Miocene upwelling events: Neritic foraminiferal evidence from southern Australia

Publisher Website
Google Scholar
Abstract
The neritic stratigraphic section in the Lakes Entrance oil shaft in Gippsland, Victoria, southeastern Australia, records four major upwelling events at the third‐order or 106 years scale. The first and second occurred during the Janjukian (latest Oligocene to Early Miocene; at about 24.5 and 22 Ma); the third in the late Longfordian (late Early Miocene; 17.5–17 Ma); and the fourth at about 9–8 Ma in the Mitchellian (Late Miocene). The evidence for upwelling consists of concurrent signals from the planktonic and the benthic foraminifera. The four upwelling events are characterized by a low specific diversity in the plankton, a high abundance of the planktonic species Globigerina bulloides, and a high sedimentation rate (> 2cm/ka). Among the benthos, such infaunal species as Uvigerina proboscidea increased in abundance and the mixing of normally separate deeper‐and shallow‐water taxa was common. The ratio of epifaunal to infaunal benthic foraminifera increases through the Miocene as a second‐order secular trend (107 a) but it is marked by four strong reversals which are the four events. It is suggested that the northerly fluctuation of the Subtropical Convergence was largely responsible for the fourth of these events. The others, however, are on the rising second‐order trends in global warming and sea level and they alternate with warming events recognized on the occurrence of larger benthic foraminifera. The ‘upwelling events’ and the ‘warming events’ occur at the same third‐order scale as the putative eustatic cycles of sequence stratigraphy and the Mi glaciations based on oceanic δ??O fluctuations. The upwelling events are neritic signals of global oceanic changes, of which the most marked is at the third, situated at the onset of the Monterey carbon isotope excursion. The Miocene climatic optimum at the zeniths of high sea level and warm climate lacks an ‘upwelling event’. At this time there was a sharp reversal to oligotrophic conditions, a reversal seen also in the shallower and more restricted sea in the Murray Basin.