orbitally forced climate signals in mid-pliocene nannofossil assemblages

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Marine Micropaleontology 51 (2004) 39–56

Orbitally forced climate signals in mid-Pliocene

nannofossil assemblages

Samantha Gibbsa,*, Nicholas Shackletona, Jeremy Youngb

aGodwin Institute for Quaternary Research, University of Cambridge, Pembroke Street, Cambridge CB2 3SA, UKbNatural 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 seasonal abundance patterns, strong vertical stratifica-

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:

0377-8398/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.marmicro.2003.09.002

* Corresponding author. Tel.: +44-1223-334877; fax: +44-

1223-334871.

E-mail address: [email protected] (S. Gibbs).

tion of populations and restricted biogeographic dis-

tributions of species (e.g., Ziveri et al., 1995; Cortes 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 Nino 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

S. Gibbs et al. / Marine Micropaleontology 51 (2004) 39–5640

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 Nino

southern oscillations (ENSO, e.g., Beaufort et al.,

2001), millennial timescales (e.g., De Garidel-

Thoran 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 latitu-

dinal distribution of solar insolation. The time-inte-

grated insolation signals are reflected in complex

longer-term climatic signals transmitted into the

geological record by their influences on sedimenta-

tion. Orbital cyclicities have been identified in a

wide variety of measurable geological parameters,

initially from lithological characters, and subse-

quently in the physical, geochemical and palaeonto-

logical characteristics of rock successions (e.g., see

reviews in Berger et al., 1984; House and Gale,

1995).

From the point of view of nannofossil accumu-

lations, investigations of climatic signals in nanno-

fossil 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; Beau-

fort 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 abun-

dances. In the Late Pliocene, cyclic patterns have

been shown for the discoasters (e.g., Backman et

al., 1986; Backman and Pestiaux, 1987; Chepstow-

Lusty et al., 1989), and for Coccolithus pelagicus

and Reticulofenestra pseudoumbilicus in the Mio-

cene (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 (Wals-

worth-Bell, 2001). It is unknown how pervasive

these signals are within nannoplankton and nanno-

fossil communities as a whole, and how much

signal is preserved in pre-Quaternary nannofossil

communities. Pre-Quaternary records are increasing-

ly 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 resolu-

tion, quantitative whole assemblage data, the aims

of this investigation are to demonstrate whether

cyclic nannofossil abundance variations in the Plio-

cene (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 con-

tact 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).

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).

S. Gibbs et al. / Marine Micropaleontology 51 (2004) 39–56 41

It is situated, as is Site 662, in intermediate depths

bathed by NADW above the modern lysocline (Tie-

demann 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., Ruhlemann 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 east-

wards, merging with the North Equatorial Counter-

current (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 com-

parable 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


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

Fig. 2. Site 926 and Site 662 age models and accumulation rates. At Site 9

shown here reversed) has been tuned to a version of the orbital target of La

bulk carbonate d18O record was graphically correlated to the d18O plankto

target. Control points are indicated by filled circles on the Site 622 bulk car

the insolation target and the Site 926 isotope curve. The tie-points for both

accumulation rates are illustrated. Isotopic stage assignments follow those

indicate the positions of the approximately synchronous isotope ‘cold’ inte

the isotope stages at Site 662 arise from the age model being dominantly ba

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 speci-

mens >3 Am were counted per sample, a statistically

significant number. For the rarer taxa (when the focal

26, the magnetic susceptibility record (Tiedemann and Franz, 1997,

skar et al. (1993), La1,1 [tilt+(precession*� 0.25)]. For Site 662, the

nic record (G. sacculifer) of Site 926 and, in part, tuned to the La1,1bonate d18O record, and the dashed lines indicate the correlation with

sites can be found in Appendix B. For each site, plots of the implied

of Tiedemann et al. (1994), after Shackleton et al. (1995). Grey bars

rvals (isotope stages Gi2–Gi14). Discrepancies in the positioning of

sed on orbital tuning rather than exact correlation of isotopic records.


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 nutrient-

supply 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,

Fig. 3. Selected high-resolution nannofossil abundance records from Site

Average abundance data can be found in Appendix C and the downcore

Records have been included here which demonstrate visually strong cyclic p

patterns observed. Many of the taxa do not show systematic patterns of

focused on selected taxa (those which show either a stratigraphic signal, a

such as Discoaster spp., Florisphaera profunda, and Sphenolithus sp. Fo

Thoracosphaera and Syracosphaera an orbital obliquity sequence is inclu

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

662. All abundances are given as number of nannofossils per mm2.

data is available from the corresponding author (in Gibbs, 2002).

atterns of abundance variation, as well as examples of other types of

variation and have therefore not been included. The longer records

strong cyclic signal or had potential palaeoecological significance)

r Florisphaera, Sphenolithus, Discoaster, Umbilicosphaera jafari,

ded. Arrows indicate trends in abundance change.


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. Assemb-

lages were dominated by placolith taxa belonging to

the family Noelaerhabdaceae (Reticulofenestra spp.,

small Gephyrocapsa, Pseudoemiliania) and the nan-

nolith species Florisphaera profunda (Appendix C).

Figs. 3 and 4 illustrate the downcore variation in

Fig. 4. Selected high-resolution nannofossil abundance records from Site 92

for Fig. 3, the longer records focused on selected taxa: those which show

palaeoecological significance. For Florisphaera, Sphenolithus and Disco

Pontosphaera, an obliquity sequence, and for Calcidiscus leptoporus a

indicate trends in abundance change. L.O.: last occurrence.

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 abun-

dances, 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 preces-

sion 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 abun-

dance are observed in other taxa though relationships

between taxa are overall few and often weak. Con-

sidering first Site 662, abundance patterns include

those of Syracosphaera, Thoracosphaera, Umbili-

cosphaera jafari and small Gephyrocapsa which

show relatively strong cyclicity dominated by an

apparently obliquity-related signal. Rhabdosphaera

6. All abundances are given as number of nannofossils per mm2. As

either a stratigraphic signal, a strong cyclic signal or had potential

aster, a precession sequence is included; for Umbilicosphaera and

longer wavelength sequence (100 ky, eccentricity, La1,1). Arrows


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 (va-

rieties 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,

Fig. 5. Cross-spectral analyses of total Discoaster record with abiotic rec

Discoaster with (a) flipped bulk carbonate d18O record from Site 662, (b) f

magnetic susceptibility record from Site 926, and (d) insolation at 65jNrelationships of Discoaster to d18O, magnetic susceptibility and insolation.

the 80% confidence limit. The amplitudes of the spectra are plotted on an a

is significant. A negative phase implies that the abiotic records (low te

Discoaster abundances. Note that the discoaster records in both sites exhib

exhibited with insolation and magnetic susceptibility. The phase relati

susceptibility maxima are in phase with insolation minima. The software

System (Oregon State University, 1973), also used in, e.g., Shackleton et al

since this time interval reflects the actual sampling interval.

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. lepto-

porus do demonstrate high amplitude, short wave-

ords (planktonic d18O and magnetic susceptibility) and insolation.

lipped planktonic (G. sacculifer) d18O from Site 926 and, (c) flipped

(target of Laskar et al., 1993). The lower plots show the phase

For each plot, the dotted/dashed line shows coherency with a line at

rbitrary log scale. Phase estimates are only plotted where coherency

mperatures, low magnetic susceptibility, low insolation) lag high

it a lead over the d18O records, in contrast to the phase relationships

onship with insolation arises from the assumption that magnetic

package used here for cross-spectral analysis is the OS-3 ARAND

. (1995). The data was interpolated at constant time intervals of 3 ky,


length variation, with C. leptoporus additionally show-

ing a possible longer wavelength cyclicity. Total Pon-

tosphaera 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 fre-

quency cyclicities generally appear uncorrelated. In

addition, longer-term trends exist, including the oppos-

ing signals of small Gephyrocapsa and small Reticu-

lofenestra 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, dis-

coasters represent modified cell coverings of phyto-

plankton (Bown, 1998). A detailed look at total

Discoaster abundances recorded at Sites 926 and

662 reveals with cross-spectral analysis that the dis-

Fig. 6. Comparison of total Discoaster abundance from Sites 662 and 926

Tiedemann and Franz (1997), and the planktonic d18O records of Sites 926

bars indicate the positions of the approximately synchronous isotope ‘col

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 produ-

ces a spectrum dominated by obliquity that closely

mirrors the d18O record (Fig. 5a). However, in both

records the discoaster abundance leads the d18Orecord which supports the idea that the d18O records

have a strong higher latitude signal (presumably an ice

volume signal) which lags low latitude directly inso-

lation-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 conse-

quence 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

(solid black plots). The published magnetic susceptibility record of

(G. sacculifer) and 662 (bulk carbonate d18O) are also shown. Grey

d’ intervals (isotope stages Gi2–Gi14).


warm-water taxa throughout their geological range,

due to their consistent occurrence at low latitudes

(e.g., Haq and Lohmann, 1976; Lohmann and Carl-

son, 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 ampli-

tude variations that could not be explained by tem-

perature variations alone (e.g., Chepstow-Lusty et al.,

1989, 1992; Chapman and Chepstow-Lusty, 1997). It

is clear that Discoaster abundances are not only sup-

pressed by decreasing temperature associated with

increasing latitude, but also by higher nutrient avail-

ability. High amplitude variations in discoaster abun-

dance 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 abun-

dance minima coinciding with high magnetic suscep-

Fig. 7. Individual Discoaster species abundance plots for Sites 662 and 926.

triradiatus and D. quadramus have not been included due to their rarity.

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 am-

plitude 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 abun-

dance 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-

All plots use the same abundance scales (except totalDiscoaster). D.


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; Cortes et al., 2001; Haidar

and Thierstein, 2001). In the fossil record, F. pro-

funda is at higher relative abundance where the

production of shallow dwelling taxa decreases with

a relatively deep thermocline and nutrient depriva-

tion 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

Fig. 8. Comparison of Discoaster variabilis abundance from Sites 662 and

and the bulk carbonate record from Site 662). Grey bars indicate the positio

to coincide with ‘cold’ intervals in the d18O record at Site 926. A weaker

abundance scales.

associated palaeoproductivity reconstructions and

has, in particular, significantly improved the under-

standing 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 relation-

ships (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

926 with oxygen isotope records (the planktonic record of Site 926

ns of isotopic ‘cold’ stages. Peaks in abundance of D. variabilis tend

signal is observed at Site 662. Both sites are plotted with the same

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.

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.


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 flour-

ishes when there is a moderate depth thermocline/

nutricline and surface water oligotrophy.

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 cone-

shaped liths form some sort of phytoplankton cell

covering. Published studies which include Spheno-

lithus 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).

S. Gibbs et al. / Marine Micropal50

In the present study, the strong downcore varia-

tion 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 publica-

tion, 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-var-

iance between Sphenolithus, F. profunda and Dis-

coaster 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.

Fig. 11. Comparison of Sphenolithus abundance from Sites 662 and 926, sh

carbonate d18O and discoaster records from Site 662. Both sites are plotted

isotopic ‘cold’ stages. Two signals dominate the records: the decline in

superimposed strongly cyclic variation. Again, Sphenolithus abundance c

926 and the d18O record at Site 662.

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 Sphe-

nolithus, 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 at-

tributed 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, preda-

tion and competition), plus species evolutionary

eontology 51 (2004) 39–56

own with the magnetic susceptibility record of Site 926, and the bulk

with the same abundance scales. Grey bars indicate the positions of

abundance associated with the extinction of Sphenolithus, and the

losely follows variation in the magnetic susceptibility record at Site

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, distribu-

tion and abundance variation can be observed

monthly, strongly seasonally, annually and on time-

scales 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 (Cortes

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 parame-

ters. In the present study, it is therefore not surpris-

ing that there are few strong linear relationships

between taxa, and also between taxa and the abiotic

environment. However, overall more of the variabil-

ity 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 assemb-

lages 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, Flori-

sphaera 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. There-

fore, these obliquity-driven nannofossil records sug-


gest an effect in the mid-Pliocene of high latitude

ice volume changes via the Southern Hemisphere-

sourced Benguela current. However, the lead ob-

served in the nannofossil abundances relative to the

d18O record, in addition to the greater relative

component of precession in the nannofossil abun-

dances, 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 abun-

dance 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 addi-

tion to the lead of the discoaster and Florisphaera

records relative to the planktonic d18O record, sug-

gests there exists significant low-latitude, local inso-

lation-driven climatic control on variability.

Nannoplankton assemblages are perhaps sensitive

to the small environmental changes directly or indi-

rectly resulting from the influence of Amazon-

sourced 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 ter-

rigenous supply and productivity suggests the fol-

lowing 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 re-

sponse by the two systems to surface current dy-

namics 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 vari-

ability 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 previ-

ously undocumented co-varying abundance records

of Discoaster, Sphenolithus, and Florisphaera dem-

onstrate strongly environmentally controlled abun-

dance patterns displaying a distinct orbitally forced

component and robust correlation with abiotic

records. Strong relationships with productivity sig-

nals confirm that productivity exerts a strong control

on discoaster abundances at low latitudes. All three

taxa are sensitive to changing oceanographic con-

ditions within both low latitude upwelling and non-

upwelling 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 envi-

ronmental 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 indica-

tors, particularly in monitoring low-latitude climatic

variability as F. profunda has in the Quaternary. This

would address the present shortage of well-under-

stood Neogene biotic proxies.

Acknowledgements

SG wishes to thank NERC, Cambridge Univer-

sity, Gonville and Caius College, and the CODENET

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 AmR. minuta Roth, 1970/R. sp. < 3 Am (small retic.)

R. pseudoumbilicus (Gartner, 1967b) Gartner, 1969c.>7 AmR. sp. 5–7 Am, noted by Backman and Shackleton (1983)

Rhabdosphaera Haeckel, 1894. Species were not differentiated

Appendix A (continued)


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.
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)

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 Muller, 1972

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 Muller, 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)

3.418 181.93 3.47 107.79 3.47 107.68

3.546 185.25 3.561 110.29 3.561 110.57

3.637 188.13 3.674 114 3.623 112.3

3.756 192.35 3.715 115.4 3.674 114.04

3.831 194.35 3.808 118.1 3.715 115.46

3.874 195.35 3.851 119.6 3.808 118.18

3.923 196.85 3.965 123.29 3.851 119.65

3.947 197.75 3.999 124.19 3.923 121.79

3.957 198.15 3.965 123.29

3.999 124


Appendix C. Average nannofossil % abundances

ODP Site 926 ODP Site 662

Species Average % abundance Species Average % abundance

3.723–3.866 Ma 3.663–3.900 Ma

Florisphaera profunda 43.0 (total Reticulofenestra) 69.2

(total Reticulofenestra) 36.2 R. minuta 35.1

R. minuta 12.8 R. haqii 23.8

Pseudoemiliania sp. 10.8 Florisphaera profunda 10.8

Gephyrocapsa sp. (small) 7.5 Pseudoemiliania sp. 9.7

(total Umbilicosphaera) 5.4 Gephyrocapsa sp. (small) 4.9

R. haqii 5.0 (total Umbilicosphaera ) 4.1

Sphenolithus abies 4.8 Sphenolithus abies 3.0

(total Discoaster) 2.9 (total Calcidiscus) 2.8

U. sibogae var. foliosa 2.8 (total Helicosphaera) 1.8

Rhabdosphaera clavigera 2.6 H. carteri 1.6

(total Helicosphaera) 1.8 C. tropicus 1.5

H. carteri 1.7 U. rotula 1.4

U. rotula 1.2 (total Discoaster) 1.3

U. jafari 1.0 C. leptoporus 1.2

(total Calcidiscus) 0.9 U. jafari 1.0

Syracosphaera pulchra 0.7 U. sibogae var. foliosa 1.0

Thoracosphaera spp. 0.7 Rhabdosphaera clavigera 0.7

D. pentaradiatus 0.6 Thoracosphaera spp. 0.6

C. tropicus 0.5 U. sibogae var. sibogae 0.5

D. brouweri 0.4 R. sp. (5–7 Am) 0.5

C. leptoporus 0.4 Syracosphaera pulchra 0.3

U. sibogae var. sibogae 0.4 D. brouweri 0.3

Oolithotus sp. 0.3 D. pentaradiatus 0.2

Coronosphaera sp. 0.3 R. pseudoumbilicus 0.2

D. surculus 0.2 Coronosphaera sp. 0.2

D. asymmetricus 0.2 D. asymmetricus 0.2

(total Pontosphaera) 0.2 C. macintyrei 0.1

Calciosolenia murrayi 0.2 G. sp. (large>3.5 Am) 0.1

P. discopora 0.1 H. carteri var. wallichii 0.1

H. carteri var. wallichii 0.1 Oolithotus sp. 0.08

D. variabilis 0.1 D. surculus 0.08

Tetralithoides sp. 0.09 H. selli 0.08

(total Scyphosphaera) 0.06 (total Pontosphaera) 0.06

R. pseudoumbilicus 0.03 Hayaster perplexus 0.05

R. sp. (5–7 Am) 0.03 P. discopora 0.03

D. tamalis 0.02 (total Scyphosphaera) 0.03

P. multipora 0.02 P. multipora 0.03

Scyphosphaera identifiable (incl. S. lagena,

S. globulata, S. pulcherrima, S. apsteinii)

0.02 D. variabilis 0.02

D. tamalis 0.02

Discosphaera tubifera 0.09 Scyphosphaera identifiable

(incl. S. lagena, S. globulata,

S. pulcherrima, S. apsteinii)

0.01

D. brouweri var. triradiatus 0.01

Ceratolithus indet 0.01 D. brouweri var. triradiatus < 0.01

P. japonica 0.01 Calciosolenia murrayi < 0.01

C. macintyrei 0.01 C. pelagicus < 0.01

D. quadramus < 0.01 Ceratolithus indet < 0.01

Hayaster perplexus < 0.01 D. quadramus < 0.01

Tetralithoides sp. < 0.01


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