Continued:
Another more recent event showed a similar and, fortunately, much smaller signal in the ice cores. On June 30th 1908, a bolide entered the Earth's atmosphere and exploded over the Tunguska river basin in central Siberia with an estimated force of around 10 megaton nuclear explosions. This comet fragment is thought to have been about 40 metres in diameter, and it flattened around 80 million trees across 2,000 km2 of forest.
Several suggestions have been made as to the origin of the Tunguska bolide, but astronomers are beginning to accept that it was part of the Beta Taurid meteor stream.
Slovak astronomer Lubor Kresak was first to suggest that the comet was a fragment of Comet Encke, a periodic comet within the orbit of Jupiter which produces a bi-annual meteor shower, one being the Beta Taurids near the end of June, the other being the South Taurids meteor shower during early November.
Vast areas were flattened by a large meteor in Tunguska in 1908
Tree ring records from Europe show a short but visible reversal in trend, with annual tree ring widths reducing after 1908.
© Büntgen et al 2012.
Tree-ring Width Chronology from Maritime French Alps (1902-1915)
Looking at the Greenland ice cores records, a corresponding spike in both nitrate and ammonium as well as sulfate is visible right around this time.
© Data from Mayewski et al 1997.
GISP2 Greenland Ice Core (1833-1951)
The Tunguska signature reveals a spike in sulfate levels - which, as we saw, is also present in the Younger Dryas layer - although these are typically associated with volcanic eruptions that inject large quantities of sulfur into the lower stratosphere, which then reacts to produce sulfate aerosol particles.
Emissions of both nitrate and ammonium are generally considered indicators of biomass burning of forest fires and grassland. These signals are usually accompanied by high levels of formate (HCOO) and organic markers including vanillic acid and p-hydroxybenzoic acid. However, other factors can also account for higher levels of these ions.
With the advancement of technology, spectral analysis of comets has been conducted to analyse the composition of cometary bodies and their tails. Among the elements identified, ammonia was found to be present in both
Comet Hale-Bopp and
Comet Alley, with an implied ammonia-to-water ratio in the range of 0.4-2%.
However, it is usually insufficient to account for the high levels of ammonia seen in ice cores matching cometary impact events. In analysing data from the GISP2 ice core,
Melott et al describe four ways that a comet can produce nitrate and ammonia when entering the atmosphere - and which would account for the spike observed at the time of the Tunguska explosion.
- Biomass Burning (resulting from the fires generated by the impact)
- Direct Deposit (from the bolide)
- Atmospheric Ionisation (as the comet enters the atmosphere)
- Ice, Atmospheric Ionization and Haber Process
Touching on the first point, Melott explains that biomass burning from the forest fires caused by the explosion cannot account for the high levels of nitrate and ammonium recorded:
A significant problem exists in using biomass burning to explain the Tunguska event. The synchronous increase in both ions in the ice core data for the winter of 1908-09 in the GISP2 signal is clear and reliably dated with high time resolution. As we shall see, biomass burning can only be a minor contributor to the signal for this event. The area of forest fire burning was only 10-20 km in diameter (Wasson 2003). If we generously assume 100,000 ha of forest burning, the surface density deposition of ammonium over the Northern Hemisphere is only 10^7 kg m-2, only somewhat larger for nitrate. Summing up the nitrate from either GISP2H or GISP2 produces ~5 × 10^6 kg m-2 for the nitrate or ammonium deposition in the wake of Tunguska, so clearly the biomass burning is insufficient. The strong signal in the wake of Tunguska is one of the highest peaks over a recent century (Dreschhoff 2002; Olivier et al. 2006).
Atmospheric ionisation, Melott's third mechanism for producing nitrate, suggests that when a comet enters the atmosphere, it ionises the surrounding air, enabling the synthesis of nitrogen oxide. This could explain the observed spikes in nitrate, however ammonium isn't known to be produced by atmospheric ionisation and, as mentioned above, the amount of NH4 in comets is often too small to account for the high levels recorded in ice cores. The last mechanism Melott outlined is thus the most likely candidate to account for the large presence of ammonia in 1908.
Melott explains:
The Haber process for ammonia synthesis was developed for fertilizer and munitions in 1909. Under conditions of high pressure, nitrogen and hydrogen react to form ammonia. Formation of ammonia is increasingly disfavored thermodynamically at higher temperatures with respect to molecular nitrogen and hydrogen because of the unfavorable entropy of reaction. However, the unfavorable free energy of reaction can be overcome by the high pressure present in the shock front of a comet entering the atmosphere. As the nitrates estimated for both the Tunguska and Younger Dryas from conventional atmospheric process are adequate to explain the data for both events, it is reasonable that the comparable amount of ammonium found in the cores could also be synthesized this way, using cometary ice.
The assumed ice mass in the Tunguska comet is insufficient to synthesize sufficient ammonia to account for Greenland ice cores. However, it was proposed that at least one fragment may have impacted a swampy, partially melted permafrost area, creating Lake Cheko (Gasperini et al., 2008). Based on the size of the probable crater lake, sufficient water would have been present as a reactant to synthesize the ammonia.
There is debate over whether Lake Cheko was
created by the impact or was already present. The presence of the lake prior to 1908, and hence of the water needed to synthesise ammonium, would lend further support to the explanation given by Melott for the signal found in the ice cores.
Examining a Possible Cometary Impact in early 1883
Now let's go back and look at the ice core signals around the time of the Krakatoa eruption in August 1883.
© Sigl et al., Nature, 2015. / Mayewski et al., 2005.
Antarctic Ammonium and Nitrate Concentrations (1880-1889)
The sulfur spike seen in the records from the non-sea-salt Sulfur (nssS) chart can be confidently associated with the eruption. However, the above chart reveals two large signals for ammonium and nitrate peaking in early 1883.
These peaks cannot then be explained by the eruption, which occurred after the spikes chronologically (see vertical dashed line). The ammonium signal you see there is actually the second-largest of the 19th century (for the WAIS Divide ice core anyway), beginning around January 1883 to peak in February of the same year.
Although communication channels were rudimentary at the time, there are no contemporary media reports about a major fire or burning event having taken place in the Southern Hemisphere in the year leading up to the signal. I therefore wonder if these spikes in ammonia and nitrate indicate
a cometary origin to the early 1883 signal, possibly by the same mechanism described by Melott for the Tunguska impact, and caused by a comet fragment - a roughly Tunguska-sized bolide - hitting the ocean or an ice sheet (or causing ablation of such via an overhead airburst).
© usap-dc.org
© usap-dc.org
Location of WAIS Divide, West Antarctica.
The ice cores also reveal a large spike in bromide:
© Data from Sigl et al., Nature, 2015.
WAIS Divide Antarctic Ice Core (1880-1889)
The spike in bromide happens to be one of the strongest signals of the 19th century for the WAIS Divide ice core. Bromide is found in relatively low concentrations in the Earth's crust, about the same concentration found in meteorites, or 1ppm. So how can we explain the large spike found in 1883?
The ocean contains much larger concentrations of bromide relative to the Earth's crust. The impact of a comet in the ocean could explain the unusually high signal in the ice core, as explained here by
Pierazzo et al:
The impact of a 500m asteroid increases the upper atmospheric water vapor content by more than 1.5 times the background over a wide region surrounding the impact point for the first month after the impact. Halogens, ClY (chlorine) and BrY (bromide), follow the water vapor distribution, with an initial increase of over 20 and 5 times normal background, respectively, in the same region surrounding the impact. The perturbations eventually spread over the northern hemisphere, where water vapor content remains about 50% above background for the first year after impact, while ClY and BrY exceed five times and twice their background values.
The strong bromide signal, together with the spike in ammonium and nitrate, supports the possibility that a bolide impacted the Earth somewhere in the southern oceans in early 1883.
Yet another stratigraphic find points to what kind of bolide it might have been. A large black carbon signal was measured in the Summit2010 Ice Core (located near the GISP2 site) for the Tunguska impact event, alongside a spike in titanium.
© Data from McConnell et al., Science, 2007.
Summit2010 Greenland Ice Core (1905-1911)
Black carbon is made up of carbonaceous spherules that are formed by the incomplete combustion of biomass and fossil fuel. It
absorbs solar energy and, unlike CO2, remains in the atmosphere only for days or weeks before falling down as precipitation. Due to its ability to reduce the reflectivity of a surface (albedo), it warms the snow on which it falls thereby increasing melting. Although
black carbon is generally associated with biomass burning, it is not a well-defined compound, comprising different physical and chemical properties from location to location. Can the presence of black carbon in the ice core be explained by another factor?
Besides coming in different shapes and sizes, meteorites have different compositions. There is a class of meteorites called carbonaceous chondrites, or C-type chondrites, that are rich in carbon compounds, water and lithophile elements like silica, including oxygen, titanium and aluminium. This class of meteorites contrasts with others that predominantly contain minerals such as magnesium. The ice core records show no strong magnesium signal in either the Antarctic WAIS Divide for 1883 nor in Greenland for the Tunguska impact. But the latter instead has a strong signal for lithophile elements such as carbon and titanium, further strengthening the hypothesis that the bolide which entered the atmosphere above Siberia in 1908 was a carbon-rich chondrite.
The WAIS Divide core also happens to show the strongest spike in black carbon in the whole 19th century, starting around January 1883 to peak in February of the same year, matching the signal observed in the other elements.
© Data from Sigl et al., Nature, 2015.
WAIS Divide Antarctic Ice Core (1800-1900)
As mentioned earlier, there is no record of a major forest fire or biomass burning event to account for this large spike in the Southern Hemisphere. But is there any evidence of a bolide impacting the ocean? Should such an event have occurred, we could expect a series of waves extending from the impact point for many miles in all directions. A look at the records from the
Australian Bureau of Meteorology shows a series of successive waves hitting the coast of Stanley in Tasmania on December 25th 1882:
Records of tsunamis affecting Australia since European settlement
Date: December 25, 1882
Australian region where tsunami effects were recorded: TAS (Tasmania)
Source Region: Unknown
Comments: Four successive waves with the third being three or four feet high reported at Stanley
While in no way conclusive, given the timing and location of the recorded event, it could point to a cometary impact as the source of the successive waves observed in Tasmania.
Unfortunately, no record of other elements such as titanium are present in the ice core data to strengthen my hypothesis of a bolide impact on or near the southern oceans around the beginning of 1883.
------------------------------------------
Continued below
