Abstract:
In order to augment our understanding of paleoclimate and paleoceanographic changes during the Middle and Late Miocene in the Southern Ocean and Antarctica the diatom record obtained from a latitudinal transect across the Atlantic sector of the Southern Ocean was studied. Geographical distribution patterns, stratigraphical occurrences and abundance fluctuations of diatom species were investigated in six Miocene sedimentary sequences recovered by the Ocean Drilling Program (Sites 689, 690, 701, 704, 1088 and 1092). The results provide evidence for the thermal development of the Southern Ocean surface waters. Biostratigraphic investigations were carried out to establish or revise age models for these sedimentary sections. Additionally, these investigations reveal latitudinal differences in stratigraphic ranges of species, which are related to latitudinal differences in surface water masses reflecting the climatic development in Antarctica. Two stratigraphic diatom zonations are erected that are applicable to the areas south and north of the Polar Front. Thermal demands of extinct Miocene diatom taxa are deduced to calculate the ratio between warm-water diatoms and those indicating cold-water masses. The resulting relative paleotemperatures are used to estimate the development of the latitudinal thermal gradient. The thermal gradient reveals a short-term thermal decoupling of the Southern Ocean from the adjacent oceans at ca. 10.5 Ma after the Middle Miocene climate optimum (ca. 17-15 Ma). An increase of the thermal gradient is observed between 9.3 and 8.6 Ma. This increase led to a strong thermal decoupling of the Southern Ocean from the adjacent oceans, which continued up to the Miocene-Pliocene boundary (5.3 Ma). This is interpreted as a consequence of strong cooling on the Antarctic continent, which might be linked to the build up of the West Antarctic Ice Sheet. The temporary closure of the Panama Isthmus during the Late Miocene might have favoured ice accumulation in Antarctica by the establishment of an ocean circulation pattern, which was possibly similar to the recent one. During the Late Miocene several warm and cold periods superimposed by a long-term cooling trend could be discovered between ca. 7 and 5.3 Ma (Messinian). Latitudinal frontal displacements are delineated for this time period. The thermal history of the Southern Ocean generally has an impact on global climate development by volume changes of the Antarctic ice sheet causing sea level fluctuations. Beside tectonic movements, These cyclic sea level fluctuations are proposed to have caused the isolation, desiccation, cyclic sedimentation and reflooding of the Mediterranean basin. The revealed Southern Ocean and ANtarctic thermal development provides evidence for the influence of glacio-eustatic processes during the isolation of the Mediterranean basin.