Vas Petrenko

Office: 101 Vaughan Hall

Phone: 858.534.4188

Fax: 858.822.3161

Email: vpetrenko@ucsd.edu

The aim of my PhD project is to constrain the methane sources during fast methane increases associated with abrupt climate warming events during the last glacial period as well as during the deglaciation. Figure 1 shows the Greenland temperature record between 10 and 60 thousand years ago reconstructed from ice cores.

Many of these large and abrupt warming events are evident in the record. Several of the warming events are associated with large increases in atmospheric methane concentrations, which can be reconstructed from ancient air that is trapped in bubbles in glacial ice in places like Greenland and Antarctica. Figure 2 shows climate records from ice cores in Greenland and Bolivia for the past 20 thousand years – a period that includes the end of the last glacial period, the deglaciation, and the present warm period, known as the Holocene.

Two large abrupt warming events, each about 10 °C in magnitude, are found in the Greenland record at about 11.5 and 14.5 thousand years ago. As can be seen, each of these warming events is accompanied by a large synchronous rise in atmospheric methane.
Methane is of interest in paleoclimatic studies because it is an important greenhouse gas (about 23 times as efficient as CO2 at absorbing infrared radiation), as well as an important player in global atmospheric chemistry. Methane concentration is also a global signal. While temperature change can vary significantly between different regions of the Earth, the global atmosphere mixes on a timescale of about 1 year; thus a significant change in methane emissions in one area of the planet will be quickly reflected in methane concentrations worldwide. At the time of the last deglaciation, methane was not being produced anywhere in the vicinity of Greenland, so a methane rise at the time of Greenland warming suggests that climate had to be changing significantly in primary methane-producing regions (mostly low latitudes at the time of deglaciation).
There are currently two main hypotheses to explain the atmospheric methane rise during such warming events. The first, known as the wetland hypothesis, states that as the global climate became warmer and wetter, methane production in the wetlands increased. Wetlands are the main natural source of methane to the atmosphere today, and have been shown to respond to warmer, wetter conditions with increased methane production.
The second hypothesis, known as the hydrate hypothesis, states that most of the rapid methane increase can be explained by massive destabilization of marine methane hydrates. Methane hydrate is an ice-like compound consisting of methane and water molecules, and is stable at low temperatures and high pressures. Methane hydrates commonly form in the oceans at intermediate depths, where abundant organic matter is available in the sediments. It is estimated that methane hydrates worldwide contain as much or more organic carbon as all the world’s oil, coal, and gas reserves taken together. This hypothesis states that as the climate was warming rapidly, some of the ocean circulation at intermediate depths may have changed, bringing significantly warmer water down to the hydrate deposits. The hydrates would have then destabilized and released large amounts of methane gas into the atmosphere. The stability of methane hydrates under climate warming conditions is important to understand in light of current global warming. Destabilization of large amounts of methane hydrates and subsequent release of methane into the atmosphere would accelerate climate warming through enhancing the greenhouse effect.
One way to distinguish between the hydrate and wetland sources is to look at the carbon-14 content of methane. C-14 is a naturally produced radioactive isotope of carbon with a half-life of about 5,700 years. Methane produced from wetlands has a C-14 content essentially identical to the C-14 content of atmospheric CO2 at the time of production. Hydrate methane, on the other hand, contains no C-14 because hydrates are generally hundreds of thousands to millions of years old, and all the C-14 has decayed away.
The main challenge in using this approach is obtaining enough ancient air from the time of these rapid warming events to perform a C-14 measurement on methane. Ancient air can be obtained from old glacial ice. However, because methane is a very minor constituent of atmosphere, it turns out that about 2 tons of ice are needed for a sample of this kind! Such a large amount of ice for one time period is not available from ice cores.
Our solution to this problem was to explore a site on the Greenland ice margin where old ice becomes exposed at the surface. We have been able to find and date ice from the time of the last deglaciation. This was accomplished by comparing records of several geochemical parameters (see Figure 3) from our site to records in the Greenland ice cores, which are well dated. We have been subsequently able to obtain several large-volume air samples for C-14 of methane analyses, and are currently developing techniques for processing these challenging samples. Our preliminary results indicate that the methane record in our samples is intact, and further suggest that the C-14 of methane in the ice at our site is also unaltered.
Several photos show our Greenland sampling site and illustrate the work that we do there.