Palaeoenvironmental reconstruction and biostratigraphy with marine palynomorphs of the Plio–Pleistocene in Tjörnes, Northern Iceland

Abstract

Deposits from the Tjörnes Peninsula in northern Iceland enable the assessment of past ocean currents and the influence of the nearby island. Most palaeoecological studies with dinoflagellate cysts from the northern and central Atlantic focus on oceanic or shelf settings and deal with outer neritic and oceanic species. Dinoflagellate cyst studies of marginal marine settings are scarce and represent only short time intervals. The Tjörnes section however accommodates 1200m of sediments that are mainly shallow marine. The sediments are attributed to the Lower Pliocene Tjörnes beds and the overlying Upper Pliocene to Pleistocene Breidavík Group. The dinoflagellate cysts and other marine palynomorphs from 68 samples from the Tjörnes beds and 20 samples from the Breidavík Group are studied. The deposits are divided into five dinoflagellate cyst assemblage zones (DAZ) and one barren interzone (BIZ). The changes in the assemblages proved to be independent of the changes in bathymetry of the sedimentary environment. Heterotrophic dinoflagellate cysts dominate in DAZ1. An abrupt impoverishment in species diversity is observed in DAZ2 when autotrophic species dominate the assemblage. Slightly preceding the entrance of Pacific molluscs in the area in unit 14 of the Serripes Zone, heterotrophic species (unit 12/13) re-enter the area in DAZ3. The marked decrease and return of the heterotrophic species does not relate to conditions of preservation, but may result from the loss of nutrients. The upper part of DAZ1 and the base of DAZ3 reflect major changes in the palaeoceanography and span a period during which the polar front may have moved temporarily from the area. The first event situated at the top of DAZ1 occurred in litholog unit 4 halfway the Tapes Zone between circa 5 and 4.8Ma. The second event at the top of litholog unit 12 around 4.5Ma has been linked to the shoaling of the Central American Seaway. Heterotrophic dinoflagellates disappear almost completely from the area during the deposition of the Pleistocene Breidavík Group (DAZ4-5). A transition from a heterotrophic dominated assemblage to an autotrophic dominated assemblage is observed in the Hörgi Formation (DAZ4a). An assemblage dominated by autotrophic cold water species, comparable to the present-day assemblage of the area north of Iceland, occurs from unit 10 in the Pleistocene Svarthamar Member on (DAZ5). This study underscores that the distribution of temperature sensitive dinoflagellate cysts is influenced by the availability of nutrients and changes in ocean currents.