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.

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In order to reconstruct the depositional environment from the Laptev Sea continental slope and shelf during the past~15,000 years BP maceral analysis was carried out on two sediment cores (PS2458-4, PS2725-5) and compared with organic-geochemical parameters. During the transition from the Last Glacial to the Holocene the environment of the Laptev Sea shelf was controlled by the post-glacial sea level rise, variations in river discharge, surface-water productivity, and Atlantic-water inflow along the Eurasian continental margin. Based on our results, we identify the following significant changes of the environment: (a) at approximately 13,500 years BP the first step of deglaciation (Termination 1a) is documented by the deposition of marine and fresh-water organic matter; (b) at approximately 10,400 years BP the first post-glacial influence of Atlantic-water inflow along the Eastern Laptev Sea continental margin is indicated by an increase in marine organic matter; (c) at the beginning of the Holocene an increased fluvial supply is documented by an increase in fresh-water alginite; and (d) since~9500±8000 years BP modern marine conditions are established at the Laptev Sea continental margin as documented in increased amounts of marine macerals, biomarkers (dinosterol, brassicasterol, short-chain fatty acids), and dinoflagellate cysts.

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