Climate Change in the Southern Ocean During the Middle Eocene to Early Oligocene: A Palynological Perspective

<p>The Antarctic and Southern Ocean is an area that was greatly affected by climatic changes during the Middle Eocene to Early Oligocene (E/O). This study aims to document climate-induced changes of the marine palynomorphs (mostly dinoflagellate cysts) by developing a Circum-Antarctic biozonation, and establishing the distribution and effects of cooling on the palynomorph assemblages and the palaeoenvironment. Samples were obtained from four sites by the Ocean Drilling Program (ODP) and Deep Sea Drilling Project (DSDP) and the palynological content was analyzed. These sites were selected as they may record effects of circulation changes and cooling trends, reflecting climate changes. ODP site 696B, was inner neritic and located in the South Orkney microcontinent, Weddell Sea. The other three sites were pelagic with ODP 699A located on the Northeast Georgia Rise, Falklands, ODP 748B located in the Western part of the Raggart Basin, Kerguelen Plateau and DSDP 277 located in Cathedral Depression on the Southern Campbell Plateau. From this analysis a comprehensive record of the local climatic transitions was ascertained, utilising assemblages of fossil organic-walled dinoflagellate cysts (dinocysts), acritarchs, prasinophyte algae, microforaminiferal linings, scoledonts and terrestrial material. The establishment of a standard nannofossil biostratigraphy provided a temporal framework of the marine palynomorphs in each site. Key dinocyst datums recognised as first or last occurrences that correlated reliably between sites, were compared with the nannofossil zones of each site. The purpose was to provide a Middle Eocene/Early Oligocene dinocyst biostratigraphy that temporally constrains the assemblages. Six primary datums and two secondary datums resulted, which lead to the recognition of four biozones and established a new biozonation in the Southern Ocean. The zones identified various dinocyst events, giving an indication of their probable palaeoenvironments. The Circum-Antarctic distribution pattern of palynomorph assemblages was documented in each site as a total abundance of grains per gram. The ranges and composition of palynomorph assemblages of each site were displayed as a percentage of total abundance. To document any cooling effects the dinocysts were separated into Gonyaulacoid or Peridinioid assemblages to best illustrate their preferred palaeoenvironment. Key findings showed that the pelagic and inner neritic sites differed greatly. Site 696B was the most climatically stable site and dominated by Peridinioid dinocysts which are predominantly heterotrophic. They are most common in an inner neritic palaeoenvironment, but are also present in nutrient upwellings and eutrophic surface waters with lower sea surface temperatures. Terrestrial material dominates all marine palynomorphs in this site which was shallow and warm with few changes up to the E/O boundary ~33.7 Ma, but became more variable in the Early Oligocene. In comparison, the pelagic sites (699, 748B, 277) were unstable, with missing or condensed sediments and no palynomorphs present at differing times. They were mostly dominated by Gonyaulacoid dinocysts, which are autotrophs and generally located in outer neritic to open ocean palaeoenvironments. The non dinocysts of Prasinophyte algae and Leiosphaeridia palynomorphs appeared in higher abundances in the pelagic sites than in the inner neritic sites. Specific dinocyst markers identified temperature changes within the pelagic sites, subsequently highlighting the climatic changes that occurred during the Middle Eocene/Early Oligocene. From ~46 Ma the pelagic sites recorded conditions that were oceanic and cooler. Between ~44 - ~41.5 Ma, a warming in site 748B indicated enhanced stratification and elevated nutrient availability. This was not the case in site 696B and may be due to warm temperatures already present. From ~41.4 Ma the pelagic sites showed that the palaeoenvironment continued to cool, indicated by the presence of Leiosphaeridia and Prasinophyte algae. The palaeoenvironment was oceanic with upwelling and offshore sea surface productivity, illustrated by the Gonyaulacoid and Peridinioid dinocysts present. During the Late Eocene from ~37 Ma a transitional and changeable palaeoenvironment was shown by the high numbers of Operculodinium spp present in sites 696B and 277. From ~33.7 Ma (E/O boundary) in the pelagic sites, most Peridinioid dinocysts had disappeared and very few Gonyaulacoid dinocysts were present. In contrast the inner neritic site (696B) Peridinioid dinocysts were still dominant, and a more gradual disappearance of all the marine palynomorphs was evident.</p>