William L. Balsam

The High-Velocity Core of the Western Boundary Undercurrent at the Base of the U.S. Continental Rise

The Western Boundary Undercurrent is a high-velocity, contour-following bottom current that flows southwesterly on the U.S. Atlantic continental margin. A high-velocity core of the Western Boundary Undercurrent is delineated by an analysis of underlying sediments, which are characterized by coarse particle sizes and efficiently aligned magnetic grains in a zone from 4440 meters at the base of the rise to 5200 meters on the adjacent abyssal plain.

Carbonate dissolution on the Muir Seamount (western North Atlantic); interglacial/glacial changes

ABSTRACT Although the history of carbonate dissolution has been studied for many years, few records of dissolution through an entire glacial cycle are available for the western North Atlantic (North American Basin). Using 14 cores from the Muir and Siboney Seamounts, located about 240 km northeast of Bermuda, changes in carbonate dissolution were examined over the last 125,000 years. The core depth range spans more than 3000 m, 1780 m-4980 m, insuring that major changes in dissolution will be recorded. Weight percent carbonate was plotted as a function of water depth for five stratigraphically defined levels to determine temporal changes in the sedimentary lysocline and calcite compensation depth (CCD). During the Holocene, the lysocline is located between 4050 m and 4350 m; the CCD is not pre ent even in the deepest sample (4980 m). The late glacial ( 18,000 years B.P.) lysocline may have shoaled to about 2500 m, whereas during early glacial time ( 72,000 years B.P) it deepened to about 3400 m. The mid-glacial carbonate maximum ( 60,000 years B.P; interstadial?) is marked by a deepening of the lysocline to about 3800 m. During the entire glacial episode, the CCD aappears to have remained fixed at 4650 m. The last interglacial ( 125,000 years B.P) was a time favorable to carbonate preservation; the lysocline was deeper than 4650 m, the deepest core containing sediments of this age Increased glacial dissolution appears to result from changes in deep circulation. The establishment of the CCD during glacial time is interpreted as indicating increased Antarctic Bottom Water flow. The lysocline, on the other hand, may reflect the degree of stagnation of North Atlantic Deep Water.

Patterns of planktonic foraminiferal abundance and diversity in surface sediments of the western North Atlantic

Abstract Seventy-two core tops and grab samples from the western North Atlantic were analyzed to determine what aspects of planktonic foraminiferal abundance and diversity are most closely related to ocean circulation. Some species appear to be reliable indicators of the Gulf Stream, a warm surface current. Both Globorotalia menardii and Globigerinoides sacculifer have their highest abundances under the main trend of the Gulf Stream. Globorotalia inflata reaches high abundances in the cold slope water north of the Gulf Stream but its distribution is not as continuous as the Gulf Stream indicators. Contoured values of species diversity, the Shannon diversity index, and species equitability also reflect surface circulation. A plot of species diversity (number of species) shows a poorly defined region of high diversity beneath the major trend of the Gulf Stream. Use of the Shannon diversity index enhances and clarifies this region of high diversity. A map of species equitability shows a broad belt of low species dominance (high equitability) beneath the Gulf Stream. North of the Gulf Stream, a tongue of high dominance (low equitability) corresponds to the increased relative abundance of Globorotalia inflata . High diversity of planktonic foraminifera in bottom sediments characterizes the warm shifting surface currents of the Gulf Stream; low diversity is typical of slope and Sargasso Sea waters. Low equitability (high species dominance) indicates either cold currents or gyre center waters. Maps of foraminiferal diversity and equitability for other intervals of geologic time may be useful in tracing the evolution of ocean circulation.

Planktonic foraminiferal diversity in the interglacial and glacial North Atlantic: A test of diversity gradients as a paleoceanographic technique

We calculated diversity indices for planktonic foraminifera in both modern and 18,000 B.P. (late Wisconsin glacial maximum) North Atlantic sediment samples. Modern diversity patterns, especially those based on the Shannon index, reflect present surface circulation. Mapped 18,000 B.P. diversity values reveal a southward migration of the polar front, a shrinking of transitional areas, a southward deflection of the Gulf Stream, and a slight expansion of Sargasso Sea waters. These changes agree with the CLIMAP reconstruction and validate the paleoceanographic utility of diversity maps in epochs prior to the origin of extant species.