Reconstructing deep ocean circulation patterns in the geologic past

Olivier Marchal

One of the major research activity of the paleoceanography group is the reconstruction of deep ocean circulation patterns in the geological past. The analysis of marine sediment properties offers the potential to produce such reconstructions. For example, sediment records indicate that the proportion in the deep Atlantic of waters coming from the northern North Atlantic and waters coming from the Southern Ocean has changed dramatically and frequently during the last ice age and the last deglaciation. The paleoceanographic and paleoclimate implications of these changes are of great interest.

A model is being developed of the circulation of waters in the deep Atlantic. In its present form the model only simulates the flow of Antarctic Bottom Water (AABW; Figure 1). This water is introduced today in the Atlantic basin through the western South Atlantic at latitudes 30°S-35°S. The dashed lines in the figure are the coastlines of South America and Western Africa. The Mid-Atlantic ridge, which is intersected by the Romanche-Chain fracture zones at the equator, is also visible in the figure.

Figure 1 (click for larger image)

The vectors (in blue) represent water fluxes in units of m²/s (the maximum transport in the Figure is 28.7 m²/s). The black solid lines represent selected isobars (lines of constant pressure). Note that, in a non-rotating fluid, the flow would normally be in the direction of the pressure force, i.e., perpendicular to the isobars. The earth, however, is rotating, so that the flow is (to the first order) perpendicular to the pressure force, i.e., parallel to the isobars (it is this feature that enables the isobars on a weather map to be representative of the pattern of atmospheric flow). The isobars in the figure (reported without labels to avoid congestion) help to visualize the flow of AABW in the western South Atlantic.

The model simulates a vigorous deep western boundary current (DWBC) flowing northward along the western margin of the Brazil Basin, a sluggish recirculation within the Brazil Basin, an outflow through the Romanche-Chain fracture zones, and a transposition of the DWBC to the western flank of the Mid-Atlantic ridge north of the Equator (see figure). These features are consistent with AABW flow patterns inferred from observations.

The model is currently being extended to include the overlying North Atlantic Deep Water. This latter water, formed in the Greenland-Iceland-Norwegian Seas and in the Labrador Sea, is a another major component of the Atlantic deep hydrography.

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