We present a comprehensive data set of organic and inorganic geochemistry from a lower Cretaceous pelagic bedded chert succession of the Shimanto accretionary belt in the Yokonami Peninsula (Kochi, Japan). Based on stable isotopic composition of total organic carbon (δ 13 C org ), in conjunction with radiolarian biostratigraphic data, we propose that a 1.3-m-thick interval within the examined section is correlative with Tethyan Selli Level (Apennines, Italy), a sedimentary expression of oceanic anoxic event (OAE) 1a. Specifically, the δ 13 C org record illustrates a discernible negative shift and subsequent positive excursions upsection, a pattern that resembles the typical δ 13 C org pattern across OAE 1a reported from various sites such as the Mediterranean Tethys and Pacific seamount flanks. Our δ 13 C org record from the deep Pacific basin supports the idea that the δ 13 C variation across OAE 1a was induced by a significant perturbation of global carbon cycle. The slight increase in total organic carbon contents of sediment deposited during OAE 1a suggests slight or no expansion of oxygen-deficient water mass in the overlying water column. Rare earth elements and lead isotopic compositions indicate relatively higher contributions of volcanic or hydrothermally altered components before and after OAE 1a. The volcanic or hydrothermal source may be associated with emplacement of the Ontong Java Plateau during the early Aptian, or tectonically induced hydrothermal alteration associated with the formation of the accretionary complex.

Among various paleoenvironmental proxy records across the lower Aptian Selli Level (Umbria-Marche basin, Italy) and its equivalents (i.e., sedimentary expressions of oceanic anoxic event [OAE] 1a), one very intriguing feature is a prominent negative δ 13 C excursion at the base of this sedimentary unit, generally by as much as a few per mil in marine carbonates. This early Aptian δ 13 C event has received special attention as an important clue to the genesis of OAE 1a, but the exact amplitude of this relatively short lived δ 13 C variation is not precisely constrained, and this fact has been an obstacle in paleoenvironmental modeling. Particularly enigmatic is the large amplitude, by −7‰, in pelagic limestones at the central Pacific Deep Sea Drilling Project (DSDP) Site 463; it may be a primary δ 13 C signal, considering its deposition in a fully open-ocean setting and a conservative burial diagenetic environment. Nevertheless, new Sr isotope data help identify a peculiar diagenetic overprint on the lower Aptian interval of this site. While the majority of examined samples represent slightly shifted, but acceptable, 87 Sr/ 86 Sr ratios for marine carbonates of this age, markedly unradiogenic 87 Sr/ 86 Sr ratios are recorded within and just below the Selli Level–equivalent interval. In particular, extremely unradiogenic 87 Sr/ 86 Sr ratios (0.70623–0.70642) are detected at exactly the same interval as the most negative δ 13 C values and a smectite-rich lithology. It is therefore proposed that hitherto unknown diagenetic process, or processes, under peculiar interstitial-water geochemistry, resulting from volcanic ash alteration, played a role for the paired δ 13 C- 87 Sr/ 86 Sr anomaly at DSDP Site 463. By removing the δ 13 C data from samples that possess the highly unradiogenic 87 Sr/ 86 Sr ratios, the amplitude of early Aptian negative δ 13 C excursion is reevaluated to be −2.7‰ or −3.3‰, facilitating comparison with published δ 13 C records from other sections. The refined Site 463 δ 13 C profile further implies that the early Aptian negative δ 13 C event is better described as a twin excursion.

The recent discovery of the direct link between Deccan volcanism and the end-Cretaceous mass extinction also links volcanism to the late Maastrichtian rapid global warming, high environmental stress, and the delayed recovery in the early Danian. In comparison, three decades of research on the Chicxulub impact have failed to account for long-term climatic and environmental changes or prove a coincidence with the mass extinction. A review of Deccan volcanism and the best age estimate for the Chicxulub impact provides a new perspective on the causes for the end-Cretaceous mass extinction and supports an integrated Deccan-Chicxulub scenario. This scenario takes into consideration climate warming and cooling, sea-level changes, erosion, weathering, ocean acidification, high-stress environments with opportunistic species blooms, the mass extinction, and delayed postextinction recovery. The crisis began in C29r (upper CF2 to lower CF1) with rapid global warming of 4 °C in the oceans and 8 °C on land, commonly attributed to Deccan phase 2 eruptions. The Chicxulub impact occurred during this warm event (about 100–150 k.y. before the mass extinction) based on the stratigraphically oldest impact spherule layer in NE Mexico, Texas, and Yucatan crater core Yaxcopoil-1. It likely exacerbated climate warming and may have intensified Deccan eruptions. The reworked spherule layers at the base of the sandstone complex in NE Mexico and Texas were deposited in the upper half of CF1, ~50–80 k.y. before the Cretaceous-Tertiary (K-T) boundary. This sandstone complex, commonly interpreted as impact tsunami deposits of K-T boundary age, was deposited during climate cooling, low sea level, and intensified currents, leading to erosion of nearshore areas (including Chicxulub impact spherules), transport, and redeposition via submarine channels into deeper waters. Renewed climate warming during the last ~50 k.y. of the Maastrichtian correlates with at least four rapid, massive volcanic eruptions known as the longest lava flows on Earth that ended with the mass extinction, probably due to runaway effects. The kill mechanism was likely ocean acidification resulting in the carbonate crisis commonly considered to be the primary cause for four of the five Phanerozoic mass extinctions.

Characteristics of host rocks, secondary minerals, and fluids would affect the transport of radionuclides from a previously proposed repository at Yucca Mountain, Nevada. Minerals in the Yucca Mountain tuffs that are important for retarding radionuclides include clinoptilolite and mordenite (zeolites), clay minerals, and iron and manganese oxides and hydroxides. Water compositions along flow paths beneath Yucca Mountain are controlled by dissolution reactions, silica and calcite precipitation, and ion-exchange reactions. Radionuclide concentrations along flow paths from a repository could be limited by (1) low waste-form dissolution rates, (2) low radionuclide solubility, and (3) radionuclide sorption onto geological media. The chief sources of radioactivity in spent nuclear fuel are americium, plutonium, and neptunium. Therefore, studies have concentrated on their geochemical mobility. Uranium-233, uranium-234, iodine-129, technetium-99, and other radionuclides also have been included in some experiments. Solubilities were determined experimentally in representative Yucca Mountain waters. Sorption coefficients were determined using water, rock, and pure mineral samples from Yucca Mountain. Batch experiments were performed at several pH levels and oxidizing conditions. Dynamic transport-column experiments, diffusion experiments, and solid-rock beaker experiments also were conducted. The batch tests gave slightly lower retardation factors than those derived from column-breakthrough experiments. This finding indicates that using batch-sorption coefficients to predict radionuclide transport will yield conservative results in a performance assessment. Understanding of unsaturated-zone transport is based on laboratory and field-scale experiments. Fractures provide advective transport pathways. Sorption and matrix diffusion may contribute to retardation of radionuclides. Conversely, sorption onto mobile colloids may enhance radionuclide transport.

New and published whole-rock major-element contents of xenolithic peridotites, combined with mineral trace-element, 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 3 He/ 4 He isotopic compositions, are used to unravel the metasomatic history of lithospheric mantle sampled by volcanic pipes in the Tanzanian section of the East African Rift. The deepest portion of the mantle beneath Labait (craton margin) exhibits high-μ (HIMU)–like 87 Sr/ 86 Sr (0.7029), 143 Nd/ 144 Nd (0.51286), and 3 He/ 4 He (5.9), which may reflect the plume in this region. Within the Mozambique belt, recent calcio-carbonatite melt metasomatism has overprinted the mantle lithosphere signature beneath Olmani, leading to high whole-rock Ca/Al and low SiO 2 , and remarkably homogeneous 87 Sr/ 86 Sr (0.7034–0.7035) and 143 Nd/ 144 Nd (0.51281–0.51283) of clinopyroxenes. Identical Sr and Nd isotope values are also reported for clinopyroxenes from peridotite xenoliths from the northern portion of the Gregory Rift in Tanzania (Pello Hill and Eledoi), which have a strong rift magma overprint. The silicate and carbonatite metasomatic melts are likely to be related to recent plume-derived magmatism of the East African Rift, and thus 87 Sr/ 86 Sr and 143 Nd/ 144 Nd values of the clinopyroxene from these samples can be used to define the rift isotopic signature beneath northern Tanzania. Some mantle regions beneath Lashaine and Labait escaped the recent rift-related overprint and have highly variable Sr-Nd isotope systematics. Labait clinopyroxenes show a near-vertical array on a 87 Sr/ 86 Sr versus 143 Nd/ 144 Nd plot, indicating highly variable time-integrated rare earth element (REE) patterns and low time-integrated Rb/Sr. Lashaine peridotites range to much higher 87 Sr/ 86 Sr at a given 143 Nd/ 144 Nd, and several plot in the right quadrants in the 87 Sr/ 86 Sr versus 143 Nd/ 144 Nd diagram, suggesting the influence of a (subducted?) Archean upper continental crust component on the lithospheric mantle beneath Lashaine. Their variable whole-rock SiO 2 and high Na 2 O contents, and clinopyroxene with high Sr/Y, low Sm/Nd, and variable Zr/Sm are consistent with this interpretation. Silicate lavas from the eastern branch of the East African Rift show increasingly evolved Sr and Nd isotope composition from north to south and hence increasing input of ancient metasomatized lithosphere (“EM1” and “EM2” components), similar to that beneath Lashaine and Labait, and well outside the suggested range in isotope compositions of the heterogeneous Kenya plume ( 87 Sr/ 86 Sr = 0.7029–0.7036; 143 Nd/ 144 Nd = 0.51275–0.51286). In the western branch, the “anomalous” Sr signature identified in Lashaine peridotites is prominent in silicate lavas and may indicate that the lithospheric mantle beneath that area was similarly enriched during ancient subduction. By contrast, the Sr-Nd isotope systematics of carbonatites reflect EM1 but not EM2 inputs, suggesting that such melts in the East African Rift neither derive from nor have interacted with subduction-modified mantle regions.

The ca. 570 Ma Catoctin volcanics, exposed in the Blue Ridge of northern Virginia, include metamorphosed rift-related basalts extruded during breakup of the supercontinent Rodinia. Field relationships, petrography, and geochemistry are used to decipher the stratigraphy for two areas of the volcanics, one at the base of the formation, and the other near its top. Geochemical characteristics of sequential flows can be explained by fractional crystallization of minerals commonly occurring in basalts. Intervening flows with slightly different geochemical features that cannot be explained by fractional crystallization from magma corresponding to an underlying flow, or by crustal contamination, most likely represent new pulses of magma. Positive Pb and negative Nb anomalies, coupled with ε Nd values of +1.5 to +4.5 and model ages that exceed crystallization ages by over 500 m.y., suggest that an enriched component was added to the mantle prior to melting. This component resulted from devolatilization of subducting sediments prior to assembly of Rodinia, likely during Grenville orogenic events. Stratigraphic control shows that lower lavas in the sequence contain more of the enriched component than upper flows.

Two Paleozoic ophiolites of different age mark the Variscan suture in the Cabo Ortegal Complex (northwestern Iberian Massif). The Moeche and Purrido allochthonous ophiolites are structurally located between an exotic terrane of continental affinity (basal units), below, and another terrane with arc affinity (upper units), above. The mafic rocks of the Moeche unit are greenschist with a composition typical of tholeiitic basalt. Their rare earth element (REE) concentrations range from 12 to 31 times the chondritic abundances, with almost flat chondrite-normalized REE patterns ([La N /Yb N ] avg = 0.83), and they have no significant Eu anomalies. According to their contents in some immobile trace elements, the geochemical composition of these metabasalts seems to be transitional between normal mid-ocean-ridge basalt (N-MORB) and island-arc tholeiites. Their abundances in the most immobile trace elements with the highest discriminating power (Th, Nb, Ce, Zr, Ti, and Y), normalized to an average tholeiitic N-MORB, depict quite flat patterns close to the average N-MORB concentrations, although they are slightly enriched. A small negative Nb anomaly points to an origin linked to a subduction zone. The Purrido unit consists of amphibolites with compositions also equivalent to tholeiitic basalt. REE compositions range from 6 to 22 times chondritic abundance, with essentially flat chondrite-normalized REE patterns ([La N /Yb N ] avg = 0.99) and very small positive Eu anomalies ([Eu/Eu*] avg = 1.08). The abundance of some trace elements in these metabasites shows an affinity with island-arc tholeiites, but, unlike the greenschist of the Moeche unit, their N-MORB–normalized trace-element patterns are slightly depleted in Nb, Zr, Ti, and Y, with a somewhat steeper negative Nb anomaly. These features are found in basalts generated in some back-arc basins. Both ophiolites seem to represent oceanic lithosphere that developed in a supra-subduction zone context. The Moeche unit was probably formed during the opening of a back-arc basin at the Cambrian-Ordovician boundary, whereas the Purrido unit was generated later, during the Early Devonian, as a result of the intraoceanic subduction of the older oceanic lithosphere represented by the Moeche unit. The two ophiolites are representative of the Variscan suture in northwest Iberia, and they preserve a significant part of the history of the oceanic realms that closed during the final amalgamation of Pangea.

Following the Variscan orogeny, the Iberian plate was affected by an extensional tectonic regime from Late Permian to Late Cretaceous time. In the central part of the plate, NW-SE–trending rift basins were created. Two rifting cycles can be identified during the extensional stage: (1) a Late Permian to Hettangian cycle, and (2) a latest Jurassic to Early Cretaceous cycle. During these cycles, thick clastic continental sequences were deposited in grabens and half grabens. In both cycles, sandstone petrofacies from periods of high tectonic activity reveal a main plutoniclastic (quartzofeldspathic) character due to the erosion of coarse-grained crystalline rocks from the Hesperian Massif, during Buntsand- stein (mean Qm 72 F 25 Lt 3 ) sedimentation and during Barremian–early Albian times (mean Qm 81 F 18 Lt 1 ). Geochemical data show that weathering was more intense during the second rifting phase (mean chemical index of alteration [CIA]: 80) due to more severe climate conditions (humid) than during the first rifting phase (mean CIA: 68) (arid climate). Ratios between major and trace elements agree with a main provenance from passive-margins settings in terms of the felsic nature of the crust. However, anomalies in trace elements have been detected in some Lower Cretaceous samples, suggesting additional basic supplies from the north area of the basin. These anomalies consist of (1) low contents in Hf, Th, and U; (2) high contents in Sc, Co, and Zr; and (3) anomalous ratios in Th/Y, La/Tb, Ta/Y, and Ni/V. Basic supplies could be related to the alkaline volcanism during Norian-Hettangian and Aalenian-Bajocian times. Geochemical composition of rift deposits has been shown to be a useful and complementary tool to petrographic deduction in provenance, especially in intensely weathered sediments. However, diagenetic processes and hydrothermalism may affect the original detrital deposits, producing changes in geochemical composition that mislead provenance and weathering deductions.

The Cosoltepec Formation consists of unfossiliferous phyllites and psammites with tectonic slices of basalt and forms a major part of the Acatlán Complex of southern México. It has traditionally been interpreted as part of either a Cambro-Ordovician accretionary prism or a passive margin deposited in the Iapetus Ocean. Although no reliable age data are available from the basaltic slices, their widespread tectonic interleaving with the Cosoltepec Formation suggests that some of the sedimentary rocks were originally deposited directly on the ocean-floor basalts. The age of the Cosoltepec metasediments ranges from Ordovician to uppermost Devonian (oldest unconformably overlying sediments: uppermost Devonian). The mafic rocks have been affected by greenschist- to sub-greenschist-facies metamorphism. The geochemistry indicates that they are mainly mid-ocean ridge basalt (MORB) tholeiites with flat or depleted rare earth element (REE) patterns and resemble MORB derived from heterogeneous sources. Another minor group consists of ocean island basalts and andesites, which have distinctly fractionated REE patterns, whereas their mantle-normalized trace element patterns do not show Nb-Ta or Ti negative anomalies. Thus, these mafic rocks appear to represent oceanic lavas that were tectonically incorporated into the clastic rocks of the Cosoltepec Formation during deformation that started immediately prior to deposition of late Fammenian sedimentary rocks and continued into the Mis-sissippian. Tectonic juxtaposition of these rocks with eclogitic rocks suggests that the deformation was related to exhumation following subduction: the Cosoltepec Formation and its mafic lenses derived from the overriding plate, whereas the eclogitic rocks represent parts of the subducting plate. The probable Middle Ordovician–Middle Devonian age of the oceanic rocks together with their initial deformation during the Devonian–Carboniferous suggest that they represent vestiges of the Rheic Ocean.