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Abstract A giant mass-transport complex was recently discovered in the eastern Arabian Sea, exceeding in volume all but one other known complex on passive margins worldwide. The complex, named the Nataraja Slide, was drilled by International Ocean Discovery Program (IODP) Expedition 355 in two locations where it is ∼300 m (Site U1456) and ∼200 m thick (Site U1457). The top of this mass-transport complex is defined by the presence of both reworked microfossil assemblages and deformation structures, such as folding and faulting. The deposit consists of two main phases of mass wasting, each consisting of smaller pulses, with generally fining-upward cycles, all emplaced just prior to 10.8 Ma based on biostratigraphy. The base of the deposit at each site is composed largely of matrix-supported carbonate breccia that is interpreted as the product of debris-flows. In the first phase, these breccias alternate with well-sorted calcarenites deposited from a high-energy current, coherent limestone blocks that are derived directly from the Indian continental margin, and a few clastic mudstone beds. In the second phase, at the top of the deposit, muddy turbidites dominate and become increasingly more siliciclastic. At Site U1456, where both phases are seen, a 20-m section of hemipelagic mudstone is present, overlain by a ∼40-m-thick section of calcarenite and slumped interbedded mud and siltstone. Bulk sediment geochemistry, heavy-mineral analysis, clay mineralogy, isotope geochemistry, and detrital zircon U-Pb ages constrain the provenance of the clastic, muddy material to being reworked, Indus-derived sediment, with input from western Indian rivers (e.g., Narmada and Tapti rivers), and some material from the Deccan Traps. The carbonate blocks found within the breccias are shallow-water limestones from the outer western Indian continental shelf, which was oversteepened from enhanced clastic sediment delivery during the mid-Miocene. The final emplacement of the material was likely related to seismicity as there are modern intraplate earthquakes close to the source of the slide. Although we hypothesize that this area is at low risk for future mass wasting events, it should be noted that other oversteepened continental margins around the world could be at risk for mass failure as large as the Nataraja Slide.

Abstract Barite (BaSO 4) is a highly stable and widely-distributed mineral found in magmatic, metamorphic, and sedimentary rocks of all ages, as well as in soils, aerosol dust, and extraterrestrial material. Barite can form in a variety of settings in the oceans (hydrothermal deposits, cold seeps, water column, or within sediments) and on the continents (soils, sulfidic springs and in the subsurface) when (1) two fluids mix – one containing barium and another containing sulfate, (2) sulfur is oxidized forming sulfate in a barium containing solution, or (3) barium or sulfate is concentrated in microenvironments where either sulfate or barium are already present. Hydrologic and biologic processes can therefore play key roles in the formation of barite and affect its geochemical composition. Characteristics of barite from various modern settings are identified here to serve as analogs for ancient systems, summarizing previous work and adding new details from the pelagic marine, hydrothermal, cold seep and continental setting. Radiogenic strontium in barite clearly identifies the source(s) of fluid forming barite with the most radiogenic values measured in continental sulfidic spring settings associated with a deep fluid component that interacts with ancient crustal rocks. Sulfur and oxygen isotopes can distinguish between sources of sulfate and identify settings where the influence of (bio)chemical processes such as sulfate reduction is prominent. There are no unique stable strontium isotopic signatures for barite formed in any of the settings investigated here, but Holocene coretop marine pelagic barite appears to have a constant offset from seawater of approximately −0.53‰ in coretop samples in contrast to the wide range of values in barite precipitated in other settings. Stable strontium mass dependent fractionation could be useful in understanding post-depositional and diagenetic processes such as authigenic precipitation and recrystallization. Graphical Abstract Unlabelled Image Highlights • Hydrologic, biologic processes play key roles in barite formation and geochemistry. • Sulfur and oxygen isotopes distinguish sulfate sources and (bio)chemical processes. • Radiogenic Sr identifies fluid sources, stable Sr isotopic signatures are not unique. • Marine pelagic barite stable Sr isotopes have consistent offset from seawater. [ABSTRACT FROM AUTHOR]

The mineral barite (BaSO4) can precipitate in a variety of oceanic settings: in the water column, on the sea floor and within marine sediments. The geological setting where barite forms ultimately determines the geochemistry of the precipitated mineral and its usefulness for various applications. Specifically, the isotopic and elemental composition of major and trace elements in barite carry information about the solution(s) from which it precipitated. Barite precipitated in the water column (marine or pelagic barite) can be used as a recorder of changes in sea water chemistry through time. Barite formed within sediments or at the sea floor from pore water fluids (diagenetic or cold seeps barite) can aid in understanding fluid flow and sedimentary redox processes, and barite formed in association with hydrothermal activity (hydrothermal barite) provides information about conditions of crust alteration around hydrothermal vents. The accumulation rate of marine barite in oxic-pelagic sediments can also be used to reconstruct past changes in ocean productivity. Some key areas for future work on the occurrence and origin of barite include: fully characterizing the mechanisms of precipitation of marine barite in the water column; understanding the role and potential significance of bacteria in barite precipitation; quantifying parameters controlling barite preservation in sediments; determining the influence of diagenesis on barite geochemistry; and investigating the utility of additional trace components in barite. [ABSTRACT FROM AUTHOR]

• Rise in Middle Ordovician global sea level suggests increased seafloor production. • Hydrothermal weathering drives inflection in marine strontium isotopes (87Sr/86Sr). • Oxygen isotope (δ 18 O) data demonstrate cooling concurrent with 87Sr/86Sr inflection. • Continental silicate weathering can drive cooling. • Cooling from weathering can counteract volcanic carbon dioxide (CO 2) degassing. The global climate of the Ordovician Period (486.9 to 443.1 Ma) is characterized by cooling that culminated in the Hirnantian glaciation. Chemical weathering of Ca- and Mg-bearing silicate minerals and the subsequent trapping of carbon in marine carbonates act as a sink for atmospheric CO 2 on multi-million-year time scales, with basaltic rocks consuming CO 2 at a greater rate than rocks of granitic composition. The oceanic Sr isotope ratio (87Sr/86Sr) can act as a geochemical proxy for the relative proportion of basaltic versus granitic weathering. Oxygen isotopes (δ 18 O) act as a proxy for paleotemperature and ice volume, providing a useful complement to 87Sr/86Sr in studies of ancient climate. Previous studies have reported stepwise cooling (increasing δ 18 O) during the Middle to Late Ordovician. Combined with Sr and C cycle models, this has led to the hypothesis that continental silicate weathering of mafic material drove Ordovician cooling (e.g., the Taconic Orogeny). However, Sr and C cycle models have not accounted for an apparent rise in sea level and seafloor production in the Middle Ordovician (Darriwilian), which would increase the hydrothermal Sr flux as well as degassing along continental volcanic arcs. Furthermore, some Ordovician studies contain temporal uncertainty between 87Sr/86Sr and δ 18 O curves if they are not based on paired analyses, which can obscure the relationship between silicate weathering and cooling. Here, we present new paired 87Sr/86Sr and δ 18 O data from conodont apatite and integrate this with both a deterministic (forward) and stochastic (reverse) modeling approach to argue that increased hydrothermal weathering played a role in driving marine 87Sr/86Sr, specifically an inflection occurring in the Pygoda serra conodont zone of the mid-Darriwilian Stage (∼ 460.9 Ma ± 1 My). This 87Sr/86Sr inflection is accompanied by an increase in δ 18 O, consistent with climate cooling. Clarifying the role of seafloor production for marine 87Sr/86Sr and the implications for Ordovician cooling allows for a more nuanced understanding of the factors that drive multi-million-year shifts in climate. [ABSTRACT FROM AUTHOR]