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& the Expedition 302 Scientists †The Palaeocene/Eocene thermal maximum, ,55 million years ago, was a brief period of widespread, extreme climatic warming [1][2][3] , that was associated with massive atmospheric greenhouse gas input 4 . Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition 5 . We show that sea surface temperatures near the North Pole increased from ,18 8C to over 23 8C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the ocean's bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations 6 , but the absolute polar temperatures that we derive before, during and after the event are more than 10 8C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms-perhaps polar stratospheric clouds 7 or hurricane-induced ocean mixing 8 -to amplify early Palaeogene polar temperatures.

The history of the Arctic Ocean during the Cenozoic era (0-65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm 'greenhouse' world, during the late Palaeocene and early Eocene epochs, to a colder 'icehouse' world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1-2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change. © 2006 Nature Publishing Group

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Except for a few discontinuous fragments of the Late Cretaceous/Early Cenozoic climate history and depositional environment, the paleoenvironmental evolution of the pre‐Neogene central Arctic Ocean was virtually unknown prior to the IODP Expedition 302 (Arctic Ocean Coring Expedition–ACEX) drilling campaign on Lomonosov Ridge in 2004. Here we present detailed organic carbon (OC) records from the entire ca. 200 m thick Paleogene OC‐rich section of the ACEX drill sites. These records indicate euxinic “Black Sea‐type” conditions favorable for the preservation of labile aquatic (marine algae‐type) OC occur throughout the upper part of the early Eocene and the middle Eocene, explained by salinity stratification due to freshwater discharge. The superimposed short‐term (“Milankovitch‐type”) variability in amount and composition of OC is related to changes in primary production and terrigenous input. Prominent early Eocene events of algae‐type OC preservation coincide with global δ13C events such as the PETM and Elmo events. The Elmo δ13C Event has been identified in the Arctic Ocean for the first time.

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...

[1] A reconstruction of Milankovitch to millennial-scale variability of sea surface temperature (SST) and sea surface productivity in the Pleistocene midlatitude North Atlantic Ocean (marine isotope stage (MIS) 16-9) and its relationship to ice sheet instability was carried out on sediments from Integrated Ocean Drilling Program (IODP) Site U1313. This reconstruction is based on alkenone and n-alkane concentrations, U 37 K 0 index, total organic carbon (TOC) and carbonate contents, X-ray diffraction data, magnetic susceptibility, and accumulation rates. Increased input of ice-rafted debris occurred during MIS 16, 12, and 10, characterized by high concentrations of dolomite, quartz, and feldspars and elevated accumulation rates of terrigenous matter. Minimum input values of terrigenous matter, on the other hand, were determined for MIS 13 and 11. Peak values of dolomite, coinciding with quartz, plagioclase, and kalifeldspar peaks and maxima in long-chain n-alkanes indicative for land plants, are interpreted as Heinrich-like events related to sudden instability of the Laurentide Ice Sheet during early and late (deglacial) phases of the glacials. The coincidence of increased TOC values with elevated absolute concentrations of alkenones suggests increased glacial productivity, probably due to a more southern position of the Polar Front. Alkenone-based SST reached absolute maxima of about 19°C during MIS 11.3 and absolute minima of <10°C during MIS 12 and 10. Within MIS 11, prominent cooling events (MIS 11.22 and 11.24) occurred. The absolute SST minima recorded directly before and after the glacial maxima MIS 10.2 and 12.2 are related to Heinrich-like event meltwater pulses, as supported by the coincidence of SST minima and maxima in C 37:4 alkenones and dolomite. These sudden meltwater pulses, especially during terminations IV and V, probably caused a collapse of phytoplankton productivity as indicated by the distinct drop in alkenone concentrations. Ice sheet disintegration and subsequent surges and outbursts of icebergs and meltwater discharge may have been triggered by increased insolation in the northern high latitudes.