Neogene dinoflagellate cysts and acritarchs from the high northern latitudes and their relation to sea surface temperature

Submitted manuscript version. Published version available at https://doi.org/10.1016/j.marmicro.2017.09.003. Submitted manuscript version, licensed CC BY-NC-ND 4.0.Organic-walled dinoflagellate cysts and acritarchs are a vital tool for reconstructing past environmental change, in particular in the Neogene of the high northern latitudes where marine deposits are virtually barren of traditionally used calcareous microfossils. Yet only little is known about the paleoenvironmental value of fossil assemblages that do not have modern analogues, so that reconstructions remain qualitative. Thus, extracting their paleoecological signals still poses a major challenge, in particular on pre-Quaternary timescales. Here we unravel the relationship between species relative abundance and sea surface temperature for extinct dinoflagellate cyst and acritarch taxa from the Neogene of the Iceland Sea using palynological assemblages and organic geochemical (alkenone) data generated from the same set of samples. The reconstructed temperatures for the Miocene to Pliocene sequence of Ocean Drilling Program Site 907 range from 3 to 26 °C and our database consists of 68 dinoflagellate cyst and acritarch samples calibrated to alkenone data. The temperature range of five extant species co-occurring in the fossil assemblage agrees well with their present-day distribution providing confidence to inferred temperature ranges for extinct taxa. The 14 extinct dinoflagellate cyst and acritarch species clearly exhibit a temperature dependency in their occurrence throughout the analysed section. The dinoflagellate cyst species Batiacasphaera hirsuta, Labyrinthodinium truncatum, Cerebrocysta irregulare, Cordosphaeridium minimum, Impagidinium elongatum and Operculodinium centrocarpum s.s., and the acritarch Lavradosphaera elongatum, which are confined to the Miocene, have highest relative abundances and restricted temperature ranges at the warm end of the reconstructed temperature spectrum. The latter five species disappear when Iceland Sea surface temperatures permanently drop below 20 °C, thus indicating a distinct threshold on their occurrence. In contrast, species occurring in both the Miocene and Pliocene interval (Batiacasphaera micropapillata, Habibacysta tectata, Reticulatosphaera actinocoronata, Cymatiosphaera? invaginata) show a broader temperature range and a tolerance towards cooler conditions. Operculodinium? eirikianum may have a lower limit on its occurrence at around 10 °C. The calibration of species relative abundance versus reconstructed sea surface temperature provides a quantitative assessment of temperature ranges for extinct Miocene to Pliocene species indicating that temperature is a decisive ecological factor for regional extinctions that may explain the frequently observed asynchronous highest occurrences across different ocean basins. It demonstrates that qualitative assessments of ecological preferences solely based on (paleo) biogeographic distribution should be treated with caution. In addition to enhancing knowledge on marine palynomorph paleoecology, this study ultimately improves the application of palynomorphs for paleoenvironmental reconstructions in the Neogene of the Arctic and subarctic seas, a region essential for understanding past global climate

Pliocene sea ice evolution in the Iceland and Labrador Sea – A biomarker approach

Sea ice plays a crucial role in the climate system. Although this is broadly acknowledged, the role of sea ice is not fully understood, especially during warmer periods such as the Pliocene (5.88–2.58 Million years (Ma) ago). Fragmentary evidence suggests that the Arctic sea ice was reduced in the Pliocene, but that it could have been transported into the Nordic Seas, when the East Greenland Current (EGC) developed, which established the modern Nordic Seas circulation. Today the EGC is the main exporter of cooler and fresher Arctic water masses into the Nordic Seas and carries 90% of the total sea ice exported from the Arctic Ocean with it. The main objectives of this thesis are to determine the presence of (seasonal) sea ice in the Pliocene Iceland and Labrador Seas and to identify the role of the EGC and sea ice on the Pliocene (sub-)Arctic climate. The Iceland Sea and the Labrador Sea are important and sensitive regions for determining the occurrence of sea ice and changes in the EGC and Greenland Ice Sheet (GIS). Therefore, Early Pliocene to Early Quaternary sediments were investigated from the Iceland Sea (ODP Site 907) and the Labrador Sea (IODP Site U1307) using biomarkers (IP25, sterols, alkenones) to reconstruct the Pliocene paleoceanography and especially the sea ice cover in both areas. Additional information was obtained from palynological analysis of the same sites. My analyses revealed, that sea ice occurred for the first time in the Pliocene Iceland Sea around 4.5 Ma, together with a cooling of the entire Nordic Seas. The development of a proto EGC replaced warmer Atlantic water masses in the Iceland Sea and either favored the local formation of sea ice or directly exported sea ice from the Arctic Ocean. At ~4.0 Ma, an extended interval of seasonal sea ice in the Iceland Sea occurred contemporaneously with the establishment of a large sea surface temperature (SST) gradient in the Nordic Seas: the Iceland Sea cooled further, whereas the Norwegian Sea warmed. Increased warming in the North Atlantic and Norwegian Sea at this time may have lead to increased moisture transport towards Siberia, which can ultimately led to a freshening of the Arctic Ocean, favoring sea ice production and export (Paper I). Frequently occurring seasonal sea ice was reconstructed between 3.5–3.0 Ma in the Iceland Sea (Paper II), while the biomarker analysis indicate dominantly ice-free conditions in the Labrador Sea for approximately the same time interval (Paper III). This may have been the result of a weak EGC influence in the Labrador Sea, whereas the EGC influence was stronger in the Iceland Sea at times when the GIS was significantly reduced. The weaker EGC influence in the Labrador Sea might be coinciding with a strong subpolar gyre (SPG) circulation in the Labrador Sea allowing for more advection of Atlantic water masses into the Labrador Sea (Paper III). Higher-than-modern alkenone-based SSTs suggest that summers in both areas were sea ice-free. After 3.0 Ma, sea ice occurred less frequently in the Iceland Sea, whereas from 2.75 Ma fluctuations in the sterol record might suggest a nearby sea ice edge (Paper II). The Labrador Sea received more polar water and a sea ice edge developed after ~3.1 Ma implying an enhanced southward flow of the EGC (Paper III). The enhanced southward penetration of polar waters might agree with a weaker SPG circulation. As such, a sea ice edge and an intensified EGC might have acted as a positive feedback for the expansion of the GIS during the Northern Hemisphere glaciation by stronger sea ice albedo feedbacks and isolation of Greenland from warm Atlantic water masses, respectively

Pliocene sea ice evolution in the Iceland and Labrador Sea – A biomarker approach

Sea ice plays a crucial role in the climate system. Although this is broadly acknowledged, the role of sea ice is not fully understood, especially during warmer periods such as the Pliocene (5.88–2.58 Million years (Ma) ago). Fragmentary evidence suggests that the Arctic sea ice was reduced in the Pliocene, but that it could have been transported into the Nordic Seas, when the East Greenland Current (EGC) developed, which established the modern Nordic Seas circulation. Today the EGC is the main exporter of cooler and fresher Arctic water masses into the Nordic Seas and carries 90% of the total sea ice exported from the Arctic Ocean with it. The main objectives of this thesis are to determine the presence of (seasonal) sea ice in the Pliocene Iceland and Labrador Seas and to identify the role of the EGC and sea ice on the Pliocene (sub-)Arctic climate. The Iceland Sea and the Labrador Sea are important and sensitive regions for determining the occurrence of sea ice and changes in the EGC and Greenland Ice Sheet (GIS). Therefore, Early Pliocene to Early Quaternary sediments were investigated from the Iceland Sea (ODP Site 907) and the Labrador Sea (IODP Site U1307) using biomarkers (IP25, sterols, alkenones) to reconstruct the Pliocene paleoceanography and especially the sea ice cover in both areas. Additional information was obtained from palynological analysis of the same sites. My analyses revealed, that sea ice occurred for the first time in the Pliocene Iceland Sea around 4.5 Ma, together with a cooling of the entire Nordic Seas. The development of a proto EGC replaced warmer Atlantic water masses in the Iceland Sea and either favored the local formation of sea ice or directly exported sea ice from the Arctic Ocean. At ~4.0 Ma, an extended interval of seasonal sea ice in the Iceland Sea occurred contemporaneously with the establishment of a large sea surface temperature (SST) gradient in the Nordic Seas: the Iceland Sea cooled further, whereas the Norwegian Sea warmed. Increased warming in the North Atlantic and Norwegian Sea at this time may have lead to increased moisture transport towards Siberia, which can ultimately led to a freshening of the Arctic Ocean, favoring sea ice production and export (Paper I). Frequently occurring seasonal sea ice was reconstructed between 3.5–3.0 Ma in the Iceland Sea (Paper II), while the biomarker analysis indicate dominantly ice-free conditions in the Labrador Sea for approximately the same time interval (Paper III). This may have been the result of a weak EGC influence in the Labrador Sea, whereas the EGC influence was stronger in the Iceland Sea at times when the GIS was significantly reduced. The weaker EGC influence in the Labrador Sea might be coinciding with a strong subpolar gyre (SPG) circulation in the Labrador Sea allowing for more advection of Atlantic water masses into the Labrador Sea (Paper III). Higher-than-modern alkenone-based SSTs suggest that summers in both areas were sea ice-free. After 3.0 Ma, sea ice occurred less frequently in the Iceland Sea, whereas from 2.75 Ma fluctuations in the sterol record might suggest a nearby sea ice edge (Paper II). The Labrador Sea received more polar water and a sea ice edge developed after ~3.1 Ma implying an enhanced southward flow of the EGC (Paper III). The enhanced southward penetration of polar waters might agree with a weaker SPG circulation. As such, a sea ice edge and an intensified EGC might have acted as a positive feedback for the expansion of the GIS during the Northern Hemisphere glaciation by stronger sea ice albedo feedbacks and isolation of Greenland from warm Atlantic water masses, respectively

Organic biomarker records and dinoflagellate cyst concentrations from the Pliocene to earliest Quaternary of ODP Site 907, Iceland Sea

Sea ice is a critical component in the Arctic and global climate system, yet little is known about its extent and variability during past warm intervals, such as the Pliocene (5.33-2.58 Ma). Here, we present the first multi-proxy (IP25, sterols, alkenones, palynology) sea ice reconstructions for the Late Pliocene Iceland Sea (ODP Site 907). Our interpretation of a seasonal sea ice cover with occasional ice-free intervals between 3.50-3.00 Ma is supported by reconstructed alkenone-based summer sea surface temperatures. As evidenced from brassicasterol and dinosterol, primary productivity was low between 3.50 and 3.00 Ma and the site experienced generally oligotrophic conditions. The East Greenland Current (and East Icelandic Current) may have transported sea ice into the Iceland Sea and/or brought cooler and fresher waters favoring local sea ice formation. Between 3.00 and 2.40 Ma, the Iceland Sea is mainly sea ice-free, but seasonal sea ice occurred between 2.81 and 2.74 Ma. Sea ice extending into the Iceland Sea at this time may have acted as a positive feedback for the build-up of the Greenland Ice Sheet (GIS), which underwent a major expansion ~2.75 Ma. Thereafter, most likely a stable sea ice edge developed close to Greenland, possibly changing together with the expansion and retreat of the GIS and affecting the productivity in the Iceland Sea

Seasonal sea ice cover during the warm Pliocene: Evidence from the Iceland Sea (ODP Site 907)

Sea ice is a critical component in the Arctic and global climate system, yet little is known about its extent and variability during past warm intervals, such as the Pliocene (5.33–2.58Ma). Here, we present the first multi-proxy (IP25, sterols, alkenones, palynology) sea ice reconstructions for the Late Pliocene Iceland Sea (ODP Site 907). Our interpretation of a seasonal sea ice cover with occasional ice-free intervals between 3.50–3.00Ma is supported by reconstructed alkenone-based summer sea surface temperatures. As evidenced from brassicasterol and dinosterol, primary productivity was low between 3.50 and 3.00Ma and the site experienced generally oligotrophic conditions. The East Greenland Current (and East Icelandic Current) may have transported sea ice into the Iceland Sea and/or brought cooler and fresher waters favoring local sea ice formation. Between 3.00 and 2.40Ma, the Iceland Sea is mainly sea ice-free, but seasonal sea ice occurred between 2.81 and 2.74Ma. Sea ice extending into the Iceland Sea at this time may have acted as a positive feedback for the build-up of the Greenland Ice Sheet (GIS), which underwent a major expansion ∼2.75Ma. Thereafter, most likely a stable sea ice edge developed close to Greenland, possibly changing together with the expansion and retreat of the GIS and affecting the productivity in the Iceland Sea

Organic biomarkers from ODP sites 151-907 and 151-911, used to reconstruct the surface temperatures and sea ice conditions in the Pliocene Nordic Seas

This dataset consists of organic biomarkers used to reconstruct the sea surface temperature and sea ice conditions in the Pliocene Nordic Seas. Specifically, it includes alkenone, IP25 and sterol data from the Pliocene of two Ocean Drilling Program sites, ODP Site 907 in the Iceland Sea and ODP Site 911 on the Yermak Plateau (Arctic Ocean)

On the causes of Arctic sea ice in the warm Early Pliocene

Scattered and indirect evidence suggests that sea ice occurred as far south as the Iceland Sea during the Early Pliocene, when the global climate was warmer than present. However, conclusive evidence as well as potential mechanisms governing sea ice occurrence outside the Arctic Ocean during a time with elevated greenhouse gas concentrations are still elusive. Here we present a suite of organic biomarkers and palynological records from the Iceland Sea and Yermak Plateau. We show that sea ice appeared as early as ~4.5 Ma in the Iceland Sea. The sea ice either occurred seasonally or was transported southward with the East Greenland Current. The Yermak Plateau mostly remained free of sea ice and was influenced dominantly by Atlantic water. From ~4.0 Ma, occurrence of extended sea ice conditions at both the Yermak Plateau and Iceland Sea document a substantial expansion of sea ice in the Arctic. The expansion occurred contemporaneous with increased northward heat and moisture transport in the North Atlantic region, which likely led to a fresher Arctic Ocean that favors sea ice formation. This extensive sea ice cover along the pathway of the East Greenland Current gradually isolated Greenland from warmer Atlantic water in the Late Pliocene, providing a positive feedback for ice sheet expansion in Greenland

Mid-Piacenzian Variability of Nordic Seas Surface Circulation Linked to Terrestrial Climatic Change in Norway

During the mid-Piacenzian, Nordic Seas sea surface temperatures (SSTs) were higher than today. While SSTs provide crucial climatic information, on their own they do not allow a reconstruction of potential underlying changes in water masses and currents. A new dinoflagellate cyst record for Ocean Drilling Program (ODP) Site 642 is presented to evaluate changes in northward heat transport via the Norwegian Atlantic Current (NwAC) between 3.320 and 3.137Ma. The record is compared with vegetation and SST reconstructions from Site 642 and SSTs from Iceland Sea ODP Site 907 to identify links between SSTs, ocean currents, and vegetation changes. The dinocyst record shows that strong Atlantic water influence via the NwAC corresponds to higher-than-present SSTs and cool temperate vegetation during Marine Isotope Stage (MIS) transition M2-M1 and KM5. Reduced Atlantic water inflow relative to the warm stages coincides with near-modern SSTs and boreal vegetation during MIS M2, KM6, and KM4-KM2. During most of the studied interval, a strong SST gradient between Sites 642 and 907 indicates the presence of a proto-Arctic Front (AF). An absent gradient during the first half of MIS KM6, due to reduced Atlantic water influence at Site 642 and warm, presumably Atlantic water reaching Site 907, is indicative of a weakened NwAC and East Greenland Current. We conclude that repeated changes in Atlantic water influence directly affect terrestrial climate and that an active NwAC is needed for an AF to develop. Obliquity forcing may have played a role, but the correlation is not consistent. Plain Language Summary At present, northward heat transport via the Norwegian Atlantic Current (NwAC) is a major reason for the mild climate in Norway. For the warmer-than-present late Pliocene (approximately 3.0-3.3Ma), it is unclear if changes in northward heat transport affected the Norwegian Sea and Scandinavian climate. We analyzed fossil dinoflagellate cysts in Ocean Drilling Program Hole 642B to reconstruct changes in the influence of the NwAC during the late Pliocene. We found that strong NwAC influence and changes in insolation are responsible for warmer-than-present climatic conditions in Norway. In contrast, reduced NwAC influence is associated with similar-to-present climatic conditions on land. These results highlight that changes in northward heat transport via the NwAC and insolation changes control late Pliocene climate changes in Norway