Clues to Past Climates

The Story: When foraminifera make their shells into calcium carbonate, CaCO₃, they record the carbon and oxygen isotopic composition of the sea-water at the time they made. Isotopes are simply different forms of the same element (contain different numbers of neutrons in their nuclei), and the ratios of each isotope depends on various factors related to climate; for example oxygen isotope ratios reflect temperature, salinity and glacial ice volume, and carbon isotope ratios can trace different water masses. During glacial periods, much of the lighter oxygen isotope, O16 is locked up in polar ice caps, and oceans are enriched in the heavier isotope, O18. Foraminfera living in the seawater during this cold glacial period should have higher levels of O18 sand lower levels of O16 stored in their shells. To date, I have found many benthic foraminifera (see images) distributed throughout a 5.5m core obtained from Hanfield Inlet ( an fjord of estimated age, between 12,000- 30,000 years old. The actual date will be known once the radiocarbon-dating of organic foraminfera fragments from the core are complete!). Once this proxy has been calibrated to modern values (measured from carbon and oxygen isotope values of modern sea water at various depths), the foraminifera for both stable isotopes can be analysed. Variations downcore can be used to reconstruct sea-surface temperatures, paleosalinities and water mass circulation which are recorded by the foraminifera at the time of calcification. This should contribute to the reconstruction of climate change over the past 12 000 years.

A scanning electron microscope image of benthic foraminifera Haynesina depressula. From a sediment grab sample in Hanfield Inlet, around 35m depth, Auckland Islands. This species is a potential candidate for stable oxygen and carbon isotope analysis. Imaged at University of Otago.

Imogen is hoping that oxygen isotope sea temperature reconstruction from foraminifera in the core will show a temperature maximum around 9000-5000 years before present, which will reflect the Holocene climatic optimum (a period of warming in the recent past, where global temperatures were higher than present). Such a record has not previously been obtained from the Auckland Islands. Carbon isotope ratios will hopefully show interesting fluctuations in the carbon composition of sea-water, which will indicate larger scale flux of CO₂ between the atmosphere and the ocean. It is currently poorly understood how warming affects absorption of atmospheric CO₂ into the oceans and this has obvious ramifications for rates and implications of global warming processes.

The Polaris II Photo: Bernard Potter

The Scientist: Imogen Browne - Otago University.

Imogen Browne is working towards a Master of Science in Geology at Otago University. She is aiming to reconstruct climate change over the past 12,000 years from sediment cores obtained from fjords at the sub-Antarctic Auckland Islands, southern New Zealand, and was lucky enough to be part of the RV Polaris II cruise in 2014 to collect data. "As paleoclimate scientists, we wish to more fully understand the last glacial to deglacial transition in Southern New Zealand, and the effects that this transition had on sea level, and on ocean and atmospheric circulation in the Southern Ocean. We know that the Auckland Islands were completely glaciated, evident by the U-shaped valleys that we currently see on the eastern side of the main island. Currently we do not know what the magnitude and timing of sea-level rise was in this area! Understanding changes in circulation patterns are needed in order to accurately predict future impacts of climate change on New Zealand and in the Southern Ocean." In order to do this, paleoclimate scientists study a range of climatic 'proxies' which are naturally preserved archives that allow us to estimate previous climatic conditions. Calcareous-shelled, single-celled organisms called foraminifera are one such proxy, and these are the most commonly preserved organism that have been found in the sediment cores. A cylindrical sample of sediment is extracted using a round barrel up to 6 metres long. A heavy weight rests on the barrel, pushing it into the sediment. These are known as gravity corers. The top of the sediment core is the youngest sediment and the bottom of the sediment core is the oldest sediment. The general rule is that one centimetre of sediment core represents 10 years. 5.5 metres represents about 5500 years of sediment deposits.