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Southern Hemisphere Fire Emissions greater during Early Medieval Period than present
Ross Edwards1,3, Joseph R. McConnell1, Marion Bisiaux1, Daniel Pasteris1, Kelley
Sterle1. Ryan Banta1, Michael Lawler2, Eric Saltzman2, Ken Taylor 1, Charles Zender2,
David Frank4 and Ian Goodwin5.
1 Desert Research Institute, Nevada System of Higher Education, Reno, NV 89512, USA
2 Department of Earth System Science, University of California, Irvine, CA 92697, USA
3 Department of Imaging and Applied Physics, Curtin University, Perth, WA 6845, Australia
4 Swiss Federal Institute WSL, Zürcherstrasse, 8903 Birmensdorf, Switzerland
5 Climate Futures and Department of Environment and Geography, Macquarie University, NSW 2109, Australia
Correspondence to: Ross Edwards3. Correspondence and requests for materials should
be addressed to R.E. (Email: [email protected]).
‘These authors contributed equally to this work’
Biomass burning is a major source of greenhouse gases, reactive gases and aerosols
affecting global climate and atmospheric chemistry1, 2, 3. Present-day biomass
burning rates vary in response to both climate variability and to land-use changes4,
5, 6. Recent ice core gas measurements have suggested that biomass burning
emissions may have been larger in the past and more variable7, 8. Here we present
a 2428 year ice core record of refractory black carbon (rBC) aerosol deposition to
the West Antarctic Ice Sheet (WAIS), reflecting the transport of fire emissions
over the Pacific sector of the Southern Ocean. The rBC record has monthly
resolution, and displays a seasonal maximum synchronous with SH dry-season
fires5 (Jun to Nov). A distinct peak in rBC during the early medieval period (MP)
2
was found with decreasing BC concentrations from ~ 1150 to 1830 (Common Era,
CE); coincident with decadal to century scale variability in global hydroclimate9,
temperature10, and fire emissions of methane8. We hypothesize that the record
reflects increased MP fire emissions from semi-arid/arid regions of Australia
resulting from increased rainfall and biomass together with a poleward Hadley
circulation and meridional transport into West Antarctica.
Highly sensitive to climate and land use, biomass burning (fire) is an important
component of the Earth system playing a key role in the evolution of the terrestrial
biosphere, biogeochemical cycles and composition of the atmosphere4, 6. Peaking
between June and November5, Southern Hemisphere (SH) fire emissions are dominated
by dry-season fires, in the tropics / sub-tropics5. Gases and aerosols from the fires are
transported over large distances perturbing the chemical and radiative properties of the
atmosphere on a hemispheric to global scale1,2,3. Knowledge of past fire emissions are
needed to understand the relationship between past variability in atmospheric
composition and its relationship to climate and human activity. Yet the history of these
emissions and their atmospheric transport are highly uncertain.
Reconstructions of fire from sedimentary charcoal and tree ring records have been used
to investigate large-scale changes in the global fire regime6,11. A limitation of these
studies is a lack of records in biomes with low woody biomass, such as savannah6,11,
which also produce significant emissions2, 6. Recently, high temporal resolution ice-core
records of rBC have been used to investigate the history of rBC in the Arctic
atmosphere from fire and fossil fuel emissions12. Solely produced by combustion, these
particles are an ideal fire proxy because of their refractory nature and short atmospheric
residence time (weeks). In contrast to individual sedimentary charcoal and tree-ring
records, which capture local fire histories, polar ice-core rBC records reflect the large-
scale impact of combustion on the atmosphere. Antarctic rBC aerosol studies13, and the
3
Byrd ice core record from WAIS14 have shown the potential for Antarctic BC ice-core
reconstructions. Here we present the most detailed record of rBC from an Antarctic ice
core.
Remote from all combustion sources, WAIS divide is a unique inland Antarctic site
(S1) with snow accumulation rates high enough to preserve sub-annual variability15. To
investigate the history of BC deposition, we developed a high temporal resolution BC
record from the top 576 m of the United States WAIS Divide project deep ice core (Fig.
S1, WDOC6A, site location 79°28.058'S, 112°05.189'W, altitude ~1806 m). The record spans the time period from -425 to 2003 CE with an effective depth
resolution of ~ 1 cm, resulting in a record with ~ monthly temporal resolution (Fig. 1A).
Monthly average rBC concentrations (S2) ranged from <0.02 to 3.8 ng g-1 with a mean of 0.136 ± 0.002 ng g-1 (95% confidence interval, n = 28665), consistent with previously reported Antarctic snow BC concentrations16, but lower than concentrations reported for the Byrd ice core14, which were from low temporal resolution samples of older ice. Large intra-annual variations in rBC concentrations were found, with a mean range of 0.277 ± 0.01 ng g-1 (95% confidence interval, n = 2350), ~204% of the mean BC concentration. This variability was highly seasonal with a maximum in August/September (austral dry-season) and a minimum in February/March (S3). Comparable seasonal variability is found in modern SH fire emissions (combined emissions from South America, Southern Africa and Australia) and atmospheric concentrations of rBC and carbon monoxide 5, 17,18. To investigate the influence of atmospheric circulation on the transport of rBC and eventual deposition on the interpreted ice core rBC seasonal cycle, an atmospheric global
4
circulation model experiment was performed with rBC emissions held constant at source regions located according to the Global Fire Emissions
Database version 2 (S4). The model results suggest that roughly half of the ice core rBC seasonal cycle amplitude results from seasonal variability in the atmospheric transport of rBC from the subtropics and mid-latitudes. Hence, approximately half of the interpreted rBC seasonal cycle in the WAIS ice core is likely a function of temporal variability in SH fire emissions, which we investigate further in this paper.
To investigate annual to centennial scale rBC variability corresponding to the austral dry-season we reconstructed a record of the June to November median rBC
concentration (rBCdry). The dry-season record (Fig. 1) reveals unexpected variability.
Specifically that rBCdry peaked during the MP with a concentration ~ 2.6 times that of
the 20th Century. Prior to the MP (450 to 1100 CE), rBCdry displayed centennial scale
oscillations, with maxima at ~ -130 and 160 CE and minima at -350, 60 and 305 CE.
Beginning at ~ 450 CE, rBCdry rose in two stages, from ~ 430 to 560 CE and ~740 to 930
CE. High rBCdry persisted during the MP until ~ 1090 CE, and then abruptly declined
until 1830 CE. The timing of the MP peak and its termination were coincident with
reconstructions of NH temperature9 (figure 2c). With the exception of ammonia, which
is also emitted by fire, similar variability was not found in other WAIS ice core
chemical species or dust particles.
Our interpretation of the rBCdry record is that on a decadal to century scale it reflects a
confluence of increased dry-season fire emissions, and changes in atmospheric transport
over the pacific sector of the Southern Ocean. This interpretation is supported by fire
signatures from isotopic measurements of the gases methane (CH4 pyro) and carbon
monoxide (COpyro) trapped in Antarctic ice cores7,8. The CH4 pyro record (figure 2b)
5
suggests that global fire emissions peaked during the MP and then decreased due to a
combination of climate change (1000 to 1500 CE), and human activity (1500 to 1700
CE)8. The COpyro record (Figure 2b) displays a “saddle” trend over the past 650 years,
with COpyro decreasing from 1300 CE to a minima at ~1620 CE before increasing up to
the end of the record in the late 1800’s. Because CO’s atmospheric lifetime is too short
for CO to mix on a global scale, the record is influenced primarily by SH fires, which
appear to have been more variable than generally assumed.
The tropical sedimentary charcoal record 11 also shows a decline in fire emissions, but
over the past two thousand years, rather than the past nine hundred, as found in the
WAIS rBCdry record. Because sedimentary charcoal records a history of woody biomass
fire, the differences between the records may reflect changes in fire emissions from SH
savannah fire regimes, which were the dominant source of rBC emissions before 1950
CE18.
Dry season fires in Australia are a potential source of rBC to WAIS because of the
relatively short atmospheric transport time19. Australia’s arid/semi arid fire regimes are
limited by herbaceous fuel, which displays a positive relationship with rainfall . Here
prolonged drought results in less fire, while prolonged wet periods, typically associated
with La Niña, result in increased fire emissions 5. Thus changes in hydroclimate may
have a greater impact on fire emissions than temperature.
Large-scale changes in global hydroclimate during the past two thousand years have
been reported by a number of studies20, 22, 23,24 with similar low frequency variability to
the rBCdry record (Figure 2 D -F). Mounting evidence suggests that changes in
hydroclimate during the MP were associated with a persistent La Niña like state in the
tropical Pacific9. Recent work is also defining the atmospheric circulation anomalies in
the subtropical to mid-latitudes of the Australian region25. This shows that the Hadley
Cell migrated poleward over the mid-latitudes in the south-east Indian to south-west
6
Pacific Ocean sector, between 600 to 1000 CE, with a blocking high pressure anomaly
that intensified over the southern Tasman Sea and New Zealand region until ~ 1150 CE.
This circulation anomaly is recorded in eastern Australian coastal strandplains through
the response to persistent easterly wave climate 26, eastern Australian coastal flood
records, and southern central Australian mega-lake shorelines27. In addition, the
increased rainfall to central Australia from 500 to 1000 CE has been shown to be
associated with an increase in the indigenous population, expansion of grasslands and
an increased flood frequency28. Air mass back trajectory analyses have shown that
aerosols reaching West Antarctica most frequently occur during periods where the
Australian region Hadley Cell is poleward with a blocking high pressure anomaly over
New Zealand. This circulation steers aerosol transport southwards over the Southern
Ocean before entrainment in the westerlies and eventual deposition over West
Antarctica29.
Hence, fires in arid and semi-arid regions of central / southern Australia are a likely
source of the MP rise in the ice-core rBCdry record. An expansion of grasslands and the
indigenous population during this period would have led to an increase in dry-season
fire emissions increasing the atmospheric loading of rBC over the southern Pacific and
ultimately over WAIS.
This hypothesis may soon be tested with “record breaking” La Nina rainfall in central
Australia during 2010 and 2011; similar to the conditions thought to be present during
the MP. When the wet period ends and the fuel cures, large fire emissions may result
and affect the atmospheric chemistry of the Southern Hemisphere providing insight into
the WAIS BC record and past fires emissions
7
Methods
rBC measurements were made using a continuous flow ice-core melting
system coupled with an intra-cavity laser-induced single particle incandescence soot
photometer12 (S2). The system was calibrated daily using rBC standards prepared from
commercially available rBC hydrosols. rBC concentrations interpolated to 10 cm depth
resolution over the top 1.5 m ranged from 0.07 to 0.3 ng g -1 comparable to the BC
concentrations of 0.1 to 0.3 ng g-1 previously determined for Antarctic snow by filtration
/ light absorption. Further details are given in S2.
The ice core chronology (WDC06A:1) was constructed by annual layer counting of a
number of seasonally varying chemical species. Results from layer counting were
confirmed since 1301 CE using comparisons of non-sea-salt sulfur concentrations with
the well-dated volcanic sequence from Law Dome in coastal East Antarctica (S2).
Uncertainty in the chronology prior to 1300 C.E. is estimated to ±3 years.
Acknowledgements
This work was supported by NSF grants OPP 0739780, 0839496, 0538416, and
0538427. Aja Ellis and Tommy Cox assisted with the ice-core sample preparation and
analysis. The authors appreciate the support of the WAIS Divide Science Coordination
Office at the Desert Research Institute of Reno Nevada for the collection and
distribution of the WAIS Divide ice core and related tasks (Kendrick Taylor, NSF
Grants 0440817 and 0230396). The NSF Office of Polar Programs also funds the Ice
Drilling Program Office and Ice Drilling Design and Operations group for coring
activities; Raytheon Polar Services for logistics support in Antarctica; and the 109th
New York Air National Guard for airlift in Antarctica. The National Ice Core
Laboratory, which archived the core and preformed core processing, is jointly funded by
8
the National Science Foundation and the United States Geological Survey.
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13 McConnell, J. R. et al. 20th-century industrial black carbon emissions altered arctic climate forcing. Science 317 (5843), 1381- 1384 (2007).14 Fiebig, M., Lunder, C. R. & Stohl, A. Tracing biomass burning aerosol from South America to Troll Research Station, Antarctica. Geophys Res Lett 36, L14815, doi:10.1029/2009gl038531 (2009).
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18 Edwards, D. P. et al. Satellite-observed pollution from Southern Hemisphere biomass burning. J Geophys Res-Atmos 111 (D14312), doi:10.1029/2005JD006655 (2006).
19 Lamarque, J. F. et al. Historical (1850-2000) gridded anthropogenic and
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biomass burning emissions of reactive gases and aerosols: methodology and application. Atmos Chem Phys 10, 7017-7039 (2010).
20 Stohl, A. & Sodemann, H. Characteristics of atmospheric transport into the Antarctic troposphere. J Geophys Res-Atmos 115 (D02305). doi:10.1029/2009jd012536 (2010).21 Zhang, P. Z. et al. A Test of Climate, Sun, and Culture Relationships from an 1810-Year Chinese Cave Record. Science 322, 940-942 (2008).
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28 Cohen et al. Continental aridification and the vanishing of Australia’s megalakes. Geology, doi:10.1130/G31518.1 (2011).
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31
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Captions
Figure 1. WAIS ice core (WDC06A) austral dry-season (June to November) median
rBC concentration. Early medieval period from 450 to 1100 CE (shaded in yellow).
Figure 2. (A) Smoothed ice-core black carbon June to November median (rBCdry). The
record has been smoothed with an 11 yr (black) and 51 yr (red) bandwidth, are gaussian
kernel regression estimates (S2). (B) Black carbon record (red) global pyrogenic
methane emissions8 (black) and SH pyrogenic carbon monoxide7 (purple). (C) Northern
Hemisphere temperature reconstructions9. (D) The Asian Monsoon record from
11
Wanxiang Cave20. (E) Reconstructions of SH hydroclimate REF. (F) Reconstructions of
global hydroclimate REF. The early Medieval Period from ~ 450 to 1100 CE is shaded
yellow.
13
Figure 2. (A) Smoothed ice-core black carbon June to November median (rBCdry). The
record has been smoothed with an 11 yr (black) and 51 yr (red) bandwidth, are gaussian
kernel regression estimates (S2). (B) Black carbon record (red) global pyrogenic
methane emissions8 (black) and SH pyrogenic carbon monoxide7 (purple). (C) Northern
Hemisphere temperature reconstructions9. (D) The Asian Monsoon record from
Wanxiang Cave20. (E) Reconstructions of SH hydroclimate REF. (F) Reconstructions of
global hydroclimate REF. The early Medieval Period from ~ 450 to 1100 CE is shaded
yellow.
Supporting online material
S1. WAIS Divide Deep Ice Core Project
Located near the West Antarctic ice divide (Figure S1), the National Science
Foundation (USA) WAIS Divide deep ice core project aims to drill a surface to bedrock
Figure S1. Location of West Antarctic Ice Sheet (WAIS) deep core project (ice core ID: WDC06A, 112.085oW, 79.467oS), figure has been modified from that of Banta et al (2008)S1.
14
ice core (~3,485 m) to study changes in atmospheric composition, climate and ice
dynamics over the past ~100,000 years. The site was chosen because of characteristics,
which preserve chemical and physical records at a high-temporal resolution comparable
to the Greenland GISP2, GRIP, and North GRIP ice cores. The site has a relatively high
ice accumulation rate (24 cm yr-1)S1, which has allowed the ice core depth range
encompassed by the rBC record (~2 to 576 m) to be dated by annual layer counting.
S2. Black Carbon Record
Black Carbon analysis The monthly average rBC record was developed from the high-
depth-resolution (~5 mm) analysis of longitudinal ice samples (30 mm by 30 mm) cut
from the ice core. rBC was determined using a continuous ice-core melter (coupled to
an ultrasonic nebuliser desolvation system (Cetac U5000 AT) and single particle soot
photometer (SP2, Droplet Measurement Technologies)S2. The ice core melter system
has been previously describedS3,S4. The ice-core melting head was made from a
monolithic block of chemical vapor composite silicon carbide (CVC-SiC, Trex
Advanced Materials). rBC blanks for the melter head were below the method detection
limit (~0.01 ng rBC g-1) and no memory effects were observed. The LII method has
been previously described in detailS5-12 and was used because of its sensitivity and
specificity to rBC. This method determines the mass of individual rBC nanoparticles
from “wavelength resolved” incandescent light emitted as the nanoparticles are heated
inside an intra-cavity ND-YAG laser (1064nm) to their boiling point (3700 to 4300 K).
A number of studies have shown that the LII method is free of positive interferences
due to the unique boiling point range of rBC S5-12. This includes positive interferences
15
from aerosolized organic compounds such as humic acids, which do not incandesce, and
incandescent inorganic species such as pure silicon and nickel, which have a lower
boiling temperature than rBCS8. The affect of organic coatings (including anthracene)
and particle morphology on the LII rBC determination have also been investigated and
do not affect the intensity of the peak incandescenceS8,S9,S11.
Ice core depth/age relationship The WDC06A:1 ice core chronology was determined
using annual layer counting of several seasonally varying elements and chemical
species analysed in parallel to rBC using the continuous ice core melter analytical
systemS4. Although nearly all elements and chemical species measured in the WDC06A
ice core showed seasonal variations, annual layers primarily were determined using
concentrations of ammonium and nitrate ions, non-sea-salt sulphur, sea salts such as
sodium, magnesium, and strontium, and components of continental dust such as cerium,
lanthanum, and non-sea-salt calcium.
Comparisons of the non-sea-salt records with the well-dated, 1301 to 1995 volcanic
sequence from Law Dome in East AntarcticaS13 were used to confirm annual layer
counting in WDC06A and to estimate uncertainty in the dating prior to 1300. Note that
similar continuous, high-depth-resolution chemical measurements of recently collected
ice cores from Law Dome revealed a one year error prior to 1818 in the earlier Law
Dome volcanic sequenceS13.
BC ice core record The monthly average WDC06A:1 BC record is shown in figures S2
and S3. The record is nearly continuous (n = 28,055), however a number of “short”
sections are missing (~1.7% of the record). In order to investigate decadal to centennial
trends in the data the “missing sections were reconstructed “filled-in” using single
16
spectral analysisS14,S15 and Kspectra software ( Spectraworks). To investigate decadal to
centennial variability the data was smoothed using R software and the R
implementation of the Nadaraya–Watson kernel regression estimate (ksmooth) using a
“normal” kernel and bandwidths of 11 and 51 years. The median seasonal cycle of the
rBC was
Figure S2. WAIS ice core (WDC06A) monthly average rBC concentration. Early medieval Period from 450 to 1100 ce is shaded in yellow.
Figure S3 WAIS rBC as for figure S2, but from 1900 to 1920 ce.
17
Figure S4. Ice-core rBC seasonal cycle. Wais monthly data points are Z-scores
calculated from monthly rBC medians for the time period -426 to 2002 ce (solid
red) and for 2002 to 1980 ce (dashed red). The 1900 to 1910 Southern
Hemisphere BC emissions estimates from Lamarque et al. (2010)S17 are shown as
solid blue. Carbon monoxide dataS19 from the Antarctic station Mawson (2000 to
1985 ce) and sub-Antarctic Macquarie Island (2001 to 1993 ce) are shown as
dashed purple and dashed green respectively.
18
constructed
from the median rBC concentration of each month binned from -425 to 2002 CE (Figure
S4). The resulting seasonal cycle has a maximum from July through to October and a
minimum in February/March. Comparable seasonality has been reported for Southern
Hemisphere fire emissions based an ensemble of satellite observations and model and
field studies S16-18. A similar seasonal cycle is found in instrumental measurements of
atmospheric carbon monoxide (CO) concentrations at remote Southern Hemisphere
sites including: Macquarie Island (sub-Antarctic), Mawson Station (East Antarctica),
South Pole and Cape Grim, AustraliaS19. The late 20th century rBC seasonal peak (2002
to 1980) (Figure 4 dashed red line) displays a shift in the seasonal peak from August
(entire record) to September/October synchronous with the instrumental CO data
(Figure S4)S19 .
S3. General Circulation Model Study
Simulations of biomass burning (BB) rBC transport to, and deposition and
accumulation at, the WAIS ice core site were conducted with the SNow, ICe, and
Aerosol Radiative modelS21embedded in the Community Atmosphere Model version
Figure S2. WAIS ice core (WDC06A) monthly average BC concentration. Early medieval Period from 450 to 1100 ce is shaded in yellow.
19
3.1S20. The geographic distribution and seasonal cycle of BB rBC sources were based on
present day climate estimates from the Global Fire Emissions Database version 2S18. In
the control simulation, smoothly varying monthly emissions were taken as the nine-year
monthly mean (1997-2005) of BB rBC emissions at each model gridpoint. A sensitivity
experiment was then performed in which the seasonal cycle of emissions was
eliminated, replaced by seasonally invariant BB rBC emissions with the same total
annual flux as the control. This removed one potential source of bias in the simulations
of rBC seasonality, since a bias in snow accumulation could impart an artificial
seasonality to the rBC deposition. The replacement of seasonally-varying by constant
rBC emissions reduced the simulated seasonal cycle of rBC deposition by about 40%.
This suggests that roughly half of the large rBC seasonal cycle amplitude found in the
WAIS ice-core results from the timing of SH dry-season fires and the short residence
time (weeks) of rBC in the atmosphere. The remaining half of the seasonal cycle
amplitude is likely driven by seasonal variations in atmospheric transport and/or BC
scavenging processes. Two caveats to this result are 1) that in both the control and the
experiment simulations, snow accumulation at WAIS was found to vary seasonally with
higher accumulation rates from April to September. This snow accumulation seasonality
is yet to be confirmed by observations at the site; and 2) The magnitude and timing of
the rBC seasonality over the past 20 years is comparable to that of instrumental
measurements of CO (which is also produced by combustion) from a variety of sites
including Southern Australia and coastal Eastern Antarctica. The sinks for CO (a
reactive gas) are very different to those for rBC; essentially wet and dry deposition
processes. The similarity between CO and rBC implicates emissions as having a larger
affect on the seasonal modulation than transport and scavenging.
21
Figure 1. Ice-core BC seasonal cycle. Monthly data points are BC concentration
averages from the entire record (n = ~ 29160pts). Non-parametric bootstrap confidence
intervals (95%) of the monthly mean are shown as grey shading. The august maximum
22
SO
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23
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