When you take a sample of sediment from the seafloor and remove all the carbonate and silica — nearly the whole sample in most cases — what are you left with?
Palynomorphs are organic microfossils that include the pollen and spores of terrestrial plants, the cellulose remains of marine phytoplankton called dinoflagellates, and other particulate organic matter. Palynology is an important part of the Antarctic Research Centre’s paleoclimate research because unlike other climate proxies, palynology allows the direct comparison between contemporaneous terrestrial (land plant succession) and marine (dinoflagellate cyst) records.
Joe Prebble recently returned to the Antarctic Research Centre to undertake a PhD project in palynology. He’s interested in quantitatively reconstructing environmental conditions of the New Zealand region during Marine Isotope Stage 11, a particularly warm and stable Pleistocene interglacial that is a good analogue for projected global temperatures of the near future. As Joe points out, the analogue falls short when considering atmospheric greenhouse gas levels: we have already gone well beyond the natural upper limit of atmospheric carbon dioxide variability during the Quaternary. By studying dinoflagellate cyst assemblages of modern seafloor sediment sample locations, and relating them to corresponding quantified ocean conditions (such as sea surface temperature), Joe will be able to generate a ‘cyst-based transfer function’ — effectively turning fossil cyst assemblages, from well-dated sediment cores, into paleo-thermometers.
A sediment trap program run by Dr. Scott Nodder, a marine geologist at NIWA, is helping to define the seasonality of ocean productivity (including dinoflagellates) in this region. Traps have been positioned to the north and south of the Chatham Rise (and the associated, overlying Subtropical Front) for the past decade. Time series data from this program will help in finding out exactly what time of the year dinoflagellates shed their cysts, allowing a more precise tie between modern cyst assemblages and empirical environmental data (variously sourced from the World Ocean Atlas and remote sensing satellites such as MODIS and SeaWIFS).
As previously reported, in late 2009 the Integrated Ocean Drilling Program research vessel JOIDES Resolution collected a record-breaking 1928 m sediment core from off the Canterbury Coast (Site 1352B). The core’s shore-proximal, continental shelf location makes it near-perfect for palynological research, with: a high sedimentation rate (thus a high resolution record); rapid transport and deposition of large amounts of terrestrial pollen and spores; and a high local production of neritic dinoflagellate cysts. However, the Subtropical Front off eastern New Zealand is effectively locked in place throughout glacial cycles by the bathymetry of the Chatham Rise, whereas in other ocean basins the front may migrate by up to four or five degrees of latitude. Therefore interpreting ocean conditions close to the South Island’s east coast can be difficult. The cyst record from Site 594 may help provide a clearer picture of local oceanographic changes at these timescales.
For the transfer function to be reliable it needs to be shown that the modern seafloor cyst assemblages are truly representative of the overlying water column, i.e. that there has been no significant lateral transport or reworking. This will be one of the main challenges of the research, but if this assumption is demonstrated to be correct, Joe’s transfer function will be a hugely valuable addition to paleoclimate science in New Zealand.
Photo (c) 2005 Matthew Wood