The Aptian deposits on Spitsbergen Island are poorly studied. Moreover, there were no published data on fossil distribution in the Aptian and Albian in the vicinity of the town of Longyearbyen. This article provides new data on ammonite-based biostratigraphy of the Carolinefjellet Formation, indicating the presence of Beds with Tropaeum arcticum (middle Aptian) and Beds with Grantziceras (lower Albian). The age of the formation was verified and the presence of lower Albian strata was justified. The results of microscopic and isotope studies of five samples of glendonites collected from the middle Aptian of the Carolinefjellet Formation section are presented. Glendonites from the Carolinefjellet Formation are composed of three calcite phases: ikaite-derived calcite and two successive types of cement, which fill cavities and develop partially after the first-phase calcite. The O and C isotope compositions of glendonites were measured in five bulk samples from the middle Aptian interval of the section. The delta O-18 isotope composition of glendonites is significantly different from that of seawater, indicating the input of diagenetic fluids; the delta C-13 values are characteristic of sedimentary organic matter and methanotrophy. For the first time, findings of ammonites allowed us to clarify the stratigraphic intervals of glendonite occurrence and associated cold-climate episodes and prove the early Albian age of glendonites from the top of the Carolinefjellet Formation.