The Future from Our Past: Molecular Fossils & Climate Change
Studying past climate is an essential part of climate prediction. By observing past climate changes scientists can use models to predict what may happen in the future. So, how do climate scientists study past climate?
Instrumental records exist of the Earth’s climate for the last 200 years. Records such as thermometer readings of air and sea surface temperature and sea level were taken by individuals and research centres across the world. However, 200 years is only a tiny fraction of Earth’s history. To study further into the past, scientists have to use proxies for climate parameters. Biomarkers are such a proxy. Biomarkers are hydrocarbon fossils left behind by plants or animals. Just like dinosaur bones imply the presence of dinosaurs existing, pollen implies the presence of plants that once covered the land. Biomarkers constitute materials such as waxes that cover leaves and the lipids structuring algal cell walls. After organisms are deceased, biomarkers are left behind and are preserved in the environment as molecular fossils.
The amazing thing about molecular fossils or biomarkers, is that they can be preserved in marine and terrestrial sediments for thousands to millions of years, making it possible to study deep into past environments and climates around the world. The Biomarkers for Environmental and Climate Science (BECS) researchers focus on terrestrial sites, to better understand regional scale climate changes. In addition to unravelling Earth’s past climates, some PhD students in the BECS lab are also researching if biomarkers can be found in the high altitude, high UV plains of the Altiplano,Chile as an analogue for life on Mars. They are also researching what biomarkers can be found in food vessels of ancient civilisations! These studies involve the detection of biomarkers, rather than their use to reconstruct palaeo-climates. Projects are taking place across the world from the study of regional droughts in Canada to investigations into changes in the North Atlantic climate oscillations in Europe.
The BECS group use a variety of biomarkers to help reconstruct past climates. Below is an introduction to a few molecular fossils and how they are used. If you really want to flex your organic chemistry knowledge you can get a full overview of biomarkers found in lake environments in a great review by Castañeda and Schouten (2011), found here: doi:10.1016/j.quascirev.2011.07.009.
Alkanes and Alkanoic acids: Straight chain hydrocarbons produced by bacteria, algae and terrestrial and aquatic plants for waxes on leaves.
Chemical Structure:
Generalised alkane structure
These tell us about: Vegetation and precipitation
How they relate to climate: The average chain length of alkanes and acids are characteristic of terrestrial and aquatic plants. The ratio of differing chain lengths can be used to identify relative inputs of terrestrial or aquatic material into a lake system, allowing the study of past environmental changes. Even more climate information can be gained from studying the carbon and hydrogen isotopes that comprise alkanes and alkanoic acid molecules. This provides insight to the kinds of vegetation growing and the climate conditions such as aridity and atmospheric carbon dioxide concentrations. Hydrogen isotope analysis of alkanes and acids from plant leaf waxes are also being researched for their use as a proxy for rainfall.
This year alkanes have been used along with other geochemical proxies to reconstruct centennial-scale vegetation change and North Atlantic Oscillations during the Holocene for Southern Spain.
Alkenones: Alkenones are lipids produced by a restricted number of haptophyte algae. They are long straight chain hydrocarbons with either 2, 3 or 4 carbon-carbon double bonds (called unsaturations) at varying positions along the chain and an oxygen attached at the start of the molecule.
Chemical Structure:
Generalised alkenone structure
These tell us about: Temperature
How it relates to climate: Algae respond to changes in climate by altering the kinds of alkenones produced. When it is warm, more alkenones with less double bonds are made, whereas when it is cold more alkenones containing more double bonds are produced. The ratio of these molecules is used in an index of ‘unsaturation’. Alkenones are successfully used to reconstruct past ocean surface temperatures but the index used for this does not function well for terrestrial temperature reconstructions. Unlike the oceans, lakes in different regions of the world can contain many species of algae which makes it difficult to produce a temperature index. The BECS lab is thus studying different regions; in Canada, Japan and the UK, to develop temperature calibrations for terrestrial sites. Work on a site in the Lake District has detected alkenone concentrations in the Bølling – Allerød time, when a rapid warming took place that could be similar to global warming today. The next step is to try to calibrate the alkenones to temperature to produce a reconstructed temperature record and an idea of the rate at which the warming occurred.
Long Chain Diols: Hydrocarbon chains with an alcohol group (oxygen and hydrogen) at the start of the molecule and at a mid-chain position. Their source is not well defined but has been shown to come from algae and aquatic plants.
Chemical Structure:
Generalised diol structure
These tell us about: Lake surface temperature
How it relates to climate: Diols are organic chemical compounds of varying lengths. Different chain lengths are found in lake sediments, and it is the abundance of these chain lengths that is related to lake surface temperature. This biomarker is new to the ‘terrestrial climate scene’ and has a lot of potential for quantitative lake temperature reconstructions across the globe. Recently, a diols duo in the lab successfully reconstructed lake temperatures in the Sierra Nevada, Spain for the past 100 or so years.
GDGTs: GDGT (glycerol dialkyl glycerol tetraether) are cell membrane lipids of archaea (single celled micro- organisms) and bacteria.
Chemical Structure:
Generalised GDGT structure
These tell us about: Temperature and soil pH
How it relates to climate: The structure of GDGT is strongly dependent on environmental parameters such as air temperature and soil pH. This relationship has been quantified in a variety of indexes which can tell us the temperature and the pH of the environment where the Archaea and Bacteria lived. A BECS researcher is studying GDGTs in over 100 lakes in Canada to investigate continental scale environmental change. Interestingly, other scientists have found GDGTs in lake sediment dating back to the late Jurassic, this means that the climate could be reconstructed from when dinosaurs existed!
Day to day BECS researchers are working in the lab, carrying out organic geochemistry experiments to analyse the presence and abundance of all these biomarkers. The molecular fossils are analysed by Gas Chromatography (GC). The laboratory’s technician is vital to all the research that takes place in BECS, aiding the analysis of hundreds of samples every month. Dr Jaime Toney, the head of BECS group, a few post-doctorates, lots of PhD students and a handful of undergraduates take part in biomarker research across the year. Sometimes field work and collaboration takes them to amazing places around the world such as the Netherlands and Japan, but undeniably the best field work location has to be Scotland in the sunshine!
Some of the research going on at the moment includes the development of proxies for palaeo-temperature and environment in the English Lake District and Scotland. A project is investigating past climate change in the transition period between the last glacial and interglacial period. Climate reconstructions for this period are of great interest to scientists because the climate changed very rapidly then. The project will investigate how the climate and environment changed, e.g. what type of vegetation grew, how arid was the climate and what was the temperature. This biomarker research will try to put quantitative values on past regional climate changes. By using these values in climate models, models will be able to better predict future climate change for the regions we live in, rather than predicting a global average for the whole Earth.
