Aerosols are Controlled by the Same Processes that Modulate Stable Isotope Ratios of Ice

Concomitant variability in high-latitude aerosols, water isotopes and the hydrologic cycle

Bradley R. Markle, Eric J. Steig, Gerard H. Roe, Gisela Winckler,  Joseph R. McConnell

Summarized by Mauricio Hollis, who is an Environmental Geology major that is also minoring in Environmental Studies at Binghamton University in New York.  He is currently a senior who will graduate in Spring 2022.  At the moment he is applying to graduate schools and plans to do research on paleoclimate.  In the future he plans to work in academia and become a professor.  Some of his hobbies aside from geology are hiking, skiing, and fitness.

What data were used?  There were three ice cores used in this article that interpreted the relationship between high latitude aerosols and water isotopes.  They were from the West Antarctic Ice Sheet Divide ice core (WDC), the Greenland Ice Core Project (GRIP), and the European Project for Ice Coring in Antarctica (EPICA) from Dome Concordia (EDC).  There were also marine sediment cores used to compare with Antarctic dust records, two being from the South Atlantic downwind of South American dust sources, and more marine sediment cores that were located just north of the WDC in the South Pacific. 

Methods:  Concentrations of δ18O, non-sea-salt calcium (nssCa), and sea-salt sodium (ssNa) were collected and analyzed from the WDC.  They were first compared on a time scale ranging from 6 thousand years ago to 67 thousand years ago comparing water isotopes with aerosol records.  Temperature, saturated mixing, δ18O, and relative atmospheric aerosol concentrations were then plotted against Southern Hemisphere latitudes.  On these figures there are three different lines representing the different trends between the Last Glacial Maxima (LGM), modern temperature, and modern temperature cooled by 3°C.  The models for the relation between high-latitude water isotopes and sea-salt amplification were then compared.  Using data from GRIP and WDC, there was a comparison of their amplifications of nssCa against δ18O concentrations.  Dust aerosol records are then compared from the WDC and EDC with marine sediment records to plot the amplification factors against the past 40 thousand years.  Lastly, dust amplification against all latitudes is predicted through the use of a rainout model.

Figure 1a. Graph comparing ssNa, nssCa, and water isotope values vs time. Shows a negative exponential correlation between aerosols and δ18O. From Markle et al. (2018), Nature Geoscience.

Results:  It was found using these data that the hydrological cycle is the primary modulator of variability for dust, sea salt, and water isotopes.  Based on Figure 1A (right) there is evidence of a negative exponential relationship between water isotopes and aerosol concentrations for ssNa and nssCa.  This is indicative that as mean global temperatures decrease and more ice is formed, δ18O water isotope values see a decrease in concentration while dusts see an increase.  It is observed that there is overall less nssCa than ssNa.  This is explained by the terrigenous source of nssCa being 20°N of the marine source for ssNa resulting in lower concentrations of nssCa from 60°S-90°S due to a longer path of travel.

It is also observed that during the LGM there were higher concentrations of dust than during warmer periods of climate.  The data indicate there is evidence of smaller glacial-interglacial change for ssNa than nssCa. This is again a result of a shorter rainout pathway for marine aerosols than terrestrial aerosols.  It was also observed that δ18O isotope values from Greenland and Antarctica are very similar as they both show a negative exponential relationship with nssCa concentration amplification factor.  This is significant because they are on opposite sides of the globe with different dust sources and different regional conditions.  The data displayed for WDC and EDC show at some times, 10-100-fold amplification in dust than that collected for marine dust, which shows an amplification of 2-3-fold.  Lastly, fthe data indicate that amplification factors at the poles and higher latitudes is exponentially higher than at lower latitudes.

Why is this study important?  This study carries a lot of significance because it concludes that dust and water isotopes are both controlled by the same process, the hydrological cycle.  This has importance because it is necessary to understand the patterns of aerosol changes to further understand past radiative forcing.  

The Bigger Picture: This study disproves the theory that dust source emissions were a primary driver in ice-core records of aerosol, but rather changes in aerosol concentration at different latitudes is a result of the rainout effect.  It provides us with relevant results that can be further used to interpret how climate has changed since the LGM. 

Citation: Markle, B.R., Steig, E.J., Roe, G.H. et al. Concomitant variability in high-latitude aerosols, water isotopes and the hydrologic cycle. Nature Geoscience, 11,853–859 (2018).

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