ALBERT

Description: One of the most puzzling characteristics of sea-floor morphology is the occurrence of anomalously shallow, fracture-zone–parallel, oceanic transverse ridges. A model is proposed for the formation of transverse ridges near lat 21° and 24°N on the Mid-Atlantic Ridge in which the differential responses of large-offset and small-offset fracture zones to recent changes in spreading direction result in the generation of normal faults that coincide with the off-axis traces of fracture zones. Numerical models of the flexural response of the lithosphere to normal faulting suggest that modest amounts of extension (〈5 km) along low-angle faults (〈45°) are responsible for the transverse ridges.

Description: The early breakup of western Pangea has been investigated by mapping the pattern of fracture zones and distribution of seismic reflectors within the sedimentary cover of the Atlantic between the Cape Verde Islands and the equator. Two distinct sets of transverse oceanic lineaments are present, separated by the Guinea Fracture Zone near lat 10°N. Lineaments to the north are associated with the formation of the central Atlantic in the Late Jurassic and Early Cretaceous; those in the south relate to the Cretaceous opening of the South Atlantic. The Guinea Fracture Zone is thus the conjugate of the Jurassic transform boundary under peninsular Florida, which linked the Atlantic with the Gulf of Mexico. The distribution of dated seismic reflectors suggests that deposition of deep-water sediments was confined to the region north of the Guinea transform until Aptian time, when the Sierra Leone Basin began to open. The latter started to widen at least 15 m.y. after the initiation of the Cape Basin off southwest Africa, an age difference that can be explained if a short-lived plate boundary developed in either Africa or South America during the Early Cretaceous. Neither the trends of the equatorial fracture zones nor the seismic stratigraphy supports the existence of a predrift gap between west Africa and Brazil.

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: Mineral assemblages in volcanic rocks record both pre-eruptive conditions and changes experienced by magma as it rises. Titanomagnetite in andesitic magmas is especially sensitive to changes in temperature and oxygen fugacity immediately prior to and during eruptions. Two end-member eruption states can be distinguished by examining titanomagnetite textures in erupted rocks. Slow-ascent eruptions—characterized by near-stagnant magma bodies and slow effusion of lava domes—show solid-state exsolution of titanohematite/ilmenite lamellae within titanomagnetite hosts. By contrast, fast-ascent eruptions—characterized by rapid chilling of magma in sub-Plinian eruptions—contain titanomagnetites without such exsolution features. This mineralogical distinction is particularly useful in examining very fine-grained distal tephra layers where other characteristic properties of the two eruptions types are not present. Such tephra records in lake deposits typically provide the most precise long-term eruption records from andesitic volcanoes. Using an example from Mount Taranaki, New Zealand, we show that by classifying eruption styles within such sequences, the underlying magmatic system processes at a volcano can be elucidated and separated from other environmental factors such as vent/crater configuration.

Description: Marine sediments contribute significantly to global element cycles on multiple time scales. This is due in large part to microbial activity in the shallower layers and abiotic reactions resulting from increasing temperatures and pressures at greater depths. Quantifying the rates of these diagenetic changes requires a three-dimensional description of the physiochemical properties of marine sediments. In a step toward reaching this goal, we have combined global data sets describing bathymetry, heat conduction, bottom-water temperatures, and sediment thickness to quantify the three-dimensional distribution of temperature in marine sediments. This model has revealed that ∼35% of sediments are above 60 °C, conditions that are suitable for petroleum generation. Furthermore, significant microbial activity could be inhibited in ∼25% of marine sediments, if 80 °C is taken as a major thermal barrier for subsurface life. In addition to a temperature model, we have calculated new values for the total volume (3.01 × 108 km3) and average thickness (721 m) of marine sediments, and provide the only known determination of the volume of marine-sediment pore water (8.46 × 107 km3), equivalent to ∼6.3% of the volume of the ocean. The results presented here can be used to help quantify the rates of mineral transformations, lithification, catagenesis, and the extent of life in the subsurface on a global scale.

Description: Logs collected while drilling measured density in situ, through the accretionary prism and decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to 〉90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5–2.0 m) intervals of anomalously low density and resistivity in the logs through the basal decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.

Description: Asteroid impacts play an important role in the evolution of planetary surfaces. In the inner solar system, the large majority of impacts occur on bodies (e.g., asteroids, the Moon, Mars) covered by primitive igneous rocks. However, most of the impacts recorded on Earth occur on different rock types and are poor proxies for planetary impacts. The Lonar crater is a 1.88-km-diameter, Quaternary age crater (Fig. 1) located on the ca. 66 Ma Deccan basaltic traps in Maharashtra (India), and is one of the very few craters on Earth emplaced directly on basaltic lava flows. We carried out 12 40Ar/39Ar step-heating experiments on 4 melt rock samples in order to (1) obtain a precise age for the Lonar crater; (2) study the response of isotopic chronometers during impacts on mafic target rocks; and (3) better understand the dating of extraterrestrial impact craters. We obtained 10 plateau and 9 inverse isochron ages on various aliquots. Combination of selected data into a global inverse isochron yielded an age of 570 ± 47 ka (MSWD = 1.1; P = 0.24). In comparison, previous nonisotopic investigations on rocks thought to be affected by secondary processes yielded a range of much younger ages (ca. 12–62 ka). The measured 40Ar/36Ar trapped values offer a direct comparison with the atmospheric benchmark value and allow us to test the inherited 40Ar* degassing capacity of basaltic impact melt rocks. The 40Ar/36Ar ratio of 296.5 ± 1.7 is indistinguishable from the atmospheric composition and suggests that inherited 40Ar* is absent from the melt rock. This result substantiates diffusion models that predict a near-complete degassing of low-viscosity melt (e.g., basalts) during impact, and demonstrates for the first time that inherited 40Ar* is less problematic for 40Ar/39Ar dating of impact events in basaltic igneous rocks compared to Si-rich rocks. These results provide direct evidence that basaltic melt rocks are excellent candidates for recording the timing of planetary impact events and, as far as dating is concerned, should be the preferred targets of sample recovery by future missions.

Description: One method of testing the concept of sequence stratigraphy is to compare it to Quaternary sediments in which chronology, stratigraphic relations, and facies geometry are more clearly understood than in older rocks. Rapid deposition rates during Quaternary glacial-eustatic cycles in large deltaic depocenters generate sequences comparable to those in the ancient stratigraphic record. In the northern Gulf of Mexico, the late Wisconsinan-Holocene Mississippi River has deposited a Type 1 sequence that includes lowstand, transgressive, and high-stand systems tracts. Characteristics of modern Mississippi River sedimentary environments support the methodology used in sequence analysis, but the short time taken for sequence generation here raises important questions about sequence time scales, correlation, and driving mechanisms.

Description: Gold colloids occur in black smoker fluids from the Niua South hydrothermal vent field, Lau Basin (South Pacific Ocean), confirming the long-standing hypothesis that gold may undergo colloidal transport in hydrothermal fluids. Six black smoker vents, varying in temperature from 250 °C to 325 °C, were sampled; the 325 °C vent was boiling at the time of sampling and the 250 °C fluids were diffusely venting. Native gold particles ranging from 〈50 nm to 2 μm were identified in 4 of the fluid samples and were also observed to precipitate on the sampler during collection from the boiling vent. Total gold concentrations (dissolved and particulate) in the fluid samples range from 1.6 to 5.4 nM in the high-temperature, focused flow vents. Although the gold concentrations in the focused flow fluids are relatively high, they are lower than potential solubilities prior to boiling and indicate that precipitation was boiling induced, with sulfide lost upon boiling to exsolution and metal sulfide formation. Gold concentrations reach 26.7 nM in the 250 °C diffuse flow sample, and abundant native gold particles were also found in the fluids and associated sulfide chimney and are interpreted to be a product of colloid accumulation and growth following initial precipitation upon boiling. These results indicate that colloid-driven precipitation as a result of boiling, the persistence of colloids after boiling, and the accumulation of colloids in diffuse flow fluids are important mechanisms for the enrichment of gold in seafloor hydrothermal systems.

Description: Numerous studies invoke weak layers to explain the occurrence of submarine mega-slides (〉100 km3), in particular those on very gentle slopes (〈3°). Failure conditions are thought to be met only within this layer, which is embedded between stable sediments. Although key to understanding failure mechanisms, little is known about the nature and composition of such weak layers, mainly because they are destroyed with the landslides. This study is the first to place detailed constraints on the weak layer for one of the submarine mega-slides that occurred on the nearly flat, subtropical, northwest African continental slopes. Integrating results from the Ocean Drilling Program with high-resolution seismic reflection data, we show that the failure surfaces traced into the undisturbed sedimentary sequence coincide with thin (〈10 m) diatom ooze layers capped by clay. As diatom oozes are common on many continental margins, we suggest a new margin-independent failure mechanism to explain submarine mega-slides at low-gradient continental slopes globally. Diatom oozes are susceptible to building up excess pore fluid during burial due to their high compressibility and water content. If a low-permeable clay cap prevents upward drainage, excess pore pressures accumulate at the ooze-clay interface, causing the shearing resistance to increase at a lower rate than the shear stress until failure can occur. Changes in global climate affect the abundance of diatoms and thus formation of diatom oozes, thereby preconditioning the sediments for failure. However, the actual timing of failure is independent of environmental changes.