172 Scientific Prospectus


Shipboard geochemical investigations and post-cruise sampling for paleoceanography will require the following modifications to normal ODP procedures:

1) Drilling a separate hole of about 50 m length (~5 cores via APC) dedicated to geochemical and paleoceanographic investigations at several sites. This replication in coring will result in complete and meaningful geochemical sampling without compromising the paleoceanographic objectives of the leg. High-resolution geochemical sampling is necessary in order to adequately document concentration profiles of key interstitial chemical species (see #2 below). Highest priority sites for geochemical sampling are Sites CS-2, BBOR-5, and BR-1.

2) Increasing interstitial water sampling of the geochemical holes to 1.5 m intervals (each section cut, 6 samples per full core). High-resolution sampling will establish meaningful chloride and sulfate profiles, which are necessary to test their response to the presence or absence of underlying gas hydrate. For example, we expect that linear sulfate profiles will occur over the shallower portions of the Blake Ridge, and that this linearity will give way to convex-upward profiles down-ridge as the influence of gas hydrate wanes. These data are important to monitor in developing the best coring strategy. Sediment "squeeze-cakes" will still be available for biostratigraphy and paleoceanographic purposes, and microfossils are not affected by the squeezing.

3) Using a Reeburgh-style, rather than Manheim-style, squeezer for pore-water sampling. Unfortunately, Manheim squeezers allow gas to escape. Pore waters (and gases) will also be collected in syringes from the Reeburgh squeezers so no other modifications to ODP procedures are needed. This will allow collection of methane gas samples directly from sediment that is analyzed for interstitial waters. (The Reeburgh squeezers will be supplied by Leg 172 geochemist Walter S. Borowski.)

4) Sampling interstitial waters at an interval of one sample per core in one hole at each site. One, 5- to 10-cm-thick, whole-round-section will be required from the bottom of the third hole at each site, depending on water content. This will enable construction of detailed chloride profiles (and other profiles, e.g., NH4+, alkalinity, delta13CsigmaCO2) to fully evaluate the influence of underlying gas hydrate.

5) Exceed ODP sample volume restrictions. Leg 172 objectives (as outlined above) differ from conventional high-resolution paleoceanography legs because the major goals are not to generate detailed time series of data, but rather to synthesize time-stratigraphic intervals from all sites in a depth transect. Thus, for the leg to succeed we must have comparable data from all sites in the transect. Because of very low abundance of benthic foraminifers (<<1/g) at some sites in intervals such as glacial maxima, it may be impossible to adequately define millennial scale variability in climate without frequent or continuous sampling, or without large volume sampling. Thus, in some holes it may be necessary to completely sample away critical intervals of core and in most situations high-resolution investigations will require individual scientists to sample 100 cm3 per meter of core. Such heavy sampling for studies that use foraminifers will occur outside dedicated intervals only if required to meet the primary leg objectives.

6) U-channel samples may be taken in the working half of some dedicated and non dedicated intervals to facilitate high-resolution measurements of magnetic properties. Magnetic studies of U-channels are nondestructive, with the exception that the magnetic remanence will likely be partially or totally removed. In cases where U-channels are necessary in nondedicated intervals, other samples within these intervals will come from the U-channel following magnetic measurements. In these cases, magnetic laboratory analysis will be completed as rapidly as possible so that the U-channels can be returned to the repository for subsampling, which will be conducted by the repository staff.

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