Forum Comment
doi:10.1130/G36539C.1 Latest Permian chars may derive from wildfires, not coal combustion
Stephen E. Grasby1,2, Hamed Sanei1,2, and Benoit
2
Beauchamp
1
Geological Survey of Canada, Calgary T2L 2A7, Canada
Department of Geoscience, University of Calgary, Calgary T2N 1N4,
Canada
2
Hudspith et al. (2014) question our interpretation that Late Permian
chars at Buchanan Lake in the Sverdrup Basin, Arctic Canada, are
derived from coal combustion (Grasby et al., 2011). However, the data
and arguments they present do not support their assertions for the
alternative wildfire origin that they suggest.
The key issue is whether the vesicular carbon particles identified and
illustrated by us might have been sourced from wildfires, and not coal
combustion, as stated in the title of the Hudspith et al. paper. Photomicrographs of vesicular chars shown by Hudspith et al. (including their
GSA Data Repository 2014312) include coals as well as degraded plant
material. They illustrate limited similarities between these two source
materials, but only to the extent that some of the features we noted as
distinctive of coal fly ash could potentially be equated with products
from wildfires.
In their Discussion, Hudspith et al. agree with us that global transport
of char is possible: “...thus enabling global dispersal of microscopic
wildfire-derived particles”. This however, undermines their initial
premise that “chars in this study originate from peat forming environment…and thus supports the formation of vesicular chars in mire ground
and/or surface fires”. Given this, and that global transport is demonstrated by modern coal combustion particles found in the most remote regions
of Earth (Rose et al., 2012), then Hudspith et al.’s observation of
vesicular char in peat swamps says nothing about their genetic origin.
Any microscopic char particle found in any peat bog can have any
source. Their observations thus have no bearing on the debate as to the
origin of the char. We appreciate, however, that the authors do open the
wider discussion around what are potential characteristics of vesicular
char that can be ascribed a unique origin.
We fully agree that all the particles that Hudspith et al. produced
experimentally look exactly like combustion products of wood. We
further agree that particles they present from recent peat bogs in Alaska
and Ireland may be char that originated from a wildfire origin. However,
we showed that particles with similar optical characteristics are formed
by modern coal burning power plants. So at best, the findings of
Hudspith et al. would suggest that presence of char in the sedimentary
record may not be open to unique interpretation. While we agree that this
may be true for some particle types, it is not true for all.
The critical observation from our study used to support the coal
combustion interpretation was the presence of vesicular cenospheres,
These were demonstrated to have optical characteristics very similar to
those formed in modern coal-burning power plants. Vesicular cenospheres are often referred to as spheroidal carbonaceous particles (SCP)
and are considered a unique signature of coal and heavy oil combustion
(Renberg and Wik, 1985; Rose, 1996). They have long been used to date
the onset of the industrial period in global sedimentary records (Rose et
al., 2012).
While Hudspith et al. did not address the origin of these particles, we
note that they used the absence of SCPs to argue that their peat samples
were pre-industrial in age (Hudspith et al.’s Data Repository 2014312).
From this, we assume that the authors agree SCPs are indeed a unique
product of coal combustion. We found it interesting that their experimental wood combustion studies did not produce SCPs, further
supporting the unique origin through coal combustion. As Hudspith et al.
have not shown alternative mechanisms for formation of SPCs, and they
apparently agree that they reflect a coal combustion origin, their study
then does not negate our interpretation. Their results may suggest a
potential dual origin of the particles we observed (wildfire and coal
combustion). Indeed a dual source in the Late Permian sediments of the
Sverdrup Basin is highly probable, as forest fires would have been
expected during onset of Siberian Trap Eruptions and associated coal
combustion.
Hudspith et al. also comment on “the lack of documented coal fly ash
elsewhere”. We do not believe the absence of observation negates our
conclusion, particularly when coal combustion particles can be easily
confused with wood combustion.
Finally, we are intrigued by the authors’ photomicrographs of Late
Permian coals from the Kuznetsk Basin of Russia. They illustrate
particles very similar to those found at Buchanan Lake. The coals studied
by Hudpsith et al. would have been peat bogs during Late Permian time,
which may well have temporally overlapped the period of coal combustion that occurred prior to the main Latest Permian Extinction (LPE)
event (Grasby et al., 2011). The swamps near the Siberian Traps may
provide the very corroborating evidence of Latest Permian coal
combustion which Hudspith et al. suggest is lacking.
In summary, while we appreciate the wider discussion on what are
unique characteristics of vesicular char that can be ascribed to a specific
origin, we believe that Hudspith et al. have over-interpreted their data.
We agree that some char from Late Permian samples at Buchanan Lake
have an ambiguous origin, and could be derived from coal or wood
combustion. However, Hudspith et al. provided no explanation for the
presence of cenospheres, which they themselves apparently agree are the
product of coal combustion. In our opinion, the work of Hudspith et al.
may provide corroborating evidence of coal combustion particles from
Late Permian coals in the Tunguska Basin.
REFERENCES CITED
Grasby, S.E., Sanei, H., and Beauchamp, B., 2011, Catastrophic dispersion of coal
fly ash into oceans during the latest Permian extinction: Nature Geoscience, v.
4, p. 104–107, doi:10.1038/ngeo1069.
Hudspith, V.A., Rimmer, S.M., and Belcher, C.M., 2014, Latest Permian chars
may derive from wildfires, not coal combustion: Geology, v. 42, p. 879–882,
doi:10.1130/G35920.1.
Renberg, I., and Wik, M., 1985, Soot particle counting in recent lake sediments an
indirect dating method: Ecological Bulletins, no. 37, p. 53–57.
Rose, N.L., 1996, Inorganic fly-ash spheres as pollution tracers: Environmental
Pollution, v. 91, p. 245–252, doi:10.1016/0269-7491(95)00044-5.
Rose, N.L., Jones, V.J., Noon, P.E., Hodgson, D.A., Flower, R.J., and Appleby,
P.G., 2012, Long-range transport of pollutants to the Falkland Islands and
Antarctica: Evidence from lake sediment fly ash particle records: Environmental Science & Technology, v. 46, p. 9881–9889.
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