Magazine article Oceanus

Details That Make the Difference

Magazine article Oceanus

Details That Make the Difference

Article excerpt

In studying the oceans, as in studying astronomy, improvements in data resolution can be crucial to identifying the natural processes at work. However, in the early years of ocean drilling, techniques were more profligate even than the Hubble space telescope in their loss of high-resolution data. Only recently have improved sediment-recovery techniques realized their full potential for revealing information about geologically rapid processes recorded in deep sea sediments.

During the first 15 years of ocean drilling, most sites really were only "drilled," but the past decade has brought increasing use of two other techniques: downhole logging and hydraulic piston coring. Logging, passing sensors down through the hole to examine surrounding sediments, allows us to learn more about the core sections and is particularly valuable where sediment recovery is poor. In the upper 200 meters (usually soft, unconsolidated sediments), drilling too often brings back homogenized slurries that have lost all but the largest-scale information about the sediment. However, advanced piston coring, which drives the core barrel through the sediment by hydraulic pressure, yields almost perfect recovery of soft, unlithified sediments that would be severely disturbed by rotary drilling. This technique's potential was first demonstrated, during DSDP's Leg 64 in 1970, when the prototype hydraulic piston corer, brought aboard Glomar Challenger halfway through the cruise, performed spectacularly well in recovering laminated sediments in perfect condition at Site 480 in the Gulf of California. No trace of the laminations had been visible in equivalent material recovered by rotary coring at nearby Site 479. More recently, similar laminated sediment was recovered from the open ocean at several of the sites cored during Leg 138, forcing us to reject the notion that laminated sediments invariably imply deposition in an anoxic water mass, such as the Gulf of California. Alan Kemp (University of Southampton) and Jack Baldauf (Texas A&M University) have shown that the laminations at the Leg 138 sites were created by mats of the diatom Thalassiothrix that episodically blanketed the seafloor during intervals of very high surface productivity, and suppressed bioturbation.

The main thrust of recent paleoceanographic research based on ocean drilling is investigation of the whole Neogene period (the past 20 million years) with the same degree of detail previously available only for the late Quaternary (a fraction of the past one million years). The conventional view of earth history holds that high-frequency environmental variability was confined to the Quaternary, with its characteristic ice-age cycles, and that variability observed in outcrops of older rocks was only of local significance. We are now learning that this was a false picture. ODP Leg 138, with author Shackleton in the scientific party, provides just one example of a drilling leg largely or entirely devoted to high-resolution paleoceanography. It was, however, enormously successful in a number of ways, and the rest of this article focuses on it as a case study in high-resolution paleoceanography.

Filling in the Blanks: Gaps in Sediment Cores

Gaps in the sequence of sediments recovered at many earlier drilling sites were disappointing. These gaps occur between successive cores as the drill string is driven further into the sediment. However, if several holes are drilled within a few tens of meters of each other, it should be possible to fill one hole's gaps using sediment from an adjacent hole, provided that the gaps in the second hole are vertically offset from those in the first. All too often, this has not been successfully achieved. The co-chief scientists on Leg 138, Larry Mayer (University of New Brunswick) and Nick Pisias (Oregon State University), made it their prime objective to recover a complete section at each site. Substantial innovation was required to speed up ship-board analysis procedures, to be certain that we did not pull pipe and sail away until the sedimentary section had indeed been fully recovered. …

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