Developing a novel proxy for Southern Hemisphere Holocene climate change: stable isotope analysis of restiad peat cellulose

The study of how and why climate has changed in the past is an important element in the scientific drive towards understanding and predicting how it may change in the future. We can answer a range of questions about past climate by studying one of a number of environments that hold a record of past changes. There are answers to our questions in ice cores, the varying width of tree rings, the growth rate of stalagmites in caves or in the make up of sediments such as those at the bottom of lakes or in peat bogs.

Peat sediments develop steadily over thousands of years and in the type of bog we study, the growth of plants on the surface is related directly to the prevailing climate. So if the climate gets wetter, the plants change in response. Then, if the climate gets drier, they change again. As the bog grows upwards, a record of all of those changes is preserved, so we can now stand on the surface, take a core back through all the layers and analyse what's been happening.

There are many methods we can use to do this. We can look at the plants themselves, or at amoebae that live on the bog surface. Another method that has only recently been applied to studies of peat bogs is to look at changes in the ratios of different isotopes captured in the plant remains.

Isotopes are atoms of the same element that contain the same number of protons, but a different number of neutrons. Isotopes can be either stable or radioactive, but here we are just interested in the stable isotopes. Oxygen, for example, has three different stable isotopes known as oxygen-16, -17 and -18. More than 99% of all oxygen is oxygen-16, which contains eight protons and eight neutrons. Only about 0.2% is oxygen-18, which has two extra neutrons, making it heavier than oxygen-16.

The isotope signal in bogs comes from the precipitation that plants use to construct cellulose, an organic compound that forms their cell structure. We can relate the record to past climate because, under different climatic conditions, lighter or heavier oxygen isotopes are more common in precipitation.

Previous studies of changing isotope ratios from peat bogs have used a particular type of moss, called Sphagnum, from which to derive their measurements. This is effective, but is also limited, both to geographical areas where Sphagnum occurs and also to the parts of a core where Sphagnum is present; nobody wants gaps in their record.

We want to address these two issues by testing the applicability of studying isotopes in a different type of peat that is found in regions where Sphagnum is less common. In the Southern Hemisphere, bogs are generally dominated by higher, or vascular, plants rather than mosses; these are plants that can actively control the movement of water and nutrients in their tissue. Bogs dominated by higher plants are widespread globally, but because of the differences in biology between them and mosses, we can't be certain that the isotope method is applicable without rigorous testing. Our initial results suggest that a reliable record of past climate can be derived from these bog types, but to be certain, more research is needed.

We will use bogs in New Zealand, dominated by a species of rush, to perform further tests. We will study the rush on different sites over the course of a year to fully understand how the isotope signal is incorporated into the plant remains. If we can demonstrate that the isotope method can be applied to this peat type, it will be a big step forward; the method would be applicable over a much wider geographical area and we will be able to address pressing research questions about past climate change more so than at present.

This project is primarily focused on developing a new technique for reconstructing atmospheric circulation patterns and moisture variability from stable isotope records in vascular plant dominated peatlands. The immediate impact of the work is therefore primarily in the academic community. If we can develop this technique, it has the potential to greatly improve our understanding of atmospheric circulation changes and moisture variability in many areas of the world because it could be applied to many regions for which suitable data are currently lacking. Ultimately this would have significant impacts in our understanding of and adaptation to future climate variability. Some of the most significant impacts of future climate change are related to changing hydrological cycles, yet this aspect of climate projection remains an area of great uncertainty and climate models show wide disagreements in the sign and strength of future hydrological change in many critical regions. The contribution of this project to the resolution of some of these problems through palaeoclimate-model intercomparisons is the greatest non-academic impact of the work, but is some way off. Nonetheless, there are a range of organisations, predominantly based in New Zealand, that can benefit more directly from the ongoing process of and outputs from our research. There are also areas of outreach to the non-science community which will be undertaken during the project. Here, we name the organisations and outline how they will benefit from our research. The specific methods of engagement are addressed in detail in the Pathways to Impact document.

The New Zealand Ministry of Science and Innovation is the lead government agency charged with driving the science and innovation sector. It supports eight Crown Research Institutes of which three have direct relevance to, and can benefit from, our research. These are GNS Science, which hosts the National Isotope Centre, Landcare Research and the National Institute for Water and Atmospheric Research. These are government owned, but function as independent environmental research and consultancy companies. In each case, aspects of our proposed research contribute to the organisations' own research goals. Engagement with these organisations will stimulate and inform both our own and their research agendas plus lead to potential future collaborations using the methods we develop here.

The International Atomic Energy Agency is a multinational organisation that hosts the Global Network for Isotopes in Precipitation. The aims of the latter organisation can be explicitly supported by provision of our new isotope data to their database.