Research

My research focuses on global climate change and the use of isotope geochemistry to understand climatic and atmospheric processes. I am interested in the parameters and mechanisms that control paleotemperature proxies; in particuler, my group develops and uses the novel proxy carbonate clumped isotopes. We examine biological parameters that affect clumped isotopes and develop its use in new archive materials; we study the effect of non equilibrium processes on clumped isotopes and oxygen isotopes. We apply clumped isotpes to reconstruct paleotemperature and paleo-rainfall in different time periods during the Cenzoic. I am also interested in the use of isotopes to understand the modern carbon cycle and the effect of the biosphere of atmospheric chemistry.

What is clumped isotopes geochemistry?

Analysis of an isotopic composition is a measurement of the relative abundance of a heavy, rare, isotope within a group of molecules. The term 'clumped isotopes' refers to the natural abundance of molecules containing two heavy isotopes, such as ¹³C¹⁸O¹⁶O, and is a measure of the preference of two heavy isotopes to clump together into a chemical bond. This preference is temperature dependent with the isotopes distributed randomly among all molecules at very high temperatures and are clustered together into a more ordered system at low temperatures.

This results in an isotopic parameter, ∆₄₇, that can record the temperature in which these bonds were formed. 'Clumped isotopes' measurements are currently applied for ¹³C-¹⁸O bonds in CO₂ molecules that are extracted either from carbonate minerals or from the atmosphere. In carbonates 'clumped isotopes' are used to determine the formation temperature of the mineral with most applications associated with reconstruction of past climatic conditions. In atmospheric CO₂ it is used as a tracer for partitioning and quantifying the different CO₂ sources and sinks of the global carbon cycle.   See also Department Newsletter article on clumped isotopes.

Current research projects:

Calibration projects:

●  Carbonate clumped isotopes thermometry calibration: 

The use of clumped isotopes in carbonates for paleo-thermometry relies on accurate determination of the relationship between clumped isotopes values and temperature of carbonate formation. The original thermometer calibration (Ghosh et al., 2006) was performed by inorganic precipitation of calcite from solution through slow CO₂ degassing. Since these measurements have been performed, though, the analytical precision of 'clumped isotopes' analysis has significantly improved, creating an opportunity for an independent test, at higher precision, of the carbonate clumped isotopes-temperature relationship. We synthesized a large number of calcite samples, covering a temperature range of 8 - 70°C, which are being analyzed for ∆₄₇ to create a high precision 'clumped isotopes' thermometer calibration.

●  Standardization of 'clumped isotopes' - inter-laboratory calibration: 

Being a new isotopic tracer, clumped isotopes in either CO₂ or carbonates does not have a set of materials to be used as accepted standards. With the growth community of laboratories measuring clumped isotopes, materials of known values are needed in order to ensure consistency of data produced in different laboratories and over time within each laboratory. We create a data base of long term measurements of a set of materials to be used as potential future laboratory standards. We also initiated an inter-laboratory calibration project that would compare standardization among the laboratories that are currently performing or setting up to perform clumped isotopes measurements. The results of these standardizations efforts have recently published in the form of a new reference frame (Dennis et al., 2011).

Terrestrial carbonates:

●  Kinetic isotope effect in speleothems:

Speleothems are cave deposits that are used to reconstruct past climatic conditions on land. Oxygen and carbon isotopes in speleothems can provide continuous records that are interpreted as variation in rainfall patterns and in vegetation, respectively. Partitioning the effect of temperature and water oxygen isotopic composition is especially difficult in terrestrial settings, making speleothems an attractive target for clumped isotopes paleo-thermometry.

My previous work in Soreq cave (Israel), in collaboration with Miryam Bar-Matthews and Avner Ayalon of the Geological Survey of Israel and Alan Matthews of the Hebrew university have revealed however non equilibrium isotope effects in speleothem formation, making the application of clumped isotopes in such systems not straightforward (Affek et al., 2008). An NSF-EAR funded research is undergoing to understand the factors controlling of kinetic isotope effects in speleothem formation in an attempt to create a speleothem specific version of clumped isotopes thermometer. Our working hypothesis is that carbonates formed in thin films of water, as is the case in speleothems, would be susceptible to kinetic isotopes effect. We examine kinetic isotope effects in speleothems geochemistry using a combination of labotaroty experiments together modern and Holocene studies in two caves - Soreq in Israel and Bunker in Germany (see Affek et al., AGU 2011 Abstract).

●  Clumped isotopes in land and fresh water snails:

In order for clumped isotopes to be useful in pursuing continental paleo-temperatures we have to verify whether there are carbonate minerals formed on land, that reflect equilibrium clumped isootpes values and air temepratures, in analogy to biogenic marine carbonates. We examine carbonate shells of land snails from a variety of locations, in an attempt to test the relationship between their growh temperature and clumped isotopes values observing a behavioural effect that leads to snail body temeperature being significantly higher than its surrounding air temperature (Zaarur et al,. 2011). We currently examine clumped isotopes in snails growing in rivers and springs, as a potential terrestrial archive material.

Paleo-temperature applications:

●  Eastern Mediterranean paleo-climate:

The current climate of the Eastern Mediterranean is semi-arid, making it highly sensitive to potential variations in climatic conditions in the future. However, models of climate change projections do not agree about the predicted climate, in particular predictions of rainfall changes. This may be aided by detailed reconstruction of the correlation between paleo-temperaturte and rainfall patterns in the region. We use a variety of carbonate archive materials to examine past temperatures. 

Soreq cave speleothems: Although our work on clumped isotopes in speleothems revealed significant non-equilibrium effects, we can use laboratory experiments to calibrate for these effects. A BSF (United States-Israel Binational Science foundation) funded project will continue this collaboration with Alan Matthews of the Hebrew University, Avner Ayalon and Miryam Bar-Matthews of the Geological survey of Israel to examine in more details paleo-temperature during the last glacial period and the Holocene. In particular the work focuses on a combination of 'clumped isotopes' and fluid inclusion δD measurements in an effort to constrain past variations in rainfall patterns in the region. 

Lake Lisan sediments: High water stands in lake Lisan (the glacial dead sea) seem to contradict interpretation of speleothem and pollen record for a dry glacial period in the region. We try to address this question by examining lake paleo-temperatures using clumped isotopes in aragonite deposited in the lake during events of fresh water influx from rivers. This work is a collaboration with Motti Stein of the Geological survey of Israel. 

Red sea corals: We complement the continental climate archives in the region with sea surface temperatures using Red sea corals. This work is done in collaboration with Thomas Felis of the University of Bremen and its first phase examines clumped isotopes over the seasonal cycle in modern corals to examine vital effects in Porites corals (see Casey Seanger AGU 2011 abstract).

●  Eocene paleo-temperatures:

The Eocene was a time period characterized with high atmospheric CO2 concentrations. Temperatures have been significantly warmer than they are today, primarily in high latitudes, resulting in a low temperature gradient between the equator and the poles. We study how warm it was by examining sea surface temperatures during the Eocene as they are recorded by clumped isotopes in mollusk shells from both high and low latitudes. This work is a collaboration with Linda Ivany of Syracuse University (Keating-Bitonti et al., 2011, and Douglas et al., AGU 2011 abstract). A significant part of this project is performed by Yale graduate student Peter Douglas.

Atmospheric CO₂:

●  Seasonal variations:

Carbon and oxygen isotopes in atmospheric CO₂ are commonly used to quantify the global carbon cycle processes that act as sources and sinks of CO₂. I study clumped isotopes in atmospheric CO₂ as a potential additional carbon cycle tracer. My previous work showed seasonal variations in clumped isotopes values in atmospheric CO2 that were related to non-equilibrium ∆₄₇ values in ecosystem respiration (Affek et al., 2007). We currently continue the work to understand kinetic and thermodynamic aspects of clumped isotopes in the isotope exchange reaction between CO₂ and H₂O, that is expected to be the main process defining atmospheric CO₂ clumped isotopes values.