The prediction, verification, and significance of flank jets at mid-ocean ridges

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 1 (2012): 277–283, doi:10.5670/oceanog.2012.26.One aspect of ocean flow over mid-ocean ridges that has escaped much attention is the capacity of a ridge to convert oscillatory flows into unidirectional flows. Those unidirectional flows take the form of relatively narrow jets hugging the ridge's flanks. In the Northern Hemisphere, the jets move heat and dissolved and particulate matter poleward on the west and equatorward on the east of north-south trending ridges. Recent measurements and a model of flow at the East Pacific Rise at 9–10°N show that these ridge-parallel flows can extend 10–15 km horizontally away from the ridge axis, reach from the seafloor to several hundreds of meters above ridge crest depth, and have maximum speeds in their cores up to 10 cm s–1. Because of their along-ridge orientation and speed, the jets can significantly affect the transport of hydrothermal vent-associated larvae between vent oases along the ridge crest and, possibly, contribute to the mesoscale stirring of the abyssal ocean. Because jet-formation mechanisms involve oscillatory currents, ocean stratification, and topography, the jets are examples of "stratified topographic flow rectification." Ridge jets have parallels in rectified flows at seamounts and submarine banks.JWL is supported by the National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory and by NOAA Vents Program. The work of other authors has been supported by National Science Foundation through grants OCE-0424953 and OCE-0425361, LADDER (LArval Dispersion along the Deep East pacific Rise)

Active positioning of vent larvae at a mid-ocean ridge

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 92 (2013): 46-57, doi:10.1016/j.dsr2.2013.03.032.The vertical position of larvae of vent species above a mid-ocean ridge potentially has a strong effect on their dispersal. Larvae may be advected upward in the buoyant vent plume, or move as a consequence of their buoyancy or active swimming. Alternatively, they may be retained near bottom by the topography of the axial trough, or by downward swimming. At vents near 9°50’N on the axis of the East Pacific Rise, evidence for active larval positioning was detected in a comparison between field observations of larvae in the plankton in 2006 and 2007 and distributions of non-swimming larvae in a two-dimensional bio-physical model. In the field, few vent larvae were collected at the level of the neutrally buoyant plume (~75 m above bottom); their relative abundances at that height were much lower than those of simulated larvae from a near-bottom release in the model. This discrepancy was observed for many vent species, particularly gastropods, suggesting that they may actively remain near bottom by sinking or swimming downward. Near the seafloor, larval abundance decreased from the ridge axis to 1000 m off axis much more strongly in the observations than in the simulations, again pointing to behavior as a potential regulator of larval transport. We suspect that transport off axis was reduced by downward-moving behavior, which positioned larvae into locations where they were isolated from cross-ridge currents by seafloor topography, such as the walls of the axial valley – which are not resolved in the model. Cross-ridge gradients in larval abundance varied between gastropods and polychaetes, indicating that behavior may vary between taxonomic groups, and possibly between species. These results suggest that behaviorally mediated retention of vent larvae may be common, even for species that have a long planktonic larval duration and are capable of long-distance dispersal.We gratefully acknowledge the support of NSF grants OCE-0424953 and OCE-0525361, which funded the Larval Dispersal on the Deep East Pacific Rise (LADDER) project. WHOI provided additional support to LSM as an Ocean Life Fellow, to DJM as the Holger Jannasch Chair for Excellence in Oceanography, and to JRL as the Edward W. and Betty J. Scripps Senior Scientist Chair. JWL was supported by the National Oceanic and Atmospheric Administration’s (NOAA) Vents Program and by NOAA’s Pacific Marine Environmental Laboratory

Hydrothermal venting at Vailulu'u Seamount : the smoking end of the Samoan chain

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 5 (2004): Q02003, doi:10.1029/2003GC000626.The summit crater of Vailulu'u Seamount, the youngest volcano in the Samoan chain, hosts an active hydrothermal system with profound impact on the ocean water column inside and around its crater (2 km wide and 407 m deep at a 593 m summit depth). The turbidity of the ocean water reaches 1.4 NTU, values that are higher than in any other submarine hydrothermal system. The water is enriched in hydrothermal Mn (3.8 ppb) and 3He (1 × 10−11 cc/g) and we measured water temperature anomalies near the crater floor up to 0.2°C. The hydrothermal system shows complex interactions with the ocean currents around Vailulu'u that include tidally-modulated vertical motions of about 40–50 m, and replenishment of waters into the crater through breaches in the upper half of the crater wall. Inside and outside potential density gradients suggest that hydrothermal venting exports substantial amounts of water from the crater (1.3 ± 0.2 × 108 m3/day), which is in good agreement with fluxes obtained from a tracer release experiment inside the crater of Vailulu'u (0.8 × 108 m3/day [Hart et al., 2003]). This mass flux, in combination with the differences in the inside and outside crater temperature, yields a power output of around 760 megawatts, the equivalent of 20–100 MOR black smokers. The Mn output of 300 kg/day is approximately ten times the output of a single black smoker

Vigorous lateral export of the meltwater outflow from beneath an Antarctic ice shelf

The instability and accelerated melting of the Antarctic Ice Sheet are among the foremost elements of contemporary global climate change1, 2. The increased freshwater output from Antarctica is important in determining sea level rise1, the fate of Antarctic sea ice and its effect on the Earth’s albedo4, 5, ongoing changes in global deep-ocean ventilation6, and the evolution of Southern Ocean ecosystems and carbon cycling7, 8. A key uncertainty in assessing and predicting the impacts of Antarctic Ice Sheet melting concerns the vertical distribution of the exported meltwater. This is usually represented by climate-scale models3–5, 9 as a near-surface freshwater input to the ocean, yet measurements around Antarctica reveal the meltwater to be concentrated at deeper levels10, 11, 12, 13, 14. Here we use observations of the turbulent properties of the meltwater outflows from beneath a rapidly melting Antarctic ice shelf to identify the mechanism responsible for the depth of the meltwater. We show that the initial ascent of the meltwater outflow from the ice shelf cavity triggers a centrifugal overturning instability that grows by extracting kinetic energy from the lateral shear of the background oceanic flow. The instability promotes vigorous lateral export, rapid dilution by turbulent mixing, and finally settling of meltwater at depth. We use an idealized ocean circulation model to show that this mechanism is relevant to a broad spectrum of Antarctic ice shelves. Our findings demonstrate that the mechanism producing meltwater at depth is a dynamically robust feature of Antarctic melting that should be incorporated into climate-scale models

FIB-SEM imaging of carbon nanotubes in mouse lung tissue

Ultrastructural characterisation is important for understanding carbon nanotube (CNT) toxicity and how the CNTs interact with cells and tissues. The standard method for this involves using transmission electron microscopy (TEM). However, in particular, the sample preparation, using a microtome to cut thin sample sections for TEM, can be challenging for investigation of regions with agglomerations of large and stiff CNTs because the CNTs cut with difficulty. As a consequence, the sectioning diamond knife may be damaged and the uncut CNTs are left protruding from the embedded block surface excluding them from TEM analysis. To provide an alternative to ultramicrotomy and subsequent TEM imaging, we studied focused ion beam scanning electron microscopy (FIB-SEM) of CNTs in the lungs of mice, and we evaluated the applicability of the method compared to TEM. FIB-SEM can provide serial section volume imaging not easily obtained with TEM, but it is time-consuming to locate CNTs in the tissue. We demonstrate that protruding CNTs after ultramicrotomy can be used to locate the region of interest, and we present FIB-SEM images of CNTs in lung tissue. FIB-SEM imaging was applied to lung tissue from mice which had been intratracheally instilled with two different multiwalled CNTs; one being short and thin, and the other longer and thicker. FIB-SEM was found to be most suitable for detection of the large CNTs (Ø ca. 70 nm), and to be well suited for studying CNT agglomerates in biological samples which is challenging using standard TEM techniques. [Figure: see text] ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00216-013-7566-x) contains supplementary material, which is available to authorized users

Basin-scale transport of hydrothermal dissolved metals across the South Pacific Ocean

Hydrothermal venting along mid-ocean ridges exerts an important control on the chemical composition of sea water by serving as a major source or sink for a number of trace elements in the ocean(1-3). Of these, iron has received considerable attention because of its role as an essential and often limiting nutrient for primary production in regions of the ocean that are of critical importance for the global carbon cycle(4). It has been thought that most of the dissolved iron discharged by hydrothermal vents is lost from solution close to ridge-axis sources(2,5) and is thus of limited importance for ocean biogeochemistry(6). This long-standing view is challenged by recent studies which suggest that stabilization of hydrothermal dissolved iron may facilitate its longrange oceanic transport(7-10). Such transport has been subsequently inferred from spatially limited oceanographic observations(11-13). Here we report data from the US GEOTRACES Eastern Pacific Zonal Transect (EPZT) that demonstrate lateral transport of hydrothermal dissolved iron, manganese, and aluminium from the southern East Pacific Rise (SEPR) several thousand kilometres westward across the South Pacific Ocean. Dissolved iron exhibits nearly conservative (that is, no loss from solution during transport and mixing) behaviour in this hydrothermal plume, implying a greater longevity in the deep ocean than previously assumed(6,14). Based on our observations, we estimate a global hydrothermal dissolved iron input of three to four gigamoles per year to the ocean interior, which is more than fourfold higher than previous estimates(7,11,14). Complementary simulations with a global-scale ocean biogeochemical model suggest that the observed transport of hydrothermal dissolved iron requires some means of physicochemical stabilization and indicate that hydrothermally derived iron sustains a large fraction of Southern Ocean export productio

Evidence of an active volcanic heat source beneath the Pine Island Glacier

Tectonic landforms reveal that the West Antarctic Ice Sheet (WAIS) lies atop a major volcanic rift system. However, identifying subglacial volcanism is challenging. Here we show geochemical evidence of a volcanic heat source upstream of the fast-melting Pine Island Ice Shelf, documented by seawater helium isotope ratios at the front of the Ice Shelf cavity. The localization of mantle helium to glacial meltwater reveals that volcanic heat induces melt beneath the grounded glacier and feeds the subglacial hydrological network crossing the grounding line. The observed transport of mantle helium out of the Ice Shelf cavity indicates that volcanic heat is supplied to the grounded glacier at a rate of ~ 2500 ± 1700 MW, which is ca. half as large as the active Grimsvötn volcano on Iceland. Our finding of a substantial volcanic heat source beneath a major WAIS glacier highlights the need to understand subglacial volcanism, its hydrologic interaction with the marine margins, and its potential role in the future stability of the WAIS