Thermocline Fertility and Upwelling

The goal of Leg 175 was to provide for the reconstruction of the history of currents and of mixing and upwelling dynamics off the margin of western Africa south of the equator. To this end, we largely use the deposits generated by biologic production in surface waters (that is, microfossils and nannofossils), assuming certain responses to varying productivity. Also, the preservation state of the fossils is useful; it can track chemical properties of bottom waters and early diagenetic reactions.

Of special interest, in the context of upwelling, is the distribution of nutrients in subsurface waters. Surprisingly, waters with high phosphate and silicate content underlie the region off Angola, where productivity is low relative to that off Namibia. The high productivity regions—off the Congo and south of Walvis Ridge—are at the very edge of a high-nutrient region, rather than in its center (Fig. F4). One may readily conclude that the nutrient content of thermocline waters is not the crucial factor, or at least not the only important factor, in determining the productivity of overlying waters. The nutrients also have to be brought upward into the sunlit zone. This is the work of upwelling and mixing. It is the combination of the two factors, high nutrient content and vigorous mixing, that results in high productivity. The general formulation of this concept may be given as follows:

PP NCT x wind,           (1)

where PP is the primary productivity, and "wind" represents mixing intensity (that is, the flux of thermocline water to the sunlit layer). NCT stands for nutrient content of the thermocline. The symbol expresses equivalence.

The obvious corollary of Equation 1 is that "wind" is equivalent to the ratio between productivity and nutrient content. For these two parameters, estimates are possible from the study of sediments, but only if flux proxies and nutrient proxies are kept separate when discussing paleoproductivity. Flux proxies represent an important fraction of the material delivered to the seafloor, while nutrient proxies contain information about the chemistry of the seawater that provides the nutrients (for a discussion of proxies see Wefer et al., 1999.) Thus, in principle, it is possible to make estimates of relative wind stress through time in the upwelling regions off Namibia. Such estimates can then be compared with the supply of dust from the Namibian Desert (which is itself a result of Namibian upwelling dynamics). This has not been done, but the first steps in this direction have been made (see Anderson et al., Chap. 21, this volume).

Temperature Anomalies and Namibia Upwelling System

Vigorous vertical mixing brings cold waters to the surface and results in temperature anomalies in surface waters. Such anomalies have been mapped in the South Atlantic since the German Meteor Expedition (1925-1927) (Fig. F5). The pattern shows large anomalies off Namibia in the vicinity of Lüderitz. Since the nutrient content of upwelled waters decreases rapidly south of 25°S (Fig. F4), optimum conditions for high productivity should occur just north of that latitude (that is, north of Lüderitz) in Walvis Bay.

We shall refer to this upwelling region as the "Namibia" or "Walvis" upwelling system because of its location. The name "Benguela upwelling system," which is commonly used to describe upwelling in regions from the coast of Angola to that of the cape is retained as a term with that broader meaning. Strictly speaking, there is no well-defined "Benguela upwelling system." The three upwelling regions (Congo-Angola, Walvis-Namibia, and Cape Province) are quite distinct (e.g., Chapman and Shannon, 1987; Hay and Brock, 1992; Berger et al., 1998a; Giraudeau et al., in press).

The front just north of the Walvis Ridge ("Angola-Benguela" Front) should be referred to, more rationally, as the "Angola-Namibia" Front. This would have the benefit of describing its location quite accurately, in contrast with present usage. In fact, there is no front off Benguela, which is close to the center of the Angola coast (12°S). The great current off Namibia turns toward the open sea and northward and westward at ~15°S (Figs. F1, F5). It never gets close to Benguela (Peterson and Stramma, 1991; Tomczak and Godfrey, 1994), whose name it carries.

The negative temperature anomaly off Namibia (Fig. F5) reflects mixing, as mentioned, and hence, is a measure of wind action, shown as "wind." In principle, it should be possible to capture the gradient of the anomaly along the Walvis Ridge (as here the seafloor is sufficiently elevated to preserve carbonate well offshore). Alternatively, or in addition, the negative anomaly within the Namibia upwelling system, as seen in the sediments, should provide a good guide to wind action, that is, intensity of upwelling and mixing. Clearly, additional information (besides a temperature gradient) is needed to define wind fields; the point is that upwelling needs to be separated conceptually from productivity to make progress in that direction.

Carbonate Preservation and NADW Production

Preservation of calcareous fossils (nannofossils and foraminifers) on the seafloor depends on the depth of the seafloor and the position of the carbonate compensation depth and the lysocline level. In turn, the lysocline is tied to the lower boundary of the North Atlantic Deep Water (NADW), where it meets the underlying Antarctic Bottom Water (AABW). The distribution of salinity nicely reflects the overall pattern of flow (Fig. F6). In fact, the hydrographic boundary near 2°C, first mapped by Georg Wüst of the Meteor Expedition, is virtually congruent with the lysocline, except where organic matter supply increases the dissolution of carbonate on the continental margin. The present distribution of NADW puts this boundary between 4 and 4.8 km west of the Mid-Atlantic Ridge (going north from the Rio Grande Rise to the equator). In the eastern trough, the situation is different for the basins north and south of the Walvis Ridge, which blocks the AABW from entering the Angola Basin. (AABW enters, but through the Romanche Trench near the equator, where it mixes with NADW.) In the Cape Basin, then, we expect a sharp boundary somewhere near 4 km. In the Angola Basin, the boundary is likely to be fuzzy and closer to 5 km.

Where the seafloor is bathed by NADW, preservation of carbonate tends to be good for a given depth of deposition. The production of NADW varies greatly through time, as can be documented by using carbonate preservation stratigraphy or carbon isotope stratigraphy or both (e.g., Bickert and Wefer, 1996; Curry, 1996). The preservation stratigraphy on the outer Walvis Ridge suggests a rapid rise of NADW production (that is, better preservation of carbonate) sometime within the late Miocene and another step of increase near the beginning of the Pliocene (e.g., Moore et al., 1984, p. 15). In essence, this pattern agrees with the great shift of silica from the North Atlantic to the North Pacific at the end of the middle Miocene (Keller and Barron, 1983; Baldauf and Barron, 1990), which is another expression of NADW production (NADW removes silicate from the deep North Atlantic). The carbonate trends and the silica shift run parallel to the overall cooling of the planet as seen in oxygen isotopes (Shackleton and Kennett, 1975; Miller et al., 1987; see review in Berger and Wefer, 1996b). Thus, increased production of NADW on this late Neogene timescale is seen to result from cooling. With the onset of glaciation in the Northern Hemisphere, that is, ice surrounding the high-latitude margins of the North Atlantic, an optimum situation is reached whereby vigorous evaporation combines with strong cooling to make plenty of deep water. However, as cooling proceeds into severe glacial periods, the optimum is exceeded, which results in a decrease of carbonate preservation upon cooling on glacial-interglacial timescales of the late Quaternary. (At this point, opaline deposits return, sporadically, to the North Atlantic; see Baldauf and Barron, 1990; their fig. 13.) Thus, we have another paradox, this time referring to the preservation of carbonate, whereby cooling first results in improved preservation, but further cooling beyond an optimum results in increased dissolution. One important task is to find out where the optimum is located along the cooling curve and within the glacial-interglacial cycles.

Concept of Optimum

We shall make use of the concept of "optimum" in several contexts: carbonate preservation, organic matter supply, and opal deposition. The simplest way to envisage an optimum situation is to think of it as a product of two factors that are varying out of phase with each other:

Optimum = max(f1a, f2b),           (2)

where f1a, f2b stands for some interaction between two variables (for example, a product). The task is to identify the two (main) factors involved. The two factors may be antagonistic, at least over some relevant range. In the case of productivity, it is thermocline fertility and mixing intensity (Berger et al., 1994; Herguera and Berger, 1994). In the case of carbonate preservation, it is evaporation and cooling of surface waters in the North Atlantic. Evaporation needs dry, warm winds; cooling needs temperatures near freezing. Best results are obtained when both conditions prevail in series (Sargasso to Iceland), or seasonally, or both. In the case of opal deposition, it is silicate content of the thermocline and deep mixing and, possibly, the presence of a shelf. Planetary cooling enhances deep mixing, but buildup of ice removes shelves.

Sediment Patterns

The sediments accumulating along the margin of southwestern Africa are well suited to study carbonate preservation and the deposition of organic matter and opal as a function of time. Surveys of sediment patterns recorded during Leg 175 are given by Wefer et al. (1998), Pufahl et al. (1998), and Berger et al. (1998a) in the Leg 175 Initial Reports volume, as well as in the site reports therein. Four different major sedimentary facies regimes characterize the region drilled during Leg 175 (see Fig. F7). Off the mouth of the Congo, sediments have a large terrigenous component. This is also true for the silts and clays recovered off Angola at sites close to the shelf edge, where very high sedimentation rates prevail. On the Walvis Ridge and in its vicinity, sediments have a strong pelagic aspect, although organic matter contents remain high. South of the ridge in Walvis Bay and near Lüderitz Bay, sediments are unusually rich in opal and organic matter, reflecting the high coastal ocean productivity here. In the southern Cape Basin away from the high-productivity regions off Namibia, sediments are dominated by pelagic carbonates.

Generally, organic carbon contents were quite high at all sites, ranging from a few percent to as much as 20% by weight. North of Walvis Ridge, values were between 1 and 5 wt%. South of the ridge, values were typically <5 wt%, but reached values well over 10 wt% at sites near the coastal upwelling areas. Black layers extremely rich in organic carbon (as much as 20 wt%) are common in sediments from the Lüderitz site (1084). There is, at all sites, an overall decrease in downhole percentages of organic carbon. Continued diagenetic destruction of organic matter and an increase in upwelling and productivity in the last 10 m.y. are thought to be responsible for this pattern. Sedimentation rates ranged from 30 to 600 m/m.y., with the most common values located between 50 and 100 m/m.y., roughly three to four times the values typical for deep-sea carbonates.

A high supply of organic matter at all but the southern Cape Basin sites drives intense diagenetic activity. Besides reactions directly affecting organogenic compounds, there is dissolution of biogenic carbonates and opal, formation of calcite and dolomite, and precipitation of glauconite, pyrite, and phosphate. One of the more conspicuous discoveries during Leg 175 was the presence of several decimeter-thick dolomite layers that were found as fragments in some cores and could be located precisely within the holes using various logging tools. If dolomites are widespread, seismic profiles will have to be reinterpreted. Also, the presence of dolomite layers in sediments <1 Ma in age indicates rapid development of such layers under the high-productivity conditions encountered (see Pufahl et al., 1998; Wefer et al., 1998).