A total of 289 belemnite guards was analyzed from the Boreal Barremian (northeastern England and northwestern Germany) for their trace elements and isotopic composition. The oxygen-isotope signature shows similar paleotemperature variations for all seven sections investigated. Three different phases of climatic evolution can be recognized. An earliest Barremian cool phase (Praeoxyteuthis pugio belemnite Zone) is followed by a distinctive warming event in the late early Barremian (Aulacoteuthis spp. belemnite Zone). This Aulacoteuthis warm pulse has a relatively short-term peak (~500 kyr), which accompanies widespread anoxic bottom-water conditions and records the highest temperature throughout the Valanginian–Barremian in the Boreal Realm. The late Barremian (Oxyteuthis brunsvicensis, O. germanica, and O. depressa belemnite Zones) is characterized by relatively positive δ¹⁸O values reflecting a cooling phase. This paleotemperature interpretation of the oxygen data is supported by the Mg/Ca trends. The carbon isotope curve of the Barremian shows an increase from 1‰ in the early to 3‰ in the late Barremian. This trend may reflect the global signal of the carbon budget of Barremian oceans and be related to the widespread deposition of TOC-rich sediments in the southern part of the Boreal Realm and the progradation of the Urgonian carbonate platforms in the Tethys. There is evidence, however, that paleobiological factors may have played a much more important role in the incorporation of stable carbon isotopes than hitherto thought. In a monospecific belemnite population collected from one 30-cm-thick interval, the δ¹³C signal shows a variation of 1.5‰. Ontogenetic changes in the metabolic activity linked to temperature or food changes may explain changes in the fractionation rate, resulting in an ontogenetically controlled biofractionation.