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Pleistocene Sedimentation in the Equatorial Atlantic

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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 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 45° 30' 15° 0° *•-. « CAPE VERDE * ISLANDS •• *' 15° o \ 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 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 CORRELATIONS OF PELAGIC CORES 285 EAST K9-5I K9-! \1 p 2 J 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 ap- parently 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 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 286 RUDDIMAN-PLEISTOCENE SEDIMENTATION, EQUATORIAL ATLANTIC 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 in- terpretations 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.
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