Magazine article Oceanus

Drilling for Sea-Level History on the New Jersey Transect

Magazine article Oceanus

Drilling for Sea-Level History on the New Jersey Transect

Article excerpt

Sediments deposited along ancient continental margins represent a significant portion of the geological record and comprise a sensitive and lengthy record of environmental change, not the least of which is a position change of the sea itself. Sea level is a complex interaction of processes that operate both locally and globally. Variations in sediment supply and adjustments to stress placed on the underlying crust are two local processes that can temporarily overwhelm global sea-level controls. For example, as the crust beneath Scandinavia continues to rebound from the weight of its last glacier, the shoreline is retreating and local sea level is falling as fast as several meters per century. Elsewhere, tide gauges detect inexorable shoreline flooding at the rate of tens of millimeters per century, and though the cause is uncertain, a strong candidate is polar ice melting.

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. …

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