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