Integrated Ocean Drilling Program (IODP) Expedition 302, “The Arctic Coring Expedition” (ACEX), unearthed the most significant find of Paleogene siliceous microfossils in nearly 2 decades. 100 m of early middle Eocene, organic-rich, finely laminated sediments contain abundant marine and freshwater siliceous microfossils allowing intriguing insights into central Arctic paleoenvironments during the start of Cenozoic cooling. Largely endemic assemblages of marine diatoms and ebridians are preserved along with very high abundances of chrysophyte cysts, the endogenously formed resting stage of freshwater algae. An overall brackish environment is invoked, but variations in group dominance suggest episodic changes in salinity, stratification, and trophic status. With the backing of inorganic geochemistry we synthesize the sediment characteristics by hypothesizing an environmental model for the cooccurrence of these diverse siliceous microfossil groups. We also report on initial insights into the composition of some of the laminations, which may help explain the formation of this rich sediment archive.