Cenozoic radiolarian paleobiogeography: Implications concerning plate tectonics and climatic cycles

Palaeogeography, Palaeoclimatology, Palaeoecology, 26(1979): 253--289

253

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

CENOZOIC RADIOLARIAN PALEOBIOGEOGRAPHY: IMPLICATIONS CONCERNING PLATE TECTONICS AND CLIMATIC CYCLES

FLORENTIN J.-M° R. MAURRASSE

Department of Physical Sciences, Florida International University, Miami, Fla. 33199 (U.S.A.) (Received November 7, 1977; revised version accepted August 17, 1978)

ABSTRACT Maurrasse, F. J.-M. R., 1979. Cenozoic radiolarian paleobiogeography: implications concerning plate tectonics and climatic cycles. Palaeogeogr., Palaeoclimatol., Palaeoecol., 2 6 : 2 5 3 - - 2 8 9 A preliminary study of the paleobiogeographic patterns of radiolarian facies during the Paleogene and subsequent time shows that: (1) Through time radiolarian assemblages display distinct faunal provincialism reminiscent of modern faunal distributions correlated with planetary temperature gradients and surface oceanic conditions. The equatorial--tropical radiolarian fauna extended apparently unrestricted across the Pacific Ocean, the Caribbean Sea and the Atlantic Ocean through Early Miocene time. In the Caribbean Sea and the Atlantic Ocean, radiolarians reached their maximum abundance in the Eocene and Oligocene. Subsequently, they gradually declined to virtual disappearance in these areas in the early Miocene. Their Pacific counterparts remained practically undisturbed, except that post early Miocene assemblages there showed a marked trend toward decreasing test thickness. This trend has since been a worldwide characteristic of Neogene radiolarian assemblages and their modern equivalents. It is postulated that the disappearance of radiolarians in the Caribbean Sea and the Atlantic Ocean at the end of the Paleogene is related to the onset of the emergence of the isthmus of Panama which interrupted the preexisting oceanic circulation between the Pacific and Atlantic Oceans. (2) Throughout the Paleogene there have been marked sequential fluctuations in the radiolarian assemblages of the Caribbean Sea which indicate intermittent incursions of higher-latitude fauna in this area. Associated with the faunal fluctuations are cyclic variations in the total carbonate of the sediment with patterns also comparable in duration to Pleistocene carbonate cycles in the equatorial Pacific known to have been induced by climatic changes. Based on similarities with Pleistocene climatic cycles in the equatorial Pacific and elsewhere, it is surmised that the faunal and lithologic fluctuations observed in Paleogene radiolarian sediments were also induced by the biologic and physico-chemical processes associated with worldwide changes in the climatic conditions of that time. INTRODUCTION

Considerable oceanographic data gathered in the past two decades have provided excellent means for defining both present and past biogeographic distributions of various microplanktonic organisms that are useful as paleoclimatic indicators. The reliability of the fossil groups stems from modern

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29 30 31 146 149 150 151 152 153 117A 10 13 14 72 73

14°47.11'N 12°52.92'N 14°56.60'N 15°06.99'N 15°06.25'N 14°30.69'N 15°01.02'N 15°52.72'N 13°58.33'N 57°20.00PN 32°37.00tN 06°02.40'N 28°19.89'N 00°26.49'N 01°54.58'S

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256 global oceanic data showing that all marine microplanktonic groups display gradients in taxonomic diversity which are consistently covariant with the relative planetary temperature gradient and oceanic water masses. Since modern assemblages are practically the same as those of the Pleistocene, the paleoecologic significance of the faunal fluctuations recorded in sediments of that epoch have been ascertained with considerable accuracy. Thus, the fluctuations in various microplanktonic groups recorded in Pleistocene sediments, compared to present population distribution patterns in the ocean's surface, have been successfully equated with past climatic fluctuations with the foraminifera (Ericson et al., 1956, 1961; Emiliani, 1964; Stehli, 1965; Ericson and Wollin, 1968; Ruddiman et al., 1970; Ruddiman, 1971; Gardner, 1973; Kellogg, 1973; Prell, 1974), the Coccolithophorida (McIntyre, 1967; McIntyre et al., 1970; Caulet and Clocchiatti, 1975), the Radiolaria (Hays, 1965; Hays et al., 1969; Duncan et al., 1970; Nigrini, 1970) and the diatoms (Kanaya and Koizumi, 1966; Donahue, 1967, 1970). Similar principles have also been successfully applied using either group for paleoclimatic and paleoceanographic reconstruction of deep-sea sediments older than Pleistocene from various parts of the world (Vella, 1967; Jenkins, 1968; Hays, 1970; Casey, 1970, 1971, 1972; Margolis and Kennett, 1971; Kennett and Vella, 1974; Kennett and Watkins, 1974; Blank and Margolis, 1975; Savin et al., 1975). Although Paleogene radiolarian biostratigraphy has developed considerably in the past eight years or so through the results of the Deep Sea Drilling Project expeditions, so far there have been no ecological inferences concerning the biogeography of the species and their potential paleoecological usefulness. The main objective of this paper is to (1) present the tectonic and climatic implications of radiolarian facies during the Paleogene and subsequent periods and (2) discuss the evidence of midPaleogene radiolarian biogeography and faunal provincialism. Furthermore, sequential fluctuations of taxa that have been determined to correlate with either low- or high-latitude assemblages also provide a basis for paleoclimatic and paleoceanographic interpretation of sequential Paleogene biofacies changes observed in cores of various deep-sea areas of the world (Figs.l, 2). METHODS The samples used in this study are from a total of 32 sites (Figs.l, 2), including eighteen piston cores from the Lamont-Doherty Geological Observatory core library and fifteen from Deep Sea Drilling Project cores. Five additional DSDP sites not shown in Figs.1 and 2 were also studied: Leg 8, Equatorial Pacific sites 72 (00°29.49'N, 138 ° 52.02'W) and (01°54.02'S, 137°28.12'W); Leg 10, Gulf of Mexico sites 86 (22°52.48'N, 90°57.75'W), 94 (24°31.64'N, 88°28.16'W), 95 (24°09.00'N, 86°23.85'W) and 96 (23°44.56'N, 86°45.80'W). Complementary data have also been taken from the various Deep Sea Drilling Project Initial Reports as listed in the

257

References. The cores chosen are considered to be located in key geographic areas of the world's ocean; secondary geographic displacement due to sea-floor spreading as shown by magnetic data (Heirtzler et al., 1968; Francheteau, 1970) has also been taken in consideration in their selection. The main cores were selected primarily on the basis of their reported radiolarian content. Most of the cores were calcareous siliceous with varying a m o u n t of carbonate. Among the cores studied, nine did not have reported Radiolaria, but their locations could indirectly give further information on the extent of radiolarian sediments at various epochs within the Tertiary. 90ow

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Sampling of the cores was systematically carried out within the various lithologies as could be deduced from color layering which was very evident in most of them. Sample intervals vary from about 50 cm in thick homogeneous sequences to less than 5 cm at boundaries of changing lithologies. Standard sample size was approximately one cubic inch (ca. 16 cm3). In order to determine state of preservation and nature of the fine matrix, a smear slide was made for every sample. Each sample was oven dried at about 100°C, weighed, disaggregated in tap water containing about two parts per thousand of calgon (sodium hexametaphosphate), then sonified for about 30 sec or less at low frequency. The sample was then washed with a weak jet of tap water in a 38-pm sieve. This process eliminated particles of medium silt and clay sizes, but ensured m a x i m u m retention of small radiolarian specimens which would otherwise have been lost. Distilled water was used for the final

258 wash. The coarse fraction was oven dried at less than 100°C for 15--30 min, then weighed. The weight was reduced to percentage of original dry sample weight. The entire coarse fraction was then examined in a foram tray under stereomicroscope for identification of total and dominant particulate elements. After this first examination, one or the other of the following steps were undertaken: (1) If the coarse fraction consisted essentially of siliceous particles, a standard radiolarian slide (Riedel, 1959) was prepared from an aliquot of the residue. (2) When calcareous components were present, the sample was treated with 2N hydrochloric acid, washed, oven dried and weighed. The noncalcareous coarse fraction was also reduced to percentage of original sample weight. The slide made from this residue was prepared following the same m e t h o d as in the preceding case. Carbonate analyses of the calcareous portions of the cores were made from dry bulk samples by use of an apparatus similar to that described by Hiilsemann (1966). Basically it involves the chemical reaction of hydrochloric acid on the carbonate fraction of the dry sample and the formation of carbon dioxide. The amount of CO2 generated is measured in a burette by the displacement of a column of mercury. Because the product of the pressure and volume of a gas is a constant which depends only upon the temperature (Boyle's law), calculations of the CO2 generated were made from this principle. Also, the total amount of gas generated is directly proportional to the amount of carbonate present in the sample. Thus, in the graphs, carbonate content is given as weight percent of the dry sample. Carbonate content for many of the Deep Sea Drilling samples was analyzed by the Scripps Institution of Oceanography for the preliminary results of Legs 4 and 15. The Scripps Laboratory used a LECO 70 Second Carbon Analyzer as described in Leg 4 (Bader et al., 1970) and Leg 9 (Hays et al., 1972). This m e t h o d uses the difference in thermal conductivity between oxygen and carbon dioxide. The Hiilsemann m e t h o d carried an absolute inaccuracy of about 1.5% whereas the LECO method as compared with the latter, gave values around 1--2% lower, particularly in results given for Leg 4.

Coarse-fraction study All particulate constituents were accounted for, and the absolute amount of benthonic and planktonic Foraminifera and ostracods (very rare) were counted and reduced to relative number per grams of sediments (Figs.6, 7). Planktonic foraminifera were generally scarce in the sites at depths below 3000 m in the present topography. Several symbols related to relative estimated frequency and state of preservation of both calcareous and siliceous microfauna are also shown in these figures.

259 -+ T R F C A D

Absent Present Present, negligible by weight or volume of total coarse fraction Rare, less than 1/100 of total coarse fraction Few, less than 1/10 of total coarse fraction Common, greater than 1/10 and less than 1/3 of total coarse fraction Between 1/3 and 1/2 of total coarse fraction Dominant, greater than 1/2 total coarse fraction, or nominal when more than two elements are equally abundant 1 Slight or no obvious dissolution effect. Planktonic foraminifera are represented by whole tests, and fragments represent less than 1/10 of the estimated total planktonic foraminifera present in the total coarse fraction larger than 38 pm. 2 Obvious etching of tests. Planktonic foraminifera fragments represent between 1/2 and 2/3 of total estimated planktonic foraminifera present in total coarse fraction larger than 38 pm. 3 Strong etching of tests. Planktonic foraminifera or radiolarian fragments predominate in their respective assemblages. 4 Whole tests very scarce or absent. Fragments dominate. The absence of a numerical subscript beside the lettered symbols indicates very good preservation of the corresponding fauna.

Smear slide study Smear slides were checked mainly for relative preservation of the calcareous nannoplanktons. They were examined with a Leitz (Wetzlar) petrographic microscope and a Wild (M20) microscope.

Radiolarian slide study These slides were studied under natural transmitted light with the same microscopes previously mentioned. Because of the scarcity of certain groups, such as Lithomitra, Bathropyramis, Peripyramis, Cornutella, the cannobotryids (Plate I, 1--7) and the diatoms referred to as Triceratium (Plate I, 8), Hemiaulus (Plate I, 14), and Pyxidicula (Plate I, 9), which appeared to be good paleoecological markers, many of the slides, particularly all those of the Caribbean samples, were studied in their full content, instead of by random count across the slide. Relative percentages were estimated for species whose abundance exceeds 5% of the total assemblage by counting 600 individuals per slide. These estimates were often double-checked by supplementary examination of the coarse fraction under stereomicroscope. In general, apart from the very rare species numbering less than one per thousand, there was good agreement b e t w e e n the assemblage represented in the slide and that of the total coarse fraction examined under the stereomicroscope. FA CTO R S I NF LUENC I N G THE OCCURRENCE OF MODERN R A D I O L A R I A N SEDIMENTS AND COMMUNITIES

Because of similarities between the Cenozoic sediments studied and present

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Artostrobium g r o u p 1. DSDP 1 5 2 - - 2 - - 2 / 6 6 - - 6 7 cm.

Lithomitra g r o u p 2. D S D P 1 5 2 - - 1 - - 2 / 1 2 8 - - 1 2 9 cm. 3. C a n n o b o t r y i d R C 9 - - - 5 8 / 4 8 8 - - 4 9 0 cm. Theocampe armadillo g r o u p 4. R C 9 - - 5 8 / 2 7 0 - - 2 7 2 cm. 5. R C 9 5 8 / 2 9 0 - - 2 9 2 cm. Peripyramis g r o u p 6. V 2 8 - - 4 3 / 3 2 0 cm. Cornutella g r o u p 7. R C 8 - - 2 1 6 8 9 - - 6 9 1 cm. 8. D i a t o m r e f e r r e d t o as Triceratium V 2 8 - - 1 2 0 / 3 9 5 - - 2 9 6 cm. 9. A s s e m b l a g e o f c o l d - w a t e r d i a t o m s r e f e r r e d to in t e x t as Pyxidicula R C 1 2 - - 2 3 7 / 2 9 2 - - 2 9 3 cm. Lophophaena cf. L. auriculaleporis g r o u p 10. V 2 8 - - 4 3 / 1 8 0 cm. 11. R C 9 - - 5 5 / 1 0 2 0 cm. Lophocyrtis biaurita g r o u p 12. R C 9 - - 1 0 7 1 4 6 6 - - 4 6 8 cm. 13. D S D P 1 0 - - 9 - - 3 1 1 4 6 - - 1 4 7 cm. 14. Stylotrochus charlestonensis V 2 8 - - 4 3 1 4 0 0 - - 4 0 2 cm. ( A r r o w at u p p e r r i g h t c o r n e r p o i n t s at d i a t o m r e f e r r e d t o in t e x t as Hemiaulus). 15. Calocyclas semipolita V 1 7 - - 1 0 7 / 4 0 0 cm. 16. Theocotyle cryptocephala nigrinae DSDP 1 0 - - 9 - - 3 / 1 4 6 - - 1 4 7 cm. 1 7 Phormocyrtis striata striata DSDP 1 0 - - 9 - - 3 / 1 4 6 - - 1 4 7 cm. 1 8 Theocampe mongolfieri R C l 1 - - 1 9 9 1 1 1 7 5 - - 1 1 7 8 cm. 19 Spongodiscus cf. S. americanus R C 9 - - 5 5 / 8 5 0 cm. 2 0 Podocyrtis chalara DSDP 1 3 - - 3 - - 1 / 9 7 - - 9 8 cm. 2 1 Podocyrtis sinuosa D S D P - - 1 4 9 - - 3 2 - - 4 1 1 0 0 - - 1 0 1 cm. 2 2 Thyrsocyrtis rhizodon D S D P 1 4 9 - - 3 2 - - 2 / 2 6 - - 2 7 cm. 23 Podocyrtis papalis DSDP 1 4 9 - - 2 2 - - 2 / 2 6 - - 2 7 cm. 24 Thyrsocyrtis triacantha R C l 1 - - 1 9 9 / 1 1 7 5 - - 1 1 7 6 cm. 25 Podocyrtis mitra DSDP 1 4 9 - - 3 3 - - 1 / 4 4 - - 4 5 cm. 26 Thyrsocyrtis tetracantha R C 9 - - 5 8 / 4 8 8 - - 4 9 0 cm. 27 C o l d - w a t e r assemblage ( L a t e E o c e n e ? ) including very large s p o n g o d i s c i d s a n d a b u n d a n t d i a t o m s o f t h e Triceratium a n d Hemiaulus groups.

262

deep-sea deposits, the factors controlling the faunal composition of m o d e m radiolarian sediments are of critical importance in the understanding of the mechanisms involved in the occurrences of the ancient analogs. A survey of the literature on modern deep-sea sediments shows that opaline silica in the form of Radiolaria occur in significant abundance in modern abyssopelagic sediments only on the floor of certain specific areas of the world oceans, mainly in the northwest and equatorial Pacific, the equatorial Indian Ocean, and around Antarctica (Fig.4). The surface waters of these areas are also known to be the sites o f dynamic divergence {Fig.l); hence, they are constantly replaced by ascending waters from intermediate depths. This upwelling mechanism, which is associated with the major wind systems and the Coriolis effect, thus leads also to a constant return of nutrients to the surface waters. Consequently, waters of upwelling areas that are greatly enriched in nutrients such as phosphate, nitrates, silica, etc., are characteristically colder and are the most productive of Radiolaria. Therefore, the distribution pattern of radiolarian sediments reflects the ecological conditions of surface waters at these locations which are conducive to greater biogenic silica productivity. These conditions are primarily dependent upon the velocity of regional wind systems which are critical in determining the magnitude of upwelling in a given area at a given time. The general relationships between atmospheric and oceanic circulations on the one hand, and oceanic circulation and upwelling on the other, are very evident in the equatorial--tropical zones. These regions are indeed well known to show remarkable seasonal changes in their circulation pattern and intensity of upwelling as a result of concomitant shifting of the major atmospheric pressure cells. Thus, n o t only does the local wind field affect the ocean currents, but changes of the general atmospheric circulation over large oceanic regions, comprising the whole hemisphere or more, also seem to affect the oceanic circulation (Neumann and Pierson, 1966). Modern distribution patterns of radiolarian sediments not only show their close relationship with upwelling areas related to dynamic divergence, but they also show distinct faunal provincialism covariant with planetary temperature gradients and water masses (Popofsky, 1908; Riedel, 1958; Hays, 1965; Petrushevskaya, 1968; Nigrini, 1967, 1970; Goll and BjCrklund, 1971). Among the various radiolarian taxa, it appears that spumellarians such as the spongodiscids, and nassellarians of the Superfamily Archipilliceae (Haeckel, 1882, amended Campbell, 1954) provide species that are often characteristic of cold-water communities. Nassellarians that show cold-water affinities include, among other taxa, the following genera: Bathropyramis, Peripyramis, Cornutella, Lophophaena, Lophocyrtis, the artostrobiids and cannobotryids (Plate I), of which some species are also cosmopolitan (Petrushevskaya, 1968). The most characteristic taxa that show warm-water affinities in modern assemblages seem to belong to the Theoperidae (Haeckel, 1881, amended

263 Riedel, 1967). Genus groups of these families, which are most typical of the warm-water realm, are usually of large size, with smooth, elongated to bellshaped abdomen, large basal opening and apical horn of varying length, but more reduced in warmer water (Plate I, 20--26). Ecophenes of these taxa which occur in cooler water seem to develop more ornamental spines on the outer surface of the shells, and often show also much greater development of apical horns. As for all other living organisms, species diversity in modern radiolarian assemblages also increases systematically toward low-latitude regions. Similarly, the n u m b e r of specimens in the assemblages, or species dominancy, increases toward high-latitude environments and vice versa toward low~ latitude environments (Hays, 1965; Petrushevskaya, 1968). Thus cold-water assemblages are significantly impoverished in number of species in comparison with those of low-latitude regions. PALEOGENE RADIOLARIAN DISTRIBUTION PATTERNS The t a x o n o m y used in this section is partly after Campbell (1954) and mostly based upon recent works by Riedel and Sanfilippo (1970, 1971), Sanfilippo and Riedel (1973), and Foreman (1973). The radiolarian biostratigraphic zonation used as time framework (Fig.3) is after Riedel and Sanfilippo (1970) subsequently completed and amended by Sanfilippo and Riedel (1973), Moore (1971), Foreman (1973), Dinkelman (1973) and Maurrasse (1973). These radiolarian zones are based essentially on tropical assemblages because they were predominant in the time range considered in this study. They would, therefore, have been of little use for most high-latitude areas if climatic fluctuations did not allow migrations of the temperate species into these regions, and vice versa. In either case further caution was exercised as it appears that the range of certain species that occur in both high- and lowlatitude cores discussed in this study may actually be diachronous, due to adaptation or permanent migration with changing environmental conditions through their time range (Maurrasse, in preparation). Fig.3 is a synoptic representation of eighty-six selected species considered to be the most reliable, biostratigraphically and paleoecologically. The great majority of the species shown at the left side of this figure does not occur in most of the high-latitude cores. Few exceptions are Calocyclas semipolita semipolita (Plate I, 15), Theocotyle cryptocephala nigrinae (Plate I, 16), and Phormocyrtis striata striata (Plate I, 17) which are cosmopolitan species. They reach their optimum abundance in the middle-latitude areas. Theocampe mongolfieri (Plate I, 18) is also cosmopolitan throughout most of its range, but it does not appear in the Late Eocene radiolarian sediments of high-latitude cores studied. In contrast to the species shown at the left side of Fig.3, most of the taxa shown at the right side (Nos. 63 to 86), for instance the spongodiscids,

264 usually reach their o p t i m u m development and abundance essentially in highlatitude sites. Their occurrence is often very sporadic in the low-latitude areas. An exception among the spongodiscids is Spongodiscus americanum {Plate I, 19) which shows significant increase in size and abundance within levels of higher-carbonate sediments of the low-latitude sites. Among the taxa represented in Fig.3, twenty-two genus and species groups have been f o u n d to characterize faunal assemblages with distribution patterns reminiscent of modern faunal provincialism correlated with planetary temperature gradients arid surface oceanic conditions.

Selected radiolaria representative o f the different paleogene faunal provinces (cf. Plate I) Group 1. Warm-water species

Podocyrtis chalara Riedel and Sanfilippo Podocyrtis mitra Ehrenberg Podocyrtis papalis Ehrenberg Podocyrtis sinuosa Ehrenberg Thyrsocyrtids: Thyrsocyrtis rhizodon Ehrenberg Thyrsocyrtis tetracantha (Ehrenberg) Thyrsocyrtis triacantha (Ehrenberg) Theocampe mongolfieri (Ehrenberg) Podocyrtids:

Group 2. Cooler-water species

Spongodiscids:

Artostrobium spp. Calocyclas semipolita semipolita Clark and Campbell Phormocyrtis striata striata Brandt Amphicraspedum murrayanum Haeckel Amphicraspedum prolixum Sanfilippo and Riedel Spongodiscus americanus Kozlova Xiphospira circularis Clark and Campbell

Group 3. Cold-water species

Bathropyramis spp. Peripyramis spp. Cornutella spp. Lopophaena spp. Lophocyrtis biaurita (Ehrenberg) group Spongodiscids: Prunopyle occidentalis Clark and Campbell Spongodiseus phrix Sanfilippo and Riedel Stylotrochus nitidus Sanfilippo and Riedel Spongodiscus cruciferus Clark and Campbell L ychnocanoma amphitrite Foreman The distribution and diversity patterns of modern and Paleogene radiolarian sediments and communities summarized herein have been used as a basic framework for paleoecological interpretations. Assuming t h a t the distribution of mid-Paleogene radiolarian productivity, and their assemblages as well,

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269

were influenced by similar environmental conditions, not only should radiolarian sediments of that time indicate past upwelling areas, b u t latitudinal fluctuation of lower- and higher-latitude assemblages should also indicate related causes. Thus, any significant variations in the oceanic circulation through that time should be recorded by means of both variations in radiolarian sediment distribution patterns and the composition of their assemblages. CENOZOIC DISTRIBUTION PATTERNS OF RADIOLARIAN SEDIMENTS: THEIR PALEOECOLOGICAL AND TECTONIC IMPLICATIONS

A compilation of data available on the occurrence of Paleogene sediments containing abundant Radiolaria comparable to their modern analogs is shown in Fig. 5. Despite the effects of continental drift and sea-floor spreading (e.g., Vine, 1966; Heirtzler et al., 1968; Pitman et al., 1971; Talwani and Eldholm, 1972; Larson and Pitman, 1972) which have modified the relative positions of the sea floor and continental masses since the Middle Paleogene, it is evident that the most extensive radiolarian sediments of that time correspond to the paleoequatorial--tropical zones. Unlike the present (Fig.4), the Middle Paleogene radiolarian facies extend nearly continuously from the Pacific Ocean across the Caribbean Sea into the Atlantic Ocean. This is shown in Fig.5, which is actually m o r e representative of the time period subsequent to the Early Eocene, Buryella clinata zone (Fig.3). Prior to that time, from the Theocorys pseudophyzella zone to the base of the Buryella clinata zone (Fig.3), radiolarian-rich sediments occurred only sporadically within the Caribbean and the Atlantic Ocean (Fig.6). In the Pacific, however, radiolarian sedimentation at the equator went uninterrupted, as is indicated b y the data from the various DSDP results concerning this area. Also, after the Early Neogene, radiolarian sediments virtually disappeared in the Caribbean Sea and the Atlantic Ocean until the present. Many factors such as global climatic conditions, regional and global tectonism may be invoked to explain the various parameters which independently or conjointly may have affected oceanic circulation and the resultant faunal and sedimentological provinces of these periods.

Climatic factors As it has been summarized in the early part of this study, dynamic divergence is the main factor that controls upwelling and the resultant radiolarian productivity patterns of the world's oceans. It is also known that the main effects of changing thermal conditions at the surface of the earth are transmitted primarily to the oceans through sea--air interactions with the shift in the global pressure cells, which consequently affects the wind systems and surface circulation. Such a relationship, whereby changing global climatic conditions affect the pre-existing oceanic circulation pattern and radiolarian

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272 productivity, has been demonstrated in the Pleistocene (Nayudu, 1964; Hays, 1965; Duncan et al., 1970). In the Northeast Pacific region, for instance, the areal expanse of radiolarian productivity fluctuated significantly throughout the Pleistocene. These fluctuations have been positively correlated with the Pleistocene climatic cycles which induced changes in the circulation pattern of the area (Nayudu, 1964; Duncan et al., 1970). Similar fluctuations of various intensities and periodicities have also been demonstrated in the equatorial Pacific (Hays et al., 1969). The following paragraphs will discuss the evidence suggesting that similar phenomena may have been of significant importance among the factors that controlled the Cenozoic radiolarian productivity patterns referred to previously. The major change in radiolarian distribution pattern that occurred between the Paleogene (Fig.5) and the present (Fig.4) appears to have been established in the Early Neogene (Creager et al., 1973). The most striking feature of this change is the abundance of Radiolaria in the North Pacific in contrast to their total absence there in the Paleogene. Recent oceanographic and sedimentologic data (Stommel, 1957; Ried, 1962; Hays, 1970) have shown that such productivity is related to the dynamic divergence associated with the boundary zone where cold subarctic and warmer subtropical waters meet at about 40°N (Fig.l). The mean circulation is wind-driven; the movement of the subarctic water is cyclonic and the subtropical water is anticyclonic. Although it is evident that a large portion of the dissolved silica available in the Paleogene oceans was being taken up by an extremely productive paleoequatorial zone, it is unlikely that absolutely no biogenic silica would occur in the Northwest Pacific if the paleo-oceanographic conditions were nearly as close to those at present. Since the paleogeographic reconstructions of that time show that this area of the Pacific was wide enough to sustain gyres similar to the present ones, it can therefore be postulated that the prevailing global climatic conditions were unfavorable to such circulation. Climatic conditions conducive to the development of dynamic divergence in the area were probably achieved because the major climatic shift which occurred in the Early Neogene. The various DSDP resultsin the Pacific area and data from different parts of the world (Devereux, 1967; Gibson, 1967; Savin et al., 1975; Berggren and Hollister, 1977; and others) substantiate the fact that a major climatic deterioration occurred in the Early Neogene. Since such an event would be expected to affect global wind systems, it is also.likely to have altered the preexisting circulation pattern of the North Pacific area. However, can the climatic factors alone explain the virtual disappearance of radiolarian sediment in the Caribbean Sea and the Atlantic Ocean since that time? One may also wonder whether the Paleogene climatic conditions alone could have totally hampered silica productivity in the North Pacific for such a long time without minor changes. The same applies for a Neogene shift that has never been modified subsequently in either ocean since the major facies change occurred, despite

273 the fact that global climatic conditions similar to those that prevailed in the Paleogene did recur in subsequent times. The only significant departure from this scheme has occurred during the short Pleistocene cold pulses when sea-air interactions caused either minor t e m p o r a r y shifts in the silica productivity pattern (Nayudu, 1964; Hays et al., 1969; Duncan et al., 1970) or intermittent recurrences of biogenic silica productivity in previously unproductive areas (Gardner, 1973). Thus, although it can be ascertained that long- and short-term global climatic conditions could have really played a role in the changing patterns of Cenozoic radiolarian productivity, they do not seem to fully account for the permanent shift in facies that occurred between the Pacific Ocean and the Atlantic Ocean in the Early Neogene. Their absolute influences on the Paleogene radiolarian productivity patterns are also questioned since global climatic conditions conducive to such an occurrence in the Early Paleogene equatorial Pacific should have had the same effects throughout the paleoequator of that time. On the contrary, in the Early Paleogene the Caribbean Sea and the Atlantic Ocean developed biogenic silica productivity only periodically (Fig.6). The faunal assemblages associated with these intermittent events will be discussed in a latter paragraph, but in the present discussion I need to mention that, on the basis of these assemblages, the periodical radiolarian productivity in the Early Paleogene has also been interpreted to be the result of short-term climatic oscillations much comparable to those of the Pleistocene (Maurrasse, 1976). Since modern physiographic conditions appear to be the main factor that limits continuous upwelling in the Caribbean and the Atlantic, except during the colder pulses, it can be postulated by analogy that paleophysiography may have also played an important role in controlling the Cenozoic radiolarian distribution patterns in these areas, and possibly in other parts of the world's oceans of that time as well.

PHYSIOGRAPHIC INFLUENCES RELATED TO TECTONISM Figs.5 and 6 give data on the relative distribution patterns of radiolarian productivity between the Paleogene and the present. In addition to the main difference pointed out earlier concerning the absence of radiolarian facies in the Caribbean Sea and the Atlantic Ocean, the wider Paleogene radiolarian belt in the equatorial Pacific contrasts markedly with its modern analog. Since radiolarian productivity had been a permanent feature of this area throughout the Cenozoic, the wider belt in this area may find a logical explanation in the west--northwest spreading of the Pacific floor beneath the highly fertile equatorial waters (Hays et al., 1969, 1972). In this case, paleoclimatic conditions had little or no impact to that effect. Nonetheless, the rhythmic pattern in the vertical record of the radiolarian facies throughout that time (Hays et al., 1972, and other DSDP results) does suggest that short-

274 LDGO: VEMA

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275 demonstrated that Radiolaria started to occur at these sites only during the Early Neogene. Because of the northward motion of the Pacific crust, one could assume that, if the same hydrographic conditions existed prior to the Early Neogene, these sites might then have been outside of the productive zone. In fact, by using (a) the extrapolated spreading rate values of 3--5 cm/ year, and an underthrusting rate of 2--5 cm/year given for the area b y Hays and Ninkovich (1970); and (b) maximum ages of ca. 70 m.y. (Maastrichtian) for site 192 and 50 m.y. (Early Eocene) for site 183 (Creager et al., 1973), their locations could have indeed been away from their present positions by as much as 1750 to 2450 km and 1250 to 1750 km, respectively. Thus, these estimated values could have been large enough to place these sites at the southernmost riras of the subarctic gyres, or even within the northern parts of the nonproductive central gyre (Fig.l), at these times. However, if conditions conducive to radiolarian productivity did exist throughout these times until the late Early Miocene, the sites should have certainly reached and passed through the productive area much earlier than is actually recorded in their sedimentological sequences. Thus, it can be deduced that the Paleogene surface circulation of the Northwest Pacific was fundamentally different from that of subsequent times. Aside from minor modifications that occurred in local circulation due to periodical climatic fluctuations, the general trends of the Cenozoic circulation appear to also have prevailed independently of the effects of global climates or plate motions that had, however, indirect worldwide impact on the oceans. It is proposed that the permanent shift in radiolarian productivity which t o o k place in the Early Neogene between the Pacific and Caribbean/Atlantic regions is mainly related to the effects of the onset of the early stage of the "Isthmus of Panama". Although this early stage is regarded only as heralding the development of the subsequent land barrier, its presence is thought to have been significant enough to interrupt deep and intermediate circulation between the two regions. Climatic fluctuations and other physiographic modifications that occurred at that time only further enhanced these effects. The Radiolaria were apparently the only biogenic group affected then, because at this early stage the proto-isthmus interrupted the deeper circulation without affecting free exchange of the shallower surface waters in between the two regions. The developmental stages of the isthmus should have in fact been expected to show different expressions of their effects in the stratigraphic record, because of the different constraints of the biologic group being affected. Evidence showing such diversity can be found indeed in the study of the various biologic groups that have been used as indicators of the presence of the isthmus (Gibson, 1967; Webb, 1976; Emiliani et al., 1972), thus the discrepancies in ages that have been given in the literature concerning the emergence or closure of the land barrier. Perhaps, one could speculate that the earliest indication of the initial disruption between the circulation of the t w o regions might be found in the

276 Late Eocene change of the carbonate compensation level as is discussed by Ramsay (1977). However, at this time relatively stable circulation patterns still prevailed as the seaway must have been deep enough to allow deeperwater circulation to go unimpeded through the Caribbean region. The disappearance of radiolarian facies in the Caribbean/Atlantic region can thus be regarded as the result of further disruption in the pre-existing oceanic circulation system, or a more developed stage but not the ultimate emergence of the istmus which apparently occurred by the end of the Pliocene (Webb, 1976). Furthermore, other Lower Neogene events such as major orogenic activities in the Greater Antilles and concurrent crustal dislocation in the Caribbean may also be considered as contributing factors in the disruption of the preexisting circulation system between the Pacific and the Atlantic Oceans. The disruption of deep-water exchanges between the Pacific and Atlantic Oceans may have thus led to a re-adjustment of the circulation patterns in the two oceans. The new physiographic conditions combined with the changing climate of that time could have thus been conducive to a circulation pattern favorable to biogenic silica productivity in the Northwest Pacific. Conversely, the circulation patterns of the Caribbean Sea and the Atlantic Ocean once more became unfavorable to sustained upwelling, except during the intermittent colder stages of the Pleistocene (Edgar et al., 1973; Gardner, 1973). The ecological conditions conducive to radiolarian productivity in the Caribbean and Atlantic since the Early Neogene are thus reminiscent, although physiographically not analogous (Francheteau, 1970), to those that prevailed in these areas during the Early Paleogene. Other changes in worldwide paleophysiography due to plate motions may be further ascertained as contributing factors in the Early Cenozoic change of radiolarian distribution patterns in the world's oceans. Such evidence is provided by the results of the various DSDP expeditions throughout the world. It has been shown in fact that crustal movements which led to the widening of the Atlantic, the gradual development of the Labrador Sea triple junction, and the accelerated opening of the Denmark Strait and Norwegian Sea during the Cenozoic (Laughton, 1971, 1972; Laughton et al., 1972; Talwani and Eldohm, 1972) also caused the developing physiographic conditions of the Atlantic to gradually increase the discharge of the North Atlantic Deep Water (NADW). Since it is the main source of water for the Circum Polar Current (CPC, Fig.l), and the Antarctic Bottom Water into the Pacific Ocean, the end results are that the continuous southward flow of NADW depletes the Atlantic of its silica contents to the benefit of the Pacific (Broecker, 1974). Evidently such effects became possible because of the development of the Drake Passage between Antarctica and South America during the Late Cenozoic (Barker et al., 1974). Thus, the major Cenozic shift in radiolarian productivity between the two main oceans, can be not only related to the onset of the emergence of the

277 proto-isthmus of Panama, b u t it may also be the result of a combination of worldwide tectonic activities which led to profound modifications of the pre-existing oceanic circulation system. Concomitant changes in global climatic conditions could further enhance or attenuate their effects. EFFECTS OF THE LATE CENOZOIC PALEOPHYSIOGRAPHIC AND PALEOOCEANOGRAPHIC CHANGES ON RADIOLARIAN SILICA METABOLISM The Early Neogene change in worldwide circulation systems appears to have n o t only affected pre-existing radiolarian productivity pattern, b u t it may also have modified the silica metabolism of Radiolaria altogether. Neogene to recent radiolarian shells throughout the world show evidence for a significant trend toward decreasing test thickness. For instance, Moore (1969) has demonstrated that the average skeletal weight of Quaternary Radiolaria is only one-fourth that of Eocene Radiolaria. Moreover, structural changes attributed to evolutionary trends of radiolarian shells (Harper and Knoll, 1975) also occurred at the same time. The latter suggested that the change in radiolarian shell thickness was related to the silica budget of the open ocean where increasingly less silica became available to the Radiolaria because of an increase in diatom abundance during the Late Cenozoic. Their suggestion remains very hypothetical as studies concerning the absolute abundance of diatoms versus Radiolaria through time are practically nonexistent. In addition, it should also be pointed o u t that the factors controlling silica metabolism in Radiolaria are still unclear. It has been shown that in Antarctica, for instance, radiolarian shell walls are very thin north of the Polar Front, compared with those to the south (Hays, 1965). Contrary to what one might have inferred from Harper and Knoll's suggestion, diatom oozes are rather scarce north of the Front as opposed to an extremely fertile diatom belt to the south. Goll and Bj~rklund (1970) also reported another peculiar phenotypic change in Atlantic Radiolaria which show significant variations in their index of refraction, depending on the area considered. Similar observations also pertain to the recurrent Eocene radiolarian assemblages of the Atlantic (Maurrasse, 1976, and in preparation). In b o t h the recent and past assemblages indicated above, radiolarian shell thickness and index of refraction vary independently of the presence or absence of diatoms. In view of these observations it is plausible that other geochemical factors such as those related to the overall chemical balance of the oceans might have been involved in modifying radiolarian silica metabolism. In this case, since the change in shell thickness is n o t confined to the Atlantic, it can be surmised that the availability of silica p e r s e may not be the predominant factor responsible for the observed change. Cenozoic radiolarian facies distribution patterns, their related assemblages and phenotypic characteristics thus show that the effects of changing global

278 paleophysiography through that time culminated with a major change in the oceanic circulation systems, and possibly some minor geochemical imbalance as well. The latter chemical changes may still have a limited inhibiting effect on radiolarian silica metabolism. PALEOECOLOGICALAND PALEOCLIMATICIMPLICATIONSOF RECURRENT RADIOLARIAN BIOFACIES As indicated in the early discussion, the Lower Paleogene abyssopelagic sediments of the Caribbean and Atlantic are characterized by alternating layers of calcareous and calcareous--siliceous cycles. Later, calcareous-siliceous cycles with varying amounts of carbonate became prevalent. In either case the biofacies fluctuations are also concurrent with sequential faunal changes (Figs.6--9). These fluctuations are further characterized by a greater frequency of radiolarian taxa, usually of high-latitude affinities (spongodiscids and other species shown at the right side of Fig.3) at the climax of the assemblages. Other significant features associated with the recurrent radiolarian facies are that (1) the acme of these assemblages typically occurs within the carbonate highs (Figs.8, 9), and (2) a greater relative percentage of high-latitude radiolaria and diatom taxa also coincides with the carbonate highs (Figs.8, 9). In the preceding discussions, periodic climatic fluctuations were proposed to be the main physical factor which intermittently enhanced upwelling, because of their direct effects on sea--air interactions. Colder periods are believed to stimulate such interactions particularly in the low-latitude regions. Although the present data cannot conclusively specify the intensity of the proposed colder periods which intensified the oceanic circulation, the duration and periodicity of the cycles show some analogy with those of the Pleistocene (Maurrasse, 1976). Unlike the Pleistocene, however, the available Lower Cenozoic paleoecological data (VeUa, 1967; Geitzenauer et al., 1968; Peterson and Abbott, 1976; Maurrasse, 1976) are insufficient to prove that true glacial periods occurred at that time. But, such possibilities cannot be totally ruled out as the evidence which will be discussed in the following paragraphs suggests. In the Caribbean Paleogene sediments the intermittent occurrence of very cold water diatom taxa referred to as Triceratium (Plate I, 8), Hemiaulus (Plate I, 14 at arrow and 27), and Pyxidicula (Plate I, 9), together with highlatitude radiolarian species do suggest periodic incursions of colder water into the lower-latitude waters of that time. Even though real glaciations cannot be invoked at present, the recurrence of such high-latitude taxa so far into the low-latitude regions could imply severe intermittent cooling periods. Other evidence that might suggest giacio-eustatic (Fairbridge, 1958) variations in the Paleogene may be taken from the Gulf Coast region. According to Fisher (1964) cyclic deposition is the most distinctive feature of the Gulf Coast Eocene sequence. The cycles are well defined and are

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generally characterized by alternations of marine transgressive (and/or regressive) and non-marine regressive units. Despite the high rate of sedimentation in such an environment, the thickness of each cycle varies only within the order of a few hundred feet or less. Approximately 39 units are present in the sequence. If 19.3 m.y. is taken as the maximum duration for the Eocene (Maurrasse, 1973, 1976), the periodicity of these cycles falls within the range of large amplitude cycles similar to those found in the Paleogene (Figs.3, 7, 8, 9) and they are also comparable to such cycles in the Pleistocene (Hays et al., 1969; Maurrasse, 1976; Hays et al., 1976). Because this pattern of cyclic sedimentation is very widespread in the Gulf Coast region (Fisher, 1964), and on account of other well-defined Eocene cycles reported in New Zealand, Europe and Australia (Vella, 1967) it is plausible that global climatic fluctuations had worldwide effects on the Paleogene sedimentation. The different Paleogene cycles may thus show various local responses to the superimposed effects of a cause of global importance, the climate. The Pleistocene sedimentary and faunal cycles lend evidence in support of this

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assertion. It has indeed been demonstrated that the various worldwide Pleistocene cycles are the results of global climatic fluctuations. The responses to the climatic factors depended on both local physiography and latitude. Thus, the nature, magnitude and frequency of these cycles at a particular location were the results of complex interacting factors. In the abyssopelagic environments, for instance, water depth played an important role in the final appearance of the cycles due to the effects of carbonate dissolution (Hays et al., 1969; Ruddiman, 1971). In that case short climatic pulses are characteristically attenuated or blurred with increasing depth. Conversely,

282 the longer, and more prominent oscillations, are the only ones preserved. Similar patterns also occur in the Paleogene sites studied. By analogy with the Pleistocene, it is thus logical to infer that periodic shifts in global climatic conditions did occur in the Paleogene. Likewise, these climatic fluctuations probably involved both minor and larger-scale oscillations of various intensities (Geitzenauer et al., 1968; Peterson and Abbott, 1976; Maurrasse, 1976). The pattern of latitudinal migrations of the different biologic assemblages, which occur concurrently with the Paleogene lithologic cycles, is also in accord with climatically induced shifts of faunal and floral provinces correlative with changes in planetary temperature gradients and surface oceanic conditions observed in the Pleistocene. Similarly, we can infer that the Paleogene taxa also had the same ecological constraints as their Pleistocene counterparts, therefore, their apparent latitudinal fluctuations can be interpreted in terms of the same controlling factors. At the present state of our knowledge on the Paleogene radiolarian paleobiogeography, the faunal fluctuations cannot be immediately interpreted as indicating glaciations (Geitzenauer et al., 1968). Nonetheless, the observed lithologic and biologic fluctuations do suggest tangible climatic oscillations at that time. CONCLUSIONS

The data discussed herein show that the Paleogene and subsequent radiolarian sediments, together with their faunal distribution patterns may be the results of the combined effects of paleophysiography and prevailing paleoclimatic conditions. For instance: (1) Recurrent radiolarian sediments characterized the Caribbean and Atlantic prior to the Middle Eocene, while at the same time the Pacific had sustained radiolarian facies. This is interpreted to indicate that at this stage of opening the Atlantic was not wide enough (Francheteau, 1970) to allow the existing circulation pattern between the two oceans to generate permanent upwelling under the prevailing climatic conditions of that time. However, upwelling and probably also other geochemical conditions conducive to radiolarian productivity were able to develop intermittently. Because of the character of the faunal assemblages which characterized the intermittent radiolarian facies, they are interpreted to have been induced by intensified circulation during colder, short-term global climatic oscillations superimposed on the longer-period prevailing climatic conditions. Similar periodic oscillations of various frequencies are also recorded in subsequent times. (2) Permanent radiolarian facies became established throughout the paleoequatorial zone from the Middle" Eocene until the Early Miocene. Subsequently they receded from the Caribbean and Atlantic, while abundant radiolarian facies occurred simultaneously as a new feature in the Northwest Pacific region. Reduced radiolarian shell thickness throughout the world further characterizes this major shift of facies. The shift is interpreted as the first significant indication of the disturbing effects of the onset of the Central

283

American isthmus on oceanic circulation. It is also suggested that some change in the overall chemistry, not necessarily related to silica balance, may have taken place concurrently and probably affected radiolarian silica metabolism. The geochemical change in the oceans may have also resulted from the cumulative effects of worldwide and regional crustal displacements which progessively disrupted pre-existing paleophysiographic and paleohydrologic conditions of the oceans and continents. Although the major shift in post-Paleogene silica productivity was also affected by the superimposed effects of concomitant climatic changes, it is postulated that the shift was mainly induced because of the disrupting effects of the early stages of the proto-isthmus on the pre-existing oceanic circulation. Carbonate layering together with distinctive faunal fluctuations also occurred throughout the Cenozoic radiolarian facies. Because of the periodicity of the Paleogene fluctuations and the relationships that exist between the lithologic and biologic changes, these periodic changes are believed to show the record of past oceanic responses to global climatic oscillations. The effects and frequencies of these oscillations are comparable to those of the Pleistocene. Similarly, the Cenozoic climatic cycles suggest that stronger temperature gradients developed in the equatorial regions during the colder periods. Although these climatic cycles are very reminiscent of those of the Pleistocene, their actual intensity remains to be determined. ACKNOWLEDGEMENTS

Most of the work presented in this paper was carried out while the author was a Graduate Research Assistant under Dr J. D. Hays of Columbia University. Sincere thanks are due to Mr William Riedel and Ms Paula Worstell for DSDP samples from Legs 2, 3, 4, 8, 10 and 12, and additional samples from Leg 15 in which the author participated. Thanks are due to Dr Robert Goll who provided samples from the Eocene of California and the island of Barbados. Drs J. D. Hays and K. Vankatharatnam were kind enough to let the author use their unpublished maps shown in Figs.4 and 5; this is greatly appreciated. Ms Diana Richardson typed the manuscript. REFERENCES Arrhenius, G., 1952. Sediment cores from the East Pacific: Rep. Swed. Deep Sea Exp. (1947--1948), 5 (G~teborg, Sweden). Bader, R. G., Gerard, R. D., Benson, W. E., Bolli, H. M., Hay, W. H., Rothwell, W. T., Jr., Ruef, M. H., Riedel, W. R. and Sayles, F. L., 1970. Initial Reports of the Deep Sea Drilling Project, 4. U.S. Government Printing Office, Washington, D.C., 753 pp. Barker, P. F., Dalziel, J. W. D., Elliott, D. H., Von der Borch, C. C., Thompson, R. W., Plafker, G., Tjalsma, R. C., Wise, S. W., Jr., Dinkelman, M. G., Gombo, A. M., Jr., Menardi, A. L. and Tarney, J., 1974. Southwestern Atlantic. Geotimes, 19: 16--18. Berger, W. H., 1968. Radiolarian skeletons: Solution at depth. Science, 159: 1237--1238.

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