Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron-and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite-to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift lowtemperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high-and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally 95% in the bottom kilometer. Hole 735B provides evidence for a strongly heterogeneous lower ocean crust, and for the inherent interplay of deformation, alteration and igneous processes at slow-spreading ridges. It is strikingly different from gabbros sampled from fast-spreading ridges and at most well-described ophiolite complexes. We attribute this to the remarkable diversity of tectonic environments where crustal accretion occurs in the oceans and to the low probability of a section of old slow-spread crust formed near a major large-offset transform being emplaced onland compared to sections of young crust from small ocean basins.

rav. Science 255. 165 11 992). 5. D.'s. Fisher, M. P. A. ~j s h e r , '~. A. Huse, Phys. Rev. 5 4 3 , 130 (1991). 6. G. Blatter, M. V. Feiqel'man, V. B. Geshkenbein, A. I. Larkin, V. M. ~inokur, Rev. Mod. M. Lelovic, P. Kr~shnaraj, N. G. Eror, U. Balachandran, ibid. 242, 246 (1 995). 11. Q. Li, H. J. Wiesmann, M. Suenaga, L. Motow~dlo, P. Haldar, Appl. Phys. Lett. 66, 637 (1 995). 12. P. Majewski, Adv. Mater. 6, 593 (1 994). 13. The problem of thermally activated flux motion is less severe In the HTSC YBa, Cu, O, (YBCO) and hence it offers better intrinsic behav~or at high temperatures and magnetic fields. The processing strategies developqd for BSCCO fail to yield viable YBCO wires as a result of poor intergranular current flow. Recent work suggests, however, that good alignment between grains can be achieved in thick films deposited on nickel tapes by Ion beam depos~tion [X. D. Wu et a/., Appl. Phys. Lett. 67, 2397 (1 99511. The commercial viabil~ty of th~s strategy remains to be demonstrated. 14. D. R. Nelson and V. M. Vinokur, Phys. Rev. Lett. 68, 2398 (1 9 9 2 ) ; , Phys. Rev. 5 48,13060 (1 993). 15. T. Hwa, P. Le Doussal, D. R. Nelson, V. M. Vinokur, Phys. Rev. Lett. 71, 3545 (1 993). 16. L. C~vale et a/., ibid. 67, 648 (1991); M. Konczykowski et a/., Phys. Rev. 5 44, 7167 (1991); R. C. 995). 25. A recent report of carbon nanotube-BSCCO composites (24) showed some evidence of J, improvement; however, the J, value of both the reference and nanorod-conta~ning sample in this report were lower than the good-quality BSCCO samples reported previously and in the present study. It is thus difficult to conclude that there is an improvement in behav~or upon adding nanotubes. In addition, th~s report and our own stud~es show that few nanotubes survive the synthesis process, leaving in doubt their ab~lity to create well-defined columnar defects in the HTSCs. 982).32. Nanorod-HTSC composites have also been successfully prepared with TI Ba, Ca, Cu, O, and T12Ba2Ca,Cu,010 materials. Preliminary measurements show that there are significant enhancements in J, for these composites (P. Yang and C. M. Lieber, unpublished results). . 34. The actual density of columnar defects that can pin flux lines may be larger than that corresponding to the dens~ty of MgO nanorods; that is, lattice strains associated w~th nanorod-BSCCO interfaces can lead to dislocations and other correlated defects that exhibit columnarlike pinning behavior. 35. The density of nanorods oriented close to the c axis was about 1 x 101° cm-'; a s~m~lar dens~ty was determined for nanorods oriented In the ab plane. Although this density is sign~ficantly lower than that obtained by heavy-ion and proton irradiation, we have not tr~ed to maximize the dens~ty of MgO nanorods and also believe that the density of correlated defects is probably significantly higher than that of nanorods (34). 36. C. P. Bean, Rev. Mod. Phys. 36, 31 (1964). 37. An inverse dependence of J, on defect size was also reported previously for Y,BaCuO, inclus~ons of 1 to 10 p m in diameter in YBC...

We construct a new seismic model for central and West Antarctica by jointly inverting Rayleigh wave phase and group velocities along with P wave receiver functions. Ambient noise tomography exploiting data from more than 200 seismic stations deployed over the past 18 years is used to construct Rayleigh wave phase and group velocity dispersion maps. Comparison between the ambient noise phase velocity maps with those constructed using teleseismic earthquakes confirms the accuracy of both results. These maps, together with P receiver function waveforms, are used to construct a new 3‐D shear velocity (Vs) model for the crust and uppermost mantle using a Bayesian Monte Carlo algorithm. The new 3‐D seismic model shows the dichotomy of the tectonically active West Antarctica (WANT) and the stable and ancient East Antarctica (EANT). In WANT, the model exhibits a slow uppermost mantle along the Transantarctic Mountains (TAMs) front, interpreted as the thermal effect from Cenozoic rifting. Beneath the southern TAMs, the slow uppermost mantle extends horizontally beneath the traditionally recognized EANT, hypothesized to be associated with lithospheric delamination. Thin crust and lithosphere observed along the Amundsen Sea coast and extending into the interior suggest involvement of these areas in Cenozoic rifting. EANT, with its relatively thick and cold crust and lithosphere marked by high Vs, displays a slower Vs anomaly beneath the Gamburtsev Subglacial Mountains in the uppermost mantle, which we hypothesize may be the signature of a compositionally anomalous body, perhaps remnant from a continental collision.

[1] The Hawaii-2 Observatory (H2O) is an excellent site for studying the source regions and propagation of microseisms since it is located far from shorelines and shallow water. During Leg 200 of the Ocean Drilling Program, the officers of the JOIDES Resolution took wind and wave measurements for comparison with double-frequency (DF) microseism data collected at nearby H2O. The DF microseism band can be divided into short-period and long-period bands, SPDF and LPDF, respectively. Comparison of the ship's weather log with the seismic data in the SPDF band from about 0.20 to 0.45 Hz shows a strong correlation of seismic amplitude with wind speed and direction, implying that the energy reaching the ocean floor is generated locally by ocean gravity waves. Nearshore land seismic stations see similar SPDF spectra, also generated locally by wind seas. At H2O, SPDF microseism amplitudes lag sustained changes in wind speed and direction by several hours, with the lag increasing with wave period. This lag may be associated with the time necessary for the development of opposing seas for DF microseism generation. Correlation of swell height above H2O with the LPDF band from 0.085 to 0.20 Hz is often poor, implying that a significant portion of this energy originates at distant locations. Correlation of the H2O seismic data with NOAA buoy data, with hindcast wave height data from the North Pacific, and with seismic data from mainland and island stations, defines likely source areas of the LPDF signals. Most of the LPDF energy at H2O appears to be generated by high-amplitude storm waves impacting long stretches of coastline nearly simultaneously, and the Hawaiian Islands appear to be a significant source of LPDF energy in the North Pacific when waves arrive from particular directions. The highest levels observed at mid-ocean site H2O occur in the SPDF band when two coincident nearby storm systems develop. Deep water, mid-ocean-generated DF microseisms are not observed at continental sites, indicating high attenuation of these signals. At near-coastal seismic stations, both SPDF and LPDF microseism levels are generally dominated by local generation at nearby shorelines.

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S U M M A R YAn L-configured, three-component short period seismic array was deployed on the Ross Ice Shelf, Antarctica during November 2014. Polarization analysis of ambient noise data from these stations shows linearly polarized waves for frequency bands between 0.2 and 2 Hz. A spectral peak at about 1.6 Hz is interpreted as the resonance frequency of the water column and is used to estimate the water layer thickness below the ice shelf. The frequency band from 4 to 18 Hz is dominated by Rayleigh and Love waves propagating from the north that, based on daily temporal variations, we conclude were generated by field camp activity. Frequency-slowness plots were calculated using beamforming. Resulting Love and Rayleigh wave dispersion curves were inverted for the shear wave velocity profile within the firn and ice to ∼150 m depth. The derived density profile allows estimation of the pore close-off depth and the firn-air content thickness. Separate inversions of Rayleigh and Love wave dispersion curves give different shear wave velocity profiles within the firn. We attribute this difference to an effective anisotropy due to fine layering. The layered structure of firn, ice, water and the seafloor results in a characteristic dispersion curve below 7 Hz. Forward modelling the observed Rayleigh wave dispersion curves using representative firn, ice, water and sediment structures indicates that Rayleigh waves are observed when wavelengths are long enough to span the distance from the ice shelf surface to the seafloor. The forward modelling shows that analysis of seismic data from an ice shelf provides the possibility of resolving ice shelf thickness, water column thickness and the physical properties of the ice shelf and underlying seafloor using passive-source seismic data.

Continuous seismic observations across the Ross Ice Shelf reveal ubiquitous ambient resonances at frequencies >5 Hz. These firn-trapped surface wave signals arise through wind and snow bedform interactions coupled with very low velocity structures. Progressive and long-term spectral changes are associated with surface snow redistribution by wind and with a January 2016 regional melt event.Modeling demonstrates high spectral sensitivity to near-surface (top several meters) elastic parameters. We propose that spectral peak changes arise from surface snow redistribution in wind events and to velocity drops reflecting snow lattice weakening near 0 ∘ C for the melt event. Percolation-related refrozen layers and layer thinning may also contribute to long-term spectral changes after the melt event. Single-station observations are inverted for elastic structure for multiple stations across the ice shelf. High-frequency ambient noise seismology presents opportunities for continuous assessment of near-surface ice shelf or other firn environments.Plain Language Summary Ice shelves are the floating buttresses of large glaciers that extend over the oceans and play a key role in restraining inland glaciers as they flow to the sea. Deploying sensitive seismographs across Earth's largest ice shelf (the Ross Ice Shelf ) for 2 years, we discovered that the shelf nearly continuously sings at frequencies of five or more cycles per second, excited by local and regional winds blowing across its snow dune-like topography. We find that the frequencies and other features of this singing change, both as storms alter the snow dunes and during a (January 2016) warming event that resulted in melting in the ice shelf's near surface. These observations demonstrate that seismological monitoring can be used to continually monitor the near-surface conditions of an ice shelf and other icy bodies to depths of several meters.