A remarkable 3-m-thick sequence of laminated diatom ooze (LDO) has been recovered in the North Atlantic Ocean. The LDO is monospecific and consists of the pennate diatom Thalassiothrix longissima. Biostratigraphy and benthic oxygen isotope stratigraphy indicate that the LDO was deposited during substage 5e (Eem). In analogy to recent observations of massive surface concentrations of diatoms in the equatorial Pacific and Neogene examples, we suggest that the LDO represents numerous depositional events of Thalassiothrix mats that were physically accumulated at a subarctic convergence front. The occurrence of the LDO, combined with other climate proxies, indicates substantial geographic stability of ocean fronts in the North Atlantic during substage 5e.