Nuclear testing and the Anthropocene, a chemostratigraphic link?

Question

The Anthropocene is described by Wikipedia as

an informal geologic chronological term for the proposed epoch that began when human activities had a significant global impact on the Earth's ecosystems

Due to its informal nature and not wide acceptance, there is often debate among adherents about when the Anthropocene is supposed to have started. Note: this is not a question about this debate.

A working group recently suggested in the article When did the Anthropocene begin? A mid-twentieth century boundary level is stratigraphically optimal (Zalasiewicz et al. 2015) has taken a different approach, looking at the effects of human activity on the chemostratigraphic record, suggesting that the Anthropocene epoch ought to have its beginning at the start of the nuclear age.

From the article's abstract:

We propose an appropriate boundary level here to be the time of the world's first nuclear bomb explosion, on July 16th 1945 at Alamogordo, New Mexico; additional bombs were detonated at the average rate of one every 9.6 days until 1988 with attendant worldwide fallout easily identifiable in the chemostratigraphic record. Hence, Anthropocene deposits would be those that may include the globally distributed primary artificial radionuclide signal, while also being recognized using a wide range of other stratigraphic criteria.

This question is not about the legitimacy of such an epoch, but looks at the main point of the working group's findings that radionuclides from nuclear testing may have a measurable imprint on the chemostratigraphic record.

Have there been any global correlation of artificial radionuclides in the recent chemstratigraphic record to justify this as a possible Holocene-Anthropocene boundary?

Answer 1

Certain anthropogenic radionuclides have pragmatic advantages as markers for the start of the anthropocene. The advantage of radionuclides over anthropogenic stable nuclides is that the primordial production of all but a few radionuclides has decayed away. Thus natural background levels are below detection by sensitive analytical methods except in a few localized situations (e.g. preCambrian natural reactors). Even radionuclides that occur naturally may be useful if the anthropogenic signal is much greater than the natural levels.

An important trait for a radionuclide stratigraphic marker is that it has a long enough half-life so the anthropogenic material remains in the sediments.

The other important trait for a stratigraphic marker is that it is not redistributed significantly in the environment and stays with the sediments of the proper age. For this reason, ¹³⁷Cs and plutonium isotopes are valuable since they have low solubility/strong adsorption onto sediments and were distributed widely by atmospheric nuclear testing. However, this does not mean that the radionuclides won't be redistributed. The radionuclides can be redistributed by water, wind, or mass wasting. Colloidal transport can move particles even in saturated groundwater and the vadose zone. In fact, levels of atmospheric fallout plutonium have been used to interpret the redistribution of other contaminants.

Even with very low solubility there can be some dissolved transport into earlier or later sediments. For example, Woods Hole Oceanographic Institute has determined that plutonium mobility may be affected by oxidation state and that more mobile ²⁴⁴Cm can be transported before it decays to ²⁴⁰Pu.

Thus, anthropogenic radionuclides, particularly those of long half life and low solubility that were produced during atmospheric testing, have properties that make for pretty good stratigraphic markers. But the limitations should be kept in mind.

An additional problem is in defining the 'start' of the anthropocene as fitting your marker rather than the other way around. Certainly, people have had had geologic and ecological impacts before 1945. There are indications that aboriginal burning regimes over 20,000 years caused nutrient export with effects that can be seen in current groundwater chemistry.

Answer 2

I will try to plainly answer your question about an artificial radionuclide production/diffusion resulting from human activities, without getting into the discussion of deciding if this is geochronologically relevant or not.

I would point Figure 1 of this paper which indicate the average tritium (³H) levels in the northern hemisphere between 1945 and 2008. Before 1945 background levels were low and are now again nearly where they were before the 40's, as Tritium half life is ~12.5 y only. Each peak were connected with a nuclear bomb detonation.

This other paper

Wolff, E. W. (2014). "Ice Sheets and the Anthropocene." Geological Society, London, Special Publications 395(1): 255-263.

have a explicit figure on the matter as well (Figure 1) showing a similar peak during the 1900's for measurement of beta radioactivity measurements in ice cores. Human made nuclear device explosions are a clear marker in this case. I assume this is only one case among many, as other posters in this thread pointed out.

Answer 3

The chapter on the Anthropocene (Zalasiewicz et al., 2012) of "The Geological Timescale 2012" quote 3 papers that used artificial radionucleotides as stratigraphic markers: Schwikowski (2004), Turetsky et al. (2004) and Marshall et al. (2007).

The first one (Schwikowski, 2004) used them in the context of dating ice cores from Alpine glaciers and the second one (Turetsky et al.,2004) dated peat deposits using various methods including "fallout isotopes from the Chernobyl accident and nuclear weapons testing".

Unfortunately I do not have access to these 2 papers and therefore can't really go in the details.

The last one (Marshall et al., 2007) used them to date salt marsh from England. They used specifically ¹³⁷Cs and ²⁴¹Am (both, according to them, can be however affected by industrial discharge from nuclear powerplant), but also more interestingly a ¹⁴C "bomb spike", following Goodsite et al. (2001). Indeed ¹⁴C records shows a substantial increase from 1950 to 1963 (USA and USSR both stopped atmospheric nuclear weapons testing in december 1962), followed by a steady decline toward nowadays values. Goodsite et al. (2001) reports that the ¹⁴C can be used for dating at least in all the Northern hemisphere.

¹⁴C bomb pulse dating seems to be since then a rather popular method in forensics as well judging by a quick google search.