ANZICE Part 3: Southern Ocean – New Zealand Responses

By Matthew Wood 05/06/2010


Sediment core 1New Zealand is a geographically lonely place. It is the only major landmass between the tropics and Antarctica at these longitudes, and shares the southern mid-latitudes with only Patagonia and Tasmania. As such, it is a fantastic natural laboratory for investigating oceanic and atmospheric change in the southern hemisphere.

Our country is perpetually being ground down by the elements in response to rapid uplift. Terrigenous sediment makes its way down river systems to be distributed far into the deep ocean, forming thick, continuous sedimentary sequences. While the sediment itself can appear outwardly unadorned, the real story is locked within the calcareous remains of plankton that have lived their short time in our seas and subsequently dropped to the seafloor to be entombed within the layers of mud. The geochemistry of their tiny shells can be used as proxy data for ocean temperature and salinity, and to assess changes in ocean currents over time.

Phytoplankton are autotrophs and thus live in surface waters of the ocean. When conditions are right — when the waters are sufficiently warm, well-lit and rich in nutrients — massive algal blooms may be initiated, often forming along major ocean fronts. These populations of microscopic plants may be so extensive as to be easily visible from space. Such blooms are thought to flourish during warm periods — evidence of an extreme case being the chalk deposits now exposed in the famous white cliffs of Dover, formed during the super hothouse world of the late Mesozoic. Such blooms appear to be increasing off New Zealand today: can this be attributed to global warming?

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This is one of the questions that the Southern Ocean — New Zealand Responses research stream of ANZICE is currently attempting to answer. Whether these blooms are driven primarily by ocean temperature, or by the amount of incoming solar radiation, is unknown. Answering this question is important because phytoplankton are the base of the marine food chain and so any changes at this level will propagate through the whole system. They also provide their own feedback into the climate system by producing atmospheric acids that act as condensation nuclei for clouds. The calcium carbonate tests of plankton also comprise a significant carbon sink in the deep ocean, particularly so in the Southern Ocean during glacial periods, as outlined in a very recent paper.

Along with marine microfossils in these seafloor sediments are the robust pollen grains of land plants, transported to their marine resting place by the vigours of water and wind. By coring these sediments from oceanographic research vessels, ARC scientists are able to identify changes in vegetation cover on land in response to climate change. This wealth of information is made even more valuable by the logistical challenges of sampling it. This work would not be possible without collaboration with GNS Science, NIWA and research groups abroad.

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A recently initiated study by the group is a classic case of uniformitarianism; the geological premise that the present is the key to the past. Beneath the central South Island glacial lakes are hundreds of metres of finely layered silts. It is not known what this layering represents and so it is prudent to first understand the modern day processes in these lakes before starting to make inferences about the past. Instruments have been deployed to measure what happens on a monthly basis in terms of water and sediment flux through the Lake Ohau system.

Gavin Dunbar returned to the Antarctic Research Centre in 2005 after working at the Australian National University (ANU), where he was interested in the climate history of the Western Pacific as shown in coral and speleothem geochemical records. Gavin now leads the Southern Ocean — New Zealand Responses group of ANZICE and supervises a number of post-graduate students who he has seen become experts in their own fields.

Satellite image from SeaWIFS. Photo (c) Matthew Wood 2006