Little is known about the potential for life in the vast, low-temperature (<100 degrees C) reservoir of fluids within mid-ocean ridge flank and ocean basin crust. Recently, an overpressured 300-meter-deep borehole was fitted with an experimental seal (CORK) delivering crustal fluids to the sea floor for discrete and large-volume sampling and characterization. Results demonstrate that the 65 degrees C fluids from 3.5-million-year-old ocean crust support microbial growth. Ribosomal RNA gene sequence data indicate the presence of diverse Bacteria and Archaea, including gene clones of varying degrees of relatedness to known nitrate reducers (with ammonia production), thermophilic sulfate reducers, and thermophilic fermentative heterotrophs, all consistent with fluid chemistry.

To determine the microbial community diversity within old oceanic crust, a novel sampling strategy was used to collect crustal fluids at Baby Bare Seamount, a 3.5 Ma old outcrop located in the north-east Pacific Ocean on the eastern flank of the Juan de Fuca Ridge. Stainless steel probes were driven directly into the igneous ocean crust to obtain samples of ridge flank crustal fluids. Genetic signatures and enrichment cultures of microorganisms demonstrate that these crustal fluids host a microbial community composed of species indigenous to the subseafloor, including anaerobic thermophiles, and species from other deep-sea habitats, such as seawater and sediments. Evidence using molecular techniques indicates the presence of a relatively small but active microbial population, dominated by bacteria. The microbial community diversity found in the crustal fluids may indicate habitat variability in old oceanic crust, with inputs of nutrients from seawater, sediment pore-water fluids and possibly hydrothermal sources. This report further supports the presence of an indigenous microbial community in ridge flank crustal fluids and advances our understanding of the potential physiological and phylogenetic diversity of this community.

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Further experiments have been performed to investigate the biasing-field dependency of alternating field demagnetization curves of anhysteretic remanent magnetization as a simple test for the domain state of magnetite and maghemite particles. The biasing-field dependency in fine-grained particles was opposite to that in coarse-grained particles. The experiments were conducted on well sized synthetic specimens in the single domain, pseudo-single domain and multi-domain grain size ranges. A single domain-like biasing-field dependency was observed in equidimensional particles up to 0 . 2 ,~ in mean grain size and up to 0 . 4~ in elongated grains.Either the single domain/pseudo-single domain boundary lies above at least 0 . 2~ grain size or this field dependency test does not distinguish between single domain and pseudo-single domain states. A multidomainlike trend was observed in very coarse magnetite. The test may possibly distinguish the change from pseudo-single domain to multi-domain states. If both fine and coarse fractions are present a confusing overlap of the demagnetization curves occurs.

Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to seismic disturbances, with observed changes in vent temperature. But these disturbances resulted from submarine volcanic activity; until now, there have been no observations of the response of a vent system to non-magmatic, tectonic events. Here we report measurements of hydrothermal vent temperature from several vents on the Juan de Fuca ridge in June 1999, before, during and after an earthquake swarm of apparent tectonic origin. Vent fluid temperatures began to rise 4-11 days after the first earthquake. Following this initial increase, the vent temperatures oscillated for about a month before settling down to higher values. We also observed a tenfold increase in fluid output from the hydrothermal system over a period of at least 80 days, extending along the entire ridge segment. Such a large, segment-wide thermal response to relatively modest tectonic activity is surprising, and raises questions about the sources of excess heat and fluid, and the possible effect on vent biological communities.

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A near-bottom geophysical survey on the Endeavour segment of the northern Juan de Fuca Ridge shows that regions of well-defined low crustal magnetization are strongly correlated with both active and extinct submarine hydrothermal vent sites. In particular, at the Main Endeavour Field, we find discrete magnetization lows associated with each cluster of vents. Magnetization lows are directly centered beneath the vent clusters and have diameters of ϳ100 m, which implies a near-vertical, narrow, pipe-like source region located directly beneath the surface expression of the vent edifices. Lows are also separated from each other by only 200 m, which further implies highly focused zones. Magnetization lows are also associated with inactive and extinct vent areas, which indicates that alteration of the magnetic minerals in the crust rather than (necessarily temporary) thermal demagnetization is the primary process responsible for the low magnetization. These narrow pipe-like bodies are highly characteristic of alteration pipes found in ophiolites and are indicative of hydrothermal fluid up-flow zones. Thus, each magnetization low may define an individual upwelling zone, with distinct subsurface plumbing and thermal structure. The crustal-magnetization patterns provide important constraints on the geometry of the subsurface plumbing beneath these hydrothermal vent systems. At the Main Endeavour Field, magnetization lows are distributed along the trend of the rift valley in a semiregular pattern with a spacing of ϳ200 m, arguing that upward flow may be partitioned into regularly spaced intervals along the axis of the rift valley.

The penetration of igneous basement in the Nazca Plate during DSDP Legs 16 and 34 provided samples of both fine-grained pillow-basalt and coarse-grained massive flow units. The magnetic mineral in these basalt samples is initially a titanomagnetite (Fe,Ti,O4) with a narrow range of composition of x = 0.62 f 0.05. Subsequent to formation, the titanomagnetite grains are generally subjected to low temperature oxidation to titanomaghemite with a corresponding rise in Curie temperature from the initial values of 120-150°C up to a maximum of 400°C. Both grain size and low-temperature oxidation state play important, and interrelated, roles in controlling the intensity and stability of magnetic remanence and other magnetic properties. Overall grain size can, in some cases, be related to oxidation state since some sections of the relatively impermeable massive flows can remain unoxidized for as long as 40 Myr while pillow basalts are extensively oxidized within 34 Myr. Low-temperature alteration in turn effects magnetic grain size since oxidation and subsequent Fe cation migration results in grain subdivision by the formation of shrinkage cracks. A five-stage sequence of the microscopic changes that are associated with progressive low-temperature oxidation is proposed and illustrated with photomicrographs from these basalt samples.A hierarchy in the intensity of magnetic remanence may exist with unoxidized pillow basalts having a much higher intensity and oxidized pillow basalts having a much lower intensity than the massive flow units. While pillow basalts are relatively immune to the addition of secondary components of magnetization, the coarse-grained massive flows readily acquire components of viscous remanence. Although they oxidize much more slowly than pillows, when oxidation does take place, components of chemical remanence can be acquired by the multi-domained grains in the massive flow units. 319319A 320 321