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Sediment and basement contact coring1

Expedition 336 Scientists2

Site summaries

This section summarizes the results of advanced piston corer (APC) sediment coring and extended core barrel (XCB) coring of the sediment–basement transition at Integrated Ocean Drilling Program (IODP) Sites U1383 (prospectus Site NP-2), U1382 (near Deep Sea Drilling Project [DSDP] Hole 395A), and U1384 (prospectus Site NP-1).

In Hole U1383D, 44.3 m of sediment was cored, of which the lowermost 1 m was XCB cored through basalt and limestone-cemented breccia (0.76 m of basement was recovered). At nearby Hole U1383E, 44.2 m of sediment and 1 m of basaltic basement was cored (0.3 m of basement was recovered). The basalts are aphyric and slightly to moderately altered. They are distinct from the uppermost basaltic flow that was cored with the rotary core barrel (RCB) in Hole U1383C and hence represent a different lithologic unit.

Hole U1382B was drilled midway between Holes 395A and U1382A. A total of 90.0 m of sediment was APC cored, and another 8.8 m was cored with the XCB, recovering a piece of basalt and countless millimeter- to centimeter-sized pebbles of completely altered plutonic and ultramafic rocks at the basement/​sediment interface. These rocks are interpreted to be part of the sedimentary breccia overlying the massive basalt of lithologic Unit 1 cored in Hole U1382A.

Coring in Hole U1384A recovered 93.5 m of sediment overlying 0.58 m of basalt and limestone-cemented breccia. The basalts are aphyric and sparsely vesicular with glassy to variolitic to microcrystalline groundmass. They are between 3% and 10% altered and display brown alteration halos along clay veins and fractures.

The sediments at all sites consist of foraminifer nannofossil ooze with layers of foraminiferal sand. The bottom several meters of the sedimentary pile are brown and appear rich in clay. Sediments from Hole U1382B show moderately rounded rock fragments concentrated in layers or dispersed in the ooze. These fragments range from coarse sand to pebble in grain size and consist of serpentinized mantle peridotite, gabbro, troctolite, and basalt. Both XCB cores from Hole U1382B also contain coarse sediment with predominantly serpentinite clasts, including soapstone and talc-tremolite schist. The occurrence of these rock fragments is consistent with the polymict sedimentary breccia recovered during basement drilling in Holes 395A and U1382A. The deformed and metasomatized lithologies encountered in Hole U1382B corroborate the hypothesis that this material was transported to the Site U1382 area in North Pond by mass wasting events and that its source is an oceanic core complex, probably in the southern rift mountains. Layers of foraminiferal sand are abundant in all holes, and many show erosional bases and normal-graded bedding, suggesting that they represent deposits of turbidity currents.

Each of the four holes cored was intensively sampled for microbiological and interstitial water analyses. The sampling program was similar for each hole. In total, we collected 167 whole-round samples for interstitial waters and 691 whole-round samples for microbiological analysis. Sampling density was increased in the bottom sections. Pore waters in these basal sediments are dominated by diffusion of components from the basement fluids into the sedimentary pile, and they allow estimation of basement fluid compositions by extrapolation. Whole-round cores were preserved for shore-based molecular analysis to provide a detailed description of the microbial community. Ship-based enrichment cultures were established to enrich for multiple metabolic functional groups. These cultures will be analyzed on shore for both metabolic activity and community composition. Sediment sections remaining after whole-round sampling were analyzed for oxygen concentration using optodes. Hard rock samples were sectioned and allocated following previous strategies established during the hard rock drilling phase of the expedition. Multiple basalt samples were provided for RNA/DNA, geochemistry, and culture analyses.

An extensive high-resolution physical properties data set was obtained for all holes. This data set includes whole-round, split-core, and discrete measurements of magnetic susceptibility, velocity, density, porosity, natural gamma radiation, and resistivity.

1 Expedition 336 Scientists, 2012. Sediment and basement contact coring. In Edwards, K.J., Bach, W., Klaus, A., and the Expedition 336 Scientists, Proc. IODP, 336: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.336.106.2012

2 Expedition 336 Scientists’ addresses.

Publication: 16 November 2012
MS 336-106