The Younger Dryas Impact Hypothesis (YDIH) posits that a meteorite or comet impact or airburst initiated the Younger Dryas (YD) period, a sudden and significant millennium-long cold snap at the end of the last glacial age (LGA), around thirteen thousand years ago. In a previous post, I came to the conclusion that the evidence presented so far in support of the hypothesis is ambiguous at best. Yes, there could have been a cosmic impact around the time of the Younger Dryas Boundary (YDB), but individual pieces of evidence presented also have other possible explanations that are completely terrestrial, and that haven’t been ruled out.

In this post, I will focus on one of the central pieces of evidence claimed in support of the YDIH, that there are spikes, or anomalies, or sudden, short-lived increases in the frequency of platinum (Pt) and other platinum group elements (PGE, such as iridium, or Ir) in soil and ice core layers around the world that date from the YDB.

PGE are sometimes associated with cosmic impacts, such as the one thought to have led to a mass extinction, notably of many dinosaur species at the Cretaceous-Tertiary boundary (K/T Boundary) about 66 million years ago. See Goderis et al. (2021) for details of the iridium signal at what is thought to have been the impact site.

The questions in this particular case is whether the elevated levels of PGE reported in some YDB soil and ice deposits are 1) anomalous, and 2) caused by a cosmic impact.

The anomaly

In their original 2007 paper that proposes the YDIH, Firestone et al. note that “Large increases in Ir and Pt occurred during the Younger Dryas as recorded in the GRIP (Greenland) ice core by Gabrielli et al. (41), who attributed these increases to increased cosmic input.” That increase in platinum and iridium in a Greenland Ice Core, just at the start of the Younger Dryas suggested to Firestone et al. that a significant cosmic impact might have been responsible for the cold snap.

In 2013, Petaev et al. reported a similar result. “We found a large Pt anomaly at the YDB, not accompanied by a prominent Ir anomaly, with the Pt/Ir ratios at the Pt peak exceeding those in known terrestrial and extraterrestrial materials.”

This was very good news for proponents of the YDIH, and to the press and the public, it seemed that “Pt spike in ice core” = “asteroid impact at the Younger Dryas Boundary”, as in the case of the K/T Boundary. But predictably, things are not so simple.

The questions here are essentially the same as for the YDIH as a whole. Did something unusual or unique happen in the YD, and does it require a cosmic impact as an explanation? Specifically, we have to ask whether the increased platinum and iridium seen in ice and soil cores around the time of the YD are special or extraordinary. Another way of asking this, is whether we are actually dealing with a platinum anomaly.

Back to the sources

Let’s go back to Firestone et al (2007) for a moment. They find the first platinum spike in a Greenland ice core in Gabrielli et al. (2004), and state that the authors attribute this early YD spike to “increased cosmic input,” i.e. to a meteor impact.

However, Gabrielli et al. don’t mention the Younger Dryas in that paper, and are not concerned with it at all in their study. They also notably don’t attribute increased Pt and Ir in the ice core to increased cosmic input.

What they do find is that during the Last Glacial Age, there are much higher levels of many trace elements in Greenland ice, including Pt and Ir, and that the post-glacial (Holocene) values are much lower, but constant:

“We find that unexpectedly constant fallout of extraterrestrial matter to Greenland occurred during the Holocene, whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust-laden atmosphere of the last glacial age.”

In other words, increased Pt and Ir values, along with increased values for lots of other trace elements during the ice age were terrestrial rather than cosmic, and were due to increased atmospheric dust in a cold, dry, and windy climate. “This period was characterized by a high dust loading in the Earth’s atmosphere, because of enhanced dry and windy conditions and the concurrent exposure of larger areas of the continental shelf because of lower sea levels.” But with reduced terrestrial input in warmer periods (like the Holocene), the cosmic input becomes more visible.

This has actually been known for a long time. In their 1983 analysis of impurities in the Vostok ice core, De Angelis et al. “observed higher continental (x37) and marine (x5.1) inputs during the last glacial age than during the Holocene stage.” In fact, their Figure 1 shows a modest but noticeable spike in aluminum, which is a good indicator of windborne terrestrial particles, and which aligns with a sudden change in Oxygen isotope 18 ratio around thirteen thousand years ago that indicates the start of the Younger Dryas cooling. The dating back then was not as good as it is today, and the resolution of their data is coarse, but the alignment is still quite visible.

Detail of Figure 1 from De Angelis et al. 1983

This is confirmed in Gabrielli et al.’s 2005a work on the Dome C ice core in Antarctica. “The large increase in dust fallout to the East Antarctic plateau during glacial periods has been shown to be largely responsible for the variation of heavy metals recorded in the deep Antarctic Vostok ice core.”

It is also consistent with Gabrielli et al.’s 2005b data from the Vostok core, also in Antarctica. “Concentrations of all elements were found to be highly variable with low values during interglacial periods and warm interstadials and much higher values during the coldest periods of the last four ice ages.”

This table from Gabrielli et al. 2005b clearly shows the change from high values for all elements measured during two of those cold periods, to low values during warm periods, such as the present day.

Detail of Table 1 from Gabrielli et al. 2005b

Although the time slices that Gabrielli et al. are looking at in their ice cores are fairly thick and wouldn’t allow precise dating of the Younger Dryas Boundary, there is indeed an increase in Pt and Ir in the area that would correspond to the sudden cooling in their 2004 paper. Given the above, however, the increase isn’t actually surprising, and in context, it doesn’t appear anomalous.

The Younger Dryas was a return to precisely the kinds of colder, dryer, and winder conditions that generated increased trace element concentrations in the ice core during the ice age. A temporary return to glacial conditions could be expected to lead to a return to glacial levels of trace element deposition in the ice core, and indeed, in ice and soil deposits worldwide.

This doesn’t mean that there was no comet or meteor impact at or near the start of the YD, but it does mean that one is not needed simply to explain higher Pt and Ir concentrations in an ice core at that time.

Even Petaev et al, who observed the platinum spike in Greenland in 2013, and who juxtaposed the terms “cataclysm” and “Younger Dryas” in the title of their paper, are not at all certain whether the spike indicates a cosmic impact. “Whereas the highly fractionated Pt/Ir ratio rules out mantle or chondritic sources of the Pt anomaly, it does not allow positive identification of the source.”

And if it did turn out to be from a cosmic impactor, Petaev et al. conclude, it would still be inconsistent with other key features of the YDIH: “Such a source could have been a highly differentiated object like an Ir-poor iron meteorite that is unlikely to result in an airburst or trigger wide wildfires proposed by the YDB impact hypothesis.”

Chronology

This is all quite apart from what we now know about the timing of the Younger Dryas Boundary and the platinum spike(s). To focus for a moment on the Greenland spike that started this whole thing, it is now becoming clear that it happens after the start of the cooling, perhaps by as much as several decades.

Dating these things is very tricky, there is always uncertainty, and correlations between different cores and other sources of evidence is anything by straightforward, but dating is constantly improving. Using some of the latest dates and correlations, Cheng et al. (2020) find that “A possible extraterrestrial impact event at ∼12,820 B.P. inferred by Pt-anomaly in the GISP2 ice core appears to lag the initial onset of the YD by ∼50 y without apparent disruption on the hydroclimate trend, suggesting that this event might not be the trigger for the YD onset.” Cheng et al. build on Rasmussen et al.’s massive 2015 dating and correlation of the Greenland ice cores.

An impact that follows the start of the YD cooling by fifty years doesn’t make sense as a trigger. It could have enhanced the cooling, or perhaps delayed a return to the Holocene warming trend. On the other hand, a platinum spike that happens after a return to glacial conditions is to be expected, along with increases in all kinds of other heavy element concentrations in ice and soil cores all over the world, including in Antarctica.

Geochemistry of the spike

In some cases, it is possible to determine whether geochemical signals (like the platinum spike) have a terrestrial (e.g. volcanic) or cosmic (e.g. meteoric) origin by looking at element and isotope ratios. But as Petaev et al. had already stated, and as Green (2019) clearly shows in a very impressive Masters thesis on the Laacher See eruption (approx. 12870 before present) as a potential source for the spike and/or trigger for the YD, the platinum spike in the Greenland core is geochemically quite different from both known cosmic and terrestrial sources.

In section 6.2 of the thesis, Green explores in great detail the various Earth-based processes that could conceivably have produced the geochemistry of the spike, and there are many, including some that are internal to the ice column itself, and not even necessarily related to what is going on in the atmosphere. As Green concludes, then, we still have much work to do before we can determine “whether the GISP2 Pt spike of 12.822 ka BP was unique or simply part of the glacial-deglacial sequence.”

Again, after all this, I am left not with any strong conviction that there is no significant cosmic impact in the Younger Dryas, but rather with the simple question: Do I need an impact to explain any of the data I have seen? So far, the answer is “not necessarily.”

One thing, though, is emerging clearly over the past few years: given the current dating frameworks, it looks increasingly like a cosmic impact that would have caused the Greenland platinum spike would not also have been the trigger for the Younger Dryas cooling, which calls into question the rest of the elaborate YDIH scenario.

References

Cheng et al 2020. Timing and structure of the Younger Dryas event and its underlying climate dynamics. PNAS 117. https://www.pnas.org/doi/epdf/10.1073/pnas.2007869117

De Angelis et al. 1983. Soluble and insoluble impurities along the 950 m deep Vostok ice core (Antarctica) — Climatic implications. Journal of Atmospheric Chemistry 1. https://link.springer.com/article/10.1007/BF00058730

Firestone et al. 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. PNAS 104. https://www.pnas.org/doi/full/10.1073/pnas.0908874106

Gabrielli et al. 2004. Meteoric smoke fallout over the Holocene epoch revealed by iridium and platinum in Greenland ice. Nature 432. https://www.nature.com/articles/nature03137

Gabrielli et al. 2005a. Trace elements in Vostok Antarctic ice during the last four climatic cycles.  Earth and Planetary Science Letters 234. https://www.sciencedirect.com/science/article/pii/S0012821X05001597

Gabrielli et al. 2005b. Variations in atmospheric trace elements in Dome C (East Antarctica) ice over the last two climatic cycles. Atmospheric Environment 39. https://www.sciencedirect.com/science/article/pii/S1352231005006333

Green CE 2019. Investigating the origin of a Greenland ice core geochemical anomaly near the Bølling-Allerød/Younger Dryas boundary. Durham Theses, Durham University. https://etheses.dur.ac.uk/13490/1/C_Green_thesis_final_CORRECTIONS.pdf

Goderis et al. 2021. Globally distributed iridium layer preserved within the Chicxulub impact structure, Science Advances 7. https://www.science.org/doi/full/10.1126/sciadv.abe3647

Petaev et al. 2013. Large Pt anomaly in the Greenland ice core points to a cataclysm at the onset of Younger Dryas. PNAS 110. https://www.pnas.org/doi/epdf/10.1073/pnas.1303924110

Rasmussen et al. 2014. A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy. Quaternary Science Reviews 106. https://www.sciencedirect.com/science/article/pii/S0277379114003485

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