Five logistical issues make the design of the Leg 199 transect (or any other Paleogene/Cretaceous transect) significantly more difficult than the design of a Pleistocene study: (1) the Neogene sediments are overburden, not the target of drilling; (2) because ocean crust is old, the sites are deep; (3) errors in plate tectonic models accumulate, and paleopositions are hard to predict with confidence; (4) the sedimentation regime is more poorly known and the type of sediments that will be encountered can be hard to predict; and (5) cherts commonly occur in the sediments and disrupt recovery, but, chert formation is poorly understood and not yet predictable. An ODP leg with Mesozoic or early Cenozoic objectives must have a large component of exploration attached to it, as well as some well-conceived studies with guaranteed payoff.
Burial of the Paleogene section by Neogene sediments has important impact on diagenesis and on the time needed to recover the interval of interest. Even minimal diagenesis (e.g., carbonate oozechalk transition) affects both recovery and the ability to perform later paleoceanographic studies. For example, at Site 1218 deeply buried lower Oligocene nannofossil oozes were converted to chalk at ~200 mbsf, which necessitated a switch from APC to XCB drilling at 189 mbsf. XCB drilling disturbs the sediments more extensively than piston coring and effectively ended our ability to develop a paleomagnetic reversal stratigraphy even though a continuous sediment column splice was completed to 263 mbsf. Completing a sediment column splice by multiple offset holes at a drill site is also more of a challenge because significant time can be lost drilling to the sediments of interest. The presence of the Neogene equatorial sediment bulge immediately to the south of the Clipperton Fracture Zone effectively limited our ability to sample the Eocene from southern tropical paleolatitudes.
Sediments over young ocean crust are relatively shallow, situated under ~3 km of overlying water. Because ocean crust cools and deepens when it ages, an older transect is under a much deeper water column. All drill sites along the 56-Ma transect of Leg 199 were deeper than 5 km. Because the water is deeper along an older transect, normal drilling operations such as recovering core take significantly longer than they would along a younger transect. Any failure of equipment leads to a larger time penalty and standard pipe trips at the beginning and end of a site take significantly more time than in shallow water. Fewer holes can therefore be drilled in the same amount of time for a Paleogene or Cretaceous transect.
Errors in plate tectonic models are relatively small for the Neogene, but even small errors accumulate with time. The resultant error may amount to several degrees of latitude in the Paleogene Pacific, and the ability to predict paleopositions becomes less reliable. In the case of Leg 199, uncertainties in the motion of the Pacific plate required that we drill a larger latitude span than may have been strictly necessary to study Paleogene equatorial Pacific circulation because prior to drilling we were not certain where the equator crossed our 56-Ma transect in the Eocene and Paleocene.
Even with prior DSDP drilling, it was only possible to predict in a very general way the type of sediments we would find. We were often surprised by the lithologies we drilled, and the scientific party had to be flexible to revise both the drilling plan and postcruise studies. During Leg 199, it was important to have alternate drill sites to choose from after new material was discovered. Any Paleogene drilling leg should have a variety of alternate sites to optimize drilling time.
Cherts are always a consideration when drilling Paleogene and older sediments. Improvements in drilling technology, such as better cutting shoes and active heave compensation, make cherts a less fearsome challenge than early in the DSDP program. Nevertheless, recovery in cherty sediments is still generally poor. In the case of Leg 199, we were able to core a continuous section through cherty chalk at Site 1218 but were generally frustrated in recovering sediments with chert near the earlymiddle Eocene boundary. We believe that these sediments were still unlithified because we were able to recover radiolarian ooze right up to the upper boundary of chert zones and recovered radiolarian ooze and carbonates below. Understanding where chert zones lie helps to minimize sediment loss but new drilling technology is still needed.
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