Description: Here, we present a first (low-resolution) biomarker sea-ice proxy record from the High Arctic (southern Lomonosov Ridge), going back in time to about 60 ka (MIS 3 to MIS 1). Variable concentrations of the sea-ice diatom specific highly branched isoprenoid (HBI) with 25 carbon atoms (“IP25“), in combination with the phytoplankton biomarker brassicasterol, suggest variable seasonal sea-ice coverage and open-water productivity during MIS 3. During most of MIS 2, the spring to summer sea-ice margin significantly extended towards the south, resulting in a drastic decrease in phytoplankton productivity. During the Early Holocene Climate Optimum, brassicasterol reached its maximum, interpreted as signal for elevated phytoplankton productivity due to a significantly reduced sea-ice cover. During the mid-late Holocene, IP25 increased and brassicasterol decreased, indicating extended sea-ice cover and reduced phytoplankton productivity, respectively. The HBI diene/IP25 ratios probably reached maximum values during the Bølling-Allerød warm period and decreased during the Holocene, suggesting a correlation with sea-surface temperature.

Description: This review paper focusses on reconstructions of the long- and short-term history of past Arctic Ocean sea-ice cover. Based on commonly used sedimentological, geochemical and micropaleontological proxies (ice-rafted debris (IRD), mineralogical composition of terrigenous sediment frac¬tion, and abundances of specific diatoms and foraminifers), three examples of reconstructions of glacial history, sea-ice cover and surface-water character¬istics are presented and discussed: (1) the onset Arctic Ocean sea-ice cover near 47 Ma and its long-term variability through Cenozoic times; (2) the Quaternary glacial/interglacial variability in Arctic Ocean ice-rafting and its relationship to sea-ice and ice-sheet history; and (3) Last Glacial Maximum (LGM), Deglacial to Holocene changes in Arctic Ocean sea-ice cover and ice-sheet decay. In the second part of this paper we concentrate on Arctic Ocean sea-ice reconstructions, using a recently developed biomarker approach that is based on the determination of sea-ice diatom-specific highly-branched isoprenoids with 25 carbon atoms (IP25; BELT et al. 2007) and IP25 in combination with phytoplankton biomarkers (PIP25; MÜLLER et al. 2009, 2011). The diene/ IP25 ratio might give additional information about sea-surface temperature (SST) in the low temperature Arctic environment (FAHL & STEIN 2012). The high potential of these novel biomarker proxies to improve reconstructions of paleo-sea-ice cover and its variability through time is demonstrated in three examples: (a) the sea-ice variability in Fram Strait over the last 30 ka, (b) the deglacial/Holocene variability of central Arctic sea-ice cover with special emphasis on the Younger Dryas Cooling Event, and (c) a comparison of historical sea-ice observations off northern Iceland over the last millennium and a corresponding high-resolution IP25 record. In a pilot study carried out in a sediment core from the Barents Sea conti-nental slope we were able to prove for the first time that IP25 is even pre-served in sediments as old as 130 to 150 ka (MIS 6), i.e., IP25 can be used for reconstruction of sea-ice variability during older glacial/interglacial intervals (MIS 6/MIS 5). In order to establish the IP25 approach as a key proxy for reconstruction of past Arctic Ocean sea-ice conditions, more basic information about produc-tion, degradation and preservation/burial of the IP25 signal is still needed. Furthermore, the hypothesis that the diene/IP25 ratio might be used as reliable proxy for SST reconstructions in the low temperature Arctic environments has to be verified by a ground-truth study including the IP25 and diene data as well as independent SST proxies like alkenone-derived SST. All these data should be obtained in future investigations of sea-ice, water column, and sediment-trap samples as well as surface sediments and sediment cores with large spatial coverage from different environments of the entire Arctic Ocean.

Description: The Mid Pleistocene Transition (MPT) constitutes a fundamental shift in Earth's climate system from a 41 ka to a 100 ka periodicity in glacial oscillations. The exact timing and mechanism(s) that caused this change from a low- to high-amplitude glacial variability are still under debate and only recently Pena & Goldstein (2014) suggested that a disruption of the thermohaline circulation at about 900 ka BP and a subsequent change in ocean circulation might have acted as a trigger for the onset of 100 ka glacial-interglacial cycles. Most studies targeting the MPT are based on Atlantic sediment records whereas only few data sets are available from the North Pacific (see e.g. Clark et al., 2006 and McClymont et al., 2013 for reviews). IODP Expedition 341 distal deep-water site U1417 in the Gulf of Alaska (subpolar NE Pacific) now provided a continuous sediment record for reconstructing Miocene to Late Pleistocene changes in the sea surface conditions and how these relate to orbital and millennial scale climate variability. Here we present organic geochemical biomarker data covering the 1.5 Ma to 0.1 Ma time interval with special focus on the MPT. Alkenone, sterol, n-alkane and C25 highly branched isoprenoid data are used to reconstruct sea surface temperatures, primary productivity and terrigenous organic matter input (via sea ice, icebergs, meltwater discharge or aeolian transport). In addition, the diatom concentration and the species composition of the diatom assemblage deliver information on changes in palaeoproductivity and nutrient (silicate) availability. A major change in the environmental setting between 1.2 and 0.8 Ma is recorded by the biomarkers. This shift seems to be associated with a significant cooling of the surface waters in the Gulf of Alaska. Matching this shift, a significant change in the main components of the diatom community occurred between 1.2 and 0.8 Ma. References Clark, P.U., Archer, D., Pollard, D., Blum, J.D., Rial, J.A., Brovkin, V., Mix, A.C., Pisias, N.G., Roy, M., 2006. Quaternary Science Reviews, 25, (23–24), 3150-3184. McClymont, E.L., Sosdian, S.M., Rosell-Melé, A., Rosenthal, Y., 2013. Earth-Science Reviews, 123, 173-193. Pena, L.D. and Goldstein, S.L., 2014. Science, 345, 318-322.

Description: Due to its strong influence on heat and moisture exchange between the ocean and the atmosphere, sea ice is an essential component of the global climate system. In the context of its alarming decrease in terms of concentration, thickness and duration, understanding the processes controlling sea-ice variability and reconstructing paleo-sea-ice extent in polar regions have become of great interest for the scientific community. In this study, for the first time, IP25, a recently developed biomarker sea-ice proxy, was used for a high-resolution reconstruction of the sea-ice extent and its variability in the western North Pacific and western Bering Sea during the past 18,000 years. To identify mechanisms controlling the sea-ice variability, IP25 data were associated with published sea-surface temperature as well as diatom and biogenic opal data. The results indicate that a seasonal sea-ice cover existed during cold periods (Heinrich Stadial 1 and Younger Dryas), whereas during warmer intervals (Bølling-Allerød and Holocene) reduced sea ice or ice-free conditions prevailed in the study area. The variability in sea-ice extent seems to be linked to climate anomalies and sea-level changes controlling the oceanographic circulation between the subarctic Pacific and the Bering Sea, especially the Alaskan Stream injection though the Aleutian passes.

Description: Coinciding with global warming, Arctic sea ice has rapidly decreased during the last four decades and climate scenarios suggest that sea ice may completely disappear during summer within the next about 50-100 years. Here we produce Arctic sea ice biomarker proxy records for the penultimate glacial (Marine Isotope Stage 6) and the subsequent last interglacial (Marine Isotope Stage 5e). The latter is a time interval when the high latitudes were significantly warmer than today. We document that even under such warmer climate conditions, sea ice existed in the central Arctic Ocean during summer, whereas sea ice was significantly reduced along the Barents Sea continental margin influenced by Atlantic Water inflow. Our proxy reconstruction of the last interglacial sea ice cover is supported by climate simulations, although some proxy data/model inconsistencies still exist. During late Marine Isotope Stage 6, polynya-type conditions occurred off the major ice sheets along the northern Barents and East Siberian continental margins, contradicting a giant Marine Isotope Stage 6 ice shelf that covered the entire Arctic Ocean.

Description: Past sea ice conditions and open water phytoplankton production were reconstructed from a sediment core taken in Disko Bugt, West Greenland, using the sea ice biomarker IP~25~ and other specific phytoplankton biomarker (i.e., brassicasterol, dinosterol, HBI III) records. Our biomarker record indicates that Disko Bugt experienced a gradual expansion of seasonal sea ice during the last 2.2 kyr. Maximum sea ice extent was reached during the Little Ice Age around 0.2 kyr BP. Superimposed on this longer term trend, we find short-term oscillations in open water primary production and terrigenous input, which may be related to the Atlantic Multidecadal Oscillation and solar activity changes as potential climatic trigger mechanisms. A direct sample-to-sample multiproxy comparison of our new biomarker record with microfossil (i.e., benthic foraminifera, dinocysts, and diatoms) and other geochemical records (i.e., alkenone biomarkers) indicates that different proxies are influenced by the complex environmental system with pronounced seasonal changes and strong oceanographic gradients, e.g., freshwater inflow from the Greenland Ice Sheet. Differences in sea ice reconstructions may indicate that the IP~25~ record reflects only the relatively short sea ice season (spring), whereas other microfossil reconstructions may reflect a longer (spring–autumn) interval.