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a b s t r a c tA reconstruction of Holocene sea ice conditions in the Fram Strait provides insight into the palaeoenvironmental and palaeoceanographic development of this climate sensitive area during the past 8500 years BP. Organic geochemical analyses of sediment cores from eastern and western Fram Strait enable the identification of variations in the ice coverage that can be linked to changes in the oceanic (and atmospheric) circulation system. By means of the sea ice proxy IP 25 , phytoplankton-derived biomarkers and ice rafted detritus (IRD) increasing sea ice occurrences are traced along the western continental margin of Spitsbergen throughout the Holocene, which supports previous palaeoenvironmental reconstructions that document a general cooling. A further significant ice advance during the Neoglacial is accompanied by distinct sea ice fluctuations, which point to short-term perturbations in either the Atlantic Water advection or Arctic Water outflow at this site. At the continental shelf of East Greenland, the general Holocene cooling, however, seems to be less pronounced and sea ice conditions remained rather stable. Here, a major Neoglacial increase in sea ice coverage did not occur before 1000 years BP. Phytoplankton-IP 25 indices ("PIP 25 -Index") are used for more explicit sea ice estimates and display a Mid Holocene shift from a minor sea ice coverage to stable ice margin conditions in eastern Fram Strait, while the inner East Greenland shelf experienced less severe to marginal sea ice occurrences throughout the entire Holocene.

a b s t r a c tIn recent years, a novel proxy for the past occurrence of Arctic sea ice has been proposed that is based on the variable marine sedimentary abundance of an organic geochemical lipid derived from sea ice diatoms in the spring. This lipid, termed IP 25 (Ice Proxy with 25 carbon atoms), is a highly branched isoprenoid mono-unsaturated alkene that appears to be sufficiently stable in sediments to permit meaningful palaeo sea ice reconstructions to be carried out over short-to long-term timescales. Since the first proposed use of IP 25 as a proxy for palaeo sea ice by Belt et al. (2007), a number of laboratories have measured this biomarker in Arctic sediments and it is anticipated that research activity in this area will increase further in the future. The content of this review is divided into a number of sections. Firstly, we describe the scientific basis for the IP 25 proxy and its initial discovery in Arctic sea ice, sedimenting particles and sediments. Secondly, we summarise the relatively few studies that have, to date, concentrated on examining the factors that influence the production and fate of IP 25 and we identify some areas of future research that need to be addressed in order to improve our understanding of IP 25 data obtained from sedimentary analyses. What is clear at this stage, however, it that the presence of IP 25 in Arctic marine sediments appears to represent a proxy measure of past seasonal sea ice rather than permanent or multi-year ice conditions. Thirdly, we highlight the importance of rigorous analytical identification and quantification of IP 25 , especially if measurements of this biomarker are going to be used for quantitative sea ice reconstructions, rather than qualitative analyses alone (presence/absence). Fourthly, we review some recent attempts to make the interpretations of IP 25 biomarker data more detailed and quantitative by combining sedimentary abundances with those of phytoplankton-and other sea ice-derived biomarkers. Thus, the bases for the so-called PIP 25 and DIP 25 indices are described, together with an overview of potential limitations, concluding that investigations into the use of these indices needs further research before their full potential can be realised. In the final section, we provide a summary of IP 25 -based palaeo sea ice reconstruction case studies performed to date. These case studies cover different Arctic regions and timescales spanning decades to tens of thousands of years.

Sea ice is a critical component of the climate system: variations in sea-ice cover affect the albedo of polar regions, and also the rate of deepwater formation 1,2 . Changes in the sea-ice cover of the North Atlantic Ocean are thought to have been related to abrupt climate changes throughout the last glacial termination 3 , but reconstructions of sea-ice conditions are rare. Here we use the sedimentary abundance of the IP 25 and brassicasterol biomarkers, produced by sea-ice-associated diatoms and open-water phytoplankton, respectively, to generate a record of sea-ice conditions in the northernmost Atlantic Ocean for the past 30,000 years. Our reconstruction shows that a stationary margin between sea-ice cover and the open ocean existed during the Last Glacial, although perennial sea-ice cover prevailed for most of the Last Glacial Maximum. An early warming about 14,000 years ago was associated with ice-free conditions; however, seasonal sea ice was present throughout the Holocene. We find temporal links between our record of sea ice and reconstructions of the amount of relatively warm Atlantic water advected into the Nordic Seas 4,5 . We therefore conclude that changes in sea-ice conditions are linked to regional and global climate anomalies and oceanographic circulation in the North Atlantic.The distribution of sea ice in Fram Strait, the only deepwater connection (∼2,600 m mean water depth) between the Arctic and Atlantic oceans, is mainly controlled by the inflow of temperate water from the North Atlantic along the western continental margin of Spitsbergen through the Norwegian and West Spitsbergen currents 1 . In contrast, the East Greenland current carries cold water (and sea ice) southward through this gateway 1 (Fig. 1). The relative contributions of these currents strongly influence the thermohaline circulation, thus contributing to global climate 1 .Previously, it has been shown that, when detected in marine sediments, a C 25 isoprenoid lipid (IP 25 ) biosynthesized by Arctic sea-ice diatoms acts as a proxy for previous spring sea-ice occurrence and subsequent melt 6,7 , whereas the phytoplanktonderived sterol brassicasterol, reflects open-ocean conditions during summer 8 . In the current study, we present organic carbon 9 (global productivity and terrigenous organic carbon input) and fluxes of IP 25 (sea ice) and brassicasterol (phytoplankton; ref. 9) for a sediment core (PS2837-5) from the western flank of the Yermak Plateau (81 • 13.99 N, 02 • 22.85 E, northwest of Spitsbergen, 1,042 m water depth; ref. 10) close to the present-day summer sea-ice margin in Fram Strait (Fig. 1). We use the previously reported age model of PS2837-5, which is based on 14 accelerator mass spectrometry 14 C ages of tests of the planktic foraminifer Neogloboquadrina pachyderma sin. and assumes a marine-reservoir correction of 400 years and linear interpolation between 14 C-dated horizons 11 . In the following, ages are given in calibrated calendar years before present. For much of the interval between 30 and 17 kyr bp (La...

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