Introduction
The long-term evolution of climate through the early to middle Palaeogene
(56–34 Ma) has been established from geochemical proxies and
palaeontological data. The primary proxy record, stable oxygen isotope
(δ18O) values of benthic foraminifera, shows a trend from an
early Cenozoic greenhouse climate to an icehouse climate with an abrupt
positive shift in benthic δ18O values of ∼ 1.2–1.5 ‰ in the earliest Oligocene
(∼ 34 Ma) (Shackleton and Kennett, 1975; Diester-Haass et al., 1996; Zachos et
al., 2001). After a prolonged period of maximum warmth during the Early
Eocene Climatic Optimum centred around 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 increase in atmospheric pCO2 (Bohaty and Zachos, 2003; Bohaty et
al., 2009; Bijl et al., 2010). Lipid biomarker-based climate proxies suggest
the southwest Pacific sea surface temperatures were tropical during the MECO
(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) (Liu et al., 2009; Bijl et
al., 2010).
The warm conditions of the Eocene indicated by geochemical proxies are
generally consistent with fossil-based reconstructions of Southern Ocean
circulation developed from high-latitude drill cores (Kennett, 1977; Nelson
and Cooke, 2001; Kennett and Exon, 2004; Houben et al., 2013), in which
subtropical waters are interpreted to have extended close to the Antarctic
margin until the late Eocene. However, the latest generation of ocean
circulation and climate modelling simulations fails to reproduce the degree
of high-latitude warmth indicated for the Eocene by these new proxies (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 with increasing evidence for
ephemeral Antarctic glaciation during the latest Eocene from both fossil and
geochemical proxies (Lazarus and Caulet, 1993; Scher et al., 2014; Barron et
al., 2015). 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, referred to as
the Priabonian Oxygen Isotope Maximum (PrOM) (Scher et al., 2014). Further
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. A negative δ18O excursion reported at Ocean Drilling
Project (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) has been interpreted to be a short-lived warming event in the late
Eocene (∼ 36.4 Ma).
Identifying the initial timing and establishment of a high-latitude fauna 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.
Kennett (1978) provided the first summary on the biogeographic development of
planktic biota in the circumpolar Southern Ocean throughout the Cenozoic. He
inferred that the development of distinct polar plankton assemblages was
related to the evolution of the Antarctic Circumpolar Current (ACC) and the
Antarctic Polar Front (AAPF). This change was linked by Kennett (1978) to
Southern Ocean circulation changes associated with the opening of Drake
Passage and Tasmanian Gateway in the late Eocene–early Oligocene and
implicated as the main causal mechanisms for Antarctic glaciation. Subsequent
deep-sea drilling campaigns have provided additional data on regional changes
in Southern Ocean plankton, which were integrated by Lazarus and Caulet
(1993) into a set of circumpolar maps across specific time intervals.
Moreover, these authors also carried out the first synthesis of radiolarian
biogeography for the region and found a pattern of increasing endemism in the
Southern Ocean across the EOT. Nelson and Cooke (2001) undertook a
comprehensive review of previous work and presented an updated synthesis on
the oceanic front development in the southwest Pacific during the Cenozoic.
According to these authors, the
proto-Subtropical Front was established in the late Eocene (ca. 35 Ma) and
an AAPF in the early Oligocene. A more detailed study of radiolarian
biogeographic patterns and trends in the southwest Pacific was done by
Lazarus et al. (2008), who found increased endemism in the radiolarian fauna
in the late Eocene (ca. 35 Ma). Further radiolarian studies from the
Atlantic sector of the Southern Ocean were performed by Funakawa and
Nishi (2008), who recorded the first expansion of an Antarctic assemblage
significantly earlier (38.5 Ma). They identified several faunal turnover
events in the Antarctic assemblage from the late middle Eocene to late
Oligocene and linked these events to migrations of the AAPF. Latest research
suggests that the ACC was not developed until ∼ 30 Ma, together with
the establishment of an AAPF (Scher et al., 2015), when the Tasmanian Gateway
aligned with the westerly wind flow (Hill et al., 2013). From the middle to
late Eocene, a westward Antarctic Slope Current is inferred to have flowed
across the gateway, driven by the polar easterlies (Bijl et al., 2013; Scher
et al., 2015).
In this paper, we document variation in radiolarian assemblages and
foraminiferal oxygen and carbon stable isotopes from middle Eocene to early
Oligocene (∼ 40 to 30 Ma) at Deep Sea Drilling Project (DSDP) Site 277 and relate these
variations to radiolarian assemblage changes at DSDP sites 280, 281, and 283 and
ODP Site 1172. DSDP Site 277 provides a unique record of pelagic
sedimentation in the southwest Pacific during late Palaeocene to Oligocene
times and the first Eocene foraminiferal δ18O record was
generated from this site (Shackleton and Kennett, 1975). Although the study by Lazarus et
al. (2008) of radiolarian assemblages included all above-mentioned
DSDP sites, this new work includes a more thorough taxonomic review of the
radiolarian assemblages at these sites and integrates the radiolarian
assemblage trends with new stable isotope data for Site 277. Our results
help to identify the extent to which tropical or warm-subtropical conditions
prevailed during the middle and late Eocene, refine the timing and nature of
the development of a distinctive Southern Ocean radiolarian fauna and
discuss implications for the palaeoceanography of the southwest Pacific from
the middle Eocene to early Oligocene.
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 Site 277, which is located on the western margin of the Campbell
Plateau (52∘13.43′ S, 166∘11.48′ E) 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 434.5 m 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 m intervals) between 301.5 and 368.0 mbsf Below 10 mbsf, a Palaeogene sequence
spanning from the late Palaeocene to middle Oligocene was recovered (Kennett
et al., 1975). We studied cores 277-35R (349.2 mbsf) to 277-15R (134.5 mbsf), which cover a middle Eocene–lower Oligocene interval. The sediment at Site
277 (palaeolatitude ∼ 55∘ S) throughout the
succession is highly calcareous, indicating a depositional environment well
above the lysocline, with a palaeodepth estimated at around 1500 m (Kennett
et al., 1975; Hollis et al., 1997).
Four additional sites were included in our study in order to acquire a
regional picture of radiolarian assemblage change and biogeography during
the middle Eocene to early Oligocene (Fig. 1). DSDP Site 280 comprises two
holes (48∘57.44′ S, 147∘14.08′ E) located
∼ 100 km south of the South Tasman Rise and drilled at a water
depth of 4176 m. We collected radiolarian assemblage data from Hole 280A,
which consists of a 201 m cored section that includes a 97.2 m middle
Eocene–middle Oligocene interval. The studied interval spans Core 280A-7R
(123.4 mbsf) to Core 280A-5R (92.54 mbsf). DSDP Site 281 on the South Tasman
Rise (47∘59.84′ S, 147∘45.85′ E), drilled at a water
depth of 1591 m, encompasses two holes (281 and 281A). We examined Hole 281,
which was cored to 169 mbsf, and recovered a 105.6 m (62.5 % recovery)
upper Eocene–Pleistocene section. The studied interval covers cores
281-16R (149 mbsf) to 281-14R (122.5 mbsf). DSDP Site 283 lies in the
central Tasman Sea (43∘54.6′ S, 154∘16.96′ E) at 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
Palaeocene–Pleistocene section that contains a hiatus from the upper Eocene to
possibly the Miocene. Samples from cores 283-8R (192.25 mbsf) to 283-5R
(87.75 mbsf) were studied from this site. ODP Site 1172 is situated west of the
East Tasman Plateau (43∘57.58′ S, 149∘55.69′ E) in a
water depth of 2622 m and was drilled during ODP Leg 189 (Exon et al.,
2004). It comprises four holes (1172A, 1172B, 1172C and 1172D). The examined
samples were from Section 1172A-39X-1 to Section 48X-CC (354.625–450.55 mbsf), spanning a middle Eocene–lower Oligocene interval, and from
Section 1172D-2R-2 to Section 1172D-2R-3 (355.225–356.875 mbsf), covering a
lower Oligocene interval.
Modern location of DSDP and ODP study sites in the southwest
Pacific. Dark grey: coastline; light grey: 2000 m isobath of continental
boundary; STF: Subtropical Front; SAF: Subantarctic Front,
SAW: Subantarctic Water.
Material and methods
Sample preparation and analysis
This study is based on 33 sediment samples from DSDP Site 277
(∼ 350 to 135 mbsf) spanning a middle Eocene–lower
Oligocene interval (17 reported by Hollis et al. (1997) and 16 new samples),
6 samples from DSDP Site 283 (new, all from the DSDP/ODP Micropaleontology
Reference Centre (MRC)), 7 from DSDP Site 281 (3 from the DSDP/ODP MRC, 4
new) and 4 from DSDP 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). To obtain a
consistent taxonomic identification across all sites, all samples previously
reported from DSDP sites 277, 280, 281 and 283 were re-examined and
re-counted as part of this study. The Supplement files include taxonomic
notes for all radiolarian species recorded in this study, plates of selected
species, and radiolarian distribution charts and sample information for DSDP
sites 277, 280, 281 and 283 (Supplement Tables 1–5). Radiolarian census
data of 41 samples from ODP Site 1172, covering a middle Eocene–lower
Oligocene interval, are provided in Supplement Table 6. The
radiolarian taxonomy, sample preparation and analysis methodology were
published in Suzuki et al. (2009).
For strewn slide preparation, 1–10 g of sample material was broken into
∼ 5 mm diameter chips and acidified with 10 % HCl to
dissolve carbonate. 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 sodium hexametaphosphate
((NaPO3)6). The residue was washed through a 63 µm sieve and
dried. Dependent on the volume of the processed residue and the abundance of
radiolarians, 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 microscope. 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 was fixed.
Strewn slides were examined using a Zeiss transmitted light microscope
fitted with a Zeiss AxioCam ERc5s digital camera. 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 ∼ 300 specimens was achieved. The proportion
of the slide examined to this point was determined and the 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 observed 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. Actinommidae undet.) to species and
subspecies. This approach allows for an accurate estimate of the abundance
of individual species, but it does result in overall diversity being
underestimated.
Radiolarian abundance was calculated using the following equation:
XR×XS×1XP/ASed,
with XR being the total number of radiolarians per slide, XS the
number of slides made of a known portion XP of the dried
material, and ASed the initial amount of dried sediment.
Additional data derived for each sample assemblages includes taxic richness,
the Fisher α diversity index and the Simpson index of evenness. The
latter two indices were calculated using the PAST software version 3.07
(Hammer et al., 2001). The Fisher α index 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
assemblages are dominated by individual taxa and ranges from 0 to 1. Since
taxic richness is correlated with preservation and is also dependent on the
sample size, we performed an individual rarefaction analysis for Site 277
samples with PAST (Supplement Table 2). This allows the comparison of
taxonomic diversity in samples of different sizes. We used 100, 200, 300 and
500 counts as sample sizes to calculate taxic richness.
Additionally, we derived a range-through taxic richness after subsampling
for Site 277 with R version 3.1.3 (www.r-project.org) (Supplement
Table 2). We chose sample sizes of 100 and 300, both with a
subsampling of 1000. This approach shows whether a diversity drop in the middle
of a series is a true diversity drop or a temporary absence due to
preservation. The diatom / radiolarian (D/R) ratio was calculated using the
counts of diatoms and radiolarians of one examined slide. In the case of very
rare diatoms, all specimens were counted on a slide; otherwise, several
transverses were counted for diatoms and the total number estimated for the
whole slide. Although this method is not an accurate measure of total diatom
abundance 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 allows us to identify significant diatom event
horizons.
Radiolarian biogeographic affinities
The assignment of biogeographic affinities to radiolarian species, subspecies
and informally defined morphotypes encountered in our study is based on a
comprehensive literature review. We focussed on published records of these
taxa or their close relatives from the southwest Pacific and Southern Ocean
(e.g. Petrushevskaya, 1975; Takemura and Ling, 1997; Sanfilippo and Caulet,
1998; Hollis, 2002, 2006; Funakawa and Nishi, 2005, 2008; Hollis et al.,
2005; Funakawa et al., 2006; Kamikuri et al., 2013). This literature review
was complemented with radiolarian occurrence data from the NSB (Neptune
Sandbox Berlin) database (Lazarus, 1994; Spencer-Cervato, 1999).
Unfortunately, this database lacks many Palaeogene radiolarians, and, for
those that are present, occurrences need to be cross-checked with the
DSDP/ODP reports. The first step was to assess the palaeolatitude of each
site for the interval of radiolarian ranges. We used
www.paleolatitude.org (van Hindsbergen et al., 2015) to extract
palaeolatitude information in intervals of 10 Ma for the past 60 Ma and
created the mean value for each site for an age range (Supplement Table 11).
We listed radiolarian taxa and their range and abundance at high-latitude
(> 45∘ N/S), mid-latitude (25–45∘ N/S) and
low-latitude sites (0–25∘ N/S) and observed that presence/absence
data are not always a reliable guide to biogeographic affinity (Supplement
Table 12). For instance, Lithomelissa ehrenbergi Buetschli, 1882 was
described from Barbados, which may indicate that this species has a tropical
or cosmopolitan ecology. However, the species is far more abundant at
high-latitude sites, and only rarely recorded at low-latitude sites.
Moreover, Haeckel (1887) found recent L. ehrenbergi from deep-water
samples at low latitudes. Therefore, we interpret this species as a
cold-water indicator, commonly found in high-latitude samples and sometimes
found in deep-water samples in low latitudes. The biogeographic affinities of
Amphicraspedum murrayanum and the A. prolixum group also
warrant some discussion. These taxa are widely reported in early and middle
Eocene sediments but occur in greater abundance in the southwest Pacific at
times of global warmth (Hollis, 2006). Liu et al. (2011) suggested that these
taxa were not valid indicators of high-latitude warming because they are
found in the Palaeocene in the North Atlantic. However, their assumption that
southwest Pacific and North Atlantic Ocean conditions would have been similar
in the Palaeogene is not supported by ocean circulation models (Huber et al.,
2003, 2004). These models indicate that oceanic conditions for the North
Atlantic and the southwest Pacific were substantially different in the early
Palaeogene: the North Atlantic was bathed in warm currents of
∼ 25 ∘C moving northwards (Huber et al., 2003), while the
southwest Pacific was influenced by a strong cyclonic gyre preventing warm
waters from penetrating southwards, except during times of extreme global
warmth (Huber et al., 2004; Hollis et al., 2012). Thus, the occurrence of
warm-water indicators throughout the Palaeocene–Eocene interval in the
mid-latitude North Atlantic is consistent with both the global circulation
model results and our interpretation of influxes of Amphicraspedum
as being indicative of warming.
DSDP Site 277
stratigraphy, NZ stages (Raine et al., 2015), Southern Hemisphere radiolarian
zones (RP), nannofossil zones (NP), lithology, core recovery, selected
bioevents (ages calibrated to the 2012 geological timescale; Gradstein et
al., 2012; Raine et al., 2015) and benthic δ18O and δ13C
data of DSDP Site 277. The dashed lines correlate with Site 277 based on the
ages of the bioevents 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). LO: lowest occurrence; LCO: lowest common
occurrence; HO: highest occurrence; MECO: Middle Eocene Climatic Optimum;
PrOM: Priabonian Oxygen Isotope Maximum; EOT: Eocene–Oligocene transition.
Tectonic reconstructions of the Australia–Antarctica–Pacific plate circuit
were undertaken in GPlates version 1.5 (Boyden et al., 2011) using finite
poles of rotation for the relative motions between Australia and East
Antarctica from Cande and Stock (2004) (0–38.13 Ma), East Antarctica and West
Antarctica from Granot et al. (2013) (30.94–40.13 Ma), and West
Antarctica and the Pacific from Croon et al. (2008) (0–47.54 Ma). Relative motions
of the Australia–Antarctica–Pacific plate circuit were tied to the
Australian palaeomagnetic apparent polar wander path of Torsvik et al. (2012)
to provide an estimate of palaeolatitude appropriate for palaeoclimate studies
(van Hinsbergen et al., 2015). The 2000 m isobath from the GEBCO bathymetric
grid (www.gebco.net) was used to approximate continental boundaries. The
continental–oceanic boundaries of Bird (2003) are also shown (dashed lines
in Figs. 1 and 8) for regions where extension has significantly thinned
continental crust. Each DSDP and ODP study site was assigned to the
appropriate plate for reconstruction.
The overlap of the North and South Island of New Zealand in these
reconstructions is a consequence of the finite poles of rotation determined
from the Adare Trough by Granot et al. (2013), which constrain the motion of
East and West Antarctica between 40 and 30 Ma. These new poles result in a
poor fit (significant overlap) of continental crust between the two islands
that is not supported by geological data. The discrepancy between geological
and palaeomagnetic data could be reconciled with the use of seafloor
spreading data from the Emerald Basin (e.g. Keller, 2003), which describes
Australia–Pacific relative motions (Sutherland, 1995) between 40 and 30 Ma,
and the Adare Trough. However, our sites lie south of New Zealand and so we
make no attempt to resolve this issue here.
DSDP Site 277 δ18O and δ13C records and
location of studied radiolarian samples within the MECO and late Eocene
warming interval (red stars) and radiolarian-rich upper Eocene–lower
Oligocene interval (blue stars).
Stable isotope analysis
Stable oxygen (δ18O) and carbon (δ13C) isotope
measurements of foraminiferal samples from Site 277 were conducted in the
stable isotope laboratories at the University of Southampton (UoS) and
University of California Santa Cruz (UCSC). Sample analyses included bulk
carbonate, benthic foraminifera (Cibicidoides spp.), and the planktic foraminifera
Subbotina spp. (thermocline) from 332.62 to 159.88 mbsf and Globigerinatheka index (mixed layer) from
332.62 to 188.58 mbsf (its last occurrence). In total, 169 samples spanning
the middle Eocene–lower Oligocene interval of DSDP Hole 277 were measured
(Supplement Tables 7–10). Stable isotope analyses at the UoS were
performed on a Europa GEO 20-20 dual-inlet mass spectrometer with CAPS
preparation oven maintained at 70 ∘C, and analyses at UCSC were
performed on a VG Prism dual-inlet mass spectrometer coupled to a carousel
preparation device with a common acid bath maintained at 90 ∘C. All
values are reported relative to the Vienna Pee Dee Belemnite (VPDB)
standard. In both labs, analytical precision, based on replicate analyses of
in-house marble standards and NBS-19, averaged ∼ 0.07 ‰ (1σ) for δ13C and
∼ 0.08 ‰ (1σ) for δ18O.
DSDP Site 277
benthic δ18O record, radiolarian abundance and diatom/radiolarian
(D/R) ratio, taxic richness (number of taxa) derived from individual
rarefaction and range-through analyses for different sample sizes, and Fisher
α index and Simpson evenness index for radiolarian assemblages. Red
bars indicate sample sizes < 100 specimens, blue bars indicate
sample sizes < 300 specimens and black bars indicate samples sizes
> 300 specimens.
Results
Site 277 biostratigraphy and stable isotope stratigraphy
Broad age control for DSDP Site 277 is based on the biostratigraphic
synthesis of Hollis et al. (1997), who correlated the succession to Southern
Hemisphere (SH) radiolarian zones RP6 to RP15. In this study we confirm the
location of the base of RP12(SH) (lowest occurrence (LO) of Lophocyrtis longiventer) at
371.2–349.2 mbsf, the base of RP14(SH) (LO of Eucyrtidium spinosum, 38 Ma) at
264.5–254.5 mbsf, the base of RP15(SH) (LO of Eucyrtidium antiquum) at 197.8–186.5 mbsf, and the base of
upper Zone RP15(SH) at 143.9–134.5 mbsf (lowest common occurrence (LCO) of
Axoprunum? irregularis). We revise the base of Zone RP13(SH) to 313.5–312.7 mbsf (LO of
Zealithapium mitra) (Fig. 2). Further refinement of the age control for Site 277 is possible
through application of several additional bioevents, which help to correlate
the discontinuous stable isotope record of this site to those from other
Southern Ocean sites (Fig. 2). The base of the local New Zealand stage
Kaiatan is defined by the highest occurrence (HO) of Acarinina primitiva (Morgans, 2009)
occurring at 280–273 mbsf based on Jenkins (1975) (39.1 Ma; Raine et al.,
2015). We set the base of the Kaiatan at 276.5 mbsf to allow for the
correlation between isotope records (Fig. 2). The base of the local
Whaingaroan Stage (latest Eocene, 34.6 Ma; Raine et al., 2015) is identified
by the HO of Globigerinatheka index; this event was identified at 189.6 mbsf by Jenkins (1975), but
in the course of preparing foraminifera for stable isotope analysis we have
determined that the event occurs slightly higher at 188.58–187.5 mbsf The
base of nannofossil zone NP17 (HO of Chiasmolithus solitus, 40.4 Ma; Gradstein et al., 2012) is
placed at 312.5–301.5 mbsf (Edwards and Perch-Nielsen, 1975). The LCO of
Chiasmolithus oamaruensis, 37.32 Ma (Gradstein et al., 2012), defines the base of NP18 at
244.5–240.6 mbsf (Edwards and Perch-Nielsen, 1975). The base of NP19-20 is
defined by the LO of Isthmolithus recurvus, 36.97 Ma (Gradstein et al., 2012), at
226.58–225.5 mbsf (Edwards and Perch-Nielsen, 1975). Within NP19-20, the HO of
Cribrocentrum reticulatum is found at 206.5–201.1 mbsf (Edwards and Perch-Nielsen, 1975), estimated
at 36.44 Ma (Raine et al., 2015). The base of NP21-22 (HO of Discoaster saipanensis) is placed at
191.6–190.1 mbsf (Edwards and Perch-Nielsen, 1975) and is dated at 34.44 Ma
(Gradstein et al., 2012). As D. saipanensis is a warm-water taxon, its disappearance is
likely to have occurred earlier at high latitudes. The Eocene–Oligocene
boundary is approximated by the HO of G. index at DSDP Site 277. More precise
location is complicated by incomplete recovery and the highly disturbed
nature of cores 277-19R, -20R, and -21R.
Although the recovery gaps in the Site 277 stable isotope record preclude
detailed correlation, the broad trends and major events such as the MECO
(∼ 40 Ma) and PrOM (∼ 37.3 Ma) can be
identified in the benthic δ18O and δ13C isotope
profiles and compared to the middle Eocene–early Oligocene benthic
isotope stratigraphy from ODP Site 689 (Maud Rise; Diester-Haass and Zahn,
1996) (Fig. 2). The EOT interval is characterised by a large
(∼ 1 ‰) positive shift in benthic oxygen
and carbon isotopes between cores 277-20R and -19R (183.64–171.28 mbsf)
(Shackleton 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).
Site 277 oxygen and carbon isotopes
Site 277 δ18O results show a typical surface-to-deep gradient
with more negative values in bulk and planktic foraminifers compared to
benthic foraminifers (Fig. 3, Supplement Tables 7–10). Foraminiferal
δ13C values also display typical gradients, with more positive
values in bulk and planktic foraminifers compared to benthic foraminifers
(Fig. 3). However, all planktic foraminifera analysed from Site 277 are
characterised by a “frosty” preservation state, indicating some diagenetic
alteration (Sexton et al., 2006). We have therefore focused our
interpretation on benthic foraminifera because their isotopic signatures are
likely less affected by diagenesis.
Several short-lived climatic events are identified in the benthic stable
isotope records at Site 277 (Figs. 2 and 3, Supplement Table 7). The
body of the MECO was not recovered due to a 16 m sampling gap between the
top of Core 277-33R and the base of Core 277-32R, but MECO onset and
recovery is well constrained by a 0.5 ‰ negative shift in
benthic δ18O values at ∼ 313 mbsf (between
samples 277-33R-2, 106–108 cm, and -33R-1, 129–130.5 cm) and a
∼ 0.4 ‰ positive shift 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 (Fig. 2). The MECO is more strongly expressed in the benthic δ18O, but this may relate to the poor recovery of the body of the event
at this site or diagenetic impacts on planktic δ18O values
(Pearson et al., 2001; Sexton et al., 2006). In agreement with other records
(Bohaty and Zachos, 2003; Bohaty et al., 2009), a positive δ13C
shift is observed in conjunction with the onset of the MECO in the benthic
and bulk carbonate records (Fig. 2).
The PrOM event (Scher et al., 2014) is well defined in the δ18O
record from DSDP Site 277 but also spans three significant recovery gaps at
the base of cores 277-26R, -25R and -24R (∼ 244.5 to 225.5 mbsf)
(Fig. 3). The ∼ 0.4 ‰ positive shift in
δ18O 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 mbsf) (Fig. 2). 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 δ18O
exceeds ∼ 0.6 ‰, with the exception of a
narrow interval 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 records show
similar trends to the benthic record in the PrOM interval but lack the
maximum excursion in uppermost Core 277-25R. At the onset of the PrOM 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 is associated with the
benthic δ18O maximum.
Directly above the PrOM event, there is a short-lived ∼ 0.4 ‰ decrease in δ18O values in Core
277-23R (210.74 to 207.41 mbsf), evident in benthic and planktic
foraminifera as well as bulk carbonate, prior to the increase in δ18O that spans the EOT (Fig. 3). Benthic and planktic δ13C also exhibit a small negative excursion at this level. This
interval may be correlated with the late Eocene warming interval reported from
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).
A large positive shift in δ18O occurs at Site 277 between the
base of cores 277-20R and 277-19R, with maximum values in benthic and
planktic δ18O and δ13C occurring in Core 277-19R
(171.28 to 169.65 mbsf). This can be correlated with the large δ18O shift across the EOT documented at many deep-sea sites, which is
characterised by two distinct steps (EOT-1 and Oi-1) in more complete
sections (e.g. Coxall et al., 2005; Katz et al., 2008).
We note that the stable isotope record at Site 277 exhibits high-amplitude
cyclical variation in the range of 0.5 ‰ for benthic
δ18O and slightly more for δ13C (Fig. 3). The
presence of at least 10 cycles within the 6 million years between the MECO
and the EOT is consistent with orbital-scale forcing. Although the record is
too incomplete to establish the frequency of these cycles, their presence in
this expanded Palaeogene section bodes well for future drilling at this
location.
Radiolarian assemblages at DSDP Site 277
In total, 16 families, 56 genera and 98 radiolarian species were identified
at DSDP Site 277 (Supplement Table 1). Radiolarian abundance is
generally low (10–100 specimens g-1) and preservation is moderate throughout
the middle Eocene–lower upper Eocene interval (349.2 to 227.2 mbsf)
(Fig. 4). In the uppermost Eocene and lower Oligocene (226.1–143.9 mbsf)
radiolarians are abundant to very abundant (> 1500 specimens g-1)
and well preserved. Diversity increases during the MECO (313.5–296 mbsf)
and in the upper Eocene (226.10–186.5 mbsf) and drops in the lower
Oligocene (162.2–134.5 mbsf) (Fig. 4). A short-lived drop in radiolarian
abundance (< 500 specimens g-1) and diversity is observed at
210.5–207.5 mbsf during the late Eocene warming event. Diversity closely
parallels trends in abundance and preservation. Simpson evenness is strongly
correlated with diversity but exhibits greater troughs where samples are
sparse (Fig. 4). Spumellarians are dominant in most samples ranging
between ∼ 44 and 96 % (∼ 71 % average). The
main families are the Actinommidae, Litheliidae, Spongodiscidae,
Artostrobiidae, Lychnocaniidae and Lophocyrtiidae (Supplement Table 1).
Three samples from the middle Eocene section of Site 277 (313.5,
312.7, 296 mbsf; cores 277-32R and -33R) that lie within the onset and
recovery of the MECO show improved preservation and a peak in diversity and
mark the first significant occurrence of diatoms (Fig. 4). Amphicraspedum murrayanum and A. prolixum gr. have
isolated occurrences in this interval, while A. prolixum gr. also has trace occurrences
in five samples in the uppermost Eocene to lowermost Oligocene (cores
277-24R to -20R at 217.70, 209, 207.5, 197.82 and
186.50 mbsf). Several species are restricted to the MECO, including Artobotrys titanothericeraos, Sethocyrtis chrysallis, Eusyringium fistuligerum and
Stichopilium cf. bicorne. Lophocyrtis jacchia hapsis, which is a high-latitude variant of L. jacchia jacchia (Sanfilippo and Caulet, 1998) and
endemic to the Southern Ocean, is also common during the MECO and uppermost
Eocene (217.7–206.83 mbsf), but is absent from the remaining middle and
lower upper Eocene. Furthermore, the LOs of several (albeit rare) species
are recorded at this site during the MECO interval (Axoprunum pierinae,
Zealithapium mitra, Periphaena spp., Larcopyle hayesi, L. polyacantha,
Zygocircus buetschli, Siphocampe? amygdala, Eucyrtidium montiparum, Lychnocanium amphitrite,
Clinorhabdus anantomus, Lophocyrtis keraspera, Lophocyrtis dumitricai, Cryptocarpium ornatum and
Lamprocyclas particollis) (Supplement Table 1).
A major change in siliceous assemblages occurs within the PrOM interval
(∼ 226 mbsf; Core 277-25R), coincident with maximum values in
benthic δ18O (Fig. 4). A pronounced increase in radiolarian
abundance (from < 50 to ∼ 4000 specimens g-1),
preservation and diversity occurs at 226.10 mbsf (Sample 277-25R-1, 60 cm).
Diatoms also become abundant at the same level as the increase in
radiolarian abundance. The most abundant nassellarian families are the
Artostrobiidae (∼ 22 %), Lophocyrtiidae (∼ 6 %) and Lychnocaniidae (∼ 2.5 %). Plagiacanthidae account
for ∼ 2 % of the total assemblage. The following taxa have
their LO within the PrOM at Site 277: Lithelius (?) foremanae,
Ceratocyrtis spp., Lithomelissa ehrenbergi, L. gelasinus, L.
sphaerocephalis, Siphocampe nodosaria, Artostrobus
annulatus, Artostrobus cf. pretabulatus, Clathrocyclas
universa, Dictyophimus? aff. archipilium, Lychnocanium
waiareka, Aphetocyrtis rossi and Theocyrtis tuberosa (Supplement Table 1).
Five samples were investigated at Site 277 that lie within the late Eocene
warming event (210.5–207.5 mbsf). During this event, radiolarian abundance
and diversity decrease significantly, as well as diatom abundance (Fig. 4). The radiolarian assemblages of these five samples differ from the other
upper Eocene samples. Lychnocaniidae are more abundant (∼ 12 %), whereas Artostrobiidae are absent. Furthermore, Lophocyrtiidae
decrease (∼ 4 %) and Plagiacanthidae and Larcopyle spp. are very rare
(0.5 and 0.9 %, respectively; Supplement Table 1).
Immediately after the warming event, a second pronounced increase in
radiolarian abundance (from < 200 to 9600 specimens g-1) and
diversity is observed at 206.83 mbsf, together with an increase in diatom
abundance (Fig. 4). In the uppermost Eocene–lowermost Oligocene
interval (206.83–186.5 mbsf), Plagiacanthidae (∼ 5 %),
Artostrobiidae (∼ 7 %) and Lophocyrtiidae (∼ 10 %) increase
again, whereas Lychnocaniidae decrease (∼ 2 %; Supplement Table 1). Theocyrtis tuberosa has a very rare occurrence from the upper
Eocene to lower Oligocene (∼ 226–143.9 mbsf; cores 277-25R to
-16R). 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).
A significant decline in radiolarian abundance and diversity is observed
through the lower Oligocene (186.5 to 134.5 mbsf; cores 277-20R to -15R)
(Fig. 4). Radiolarian abundance declines from 6400 to 750 radiolarians g-1. Many nassellarian taxa decline or disappear, especially
within the Lophocyrtiidae and Plagiacanthidae. Spumellarians increase from
∼ 73 to ∼ 97 % of the total fauna, with
Litheliidae and Actinommidae being the most abundant families (Supplement Table 1).
Rarefaction analysis of Site 277 radiolarian data (Fig. 4) indicates that
counts of at least 300 specimens are required to achieve a reliable measure
of diversity and taxic richness. However, poor preservation in the middle
Eocene and lower upper Eocene intervals (∼ 350 to
∼ 227 mbsf) has resulted in poor recovery of radiolarians, with
9 samples containing < 300 specimens and 9 samples of < 100
specimens. Because these samples span an interval in which significant
changes in diversity and assemblage composition occur, we include metrics
for all samples in Fig. 4 (samples of < 100 specimens, < 300 specimens and > 300 specimens are highlighted) and metrics
for samples with > 100 specimens in Figs. 6 and 7. To
investigate whether the diversity drop between ∼ 292 and
∼ 227 mbsf is a preservational artefact or a real feature of
the assemblage, we also determined range-through taxic richness (Fig. 4).
We have chosen sample sizes of 100 and 300 (both with a subsampling of
1000), which show a similar pattern to the original
observation. The decrease in range-through taxic richness at the top and
bottom of the record is due to edge effects. According to this analysis,
range-through taxic richness is higher than observed in cores 277-32 to -26
(292.2–235.5 mbsf). Chert nodules are present down-core from
∼ 246 mbsf, so the scarcity of taxa in the interval between
∼ 350 and 246 mbsf is likely to be an artefact of diagenesis.
However, the increase in taxic richness in the MECO appears to be supported
by this analysis, at least for the uppermost sample. The analysis also
indicates that there is a distinct increase in diversity related to the PrOM
event around ∼ 226 mbsf, although it is more muted than the
raw data suggest. It is notable that the decrease in diversity evident in
the raw data during the late Eocene warming event is not shown in the
range-through data. In fact, there may be a further increase in taxic
richness within this interval. We conclude that range-through taxic richness
is a helpful tool for determining whether diversity changes are due to diagenesis
or environmental variation, especially when coupled with consideration of
the lithologic changes (e.g. chertification).
Stratigraphy,
Southern Hemisphere radiolarian zones (RP), lithology and core recovery at
DSDP sites 280, 281 and 283. Variation in radiolarian abundance,
diatom / radiolarian (D/R) ratio, Fisher α index and Simpson
evenness for radiolarian assemblages at all sites.
Radiolarian assemblages at other southwest Pacific sites
To establish the significance and nature of radiolarian faunal turnover
associated with the PrOM event regionally, we investigated the upper
Eocene–lower Oligocene intervals of DSDP sites 280, 281 and 283 and ODP
Site 1172.
DSDP Site 280
Four samples were investigated at DSDP Site 280 from cores 280-7R, -6R and -5R
(123.4 to 92.54 mbsf). In previous work, the Eocene–Oligocene
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 lower Oligocene, we place the studied interval
(123.4–92.54 mbsf) in lower Oligocene Zone RP15(SH) (Fig. 5,
Supplement Table 3). This is in agreement with O'Connor (2000), who found
upper Eocene assemblages were restricted to cores 280-10R to -8R (205.5 to
139 mbsf). The absence of the zonal marker Axoprunum? irregularis indicates correlation with lower
RP15(SH) Eucyrtidium spinosum, which according to Funakawa and Nishi (2005) has its HO in the
lower Oligocene, is absent in the Site 280 study interval. However, the HO
of this species is recorded within the upper Eocene interval at Site 277,
suggesting a diachronous HO between the southwest Pacific and the South
Atlantic.
In total, 15 families, 35 genera and 50 radiolarian species were identified
at Site 280. Radiolarians are abundant (1000–2500 specimens g-1) and well
preserved in all samples. Diatoms are also very abundant (D/R ratio
∼ 10) (Fig. 5). Diversity and evenness are stable and high in
all samples. Spumellarians are slightly more abundant than nassellarians
(52–66 % of the assemblage). The most abundant families are Litheliidae
(20–37 %), Plagiacanthidae (14–22 %), Actinommidae (4–12 %),
Spongodiscidae (5–9 %), Eucyrtidiidae (4–8 %) and Lophocyrtiidae
(3–8 %) (Supplement Table 3). Compared to DSDP Site 277, this site has
higher diatom abundance and better overall preservation, which may explain
the higher diversity. More species of the genera 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 Lychnocanium is absent (Supplement Table 3).
DSDP Site 281
Seven samples were investigated from DSDP Site 281 in the interval between
149 and 122.5 mbsf (cores 281-16R to -14R) (Fig. 5). Results from three of
these samples were previously 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 correlate the Site 281 study interval with lower Zone
RP14(SH) (∼ Kaiatan local stage). A hiatus spanning the
uppermost 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.
Radiolarians are abundant (2000–4000 specimens g-1) and well preserved.
Diversity is lower than at Site 280, but evenness is still high and similar
to the other sites (Fig. 5). The D/R ratio is also 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 (10–20 %) the most abundant families. The most common
nassellarians belong to the Plagiacanthidae (1–15 %), Lophocyrtiidae
(3–7 %) and Eucyrtidiidae (1–7 %) (Supplement Table 4). Although
sites 280 and 281 were relatively close to each other (Fig. 1), the
radiolarian assemblages are distinctly different, indicating different
oceanographic conditions. Crouch and Hollis (1996) concluded that Site 281
was shallower and closer to terrigenous influx than Site 280. The
depositional environment 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). Compared to the early upper Eocene
assemblage of Site 277, where radiolarian abundance and diversity is very
low, with several samples containing less than ∼ 100
specimens, Site 281 contains more Spongidiscidae (∼ 20 %),
Plagiacanthiidae (∼ 7 %) and Litheliidae (∼ 20 %), whereas the genus Lychnocanium is absent at Site 281.
DSDP Site 283
Six samples were examined from Site 283 between 192.25 and 87.75 mbsf (cores
283-8R to -5R) (Fig. 5). The lowermost sample at 192.25 mbsf is correlated
with RP13(SH) 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 contain Eucyrtidium nishimurae: at Site 283 it is found in all samples, and 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 upper middle to upper Eocene
and the absence (or very rare) occurrence of foraminifera suggests a deep
oceanic setting close or below the calcite compensation depth (CCD) for Site
283.
Biogeographic
affinities of radiolarian assemblages at DSDP Site 277 and the abundance of
high-latitude taxa/families. MECO: Middle Eocene Climatic Optimum; PrOM:
Priabonian Oxygen Isotope Maximum; EOT: Eocene–Oligocene transition.
A total of 16 families, 50 genera and 81 radiolarian species were recorded
at Site 283. Radiolarians are very abundant
(4700–21 150 radiolarians g-1) – with the highest abundance in cores 283-6R and -5R – well preserved, and
diverse (59–77 taxa per sample, Fisher α index of 10–13, evenness
of 0.75–0.89). Diatoms are present in low abundance with D/R ratios
< 1 (Fig. 5). Spumellarians account for 59–87 % of the
assemblage, with the Litheliidae (23–38 %), Actinommidae (5–19 %) and
Spongodiscidae (2–8 %) the most abundant families. The Trissocyclidae
(2–11 %), Eucyrtidiidae (2–11 %), Lophocyrtiidae (3–8 %) and
Plagiacanthidae (2–8 %) are the most common nassellarian families
(Supplement Table 5). Theocyrtis tuberosa is very abundant in the uppermost sample. The acme
of this taxon might be correlated with its rare occurrence at Site 277 in the
upper Eocene. Several taxa appear earlier at Site 283 than at Site 277.
These include the following taxa that occur in the upper middle Eocene (e.g.
Axoprunum bispiculum, Amphicentria sp. 1 sensu Suzuki,
Ceratocyrtis spp., Lithomelissa 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, Lychnocanium waiareka,
Aphetocyrtis gnomabax) or lower upper Eocene
(Spirocyrtis greeni, Eurystomoskevos cauleti,
Lophocyrtis jacchia hapsis, Lamprocyclas particollis) at Site 283.
ODP Site 1172
Forty-one samples were analysed from ODP Site 1172 spanning the middle
Eocene–lower Oligocene interval, including four samples from Core
1172D-2R (356.875–355.675 mbsf) and 37 from cores 1172A-48X to
-39X (445.01–354.625 mbsf). The faunal assemblages of ODP Site 1172 were
described by Suzuki et al. (2009), who did not correlate them to RP zones.
Many taxa used to define Southern Hemisphere RP zones at Site 277 are absent
at Site 1172 or have diachronous ranges. Eucyrtidium spinosum, the marker for Zone RP14(SH), has
its LO at 373.75–371.21 mbsf, but Lithomelissa tricornis and Pseudodictyophimus gracilipes are absent. Eucyrtidium antiquum has a single LO at
365.21 mbsf, but is absent in the early Oligocene. E. nishimurae is present within the
middle and upper Eocene. Axoprunum irregularis is very abundant in the lower Oligocene interval
at this site (356.875–354.625 mbsf), which we correlate to the upper
RP15(SH) zone.
Spumellarians dominate the Site 1172 assemblages throughout the middle
Eocene to lower Oligocene (∼ 82 %). The Litheliidae are the
most abundant family, comprising about 20 % on average in the middle
Eocene, 35 % in the upper Eocene, and 25 % in the lower Oligocene.
Plagiacanthidae (0.5–2.5 %), Eucyrtidiidae (0.5–3 %), Lophocyrtiidae
(1.5–8 %) and Lychnocaniidae (0.5–2.7 %) account for most of the
nassellarians. Fisher α diversity and Simpson evenness are very high
throughout the succession, ranging between ∼ 10 and 20 and between
0.82 and 0.96, respectively. Similar to Site 277, diversity and evenness
decrease in the lower Oligocene (Supplement Table 6).
Eocene sediments at Site 1172 consist of silty claystone with abundant
diatoms. This sequence is overlain by a transitional unit in the uppermost
Eocene consisting of glauconitic siltstones, which indicate increased
bottom-water current activity in the uppermost Eocene (Kennett and Exon,
2004; Stickley et al., 2004). There is a sharp transition in the lowermost
Oligocene to a pelagic carbonate sequence consisting of nannofossil chalk
(Exon 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 indicate 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).
Variation in
faunal affinities for radiolarian assemblages and Fisher α diversity
at all sites. Dashed black lines indicate correlation between sites. The
location of the MECO at Site 1172 is taken from Bijl et al. (2010).
Trends in biogeographic affinities
The radiolarian assemblages at our five sites include 92 species or species
groups that can be assigned to one of three biogeographic categories:
high latitude (50 taxa), cosmopolitan (38 taxa), and low latitude (4 taxa)
(Table 1, Supplement Table 12). Biogeographic affinities remain poorly
known for the remaining 39 taxa encountered at DSDP sites 277, 280, 281 and
283, and for ∼ 100 taxa at Site 1172 reported by Suzuki et al. (2009).
Within the high-latitude group, six taxa are bipolar (Artostrobus annulatus, Axoprunum bispiculum,
Ceratocyrtis spp., Cycladophora cosma cosma, Pseudodictyophimus gracilipes
gr. and Spongopyle osculosa), whereas 45 taxa are inferred to be endemic to the
Southern Ocean. Almost all species in the Litheliidae, Lophocyrtiidae and
Plagiacanthidae are high latitude. The biogeographic affinity of Lithelius minor gr. is
considered to be cosmopolitan, but because this group is very abundant in
some assemblages, we separate it out in Figs. 6 and 7. For Site 277, we
also differentiate key high-latitude taxa within the three families noted
above, namely Larcopyle spp., Lophocyrtis longiventer and Lithomelissa spp., and the actinommid Axoprunum irregularis (Fig. 6).
At Site 277, taxa with high-latitude affinities are present from the base of
the study section in the middle Eocene (Fig. 6). The MECO is characterised
by the presence of high-latitude taxa of ∼ 23 % (Larcopyle spp.,
Lophocyrtis jacchia hapsis, L. longiventer), but also the appearance of low-latitude species Amphicraspedum murrayanum and A. prolixum gr. (up to
∼ 10 %). Lophocyrtis jacchia hapsis is considered to be a high-latitude variant of L. jacchia jacchia
and has a short stratigraphic range in the middle to late Eocene in the
Southern Ocean (Sanfilippo and Caulet, 1998). In our study this taxon has a
common appearance only during the MECO and in the upper Eocene (Fig. 6).
In the middle of the PrOM event (∼ 225 mbsf), diversity and
high-latitude taxa increase (average of 28 %) in conjunction with the
appearance of Lithomelissa spp. and other high-latitude Lophocyrtiidae.
Summary of species for which biogeographic affinities have been
established and their presence (x) at sites 277, 280, 281, 283, and 1172.
H: high latitude (> 45∘ N/S); L: low latitude
(< 25∘ N/S); and C: cosmopolitan. Location of photographic
images on plates for selected species.
Taxa
Biogeogr.
Site 277
Site 280
Site 281
Site 283
ODP1172
Plate
affinity
Amphicentria sp. 1 sensu Suzuki
H
x
x
x
x
Pl. 2, Fig. 1
Amphicraspedum murrayanum Haeckel
L
x
Pl. 1, Fig. 14
Amphicraspedum prolixum Sanfilippo and Riedel gr.
L
x
x
Pl. 1, Figs. 15–17
Amphisphaera coronata (Ehrenberg) gr.
C
x
x
x
Pl. 1, Fig. 2
Amphisphaera spinulosa (Ehrenberg)
C
x
x
Pl. 1, Fig. 5
Amphymenium splendiarmatum Clark and Campbell
C
x
x
x
x
Pl. 1, Figs. 18, 19
Antarctissa cylindrica Petrushevskaya
H
x
Antarctissa robusta Petrushevskaya
H
x
Aphetocyrtis bianulus (O'Connor)
H
x
x
x
Pl. 5, Fig. 1
Aphetocyrtis gnomabax Sanfilippo and Caulet
H
x
x
x
x
Pl. 5, Figs. 2–7
Aphetocyrtis rossi Sanfilippo and Caulet
H
x
x
x
Pl. 5, Figs. 8–11
Artobotrys auriculaleporis (Clark and Campbell)
C
x
x
Artostrobus annulatus (Bailey)
H
x
x
Artostrobus cf. pretabulatus Petrushevskaya
H
x
Pl. 3, Fig. 13
Aspis sp. A sensu Hollis
H
x
x
x
Pl. 3, Figs. 14–16
Axoprunum bispiculum (Popofsky)
H
x
x
Axoprunum pierinae (Clark and Campbell) gr.
C
x
x
x
x
x
Pl. 1, Figs. 10, 11
Axoprunum? irregularis Takemura
H
x
x
Pl. 1, Fig. 12
Ceratocyrtis spp.
H
x
x
x
x
Pl. 2, Figs. 3–5
Cinclopyramis circumtexta (Haeckel)
C
x
x
x
x
x
Clathrocyclas universa Clark and Campbell
C
x
x
x
x
Clinorhabdus anantomus Sanfilippo and Caulet
H
x
x
x
Pl. 5, Figs. 12, 13
Clinorhabdus robusta (Abelmann)
H
x
Cornutella profunda Ehrenberg
C
x
x
x
x
x
Cryptocarpium bussonii (Carnevale) gr.
C
x
x
x
x
x
Pl. 5, Figs. 25a, b, 26a, b
Cryptocarpium ornatum (Ehrenberg)
C
x
x
Cycladophora cosma cosma Lombari and Lazarus
H
x
Pl. 3, Fig. 17
Cycladophora humerus (Petrushevskaya)
H
x
x
x
Pl. 3, Fig. 18
Cycladophora spp.
H
x
x
x
Cyrtolagena laguncula Haeckel
C
x
x
Dictyophimus pocillum Ehrenberg
C
x
Dictyophimus? aff. archipilium Petrushevskaya
H
x
x
x
Pl. 4, Fig. 3a, b–8
Dictyophimus? archipilium Petrushevskaya
H
x
x
x
Pl. 4, Figs. 1a, b, 2
Eucyrtidium antiquum Caulet
H
x
x
x
Pl. 3, Fig. 19
Eucyrtidium mariae Caulet
H
x
Eucyrtidium nishimurae Takemura and Ling
H
x
x
x
Pl. 3, Fig. 20a, b
Eucyrtidium spinosum Takemura
H
x
x
x
x
Pl. 3, Fig. 21
Eucyrtidium montiparum Ehrenberg
C
x
x
Pl. 3, Fig. 22
Eurystomoskevos cauleti O'Connor
H
x
x
x
x
Pl. 3, Fig. 23a, b
Eurystomoskevos petrushevskaae Caulet
H
x
x
x
x
x
Pl. 3, Fig. 24
Eusyringium fistuligerum (Ehrenberg)
C
x
x
Pl. 3, Fig. 25
Eusyringium lagena (Ehrenberg)
C
x
Glycobotrys nasuta (Ehrenberg) gr.
C
x
x
x
x
x
Pl. 3, Figs. 5–7
Lamprocyclas particollis O'Connor
H
x
x
x
x
Pl. 5, Fig. 27
Larcopyle cf. pylomaticus (Riedel)
H
x
x
Pl. 1, Fig. 25a, b
Larcopyle frakesi (Chen)
H
x
Pl. 1, Fig. 20
Larcopyle hayesi (Chen)
H
x
x
x
x
Pl. 1, Fig. 21
Larcopyle labyrinthusa Lazarus
H
x
Pl. 1, Fig. 22
Larcopyle polyacantha (Campbell and Clark) gr.
H
x
x
x
x
Pl. 1, Figs. 23, 24
Larcopyle spp.
H
x
x
x
Lithelius minor Jörgensen gr.
C
x
x
x
x
x
Pl. 1, Figs. 26–28
Lithomelissa challengerae Chen
H
x
Pl. 2, Figs. 6–8
Lithomelissa ehrenbergi Bütschli
H
x
x
x
x
x
Pl. 2, Figs. 10, 11
Lithomelissa gelasinus O'Connor
H
x
x
x
x
Pl. 2, Figs. 12, 13
Lithomelissa robusta Chen
H
x
x
Pl. 2, Fig. 16
Lithomelissa sphaerocephalis Chen
H
x
x
x
x
Pl. 2, Fig. 17
Lithomelissa spp.
H
x
x
x
x
Lithomelissa tricornis
H
x
x
x
x
Pl. 2, Fig. 18
Lithomelissa? sakaii O'Connor
H
x
Pl. 2, Fig. 19
Lophocyrtis (Apoplanius) aspera (Ehrenberg)
H
x
x
x
Pl. 5, Figs. 14a, b–16
Lophocyrtis (Apoplanius) keraspera Sanfilippo and Caulet
H
x
x
x
Pl. 5, Figs. 17–19
Lophocyrtis (Lophocyrtis) jacchia hapsis Sanfilippo and Caulet
H
x
x
Pl. 5, Figs. 20–22
Lophocyrtis (Paralampterium) dumitricai Sanfilippo
C
x
x
Continued.
Taxa
Biogeogr.
Site 277
Site 280
Site 281
Site 283
ODP1172
Plate
affinity
Lophocyrtis (Paralampterium) longiventer (Chen)
H
x
x
x
x
x
Pl. 5, Figs. 23, 24
Lophocyrtis spp.
H
x
Lophophaena capito Ehrenberg
C
x
x
x
Lychnocanium amphitrite (Foreman)
C
x
x
x
Pl. 4, Figs. 11a, b, c, 12
Lychnocanium babylonis (Clark and Campbell)
C
x
x
Pl. 4, Figs. 13a, b, 14
Lychnocanium bellum Clark and Campbell
C
x
x
x
Pl. 4, Figs. 15, 16
Periphaena decora Ehrenberg
C
x
x
x
x
x
Periphaena heliastericus (Clark and Campbell)
C
x
x
x
x
x
Plectodiscus circularis (Clark and Campbell)
C
x
x
x
x
x
Pseudodictyophimus galeatus Caulet
H
x
Pl. 2, Fig. 20
Pseudodictyophimus gracilipes (Bailey) gr.
H
x
x
x
x
Pl. 2, Figs. 21–23
Pseudodictyophimus spp.
H
x
Pl. 2, Figs. 24–27
Sethocyrtis chrysallis Sanfilippo and Blome
C
x
Pl. 3, Fig. 26a, b
Siphocampe nodosaria (Haeckel)
C
x
x
x
x
Siphocampe quadrata (Petrushevskaya and Kozlova)
C
x
x
x
x
Siphocampe? amygdala (Shilov)
C
x
x
Pl. 3, Figs. 11, 12
Sphaeropyle tetrapila (Hays)
H
x
Pl. 1, Fig. 29
Spirocyrtis joides (Petrushevskaya)
C
x
x
x
x
Spongodiscus cruciferus (Clark and Campbell)
C
x
x
x
Spongodiscus festivus (Clark and Campbell)
C
x
x
Spongopyle osculosa Dreyer
H
x
x
x
x
x
Pl. 1, Fig. 13
Spongurus bilobatus Clark and Campbell
C
x
x
x
x
Stylosphaera minor Clark and Campbell gr.
C
x
x
x
x
Pl. 1, Fig. 7
Theocampe amphora (Haeckel)
C
x
Theocampe urceolus (Haeckel)
C
x
x
x
x
Theocyrtis tuberosa Riedel
L
x
x
Pl. 5, Fig. 30
Thyrsocyrtis pinguisicoides O'Connor
L
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
Average of total % of high-latitude species, groups, genera and
high-latitude members of families for five time slices: Middle Eocene
Climatic Optimum (MECO, ∼ 40 Ma), early late Eocene/PrOM
(∼ 38–37 Ma), late Eocene warming event (∼ 36 Ma), latest Eocene–earliest Oligocene (∼ 35–32 Ma) and early
Oligocene (∼ 30 Ma).
Site 277
Site 280
Site 281
Site 283
Site 1172
40 Ma
38–37 Ma
36 Ma
35–32 Ma
30 Ma
earliest
38–37 Ma
38–37 Ma
36 Ma
40 Ma
late
30 Ma
Olig.
Eoc.
% high-latitude species
23.2
28.9
13.7
39.0
100.0
62.6
61.2
28.0
25.6
7.8
26.8
66.1
Larcopyle spp. %
6.9
2.9
2.5
6.2
–
18.4
26.5
3.0
1.8
–
–
7.0
Lithomelissa spp. %
0.1
1.8
0.1
5.9
–
16.4
11.8
4.1
4.8
0.4
0.7
0.8
High-lat. Lophocyrtiidae %
14.9
20.8
8.0
16.7
3.3
10.4
14.2
8.5
6.9
4.1
19.3
5.7
High-lat. Eucyrtidiidae %
–
0.4
0.5
1.8
8.8
6.3
7.4
9.1
3.3
5.3
–
Other high-lat. Plagiacanthidae %
–
0.2
–
1.4
6.5
1.4
1.8
1.5
–
–
0.2
Other high-lat. species %
1.3
2.8
2.7
7.0
96.7
2.0
1.1
3.1
1.5
0.1
1.6
52.5
% cosmopolitan species
72.6
71.1
80.9
59.9
–
37.3
38.8
71.8
65.2
92.2
73.2
33.9
% low-latitude species
4.2
0.1
5.4
1.0
–
0.1
–
0.1
9.2
–
–
–
During the late Eocene warming event, high-latitude taxa decrease to
∼ 13 % at Site 277 and only rare occurrences of
Lithomelissa spp. and high-latitude Lophocyrtis spp. are noted (Fig. 6, Table 2). Late Eocene
warming, however, coincides with the abundant occurrence of the low-latitude
taxon Thyrsocyrtis pinguisicoides (up to 20 %) and the trace occurrence of A. prolixum. Cosmopolitan taxa are
dominated by Lychnocanium spp., but general diversity also decreases within the warming
event (Supplement Table 1). After this event, high-latitude taxa increase
to up to ∼ 50 % in the uppermost Eocene and lowermost
Oligocene with the reappearance of all high-latitude taxa and an overall
diversification (Fig. 6, Table 2). During the lower Oligocene, diversity
declines and especially the Plagiacanthidae and Lophocyrtiidae decrease in
abundance. Lithelius minor gr. is dominant until ∼ 144 mbsf Above 144 mbsf,
Lithelius minor gr. decreases in abundance and high-latitude actinommids Axoprunum bispiculum and A. irregularis make up
∼ 97 % of the high-latitude assemblage (Fig. 6,
Supplement Table 1).
At Site 1172, high-latitude taxa are present in the middle and upper Eocene,
although varying between ∼ 3 and 40 % of the assemblage for
which biogeographic affinities have been established (Fig. 7). The MECO
interval at Site 1172 (Core 1172D-45X; Bijl et al., 2010) corresponds to a
minimum in high-latitude taxa, which is part of a longer minimum in
high-latitude taxa from 430 to 410 mbsf The most profound increase in
high-latitude taxa at Site 1172 occurs in the lower Oligocene
(∼ 50–80 %) with an increase in abundance of A. irregularis to dominant
levels, similar to Site 277. None of the low-latitude taxa found at the
other sites are present at Site 1172.
At Site 283, high-latitude taxa are present from the middle Eocene and range
between ∼ 12 and 35 %. Lithelius minor gr. is very abundant and varies
between ∼ 20 and 40 % in all samples (Fig. 7). We tentatively
correlate the relatively high abundance in the low-latitude species
Theocyrtis tuberosa (∼ 9 %) in the upper part of the studied section
(87.75 mbsf) to the late Eocene warming event at Site 277. Sites 280 and 281 both
have a higher proportion of high-latitude taxa in the lower upper Eocene to
lower Oligocene than all other sites. High-latitude taxa range between
∼ 40 and 73 % in the lower upper Eocene at Site 281 and
between ∼ 50 and 73 % in the lower Oligocene at Site 280 (Fig. 7). Several taxa that are present in the lower
Oligocene at Site 280 are absent at Site 277, including Lithomelissa challengerae, Larcopyle frakesi, Lithomelissa sakaii, and Antarctissa spp. The
abundance of Lithelius minor gr. is also high at sites 280 and 281, ranging between
∼ 20 and 40 %.
Conclusions
Middle Eocene–early Oligocene radiolarian assemblages from DSDP sites
277, 280, 281, 283 and ODP Site 1172 were examined to investigate the
relative influence of low- and high-latitude water masses in the southern
southwest Pacific Ocean as global climate cooled and ice sheets expanded in
Antarctica. 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),
Eocene radiolarian assemblages in this region lack significant numbers of
warm-water taxa. Furthermore, we show that many high-latitude and taxa
endemic to the Antarctic are already present in the middle Eocene. The MECO
has been identified at Site 277 from foraminiferal δ18O records
and is associated with a short-lived incursion of two taxa with low-latitude
affinities, Amphicraspedum prolixum gr. and A. murrayanum. The absence of definitive tropical taxa suggests
warm temperate rather than tropical conditions during this short-lived event.
Radiolarians are very abundant and well preserved at high-latitude sites
281, 283 and 1172 during the early late Eocene and at Site 280 during the
early Oligocene. For taxa with identified biogeographic affinities, those
with high-latitude affinities comprise ∼ 60 % at sites 280
and 281 and ∼ 30 % at sites 283 and 1172. During the early
late Eocene (∼ 37 Ma), a positive shift in foraminiferal
δ18O values at Site 277 marks the onset of the PrOM event. A
pronounced increase in diversity, abundance and preservation of radiolarians
occurs in conjunction with this event at Site 277 in addition to a marked
increase in diatom abundance. Many high-latitude taxa that are very abundant
at sites 281 and 283 in the late middle Eocene and early late Eocene become
abundant or have their LOs at Site 277 at ∼ 37 Ma, including
Lithelius minor gr., Larcopyle hayesi, L. polyacantha,
Spongopyle osculosa, Lithomelissa sphaerocephalis, L. gelasinus,
L. ehrenbergi, Ceratocyrtis spp., Dictyophimus aff. archipilium,
Lamprocyclas particollis, and Antarctic morphotypes of
Aphetocyrtis gnomabax, A. rossi, Lophocyrtis aspera, L. keraspera
and L. longiventer. This northward extension of high-latitude taxa onto the Campbell
Plateau appears to have been triggered by cooling during the PrOM event,
which may have been associated with a short-lived development of an
Antarctic ice sheet.
A late Eocene warming event at ∼ 36 Ma is accompanied by a
decrease in radiolarian diversity, high-latitude taxa and low diatom
abundance at Site 277. Two low-latitude taxa, Theocyrtis tuberosa and Thyrsocyrtis pinguisicoides, make short-lived
incursions into the southwest Pacific at this time. After this event,
radiolarian diversity increases again with the reappearance of high-latitude
taxa and abundant diatoms at Site 277. Through the EOT, radiolarians
decrease in abundance and diversity at Site 277. Most nassellarian taxa
within the Plagiacanthidae and Lophocyrtiidae decline, whereas Lithelius minor gr. and
Actinommidae become dominant. Together with the scarcity of diatoms, we
infer that conditions over the Campbell Plateau became nutrient-depleted as
a consequence of the development of the ACC. The establishment of the ACC at
around 30 Ma is inferred to have caused widespread non-deposition in the
southwest Pacific and restricted the northward flow of Ross Gyre.