Orbitally forced climate signals in mid-Pliocene nannofossil assemblages

Marine Micropaleontology 51 (2004) 39 – 56 www.elsevier.com/locate/marmicro

Orbitally forced climate signals in mid-Pliocene nannofossil assemblages Samantha Gibbs a,*, Nicholas Shackleton a, Jeremy Young b a

Godwin Institute for Quaternary Research, University of Cambridge, Pembroke Street, Cambridge CB2 3SA, UK b Natural History Museum, South Kensington, London, UK Received 28 July 2003; received in revised form 15 September 2003; accepted 16 September 2003

Abstract Downcore cyclic variation in high-resolution nannofossil abundance records from mid-Pliocene equatorial Atlantic ODP Sites 662 and 926 demonstrate the direct response by several Pliocene taxa (notably Discoaster, Sphenolithus and Florisphaera profunda) to orbitally forced climatic variation. In particular, these records display strong obliquity and precessional signals reflecting primarily high latitude, Southern hemisphere changes influencing upwelling intensity and local low-latitude, insolation-driven climatic changes (via the productivity and/or turbidity influence of Amazon-sourced terrigenous material) at Sites 622 and 926 respectively. In seasonal studies of coccolithophorid assemblages, only part of the variation observed can be explained by abiotic processes, so it is perhaps not surprising that in this study few Pliocene nannofossil taxa demonstrate significant correlations with each other or with physical environmental parameters. Only some variance in nannofossil abundances can be explained by the primary controls of temperature and productivity. The rest is attributed to nonlinear responses to climatic changes; biotic processes such as grazing, predation, viral infection and competition, and/or, abiotic factors for which there is no readily available proxy (e.g. salinity). The lack of strong, consistent intra- and inter-relationships of the nannoflora and the environment reflects an ecologically complex, differentiated original community producing a complex integrated signal transmitted into the fossil record. D 2003 Elsevier B.V. All rights reserved. Keywords: Pliocene; central atlantic; calcareous nannofossils; biological cycles; environmental parameters

1. Introduction 1.1. Timescales of biotic change Studies of living nannoplankton species reveal that, like most organisms, they have very limited ecological tolerances, giving rise to distribution heterogeneities: * Corresponding author. Tel.: +44-1223-334877; fax: +441223-334871. E-mail address: [email protected] (S. Gibbs). 0377-8398/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2003.09.002

seasonal abundance patterns, strong vertical stratification of populations and restricted biogeographic distributions of species (e.g., Ziveri et al., 1995; Corte´s et al., 2001; Haidar and Thierstein, 2001). Longer-term environmental changes influence these distributions. For example, direct observation of modern oceans demonstrates variability due to El Nin˜o productivity effects on sub-decadal timescales (observed in the SeaWiFs time-series of chlorophyll content of surface waters, e.g., Volumes 1– 8: September 4, 1997 to June 30, 2001, SeaWiFS Project, NASA/Goddard Space

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Flight Center). Questions which naturally arise are: how much of this variation in modern oceans on observable timescales would be transmitted to the geological record, on what geological timescales can ecological signals be observed, and how much of this signal is destroyed by time integration? 1.2. Milankovitch cyclicity and nannofossils It is perhaps reasonable to extrapolate variation on observable modern timescales to longer records, and variability has been observed in nannofossil populations on a variety of temporal scales: El Nin˜o southern oscillations (ENSO, e.g., Beaufort et al., 2001), millennial timescales (e.g., De GaridelThoran et al., 2001), and on the Milankovitch scale (see below). On Milankovitch timescales of tens to hundreds of thousand years, variations in the Earth’s orbital parameters alter the physical environment of the Earth by varying the seasonal cycle and latitudinal distribution of solar insolation. The time-integrated insolation signals are reflected in complex longer-term climatic signals transmitted into the geological record by their influences on sedimentation. Orbital cyclicities have been identified in a wide variety of measurable geological parameters, initially from lithological characters, and subsequently in the physical, geochemical and palaeontological characteristics of rock successions (e.g., see reviews in Berger et al., 1984; House and Gale, 1995). From the point of view of nannofossil accumulations, investigations of climatic signals in nannofossil biotic records have generally been restricted to the Late Quaternary and have focused on key taxa, notably Florisphaera profunda (e.g., Molfino and McIntyre, 1990; McIntyre and Molfino, 1996; Beaufort et al., 1997, 2001; Bassinot et al., 1997). In older records, and in other taxa, there have been fewer studies of orbital signals in nannofossil abundances. In the Late Pliocene, cyclic patterns have been shown for the discoasters (e.g., Backman et al., 1986; Backman and Pestiaux, 1987; ChepstowLusty et al., 1989), and for Coccolithus pelagicus and Reticulofenestra pseudoumbilicus in the Miocene (Beaufort and Aubry, 1990). In the Mesozoic, possible periodicities in nannofossil abundances have been observed in several studies, including

notably in the Aptian/Albian of southern England, northern Germany and southeast France (Erba et al., 1992; Weber et al., 2001; Herrle et al., 2003a,b) and in the Pliensbachian of southern England (Walsworth-Bell, 2001). It is unknown how pervasive these signals are within nannoplankton and nannofossil communities as a whole, and how much signal is preserved in pre-Quaternary nannofossil communities. Pre-Quaternary records are increasingly the focus of high-resolution palaeoceanographic study and the identification and understanding of climatic signals within nannofossil communities may prove significant as a source of climatic information, just as Florisphaera has been for palaeoproductivity investigations in the Quaternary. Using high resolution, quantitative whole assemblage data, the aims of this investigation are to demonstrate whether cyclic nannofossil abundance variations in the Pliocene (3.55 – 3.95 Ma) reflect orbitally forced climate signals, which Pliocene taxa (beyond discoasters) display these signals, and what proportion of the nannofossil assemblage exhibits them.

2. Material and methods 2.1. ODP Sites 662 and 926 and their oceanographic settings Sites chosen for this study provide an east and west tropical Atlantic comparison, and also a more oligotrophic and more eutrophic site comparison. Site 662 (Leg 108) is situated off northwest Africa (Fig. 1) and lies at present in the region most affected by seasonal equatorial divergence. The contact zone of the equatorial undercurrent (EUC) and the Southern Equatorial Current (SEC) forms the divergence where colder waters are upwelled from the thermocline (summarised in Baumann et al., 1999). In addition, the site is strongly influenced by the Benguela current, which transports cooler water north into the equatorial region. Site 662 therefore lies in the southern hemisphere thermal regime (i.e. south of the thermal equator) (Ruddiman et al., 1988). Site 926 lies on Ceara Rise, a bathymetric high located northeastward of the Amazon delta, out of the influence of strong equatorial divergence (Fig. 1).

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Fig. 1. Central Atlantic map showing the positions of Leg 154 Site 926 and Leg 108 Sites 662, and the general modern surface circulation based on Billups et al. (1998) and Peterson and Stamma (1991) in Baumann et al. (1999). Abbreviations: North Equatorial Current (NEC), South Equatorial Current (SEC), South Equatorial Counter Current (SECC), North Brazil Coastal Current (NBCC). Surface water currents (black arrows). The shaded area indicates the main areas of high productivity, resulting from equatorial divergence, coastal upwelling and Amazon outflow. Cartography uses Online Map Creation (OMC).

It is situated, as is Site 662, in intermediate depths bathed by NADW above the modern lysocline (Tiedemann and Franz, 1997), and nannofossil (as well as foraminiferal) preservation is excellent. Site 926 allows us to monitor a warm water, oligotrophic site at high resolution: sedimentation rates are unusually high because of Amazon outwash accumulation. Climate in the modern equatorial Atlantic is primarily controlled by variations in the intensity of low level winds over Africa and the equatorial Atlantic, the NE and SE trade winds, which are separated by the Intertropical Convergence Zone (ITCZ, Ruddiman et al., 1989). The position of the ITCZ shifts seasonally and extrapolation of this seasonal pattern can be used as a first approximation to represent glacial –interglacial patterns (Ruddiman et al., 1989). It is thought that strong trade winds are associated with a deep western tropical Atlantic thermocline (e.g., Ru¨hlemann et al., 2001). This is concurrent with a strengthening of the North Brazil Current retroflection, in which the NBC (instead of flowing uninterrupted north) turns and flows eastwards, merging with the North Equatorial Countercurrent (NECC, Fig. 1), and bringing with it increased suspended terrigenous load to Ceara Rise.

2.2. Sampling, age models and site correlation Samples were selected from the mid-Pliocene, 3.55 to 3.95 Ma (mean sampling interval f 3 ky, every 10 cm at both sites), an interval which exhibits no significant bioturbation at either site and in which nannofossil preservation is excellent (confirmed by Gibbs et al., submitted for publication a,b). Reliable age models are an essential requirement for assessing the nature of climatic variability present in records, particularly climatic variability at orbital frequencies. The age models used here utilise recently calculated orbital targets (Laskar et al., 1993) to produce comparable age models for detailed site correlation. In this time interval, the effects of a more recent orbital calculation are negligible (Laskar, 2001). Age models were constructed by oxygen isotope record correlation and using the astronomical solution La1,1; the target that was used for the Leg 154 tuning of records from 5 to 34 Ma. The published age model for Site 926, based on tuning the magnetic susceptibility record to the northern hemisphere insolation curve of Laskar (1990) in Tiedemann and Franz (1997), was herein adjusted by tuning to the La1,1 based target. Maxima in the magnetic susceptibility records were tuned to

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target minima, assuming no lag (Fig. 2). At Site 662, the age model used principally graphic correlation to Leg 154 and tuning of the bulk carbonate d18O record (Fig. 2). Tie-points for these age models can be found in Appendix B. 2.3. Quantitative analyses Quantitative (number of specimens per mm2) whole assemblage counts were made from smear

slides, in part following counting strategies employed by Backman and Shackleton (1983) and Flores et al. (1995), and the taxonomy of Young (1998) (Appendix A). All specimens were counted on a minimum of 10 fields of view (FOV) which had approximately the same density of particles (averaging approximately 40 nannofossils >3 Am with a highly variable number of smaller nannofossils). Therefore at least 400 specimens >3 Am were counted per sample, a statistically significant number. For the rarer taxa (when the focal

Fig. 2. Site 926 and Site 662 age models and accumulation rates. At Site 926, the magnetic susceptibility record (Tiedemann and Franz, 1997, shown here reversed) has been tuned to a version of the orbital target of Laskar et al. (1993), La1,1 [tilt+(precession* 0.25)]. For Site 662, the bulk carbonate d18O record was graphically correlated to the d18O planktonic record (G. sacculifer) of Site 926 and, in part, tuned to the La1,1 target. Control points are indicated by filled circles on the Site 622 bulk carbonate d18O record, and the dashed lines indicate the correlation with the insolation target and the Site 926 isotope curve. The tie-points for both sites can be found in Appendix B. For each site, plots of the implied accumulation rates are illustrated. Isotopic stage assignments follow those of Tiedemann et al. (1994), after Shackleton et al. (1995). Grey bars indicate the positions of the approximately synchronous isotope ‘cold’ intervals (isotope stages Gi2 – Gi14). Discrepancies in the positioning of the isotope stages at Site 662 arise from the age model being dominantly based on orbital tuning rather than exact correlation of isotopic records.

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taxa abundances were less than 2 in 10 FOVs), 20 to 50 extra fields of view were counted. 2.4. Non-nannofossil indicators of environmental change: temperature/ocean circulation and nutrientsupply indicators Planktonic d18O records were produced for Sites 662 (from bulk carbonate analysis) and 926 (from G. sacculifer >450 Am). A detailed one-cycle study of Site 662 shows that the planktonic G. sacculifer d18O record resembles the bulk carbonate d18O curve and therefore the bulk carbonate curve is assumed to be primarily reflecting a planktonic signal (Gibbs, 2002). However, the very positive isotope ratios indicate that this is not solely a surface water record,

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despite being a useful first approximation for a planktonic record. Magnetic susceptibility, as a proxy for terrigenous supply at Ceara Rise, and terrigenous particle counts may be a record of varying nutrient supply and hence, related to productivity. An independent count of terrigenous particles (number particles >1 Am/mm2) shows a clear correlation with the published magnetic susceptibility record of Tiedemann and Franz (1997), as expected (Gibbs, 2002). An increased terrigenous flux may be accompanied by an increase in the nutrient supply and organic matter. In addition, the fluctuating terrigenous accumulation may be related to wind variability and therefore may be linked to upwelling intensity. The terrigenous records reflect the more local climate change in contrast to the d18O

Fig. 3. Selected high-resolution nannofossil abundance records from Site 662. All abundances are given as number of nannofossils per mm2. Average abundance data can be found in Appendix C and the downcore data is available from the corresponding author (in Gibbs, 2002). Records have been included here which demonstrate visually strong cyclic patterns of abundance variation, as well as examples of other types of patterns observed. Many of the taxa do not show systematic patterns of variation and have therefore not been included. The longer records focused on selected taxa (those which show either a stratigraphic signal, a strong cyclic signal or had potential palaeoecological significance) such as Discoaster spp., Florisphaera profunda, and Sphenolithus sp. For Florisphaera, Sphenolithus, Discoaster, Umbilicosphaera jafari, Thoracosphaera and Syracosphaera an orbital obliquity sequence is included. Arrows indicate trends in abundance change.

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records which are likely to contain a significant component of ice volume influence, primarily a higher latitude, more geographically distant signal.

3. Results and discussion 3.1. Downcore variability in abundance patterns at Sites 662 and 926 Approximately 45 species were differentiated in mid-Pliocene assemblages of Sites 926 and 662 though, despite this relatively high species diversity, the first four most abundant taxa contribute f 75% of the nannofossil abundance at both sites. Assemblages were dominated by placolith taxa belonging to the family Noelaerhabdaceae (Reticulofenestra spp., small Gephyrocapsa, Pseudoemiliania) and the nannolith species Florisphaera profunda (Appendix C). Figs. 3 and 4 illustrate the downcore variation in

selected taxa from these assemblages, demonstrating the different types of abundance pattern encountered. Perhaps the most obvious patterns at both sites are the high amplitude, strongly cyclic abundance records reflecting a strong climatic control on abundances, dominated by the discoasters, in addition to co-varying Sphenolithus and F. profunda records. These taxa are discussed in detail below. All three visually demonstrate a combined pattern of precession and obliquity, confirmed by spectral analysis (see below). Beyond this orbitally related cyclicity and marked co-variance of Discoaster, Sphenolithus and F. profunda, varying strengths of cyclic abundance are observed in other taxa though relationships between taxa are overall few and often weak. Considering first Site 662, abundance patterns include those of Syracosphaera, Thoracosphaera, Umbilicosphaera jafari and small Gephyrocapsa which show relatively strong cyclicity dominated by an apparently obliquity-related signal. Rhabdosphaera

Fig. 4. Selected high-resolution nannofossil abundance records from Site 926. All abundances are given as number of nannofossils per mm2. As for Fig. 3, the longer records focused on selected taxa: those which show either a stratigraphic signal, a strong cyclic signal or had potential palaeoecological significance. For Florisphaera, Sphenolithus and Discoaster, a precession sequence is included; for Umbilicosphaera and Pontosphaera, an obliquity sequence, and for Calcidiscus leptoporus a longer wavelength sequence (100 ky, eccentricity, La1,1). Arrows indicate trends in abundance change. L.O.: last occurrence.

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also displays a relatively marked cyclicity but does not obviously correlate with the cyclicity observed in the other taxa. Calcidiscus leptoporus, C. tropicus, Helicosphaera carteri, Umbilicosphaera sibogae (varieties sibogae and foliosa) and U. rotula, though they show high amplitude abundance variations, display no discernible regularity in these variations. There are also taxa which display relatively constant abundance, namely Pseudoemiliania at both sites,

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and Helicosphaera spp. (in Site 926), perhaps reflecting a lack of long-term climatic control on their abundances and a lack of relative sensitivity to environmental conditions. At Site 926, in general there is little discernible regularity to nannofossil abundance fluctuations, other than in the Discoaster, Sphenolithus and Florisphaera records. However, Calcidiscus tropicus and C. leptoporus do demonstrate high amplitude, short wave-

Fig. 5. Cross-spectral analyses of total Discoaster record with abiotic records (planktonic d18O and magnetic susceptibility) and insolation. Discoaster with (a) flipped bulk carbonate d18O record from Site 662, (b) flipped planktonic (G. sacculifer) d18O from Site 926 and, (c) flipped magnetic susceptibility record from Site 926, and (d) insolation at 65jN (target of Laskar et al., 1993). The lower plots show the phase relationships of Discoaster to d18O, magnetic susceptibility and insolation. For each plot, the dotted/dashed line shows coherency with a line at the 80% confidence limit. The amplitudes of the spectra are plotted on an arbitrary log scale. Phase estimates are only plotted where coherency is significant. A negative phase implies that the abiotic records (low temperatures, low magnetic susceptibility, low insolation) lag high Discoaster abundances. Note that the discoaster records in both sites exhibit a lead over the d18O records, in contrast to the phase relationships exhibited with insolation and magnetic susceptibility. The phase relationship with insolation arises from the assumption that magnetic susceptibility maxima are in phase with insolation minima. The software package used here for cross-spectral analysis is the OS-3 ARAND System (Oregon State University, 1973), also used in, e.g., Shackleton et al. (1995). The data was interpolated at constant time intervals of 3 ky, since this time interval reflects the actual sampling interval.

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length variation, with C. leptoporus additionally showing a possible longer wavelength cyclicity. Total Pontosphaera may demonstrate a weak obliquity-related signal. Among the other taxa, the main feature is the variation in amplitude of abundance changes observed. For example, Umbilicosphaera species show high amplitude abundance variations though these low frequency cyclicities generally appear uncorrelated. In addition, longer-term trends exist, including the opposing signals of small Gephyrocapsa and small Reticulofenestra as well as the decline and disappearance of Sphenolithus and R. pseudoumbilicus. 3.2. Taxa which demonstrate strongly orbitally forced cyclic abundance patterns 3.2.1. Discoaster Discoasters are an important component of the Palaeogene and Neogene nannofossil assemblages and although their relationship with coccolithophores is uncertain, it is believed that, like coccoliths, discoasters represent modified cell coverings of phytoplankton (Bown, 1998). A detailed look at total Discoaster abundances recorded at Sites 926 and 662 reveals with cross-spectral analysis that the dis-

coaster record at Site 926 closely follows the magnetic susceptibility record (rather than d18O or insolation, Fig. 5b, c and d), supporting visual assessments of cyclic variation (Fig. 6). Dominant spectral peaks are clearly observed in both the obliquity and precession bands in addition to a possible 100-ky signal (Fig. 5d). This contrasts with Site 662 where cross-spectral analysis with the bulk carbonate d18O records produces a spectrum dominated by obliquity that closely mirrors the d18O record (Fig. 5a). However, in both records the discoaster abundance leads the d18O record which supports the idea that the d18O records have a strong higher latitude signal (presumably an ice volume signal) which lags low latitude directly insolation-forced changes. In contrast, at Site 926 the discoasters are in phase with magnetic susceptibility, and therefore also insolation, presumably reflecting the strength of the effects of local climate change on the nannoplankton abundances. Note that magnetic susceptibility and insolation are in phase as a consequence of assumptions made in the tuning of the magnetic susceptibility record. Ecologically, these orbitally forced cyclic signals reflect strong local climatic control on discoaster abundances. Discoasters have long been considered

Fig. 6. Comparison of total Discoaster abundance from Sites 662 and 926 (solid black plots). The published magnetic susceptibility record of Tiedemann and Franz (1997), and the planktonic d18O records of Sites 926 (G. sacculifer) and 662 (bulk carbonate d18O) are also shown. Grey bars indicate the positions of the approximately synchronous isotope ‘cold’ intervals (isotope stages Gi2 – Gi14).

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warm-water taxa throughout their geological range, due to their consistent occurrence at low latitudes (e.g., Haq and Lohmann, 1976; Lohmann and Carlson, 1981) and, therefore, are believed to sensitively record temperature variations (e.g. Backman et al., 1986; Backman and Pestiaux, 1987). However, at low latitudes, later studies have demonstrated high amplitude variations that could not be explained by temperature variations alone (e.g., Chepstow-Lusty et al., 1989, 1992; Chapman and Chepstow-Lusty, 1997). It is clear that Discoaster abundances are not only suppressed by decreasing temperature associated with increasing latitude, but also by higher nutrient availability. High amplitude variations in discoaster abundance at both Sites 662 and 926 clearly support this low latitude productivity-control hypothesis (Fig. 6). In particular, at Site 926, water transparency/possible productivity associated with terrigenous input from the Amazon may reflect this important productivity control, an observation supported by discoaster abundance minima coinciding with high magnetic suscep-

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tibility intervals/high terrigenous intervals (Fig. 6). At Site 662, abundances are overall lower due to the interplay of both higher productivity and temperature variations at this upwelling site. However, high amplitude variation is still observed, with abundance minima associated with the colder/higher productivity intervals (the oxygen isotope troughs). Considering the discoaster species individually, the majority of the strong cyclic signal is focused in the abundance variations of Discoaster pentaradiatus and D. brouweri (Fig. 7). These taxa, in addition to D. asymmetricus and D. surculus, dominate discoaster abundances and show the typical discoaster abundance sensitivity to low latitude productivity (plus or minus water temperature) with higher abundances in ‘warmer’/more productive intervals. In contrast, D. variabilis and D. surculus show a possible sensitivity to the subtle temperature changes at Site 926 (a better visual correlation than with the productivity signal inferred from the magnetic susceptibility record), with generally higher abundances in colder intervals (illus-

Fig. 7. Individual Discoaster species abundance plots for Sites 662 and 926. All plots use the same abundance scales (except total Discoaster). D. triradiatus and D. quadramus have not been included due to their rarity.

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trated here using the D. variabilis record, Fig. 8). In contrast, at Site 662 the effect of a significantly stronger superimposed productivity signal perhaps overwhelms the temperature signal, since the two parameters would be acting in opposite directions on D. variabilis and D. surculus abundance. 3.2.2. Florisphaera profunda Florisphaera profunda is an extant species which is today restricted to low and middle latitudes warmer than 10 jC, to the lower photic zone at depths of f100 –150 m, and whose abundances are closely tied to light intensity (Okada and Honjo, 1973; Ziveri et al., 1995; Corte´s et al., 2001; Haidar and Thierstein, 2001). In the fossil record, F. profunda is at higher relative abundance where the production of shallow dwelling taxa decreases with a relatively deep thermocline and nutrient deprivation of the upper photic zone. Conversely, when the thermocline is shallow and nutrient availability in surface waters is greater, the relative abundance of F. profunda decreases (Molfino and McIntyre, 1990). Florisphaera profunda is frequently cited as a successful nannofossil oceanographic proxy and though it first appeared in the Middle Miocene the majority of studies are based on the Late Quaternary. It has been used extensively for thermocline and

associated palaeoproductivity reconstructions and has, in particular, significantly improved the understanding of the influence and intensity of equatorial trade wind systems (e.g., Molfino and McIntyre, 1990; Ahagon et al., 1993; McIntyre and Molfino, 1996; Bassinot et al., 1997; Beaufort et al., 1997, 2001; De Garidel-Thoran et al., 2001). At Site 662, Florisphaera profunda abundance shows a strong visual correlation with planktonic d18O, as observed for Discoaster and Sphenolithus (see below), with abundance minima of these taxa coinciding with (though with a slight lead) the cold/ more productive intervals (Fig. 9). A relationship between F. profunda abundance and Quaternary planktonic d18O has been documented by a number of studies with varying complexities of phase relationships (e.g., Ahagon et al., 1993; Beaufort et al., 1997; Bassinot et al., 1997). In contrast, at Site 926 F. profunda (again, as observed with Discoaster and Sphenolithus abundances) follow most strongly the terrigenous record supported by spectral analysis of the F. profunda and flipped magnetic susceptibility records (Figs. 9 and 10). The phase plot shows that high F. profunda abundance is associated with low magnetic susceptibility, again reflecting a strong local low latitude insolation-forced signal in the record at Site 926. The F. profunda record does, however, differ

Fig. 8. Comparison of Discoaster variabilis abundance from Sites 662 and 926 with oxygen isotope records (the planktonic record of Site 926 and the bulk carbonate record from Site 662). Grey bars indicate the positions of isotopic ‘cold’ stages. Peaks in abundance of D. variabilis tend to coincide with ‘cold’ intervals in the d18O record at Site 926. A weaker signal is observed at Site 662. Both sites are plotted with the same abundance scales.

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Fig. 9. Comparison of Florisphaera profunda abundances from Sites 662 and 926 shown with the magnetic susceptibility record of Site 926 and the bulk carbonate d18O record from Site 662. Grey bars indicate the positions of isotopic ‘cold’ stages. Note the different abundance scales. Florisphaera shows a strong visual correlation with magnetic susceptibility at Site 926, whereas at Site 662, variations in abundance amplitude follow very closely those observed in the d18O record.

from the records of Sphenolithus and Discoaster by demonstrating the closest correlation with d18O in terms of both varying abundance amplitude and also the lead it demonstrates visually over d18O, which is the smallest lead of the three taxa. Overall, the observations fit well with the model of F. profunda as a deeper-water taxa (recording a greater ice volume signal than Sphenolithus and Discoaster) which flourishes when there is a moderate depth thermocline/ nutricline and surface water oligotrophy.

Fig. 10. Cross-spectral analysis of the Florisphaera profunda record at Site 926 with magnetic susceptibility. The lower plots show the phase relationships between F. profunda and flipped magnetic susceptibility. The amplitude of the spectra is on an arbitrary log scale (see comments on Fig. 5). F. profunda abundances are in phase with magnetic susceptibility in the precession band.

3.2.3. Sphenolithus Sphenolithus, like Discoaster, is a long ranging extinct nannolith genus whose taxonomic affinities are uncertain. Again, it is assumed that the coneshaped liths form some sort of phytoplankton cell covering. Published studies which include Sphenolithus data rarely comment on the detailed ecology of this group. In general, it has been grouped with Discoaster as characteristic of low latitude, warm water assemblages (e.g., Haq and Lohmann, 1976; Haq, 1980; Lohmann and Carlson, 1981), perhaps with a shallower water preference (Perch-Nielsen, 1985).

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In the present study, the strong downcore variation of this taxon produces a pattern very similar to the surface nutrient availability-controlled pattern of Florisphaera profunda and Discoaster (Figs. 11 and 12). In addition, the records of Sphenolithus are here dominated by a pattern of decline prior to their extinction, superimposed on this strong climatic variability (see Gibbs et al., submitted for publication, b). The climate-forced pattern at Site 926 again reflects a predominantly local productivity control on abundances relatively independent of glacial – interglacial temperature variation. The close co-variance between Sphenolithus, F. profunda and Discoaster at high resolution (Fig. 12, an observation which has not been made previously) suggests a close ecological relationship at low latitudes which may reflect the strong response to productivity, but perhaps from different parts of the water column. F. profunda could be recording from the lower photic zone the nutrient availability to surface waters, whilst Discoaster could be responding primarily in warm waters at the surface.

3.3. Biological versus climatically forced variability: variance explained by abiotic control An interesting conclusion of this investigation is the relative scarcity of robust correlations between nannofossil taxa abundances and abiotic records. The variance observed in nannoplankton records that can be explained by direct orbitally forced climatic changes is concentrated within a number of key taxa. Systematic high amplitude variations are observed in Discoaster, Florisphaera and Sphenolithus, but relationships outside this group are often weak. The level of unexplained variance in the data (aside from the variation intrinsic to preparation and counting techniques) could be attributed to forcing by other abiotic controls, for which there is no measurable record (e.g., salinity, trace metals, turbidity, etc.), or non-systematic responses to climatic change. In addition, biotic forcing is likely to be significant (factors such as nutrient availability, grazing, viral infection, predation and competition), plus species evolutionary

Fig. 11. Comparison of Sphenolithus abundance from Sites 662 and 926, shown with the magnetic susceptibility record of Site 926, and the bulk carbonate d18O and discoaster records from Site 662. Both sites are plotted with the same abundance scales. Grey bars indicate the positions of isotopic ‘cold’ stages. Two signals dominate the records: the decline in abundance associated with the extinction of Sphenolithus, and the superimposed strongly cyclic variation. Again, Sphenolithus abundance closely follows variation in the magnetic susceptibility record at Site 926 and the d18O record at Site 662.

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Fig. 12. Sites 662 and 926 comparison of co-varying Sphenolithus, Florisphaera, Discoaster abundance records. Grey bars indicate the positions of isotopic ‘cold’ stages. The amplitude of abundance variation varies between the taxa but abundance maxima and minima consistently coincide.

signals either as a response to longer term climate change or changes that are climate independent. In living coccolithophorid populations, distribution and abundance variation can be observed monthly, strongly seasonally, annually and on timescales of several years (see Introduction). Variation clearly occurs from the daily scale upwards and this variability is controlled by both abiotic and biotic factors. However, Haidar and Thierstein (2001) and Renaud and Klaas (2001) found that very little variability in populations can be directly explained by abiotic controls. Simple comparisons of cell densities with environmental parameters generally produce only low to moderate correlations (Corte´s et al., 2001; Haidar and Thierstein, 2001). This was also the conclusion reached in the work by Margalef (1958, 1967) on phytoplankton succession. The model outlined by Margalef does imply a strong control of assemblages by abiotic factors (nutrients, turbulence, temperature), but does not predict linear relationships between abundances and these parameters. In the present study, it is therefore not surprising that there are few strong linear relationships between taxa, and also between taxa and the abiotic environment. However, overall more of the variability in the fossil record may be attributed to abiotic causes than can be explained in seasonal records, because of the sedimentological time integration of some of the biotic signal. In addition, the Pliocene

data presented here shows stronger productivity patterns than Holocene core-top assemblages (e.g., Roth, 1994), this is probably because discoasters and sphenoliths had a much higher preservation potential than the typical modern surface-water oligotrophs Umbellosphaera and Discosphaera. 3.4. The use of nannoplankton climatic signals: important ecological controls on nannofossil assemblages at each site Patterns observed in those nannofossil taxa which exhibit strong climatic signals allow us to say something about the dominant environmental controls operating at the two sites. In the Pliocene at Site 662, selected nannofossil records appear to be responding to the relatively high degree of temperature variability coupled with productivity variation in surface waters associated with the intensity of equatorial upwelling. Discoaster, Florisphaera and Sphenolithus display strong climatic signals which co-vary with the obliquity-driven upwelling cycles reflected in the bulk carbonate d18O record. Site 662 lies south of the thermal equator (i.e. in the southern hemisphere thermal regime) and Ruddiman et al. (1988) proposed that long-term SST changes recorded here would be responding to southern hemisphere forcing. Therefore, these obliquity-driven nannofossil records sug-

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S. Gibbs et al. / Marine Micropaleontology 51 (2004) 39–56

gest an effect in the mid-Pliocene of high latitude ice volume changes via the Southern Hemispheresourced Benguela current. However, the lead observed in the nannofossil abundances relative to the d18O record, in addition to the greater relative component of precession in the nannofossil abundances, would suggest that insolation-driven local climate variability is also a significant control on nannofossil abundances at this site. In contrast, Site 926 lies outside the main region of equatorial upwelling and experienced more stable temperatures and productivity. In particular, in the Pliocene, global ice volume was lower than today and amplitudes of ice volume fluctuations were smaller (see Billups et al., 1998, and references therein). However, despite this presumed relative stability, a comparatively high amplitude of abundance variation was observed within some of the taxa which is in some ways unusual for an equatorial oligotrophic site. The close relationship with the precessionally controlled terrigenous record, in addition to the lead of the discoaster and Florisphaera records relative to the planktonic d18O record, suggests there exists significant low-latitude, local insolation-driven climatic control on variability. Nannoplankton assemblages are perhaps sensitive to the small environmental changes directly or indirectly resulting from the influence of Amazonsourced terrigenous material and associated nutrients, +/ very limited precessionally driven temperature changes associated with divergence (observed in Ceara Rise planktonic d18O records, Billups et al., 1998). Discoaster, Florisphaera and Sphenolithus are more oligotrophic-favouring taxa and at Site 926 display higher abundances in the low terrigenous accumulation intervals. This covariance between terrigenous supply and productivity suggests the following possibilities: (1) A direct productivity response to the terrigenous input (by the introduction of nutrients or a turbidity influence), as reflected in the temporal distribution of the more oligotrophic assemblages. (2) An independent co-varying response by the two systems to surface current dynamics controlled by wind systems. It is not possible to select between these possibilities other than to note that fertilisation effects are implied by the suppressed discoaster abundances in addition to the strong coherence of the discoaster and magnetic

susceptibility spectra. Nannofossil abundance variability could be reflecting humidity cycles over south America controlling Amazon outwash, or surface water current strength controlling the amount of Amazon-sourced material reaching Ceara Rise and/ or wind-driven thermocline depth.

4. Conclusions The nannofossil abundance patterns identified in this study are often difficult to interpret, with few nannofossil taxa demonstrating strong correlations with each other or with environmental parameters. This is expected given that living populations exhibit very little variance that can be explained by simple correlation with abiotic factors. However, the previously undocumented co-varying abundance records of Discoaster, Sphenolithus, and Florisphaera demonstrate strongly environmentally controlled abundance patterns displaying a distinct orbitally forced component and robust correlation with abiotic records. Strong relationships with productivity signals confirm that productivity exerts a strong control on discoaster abundances at low latitudes. All three taxa are sensitive to changing oceanographic conditions within both low latitude upwelling and nonupwelling areas. Significantly, it can be shown here that few Pliocene nannoplankton taxa have this character. Therefore, even though the controls on the distributions of Discoaster and Sphenolithus are less well understood than those for F. profunda, they are still clearly unusually sensitive to Pliocene environmental change, and have been undervalued as indicators of surface or subsurface change. Selected abundance records that yield strong climatic signals could permit the further development of the use of Pliocene nannofossils as palaeoceanographic indicators, particularly in monitoring low-latitude climatic variability as F. profunda has in the Quaternary. This would address the present shortage of well-understood Neogene biotic proxies.

Acknowledgements SG wishes to thank NERC, Cambridge University, Gonville and Caius College, and the CODENET

S. Gibbs et al. / Marine Micropaleontology 51 (2004) 39–56

project (Coccolithophorid Evolutionary Biodiversity and Ecology Network, EC TMR project) for their financial assistance. The Ocean Drilling Program kindly provided samples. Many thanks to Mike Hall, James Rolfe, and Benoir Vautravers of the Godwin Laboratory for their technical assistance. Thanks also to Simon Crowhurst for statistical analyses and his constructive review of the manuscript. Thanks also to Simon Crowhurst for statistical analyses and Jan Backman and Jens Herrle for their constructive reviews of the manuscript.

Appendix A . Taxa list and notes Taxonomy in general follows that outlined in Young (1998) and Perch-Nielsen (1985). Calcidiscus leptoporus (Murray and Blackman, 1898) Loeblich and Tappan, 1978 C. macintyrei (Bukry and Bramlette, 1969a) Loeblich and Tappan, 1978 (>10 Am) C. tropicus Kamptner, 1956 sensu Gartner, 1992 (< 10 Am) Calciosolenia murrayi Gran, 1912 Ceratolithus Kamptner, 1950. Species were not differentiated; however, most were likely to be C. cristatus Kamptner, 1950 Coccolithus pelagicus (Wallich, 1871) Schiller, 1930 Coronosphaera sp. Gaarder in Gaarder and Heimdal (1977). There appears to be only one species present Discoaster asymmetricus Gartner, 1969c D. brouweri Tan, 1927b emend. Bramlette and Reidel, 1954 D. brouweri var. triradiatus Tan, 1927b sensu Backman and Shackleton, 1983 D. pentaradiatus Tan, 1927b D. quadramus Bukry, 1973b D. surculus Martini and Bramlette, 1963 D. tamalis Kamptner, 1967 D. variabilis Martini and Bramlette, 1963 Discosphaera tubifera (Murray and Blackman, 1898) Ostenfeld, 1900 Florisphaera profunda Okada and Honjo, 1973 Gephyrocapsa Kamptner, 1943 Gephyrocapsa < 3.5 Am. Arbitrary size subdivision following Rio et al. (1990) Gephyrocapsa >3.5 Am. Arbitrary size subdivision following Rio et al. (1990) Hayaster perplexus (Bramlette and Riedel, 1954) Bukry, 1973b Helicosphaera carteri (Wallich, 1877) Kamptner, 1954 H. carteri var. wallichii (Lohmann, 1902) Theodoridis, 1984 H. sellii (Bukry and Bramlette, 1969b) Jafar and Martini, 1975 Oolithotus fragilis (Lohmann, 1912) Martini and Mu¨ller, 1972

53

Appendix A (continued) Pontosphaera discopora Schiller, 1925 P. japonica (Takayama, 1967) Nishida, 1971 P. multipora (Kamptner, 1948) Roth, 1970 Pseudoemiliania Gartner, 1969c. Only one species appeared to be present Reticulofenestra Hay et al, 1966. These are generally size-defined following the taxonomy outlined in Young (1998) R. haqii Backman, 1978/R. sp. 3 – 5 Am R. minuta Roth, 1970/R. sp. < 3 Am (small retic.) R. pseudoumbilicus (Gartner, 1967b) Gartner, 1969c.>7 Am R. sp. 5 – 7 Am, noted by Backman and Shackleton (1983) Rhabdosphaera Haeckel, 1894. Species were not differentiated Scyphosphaera apsteinii Lohmann, 1902 S. globulata Bukry and Percival, 1971 S. lagena Kamptner, 1955 S. pulcherrima Deflandre, 1942 Sphenolithus abies Deflandre in Deflandre and Fert, 1954. Morphometrically, there is not evidence here for the presence of two sphenolith species (Gibbs, 2002) Syracosphaera pulchra Lohmann, 1902 Tetralithoides Theodoridis, 1984 emend. Jordan et al., 1993. Likely to have been the species T. quadrilaminata (Okada and McIntyre, 1977) Jordan et al., 1993 Thoracosphaera Kamptner, 1927 Calcareous dinocysts were not differentiated though at least 2 species existed Umbilicosphaera jafari Mu¨ller, 1974b U. rotula (Kamptner, 1956) Varol, 1982 U. sibogae var. foliosa (Kamptner, 1963) Okada and McIntyre, 1977 U. sibogae var. sibogae (Weber-van Bosse, 1901) Gaarder, 1970

Appendix B . Age model tie points Site 662a

Site 926a

Site 926c

Age (Ma)

Depth (mbsf)

Age (Ma)

Depth (mcd)

Age (Ma)

Depth (mcd)

3.418 3.546 3.637 3.756 3.831 3.874 3.923 3.947 3.957

181.93 185.25 188.13 192.35 194.35 195.35 196.85 197.75 198.15

3.47 3.561 3.674 3.715 3.808 3.851 3.965 3.999

107.79 110.29 114 115.4 118.1 119.6 123.29 124.19

3.47 3.561 3.623 3.674 3.715 3.808 3.851 3.923 3.965 3.999

107.68 110.57 112.3 114.04 115.46 118.18 119.65 121.79 123.29 124

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S. Gibbs et al. / Marine Micropaleontology 51 (2004) 39–56

Appendix C . Average nannofossil % abundances ODP Site 926 Species

ODP Site 662 Average % abundance

Species

3.723 – 3.866 Ma Florisphaera profunda (total Reticulofenestra) R. minuta Pseudoemiliania sp. Gephyrocapsa sp. (small) (total Umbilicosphaera) R. haqii Sphenolithus abies (total Discoaster) U. sibogae var. foliosa Rhabdosphaera clavigera (total Helicosphaera) H. carteri U. rotula U. jafari (total Calcidiscus) Syracosphaera pulchra Thoracosphaera spp. D. pentaradiatus C. tropicus D. brouweri C. leptoporus U. sibogae var. sibogae Oolithotus sp. Coronosphaera sp. D. surculus D. asymmetricus (total Pontosphaera) Calciosolenia murrayi P. discopora H. carteri var. wallichii D. variabilis Tetralithoides sp. (total Scyphosphaera) R. pseudoumbilicus R. sp. (5 – 7 Am) D. tamalis P. multipora Scyphosphaera identifiable (incl. S. lagena, S. globulata, S. pulcherrima, S. apsteinii) Discosphaera tubifera

D. brouweri var. triradiatus Ceratolithus indet P. japonica C. macintyrei D. quadramus Hayaster perplexus

43.0 36.2 12.8 10.8 7.5 5.4 5.0 4.8 2.9 2.8 2.6 1.8 1.7 1.2 1.0 0.9 0.7 0.7 0.6 0.5 0.4 0.4 0.4 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.09 0.06 0.03 0.03 0.02 0.02 0.02

0.09

0.01 0.01 0.01 0.01 < 0.01 < 0.01

Average % abundance 3.663 – 3.900 Ma

(total Reticulofenestra) R. minuta R. haqii Florisphaera profunda Pseudoemiliania sp. Gephyrocapsa sp. (small) (total Umbilicosphaera ) Sphenolithus abies (total Calcidiscus) (total Helicosphaera) H. carteri C. tropicus U. rotula (total Discoaster) C. leptoporus U. jafari U. sibogae var. foliosa Rhabdosphaera clavigera Thoracosphaera spp. U. sibogae var. sibogae R. sp. (5 – 7 Am) Syracosphaera pulchra D. brouweri D. pentaradiatus R. pseudoumbilicus Coronosphaera sp. D. asymmetricus C. macintyrei G. sp. (large>3.5 Am) H. carteri var. wallichii Oolithotus sp. D. surculus H. selli (total Pontosphaera) Hayaster perplexus P. discopora (total Scyphosphaera) P. multipora D. variabilis D. tamalis Scyphosphaera identifiable (incl. S. lagena, S. globulata, S. pulcherrima, S. apsteinii) D. brouweri var. triradiatus Calciosolenia murrayi C. pelagicus Ceratolithus indet D. quadramus Tetralithoides sp.

69.2 35.1 23.8 10.8 9.7 4.9 4.1 3.0 2.8 1.8 1.6 1.5 1.4 1.3 1.2 1.0 1.0 0.7 0.6 0.5 0.5 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.08 0.08 0.08 0.06 0.05 0.03 0.03 0.03 0.02 0.02 0.01

< 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01

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