Oligocene to Early Pliocene climate evolution in the Nordic area
T. Eidvin, F. Riis, E. S. Rasmussen & Y. Rundberg, 2013. New layout 2021
Early Oligocene climate and environment
The long-term trend in the global δ18O record, based on carbonate from deep-sea calcareous benthic foraminifera, shows that the Early Eocene Climatic Optimum was followed by a 17-My-long trend towards cooler conditions with most of the changes occurring during the Early-Middle Eocene (50-48 Ma), Late Eocene (40-36 Ma) and Early Oligocene (35-34 Ma, Fig. 5). This was a profound change in regime from a greenhouse climate, prevailing since the Mesozoic, to a modern ice-house climate (Zachos et al. 2001). A generally lowered, but fluctuating eustatic sea level was caused by growing and waning ice sheets primarily in Antarctica, but probably also in Greenland (see below). A δ18O-record, based on carbonate from molluscs collected from strata in southern England, Holland, Germany, Denmark and southern Sweden, shows that the climate became much cooler also in Scandinavia (Buchardt 1978).
Eldrett et al. (2007) studied cores from ODP Site 913 in the Norwegian-Greenland Sea (Fig. 5). To test the shipboard interpretations, they undertook an investigation of the Eocene-Oligocene succession at Site 913B. The core observations revealed the presence of in situ macroscopic gneiss clasts up to 3.5 cm in length. According to Eldrett et al. (2007), their data demonstrate that ice rafting into the Norwegian-Greenland Sea occurred at least intermittently between 38 and 30 Ma at ODP Site 913, and they pointed to East Greenland as the likely source. Previously, the existence of Northern Hemisphere ice sheets had been demonstrated back to the Mid Miocene (approximately 15 Ma, Winkler et al. 2002, Helland & Holmes 1997), but these findings document the first occurrence of ice-rafted debris some 20 million years earlier in the Norwegian-Greenland Sea (Moran et al. 2006).
According to DeConto et al. (2008) and Pekar (2008), the findings of Eldrett et al. (2007) indicate that small, isolated sheets of glacial ice could have formed in the Northern Hemisphere during the cooler intervals of the Eocene and Oligocene, especially during periods when variations in the Earth’s orbit produced relatively cold northern summers. However, they stressed that there is currently only scant evidence to suggest that large amounts of glacial ice existed in the Northern Hemisphere before the Late Miocene.
Erosion has removed any terrestrial, palynological evidence from the Oligocene to the Pliocene on and close to the Fennoscandian Shield area and the Barents Sea. However, Boulter & Manum (1996) have recorded organic assemblages (pollen, spores, dinoflagellate cysts, plant debris) in mid Oligocene sediments from the Hovgård Ridge (ODP Site 908, Fig. 8). The assemblages are dominantly of terrestrial origin, and their present position, in the middle of the Fram Strait (Greenland Sea), can be explained by a tectonic model for the origin of the ridge as a sliver rifted from the Svalbard Platform since anomaly 13 time. The dominance of pollen and plant-tissue fragments and the low proportion of dinoflagellate cysts indicate relatively short distances to a swampy, forested lowland with prolific humic productivity. The pollen flora in the Hovgård Ridge sediment presents a unique glimpse into previously unknown vegetation in high northern latitudes during mid Oligocene times. The pollen indicates forests of conifers related to Pinus, Picea, Tsuga and Taxodium, with a minor element of angiosperms but relatively common ferns. This is different from the well-known Paleocene-Eocene floras on adjacent Spitsbergen that were also rich in conifers, but had a richer and more diverse angiosperm element and lacked Tsuga relatives.
Manum (1962) has also described the pollen found in Cenozoic deposits from Sarsbukta (Forlandsundet, Spitsbergen). Manum & Throndsen (1986) gave a Late Eocene age to these deposits based on dinoflagellate cysts, but Feyling-Hanssen & Ulleberg (1984) interpreted the deposits to be of mid Oligocene age. The last age interpretation was verified by Eidvin et al. (1994, 1998b) by means of Sr isotope analyses (latest Early Oligocene). Boulter & Manum (1996) supported an Oligocene age for the Sarsbukta deposits and considered the pollen flora to be very similar to that recorded on the Hovgård Ridge, with a pronounced but small angiosperm component.
Late Oligocene climate and environment
The global deep-sea δ18O record shows that a cool climate prevailed early in the Late Oligocene, but a warming trend started in the late part of Late Oligocene (Fig. 5). The Antarctic continental ice-sheet that had built up during the Early Oligocene persisted until the later part of the Oligocene (27 to 26 Ma), when the warming trend reduced the extent of the Antarctic ice (the Northern Hemisphere ice sheets may have disappeared). From this time until the Mid Miocene (approximately 15 Ma), the global ice volume remained low and bottom-water temperatures trended slightly higher, with the exception of several brief periods of glaciation. This warm phase peaked in the late Mid Miocene climatic optimum (17-15 Ma, Zachos et al. 2001). The latest Oligocene warming trend is also seen in the δ18O record of Buchardt (1978) from northern Europe, but it is considerable less distinct than the Mid Miocene optimum warming trend.
Early Miocene climate and environment
According to the global deep-sea δ18O record, the warming trend that started in the late part of Late Oligocene levelled out in the Early Miocene, and cooled somewhat early in the period (Fig. 5, Zachos et al. 2001).
According to Larsson et al. (2010), two exposures in Jytland encompassing beds of latest Oligocene to earliest Miocene age yield well-preserved palynofloras. The assemblages indicate that Jutland was covered by Taxodiaceae swamp forests at the time. Besides a Taxodiaceae-Cupressaceae association, which was overwhelmingly dominant, other common plants in the habitat were Alnus, Nyssa, Betula, Salix, Cyrilla and Myrica. Most of the trees and shrubs are well adapted to swamps and thrive under more or less flooded conditions in modern, bald, cypress swamps of southeastern North America. Vegetation composition indicates that a warm-temperate climate prevailed in Denmark during the Oligocene-Miocene transition. According to calculations using the Coexistence Approach of Mosbrugger & Utescher (1997), the mean annual temperature during this time span ranged from 15.6 to 16.6 °C. An increase to 16.5-21.1°C is inferred from the palynoflora in the upper part of the section. The earlier, cooler period possibly reflects global cooling associated with the Mi-1 glaciation event at the Oligocene-Miocene boundary.
Larsson et al. (2006) performed a palynological analysis of a Lower Miocene (upper Aquitanian) section from Sønder Vium in southwestern Jytland, Denmark. Terrestrial pollen and spores dominated their samples, with lesser contents of dinoflagellates, indicating a substantial fluvial input into a marine setting. The pollen record suggests that swamp forests dominated the onshore region, which is consistent with previous results from central and northern Europe. The swamp forest also contained several angiosperm taxa. Elevated or better drained hinterland areas hosted a diverse mesophytic forest, which included evergreen conifers and deciduous angiosperms. A decrease in the relative abundances of thermophilous elements in the middle part of the studied succession indicates a possible correlation with the Early Miocene climatic cooling. The composition of the palynological assemblages suggests a warm, frost-free, temperate climate during the Early Miocene, culminating with a subtropical climate in the latest part of the Early Miocene in Denmark (Friis 1975, Larsson et al. 2006).
Middle Miocene climate and environment
According to the global deep-sea δ18O record, the warming trend which started in the late part of Late Oligocene peaked in the early Middle Miocene climatic optimum (17-15 Ma, Fig. 5, Zachos et al. 2001). This peak is also distinct in the δ18O record of Buchardt (1978) from northern Europe.
Utescher et al. (2000) made a reconstruction of the continental paleoclimate evolution of Northwest Germany during Late Oligocene to Pliocene time. The paleoclimate data are derived from the paleobotanical record of twenty-six megafloras (fruits and seeds, leaves, wood) from the Lower Rhine Basin and neighbouring areas. The temperature curves show a comparatively cooler phase in the Late Oligocene, a warm interval in the Middle Miocene, and a cooling starting at 14 Ma.
Grimsson et al. (2007) described two Middle Miocene macrofloras (15 and 13.5 Ma) from plant-bearing sediments, sealed off by lava, in northwestern Iceland. In the case of the older flora, differences in environments are reflected in plants derived from high elevations and from lowland alluvial plains. The former flora is characterised by Fagus, and the latter by conifers inhabiting swamps and hummocks. The younger flora is poorer and more similar to the older high-elevation flora. Both floras suggest a humid warm temperate climate with a number of exotic elements.
Late Miocene to Early Pliocene climate and environment
According to the global deep-sea δ18O record, the late Middle Miocene climatic optimum was followed by a gradual cooling and reestablishment of a major ice-sheet on Antarctica by 10 Ma. Mean δ18O values then continued to rise gently through the Late Miocene until the Early Pliocene (6 Ma), indicating additional cooling and small-scale ice-sheet expansion on West Antarctica and in the Arctic. The Early Pliocene is marked by a subtle warming trend until approximately 3.2 Ma, when δ18O again increased reflecting the onset of large-scale northern hemisphere glaciations (Fig. 5, Zachos et al. 2001). This trend is also quite distinct in the δ18O record of Buchardt (1978) from northern Europe.
Frondval & Jansen (1996) studied continuous, late Neogene, sediment sections from ODP Site 907 on the Iceland Plateau and ODP Sites 642, 643 and 644 on the Vøring Plateau (Norwegian Sea) by using stable isotope stratigraphy and sedimentological methods. They described an overall increase in δ18O values in benthic calcareous foraminifera from 12 to 1 Ma which documents a gradual cooling of the Iceland-Norwegian Sea deep water with major cooling events at approximately 11 and 6.4 Ma. The oldest ice-rafted debris detected is dated to approximately 12.6 Ma (an event also recorded in borehole 6704/12-GB1, see also Eidvin et al. 1998c). This coincides with a decrease in mean annual temperature at middle and high latitudes, an intensification of North Atlantic deep-water production, and a change in circulation patterns within the Iceland-Norwegian Sea, as indicated by a shift from extensive biogenic opal deposition to carbonate accumulation on the Vøring Plateau. IRD records from southeast Greenland (Larsen et al. 1994), the Iceland and the Vøring Plateau suggest further intensifications of the Northern Hemisphere glaciations at approximately 7-6 Ma (Messinian). Between 6 and 3 Ma, small-scale ice sheets periodically existed around the Iceland-Norwegian Sea, interrupted by intervals with lesser local ice volumes as indicated by reduced ice rafting. The onset of the large-scale Northern Hemisphere glaciations is dated to 2.75 Ma on the Vøring Plateau and Svalbard/Barents Sea (Knies et al. in press) and 2.9 Ma at the Iceland Plateau.
Besides terrestrial, palynological evidence, at ODP Site 908 on the Hovgård Ridge (Fram Strait, Greenland Sea, Fig. 8) from the Oligocene, Boulter & Manum (1996) also reported such evidence from the Upper Miocene. They stated that in the Upper Miocene, all the pollen taxa present in the Oligocene sections continue to be present, which showed that the vegetational source was fundamentally similar over a hiatus of about 18-15 My. They suggested that the high Arctic (Spitsbergen area) would have served as a stagnant genetic pool with little evolutionary activity during the late Paleogene and early Neogene, quite different than in areas at lower latitudes.
Denk et al. (2005) studied a large number of plant macrofossils from several localities exposing Middle to Upper Miocene successions on Iceland. Their main finding is that the Miocene flora of Iceland belongs to a widespread, Neogene, northern hemispheric floral type whose representatives are restricted to East Asia, North America and western Eurasia at the present time. The type of vegetation in four plant-bearing sedimentary formations from the late Middle Miocene to Late Miocene, at respectively 12, 10, 9-8 and 7-6 Ma, corresponds to humid temperate broad-leaved (deciduous) to coniferous mixed forest. Compositional changes in the species in the sedimentary formations reflect a shift from warm temperate to cool temperate conditions from the late Mid Miocene to the latest Miocene.
From eastern Iceland, Mudie & Helgason (1983) have obtained palynological data from clastic units between lava formations. The data they obtained from the Holmatindur Tuff Formation records a floral spectrum ranging up-section from a strongly thermophilic swamp forest to cool temperate deciduous-boreal forest, then microthermal spruce forest, and finally, subarctic woodland. According to Mudie & Helgason (1983), this suggests the occurrence of a major climatic cooling event during the time interval from 10.3 and 9.5 Ma.
From an extensively karstified landscape at Pollnahallia, western Ireland, Coxon (2005) and Coxon et al. (2005) have obtained palynological data from a borehole in preglacial Pliocene lignite. The lignite is filling a limestone gorge, and is covered with silica-rich sand and lodgement till. The lignite contains a richly diverse pollen assemblage characteristic of the preglacial Pliocene including fir, maple, sweet chestnut, swamp cypress (abundant), hazel, beech, walnut, tulip tree, sweetgum, sourgum, pines, spruce, oak, Japanese umbrella pine, wingnut, oak, redwood, yew, hemlock and heather. This indicates a forested, swampy landscape on an undulating intensely karstified surface. The upper pollen assemblages in the lignite indicate a deterioration of the climate.