to 1440 to 1400 Ma possibly accompanied by fluid flow, with subsequent enhanced cooling possibly due to thermal relaxation or further crustal exhumation. The youngest thermal and/or fluid-flow event at 1250 to 1150 Ma is recorded mainly to the cast of the Tern-lite Range fault and may be related to the assembly of the Rodinian supercontinent. Fluids in equilibrium with illite that formed over a range of temperatures, at different times in different parts of the platform. have relatively uniform oxygen isotope compositions and more variable hydrogen isotope compositions (delta O-18 = 3.5-9.7 parts per thousand V-SMOW; delta D = -94 to -36 parts per thousand V-SMOW). The extent of the 180 enrichment and the variably depleted hydrogen isotope compositions suggest the illite interacted with deep-basin hypersaline brines that were composed of evaporated seawater and/or highly evolved meteoric water. Siderite is the most abundant iron-rich gangue phase in the Century Zn-Pb deposit, which is surrounded by all extensive ferroan carbonate alteration halo. Modeling suggests that the ore siderite formed at temperatures of 120 degrees to 150 degrees C, whereas siderite and ankerite in the alteration halo formed at temperatures of 150 degrees to 180 degrees C. The calculated isotopic compositions of the fluids are consistent with O-18-rich basinal brines and mixed inorganic and organic carbon Sources (6180 = 3-10 parts per thousand V-SMOW, delta C-13 = -7 to -3 parts per thousand V-PDB). in the northeast Lawn Hill platform carbonate-rich rocks preserve marine to early diagenetic carbon and oxygen isotope compositions, whereas ferroan carbonate cements in siltstones and shales in the Desert Creek borehole are O-18 and C-13 depleted relative to the sedimentary carbonates. The good agreement between temperature estimates from illite crystallinity and organic reflectance (160 degrees-270 degrees C) and inverse correlation with carbonate delta O-18 values indicates that organic maturation and carbonate precipitation in the northeast Lawn Hill platform resulted from interaction with the 1250 to 1150 Ma fluids. The calculated isotopic compositions of the fluid are consistent with evolved basinal brine (delta O-18 = 5.1-9.4 parts per thousand V-SMOW; delta C-13 = -13.2 to -3.7 parts per thousand V-PDB) that contained a variable organic carbon component from the oxidation and/or hydrolysis of organic matter in the host sequence. The occurrence of extensive O-18- and C-13-depleted ankerite and siderite alteration in Desert Creek is related to the high temperature of the 1250 to 1150 Ma fluid-flow event in the northeast Lawn Hill platform, in contrast to the lower temperature fluids associated with the earlier Century Zn-Pb deposit in the central Lawn Hill platform.

Sulphur mineralogy and geochemistry of serpentinites and gabbros from the Atlantis Massif

In-situ uplifted portions of oceanic crust at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were drilled during Expeditions 304 and 305 of the Integrated Ocean Drilling Program (IODP) and a 1.4 km section of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered. Here we characterize variations in sulfur mineralogy and geochemistry of selected samples of serpentinized peridotites, olivine-rich troctolites and diverse gabbroic rocks recovered from Hole 1309D. These data are used to constrain alteration processes and redox conditions and are compared with the basement rocks of the southern wall of the Atlantis Massif, which hosts the Lost City Hydrothermal Field, 5 km to the south. The oceanic crust at the central dome is characterized by Ni-rich sulfides reflecting reducing conditions and limited seawater circulation. During uplift and exhumation, seawater interaction in gabbroic-dominated domains was limited, as indicated by homogeneous mantle-like sulfur contents and isotope compositions of gabbroic rocks and olivine-rich troctolites. Local variations from mantle compositions are related to magmatic variability or to interaction with seawater-derived fluids channeled along fault zones. The concomitant occurrence of mackinawite in olivine-rich troctolites and an anhydrite vein in a gabbro provide temperature constraints of 150-200 °C for late circulating fluids along local brittle faults below 700 m depth. In contrast, the ultramafic lithologies at the central dome represent domains with higher seawater fluxes and higher degrees of alteration and show distinct changes in sulfur geochemistry. The serpentinites in the upper part of the hole are characterized by high total sulfide contents, high d34Ssulfide values and low d34Ssulfate values, which reflect a multistage history primarily controlled by seawater-gabbro interaction and subsequent serpentinization. The basement rocks at the central dome record lower oxygen fugacities and more limited fluid fluxes compared with the serpentinites and gabbros of the Lost City hydrothermal system. Our studies are consistent with previous results and indicate that sulfur speciation and sulfur isotope compositions of altered oceanic mantle sequences commonly evolve over time. Heterogeneities in sulfur geochemistry reflect the fact that serpentinites are highly sensitive to local variations in fluid fluxes, temperature, oxygen and sulfur fugacities, and microbial activity.

Chemical and isotopic composition of carbonate veins from ODP Leg 209

Carbonate veins hosted in ultramafic basement drilled at two sites in the Mid Atlantic Ridge 15°N area record two different stages of fluid-basement interaction. A first generation of carbonate veins consists of calcite and dolomite that formed syn- to postkinematically in tremolite-chlorite schists and serpentine schists that represent gently dipping large-offset faults. These veins formed at temperatures between 90 and 170 °C (oxygen isotope thermometry) and from fluids that show intense exchange of Sr and Li with the basement (87Sr/86Sr = 0.70387 to 0.70641, d7Li L-SVEC = + 3.3 to + 8.6 per mil). Carbon isotopic compositions range to high d13C PDB values (+ 8.7 per mil), indicating that methanogenesis took place at depth. The Sr-Li-C isotopic composition suggests temperatures of fluid-rock interaction that are much higher (T > 350-400 °C) than the temperatures of vein mineral precipitation inferred from oxygen isotopes. A possible explanation for this discrepancy is that fluids cooled conductively during upflow within the presumed detachment fault. Aragonite veins were formed during the last 130 kyrs at low-temperatures within the uplifted serpentinized peridotites. Chemical and isotopic data suggest that the aragonites precipitated from cold seawater, which underwent overall little exchange with the basement. Oxygen isotope compositions indicate an increase in formation temperature of the veins by 8-12 °C within the uppermost ~ 80 m of the subseafloor. This increase corresponds to a high regional geothermal gradient of 100-150 °C/km, characteristic of young lithosphere undergoing rapid uplift.

(Table) Argon isotope ratios and argon ages of rocks obtained from the Chernorud zone, western Baikal area

Structural-petrologic and isotopic-geochronologic data on magmatic, metamorphic, and metasomatic rocks from the Chernorud zone were used to reproduce the multistage history of their exhumation to upper crustal levels. The process is subdivided into four discrete stages, which corresponded to metamorphism to the granulite facies (500-490 Ma), metamorphism to the amphibolite facies (470-460 Ma), metamorphism to at least the epidote-amphibolite facies (440-430 Ma), and postmetamorphic events (410-400 Ma). The earliest two stages likely corresponded to the tectonic stacking of the backarc basin in response to the collision of the Siberian continent with the Eravninskaya island arc or the Barguzin microcontinent, a process that ended with the extensive generation of synmetamorphic granites. During the third and fourth stages, the granulites of the Chernorud nappe were successively exposed during intense tectonic motions along large deformation zones (Primorskii fault, collision lineament, and Orso Complex). The comparison of the histories of active thermal events for Early Caledonian folded structures in the Central Asian Foldbelt indicates that active thermal events of equal duration are reconstructed for the following five widely spiced accretion-collision structures: the Chernorud granulite zone in the Ol'khon territory, the Slyudyanka crystalline complex in the southwestern Baikal area, the western Sangilen territory in southeastern Tuva, Derbinskii terrane in the Eastern Sayan, and the Bayankhongor ophiolite zone in central Mongolia. The dates obtained by various isotopic techniques are generally consistent with the four discrete stages identified in the Chernorud nappe, whereas the dates corresponding to the island-arc evolutionary stage were obtained only for the western Sangilen and Bayankhongor ophiolite zone.