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measurements show that there are significant enhancements in J, for these composites (P. Yang and C. M. Lieber, unpublished results).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...

Below water depths of about 300 metres, pressure and temperature conditions cause methane to form ice-like crystals of methane hydrate. Marine deposits of methane hydrate are estimated to be large, amassing about 10,000 gigatonnes of carbon, and are thought to be important to global change and seafloor stability, as well as representing a potentially exploitable energy resource. The extent of these deposits can usually be inferred from seismic imaging, in which the base of the methane hydrate stability zone is frequently identifiable as a smooth reflector that runs parallel to the sea floor. Here, using high-resolution seismic sections of seafloor sediments in the Cascadia margin off the coast of Vancouver Island, Canada, we observe lateral variations in the base of the hydrate stability zone, including gas-rich vertical intrusions into the hydrate stability zone. We suggest that these vertical intrusions are associated with upward flow of warmer fluids. Therefore, where seafloor fluid expulsion and methane hydrate deposits coincide, the base of the hydrate stability zone might exhibit significant roughness and increased surface area. Increased area implies that significantly more methane hydrate lies close to being unstable and hence closer to dissociation in the event of a lowering of pressure due to sea-level fall.

No abstract

Seafloor properties, including total organic carbon (TOC), are sparsely measured on a global scale, and interpolation (prediction) techniques are often used as a proxy for observation. Previous geospatial interpolations of seafloor TOC exhibit gaps where little to no observed data exists. In contrast, recent machine learning techniques, relying on geophysical and geochemical properties (e.g., seafloor biomass, porosity, and distance from coast), show promise in making comprehensive, statistically optimal predictions. Here we apply a nonparametric (i.e., data‐driven) machine learning algorithm, specifically k‐nearest neighbors (kNN), to estimate the global distribution of seafloor TOC. Our results include predictor (feature) selection specifically designed to mitigate bias and produce a statistically optimal estimation of seafloor TOC, with uncertainty, at 5 × 5‐arc minute resolution. Analysis of parameter space sample density provides a guide for future sampling. One use for this prediction is to constrain a global inventory, indicating that just the upper 5 cm of the seafloor contains about 87 ± 43 gigatons of carbon (Gt C) in organic form.

A laterally extensive, high-resolution travel time velocity analysis and acoustic wave form. inversion were used to quantitatively determine methane hydrate content in deep water sediments of the Blake Ridge off the southeast U.S. coast. The interval acoustic velocity (Vp) analyses were performed in the x-p domain by interactively picking the x-p trajectories of prominent reflections in each of 50 plane wave-decomposed common midpoint gathers. The reflections correspond to seismic stratigraphic boundaries so that lateral Vp changes due to lithology changes are mitigated, and Vp changes due to changing hydrate content are enhanced. Two separate interval Vp analyses were performed, one with thick (4).4 km) layers which yielded lower uncertainty but also lower resolution, and one with thinner layers (-0.1 km), yielding higher resolution but slightly larger uncertainties. Results show no correlation between lowsediment reflectivity and Vp. However, in the areas exhibiting a bottom simulating reflector (BSR) a high Vp interval (-2.0 km/s and 0.15 km thick) is seen immediately above the BSR. Where the BSR is strongest a 256-layer, least squares acoustic wave form inversion reveals the BSR to be caused by a Vp decrease from -2.0 to -1.5 km/s, with little or no change in density. The inversion also reveals a thin (0.025 km) layer of anomalously low Vp lying immediately below the BSR. Two models of methane hydrate distribution are tested, each indicating that the volume of methane hydrate in the intervals of elevated Vp is up to -25% of the total volume. have been sampled directly off the Pacific coast of Central America [Shipley and Didyk, 1982; Mathews and von Huene, 1985] and off the southern Atlantic coast of the United States [Kvenvolden and Barnard, 1983]. The sharp sediment property contrast between a high V•, hydrate and either low V•,, gascharged sediments or just normal nonhydrated sediment below, sometimes make the bottom of the hydrate zone detectable as a bottom simulating reflector (BSR) in seismic data. The top of this zone is not as easily detected. BSRs are found in many places in the world's oceans [Tucholke et al., 1977; Shipley et al., 1979; Field and Kvenvolden, 1985 ] and the amount of gas stored worldwide in these hydrates is probably quite large. The chief component gas of the hydrates is methane, present in excess of 98% [Claypool and Kaplan, 1974; Kvenvolden, 1984]. The objectives of this study include developing seismic techniques to estimate the vertical and lateral extent of the methane by analyzing its effect on sediment physical properties, primarily P-wave velocity V•,. Recently, Hyndman and Spence [1992] have performed a similar study of a BSR in the northern Cascadia subduction zone.Whether the methane is stable as a free gas or in a solid form combined with water as a gas hydrate is dictated by the surrounding pressure and temperature (PT) conditions. Figure 1 shows a temperature and pressure profile for a typical deep water environment. Note that increasing depth (pressure) drives the m...