Expansion and diversification of high-latitude radiolarian assemblages in the late Eocene linked to a cooling event in the Southwest Pacific

Abstract

The Eocene was characterised by "greenhouse" climate conditions that were gradually terminated by a long-term cooling trend through the middle and late Eocene. This long-term trend was determined by several large-scale climate perturbations that culminated in a shift to "ice-house" climates at the Eocene–Oligocene Transition. Geochemical and micropaleontological proxies suggest that tropical-to-subtropical sea-surface temperatures persisted into the late Eocene in the high-latitude Southwest Pacific Ocean. Here, we present radiolarian microfossil assemblage and foraminiferal oxygen and carbon stable isotope data from Deep Sea Drilling Project (DSDP) Sites 277, 280, 281 and 283 from the middle Eocene to early Oligocene (~ 40–33 Ma) to identify oceanographic changes in the Southwest Pacific across this major transition in Earth's climate history. The Middle Eocene Climatic Optimum at ~ 40 Ma is characterised by a negative shift in foraminiferal oxygen isotope values and a radiolarian assemblage consisting of about 5 % of low latitude taxa Amphicraspedum prolixum group and Amphymenium murrayanum. In the early late Eocene at ~ 37 Ma, a positive oxygen isotope shift can be correlated to the Priabonian Oxygen Isotope Maximum (PrOM) event – a short-lived cooling event recognized throughout the Southern Ocean. Radiolarian abundance, diversity, and preservation increase during the middle of this event at Site 277 at the same time as diatoms. The PrOM and latest Eocene radiolarian assemblages are characterised by abundant high-latitude taxa. These high-latitude taxa also increase in abundance during the late Eocene and early Oligocene at DSDP Sites 280, 281 and 283 and are associated with very high diatom abundance. We therefore infer a~northward expansion of high-latitude radiolarian taxa onto the Campbell Plateau towards the end of the late Eocene. In the early Oligocene (~ 33 Ma) there is an overall decrease in radiolarian abundance and diversity at Site 277, and diatoms are absent. These data indicate that, once the Tasman Gateway was fully open in the early Oligocene, a frontal system similar to the present day was established, with nutrient-depleted subantarctic waters bathing the area around DSDP Site 277, resulting in a more oligotrophic siliceous plankton assemblage.

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11, 2977–3018, 2015
Expansion and
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Clim. Past Discuss., 11, 2977–3018, 2015
www.clim-past-discuss.net/11/2977/2015/
doi:10.5194/cpd-11-2977-2015
© Author(s) 2015. CC Attribution 3.0 License.
This discussion paper is/has been under review for the journal Climate of the Past (CP).
Please refer to the corresponding final paper in CP if available.
Expansion and diversification of
high-latitude radiolarian assemblages in
the late Eocene linked to a cooling event
in the Southwest Pacific
K. M. Pascher1,2, C. J. Hollis1, S. M. Bohaty3, G. Cortese1, and R. M. McKay2
1GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand
2Victoria University Wellington, Antarctic Research Centre, P.O. Box 600, Wellington 6140,
New Zealand
3Ocean and Earth Science, National Oceanography Centre Southampton, University of
Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, UK
Received: 16 June 2015 – Accepted: 18 June 2015 – Published: 09 July 2015
Correspondence to: K. M. Pascher (k.pascher@gns.cri.nz)
Published by Copernicus Publications on behalf of the European Geosciences Union.
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Abstract
The Eocene was characterised by “greenhouse” climate conditions that were gradually
terminated by a long-term cooling trend through the middle and late Eocene. This long-
term trend was determined by several large-scale climate perturbations that culminated
in a shift to “ice-house” climates at the Eocene–Oligocene Transition. Geochemical5
and micropaleontological proxies suggest that tropical-to-subtropical sea-surface tem-
peratures persisted into the late Eocene in the high-latitude Southwest Pacific Ocean.
Here, we present radiolarian microfossil assemblage and foraminiferal oxygen and car-
bon stable isotope data from Deep Sea Drilling Project (DSDP) Sites 277, 280, 281
and 283 from the middle Eocene to early Oligocene (∼40–33 Ma) to identify oceano-10
graphic changes in the Southwest Pacific across this major transition in Earth’s climate
history. The Middle Eocene Climatic Optimum at ∼40 Ma is characterised by a negative
shift in foraminiferal oxygen isotope values and a radiolarian assemblage consisting of
about 5 % of low latitude taxa Amphicraspedum prolixum group and Amphymenium
murrayanum. In the early late Eocene at ∼37 Ma, a positive oxygen isotope shift can15
be correlated to the Priabonian Oxygen Isotope Maximum (PrOM) event – a short-lived
cooling event recognized throughout the Southern Ocean. Radiolarian abundance, di-
versity, and preservation increase during the middle of this event at Site 277 at the
same time as diatoms. The PrOM and latest Eocene radiolarian assemblages are
characterised by abundant high-latitude taxa. These high-latitude taxa also increase20
in abundance during the late Eocene and early Oligocene at DSDP Sites 280, 281 and
283 and are associated with very high diatom abundance. We therefore infer a north-
ward expansion of high-latitude radiolarian taxa onto the Campbell Plateau towards the
end of the late Eocene. In the early Oligocene (∼33 Ma) there is an overall decrease in
radiolarian abundance and diversity at Site 277, and diatoms are absent. These data25
indicate that, once the Tasman Gateway was fully open in the early Oligocene, a frontal
system similar to the present day was established, with nutrient-depleted subantarc-
2978

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tic waters bathing the area around DSDP Site 277, resulting in a more oligotrophic
siliceous plankton assemblage.
1 Introduction
The climate history of the early Paleogene has been established by geochemical prox-
ies for temperature, loosely linked to paleontological data. The primary proxy record,5
stable oxygen isotope (δ18O) values of benthic foraminifera, shows a trend from an
early Cenozoic greenhouse climate to an icehouse climate with the major shift in ben-
thic δ18O values of ∼+1.5 ‰ in the earliest Oligocene (∼34Ma) (Shackleton and Ken-
nett, 1975; Diester-Haass et al., 1996; Zachos et al., 2001). After a prolonged period of
maximum warmth during the Early Eocene Climatic Optimum (EECO) centred around10
53–51 Ma, long-term cooling was interrupted by the Middle Eocene Climatic Optimum
(MECO), a ∼500 kyr period of warmth peaking ∼40 Ma that has been linked to an in-
crease in atmospheric pCO2(Bohaty and Zachos, 2003; Bohaty et al., 2009; Bijl et al.,
2010). Organic biomarker-based climate proxies (Liu et al., 2009; Bijl et al., 2010) sug-
gest the Southwest Pacific sea surface temperatures were tropical during the MECO15
(28 ◦C) and continued to be warm throughout the late Eocene (24–26 ◦C), cooling only
slightly across the Eocene–Oligocene transition (EOT, ∼22 ◦C). Following the MECO
event, benthic δ18O values increased to their maximum Eocene values of ∼2.3 ‰ at
about 37.3 Ma during a short-lived cooling episode in the early late Eocene, designated
as the Priabonian Oxygen Isotope Maximum (PrOM) event (Scher et al., 2014). Further20
climate oscillations are reported for the late Eocene (Vonhof et al., 2000; Pälike et al.,
2001; Bohaty and Zachos, 2003; Villa et al., 2008; Westerhold et al., 2014) prior to the
expansion of Antarctic ice that defines the EOT.
The generally warm conditions of the Eocene are consistent with fossil-based re-
constructions of Southern Ocean circulation developed from high-latitude drill cores25
(Kennett, 1977; Nelson and Cooke, 2001; Kennett and Exon, 2004), in which sub-
tropical waters are interpreted to extend close to the Antarctic margin. However, both
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geochemical proxy data and these paleoecological reconstructions are at odds with the
latest generation of ocean circulation and climate modelling simulations (Hollis et al.,
2012; Lunt et al., 2012). Even under hyper-greenhouse conditions, the models produce
a cyclonic gyre that blocks subtropical waters from penetrating southward beyond 45◦S
(Huber and Sloan, 2001; Huber et al., 2004). High-latitude warmth also conflicts with5
evidence for the initiation of Antarctic glaciation in the latest Eocene from both fossil
and geochemical proxies (Lazarus and Caulet, 1993; Scher et al., 2014; Barron et al.,
2015).
Paleobiogeographic changes in marine biota may help to delineate general climate
trends and events. Identifying the initial timing and development of a high-latitude fauna10
in the Southern Ocean helps to constrain the development of the Southern Ocean
frontal systems and, in turn, heat transfer between low and high latitudes. The timing of
the establishment of a distinct Southern Ocean surface-water mass is inferred to have
occurred within the middle-to-late Eocene interval, triggered by the opening of the Tas-
man Gateway or changes in carbon cycling (Stickley et al., 2004; Lazarus et al., 2008;15
Bijl et al., 2013), or abruptly at the E-O transition, associated development of a proto-
Antarctic Circumpolar Current (ACC) and implicated as the main causal mechanism for
Antarctic glaciation (Kennett, 1978; Nelson and Cooke, 2001; Houben et al., 2013). Im-
proved understanding of the timing of major changes in the early Cenozoic evolution of
the Southern Ocean will help to resolve the relative importance and inter-relationships20
between tectonism, biological evolution and long-term trends in atmospheric CO2con-
centration.
In this paper, we document variation in radiolarian assemblages and foraminiferal
oxygen and carbon stable isotopes from the middle Eocene-to-early Oligocene interval
(∼40 to 33 Ma) at DSDP Site 277 and relate these variations to radiolarian assem-25
blage changes at DSDP Sites 280, 281, 283 and to a previously published study of
Eocene radiolarian assemblages from ODP Site 1172 (Suzuki et al., 2009). DSDP Site
277 provides a unique record of pelagic sedimentation in the Southwest Pacific from
the late Paleocene to Oligocene times and the first Eocene benthic δ18O record was
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generated from this site (Shackleton and Kennett, 1975). We use these data to test if
(i) a distinct Southern Ocean fauna was established prior to the major shift in oxygen
isotopes in the earliest Oligocene and (ii) if tropical-subtropical conditions persisted in
the Southwest Pacific until at least the late Eocene. Our results will help to identify
the timing and nature of the development of a distinctive Southern Ocean fauna and5
discuss implications for the oceanographic history of the SW Pacific from the middle
Eocene to early Oligocene.
2 Study sites
Deep Sea Drilling Project (DSDP) sites 277, 280, 281 and 283 were drilled during
DSDP Leg 29 (Kennett et al., 1975) (Fig. 1). The main focus of our study is Site10
277, which is located on the western margin of the Campbell Plateau (52◦13.430S;
166◦11.480E) at a water depth of 1214 m. Forty-six cores were drilled with a maximum
penetration of 472.5 m below sea floor (mbsf), but with total length of 434.5m of cored
section and only 59.6 % recovery. Poor recovery was due to 9.5 m coring runs being
conducted every 19 m (i.e. alternate drilling and coring at 9.5 intervals) between 301.515
and 368.0 mbsf. Below 10mbsf, a Paleogene sequence spanning from the middle Pa-
leocene to middle Oligocene was recovered (Kennett et al., 1975). We studied Cores
277-35R (349.2 mbsf) to 277-15R (134.5 mbsf) that cover a middle Eocene-to-lower
Oligocene interval. The sediment at Site 277 (paleolatitude ∼60◦S) throughout the
succession is highly calcareous indicating a depositional environment well above the20
lysocline, with a paleodepth estimated at around 1500 m (Kennett et al., 1975; Hollis
et al., 1997).
Three additional sites were included in our study in order to acquire a regional pic-
ture of radiolarian assemblage change and biogeography during the middle Eocene to
early Oligocene. DSDP Site 280 comprises two holes (48◦57.440S; 147◦14.080E) and25
is located ∼100 km south of the South Tasman Rise and was drilled at a water depth
of 4176 m. We collected radiolarian assemblage data from Hole 280A, which consists
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of a 201 m cored section that includes a 97.2 m middle Eocene-to-middle Oligocene
interval. The studied interval spans Core 7R (123.4 mbsf) to Core 5R (92.54 mbsf).
DSDP Site 281 on the South Tasman Rise (47◦59.840S; 147◦45.850E), drilled at a wa-
ter depth of 1591 m, encompasses two holes (281 and 281A). We examined Hole
281 which was cored to 169 mbsf and recovered a 105.6m (62.5 % recovery) late5
Eocene-to-Pleistocene section. The studied interval covers Core 16R (149 mbsf) to
Core 14R (122.5 mbsf). DSDP Site 283 lies in the Central Tasman Sea (43◦54.60S;
154◦16.960E) in a water depth of 4729 m and also comprises two holes (283 and 283A).
We examined Hole 283 which was drilled to 156 mbsf (39 % recovery) and recovered
a Paleocene-to-Pleistocene section that contains a late Eocene-to-(?)Miocene hiatus.10
Core 8R (192.25 mbsf) to Core 5R (87.75 mbsf) were studied from this site.
3 Material and methods
This study is based on 28 sediment samples from DSDP Site 277 from ∼350 to
135 mbsf spanning a middle Eocene-to-lower Oligocene interval (17 reported by Hollis
et al. (1997) and 11 new samples), 6 samples from DSDP Site 283 (new, all from the15
DSDP/ODP Micropaleontology Reference Centre (MRC)), 7 from Site 281 (3 from the
DSDP/ODP MRC, 4 new) and 4 from Site 280 (new). Due to incomplete core recovery
in all study sections, the sampling resolution of our study is variable (∼0.5 to ∼30 m
sample spacing; Supplement). To obtain a consistent taxonomic identification across
all sites, all samples previously reported from DSDP sites 277, 280, 281 and 283 were20
re-examined and re-counted as part of this study.
For strewn slide preparation, 1–10g of sample material was broken into ∼5 mm-
diameter chips and leached in 10 % HCl to dissolve carbonate until the reaction ceased.
Samples were then washed through a 63-µm sieve and the >63 µm residue was
cleaned by gently heating in a 1 : 1 solution of 10 % hydrogen peroxide and sodium25
hexametaphosphate ((NaPO3)6). The residue was washed though a 63µm sieve and
dried. Dependent on the volume of the processed residue and the abundance of ra-
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diolarians, 1–5 strewn slides were prepared for each sample. If the radiolarians were
sparse, specimens were individually picked from the dried residue under a stereo mi-
croscope. For strewn slides, a known portion of dried residue was evenly distributed on
a pre-glued coverslip, which was inverted and placed gently on a glass slide with a thin
coating of Canada Balsam. The slide was placed on a hot plate until the balsam was5
fixed.
Strewn slides were examined using a Zeiss transmitted light microscope fitted with
a Zeiss AxioCam ERc5s digital camera. The Supplement include taxonomic notes for
all radiolarian species recorded in this study, plates of selected species, and radiolar-
ian distribution charts and sample information for DSDP sites 277, 280, 281 and 283.10
Radiolarian census data were derived along vertical slide traverses under transmitted
light following the method of Hollis (2006). For samples with sparse radiolarians (<300
specimens per slide), all radiolarians on the prepared slide(s) were counted. For richer
samples, all specimens were counted until a total number of about 300 specimens was
achieved. The proportion of the slide examined to this point was determined and the15
abundance of common taxa (>15 observed specimens) estimated for the rest of the
slide. The remaining portion was then examined and rare taxa (<15 specimen ob-
served in initial count) recorded. All intact tests were assigned to a counting group that
range from undifferentiated order (e.g. Nassellaria undet.) and family (e.g. Actinommi-
dae undet.) to species and subspecies. This approach allows for an accurate estimate20
of the abundance of individual species, but does result in overall diversity being under-
estimated.
Radiolarian abundance was calculated using the following equation:
XR×XS×1
XP/ASed (1)
With XRbeing the total number of radiolarians per slide, XSthe number of slides made25
of a known portion XPof the dried material, ASed is the initial amount of dried sediment.
Additional data derived for each sample assemblages includes taxic richness, the
Fisher αDiversity index and the Simpson index of Evenness. The latter two indices
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were calculated using the PAST software (Hammer et al., 2001). The Fisher αindex is
a general guide to diversity, calculated from the number of taxa and the total number of
individuals. The Simpson index of Evenness determines the degree to which assem-
blages are dominated by individual taxa and ranges from 0 to 1. The diatom/radiolarian
(D/R) ratio was calculated using the counts of diatoms and radiolarians of one exam-5
ined slide. In case of very rare diatoms, all specimens were counted on a slide, oth-
erwise several transverses were counted for diatoms and the total number estimated
for the whole slide. Although this method is not an accurate measure of diatom abun-
dance as most pelagic diatoms are smaller than the 63 µm screen used in this study, it
serves to identify the order of magnitude in changes in diatom abundance that allows10
us to identify significant diatom event horizons. We also determined diversity, even-
ness and biogeographic affinities for the Eocene radiolarian assemblages described
from ODP Site 1172 (Suzuki et al., 2009) using unpublished distribution data provided
by N. Suzuki (personal communication, 2013, Tohoku University, Japan).
The biogeographic affinities of the radiolarian species, subspecies and informally de-15
fined morphotypes encountered in our study were assigned using information from pre-
vious paleobiogeographic studies (Lazarus and Caulet, 1993), distributions reported in
taxonomic studies (Petrushevskaya, 1975; Sanfilippo and Caulet, 1998) and our own
assessment based on published records of the recorded taxa or closely related taxa
(e.g. Takemura and Ling, 1997; Hollis, 2002; Funakawa and Nishi, 2005, 2008; Fu-20
nakawa et al., 2006; Kamikuri et al., 2013) (Table 1). We quantified trends in biogeo-
graphic affinity to determine how the relative influences of high- and low-latitude water
masses varied through the middle Eocene to early Oligocene.
At the University of California Santa Cruz (UCSC) and the University of Southamp-
ton (UoS), stable oxygen (δ18O) and carbon (δ13C) isotope ratios were determined25
for bulk carbonate, Cibicidoides spp., Subbotina spp. (Core 277-34R (332.62mbsf) to
18R (159.88 mbsf) and Globigerinatheka index (Core 277-34R (332.62 mbsf) up to its
last occurrence in Core 277-21R (188.58 mbsf)). In total, set of 157 samples span-
ning the middle Eocene-to-lower Oligocene interval of DSDP Hole 277 was measured.
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Stable isotope analyses at UCSC were performed on a VG Prism dual-inlet mass spec-
trometer coupled to carousel preparation device with common acid bath maintained at
90 ◦C. Analyses at the UoS were performed on a Europa GEO 20–20 dual-inlet mass
spectrometer with CAPS preparation oven maintained at 70◦C. All values are reported
relative to the Vienna Pee Dee Belemnite (VPDB) standard. In both labs, analytical5
precision based on replicate analyses of in-house marble standards and NBS-19 av-
eraged ∼0.05 ‰ (1σ) for δ13C and ∼0.08 ‰ (1σ) for δ18O. All planktic foraminifera in
this record appeared to have a “frosty” preservation.
4 Results
4.1 Site 277 biostratigraphy and stable isotope stratigraphy10
Broad age control for DSDP Site 277 is based on the biostratigraphic review of Hollis
et al. (1997) who correlated the succession to radiolarian Zones RP6 to RP15. In this
study we confirm the location of the base of Zone RP14 (Lowest Occurrence (LO)
of Eucyrtidium spinosum) at 264.5–254.5 mbsf, the base of RP15 (LO of Eucyrtidium
antiquum) at 197.8–186.5 mbsf, and the base of upper Zone RP15 at 143.9–134.5 mbsf15
(lowest common occurrence (LCO) of Axoprunum? irregularis) (Fig. 2). We revise the
base of Zone RP12 to 371.2–349.2 mbsf (LO of Lophocyrtis longiventer) and the base
of RP13 to 313.5–312.7 mbsf (LOs of Eusyringium fistuligerum and Zealithapium mitra)
(Fig. 2). The Eocene–Oligocene boundary is poorly defined by biostratigraphy at DSDP
Site 277. The base of the local Whaingaroan Stage (latest Eocene, 34.6 Ma, Raine20
et al., 2015) is identified by the Highest Occurrence (HO) of Globigerinatheka index.
This event was identified at 189.6 mbsf by Jenkins (1975) but we have determined that
the event occurs slightly higher at 188.58mbsf.
Further refinement of the age control for Site 277 is possible through correlation of
the stable isotope records to those from other Southern Ocean sites (Fig. 2). Although25
the gaps in the Site 277 isotope record preclude detailed correlation, the broad trends
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and major events such as the MECO (∼40Ma) and PrOM event (∼37.3 Ma) can be
identified in the benthic δ18O and δ13 C isotope profiles and compared to the middle
Eocene-to-early Oligocene benthic isotope stratigraphy from ODP Site 689 (Diester-
Haass and Zahn, 1996) (Fig. 2). The EOT is expressed as a large (∼1 ‰) positive shift
in benthic oxygen and carbon isotopes between Cores 277-20R and -19R (Shackleton5
and Kennett, 1975; Keigwin, 1980), which is slightly lower than the full magnitude of the
benthic δ18O shift seen at other Southern Ocean sites on the Kerguelen Plateau and
Maud Rise (Diester-Haass and Zahn, 1996; Zachos et al., 1996; Bohaty et al., 2012).
Foraminiferal δ18O values show a normal planktic–benthic gradient with more posi-
tive values in the benthic foraminifers compared to bulk and planktic foraminifera with10
some crossover in the latter two (Fig. 3). Foraminiferal δ13C values also show a typical
positive benthic–planktic gradient. Therefore, we interpret relatively robust stable iso-
tope signals representative of deep (intermediate), upper (thermocline) and uppermost
(mixed/surface) waters, although it is likely that the δ18O gradients are attenuated by
diagenetic effects on planktic foraminifera (Sexton et al., 2006) as they show a “frosty”15
preservation.
Several short-lived climatic events are identified in the benthic stable isotope records
at Site 277 (Fig. 3). The body of the MECO was not recovered (due to a 16m sampling
gap between the top of Core 277-33R and the base of Core 277-32R), but its onset
and recovery is well constrained by a 0.5‰ negative excursion in benthic δ18O values20
at ∼313 mbsf (between Samples 277-33R-2, 106–108 cm and -33R-1, 129–130.5 cm)
and a ∼0.4 ‰ positive excursion in δ18O values at ∼296 mbsf (between samples 32R-
3, 107–109 cm and 32R-3, 77–79 cm), indicating that the MECO spans ∼17 m. The
MECO is more strongly expressed in the benthic δ18O than in the planktic record but
this may relate to the poor recovery of the body of the event at this site or diagenetic25
impacts on planktic δ18O values (Pearson et al., 2000; Sexton et al., 2006). In agree-
ment with other records (Bohaty and Zachos, 2003; Bohaty et al., 2009), a positive
δ13C excursion is observed at the onset of the MECO in the benthic and bulk carbon-
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ate records, although the δ13C record is also compromised by the missing core of the
event.
The PrOM event (Scher et al., 2014) is well-defined in the δ18O record from DSDP
Site 277 but also spans two significant recovery gaps between the base of Cores 277-
26R, 25R and 24R (∼244.5 to 225.5 mbsf) (Fig. 3). The ∼0.4 ‰ positive shift in δ18O5
that marks the onset of the PrOM, spans upper Core 277-26R and lower Core 277-
25R (∼240–230 mbsf), and is followed by an interval of relatively low δ18O values in
upper Core 277-25R, prior to reaching maximum values in uppermost Core 277-25R
(∼226 m). A gradual decrease in δ18O occurs through Core 277-24R. We define the
PrOM at DSDP Site 277 as the interval within these three cores in which benthic δ18O10
exceeds 1.25 ‰, with the exception of the interval noted above in upper Core 277-25R.
These benthic δ18O values are lower than those reported by Scher et al. (2014), but it
is likely that peak δ18O values are not captured at Site 277. Consequently the PrOM
is placed between 240.62 and 219.57 mbsf (spanning a ∼21 m section). The planktic
δ18O record is similar to the benthic, but lacks the maximum excursion in uppermost15
Core 277-25R. At the onset of the event, short-lived negative δ13C excursions are
evident in the benthic, bulk and planktic records. However, a longer-term positive trend
for planktic and benthic δ13C values becomes apparent simultaneously to the benthic
δ18O maximum.
Directly above the PrOM event, δ18O values decrease by ∼0.5 ‰ in upper Core 277-20
24R and -23R (217.37 to 207.41 mbsf), evident in benthic and planktic foraminifera as
well as bulk carbonate. This interval can be correlated to the late Eocene warming inter-
val interpreted at ODP Sites 689 (Maud Rise), 738, 744, and 748 (Kerguelen Plateau)
(Diester-Haass and Zahn, 1996; Bohaty and Zachos, 2003; Villa et al., 2008, 2014).
The large positive shift in δ18O defines the E-O transition at Site 277 between the25
base of Core 277-20R and Core 277-19R, with the most positive values in benthic and
planktic δ18O and δ13 C occurring in Core 277-19R (171.28 to 169.65 mbsf), within the
earliest Oligocene.
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4.2 Radiolarian assemblages at DSDP Site 277
In total, 16 families, 56 genera and 98 radiolarian species were identified at DSDP Site
277. Radiolarian abundance is generally low (10–100 specimens g−1) and preserva-
tion is moderate throughout the middle Eocene-to-early late Eocene interval (349.2 to
227.2 mbsf) (Fig. 4). In the latest Eocene and early Oligocene radiolarians are abun-5
dant to very abundant (>1500 specimens g−1) and well preserved. Diversity is strongly
correlated to abundance, which is lower in the middle and early late Eocene and high
thereafter (Fig. 4). Simpson Evenness is strongly correlated to diversity but exhibits
greater troughs where samples are sparse (Fig. 4). Spumellarians are dominant in most
samples ranging between ∼45 and 96 % (∼70 % average). The main families are the10
Actinommiidae, Litheliidae, Artostrobiidae, Spongodiscidae, Lophocyrtiidae and Lych-
nocaniidae (Supplement Table Site 277).
Three samples from the middle Eocene (313.5, 312.7, 296 mbsf; Cores 277-32R and
-33R) that lie within the onset and recovery of the MECO at Site 277, show improved
preservation, a peak in diversity, and mark the first significant occurrence of diatoms15
(Fig. 4). The low-latitude species Amphymenium murrayanum and Amphycraspedum
prolixum gr. have short-lived occurrences in this interval, with only A. prolixum gr. also
very rare in the latest Eocene. Several species are restricted to the MECO: Artobotrys
titanothericeraos, Sethocyrtis chrysalis,Eusyringium fistuligerum and Stichopilium cf.
bicorne.Lophocyrtis jacchia hapsis, which is a high-latitude variant of L. jacchia jacchia20
(Sanfilippo and Caulet, 1998) and endemic to the Southern Ocean, is also common
during the MECO, but is absent from the remaining middle Eocene and very rare in the
late Eocene. Furthermore, the LOs of several species are recorded (albeit very rare)
at this site during the MECO interval (Axoprunum pierinae,Zealithapium mitra, Pe-
riphaena spp., Larcopyle hayesi, L. polyacantha, Zygocircus buetschli, Siphocampe?25
amygdala, Eucyrtidium ventriosum, Lychnocanium amphitrite,Clinorhabdus ananto-
mus, Lophocyrtis keraspera, Lophocyrtis dumitricai, Cryptocarpium ornatum and Lam-
procyclas particollis) (Fig. 2 and Supplement Table Site 277).
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A major change in siliceous assemblages occurs within the PrOM interval (∼
226 mbsf; Core 25R), coincident with maximum values in benthic δ18O (Fig. 4). A pro-
nounced increase in radiolarian abundance (from <50 to ∼4000 radiolarians g−1),
preservation and diversity occurs at 226.10 mbsf (Sample 277-25R-1, 60 cm). The fol-
lowing taxa have their LO within the PrOM at Site 277: Lithelius foremanae,Cera-5
tocyrtis spp., Lithomelissa ehrenbergi,L.gelasinus,L.sphaerocephalis,Siphocampe
nodosaria,Artostrobus annulatus,Artostrobus cf. pretabulatus,Clathrocyclas universa,
Dictyophimus? aff.archipilium,Lychnocanium waiareka,Aphetocyrtis rossi and Theo-
cyrtis tuberosa (Fig. 2 and Supplement Table Site 277). Diatoms become abundant at
the same level as the increase in radiolarian abundance and remain abundant through10
the latest Eocene, decreasing in the Oligocene. The most abundant radiolarian fam-
ilies in the PrOM and latest Eocene are the Actinomiidae (∼11–36 %), Litheliidae
(∼16–28 %), Spongodiscidae (∼5–13 %), Lophocyrtiidae (∼3–15 %), Lychnocaniidae
(1–11 %) and Plagiacanthidae (1–6 %). Theocyrtis tuberosa has a very rare occur-
rence from the late Eocene to early Oligocene (∼226–143.9 mbsf; Core 25R to 16R).15
This species is also known to have had isolated occurrences in the southern Atlantic
and southern Indian oceans in the late Eocene (Takemura, 1992; Takemura and Ling,
1997) and is common in latest Eocene to early late Oligocene assemblages from low
to middle latitudes of all ocean basins (Sanfilippo et al., 1985). As none of our samples
lie within the late Eocene warming interval (Fig. 3), we cannot assess how radiolarian20
assemblages responded to this warming. However, closer to New Zealand, the latest
Eocene Runangan stage is associated with incursions of warm-water taxa, including
larger benthic foraminifera and the short-lived occurrence of the low-latitude genus
Hantkenina (Hornibrook et al., 1989).
A significant decline in radiolarian abundance and diversity is observed through the25
early Oligocene (186.5 to 134.5 mbsf; Cores 20R to 15R) (Fig. 4). The fauna is dom-
inated by spumellarians that increase from ∼73 to ∼97 %, with Litheliidae and Acti-
nommidae being the most abundant families (Supplement Table Site 277).
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4.3 Radiolarian assemblages at other SW Pacific sites
To establish the significance and nature of radiolarian faunal turnover associated with
the PrOM event regionally, we investigated the late Eocene to early Oligocene intervals
of DSDP Sites 280, 281 and 283.
4.3.1 DSDP Site 2805
Four samples were investigated at DSDP Site 280 from Cores 7R, 6R and 5R (123.4
to 92.54 mbsf). In previous work, the E-O boundary in Hole 280 was placed at the
base of Core 280-6R (110.5 mbsf) (Crouch and Hollis, 1996). However, due to the
presence of Eucyrtidium antiquum (Caulet, 1991) and Larcopyle frakesi (Chen, 1975),
both of which have LOs in the early Oligocene, we place the studied interval (123.4–10
92.54 mbsf) in early Oligocene Zone RP15 (Fig. 5 and Supplement Table Site 280).
This is in agreement to O’Connor (2000), who found late Eocene assemblages were re-
stricted to Cores 280-10R to 8R (205.5 to 139 mbsf). The absence of the zonal marker
Axoprunum? irregularis indicates correlation with lower RP15. Eucyrtidium spinosum,
which according to Funakawa and Nishi (2005) has its HO in the early Oligocene, is15
absent in the Site 280 study interval. However, the HO of this species is recorded
within the late Eocene interval at Site 277, suggesting a diachronous HO between the
Southwest Pacific and the South Atlantic (Supplement Table Site 277).
In total, 15 families, 35 genera and 50 radiolarian species were identified at Site
280. Radiolarians are abundant (1000–2500 specimensg−1) and well preserved in all20
samples. Diatoms are also very abundant (D/R ratio ∼10) (Fig. 5). Diversity and Even-
ness is stable and high in all samples. Spumellarians are slightly more abundant than
nassellarians (52–66 % of the assemblage). The most abundant families are Litheli-
idae (20–37 %), Plagiacanthidae (14–22 %), Actinommidae (4–12 %), Spongodiscidae
(5–9 %), Eucyrtiidae (4–8 %) and Lophocyrtiidae (3–8 %) (Supplement Table Site 280).25
Compared to DSDP Site 277, this site has a higher diatom abundance and better
overall preservation, which might explain the higher diversity. More species of the gen-
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era Lithomelissa (7) and Larcopyle (5) are present, as well as a higher abundance of
Lophocyrtiidae. Lychnocaniids are very rare at this site (<1 %) and the genus Lychno-
canium is absent (Supplement Table Site 280).
4.3.2 DSDP Site 281
Seven samples were investigated from DSDP Site 281 in the interval between149 and5
122.5 mbsf (Cores 16R to 14R) (Fig. 5). Results from three of these samples were pre-
viously reported in Crouch and Hollis (1996) but have been re-examined for this study.
Due to the presence of Eucyrtidium spinosum and Eucyrtidium nishimurae, the latter
with a HO in the late Eocene at ∼36.9–36.7 Ma (Funakawa and Nishi, 2005), we corre-
late the Site 281 study interval with lower Zone RP14 (∼Kaiatan local stage). A hiatus10
spanning the latest Eocene and Oligocene is inferred from the presence of abundant
glauconite in the upper part of Core 281-14R as well as from common Cyrtocapsella
tetrapera in Core 281-13R, which indicates a Miocene age (Crouch and Hollis, 1996).
In total, 14 families, 34 genera and 46 species were identified at Site 281. Radiolar-
ians are abundant (2000–4000 specimens g−1) and well preserved. Diversity is lower15
than at Site 280A, but Evenness is still very high and similar to the other sites (Fig. 5).
The D/R ratio is very high and comparable to Site 280, except in the upper two samples
in Core 281-14R (125.5–122.5 mbsf). The radiolarian assemblages are dominated by
spumellarians (55–93 %), with Litheliidae (17–42 %), Spongodiscidae (12–30 %) and
Actinommidae (10–0 %) the most abundant families. The most common nassellarians20
belong to the Plagiacanthidae (1–15 %), Lophocyrtiidae (3–7 %) and Eucyrtiidae (1–
7 %) (Supplement Table Site 281). Although Sites 280 and 281 were relatively close to
each other (Fig. 1), the radiolarian assemblages are distinctly different, indicating differ-
ent oceanographic conditions. Crouch and Hollis (1996) concluded that Site 281 was
shallower and closer to terrigenous influx than Site 280. The depositional environment25
of Site 280 is interpreted as more oceanic. The greater abundance of Spongodiscidae
at Site 281 supports a shallower oceanic setting for this locality (Casey, 1993). Com-
pared to the early late Eocene assemblage of Site 277, where radiolarian abundance
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and diversity is very low, with several samples containing less than ∼100 specimens,
Site 281 contains more Spongidiscidae (∼20 %), Plagiacanthiidae (∼7 %) and Litheli-
idae (∼20 %), whereas the genus Lychnocanium is absent at Site 281.
4.3.3 DSDP Site 283
Six samples were examined from Site 283 between 192.25 and 87.75mbsf (Cores5
8R to 5R) (Fig. 5). The lowermost sample at 192.25 mbsf is correlated to RP13 due
to the absence of Eucyrtidium spinosum. The uppermost five samples are of early
late Eocene age based on the presence of E. spinosum and nannofossil age control
(Edwards and Perch-Nielsen, 1975). The age of the Site 281 and 283 successions are
poorly defined and the PrOM event cannot be located at these sites. Both sites contain10
Eucyrtidium nishimurae: at Site 283 in all samples, at Site 281 its HO is in 125.5–
122.5 mbsf. According to Funakawa and Nishi (2005) its HO is in C17n1n (∼36.7 Ma,
Gradstein et al., 2012). E. nishimurae is absent at Site 277. The deposition of siliceous
ooze in the late middle to late Eocene and the absence (or very rare) occurrence of
foraminifera suggests a deep oceanic setting close or below the Calcite Compensation15
Depth (CCD) for Site 283.
A total of 16 families, 50 genera and 81 radiolarian species were recorded at Site
283. Radiolarians are abundant (4700–21 150 radiolariansg−1), with the highest abun-
dance in Cores 283-6R and 5R, well preserved, and diverse (59–77 taxa per sam-
ple, Fisher αIndex of 10–13, Evenness of 0.75–0.89). Diatoms are present in low20
abundance with D/R ratios <1 (Fig. 5). Spumellarians account for 59–87 % of the as-
semblage, with the Litheliidae (23–38 %), Actinommidae (5–19 %) and the Spongodis-
cidae (2–8 %) the most abundant families. The Trissocyclidae (2–11 %), Eucyrtiidae
(2–11 %), Lophocyrtiidae (3–8 %) and Plagiacanthidae (2–8 %) are the most common
nassellarian families (Supplement Table Site 283). Theocyrtis tuberosa is very abun-25
dant in the uppermost sample. The acme of this taxon might be correlated to its rare
occurrence at Site 277 in the late Eocene. Several taxa appear earlier at Site 283 than
at Site 277. These include the following taxa that occur in the late middle Eocene (e.g.
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Axoprunum bispiculum,Amphicentria sp. 1 sensu Suzuki, Ceratocyrtis spp., Lithome-
lissa ehrenbergi,L. cf. haeckeli,L.sphaerocephalis,L. tricornis,Pseudodictyophimus
gracilipes gr., Tripodiscinus clavipes,Siphocampe nodosaria,Spirocyrtis joides,Aspis
sp. A sensu Hollis, Clathrocyclas universa,Eurystomoskevos petrushevskaae,Lych-
nocanium waiareka,Aphetocyrtis gnomabax) or early late Eocene (Spirocyrtis greeni,5
Eurystomoskevos cauleti,Lophocyrtis jacchia hapsis,Lamprocyclas particollis) at Site
283.
4.3.4 ODP Site 1172
Forty samples were considered from ODP Site 1172 spanning a middle Eocene-to-
lower Oligocene interval. Four samples from Hole D, Core 2R (356.875–355.675 mbsf)10
and thirty-six from Hole A, Core 48X to 39X (445.01–354.625 mbsf). The faunal as-
semblages of ODP Site 1172 were described by Suzuki et al. (2009), who did not
correlate them to RP Zones. We identified key radiolarian index species and corre-
lated the interval to RP Zones 10–15. The absolute age of the succession is based
on the age-depth plot of Site 1172 by Stickley et al. (2004). Many taxa used to define15
RP zones at Site 277 are absent at Site 1172 or have diachronous ranges. We place
the base of Zone RP10–12 (LO of Theocampe mongolfieri) at 450.55–445.01 mbsf
(43.14–42.79 Ma). The base of Zone RP13 (LO of Eusyringium fistuligerum) can be lo-
cated at 419.21–417.71 mbsf (40.48–40.35 Ma), however Zealithapium mitra is absent.
Eucyrtidium spinosum, the marker for Zone RP14, has its LO at 373.75–371.21 mbsf20
(38.05–37.2 Ma) and Lithomelissa tricornis and Pseudodictyophimus gracilipes are ab-
sent. Eucyrtidium antiquum has a single LO at 365.21 mbsf (35.15 Ma), but is absent
in the early Oligocene. E. nishimurae is present within the middle and late Eocene.
Diversity and Evenness are very high throughout the succession.
Spumellarians dominate the Site 1172 assemblages throughout the middle Eocene25
to early Oligocene (∼80 %). The Litheliidae are the most abundant family comprising
about 20 % on average in the middle Eocene, 35 % in the late Eocene, and 25 % in the
early Oligocene.
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Eocene sediments at Site 1172 consist of silty claystone with abundant diatoms.
This sequence is overlain by a transitional unit in the latest Eocene consisting of
glauconitic siltstones, which indicate increased bottom-water currents near the E-O
boundary (Kennett and Exon, 2004; Stickley et al., 2004). In the lowermost Oligocene,
a pelagic carbonate sequence consisting of nannofossil chalk appears abruptly (Exon5
et al., 2004). Diatoms are more abundant and of inner neritic nature in the middle
Eocene until ∼408 mbsf (∼39 Ma), where they become more oceanic and may indi-
cate a change to a more outer neritic regime. Above ∼376 mbsf (∼38 Ma) the diatom
assemblage indicates an inner to outer neritic regime (Röhl et al., 2004).
4.4 Trends in biogeographic affinities10
Using the Eocene–Oligocene assemblage data collected at the four Southwest Pacific
study sites, radiolarian taxa were grouped according to their biogeographic affinity:
high-latitude (58), cosmopolitan (39), low-latitude (3) and unknown (31). Within the
high-latitude group, several taxa are bipolar (6), whereas 52 taxa are currently only
known to be endemic to the Southern Ocean (Table 1). Almost all species in the Lithe-15
liidae, Lophocyrtiidae and Plagiacanthidae are high-latitude. The biogeographic affinity
of Lithelius minor gr. is uncertain, as some members may be confined to the high lat-
itudes and others may be cosmopolitan. Because this group is a major component in
some assemblages, we consider it as part of the high-latitude complex but separate
it out in Figs. 6 and 7. For Site 277, we also differentiate key high-latitude elements20
within the three families noted above, namely Larcopyle spp., Lophocyrtis longiventer
and Lithomelissa spp. (Fig. 6).
At Site 277, taxa with high-latitude affinities are present from the middle Eocene
(Fig. 6). The MECO is accompanied by an increase in high-latitude taxa to ∼19 %
(Larcopyle spp., Lithelius minor gr., Lophocyrtis jacchia hapsis), but also the appear-25
ance of low-latitude species Amphicraspedum murrayanum and A. prolixum gr. (5 % of
total assemblage). The abundance of high-latitude taxa further increases at the start
of the late Eocene, with increasing numbers of lophocyrtids, dominated by L. longiven-
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ter (Fig. 6), and the radiolarian diversification during the PrOM event is marked by an
increase Lithomelissa spp. Amphycraspedum prolixum gr. has a trace occurrence in
the latest Eocene. During the early Oligocene, overall diversity declines and especially
the delicate plagiacanthiids and lophocyrtiids decrease. Lithelius minor gr. becomes
dominant until ∼144 mbsf, then this group decreases and high-latitude actinommid5
Axoprunum bispiculum and A. irregularis make up ∼75 % of the assemblage (Fig. 6).
At Sites 1172 and 283, high-latitude taxa are present from the middle Eocene, com-
prising 20–30 % of the assemblage at Site 1172 and ∼40 % at Site 283 (Fig. 7). The
MECO at Site 1172 corresponds to a decline in high-latitude taxa and an increase in
cosmopolitan taxa. In the early late Eocene (∼38–37 Ma), high-latitude taxa increase10
at Site 1172, from ∼30 to ∼50 %. High-latitude taxa at Site 281 range between 20
and 40 % in the early late Eocene. At Site 283 high-latitude taxa are more abundant
ranging between 40 and 55 %. However, this is mainly due to the high abundance of
a single taxon, Lithelius minor gr. Several taxa that are present in the early Oligocene at
Site 280 are absent at Site 277, including Lithomelissa challengerae, Larcopyle frakesi,15
Lithomelissa sakai, and Antarctissa spp. The percentage of high-latitude taxa at Site
280 is between 45 and 55 %, with Lithelius minor gr. of 10–20 %. Amphycraspedum
prolixum gr. has a trace occurrence at ∼103 mbsf at Site 280.
5 Discussion
5.1 Comparison with geochemical temperature proxies20
The radiolarian assemblages documented at Site 277 and 1172 within the MECO in-
terval lack typical tropical taxa such as Thyrsocyrtis spp. (e.g. Kamikuri et al., 2013),
and the low-latitude taxa Amphycraspedum murrayanum and A.prolixum gr. account
for only 5 % of the total assemblage at Site 277 and are absent at Site 1172. The
persistence of high-latitude taxa and the variety of cosmopolitan species at both sites25
suggests a warm-temperate climate of ∼15–20 ◦C, in contrast to geochemical proxies
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suggesting >25 ◦C for the MECO at Site 1172 (Bijl et al., 2010) and ∼27 ◦C for the late
Eocene at Site 277 (Liu et al., 2009).
5.2 Nature of the Antarctic assemblage
High-latitude taxa existed from at least the middle Eocene at sites 277, 283 and 1172.
Many taxa that are present from the earliest late Eocene (∼38 Ma) at Sites 281 and 2835
appear later at Site 277 (∼37–36 Ma), during the PrOM event. This appearance coin-
cides with an increase in radiolarian abundance, diversity and preservation. A compar-
ison of all high-latitude groups is shown in Table 2. We assigned all Lithomelissa spp.
and Larcopyle spp. to the high-latitude group as they are more abundant at higher-
latitude sites. The ecological and biogeographic affinity of Lithelius minor gr. is not10
yet fully understood. This group has a cosmopolitan distribution but tends to be most
abundant at high-latitude sites. The sudden appearance of Lithomelissa spp., other
high-latitude taxa and diatoms at Site 277 indicates the expansion of high-latitude wa-
ter masses across the southern Campbell Plateau during the PrOM event.
5.3 High-latitude cooling and eutrophication during the PrOM event15
5.3.1 Diagenesis
One possibility is that the pronounced increase in radiolarian abundance and diver-
sity observed in the Late Eocene of Site 277 is an artefact of biogenic opal diagenesis.
Chert nodules are recorded throughout the upper Paleocene-to-middle Eocene section
of the cored sequence at Site 277, with a transition between chert-bearing nannofos-20
sil chalk and overlying nannofossil recorded at 246mbsf (early late Eocene) (Kennett
et al., 1975). The presence of chert combined with the generally poorer preservation
of radiolarians in the lower Paleogene interval indicates some degree of diagenesis.
However, the radiolarian turnover event occurs ∼20 m above the lithological transition
within the succession of nannofossil oozes, which implies that the event represents25
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a real increase in radiolarian and diatom abundance and not an artefact of preserva-
tion.
5.3.2 Climate cooling
The long-term cooling trend through the middle and late Eocene, which was interrupted
by the short-lived MECO warming event, cannot explain the sudden radiolarian diver-5
sification in the late Eocene at Site 277. If gradual, long-term cooling was the driver of
the expansion of high-latitude taxa, a progressive increase in such taxa would be ex-
pected over a longer time period. A gradual increase of high-latitude taxa is observed
at Site 1172 from the middle Eocene but not at Site 277. Instead, the short-lived PrOM
event was likely the trigger for the sudden expansion of high-latitude taxa towards the10
north onto the Campbell Plateau. Whether that event was caused by a sudden drop
in atmospheric CO2concentrations or was related to the opening of the Tasmanian
Gateway, which may have been open to surface circulation in early middle Eocene (Bijl
et al., 2013), cannot be determined. Furthermore, astronomical induced changes also
have to be considered. Reconstructions from Laskar et al. (2004) show that nodes in15
the amplitude modulation of eccentricity and obliquity are present at ∼37 Ma. Addition-
ally, Röhl et al. (2004) found evidence at Site 1172 for the increasing dominance of the
100 kyr eccentricity cycle at ∼37 Ma. Although there are nodes in amplitude modula-
tion throughout the Eocene (Laskar et al., 2004), it is likely that only the combination
of all parameters (CO2drop, gateway opening and nodes in amplitude modulation)20
crossed a certain threshold for a cooling event. The PrOM event may have been asso-
ciated with the formation of small Antarctic ice sheets (Scher et al., 2014), which would
have resulted in an intensification of currents. Several radiolarian turnover events are
recorded from the South Atlantic (Maud Rise) by Funakawa and Nishi (2008) during the
late Eocene to early Oligocene. At ∼38.5 Ma they identified a shift from subantarctic to25
Antarctic bioprovinces with an increase in Antarctic taxa. At ∼36.3 Ma a decrease in
Antarctic taxa was observed and was related to the late Eocene warming (Bohaty and
Zachos, 2003). Both events were explained by the northward and southward shift of
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a proto-Antarctic Polar Front, respectively, however, the first event is not identical with
the PrOM event.
5.3.3 Radiolarian biogeographic reconstruction
During the middle Eocene, high-latitude radiolarian taxa were present at sites 277,
283, and 1172 (Fig. 7). The short-lived increase in abundance, diversity and the influx5
of low-latitude radiolarian Amphycraspedum murrayanum and A. prolixum gr. during
the MECO at Site 277 and a high percentage of cosmopolitan taxa at Site 1172 during
the late middle Eocene suggest moderately warm temperatures at both sites, which
may have been the result of a slightly stronger influence of an East Australian Current
(Fig. 8a). However, radiolarians and diatoms were abundant only at Site 1172 during10
the middle Eocene, which suggests a higher productivity region, perhaps a conse-
quence of local of upwelling.
During the middle to early late Eocene (∼39–38 Ma, Fig. 8b), the abundance of
high-latitude taxa increases at Site 1172. Additionally, Sites 281 and 283 show high
radiolarian abundance, with ∼25 to almost 50 % high-latitude taxa. The region of high-15
productivity is expanding, with the southernmost sites having the highest D/R ratio
in the interval ∼39–38 Ma (Fig. 8b). This could have resulted from further gateway
opening and an intensified cold-water proto-Ross gyre. Radiolarian abundance is still
low at Site 277.
In the late Eocene (∼37–35 Ma, Fig. 8c), radiolarians abruptly diversify and increase20
in abundance at Site 277. High-latitude taxa appear (Lithomelissa spp., Larcopyle spp.,
Lophocyrtiidae, Table 2), together with diatoms, resulting from cooling and eutrophi-
cation at Site 277. High-latitude taxa increase at Site 1172 from ∼36.5 Ma (Fig. 7),
whereas Site 281 contains a late Eocene hiatus, implying that increasing bottom water
currents were established across the Tasmanian Gateway.25
During the early Oligocene (∼33 Ma, Fig. 8d), the area of non-deposition widened
across the Tasmanian Gateway, suggesting the fully open gateway and deep-water
connection between ocean basins was established. Only Site 280 has a radiolarian
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and diatom-rich record in the early Oligocene indicating a high primary productivity
region. About 50 % of the radiolarian fauna are high-latitude taxa at that site. Site 277
also shows high radiolarian abundance and increasing high-latitude portion (∼40 %) at
∼33 Ma (Fig. 8d). The diversity, however, declines and diatoms are rare or absent. The
radiolarian fauna becomes dominated by Lithelius minor gr. and Actinommidae and5
many other high-latitude taxa disappear (e.g. Lithomelissa spp.). This may indicate
the establishment of a cold-water nutrient-depleted environment, similar to the modern
setting (Hollis and Neil, 2005), with a proto-Subantarctic Front being established to the
south of the Campbell Plateau.
6 Conclusions10
Middle Eocene to early Oligocene radiolarian assemblages from DSDP sites 277, 280,
281, 283 and ODP Site 1172 were examined to identify the distribution of Antarc-
tic assemblages in the Southwest Pacific. In contrast to temperature reconstructions
based on geochemical proxies that indicate subtropical-tropical temperatures at high-
latitudes during the middle and late Eocene (Liu et al., 2009; Bijl et al., 2010), Eocene15
radiolarian assemblages in this region lack significant numbers of low-latitude taxa.
Furthermore, we show that many high-latitude and taxa endemic to the Antarctic are
already present in the middle Eocene. The MECO event, although truncated by poor
recovery, has been identified at Site 277 within foraminiferal oxygen isotope records,
and is associated with a short-lived incursion of two low-latitude taxa, Amphycraspe-20
dum prolixum gr. and Amphycraspedum murrayanum, in low numbers. The absence of
definitive tropical taxa suggests warm temperate rather than tropical conditions during
this short-lived event. However, the peak warming interval is likely missing due to poor
core recovery. Radiolarians are very abundant and well preserved at high-latitude sites
281, 283 and 1172 during the early late Eocene with about 30–50 % of the assemblage25
consisting of high-latitude taxa. During the early late Eocene (∼37 Ma), a positive ex-
cursion in foraminiferal δ18O values at Site 277 marks the PrOM event. A pronounced
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increase in diversity, abundance and preservation of radiolarians occurs in conjunction
with this event. It is also accompanied by a pronounced increase in the abundance
of diatoms. Many high-latitude taxa that are very abundant at Site 281 and 283 in
the late middle Eocene and early late Eocene become abundant or have their LOs at
Site 277 at ∼37 Ma, respectively: Lithelius minor gr., Larcopyle hayesi,L. polyacan-5
tha,Spongopyle osculosa,Lithomelissa sphaerocephalis,L. gelasinus,L. ehrenbergi,
Ceratocyrtis spp., Dictyophimus aff.archipilium,Lamprocyclas particollis, and Antarc-
tic morphotypes of Aphetocyrtis gnomabax,A. rossi,Lophocyrtis aspera,L. keraspera
and L. longiventer. This northward extension of high-latitude taxa on the Campbell
Plateau appears to have been triggered by the PrOM event, which is inferred to have10
been a short-lived expansion of the Antarctic ice sheet. Through the EOT, radiolarians
remain abundant at Site 277, but decline in diversity. Delicate forms such as Plagiacan-
thidae decline, whereas Lithelius minor gr. and Actinommidae became dominant. The
disappearance of diatoms indicates that conditions over the Campbell Plateau became
nutrient-depleted. We infer that the Tasmanian Gateway was fully open by the earli-15
est Oligocene and a strong circumpolar current was established causing widespread
non-deposition in the Southwest Pacific. At the same time, a proto-Subantarctic Front
developed supplying nutrient-depleted Subantarctic waters onto the Campbell Plateau
resulting in a decline in radiolarian and diatom productivity.
The Supplement related to this article is available online at20
doi:10.5194/cpd-11-2977-2015-supplement.
Acknowledgements. This study has used bulk material and reference slides stored in the
DSDP/ODP Micropaleontology Reference Centre, which is located at the Institute of Geological
and Nuclear Sciences, Lower Hutt, New Zealand. We thank Noritoshi Suzuki (Tohoku Univer-
sity, Japan) for providing unpublished radiolarian data for ODP Site 1172. We acknowledge25
the support of Hannu Seebeck (GNS Science) in generating the paleogeographic maps. This
project is funded by the New Zealand Marsden Fund (Contract GNS1201).
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Table 1. Summary of species encountered at sites 277, 280, 281, and 283, their biogeo-
graphic affinitiy (A =Antarctic, B =bipolar, L =low-latitude and C =cosmopolitan), and location
on plates for selected species.
Taxa Biogeographic
affinity
Site 277 Site 280 Site 281 Site 283 Plate
Actinommidae sp. A sensu Hollis x Pl. 1, Fig. 1
Amphicentria sp. 1 sensu Suzuki A x x x Pl. 2, Fig. 1
Amphicraspedum murrayanum Haeckel T x Pl. 1, Fig. 14
Amphicraspedum prolixum Sanfilippo and Riedel gr. T x x Pl. 1, Figs. 15–17
Amphisphaera aff.radiosa (Ehrenberg) x Pl. 1, Fig. 4a and b
Amphisphaera coronata (Ehrenberg) gr. C x x Pl. 1, Fig. 2
Amphisphaera radiosa (Ehrenberg) x Pl. 1, Fig. 3
Amphisphaera spinulosa (Ehrenberg) C x x Pl. 1, Fig. 5
Amphisphaera?megapora (Ehrenberg) x x x x Pl. 1, Fig. 6
Amphymenium splendiarmatum Clark and Campbell C x x x x Pl. 1, Figs. 18 and 19
Anomalocantha dentata (Mast) x x x x
Antarctissa cylindrica Petrushevskaya A x
Antarctissa robusta Petrushevskaya A x
Aphetocyrtis bianulus (O’Connor) A x x Pl. 5, Fig. 1
Aphetocyrtis gnomabax Sanfilippo and Caulet A x x x x Pl. 5, Figs. 2–7
Aphetocyrtis rossi Sanfilippo and Caulet A x x x Pl. 5, Figs. 8–11
Archipilium macropus (Haeckel) x x
Artobotrys auriculaleporis (Clark and Campbell) C x
Artobotrys titanothericeraos (Clark and Campbell) x x
Artostrobus annulatus (Bailey) B x x
Artostrobus cf. pretabulatus Petrushevskaya A x Pl. 3, Fig. 13
Aspis sp. A sensu Hollis A x x x Pl. 3, Figs. 14–16
Axoprunum bispiculum (Popofsky) A x x
Axoprunum pierinae (Clark and Campbell) gr. C x x x x Pl. 1, Figs. 10 and 11
Axoprunum?irregularis Takemura A x Pl. 1, Fig. 12
Botryocella? sp A sensu Apel x Pl. 3, Figs. 1–4
Buryella granulata (Petrushevskaya) A x
Callimitra? aff.atavia Goll x Pl. 2, Fig. 2
Calocycloma ampulla (Ehrenberg) x
Ceratocyrtis spp. B x x x Pl. 2, Figs. 3–5
Cinclopyramis circumtexta (Haeckel) C x x x x
Cladoscenium ancoratum Haeckel x x
Clathrocyclas universa Clark and Campbell C x x x
Clinorhabdus anantomus Sanfilippo and Caulet A x x x Pl. 5, Figs. 12 and 13
Cornutella profunda Ehrenberg C x x x x
Corythomelissa adunca (Sanfilippo and Riedel) x
Cryptocarpium bussonii (Carnevale) gr. C x x x x Pl. 5, Figs. 25a and b, 26a and b
Cryptocarpium ornatum (Ehrenberg) C x x
Cycladophora cosma cosma Lombari and Lazarus A x Pl. 3, Fig. 17
Cycladophora humerus (Petrushevskaya) A x x x Pl. 3, Fig. 18
Cycladophora spp. A x x x
Cymaetron sinolampas Caulet x x
3007

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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Table 1. Continued.
Taxa Biogeographic
affinity
Site 277 Site 280 Site 281 Site 283 Plate
Cyrtolagena laguncula Haeckel C x x
Dictyophimus? aff.constrictus O’Connor x Pl. 4, Figs. 9 and 10
Dictyophimus infabricatus Nigrini C x
Dictyophimus? aff.archipilium Petrushevskaya A x x x Pl. 4, Figs. 3a, b–8
Dictyophimus?archipilium Petrushevskaya A x x x Pl. 4, Figs. 1a and b, 2
Eucyrtidium antiquum Caulet A x x Pl. 3, Fig. 19
Eucyrtidium mariae Caulet A x
Eucyrtidium microporum Ehrenberg x
Eucyrtidium nishimurae Takemura and Ling A x x Pl. 3, Fig. 20a and b
Eucyrtidium spinosum Takemura A x x x Pl. 3, Fig. 21
Eucyrtidium spp. A x
Eucyrtidium ventriosum O’Connor A x x Pl. 3, Fig. 22
Eurystomoskevos cauleti O’Connor A x x x x Pl. 3, Fig. 23a and b
Eurystomoskevos petrushevskaae Caulet A x x x x Pl. 3, Fig. 24
Eusyringium fistuligerum (Ehrenberg) C x Pl. 3, Fig. 25
Eusyringium lagena (Ehrenberg) C x
Glycobotrys nasuta (Ehrenberg) gr. C x x x x Pl. 3, Figs. 5–7
Heliodiscus inca Clark and Campbell x x
Lamprocyclas particollis O’Connor A x x x x Pl. 5, Fig. 27
Larcopyle cf. pylomaticus (Riedel) A x x Pl. 1, Fig. 25a and b
Larcopyle frakesi (Chen) A x Pl. 1, Fig. 20
Larcopyle hayesi (Chen) A x x x x Pl. 1, Fig. 21
Larcopyle labyrinthusa Lazarus A x Pl. 1, Fig. 22
Larcopyle polyacantha (Campbell and Clark) gr. A x x x x Pl. 1, Figs. 23 and 24
Larcopyle spp. A x x x
Lithelius foremanae Sanfilippo and Riedel x
Lithelius minor Jörgensen gr. B x x x x Pl. 1, Figs. 26–28
Lithomelissa cf. challengerae Chen A x Pl. 2, Fig. 9
Lithomelissa cf. haeckeli Bütschli A x x Pl. 2, Fig. 14
Lithomelissa challengerae Chen A x Pl. 2, Fig. 6–8
Lithomelissa ehrenbergi Bütschli A x x x x Pl. 2, Figs. 10 and 11
Lithomelissa gelasinus O’Connor A x x x x Pl. 2, Figs. 12 and 13
Lithomelissa macroptera Ehrenberg A x Pl. 2, Fig. 15a and b
Lithomelissa robusta Chen A x x Pl. 2, Fig. 16
Lithomelissa sphaerocephalis Chen A x x x x Pl. 2, Fig. 17
Lithomelissa spp. A x x x x
Lithomelissa tricornis A x x x x Pl. 2, Fig. 18
Lithomelissa?sakai O’Connor A x Pl. 2, Fig. 19
Lophocyrtis (Apoplanius) aspera (Ehrenberg) A x x x Pl. 5, Figs. 14a, b–16
Lophocyrtis (Apoplanius) keraspera Sanfilippo and Caulet A x x Pl. 5, Figs. 17–19
Lophocyrtis (Lophocyrtis) jacchia hapsis Sanfilippo and Caulet A x x Pl. 5, Figs. 20–22
Lophocyrtis (Paralampterium)dumitricai Sanfilippo C x
Lophocyrtis (Paralampterium)longiventer (Chen) A x x x x Pl. 5, Figs. 23 and 24
Lophocyrtis spp. A x
3008

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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Table 1. Continued.
Taxa Biogeographic
affinity
Site 277 Site 280 Site 281 Site 283 Plate
Lophophaena capito Ehrenberg C x x x
Lophophaena simplex Funakawa x x
Lychnocanium aff.carinatum Ehrenberg x Pl. 4, Fig. 17
Lychnocanium amphitrite (Foreman) C x x Pl. 4, Figs. 11a–c and 12
Lychnocanium babylonis (Clark and Campbell) C x x Pl. 4, Figs. 13a and b, 14
Lychnocanium bellum Clark and Campbell C x x Pl. 4, Figs. 15 and 16
Lychnocanium conicum Clark and Campbell C x
Lychnocanium continuum Ehrenberg x
Lychnocanium tetrapodium Ehrenberg T x Pl. 4, Fig. 18a and b
Lychnocanium waiareka O’Connor x x
Perichlamydium limbatum Ehrenberg x
Periphaena decora Ehrenberg C x x x x
Periphaena heliastericus (Clark and Campbell) C x x x x
Phormocyrtis striata striata Brandt C x
Plectodiscus circularis (Clark and Campbell) C x x x x
Pseudodictyophimus galeatus Caulet A x Pl. 2, Fig. 20
Pseudodictyophimus gracilipes (Bailey) gr. B x x x x Pl. 2, Figs. 21–23
Pseudodictyophimus spp. A x Pl. 2, Figs. 24–27
Pterocodon apis Ehrenberg x Pl. 4, Figs. 19 and 20a, b
Pteropilium aff.contiguum (Ehrenberg) x Pl. 4, Fig. 21
Saturnalis circularis Haeckel x
Sethocyrtis chrysallis Sanfilippo and Blome C x Pl. 3, Fig. 26a and b
Siphocampe lineata (Ehrenberg) C x
Siphocampe nodosaria (Haeckel) C x x x
Siphocampe quadrata (Petrushevskaya and Kozlova) C x x x
Siphocampe?acephala (Ehrenberg) gr. x x Pl. 3, Figs. 8–10
Siphocampe?amygdala (Shilov) A x x Pl. 3, Figs. 11 and 12
Sphaeropyle tetrapila (Hays) A x Pl. 1, Fig. 29
Spirocyrtis greeni O’Connor x x x
Spirocyrtis joides (Petrushevskaya) C x x x x
Spongatractus pachystylus (Ehrenberg) x
Spongodiscus craticulatus (Stöhr) x
Spongodiscus cruciferus (Clark and Campbell) C x
Spongodiscus festivus (Clark and Campbell) C x
Spongopyle osculosa Dreyer B x x x x Pl. 1, Fig. 13
Spongurus bilobatus Clark and Campbell C x x x
Stichopilium cf. bicorne (Haeckel) x x x x Pl. 5, Figs. 28a and b, 29a and b
Stylosphaera minor Clark and Campbell gr. C x x x Pl. 1, Fig. 7
Theocampe amphora (Haeckel) C x
Theocampe urceolus (Haeckel) C x x x x
Theocyrtis tuberosa Riedel C x x Pl. 5, Fig. 30
Thyrsocyrtis pinguisicoides O’Connor B x x Pl. 3, Fig. 27
Tripodiscinus clavipes (Clark and Campbell) C x x x
Zealithapium mitra (Ehrenberg) C x x Pl. 1, Fig. 8
Zygocircus bütschli Haeckel x x
3009

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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Table 2. Average of total % of high-latitude species, groups, genera and high-latitude members
of families for four time slices: MECO (∼40 Ma), middle/late Eocene (∼39–38 Ma), late Eocene
(∼37–35 Ma) and early Oligocene (∼33 Ma).
Site 280 Site 281 Site 283 Site 277 Site 1172
E. Olig. m/l Eoc. m/l Eoc. MECO m/l Eoc. late Eoc. E. Olig. MECO m/l Eoc late Eoc.
% total high-lat. species 49 27 48 14 9 18 40 23 26 46
Lithelius minor gr. % 15.0 2.1 31.5 4.2 1.9 5.3 30.7 13.7 12.4 22.0
Larcopyle spp. % 10.0 10.5 1.7 2.9 1.93 1.88 1.5 6.0 5.4 12.8
Lithomelissa spp. % 8.9 4.8 2.5 0.06 0.1 2.15 1.2 0.5 1.4 0.5
High-lat. Lophocyrtiidae % 5.6 5.6 5.0 6.2 3.8 5.5 3.60 1.0 2.8 5.3
High-lat. Eucyrtiidae % 4.9 2.7 4.8 0.1 1.0 1.2 0.2 1.4 1.9 1.5
Other high-lat. Plagiacanthidae % 3.5 0.6 1.0 0 0.1 0.34 0.25 0 0.02 0
Other high-lat. species % 1.2 0.4 1.5 1.0 0.1 2.1 2.8 0.6 1.9 4.2
3010

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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!
!
!
!
-30°
-40°
280A
281
283
277
1172 Campbell Plateau
Tasman Rise
180°
160°
180°160°140°
-40°
-50°
Tasman Sea
STF
SAF
SAW
Figure 1. Modern location of DSDP and ODP study sites in the Southwest Pacific;
STF =Subtropical Front, SAF =Subantarctic Front, SAW=Subantarctic Water.
3011

CPD
11, 2977–3018, 2015
Expansion and
diversification of
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radiolarian
assemblages
K. M. Pascher et al.
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nanno chalk
nanno ooze sandstone
no core recovery
chert nodules
Lithology
Age
RP(SH)
Log
Core
NZ Stages
mbsf
G. index
E.spinosum
L.tricornis
P.gracilipes
E.antiquum
Z. mitra
E. fistuligerum
A. rossi
L. longiventer
A. pierinae gr.
S. chrysallis
L. jacchia hapsis
T. tuberosa
L. ehrenbergi
L. sphaerocephalis
A. irregularis
L. particollis
A. gnomabax
DSDP 277
Early Oligocene
Late EoceneMiddle Eocene
lRP15
uRP15
uRP14
lRP14RP12
early WhaingaroanKaiatan
Bortonian Runangan
350
300
250
200
150
RP13
14
20
17
34
31
30
29
28
26
23
22
21
16
15
18
32
27
25
24
18
Cibicidoides O δ
(‰, v-PDB)
13
Cibicidoides C δ
(‰, v-PDB)
ODP 689
18
Cibicidoides O δ
(‰, v-PDB)
ODP 689
13
Cibicidoides Cδ
(‰, v-PDB)
Age (Ma)
0 0.8 1.6
48
44
40
36
32
28
24
3 2 1 0 -1
E-O transition
MECO
PrOM
34
33
19
δ13C
δ18O
35
2 1.5 1 0.5 0 -0.5
0 0.8 1.6
Figure 2. DSDP Site 277 stratigraphy, lithology, Southern Ocean radiolarian zones, core re-
covery, and ranges of Globigerinatheka index and selected radiolarians. Benthic stable oxy-
gen and carbon isotope data of DSDP Site 277 correlated to Southern Ocean Cibicidoides
data of ODP Site 689 Hole B (Maud Rise) (Diester-Haass and Zahn, 1996) calibrated to the
GTS2012 timescale using the magnetostratigraphy data of Florindo and Roberts (2005) and
Spiess (1990).
3012

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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δ18O (‰, v-PDB)
δ13C(‰, v-PDB)
MECO
E-O transition
Cibicidoides
Bulk
G.index
Subbotina
Rad samples
PrOM
Late Eoc. warming
Early Oligocene
Late Eocene
Middle Eocene
early Whaingaroan
Kaiatan
Bortonian Runangan
350
300
250
200
150
20
17
31
30
29
28
26
23
22
21
16
15
18
32
27
25
24
35
Age
Core
NZ Stages
mbsf
DSDP 277
34
33
19
0 1 2 3 4
2 1 0 -1
Figure 3. DSDP Site 277 oxygen and carbon stable isotope records and position of studied ra-
diolarian samples within MECO interval (red stars) and radiolarian-rich late Eocene–Oligocene
interval (blue stars).
3013

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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18
Cibicidoides O δ
(‰, v-PDB)
MECO
PrOM
10
100
1000
10000
Radiolarians/
gram Taxic richness
Fisher α
diversity
Simpson
Evenness
rads/g
D/R
D/R ratio
0 10 20
0.2 0.6 1
0 50 100
Early Oligocene
Late EoceneMiddle Eocene
early WhaingaroanKaiatan
Bortonian Runangan
350
300
250
200
150
Age
Core
NZ Stages
mbsf
DSDP 277
E-O transition
.0001
.001
.01
0.1
1
2 1.5 1 0.5 0 -0.5
20
17
34
31
30
29
28
26
23
22
21
16
15
18
32
27
25
24
34
33
19
35
Figure 4. DSDP Site 277 benthic δ18O record; radiolarian abundance and Diatom/Radiolarian
(D/R) ratio; Taxic Richness (number of taxa), Fisher αIndex and Simpson Evenness Index for
radiolarian assemblages. Red arrows indicate samples with total specimen counts less than 99,
which may be statistical insignificant but are included in all figures for the sake of completeness.
3014

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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Age
Core
5
6
7
DSDP 280
RP(SH)
RP15
Early Oligocene
Radiolarians/
gram
130
120
110
100
90
mbsf
Lithology
no core recovery
silty diatom ooze
silty diatom clay
Log
14
Late Eocene
15
16
RP14
calc.&silic.
fossil-rich glauconitic,
sandy to silty clays
Lithology
150
140
130
120
Fisher α
diversity
Age
Core
DSDP 281
RP(SH)
mbsf
Log
M.E.
Late Eocene
RP13
RP14
80
200
160
120
Core
DSDP 283
RP(SH)
mbsf
Log
Lithology
siliceous fossil-rich
diatom ooze
siliceous fossil-rich
detrital silty clay
Diversity
Evenness
Fisher α
diversity
rads/g
D/R siliceous fossil-rich
diatom-nanno ooze
no core recovery
no core recovery
.01
0.1
1
10
D/R ratio
0 10 20
Simpson
Evenness
0.7 1
1000
10000
Radiolarians/
gram
1000
10000
.01
0.1
1
10
D/R ratio
0 10 20
Simpson
Evenness
0.7 1
rads/g
D/R
Diversity
Evenness
rads/g
D/R
Radiolarians/
gram
1000
10000
.01
0.1
1
10
D/R ratio
Fisher α
diversity
0 10 20
Simpson
Evenness
0.7 1
Diversity
Evenness
5
8
6
7
Age
Olig.
Figure 5. Variation in radiolarian abundance, Diatom/Radiolarian (D/R) ratio, Fisher αIndex
and Simpson Evenness for radiolarian assemblages at DSDP sites 280, 281 and 283.
3015

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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0 50 100
Biogeographic affinity %
Early Oligocene
Late EoceneMiddle Eocene
lRP15
uRP14
lRP14RP12
early WhaingaroanKaiatan
Bortonian Runangan
350
300
250
200
150
RP13
DSDP 277
Age
RP(SH)
mbsf
NZ stages
MECO
E-O
0 50 100
Taxic richness
0 40 80
% Litheliidae
048
% Plagiacanthidae
% Lophocyrtiidae
010 20
Larcopyle spp.
Lithelius minor gr.
Litheliidae
Lithomelissa spp.
Plagiacanthidae
L. longiventer
Lophocyrtiidae
uRP15
PrOM
L. minor gr.
Cosmopolitan
Low latitude
Unknown
Biogeographic affinity
High latitude
without L. minor gr.
Figure 6. Biogeographic affinities of radiolarian assemblages at DSDP Site 277; Taxic rich-
ness; most abundant families with high-latitude affinity. Red arrows indicate samples with total
specimen counts less than 99.
3016

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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Faunal affinities %
Late Eocene
Middle Eocene
IODP 1172
RP14
RP10-12
0 50 100
Faunal affinities %
Late EoceneMiddle Eocene
lRP15
uRP14
lRP14
RP12
early Whaingaroan
Kaiatan
Bortonian Runangan
350
300
250
200
150
RP13
DSDP 277
Age
RP(SH)
Age
RP(SH)
mbsf
M.E.
early Late Eocene
DSDP 283
RP
13
RP14
0 50 100
Faunal affinities %
200
160
120
80
Age
RP(SH)
mbsf
NZ stages
0 50 100
Taxic richness
MECO
E-O
0 50 100
Taxic richness
PrOM
L. minor gr.
Cosmopolitan
Low latitude, =trace tr
Unknown
Biogeographic affinity
DSDP 280
DSDP 281
early Late Eocene
Olig.
RP14
Age
90
100
110
120
RP(SH)
RP15
Early Oligocene
0 50 100
Faunal affinities %
150
140
130
120
mbsf
Age (Ma)
uRP15
RP13
tr
tr
0 50 100
Taxic richness
0 50 100
Taxic richness
High latitude without L. minor gr.
*
*tr
*tr
*tr
*tr
a
*tr
*tr
*tr
T. tuberosa (tr=trace, a=abundant)
*
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
050 100
Early Oligocene
Early Oligocene
RP15
Figure 7. Variation in faunal affinities for radiolarians assemblages at all sites. Dashed black
lines indicate correlation between sites, which is hampered by hiatuses and poorly defined
ages, respectively. The age model of ODP Site 1172 is based on the age-depth plot of Stickley
et al. (2004).
3017

CPD
11, 2977–3018, 2015
Expansion and
diversification of
high-latitude
radiolarian
assemblages
K. M. Pascher et al.
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MECO (~40 Ma)
!
!
!
!
!
!
-160°
-160°
180°
180°
160°
160°
-140°140° -120°120°
-40° -40°
-50°
-50°
-60°
-60°
277
281
280A
283
1172
EAC
proto-Ross
gyre
EAC
proto-Ross
gyre
Late Eocene (~37-35 Ma)
!
!
!
!
-160°
-160°
180°
180°
160°
160°
-140°140° -120°120°
-40° -40°
-50° -50°
-60°
-60°
277
283
1172
!
!
!
!
!
!
Early Oligocene (~33 Ma)
ACC
cold, nutrient-depleted waters
EAC
-160°180°160° -140°140° -120°120°
-160°180°160°
-40°
-50°
-60°
-40°
-50°
-60°
277
281
280A
283
1172
non-deposition
proto-SAF
area of primary productivity/common diatoms
EAC: East Australian Current
ACC: Antarctic Circumpolar Current
proto-SAF: proto Subantarctic Front
A
CD
% high-latitude (blue)
% cosmopolitan (yellow)
% low-latitude (red)
% others (grey)
Biogeographic affinity
high abundance (>1000rads/gr)
low abundance (<100rads/gr)
middle/late Eocene (~39-38 Ma)
!
!
!
!
!
!
-160°
-160°
180°
180°
160°
160°
-140°140° -120°120°
-40° -40°
-50°
-50°
-60°
-60°
277
EAC
proto-Ross
gyre
B
280A
281
283
1172
281
poorly fossiliferous silty clay
280A
10.5%
Figure 8. Paleogeographic reconstructions (GPlates, using the latest hotspot trace reference
frames, Seton et al., 2012; Matthews et al., 2015) and biogeographic affinities at investigated
sites during the MECO, middle/late Eocene (∼39–38 Ma), PrOM and latest Eocene (∼37–
35 Ma) and early Oligocene (∼33 Ma).
3018