The Greenland‐Norwegian Seaway: A key area for understanding Late Jurassic to Early Cretaceous paleoenvironments

Abstract

The paleoclimatology and paleoceanology of the Late Jurassic and Early Cretaceous are of special interest because this was a time when large amounts of marine organic matter were deposited in sediments that have subsequently become petroleum source rocks. However, because of the lack of outcrops, most studies have concentrated on low latitudes, in particular the Tethys and the “Boreal Realm,” where information has been based largely on material from northwest Germany, the North Sea, and England. These areas were all south of 40°N latitude during the Late Jurassic and Early Cretaceous. We have studied sediment samples of Kimmeridgian (∼154 Ma) to Barremian (∼121 Ma) age from cores taken at sites offshore mid‐Norway and in the Barents Sea that lay in a narrow seaway connecting the Tethys with the northern polar ocean. During the Late Jurassic‐Early Cretaceous these sites had paleolatitudes of 42–67°N. The Late Jurassic‐Early Cretaceous sequences at these sites reflect the global sea‐level rise during the Volgian‐Hauterivian and a climatic shift from warm humid conditions in Volgian times to arid cold climates in the early Hauterivian. The sediments indicate orbital control of climate, reflected in fluctuations in the clastic influx and variations in carbonate and organic matter production. Trace element concentrations in the Volgian‐Berriasian sediments suggest that the central part of the Greenland‐Norwegian Seaway might have had suboxic bottom water beneath an oxic water column. Both marine and terrigenous organic matter are present in the seaway sediments. The Volgian‐Berriasian strata have unusually high contents of organic carbon and are the source rocks for petroleum and gas fields in the region. The accumulation of organic carbon is attributed to restricted conditions in the seaway during this time of low sea level. It might be that the Greenland‐Norwegian segment was the deepest part of the transcontinental seaway, bounded at both ends by relatively shallow swells. The decline in organic matter content of the sediments in the Valanginian‐Hauterivian indicates greater ventilation and more active flow through the seaway as the sea level rose. The same benthic foraminifera assemblages are encountered throughout the seaway. Endemic assemblages of arenaceous foraminifera in the Volgian‐Berriasian give way to more diverse and cosmopolitan Valanginian‐Hauterivian benthic communities that include calcareous species. The foraminiferal assemblages also suggest low oxygen content bottom waters during the earlier Cretaceous, changing to more fully oxygenated conditions later. The calcareous nannoplankton, particularlyCrucibiscutum salebrosum, which is rare at low latitudes and abundant in high latitudes, reflect the meridional thermal gradient. They indicate that the Greenland‐Norwegian segment of the seaway was north of a subtropical frontal zone that acted as a barrier between the Tethyan and Boreal Realms. This implies the existence of stable climatic belts during the early Valanginian and Hauterivian, significant meridional temperature gradients, and moderate “ice house” conditions.