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Melissa
Headly
Office: 233 Vaughan Hall
Phone: 858.534.4198
Fax: 858.822.3310
Email: mheadly@ucsd.edu
For my Ph.D. thesis, I am developing a technique to measure krypton and xenon in air bubbles trapped in ice cores. Specifically, I measure Kr/N2 and Xe/N2 ratios in the air bubbles in the ice. I have measured Kr/N2 and Xe/N2 in GISP2 ice from the late Holocene and the LGM. This is the first reconstruction of atmospheric Kr/N2 and Xe/N2 during the last glacial period ~20,000 years ago. Because of their high solubility, Kr/N2 and Xe/N2 measurements in ice cores should reflect past changes in mean ocean temperature. At this point, the Kr/N2 measurements show a clear change in ocean temperature, but the Xe/N2 precision is not high enough to support the Kr/N2 findings. I am currently working to improve the measurement precision of both Kr/N2 and Xe/N2, so that I will have two independent indicators of past ocean temperature change.
Knowing the mean ocean temperature in the past will give insight into past variations in deep ocean temperature, which remain poorly understood. Deep ocean temperature is a fundamental parameter of the climate system. Therefore, it is important to constrain deep ocean temperature in the past to understand past climate change. Because krypton and xenon’s solubilities are sensitive to temperature, past atmospheric Kr/N2 and Xe/N2 ratios should reflect past ocean temperature change.
How does this new proxy work?
We use a mass balance approach, conserving Kr, Xe, and N2 inventories in the ocean-atmosphere system, to estimate the mean ocean temperature change between the LGM and today. Kr and Xe are very soluble gases, and they are more soluble at colder temperatures. This solubility-temperature relationship is shown in Figure 1, where the solubilities of Kr and Xe are plotted vs. water temperature.
A colder ocean during the LGM would therefore hold more Kr and Xe than today’s ocean. Because the total amount of krypton in the ocean and atmosphere together is constant, the increase in the krypton inventory in the glacial ocean should cause a resultant decrease in the atmospheric inventory of krypton. Krypton is measured as a ratio to nitrogen concentration because nitrogen does not respond as sensitively to ocean temperature due to its low solubility in water, and direct measurement of the absolute krypton inventory is impractical. The results of the mass balance model using Kr/N2 are shown in Figure 2. Xe/N2 can be modeled in a similar way, and it is actually more sensitive to temperature change than krypton.
What measurements have been done so far?
We have used Kr/N2 ratios to estimate the average ocean temperature during the last glacial maximum (LGM), 20,000 years ago. We measured dKr/N2 and dXe/N2 in air bubbles in GISP2 ice from both the late Holocene and the LGM, using the present atmosphere as a standard. Delta (d) values refer to the deviation (per mil) of a ratio from a standard ratio. By comparing our measurements of dKr/N2 in the air bubbles with the mass balance model results, we estimate that the mean temperature change between the LGM ocean and today’s ocean is 2.7 ± 0.6°C. (Figure 3) The dXe/N2 measurement precision is insufficient at this point to use it as a meaningful proxy.
Current and future work
I am currently developing a method to use bigger ice samples (~700-800g of ice, compared to 50g used previously) to measure dKr/N2 and dXe/N2. The use of bigger ice sample sizes will potentially improve the precision of both dKr/N2 and dXe/N2 by averaging over inhomogeneities in the ice. Krypton and xenon are present in very small amounts in both the present and LGM atmosphere (~1.14 ppm and 0.087ppm, respectively, in today’s atmosphere), so using a larger sample will also allow for a stronger voltage signal to be measured on the mass spectrometer. Improved dKr/N2 precision and an additional proxy, dXe/N2, will allow for a more robust estimate of mean ocean temperature. We will use the remaining late Holocene ice from the GISP2 core to measure dKr/N2 and dXe/N2 in bigger samples of ice. Once improved measurementprecision has been established, we will measure a time series of Kr/N2 and Xe/N2 in ice from the Vostok ice core.