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Biogenic Silica Accumulation in the Central Equatorial Pacific and its Implications for Cenozoic Paleoceanography
Abstract
Since the Deep Sea Drilling Project (DSDP) has made available long cores through Cenozoic deep-sea sediments, geologists have begun to study the history of sedimentation in the world ocean and to infer the oceanographic conditions which have influenced deposition of the sediment. During this time, the central equatorial Pacific Ocean has become one of the most intensively studied geologic provinces in the world ocean. This area lies beneath the biologically productive equatorial current system which supplies great quantities of calcium carbonate and amorphous silica (opal) to the sediment in the form of skeletal tests. This biogenic debris, together with terrigenous, authigenic, and hydrothermal sediment, has produced an unusually thick and complete marine record which was cored by four DSDP Legs (5, 8, 9, and 16). Because the productivity of the equatorial Pacific is a direct consequence of large nutrient concentrations associated with upwelling in the complex equatorial current system, the sedimentary section can be used to interpret the paleoceanography as well as the history of productivity in the equatorial Pacific. The history of carbonate sedimentation in the central equatorial Pacific is well known from the Initial Reports of DSDP Legs 8, 9, and 16 (Tracey and others, 1971; Hays and others, 1972; van Andel and Heath, 1973), as well as from a wealth of published information (Hays and others, 1969; McManus and others, 1972; Berger, 1973; Winterer, 1973; van Andel and others, 1975). There are limitations, however, to the use of carbonate sediments for interpreting the paleoceanography and productivity of surface waters. The patterns of distribution and accumulation of calcareous sediments in the deep ocean are modified by the dissolution of carbonate in undersaturated bottom Waters at rates which vary with depth (Peterson, 1966; Berger, 1973) and with time (van Andel and others, 1975). Because we are, at present, unable to determine past rates of carbonate dissolution in bottom waters unambiguously (van Andel and others, 1975; Heath and others, 1977), we are also unable to assess accurately the rate of supply of carbonate to an area in the past.
... Rea and Leinen (1985) pointed out that such a conclusion is consistent with the model of Broecker and Peng (1982) that directly relates the thickness of this sublysocline zone (i.e., the vertical separation between the top of the lysocline and the CCD) to the downward flux of carbonate from the surface. Their conclusion is supported by evidence for increased equatorial Pacific productivity of both calcareous and siliceous organisms in the early Miocene (van Andel et al., 1975;Leinen, 1979). ...
... If real, these differences would seem to indicate that the first-order controls on the depth of the CCD and on carbonate accumulation patterns above may be somewhat uncoupled. Leinen (1979), for example, showed that equatorial Pacific productivity was already fairly high in the early Miocene, with increasing biogenic silica accumulation after about 17-18 Ma. At our Leg 115 sites, the reappearance of silica and the first evidence for high Neogene productivity substantially postdates this equatorial Pacific event (i.e., late Miocene; -8-9 Ma). ...
... Vincent (1974), for example, concluded from Leg 24 data that the modern high-productivity belt in the equatorial Indian Ocean formed in the middle to late Miocene after the passage of India across the equator. However, high accumulation rates in the late Miocene have been reported from the Caribbean and equatorial Pacific as well (e.g., Leinen, 1979;Prell, Gardner, et al., 1982;Theyer et al., 1984). Leinen (1979) showed that biogenic silica deposition in the equatorial Pacific, which began to increase in the early Miocene, reached a maximum between about 10 and 8 Ma and suggested that this event recorded an intensification of the southeast trade winds. ...
... Rea and Leinen (1985) pointed out that such a conclusion is consistent with the model of Broecker and Peng (1982) that directly relates the thickness of this sublysocline zone (i.e., the vertical separation between the top of the lysocline and the CCD) to the downward flux of carbonate from the surface. Their conclusion is supported by evidence for increased equatorial Pacific productivity of both calcareous and siliceous organisms in the early Miocene (van Andel et al., 1975;Leinen, 1979). ...
... If real, these differences would seem to indicate that the first-order controls on the depth of the CCD and on carbonate accumulation patterns above may be somewhat uncoupled. Leinen (1979), for example, showed that equatorial Pacific productivity was already fairly high in the early Miocene, with increasing biogenic silica accumulation after about 17-18 Ma. At our Leg 115 sites, the reappearance of silica and the first evidence for high Neogene productivity substantially postdates this equatorial Pacific event (i.e., late Miocene; -8-9 Ma). ...
... Vincent (1974), for example, concluded from Leg 24 data that the modern high-productivity belt in the equatorial Indian Ocean formed in the middle to late Miocene after the passage of India across the equator. However, high accumulation rates in the late Miocene have been reported from the Caribbean and equatorial Pacific as well (e.g., Leinen, 1979;Prell, Gardner, et al., 1982;Theyer et al., 1984). Leinen (1979) showed that biogenic silica deposition in the equatorial Pacific, which began to increase in the early Miocene, reached a maximum between about 10 and 8 Ma and suggested that this event recorded an intensification of the southeast trade winds. ...
Sediments were recovered from a closely spaced depth transect (1541-4428 m) of five sites (Sites 707 through 711) from on and around the Mascarene Plateau that record the last 50 my of pelagic deposition. More than 2200 measurements of carbonate content are combined here with bio- and magnetostratigraphy to produce the first detailed compilation of bulk, carbonate, and noncarbonate mass accumulation rates (MARs) from the Indian Ocean. These results allow us to recognize three major depositional intervals, each characterized by a distinct depth-dependent pattern of carbonate accumulation: 1) the Paleogene; 2) the early and middle Miocene; and 3) the late Miocene to Holocene. The fundamentally different character of the carbonate system during each of these intervals must represent a regional response to the complex evolution of late Cenozoic oceans and climate. -from Authors
... Ocean circulation was significantly different when the seaway was open (Maier-Reimer et al., 1990), as discussed below. Tectonic closure of the Indonesian Passage, between 8.7 and 10.5 Ma, probably created, or at least intensified, the EUC (Leinen, 1979;Kennett, 1985). ...
... Maps of the 0-1 Ma interval (Fig. 16) are generally similar to those showing Holocene and Quaternary (Snoeckx and Rea, this volume; Leinen, 1979) sedimentation patterns. Highest concentrations of CaCO 3 are beneath the equator, and farthest from the continental margin, whereas the reverse is true of terrigenous and opal concentrations. ...
... On average, the CaCO 3 MAR increased at this time by 360% and the opal MAR by 270%, compared to those during the mid-Pliocene time slice (3-4 Ma) ( Table 4). Our values for opal MARs are somewhat higher than those observed by Leinen (1979) for the 4-to 7-Ma interval, most likely because of our higher sedimentation rates. Despite the overall increase in MAR, the spatial patterns of sedimentation remained the same, with two key exceptions. ...
... All coring at Site 575 was done with the HPC, and the oldest sediments recovered are about 22 Ma old (Barron, this volume). The middle and late Miocene were times of maximum diatom abundance in the central equatorial Pacific (van Andel et al., 1975; Leinen, 1979). Major global cooling occurred in the middle Miocene (Shackleton and Kennett, 1975;), and modern patterns of surface-water circulation were largely established (van Andel et al., 1975; Keller and Barron, 1983 ), although circulation of surface waters across the Central American Seaway did not cease until the Pliocene. ...
... Presumably, warming of tropical surface waters, coupled with high-latitude cooling at about 11 Ma, was responsible for the pronounced increases in provincialism—between middle-and low-latitude planktonic foraminifers (Ingle, 1973; Keller, 1981; Srinivasan and Kennett, 1981 ), calcareous nannofossils (Haq, 1980; Bukry, 1981b), silicoflagellates (Bukry, 1981a), and diatoms (Burckle, 1978; Keller and Barron, 1981)—that occurred in the early late Miocene. Keller (in press) and Leinen (1979) both argue that the Equatorial Undercurrent intensified in the earliest late Miocene. Leinen (1979) reports that opal accumulation rates along the equator became markedly higher at the eastern end of the Pacific in the early late Miocene, presumably reflecting increased upwelling associated with the Equatorial Undercurrent. ...
... Warming of tropical Pacific surface waters may have also resulted if the equatorial countercurrents were also intensified. The increased abundance of Thalassionema nitzschioides from 11.3 to 11.0 Ma (Fig. 4), which occurred in the eastern equatorial Pacific immediately after the decline of D. hustedtii, may have reflected increased productivity associated with the Equatorial Undercurrent, as suggested by Leinen (1979) , rather than intensification of the Peru- Chile Current, as suggested by Burckle et al. (1982). ...
Diatom assemblages in the central and eastern equatorial Pacific record the effects of polar cooling in the middle Miocene and across the middle/late Miocene boundary. Diatom evolutionary events (appearances and extinctions) were numerous between 14.5-13.5Ma, presumably reflecting reorganization of equatorial surface-water masses in response to polar cooling. Increasing numbers of the cold-water diatom Denticulopsis hustedtii indicate cooling of equatorial Pacific surface waters during the middle Miocene, especially between 12-11.5Ma. The onset of severe carbonate dissolution at 12Ma marked the beginning of the late Neogene period of high-amplitude fluctuations of carbonate in the equatorial Pacific. Initiation or a substantial increase in the production of North Atlantic Deep Water (NAFW) in the North Atlantic at 12Ma is believed to have been a possible triggering event for these middle/late Miocene changes. Increased polar cooling at 11.3Ma contributed to steepening of latitudinal thermal gradients by altering circulation patterns. Tropical surface waters became warmer, cold-water diatoms were excluded from the equatorial Pacific, and latitudinal provincialism was enhanced in all planktonic groups. Most of the modern-day features of equatorial Pacific surface-water circulation were probably present by 11Ma.-Author
... The abundance of biogenic silica (opal) in the surface sediments of the world oceans is high in areas of upwelling and primary productivity (Leinen, 1979;Molina-Cruz and Price, 1977;Leinen et al., 1986;Pokras and Molfino, 1986;Pokras, 1986). Surface-water productivity exerts a primary control on the abundance of siliceous micro fossils in the sediments (Schink et al., 1974;Broecker and Peng, 1982;Pokras and Molfino, 1986) by controlling the supply rate of siliceous valves to the seafloor and the silica saturation level of the pore waters. ...
... It is witnessed by a decrease in the abundance of diatoms and in particular by a decrease of the Thalassionema group. A similar decline, which continues into the Quaternary, is described for the equatorial Pacific Ocean (Leinen, 1979). This decline was caused by an early Pliocene shift in the site of opal accumulation from equatorial regions to the Antarctic Convergence, where opal is trapped in the area of the Polar Front (Brewster, 1980). ...
... The abundance of biogenic silica (opal) in the surface sediments of the world oceans is high in areas of upwelling and primary productivity (Leinen, 1979;Molina-Cruz and Price, 1977;Leinen et al., 1986;Pokras and Molfino, 1986;Pokras, 1986). Surface-water productivity exerts a primary control on the abundance of siliceous micro fossils in the sediments (Schink et al., 1974;Broecker and Peng, 1982;Pokras and Molfino, 1986) by controlling the supply rate of siliceous valves to the seafloor and the silica saturation level of the pore waters. ...
... It is witnessed by a decrease in the abundance of diatoms and in particular by a decrease of the Thalassionema group. A similar decline, which continues into the Quaternary, is described for the equatorial Pacific Ocean (Leinen, 1979). This decline was caused by an early Pliocene shift in the site of opal accumulation from equatorial regions to the Antarctic Convergence, where opal is trapped in the area of the Polar Front (Brewster, 1980). ...
Sites 705-716 of Ocean Drilling Program Leg 115 form a transect from 13°S to 5°N in the equatorial region of the western Indian Ocean. Two major cycles of biogenic silica accumulation appear in the sedimentary record from middle Eocene to early Oligocene and again from middle Miocene to Holocene. Diatoms are rare and poorly preserved in the recovered biosiliceous sediments of Paleogene age, whereas radiolarians and siliceous sponge spicules are well developed in these sequences. Diatoms are absent from sediments of early Oligocene to late middle Miocene age. They reappear around 8 Ma in the sites south of the equator. In contrast, well-developed middle Miocene diatom assemblages are present beginning at 13 Ma at Site 714 north of the equator. Two pulses of high diatom productivity appear in the Neogene sedimentary record. Between early Pliocene times and the present, the fertile belt of the western Indian Ocean shifted a few degrees north to the present-day location of the opal-producing sector confined to a belt between 3°S and 5°S. -from Author
... Subsequently, a series of cooling events beginning in the mid-Miocene influenced global and regional patterns of biosiliceous sedimentation. Coincident with decreased biosiliceous accumulation in the equatorial and northeastern Pacific in the early Pliocene (Leinen, 1979;Barron, 1981), the Southern Ocean emerged as an additional depocenter for biogenic opal as the Antarctic Circumpolar Current system strengthened (Brewster, 1980). Associated with continued cooling and development of the Northern Hemisphere ice sheets, biosiliceous deposition reappeared in limited areas of the North Atlantic during the late Pliocene (Baldauf, 1986). ...
... While a fairly comprehensive view of Neogene sedimentation in the equatorial and northeast Pacific is emerging (Ingle, 1973;Leinen, 1979;Barron, 1985Barron, , 1986; and many others), considerably less is known about the response of the marginal seas of the northwest Pacific to Miocene-Holocene tectonic events and climatic changes. Barron and Baldauf (1990) suggested that biosiliceous sedimentation in the Sea of Okhotsk and the Bering Sea increased significantly between 6.5 and 4.5 Ma. ...
Upper Pliocene through Holocene sediments recovered at Site 798 in the Japan Sea (Oki Ridge) exhibit rhythmic variation in weight percent biogenic opal at intervals of ~5 m and periods equivalent to the 41-k.y. obliquity cycle. Variance at 17 and 100 k.y. is observed prior to 1.3 Ma. These cycles are also clearly defined by log data and correspond to clusters of decimeter-scale dark-colored sediment units alternating with clusters of light-colored units. Opal content varies between 3% and 22% between 0 and 1.3 Ma and from 3% to 43% between 1.3 and 2.6 Ma. Long-term opal accumulation rates average 1.8 g/cm2/k.y. in the late Pliocene/early Pleistocene and decrease by about 60% at ~1.3 Ma. Rough calculations suggest that opal accumulation rates increased and terrigenous flux decreased during the Holocene relative to the last glacial period. Our age control is not yet sufficient to allow a similar analysis of the 41-k.y. cyclicity in opal content throughout the Pleistocene. -from Authors
... The range of surface-and deep-water processes involved and the steep gradients in temperature, productivity, and sediment flux in the eastern equatorial Pacific Ocean have combined to produce diverse pelagic sedimentary facies. Previous studies of changes in the accumulation patterns of these sediments highlighted several significant spatial and temporal changes during Neogene and Quaternary time (e.g., van Andel et al., 1975;Leinen, 1979;. The principal objective of Leg 138 was to generate a series of continuous sediment sections (Fig. 1), both along and across the equator that would facilitate a new definition and understanding of these spatial and temporal variations. ...
... The record of these processes is most commonly preserved as a variation in the relative proportions of biogenic carbonate and silica in sediments. Small (decimeter to meter>scale, high-frequency (10,000 yr) variations in the relative abundance of carbonate and silica are discussed above and illustrated in Figure 3 and Plates 1 and 2. Major, longer time period changes in carbonate and silica in the equatorial Pacific Ocean have been the subject of extensive discussion, both in terms of perceived carbonate dissolution events and their implications for deep-water circulation (e.g., , and in relation to major shifts in the locus of silica cycling and deposition within the ocean basins (Leinen, 1979;Brewster, 1980). With the Some of the largest and most widespread variations in carbonate occurred during early late Miocene time (Fig. 8). ...
Analysis of the variation in Neogene and Quaternary sediment types in the eastern Equatorial Pacific Ocean reveals distinctive spatial and temporal patterns. Miocene sediments are nannofossil oozes with increased diatom contents in equatorial sites. All the major Miocene carbonate-silica variations within near equatorial sites and in the early and middle Miocene age sediments of Site 844, result from the episodic and widely correlatable sedimentation of laminated diatom mat deposits. In sites farther from the equator, the only major middle-late Miocene carbonate/silica variation is a period of carbonate dissolution that developed over several hundred thousand years and had a maximum intensity at 8.92 to 8.71 Ma. The complex and varied causes and expressions of carbonate/silica (and, hence, seismic impedance contrast) variation during the early late Miocene suggest that great care will be required in the use of seismic reflectors as agents for correlating Oceanographic events. Diatom mat deposits and their bioturbated remnants are found in some sites north of the equator, and in some to the south, suggesting a link with the Equatorial Current systems. Major changes occurred during the Pliocene, with (1) no preservation of diatom mat deposits after 4.37 Ma; (2) a major carbonate dissolution episode between 3.88 and 3.18 Ma; and (3) the inclusion of foraminifers as an important sediment component in middle Pliocene through Quaternary sediments. In contrast to the diatom mat deposits, a major Pliocene-Pleistocene interval of elevated diatom contents and diatom-rich interbeds is best developed to the south at Site 846 and fades to the west and north, suggesting a strong link to the Peru Current/South Equatorial Current (PC-SEC) system.
... As a corollary, it is conceivable that a thick intermediate water layer was continuously stripped of its nutrients in its outcrop region, perhaps due to increased supply of Fe, since Fe may be a limiting factor in the region (Martin and Fitzwater 1988;Young et al. 199 1). It must be admitted, however, that the concept of Fe limitation in this area is in contention (Banse 199 1) and that there is no clear evidence for greatly increased dust supply in the crucial region during the last glacial period (Leinen 1989). ...
... The paradox can be resolved if it is assumed that the nutrients are extracted more vigorously during cold periods when the thermocline rises. There is also an indication for such a process in the eastern equatorial Pacific where the supply of opal greatly decreased during the last 3 X 106 yr (Leinen 1979). In contrast, the mean flux of opal increases greatly in the late Pliocene off NW Africa (Stein 198 5) and in the equatorial Atlantic (Ruddiman and Janecek 1989) in consequence of increased upwelling; the dust influx from the Sahara and the Sahel also increased at this time (see Sarnthein and Fenner 1988). ...
An increase in the productivity of the glacial-age Southern Ocean has been postulated to explain the decrease in pCO2 of the atmosphere observed in ice cores. A plausible mechanism has been proposed elsewhere that productivity is limited by the availability of Fe in this region and that the greater supply of eolian dust during glacial time removed this limit. Recently published evidence from cores from the Southern Ocean suggests that in fact there was no change in productivity in the assumed manner. Glacial-age productivity was indeed greatly increased in the equatorial Pacific and in the eastern boundary upwelling systems. The cause, presumably, was the mechanical action of glacial-age winds rather than a greater supply of Fe. However, a role of increased supply of micronutrients from the continents in the increase of equatorial productivity during glacial time cannot be excluded. -from Authors
... To test the sensitivity of diatom diversity to climate state, the resultant diversity estimates are compared to the global compilations of Cenozoic marine benthic foraminiferal isotope data for h 18 O and h 13 C of Zachos et al. [22], and to the Cenozoic record of biogenic opal in marine sediments [23][24][25][26]. Benthic h 18 O is an often used proxy for Cenozoic climate in studies of climate and evolution, e.g. ...
... Our first order trend in diatom diversity is of major increase over the Cenozoic, suggesting a similar large increase in diatom export productivity and the relative abundance of biogenic opal in sediments. Prior compilations of biogenic opal abundance in Cenozoic deep-sea sediments [23][24][25] suggest that siliceous sediments have indeed become more common over the Cenozoic, although as these compilations were of a qualitative nature no numeric comparison is possible. Studies over shorter time intervals exist but are often difficult to evaluate. ...
Marine planktonic diatoms export carbon to the deep ocean, playing a key role in the global carbon cycle. Although commonly thought to have diversified over the Cenozoic as global oceans cooled, only two conflicting quantitative reconstructions exist, both from the Neptune deep-sea microfossil occurrences database. Total diversity shows Cenozoic increase but is sample size biased; conventional subsampling shows little net change. We calculate diversity from a separately compiled new diatom species range catalog, and recalculate Neptune subsampled-in-bin diversity using new methods to correct for increasing Cenozoic geographic endemism and decreasing Cenozoic evenness. We find coherent, substantial Cenozoic diversification in both datasets. Many living cold water species, including species important for export productivity, originate only in the latest Miocene or younger. We make a first quantitative comparison of diatom diversity to the global Cenozoic benthic ∂(18)O (climate) and carbon cycle records (∂(13)C, and 20-0 Ma pCO2). Warmer climates are strongly correlated with lower diatom diversity (raw: rho = .92, p<.001; detrended, r = .6, p = .01). Diatoms were 20% less diverse in the early late Miocene, when temperatures and pCO2 were only moderately higher than today. Diversity is strongly correlated to both ∂(13)C and pCO2 over the last 15 my (for both: r>.9, detrended r>.6, all p<.001), but only weakly over the earlier Cenozoic, suggesting increasingly strong linkage of diatom and climate evolution in the Neogene. Our results suggest that many living marine planktonic diatom species may be at risk of extinction in future warm oceans, with an unknown but potentially substantial negative impact on the ocean biologic pump and oceanic carbon sequestration. We cannot however extrapolate our my-scale correlations with generic climate proxies to anthropogenic time-scales of warming without additional species-specific information on proximate ecologic controls.
... Nearly 30 yr ago, DSDP rotary drilling and coring of the central Pacific equatorial mound of sediments (e.g., DSDP Legs 5,8,9,and 16) (Fig. F1) established the general pattern of equatorial sediment accumulation and plate migration through the Neogene and late Paleogene (e.g., van Andel et al., 1975;Berger and Winterer, 1974;Leinen, 1979). However, the rotary coring technology available to these early legs could not provide undisturbed sections or complete recovery and was utterly defeated by middle Eocene chert layers encountered in some of the more deeply buried sections. ...
Ocean Drilling Program (ODP) Leg 199, "The Paleogene Equatorial Transect" (Sites 1215-1222), was designed to study the evolution of the equatorial Pacific current and wind system as Earth went from maximum Cenozoic warmth to initial Antarctic glaciations. The drilling program was primarily devoted to a transect along the 56- to 57-Ma crust, old enough to capture the Paleocene/Eocene boundary in the basal, more carbonate-rich sediments. The Leg 199 transect extends from a paleolatitude of ~4°N- ~4°S to encompass a relatively thick lower Eocene sediment section perhaps 8° north of the paleoequator. One site (1218) was also drilled on ~40-Ma crust to collect a near-equatorial sediment sequence from the middle Eocene to the Oligocene in order to investigate the transition in global climate from the Eocene "greenhouse" to Oligocene "icehouse." The Pacific plate has drifted northward through Cenozoic time transporting biogenic sediments deposited under the high-productivity equatorial belt into a zone of extremely slow sediment (red clay) accumulation. Thus, the central tropical North Pacific Ocean is an ideal region in which to sample shallowly buried Paleogene sequences of equatorially deposited biogenic sediments. The thin Neogene cover of red clay in the area means that the entire Paleogene sediment section is potentially drillable by ODP advanced piston coring and extended core barrel methods.
... Earlier studies (Maurrasse, 1973: Maurrasse and Keens-Dumas, 1988 concluded that the production of widespread pre-Miocene biogenic silica in the deep Caribbean basins is compatible with upwelling in the area, as can be related to dynamic divergence associated with a large-scale global equatorial circulation (Sverdrup et al., 1970) flowing through the Caribbean area. The oceanographic mechanism inferred to have led to the earlier silica deposits is analogous to present circulation systems in the equatorial Pacific (Hays et al., 1969;Molina-Cruz, 1977;Leinen, 1979). It was also argued that cessation of biogenic silica accumulation in the Caribbean deep-sea basins at the onset of the early Miocene, Lychnocanoma elongata Zone (Fig. 2) resulted from increasing disruptive effects of the large-scale circulation system between the Atlantic and Pacific Oceans caused by the emerging Isthmus of Panama (Maurrasse, 1973(Maurrasse, ,1976(Maurrasse, ,1979. ...
Rock sequences of the Montpelier Formation exposed on the northeastern coast of Jamaica east of Buff Bay include an interval of calcareous-siliceous facies composed of abundant sponge spicules with varying amounts of Radiolaria. Hexactinellid spicules of the Hyalospongea group (60 to 99%) dominate the biogenic silica components, whereas radiolarian assemblages vary from impoverished to well diversified. Liriospyris globosa, Calocycletta costata, and Dorcadospyris dentata occur at the base of the exposed outcrop, thus indicating a minimum age of late early Miocene (Calocycletta costata zone) for the onset of the calcareous-siliceous facies. This age is correlative with the Globigerinatella insueta zone as indicated by the co-occurrence of Globigerinatella insueta, Globorotalia peripheroronda, and Globigerinoides sicanus (without Praeorbulina glomerosa). Thus, the base of the silicabearing sequence lies within Zone N7 of Banner and Blow (1965). The upper limit of the calcareous-siliceous facies lies within the upper part of the Dorcadospyris alata zone, or early middle Miocene. This age is indicated by the presence of the nominate radiolarian taxon whose full range characterizes the zone, and the disappearance of distinctive taxa such as Didymocyrtis violina and Calocycletta costata, which occur up to the base of this zone. This radiolarian stratigraphic level corresponds to the Globorotalia fohsifohsi zone, as indicated by the presence of the nominate taxon and Sphaeroidinellopsis disjuncta and the absence of Praeorbulina glomerosa. These foraminifera are also indicative of Zones N10/N11 of Banner and Blow (1965). The overwhelming abundance of sponge spicules in the calcareous-siliceous facies at Buff Bay, Jamaica, implies that a significant subsurface current flowed through the area for more than four million years, between late early and middle Miocene time. Sponge spicule abundance remained continuous during that time while Radiolaria occurred with large variation in frequencies. Radiolarian recurrences are interpreted to indicate fluctuating productivity in relation to variations in climatic forcing superimposed on the long-lasting subsurface flow in the area. Physiographic and oceanic conditions that led to the calcareous-siliceous facies at Buff Bay are compared to environmental conditions in the Hatton/Rockall Bank area, in northeastern Atlantic. Sedimentation on the bank surface is strongly influenced by variations in the subsurface transport of North Atlantic Deep Water (NADW) over the bank. The subsurface flow may result in unusually high productivity of Hyalospongea similar to the Buff Bay sediments. Such a mechanism may apply to the Jamaican Miocene Series when paleophysiographic conditions in the northern Caribbean allowed the flow of NADW into the Caribbean basin through the incipient Jamaican island.
... Ma) when the site crossed the equator and in the middle Miocene (13.0-11.8 Ma), which was an interval of high biogenic silica accumulation throughout the equatorial Pacific (Leinen, 1979). The occurrence of upwelling diatom assemblages has been related to periods of expanded Antarctic glaciation causing increased gyral circulation and increased upwelling throughout the eastern equatorial Pacific (Keller et al., 1982). ...
Samples from DSDP Leg 85 Sites 572-575 in the central equatorial Pacific were analyzed for palynomorph content; only those from Site 572 consistently yielded assemblages of Neogene-Quaternary dinoflagellate cysts. These assemblages are described and compared with coeval material from elsewhere in the world. Several of the 15 species identified are stratigraphically restricted, and comparison with published ranges suggests that they may provide worldwide stratigraphic indices. Nematosphaeropsis labyrinthea (Ostenfeld) Reid and identified species of the genus Impagidinium range from middle Miocene to Quaternary. Palaeocystodinium golzowense Alberti disappears in the middle Miocene and Pyxidiella simplex Harland is restricted to the basal upper Miocene. Operculodinium cf. israelianum Wall and Spiniferites bentorii truncatus (Rossignol) Lentin and Williams are Quaternary species. An increased abundance of dinoflagellate cysts in the uppermost Miocene at Site 572 is associated with the common occurrence of the diatom genera Thalassionema and Thalassiothrix, which are regarded as indicators of upwelling. The higher concentration of dinoflagellate cysts in this interval is related to the location of the site below the equatorial high productivity belt at a time of enhanced local upwelling in the E equatorial Pacific.-Authors
... (e.g., Tracey, Sutton, et al., 1971;Hays et al., 1972, pp. 43-61;van Andel and Heath, 1973;Winterer, 1973; van Andel et al., 1975;Leinen, 1979;Kennett, in press), but truly detailed studies on this older material have been frustrated by incomplete coring and by core disturbance. Deep Sea Drilling Project Leg 85, however, using the hydraulic piston corer (HPC) in the upper, unconsolidated part of the section and rotary coring in the deeper parts of the section, returned to the central equatorial Pacific and collected nearly complete and relatively undisturbed sedimentary records of the past 40 m.y. ...
The analysis of high-resolution watergun seismic profiles collected in support of DSDP Leg 85 drilling reveals several major, regionally traceable reflectors that can be correlated over <360 000km2 in the central equatorial Pacific. The reflectors can be dated (+ or -0.5Ma) as follows: Orange, 21.5-22.5Ma; Yellow, 20.5-21.5Ma; Lavender, 16-17Ma; Red, 13.5-14.5Ma; Purple, 11-12Ma; Brown, 7-8Ma; and Green, 3-4Ma. Similar analyses at the other Leg 85 sites results in identical ages. The reflectors are thus time surfaces; this chapter relates them to major paleoceanographic events and changes in the relative sea-level curve.-from Authors
... Cooling and an increase in trade winds, and hence in equatorial upwelling, is likely as well, an interpretation supported by the optimal preservation of diatoms in the interval. A strong eastwest gradient in productivity developed during this time in the equatorial Pacific (Leinen, 1979;Kennett, 1985), an asymmetry that has implications for the dynamics of the equatorial undercurrent (Barron and Baldauf, 1989) and, indeed, for the entire equatorial system (Flohn, 1985). The remarkable decrease in carbonate accumulation in the Pliocene, and especially in the Quaternary, was accompanied by decreasing diatom and radiolarian preservation. ...
... The central equatorial Pacific has received considerable attention for biostratigraphic and paleoceanographic studies in recent years (van Andel et al., 1975;Leinen, 1979;Savin et al., 1981;Keller and Barron, 1983). DSDP Site 77 has been thoroughly studied for planktonic foraminifers (Jenkins and Orr, 1972;Keller, 1980Keller, , 1981Keller, , 1983a, calcareous nannofossils (Bukry, 1972;Gartner, 1972;Lohmann and Carlson, 1981), radiolarians (Goll, 1972(Goll, , 1979Westberg and Riedel, 1978), and diatoms (Milow in Hays et al., 1972;Keller et al., 1982;Barron, 1981Barron, , 1983Harwood, 1982). ...
Biostratigraphic studies using planktonic and benthic foraminifers, calcareous nannofossils, radiolarians, diatoms, and silicoflagellates are summarized for the four main sequences (sites 572 to 575) collected during DSDP Leg 85 in the central equatorial Pacific. Upper Eocene through Quaternary reference sections were recovered at Sites 573 and 574, and middle Miocene through Quaternary and lower Miocene through Quaternary reference sections were cored at Sites 572 and 575, respectively. Detailed graphic correlation plots suggest that Site 573 may contain as many as nine short hiatuses. Site 572 in the fertile eastern equatorial Pacific is apparently free of hiatuses, and Sites 574 and 575 have hiatuses (two each) only in the compressed upper sections. Few inconsistencies exist in the various microfossil stratigraphies. Diachronous species occurrences are identified, and age estimates for 66 late Eocene through Quaternary microfossil datums have been extrapolated from age-vs.-depth plots constructed from 63 datums with direct and/or indirect ties to paleomagnetic stratigraphy. An addendum presents an alternative biostratigraphic time scale for the middle and late Miocene based on a correlation of paleomagnetic Anomaly 5 with paleomagnetic Chron 11. -Authors
... The cessation of laminated sediment deposition after 4.37 Ma coincides with a shift in silica production from the equatorial Pacific to the Antarctic circum-polar ocean (Leinen, 1979;Brewster, 1980) with the continued growth of the ice sheets and approximates the closure of the Pan-American Seaway (Keigwin, 1982). At present, one can only speculate as to the cause of this cessation. ...
... Several DSDP and ODP expeditions in the Pacific and northecentral Indian Oceans have documented significant increases in the mass accumulation rates (MAR) of biogenic sediments during the late MioceneePliocene (e.g., Leinen, 1979;Woodruff, 1985;Farrell et al., 1995;Dickens and Owen, 1996). These changes mark important shifts in planktonic and benthic productivity, and are especially well developed in oceanographic divergence zones (Peterson et al., 1992;Berger et al., 1993;Dickens and Owen, 1994). ...
... This local change in sedimentation patterns in the Panama Basin fits into the global change in opal accumulation at this time. Near the Miocene/Pliocene boundary the accumulation of biogenic opal shifted its locus from the equatorial Pacific to the Southern Ocean (Leinen, 1979;Brewster, 1980). These changes were associated with the intensification of the Polar Front and led to the Southern Ocean becoming the dominant "sink" for oceanic silica. ...
The calcareous nannofossil assemblages recovered from Site 677A range in age from latest Miocene (NN11) to latest Pleistocene. The consistently high sediment-accumulation rate (mean of 48 m/m.y.) coupled with little dissolution of carbonate has ensured that generally rich, well-preserved coccolith assemblages occur at this site. Exceptions are the Pli-ocene/Pleistocene boundary sediments, which show evidence of dissolution, and upper Miocene basement sediments, which show evidence of diagenetic overgrowths. The apparent paradox of the occurrence of large numbers of the cool-water species Coccolithus pelagicus together with subtropical and tropical nannofossil species is thought to represent increased seasonality in the degree of upwelling in the Panama Basin. Seasonal upwelling was particularly intense during the late Miocene, when common C. pelagicus occurred with Thalassionema diatoms. Populations of C. pelagicus probably bloomed during a winter period of upwell-ing that was associated with a shallow thermocline and depressed surface-water temperatures.
... One lower abundance peak is located at 66 mbsf . SILICEOUS FOSSIL STUDIES Surface - water productivity acts as a primary control on the abundance of diatoms in sediments by controlling the silica saturation level of pore waters ( Molina - Cruz and Price , 1977 ; Leinen , 1979 ; Pokras and Molfino , 1986 ...
Combined data on benthic foraminifers, siliceous fossils, and stable isotopes depict times of enhanced organic car-bon oxidation in the sediments and high primary productivity in the southern Indian upwelling zone during the Mio-cene. Increased abundance of the diatom productivity index, the Thalassionema group, elevated diatom and uvigerinid abundances, and bolivinid diversity all suggest heightened primary productivity and the development of a mid-depth oxygen minimum between ~ 17 and 10 Ma. The abundance of bolivinids with large pores and crenulate chamber surfaces may indicate more aerated pore waters in the upper few centimeters of the partly siliceous sediments deposited during the episode of higher primary productiv-ity and increased organic carbon flux around 10 Ma.
... Leinen et al. (1986), for example, measured percent biogenic opal in surface sediments and showed a close correspondence to biogenic (largely diatom) productivity in the overlying waters. A number of authors (Molina-Cruz and Price, 1977;Leinen, 1979;Brewster, 1980;Heath et al., 1983;Banahan and Goering, 1986) demonstrated temporal and spatial variations in opal accumulation related to shifts in productivity. Mortlock and Froelich (in press) concluded that the percent opal content in sediment carries a valuable paleoceanographic message related to surface water productivity. ...
We examined diatom preservation patterns in Pliocene age sediments of Jane Basin (ODP Site 697) and compared them with diatom distribution in more northerly sites at various sectors of the Southern Ocean. Our data from Site 697, as well as other sites from around the Southern Ocean, support the view that there was significant growth on Antarctica during the late Pliocene. Our view that there was ice growth on Antarctica during the late Gauss Chron is supported by the results from ODP Site 697. While diatoms are present and percent opal is high in the early and middle Gauss Chron (suggesting more open-ocean conditions), late Gauss sediments contain low percentages of opal and few or no diatoms. This is also true for the early Matuyama Chron. If we accept spring and summer sea-ice cover as the major suppressant of diatom productivity in the Southern Ocean, then we conclude that sea-ice covered the region around Site 697 through much of the year during this interval. Although warmer open-ocean intervals are inferred for intervals near the Olduvai and Jaramillo Subchrons, most of the Matuyama Chron was marked by extensive sea-ice cover with low seasonal contrast. Our results for the early part of the Brunhes Chron are similar, at least for the Jane Basin. During this time, sea-ice cover over the basin apparently extended well into the growing season. In contrast, the later Brunhes Chron is marked by alternating open water (during the growing season) and extensive, almost year-round, sea-ice. -from Authors
... Although limited by diagenetic overprinting, consideration of microfossil assemblages indicates that the Tallahatta siliceous mud accumulated in inner to outer shelf settings (Weaver and Wise, 1974;Laws and Thayer, 1992). Deposition presumably occurred beneath nutrient-rich waters (Ivany, 1998) and perhaps is tied to the Eocene maximum in oceanic biogenic silica accumulation (Weaver and Wise, 1974;Leinen, 1979;McGowran, 1989). ...
The Eocene Tallahatta Formation exposed in the eastern United States Gulf coastal plain includes enigmatic siliceous facies characterized by variably indurated claystone, porcelanite, and subordinate thin sandstone and siltstone beds. Ichnosedimentologic studies at localities in eastern Mississippi and Alabama indicate that these strata accumulated in middle-shelf settings (offshore transition). Sandstone and siltstone beds record storm events of variable magnitude. Moderately diverse Cruziana ichnofacies assemblages that reflect overall hospitable fair-weather conditions characterize most mudrock intervals; however, periodically oxygen-deficient or otherwise unfavorable conditions are indicated locally by unbioturbated mudrock. Storm beds associated with bioturbated mud typically contain mixed Cruziana-Skolithos ichnofacies assemblages reflecting the work of both opportunistic(?) and fair-weather tracemakers. In contrast, ichnofabrics of storm beds that were not disrupted by fairweather bioturbation likely record short-term colonization of substrates by organisms that were transported, sorted, and redeposited by storm currents. Ichnofabrics and diagenesis of the siliceous facies are linked. Ichnofossils locally influenced silica cementation in some storm beds, whereas preferential conversion to porcelanite of siliceous clay intervals immediately below and above many storm beds dramatically enhanced the visibility of fair-weather ichnofabrics.
... The differentiation of radiolarian faunas between the Indian Ocean and western Pacific at about 11 Ma is one example suggesting some restriction of water exchange through the Indonesian Passage to develop different water masses on either side of the Indonesian Gateway [66,67]. However, Leinen [68] and Keller [69] proposed that the Equatorial Undercurrent initiated by circa 11 Ma in the equatorial Pacific could have permitted a westward flow of the Pacific water into the Indian Ocean subsequent to the partial closure of the Indonesian Seaway. This is exemplified by the wide of geographical distribution of Hibiscus tiliaceus, a semi-mangrove with water-buoyant and salt-tolerant seeds, which has dispersed long distance via ocean current throughout the northwestern Pacific Ocean and the Indian Ocean [70]. ...
Mangroves are key components of coastal ecosystems in tropical and subtropical regions worldwide. However, the patterns and mechanisms of modern distribution of mangroves are still not well understood. Historical vicariance and dispersal are two hypothetic biogeographic processes in shaping the patterns of present-day species distributions. Here we investigate evolutionary biogeography of mangroves in the Indo-West Pacific (IWP) and western Atlantic-East Pacific (AEP) regions using a large sample of populations of Rhizophora (the most representative mangrove genus) and a combination of chloroplast and nuclear DNA sequences and genome-wide ISSR markers.
Our comparative analyses of biogeographic patterns amongst Rhizophora taxa worldwide support the hypothesis that ancient dispersals along the Tethys Seaway and subsequent vicariant events that divided the IWP and AEP lineages resulted in the major disjunctions. We dated the deep split between the Old and New World lineages to early Eocene based on fossil calibration and geological and tectonic changes. Our data also provide evidence for other vicariant processes within the Indo-West Pacific region in separating conspecific lineages of SE Asia and Australia-Pacific at the Oligocene-Miocene boundary. Close genetic affinities exist between extant Fijian and American lineages; East African and Australian lineages; and Australian and Pacific lineages; indicating relatively more recent oceanic long-distance dispersal events.
Our study demonstrates that neither vicariance nor dispersal alone could explain the observed global occurrences of Rhizophora, but a combination of vicariant events and oceanic long-distance dispersals can account for historical diversification and present-day biogeographic patterns of mangroves.
[see: http://www.biomedcentral.com/1471-2148/14/83]
... Second, the "biogenic bloom" event noted by Pisias et al. (1995) and Farrell et al. (1995a) occurs during this interval. Carbonate accumulation rates peak in many ocean basins during this interval (Pacific- Berger et al., 1993;Pisias et al., 1995;Farrell et al., 1995b;Indian-Peterson et al., 1992), and opal accumulation rates are also high (Leinen, 1979;Kemp and Baldauf, 1993;Farrell et al., 1995b;Rea and Snoeckx, 1995). For all of these accumulation rate records, a net increase in global oceanic burial rate on these time scales is likely driven by an increase in the dissolved input of these elements from continental weathering. ...
Several sedimentary, geochemical, and isotopic records indicate that the
hypothesized intensification of the Asian monsoon at about 8 Ma
triggered a physical and chemical weathering event in the
Himalayan-Tibetan Plateau. Records of sediment input and clay
composition from the northern Indian Ocean reveal clear weathering
changes in the plateau at this time, and Ge/Si ratios of opaline silica
as well as biogenic sedimentation rates indicate that increased
dissolved element fluxes from this weathering event had an oceanwide
effect. It is likely that weathering intensity also increased in the
Andes and Amazon Basin in this interval as well. Perhaps the most
important impact of this weathering event was to increase the net flux
of the biolimiting nutrient phosphorus to the ocean, as evidenced by a
peak in phosphorus accumulation rates at this time. The temporary
increase in nutrient inputs to the ocean triggered increased oceanic
productivity and organic carbon burial (recorded in carbon isotopic
records and paleooxygen concentrations). The net result of this
weathering event may have been an increase in the rate of drawdown of
atmospheric CO2 through the late Miocene early Pliocene, via
direct silicate weathering reactions and increased burial of organic
carbon in the ocean. This increased rate of atmospheric CO2
drawdown may have destabilized the climate system by the late Pliocene,
thus initiating a period of intense cooling and ice buildup leading to
the present.
... During the middle Miocene, the Indonesian Seaway progressively closed, causing a buildup of westward flowing waters in the western Pacific. By ca. 1 1 Ma the eastward-flowing, equatorial undercurrent system was established (Leinen 1979, Keller 1985, resulting in intensification of east-west differences in upwelling and sea surface temperatures in the equatorial Pacific. The disappearance of Denticulopsis simonsenii (= D. hustedtii of earlier reports) in the equatorial Pacific at ca. 9.35 Ma marked an end to the migration of North Pacific planktonic diatom species into the equatorial Pacific (Barron I985c, 1986). ...
Recently published diatom biochronologies provide accurate (to 0.1 m.y.) determination of the ages of appearances and disappearances of planktonic diatoms during the past 18 m.y. in the equatorial Pacific, North Pacific, and Southern Ocean. Comparisons of these records reveal the age of evolutionary appearance and extinction of species and their region of origin. Extinct planktonic diatom species have a mean longevity of 3.4 ± 2.8 m.y. (SD, n = 53) in the equatorial Pacific, 2.5 ± 2.1 m.y. (n = 52) in the North Pacific, and 2.9 ± 2.3 m.y. (n = 38) in the Southern Ocean. The relatively large standard deviations are likely due to the inclusion of taxa that probably could be subdivided into two or more species. In the equatorial Pacific, evolutionary turnover of diatom species was relatively high between 18.0 and 6.0 Ma compared with the period after 6.0 Ma, presumably reflecting changing oceanic circulation and evolving water masses. In the North Pacific, evolutionary turnover speaked between 10.0 and 4.5 Ma, with increasing high-latitude cooling and enhanced provincialism. In the Southern Ocean, evolutionary turnover of endemic diatoms was greatest between 5.0 and 1.6 Ma, which provides evidence for the strong provincial character of Pliocene diatom assemblages. Taken as a whole, oceanic diatom assemblages became increasingly provincial in character during the late Miocene and Pliocene, as pole-to-equator thermal gradients increased and oceanic frontal systems were strengthened.
... [4] The problems facing many of these earlier works in addressing questions of silica flux to the sea floor were twofold. First, in the Pacific at least, the time control on sediment accumulation rates was rather poor [van Andel et al., 1975;Leinen, 1979aLeinen, , 1979b and the practical recovery of complete Eocene sections was inhibited by pervasive chert deposits. Time control in the Atlantic was somewhat better [Thiede et al., 1981;van Andel et al., 1977]; however, cherts often impeded good recovery there as well. ...
A synthesis of Eocene biogenic silica accumulation rates in the equatorial zone of the Pacific shows several relatively broad maxima spaced a few million years apart and extending from the uppermost Eocene into the lower Eocene. There is a distinct, step-like decrease in biogenic silica mass accumulation rate at 38.5 Ma (just below the top of C18n.1n),which appears to be related to changes in oceanographic conditions in the eastern equatorial region. Eocene biogenic silica accumulation rates in the equatorial Pacific are distinctly lower than those in the Neogene. A comparison of Eocene accumulation rates and known areas of open ocean biogenic silica deposition with those in the more modern world leads us to conclude that there is no need to call upon an increased supply of dissolved silica to the world ocean to account for the siliceous-rich deposits of the Eocene.
... The cessation of laminated sediment deposition after 4.37 Ma coincides with a shift in silica production from the equatorial Pacific to the Antarctic circum-polar ocean (Leinen, 1979;Brewster, 1980) with the continued growth of the ice sheets and approximates the closure of the Pan-American Seaway (Keigwin, 1982). At present, one can only speculate as to the cause of this cessation. ...
Laminated diatom ooze (LDO) has been recovered from several ODP Leg 138 sites and now is also recognized from several DSDP Leg 85 sites. These remarkable sediments are the result of massive and episodic flux of mats of the diatom Thalassiothrix longissima. By analogy with the Rhizosolenia diatom mat-forming events monitored by JGOFS (Joint Global Ocean Flux Study) in the equatorial Pacific Ocean during cooling conditions in late 1992, these episodes of massive flux of T. Iongissima mats may represent the 'fall out' from major frontal systems generated during La Nina (anti-El Niño) events. Laminations were preserved in the mat deposits because of the rapid mat deposition and high strength of the diatom mat meshwork, that subjugated benthic activity. This new mechanism of preservation of lamination in marine sediments has wide implications for other laminated sequences. The sustained periods of mat deposition documented in Neogene sediments of the eastern equatorial Pacific Ocean are part of the major cycles in the relative abundance of carbonate and silica in the region and, possibly, in the case of some intervals, also in the Atlantic Ocean.
Unlike most organisms, marine phytoplankton experience nearly deterministic rates of biotic interactions mediated by stochastic rates of turbulence. Yet studies of phytoplankton evolution have largely ignored biotic interactions, whereas studies of phytoplankton ecology have largely ignored evolutionary constraints. This review will attempt to integrate the opposing and complementary selective forces experienced by phytoplankton due to biotic and abiotic parameters over ultimate (evolutionary) and proximate (ecological) time scales. Attention will be focused on biotic interactions experienced by siliceous phytoplankton and the influence of these interactions on coexistence and local extinction (the ecological time framework), and diversification and species selection (the evolutionary time framework). The principal topics of discussion include the significance of biotic interactions among phytoplankton to the mechanisms of species displacement, species coexistence, the maintenance of species diversity, the selection of life history characteristics, and the mode and rate of evolution. A case study developed from a Paleogene phytoplankton assemblage is presented to illustrate that understanding the mechanisms of species interactions is requisite to a comprehensive interpretation of temporal changes in morphology, species dominance, and diversity. [For reviews of the physiology, ultrastructure, and distribution of siliceous marine phytoplankton, the reader is referred to Werner (1977), Bold and Wynne (1978), Morris (1980), Falkowski (1980), and Tappan (1980)].
Paleogeography is established using mainly plate kinematics and unfolding mountain belts, but basinal paleodepths are defined using specific facies, among which radiolarites can provide key information. The common equation radiolarites = deep oceanic basin is obviously too simple to be correct; and to infer from the geological message (e.g., radiolarites), the original, environmental, biological signal requires a good knowledge of the successive filters that have changed it and the originators of these signals (e.g., the radiolarians).
Being of especially high quality, the Neogene fossil record of planktonic foraminifera offers special opportunities for assessing patterns of extinction and speciation. A variety of metrics indicates that within this group the mean duration of lineages has been much shorter (rate of extinction has been higher) for the globorotaliid clade than for the globigerinid clade. Furthermore, in the globorotaliid clade rates of extinction and speciation have not been closely linked to changes in diversity, but rather have been relatively high even at times when diversity has undergone little change. Thus, the globorotaliid clade has undergone more rapid evolutionary turnover than the globigerinid clade. Data for living species reveal that neither geographic range nor temperature tolerance is the primary factor controlling lineage duration. -from Authors
Introduction Diatomite, a soft, porous, fine-grained, lightweight, siliceous sedimentary rock, is produced by the accumulation and compaction of diatom (Class Bacillariophyceae) remains. The cell wall of living diatoms is impregnated with silica (amorphous hydrous, or opaline (SiO2.nH2O)), which preserves ornate and highly porous structures (Armbrust, 2009). Upon death of the organism, these siliceous elements become sedimentary particles in a diatomite deposit. The intricate structure of diatom frustules, and packing of the myriad diatom shapes into rock-forming sedimentary layers, gives diatomite deposits properties that are useful in many industrial and commercial applications. Most diatoms are 10 μm to 100 μm in size, although larger species reach more than 1 mm (Tappan, 1980) and smaller ones are <2 μm. This small size results in large concentrations of diatoms; a cubic inch of diatomite may contain 40 to 70 million diatoms (Crespin, 1946). Although the specific gravity (density) of the SiO2 that comprises the diatom particles is nearly twice that of water, perforations and open structures in the frustule renders diatomite a considerably lower effective density (between 0.12 g cm–3 and 0.25 g cm–3) with high porosity (from 75 to 85 percent). This sedimentary rock is able to absorb and hold up to 3.5 times its own weight in liquid (Cleveland, 1966). These properties bring industrial utility and commercial value to rock-forming accumulations of diatom remains. Diatomite deposits of varying quality are known from freshwater and marine sedimentary environments (Moyle & Dolley, 2003). Other names for diatomite and diatomaceous earth include tripoli, kieselguhr, and infusorial earth.
The biogenic opal content of Antarctic deep sea sediments is estimated by subtracting isolated non-biogenic silica fractions from the total silica content of the samples. Quantitatively significant sources of non-biogenic silica in Antarctic Ocean sediments are clay-associated silica and detrital quartz. in this way, the relation is derived:. Biogenic Opal = Total Sio2 - Clay-Associated Silica - Detrital Quartz. Clay-associated silica is estimated from aluminum and magnesium concentrations in the sediment, using a normative calculation. Detrital quartz content is determined by X-ray diffraction.
The benthic environmental impact experiment addresses environmental impacts at a specific site related to deep-sea mineral resource development. We have acquired several tens of multi- or box core samples at 31 sites within the Benthic environmental Impact Site (BIS) since 2010, aiming to examine the basic properties of surficial deep-sea sediment as a potential source for deep-water plumes. In this study, we present the geochemical properties such as major elements, rare earth elements (REEs), and heavy metal contents at the BIS. Such proxies vary distinctly according to the Facies association. The lithology of all core sediments in the BIS corresponds to both Association Ib and Association IIIb. The vertical profiles of some major elements (SiO2, Fe2O3, CaO, P2O5, MgO, MnO) show noticeable differences between Association Ib and IIIb, while others (Al2O3, TiO2, Na2O, and K2O) do not vary between Association Ib and IIIb. REEs are also distinctly different for Associations Ib and IIIb; in Association Ib, REY and HREE/LREE are uniform through the sediment section, while they increase downward in Association IIIb like the major elements; below a depth of 8 cm, REY is over 500 ppm. The metal enrichment factor (EF) evaluates the anthropogenic influences of some metals (Cu, Ni, Pb, Zn, and Cd) in marine sediments. In both Associations, the EF for Cu is over 1.5, the EF for Ni and Pb ranges from 0.5 to 1.5, and the EF for Zn and Cd are less than 0.5, indicating Cu is enriched but Zn and Cd are relatively depleted in the BIS. The vertical variations of geochemical properties between Association Ib and IIIb are shown to be clearly different, which seems to be related to the global climate changes such as the shift of Intertropical convergence zone (ITCZ). © 2014, Korea Ocean Research and Development Institute. All rights reserved.
Interpretation of SEASAT geoid anomaly data and improved seafloor mapping of the south-central Pacific suggest a complex tectonic history for the islands of the Pitcairn ile Beno atoll formed at - 16m.y.BP at a 'hotspot' now south of the Easter micro- sequent progressive island development at Henderson (i3m.y.), Ducie (8m.y.) and Crough seamount (4m.y.) resulted from the lateral leakage of magma from the Beno lineation along an old fracture zone, itself originating during the Tertiary reorientation of the Pacific plate. At all four islands cessation of volcanism was followed by subsiden- and the development of a carbonate cap. By comparison, Pitcairn has been the product of recent (
The biogenic silica analysis was performed on 463 samples from ODP 184 Site 1143, the southern South China Sea. The results show that the opal content and MAR evidently increased between 12.3 and 5.7 Ma, which was contemporary with the high carbonate and total MARs, reflecting the high biogenic productivity in the late Miocene. This demonstrates the occurrence of the late Miocene “ Biogenic Bloom Event” in the southern South China Sea, corresponding to the late Miocene-early Pliocene “ Biogenic Bloom Event” in the Indian-Pacific Ocean. The increases of opal content and MAR after the middle Pleistocene (about 0.7 Ma) is inferred to result from the enhanced upwelling and nutrient supply, which was induced by the intensified monsoon circulation after the “ Mid-Pleistocene Revolution”.
In this study radiolarians are useful paleoceanographic indicators. The long-term stratigraphic changes (those lasting longer than 100 000 yr) in relative abundances of radiolarian taxa reflect a strong upwelling and temperature signal. Abundance fluctuations on shorter time scales (1000-900 000 yr) are also prominent in this section but more difficult to interpret. Taxa with modern equivalents living in cold, subsurface water increase in relative abundance during the late Miocene and dominate in the early Pliocene. These changes suggest that upwelling of cold, subsurface water increased during the deposition of the Monterey Formation. This paleoceanographic interpretation is consistent with isotopic data and lithologic studies, indicating an intensification of oceanic circulation and biological productivity around the north Pacific rim during the late Miocene.-after Authors
The phosphorites occur in a wide variety of forms, but most commonly carbonate fluorapatite (CFA) replaced middle Eocene and older carbonate sediment in a deep water environment (>1000 m). Element ratios distinguish seamount phosphorites from continental margin, plateau, and insular phosphorites. Uranium and thorium contents are low and total rare earch element (REE) contents are generally high. The paleoceanographic conditions initiated and sustained development of phosphorite by accumulation of dissolved phosphorus in the deep sea during relatively stable climatic conditions when oceanic circulation was sluggish. Fluctuations in climate, sealevel, and upwelling that accompanied the climate transitions may have driven cycles of enrichment and depletion of the deep-sea phosphorus reservoir. -from Authors
This introductory chapter briefly summarizes many aspects of siliceous deposits, from the introduction of silica into the marine environment to diagenesis and evolution of depositional basins. We also describe and contrast the Mesozoic Tethyan and Pacific depositional settings of siliceous deposits.
Silica enters the oceans primarily from rivers and is removed primarily in siliceous deposits via silica uptake by siliceous plankton. Less than one percent of the biogenic silica fixed in surface waters makes it into the geologic record. The solubility of the various silica polymorphs varies, and controls the dissolution or reprecipitation of silica in sea water, bottom sediments, and during late diagenetic stages. Recent studies in the geochemistry of siliceous deposits are shedding light on many long outstanding questions related to siliceous deposits. K-Ar and Rb-Sr age dating of cherts offer new and significant tools to better understand the timing of sedimentation and tectonics of chert-bearing sequences.
The plate tectonic regimes and depositional basins of the Mesozoic Tethyan seaway and the circum-Pacific region were fundamentally different. These differences resulted in deposition of siliceous sequences with different lithologic associations and lithologic successions.
Assemblages of clay-sized minerals in twelve deep-sea cores from the northern part of the Magellan Trough in the Central Pacific Basin clearly show that authigenic smectite together with clinoptilolite formed subsea in situ and predominated throughout Palaeogene time. Clastic clay minerals such as chlorite, illite and kaolinite increased at the expense of smectite in sediments deposited since the Pliocene. Most of the authigenic smectites are of the di-octahedral, iron-rich type. The most probable origin of the authigenic smectites is formation from precursors such as volcanic glass. As part of this process biogenic silica evidently contributed to the formation of smectite and zeolites as clinoptilolite. The presence of a tri-octahedral saponite and the TEM morphology of some smectites, on the other hand, suggests that not all of the smectite studied was formed subsea in situ but that some had an allogenic origin. Systematic vertical changes in the assemblage of clay-sized minerals suggest that the authigenesis of submarine smectite and clinoptilolite prevailed in conditions of reduced aridity and weak wind activity throughout the Palaeogene in the study area. However, allogenic clay minerals such as chlorite and illite increased since the Olduvai event under conditions of severe aridity and strong wind activity caused by global climatic change. The west-northwest movement of the Pacific plate must have partly played a role in deposition of the clastic clay-sized minerals from the Asian continent by westerlies in the study area. The widespread occurrence of Cenozoic hiatuses in the study area is attributed to the influence of Antarctic Bottom Water (AABW). The latter must have influenced non-deposition of the clay-sized minerals at times during the Cenozoic.
Distinctive acoustic reflectors have been identified and traced over hundreds of kilometers on both low-frequency (air gun) and high-frequency (3.5 kHz) seismic profiles from the region immediately to the north of the Clipperton Fracture Zone in the equatorial Pacific. The reflectors result from impedance contrasts between carbonate-rich layers (up to 90% CaCO3) and siliceous clays, and they represent sharp decreases in porosity, increases in wet bulk density, and decreases in sound velocity. The reflectors have northern limits which increase with increasing age: reflector R0 (late Pleistocene) occurs only south of ~5°N, R1 (upper Miocene) occurs south of ~6°N, and R2 (middle to lower Miocene) extends to at least 10°N. Radiolarian biostratigraphy of the piston cores shows that the most extensive of the reflectors, R2, is clearly time transgressive within the middle and lower Miocene. The variable geographic extent and the time-transgressive nature of individual acoustic reflectors are in part a consequence of the northward component of motion of the Pacific plate during the Neogene, with carbonate sedimentation generally restricted to a narrow equatorial region south of ~5°N. There are no detectable unconformities within core intervals corresponding to the three major reflectors; hence the reflectors apparently do not represent significant erosional episodes. Superimposed on the effect of plate motion may be an abrupt narrowing of the equatorial productivity belt during the middle Miocene at 11-12 m.y. B.P. This abrupt transition from carbonate-rich to carbonate-poor sedimentation occured over a widespread region (approximately 3° of latitude) and corresponds with reflector R2. Similar events may have been responsible for the shallower reflectors R0 and R1.
Continental sedimentation reflects a complex interplay of tectonics and climate. A 2000-km transect from coastal California to the western Great Plains documents a major increase in sedimentation (ca. 16-6 Ma) coeval with deposition of the hemipelagic Monterey Formation along the California coast. Basin and Range-style regional extension following elongation of the Pacific-North American transform boundary at ca. 17.5 Ma provided fault-bounded basins for accommodation space, but sedimentation also occurred on unextended erosional surfaces of the Great Plains and Colorado Plateau. Two global climate transitions bracket this sedimentary interval. The middle Miocene transition (ca. 17-12 Ma) records the global change from equatorial to meridional circulation caused by: (1) closing of the eastern Tethys Seaway (ca. 18 Ma); (2) opening of the Arctic-North Atlantic connection (ca. 17.5 Ma); (3) growth of the East Antarctic Ice Sheet (ca. 14 Ma); and (4) closing of the Indonesian Seaway (ca. 12 Ma). Upwelling of cold waters along the California coast, abetted by domination of La Niña phases of El Niño-Southern Oscillation (ENSO), progressively aridified the Southwest as reflected in sedimentary and biologic records. The second climate transition occurred as opening of the Gulf of California (ca. 6 Ma) intensified the North American monsoon, resulting in integration of drainages, incision of uplifts, and exhumation of basin fills. The Miocene ended with the driest climate of the Tertiary (both regional and global) accompanied by conversion of savanna to steppe or scrub desert, spread of C4 grasses, and the greatest mammal extinction of the Neogene.
The Mino-Tanba belt in southwest Japan, a segment of the
Cordilleran-type orogenic chain of Jurassic east Asia, is composed
mainly of a Middle-Upper Jurassic subduction-accretion complex in which
Triassic and Lower Jurassic bedded radiolarian cherts occur as large
allochthonous units structurally interlayered with Middle-Upper Jurassic
clastic rocks. High-resolution microfossil (conodont and radiolaria)
research has identified very low average sedimentation rates of about
0.5 g/cm²/1000 yr in the chert units, similar to those of modern
pelagic sediments accumulated in open ocean environments. Judging from
the low average sedimentation rate, high purity of biogenic silica, long
duration of continuous deposition (>50 m.y.), and wide along-strike
extent (>1000 km), the bedded radiolarian cherts in the Mino-Tanba
belt are best understood as ancient pelagic sediments that accumulated
in open ocean environments; accordingly, the alleged origin in smaller
marginal basins is untenable. Upward lithologic change from bedded chert
to overlying siliceous mudstone in the uppermost portion of chert
sequences suggests the gradual landward approach of the oceanic plate
toward a trench. The tectonic interlayering of these cherts and
coarse-grained terrigenous elastics is a secondary feature that was
added through duplexing-underplating in the subduction zone. On the
basis of the primary stratigraphy and field occurrence of Triassic
bedded chert in the Mino-Tanba belt, newly proposed are an idealized
oceanic plate stratigraphy and a generalized travel history of a
Cordilleran-type bedded chert from its birth at a mid-oceanic ridge to
its demise at a subduction zone.
Previous modeling studies suggested that changes in ocean gateways may have exerted a dramatic influence on the ocean circulation. In this pilot study we extend those results to examining the potential ramifications of circulation changes on the sedimentary record. A version of the Hamburg carbon cycle/sediment model is used in these sensitivity experiments. Results indicate that internal reorganization of the ocean circulation can potentially cause very large regional changes in lysocline depth (1500-3000 m) and opal deposition. These shifts are sometimes comparable in magnitude to those imposed by changes in external forcing (e.g., climate, sea level, and weathering). Comparisons of the model response with the geologic record indicate some significant levels of first-order agreement. This exercise suggests that opportunities now exist for physically based modeling of past sediment responses to circulation and climate changes.
Presents P accumulation rates from the eastern (Neogene) and western (Cenozoic) equatorial Pacific. The influences of surface productivity patterns, site migration through time relative to equatorial upwelling, and water depth are observed in the P accumulation rate records. These site-specific effects are relatively minor, however, compared to synchronous, significant trends in P accumulation rates in these equatorial Pacific sites. The most notable event occurred in the late Miocene. The P accumulation rate record is markedly different from the unidirectional increases in continental weathering rates over the last 40 m.y. interpreted from the Sr isotope record, suggesting a decoupling of nutrient input fluxes from input fluxes of other dissolved constituents. -from Authors
A new method has been developed for measuring biogenic silica (amorphous) in marine sediments. It is based on the Högbom–Urey reactionCaCO3+SiO2(biogenicsilica)→CaSiO3(wollastonite)+CO2.The reaction is carried out in evacuated sealed glass tubes with added excess reagent grade calcite at 500°C for 3d. The product CO2 is measured in a calibrated manometer. The dependence of the reaction on time, temperature, product CO2 pressure, and other factors is examined.
At sites 89 and 90 of the Korea Deep Ocean Study (KODOS), located in the western part of the area between the Clarion and Clipperton fracture zones, the topmost sediment column can be divided into two layers by a sharp color boundary: an upper pale brown layer (Unit A) and a lower dark brown layer (Unit B). Geochemical and mineralogical properties in Units A and B are clearly different across the color boundary, which can be interpreted to be a hiatus with an age gap of at least 3 Ma. The large gap in sediment ages results in an overall difference in sediment properties between Units A and B. That is, older Unit B sediment is higher in smectite contents, more severe in fossil dissolution, and lower in POC contents, commonly due to prolonged diagenesis. Disappearance of the coarse mode (50–60 μm) of the size distribution in Unit B is a result of prolonged dissolution of siliceous fossils. However, other phenomena occurring just below the color boundary (the most severe dissolution of fossils, the most frequent occurrence of coarse-grained micronodules and smectite aggregates, maximum value of metals (Mn, Cu, Ni), and increased water contents) can be explained by the active stirring of bottom sediments by intensified paleo-bottom currents present during the hiatus. That is, turbulence as well as winnowing action caused by the paleo-currents had, respectively, accelerated the dissolution of siliceous fossils and the concentration of coarse grains at the hiatus. In addition, micronodule formation was promoted by the efficient formation of oxygen-depleted micro-environments around in-situ fauna remains, which were buried efficiently by increased reworking and resedimentation of bottom sediments by the paleo-current activity. Meanwhile, increases in metal and water contents just below the color boundary are the results of the concentration of micronodules enriched in metals and probable formation of ‘woodpile’ fabric by abundant spicules and spines, respectively. Active stirring of bottom sediments by the intensified paleo-bottom currents as well as the large gap in sediment age between Units A and B are mostly responsible for the vertical variations of geochemical and mineralogical properties in two-color core sediments.
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