Project

Discipline

Compound-specific 14C; organic geochemistry; radiocarbon; paleolimnology; soil carbon; Holocene; Carbon cycle

Lead
Lay summary
Over the past decades, much progress hasbeen made in the understanding of past climate and environments, and much ofthis information has come from the bottom of lakes and oceans, where remains ofplants, algae and eroded soil accumulate over time. However, there are alsostill many unknowns about how global climate has changed through time, animportant information if we want to predict future. There are practical issuesthat make research in this direction difficult. In this project, we try tosolve two of these issues, using the relatively new technique ofcompound-specific radiocarbon analysis. That is, determining the age ofmolecules that derive from living organisms with radiocarbon dating. In theClimate Geology group at the ETH we are able to do such molecular-scaleseparations, and small-scale radiocarbon measurements can be performed at theLaboratory of Ion Beam Physics of the ETH. The first goal of the project is toimprove understanding of soil organic carbon dynamics (SOC) and how it haschanged over the course of centuries and millennia. Such knowledge is importantfor predictions of the carbon dioxide (CO2) content in the atmosphere, becauseSOC is an important reservoir of the global carbon cycle. SOC consists mainlyof dead plant material that has been sitting in the soil for a certain amountof time. This carbon has thereby 'escaped' from the atmosphere when it was usedby the plants to grow. However, soil bacteria also slowly re-oxidize the SOC toCO2, and the total amount of SOC preserved in soils is thus result of thebalance between input of plant material and output by bacteria. By looking atthe age of soil-derived components that have accumulated in lake sediments andcomparing that to the age of the lake sediments itself, we gain insight in theaccumulation of SOC in the soils and how the soil carbon pool has behavedthrough time. In the second part of the project, we usemolecular-scale radiocarbon analysis for dating. Often, the age of a sediment layerin a lake or ocean can be determined only based on allochtonous material like treeleaves that were produced the same year they were sedimented. Their radiocarbonage is then representative for the time of deposition of that layer. However,sometimes there are no recognizable leaf remains and dating of the sediments isnot possible. In this project we develop further the technique of radiocarbondating on a molecular scale. The origin of molecules (for instance algae thatgrow in the lake, die the same year, and end up in the sediment) can bedetermined using chemical analysis. In this way, dating may be possible onsediments while it is not possible by traditional methods. In addition we are usingthe same technique to date pottery from the Maya, in collaboration with Dr. DouglasKennett, University of Oregon.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Self-organised

Number	Title	Start	Funding scheme

With this proposal we seek funding for the continuation of SNF-grant 200021-11950, titled "Compound-Specific Radiocarbon Analysis of Lake Sediments: A New Tool for Dating and Reconstruction of Carbon Dynamics of Soils Through the Holocene". The funding provided three years salary for one PhD student and 2 years salary for a second PhD student, who began one year after the project started. The research focuses on the practical aspects of extraction and purification of individual organic compounds from the complex mixture of sedimentary organic matter, soils and other matrices, and their subsequent radiocarbon analysis. For this, the modern techniques of preparative gas chromatography and high-performance liquid chromatography - mass spectrometry are used, while radiocarbon contents are measured on the recently developed Miniaturized Carbon Dating System (MICADAS) equipped with a gas-source, allowing measurements down to a few micrograms of C only. The underlying research question falls under the overarching goal of better understanding of past climate, environments, and the global carbon cycle. One of the outstanding questions concerns the dynamics of the large terrestrial soil organic carbon (SOC) pool. This constitutes one of the largest active pools of C on the planet, but knowledge about its long-term dynamics upon climate change is still very limited. The first doctoral student started to work on this topic by measuring radiocarbon ages of soil-derived molecular compounds and organic matter fractions, which were preserved in well-dated and well-constrained sedimentary records. Comparison between those ages and the actual age of deposition of the sediment will allow reconstruction of temporary changes in the average age of soil carbon. This in turn will be the basis from which the extent of storage or loss of SOC in the surrounding landscape can be derived. The goal of the second doctoral student is further development of radiocarbon dating using compound-specific analysis. Chronologies of natural archives such as lake sediments are often based on radiocarbon ages of recognizable terrestrial macrofossils that are deposited soon after their biosynthesis. However, sediments often lack terrigenous macrofossils making radiocarbon dating a difficult task. In these cases, dating is only possible using specific compounds that contain atmospheric 14C signature of the organic matter contemporary to sedimentary deposit. Compound-specific radiocarbon analysis may also prove to be an important method in other fields that heavily rely on 14C dating e.g., archaeology. One aspect of the instrumental set up that was not considered in the original proposal is the transfer of the purified compounds to the MICADAS analysing system. The original method of an off-line combustion and sample transfer is used. Coupling of an element analyzer (EA), where carbon is oxidized on-line, to the AMS gas source system was recently explored and showed that this method holds great promises. However, finalizing this development requires considerable effort and time of a specialist, for which we request also some funding.The research has the potential to provide many new insights and to produce results with a wide range of applications. One outcome will be a thorough and rigorous test of compound-specific radiocarbon dating for sedimentary records, but also other matrices. Secondly, more insight will be gained into the development and evolution of refractory soil organic carbon over time. Finally, through this research we are further pushing the limits of small-scale radiocarbon analysis, which has applications in various research areas including earth sciences, archaeology, environmental studies, or even medicine.