Chapter 9
Downloaded by DUKE UNIV on June 28, 2012 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1025.ch009
Isotopes Illuminate Chemical Change: Boron Isotope pH Proxy N. Gary Hemming1,2 1
School of Earth and Environmental Sciences, Queens College, Flushing, NY 11367 2 Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964
A driving force for the evolution of life on Earth is adaptation to environmental change. Basic chemical principles are used by geochemists to study the evolution of the Earth system, including ancient climate. Because there is no way to obtain climate information directly, geochemists rely on proxies, and isotopes have been particularly useful tools. Boron isotopes are promising as a proxy for ancient ocean pH and for understanding natural variations in atmospheric CO2 concentrations, due to the tight coupling between the atmosphere and surface ocean. Proxies for CO2, combined with temperature proxies, are important for understanding the natural relationships between global warming and atmospheric CO2, and thus will aid models that seek to predict future warming. The development of any proxy requires an understanding of fundamental chemical principles including atomic structure, vibrational energies, aqueous speciation, and isotopic fractionation, all within a dynamic system that is perturbed by biological influences.
© 2009 American Chemical Society In Chemical Evolution II: From the Origins of Life to Modern Society; Zaikowski, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
157
158
Downloaded by DUKE UNIV on June 28, 2012 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1025.ch009
Introduction Presented here are the basic chemical principles behind the development of a new tool for understanding changes in ocean chemistry in the past. The boron isotope paleo-pH proxy allows a measure of the pH of ancient oceans, and because there is a coupling between the surface ocean and the atmosphere, it also allows an estimation of atmospheric CO2 concentrations in the past. This is essential information because the better we understand the natural fluctuations and controls on Earth’s carbon budget, the better we can understand, predict, and ameliorate the consequences of the post-industrial increase in atmospheric CO2. Humans are having a profound impact on Earth’s chemistry, and the recent increase in atmospheric CO2 concentration is alarming. David Keeling began monitoring atmospheric CO2 concentrations in 1958, so there are direct measurements showing the exponential rise of this gas (1) (Figure 1). Because CO2 is a greenhouse gas, it is expected to impact on our future climate. By studying ancient climate, we can see the coupling between atmospheric CO2 concentrations and major climate fluctuations. A remarkable record has been produced by scientists who extract bubbles of air trapped in Antarctic ice cores to produce a record that may go back as far as 1 million years. Figure 2 shows data to 350 thousand years before present. The cyclic changes in atmospheric CO2 reflect glacial-interglacial cycles, with high CO2 during warmer interglacial times, and low CO2 during cold glacial times. Boron isotopes may allow us to extend the atmospheric CO2 record back beyond the extent of ice core records. While not a direct measurement of atmospheric CO2 concentration, it will give us the magnitude and frequency of past, pre-anthropogenic changes that can be related to climatic conditions determined by other proxies. Understanding of Earth history and geological materials has been greatly facilitated by the discovery of fundamental chemical principles. It has been more than 200 years since Dalton first theorized on the structure of the atom and more than 100 years since Mendelev’s 1870 version of the periodic table had reached the form close to that we use today. But it was Rutherford’s 1911 “planetary” model and Neil Bohr’s 1913 electrostatic model of the atom that allowed geologists to become geochemists and rigorously apply these new ideas to the study of the earth. Geologic applications began with investigations of compounds that make up Earth materials. These are the minerals and rocks whose formation follows the basic chemical principles set forth by those pioneering scientists. V. M. Goldschmidt was a chemist who used the contemporary knowledge of these basic principles towards understanding crystal structures. His pioneering work to determine ionic radii and coordination number of elements in minerals led to many of the basic principles we use today in geochemistry, particularly in understanding the partitioning of elements in the Earth (see review in (2)). Goldschmidt is often called the “father of geochemistry” and is honored by the annual international meeting of the Geochemical Society, the most respected meeting for geochemists, the V.M. Goldschmidt Conference.
In Chemical Evolution II: From the Origins of Life to Modern Society; Zaikowski, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
Figure 1. Atmospheric CO2 concentrations measured on air samples from Mauna Loa. The dramatic increase since measurements began in 1958 are due to anthropogenic CO2 additions predominantly from fossil fuel burning. Also captured in this record are seasonal cycles resulting from changes in northern hemisphere biomass (1, 3). It is remarkable how quickly the field of isotope geochemistry evolved, considering the neutron was not identified until 1932! (4). Henri Becquerel was a physicist and mineralogist who identified products of radioactive decay in 1896, although this was not identified as radioactivity until Marie and Pierre Curie’s work in Becquerel’s lab. Urey’s 1947 work on the thermodynamic basis for isotope exchange set the stage for the rapidly advancing field of stable isotope geochemistry. Calculation of the temperature dependence of oxygen isotope fractionation (5, 6) preceded the ability to measure isotopes precisely. Progressing from theory to the widespread application of these new concepts could not have happened without the development of analytical techniques to make the extremely precise measurements necessary to determine the small range in isotope compositions found in nature. While physicists led the way in developing early isotope instrumentation, Alfred Neir, who was also a geochemist, developed the isotope ratio mass spectrometer we know today (7). 400
Mauna Loa Law Dome, Antarctica
350
CO 2 (ppmv)
Downloaded by DUKE UNIV on June 28, 2012 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1025.ch009
159
EPICA, Antarctica
2005 level 377.5ppm
300
250
200
150 350
250
150
50
0
-50
Years before Present x 1000
Figure 2. Atmospheric CO2 concentrations over the last 350,000 years compiled from the Mauna Loa record (1957-2005) (1, 3), Law Dome ice core record (1010-1975) (8), Vostok ice core record (9).
In Chemical Evolution II: From the Origins of Life to Modern Society; Zaikowski, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
Downloaded by DUKE UNIV on June 28, 2012 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1025.ch009
160 Geochemists deserve much of the credit for advances in analytical techniques that pushed the limits of small sample size and improved analytical precision for Earth materials. Applications of isotopes in geology fall into two broad categories: radiogenic isotope studies, where an unstable parent isotope decays to a daughter product, thus giving “age” information when analyzed in rocks and minerals, and stable isotope studies that generally rely on fractionation due to mass difference, and thus have been traditionally restricted to light isotope systems (typically elements with an atomic number
