Produced by the incomplete combustion of fossil fuel and biomass, black
carbon (BC) contributes to Arctic warming by reducing snow albedo and thus
triggering a snow-albedo feedback leading to increased snowmelt. Therefore,
it is of high importance to assess past BC emissions to better understand and
constrain their role. However, only a few long-term BC records are available
from the Arctic, mainly originating from Greenland ice cores. Here, we
present the first long-term and high-resolution refractory black carbon (rBC)
record from Svalbard, derived from the analysis of two ice cores drilled at
the Lomonosovfonna ice field in 2009 (LF-09) and 2011 (LF-11) and covering
800 years of atmospheric emissions. Our results show that rBC concentrations
strongly increased from 1860 on due to anthropogenic emissions and reached
two maxima, at the end of the 19th century and in the middle of the 20th
century. No increase in rBC concentrations during the last decades was
observed, which is corroborated by atmospheric measurements elsewhere in the
Arctic but contradicts a previous study from another ice core from Svalbard.
While melting may affect BC concentrations during periods of high
temperatures, rBC concentrations remain well preserved prior to the 20th
century due to lower temperatures inducing little melt. Therefore, the
preindustrial rBC record (before 1800), along with ammonium (NH
In the last decades, the Arctic region has experienced the strongest surface air temperature increases globally, referred to as the Arctic amplification (Serreze and Barry, 2011), leading to a range of severe consequences for glaciers, sea ice, wildlife, and local human societies and partially explained by strong snow and sea ice feedbacks implying surface albedo changes. Black carbon (BC) is one of the substances involved in this process. BC consists of aggregates of carbonaceous spherules produced in the form of aerosols by the incomplete combustion of fossil fuel and biomass. BC does not refer to a single well-defined compound because carbonaceous aerosols are emitted in the form of a continuum of compounds with different physical and chemical properties (Goldberg, 1985), leading to a complex terminology depending on the method used for its quantification. Here we follow the recommendations given by Petzold et al. (2013) and will use the term rBC (refractory black carbon) when referring to our measurements carried out with the laser-induced incandescence method. BC possesses some unique properties: it is highly refractory, strongly absorbs visible light and has a very low chemical reactivity (AMAP, 2011a; Bond et al., 2013). Its strong absorptive ability impacts the Earth radiative budget and contributes to global warming via three main effects: a direct radiative forcing by sunlight absorption in the atmosphere, a modification of cloud properties whose mechanisms remain poorly understood, and a snow and ice forcing when BC is deposited on those surfaces, thus lowering their albedo and triggering melting (Bond et al., 2013; Hansen and Nazarenko, 2004). This latter effect is of great importance in the Arctic because most of the surface is permanently covered with snow and ice and BC concentrations in snow normally peak in spring, due to the Arctic haze phenomenon (Quinn et al., 2007; Shaw, 1995), when daylight hours increase considerably and mean surface air temperatures rise (Flanner et al., 2007). BC could be the second largest contributor to global warming after carbon dioxide (Ramanathan and Carmichael, 2008). However, given its short atmospheric lifetime from days to weeks, BC impacts can be considerably lowered when mitigation strategies are implemented (Bond et al., 2013).
Current global BC emissions are dominated by anthropogenic sources including industry, energy production, diesel engines and residential biofuel uses. While Western countries were responsible for most of the BC emissions until the mid-20th century, emerging economies in Asia are currently the major contributors (Bond et al., 2007, 2013). Conversely, before the beginning of the Industrial Revolution, biomass burning sources were largely predominant (Bond et al., 2013), encompassing wildfires and wood burning for heating, cooking and agricultural purposes. These general trends have been confirmed by recent ice core records from Greenland (Keegan et al., 2014; McConnell et al., 2007; Sigl et al., 2013), the Himalayas (Jenkins et al., 2016; Kaspari et al., 2011), the Caucasus (Lim et al., 2017) and the Alps (Jenk et al., 2006). However, detailed source attribution remains difficult because every record is the synthesis of a wide range of BC emission sources, transport, deposition and post-deposition processes. Therefore, more ice core records are needed to achieve a finer spatial and temporal representativeness of BC in the Arctic, which can be used to better constrain climate–aerosol model simulations (Bauer et al., 2013; Lee et al., 2013).
rBC or EC concentrations (italic) from different ice core and snow studies in the Arctic and in Europe.
The Svalbard archipelago, located 700 km north of mainland Norway, is of great interest within the Arctic because it is subject to air masses originating from different sources compared to Greenland (Fig. 1). While it is commonly assumed that North America is the dominant source region of air masses reaching Greenland (Fuhrer et al., 1996; Legrand et al., 2016; Shindell et al., 2008), an attribution supported by ammonium (Fischer et al., 2015) and BC records (McConnell et al., 2007) from Greenland ice cores, atmospheric and ice core data from Svalbard rather reflect emissions from Eurasia (Eleftheriadis et al., 2009; Goto-Azuma and Koerner, 2001; Tunved et al., 2013). Hirdman et al. (2010a) showed that northern Eurasia is the dominating source of the BC detected at Zeppelin station in Ny-Ålesund (Fig. 1a) over the entire year, with an influence from Siberian boreal forest fires in summer. Therefore BC data from Svalbard are useful to better disentangle the sources of Arctic BC. Several snow studies have already been conducted in Svalbard in order to assess the BC impact on surface albedo (Clarke and Noone, 1985; Doherty et al., 2010; Forsström et al., 2009, 2013) and the contribution from local BC sources such as coal mining (Aamaas et al., 2011) (Table 1). Atmospheric BC concentrations at Zeppelin station show a decreasing trend in the most recent years (Eleftheriadis et al., 2009; Hirdman et al., 2010b), confirmed elsewhere in the Arctic (Dutkiewicz et al., 2014; Gong et al., 2010; Sharma et al., 2004). However, only one long-term ice core record, drilled at Holtedahlfonna (HDF) (Fig. 1a), is available from Svalbard, based on the analysis of elemental carbon (EC), a proxy for BC obtained by thermal-optical measurements (Ruppel et al., 2014). Like the Greenland BC records, the record from HDF shows anthropogenic BC emissions starting in the second half of the 19th century and peaking around 1910. A recent and unexpected EC increase is also visible from 1970 onwards, contradicting atmospheric data and remaining partially unexplained (Ruppel et al., 2014). Several hypotheses have been discussed, such as increased flaring emissions from Siberia (Stohl et al., 2013) or changes in BC scavenging efficiencies due to higher temperatures. However, in a more recent study on a shallow firn core from the same ice field, no comparable recent increase could be detected (Ruppel et al., 2017).
Like other low elevation sites in the Arctic, Svalbard glaciers experience recurrent summer melting, which can alter the ice core records due to water percolation through the snowpack, leading to relocation of chemical compounds or even runoff in the warmest years. Pohjola et al. (2002) and Vega et al. (2016) concluded that most of the atmospheric signal was preserved at an annual, or in the worst cases, at a biannual resolution in the Lomonosovfonna 1997 (LF-97) and 2009 (LF-09) ice cores. Moore et al. (2005) also confirmed that chemical stratigraphy remained preserved despite high melt ratios. More recently, the impact of melting on the HDF ice core was assumed to be low compared to the EC deposition signal (Ruppel et al., 2014, 2017). Similar findings were postulated for the Lomonosovfonna 2009 (LF-09) ice core in which melting impact was negligible on ionic species at a decadal resolution (Wendl et al., 2015). However, it can become an issue when dealing with high-resolution records: Kekonen et al. (2005) found percolation lengths of up to 8 years for the warmest periods and half of the variance of the chemical dataset at those sites can be explained by post-depositional effects (Beaudon et al., 2013).
Other covarying proxies can also be used to help disentangle the BC
origin. Non-sea-salt sulfate (
Here we present the first long-term and high-resolution rBC record from Svalbard, obtained by single-particle soot photometer (SP2) analysis of two ice cores drilled on the Lomonosovfonna ice field in 2009 (LF-09) and 2011 (LF-11), further referred to as LF when both records are combined. After focusing on the anthropogenic imprint and its source attribution since the mid-19th century, we will discuss the impact of snowmelt on the record during the 20th century and finally we will reconstruct paleofire trends by using the preindustrial rBC record along with other biomass burning proxies.
Lomonosovfonna is one of the highest ice fields in Svalbard (Fig. 1), reaching 1250 m a.s.l. in its accumulation area (Isaksson et al., 2001). For this reason, it is less affected by summer melting and meltwater percolation than other low-elevation glacier sites in Svalbard (Gordiyenko et al., 1981; Pohjola et al., 2002), making it suitable for ice core studies. Therefore this site has already been regularly studied in the past. Two deep ice cores were retrieved in 1976 and 1982 by pioneering Soviet expeditions, mainly for stratigraphic purposes (Gordiyenko et al., 1981; Zagorodnov et al., 1984). The first extensive study that retrieved both physical and chemical records from the ice was conducted on a deep ice core drilled in 1997 by an international team (Isaksson et al., 2001).
In March 2009, using the Fast Electromechanical Lightweight Ice Coring System
(FELICS) (Ginot et al., 2002), a Norwegian–Swedish–Swiss team drilled a
149.5 m long ice core on Lomonosovfonna at 1202 m a.s.l.
(78
The LF-09 ice core was processed in a
The LF-11 ice core was cut at 4 cm resolution following clean protocols,
resulting in a total of 155 samples (Vega et al., 2015b). The dating was
performed by counting
The entire LF-09 core and the LF-11 core were analyzed for rBC at PSI in
several campaigns between 2012 and 2016 and in April–May 2016, respectively,
following the procedure established by Wendl et al. (2014) for liquid samples
and further evaluated by Lim et al. (2014). Discrete rBC samples were melted
at room temperature, sonicated in a ultrasonic bath for 25 min, and
immediately analyzed using a SP2 (Droplet Measurement Technologies, USA)
(Schwarz et al., 2006; Stephens et al., 2003) coupled with a jet nebulizer
(APEX-Q, Elemental Scientific Inc., USA). External calibrations from 0.1 to
50 ng g
Only the first 900 LF-09 samples were manually analyzed. Then a CETAC ASX-520
autosampler (CETAC Technologies, USA) was implemented in order to speed up
the measurements and improve their reproducibility. Total rBC particle
counting was kept to 10 000 as recommended (Schwarz et al., 2012), the
limiting condition being a measuring time between 1 and 30 min. The
autosampler probe was rinsed with ultrapure water for 45 s between each
sample and the waiting time in each vial before data acquisition was set to
1 min 45 s, which turned out to be sufficient for the background signal to
become stable. However, some difficulties arose from the fact that rBC
concentrations tend to decrease with time due to particles sticking to the
walls and agglomerating beyond the SP2 detection range, which implies that
rBC samples have to be measured as fast as possible after sonication (Lim et
al., 2014; Wendl et al., 2014). We therefore studied the rBC degradation with
time by using 24 ice core samples from Lomonosovfonna and the Swiss Alps
(Colle Gnifetti and Fiescherhorn ice cores). Each sample was measured between
5 and 14 times (depending on the concentration) over 24 h. They all showed a
similar decreasing trend, largely independent of the ice core site, the rBC
or dust concentrations. On average, the relative apparent rBC loss was
rBC raw data from the LF-09 and LF-11 cores,
Historical BC emission inventories reconstructing past emissions and atmospheric loading are used to compare the LF rBC record with estimated trends in anthropogenic BC produced by fossil fuel and biomass combustion in order to carry out source apportionment. Here, we use the BC emission inventory from Bond et al. (2007) available at 5-year resolution, between 1850 and 2000, for countries or areas identified as potential BC source regions: Canada, the USA, OECD Europe, eastern Europe, and the former USSR. This inventory includes emissions from fossil fuel and biofuel combustion, but does not include open burning such as wildfires, which contribute to a substantial part of the Arctic BC burden in summer (Stohl, 2006).
Our approach to detect years with increased forest fire
activity in the LF-09 ice core follows the methodology proposed by Fischer et al. (2015), which
basically uses an outlier detection approach. From the annual averages,
31-year moving medians were created. For each year, the residues between the
median and average were calculated. Median absolute deviations were obtained
by averaging the residues over the whole LF-09 time period for specific fire
proxies (VA,
Here we present the long-term and high-resolution rBC record from Svalbard
derived from the combination of the LF-09 and LF-11 ice cores spanning the
time periods 1222–2004 and 2004–2011, respectively (Fig. 2a). In the LF-09
ice core, rBC concentrations are generally low with a range between the LOD
(i.e., 0.051 ng g
The rBC concentrations in the LF-11 ice core are comparable to preindustrial
values (before 1800) measured in the LF-09 ice core, with an average of
The rBC long-term trends from the combined LF-09 and LF-11 cores,
expressed as
LF rBC concentrations are very similar to those observed in Greenland and
Canadian Arctic ice cores obtained by SP2 analyses (Keegan et al., 2014;
McConnell et al., 2007; Sigl et al., 2013; Zdanowicz et al., 2017) (Table 1).
However, EC concentrations in Svalbard snow (Aamaas et al., 2011; Doherty et
al., 2010; Forsström et al., 2009, 2013) as well as in the HDF and
Fiescherhorn ice cores (Jenk et al., 2006; Ruppel et al., 2014, 2017) are 1
order of magnitude higher than rBC concentrations in the topmost part of the
LF core. This can be mainly explained by the different analytical methods
employed, which do not measure the same fraction of the carbonaceous
compounds, as discussed by Ruppel et al. (2014). Whereas the SP2 does not
detect rBC particles larger than 500 nm, the optical and thermal-optical
methods include a filtration step in which the smallest fraction of EC particles
is generally lost. Lim et al. (2014) reported significant variations in the
EC
In Fig. 3a rBC annual averages and 11-year moving averages are presented to
document long-term trends in the LF ice core record. The most striking
feature is the increase in rBC concentrations and variability from 1800 on
that we attribute to rising anthropogenic BC emissions. Before 1800, annual
rBC concentrations were low, with an average of
To account for potential biases due to changes in accumulation rates, annual rBC fluxes were calculated by multiplying annual rBC concentrations by annual snow accumulation (Fig. 3b). Trends in the rBC flux and concentration records are almost the same (except that the highest fluxes were recorded in the 1870s), implying that accumulation has low variability and little impact on rBC long-term trends. Consequently, fluxes will not be considered in the remaining part of the study.
We attribute an anthropogenic origin to the higher rBC concentrations after
1860, supported by a significant correlation (at the 0.05 confidence level)
between rBC and other proxies for anthropogenic emissions in the LF-09 core,
namely
As seen in Fig. 4, the broad peak between 1940 and 1980, the decline in
concentrations after 1980, and the low concentrations observed in the 1920s
and 1930s are present for all the species. The double peak at the end of the
19th century is also visible, mainly for non-sea-salt
In order to interpret the anthropogenic rBC trend in the LF ice core and
assess the source regions of anthropogenic rBC, we compare the LF record to
other ice core rBC records and emission inventories. All rBC ice core records
from Greenland show a similar broad concentration maximum (Fig. 5b to 5d)
with values strongly increasing after 1880, peaking around 1910, followed by
a clear decline close to preindustrial levels reached after the 1950s (Keegan
et al., 2014; McConnell et al., 2007; Sigl et al., 2013). These records are
widely interpreted as proxies for North American BC emissions and they
closely follow the main trends in emission inventories for this region
(Fig. 5g; Bond et al., 2007). Atmospheric back-trajectory studies
corroborated that North America is the dominant source of BC deposited in
Greenland (Shindell et al., 2008). For Svalbard, the rBC record is notably
different with two maxima and contrasting timing. The rBC concentrations
sharply increased already from 1860 onwards and peak values were also reached
earlier, whereas they were low during times (1920–1940) when rBC
concentrations in Greenland were strongly enhanced. The most striking
difference is the second maximum observed after 1940 in the LF core, which
does not appear in any ice core from Greenland. We argue that this
discrepancy is in part related to different source areas of air masses
reaching Svalbard and Greenland. Air mass back-trajectory analysis obtained
with the Lagrangian HYSPLIT model showed that Siberia followed by northern
Europe were the dominant source regions for the LF site, while North America
was only occasionally the origin of air masses reaching the site (Grieman et
al., 2018). Contributions from Siberia were higher in spring and fall,
whereas European sources dominated in summer. Contrary to the HDF EC record
(Fig. 5e; Ruppel et al., 2014), we
do not observe any recent increase in rBC, which would support their
hypotheses of increased BC scavenging efficiency due to higher air
temperatures or stronger flaring emissions from Russia. On the contrary, rBC
concentrations in the LF record started declining in the 1970s and further
decreased from the end of the 1980s until rBC levels were comparable to
preindustrial values. Compared to the highest 1950–1970 rBC concentration
average of 3.3 ng g
Comparison of
We therefore postulate that a clear anthropogenic signal is present in the LF record from 1860 on (Fig. 5a) due to the start of the Industrial Revolution in Europe. This period of extensive coal burning would be responsible for the double peak observed at the end of the 19th century in the rBC record. The second peak period starting around 1940 would reflect the Eurasian economic growth after World War II and the extensive use of coal and oil for industry, transport, and energy production. The decline starting in the 1970s is consistent with emission inventories showing decreasing BC emissions for Europe due to the implementation of cleaner technologies and stricter environmental policies, and, from the 1990s on, for the former Soviet Union due to the collapse of the USSR and the subsequent economic crisis. However, some features of the LF rBC record remain unexpected. First, the sharp increase around 1860 is surprising since a smoother trend is observed at other Arctic sites (e.g., Greenland, HDF) in emission inventories and also in other anthropogenic proxies such as sulfate and nitrate from Arctic ice cores. The increase in rBC also occurs slightly earlier than in Greenland ice cores (1880s, Fig. 5b to d; Keegan et al., 2014; McConnell et al., 2007; Sigl et al., 2013) and an ice core from the Swiss Alps (1870s, Fig. 5f; Jenk et al., 2006). This earlier increase in rBC concentrations observed in the LF record supports our hypothesis that European BC emissions, probably from the early industrialized British Empire, might have dominated the LF-09 rBC record at that time. Indeed, the Industrial Revolution began in the second half of the 18th century in England (Deane, 1965) and spread to western Europe by 1850 (Spielvogel, 2010). In the LF core, the first increase in rBC background concentrations appeared around 1800. In the HDF ice core, stronger acidity from 1850 on was attributed to the Industrial Revolution (Beaudon et al., 2013). Local sources of contamination from coal mining in Svalbard can be excluded because the first industrial mines did not open until around 1900 (Catford, 2002; Hisdal, 1998). Second, our record displays two local minima around 1885 and between 1910 and 1940, which cannot be explained only by lower emissions as reflected by emission inventories (Bond et al., 2007). The economic crisis in the 1920s and 1930s might have contributed to these lower values, as also seen in the Fiescherhorn ice core, but it is unlikely to cause such a long and clear drop in rBC concentrations, as it started earlier and showed low rBC values similar to early 19th century levels. In addition, to our knowledge, no anthropogenic cause can explain the 1885 concentration drop. Interestingly, lower values are also found in the HDF EC record in the 1880s and 1920s, but the minima are less noticeable (Ruppel et al., 2014). As discussed in the next section, we suggest that post-depositional effects induced by summer melting are mostly responsible for these features.
Up to now, it remains unclear what happens to BC when melting occurs at the surface of the snowpack. BC can be enriched at the surface due to its low solubility in water or it can be eluted with the meltwater and percolate downward through the snowpack. When the water refreezes further down, forming an ice lens, BC is trapped. If melting is considerable, runoff can occur, leading to a net loss of BC. Doherty and al. (2013) showed that BC particles tend to be retained at the snow surface when melting occurs and that only 10–30 % of the BC is eluted with meltwater through the snowpack. However, Xu et al. (2012) observed that BC concentrations were higher not only at the surface but also in firn, at the bottom part of the snowpack, due to BC percolation and enrichment on top of superimposed ice, hindering further penetration of meltwater, while the intermediate snowpack zone was depleted in BC. Moreover, fresh snow displayed higher BC concentrations compared to snow experiencing summer melting. If BC concentrations are high, percolation can even be the dominant process (Conway et al., 1996).
In Fig. 6 a qualitative assessment of the melting impact on the LF-09 record
is made. Quantitative values cannot be obtained, especially as runoff might
have occurred. In addition to the annual melt percent (Fig. 6b) calculated by
Wendl et al. (2015), we use the melt index (Fig. 6c) defined as
Influence of melting on the LF-09 rBC record since 1800. Comparison
of
The melt index shows two periods of enhanced melting in the LF-09 ice core, from the 1910s to 1930s and in the 1980s–1990s. The first one is associated with the well-known early 20th century pan-Arctic warming (Bengtsson et al., 2004), also clearly visible in the temperature series from Svalbard Airport in Longyearbyen (Fig. 6d, Nordli et al., 2014), while the second one is explained by the current global warming trend (AMAP, 2011b; IPCC, 2013). The existence of local algae in the LF-97 core only between 1900 and 1940 (Hicks and Isaksson, 2006) underlines the fact that wet surface snow was present at that time at the drill site, another clear indication of summer melting. In contrast, the annual melt percent displays the highest values around 1905 and some local maxima around 1955 and in the 1980s. Our hypothesis is that the strong 1920s melting peak was responsible for extensive water percolation through the snowpack, leading to the formation of ice lenses and producing the melt percent peak around 1905. This would correspond to a percolation length of over 15 years, which strongly exceeds the up to 8 years postulated by Kekonen et al. (2005). A nonnegligible fraction of the rBC particles might have been eluted with the meltwater. Another substantial fraction might have been lost by runoff, which could explain the rBC minima from the 1910s to 1930s. Regarding the melt index peak in the 1980s and 1990s, as no clear increase can be seen in the annual melt percent record in the previous years, percolation must have been overwhelmed by surface runoff responsible for rBC losses. The dramatic decline of the melt percent since the 1990s also confirms the dominant contribution from runoff over percolation due to increasingly warm temperatures. Kekonen et al. (2005) also noted ion losses due to runoff since the 1990s in the LF-97 ice core. Nevertheless, we suggest that the decreasing trend in rBC since the 1970s is not only an artefact due to melting issues but is primarily driven by reductions in source emissions as confirmed by atmospheric measurements throughout the Arctic.
The case of the 1885 rBC minimum is more puzzling as there is no evidence of strong melting either in the melt index or in the annual melt percent record, which could indicate that losses happened only by runoff. Melting occurrence is supported by red layers typical of algae growing only in the presence of liquid water in LF-09 ice core sections around 1879–1881 and 1883–1885. Interestingly, Keegan et al. (2014) described a widespread melting event in Greenland associated with a prominent ice layer corresponding to the year 1889, which would lie within our dating uncertainties.
Furthermore, we cannot fully exclude that the apparent loss of rBC due to
melting is an artefact of the SP2 analytical method. Losses of rBC from
samples which were melted and refrozen in laboratory tests can reach
Results of the principal component analysis (PCA) for the LF-09
preindustrial record (1222–1859) after VARIMAX rotation. Data are
log-transformed annual averages. Values above 0.5 are in bold.
MSA: methanesulfonate (
Even if the impact of summer melting and anthropogenic emissions hampered the
use of rBC as a biomass burning proxy since the beginning of the Industrial
Revolution, it is still possible to reconstruct past biomass burning trends
in the preindustrial times (before 1800). This part of the record has a limited
melting effect due to low air temperatures and presumably no anthropogenic
input. In this period every rBC peak is assumed to correspond to a biomass
burning episode whose emissions were transported to and deposited at the
drilling site. Natural rBC emissions (wildfires) may also have contributed to
the rBC record since the beginning of the Industrial Revolution but their
signature is largely masked by the anthropogenic signal. For instance, the
highest rBC concentrations of the record in 1980 and 1981 could be linked
with strong biomass burning seasons in Canada (4.8 and 6.1 Mha,
respectively; Stocks et al., 2003) potentially related to the ammonium spikes
noted in Greenland ice cores (Legrand et al., 2016). The clear rBC peak
visible in the LF record in summer 1994 could reflect the high fire activity
in Canada for the year 1994, when 6.1 Mha burned (Stocks et al., 2003). Dibb
et al. (1996) documented the advection of a biomass burning plume from the
Hudson Bay lowlands, Canada, to Greenland on 5 August 1994, suggested to be
responsible for an increase in
To identify common variability among the chemical species in the LF ice core
and isolate biomass burning proxies, we performed a principal component
analysis (PCA) (Table 2). We used normalized annual averages and restricted
our analysis to the preindustrial period (1222–1859) as many compounds
(e.g., sulfate, rBC, ammonium, and nitrate) are influenced by anthropogenic
activities. Four principal components (PCs) were retrieved. PC1 has high
loadings of sodium, magnesium, potassium, calcium, nitrate, sulfate,
chloride, and methanesulfonate, representing 52 % of the total variance
and can be explained by mineral dust and marine sources (Wendl et al., 2015).
PC2 isolates light carboxylic acids (formate, acetate, and oxalate),
accounting for 19 % of the total variance. These compounds are well-known
proxies for biomass burning in Greenland ice cores, especially formate
(Legrand and De Angelis, 1996; Legrand et al., 2016). PC3 contains high
loadings of rBC and ammonium, contributing to 15 % of the total variance
and forming another group of biomass burning proxies. It is interesting to
note that the preindustrial LF rBC record shows the highest correlation
coefficient with ammonium independent of the resolution (significant at the
0.01 level). Most of the rBC peaks correspond to ammonium peaks (see below),
reflecting a similar sensitivity to biomass burning emissions, transport, and
deposition. We therefore suggest that ammonium is not only a proxy for
Eurasian biogenic emissions as stated by Wendl et al. (2015) but also
reflects a contribution from biomass burning, with biogenic emissions driving
background variations while sharp peak events are associated with forest
fires. Lastly, PC4 shows high loadings of specific organic markers of biomass
burning (VA and
Paleofire trends in the LF-09 ice core using
Number of peaks matching within
These discrepancies are also reflected in the diverging long-term trends. rBC
(Fig. 7a) and ammonium (Fig. 7b) display a relatively flat background over
the preindustrial time period, meaning that they do not indicate any
significant change in biogenic emissions and biomass burning.
Only slight increases in background concentrations are visible around 1370
and 1545 for ammonium and around 1290–1340, 1470, 1545–1565, and after 1750
(possibly already influenced by anthropogenic emissions) for rBC. Conversely, formate, VA, and
The frequency counting of forest fire episodes enables to focus on episodic
biomass burning plumes reaching Svalbard and can provide complementary
information in addition to long-term variations. For rBC and ammonium, the
centennial frequency of biomass burning episodes did not show a systematic
trend throughout the Little Ice Age but showed fewer biomass burning episodes in
the 13th and 15th centuries and more in the 18th century, despite colder
temperatures, though for the latter a contribution of early anthropogenic
emissions cannot be fully excluded. The VA,
Summer temperature (JJA) anomalies and drought (PDSI: Palmer drought
severity index) reconstructions along with periods of enhanced biomass
burning (colored lines between the two panels) and fire peaks (colored dots)
in the LF ice core,
Interestingly, severe droughts were reported over central Europe in 1540 (Wetter et al., 2014) and over northern central Europe between 1437 and 1473 (Cook et al., 2015). The case of the 1797 peak in the rBC and VA records is also remarkable as outstanding values in various biomass burning proxies were detected in several ice cores from Greenland during the last decade of the 18th century. In the NEEM ice core, rBC and levoglucosan were greatly enhanced between 1787 and 1791 (Sigl et al., 2013; Zennaro et al., 2014), while a very strong peak was visible in 1794 in the D4 ice core (McConnell et al., 2007) and in 1799 in the Summit 2010 ice core (Keegan et al., 2014). Ammonium records from the ice cores mentioned above all showed peak values in the same decade (Legrand et al., 2016), supporting the fact that this period of enhanced biomass burning could originate from the same decadal-scale climatic event. Severe drought conditions prevailed in central Asia during this decade due to South Asian monsoon failure (Cook et al., 2010). According to the dust proxy records from the Dasuopu ice core (Tibet), this decade experienced the driest conditions of the last millennium for this part of the globe (Thompson et al., 2000). In the Altai region, 9 out of 10 years between 1783 and 1792 belonged to the 10 % of the coldest years of the time period 1200–1850, while the following summers between 1793 and 1811 were clearly warmer (Büntgen et al., 2016). Those cold and dry conditions likely promoted dry dead wood accumulation, which then facilitated fire spread when temperatures rose later in the 1790s, a situation in agreement with the findings from Eichler et al. (2011). In Fig. 8 an exhaustive comparison is made between enhanced background concentration periods and peak years versus summer temperature anomalies and drought reconstructions from two regions of northern Eurasia for which datasets are available, namely northern Europe (Cook et al., 2015; Esper et al., 2014) and the Altai (Büntgen et al., 2016; Cook et al., 2010). It appears that biomass burning episodes frequently occurred in concert with decadal-scale summer temperature increases. Conversely, the link with moisture variations seems less consistent as conditions were either drier or wetter than average depending on the period, suggesting that summer temperature is the controlling factor for biomass burning activity in these regions. While dry conditions can lead to dead fuel accumulation (Eichler et al., 2011), wet conditions promote biomass productivity, especially for grasslands (Pederson et al., 2014). Both of these different mechanisms can enhance fire severity in the context of decadal-scale temperature increases.
Refractory black carbon (rBC) was analyzed in two ice cores from the Lomonosovfonna ice field, Svalbard, spanning 1222–2011. Long-term trends were discussed and compared to other ice core records and climate proxies in order to assess the representativeness of the rBC signal archived in the LF ice core in terms of anthropogenic and biomass burning inputs. Our results show that a clear anthropogenic imprint is present since the beginning of the Industrial Revolution, thus hindering the identification of natural biomass burning trends in the most recent 2 centuries. Concentrations of rBC we attributed to predominantly industrial emissions show two maxima, at the end of the 19th century and in the middle of the 20th century. This profile differs from those observed in Greenland ice cores and we suggest that Eurasian emissions account for most of the rBC deposition in the LF ice core in contrast to Greenland where North American emission sources appear more important. Contrary to the Holtedahlfonna EC record, LF rBC concentrations decreased in the last 40 years, in agreement with atmospheric measurements throughout the Arctic. However, during the warm climate regime over most of the 20th century, parts of the record experienced high levels of melting, which potentially disturbed the preservation of the rBC signal due to percolation and runoff. We thus advocate for a careful interpretation of the trends, especially for the time period with low concentrations between 1910 and 1940, which could be an artefact resulting from the early 20th century Arctic warming.
Before the 19th century, as both the melting and the anthropogenic influence
are shown to be low, the LF rBC record can be used to reconstruct past
biomass burning trends. No obvious long-term variability is evident in both
the rBC and ammonium records. Formate, VA, and
The rBC data are available at the US National Oceanic and
Atmospheric Administration (NOAA) data center for paleoclimate (ice core
sites) at the following address:
DO performed SP2 measurements, analyzed the data, and wrote the paper. IAW cut the LF-09 ice core, performed the dating, and designed the analytical method. LS optimized the method and performed SP2 analyses. MiS helped with the data interpretation. CPV processed and dated the LF-11 ice core. EI organized the field campaigns in Svalbard. MaS designed and led the project, organized and conducted ice core drilling, and led the paper writing.
The authors declare that they have no conflict of interest.
This study was supported by the Swiss National Science Foundation through the
Sinergia project “Paleo fires from high-alpine ice cores”
(CRSII2_154450/1). The authors would like to thank Sabina Brütsch for
ion chromatography analyses; Philipp Steffen, Denis Alija, and Susanne
Haselbeck for helping with the SP2 measurements; Joel Corbin, Robin Modini,
Jinfeng Yuan, and Susan Kaspari for their support regarding technical issues
with the SP2; the NPI, Utrecht University, and Uppsala University joint team
that retrieved the LF-11 ice core; Mackenzie Grieman for the VA and