Pleistocene Sedimentation in the Equatorial Atlantic

WILLIAM F. RUDDIMAN U.S. Naval Oceanographic Office, Chesapeake Beach, Maryland 20732

Pleistocene Sedimentation in the Equatorial Atlantic: Stratigraphy and Faunal Paleoclimatology ABSTRACT deposited at sufficiently rapid rates to allow a detailed unraveling of past climates. The A suite of 15 piston cores up to 23 m long equatorial belt of high productivity provides taken in an eastern equatorial Atlantic frac- an influx of fossiliferous carbonate, but with- ture zone at 8°N. and in the nearby Sierra in deep troughs situated in fracture zones, Leone basin document climatic variations this is largely dissolved before burial. Lesser over the last 1.8 m.y. Foraminiferal and paleo- but useful amounts of siliceous microfossils, magnetic stratigraphies were used to correlate including occasional diatom blooms, con- the cores and select the most representative tribute to the sedimentary record. Aside from pelagic record. the more strictly "pelagic" clays, strong trade "Total fauna" analysis of foraminiferal winds blowing from Africa shower dust and variations in a suite of shorter cores spanning terrestrial plant debris upon the eastern equa- the last 200,000 yrs substantiates in detail torial ocean. Continentally derived turbidity the oxygen-isotope trends over that interval. currents flowing along the fracture zone axes When applied to two cores containing 1.8 m.y. and local slumps off topographic highs spora- of equatorial sedimentary history, this analysis dically spread across the fracture zone floors, pinpoints two prominent, large-scale climatic while bottom currents may rework or erode shifts: (l) at 1.3 m.y. B.P., the mean cli- the pelagic and turbidite deposits. Tectonic matic situation deteriorated, and short but movements in the fracture zone cause micro- severe cold pulses began to punctuate the faulting and contorted bedding. previous moderate warmth of the late Matuyama; (2) following 900,000 yrs B.P., LOCATION AND DESCRIPTION the duration of cold intervals increased. Prior OF CORES to the Jaramillo, no cold pulse exceeded During the fall of 1968, USNS Kane 30,000 yrs; three post-Jaramillo cold intervals collected piston cores in the stratified sedi- ranged in duration from about 50,000 to ments of a fracture zone at 8°N. in the equa- 150,000 yrs. The shortest and most recent of torial Atlantic Ocean (Fig. 1; Table 1; Heezen these correlates with the Wisconsin glaciation. and others, 1969). Additional cores were In addition to pelagic carbonates, con- obtained in a short run to the southeast in tinental sediment is introduced into these the Sierra Leone Basin (Fig. l). Cores in this cores by turbidity currents flowing down the suite vary in length from 11.4 to 23.4 m axis of the fracture zone and by wind blow- and, in pelagic sections, contain stratigraphic ing off Saharan and equatorial Africa. Al- records of the last 0.5 to 1.8 m.y. though the absolute input rate of pelagic The basic sediment types are summarized carbonate to these sediments increases dur- in Figure 2. Cores K.9-55 and 59 consist ing cold intervals, the glacial carbonate per- dominantly of fine-grained turbidites origi- centages tend to decrease due to even greater nating from the African continental margin, influxes of continental detritus. Beginning in whereas in K9-62 the turbidites bottom in the Jaramillo event at roughly 900,000 yrs graded foraminiferal sands that are derived B.P., this terrigenous dilution depresses car- from local topographic highs near the rift bonate percentages in these cores, often to valley (van Andel and others, 1965). The very low values. Pre-Jararnillo sections are other cores have only minor layers of slump generally calcareous oozes. or rurbidite origin. The foraminiferal or INTRODUCTION mineral sand turbidites can be easily detected by coarseness and graded bedding. The fine- Fracture zones in the equatorial Atlantic grained sediments interpreted as lutite turbi- are marked by contrasting sediment types dites were recognized by the absence of

Geological Society of America Bulletin, v. 82, p. 283-302, 14 figs., February 1971 283

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*•-. « CAPE VERDE * ISLANDS •• *' 15°

\ A • 56 \ V

KANE 9 EQUATORIAL CORES

45" Figure 1. Location of Kane cores examined, in and on the flanks of a fracture zone at 8° N. Except for southeasternmost cores (K9-52,K9- Fracture zones stippled lightly; axis of Mid- 5 3, K.9-54, K9-5 5, K9-56), the suite was cored Ocean Ridge system shown by solid lines.

TABLE 1. CORE LOCATIONS AND DEPTHS

Core Lat. (N.) Long. (W.) Depth (m) K9-47 8°11 27°18 4980 K9-48 7°58 27°20 5073 K9-49 8°00 25°53 4767 K9-50 8°19 23°46 4913 K9-51 8°18 20°03 4289 K9-52 7°45 19°22 4402 K9-53 7°37 19°24 4451 K9-54 6°20 19°20 3162 K9-55 5°08 18°07 4998 K9-56 3°23 15°30 4829 K9-57 8°38 22°02 4479 K9-58 8°15 25°06 5072 K9-59 8°08 26°27 5130 K9-60 7°56 29°14 4652 K9-62 8°02 34°12 5034 V19-297 2°37 12°00 4122 V22-186 3°23 20°07 4471 A180-73 0°10 23°00 3749 A180-76 0°46 S. 26°02 3512 236A 2°06 20°07 5065 243A 0°27 27°45 3740 246A 0°48 31°28 3210

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EAST

K9-5I K9-!

\1 p 2

Left - coiling pulses of 8? /G. truncotulinoides ^Turbidite deposits ]-— Pre- Pleistocene contamination

Abundance curves (abundant to left, absent to right) ^Paleomagnetic stratigraphy (Shaded means normal polarity ) J Figure 2. Foraminiferal and paleomagnetic Wollin (1956b). Magnetic stratigraphy inter- stratigraphy of Kane equatorial core suite. preted by declination changes. Suggested cor- Lettered zones based on presence-absence of relations shown by solid and dotted lines. foraminiferal species discussed by Ericson and

bedding or other structures (resulting in an in this suite contain sufficient carbonate for unusually homogeneous appearance) and by the construction of gross foraminiferal stra- the presence of a substantial amount of tigraphies, although the carbonate may be Tertiary Foraminifera, Discoasters, or other- diluted or dissolved to low concentrations. wise mixed faunal assemblages. These homo- The cores came from depths of as much as geneous lutite beds strongly contrast with 5130 m, but the relatively noncorrosive the highly burrowed and finely bedded bottom waters of the eastern equatorial pelagic layering. Atlantic have left intact biogenous fractions Core K9-49 is Mio-Pliocene in age with substantial correlative elements. throughout most of its length. Several apparently Pleistocene cores are sprinkled at CORRELATIONS OF PELAGIC intervals by Miocene-Pliocene microfossil CORES contamination (usually 5 percent or less of Because evolutionary appearances and ex- the foraminiferal population). tinctions of planktonic Foraminifera are rare Cores with detailed pelagic correlations are in the Pleistocene, deep-sea stratigraphers rare in the deep sea (Ericson and Wollin, have sought other means of correlation. In 1956a). In addition to layers of apparent the tropical-subtropical Atlantic, Eticson and turbidite or bottom-current origin, there are associates have utilized a semiquantitative depositional hiatuses within otherwise valid evaluation of the relative abundance of one pelagic sequences (Glass and others 1967). gtoup of related species or subspecies, the These often can be detected only by a net- Globorotalia menardii complex (Ericson and work of intercorrelated cores. All pelagic cores Wollin, 1956a, 1968). The G. menardii zones

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are designated Z-Y-X-W, and so on, in order CORE K9-58 of increasing age. Morphologic variations PALEOMAGNETIC DECLINATION within the G. menardii complex add diag- STRATIGRAPHY nostic parameters to this scheme (Ericson and others, 1961; Emiliani, 1969). Glass and others (1967) have added paleomagnetic zonations which correct previous miscorrela- tions and provide an absolute time scale. While one may challenge the climatic interpretations developed from this technique, B it has a clear practical validity for low-latitude R stratigraphic zonation in the Atlantic. The U G. menardii zones are chronologically N bracketed within rather narrow limits of un- H certainty by paleomagnetic reversal ages and E by C14 and Th230/Pa231 radiometric dates S (Glass and others, 1967; Ericson and Wollin, 1968; Broecker and Ku, 1969; Emiliani, 1970). The major problem in the absolute age scale is the conflicting interpretations of paleomagnetic stratigraphies. An event with normal magnetic polarity occurs prominently M within the lower Maruyama reversed epoch, A and there is a difference of 200,000 yrs in two T interpretations of its age. Lament studies U (Opdyke and others, 1966; Hays and others, Y 1969) have linked this event with the Olduvai A normal polarity lavas, although they have M occasionally extrapolated its age to a some- A what younger figure (Ninkovich and others, 1966). From extensive lava dating, Cox (1969) Figure 3. Stratigraphic interpretation of dec- has proposed an event at 1.79 to 1.61 m.y. lination variations in core K9-58. B.P. called the Gilsa. The Olduvai lavas may then represent a pair of much shorter 2 m.y. Of the two longest stratigraphic sections, B.P. events which are less frequently detected K9-58 reaches the top of the Gilsa event, in deep-sea cores. Sea-floor spreading evidence while K9-57 extends to just below it. K9-57 (Emilia and Heinrichs, 1969) supports this and K9-58 both penetrate into the "Q" zone, interpretation. This revised stratigraphy re- the former reaching the level of Discoaster duces the previous (extrapolated) dates of extinction (J. R. Conolly, 1969, personal pre-Brunhes G. menardii zones (Glass and commun.). Several other cores bottom deep others, 1967; Ericson and Wollin, 1968). The in the "T" zone near the Jaramillo event, at Plio-Pleistocene boundary of Ericson and almost 1 m.y. B.P. Core K9-56 reaches into others (1963) then occurs at roughly 1.9 m.y. the Jaramillo event, and contains abundant B.P. Ivory Coast microtektites in its flow-in and Plotted in Figure 2 for several cores are lowest undisturbed layers (Gentner and paleomagnetic stratigraphies. Since the en- others, 1970). Only K9-51, among the con- tire suite lies within 8°30' of the equator, it tinuous pelagic cores, fails to penetrate into was necessary to use declination to detect Matuyama-age sediments, reaching into the magnetic reversals (Fig. 3). Hays and others, Ericson "U" zone at about 500,000 yrs B.P. (1969) have discussed the problems involved The temperate foraminifer, Globorotalia in studying declination in equatorial cores. truncatulinoides, occurs sporadically in equa- In addition, until core K9-57, insufficient torial Pleistocene sediments and is valuable precautions were taken during shipboard ex- for correlations. During most of the time trusion to facilitate later realignment. spanned by these cores, it coils 90 to 95

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percent to the right (Ericson and Wollin, PELAGIC CORES 1956a, 1968; Emiliani, 1966). Short pulses of K9-S7 K9-M K930 K9-S6 K9-5I left-coiling punctuate the record twice in the Brunhes Epoch, within the "X" and "U" zones (Fig. 2). Prior to and during the Jara- millo event, there were additional left-coiling zones in the lowermost "T" zone and in the "S." However, G. truncatulinoides was too rare at this time in equatorial areas to be of use in correlation, and it graded complexly .6-1 into G. tosaensis within the Gilsa (Berggren, 1968). ,« i .„ Cores K9-52, K9-53, and K9-48 seem to contain pelagic lithologies, yet their pelagic a foraminiferal sequences fail to fit the most clearly developed stratigraphy in Figure 2. Each contains a normal Z-Y-X-W section that echoes the basic sequence, but in each, the "V" sections and below are anomalous. The G. menardii curves are erratic in the "V," while the "U" zones are indistinct, brief, or not distinguishable. Moreover, the G. (1 truncatulinoides coiling curves display extra pulses of left-coiling at various levels within 1.6 <3 what appears to be the "V," whereas none 1.7 •» exist in the reliable pelagic cores. These anomalous zones are also deficient in the Figure 4. Foraminiferal and paleomagnetic menardii complex and may represent lateral correlations of the five most representative contamination by lower Pleistocene material pelagic cores, using the criteria described in (necessarily either "U" or "S" in age). The Figure 2. Core lengths in meters indicated by "V" zones in cores K9-52 and K9-53 are scale to right. marked by a striking excess of sediment, compared to the reliable cores. K9-48, on the other hand, is deficient in "V" sediment. Also of potential value is the banded nature This suggests redistribution of sediment on of the carbonate layers. Many layers are car- the sea floor during "V" time, probably by bonate oozes or highly calcareous lutites, bottom currents. which sharply contrast with the carbonate- From among the nominally pelagic cores, poor layers (Fig. 5). Some correlation of five emerge as sufficiently complete to war- carbonate bands occurs above the "V" and rant investigation of other potential means within the "U" and "T." Lithologies in the of correlation (Fig. 4). "V" zone are poorly resolved even in these five cores. Otherwise, the similarities suggest ADDITIONAL PELAGIC that a basic climatic history of the equatorial CORRELATIONS Atlantic for almost 2 m.y. must lie in these five Certain layers contain floods of the large cores, only one of which (K.9-57) has sufficient diatom, Ethmodiscus rex, which may be de- carbonate for foraminiferal paleoclimatology. tected megascopically. When such samples are washed through 74p. screens for foram- METHODS OF ANALYSIS OF iniferal processing, the diatom mantles form PLEISTOCENE CLIMATES fibrous interlocking mats on the sieves. Kolbe Two methods involving Foraminifera best (1955) and Ericson and Wollin (I956a) have apply to the study of Pleistocene paleocli- shown correlations of these layers in calcar- mates: faunal methods and oxygen isotope eous ooze cores in the equatorial Atlantic. analyses. Neither technique may be legiti- Several suggested correlations of diatom-rich mately claimed to give an unambiguous horizons are marked in Figure 5. interpretation of paleotemperatures.

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LITHOLOGIES periods (Ericson and Wollin, 1956b; Emiliani, 1966). This method is semi-quantitative and it disregards over 90 percent of the foraminiferal information available in each sample. Phlegerand others (1953) and Parker (1955, 1958) have followed the classical paleontolo- gical approach of assigning to each species broad temperature preferences from surface sediment distributions and constructing curves into which warm, cool, cold, and cosmopolitan species are lumped as separate groups. This is a sound approach, the only disadvantage being visual, since two or three curves are somewhat difficult to evaluate simultaneously. By using the ratios of various warm to cool species, Lidz (1966) obtained curves closely approximating the oxygen isotope curve for the same core. TOTAL FAUNA PALEOCLIMATIC ANALYSIS A broadly-based paleoclimatic technique should incorporate all relevant faunal information. Several previous studies (Ericson and Wollin, 1956a, 1956b; Schott, 1935, 1966; Figure 5. Lithologies of rive pelagic equa- Phleger and others, 1953; Wiseman, 1965) torial cores, showing zones of carbonate oozes of planktonic Foraminifera in equatorial and bands of high diatom concentrations. Cor- relations suggested by dashed lines, with solid Atlantic cores have provided valuable strati- and dotted lines connecting foraminiferal zona- graphic information that indicates whether a tions from Figure 4. core has a complete late Pleistocene record. For several cores in the upper Pleistocene and The oxygen isotope method has a rigorous Recent (G. menardii zones Z through W), physical-chemical basis (Urey, 1947); the de- 18 I6 faunal and oxygen isotope data have been pendence of the 0 /O ratio on temperature published (Phleger and others, 1953; Emiliani, is a demonstrated fact (Epstein and others, 1955; Ericson and Wollin, 1956b). This in- 1953). However, isotopic variations caused 16 terval, within which these two methods are by preferential storage of O in glacial ice in closest harmony, may be used to test a may outweigh the temperature control. simple faunal technique. Initially, percentage Emiliani (1955) states that his curves are con- counts of the Foraminifera are made at close trolled 70 percent by temperature changes intervals (generally 10 cm). Following Phleger and 30 percent by glacial ice storage, where- and others (1953), the species are designated as Olausson (1965) and Shackleton (1967) as warm-water or cool-water types, according feel that the latter factor explains the entire to present surface sediment distributions. A variation. Dansgaard and Tauber (1969) few species are relegated to nondiagnostic arrived at an intermediate interpretation. In cosmopolitan forms. The warm and cool types any case, if the two controlling factors act are summed into respective totals, either of in unison, oxygen isotope curves are valid which may dominate at a given level in a core. curves of "total glaciation effect" (Shackleton, Although the equatorial Atlantic has prob- 1967). ably been the most thermally conservative Among the faunal techniques is that of sector of the Atlantic Ocean through the Ericson and Wollin (I956b). The stratigra- Pleistocene, its sediments are subject to cold phically useful G. menardii zones, when given faunal influences even at present, particularly alternating warm-cold designations, broadly in its eastern half. The cool Canaries Current support oxygen isotope curves for the last carries deep-dwelling rnid-latitude foramini- 150,000 years or so, but diverge for earlier fers southward near the coast of Africa, in-

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traducing a cooler faunal aspect (Phleger and tions (Fig, 7). Based on North Atlantic sur- others, 1953). Furthermore, divergence and face sediment distributions in Ruddiman coastal upwelling create cooler surface waters, (1969), temperature preferences were assigned spurring development of non-equatorial to the species as follows: Warm species: species. Such tendencies may have been re- Globoratalia menardii, G. tumida, G. flexuosa, inforced during glaciations (Berger, 1968). Globigerinoides ruber, G. samlifer, and Pulknia- Consequently, during cold intervals the non- tina obliquiloculata. Cool species: Globigerina equatorial fauna may outnumber warm species bulloides, Globorotalia inflata, G. truncatuli- in equatorial cores. noides, and Globoquadrina dutertrei. For this reason, the two faunal groups have For sake of rapidity all other species were been combined into a single plot of the per- lumped as "other." These averaged about 15 centage excess of warm over cool (or cool percent of the fauna, of which roughly two- over warm) species. Samples with equal per- thirds belonged to the nondiagnostic cosmo- centages of warm and cool foraminifers plot politan forms, Globigerinita bumilis, Globi- on a zero line which constitutes a natural gerinita glutinata and Orbulina universa. The boundary between warm and cold paleocli- remaining 5 percent of faunal information mates. Only the climatically insensitive cos- was sacrificed in this initial analysis.1 mopolitan species are not included in this Not all species variations need be ascribed analysis, which results in a "total fauna" directly to temperature. The most abundant curve. It should give a good first approxima- "cool" species, Globoquadrina dutertrei, re- tion of past climates, but is admittedly some- sponds sensitively to salinity, preferring lower what simplistic in considering all species as values and specifically avoiding high salinities equally "warm" or "cold," when gradations (Parker, 1955, 1958; Ruddiman, 1969). Yet exist. More sophisticated techniques to this species clearly echoes the oxygen isotope quantify such factors are being developed by curve over the last 200,000 yrs (Fig. 8; Imbrie and Kipp (1969). Emiliani, 1966). While Phleger and others To test this method in the equatorial At- (1953) termed it a cool species, it should be lantic, three previously studied Swedish Deep- considered a low-salinity species with a cool- Sea Expedition cores (Phleger and others, water preference. Despite the erratic abun- 1953) were combined with three Lamont- dance patterns of some species in the piston Doherty cores from this study (Fig. 6, Table cores, the surface sediment distributions pro- 1). Methods of sample preparation and anal- vide the best paleoecologic guidelines and ysis are outlined in Ruddiman (1969). Per- have been followed for consistency through- centage plots of the G. menardii complex in out. these cores give Z-Y-X-W zones for correla- There appears to be strikingly good agree-

45° 40- 35° 30- 25' 20° 15° ment of the "total fauna" curves thus derived with the oxygen isotope curves, even in second-order fluctuations (Fig. 9). The "total EQUATORIAL \ fauna" curves, like the oxygen isotope curves, ATLANTIC are indices not of absolute temperature, but OCEAN of relative intensity of glaciation. Two of the six cores (236A and V22-186) are excluded from Figure 9 because they were taken in depths at which selective solution « makes them suspect for paleoclimatic pur- poses (Ruddiman and Heezen, 1967; Berger, 1968). However, the shallow cores establish an undissolved carbonate ooze standard, against which solution effects can be traced Figure 6. Location of six short equatorial into the eastern Atlantic Sierra Leone Basin. cores used in preliminary climatic interpretation of foraminiferal data. Four contain relatively 1 The counts from these three cores and two to shallow (less than 4000 m) carbonate oozes; follow later are available from the National Ocean- the two easternmost are foraminiferal lutites. ographic Data Center upon request: file no. 70-0878, Cores 243, 246, and 236 taken from Phleger National Oceanographic Data Center, Washington and others (1953). Navy Yard, Washington, D. C.

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246 243 A160-76 AI80-73 V22-186 O'48'N 2'34'N srzs w I9'I4'W 0 4567nn 50

PERCENTAGE ABUNDANCE /N OF Glmbonttalia meatrtfh' & Globorottlis tumids Figure 7. Percentage abundance of the where G, truncatulinoides coils more than 50 Glaborotalia menardii group in six equatorial percent left noted by stippled boxes. Core Atlantic cores. These species provide the basic lengths indicated in meters. Z-Y-X-W zonations used for correlations. Levels Solution acts selectively on the various interval. Thus, the apparently greater net so- Foraminifera, dissolving fragile species in the lution during warm climates may be merely deep cores (Fig. 10) and residually concen- an artifact of this initial inequality in the trating resistant forms (Olausson, 1965; Figs. assemblages. More exact quantification of 7, 8). Core V22-186 is obviously only slightly net solution will only be possible when de- affected, since G. ruber is little diminished tailed absolute data on average species resis- relative to the shallow cores. Most foraminif- tance to solution are available. At this point, eral assemblages in core 236A are strongly differential solution is recognized as a factor altered. The greatest increases in net differ- that can invalidate climatic interpretations, ential solution seem to occur in periods of particularly at depths exceeding 4500 m in warm climate. However, the net amount of the eastern equatorial Atlantic. It can be de- differential solution also depends upon the tected by its effects on the fauna and taken original undissolved assemblage. Variations into account in climatic reconstructions. in the net resistance of the original population, and in the amount of contrast of its "TOTAL FAUNA" component species, will yield differing ap- (FORAMINIFERAL) ANALYSIS parent net solution effects. Solution during OF LONG CORES warm periods will quickly attack the two From these preliminary analyses, the "total abundant but very fragile warm species (G. fauna" technique has been applied to two ruber, G. sacculifer) and more rapidly change cores reaching an order of magnitude farther the warm-water fauna than will the same net back in time. During most of this interval, no solution of the more homogeneous and mod- oxygen isotope curves exist for comparison. erately resistant cold assemblage during a cold For the long cores, Foraminifera were counted

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PERCENTAGE ABUNDANCE OF

Figure 8. Percentage abundance of Globoquadrina dutertrei in six equatorial cores.

CORE 24S CORE A180-73 CARIBBEAN OXYGEN ISOTOPE FAUMAL CURVE FAUNAL CURVE ISOTOPE CURVE Z2 Z3 » ZS 26 Z7 C'

TOTAL FAUNA" CLIMATIC CURVES

Figure 9. Total fauna (foraminiferal) cli- flanked by oxygen isotope curves from Emiliani matic curves for four shallow equatorial cores, (1955, 1966).

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PERCENTAGE ABUNDANCE /\ OF ruber

Figure 10. Percent abundance of Globigeri- fragile and thus is selectively removed from the noides ruber. This warm-water species is very two deeper cores.

using the same techniques (Ruddiman, 1969), Both reach into the "Q" zone, K9-57 to a with the total cuttailed to about 300 to 350. somewhat deeper level. Core K9-57 was For the rarer species not noted in the prelim- plotted on a time scale by aligning three inary study, temperature preferences based on paleomagnetic boundaries (lower Jaramillo, surface-sediment distributions were assigned upper and lower Gilsa) with the proper chro- as follows: Warm species: Globigerinita aequi- nologic levels and extrapolating between lateralis, Sphaeroidinella dehiscens, Globigeri- paleomagnetic datum points. Since the sedi- noides conglobatus, G. tenellus, Candeina nitida, mentation rate in K9-57 (determined both Globigerina rubescens, G. digitata, Globoquadrina from paleomagnetic horizons and published hexagona, and Hastigerina pelagica. Cold spe- isotopic dating of G. menardii zones) ap- cies: Globoratalia hirsuta, G. punctulata, G. peared relatively constant down to the bottom scitula, and Globigerina quinqueloba. of the Jaramillo at 1420 cm, core K.9-57 was Core K9-57 was selected both for its ap- plotted assuming a rate of 1.5 cm/1000 yrs to parently complete record and for the long that level. The top and bottom of the Gilsa time interval spanned. Samples from core event (1.61 and 1.79 m.y. B.P.) provided the V19-297 were provided for a more reliable other chronologic levels (Figs. 2,4). Constant carbonate ooze standard by Lamont-Doherty sedimentation rates were assumed between Geological Observatory. Glass and others paleomagnetic boundaries. (1967) and Ericson and Wollin (1968) previ- Only the Brunhes-Matuyama boundary ously had run paleomagnetic and frequency- was paleomagnetically detected in V19-297 to-weight G. menardii curves on the latter (Glass and others, 1967). That core was core. It is located near K9-57 (Table l) and plotted by aligning the bottom boundary of contains a carbonate ooze lithology through- the "T" zone (virtually equivalent to the base out. of the Jaramillo) against the same level in The G. menardii percent abundance curves K.9-57, and assuming a constant sedimenta- for these two cores are shown in Figure 11. tion rate to the top. From the T/S boundary

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to the bottom of V19-297, the constant sedi- Jaramillo portion of the Matuyama Epoch mentation rate that best juxtaposed the bases was a long period of moderate warmth, inter- of the "R" zones of the two cores was used. rupted only by brief intervals of colder con- With these alignments, the reinterpreted ditions until the Jaramillo. The longest and chronology of the pre-Btunhes G. menardii most extensive cold faunal episodes (deter- zones is shown in Figure 11. mined largely from the more reliable core, A similar time plot was made of Sphaeroidi- V19-297) occurred between the Jaramillo and nella dehiscens (Fig. 11). Glass and others Brunhes, roughly 900,000 to 775,000 yrs B.P., (1967) detected diminishing percentages of and then in and before the "U" zone, rough- S. dehiscens across the Brunhes-Matuyama ly 600,000 to 425,000 yrs. B.P. Other shorter boundary, from which Bandy and others but equally intense cold periods are evident. (1969) inferred a possible climatic deteriora- Although the longest cold intervals postdate tion. This change occurs a short distance the Jaramillo, there is a shift in the mean above the boundary in both cores. position of the climatic curve toward colder The "total fauna" foraminiferal climatic values at the R/S boundary at 1.3 m.y. B.P. curves for K9-57 and V19-297 are shown in Prior to this time, virtually no cold periods Figure 12. As a first approximation, late existed back to the 1.8 m.y. B.P. maximum Jaramillo and post-Jaramillo time (the last age of core K9-57. 900,000 yrs) encompasses the most long- As a second indicator of climatic variations lasting of the cold faunal intervals. The pre- in these cores, the percentages of Globorotalia

PERCENT ABUNDANCE PERCENT ABUNDANCE GloborotQliq rnenqrdii Sghaerojdineno AGE IN complex dehiscens MY. B.P. V19-297 K9-57 V19-297 K9-57 0% 20% 40% 0% 20% 40% 60% 0% 10% 20% 0% 10% 20% F=z=l f?: V ' ~-

I-

IK fI S3

1.5'

Figure 11. Percentage abundance of the G. core lengths in meters; the cores were adjusted menardii complex and of S. debiscens in two to a time scale. Pleistocene cores. The shaded triangles denote

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inflata are also shown in Figure 12. This cold- above the Jaramillo event, thus dividing the water species generally does not exceed 10 Pleistocene into roughly 900,000 yrs of "gla- percent of the total population; thus it has a cial" time and an earlier but equal span of relatively minor influence on the total fauna preglacial Pleistocene. These equatorial cores curve. The increases of G. inflata at 1.3 m.y. strongly confirm that first-order determina- B.P., and near the Jaramillo event, also sug- tion, but also suggest an earlier deterioration. gest fundamental coolings at those levels. Mclntyre and Jantzen (1969) found nine Other studies have previously inferred a prominent coccolith carbonate minima in a climatic deterioration near the Jaramillo. western North Atlantic core. The first mini- Berggren (1968) noted a general cooling in mum occurred at the top boundary of the the lower half of the Pleistocene based on the Jaramillo, and the other eight fell within the approximate proportion of Pulleniatina oh- Brunhes Epoch. They attributed these minima liquiloculata and Sphaeroidinella dehiscens to to the proximity of the North Atlantic Polar Globorotalia inflata and G. hirsuta. Specifically, Front during glacials. Since extremely cold he inferred a significant cooling within and temperatures are known to inhibit the cocco-

TOTAL FAUNA CLIMATIC CURVES AGE IN MY. B.P. G. INFLATA EXCESS EXCESS COOL WARM SPEC ES SPECIES rO

BRUNHES

JARAMILLO

M A T U Y A M A

V19-297

K9-57

Figure 12. Total fauna climatic curves, de- darkened zones. The most prominent cold zones termined from the foraminiferal population, occur above the Jaramillo. Cores are plotted Zones in which cool non-equatorial species out- against time, number warm tropical forms are marked by

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lith flora, each major southward displacement Before relating these curves to other paleo- of polar waters may have caused a coccolith climatic reconstructions, it is necessary to carbonate minimum. examine the problem of carbonate variations Hays and others (1969) dated fluctuations in deep-sea cores. in calcium carbonate content which had been reported by Arrhenius (1952) from the equa- CARBONATE VARIATIONS IN torial Pacific. They observed eight fluctua- EQUATORIAL ATLANTIC CORES tions in calcium carbonate percentages during Three major variables determine the car- the Brunhes Epoch. Since the last carbonate bonate content of deep-sea cores: (l) the maximum correlated with the Wisconsin biogenous input (productivity of carbonate glaciation, they inferred that the previous and silica microfossils); (2) the terrigenous Brunhes peaks were of glacial origin. The input (due to wind, bottom current redistri- pre-Brunhes peaks were sufficiently lower in bution, and turbidites which leave clay in intensity to suggest a warmer average Maru- suspension); and (3) carbonate removal by yama climate. dissolution on the sea floor (dependent main- Other studies support the concept of a ly upon the interplay of sedimentation rate fundamental cooling somewhere between the against chemical aggressiveness of the bot- base of the Brunhes and the top of the tom water). Jaramillo (Bandy and others, 1969; Hermann, It is characteristic of pelagic equatorial 1970; Lamb, 1969; Beard, 1969). Lamb and Atlantic cores that the postglacial section is Beard also infer an earlier climatic deteriora- carbonate-rich while the late Wisconsin gla- tion in the Kaena event of the Gauss Normal cial sediments are lutite-rich2 (Correns, 1937; Epoch at 2.8 m.y. B.P. Schott, 1935). Schott (1935) supposed this to Viewed in detail, not all individual cold be a consequence of lower carbonate and periods in these two cores can be clearly cor- higher clay input rates during the Wisconsin. related. This is due in part to the faunal record Broecker and others (1958), from a combined in K9~57 being badly altered by solution at radiocarbon and chemical mass analysis on some levels. Sphaeroidinella dehiscent is a useful core A180-74, found that the net carbonate differential solution indicator, levels in K9-57 input actually increased in the Wisconsin that are strongly affected having much higher (mainly in the coccolith fraction), but that a percentages than comparable zones in Vl9~ proportionally even higher detrital lutite in- 297 (Fig. 11). Extreme dissolution of levels flux diluted the increased carbonate. Need- containing abundant Globorotalia tumida will ham and others (1969; and in prep.) have residually concentrate this warm species and stressed the potential paleoclimatic signifi- increase the apparent "warmth" of the total cance of variable inputs of continental detritus fauna curve (Olausson, 1965). Conversely, in this region. where G. tumida is rare (and the resistant but Both cores K.9-57 and K9-58 have rela- cool species Globoquadrina dutertrei is abun- tively low sedimentation rates prior to the dant), solution will probably enhance the cool Jaramillo event (l.O and 0.7 cm/1000 yrs, aspect of a faunal assemblage. respectively). Above the Jaramillo, the rate in The levels of greatest apparent solution K9-57 increases to 1.5 cm/1000 yrs, and in effects in K9-57 do not coincide solely with K9-58 to 1.8 cm/1000 yrs. Within the zones either warm or cold faunal periods. The bot- of higher sedimentation rates, the average tom waters are probably colder and more carbonate content is lower, falling to negli- aggressive in glacial times, but the sedimen- gible values in many layers. Of the three tation rates are more rapid, thus tending factors influencing CaCOs contents, only ter- to cover and protect the carbonate shells rigenous lutite input could produce this pat- (Broecker and others, 1958). The delicate tern. Neither of the other two could both balance of these factors, combined with local increase the sedimentation rate and decrease effects at the depositional site, precludes any the carbonate percentages. The only other clear dependence of selective solution on conceivable explanation is that bottom-cur - climate. However, because of differential solution and other effects, there are decided 2 The term "lutite" is a size term in strict usage. differences in the relative intensities of some As used here, terrigenous lutite will refer only to climatic oscillations between cores K9-57 noncalareous detrital material from the continents. and V19-297. Some terrigenous lutite is, however, carbonate.

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rent winnowing of sediments on a local scale tan-Saharan dust zone each summer com- has somehow altered the original pattern of pletely vacates the area of core V19-297, and sedimentation. There is no way to evaluate only barely continues to overlap the 8° N. local variability with such a small sample. fracture zone from which K9-57 was cored. Although based on cores spanning a broader Prior to the Jaramillo, the generally warm time scale than that analyzed by Broecker and climate may have resulted in a nearly constant others (1958), this study points to the same northern location of the dust belt, perhaps in basic conclusion: lutite influxes controlled the same zone now occupied only in summer. carbonate variations in the equatorial Atlantic Alternatively, a less intense atmospheric cir- during the coldest portions of the Pleistocene culation at that time might not have covered (the last 900,000 yrs). as large an area with windblown terrigenous Core V19-297 lies near the equator (Table debris. In either case, very little windblown 1; Fig. 13), somewhat south of the region of dust reached core V19-297, and only minor highest atmospheric dust concentrations in amounts influenced the 8° N. fracture zone winter (Hustedt,1921; Pratje,1934; Arrhenius, cores, K9-57 and K9-58. Beginning at 1.3 1963; Folger and others, 1967). Core K9-57, m.y. B.P. and culminating at the Jaramillo on the other hand, lies along the latitude of event, the prominent cooling may have shifted highest concentration, directly downwind this belt southward to a mean position nearer from the main source area (Needham and that presently occupied in winter (Fig. 13), others, 1969; Bowles, 1970; see also Fig. 13). or else caused an enhanced atmospheric cir- The seasonal shift northward of the Harmat- culation which increased its regional extent.

45° 30° 15'

Harmattan S^f dust /' O

WINTER HAZE FREQUENCY EQUATORIAL ATLANTIC OCEAN

Figure 13. Winter haze frequency over areas Folger and others, 1967). Contours show of the equatorial Atlantic subject to windblown average frequency of occurrence of winter haze. dust off Africa (from MacDonald, 1938; after

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Climatically induced changes in the nature of Kennett. The Antarctic seas seem generally the source area for the windblown dust may to have been out of phase with, or indepen- also be expected to have occurred. dent of, northern hemisphere and equatorial A different alternative is that the very fine variations. On a gross scale, the Brunhes distal turbidites running down the fracture appears most noteworthy in the Antarctic for zone axis from Africa contributed detrital the prominent warming episodes relative to lutite preferentially during glacial times to the uniformly cold latter portion of the the area from which the deep Kane cores were Matuyama (also noted by Goodell and others, taken. Bottom currents could rework this fine 1968); in contrast, the northern hemisphere fraction into the "pelagic" record. Evidence data point to a nearly coincident climatic for more frequent glacial turbidites has been deterioration. On a finer scale, too little evi- noted farther south along the African conti- dence is available to compare Antarctic with nental margin (Heezen and others, 1964). northern hemisphere variations. In any case, carbonate variations in the Kane The similarity of the Emiliani, Mclntyre cores, and in equatorial Atlantic cores in gen- and Jantzen, and Ruddiman curves for the eral, are due primarily to variations in the Atlantic-Caribbean is substantial; aside from supply of detrital terrigenous lutite (wind- the broad 900,000-yr period of cooling, there blown or other) rather than to changes in is fair agreement in the detailed fluctuations. carbonate productivity or net carbonate so- Although peak-to-peak correlations are not lution. possible for every pulse, suggested connec- tions are dotted lightly in Figure 14. PLEISTOCENE CLIMATIC RECORDS A major discrepancy between these At- IN DEEP-SEA CORES lantic paleoclimatic interpretations occurs in Attempts to reconstruct paleoclimates from the lower Brunhes. Mclntyre and Jantzen deep-sea Pleistocene sections have focused (1969) found the strongest, most prominent upon variations in percent carbonate, fora- North Atlantic carbonate maximum (inferred miniferal fauna, and oxygen isotope ratios to reflect warm climates) at about 450,000 yrs (Fig. 14). The cores shown in Figure 14 were B.P. (Fig. 14). Clearly bracketing that point aligned along five paleomagnetic boundaries: in time, this study found the longest cold the base of the Brunhes, and the upper and faunal episode in the "U" zone. Imbrie and lower limits of both the Jaramillo and the Kipp (1969) also noted unusually cold "U" Gilsa. conditions. Lithologically, however, there is The G. menardii Atlantic curves fluctuate a very prominent carbonate maximum in the sympathetically with the "total fauna" curves equatorial Atlantic cores at this level (see Fig. during the Brunhes, but the earlier trends 5). Using oxygen isotopes, Emiliani (1966) diverge. This group of species or subspecies, found a long warm peak at a correlative level when viewed over broad time spans, evidences in Caribbean cores. Instead of the usual iso- a profound morphological instability through topic pattern of gradually declining tempera- the Pleistocene (Emiliani, 1969). The Pacific tures from an abrupt initial maximum, this Pleistocene curve from Ericson and Wollin peak reverses that trend and slowly increases (1970) is actually determined by the G. in intensity. The lightly dashed line in Figure tumida-G.flexuosa members of the G. menardii 14 connects levels from each curve inferred to complex, while the Atlantic curve incorpor- represent this anomalous climatic pulse. ates all three members of the group, but in This discrepancy requires one of two alter- continuously changing proportions (Hays natives: either (l) otherwise climatically in- and others, 1969; Emiliani, 1969). Such in- dicative foraminiferal species in the equatorial stability may be presumed to mirror variations Atlantic and Caribbean registered a false cold in paleoecological behavior. Of the five re- period, or (2) an influx of Olfi-rich water into maining curves in Figure 14, those from the the Caribbean coincided with a temporarily Caribbean-Atlantic area are most similiar, reversed phasing of the usual carbonate mini- while the Kennett (1970) Antarctic curve mum/glacial maximum pairing in the equa- clearly differs from the others. Faunal trends torial and North Atlantic. Possibly relevant is from Hays (1965), later dated paleomagneti- the dominant Antarctic warm climatic pulse cally by Opdyke and others (1966), present a indicated by Kennett (1970) and Hays (1965). climatic picture similar to that described by This strongest of Antarctic warm fluctuations

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M. Y. KENNETT E.MILIANE RUDDIMAN MCINTYRE AND HAYS AND ERICSON AND ERICSON AND MY. (1970) (1966) {THIS PAPER) JANTZEN (1969) OTHERS (1969 WOLLIN (1970) WOLLIN (1968) FORAM FAUNA OXYGEN ISOTOPE FORAM FAUNA CARBONATE CARBONATE FORAM FAUNA FORAM FAUNA

BRUNHES

EOUAT ATLANTIC EQUAT. ATLANTIC NORTH ATLANTIC

Figure 14. Comparison of deep-sea Pleisto- bonate in fine fraction ("coccolith carbonate")- (/Olduvai) event. Shaded portions of curves cenepaleoclimaticcurves. Mclntyreand Jantzen All curves aligned with reinterpreted Gilsa arbitrarily inserted for visual aid. (1969) carbonate curve based on percent car-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/2/283/3428179/i0016-7606-82-2-283.pdf by guest on 28 September 2021 REFERENCES CITED 299 (occurring near Hays' U-^ boundary) may tocene encompassed the most long-lasting have intruded upon and influenced the north- glaciations, with an earlier cooling trend indi- ern hemisphere cycles (Fig. 14). cated at 1.3 m.y. B.P. Pre-Jaramillo climates None of the long-term 1.8 m.y. climatic were on the average warmer, interrupted only curves can be considered definitive. The by numerous short, but often intense, cold faunal curves are subject to nonthermal de- pulses. The longest periods of unrelieved cold terminant factors acting on the species; the were from 600,000 to 425,000 yrs B.P. and carbonate curves are subject to the intricate from 900,000 to 775,000 yrs B.P. Several interplay of the parameters determining per- shorter post-Jaramillo intervals of severe cold cent carbonate. Whether the detailed dis- occurred, the most prominent of which cor- crepancies among the equatorial and northern relates with the Wisconsin continental glaci- hemisphere lithologic/paleontologic studies ation. The opposed phasing of carbonate are due to inadequate correlations between minima and glacial maxima in the eastern paleomagnetic horizons, climatic phase dif- equatorial Atlantic is primarily a consequence ferences from area to area, or to the collapse of influxes of terrigenous lutite from Africa, of the central assumptions of one or more of which mask variations in net carbonate de- the methods, is uncertain. An exact chronol- position. Terrigenous dilution may also ex- ogy of Pleistocene glacial episodes and a plain the opposite timing of carbonate cycles regionally unified interpretation of that se- between the Atlantic and Pacific Oceans. quence remain unavailable. Within the available interpretations of ACKNOWLEDGMENTS glacial-interglacial alternations, Olausson The fracture zone cores were taken aboard (1967) noted the opposed phasing of car- USNS Kane (TAGS-27) under command of bonate peaks between the Atlantic and Pacific J. McCandless. B. C. Heezen, P. J. Fox, and Oceans and ascribed it to a massive net trans- J. A. Ballard directed scientific efforts on three port of carbonate from ocean to ocean. How- legs of the cruise, which was manned by U.S. ever, net carbonate deposition probably in- Naval Oceanographic Office personnel. creased in both equatorial oceans during I owe thanks to F. Bowles, J. Conolly, and glacials and during post-Jaramillo time in D. Needham for discussions, and to P. Lanasa, general (Broecker and others, 1958; Hays and Gloria Williams, Doris Newton, D.Johnson, others, 1969). This study suggests that detrital Barbara Grovesnor, J. White, and T. Wolaver lutite diluted and overwhelmed these infor their assistance. creases in the Atlantic but failed to do so in Portions of this research were conducted at the Pacific. Previous studies have defined Columbia University while the author received great excesses of glacial lutite input else- a Duke University Traineeship in the Co- where in the North Atlantic (Ericson and operative Research and Training Program in others, 1961; Heezen and others, 1966). This Biological Oceanography, supported through explanation applies both to the detailed scale National Science Foundation Grant Nos. GS- of glacial-interglacial cycles and to the broad 5529, GB-6868, and GS-8189 to Duke Uni- scale of pre-Jaramillo and post-Jaramillo Pleis- versity. The Lamont-Doherty Observatory tocene comparisons. cores were processed under Naval Research Mass carbonate transport from the Atlantic Grant N0014-67-A-0108-0004. to the Pacific is thus not necessary to explain REFERENCES CITED their opposed phasing of percent carbonate peaks with glacial episodes. Until absolute Arrhenius, G. Sediment cores from the East sedimentation rates of the component frac- Pacific: Swedish Deep-Sea Expedition (1947-1948) Repts., Vol. 5, fasc. 1, 89 p., tions are determined for cores in other areas 1952. as done by Broecker and others (1958), in- Arrhenius, G. Pelagic sediments, in The sea, terpretations of carbonate changes should be Vol. 3: Interscience, New York, p. 655- carefully considered in the light of possible 727, 1963. terrigenous lutite dilution, carbonate dissolu- Bandy, O. L.; Casey, R. E.; and Wright, tion, and carbonate productivity variations. R. C. Climatic deterioration near the Brunhes-Matuyama boundary: 7th Cong. SUMMARY AND CONCLUSIONS Inqua, Resumes Communications, Sec. 2, Faunal climatic curves from the equatorial p. 61-62, 1969. Beard, J. H. Pleistocene paleotemperature Atlantic suggest that the post-Jaramillo Pleis-

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