Sediment Accumulation Rates and Fluxes

17

Sediment Accumulation Rates and Fluxes

The purpose here is to introduce the nature of overall sedimentation rates as well as the individual contributions of terrigenous and also biogenic components to the geological record. Despite the widely varying density of the results, the aim is to attempt to evaluate relationships between the various records and to summarise the salient aspects of the processes. For long-term studies of temporal variations, the only mass/age distributions were recovered at deep-water sites with continuous deposition but with generally low temporal resolution. This low resolution is compensated by frequent covarying forcings and, during Pleistocene times, the terrigenous record is largely related to the aeolian supply (aeolo-marine sediment). The major sedimentary changes preceded the major change in 8180 and the onset of North Atlantic ice rafting by about 0.5 Myr. This age difference suggests that the two responses are not causally linked but are instead independent responses to some other kind of forcing such as the climatic consequence linked to the tectonic uplift of the Tibetan Plateau (Ruddiman et al., 1989). Particularly, the influx of early Pliocene, terrigenous sediment cannot be induced by Northern Hemisphere ice sheets, which first appeared about 1.5 Myr later. On other side, shallower sides will provide higher resolution of the records, but with a more modest hindsight.

increase about 3 Ma that seems to indicate the main onset of coastal upwelling fertility and enhanced trade winds in northern summer. At site 658, the trend of the organic carbon flux is almost reverse to the opal one, in that it shows an abrupt decrease at 3.1 Ma. This decrease is considered as a decrease in river-borne sediments and nutrients induced by the aridification of adjacent sections of the West Sahara. The records of opal and terrigenous deposition at ODP Sites 662/663 and 664 have some similarities to the trend of Northern Hemisphere ice sheets (and high-latitude North Atlantic cooling). Both signals show large increases in abundance (and burial fluxes) near 2.5 Ma (i.e., at the beginning of variations of moderate amplitude at the 41,000-yr period), as well as earlier less-prominent increases at 3.6-3.4 Ma. This 2.5-Ma increase is best developed in the opal records and in the dust records at more distal Site 664 (Ruddiman et al., 1989). At equatorial Atlantic sites high-resolution analyses show that the mean flux of opal increased abruptly by 60%-70% near 2,5 Ma due mainly to increased upwelling. The mean winter-plume dust influx from Sahelian and Saharan Africa also increased at this time by between 35% and 75% following smaller increases earlier during the late Pliocene. In this equatorial divergence zone, the increasing opal flux implies a stronger zonal component of the southern trade winds during Southern Hemisphere winters. The highest dust flux suggests a weaker southwesterly monsoonal flow into western Africa during boreal summers. The study of deposition at Site 663 in the equatorial divergence zone supports previous studies, which indicate terrigenous variability prior to 2.4 Ma occurring at periodicity near 23-19-kyr (de Menocal et al., 1993). It was suggested that African climate was forced by precessional insolation modulation of the monsoonal intensity. The absence of significant high-latitude climate variability was emphasised. Consequently, it was proposed that "pre-ice" and "syn-ice" sensitivities of African terrestrial climate be separated. In such a process, high-and low-latitude climate systems are independent when ice-sheet variability is low, but when ice sheets grow such that major climate oscillations (100-kyr and 40-kyr) are sustained (i.e., after circa 2.4 Ma), even low latitude climate change becomes controlled by high latitude climate variability. In the Southern Hemisphere, terrigenous supply and especially quartz-rich dust play a secondary role in the deep-sea fluxes of the last million years. DSDP Site 532 drilling on the Walvis Ridge Abutment Plateau (Hay and Brock, 1992) has recovered a sequence of late Miocene, Pliocene, and Pleistocene hemipelagic sediments rich in organic matter and opaline silica. Even if several massive

1. Long-Term Records of Mass Fluxes and Sediment Accumulation Rates The available data are distributed through the West Sahara margin, the Atlantic low latitudes and the southwestern Africa margin. Five ODP sites (657-661) off the western periphery of the Sahara provide a record on the long-term history of climatic changes of Saharan-Sahelian region (Tiedemann et al., 1989). Latitudinal variability of quartz accumulation rates indicates the zonal aeolian dust discharge was centred near 18~ at site 659. The aridification of the southern Sahara and Sahel is evident especially at 4.0, 3.6, and 2.1 Ma, and again, at 0.8 Ma, meaning that at this latitude the late Miocene and early Pliocene were humid. The long-term aridification farther south followed a different process with a generally lower dust supply and a delayed increase after 1 Ma. This spatial distribution of aeolian quartz suggests that dust outbreaks linked to the ITCZ during summer did not shift in latitude over at least the past 4.0 Myr. At the same time, the input of fluvial clay strongly decreased suggesting, despite pulses of fluvial runoff linked to interglacial stages, a general drying up of central Sahara rivers. During the same phase, the mean flux of opal, especially off Cape Blanc, showed an abrupt DEVELOPMENTS IN QUATERNARY SCIENCES VOLUME 10 ISSN 1571-0866

9 2008 ELSEVIER B.V. ALL RIGHTS RESERVED 213

214

Tropical and Sub-Tropical West Africa

debris-flow and a number of turbidites interrupted this hemipelagic deposition, accumulation rates of terrigenous matter are modest. However, some terrigenous (namely clay) accumulation rates indicated a significant decrease from 2.0-5.9 Ma interval to 0-2.0 Ma interval. The authors show that the amounts of opaline silica and organic carbon in the sediment increase from latest Miocene to latest Pliocene, and then decline to present. During the late Pliocene, opaline silica accumulated ten times faster than during the late Pleistocene (Fig. 1). Before the Pliocene, the

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maximum biological productivity in this region occurred during glacials (i.e., first Miocene glacials) but since then it has occurred during interglacials, an effect or sea-level change of productivity. CaCO3, the third major biogenic component indicates pronounced diminution from the latest Miocene to latest Pliocene. Various factors causing changes in sedimentation are suggested and involve oceanic processes in the Atlantic. 1. The production of relatively nutrient-rich AAIW was a prerequisite for the long-term increase in productive upwelling recorded by late Miocene to latest Pliocene sediments. This production was controlled by the closing of the Central American Isthmus resulting in salinisation of the North Atlantic and in a shallower pycnocline facilitating upwelling of nutrient-rich water. 2. Southward migration of the ITCZ as the Earth changed from unipolar to bipolar glaciation would have enhanced the Angola Coastal Current and subsequent upwelling from the Angola Thermal Dome (see Chapter 6). Consequently, the decrease in strength of upwelling since 1.7 Ma recorded on the Walvis Ridge may reflect a southward shift of the upwelling centre to its present location (near Ltideritz) in response to the growth of northern hemisphere ice sheets. 3. Lastly another possible factor of the general decline in productivity since 1.7 Ma would be the increasing Mediterranean saline outflow resulting in progressive spreading of NAIW into the South Atlantic during glacials. Several ODP sites between 20 ~ and 30~ in the Benguela Current System show similar trends during the last 3 Myr (Lange et al., 1994). A distinct opal (and diatom) maximum is recorded during the lower half of the Matuyama reversed Chron (around 2.0-2.6 Ma), called in the Matuyama Diatom Maximum (MDM). This MDM probably reflects a period of seasonally pulsed continuous advection of subantarctic waters resulted in subsurface waters enriched in silicate. The onset and cessation of the MDM would be related, as for the Walvis Ridge site, to the change from unipolar to bipolar glaciation and to the consequent southward shift of the ITCZ.

(f) 9 ""

0 0.4

Site 530

1.7

3.0

2. Mass Fluxes and Sediment Accumulation Rates in the Pleistocene

Opal

4.4

5.8 Ma

Fig. 1. Mass~age distribution for the three major biogenic components (CaC03, Corg, Opal) in latest Miocene (5.8 Ma) to Recent sediments recovered at DSDP Sites 530 and 532. Vertical axis is accumulation rate (mass/ area x time). Mass is proportional to the area of each bar; (a) CaC03 at DSDP Site 532; (b) CaCOs, at DSDP Site 530; (c) Corg at DSDP site 532, (d) Corg at DSDP Site 530; (e) Opaline silica at DSDP Site 532; (f) Opaline silica at DSDP Site 530 (after Hay and Brook, 1992).

Stein (1985) observed an upward decreased opal flux off the Northwest African margin (ODP"site 397) over Pleistocene. This change along the continental margin could be attributed either to reduced fluvial delivery of nutrients linked to aridification of the continent or to a lower silica content of upwelled waters. The northernmost complete long-term record (last 750 kyr) is located on the Sierra Leone Rise (Sarnthein et al., 1984). As indicated above, aeolian dust is the dominant terrigenous component. It is considered to be fairly constant varying by less than a factor of 1.5 over 750 kyr. The other determining factors consist in the variable porosity and CaCO3 content, which

Sediment Accumulation Rates and Fluxes has varied in the past because of dissolution. The average rates of overall sedimentation vary allowing a separation of three major phases of deposition subsequent to the hiatus ending about 775 kyr BP. Low average rates of 1.27 cm/kyr are recorded in the oldest section until about 540 kyr B P. From 540 to 265 kyr B P, average sedimentation rates increased to 1.45 cm&yr. Later sedimentation rates fall to 1.32 cm/kyr until the Recent. In the same way, the most extreme values of the paleo-productivity curve and of organic matter content have also increased almost suddenly after 540 kyr. Short-term variations of sedimentation rates (0.8 to 4.0 cm&yr) are superimposed on the long-term variations with a periodicity of 10-45 kyr, not 100 kyr. These variations are controlled by variable CaCO3 dissolution for large areas of the eastern North Atlantic. Curry and Lohman (1986) found that CaCO3 productivity in surface waters near the Sierra Leone Rise decreased by a factor 2 or more during late Pleistocene glacial stages but various factors could have resulted in this decrease, such as productivity suppression and dissolution.

215

At both ODP equatorial Atlantic sites (662, 663, and 664), average CaCO3 fluxes show a decrease over the last 0.8-1.2Myr by more than half (Ruddiman and Janecek, 1989) (Fig. 2). In some cases, a late Pleistocene decrease in long-term average of CaCO3 flux appears to be characteristic of deeper Atlantic sites (Stein and Sarnthein, 1984; Ruddiman et al., 1989). Various causes could induce this decreasing trend. Late Pleistocene 513C records in the deep Atlantic have shown that the formation of deep water in the North Atlantic was suppressed during glaciations. Consequently, waters of southern origin spread farther northward bringing higher nutrient concentrations and a greater ability for dissolving CaCO3 (Oppo and Fairbanks, 1987). Theses changes resulted in high CaCO3 dissolution in equatorial regions in the Late Pleistocene, driven mainly at the 100-kyr periods of the Northern Hemisphere ice sheets. However, long-term 613C records indicate that the greatest suppression of northern-source water fluxes may have developed between 0.85 and 0.4 Ma, namely prior to the large decrease in CaCO3 flux observed here.

Holes 662A (3,824 m) and 663A (3,708 m) CaCO3 (g/cm2/kyr) 1

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Sedimentation rate (cm/kyr)

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Fig. 2. Mass fluxes of CaC03, opal, and terrigenous sediment in the eastern equatorial Atlantic (Holes 662A-663A and 664B-664D). Bar graphs indicate average fluxes between datum levels used as absolute time constraints. Dashed lines show fluxes calculated by downward extrapolation of sedimentation rates from the overlying layer (after Ruddiman and Janecek, 1989).

216

Tropical and Sub-Tropical West Africa

Simultaneously or a little before the CaCO3 fall, mean opal fluxes decreased by around half over the last 0.8 Myr (Fig. 2). In the equatorial divergence zone, this implies a weaker zonal component of southerly trade winds flowing along the equator during Northern Hemisphere summers. Equally, terrigenous (mostly dusts) fluxes decreased by as much as half in the last 0.7 Myr at both equatorial vertical profiles (Fig. 2). There are various lines of evidence (see Chapter 11) for longterm aridification over Saharan and Sahelian Africa that oppose the decreased winter-plume dust fluxes after 0.47 Ma. A possible explanation is that changes in atmospheric circulation diminished the influence of the winter dust plume at the equator by redirecting the dust flux farther north. This redirection of the winter dust plume away from the equator would be documented by an increase in siliciclastic flux during the late Pleistocene at Site 660. Another possibility is that maxima in dust transport occur during maximum source area destabilization rather than during times of absolute maximum aridity. Influences of high- and low-latitude processes on African terrestrial climate were especially considered by DeMenocal et al. (1993) in a high-resolution record of core 663. They show that the opal, terrigenous components (aeolian dust) and phytoliths (savanna grass cuticles) are highly covariant and have maximum values during glacial stages whereas carbonate and Melosira (freshwater diatom) occur within interglacial stages. Accumulation rates of opal, terrigenous and phytoliths vary predominantly at 100- and 41-kyr periodicities implying high latitude forcing. Freshwater diatoms abundance varies coherently at 23-19-kyr orbital periodicities linked to low-latitude precessional monsoon forcing. Although carbonate percentage exhibits strong spectral power at the 100-kyr and 41-kyr periodicities, the carbonate accumulation rate record also varies at a 69-kyr periodicity. All components that have maximum concentration during glacial intervals tend to have two- to three-fold higher sedimentation rates. Phytolith abundance is in phase with 6]80 whereas terrigenous percent leads 6180 by 5 kyr. Consequently, changes in aeolian supply precede changes in presumed high-latitude forcing or the migration of Sahelian grassland. It seems that carbonate dissolution has not been the dominant factor controlling the observed carbonate-terrigenous variation at this site (DeMenocal et al., 1993). Glacial-interglacial variations in the intensity of deep circulation could influence sediment redistribution and accumulation. But this is not apparently the case for Site 663 carbonate accumulation rate that varies with a dominant periodicity of 69-kyr precessional periodicities (Molfino and McIntyre, 1990). Various factors were suggested (DeMenocal et al., 1993): (1) glacial-interglacial changes in nutrient content of intermediate water can affect the nutrient content of the upwelled water (during glacial, intermediate water had lower nutrient levels); (2) the supply of aeolian dust can increase the nutrient uptake efficiency of phytoplankton. These records demonstrate the difference in phase between surface productivity and upwelling and indicate a possible cause of the strong covariance between the

terrigenous and opal records. This detailed study indicates that both high-and low-latitude patterns of African climate variability are recorded at Site 663. In the Southern Hemisphere, a long term climatic change 4.0 • 105 to 3.0 • 105 years ago is recorded in deep-sea sediments of the eastern Angola Basin (Congo Fan) by Jansen et al., (1989). This change is indicated by a distinct break in sediment accumulation rate and mineral composition (Fig. 3). The decrease in the rate of terrigenous accumulation, in the supply of high-crystalline-smectite, the disappearance of silts and micas, the cessation of large scale turbidite sedimentation reflects a long-term change from arid to more humid conditions in south equatorial Africa. At present, there are no land-based records to characterise this trend in Africa over the last 500 kyr. Off Gabon coastline, at some 500 km from the Congo River mouth, there is no clear evidence for such a warming (Bonifay and Giresse, 1992). A comparative study of the records from the Canary Basin was carried out. Between 3.7 • 105 and 3.1 • 105 years ago, tropical and sub-tropical foraminifer species became absent. This opposite trend is confirmed by comparison with various marine and continental records. Today the locations of extra tropical low-pressure centres in the Southern Hemisphere are associated with the position of the South Atlantic polar front and a stronger atmospheric circulation there favours zonal circulation in the Northern Hemisphere. During the early Brunhes, according to the same relation between atmosphere and ocean surface circulation in the Southern Hemisphere, circulation was probably more zonal in the Northern Hemisphere. During the early

Noncarbonate accumulation rate (g/cm2/lO3years) oi5 1;o

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5~176/ Fig. 3. Non-carbonate accumulation rates, mica abundance, and smectite crystallinity in the pelagic core T78-38 from the Congo Fan, F, C, and A indicate few, common, and abundant, respectively. (1) Dictyocha perlaevis perlaevis acme zone; (2) 23~ dating of 300,000 years ago; and (3) Stylatractus universus. Time interval of climatic change is shaded. The stages are calcium carbonate preservation stages; 37 radiocarbon dates from 14 cores from the fan prove that the stage boundaries fit well with oxygen isotope boundaries (after Jansen et al., 1989).

Sediment Accumulation Rates and Fluxes Brunhes, as during glacial stages, polar fronts were situated more to the north. The magnitude of the displacement in the Canary Basin and South Atlantic suggests that oceanic and climatic zones migrated toward the south over a few degrees of latitude after the early Brunhes. Middle and late Quaternary paleooceanography of the eastern Angola Basin was considered through carbonate accumulation on the Congo Fan (Jansen, 1990). The interglacial maxima of the Stages 1, 5.5, and 7.1 are distinguished by high carbonate accumulation. The peaks represent increased production of carbonate by coccolithophores and foraminifers and not due simply to better preservation. There is a special influence of the nutrients from the Congo River: the river plumes high carbonate production is attributed to more intense zooplankton activity in an area of normal oceanic phytoplankton productivity. Non-carbonate sedimentation rates are higher during glacials suggesting that the deposits of the continental shelf were an important source. However, at water depths below 4,000 m, the concentrations of carbonate at the interglacial maxima approximate glacial values. This is possibly caused by carbonate dissolution as the glacial CCD and lysocline lie at approximately 4,300-4,500m and 3,800-4,000 m. For the interval covering the last 225 kyr, accumulation of opal was measured by Jansen and Van der Gaast (1988) in a core from the same zone. The opal accumulation primarily reflects variations in biogenic opal production in the water column: maxima occurring in the cold stages 2, 5.1, and 5.4 and, in Stage 6. The concentration curves show an overall decrease downcore in opal to a depth of about 800 cm in the core. This decrease is estimated to about 50% of the present diffusive silica flux out of the sediment surface layer. This longterm loss, which is the same for most of the data, was calculated and applied to construct the curve of the original opal concentrations. A significant positive correlation was observed between the original opal concentration and the percentage of Cycladophora davisania a marker species for increased fertility. Both corrected and uncorrected opal concentration curves show the same distinct fluctuations. Dilution, thus, can be effectively excluded as an important cause for the major downcore variations in opal concentration. Quaternary sedimentation and paleoenvironmental studies off Southwest Africa indicate only the broad aspects of the fluxes trends and do not allow to determination of precise accumulation rates during glacial intervals and interglacial intervals (Embley and Morley, 1980). These rates range between 2-4 cm/103 years over the past 0.5 to 1 x 106 years for the slope and continental rise. A similar average rate of 3 cm/103 years was calculated from DSDP Site 362 taken at 19~ t, 10~ t E (Bolli et al., 1978). As most sediment accumulation from the continents and from biogenic sources was confined to the continental shelf, slope and rise, the rate of sedimentation decreases drastically below 4,800 m where rates are less than 0.1 cm/103 yr. In contrast to the Angola Margin, this Southwest Africa margin does not contain terrigenous turbidites: the only dynamic sedimentation processes active in this Basin during Quaternary have been slides and slumps and possibly sea-floor erosion by AABW (Embley and Morley, 1980).

217

3. Late Quaternary Mass Fluxes and Sediment Accumulation Rates This high-resolution analysis focuses the changing depositional regimes that characterise the distinct last glacial and interglacial phases. This general evolution is reflected in deep-sea deposition. Another focus is the comparison of these deep-sea deposits with shallower environments in order to document the changing style of terrigenous sedimentation or specific hydrologically induced processes.

3.1. General Deep-Sea Evolution In a large synthesis of global variations of surface ocean productivity during the last 21,000 years, Sarnthein and Winn (1989) emphasised that in almost all parts of lowand mid-latitude oceans, the restricted but crucial highproductivity zone acted in phase with a drastic decrease in new production during Termination I. As a general rule, this productivity reversal appeared to be controlled by a rapid reduction in the strength of meridional trade winds induced by a decrease in the extent of sea ice caused by a major increase in high-latitude insolation. A sediment core from the coastal upwelling zone off Northwest Africa exhibits sedimentation rate that is about 10 cm/1000 years; the shift in ~13Corg starts as exactly the same core depth as the 6180 shift associated with Termination 1 A, i.e., 15 cm or 1500 years above the onset of the paleoproductivity shift. On the basis of planktonic foraminiferal stratigraphy of late Quaternary off Mauritania and Senegal, Pflauman (1975) showed evidence that during cold periods bulk sedimentation rates were higher than during warmer periods, even if different rates were observed within the warm and cold intervals. Comparing the sedimentation rates of cores from 3,527 to 100m water depth and plotting them versus water depth and distance from the coast, it can be noted that there is a tendency for increasing sedimentation rates, especially during cold intervals, toward the coast. However, rough estimates of the accumulation rates of carbonate show no relationship to distance from the coast, the only observed trend being a decreasing sedimentation rate from north to south for both the X- and the Y-zones (Fig. 4). The deeper core (3,527 m water depth) indicates too low accumulation in the Y-zone. Thus, it is suggested that there was erosion by the turbidity current. The lowered sea level may have given rise to turbidity currents originating from the outer shelf and the uppermost slope when the coastal currents and waves were active erosion agents. Tropical Atlantic sediments were especially studied because high pelagic sedimentation rates in this area typically 3-5 cm/kyr allow high-resolution analyses (Mix and Ruddiman, 1985). During the last 30 kyr, the rate of sedimentation is essentially constant in the shallower cores with higher sedimentation rates. At sites where sedimentation rates are less than 3cm/kyr, there is a general rate decrease after 15 kyr B P. A study of tropical Atlantic fluxes since 25,000 yr BP was carried out by Ruddiman (1997). Twelve cores were

218

Tropical and Sub-Tropical West Africa 20~ 5 cm/1,000 a

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Fig. 4. Quaternary accumulation rates of some deep-sea cores off West Africa. Note the high amount of missing sediment in core 782, due to erosion (after Pflauman, 1975).

analysed for burial fluxes of terrigenous (mostly aeolian) sediment and biogenic (opal and CaCO3) sediment over the last 25,000 years (Fig. 5). Bulk sedimentation rates are relatively high, averaging 4 cm/103 yr and ranging from 2.3 cm/103 yr to 7.3 cm/1000 yr. Generally, increased terrigenous dust fluxes observed during the LGM are consistent with previous studies. This basic trend is coupled with that of opal and is opposite to an increase in CaCO3 from the last glacial to the Holocene. This trend is generally less evident or absent in cores along the south of the equator, where percentages fluctuate less (V29-144, PC24-16, V22-182, V30-41K). The flux changes of the three components show relatively little independence: because of the overall control by sedimentation rates, the fluxes of the three main components generally co-vary through time within each core, despite the widely differing percentages. Dust flux was a primary focus of this paper. The cores showing the best-developed dust flux maxima during the last glaciation lie preferentially along and just north of the equator, but there is no evident relationship between the timing of these maxima and the geographic location of the cores. In this study, two different kinds of aeolian influxes were considered: one with a "glacial" tempo, and the other with a "monsoonal" (or more accurately "post-monsoonal") tempo. The lack of significant dust flux maxima following the "post-monsoonal" Holocene arguing against simple changes in source-area humidity being a predominant

factor in aeolian dust transport and deposition. Various previous studies showed that aeolian fluxes are generally highest in semi-arid, rather than hyperarid climates (Pye, 1987; Rea, 1994). This suggests that changing wind transport is the best remaining explanation because, all of the cores with increased glacial aeolian fluxes are located beneath, or just south, of the present boreal African winter or in areas likely to be affected by winter trade winds along the coast of North Africa. As we have seen previously (Chapter 13), stronger winter highs over both the eastern Atlantic and West Central North Africa would have increased the strength of the surface trade winds along the coast (Rind, 1987, DeMenocal et al., 1993). Concerning increased burial of CaCO3 and opal, the analysed core sections suggest that neither productivity nor dissolution is a good explanation. Ruddiman considered that "although there is significant precessional power in percentage CaCO3 records from shallow-water tropical cores (Verardo and McIntyre, 1994), this period is less apparent in % opal signals from the same cores, and even less obvious in mass accumulation rates of these two biogenic components in deeper cores". Equally, an increase in dissolution of CaCO3 during glaciation was inferred due to the northward movement of a benthic water mass front that introduced highly corrosive AABW during glaciations, replacing less-corrosive NADW (Curry and Lohman, 1990). The increased burial of opal during glaciations appears problematic. This increased burial suggests that post-depositional redistribution of sediment is the strongest control on burial fluxes along the equator in the tropical Atlantic. The tempo of this sediment redistribution must integrate fluxes and large-scale redistribution and consequently is not exactly controlled by a "glacial" tempo. DeMenocal et al. (1993) showed that the main fluctuation of sediment (mainly CaCO3) burial occurred during a periodicity centred near 70 kyr rather than at glacial-interglacial periods of 100-kyr and 41-kyr. Two cores from the continental slope off Gabon (2,330m of water) and one core from the Mid-Guinean margin (4,000 m water depth) were analysed (Bonifay and Giresse, 1992). The resulting sedimentation rates for the two shallower cores are similar (3-8.5cm/103/yr) are roughly half of the rates measured at an equivalent water depth, on the mid-Congo Fan (Jansen et al., 1989). These rates remain constant through time, and do not show any significant acceleration such as observed off the river mouths or in the northern area of aeolian dust deposition. Sedimentation rates for a 4,000 m water depth core (2.1 to 3.6 cm/103/yr) are also lower than in cores from the Congo Fan at about the same water depth (Jansen et al., 1989). The cumulative curves of carbonate particle flux rates for the two shallower cores show several slight reduction which generally correspond to warm isotopic stages 5.0, 5.4, 7 for core KW 23, and 4, 5.0, 5.4 for core KW 24 while slight increases correspond to cool stages (5.2, 2 for KW 23 and 8 for core KW 24) (Fig. 6). These changes in carbonate accumulation do not appear to affect the total flux rate that is largely constant. The relative regularity of the curves reflects the large distance of the cores from the main river discharges and from areas of massive aeolian dust flux It should be noted that there are some quartz concentrations within various cold isotopic stages suggesting there was

Sediment Accumulation Rates and Fluxes V30-51 K Sed. rate (cm/kyr) 140 (-.... )

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& 15 10 ~

10 ~ < 20

s

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V29-144

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9 RC24-07

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e•

Sed. rate (cm/kyr)

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o

A

RC24-16

40 ~

RC13-189

30 ~ Sed. rate

20 ~ Fluxes

(cm/kyr)

(g/cm2/kyr)

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,

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Sed. Rate (cm/kyr) 14C (- .... )

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Fluxes (g/cm2/kyr) Opal ( m )

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(g/cm2/kyr)

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Opal(~)

U/Th ( ~ )

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Terrig. ( .... ) 0 1 I_a._l

0 Total ( q )

6

0

1

4 5

GAG03 (.... )

RC24-16 Sed. rate

~2~

',

V22-182 Sed. rate (cm/kyr)

25

..~ lO

Fluxes

(cm/kyr)

(g/cm2/kyr)

14C (..... ) Ufl'h ( ~ )

Opal ( ~ ) Terrig. (.... )

0 5 0 I I--'a'----'l :

_
15

0

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.

V30-40 3

(cm/kyr) 14 0 (- .... )

o

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;

1

25 0

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Total ( m ) ii

GAG~

(.... )

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9

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:

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12

0 ~

2o

5 r

25

CaCO3 (.... ) Fluxes (g/cm2/kyr)

U/Th ( - - ) 0

'..

20~.

4

140 (..... )

l't

i

20

15

Total ( ~ ) V25-59 Sed. rate (cm/kyr)

5

o~

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U/Th ( ~ ) 0 6 ~

i

i

Opal ( m ) Terrig. (.... )

15

<

Opal ( - - ) Terrig. (.... )

U/Th ( - - )

14C (..... ) U/Th ( ~ )

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oo

10 ~

V30-41

146 (..... )

9 Total ( m ) C a C O 3 (.... )

5

V22-177

10 ~

o

Fluxes (g/cm2/kyr)

v22-177

15

< Fluxes " (" g / c m 2 / K"y"

Sed. rate (cm/kyr)

20 r)

Opal ( ~ ) Terrig. (.... ) 0 1 u-,,.J

25" (,q

~ 0 4 Total ( - - ) C a C O 3 (----)

0

0

Fluxes (g/cm2/kyr)

14 C ( ..... )

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10

5 &15

"

20 :

" ........

,

:

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25

_. '

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lo

?


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20

25

25 (o~

0 4 Total ( ~ ) CaCO3 (.... )

~ 1~

0

6 Total ( - - )

1,~

#,~,f,,%

~'a~-'u3

(

)

Fig. 5. Mean sedimentation rates between datum levels (14C, (~180) based on 14C and U/Th dating and fluxes of total sediment, CaC03, opal and terrigenous fractions. Asterisks indicate position of datum levels (after Ruddiman, 1997).

220

Tropical and Sub-Tropical West Africa

10 -

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. . . .

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t

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--

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l

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fluxrates

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.

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flux rates

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.

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50

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0 •

Fig. 6. Carbonate, and total sediment accumulation rates of three cores located between the continental slope off Gabon and the Mid-Cameroon margin (after Bonifay and Giresse, 1992).

occasional dust supply from the Chad area funnelled through the Benoue Valley (see Chapter 13). In the southeastern Atlantic, taking into account the relatively low terrigenous supply, sediment input is believed to be the same at each site along the margin, while the only changing factor is the water depth (Bickert and Wefer, 1996). Variations through time are consequently interpreted as changes in deep water chemistry and/or circulation. However, if changes in the influx of terrigenous material and changes in the intensity of the upwelling have a significant influence on the carbonate dilution and dissolution, the precise nature of these interactions interpretation appears difficult or impossible to interpret.

3.2. Shallower Environments and Specific Processes The western equatorial Atlantic and especially the Amazon cone provide us with a basis of comparison for fluvial

impact on deep-water deposition (B6 et al., 1976). Terrigenous sediment accumulated continuously on the continental margin and abyssal basins throughout the last glaciation (4 to more than 25 cm/103 yr on the continental rise). Rivers discharged their sediment directly into the heads of submarine canyons where it could easily be transported to abyssal plains by gravity-controlled sediment flows. During postglacial sea level rise, the supply of terrigenous sediment was abruptly shut off. This break was marked by the formation of a thin 10-cm rustcoloured, iron-rich crust on most of the continental margin and abyssal plain. Comparison of the last glacial sedimentation rates with those of the Holocene clearly reveals that increased accumulation rates were prevalent during glacial. Late Quaternary climatic changes were deduced from deep-sea sedimentation off the Niger delta (core at 1,181-m water depth) (Pastouret et al., 1978). Sedimentation rates were constant, 30-35 cm/103 yr from 25,000 to 11,500 yr BP.

Sediment Accumulation Rates and Fluxes The beginning of the rain increase of the post-glacial phase is characterised by a strong rise of the flux occurring from 11,500 to 10,900 yr BP when the sedimentation increased to 640 cm/103 yr. Following this pulse, the Holocene sedimentation rate (31-cm/103 yr) was of the same magnitude as the mean Pleistocene rate. Precipitation during this period (11,500 to 10,900 yr BP) must have washed large amounts of terrigenous materials with relatively high concentrations of quartz into the Gulf of Guinea. High quartz concentrations are especially typical of the transition between oxygen isotope Stages 1 and 2 at the beginning of heavy precipitation in the southern Sahel zone. Highest quartz concentrations occurred just after the maximum dry period between 14,000 and 11,500 yr BP just before the sedimentation rate acceleration. Off the Congo River mouth, the occurrence of intense washing of soil formations was also recorded at the onset of the Holocene pluvial period (Giresse et al., 1982). Sedimentation rates of a set of gravity cores were measured based on distance (145-345 kin) from the river mouth. The mean sedimentation rate decreases in the most distal cores. Around 11,000 yr BP, an increased rate (160 cm/103 yr) was measured in the most proximal core, this maximum diminished rapidly in more distal cores (Fig. 7). The comparison between the Niger and Congo continental slopes emphasises the greater impact of fluvial washing action during a period of higher precipitation but when vegetation cover was still in the first stages of re-establishment. It is clear that the protective role of vegetation acted very gradually in the savanna landscape of the northern Niger River watershed, which is partly included in the southern Sahel zone. Today, the water discharge of the Niger River is five times lower than that of the Congo and the seasonal contrast is higher. With a more homogeneous distribution of precipitation and discharge through the year, the Congo River showed lower effects of erosion as a result of climatic change. It was generally found that the whole Atlantic was more productive during glacial intervals than during interglacials (Mix, 1989). This is particularly evident in the northwest African coastal upwelling-system where

5 ~ , ~ _ t ,

~'t~',114

I , , , ,

\ ^_ ~m,,

25

I , , , ,

0m/103Yr ~L~m/103_y

5

productivity also increased during the glacials (Sarnthein et al., 1988). However, some sites are an exception this generality. In their study of the Benguela upwelling system, Summerhayes et al., (1995) observed that during the last 70,000 yr, Stage 3 was the most productive period. Glacial Stages 4 and 2 were characterised by advection of organic matter from the shelf during lowstands and were less productive. They suggested that these productivity variations were induced by changes in wind direction. Specifically, easterlies were dominant during glacial times and were less favourable to upwelling processes than the southerly trade winds that prevailed during Stage 3. On the North-West African margin, Bertrand et al., (1996) documented large heterogeneities in organic carbon fluxes between two sites at two latitudes next to each other: off Cape Blanc (~21~ productivity increased during Stage 3 whereas off western Sahara (~25~ the expected glacial maximum was recorded. These authors suggested that these differences could be a result of counteracting influences of changes in wind stress and sea level change. Off Cape Blanc, another core offered evidence for enhanced productivity at the end of the glacial termination and not exactly during the last Glacial (Harris et al., 1996). The core 11 K off Cape Blanc (1,200 m water depth) has been used for a detailed reconstruction of past variations in local upwelling intensity and oceanic productivity (Martinez et al., 1999). The last 70 kyr show a mean sedimentation rate value of 10 cm/kyr. Glacial stages 4 and 2 have low Corg fluxes (0.05 g/cmZ/kyr) whereas the interglacial stages 1 and 3 have high mean values (~2 g/cmZ/kyr). Especially after 15,000 yr BP, Corg fluxes have continuously increased with a peak around 2,000 yr BP (Fig. 8). Planktonic carbonate flux maxima are observed between 14 and 5 kyr BP during the transition between stage 1 and 2 and between 45-50 and 55-60 kyr in stage 3 whereas minima occurred between 25 and 15 kyrBP and during the last 5 kyr. It was low during the last glacial. The high values recorded during the last 5,000 yr suggest an advective supply from the shelf. As biological productivity was

15 ,,,

221

I,

~ f,

I

35 f,,

L I , , , ,

I I

x l 03 years

, , ,

~'----="~-':" ~ - = :----':----:----:-':-----.-~.._z= - --._._-.~ :..~ ..._.7_ T80-7 [............

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, " ~ C 237

......

u" yr

I-. . . . . " " . . . . "T "8t0-10

Niger

15-m

Fig. 7. Compared sedimentation rates in core KW 31 (off Niger River delta) and various cores off the Congo River mouth (increasing distance from the mouth from C237 to BT4); 14C ages are plotted against sample depth (after Giresse et al., 1982).

Tropical and Sub-Tropical West Africa

222

oc (%) 1

C a C O 3 (%)

2

3

35

40

45

OC/CaCO 3

50

55

0.01

0.03

0.05

Mo/AI

0.07

1

2

3

Mo/AI

~

4

5

0 5 10 15 20 25 -"c>,, 30 "~ 35 <

2:,~p-..- n/oc

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%

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3

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t

olp

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0

O. 1 O C Flux

0.2

0.3

(g/cm2/kyr)

2

3

4

5

6

7

CaCO 3 Flux (g/cm2/kyr)

0

50

1O0

150

I/OC

Fig. 8. Records of organic carbon (OC) and calcium carbonate (CaCO~) contents in % and fluxes in g/cm2/kyr (thick curves), as well as OC to CaC03 flux ratio, molybdenum to aluminium (Mo/Al; ppm/%) and iodine to organic carbon (I/OC; ppm/%). The fact that linear sedimentation rates do not present great variations explains why contents and fluxes curves (OC and CaCO~) are rather similar. Shaded areas indicate glacial isotope Stages 2 and 4 (after Martinez et al., 1999).

not recorded in the sediments, these high values are attributed to refractory organic matter redistributed after winnowing. The ratio of molybdenum to aluminium is well-correlated with the Corg fluxes except for the last 5,000 yr confirming the lack of oxygen consumption during this time. The ratio of iodine to organic carbon indicates the degree of bottom water oxygenation: it shows a decrease during Stage 3 and at the boundary between Stage 2 and 1 suggesting oxygen deficiency when organic carbon supply increased. The authors suggest an explanation related to variability in monsoon pressure intensity. During the last glacial, the winter wind system did not allow nutrient-rich intermediate waters to upwell, leading to low productivity. During Stage 3 and the transition Stage 2-Stage 1, spring-summer conditions were prevalent: maritime trade winds were controlled by enhanced monsoon pressure in response to the northward shift of the ITCZ.

3.3. Controlling Factor for Particulate Fluxes of Terri genous Non-Carbonate Material Analytical data were studied to evaluate the various effects of terrigenous fluxes and sedimentation rates linked to the last eustatic and climatic oscillations (Giresse and Barusseau (1989). All the cores collected below 2,100 m, and most of those above extended through both foraminiferal biozone Z and Y, and thus allow a statistical approach to the study of average sedimentation rates. By comparing these last two Ericson biozones, Z and Y, an increase by a factor of 2 or 5 is shown during Z on most shelves and slopes. This ratio tends to lessen, to

equalise or even to reverse with the depth. The Z/Y ratio slowly decreases off Mauritania, off Senegal and, to a lesser extent, in the Trou-sans-fond Canyon (Ivory Coast) and off Gabon. A reverse ratio is observed off Morocco (Z/Y - 1/3) because of the aeolian fluxes during Y (see Chapter 13). So, it stands to reason that in the intertropical zone, the accumulation rate during biozone Z during which the continent received maximum precipitation, was 2 or 3 times higher than in Y, which corresponds to a fairly arid terrestrial interval, coinciding with a large reduction in the extent of the great ombrophilous forest of the area (Maley, 1987). Study of evolving terrigenous sedimentation rates based on isotopic data and ~4C dates allow high-resolution definition of three main stages of the most recent period of major climatic change. (1) From 21,000 to 16,000 yr BP, a relatively stable low sea level phase centred at about 110-130 m; (2) From 16,000 to 6,000 yr BP, an episode of active eustacy; (3) From 6,000 yr to present, a relatively stable high sea level, very close to the present-day mean sea level. Taken as a whole, the data for the Northwest African margin allow a comparison between the main physiographic units. Sedimentation rates are always higher during active eustatic changes than at lowstands on the slope as well as on the abyssal plain. The comparison, as a function of latitude, shows intensification of Holocene precipitation close to the intertropical zone only has the effect of increasing sedimentation rates (Fig. 9a). Gravitational phenomena sometimes accelerated either during lowstand or during sea level change. However, in both phases, these processes depend on topographic conditions and the nature of the submarine valleys. On the south Moroccan margin (25-27~ the

Sediment Accumulation Rates and Fluxes

223

(a) 9 Low sea-level ,, Sea-level change i High sea-level

g/cm2/10 3 yrs

10 (c)

g/cm2/10 3 yrs

"••\

30 "

Ii

II

'"

i

,

'

25 ~

9

'

'

'I

....

20 ~

J

I

15 o 7 ~

CM

I '

N

I

"

|

30

\ t

g/cm2/10 3 yrs

p.

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I

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\

/

I'

"

'

6-16 kyr O KW 13 (1,181 m) Nigeria

"=

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\

/

'

1 '

\

30

!

'

16-6 kyr Sea-level change

\

0

0-6 kyr

'

1

19 ~

(b)

30

21-16 kyr Low sea-level

'

,

'1'

16-21 kyr

,A" Core nb 13 (842 m) Ivory Coast

\

sh

>10

.,

10-4

, ,-..

4-2

,--'Tm~~

2-1

1-0.5

sh: Shelf, CM: Congo Margin, GM: Garbon Margin

Fig. 9. (a) Relationship between sedimentation rate during the three main stages of the last glacial-interglacial sea level oscillations on the Northwest African margin; (b) Same relationship on the margin of the Gulf of Guinea; (c) Comparison between sedimentation rates and water depths on the Gabon and Congo margins. Physiographic zones were plotted according to their depositional rate areas (after Giresse and Barusseau, 1989).

sedimentation rate on the slope, which is fairly low, rises to its highest value during the period of lowest sea level. Then, it decreases slightly during transgression and reaches its present low level at the highest sea level stand. Farther south, at 23~ the higher rates are of the same order of magnitude during lowstands as during active eustatic change. Similarly, on the Mauritanian slope (19~176 sedimentation slowed during high sea level while the rates during the previous lowstand and highstand were fairly similar. During active eustatic change, they were perhaps slightly higher. Off Senegal (15~ the top of the slope (945m water depth) also shows depositional rates, which were higher during low sea levels, and highest during active sea level change. There is also a relationship between sedimentation rate and bathymetry (right of Fig. 9a): lower slope rates are almost two times lower than those at the top of the slope, and compared to highstands they are twice as high during lowstands and times of active sea level change.

The same comparison was applied to the Gulf of Guinea (Fig. 9b). The Trou-sans-fond Canyon (Ivory Coast), which receives no runoff from the continent shows it highest sedimentation rate during lowstands, decreasing markedly during transgressions and reaching a minimum at highstands. On the slope off the Niger delta (-1,181 m water depth), the pattern is similar to that of the northwestern margin with strong acceleration in sedimentation during the transgression, reflected in a significant increase around l l,000yrBP (Pastouret et al., 1978). At the highstand, the noticeable reduction in sedimentation can be considered not only a function of the new eustatic conditions, and the resulting physical processes but also of a relative fall in precipitation since about 3,000 yr BP (see previous Chapter C9). Based on the number of cores, the Gabon and Congo margins are the best-studied regions; with respect to deposition rates. Both margins were divided in various bathymetric sub-regions that allowed a comparison in terms of the three reference periods (Fig. 9c). The Congo

224

Tropicaland Sub-Tropical West Africa

continental shelf has greater sedimentation than off Gabon, a result of the proximity of the mouth of the Congo River. The rate was high during active eustacy as well as during lowstands. On the slope (zones > 10 and 104 g/cmZ/yr3) the maximum accumulation rate always coincided with he period of active eustacy in which the increasing role of the Congo Canyon in the distribution of the solid load accounts for a rate on the margin that is higher than on the Gabon margin. At greater water depths (zones 4-2, 2-1, and 1-0.5 g/cmZ/yr3), with longer

transport distances, the differences are far smaller. In the two regions, the rates during highstand remain still lower than during the two previous eustatic phases. At lowstand, the fairly high rates, in part associated with the prevalence of gravitational phenomena are nearly similar on the two margins although the inputs of the Congo River were carried out to sea more easily and spread more evenly on the adjacent slopes. Consequently, the slightly faster sedimentation from the Congo River was carried out to sea more easily and spread evenly on the adjacent slope.