124 Effects of Waves and Currents
above bottom. During these storms, erosion may remove the uppermost 5-10 cm of
sediment, leaving a smooth bottom, but after a few weeks, bioturbation re-establishes
a rough microtopography. This activity by benthic organisms facilitates erosion during the next storm.
Drift-like storm deposits hundreds of meters high, as well as hiatuses in deep-sea
sediments, may result from these processes. Obviously, the existence and nature of
these storms are of great interest in connection with plans to dump noxious waste into
the deep sea (Sect. 10.5.5).
At this point, it seems that these dramatic abyssal storms are restricted to the
neighborhood of the axes of very cold bottom-water flows, as on the west side of the
North and South Atlantic Basins, around South Africa, in Circum-Antarctic waters.
Also, benthic storms occur in regions with strong large eddies in surface waters.
If confirmed by further invetigations, these results raise the exciting possibility of
a direct link between surface and deep-sea currents.
During the last Ice Age the production of NADW was greatly reduced, for at least
two reasons. One, there was less evaporation in the North Atlantic and less export of
water vapor to the North Pacific. Thus, the necessary high salinities to make the
water heavy enough to displace existing bottom water were not achieved.
Two, the Norwegian Sea probably was largely covered with pack ice which blanketed the water surface and prevented rapid cooling. Thus, the deep circulation in the
Atlantic (and probably also in the Pacific) was entirely different during glacials from
what it is today.
On top of the cold, deep waters there is another layer before we reach the warm
surface waters. This is the intermediate water, which sinks in the subarctic and
subantarctic open ocean convergences, and forms the bottom for the subtropical
gyres. Along the ocean margins, where the intermediate waters intersect the continental slope, the flow tends to be in a direction opposite to that of the surface waters.
In general, the frictional interaction between bottom currents and the sea floor
results in a "benthic boulldary layer" which is on the order of a few hundred meters
thick. Compared with the overlying deep water, it has a high content of suspended
matter, and is characterized by turbulent motion. Also, its chemistry is distinct. This
layer, and the bioturbated mixed layer at the sea floor, form a system within which
seawater/sea-floor interaction is a dominant process controlling sedimentation patterns.
4.3.5 Exchange Currents. Geologically highly significant are the currents which
provide for the exchange of waters between marginal, semi-enclosed seas, and the
open ocean. The exchange entirely dominates the chemistry and fertility of the marginal basin and hence its sedimentation (see Sect. 7.6 and Fig. 7.12).
In arid zones, excess of evaporation over precipitation produces heavy water in the
marginal sea; as it flows out over the sill, it is replaced by surface water from the
open ocean. This is the case in the Mediterranean, and this circulation is called
allti-estuarine. The salty Mediterranean waters are found at 1500 m depth over the
entire central Atlantic, and profoundly affect the development of abyssal and deep
water masses (Fig. 4.21 a). A similar situation obtains, on a smaller scale, for the Red
above bottom. During these storms, erosion may remove the uppermost 5-10 cm of
sediment, leaving a smooth bottom, but after a few weeks, bioturbation re-establishes
a rough microtopography. This activity by benthic organisms facilitates erosion during the next storm.
Drift-like storm deposits hundreds of meters high, as well as hiatuses in deep-sea
sediments, may result from these processes. Obviously, the existence and nature of
these storms are of great interest in connection with plans to dump noxious waste into
the deep sea (Sect. 10.5.5).
At this point, it seems that these dramatic abyssal storms are restricted to the
neighborhood of the axes of very cold bottom-water flows, as on the west side of the
North and South Atlantic Basins, around South Africa, in Circum-Antarctic waters.
Also, benthic storms occur in regions with strong large eddies in surface waters.
If confirmed by further invetigations, these results raise the exciting possibility of
a direct link between surface and deep-sea currents.
During the last Ice Age the production of NADW was greatly reduced, for at least
two reasons. One, there was less evaporation in the North Atlantic and less export of
water vapor to the North Pacific. Thus, the necessary high salinities to make the
water heavy enough to displace existing bottom water were not achieved.
Two, the Norwegian Sea probably was largely covered with pack ice which blanketed the water surface and prevented rapid cooling. Thus, the deep circulation in the
Atlantic (and probably also in the Pacific) was entirely different during glacials from
what it is today.
On top of the cold, deep waters there is another layer before we reach the warm
surface waters. This is the intermediate water, which sinks in the subarctic and
subantarctic open ocean convergences, and forms the bottom for the subtropical
gyres. Along the ocean margins, where the intermediate waters intersect the continental slope, the flow tends to be in a direction opposite to that of the surface waters.
In general, the frictional interaction between bottom currents and the sea floor
results in a "benthic boulldary layer" which is on the order of a few hundred meters
thick. Compared with the overlying deep water, it has a high content of suspended
matter, and is characterized by turbulent motion. Also, its chemistry is distinct. This
layer, and the bioturbated mixed layer at the sea floor, form a system within which
seawater/sea-floor interaction is a dominant process controlling sedimentation patterns.
4.3.5 Exchange Currents. Geologically highly significant are the currents which
provide for the exchange of waters between marginal, semi-enclosed seas, and the
open ocean. The exchange entirely dominates the chemistry and fertility of the marginal basin and hence its sedimentation (see Sect. 7.6 and Fig. 7.12).
In arid zones, excess of evaporation over precipitation produces heavy water in the
marginal sea; as it flows out over the sill, it is replaced by surface water from the
open ocean. This is the case in the Mediterranean, and this circulation is called
allti-estuarine. The salty Mediterranean waters are found at 1500 m depth over the
entire central Atlantic, and profoundly affect the development of abyssal and deep
water masses (Fig. 4.21 a). A similar situation obtains, on a smaller scale, for the Red