ALBERT

Description: Submarine hydrothermal vents and associated sealfoor mineralization on the Tonga arc have been found for the first time, in the summit calderas of two shallow-water volcanoes, greatly extending the known areas and diversity of seafloor hydrothermal activity in the western Pacific region. The highest temperature vents (245-265 degrees C) occur at water depths of 385-540 m near the summit of one volcano at 24 degrees S. The vents are spatially related to basaltic dike swarms exposed at a summit cone and in the caldera walls. Clusters of large (to 10 m high) barite, anhydrite, and sulfide chimneys on the summit cone are vigorously discharging clear hydrothermal fluids with temperatures on the seawater boiling curve. There is abundant evidence of phase separation, which can be seen as flame-like jets of steam (H2O vapor) at the chimney orifices. Pyrite, marcasite, sphalerite, and chalcopyrite line the interiors of the highest temperature vents, similar to black smoker chimneys on the mid-ocean ridges.

Description: The thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior. We have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges. We describe a general geometric model for subduction thrust décollements, which includes multiple simultaneously active, anastomosing fault strands tens of meters thick. The total thickness encompassing all simultaneously active strands increases to ~100–350 m at ~1–2 km below seafl oor, and this thickness is maintained down to a depth of ~15 km. Thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens-ofmeters- thick fault strands. Although fl attening, primary inherited chaotic fabrics, and fault migration through subducting sediments or the frontal prism may build mélange sections that are much thicker (to several kilometers), this thickness does not describe the active fault at any depth. These observations suggest that models should treat the subduction thrust plate boundary fault as 〈1–20 cm thick during earthquakes, with a concentration of postseismic and interseismic creep in single to several strands 5–35 m thick, with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands.

Description: Volcanic ash layers preserved within the geologic record represent precise time markers that correlate disparate depositional environments and enable the investigation of synchronous and/or asynchronous behaviors in Earth system and archaeological sciences. However, it is generally assumed that only exceptionally powerful events, such as supereruptions (≥450 km3 of ejecta as dense-rock equivalent; recurrence interval of ∼105 yr), distribute ash broadly enough to have an impact on human society, or allow us to address geologic, climatic, and cultural questions on an intercontinental scale. Here we use geochemical, age, and morphological evidence to show that the Alaskan White River Ash (eastern lobe; A.D. 833–850) correlates to the “AD860B” ash (A.D. 846–848) found in Greenland and northern Europe. These occurrences represent the distribution of an ash over 7000 km, linking marine, terrestrial, and ice-core records. Our results indicate that tephra from more moderate-size eruptions, with recurrence intervals of ∼100 yr, can have substantially greater distributions than previously thought, with direct implications for volcanic dispersal studies, correlation of widely distributed proxy records, and volcanic hazard assessment.

Description: During the past decades, remarkable changes in sea-surface temperature (SST) and sea-ice extent have been observed in the marginal seas of the subarctic Pacific. However, little is known about natural climate variability at millennial time scales far beyond instrumental observations. Geological proxy records, such as those derived from marine sediments, offer a unique opportunity to investigate millennial-scale natural climate variability of the Artic and subarctic environments during past glacial-interglacial cycles. Here we provide reconstructions of sea-ice variability inferred from IP25 (Ice Proxy with 25 carbon atoms) sea-ice biomarker and SST fluctuations based on alkenone unsaturation index (Graphic) of the subarctic Pacific realm between 138 and 70 ka. Warmest sea-surface conditions were found during the early Eemian interglacial (128 to 126 ka), exceeding modern SSTs by ∼2 °C. The further North Pacific climate evolution is marked by pronounced oscillations in SST and sea-ice extent on millennial time scales, which correspond remarkably well to short-term temperature oscillations known from Greenland and the North Atlantic. These results imply a common forcing, which seems to be closely coupled to dynamics of the Atlantic meridional overturning circulation. However, immediate propagation of such climate fluctuations far beyond the North Atlantic basin suggests a rapid circumpolar coupling mechanism probably acting through the atmosphere, a prerequisite to explain the apparent synchronicity of remote climatic reorganizations in the subarctic Pacific.

Description: Hydrothermal venting, an important cooling mechanism of the Earth, supports a diverse array of seafloor and sub-seafloor ecosystems that are sustained by large thermal and chemical fluxes. Vents have been found along even the slowest and coldest spreading centers, calling into question the driving heat source for these vents. The ultraslow-spreading Mid-Cayman Spreading Center in the Caribbean Sea, which hosts the axial-flank Von Damm Vent Field (VDVF), provides an opportunity to probe the mechanisms for venting at ultraslow spreading rates. Using active-source seismic data from the 2015 CaySeis (Cayman Seismic) experiment, we determined the seismic velocities in the large massif beneath the VDVF. We propose that this massif was produced by a pulse of on-axis magmatism at ca.2 Ma, which was then followed by exhumation, cooling, and fracturing. A low seismic velocity anomaly 5 km below the VDVF is evidence for either a cracking front mining lithospheric heat or intrusive magmatic sills, both of which could drive ongoing deep hydrothermal fluid circulation. We conclude that the transient magmatism and variable crustal thickness at ultraslow-spreading centers create conditions for long-lived hydrothermal venting that may be widespread, and other VDVF-like vents may be common in these areas.

Description: North Atlantic climate is very sensitive to overturning in the Greenland-Iceland-Norwegian (GIN) Seas, overflow of deep water into the North Atlantic via the Greenland-Iceland-Scotland Ridge, and compensating northward flow of warm surface water. Physical models suggest that, in the absence of such overturning, oceanic heat transport to the Northern Hemisphere is reduced by as much as 50%, open North Atlantic sea-surface temperatures are as much as 6 °C lower, and the winter sea-ice limit migrates as far south as 45°N. Although simulations of the equilibrium climate state for the Last Glacial Maximum (LGM) suggest the absence of GIN Seas overflow, tests of these model results have been hampered by ambiguity in sedimentary proxies. Here we present a bottom-water neodymium (Nd) isotope record from the Rockall Trough to investigate changes in the sources of circulating waters over the past 43 k.y. Today and throughout most of the Holocene, water from the GIN Seas, along with water from the North Atlantic Current (NAC) entrained during overflow, sets the bottom-water Nd isotope composition of the Rockall Trough to ∼–10. Our results suggest the persistence of this scenario back into the LGM and beyond to mid-Marine Isotope Stage 3. Periodic radiogenic excursions punctuate the record at times of meltwater events, implying either continued GIN Seas overflow without NAC entrainment, or millennial-scale interruptions in the overflow and shoaling of Southern Source Water. We conclude that overflow was at least intermittently present during the LGM, if not continuous, and that the GIN Seas have remained a source of deep water to the North Atlantic during the last glacial cycle.

Description: Large negative carbon (δ13C) and boron (δ11B) isotope excursions (both 〉6‰) within the widely distributed Neoproterozoic carbonates associated with the Marinoan "snowball Earth" event are interpreted to represent considerable perturbations of the carbon cycle and the accompanying reduction in global ocean pH. Yet this interpretation is predicated on these isotopic signals being primary in origin. Recent studies of Pleistocene carbonate platform sediments from the Great Bahama Bank (western Atlantic Ocean; Clino core, drilled by the Bahamas Drilling Project) and elsewhere demonstrate that δ13C excursions of similar magnitude and global distribution to the snowball Earth excursions are formed following eustatic sea-level fall and exposure of shelf carbonates to meteoric diagenesis. Here we present δ11B and trace element data (B/Ca, Na/Ca, Mg/Ca, and Sr/Ca) from the same Clino core carbonate sediments in order to explore the influence of this diagenetic process on the boron system. We find that within the interval of meteoric diagenesis the δ11B of bulk carbonate is reduced by ~6‰ in conjunction with a drop in the B/Ca ratio of 90%. Our results clearly demonstrate that the boron system is impacted by meteoric diagenesis, implying that a rigorous assessment of the diagenetic history of all ancient carbonates is required to ensure any paleoceanographic interpretation based on δ11B and/or B/Ca are robust.

Description: Ecological and taxonomic study of the mollusk-rich fauna of the Golfe d’Arguin, North Mauritania, investigates the various environmental influences affecting this tropical shelf. The upwelling of nutrient-rich waters leads to a highly productive environment under tropical conditions. The resulting mixed carbonate-siliciclastic sediment contains a large portion of calcareous components produced by heterotrophic organisms— e.g., mollusks, foraminifers, worms, barnacles—that are reworked on the open shelf. On the basis of mollusk assemblages, six taphocoenoses are defined, all being characterized by a mixed fauna of tropical (e.g., Tellina densestriata), subtropical (e.g., Macoma cumana) and temperate (e.g., Spisula subtruncata) species. Differences between the assemblages are related to the medium—grain size ranging from mud to gravel—that results from local hydrodynamic conditions and water depth. Among carbonate grains, Donax burnupi shells are very abundant in the swellexposed, northern part of the Golfe d’Arguin and reflect the tropical to subtropical, high-energy, and high-nutrient waters. Mollusk assemblages are demonstrated to be a sensitive tool for deciphering complex environmental conditions in sedimentary archives.

Description: The venting at the northern Peru convergent margin, unlike at other margins, has produced large barite deposits, which have not been observed outside the vents. The 87Sr/86Sr isotopic ratios of the fluids are more radiogenic than seawater. To explain these elevated values, we propose either the influence of a fluid characterized by a more radiogenic signature originating from the continent, or a reaction between seawater and the underlying continental metamorphic basement. The presence of this nonlocal radiogenic component is marked more strongly on the 87Sr/86Sr ratios measured in the barite deposits. We assume that the fluid sampled at the venting site and the fluid responsible for the barite deposit sampled at the same site originated from the same source, i.e., the Paleozoic metamorphic basement of the Andean continental margin.

Description: The spreading axis at many slow-spreading mid-ocean ridges is marked by an axial volcanic ridge. In this study, we use a combination of high-resolution remote sensing methods to elucidate the detailed nature of volcanoes in such a ridge. We fi nd that the “hummocks” described in previous sidescan sonar studies are dome- or cone-shaped edifi ces, 5–150 m high with diameters of 30–330 m. We estimate they form quickly, in single eruptions, each of which may produce several hummocks. Hummock collapse is common and hummocks of all heights are prone to failure. Collapses generally occur down the regional seafl oor slope, suggesting control by local topography. Approximately 33% of hummocks lose ~40% of their volume by collapse, so ~12% of all material erupted on the axial volcanic ridge is rapidly converted to talus. The higher porosity of these deposits may increase average upper crustal porosity by several percent, contributing 〉0.5 km s–1 to seismic velocity decrease in the upper oceanic crust, and may be one of the dominant mechanisms for increasing porosity in upper slow-spreading oceanic crust.