climatic optimum
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Malakofauna navrhované přírodní rezervace Houžetín v CHKO České středohoří [Mollusc fauna of the proposed Houžetín Nature Reserve in the České středohoří Protected Landscape Area]
Although the České středohoří PLA is one of the most malacologically explored areas in Central Europe, ongoing research shows that there are still previously unexplored places with rich malacofauna. The Houžetín Hill near the village of Skalice hosts extremely rich flora and its research has also shown relatively rich mollusc fauna (45 species). Unexpectedly, many strictly forest species were found there in 2019, such as Discus perspectivus, Isognomostoma isognomostomos, Aegopinella nitidula and many others. It is unusual for this part of the PLA and it proves that the forest environment has been maintained there continuously for a long time, probably since the forest climatic optimum of the Holocene.
Abstract. As atmospheric carbon dioxide (CO2) and temperatures increase with modern climate change, ancient hothouse periods become a focal point for understanding ecosystem function under similar conditions. The early Eocene exhibited high temperatures, high CO2 levels, and similar tectonic plate configuration as today, so it has been invoked as an analog to modern climate change. During the early Eocene, the greater Green River Basin (GGRB) of southwestern Wyoming was covered by an ancient hypersaline lake (Lake Gosiute; Green River Formation) and associated fluvial and floodplain systems (Wasatch and Bridger formations). The volcaniclastic Bridger Formation was deposited by an inland delta that drained from the northwest into freshwater Lake Gosiute and is known for its vast paleontological assemblages. Using this well-preserved basin deposited during a period of tectonic and paleoclimatic interest, we employ multiple proxies to study trends in provenance, parent material, weathering, and climate throughout 1 million years. The Blue Rim escarpment exposes approximately 100 m of the lower Bridger Formation, which includes plant and mammal fossils, solitary paleosol profiles, and organic remains suitable for geochemical analyses, as well as ash beds and volcaniclastic sandstone beds suitable for radioisotopic dating. New 40Ar / 39Ar ages from the middle and top of the Blue Rim escarpment constrain the age of its strata to ∼ 49.5–48.5 Myr ago during the “falling limb” of the early Eocene Climatic Optimum. We used several geochemical tools to study provenance and parent material in both the paleosols and the associated sediments and found no change in sediment input source despite significant variation in sedimentary facies and organic carbon burial. We also reconstructed environmental conditions, including temperature, precipitation (both from paleosols), and the isotopic composition of atmospheric CO2 from plants found in the floral assemblages. Results from paleosol-based reconstructions were compared to semi-co-temporal reconstructions made using leaf physiognomic techniques and marine proxies. The paleosol-based reconstructions (near the base of the section) of precipitation (608–1167 mm yr−1) and temperature (10.4 to 12.0 ∘C) were within error of, although lower than, those based on floral assemblages, which were stratigraphically higher in the section and represented a highly preserved event later in time. Geochemistry and detrital feldspar geochronology indicate a consistent provenance for Blue Rim sediments, sourcing predominantly from the Idaho paleoriver, which drained the active Challis volcanic field. Thus, because there was neither significant climatic change nor significant provenance change, variation in sedimentary facies and organic carbon burial likely reflected localized geomorphic controls and the relative height of the water table. The ecosystem can be characterized as a wet, subtropical-like forest (i.e., paratropical) throughout the interval based upon the floral humidity province and Holdridge life zone schemes. Given the mid-paleolatitude position of the Blue Rim escarpment, those results are consistent with marine proxies that indicate that globally warm climatic conditions continued beyond the peak warm conditions of the early Eocene Climatic Optimum. The reconstructed atmospheric δ13C value (−5.3 ‰ to −5.8 ‰) closely matches the independently reconstructed value from marine microfossils (−5.4 ‰), which provides confidence in this reconstruction. Likewise, the isotopic composition reconstructed matches the mantle most closely (−5.4 ‰), agreeing with other postulations that warming was maintained by volcanic outgassing rather than a much more isotopically depleted source, such as methane hydrates.
The Middle Eocene Climatic Optimum (MECO; ~40 Ma), which interrupted for ~500–600 kyr the long-term cooling trend culminating at the Eocene/Oligocene boundary, still requires a comprehensive understanding of the biotic resilience. Here we present a high-resolution integrated foraminiferal and calcareous nannofossil study across the MECO from the expanded and continuous Tethyan Baskil section (eastern Turkey) that offers a complete magneto-biostratigraphic and geochemical framework. The five MECO phases identified reveal a transition from oligotrophic (pre-MECO) to eu-mesotrophic conditions, possibly related to accelerated hydrological cycle, during the initial MECO and MECO δ13C negative excursion phases. The MECO WARMING PEAK phase, marking the highest carbonate dissolution interval, records the most striking biotic changes, such as peak in warm and eutrophic nannofossils, virtual disappearance of the oligotrophic planktic foraminiferal large Acarinina and Morozovelloides, and peak in eutrophic deep dwellers Subbotina. Benthic foraminifera suggest in this phase an improvement in the quality of organic matter to the seafloor. The post-MECO phase shows only a partial recovery of the pre-event conditions. Large Acarinina and Morozovelloides did not recover their abundance, possibly due to cooler conditions in this phase. Our reconstruction reveals how paleoenvironment and marine biota from the studied Neo-Tethyan setting reacted to the MECO perturbations.
<p>This thesis investigates the effect of climatic and oceanographic changes on the distribution of fossil radiolarian assemblages from the early Eocene to early Oligocene (~56–30 Ma) in the Southwest Pacific. Radiolarian assemblages have been analysed from a series of archived cores collected by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP). The selected cores form a latitudinal transect designed to investigate the ecological change associated with the transition from the warm ‘greenhouse’ climate of the Eocene into the cooler Oligocene, when continental-scale glaciation is believed to have intiated in Antarctica. High-latitude sites were sampled on the Campbell Plateau (DSDP Site 277), Tasman Rise (DSDP sites 280 and 281) and the Tasman Sea (DSDP Site 283 and ODP Site 1172), while mid-latitude sites were sampled both to the west of New Zealand (DSDP sites 207, 206, 592) and east of New Zealand (ODP Site 1123). New foraminifer oxygen (δ¹⁸O) and carbon (δ¹³C) stable isotope data from DSDP sites 277, 207 and 592 are presented and provide additional age control and insights in the climatic and oceanographic changes in the Southwest Pacific during the early Eocene to early Oligocene. This thesis contributes a comprehensive taxonomic review of Eocene radiolarian taxa with the intention of standardising nomenclature and to resolve synonymies. 213 out of 259 counting groups have been reviewed and assigned to species or subspecies level and 7 new species are yet to be described. All sites have been correlated to the Southern Hemisphere radiolarian zonation, from the upper Paleocene to upper Oligocene (RP6SH to RP17SH). Alternative datums for the base of RP10SH (LO of Artobotrys auriculaleporis) and the base of RP12SH (LO of Lophocyrtis longiventer) are proposed. The early Eocene climatic optimum (EECO, ~53–49 Ma) can be identified by a negative excursion in foraminiferal δ¹⁸O values at Site 207. The radiolarian assemblages at sites 207 (paleolatitude ~46°S) and 277 (paleolatitude ~55°S) during the EECO are dominated by taxa with low-latitude affinities (Amphicraspedum spp. represents up to 89% of total fauna), but many typical low-latitude genera (e.g. Thyrsocyrtis, Podocyrtis, Phormocyrtis) are absent. Following the EECO, low-latitude taxa decrease at Site 207 and disappear at Site 277. Radiolarians are abundant and very diverse at mid-latitude sites 207 and 206 (paleolatitude ~42°S) during the middle Eocene, and low-latitude taxa are common (up to ~15% of the total fauna at Site 207 and ~10% at Site 206). The middle Eocene climatic optimum (MECO, ~40 Ma), although truncated by poor drilling recovery at Site 277, is identified by a negative shift in foraminiferal δ18O values at this site and is associated by a small increase in radiolarian taxa with low-latitude affinities (up to ~5% of total fauna). Early in the late Eocene (~37 Ma), a positive shift in δ¹⁸O values at Site 277 is correlated with the Priabonian oxygen isotope maximum (PrOM). Within this cooling event, radiolarian abundance, diversity and preservation, as well as diatom abundance, increase abruptly at Site 277. A negative δ¹⁸O excursion above the PrOM is correlated to a late Eocene warming event (~36 Ma) and is referred to as the late Eocene climatic optimum (LECO). The LECO is identified using stable isotopes at sites 277 and 592. Radiolarian abundance and diversity decline within this event at Site 277 although taxa with low-latitude affinities increase (up to ~10% of total fauna). At Site 592, radiolarian-bearing sediments are only present during this event with up to ~6% low-latitude taxa. Apart from the LECO, late Eocene radiolarian assemblages at Site 277 are characterised by abundant high-latitude taxa. High-latitude taxa are also abundant during the late Eocene and Oligocene (~38–27 Ma) at DSDP sites 280, 281, 283, and ODP sites 1172 and 1123 and are associated with very high diatom abundance. Radiolarian assemblages are used for reconstructing the evolution of oceanic fronts. The composition of the assemblages suggests that the oscillation between warm subtropical and cool subtropical conditions can be explained by the varying influence of the warm proto-East Australian Current and cold proto-Ross Gyre. In contrast to temperature reconstructions based on geochemical proxies (TEX₈₆, UK’₃₇ and Mg/Ca), which indicate tropical temperatures throughout most of the Eocene, radiolarians indicate warm subtropical conditions during the EECO. Warm surface water masses may have been transported by the proto-East Australian Current to ~55°S during the EECO. During the middle to late Eocene, cool subtropical conditions prevailed in the Southwest Pacific. Localised occurrences of abundant diatoms indicate upwelling areas close to the Tasman Rise in the middle Eocene. The proliferation of radiolarian assemblages and expansion of high-latitude taxa onto the Campbell Plateau in the latest Eocene is explained by a northward expansion of proto-Ross Gyre. In the early Oligocene (~32 Ma), there is an overall decrease in radiolarian abundance and diversity on the Campbell Plateau (Site 277) and diatoms disappear. Major hiatuses in the region indicate intensified bottom-water currents associated with the establishment of the Antarctic Circumpolar Current. A frontal system similar to present day developed in the early Oligocene, with nutrient-depleted subantarctic waters bathing the southern Campbell Plateau, resulting in a more restricted radiolarian assemblage at Site 277.</p>
<p>This thesis investigates the effect of climatic and oceanographic changes on the distribution of fossil radiolarian assemblages from the early Eocene to early Oligocene (~56–30 Ma) in the Southwest Pacific. Radiolarian assemblages have been analysed from a series of archived cores collected by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP). The selected cores form a latitudinal transect designed to investigate the ecological change associated with the transition from the warm ‘greenhouse’ climate of the Eocene into the cooler Oligocene, when continental-scale glaciation is believed to have intiated in Antarctica. High-latitude sites were sampled on the Campbell Plateau (DSDP Site 277), Tasman Rise (DSDP sites 280 and 281) and the Tasman Sea (DSDP Site 283 and ODP Site 1172), while mid-latitude sites were sampled both to the west of New Zealand (DSDP sites 207, 206, 592) and east of New Zealand (ODP Site 1123). New foraminifer oxygen (δ¹⁸O) and carbon (δ¹³C) stable isotope data from DSDP sites 277, 207 and 592 are presented and provide additional age control and insights in the climatic and oceanographic changes in the Southwest Pacific during the early Eocene to early Oligocene. This thesis contributes a comprehensive taxonomic review of Eocene radiolarian taxa with the intention of standardising nomenclature and to resolve synonymies. 213 out of 259 counting groups have been reviewed and assigned to species or subspecies level and 7 new species are yet to be described. All sites have been correlated to the Southern Hemisphere radiolarian zonation, from the upper Paleocene to upper Oligocene (RP6SH to RP17SH). Alternative datums for the base of RP10SH (LO of Artobotrys auriculaleporis) and the base of RP12SH (LO of Lophocyrtis longiventer) are proposed. The early Eocene climatic optimum (EECO, ~53–49 Ma) can be identified by a negative excursion in foraminiferal δ¹⁸O values at Site 207. The radiolarian assemblages at sites 207 (paleolatitude ~46°S) and 277 (paleolatitude ~55°S) during the EECO are dominated by taxa with low-latitude affinities (Amphicraspedum spp. represents up to 89% of total fauna), but many typical low-latitude genera (e.g. Thyrsocyrtis, Podocyrtis, Phormocyrtis) are absent. Following the EECO, low-latitude taxa decrease at Site 207 and disappear at Site 277. Radiolarians are abundant and very diverse at mid-latitude sites 207 and 206 (paleolatitude ~42°S) during the middle Eocene, and low-latitude taxa are common (up to ~15% of the total fauna at Site 207 and ~10% at Site 206). The middle Eocene climatic optimum (MECO, ~40 Ma), although truncated by poor drilling recovery at Site 277, is identified by a negative shift in foraminiferal δ18O values at this site and is associated by a small increase in radiolarian taxa with low-latitude affinities (up to ~5% of total fauna). Early in the late Eocene (~37 Ma), a positive shift in δ¹⁸O values at Site 277 is correlated with the Priabonian oxygen isotope maximum (PrOM). Within this cooling event, radiolarian abundance, diversity and preservation, as well as diatom abundance, increase abruptly at Site 277. A negative δ¹⁸O excursion above the PrOM is correlated to a late Eocene warming event (~36 Ma) and is referred to as the late Eocene climatic optimum (LECO). The LECO is identified using stable isotopes at sites 277 and 592. Radiolarian abundance and diversity decline within this event at Site 277 although taxa with low-latitude affinities increase (up to ~10% of total fauna). At Site 592, radiolarian-bearing sediments are only present during this event with up to ~6% low-latitude taxa. Apart from the LECO, late Eocene radiolarian assemblages at Site 277 are characterised by abundant high-latitude taxa. High-latitude taxa are also abundant during the late Eocene and Oligocene (~38–27 Ma) at DSDP sites 280, 281, 283, and ODP sites 1172 and 1123 and are associated with very high diatom abundance. Radiolarian assemblages are used for reconstructing the evolution of oceanic fronts. The composition of the assemblages suggests that the oscillation between warm subtropical and cool subtropical conditions can be explained by the varying influence of the warm proto-East Australian Current and cold proto-Ross Gyre. In contrast to temperature reconstructions based on geochemical proxies (TEX₈₆, UK’₃₇ and Mg/Ca), which indicate tropical temperatures throughout most of the Eocene, radiolarians indicate warm subtropical conditions during the EECO. Warm surface water masses may have been transported by the proto-East Australian Current to ~55°S during the EECO. During the middle to late Eocene, cool subtropical conditions prevailed in the Southwest Pacific. Localised occurrences of abundant diatoms indicate upwelling areas close to the Tasman Rise in the middle Eocene. The proliferation of radiolarian assemblages and expansion of high-latitude taxa onto the Campbell Plateau in the latest Eocene is explained by a northward expansion of proto-Ross Gyre. In the early Oligocene (~32 Ma), there is an overall decrease in radiolarian abundance and diversity on the Campbell Plateau (Site 277) and diatoms disappear. Major hiatuses in the region indicate intensified bottom-water currents associated with the establishment of the Antarctic Circumpolar Current. A frontal system similar to present day developed in the early Oligocene, with nutrient-depleted subantarctic waters bathing the southern Campbell Plateau, resulting in a more restricted radiolarian assemblage at Site 277.</p>
<p>Little is known about how mid-latitude Southern Hemisphere terrestrial vegetation responded during glacial terminations and the warmer phases of the Late Quaternary, especially beyond the last glacial cycle where records are commonly fragmentary and poorly-dated. The timing, magnitude and sequence of environmental changes are investigated here for terminations (T) I, II and V and their subsequent warm interglacials of MIS 1, 5e and 11 by direct correlation of terrestrial palynomorphs (pollen and spores) and marine climate indicators in marine piston cores MD06-2990/2991 recovered from the East Tasman Sea, west of South Island, New Zealand. The climate there is strongly influenced by the prevailing mid-latitude westerly wind belt that generates significant amounts of orographic rainfall and the proximity of the ocean which moderates temperature variability. Chronological constraint for the cores is provided by δ¹⁸O stratigraphy, radiocarbon chronology and the identification of two widespread silicic tephra horizons (25.6 ka Kawakawa/Oruanui Tephra (KOT); ~345 ka Rangitawa Tephra (RtT)) sourced from the central North Island. Similar vegetation changes over the last two glacial cycles at MD06-2991 and in the adjacent nearby on land record of vegetation-climate change from Okarito Bog permit transfer of the well resolved Marine Isotope Stage (MIS) chronology to Okarito for the pre radiocarbon dated interval (~139-28 ka). Placing both sequences on a common age scale nonetheless assumes there is minimal lag between pollen production and final deposition on the seafloor. However, the timing of Late Pleistocene palynomorph events and KOT between independently dated marine and terrestrial sedimentary sequences are found in this study to be indistinguishable, which supports the direct transfer of terrestrially derived ages to the marine realm and vice versa. Vegetation change in southwestern New Zealand is of similar structure during T-I and T-II, despite different amplitudes of forcing (i.e., insolation rise, CO₂ concentrations). In a climate amelioration scenario, shrubland-grassland gave rise to dominantly podocarp-broadleaf forest taxa, with accompanying rises in mean annual air temperature (MAAT) estimated from Okarito pollen typically synchronous with nearby ocean temperatures. The T-II amelioration commenced after ~139 ka in response to increasing boreal summer insolation intensity, with prominent ocean-atmosphere warming over the period from ~133-130 ka. In contrast, northern mid-high latitude paleoclimate records display cooling over Heinrich Stadial 11 (~135-130 ka), and are prominently warm from ~130-128 ka, while southwestern New Zealand and the adjacent ocean displays cooling. Such millennial-scale climate asynchrony between the hemispheres is most likely a result of a systematic, but non-linear re-organisation of the ocean-atmosphere circulation system in response to orbital forcing. The subsequent MIS 5e climatic optimum in Westland was between ~128-123 ka, with maximum temperatures reconstructed in the ocean and atmosphere of 2.5°C and 1.5°C higher than present. Similarities revealed between land and sea pollen records in southwestern New Zealand over the last ~160 ka offer confidence for assessing vegetation and climate for older intervals, including T-V/MIS 11, for which no adjacent terrestrial equivalents currently exist. Vegetation change over T-V is similar to T-II and T-I, with southern warming antiphased with northern mid-high latitude cooling. Tall trees and the thermophilous shrub Ascarina lucida define interglacial conditions in the study region between ~428-396 ka. East Tasman Sea surface temperatures rose in two phases; 435-426 ka (MIS 12a-MIS 11e) and 417-407 ka (MIS 11c climatic optimum), reaching at least ~1.5-2°C warmer than present over the latter. Similarly, Ascarina lucida dominance over MIS 11c is akin to that displayed during the early Holocene climatic optimum (11.5-9 ka) in west-central North Island, where MAAT average ~3°C higher today. This contrasts markedly with the dominance of the tall tree conifer Dacrydium cupressinum for the Holocene (MIS 1) and last interglacial (MIS 5e) in southwestern New Zealand. Biogeographic barriers are proposed to have inhibited the migration of species from more northerly latitudes better adapted to warmer climatic conditions over MIS 5e and MIS 11.</p>
<p>Little is known about how mid-latitude Southern Hemisphere terrestrial vegetation responded during glacial terminations and the warmer phases of the Late Quaternary, especially beyond the last glacial cycle where records are commonly fragmentary and poorly-dated. The timing, magnitude and sequence of environmental changes are investigated here for terminations (T) I, II and V and their subsequent warm interglacials of MIS 1, 5e and 11 by direct correlation of terrestrial palynomorphs (pollen and spores) and marine climate indicators in marine piston cores MD06-2990/2991 recovered from the East Tasman Sea, west of South Island, New Zealand. The climate there is strongly influenced by the prevailing mid-latitude westerly wind belt that generates significant amounts of orographic rainfall and the proximity of the ocean which moderates temperature variability. Chronological constraint for the cores is provided by δ¹⁸O stratigraphy, radiocarbon chronology and the identification of two widespread silicic tephra horizons (25.6 ka Kawakawa/Oruanui Tephra (KOT); ~345 ka Rangitawa Tephra (RtT)) sourced from the central North Island. Similar vegetation changes over the last two glacial cycles at MD06-2991 and in the adjacent nearby on land record of vegetation-climate change from Okarito Bog permit transfer of the well resolved Marine Isotope Stage (MIS) chronology to Okarito for the pre radiocarbon dated interval (~139-28 ka). Placing both sequences on a common age scale nonetheless assumes there is minimal lag between pollen production and final deposition on the seafloor. However, the timing of Late Pleistocene palynomorph events and KOT between independently dated marine and terrestrial sedimentary sequences are found in this study to be indistinguishable, which supports the direct transfer of terrestrially derived ages to the marine realm and vice versa. Vegetation change in southwestern New Zealand is of similar structure during T-I and T-II, despite different amplitudes of forcing (i.e., insolation rise, CO₂ concentrations). In a climate amelioration scenario, shrubland-grassland gave rise to dominantly podocarp-broadleaf forest taxa, with accompanying rises in mean annual air temperature (MAAT) estimated from Okarito pollen typically synchronous with nearby ocean temperatures. The T-II amelioration commenced after ~139 ka in response to increasing boreal summer insolation intensity, with prominent ocean-atmosphere warming over the period from ~133-130 ka. In contrast, northern mid-high latitude paleoclimate records display cooling over Heinrich Stadial 11 (~135-130 ka), and are prominently warm from ~130-128 ka, while southwestern New Zealand and the adjacent ocean displays cooling. Such millennial-scale climate asynchrony between the hemispheres is most likely a result of a systematic, but non-linear re-organisation of the ocean-atmosphere circulation system in response to orbital forcing. The subsequent MIS 5e climatic optimum in Westland was between ~128-123 ka, with maximum temperatures reconstructed in the ocean and atmosphere of 2.5°C and 1.5°C higher than present. Similarities revealed between land and sea pollen records in southwestern New Zealand over the last ~160 ka offer confidence for assessing vegetation and climate for older intervals, including T-V/MIS 11, for which no adjacent terrestrial equivalents currently exist. Vegetation change over T-V is similar to T-II and T-I, with southern warming antiphased with northern mid-high latitude cooling. Tall trees and the thermophilous shrub Ascarina lucida define interglacial conditions in the study region between ~428-396 ka. East Tasman Sea surface temperatures rose in two phases; 435-426 ka (MIS 12a-MIS 11e) and 417-407 ka (MIS 11c climatic optimum), reaching at least ~1.5-2°C warmer than present over the latter. Similarly, Ascarina lucida dominance over MIS 11c is akin to that displayed during the early Holocene climatic optimum (11.5-9 ka) in west-central North Island, where MAAT average ~3°C higher today. This contrasts markedly with the dominance of the tall tree conifer Dacrydium cupressinum for the Holocene (MIS 1) and last interglacial (MIS 5e) in southwestern New Zealand. Biogeographic barriers are proposed to have inhibited the migration of species from more northerly latitudes better adapted to warmer climatic conditions over MIS 5e and MIS 11.</p>
<p>This thesis investigates the effect of climatic and oceanographic changes on the distribution of fossil radiolarian assemblages from the early Eocene to early Oligocene (~56–30 Ma) in the Southwest Pacific. Radiolarian assemblages have been analysed from a series of archived cores collected by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP). The selected cores form a latitudinal transect designed to investigate the ecological change associated with the transition from the warm ‘greenhouse’ climate of the Eocene into the cooler Oligocene, when continental-scale glaciation is believed to have intiated in Antarctica. High-latitude sites were sampled on the Campbell Plateau (DSDP Site 277), Tasman Rise (DSDP sites 280 and 281) and the Tasman Sea (DSDP Site 283 and ODP Site 1172), while mid-latitude sites were sampled both to the west of New Zealand (DSDP sites 207, 206, 592) and east of New Zealand (ODP Site 1123). New foraminifer oxygen (δ¹⁸O) and carbon (δ¹³C) stable isotope data from DSDP sites 277, 207 and 592 are presented and provide additional age control and insights in the climatic and oceanographic changes in the Southwest Pacific during the early Eocene to early Oligocene. This thesis contributes a comprehensive taxonomic review of Eocene radiolarian taxa with the intention of standardising nomenclature and to resolve synonymies. 213 out of 259 counting groups have been reviewed and assigned to species or subspecies level and 7 new species are yet to be described. All sites have been correlated to the Southern Hemisphere radiolarian zonation, from the upper Paleocene to upper Oligocene (RP6SH to RP17SH). Alternative datums for the base of RP10SH (LO of Artobotrys auriculaleporis) and the base of RP12SH (LO of Lophocyrtis longiventer) are proposed. The early Eocene climatic optimum (EECO, ~53–49 Ma) can be identified by a negative excursion in foraminiferal δ¹⁸O values at Site 207. The radiolarian assemblages at sites 207 (paleolatitude ~46°S) and 277 (paleolatitude ~55°S) during the EECO are dominated by taxa with low-latitude affinities (Amphicraspedum spp. represents up to 89% of total fauna), but many typical low-latitude genera (e.g. Thyrsocyrtis, Podocyrtis, Phormocyrtis) are absent. Following the EECO, low-latitude taxa decrease at Site 207 and disappear at Site 277. Radiolarians are abundant and very diverse at mid-latitude sites 207 and 206 (paleolatitude ~42°S) during the middle Eocene, and low-latitude taxa are common (up to ~15% of the total fauna at Site 207 and ~10% at Site 206). The middle Eocene climatic optimum (MECO, ~40 Ma), although truncated by poor drilling recovery at Site 277, is identified by a negative shift in foraminiferal δ18O values at this site and is associated by a small increase in radiolarian taxa with low-latitude affinities (up to ~5% of total fauna). Early in the late Eocene (~37 Ma), a positive shift in δ¹⁸O values at Site 277 is correlated with the Priabonian oxygen isotope maximum (PrOM). Within this cooling event, radiolarian abundance, diversity and preservation, as well as diatom abundance, increase abruptly at Site 277. A negative δ¹⁸O excursion above the PrOM is correlated to a late Eocene warming event (~36 Ma) and is referred to as the late Eocene climatic optimum (LECO). The LECO is identified using stable isotopes at sites 277 and 592. Radiolarian abundance and diversity decline within this event at Site 277 although taxa with low-latitude affinities increase (up to ~10% of total fauna). At Site 592, radiolarian-bearing sediments are only present during this event with up to ~6% low-latitude taxa. Apart from the LECO, late Eocene radiolarian assemblages at Site 277 are characterised by abundant high-latitude taxa. High-latitude taxa are also abundant during the late Eocene and Oligocene (~38–27 Ma) at DSDP sites 280, 281, 283, and ODP sites 1172 and 1123 and are associated with very high diatom abundance. Radiolarian assemblages are used for reconstructing the evolution of oceanic fronts. The composition of the assemblages suggests that the oscillation between warm subtropical and cool subtropical conditions can be explained by the varying influence of the warm proto-East Australian Current and cold proto-Ross Gyre. In contrast to temperature reconstructions based on geochemical proxies (TEX₈₆, UK’₃₇ and Mg/Ca), which indicate tropical temperatures throughout most of the Eocene, radiolarians indicate warm subtropical conditions during the EECO. Warm surface water masses may have been transported by the proto-East Australian Current to ~55°S during the EECO. During the middle to late Eocene, cool subtropical conditions prevailed in the Southwest Pacific. Localised occurrences of abundant diatoms indicate upwelling areas close to the Tasman Rise in the middle Eocene. The proliferation of radiolarian assemblages and expansion of high-latitude taxa onto the Campbell Plateau in the latest Eocene is explained by a northward expansion of proto-Ross Gyre. In the early Oligocene (~32 Ma), there is an overall decrease in radiolarian abundance and diversity on the Campbell Plateau (Site 277) and diatoms disappear. Major hiatuses in the region indicate intensified bottom-water currents associated with the establishment of the Antarctic Circumpolar Current. A frontal system similar to present day developed in the early Oligocene, with nutrient-depleted subantarctic waters bathing the southern Campbell Plateau, resulting in a more restricted radiolarian assemblage at Site 277.</p>
<p>This thesis investigates the effect of climatic and oceanographic changes on the distribution of fossil radiolarian assemblages from the early Eocene to early Oligocene (~56–30 Ma) in the Southwest Pacific. Radiolarian assemblages have been analysed from a series of archived cores collected by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP). The selected cores form a latitudinal transect designed to investigate the ecological change associated with the transition from the warm ‘greenhouse’ climate of the Eocene into the cooler Oligocene, when continental-scale glaciation is believed to have intiated in Antarctica. High-latitude sites were sampled on the Campbell Plateau (DSDP Site 277), Tasman Rise (DSDP sites 280 and 281) and the Tasman Sea (DSDP Site 283 and ODP Site 1172), while mid-latitude sites were sampled both to the west of New Zealand (DSDP sites 207, 206, 592) and east of New Zealand (ODP Site 1123). New foraminifer oxygen (δ¹⁸O) and carbon (δ¹³C) stable isotope data from DSDP sites 277, 207 and 592 are presented and provide additional age control and insights in the climatic and oceanographic changes in the Southwest Pacific during the early Eocene to early Oligocene. This thesis contributes a comprehensive taxonomic review of Eocene radiolarian taxa with the intention of standardising nomenclature and to resolve synonymies. 213 out of 259 counting groups have been reviewed and assigned to species or subspecies level and 7 new species are yet to be described. All sites have been correlated to the Southern Hemisphere radiolarian zonation, from the upper Paleocene to upper Oligocene (RP6SH to RP17SH). Alternative datums for the base of RP10SH (LO of Artobotrys auriculaleporis) and the base of RP12SH (LO of Lophocyrtis longiventer) are proposed. The early Eocene climatic optimum (EECO, ~53–49 Ma) can be identified by a negative excursion in foraminiferal δ¹⁸O values at Site 207. The radiolarian assemblages at sites 207 (paleolatitude ~46°S) and 277 (paleolatitude ~55°S) during the EECO are dominated by taxa with low-latitude affinities (Amphicraspedum spp. represents up to 89% of total fauna), but many typical low-latitude genera (e.g. Thyrsocyrtis, Podocyrtis, Phormocyrtis) are absent. Following the EECO, low-latitude taxa decrease at Site 207 and disappear at Site 277. Radiolarians are abundant and very diverse at mid-latitude sites 207 and 206 (paleolatitude ~42°S) during the middle Eocene, and low-latitude taxa are common (up to ~15% of the total fauna at Site 207 and ~10% at Site 206). The middle Eocene climatic optimum (MECO, ~40 Ma), although truncated by poor drilling recovery at Site 277, is identified by a negative shift in foraminiferal δ18O values at this site and is associated by a small increase in radiolarian taxa with low-latitude affinities (up to ~5% of total fauna). Early in the late Eocene (~37 Ma), a positive shift in δ¹⁸O values at Site 277 is correlated with the Priabonian oxygen isotope maximum (PrOM). Within this cooling event, radiolarian abundance, diversity and preservation, as well as diatom abundance, increase abruptly at Site 277. A negative δ¹⁸O excursion above the PrOM is correlated to a late Eocene warming event (~36 Ma) and is referred to as the late Eocene climatic optimum (LECO). The LECO is identified using stable isotopes at sites 277 and 592. Radiolarian abundance and diversity decline within this event at Site 277 although taxa with low-latitude affinities increase (up to ~10% of total fauna). At Site 592, radiolarian-bearing sediments are only present during this event with up to ~6% low-latitude taxa. Apart from the LECO, late Eocene radiolarian assemblages at Site 277 are characterised by abundant high-latitude taxa. High-latitude taxa are also abundant during the late Eocene and Oligocene (~38–27 Ma) at DSDP sites 280, 281, 283, and ODP sites 1172 and 1123 and are associated with very high diatom abundance. Radiolarian assemblages are used for reconstructing the evolution of oceanic fronts. The composition of the assemblages suggests that the oscillation between warm subtropical and cool subtropical conditions can be explained by the varying influence of the warm proto-East Australian Current and cold proto-Ross Gyre. In contrast to temperature reconstructions based on geochemical proxies (TEX₈₆, UK’₃₇ and Mg/Ca), which indicate tropical temperatures throughout most of the Eocene, radiolarians indicate warm subtropical conditions during the EECO. Warm surface water masses may have been transported by the proto-East Australian Current to ~55°S during the EECO. During the middle to late Eocene, cool subtropical conditions prevailed in the Southwest Pacific. Localised occurrences of abundant diatoms indicate upwelling areas close to the Tasman Rise in the middle Eocene. The proliferation of radiolarian assemblages and expansion of high-latitude taxa onto the Campbell Plateau in the latest Eocene is explained by a northward expansion of proto-Ross Gyre. In the early Oligocene (~32 Ma), there is an overall decrease in radiolarian abundance and diversity on the Campbell Plateau (Site 277) and diatoms disappear. Major hiatuses in the region indicate intensified bottom-water currents associated with the establishment of the Antarctic Circumpolar Current. A frontal system similar to present day developed in the early Oligocene, with nutrient-depleted subantarctic waters bathing the southern Campbell Plateau, resulting in a more restricted radiolarian assemblage at Site 277.</p>
