SPECIAL PROBLEMS AND OPPORTUNITIESPaleoceanographic studies require good positional information for the best paleoenvironmental interpretations. For intervals older than the Pleistocene, sites must be backtracked by a plate tectonic model to locate drill sites with respect to important geographic features. For Leg 199, it is important to locate the equator in the early Eocene and Paleocene with respect to each of the drill sites.
We backtracked positions of the proposed drill sites by using a model in which hotspots have remained fixed with respect to the Earth's spin axis. The hotspot reference frame we used was Gripp and Gordon (1990) for 0- to 5-Ma Pacific hotspot rotation pole and Engebretson et al. (1985) for older poles. Because the hotspots may have moved, the drill sites may be systematically mislocated (Tarduno and Cottrell, 1997). In addition to the possibility of hotspot motion leading to inaccurate paleopositions, there is also significant disagreement between plate reconstructions using hotspots and those following global plate circuits and controversy over whether hotspots have moved or whether plate reconstructions are incomplete (e.g., Acton and Gordon, 1994; Cande et al., 1995; Tarduno and Cottrell, 1997). There is an additional concern that true polar wander (the shift of Earth's rotation pole through time) (Besse and Courtillot, 1991, Steinberger and O'Connell, 1997) may have caused a shift in the position of the equator over time. This presents both a problem for designing the leg and an opportunity to improve the Pacific plate tectonic model by paleomagnetic studies on Leg 199 recovered cores.
We have already combined seismic reflection data from the EW9709 site survey with the Neogene seismic stratigraphy of Mayer et al. (1985, 1986) to determine that the early Miocene equator was about 2° farther south than predicted by the backtrack model we used (Fig. 3) (Knappenberger, 2000). This illustrates the necessity of locating the equatorial position in the Paleogene.
To deal with the uncertain location of the late Paleocene equator position, we have proposed
drill sites that adequately cover possible major errors in latitude. The proposed drilling transect
spans a paleolatitude range of more than 16° around the estimated hotspot equatorial position,
from 5°S to 11°N.
The span of drilling latitudes presents an opportunity to use paleomagnetic techniques to constrain this important plate tectonic problem. The Leg 199 paleomagnetic studies will complement Leg 197 (Motion of the Hawaiian Hotspot During Formation of the Emperor Seamounts: a Paleomagnetic Test) objectives and will provide further constraints on Pacific plate motion. Paleomagnetic determinations over the same time period during Leg 198 (Exploring Extreme Warmth in the Cretaceous and Paleogene: a Depth Transect on Shatsky Rise, Central Pacific) when combined with Leg 199 data will be used to better define Paleogene poles of rotation for the Pacific plate.
Estimated equatorial crossings of Sites PAT-6 (~22 Ma), PAT-17 (~29 Ma), PAT-9 and PAT-8 (~41 Ma), and PAT-10 (~50 Ma) all should occur within the APC section and should be useful for paleomagnetic analysis. In addition, the more northern drill sites (Sites PAT-12, PAT-19, PAT-13, PAT-15, and PAT-26) will provide an array of paleomagnetic data spanning an additional 13° in latitude that should also have good paleomagnetic profiles contained in the APC section of the sediment column.
Comparisons Between Seismic Profiles and Drilling | Table of Contents