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International audience; The Upper Cretaceous sedimentary sequence at International Ocean Discovery Program Site U1512 in the Ceduna Sub-basin of the Great Australian Bight represents a continuous, > 690 m thick interval of black silty clay and claystone spanning the lower Turonian through Lower Campanian (~10 million years). Sediments were deposited in an elongate, ~E-W oriented, ~2500 km long rift system that developed between Australia and Antarctica with an open-ocean connection to the west and a continental bridge to the east. Site U1512 cores provide a unique, continuous record of Late Cretaceous deposition in the Ceduna Sub-basin on the hanging wall of the Wallaroo Fault Zone. Study of U1512 samples could provide both an important high-latitude, southern hemisphere perspective on climatic evolution during the peak and demise of the Cretaceous hothouse and an offshore record of the sedimentation history in the basin during the Late Cretaceous portion of the Gondwanan breakup. The Upper Cretaceous sequence at Site U1512 is notable for its lithologic uniformity. Burrow-mottled to massive claystone and silty claystone make up the majority of the almost 700 m section, while rare (n = 28) isolated, 2 to 21 cm thick medium to fine sandstone beds are a minor lithology. Macrofossils present include common inoceramids and rare occurrences of other bivalves and ammonites. Microfossils include common occurrences of calcareous nannofossils, agglutinated and calcareous benthic foraminifera, radiolaria and organic-walled dinoflagellate cysts as well as rare small, surface dwelling planktonic foraminifera. Carbonate (

金沢大学理工研究域地球社会基盤学系, Detailed reconstruction of Indian summer monsoons is necessary to better understand the late Quaternary climate history of the Bay of Bengal and Indian peninsula. We established a chronostratigraphy for a sediment core from Hole 19B in the western Bay of Bengal, extending to approximately 80kyr BP and examined major and trace element compositions and clay mineral components of the sediments. Higher δ 18 O values, lower TiO 2 contents, and weaker weathering in the sediment source area during marine isotope stages (MIS) 2 and 4 compared to MIS 1, 3, and 5 are explained by increased Indian summer monsoonal precipitation and river discharge around the western Bay of Bengal. Clay mineral and chemical components indicate a felsic sediment source, suggesting the Precambrian gneissic complex of the eastern Indian peninsula as the dominant sediment source at this site since 80kyr. Trace element ratios (Cr/Th, Th/Sc, Th/Co, La/Cr, and Eu/Eu*) indicate increased sediment contributions from mafic rocks during MIS 2 and 4. We interpret these results as reflecting the changing influences of the eastern and western branches of the Indian summer monsoon and a greater decrease in rainfall in the eastern and northeastern parts of the Indian peninsula than in the western part during MIS 2 and 4. © 2018. American Geophysical Union. All Rights Reserved.

The South China Sea is the largest marginal basin in the world, and is located in the confluence of three major tectonic plates (Pacific-Philippine Sea, Indo-Australia, Eurasia). Its tectonic history and evolution throughout the Cenozoic has been the subject of much discussion with regards to its mechanism, timing, and relationship to neighboring geological features. We conducted the first Re-Os isotope study of the South China Sea cores recovered from ocean drilling (ODP-Ocean Drilling Program and IODP-International Ocean Discovery Program) and from Philippine oil exploration wells in order to investigate whether or not the basin's tectonic evolution will be reflected in the seawater Os isotope record. These cores represent the northern and southern conjugate margins of the South China Sea, and should have complementary records of the tectonic events in the basin. We found three important negative anomalies in the South China Sea's seawater Os isotope record at 35–33 Ma, 26–23 Ma, and ~ 13–6 Ma, which are temporally related to tectono-magmatic events. At the northern margin, the record starts with the lowest (187Os/188Os)i values of 0.37 and 0.32 for the oldest samples, which then progresses to more radiogenic values for younger sediments. The trend is consistent with the start of seafloor spreading at ~ 33 Ma that saw the peak in the flux of mantle-derived material, which later slowed down until the second peak between 26 and 23 Ma. However, these unradiogenic values also mark the Eocene-Oligocene Transition and more likely implies South China Sea connectivity to global ocean circulation since pre-Oligocene. In both margins, slight to marked decrease in (187Os/188Os)i values deviate from the global trend and coincides with the ridge jump at ~ 23 Ma. In the southern conjugate margin, (187Os/188Os)i show a general decreasing tendency opposite that of the northern margin and the global trend starting from Mid- to Late Miocene (~ 13–6 Ma). Mass balance modeling suggests that the two younger negative excursions may be accounted for by mantle and hydrothermal Os inputs of ~ 372 mol/my at ~ 23 Ma and post ~ 13 Ma. We suggest that the post ~ 13 Ma protracted decrease in (187Os/188Os)i values for the southern margin reflects localized input of mantle-derived lithogenic Os from the Luzon Arc combined with post-spreading magmatic intrusions, and possibly gradual changes in paleoceanographic condition at the southern East Sub-basin.

Shatsky Rise in the Northwest Pacific is the best example so far of an oceanic plateau with two potential hotspot tracks emanating from it: the linear Papanin volcanic ridge and the seamounts comprising Ojin Rise. Arguably, these hotspot tracks also project toward the direction of Hess Rise, located ∼1200 km away, leading to speculations that the two plateaus are connected. Dredging was conducted on the massifs and seamounts around Shatsky Rise in an effort to understand the relationship between these plateaus and associated seamounts. Here, we present new 40Ar/39Ar ages and trace element and Nd, Pb, and Hf isotopic data for the recovered dredged rocks and new trace elements and isotopic data for a few drill core samples from Hess Rise. Chemically, the samples can be subdivided into plateau basalt-like tholeiites and trachytic to alkalic ocean-island basalt compositions, indicating at least two types of volcanic activity. Tholeiites from the northern Hess Rise (DSDP Site 464) and the trachytes from Toronto Ridge on Shatsky’s TAMU massif have isotopic compositions that overlap with those of the drilled Shatsky Rise plateau basalts, suggesting that both Rises formed from the same mantle source. In contrast, trachytes from the southern Hess Rise (DSDP Site 465A) have more radiogenic Pb isotopic ratios that are shifted toward a high time-integrated U/Pb (HIMU-type mantle) composition. The compositions of the dredged seamount samples show two trends relative to Shatsky Rise data: one toward lower 143Nd/144Nd but similar 206Pb/204Pb ratios, the other toward similar 143Nd/144Nd but more radiogenic 206Pb/204Pb ratios. These trends can be attributed to lower degrees of melting either from lower mantle material during hotspot-related transition to plume tail or from less refractory shallow mantle components tapped during intermittent deformation-related volcanism induced by local tectonic extension between and after the main volcanic-edifice building episodes on Shatsky Rise. The ocean-island-basalt-like chemistry and isotopic composition of the Shatsky and Hess rise seamounts contrast with those formed by purely deformation-related shallow mantle-derived volcanism, favoring the role of a long-lived mantle anomaly in their origin. Finally, new 40Ar/39Ar evidence indicates that Shatsky Rise edifices may have been formed in multiple-stages and over a longer duration than previously believed.

International audience; Coring/logging data and physical property measurements from International Ocean Discovery Program Expedition 349 are integrated with, and correlated to, reflection seismic data to map seismic sequence boundaries and facies of the central basin and neighboring regions of the South China Sea. First-order sequence boundaries are interpreted, which are Oligocene/Miocene, middle Miocene/late Miocene, Miocene/Pliocene, and Pliocene/Pleistocene boundaries. A characteristic early Pleistocene strong reflector is also identified, which marks the top of extensive carbonate-rich deposition in the southern East and Southwest Subbasins. The fossil spreading ridge and the boundary between the East and Southwest Subbasins acted as major sedimentary barriers, across which seismic facies changes sharply and cannot be easily correlated. The sharp seismic facies change along the Miocene-Pliocene boundary indicates that a dramatic regional tectonostratigraphic event occurred at about 5 Ma, coeval with the onsets of uplift of Taiwan and accelerated subsidence and transgression in the northern margin. The depocenter or the area of the highest sedimentation rate switched from the northern East Subbasin during the Miocene to the Southwest Subbasin and the area close to the fossil ridge in the southern East Subbasin in the Pleistocene. The most active faulting and vertical uplifting now occur in the southern East Subbasin, caused most likely by the active and fastest subduction/obduction in the southern segment of the Manila Trench and the collision between the northeast Palawan and the Luzon arc. Timing of magmatic intrusions and seamounts constrained by seismic stratigraphy in the central basin varies and does not show temporal pulsing in their activities..

The Louisville Seamount Chain (LSC) is, besides the Hawaiian-Emperor Chain, one of the longest-lived hotspot traces. We report here the first Re-Os isotope and platinum group element (PGE) data for Canopus, Rigil, and Burton Guyots along the chain, which were drilled during IODP Expedition 330. The LSC basalts possess (187Os/188Os)i = 0.1245–0.1314 that are remarkably homogeneous and do not vary with age. A Re-Os isochron age of 64.9 ± 3.2 Ma was obtained for Burton seamount (the youngest of the three seamounts drilled), consistent with 40Ar-39Ar data. Isochron-derived initial 187Os/188Os ratio of 0.1272 ± 0.0008, together with data for olivines (0.1271–0.1275), are within the estimated primitive mantle values. This (187Os/188Os)i range is similar to those of Rarotonga (0.124–0.139) and Samoan shield (0.1276–0.1313) basalts and lower than those of Cook-Austral (0.136–0.155) and Hawaiian shield (0.1283–0.1578) basalts, suggesting little or no recycled component in the LSC mantle source. The PGE data of LSC basalts are distinct from those of oceanic lower crust. Variation in PGE patterns can be largely explained by different low degrees of melting under sulfide-saturated conditions of the same relatively fertile mantle source, consistent with their primitive mantle-like Os and primordial Ne isotope signatures. The PGE patterns and the low 187Os/188Os composition of LSC basalts contrast with those of Ontong Java Plateau (OJP) tholeiites. We conclude that the Re-Os isotope and PGE composition of LSC basalts reflect a relatively pure deep-sourced common mantle sampled by some ocean island basalts but is not discernible in the composition of OJP tholeiites.

Mantle-derived spinel peridotite and spinel plagioclase pyroxenite xenoliths hosted in Cenozoic alkali basalts (20 Ma) are found in the Xikeer area, western Tarim Block, NW China. Based on petrographic and geochemical characteristics, the peridotite xenoliths are divided into three groups. Group 1 peridotite xenoliths have experienced high degree melt extraction (~ 17% fractional melting) and weak, incipient metasomatism. Group 2 and 3 peridotite xenoliths have undergone extensive silicate melt metasomatism, resulting in clinopyroxenes with spoon-shaped and highly fractionated REE patterns respectively. Although their present texture is metamorphic, the pyroxenite xenoliths were initially of metasomatic origin, with high pressure protolith mineral assemblage of orthopyroxene + clinopyroxene + spinel ± olivine. Numerical modeling of the Mg-number shows that the Xikeer pyroxenites may have resulted from reaction between Group 1 peridotite and a primitive Permian picritic melt at a high melt/rock ratio (> 15) and that the host basanite is incapable of being the metasomatic melt. The Re–Os isotopic systematics of the Xikeer peridotites and pyroxenites yield an apparent isochron of ~ 290 Ma, virtually identical to the age of Tarim flood basalts. Their eNd(t = 290 Ma) ranges from + 9.5 to + 14.3, typical of convecting mantle. We propose that the Xikeer xenolith suite may have been initially formed by melt extraction from the convecting mantle and, shortly after, was refertilized by Tarim mantle plume melts during the Early Permian, a process which is referred to as mantle ‘auto-refertilization’.

We present initial isotopic ratios of lead for Early Cretaceous (Barremian–Aptian) sections from Shatsky Rise (Pacific) and Gorgo a Cerbara (Italy). Our Pb isotopic data track an interval representing Oceanic Anoxic Event (OAE)-1a, which is characterized by quasi-global deposition of organic carbon-rich black shale. Pb isotopic compositions of sediments from Shatsky Rise decrease at the end of Barremian time, from radiogenic continental values to unradiogenic values, and subsequently remained less radiogenic until the end of early Aptian time. We explain the isotopic shift by a significant increase in supply rate of unradiogenic Pb, most likely due to massive volcanism. In contrast, the Pb isotopic compositions from the Italian section, which was situated at the western end of Tethys, are mostly identical to those of upper continental crust, showing no significant change in supply rate of unradiogenic Pb. The discrepancy between two sites is attributed to quiescent deep-submarine eruptions of Pacific large igneous provinces (LIPs) such as the Ontong Java Plateau (OJP), which severely limited dispersion of Pb-carrying particles out of the Pacific Ocean. Published Os isotopic data from the Italian section indicate two episodes of massive eruptions of OJP or contemporaneous Manihiki and Hikurangi plateaus starting from earliest Aptian time, slightly later than that indicated by the sedimentary Pb isotopic record from Shatsky Rise. Differences in isotopic variations between Pb and Os likely reflect differences in their chemical behaviors in the oceans, i.e., Pb isotopic compositions would have varied in response to local or regional changes in sediment provenances, whereas large-scale changes in Os inputs are required to explain variations in seawater Os isotopic compositions. Our Pb isotopic data, together with the published Os isotopic record, provide new evidence for the eruptive history of OJP together with contemporaneous Pacific plateaus and its environmental consequences, starting from end-Barremian time and extending through early Aptian time.

The Early Cretaceous Ontong Java Plateau was emplaced at almost the same time as marine biotic changes that culminated in oceanic anoxic event 1 (OAE1a). A causative link between these events has been suggested, but direct evidence has been lacking until now. New Os isotope measurements across the Lower Aptian “Selli Level” black shale deposited during OAE1a in central Italy reveal two negative excursions in marine 187 Os/ 188 Os ratios within a period of 2 Ma starting above the Barremian-Aptian boundary and ending just above the Selli Level horizon, suggesting an order-of-magnitude increase in the global fl ux of unradiogenic Os. The results are consistent with early and major phases of eruption of the Ontong Java Plateau. The latter phase is estimated to have been as short as ~1 Ma and may have induced widespread oceanic stratifi cation that triggered OAE1a.