Mass spectrometry and climate science. Part I: Determining past climates

by Roland Hirsch

Mass spectrometry is essential for research in climate science.

Understanding climate requires having sufficient knowledge about past climate and about the important factors that are influencing climate today, so that reliable models can be developed to predict future climate.

Analytical chemistry enables measurement of the chemical composition of materials, from the amounts of elements and their isotopes in a sample to the identity and concentrations of substances in the most complex biological organisms.

This two-part series covers the application of a powerful analytical chemistry technology — mass spectrometry — to two important areas in climate science:

• Obtaining reliable information about past climate
• Understanding composition and behavior of aerosols, which have a large impact on climate

The examples that are included for each topic were selected out of many published papers on the study of climate using mass spectrometry, partly because they feature a very wide range of types of these instruments. The authors were very helpful in providing me with information on their work.[1]

The technology described in this essay may at times be quite complicated! However, I hope that the results of each study will be understandable.

Part 1: Determining past climate

Information about past climate is quite limited. The atmospheric temperature records obtained using satellites and covering nearly all of the earth start only in 1979. Surface temperature records cover only a small portion of the earth, perhaps 15% going back to about 1900, and much less before that. The Argo buoys were deployed ~15 years ago and cover much of the oceans, which had minimal coverage before then. Information about aspects of past climate other than temperatures is even more limited.

Analytical chemistry is providing improved information about past climate, for example:

• Temperatures, changes in climate, and extreme weather events
• Concentrations of CO2 and other atmospheric components
• Extent of sea ice and glaciers over time
• Impact of geological events such as volcanic eruptions and earthquakes

The first step in studying a sample that was formed in the past (such as a fossil or a layer of sediment under the ocean materials that is more than a few hundred years old and have no attached information) is to determine its age.

The most common way to do this is using mass spectrometry to measure an elemental isotope ratio that is dependent on age. Carbon is often used for dating once-living samples, such as plants or artifacts made of wood, as its main isotopes, carbon-12 (¹²C) and carbon-13 (¹³C), are stable, while carbon-14 (¹⁴C) is radioactive, with a half-life of about 5700 years. The supply of ¹⁴C in the air is constantly replenished by cosmic rays hitting nitrogen-14, but once an organism dies the ¹⁴C fraction will steadily go down. [i]

Mass spectrometry is a preferred technique for measuring ratios of isotopes of elements, and reliably determines the fraction of ¹⁴C in the once-living sample and thus the age at the end of its life for up to about 20,000 years ago, and possibly somewhat earlier. For older samples other elements must be used, for the ¹⁴C fraction is too small to be reliably measured.

Several other elements have naturally-occurring long-lived radioactive isotopes that could be used to determine the age of an older sample. Potassium, for example, is a widely-distributed element with substantial concentrations throughout the earth. Potassium-40 (⁴⁰K) is a very long-lived isotope (1.25 billion years) with (unusually) two forms of decay, one to stable argon-40 and the other to stable calcium-40.

Argon in the atmosphere contains the ⁴⁰Ar produced over the lifetime of the earth. However, if the decay occurs in a solid that did not contain any air when it solidified and does not allow the argon produced by potassium decay to escape, then the amount of ⁴⁰Ar reflects the date when the solidification took place.[ii]

The reverse also can be measured: the amount of ⁴⁰Ar in air trapped in ice will reflect the age at which the ice formed. [iii] This mass spectrometric technique has recently been used to determine the age of samples at different depths in an Antarctic ice core (Figure 1).[iv] [v] The authors of these studies then measured the amounts of different atmospheric gases in those samples and plotted them as a function of sample age (Figure 2).

Figure 1: Age of samples taken at indicated depth below surface of ice core

Figure 2: Results for key atmospheric gases as function of ice core age (various techniques were used for measuring the gases)

Mass spectrometry is often used to study past temperatures. The ratio of the trace isotope oxygen-18 (¹⁸O) to the most common isotope oxygen-16 (¹⁶O) in a once living organism depends on the temperature of the air at the time that the oxygen was incorporated into the organism by metabolism. The ratio is measured using mass spectrometry. The higher the temperature, the lower the fraction of ¹⁸O in the once-living organism.

A representative study was carried out on mollusks found in layers of sediments near the northwest shore of Iceland, covering the period 350 B.C. to A.D. 1600. The layers in the shells reflected the year-round temperatures in which the mollusks had lived (Figure 3). From such data on many samples at different layers in the sediments the authors constructed a chart of temperatures over that time period. Notably, they were able to correlate these temperatures with historical records in Iceland from 865 to 1600 (Figure 4). The authors pointed out that “On the basis of δ¹⁸O data, reconstructed water temperatures for the Roman Warm Period in Iceland are higher than any temperatures recorded in modern times.”[vi]

Figure 3: Example of temperatures derived from a shell that lived through four summers (S) and three winters (W)

Figure 4: Variation of temperatures from the Roman Warm Period to ~AD 1800.Information from historical documents is at the top on the right side.

 

Another study using mass spectrometry to determine temperatures using oxygen isotope ratios was carried out over in fjords in Sweden, covering a 2500 year period. One of the fjords is on the north coast of Sweden (Atlantic Ocean) and the other on the southwest (North Sea near Denmark). The authors state:

“The record demonstrates a warming during the Roman Warm Period (~350 BCE – 450 CE), variable BWT [bottom water temperatures] during the Dark Ages (~450 – 850 CE), positive BWT anomalies during the Viking Age/Medieval Climate Anomaly (~850 – 1350 CE) and a long-term cooling with distinct multidecadal variability during the Little Ice Age (~1350 – 1850 CE). The fjord BWT record also picks up the contemporary warming of the 20th century (presented here until 1996), which does not stand out in the 2500-year perspective and is of the same magnitude as the Roman Warm Period and the Medieval Climate Anomaly.[vii]”

The authors of this study include a chart relating their temperature information with others in the North Atlantic (Figure 5).

Figure 5: Bottom water temperatures for different locations in the North Atlantic Ocean going back to about 350 BCE. Abbreviations shown at the top of the chart: RWP represents the Roman Warm Period, DA represents the Dark Ages, VA/MCA represents the Viking Age/Medieval Climate Anomaly and LIA represents the Little Ice Age.

The oxygen ratios thus allow estimating temperatures for once-living organisms. Importantly, they allow doing so year-round. Tree ring diameters, which are sometimes used for estimating past temperatures, primarily reflect temperatures during the growing season, and are also influenced by factors such as rainfall and location (such as above or below the treeline at a given time).

A recent study provides a different use of the oxygen isotope ratios for studying past climate: do volcanos have an impact on major climate factors such as the El Niño – Southern Oscillation (ENSO)? Some studies have suggested that major volcanic eruptions can impact the ENSO cycle, but only a few such events have relevant weather data. Fossil corals in the regions impacted by ENSO dating back centuries have well-defined monthly layers. They were dated by mass spectrometry using U/Th ratios. Then oxygen-18 measurements for these layers allow estimation of temperatures coinciding with major volcanos.

The results were combined with prior studies to produce a temperature record covering from ~1100 to ~2000 CE. Six major volcanos in this time period were charted against the temperatures (Figure 6). No evidence was found that the volcanos had caused an ENSO event. [viii]

Figure 6: Coral δ18O measurements of temperatures for six major volcanic eruptions in the last 900 years, with lines at the bottom showing the degree to which stratospheric aerosols reduced downwelling sunlight during the volcanos

Conclusion for Part One

This completes the first post on applications of mass spectrometry to climate science. The second post will focus on studies of factors that need to be understood to be able to develop reliable models of climate, with emphasis on research on aerosols.

Endnotes

[1] The figures and charts and other information from the papers are the property of the authors and publishers.

[i] https://en.wikipedia.org/wiki/Carbon-14

[ii] https://www.radioactivity.eu.com/site/pages/Potassium_40.htm

[iii] M.L. Bender, et al., “The contemporary degassing rate of ⁴⁰Ar from the solid Earth”, PNAS (2008) 105, 8232-8237 https://www.pnas.org/content/105/24/8232

[iv] J.A. Higgins, et al., “Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica”, PNAS (2015) 112, 6887-6891   https://www.pnas.org/content/112/22/6887

[v] Y. Yan , et al., “Two-million-year-old snapshots of atmospheric gases from Antarctic ice”, Nature (2019) 574, 663-663   https://www.nature.com/articles/s41586-019-1692-3   and https://www.nature.com/articles/d41586-019-03199-8

[vi] W.P. Patterson, K.A. Dietrich, C. Holmden and J.T. Andrews, “Two millennia of North Atlantic seasonality and implications for Norse colonies” PNAS (2010) 107, 5306-5310 https://www.pnas.org/content/107/12/5306

[vii] I.P. Asteman, H.L. Filipsson, and K. Nordberg, “Tracing winter temperatures over the last two millennia using a north-east Atlantic coastal record”, Climate of the Past (2018) 14, 1097–1118. https://doi.org/10.5194/cp-14-1097-2018

[viii] S.G. Dee, et al., “No consistent ENSO response to volcanic forcing over the last millennium”  Science (2020), 367, 1477-1481   https://science.sciencemag.org/content/367/6485/1477.full

Biosketch

Roland Hirsch has served the field of analytical chemistry in a 52-year career that spans teaching, research, and leadership at Seton Hall University, and 33 years of government service at the National Institutes of Health and the U.S. Department of Energy.  Roland has been a leader of the ACS Division of Analytical Chemistry, as Councilor for 25 years, as Division Secretary for 4 years, Chair-Elect, Program Chair, and Chair, and as its Web Editor for 22 years. Roland organized the 50th-anniversary celebration of the Division and 25 years later, wrote the definitive history of the first 75 years of the Division, published in Analytical Chemistry in 2013.  Roland has also been active in ACS Governance, including Chair of the Committee on International Activities, Secretary of the Committee on Nominations and Elections, Member of the Committee on Divisional Activities, Senior Chemists Task Force, Committee on Committees, and Liaison to the ACS Committee on Professional Training.

Based on a presentation prepared for the American Chemical Society National Meeting in Philadelphia in March 2020. It was to have been in the Division of Analytical Chemistry’s session “Advances in Mass Spectrometry”. The meeting was canceled, but this presentation was revised and made available on the web site for the meeting: https://www.morressier.com/article/mass-spectrometry-essential-research-climate-science/5e735e33cde2b641284a879e