The success of Leg 187 depended on our ability to locate the Indian/Pacific mantle isotopic boundary through its expression in the geochemistry of MORB lavas from 14- to 28-Ma seafloor north of the AAD. Because the isotopic analyses required to define the IMM/PMM boundary cannot be performed at sea, we employed diagnostic elemental analyses as a proxy to distinguish IMM, PMM, and transitional lava compositions using inductively coupled plasma–atomic emission spectrometry (ICP-AES)—the first such analyses performed during an ODP cruise. At critical times during the leg, onboard analyses were completed within 12 hr of core recovery to optimize selection of the next drilling target and ensure optimal coverage of the boundary trace.
Based on existing data for 0- to 7-Ma dredge samples collected from the AAD prior to Leg 187 (Pyle et al., 1992, 1995; D. Pyle, unpubl. data, 2004), we were able to identify two discrimination diagrams, Ba vs. Zr/Ba and MgO vs. Na2O/TiO2, that were within the onboard analytical capabilities of the JOIDES Resolution and could reliably assign >95% of existing axis and near-axis dredge samples to their correct, isotopically defined mantle source type. For reliability, these analyses must be performed on fresh basaltic glass fragments, as whole-rock samples are subject to chemical alteration, especially for Ba. In order to obtain reliable Ba and Zr data from small basaltic glass samples, it was essential to accelerate the planned installation of the ICP-AES instrument aboard the JOIDES Resolution. Although the instrument was not installed until immediately prior to our departure, we were able to develop analytical protocols and obtain satisfactory analytical performance throughout the leg. The onboard Zr/Ba data enabled us to assign a source affinity at most sites. Of the 13 sites drilled, 8 were correctly (as determined by later isotopic analysis) assigned to a PMM, TMM, or IMM source. Glasses from four sites were designated onboard as "Transitional Pacific" and subsequently proved to be of IMM affinity. One lava from Site 1164 that plotted just inside the PMM boundary (on the Zr/Ba graph shown in Fig. F8) proved subsequently to be of IMM affinity, leading to a "mixed" designation for that site in the Leg 187 Initial Reports volume (Christie, Pedersen, Miller, et al., 2001). Na2O/TiO2 data were not immediately useful on board, as Leg 187 samples have systematically lower Na2O than their younger (dredged) counterparts. In retrospect, Na2O/TiO2 ratios appear to be reliable source indicators for the Leg 187 data alone; the PMM/IMM boundary is well defined at lower Na2O/TiO2 values than for the near-zero–age samples.
In order to locate the Indian/Pacific mantle isotopic boundary as precisely as possible over as long a seafloor age interval as possible, our drilling strategy had two key elements. The first was to maximize the number of sites drilled, limiting penetration depth into basaltic basement to a nominal objective of 50 m. Even this limited penetration was achieved at only five sites due to poor drilling conditions in broken pillow flows and talus. The second element of our strategy was one of responsive site selection (from a slate of 19 preapproved sites) based on rapid chemical analysis within 24 hr of sample recovery. At key decision points throughout the leg, we reviewed and modified our drilling strategy on the basis of shipboard geochemical analyses of newly recovered basalt glasses.
Selection of the initial suite of approved potential drill sites was complicated by the absence of measurable sediment cover across much of the region. The paucity of sediment most likely reflects winnowing by strong bottom currents associated with the Circum-Antarctic Current as it crosses the SEIR through the deep AAD. Drillable sites were selected on localized sediment pockets detected by single-channel seismic imaging during the site survey cruises, Boomerang 5 (BMRG05) and Sojourn 5 (SJRN05) of the Melville. Three additional sites were surveyed by the JOIDES Resolution during the transit from Site 1158 to Site 1159; two of these were drilled as Sites 1161 and 1162. Because we expected sediments across the region to be reworked and of little use for stratigraphy and because we needed to maximize the number of sites drilled, we chose to wash through the sediment sections. Wash cores covering significant sediment intervals at 10 sites were described on board but have not been studied further.
An investigation of the nature and geological impacts of microbial life on and beneath the ocean floor was an important goal of Leg 187. Much attention has been focused on the nature of microbes that live on and contribute to the alteration of oceanic basalts, especially the glassy rims of pillow lavas (Thorseth et al., 1995; Furnes et al., 1996; Fisk et al., 1998; Torsvik et al., 1998). This phenomenon has been documented from Iceland (Thorseth et al., 1991), ODP Hole 896A at the Costa Rica Rift, and the Arctic regions of the Mid-Atlantic Ridge (Thorseth et al., 1999). During Leg 187, considerable effort was made to obtain rock, sediment, and water samples with minimal contamination from the shipboard environment for onshore deoxyribonucleic acid (DNA) analysis of cultured and uncultured materials. A large suite of altered basaltic samples was also collected to enable an electron and optical microscopic search for evidence of microbial activity.
Uncontaminated rock and sediment samples collected during Leg 187 yielded a variety of viable microbial cultures. Lysnes et al. (this volume) performed extensive DNA sequence analysis of material extracted from basalt samples and compared the results with control samples of sediment, drilling mud, and seawater. They also performed extensive culture experiments designed to enhance the growth of microbial populations from basalt samples.
Lysnes et al. showed that the basalt samples contain diverse microbial populations that are distinct from those of Leg 187 sediment samples and from local seawater. Only a relatively small subset of the native basalt population grew successfully in the various enrichment cultures. The native basalt populations and the enrichment cultures were both dominated by gamma Proteobacteria, with five clusters of closely related microbes responding to the culture media. Additional microbes that responded well to the culture media clustered within the CFB (Cytophaga/Flavobacterium/Bacteroides) and Bacillus/Clostridium groups.
The existence of culturable microbes in Leg 187 samples strongly suggests that there is significant microbial activity in shallow ocean crust as old as 28 Ma. However, neither conventional nor scanning electron microscopic examination revealed any evidence of microbial morphologies in alteration zones abutting fresh volcanic glass from any Leg 187 site (Thorseth et al., 2003). This absence of morphological evidence contrasts with parallel studies of 0- to 2-Ma dredge samples from the AAD, which revealed several morphological microbial forms associated with similar alteration fronts and with zeolite mineral surfaces within fractures in the glass. This suggests that microbial activity ceased along the glass front between 2 and 14 Ma. After this time, microbial activity continued (or possibly relocated) elsewhere within the samples, most likely in fractures within the pillow basalts. This shift in the dominant mode of microbial activity may coincide with the cessation of active seawater circulation due to burial, perhaps reflecting a change in pore fluid chemistry at this time. Such a change may also have led to an observed change in the style of chemical alteration at the glass front (see "Mineralogical and Chemical Aging of Basalts," in "Alteration Studies").