Woods Hole Oceanographic Institution

About: Woods Hole Oceanographic Institution is a nonprofit organization based out in Falmouth, Massachusetts, United States. It is known for research contribution in the topics: Population & Mantle (geology). The organization has 5685 authors who have published 18396 publications receiving 1202050 citations. The organization is also known as: WHOI.

The three-dimensional seismic velocity structure of the East Pacific Rise near latitude 9° 30′ N

Abstract: Three-dimensional images of crustal seismic structure beneath the East Pacific Rise show pronounced axial heterogeneity over distances of a few kilometres. A linear high-velocity anomaly, approximately 1–2 km in width and restricted to the uppermost 1 km of the crust, is centred on the rise axis. An axial low-velocity anomaly at depths of 1–3 km varies in amplitude along the axis, consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis. This apparent thermal segmentation along axis is consistent with injection of mantle-derived melt midway along a locally linear, 12-km-long segment of the rise.

A mechanism for magmatic accretion under spreading centres

Abstract: The most active volcanic zone on Earth occurs along the global mid-ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km Myr−1. This volcanism is segmented, and is concentrated along the axial valleys and highs of slow- and fast-spreading ridges respectively, with transform fault zones separating, and generally offsetting, each active ridge segment. Volcanic activity may be nearly absent at the ridge transform intersection where a slow-spreading ridge is offset, and greatly reduced where a fast-spreading ridge is offset, implying thinner crust in these regions1. The source of magmas erupted along ocean ridges is the underlying mantle, which undergoes decompression melting when rising to fill the gap between the spreading plates. The resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock. The melt is believed to pool in crustal magma chambers—whence it periodically erupts to the surface. Typically, the formation, ascent and aggregation of magmas along the mid-ocean ridges is regarded two-dimensionally and modelled in a section through the crust and mantle across the strike of the ridges (see ref. 2). Here, however, we consider the problem with a three-dimensional model for which we provide supporting geological evidence, in which the magmas rise as a result of a gravitational instability (modified Rayleigh–Taylor) in the underlying partial melt zone.

Biogeography of toxic dinoflagellates in the genusAlexandrium from the northeastern United States and Canada

Abstract: Twenty-eight strains of toxic dinoflagellates in the genusAlexandrium from the northeastern United States and Canada were characterized on the basis of morphology, bioluminescence capacity, mating compatibility, and toxin composition. The distributions of these characters were evaluated in the context of regional patterns of paralytic shellfish poisoning (PSP) and coastal hydrography. Two morphospecies were identified-A. tamarense Lebour andA. fundyense Balech. The two are interspersed geographically though there are areas, such as the Gulf of Maine, where apparently onlyA. fundyense occurs. Southern waters (Cape Cod, Connecticut, and Long Island) have especially diverse populations. The two species are sexually compatible. Virtually all northern isolates are bioluminescent, whereas southern isolates include bioluminescent and non-bioluminescent strains. Cluster analyses, based on high performance liquid chromatography (HPLC) determinations of the suite of toxins produced by each isolate, revealed two and perhaps three distinct groups. One is comprised almost exclusively of northern strains, and the other of southern strains. A Cape Cod cluster may be separable from the southern group. These analyses explain a previously reported north-to-south trend of decreasing toxicity, as the northern isolates produce greater proportions of the more potent toxins than do southern forms. The overall perspective is that the biogeography of toxicAlexandrium spp. in the study region is not that of a single, widespread, homogeneous population, but rather is comprised of several sub-populations, each with its own physiological characteristics and history. Two scenarios are considered with respect to this regional biogeography. The first invokes recent and continuing dispersal of isolates to the south from a center of origin in the north, followed by recombination and strong selection. The second holds that the northern and southern populations diverged from a common ancestor (vicariance), but now represent localized populations with little mixing of genotypes. Neither hypothesis can be completely refuted by the data presented here, though the weight of the evidence favors the latter. The correct scenario may be a combination of both, with recent and continuous speading occuring within the Gulf of Maine and perphaps the Gulf of St. Laerence, but with endemic localized populations persisting without genetic exchange in most southern locations. These data also indicate that although morphological criteria separate toxicAlexandrim isolates from the study region into two morphospecies, these assignments do not coincide with clusterings based on toxin composition or allozyme electrophoresis, and they are further violated by mating results. A revision of taxon designations to the varietal level could be justified.

The δ15N of nitrate in the Southern Ocean: Nitrogen cycling and circulation in the ocean interior

Abstract: We report analyses of the nitrogen isotopic composition of nitrate in the eastern Indian and Pacific sectors of the Southern Ocean. In this paper, we focus on the subsurface data as well as data from the deep waters of other ocean basins. Nitrate δ15N is relatively invariant in much of the abyssal ocean (i.e., below 2.5 km), with a value of 4.8±0.2‰ observed in Lower Circumpolar Deep Water, North Atlantic Deep Water, and central Pacific deep water. The isotopic invariance of deep ocean nitrate stems fundamentally from the completeness of nitrate utilization in most of the global surface ocean, the Southern Ocean surface being an important exception. In the Subantarctic Zone (north of the Polar Frontal Zone) the nitrate δ15N of Upper Circumpolar Deep Water is ∼0.7‰ greater than that of Lower Circumpolar Deep Water. This isotopic enrichment appears to result from denitrification in the low-latitude water masses with which Upper Circumpolar Deep Water communicates. The isotopic enrichment of Upper Circumpolar Deep Water is diminished in the Antarctic, probably because of the remineralization of sinking organic N, which has a low δ15N in the Antarctic. Relative to the other water masses of the Southern Ocean, the Subantarctic thermocline has a very low nitrate δ15N for its nitrate concentration because of exchange with the low-latitude thermocline, where this isotopic signature appears to originate. This signature of the low-latitude thermocline has two probable causes: (1) mixing with low-nitrate surface water and (2) the oxidation of newly fixed N.

Rapid reequilibration of H2O and oxygen fugacity in olivine-hosted melt inclusions

Abstract: The solubility of H 2 O in silicate melt drops substantially with decreasing pressure, so that a magma initially containing several weight percent H 2 O in a crustal magma reservoir is left with only a few thousand parts per million following ascent and eruption at the Earth’s surface. This rapid release of volatiles makes determining the pre-eruptive H 2 O contents of magmas very difficult. Olivine-hosted melt inclusions are thought to retain their H 2 O because they are protected from decompression by the strength of the host crystal, and pre-eruptive concentrations obtained from melt inclusions have been used to both estimate the amount of H 2 O in the upper mantle and investigate its role in the melt generation process. The greatest uncertainty involved in constraining upper mantle conditions from melt inclusions is the potential for rapid diffusive loss or gain of H + (protons) through the host olivine. Here we present results from hydration and dehydration experiments that demonstrate that, contrary to the widely held view, H 2 O loss or gain in melt inclusions is not limited by redox reactions and significant fluxes of H + through the host olivine are possible on very short time scales. We also show that the Fe 3+ /ΣFe of an olivine-hosted melt inclusion maintains equilibrium with the external environment via diffusion of point defects through the host olivine. Our results demonstrate that, while pre-eruptive H 2 O and Fe 3+ /ΣFe can be reliably estimated, olivine-hosted melt inclusions do not necessarily retain a record of the H 2 O and O 2 fugacity conditions at which they formed. High-H 2 O melt inclusions are particularly susceptible to diffusive dehydration, and therefore are not reliable proxies for the state of the upper mantle.