1

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: r.edwards@curtin.edu.au).

‘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.

References

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23 Andreae, M. O. & Merlet, P. Emission of trace gases and aerosols from

biomass burning. Global Biogeochem Cycles 15, 955-966 (2001).4 Penner, J. E., Dickinson, R. E. & Oneill, C. A. Effects of Aerosol from

Biomass Burning on the Global Radiation Budget. Science 256, 1432-1434 (1992).

5 Bowman, D. M. J. S. et al. Fire in the Earth System, Science 324 (5926), 481-484 (2009).

6 van der Werf, G. R. et al. Climate controls on the variability of fires in the tropics and subtropics. Global Biogeochemical Cycles 22 (3) GB3028, doi:10.1029/2007GB003122 (2008).

7 Scott, A. Terrestrial biosphere: The burning issue. Nature Geoscience 1, 643-644 (2008).

8 Wang, Z. et al. Large Variations in Southern Hemisphere Biomass Burning During the Last 650 years. Science 330 (6011), 1663-1666 (2010).

9 Ferretti, D. F. et al. Unexpected changes to the global methane budget over the past 2000 years. Science 309 (5741), 1714-1715 (2005).

10 Seager, R. et al. Blueprints for Medieval hydroclimate. Quaternary Sci Rev 26, 2322-2336 (2007).

11 Frank, D. C. et al. Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate. Nature 463 (7280), 527-530 (2010).

12 Marlon, J. R. et al. Climate and human influences on global biomass burning over the past two millennia. Nature Geoscience 1 (10), 697-702 (2008).

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).

15 Chylek, P., Johnson, B. & Wu, H. Black Carbon Concentration in Byrd Station Ice Core - from 13,000 to 700 Years before Present. Ann Geophys 10, 625-629 (1992).

16 Banta, J. R. et al. Spatial and temporal variability in snow accumulation at the West Antarctic Ice Sheet Divide over recent centuries. J Geophys Res-Atmos 113 (D23102), doi:10.1029/2008JD010235 (2008).

17 Flanner, M. G. et al. Present day climate forcing and response from black carbon in snow. J Geophys Res-Atmos 112 (D11202), doi: 10.1029/2006JD008003 (2007).

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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2526 Goodwin, I.D., Cohen, T., Mayewski, P., Lorrey, A., Browning, S.,

Curran, M., van Ommen, T., and Renwick, J. 2010. The Medieval Climate Anomaly – A view from down under. Abstract PP34B-06 presented 2010 Fall meeting, American Geophysical Union, Francisco, Calif., Dec. 13-17.

27 Goodwin, I.D., Stables, M. A. & Olley, J. Wave climate, sediment budget and shoreline alignment evolution of the Iluka-Woody Bay sand barrier, northern NSW, Australia, since 3000 yr BP. Marine Geology 226, 127-144 (2006).

28 Cohen et al. Continental aridification and the vanishing of Australia’s megalakes. Geology, doi:10.1130/G31518.1 (2011).

29 Williams, A. N. et al., Hunter-gatherer response to late Holocene climatic variability in northern and central Australia. Journal of Quaternary Science 25, (6), 831-838 (2010).

30 De Deckker, P. et al. Lead isotopic evidence for an Australian source of aeolian dust to Antarctica at times over the last 170,000 years. Palaeogeography, Palaeoclimatology, Palaeoecology 285, 205-223, doi:10.1016/j.paleo.2009.11013 (2010).

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10

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.

12

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.

20

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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Figure S4. Ice-core BC seasonal cycle. Monthly data points are BC

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23

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24

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