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

The long-term cooling trend from middle to late Eocene was punctuated by several large-scale climate perturbations that culminated in a shift to “icehouse” climates at the Eocene–Oligocene transition. 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 and Ocean Drilling Project (ODP) Site 1172 to identify significant oceanographic changes in the southwest Pacific through this climate transition (∼ 40– 30 Ma). We find that the Middle Eocene Climatic Optimum at ∼ 40 Ma, which is truncated but identified by a negative shift in foraminiferal δO values at Site 277, is associated with a small increase in radiolarian taxa with low-latitude affinities (5 % of total fauna). In the early late Eocene at ∼ 37 Ma, a positive oxygen isotope shift at Site 277 is correlated with the Priabonian Oxygen Isotope Maximum (PrOM). Radiolarian abundance, diversity, and preservation increase within this cooling event at Site 277 at the same time as diatom abundance. A negative δO excursion above the PrOM is correlated with a late Eocene warming event (∼ 36.4 Ma). Radiolarian abundance and diversity decline within this event and taxa with low-latitude affinities reappear. Apart from this short-lived warming event, the PrOM and latest Eocene radiolarian assemblages are characterised by abundant high-latitude taxa. High-latitude taxa are also abundant during the late Eocene and early Oligocene (∼ 38–30 Ma) at DSDP sites 280, 281, 283 and 1172 and are associated with very high diatom abundance. We therefore infer a northward expansion of high-latitude radiolarian taxa onto the Campbell Plateau in the latest Eocene. In the early Oligocene there is an overall decrease in radiolarian abundance and diversity at Site 277, and diatoms are scarce. These data indicate that, once the Antarctic Circumpolar Current was established in the early Oligocene (∼ 30 Ma), a frontal system similar to present day developed, with nutrient-depleted Subantarctic waters bathing the area around DSDP Site 277, resulting in a more restricted siliceous plankton assemblage.

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 can

Introduction
The climate history of the early Paleogene has been established by geochemical proxies for temperature, loosely linked to paleontological data. The primary proxy record, 5 stable oxygen isotope (δ 18 O) values of benthic foraminifera, shows a trend from an early Cenozoic greenhouse climate to an icehouse climate with the major shift in benthic δ 18 O values of ∼ +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 (EECO) centred around 10 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 pCO 2 (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) suggest the Southwest Pacific sea surface temperatures were tropical during the MECO 15 (28 • C) and continued to be warm throughout the late Eocene (24-26 • C), cooling only slightly across the Eocene-Oligocene transition ( 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 reconstructions of Southern Ocean circulation developed from high-latitude drill cores Introduction 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 with 5 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 fauna 10 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 Tasman Gateway or changes in carbon cycling (Stickley et al., 2004;Lazarus et al., 2008 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 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 and 5 discuss implications for the oceanographic history of the SW 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 10 277, 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 intervals) between 301.5 15 and 368.0 mbsf. Below 10 mbsf, a Paleogene sequence spanning from the middle Paleocene 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 the 20 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 picture of radiolarian assemblage change and biogeography during the middle Eocene to early Oligocene. DSDP Site 280 comprises two holes (48 • 57.44 S; 147 • 14.08 E) and 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 re-examined and re-counted as part of this study. For strewn slide preparation, 1-10 g of sample material was broken into ∼ 5 mmdiameter 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 sodium Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 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 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 5 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 radiolarian 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 the 15 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 20 of the abundance of individual species, but does result in overall diversity being underestimated.
Radiolarian abundance was calculated using the following equation: With X R being the total number of radiolarians per slide, X S the number of slides made 25 of a known portion X P of the dried material, A Sed is 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 2983 Introduction were calculated using the PAST software (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. 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, 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 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. We also determined diversity, evenness 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. The biogeographic affinities of the radiolarian species, subspecies and informally de- 15 fined morphotypes encountered in our study were assigned using information from previous 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 andNishi, 2005, 2008;Fu-20 nakawa et al., 2006;Kamikuri et al., 2013) (Table 1). We quantified trends in biogeographic affinity to determine how the relative influences of high-and low-latitude water masses varied through the middle Eocene to early Oligocene. and major events such as the MECO (∼ 40 Ma) and PrOM event (∼ 37.3 Ma) can be identified in the benthic δ 18 O and δ 13 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 (Shackleton 5 and Kennett, 1975;Keigwin, 1980), which is slightly lower than the full magnitude of the benthic δ 18 O 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). 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 16 m 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 δ 18  ate records, although the δ 13 C record is also compromised by the missing core of the event.
The PrOM event (Scher et al., 2014) is well-defined in the δ 18 O 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)

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 preservation 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 abundant 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 the 10 Actinommiidae, Litheliidae, Artostrobiidae, Spongodiscidae, Lophocyrtiidae and Lychnocaniidae (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 diatoms 15 (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 jacchia 20 (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 ( A major change in siliceous assemblages occurs within the PrOM interval (∼ 226 mbsf; Core 25R), coincident with maximum values in benthic δ 18 O (Fig. 4). A pronounced increase in radiolarian abundance (from < 50 to ∼ 4000 radiolarians g −1 ),  (1-11 %) and Plagiacanthidae (1-6 %). Theocyrtis tuberosa has a very rare occurrence 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 radiolarian 20 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 the 25 early Oligocene (186.5 to 134.5 mbsf; Cores 20R to 15R) (Fig. 4). The fauna is dominated by spumellarians that increase from ∼ 73 to ∼ 97 %, with Litheliidae and Actinommidae being the most abundant families (Supplement Table Site

DSDP Site 283
Six samples were examined from Site 283 between 192.25 and 87.75 mbsf (Cores 5 8R to 5R) (Fig. 5) 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 the Spongodiscidae (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  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 (Exon  10 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 latitudes 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 elements 20 within the three families noted above, namely Larcopyle spp., Lophocyrtis longiventer and Lithomelissa spp. (Fig. 6).

Trends in biogeographic affinities
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.  (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 actinommid 5 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, comprising 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 increase 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, 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. 20 The radiolarian assemblages documented at Site 277 and 1172 within the MECO interval 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 sites Introduction  (Bijl et al., 2010) and ∼ 27 • C for the late Eocene at Site 277 (Liu et al., 2009).

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 283 5 appear later at Site 277 (∼ 37-36 Ma), during the PrOM event. This appearance coincides with an increase in radiolarian abundance, diversity and preservation. A comparison 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 higherlatitude sites. The ecological and biogeographic affinity of Lithelius minor gr. is not 10 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 water masses across the southern Campbell Plateau during the PrOM event.

Diagenesis
One possibility is that the pronounced increase in radiolarian abundance and diversity 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 246 mbsf (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 represents Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | a real increase in radiolarian and diatom abundance and not an artefact of preservation.

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 expected 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 the 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 2997 Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | a proto-Antarctic Polar Front, respectively, however, the first event is not identical with the PrOM event.

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 influx 5 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 during the middle Eocene, which suggests a higher productivity region, perhaps a consequence 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 increase 20 in abundance at Site 277. High-latitude taxa appear (Lithomelissa spp., Larcopyle spp., Lophocyrtiidae, Table 2), together with diatoms, resulting from cooling and eutrophication 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 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 and 5 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.

10
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 Antarctic assemblages in the Southwest Pacific. In contrast to temperature reconstructions based on geochemical proxies that indicate subtropical-tropical temperatures at highlatitudes during the middle and late Eocene (Liu et al., 2009;Bijl et al., 2010), Eocene 15 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. 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. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | chos, J. C.: Early Paleogene temperature history of the Southwest Pacific Ocean: reconciling proxies and models, Earth Planet. Sc. Lett., 349, 53-66, doi:10.1016Lett., 349, 53-66, doi:10. /j.epsl.2012Lett., 349, 53-66, doi:10. .06.024, 2012 Hornibrook, N. de B., Brazier, R. C., and Strong, C. P.: Manual of New Zealand Permian to Pleistocene foraminiferal biostratigraphy, Paleontological bulletin/New Zealand Geological Survey,   Figure 3. DSDP Site 277 oxygen and carbon stable isotope records and position of studied radiolarian samples within MECO interval (red stars) and radiolarian-rich late Eocene-Oligocene interval (blue stars).