Drilling for sea-level history on the New Jersey transect.
Many researchers in the academic community are striving to understand the history of sea-level change on geological time scales (10,000 to 10,000,000 years) because of its profound influence on fundamental elements of the earth system, such as: particle, chemical, and nutrient flux into the ocean; distribution and character of near-shore ecosystems; and air-sea-land interactions and their relationship to global climate. Consequently, studies are focused on extracting the global sea-level signal that is locked in the sedimentary record of the coastal plain, shelf, and slope in key regions of the world.
The industrial community has long had an interest in understanding what controls the character and distribution of sediment deposited in shallow water (less than 200 meters deep), particularly as this understanding helps to predict the occurrence of hydrocarbons. Peter Vail and his colleagues at the Exxon Production Research Company published a watershed monograph in 1977 that described how to read the history of local sea-level change in seismic reflection profiles collected along continental margins. They argued that angular relationships between reflectors are the key to identifying times of local sea-level change, and that when profiles are compared around the world, common signals emerge to form a truly global sea-level record. The work met with immediate controversy that was based, in part, on the challenging argument that the effects of local processes typically swamp the sedimentary record.
The Deep Sea Drilling Project entered into the conflict by conducting three legs in search of the imprint of sea-level changes along continental margins: Leg 80 drilled on the Irish continental slope, and Legs 93 and 95 drilled on the New Jersey slope and rise. The results of all three programs provided tantalizing support for the times of sea-level change Vail and his associates had proposed back several tens of millions of years into the past. Unfortunately, all drill cores encountered long stratigraphic gaps and were located in relatively deep water (more than 1,000 meters), where the record of sea-level change is indirect at best. The results swayed few opinions, and the "Vail curve" remained controversial.
Beginning in 1987, Exxon again revolutionized the search for a record of global sea level. This advance was achieved in part by improved technologies, and in part by improved insight into how these technologies can be integrated. A series of publications described the use of outcrops, cores, wireline logs, and seismic profiles for detailing sedimentary, histories at previously unattainable spatial and temporal scales. Ironically, a continuously cored hole is rare in the oil industry, so the potential of this technique cannot always be achieved with commercial data.
The academic community soon realized that it had in JOIDES Resolution a unique and valuable tool to probe continental margins for evidence of sea-level changes. Continuously cored and logged boreholes are routinely collected by this vessel, though to date it has not drilled in typical continental shelf water depths. In a series of meetings between 1987 and 1991, the scientific drilling community developed the strategy needed to address sea-level change based on transects of boreholes extending from the coastal plain to the continental slope. To build on the success of Legs 93 and 95, we proposed a transect of the New Jersey margin. The first two of three steps in this effort began in 1993: 1) during Leg 150, sponsored by the Ocean Drilling Program, four boreholes on the continental slope and one on the rise were completed; and 2) the National Science Foundation Continental Dynamics Program along with ODP funded the drilling of two boreholes on the onshore coastal plain. Step three requires drilling on the continental shelf, but has been postponed until sufficient data are collected to evaluate risks posed by the chance of encountering hydrocarbons.
Several characteristics make New Jersey an ideal margin for this transect: We know there have been few local tectonic disturbances in this well-studied region; its mid-latitude setting maximizes the chance for excellent age control built on the integration of biostratigraphy and chemical isotopic and paleomagnetic stratigraphies; and high sedimentation rates over the last 30 million years promise a record with especially high resolution. We focus on this time interval for an important reason: Oscillations in the marine-oxygen isotopic record detail a 30-million-year history of glacial ice growth and decay. This geological interval represents a starting point for a detailed study of the stratigraphic response to known changes in global sea level. Conclusions about the mixed local/global record along the New Jersey margin will be evaluated by future studies on other margins that focus on this same time interval in places where local conditions such as the age of continental rifting are different, and the global signal can be more confidently extracted.
We began our study in fall 1990 by collecting a grid of seismic reflection profiles across the New Jersey margin. Based on these data and background information provided by Exxon Production Research, we laid out a transect of drill sites needed to document the age and character of discontinuities recognized in these profiles. We anticipated that the most dramatic discontinuities would have formed when local sea level fell rapidly and little sediment could be retained on the shelf. By contrast, intervals of widespread shelf deposition would indicate times of rapid sea-level rise.
We led ODP Leg 150 last summer and drilled four sites on the slope and another site several tens of kilometers out on the continental rise. Water depth at the slope sites ranges from 450 to 1,130 meters. We are able to trace over a dozen reflectors to all four sites and correlate each to the rock record. In most cases the reflectors match debris swept off the adjacent shelf; in others they match especially well-cemented intervals that developed during times of especially slow sediment accumulation. We conclude that the former occurrences mark times when local sea level fell, and the latter, times of local sea-level rise, when a wide continental shelf--not the slope--was the primary depository for sediment washed in by rivers. Preliminary analyses suggest that the ages of many falling sea-level trends that we found match the oxygen-isotopic record that is assumed to be a proxy indicator of glacial ice growth and, consequently, of global sea-level minima.
Two more holes were drilled onshore of the New Jersey shelf in 1993 to sample shallow-water (less than 200-meter) marine environments now beneath the coastal plain. Another hole is planned for 1994. The onshore boreholes recovered an excellent record of sedimentary environments that are especially sensitive to sea-level changes. With this sensitivity, however, comes a challenge: These sediments typically lack the fossils found in deep-water sediments, and biostratigraphic control is often too coarse to be useful for sea-level studies. Fortunately, we have recovered numerous shell beds that can be dated with strontium isotope techniques. As a result, we are confident that we will be able to establish time planes that tie the continental slope record to the coastal plain record.
We have thus continued a transect begun by DSDP Legs 93 and 95. We are completing the onshore drilling and integrating the results with those of the five continental slope and rise boreholes. Our most challenging work awaits us: determining that drilling can be done safely on the shelf, completing this bold effort, and integrating these results with existing data.
After two years of describing cores at the Woods Hole Oceanographic Institution as a Research Assistant, Greg Mountain concluded that most cores are cylindrical and furl of mud. With that foundation he enrolled in graduate school in 1974 at Columbia University's Lamont-Doherty Earth Observatory; he is still there, now as a Research Scientist studying the effects of sea-level change. He has learned that one such effect--rarely mentioned--is a rising tide of planning documents, meetings, and ancillary activities that accompany such interdisciplinary efforts. When not treading in this sea of paperwork, Greg makes his home in Westwood, New Jersey, where he and his wife are raising two boys at 176 meters above sea level.
Ken Miller is a Professor at Rutgers, the State University of New Jersey, an Adjunct Scientist at Lamont-Doherty Earth Observatory, and a 1982 graduate of the MIT/WHOI Joint Program. When not teaching, going to sea, or attending meetings, he can be located somewhere on the New Jersey Turnpike, caught in traffic during one of his frequent commutes to Lamont-Doherty. Otherwise, Ken can be found at the Jersey shore, keeping a diligent watch on the inexorable rise in sea level from the deck of his house at 4 meters above sea level.
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|Title Annotation:||25 Years of Ocean Drilling|
|Author:||Mountain, Gregory S.; Miller, Kenneth G.|
|Date:||Dec 22, 1993|
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