Introduction
Biomass burning influences the biosphere, atmospheric
chemistry, and the climate system on both regional and global scales. Fire
influences ecosystem dynamics, ecohydrology, surface albedo, and emissions of
chemically and radiatively active aerosols and gases
. In boreal regions,
fire plays a stabilizing role in circumboreal successional dynamics,
influencing forest age structure, species composition, and floristic
diversity . Boreal forest burned area, fire frequency, fire
season length, and fire severity will likely increase with continued warming
. Arctic tundra fires are of particular concern
because of their potential to release large amounts of ancient permafrost
carbon into the atmosphere .
Understanding the role of fire in the climate system requires a knowledge of
past regional and temporal variations on decadal, centennial, and millennial
timescales. A number of proxy fire records have been developed from sediment
cores and ice cores but systematic reconstruction of fire history remains a
major challenge. Terrestrial sedimentary charcoal records are inherently
local in extent, but regional and even global trends in burning have been
developed from these records using various normalizing and averaging methods
. The global charcoal database
GCD: is spatially and temporally inhomogeneous across
the Northern Hemisphere boreal and Arctic regions, with good coverage in
regions of North America and western Europe, and poor coverage in Asia.
Dissolved and particulate constituents in ice cores have also been used as
burning proxies. These include cations (ammonium, potassium), anions
(formate, acetate, nitrate), and black carbon seefor recent
reviews. One of the challenges of interpreting these
records is that most of the dissolved ions have multiple sources, in addition
to burning. For example, ammonium is also derived from biogenic marine and
terrestrial sources, agriculture, and livestock
. Efforts
to isolate the fire-derived contributions to these records have employed
principle component analysis and peak counting methods.
examined a Siberian Altai ice core using a multiproxy approach, and concluded
that potassium, nitrate, and charcoal were fire-related while ammonium and
formate were biogenic in origin. The detailed interpretation of ice core
chemical proxies is complicated by the fact that black carbon is emitted
primarily during the flaming conditions, while ammonium and many organic
aerosol-borne constituents are emitted primarily during smoldering. Ice core
gas measurements of methane and carbon monoxide, and their isotopomers, have
also been used to derive histories of pyrogenic emissions
. These gases have sufficiently long atmospheric
lifetimes in that they integrate emissions over hemisphere/global
scales.
A variety of organic aerosols are emitted from the burning of vegetation
under smoldering conditions. Levoglucosan, a combustion product of cellulose,
is considered a universal biomass burning tracer because it is emitted in
greater quantities than most other burning-derived organic aerosols and is
uniquely produced by the burning of plant matter .
Levoglucosan has been detected in ice cores from Antarctica, Greenland, and
northeast Asia . It is considered a
qualitative tracer because it degrades rapidly in the atmosphere
.
Aromatic acids are among a wide range of phenolic compounds generated by
lignin pyrolysis. These compounds are ubiquitous constituents of biomass
burning aerosols and have been detected in polar ice cores. Lignin is
produced from three precursor alcohols (p-coumaryl alcohol, coniferyl
alcohol, and sinapyl alcohol), and the resulting phenolic compounds retain
the structure of these alcohols. The aromatic acids analyzed in this study
are vanillic acid (VA) and p-hydroxybenzoic acid (p-HBA). VA is
predominantly associated with conifer and deciduous boreal forest tree
species, while tundra grasses and peat generate primarily p-HBA with lesser
amounts of VA . p-HBA is
also produced from boreal conifer tree species .
Burning is the only known source of these aromatic acids in aerosols or ice
cores. Quantitatively, the ice core levels of these compounds result from the
combined effects of emissions, atmospheric transport, depositional, and
perhaps post-depositional processes. Aromatic acids can undergo
re-volatilization at the snow surface. Laboratory experiments have shown that
Arctic snow samples containing lignin-derived compounds photochemically react
to produce formaldehyde and acetaldehyde . Melting and
refreezing processes have the potential to redistribute aromatic acids to
lower depths. Meltwater at the surface percolates to deeper snow layers and
water-soluble compounds are concentrated when the meltwater is refrozen
.
Prior studies of pyrogenic aromatic acids in ice cores include shallow cores
from Greenland, northeast Asia (Kamchatkan Peninsula), and Europe (the Swiss
Alps) . The record from Greenland
showed that the timing of variability in the VA and black carbon records was
similar over the past 200 years until around 1890 CE . VA
and p-HBA were elevated from the 1950s to the 1970s in the 60-year ice core
record from the Swiss Alps . VA and p-HBA were elevated in
the 1700s and in the twentieth century in the ice core from northeast Asia over
the past 300 years. There is only one millennial timescale ice core record
of VA and p-HBA: a 2600-year Akademii Nauk ice core record from the Eurasian
Arctic . This record showed three major multi-century pulses
of burning-derived aromatic acids, including one during the Little Ice Age
(LIA; 1450–1700 CE).
Here we present measurements of vanillic acid (VA) and p-hydroxybenzoic acid
(p-HBA) in an Arctic ice core from the Lomonosovfonna ice cap in central
Spitsbergen, Svalbard, which is located northeast of Greenland, in the
Atlantic sector of the Arctic Ocean (Fig. ). The goal of this
study was to generate a record sensitive to conditions in northern
Europe/northern Eurasia. Air mass back trajectories are used to examine the
distribution and ecology of likely source regions for biomass burning
aerosols transported to Svalbard. We discuss the variability observed in the
ice core records of these compounds over the past 750 years, and compare the
records to other proxy records of Northern Hemisphere climate and biomass
burning.
Location of Lomonosovfonna ice core drilling site on the island
of Spitsbergen in Svalbard (78∘49′24.4′′ N,
17∘25′59.2′′ E) and the Akademii Nauk ice core drilling site on
Severnaya Zemlya (80∘31′ N, 94∘ 49′ E). The map was
produced in the Python “matplotlib” graphics environment .
Methods
Ice core site characteristics and dating
The Lomonosovfonna ice core site is 1202 m above sea level (a.s.l.)
(78.82∘ N, 17.43∘ E) (Fig. ). The ice core was
drilled in 2009 to a depth of 149.5 m by a team from the Paul Scherrer
Institute and the Norwegian Polar Institute. The core did not reach bedrock
and contains a continuous 750-year record . The near-surface
annual average temperature of Lomonosovfonna is -12.5 ∘C at
1020 m a.s.l. . The annual
average accumulation rate is 0.58±0.13 m w.e. year-1
. The firn/ice transition occurs at 13 m depth, at
approximately 1997 CE.
The Lomonosovfonna ice core was dated by , yielding a time
span for the core of 1222–2009 CE (Fig. S1 in the Supplement). Annual
layers were counted using seasonal δ18O and Na+
variations to a depth of 97.7 m (79.7 m w.e.), giving an age of 1750 CE
at that depth. The chronology of the upper section of the core was also
constrained by the 210Pb profile and the 1963 3H horizon.
The age scale below 97.7 m was developed using a simple glacier ice flow
model , assuming an average accumulation rate of
0.58±0.13 m w.e. year-1 . The age scale was
adjusted to match seven volcanic reference horizons. The oldest of these is
the Samalas volcanic eruption of 1257/8. Dating uncertainty was estimated by
comparing purely modeled reference horizon years to known volcanic eruption
years. Above 68 m w.e., the dating uncertainty is ±1 year within
10 years of reference horizons, and ±3 years otherwise. Between
68 and 80 m w.e., the dating uncertainty is ±3 years, and below
80 m w.e., the dating uncertainty is ±10 years .
The Lomonosovfonna site experiences summer surface melting and winter
refreezing . examined the distribution of melt
layers in the Lomonosovfonna ice core, concluding that most summer melt
layers are refrozen within the year, with some extending over 2 or 3 years. The
frequency of melt layers increases after 1800 CE
Fig. S2;. During the warmest years of the twentieth
century, percolation length reached 8 years. Due to possible redistribution
of soluble compounds by melt, percolation, and refreezing, interannual
variability of the aromatic acid signals is not interpreted in this study.
Ten-year bin averages are used to illustrate short-term variability in the
data (see Sect. 3.1).
Potential source regions and ecological types using air mass back trajectories
Air mass back trajectories were used to identify potential source regions and
eco-floristic zones from which biomass burning aerosols are likely to reach
the Lomonosovfonna ice core site. This analysis assumes modern-day
meteorological conditions. Ten-day air mass back trajectories were computed
using the HYSPLIT model with NCEP/NCAR reanalysis data
. The 10-day back trajectories were started
at 100 m above the ice surface at 00:00 and 12:00 LT (local time) for three
separate 10-year periods, 1948–1957, 1970–1979, and 2006–2015 CE. The
fraction of trajectories originating in or transecting various geographic
regions and eco-floristic zones was calculated for spring (1 March–31 May),
summer (1 June–31 August), and fall (1 September–30 November). The
geographic regions included in the study were North America, Siberia
(> 42∘ E), and Europe (< 42∘ E). The boundaries of
North America, Siberia, and Europe were defined using global self-consistent,
hierarchical, high-resolution geography database GIS shapefiles
. These regions were subdivided into eco-floristic zones
defined by the Food and Agriculture Organization (FAO; Fig. S3;
,
http://cdiac.ornl.gov/epubs/ndp/global_carbon/carbon_documentation.html,
last access: 1 February 2017).
The Siberian region has the highest fraction of trajectories to the
Lomonosovfonna site, accounting for 39, 15, and 38 % of the trajectories
in spring, summer, and fall from 2006 to 2015 CE, respectively. Siberian
trajectories were most commonly from boreal tundra woodlands, boreal conifer
forests, and boreal mountain systems (Fig. S4; Table S1 in the Supplement).
Fewer than 3 % of the trajectories transected other Siberian eco-floristic
zones. Europe contributed fewer than 11 % of the trajectories arriving at
the site in any season. Those trajectories most commonly encountered boreal
coniferous forests, boreal mountain systems, and temperate oceanic forests
(Fig. S5; Table S1). Pollen species in the Lomonosovfonna ice core covering
the past 150 years drilled in 1997 match northern boreal taxa from
Fennoscandia . Biomass burning aerosols from eastern European
agricultural fires in 2006 reached Svalbard within a few days
. Other European eco-floristic zones contributed to fewer than
5 % of the trajectories in any season. North America contributed fewer
than 4 % of the trajectories for any season. This analysis does not rule
out contributions from North America, but it does suggest that such input
would likely require considerably longer atmospheric transport times.
Ice core sample preparation and analysis
For this study, we resampled discrete ice core samples previously analyzed
for major ions . The original ice core samples were
1.8×1.9 cm in cross section and 3–4 cm long, melted and stored
frozen in polypropylene vials. For analysis of VA and p-HBA, the ice was
re-melted, 1 mL was withdrawn from each vial, and the samples from four
adjacent vials were combined into one. This resulted in a total of 993
samples. As discussed below, this sampling procedure resulted in decreasing
temporal resolution with increasing depth from sub-annual samples at the
surface to about 2-year samples at the bottom of the core.
VA and p-HBA in the ice core samples were measured using anion exchange
chromatographic separation and tandem mass spectrometric detection in
negative ion mode with an electrospray ionization source (IC-ESI-MS/MS). The
analytical methods and standards used in this study are described in detail
by . The experimental system consisted of a Dionex AS-AP
autosampler, ICS-2100 integrated reagent-free ion chromatograph, and
ThermoFinnigan TSQ Quantum triple quadrupole mass spectrometer. VA was
detected at two mass transitions (m/z 167 → 108 and
m/z 167 → 152) and p-HBA was detected at
m/z 137 → 93.
Limits of detection for single measurements were estimated using three times
the standard deviation of the Milli-Q water blank. The limits of detection for
the VA m/z 167 → 108 and 167 → 152 transitions
were 0.010 and 0.006 ppb (n=80), respectively. The limit of detection
for p-HBA was 0.012 ppb (n=80). The mass spectrometer signals for VA
at the two mass transitions were highly correlated and either mass transition
can be used to measure ice core VA (Fig. S6). Data from the
m/z 167 → 152 transition is reported here because of the
slightly better detection limit.
Aromatic acids in the Lomonosovfonna, Svalbard ice core.
(a) Vanillic acid, (b) p-hydroxybenzoic acid. Arrows are
the detection limits. The black horizontal lines are the Medieval Climate
Anomaly (MCA) and the Little Ice Age (LIA) . The dashed
horizontal line is the extended LIA in the Svalbard region .
Lomonosovfonna ice core records of vanillic acid (a) and
p-hydroxybenzoic acid (b) and the ratio of vanillic
acid / p-hydroxybenzoic acid (c). For all plots: solid lines are
10-year bin averages, and gray shaded areas are 40-year bin averages of ±1
standard error. The dashed line on the bottom plot indicates a ratio of 1.
The black horizontal lines are the Medieval Climate Anomaly (MCA) and the
Little Ice Age (LIA) . The dashed horizontal line at the top is
the extended LIA in the Svalbard region .
Lomonosovfonna ice core records of vanillic acid (a) and
p-hydroxybenzoic acid (b) for the twentieth century. Gray shaded areas
are 10-year bin averages with ±1 standard error.
Results and discussion
Analytical results and data processing
In this study, 993 samples were analyzed for VA and p-HBA (Fig. ).
VA and p-HBA levels range from below detection to 0.2 and 0.07 ppb,
respectively. A substantial fraction of the VA and p-HBA data (67 and
58 %, respectively) is below the limits of detection. Data below the
limits of detection are reported as 0.5 times the limit of detection
(0.003 ppb for VA and 0.006 ppb for p-HBA). Smoothing of the data was
carried out using time bin averaging (10, 40, and 100 year), loess smoothing,
and moving averages. All smoothing treatments reveal similar multi-decadal
and centennial-scale features in the records and the choice of smoothing
technique does not influence the interpretation of the data (Figs. S7, S8).
Geometric means and standard deviations of the transformed data were used for
all statistics because frequency distributions of the data show skewness
towards lower concentrations. Time-averaging compensates for the decrease in
frequency of sampling with depth in the core due to layer thinning.
The Lomonosovfonna vanillic and p-hydroxybenzoic acid time series
The Lomonosovfonna VA and p-HBA time series exhibit variability on a wide
range of timescales. There is abundant annual to decadal variability in both
records. The amplitude of individual peaks in the raw data is roughly similar
across the whole record for both compounds, ranging from 0.1 to 1.2 ppb for
VA and 0.1–0.8 for p-HBA (Fig. ). The peaks appear to be of
longer duration during the older portions of both records. Both of these
aspects of the raw data are likely artifacts due to the combined effect of
annual layer thinning with depth and the sampling strategy of analyzing
individual ice core samples of constant thickness (12–16 cm). The time span
integrated by individual samples ranged from 1.7 years near the bottom of the
core, to 0.5 years at mid-depth (∼ 80 m), to 0.07 years near the top
(Fig. S1). This thinning effect is eliminated when the data are time
bin-averaged. Peaks in the 10-year bin-averaged records are roughly similar
in duration across the whole record (Fig. ). In the bin-averaged
data, the magnitudes of the peaks are no longer constant across the record.
For VA, the two early peaks (1250–1280 and 1360–1390 CE) are much larger
than all subsequent peaks. p-HBA exhibits three major multi-decadal peaks.
One is simultaneous with the early VA peak (1250–1280 CE) and the others
occur at 1520–1570 and 1610–1640 CE.
Both compounds exhibit long-term decreasing trends over the 800-year record,
as illustrated by the 40-year bin-averaged data (Fig. ). Forty-year
bin-averaged VA levels decline by about a factor of 3 over the first half
of the record (1200–1600 CE), then remain relatively steady for the
remainder of the record. It is possible that the decline in VA continued
after 1600 CE but much of the data after this time
are near or below the limit of
detection. Forty-year bin-averaged p-HBA levels decline by about a factor of
2 over the whole 800-year record.
Centennial-scale variability is observed as pronounced maxima early in the
record (1300–1500 CE), as illustrated by the 40-year bin averages
(Fig. ). There are hints of continued centennial-scale variability
in VA in the remainder of the record. Centennial-scale variability is evident
throughout the p-HBA record, with maxima coinciding with the VA maxima early
on and with additional maxima in the 1500s, 1600s, and 1800s.
The twentieth century levels of VA and p-HBA are not anomalous relative to the
rest of the ice core record. VA exhibits a slight increase after 1970 and the
largest single peak in the record occurs from 2000 to 2008 CE
(Fig. ). p-HBA levels also appear to increase after 1970, although
to a lesser degree than VA. The 2000–2008 period is slightly elevated in
p-HBA but not to the extent observed in VA. The samples from 1997 to 2009 CE
are within the firn layer. It is possible that firn samples could be
contaminated with biomass burning aerosols during handling in the field but
we have no reason to suspect that the aromatic acids in these samples are
influenced by contamination. We have not observed laboratory contamination
for these compounds as a significant issue in our laboratory.
Wavelet analysis was used to illustrate temporal variations in the spectral
content of the signals. Lomonosovfonna VA and p-HBA time series exhibit
non-stationary periodic variability, meaning that the spectral
characteristics vary with time (Fig. S9).
Potential for post-depositional modification of VA and p-HBA
There have been no field studies of atmosphere–snow interactions for aromatic
acids like VA and p-HBA, so little is known about post-depositional effects.
The following three types of effects should be considered: (1) re-volatilization after
deposition to the snowpack; (2) vertical redistribution associated with
melting, percolation, and refreezing; and (3) degradation due to chemical or
microbiological processes. All of these effects would likely occur to a
greater extent at relatively warm sites like Lomonosovfonna (mean annual
temperature -12.5 ∘C), and during warmer periods like the Medieval
Climate Anomaly (MCA) or the twentieth century. Redistribution associated with melt
layers has been discussed in detail for other ions , and one
would expect that the influence of these processes on aromatic acids would be
similar. used principal component analysis to determine that
melt layers did not have a major influence on the distribution of ions on
decadal timescales. Finally, the VA and p-HBA data from Lomonosovfonna and
Akademii Nauk argue against chemical degradation as an important process,
since there is clearly no monotonic decrease in VA or p-HBA levels downcore.
The fact that VA and p-HBA are commonly observed in atmospheric aerosols,
even after long distance transport, suggests that the volatility of these
compounds in aerosols might be considerably lower than that of the pure
substance . The vapor
pressures for VA and p-HBA are 0.0023 and 2.5×10-5 Pa
(https://chem.nlm.nih.gov/chemidplus/rn/121-34-6, last access:
15 May 2017, ) at 25 ∘C, respectively. Ionic
interactions with salts or hydrophobic interactions with soot or complex
organics may stabilize aromatic acids in aerosols or snow. Laboratory and
aerosol field studies have demonstrated reduced volatility of low molecular
weight organic acids (relative to the vapor pressure of the pure compound)
due to interaction with cations derived from sea salt or other sources, but
this effect has not been studied for aromatic acids
.
If re-volatilization of aromatic acids from the snowpack does occur, one might
expect loss to be enhanced in ice acidified by high levels of nitrate and
sulfate from volcanic or pollutant inputs. There is no obvious evidence that
acidification is a dominant control on VA or p-HBA levels in the ice core
(Fig. S10). It is particularly notable that VA and p-HBA levels are not
anomalously low during the twentieth century, when ice core sulfate and
nitrate levels increased several fold compared to the preindustrial era
(Figs. S10, S11). Based on the ice core signals alone, we conclude that
re-volatilization does not appear to be the predominant factor controlling
ice core aromatic acid levels, although this cannot be ruled out. Further
investigation of this issue is needed.
Relationship to ammonium record
Here we compare the variability of Lomonosovfonna VA and p-HBA to the
previously published ammonium record from the same ice core .
That study concluded that prior biogenic sources were the major contributor
to ammonium in the ice core prior to the mid-1800s, and agriculture became a
major source during the twentieth century. Prior studies have suggested that
episodic ammonium peaks in ice cores represent a fire signal, while
longer-term variability reflects the biogenic signal
. Following , we used singular
spectrum analysis (SSA) to decompose the Lomonosovfonna VA, p-HBA, and
ammonium records into these two components.
The analysis was done by computing 30 principle components (PCs) using
3-year bin-averaged data for VA, p-HBA, and ammonium . The
low frequency component (PC-1) of the ammonium record shows little similarity
to PC-1 for either of the organic acids. VA and p-HBA exhibit decreasing
trends over the record while ammonium increases (Fig. ). To
compare the higher frequency components, we used a peak detection method
. This was done by summing PCs 2–30 and counting the
frequency of peaks above a threshold (75th percentile) in a 40-year moving
window. The resulting signals for VA and p-HBA exhibit centennial-scale
variability which is consistent with that obtained from bin-averaging
(Fig. ) and ammonium exhibits similar variability on these timescales. The correlation coefficient between VA-ammonium and p-HBA-ammonium
was computed from the peak frequency data using a 200-year moving window. The
95 % confidence intervals for
these correlation coefficients are shown in Fig. . Based on this
analysis, ammonium and VA are positively correlated for three time periods
(1300–1450, 1675–1725, and post-1880). Ammonium and p-HBA exhibit
positive correlations for two time periods (1425–1650 and 1825–1875).
Interestingly, the positive correlations for ammonium with VA and with
p-HBA occur at different intervals. The fact that some extended periods of
correlation between VA, p-HBA, and ammonium are present in the
Lomonosovfonna record suggests that there may be a fire-derived contribution
to the ammonium signal in this ice core. However, the relationships are
obviously complex and worthy of further study.
Relationship to sedimentary charcoal records
Sedimentary charcoal records in the Global Charcoal Database (GCD) from
Siberia (50–70∘ N, 50–150∘ E) and Fennoscandia
(50–70∘ N, 0–50∘ E) were analyzed using the paleofire R
package GCD:. Only 3 of the 12 Siberian records in the
GCD have sufficient data from 1200 to 2000 CE for comparison to the
Lomonosovfonna ice core record. These regions are Chai-ku Lake in eastern
Siberia, and Zagas Nuur and Lake Teletskoye in southern Siberia. The Siberian
region as a whole is therefore not well-represented. Six of the 19
Fennoscandian records in the GCD have enough data from 1200 to 2000 CE for
comparison to the Lomonosovfonna ice core record. Siberia and Fennoscandia
are primarily boreal tundra woodlands, boreal conifer forests, and boreal
mountain systems Fig. S3;. One important caveat to this
comparison is that the dating of sedimentary charcoal records is often based
on linear interpolations between a few 14C ages. Hence their age scales
are typically less well-constrained than Lomonosovfonna or other ice cores.
Four of the six records from Fennoscandia exhibit increased charcoal influx
from 1200 to 1400 CE Fig. S12;. Lomonosovfonna VA and
p-HBA are both elevated during this period. Three of the six charcoal records
are elevated around 1600 CE when Lomonosovfonna p-HBA is also elevated. Two
of the records also show a long-term decline from 1200 to 2000 CE similar to
the Lomonosovfonna VA and p-HBA records. Two of the Siberian records exhibited
increased charcoal influx from 1200 to 1600 CE relative to 1600 CE to
present (Fig. S13). The Lomonosovfonna VA and p-HBA are also higher early in
the record. The Fennoscandian records are clearly most similar to the
Svalbard ice core record, but the database is too limited to determine definitively the source region for the VA and the p-HBA in the Lomonosovfonna
ice core from so few charcoal records.
Relationships between Lomonosovfonna vanillic acid (VA),
p-hydroxybenzoic acid (p-HBA), and ammonium (NH4) using 3-year bin-averaged data. (a) First component from the singular spectrum
analysis of VA (blue solid line), p-HBA (blue dashed line), and NH4
(orange line) (PC1) reconstructed into concentration units (RC1, ppb),
(b, c, d) Frequency of peaks in the ice core signals reconstructed
using singular spectrum components 2–30 and peak threshold of 75th
percentile, smoothed with a 40-year running window. (e) Correlation
coefficients for the ice core peak frequencies using a 200-year running
window (p<0.001). Bands are the 95 % confidence intervals of the
correlation coefficients of VA and ammonium (blue) and p-HBA and ammonium
(green).
Aromatic acids in the Lomonosovfonna, Svalbard, and Akademii Nauk ice
cores. (a) Vanillic acid and (b) p-hydroxybenzoic acid. Violet
lines are 10-year bin averages of the Lomonosovfonna ice core. Green lines
are the 10-year bin averages of the Akademii Nauk ice core measurements
. The black horizontal lines are the Medieval Climate
Anomaly (MCA) and the Little Ice Age (LIA) . The dashed
horizontal line is the extended LIA in the Svalbard region .
Comparison between Svalbard and Siberian ice core records of vanillic acid and p-hydroxybenzoic acid
The only other millennial-scale ice core record of VA and p-HBA is the
Akademii Nauk ice core from the Severnaya Zemlya Archipelago in the Arctic
Ocean north of central Siberia . The Akademii
Nauk ice core covers a considerably larger time range than Lomonosovfonna,
extending over the past 2600 years. Here we discuss only the period of
temporal overlap between the two ice cores (1200–2000 CE).
The two ice core records exhibit similar trends and levels during the early
part of the record (1220–1400 CE; Fig. S14). During this period,
Lomonosovfonna exhibits declining levels of both aromatic acids. In the
Akademii Nauk core, this period represents the tail end of an earlier peak
in both aromatic acids with a maximum around 1200 CE. The two cores diverge
markedly after 1400 CE for the remainder of the records (Fig. ).
Akademii Nauk VA exhibits a period of highly elevated levels from
1460 to 1660 CE. During this period, Akademii Nauk VA reaches levels more than
an order of magnitude above those of Lomonosovfonna VA. Akademii Nauk p-HBA
exhibits a period of elevated levels from 1460 to 1550 and 1780 to 1860 CE.
There are multi-decadal peaks in p-HBA in the Lomonosovfonna record that
overlap in time with the large Akademii Nauk VA peaks, although not nearly as
large in amplitude or duration. Interestingly, these peaks do not appear at
all in the Lomonosovfonna VA record.
Ten-day back trajectories were computed for the Akademii Nauk site using the
same methods as those described above from 2006 to 2015 CE
Sect. 2.2;. The trajectories show that both of the
Lomonosovfonna and Akademii Nauk sites are influenced by air masses
transecting Eurasian forested regions (Fig. ; Table S1). The
largest fraction of trajectories transects Siberian boreal tundra woodland, boreal coniferous forests, and
boreal mountain systems for both ice core sites, particularly in the summer
and fall. Given this similar transport pattern, we would have expected the
two large multi-century peaks in Siberian aromatic acids after 1400 CE to be
exhibited in the Lomonosovfonna record as well. The sharp divergence between
the two records around 1400 CE and the subsequent dramatic increase in
aromatic acids only in the Siberian ice core suggest a change in transport
patterns to the sites after 1400 CE. The fraction of back trajectories
transecting vegetated regions of Siberia for Akademii Nauk
was about twice that for
Lomonosovfonna. Conversely, trajectories from European forests comprise a
smaller contribution to Akademii Nauk than to Lomonosovfonna. Air masses from
European regions are more likely to reach the Lomonosovfonna site than the
Akademii Nauk site. We speculate that the divergence between the two ice
cores reflects a shift in large-scale atmospheric circulation patterns, as
discussed below.
Ten-day back trajectories from 2006 to 2015 reaching the boreal
ecosystems starting from the Lomonosovfonna and Akademii Nauk ice core
locations. Blue is Lomonosovfonna and red is Akademii Nauk. Trajectories
reaching: Siberia (a, c), Europe (b, d), boreal coniferous
forest (a, b), and boreal mountain system (c, d).
Relationship to atmospheric circulation and climate
The general climate context for the last millennium is late Holocene cooling,
with superimposed centennial-scale climate variability associated with the
MCA (950–1250 CE), the LIA (1400–1700 CE), and the twentieth century warming
. Svalbard δ18O ice core records
show that cooling continued in the region through the nineteenth century
. suggest that the extended LIA at Svalbard
could reflect the climatic influence of regional sea ice conditions. In that
case, the extended LIA was probably not characteristic of the biomass burning
source regions in Europe and Siberia influencing the Lomonosovfonna ice core.
For recent decades, increased burning of wildfires is generally associated
with higher summer temperatures . On that basis alone, one
might expect to see a long-term decrease in aromatic acid signals over the
last millennium, and that is generally the case for both VA and p-HBA in the
Lomonosovfonna ice core. However, on multi-century and centennial timescales, the variability in the aromatic acids in the Lomonosovfonna ice core
is also large and somewhat complex. Both VA and p-HBA levels were high during
the MCA. VA declines into the LIA. p-HBA exhibits elevated levels during the
latter half of the LIA but VA does not (Fig. ). This dissimilarity
could be due to a shift in spatial patterns of either biomass burning or
atmospheric transport after 1400 CE.
It seems likely that regional changes in burning proxies on multi-century and
centennial timescales are strongly linked to changes in large-scale
atmospheric circulation and the resulting impacts on regional climate and
atmospheric transport. For the source regions influencing the Svalbard
Lomonosovfonna ice core, one might expect that changes in the North Atlantic
Oscillation (NAO) might play an important role. The NAO is a major mode of
climate variability in the North Atlantic region, characterized by changes in
the pressure gradient between the Icelandic low and the Azores high during
winter months . Strong pressure gradients (positive NAO
index) are associated with strong zonal flow, enhanced westerlies
transporting warm air to Europe, increased precipitation in northwest Europe,
and decreased precipitation in southern Europe . Weaker
pressure gradients (negative NAO index) are associated with stronger
meridional flow and cooling of the North Atlantic region .
Proxy NAO records have been developed from variations in wintertime sea salt
sodium in the GISP2 ice core, from Moroccan tree rings and speleothem records
in Scotland, and from lake sediments in southwestern Greenland
.
Comparison of the timing of aromatic acid signals in the
Lomonosovfonna ice core over the past 800 years compared to other
climate-related proxy records. From top: (a) 10-year bin averages of
Lomonosovfonna vanillic acid (violet line), ratio of vanillic
acid / p-hydroxybenzoic acid (gray line), and (b) p-hydroxybenxoic
acid; (c) 10-year bin averages of the oxygen isotope record from the
Lomonosovfonna ice core ; (d) North Atlantic
Oscillation (NAO) index (red > 0; blue < 0; ); and
(e) 10-year bin averages of the summer North Atlantic Oscillation
(SNAO) index . The black horizontal lines are the timing
of the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA)
. The dashed horizontal line is the extended LIA in the
Svalbard region .
The proxy NAO records show a marked change in phase at the onset of the LIA
(around 1400 CE) from several hundred years of positive NAO index to a more
negative and variable NAO state that continued throughout and after the LIA
(Fig. ). The Lomonosovfonna oxygen isotope (δ18O)
record shows a cooling trend at this time, consistent with the NAO shift
. The change in NAO behavior at this time was accompanied by
a decline in VA in the Lomonosovfonna record, a decline in the VA / HBA
ratio, and a sudden divergence between the Lomonosovfonna and Akademii Nauk
ice cores (1400 CE). We suggest that a change of high latitude northern
hemispheric atmospheric circulation patterns occurred at this time, resulting
in (1) a cooler, wetter northern Europe with less burning and (2) reduced
zonal transport, resulting in “decoupling” of the two ice core signals.
Central Siberian burning likely increased at this time, as evidenced by
sharply increased aromatic acids in the Akademii Nauk ice core. The Siberian
ice core signals are similar in timing to changes in the strength of the
Asian monsoon, as recorded in speleothem proxy records
. We speculate that during the LIA, central Siberia
was influenced primarily by conditions in the Pacific rather than the
Atlantic Ocean.
The summertime NAO (SNAO) is defined as the leading mode of July–August sea
level pressure (SLP) variability in the North Atlantic sector
. The SNAO affects temperatures,
precipitation, and cloudiness in Europe during summer, and one might expect
that variations in burning are even more directly linked to the SNAO than the
NAO. The SNAO has a slightly different spatial pattern than the NAO, with a
relatively small Arctic node and a southern node over northwestern Europe.
The positive (negative) mode of the SNAO is characterized by a warmer and
drier (cooler, wetter) northern Europe . The
influence of the SNAO extends to central Asia, and could therefore influence both
major source regions for the Lomonosovfonna ice core.
In order to illustrate the possible influence of the SNAO, we compared back
trajectories from the Lomonosovfonna site for recent periods when the SNAO
index was positive (1970–1979 CE, mean SNAO index: 6.3) and negative
(1948–1957 CE; mean SNAO index: -2.0) .
Figure shows the major spatial clusters of 10-day air mass back
trajectories for each time period (computed using HYSPLIT) superimposed on
the SLP anomalies relative to mean SLP from 1948 to 2017.
The high SNAO period is characterized by (1) high pressure over Scandinavia,
favoring drier conditions, and (2) trajectories generally originating at
lower latitudes, with a larger fraction of transport from Scandinavia. These
results suggest that SNAO-driven variability in atmospheric transport could
contribute to variability in burning signals in the Lomonosovfonna record.
SNAO variability has been reconstructed for the past 500 years using
historical documents and tree rings
. The SNAO record
is primarily negative over the past 500 years, with brief positive excursions
until the start of the twentieth century when it shifted into its positive phase
. The long-term trends in the SNAO and NAO
reconstructions are generally similar, and there are some common features on
centennial timescales Fig. ;. Both NAO and
SNAO records exhibit a positive excursion from 1500 to 1650 CE, during a
period of elevated p-HBA in Lomonosovfonna. After 1400 CE, VA remains low
and does not show similar variability to p-HBA. This incoherence between the
records could be due to the change in atmospheric circulation patterns after
1400 CE when the Svalbard and Siberia ice core records diverge.
Lomonosovfonna ice core VA / p-HBA ratios
The mean VA / p-HBA ratio for the Lomonosovfonna ice core using the
10-year bin averages of each record is 0.40±0.25 (n=79). Both
compounds are produced during the smoldering phase of burning
and both are produced from combustion of
major boreal forest tree species. Short-term changes in the ratio most likely
reflect the changing contributions from various source regions with different
ecosystems. Longer-term changes in the ratio could reflect changes in
ecology and biogeography (i.e., shifts between conifer and broadleaf forests or
grasslands) or changes in atmospheric transport patterns. As noted earlier,
analysis of back trajectories suggests that boreal forests are the principle
source regions for this ice core, with minor contributions from tundra and
temperate forests.
Ten-day clustered back trajectories starting from the Lomonosovfonna
ice core location superimposed on sea level pressure anomalies for summer
(June–August) of 1948–1957 (negative SNAO) and 1970–1979 (positive SNAO).
Anomalies are relative to the 1948–2017 mean of NCEP/NCAR reanalysis data.
The range of VA / p-HBA ratios observed in the Lomonosovfonna ice core
is consistent with the laboratory combustion studies of boreal forest tree
species. Combustion studies have been conducted on several conifers
characteristic of North American and European boreal forests. North American
conifer (lodgepole pine, Sitka spruce, Douglas fir, and mountain hemlock)
combustion yielded VA / p-HBA weight ratios ranging from 0.40 to 0.99
(Table S2). Specific North American conifers produce only one
of the two compounds. For example, eastern white pine produced only VA, and
noble fir produced only p-HBA . European conifers and peat
burning produced VA / p-HBA weight ratios ranging from 0.07 to 8.75
. Combustion of a German peat sample yielded a low
VA / p-HBA ratio of 0.08 . Combustion of a
tundra grass sample from the Canadian Yukon produced p-HBA only,
but at rates 1000-fold less than conifers . Deciduous tree
species produced roughly 1000-fold more VA than conifers (mg VA kg-1
fuel burned), and deciduous species did not produce detectable levels of
p-HBA . Thus even a small fraction of air mass
trajectories from temperate forests could influence the VA / p-HBA
ratio.
We are not aware of laboratory combustion studies of the actual species
comprising Siberian forests or tundra. This is a major gap in the knowledge
base needed to interpret Arctic ice core data. Similarly, very few studies to
date have reported VA / p-HBA ratios for ambient Arctic aerosols.
reported ratios ranging from 0.16 to 2.2 in weekly aerosol samples
collected at Alert, covering the range observed in the ice core.
There are significant long-term changes in the Lomonosovfonna
VA / p-HBA ratio over time. The ratio is relatively high during the MCA
(0.8), decreases by a factor of 2 from 1200 to 1400 CE,
and remains low through the LIA until
around 1800 CE (Fig. ). There is also an increase in
VA / p-HBA after 1800, although VA is close to the detection limit and
the uncertainty in the ratio is consequently large. Interestingly, the
changes in the VA / p-HBA ratio broadly mirror changes in the phase of
the paleoreconstructions of the NAO and SNAO (Fig. ). One might
speculate that the associated changes in climate and transport mentioned
earlier contribute to the variations in the VA / p-HBA ratio but the
specific causes are not understood at this time.
There are several multi-decadal excursions in the VA / p-HBA ratio.
Ratios greater than one occur in the VA peaks around 1270 and 1370 CE. The
second of these peaks is a notable increase in VA, with no corresponding peak
in p-HBA. Conversely, around 1540 and 1620 CE, there are p-HBA peaks without
a corresponding peak in the VA record. These events are probably too short to
represent ecological changes, but too long to represent single fire events or
seasons. Such events are worthy of further investigation.