Tertiary carbonate-dissolution cycles on the Sierra Leone Rise, eastern equatorial Atlantic Ocean

Marine Geology, 39 (1981) 81--101 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

81

TERTIARY CARBONATE-DISSOLUTION CYCLES ON THE SIERRA LEONE RISE, EASTERN EQUATORIAL ATLANTIC OCEAN

WALTER E DEAN ~, JAMES V GARDNER 2 and PAVEL CEPEK ~

'U.S. Geologzcal Survey, Denver, Colo 80225 (U.S.A.) :U_S. Geological Survey, Menlo Park, Calif 94025 (U.S A ) ~Bundesanstalt fur Geow~ssenschaften und Rohstoffe, Hannover (F R.G ) (Received July 24, 1979;revised version accepted February 1, 1980)

ABSTRACT Dean, W.E , Gardner, J.V. and Cepek, P., 1981. Tertiary carbonate-dissolution cycles on the Sierra Leone Rise, eastern equatorial Atlantic Ocean. Mar Geol, 39: 81--101. Most of the Tertiary section on Sierra Leone Rise off northwest Africa consists of chalk, marl, and limestone that show cychc alterations of clay-rmh and clay-poor beds about 20--60 cm thick. On the basis of bIostratigraphm accumulation rates, the cycles in Oligocene and Miocene chalk have periods which average about 44,000 years, and those in Eocene siliceous limestone have periods of 4000--27,000 years Several sections were sampled m detail to further define the cycles in terms of content of CaCO3, clay minerals, and relative abundances of calcareous nannofosslls Extending Information gamed by analyses of Pleistocene cores from the continental margin of northwest Africa to the Tertiary cycles on Sierra Leone Rise, both dilution by noncarbonate material and dissolution of CaCO~ could have contributed to the observed relative varlahons in clay and CaCO 3 However, dissolution of CaCO 3 as the main cause of the carbonate-clay cycles on the Sierra Leone Rise, rather than dilution by clay, IS suggested by the large amount of change (several thousand percent) in terrigenous influx required to produce the observed variations in amount of clay and by the marked increase in abundance of dissolutionresistant discoasters relative to more easily dissolved coccoliths in low-carbonate parts of cycles. The mare cause of dmsolutlon of CaCO~ was shoaling of the carbonate compensation depth (CCD) during the early Neogene and climatmally induced fluctuations in the thickness of Antarctic Bottom Water

INTRODUCTION A c o n t m u o u s C e n o z o i c record consisting of 850 m of pelagic c a r b o n a t e s e d t m e n t w i t h o u t h i a t u s e s w a s c o r e d a t D e e p S e a D r i l l i n g P r o j e c t Site 3 6 6 o n t h e e a s t e r n e d g e o f t h e S m r r a L e o n e Rise o f f n o r t h w e s t A f r i c a ( F i g . l ) ( L a n c e l o t , S e i b o l d , e t al., 1 9 7 7 ) . T h e S i e r r a L e o n e Rise a p p e a r s t o h a v e b e e n a p r o m i n e n t t o p o g r a p h i c rise t h r o u g h o u t m o s t o f its h i s t o r y . T h e C a p e V e r d e Rise o r i g i n a t e d a b o u t 8 0 m . y . ago as a 2 - k m - t h i c k a c c u m u l a t i o n o f v o l c a n i c e x t r u s i v e s t h a t w a s s p l i t i n t o two segments by spreading along the Mid-Atlantic Ridge (Kumar and E m b l e y , 1 9 7 7 ) . T h e w e s t e r n s e g m e n t f o r m e d t h e Cearfi Rise w h i c h is b u r i e d 0025-3227/81/0000--0000/$02 50 © 1981 Elsevmr Scientific Publishing Company

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under several kilometers of sediment from the Amazon Cone. The eastern segment f o r m e d the Cape Verde Rise which now stands 1500--2500 m above the surrounding abyssal plato and is bounded on all sides by relatively steep slopes. The completeness of the section, together with the abundance of siliceous and calcareous planktonic fossils and the relative Isolation of the site from terrigenous influences, makes this site an ideal reference section for the low-latitude Cenozoic record in the Atlantic• The section at Site 366 consists of nannofossil ooze, grading downward into chalk and marl, and then into hmestone and marlstone. The middle and lower Eo cen e sections contam a bunda nt chert and porcellanite characteristm of the Eocene reflector Horizon " A " in the Atlantm. Most of the chalk, marl, and limestone show cyclic alternations of clay-rich and clay-poor calcareous beds in cycle thmknesses between 20 and 70 cm. Concentrations of CaCO3 are generally a b o u t 80% in the clay-poor portions of cycles and 60--70% in

83 the "clay-rich" portions, although individual samples may vary from greater than 90% to less than 20%. The average of 121 analyses of CaCO3 for the sectmn above the beginning of sihca dlagenesis (core 16; 480 m) is 74%. More detailed descriptmns of the lithologies are m Dean et al. (1977). ANALYTICAL RESULTS Core sections from Site 366 on the Smrra Leone Rise were sampled at 10-cm intervals through several well-developed cycles of chalk and marl of early Miocene age that show extreme variations m a m o u n t of CaCO~ (core 23A, sections 1 and 2; 205.5--208.5 m). The samples were split for analyses of CaCO3, clay mineralogy, and quantitative and qualitative variations in nannofossll content. Fig.2 shows that there are no qualitative differences in carbonate-free clay mineralogy between high-carbonate and low-carbonate portions of the cycles. quartz and kaolinite are the dominant crystalline materials and high but variable amounts of X-ray amorphous material are present. The mare variability appears to be in the degree of crystaUinity of the clay-size material. In general, the high-carbonate portions of the cycles contain the poorest crystallme material, as indmated by the lower intensities of the quartz peak at 26.5 ° 28 and the (020), (110), and (111) kaolinite peaks around 20 ° 20, and the high X-ray amorphous " h u m p " between 16 ° and 34 ° 20. The most crystalhne clay-size material (greatest peak mtensltms, lowest amorphous " h u m p " ) occurs in the low-carbonate portions of the cycles. Variations in total numbers of calcareous nannofossils withm a standardized sample and percentage of dissolution-resistant dlscoasters relative to total nannofossils are plotted along with percent CaCO3 m Fig.3. A positive but poor correlatmn (r = 0.45; n = 8) exists between a m o u n t of CaCO3 and numbers of nannofossils, and a good negative correlation (r = --0.81; n = 27) between percent CaCO3 and relative abundance of discoasters. Fig.4. shows further that coccoliths m the low-carbonate portions of cycles are more corroded than those in the high-carbonate portions. This means that with a decrease in relative abundance of CaCO3, there is a marked reduction in the total nannofossfl content; further, the nannofossils that are present are characterized by a relative decrease in abundance and degree of preservation of the more easily dissolved coccoliths and a relative increase in the more dissolutmn-reslstant dlscoasters. ORIGIN OF THE CARBONATE-CLAYCYCLES In this section we revmw the recent literature on the origin of late Pleistocene cyclic sedunentation with the objective that possibly the same logic and conclusions can be applied to cycles m the Tertiary section on the Sierra Leone Rise.

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Calculation o f amount of dzssolutzon and dUutzon Possible causes for eychc variation m relative abundance of CaCO3 include: (1) variation in rate of production of CaCO~, largely by calcareous plankton; (2) variation m amount of dissolution of CaCO3 at depth; and (3) variation m non-carbonate mput (dilution). For the moment, we will assume that production of CaCO3 remains constant and examine the evidence for carbonate dissolution and noncarbonate dilution. The greatest variations m carbonate content observed m the carbonate--clay

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Fig.3. Curves o f % CaCO3, % organic-carbon, % dlscoasters, and histograms o f total nannofosslls, Site 3 6 6 , core 2 3 A , s e c t i o n s 1 and 2. L o c a t i o n s o f samples for s c a n m n g electron micrographs in Fig.4 are indicated b y arrows to the left o f the curve o f % CaCO3.

cycles on the Sierra Leone Rise occur in core 23A, sections 1 and 2 (205.5-208.5 m; Figs.3 and 4). If we assume that the maximum value of 85% CaCO3 represents a baseline for normal pelagic rain on the Sierra Leone Rise during the early Miocene, and if we assume further that the amount of CaCO3 produced in the surface waters does not change, then we can calculate the amount of CaCO3 that would have to be dissolved or the amount of clay that would have to be added to produce CaCO3 values less than 85%. For dissolution only, the percentage of initial CaCO3 that would have to be dissolved to give the observed percentage of CaCO3 is gwen by the followmg equation (Berger, 1971; Gardner, 1975): % loss (L) = [(1 -- ( Y l / N f ) × 100]/Ci,

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where N 1 and Nf are the initial and final noncarbonate fractions, and C1 is the initial carbonate fraction. For dilution with clay only, the percentage of the initial clay content that would have to be added to dilute the initial carbonate percentage to the observed percentage ts given by the following equation (Gardner, 1975): % addition (A = [ [ ( C , / C f ) - - 1 ] / N , ] X 100

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86

where C. and Ni, as in the dissolution-loss equation, are the initial carbonate and noncarbonate fractions, and C~ is the final (observed) carbonate fraction. Curves for L and A for the range 1--100% final (observed) CaCO~ content,

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87 assuming an initial c o n t e n t of 85% CaCO3 and 15% clay, are shown m Fig.5. As an example, to decrease the pelagic carbonate c o n t e n t from 85% to 50%, either 82% of the initial CaCO3 must be dissolved, or 470% more clay must be added by dilution. The percentages of CaCO3 pl ot t ed in Figs.2, 3, and 4 were d e t e r m m e d by analyses o f I cm think samples collected every 10 cm. Assuming that the c o n t e n t o f CaCO3 remains constant over each 10-cm interval, the CaCO3 curve m Fig.3 can be converted to a CaCO3 percentage histogram (Fig.6, A). Carbonate-dissolution (L) and clay dilution (A) percentages were then determined for each 10-cm interval, using the curves m Fig.5, and plotted as histograms in Fig.6, B and 6, D. Notice t ha t bot h the A- and L-scales are hnear, b u t th at the L-scale ranges from 1--100% and the A-scale ranges from 0--9000%. In order to obtain the low carbonate percentages m the clay-rich portions o f cycles, a large percentage of the initial CaCO3 must be dissolved if only dissolutmn is assumed, but a t r em e nd ous increase in clay is required if we assume only dflutmn. For example, to get 15% CaCO3 in the final product, 97% of the pelagic CaCO3 rain would have to be dissolved, or the clay m p u t would have to increase by over 3000% (1.e., by a factor of more than 30).

Mechamsms for dissolution and dilutton The zone within the water column having the greatest rate of change i n carbonate dissolution is defined as the lysocline (Berger, 1968, 1970; L1 et al., 1969; Heath and Culberson, 1970). The top of the lysocline, particularly in

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89 the Atlantm, is correlated with the top of Antarctic B o t t o m Water (AABW) (e.g., Berger, 1970), and the sharpness of the lysocline is t h o u g h t to be a function of the strength of bottom-water supply. The top of the lysochne is presently at about 4250 m water depth m the western equatorial Atlantic, and at about 4500 m water depth m the eastern equatorial Atlantic (Berger, 1970; Gardner, 1975). The difference m depth correlates with the top of AABW, which is shallower on the western side of the Mid-Atlantic Ridge than on the eastern side. The CaCO3 content of the sediments withm the lysocline can decrease drasticall~ within a few hundred meters water depth, and eventually a depth is reached at which no CaCO3 is preserved. This depth IS called the carbonate compensation depth (or more correctly, the calcite compensation depth, CCD). Therefore, if a portion of the sea floor is within the lysoclme, the CaCO3 content of the sediment whmh has accumulated at that site could be greatly changed simply by raising or lowering the lysocline several hundred meters. The vertical changes in the position of the lysochne required to produce these changes would depend upon the steepness of the lysoclme. Temporal variation of several hundred meters in depth of the lysochne are well documented (e.g., Hsti and Andrews, 1970; Hay, 1970; Berger and Von Rad, 1972; Kennett and Huddleston, 1972; Ramsay, 1974; Van Andel and Moore, 1974; Damuth, 1975; Diester-Haass, 1975, Luz and Shackleton, 1975, Gardner, 1975; Van Andel et al., 1975). Dilution by noncalcareous material can be accomphshed by wlndborne material, a nepheloid layer within the bottom water, or river-derived material rejected into the water column at different depths. Data on the quantitative load carried by a nepheloid layer are n o t generally known, although Blscaye and Eittreim (1977) suggested a flux on the order of 100 ug/1. Nepheloid layers exist along the West-African continental margin, but the Sierra Leone Rise is above and west of any reported layers. Presumably, any increase in terrestrial runoff would increase the density of the nepheloid layer. Increase in terrestrial runoff would also increase the amount of river-derived material that would then be transported away from shore by currents. However, we fred it difficult to ~magine clay-transporting b o t t o m currents, even turbidity currents, capable of flowing across the abyssal passage separating the Sierra Leone Rise from Africa and then up the steep sides of the rise for more than 1500 m. Some current-transported clay may be thrown into suspension at a sufficient elevatmn above the sea floor to be incorporated as part of the pelagic rain on top of the Sierra Leone Rise, but this contribution would be minor. The importance of windblown, Sahara-derived dust as a contributor to pelagic clay in the North Atlantic is well known (e.g., Delany et al., 1967; Folger et al., 1967; Folger, 1970; Carlson and Prospero, 1972; Chester, 1972; Parkin and Shackleton, 1973; Parmenter and Folger, 1974). This is particularly true immediately offshore of west Africa between about 5 ° and 20°N latitude in the shadow of the Sahara-derived Harmatton winds. Ruddiman (1971) and Parkin and Shackleton (1973) suggested that increases m Saharan windblown dust and (or) distal turbidites are the mare reasons for lower

90 concentration of CaCO3 in sediments deposited during Pleistocene glacial periods in cores from the equatorial Atlantic. This conclusion is based on the observation that the percentage of CaCO3 in cores decreased b u t absolute sedimentation rates increased during glacial periods. Dmster-Haass (1976) concluded that increases in amounts of terngenous material and decreases in amounts of CaCO3 during glacial periods m cores collected off the coast of west Africa south of 20°N latitude were the result of increased aridity and wind strength during glacial periods and hence a greater supply of aeolian material from the contment. Additional evidence for aridity on the tropical African continent and increased strength of trade winds during the Wi]rm glacial period, relative to the Holocene, was presented by Parmenter and Folger (1974}, Sarnthein and Walger (1974), Bowles (1975), Sarnthem and Diester-Haass (1977), Dlester-Haass and Chamley (1978), and Sarnthem (1978). Data of Sarnthein and Walger (1974) indicated that the a m o u n t of aeohan material transported to the Atlantic durmg the Wfirm glacial period was as much as 5 times greater than dunng the Holocene. Hays and Perruzza (1972) concluded that windblown dust is the main cause of Pleistocene clay--carbonate cycles that they observed in cores collected within the heart of the Sahara wind shadow just offshore from Dakar. Their cores are from fairly shallow depths (about 3200 and 3700 m) but contain very little CaCO3 (average of a b o u t 15%). Cores from nearby areas at the same approximate depths, b u t outside the main Sahara wind shadow area, contain averages of a b o u t 70% CaCO3 (Gardner, 1975). Accumulation rates of noncarbonate material, calculated from data of Hays and Perruzza (1972) and Gardner (1975), are on the order of 5 cm/103 years for the wind shadow (Dakar) cores and 0.5--1.5 cm/103 years for non-windshadow cores from comparable depths (Sierra Leone Rise and flank of the Mid-Atlantic Ridge). It therefore appears that the Dakar cores do have higher clastic input, presumably from windblown dust. Although Hays and Perruzza (1972) concluded that CaCO3 input was constant, Gardner (1975) has shown that at least one of the Dakar cores shows marked variations in amount of CaCO3 dissolution, based on ratios of planktonic to benthonic foraminifera, and that dissolution was greatest during glacial periods. The concentrations of CaCO3 m 21 cores from the eastern equatorial Atlantic, including the two Dakar cores studied by Hays and Perruzza (1972), indicate that both dissolution and dilution contributed to variations in proportions of CaCO3 and clay (Gardner, 1975). Extending the above reformation gained from analyses of Pleistocene cores to the Eocene to Mmcene cycles on the Sierra Leone Rise, both dilution by noncarbonate material (mainly by aeolian transport, possibly reded by nepheloid and current transport) and dissolution of CaCO3 could have contributed to the observed relative variations in CaCO3 and clay. Because of the magnitude of some of the CaCO3 fluctuations and the relative magnitudes of the dissolution and dilutmn processes required (Fig.6), we favor dissolution over dilution as the dominant process forming the cycles at Site 366. Our best indmators of dissolution as the main cause of the carbonate--clay

91

cycles on the Sierra Leone Rise are the quantitative and qualitatwe changes in the calcareous microfossils. Dissolution of these organisms would be particularly sensitive to climatic changes if the top of the Sierra Leone Rise were near the lysocline at the time the sediments were accumulating. The top of the present calcite lysochne in the eastern equatorial Atlantic is approximately 4500 m, with the CCD about 500 m deeper. During the Miocene, however, the CCD, and presumably the lysocline, was as much as 1500 m shallower (Fig.7), which would probably place the top of the Sierra Leone Rise below the top of the lysocline and therefore subject to variations in dissolution of CaCO3. A marked change m abundance and degree of preservation of calcareous microfossils occurs in the upper Miocene part of the core (core 13A, 100 m) at DSDP Site 366. Coccoliths are common from the Paleocene through the upper Miocene, but preservation is poor to moderate (Fig.8, and DiesterHaass, 1977), from the upper Miocene upward, coccoliths are abundant and well preserved. These changes in abundance and degree of preservation suggest that the Sierra Leone Rise has been above the lysocline only since the late Mmcene. Foraminifera, when present, are found in the carbonate portions of the cycles, and are rare or absent m marl and clay. Diester-Haass (1977) found that the degree of fragmentation of foraminifera and the proportions of ben-

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Fig.8. Histograms of relative degrees of preservation of foraminifera and nannofossds, Site 366, based on shipboard estlrnates from core catcher samples_ P = poor; M = moderate, G = good. t h o m c foraminifera relative to p l a n k t o n i c foraminifera increase in the lower M i o c e n e section at D S D P Site 3 6 6 . She c o n c l u d e d that these changes in degree o f preservation o f foraminifera are the result o f greater dissolution as the CCD shoaled durmg the early Miocene. The degree o f fragmentation and p r o p o r t i o n o f b e n t h o n i c foraminifera then decrease f r o m middle M i o c e n e through the Pliocene and Pleistocene, presumably in response to a deepening CCD. Strnilar variations in CCD are interpreted b y M c C o y and Zmamerman ( 1 9 7 7 ) o n t h e basis o f relative abundances o f foraminifera and bloslliceous material.

93 Figs.3 and 4 show that the absolute number of nannofossils Is markedly reduced in the low-carbonate portions of cycles and that the coccoliths generally show signs of corrosion. What is perhaps more sigmfmant is the variation m abundance of discoasters, which are more resistant to dissolution than the coccoliths (Bukry, 1971). The discoasters in the high-carbonate parts of cycles are generally less than 5% of total nannofossils (Fig.3). However, with decreasmg CaCO~ content, relative abundance of discoasters increases to as much as 30% m the low-carbonate parts of the cycles, presumably due to selective removal of the more soluble coccohths. The extreme values m Fig.3 indmate that the undissolved carbonate contains about 1% discoasters and that the most dissolved carbonate contains about 30% discoasters. Percentages of dissolved nannofossils can be calculated by using an initial value of 1% dmcoasters m undissolved carbonate and final percentages of discoasters from Fig.3 mstead of the a m o u n t of noncarbonate material in eq.1. Results of these calculations for core 23A, sections 1 and 2, are plotted as a percentage histogram in Fig.6, C. For the extreme of 30% discoasters, Fig.6, C shows that 98% of the nannofossfls (presumably the more easily dissolved coccoliths) were dissolved. This value Is the same as that calculated for a m o u n t of dissolution of bulk carbonate (Fig.6, B) and further suggests that dissolution was the d o m i n a n t process that formed the observed carbonate-clay cycles. The same dissolution pattern holds true in the more slhceous Eocene cycles. For example, m the green-chalk--cherty-chalk cycles of middle Eocene age, nannofossils are abundant and well preserved, with coccollths much more abundant than discoasters in higher-carbonate (ca. 90% CaCO3) cherty portions of the cycles. However, in the higher-clay, green-chalk portmns of the cycles (ca. 80% CaCO3), nannofossils are rare, and the more resistant discoasters are m greater abundance than coccohths which are corroded and fragmented. Dissolution of CaCO3 has often been assumed to take place in the water column. However, recent work on the transport of nannoplankton in fecal pellets of zooplankton suggests that m u c h of the CaCO3 is transported to the b o t t o m faster than formerly suspected, and that much of the dissolution may take place on the sea floor (e.g., Honjo, 1975, 1976; Roth et al., 1975). Regardless of where the dissolution occurs, the result would be the same: decrease in total numbers of nannofossils and relative enrichment in discoasters. Until now, we have assumed that productivity of calcareous plankton remained more or less constant; however, variations in abundance of CaCO3 could be due to changes m rates of production of calcareous plankton. For example, Prell and Hays (1976) found that portmns of cores from the Colombm Basin containing low contents of CaCO3 and the coarse fractmn (> 62 pm) correlate with the presence of a low-productivity Sargasso assemblage of planktonic foraminifera. High contents of CaCO3 and the coarse fraction correlate with the presence of a high-productivity equatorial foraminiferal assemblage. These relationships suggest that the CaCO3 and coarse-

94 fraction curves, at least for Caribbean cores, reflect productivity of foraminifera in the surface water. Adelseck and Anderson (1978) showed that lowCaCO3 portions of shallow cores from the equatorial Pacific do not exhibit any decreases in degree of preservation of calcareous microfossils or increases in dilutmn by noncarbonate material. They concluded, therefore, that decreases in a m o u n t of CaCO3 were the result of decreases m productivity of calcareous plankton. However, the cores studied by Prell and Hays (1976) and Adelseck and Anderson (1978) are from above the lysocline. Cores collected from below the lysocline in the equatorial Pacific usually exhibit marked changes in the degree of preservation of calcareous mmrofossfls that correlate with changes in the a m o u n t of CaCO3 (e.g., Berger, 1973; Thompson and Saito, 1974). The relative abundances of coccohths and discoasters in the carbonate--clay cycles on the Sierra Leone Rise show exact opposite fluctuations. If these fluctuations were the result of variations m productivity, then opposite variations in productivity of discoasters and coccoliths would be required. This is unhkely because both organisms respond similarly to environmental changes. This leaves dissolutmn of CaCO3 as the only process that could have produced the observed cyclic variations in abundances of nannofossfls and as the most probable process responsible for the observed cyclic variatmns in CaCO3 and clay. Cyclic fluctuations in rate of supply of terrigenous material m a y well have occurred, and to the extent that these fluctuations were in phase with cychc fluctuations in dissolution of CaCO3, the two processes could have enhanced differences m the relabve abundances of CaCO3 and clay. PALEOENVIRONMENTAL IMPLICATIONSFOR THE SIERRA LEONE RISE If dissolution of CaCO3 was an active process in the formation of the carbonate--clay cycles of Eocene to Miocene age on the Sierra Leone Rise, as our data suggest, then the cause of dissolution still remains to be accounted for. Agam we borrow from Pleistocene dissolution-cycle analogues, using the assumption that the climatic variations that resulted in glacial--interglacial cycles in the Pleistocene were present at least through the remainder of the Neogene and probably back into the Paleogene. It has been suggested that the cause of late Pleistocene fluctuations of the CCD m the equatorial Atlantic is the changing volume and (or) strength of c~rculation of AABW (Berger, 1973; Damuth, 1975; Gardner, 1975; B~, et al., 1976; Gardner and Hays, 1976; Diester-Haass and Chamley, 1978). The CCD and presumably the lysocline were much shallower during the earlier part of the Neogene than at present (Fig.7), and so any climatmally reduced fluctuations in activity of AABW would result in fluctuations in the rate of dissolution at shallower depths than at present. We therefore believe that the combined effects of shoaling of the CCD (and lysocline) during the early Neogene and of climatmally induced fluctuations in the position of the top of AABW (and therefore the position of the lysocline) were the main causes of the cyclic variations in CaCO3 and clay observed on the Sierra Leone Rise.

95 Dilution by terrigenous material, particularly windblown Saharan dust, cannot be ruled o u t and m a y have combined with dissolution to enhance the carbonate--clay cycles, as suggested for the Pleistocene cycles of CaCO~ and clay in the same region (Gardner, 1975). TIMING OF THE DISSOLUTION CYCLES The timing of the Oligocene and Miocene CaCO~-clay cycles on the Sierra Leone Rise also supports the Pleistocene analogue of episodes of climatically induced dissolution of CaCO3. The timing of the Eocene cycles, however, is more questionable. The cycles of Oligocene to early Miocene age have periods, based on average accumulation rates between foraminiferal biostratlgraphic zones, which range from 27,000 to 67,000 years and average a b o u t 44,000 years (Table I). The middle to late Eocene cycles range from 15,000 to 27,000 years and average a b o u t 19,000 years; the early to middle Eocene cycles range from 4000 to 14,000 years and average about 7000 years. Three separate and distinct periodicities seem to occur in these sections. Climatic changes related to the earth's orbital cycles of precession, obliquity, and eccentricity (with periods of a b o u t 23,000, 41,000, and 100,000 years, respectively) have been suggested as the main driving forces behmd long-term climatic change (e.g., Mllankovitch, 1930; Anderson, 1964; Broecker and Van Donk, 1970; Matthews, 1974; Hays et al., 1976). Matthews (1974) referred to these cycles as the "tuning forks" of geologic time, and Hays et al. (1976) went so far as to state that the sedimentary records of these cycles provide more accurate dating than present radiometric dating techniques, at least for the past 400,000 years. Other investigators (e.g., Emiliani, 1978, and Kominz and Pislas, 1979), however, have cautioned that although there appears to be a relationship between the earth's orbital cycles (especially those of precession and obliquity) and cycles of sedimentary climatic indicators, a large c o m p o n e n t of the sedimentary record of climatic variation is due to other factors that are more random. The relationships between orbital cycles and cycles of sedimentary climatic indicators suggested by Matthews (1974), Hays et al. (1976) and others imply periodicities that are more regular than are actually observed. In addition, different climatic indicators may have different periodicities. For example, Morley and Hays (1978) found periods of a b o u t 150,000, 30,000, and 19,000 years for estimates of sea-surface temperature, pemods of about 20,000, 15,000, and 12,000 years for relative abundance of a radiolarian species, and periods of 37,500, 23,000, 19,000, and 14,500 years for variation m percent CaCO3 in a core from the subtropical Atlantic Ocean. The oxygen-isotope paleotemperature record for the past 730,000 years in a composite core described by Emiliani (1978) shows ten maxima of 51sO, indmating glaciation, of which seven coincide with minima in the earth's obliquity. The durations of nine of the ten ~180 maxima, however, range from 49,000 to 122,000 years (the most recent maxLrna, Emiliani's stage 1, has a duratmn of 23,000 years but has n o t yet reached a mmLmum).

96 TABLE I Ages, thicknesses, accumulation rates, and periods of typical cycles of CaCO3 and clay at DSDP Site 366 on the Smrra Leone Rise Core

Age

Average cycle thickness (era)

Accumulation rate based on Foraminifera (cm/10 ~ yrs)

Cycle period (yrs)

Average periodicity (yrs)

18A 23A 27A 34A 37A 38A 39A 5 10

Early Miocene Early Miocene Early Miocene Ollgocene Oligocene Ohgocene Ollgocene Ohgocene Ohgocene

40 65 63 70 68 46 50 60 47

1.5 1.5 1.6 14 1.4 1.4 09 0.9 1.6

26,700 43,300 39,400 50,000 48,600 32,900 55,600 66,700 29,400

44,000

21

Middle--late Eocene Middle--late Eocene Mldd le--late Eocene Middle--late Eocene Middle--late Eocene Middle--late

30

1.4

21,400

33

1.4

21,400

25

1.4

17,900

25

1.4

17,900

23

1.4

16,400

28

14

20,000

21--38

14

15,000--27,100

22 23 24 25 26 27

Eocene Middle--late Eocene

28

Middle-late Eocene

25

14

17,900

29

Early--middle Eocene Early Eocene Early Eocene Early Eocene Early Eocene Early Eocene

30

52

5,800

21--38 25--38 30 30--50 38--75

53 5.2 5 2 52 52

4,000--7,310 4,800--7,310 5,800 5,800--9,600 7,310--14,400

32 33 34 35 36

l ( [

19,000

-~ 7,000

We believe that our data indicate that Tertiary cycles of sedimentary c h m a t l c i n d i c a t o r s o n t h e S i e r r a L e o n e R i s e , p a r t i c u l a r l y t h o s e in t h e O l l g o cene and Miocene sections, are recording the same relationships to orbital p a r a m e t e r s a s t h e P l e i s t o c e n e c y c l e s , i.e., t h e g l o b a l c l i m a t e s y s t e m w a s r e a c t ing to orbital variations but the periodicities of the responses varied because o f o t h e r i n f l u e n c e s . We, t h e r e f o r e , s u g g e s t t h a t t h e i n t e r p r e t a t i o n s o f t h e Pleistocene paleoclimatlc record can be extended at least into the Oligocene and possibly into the Eocene.

97

T E R T I A R Y CLIMATES OF NORTHWEST AFRICA

By further apphcation of our uniformitarian analogue that the Pleistocene is the key to the Miocene and m a y b e the Eocene, we can speculate on the continental climatic regLrne of northwest Africa. If the pre-Plelstocene carbonate cycles are a response to c o l d : w a r m pulses as they apparently are in the Pleistocene, then we might expect continental climates to vary in much the same way as they did during the Pleistocene. The North African continent was colder and dryer during Pleistocene cold periods than during warm pemods (Gates, 1976). We might expect under these conditions to have an increase m rate of wind erosion associated with a southern expansion of the Sahara. This would be particularly true if equatorial trade-wind intensity also increased, as suggested by Ruddiman (1971), Hays and Perruzza (1972), Parkin and Shackleton (1973), Parmenter and Folger (1974), Sarnthein and Walger {1974), Bowles (1975), Diester-Haass (1976), Gardner and Hays (1976), Sarnthein and Dmster-Haass (1977), and Sarnthein (1978). Indeed, Gates' (1976) model for ice-age atmospheric conditions predicted that present wind patterns shift to lower latitudes and increase somewhat in strength durmg glacial periods. Data of Santhein and Walger (1974) suggested that the amount of material transported by wind to the Atlantic was greater during the WiSrm glacial period b y as much as five times relative to the Holocene interglacial period. Increased wind erosion would tend to enhance differences in relative concentrations of clay and CaCO3 brought a b o u t by dissolution. Under cold/dry conditions, we would also expect less intense chemical weathermg due to lower temperature and decreased available moisture. The interrelationships are extremely complex, but decreased decomposition of soil materials would explain the observed increase in clay crystallimty during periods of minimum CaCO3, presumed to correspond to climatic cold periods (Fig.2). Soil materials would experience greater decomposition with increases m temperature and available moisture, which would result in a decrease in crystalline material and an increase m X-ray amorphous clay. Our observations and interpretations are summarized in Fig.9. On the basis of observations of cyclic variations in % CaCO3, % clay, clay mineralogy, and nannofossil abundance and preservation, we have interpreted mid-Tertiary sedimentation of the Sierra Leone Rise in terms of activity of Antarctic B o t t o m Water, position of the calcite lysochne, rate of carbonate dissolution, and rate of dilution by windblown terrigenous clastic material. Conditions on the African continent are interpreted in terms of temperature, available moisture, rates of weathering and erosion, and intensity of trade winds. CONCLUSIONS

(1) The carbonate section at DSDP Site 366 on the Sierra Leone Rise contams a complete fossiliferous record of the Cenozoic. The completeness of the section, together with the abundance of siliceous and calcareous microfossils and the relative isolation of the site from terrigenous influences,

98 Eocene-Miocene CaCO3-Clay Cycles, DSDP Site 366, Sierra Leone Rise Low-carbonate part of cycle

INTERPRETATIONSIERRA LEONE R I S E Colder

Less CaC03

7f

Greater AABW achv~ty Shallower Lysochne

INTERPRETATIONNORTHWESTAFRICA

OBSERVATIONS

Colder and dryer

J

Fewer Nannofosslls

More eros=on

J (Coccohths> Discoasters) / 7

~,~, More clay~¢,~

Greater dlssolut,on-

InC::y~tael~nlt~ ay

+

Less decomposition

Stronger Trade

1

W,°ds

,ocrease0 c,ay

~More

w!ndblown

crystalhn,ty

clashcs B Htgh-carbonate part of cycle

INTERPRETATIONSIERRA LEONE R I S E

OBSERVATIONS

Warmer

/

More CaCO3

Less AABW activity

J_

More Nannotosslls

Deeper ~Lysochne

( C/./~ o c c j j ~l,t~;s:> ~,~oasters)

INTERPRETATIONNORTHWESTAFRICA JWarmer and wetter Less erosion

More decomposition

Weeks: Trade

Less d ! s o l u t , o n ~ " ~ D e c r e a s e d cla~LessWw!~blown crystalhmty clast~cs

Decreased clay crystalhmty

Fig.9.

Summary of observations and interpretations for the lower Eocene to middle Miocene cycles of C a C O 3 and clay on the Sierra Leone Rise.

makes this site an ideal reference section for the low-latitude Cenozoic paleoenvironmental record in the equatorial Atlantic. (2) Well-developed cycles of CaCO3 and clay at Site 366 occur in sediments and rocks of early Eocene to middle Miocene age. The cycles in the Eocene section are complicated by chert and porcellanite. (3) No differences in clay-mineral content are found between clay-rich and clay-poor portions of the cycles, but the clays in the higher-clay portions are more crystalline than in the lower-clay (higher-carbonate) portions. (4) Dissolution of CaCO3, rather than dilution by terrigenous clastic material, as the main cause of the cyclic variations in CaCO3 and clay is suggested by: (a) the relatwely large amount of change in terrigenous influx required to produce the observed variation in CaCO3 and clay, and (b) the decrease in total number of nannofossfls and marked increase in the relative abundance of dissolution-resistant discoasters in low-carbonate portions of cycles. (5) The main causes for the dissolution of CaCO3 on the Sierra Leone Rise are combined effects of shoaling of the lysocline during the early Neogene and climatically induced fluctuations in the position of the top of Antarctic Bottom Water (and, therefore, the top of the lysocline). Cyclic fluctuation in amount of dilution by terrigenous material, particularly windblown Saharan

99

dust, also may have occurred and enhanced differences in the relative abundances of CaCO3 and clay. (6) The Oligocene to middle Miocene chalk--marl cycles have an average period of about 44,000 years. The Eocene siliceous cycles have two distinct periods that average a b o u t 7000 and 19,000 years. The 44,000- and 19,000year periodicities of these cycles are similar to periodicities of cycles of sedimentary climatic indicators and carbonate dissolution cycles observed in Pleistocene sediments m the eastern equatorial Atlantic. This suggests that cycles of the earth's movement, beheved to be the causes of long-term climatic change in the Pleistocene, extended back at least into the Oligocene and possibly into the Eocene. (7) Mid-Tertiary paleoenvironmental conditions on the Sierra Leone Rise can be interpreted m terms of fluctuation in Antarctic B o t t o m Water, position of the lysocline, rate of dissolution of CaCO3, and rate of dilution by terrigenous clastic material (Fig.9). Conditions on North Africa can be interpreted in terms of temperature, available moisture, rates of weathering and erosion, and trade-wind intensity. These paleoen-Aronmental parameters are inferred from observed cyclic variations in CaCO3 and clay, clay crystallinity, and nannofossil abundance and preservation. ACKNOWLEDGEMENTS

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