Organic
Geochemistry
Organic Geochemistry 38 (2007) 16–27
www.elsevier.com/locate/orggeochem
A novel chemical fossil of palaeo sea ice: IP25
Simon T. Belt
a
a,*
, Guillaume Massé a, Steven J. Rowland a, Michel Poulin b,
Christine Michel c, Bernard LeBlanc c
Petroleum and Environmental Geochemistry Group, School of Environmental Sciences, University of Plymouth, Drake Circus,
Plymouth, Devon PL4 8AA, UK
b
Research Division, Canadian Museum of Nature, P.O. Box 36, Ottawa, Ont., Canada K1P 6P4
c
Freshwater Institute, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Man., Canada R3T 2N6
Received 12 July 2006; received in revised form 20 September 2006; accepted 26 September 2006
Abstract
A unique and novel organic compound has been detected in sea ice samples from three locations in the Canadian Arctic,
thousands of kilometers apart. It is likely that this biomarker is produced by diatoms living in the sea ice and we provide
evidence which suggests that the compound, a C25 monounsaturated hydrocarbon (IP25), may be a specific, sensitive and
stable proxy for sea ice in sediments over at least the Holocene. Since it has not been reported before, we confirmed its
identity by synthesis and used the synthesized compound as a reference for quantifying IP25 in a range of sediments from
an East-West transect in the Canadian Arctic. The recent sediments are either seasonally covered with ice or have permanent ice cover. Older sediments containing IP25 have been dated using radiocarbon methods to at least 9000 yr. If the concept proves generally applicable, monitoring IP25 in further sediment cores along with other accepted proxies, should allow
movements in the position of the ice edge throughout the Holocene (at least) to be better determined, which is essential for
accurate calibration of climate prediction models. A similar approach for Antarctic samples of ice and sediments would
also seem worthwhile.
2006 Elsevier Ltd. All rights reserved.
1. Introduction
1.1. Polar sea ice and climate change
Polar sea ice plays a fundamental role in determining the rate of global climate change by influencing the exchange of heat and moisture between
polar oceans and the atmosphere, since it reflects
*
Corresponding author. Tel.: +44 1752 233042; fax: +44 1752
232406.
E-mail address: [email protected] (S.T. Belt).
much of the incoming solar radiation (the albedo
effect; Thomas and Dieckmann, 2003). In addition,
it contributes to circulation models of numerous
oceanic processes (e.g. thermohaline circulation),
many of which have further positive feedback mechanisms in relation to sea ice extent (Thomas and
Dieckmann, 2003). Consequently, the retreat of
sea ice from the Arctic regions during the last 30
years has been receiving considerable attention
(Johannessen et al., 1995, 1999; De la Mare, 1997;
Vinnikov et al., 1999; Francis et al., 2005; Stroeve
et al., 2005; Divine and Dick, 2006) It is therefore
0146-6380/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.orggeochem.2006.09.013
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
essential to improve our knowledge of historical sea
ice fluctuations and the associated variations in climate in order to better refine and calibrate models
of future climate change. In recent times (the last
30 years), satellite imaging methods have enabled
sea ice coverage determination to become both rigorous and routine (Johannessen et al., 1995, 1999;
Vinnikov et al., 1999; Francis et al., 2005; Stroeve
et al., 2005), while historical sealing and whaling
records provide some information for the last 300
years (De la Mare, 1997; Divine and Dick, 2006).
However, there is a paucity of meaningful records
of sea ice over longer timescales, principally since
the melt process leaves behind a relatively poor geological record. Of the proxy measures that exist for
the determination of palaeo sea ice cover, one
method relies on the preservation in sediments of
fossil siliceous frustules (or skeleta) of diatomaceous
sea ice algae (Thomas and Dieckmann, 2003; Gersonde and Zielinski, 2000). These microalgae can
be distinguished from open water diatoms or flora
found in multi-year ice. The sea ice diatoms inhabit
the underside of first year ice at the interface with
nutrient-rich seawater, where light penetration
through the ice is adequate and where microscopic
capillaries and brine pockets provide a unique ecological niche (Thomas and Dieckmann, 2003).
When the ice melts, or the microalgae die or are
grazed by zooplankton, the silica and organic compounds of which these microalgae (diatoms) are
comprised, settle through the water column before
reaching the sediment. However, poor preservation
of the thin siliceous frustules of diatoms sometimes
makes it difficult to carry out unambiguous identification, and the majority of organic compounds
found in sea ice diatoms are not preserved well
and suffer biodegradation in the water column or
sediment. Of those which do survive, most are
non-specific for sea ice diatoms, rendering them
unsuitable as specific sea ice proxies.
In order to be useful as an indicator of the presence of former sea ice, a chemical biomarker must
originate within the sea ice and be absent from open
waters (i.e. be specific), be sufficiently resistant to
degradation in the water column and to other diagenetic changes in the sediment (i.e. be stable) and be
produced in sufficient quantity for straightforward
detection, both amongst the numerous other compounds in sediments and with meaningful geological
resolution (i.e. be sensitive). The ketones measured
0
in the well known Uk37 alkenone index, which has
provided a useful and widely used chemical proxy
17
for the determination of mean summer or spring
sea surface temperatures of open waters (e.g. Brassell et al., 1986), have many of these attributes.
1.2. Diatom specific isoprenoids as a new chemical
proxy
C25 and C30 highly branched isoprenoid (HBI)
alkenes, known as haslenes and rhizenes, respectively, are biosynthesised by a limited number of
diatom genera; notably some Haslea spp., Rhizosolenia spp., Pleurosigma spp. and some Navicula
spp. (Volkman et al., 1994; Massé et al., 2004; Sinninghe Damsté et al., 2004). In Haslea ostrearia
(Gaillon) Simonsen, the extent of unsaturation in
the haslenes co-varies with culture temperature.
Thus, tetra- and tri-unsaturated haslenes are produced as the major isomers at 25 C and 15 C
respectively, while at 5 C, diunsaturated haslenes
(hasladienes) are biosynthesized as the major isomers (Rowland et al., 2001). Haslea spp. have been
identified as minor members of both Arctic (Poulin,
1990; Booth and Horner, 1998; Riedel et al., 2003)
and Antarctic (Pakhomov et al., 2001) sea ice diatom populations and hasladienes have been found
in mixed diatom populations obtained from Antarctic sea ice (Nichols et al., 1988; Johns et al., 1999).
We hypothesised that a similar dependence of
unsaturation on temperature might lead to the production of monounsaturated haslenes in sea ice populations of Haslea spp., and that, on the melting or
grazing of the ice, such compounds might be better
preserved in sediments than the more unsaturated
HBIs, including trienes and tetraenes. Previously,
synthesized haslane (Robson and Rowland, 1986)
and mono-unsaturated haslenes have been shown
to be relatively resistant to biodegradation compared to unbranched alkanes and alkenes (Robson
and Rowland, 1988) and, even hasladienes undergo
little diagenetic reaction compared with the more
unsaturated HBIs (Belt et al., 2000b), suggesting
that the monoenes would be even more stable. As
such, HBI monoenes should have the potential to
serve as proxy measures of contemporary and historical sea ice when detected in polar ice and sediments respectively. Although we were aware of a
limited number of reports of haslene monoenes of
unknown biological origin in sediments from temperate and sub-tropical environments (Dunlop and
Jefferies, 1985; Xu et al., 2006), the specificity of
the biosynthetic process in the genus Haslea Simonsen (Massé et al., 2004), encouraged us to postulate
18
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
that monoenes in sea ice and/or polar sediments
might be structurally specific and different from
those found in non-polar regions. In order to test
this hypothesis, we examined the hydrocarbon content of sea ice obtained from three distinct regions
of the Canadian Arctic, together with recent and
ancient (Holocene) sediments collected from across
the Canadian sub- and High Arctic regions, including those seasonally covered with ice. We also examined the hydrocarbon extracts from assemblages of
open water phytoplankton in order to determine
whether the chemicals present in sea ice were sea
ice restricted or not.
2. Experimental
2.1. Sea ice and sediment sampling
Sea ice sampling (Fig. 1) was performed in
McDougall Sound (May 2003; Site 1), Franklin
Bay (May 2004; Site 2) and Button Bay (April
2005; Site 3). Ice samples were collected using a
MARK II ice corer at a site where snow cover
was at a minimum. For diatom analysis, the bottom
sections (5 cm) of three cores were combined in a
dark isothermal container. In the laboratory, these
sections were allowed to melt at room temperature
(15 C) and two sub-samples (25 ml) were preserved
with 1% formaldehyde (1 ml) prior to analysis. For
each sampling site, a further three ice cores were collected and kept frozen for hydrocarbon analysis.
Sediment material was obtained in three stages.
First, surface and down-core material (seven locations, NOW-1 – NOW-5, WEI-1,2; Fig. 1, Table
1) was obtained from the Geological Survey of Canada (Atlantic Division, Bedford Institute of Oceanography). Second, surface sediment samples were
collected from Button Bay (April 2005; ARC-8;
Fig. 1, Table 2) using an Ekman grab sampler.
Third, combinations of surface sediment (box
core; 50 cm · 50 cm · 80 cm), trigger weight cores
(99.2 mm internal diameter) and piston cores
(99.2 mm internal diameter) were obtained from
seven locations (ARC-1 to ARC-7; Fig. 1, Table
2) during the CCGS Amundsen Leg 1 ArcticNet
cruise (August/September 2005). Sub-samples of
surface and core material (20 C) and the remaining core material (< 4 C) were stored in the dark
prior to analysis.
2.2. Diatom analysis
Diatom material from sea ice was prepared
according to Hendey (1974). Cleaned diatom frus-
Fig. 1. Map of sampling locations for sea ice and sediment samples. , Sea ice; , NOW sediments; , WEI sediments; , ARC sediments.
Individual numbering corresponds to specific locations cited in the text.
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
19
Table 1
Summary of Arctic sediments corresponding to North Water (NOW) and Ellesmere Island (WEI) regions of the Canadian High Arctic
Sample
Cruise
number
Hole
Water
depth (m)
Ice cover
Date of
collection
Latitude (N)
Longitude
(W)
IP25 (lg/g
w.w.)
NOW-1
84015
71
538
23-09-1984
7523.58 0
7921.30 0
0.53
NOW-2
84015
61
578
22-09-1984
7612.90 0
8303.18 0
0.56
NOW-3
84015
67
677
22-09-1984
7601.38 0
8235.70 0
0.33
NOW-4
91039
8twc
663
27-08-1991
7715.96 0
7419.92 0
0.29
NOW-5
91039
26d
450
06-09-1991
7529.52 0
7810.98 0
0.16
WEI-1
WEI-2
85200
85200
38
50
134
115
Annual/
seasonal
Annual/
seasonal
Annual/
seasonal
Annual/
seasonal
Annual/
seasonal
Permanent
Permanent
07-08-1985
10-08-1985
8101.80 0
8102.58 0
9638.28 0
9628.38 0
n.d.
n.d.
n.d., not detected.
Table 2
Summary of Arctic sediments obtained during the ArcticNet 2005 cruise and from Churchill, Hudson Bay (NB IP25 concentrations for
ARC-5 have not been determined)
Sediment
ArcticNet stations
Water
depth (m)
Ice cover
Date of
collection
Latitude
(N)
Longitude
(W)
IP25
(lg/g w.w.)
ARC-1
ARC-2
ARC-3
ARC-4
ARC-5
ARC-6
ARC-7
ARC-8
2 – North Water
3 – Lancaster Sound
4 – Barrow Strait
6 – Victoria Strait
7 – Dease Strait
11 – Franklin Bay
12 – Amundsen Gulf
Button Bay, Churchill
703
811
347
61
112
253
219
15
Annual/seasonal
Annual/seasonal
Annual/seasonal
Annual/seasonal
Annual/seasonal
Annual/seasonal
Annual/seasonal
Annual/seasonal
20-08-2005
23-08-2005
24-08-2005
27-08-2005
30-08-2005
10-09-2005
13-09-2005
03-05-2005
7749.75 0
7402.95 0
7416.05 0
6909.94 0
6859.45 0
7019.49 0
6954.81 0
5850.00 0
7439.97 0
7954.41 0
9106.38 0
10041.72 0
10634.27 0
12623.50 0
12257.96 0
9420.00 0
0.31
0.33
1.52
0.37
–
0.14
0.16
0.15
tules were examined with light microscopy and, to
achieve unambiguous identification of some of the
species, some preparations were examined using
scanning electron microscopy (Jeol 6400F).
2.3. Lipid analysis
Sea ice sections (200–300 ml) were freeze-dried
and an internal standard was added to permit quantification using GC–MS (7-hexylnonadecane,
1.5 lg sample1). The freeze-dried samples were
extracted using a mixture of CH2Cl2/CH3OH (50/
50; 5 ml) to yield a total organic extract (TOE).
The TOE was saponified with 2 ml 5% KOH in
CH3OH/water (80/20) at 80 C for 30 min to hydrolyse triglyceride esters of fatty acids. The nonsaponifiable lipids (NSLs) were re-extracted into
hexane (3 · 1 ml) and dried over anhydrous
Na2SO4. Following derivatisation (100 ll BSTFATMCS (99/1), 30 min at 70 C), these NSLs were
analysed using GC–MS and purified using a two
stage chromatography procedure if necessary
(Section 2.4).
Individual compounds were identified on the
basis of comparison between their GC retention
indices and mass spectra with those of authentic
standards. A similar procedure was used for sediment samples except that the internal standard
(1.5 lg) was added to the wet sediment material
(5 g) immediately before extraction.
2.4. Purification of individual compounds
Hydrocarbons were separated from phytol, sterols and other compounds in the NSLs using open
column chromatography (50:1 SiO2:NSLs (w/w)).
The hydrocarbons were eluted with hexane (5 column volumes) while the more polar compounds were
obtained as a single fraction using CH2Cl2 (2 column
volumes) followed by (CH3)2CO (2 column volumes). In some cases, individual lipids from the
hydrocarbon fraction were then separated with silver
ion chromatography (5:95 AgNO3:SiO2). Saturated
hydrocarbons were eluted with hexane (5 column
volumes), monoene 1 was obtained using CH2Cl2
20
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
5970 Series mass selective detector (MSD) was used
to perform gas chromatography–mass spectrometry
(GC–MS) analysis. The GC oven temperature was
programmed from 40 to 300 C at 5 C min1 and
held at the final temperature for 10 min. Operating
conditions for the mass spectrometer were 280 C
for the ion source temperature and 70 eV for the
ionisation energy. Spectra (m/z 40–550) were collected using Hewlett-Packard MS-Chemstation
software.
2
1
3
Fig. 2. Chemical structures of IP25 (1), C25 HBI monoene (2) and
C25 HBI diene (3). The double bond in IP25 is located at C23–24.
2.6. Synthesis of C25 HBI monoenes 1 and 2
2.6.1. Monoene 1
Synthesis of 1 was performed via a 5 step procedure as shown in Fig. 3. To diene 3 (5 mg,
14.4 lmol) in CH2Cl2 (1 ml) was added m-chloroperoxybenzoic acid (MCPBA) (6.25 mg, 14.4 lmol)
dissolved in CH2Cl2 (1 ml) and the resulting solution was heated at 50 C for 4 h. Following solvent
evaporation (N2), the reaction mixture was redissolved in hexane and subjected to column chromatography, whereby unreacted 3 and epoxide 4
(3 mg) were obtained by elution with hexane and
(2 column volumes) and the more unsaturated
alkenes were eluted using a mixture of CH2Cl2/
(CH3)2CO (60/40, 2 column volumes). Structures of
the C25 HBI alkenes discussed are shown in Fig. 2.
2.5. Lipid analysis
A Hewlett-Packard 5890 Series II gas chromatograph (GC), fitted with a 30 m fused silica HP-1 column (0.25 mm i.d., 0.25 lm film) and coupled to a
O
(i)
3
4
(ii)
HO
O
HO
6
Br
(iii)
Br
Br
5
Br
(iv)
(v )
Br
7
1
Br
Fig. 3. Synthetic scheme for the preparation of the C25 monoene (IP25) found in Arctic sea ice and sediments: (i) MCPBA, CH2Cl2, 4 h,
50 C; (ii) TBABr3, CH2Cl2, 0.5 h, 25 C; (iii) HClO4, THF/H2O, 1 h, 25 C; (iv) PtO2/H2, CH3CO2H, 5 h, 25 C; (v) Zn, t-BME,
CH3CO2H, 12 h, 25 C.
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
CH2Cl2, respectively. Epoxide 4 (3 mg, 8.2 lmol) in
CH2Cl2 (1 ml) was added to 2 equiv. of tetra-n-butylammonium tribromide in CH2Cl2 (1 ml) and the
resulting solution left at room temperature (1 h).
After solvent evaporation (N2), the reaction mixture
was redissolved in hexane and subjected to column
chromatography, whereby the dibromo-epoxide 5
(4.1 mg) was obtained by elution with hexane. Following reaction of dibromo-epoxide 5 (4.1 mg,
7.8 lmol; H2O/THF (3 ml, 40/60)) with HClO4
(75 ll; 3 h), the reaction was quenched with saturated NaHCO3 (3 ml) and the products re-extracted
into hexane (2 · 3 ml). Solvent removal yielded the
dibromo-diol 6 (3.5 mg). H2 was bubbled through
a solution of 6 (3.5 mg, 6.4 lmol) in CH3CO2H
(1 ml) in the presence of PtO2 Æ 2H2O (0.05 g). After
5 h, the sample was filtered through a glass Pasteur
pipette containing anhydrous sodium Na2SO4. Following solvent evaporation (N2), the reaction mixture was redissolved in hexane and subjected to
column chromatography, whereby dibromide 7
(1.9 mg) and unreacted 6 were obtained by elution
with hexane and CH2Cl2, respectively. Dibromide
7 (1.9 mg, 3.7 lmol) was dissolved in a mixture of
tert-butyl methyl ether (2 ml) and CH3CO2H
(1 ml); 20 mg of zinc dust was added and the mixture was stirred at room temperature (12 h). After
filtering (Na2SO4) and removal of the solvent (N2),
the reaction mixture was redissolved in H2O (2 ml)
and the products reextracted into hexane
(2 · 3 ml). Removal of the hexane yielded 1.1 mg
of 1.
21
2.6.2. Monoene 2
H2 was bubbled through a solution of diene 3
obtained previously by extraction from H. ostrearia
(5 mg, 14.4 lmol) in hexane (5 ml) in the presence of
PtO2 Æ 2H2O (0.1 g). Aliquots were removed at regular intervals (each 30 s) to monitor the hydrogenation
progress
with
GC–MS.
Following
hydrogenation, samples were filtered through a glass
Pasteur pipette containing anhydrous Na2SO4 and
the solvent was removed under N2 prior to analysis.
Monoene 2 was separated from 3 and its saturated
analogue using silver ion chromatography as
described above to yield 0.7 mg (2 lmol) of pure 2.
2.6.3. NMR characterisation of C25 monoene 1
1
H and 13C NMR spectroscopy were used to
characterise the C25 monoene 1. Spectra were
obtained using a Jeol EX 270 spectrometer operating at 270 and 68 MHz for 1H and 13C respectively,
and processed using DELTA software. Spectra were
recorded in CDCl3 and chemical shifts were referenced relative to residual proton signals or to the
central line of 13C multiplets of the solvent (1H:
7.24 ppm, 13C: 77.0 ppm). Selected NMR data: 1H
(d/ppm): 5.69 (m, 1H, –CH@CH2), 4.92 (m, 2H, –
CH@CH2); 13C (d/ppm): 145.1 (CH@CH2), 114.3
(CH@CH2).
2.7. Dating of core material
Determination of the age of the longest core
obtained from the ArcticNet cruise was achieved
Table 3
Summary of phytoplankton samples collected across the Canadian High Arctic during the ArcticNet cruise in 2005
Sample
ARCTICNET station
Date of collection
Water depth (m)
Latitude
Longitude
PHK-1
PHK-2
PHK-3
PHK-4
PHK-5
PHK-6
PHK-7
PHK-8
PHK-9
PHK-10
PHK-11
PHK-12
PHK-13
PHK-14
PHK-15
PHK-16
PHK-17
PHK-18
Pond Inlet
BA01 – North Water
BA02 – North Water
BA03 – North Water
BA04 – North Water
3 – Lancaster Sound
4 – Barrow Strait
6 – Victoria Strait
7 – Dease Strait
204
CA-05-05
10 – Beaufort Sea
CA-04-05
CA-08-05
11a – Franklin Bay
11b – Franklin Bay
CA-18-04
12 – Amundsen Gulf
14-08-2005
16-08-2005
17-08-2005
18-08-2005
21-08-2005
22-08-2005
24-08-2005
27-08-2005
30-08-2005
02-09-2005
03-09-2005
05-09-2005
06-09-2005
09-09-2005
11-09-2005
11-09-2005
12-09-2005
13-09-2005
751
676
446
348
478
817
332
65
120
65
211
220
335
414
263
256
609
219
72.47 0 03900
76.17 0 92400
76.15 0 49100
76.23 0 11400
75.14 0 28000
74.02 0 89500
74.15 0 51900
69.10 0 09200
69.00 0 05400
71.08 0 47300
71.16 0 72700
71.33 0 50000
71.05 0 02700
71.00 0 00000
70.20 0 26400
70.20 0 31000
70.39 0 17400
69.54 0 81300
78.07 0 50500
71.24 0 52500
74.36 0 32100
77.21 0 20800
74.59 0 07500
79.55 0 68400
91.11 0 27300
100.42 0 14500
106.35 0 00900
128.11 0 28300
127.28 0 98800
140.04 0 20000
133.34 0 28900
125.55 0 00000
126.22 0 10600
126.21 0 67500
122.59 0 45000
122.57 0 95700
22
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
using radiocarbon (14C) dating of foraminifera
shells [Neogloboquadrina pachyderma (s)] isolated
from the base of the core obtained from Lancaster
Sound (641 cm). A sub-sample of the sediment
(50 g) was air-dried, sieved (63 lm) and the foraminifera isolated by hand under a microscope.
Approximately 200 specimens were analysed using
accelerator mass spectrometry (Beta Analytic Inc.,
USA).
2.8. Phytoplankton collection
Surface and near surface (1–50 m water depth)
samples of phytoplankton were obtained via horizontal and vertical trawling (75 lm) of late summer
ice-free waters across the entire East-West transect
of the Canadian High Arctic during the ArcticNet
cruise (August/September 2005, Table 3). Aliquots
of the phytoplankton concentrates were preserved
(Lugol) prior to diatom analysis. The remaining
concentrates were extracted and analysed as per
other lipid analyses described previously.
Johns et al., 1999) but not, to date, in sea ice. In
addition, a more abundant haslene isomer was identified, which had one degree of unsaturation (Figs. 4
and 5). In order to confirm that it was a monounsaturated haslene (rather than a related monocyclic
compound; Belt et al., 2001), the isomer was hydrogenated to the parent haslane (Robson and Rowland, 1986). Comparison of the retention index
and mass spectrum of this monoene with those
reported previously for HBI monoenes (Dunlop
and Jefferies, 1985; Xu et al., 2006) demonstrated
clearly that the sea ice derived compound was structurally different from any of the other HBI monoenes identified previously in sediments (e.g. 2,
Fig. 2). Given the structural relationships of the
a
* + (Δ 6,17) C25:2
(Δ 5,6) C25:2
*
(Δ 5,6) C25:3
n-C21:6
3. Results and discussion
3.1. Analysis of Arctic sea ice and phytoplankton
assemblages
20
As part of an initial study, sea ice samples from
three well-separated regions of the Canadian suband High Arctic (McDougall Sound, Franklin Bay
and Button Bay; Fig. 1) were obtained. In all three
cases, analysis with light and electron microscopy
revealed the presence of characteristic Arctic sea
ice diatoms, including members of the common genera Nitzschia Hassall, Fragilariopsis Hustedt,
Entomoneis Ehrenberg, Navicula Bory and Pseudogomphonema Medlin (Thomas and Dieckmann,
2003; Poulin, 1990). Significantly, and as expected
from previous reports (Poulin, 1990; Booth and
Horner, 1998; Riedel et al., 2003), low abundances
(1–3%) of a number of Haslea spp. were present in
each case. Thus, Haslea crucigeroides (Hustedt)
Simonsen, Haslea vitrea (Cleve) Simonsen and
Haslea kjellmanii (Cleve) Simonsen were readily
detected.
Examination of the hydrocarbon fractions from
the extracted sea ice samples from each of the three
Arctic locations (Fig. 1) using GC–MS revealed the
occurrence of C25 HBI dienes and trienes, one of the
former having also been identified in Antarctic sea
ice diatoms and sediments (Nichols et al., 1988;
30
40
50
b
* + (Δ 6,17) C25:2
15
c
n-C21:6
(Δ 7,20) C 25:3
(Δ 6,17) C 25:3
*
24.5
29.5
34.5
Time
Fig. 4. Partial total ion current chromatograms of hydrocarbon
fractions from extracted Arctic sea ice: (a) McDougall Sound
(Site 1); (b) Franklin Bay (Site 2); (c) Button Bay (Site 3). The
peak labelled * is due to the C25 HBI monoene, IP25. Filled circles
denote n-alkanes, C20–C23.
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
23
140
196
50
100
150
224
210
200
266 280
250
350
300
350
m/z
Fig. 5. Mass spectrum of IP25 corresponding to the D23, 24 HBI monoene found in the hydrocarbon fractions of Arctic sea ice and
sediment extracts. RIHP1 = 2090; RIZB–WAX = 2056.
co-occurring HBI dienes and trienes, along with the
structures of previously reported HBI monoenes,
the most likely isomeric form of the new compound
appeared to be 1, with a double bond at C23-24
(Fig. 2). Confirmation of this assignment was
achieved by independent synthesis of 1 and 2 from
the previously characterised diene 3. With the structure of synthetic 1 firmly established using NMR
spectroscopy, gas chromatographic and mass spectral analysis showed it to be the same isomer as that
found in the Arctic sea ice samples (Fig. 5).
In contrast to these observations of microflora
and hydrocarbons found in Arctic sea ice, both
Haslea spp. and the new monoene were absent or,
at least, were below our limits of detection, from
samples of open water phytoplankton obtained
from eighteen sampling locations across the entire
East-West transect of the Canadian High Arctic
(Table 3). As such, the novel C25 HBI monoene
appears to be specific to Arctic sea ice. Given the
ubiquity of haslene production by members of the
genus Haslea, we suggest that species such as H.
vitrea and H. crucigeroides, present in all the sea
ice samples we examined, are the probable sources
of this novel biomarker. Confirmation of this
hypothesis will require culturing of individual species under sea ice conditions. However, regardless
of the exact species of diatom which is/are the
source(s) of the alkene, we propose the term IP25
as a suitable reference term for this compound, since
it corresponds to an Ice Proxy with a 25 carbon
atom skeleton.
3.2. C25 HBIs in Arctic sediments
We have also carried out core top and partial
down-core analyses of hydrocarbon fractions from
sediment extracts obtained from a total of fifteen
locations across the Canadian Arctic (Fig. 1). Initially, we analysed sediments from locations
between Greenland and Ellesmere Island in the
northwest region of Baffin Bay (Table 1). This area,
known as the North Water Polynya (NOW), is one
of the most productive areas of the Arctic, with an
annual phytoplankton production of up to
250 g C m2 (Klein et al., 2002). When the hydrocarbon fractions from the NOW surface sediments
(NOW-1 to NOW-5) were examined using GC–
MS, the same HBI isomers found in sea ice were
present, with IP25 occurring as the most abundant
isomer in all cases (Fig. 6). Further, in two out of
the five sediments from this region (NOW-1,
NOW-2), IP25 was present as the most abundant
compound in the hydrocarbon fraction. In contrast,
haslenes were absent from surface sediments from
West Ellesmere Island (WEI-1, WEI-2). Thick,
multi-year ice occurs to the West of Ellesmere
Island and so little biological activity occurs in this
24
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
a
b
corresponding to an East-West transect of the
Canadian High Arctic, together with some surface
sediment material from Churchill, South-West Hudson Bay (ARC-1–ARC-8; Fig. 1, Table 2). Consistent with the findings from the NOW sediments,
haslenes were present in all core top material, with
IP25 occurring as the most abundant isomer in all
cases (Fig. 6). For Barrow Strait (ARC-3), Victoria
Strait (ARC-4) and Dease Strait (ARC-5), we also
carried out partial down-core analysis and found
IP25 at 1 m intervals in each of the three cores.
For the longest core (ARC-3), accelerator mass
spectrometric (14C) dating of planktonic foraminifera [Neogloboquadrina pachyderma (s)] isolated from
the core bottom (641 cm) yielded a date of
9150 ± 200 yr BP.
c
4. Discussion
d
e
Fig. 6. Partial total ion current chromatograms of hydrocarbon
extracts from Arctic sediments: (a) ARC-1; (b) ARC-3; (c) ARC4; (d) ARC-7; (e) WEI-1. The peaks labelled * and are due to
IP25 and the internal standard respectively. Filled circles denote
n-alkanes, C20–C24. Note, the absence of IP25 in (e).
region and the presence of the sea ice derived chemical would not be expected.
In measurements designed to investigate the
wider occurrence of the haslenes in Arctic sediments, we obtained a series of seven sediment cores
Reconstruction of the palaeo-glacial ocean surface is clearly a crucial and challenging task which
has occupied scientists for at least 30 years (Hays
et al., 1976; Kucera et al., 2005). To date, the most
reliable reconstructions of sea ice boundaries have
relied on counting at least 300 diatom valves or
radiolarian skeletons in each sediment sub-sample
using high quality photomicroscopes and skilled,
specialist operators (e.g. Gersonde et al., 2005).
The species distributions and abundance data, corrected for the dominance of some species, are compared with modern analogues where possible and
treated by a variety of statistical methods and equations (Gersonde et al., 2005). Even so, in some environments, dissolution of the siliceous diatom
frustules prevents identification or diatom counting.
Thus, for instance, major uncertainties still exist
concerning the reconstruction of summer sea ice
extent in the Antarctic. The presence or absence of
calcareous microfossils such as planktonic foraminifera has also been used as a proxy for previous
sea ice cover (e.g. Rahman and de Vernal, 1994),
though abundances and species diversity are often
very low, with only Neogloboquadrina pachyderma
being present at higher latitude. Dissolution of aragonite/carbonate microfossils is a further limitation
with this method. Finally, the occurrence of dinoflagellate cysts has been used as a proxy measure of
Arctic sea ice (De Vernal et al., 2000; Mudie et al.,
2001), partly due to their stability under a wide
range of environmental conditions. However, further work is needed to understand the controls governing cyst formation and, rather like other proxy
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
measures, the strict association of cysts with sea ice
is implied rather than known. Indeed, few current
proxy methods are direct indicators of sea ice, but
rather indicate the presence or absence of permanent ice or of ice free waters. Clearly, complementary sea ice proxies are desirable.
Our studies have shown that the C25 haslene,
IP25, is present in Arctic sea ice from at least three
diverse locations, yet absent from the phytoplankton. Previous studies of HBI-producing diatoms
(e.g. Haslea spp.) suggest that haslenes are probably
produced in the chloroplasts of the algae (Massé
et al., 2004), but the roles of the compounds are
unknown. Nevertheless, whilst the specific source(s)
of IP25 have not been firmly identified and, indeed,
such species may not necessarily be particularly
abundant, it is known that Haslea spp. produce very
high concentrations of lipids, including haslenes. As
such, haslenes may be readily detected and measured in sea ice and sediments even if the source
organisms are relatively low in abundance. In addition, the unique isomeric form of IP25, which
enables it to be distinguished from other sedimentary HBI monoenes detected in temperate and tropical environments, ensures that its detection can be
strongly associated with a sea ice origin. In our
experience, IP25 can be detected from < 1 g of sediment, which should permit a detailed and high resolution sediment core analysis to be carried out,
possibly with sub-decadal resolution.
Having confirmed the occurrence of IP25 in Arctic sea ice and sediments, it will be important to
establish how its concentration in sediments can
be used to obtain a quantitative assessment of previous ice cover. To date, the specificity of the biomarker to sea ice (cf. open waters) means that its
detection in sediments can be used as a presence/
absence indicator of previous sea ice cover, as evidenced by its occurrence in core top sediments from
regions of known recent sea ice cover (Tables 1 and
2) and its absence from regions of permanent ice
cover (WEI sediments, Table 1) or ice free conditions (Rowland and Robson, 1990; Belt et al.,
2000a). A more quantitative measure of sea ice
cover may only be achievable through use with
other proxies and/or an improved understanding
of fluxes from the sea ice to sediments and subsequent sedimentary processes. This, in part, depends
on the ecology of the source organism(s) of IP25.
Currently, little is known about the strict ecological
cycle of the so-called ice-restricted diatoms, in particular, the fate of such species under ice-free condi-
25
tions and the subsequent seeding of sea ice during
winter/spring. If such species (perhaps confusingly
referred to as benthic species since they are normally
found to be present at the sea ice surface rather than
in the benthos), seed the ice from the benthos of
shallow waters of coastal regions then, potentially,
biomarkers such as IP25 may be present in sediments
as a result of direct deposition in addition to transfer from the sea ice. However, this cannot be the
case for most of the sediments studied here since
IP25 has been readily detected in core top sediments
collected from regions with water depths considerably greater than the photic zone. In addition, the
ubiquity and uniformity of diatom populations
across the Arctic is not consistent with a model of
re-suspension of diatoms from coastal sediments.
Alternatively, the seeding of sea ice by diatoms
may occur more directly from species, residual in
the water column, that originate from the melt process during the summer, but survive sediment deposition or grazing. Although both Haslea spp. and
IP25 were not found in our numerous open water
samples, this may be attributable to procedural
detection limits rather than strict absence. Diatom
populations of so-called ice restricted species have
been detected in Arctic open waters (Riedel et al.,
2003; Von Quillfeldt, 2000) although, even when
detected, their concentrations are typically at least
three orders of magnitude lower than for the corresponding sea ice, and the values are (not surprisingly) at a maximum during the most rapid melt
periods (e.g. July). It is also noteworthy that concentrations of diatoms in laboratory cultures are
amplified typically by six or seven orders of magnitude during the exponential growing phase (Wraige
et al., 1997, 1999). Thus, seeding of sea ice from
ultra-low concentrations of diatoms in the plankton
remains a reasonable alternative to re-suspension
from the benthos. In either case, our studies indicate
that IP25 is highly specific to sea ice even if its
sources, in the strictest sense, may not be.
In terms of sedimentary processes, it will be
important to evaluate the diagenetic properties of
IP25 in order that more detailed down-core concentrations can be interpreted more quantitatively for
determination of previous sea ice cover. This will
require laboratory simulations of diagenesis of
IP25 and/or selection of appropriate reference biomarkers with similar degradation properties. Similarly, since sediments are susceptible to advection
and re-working, it will be necessary to establish
the extent of these processes in order that spatial
26
S.T. Belt et al. / Organic Geochemistry 38 (2007) 16–27
and temporal sea ice reconstruction can be
achieved. Feasibly, this could be performed by the
parallel dating of IP25 (14C) and planktonic foraminifera where both are sufficiently abundant. To conclude, our next step will be to quantify IP25 in
sediments in order to make comparisons with satellite and historical records and with other palaeo sea
ice proxy measurements for the global Holocene
(Bond and Lotti, 1995; Andrews et al., 1999; De
Vernal and Hillaire-Marcel, 2000; Gersonde and
Zielinski, 2000; Levac et al., 2001).
5. Conclusions
In summary, the newly characterised HBI monoene represents a potentially specific, stable and sensitive proxy measure of Arctic sea ice and its
presence in Arctic sediment cores may well prove
to be a reliable indicator of the presence of the palaeo-ice edge and of sea ice duration. Such data will
be invaluable in the reconstruction of the former
extent of the Arctic polar ice cap, a necessary prerequisite for the accurate calibration of global climate models. Application of the same principles to
reconstruction of Antarctic sea ice edge would seem
an obvious further application since a related compound has been reported to occur in sediments and
mixed sea ice diatoms, but not, as yet, in individual
species of sea ice diatoms.
Acknowledgements
This work was supported by the (UK) Natural
Environment Research Council (NE/D000068/1),
the Seale-Hayne Educational Trust, Fisheries and
Oceans Canada (Strategic Science Fund to C. Michel), the Canadian Museum of Nature Research
Fund, and the Natural Sciences and Engineering Research Council of Canada Network Grant (Canadian Arctic Shelf Exchange Study). The authors
are members of ArcticNet Networks of Centres of
Excellence Canada funded by the Natural Sciences
and Engineering Research Council, the Canadian
Institutes of Health Research, the Social Sciences
and Humanities Research Council, and Industry
Canada. Logistical support was provided by the Polar Continental Shelf Project (Natural Resources
Canada), the Churchill Northern Research Centre,
and the Centre for Earth Observation Science
(CEOS, University of Manitoba). We thank the officers and crew of the CCGS Amundsen for invaluable
support, A. Rochon (ISMER-UQAR) and co-work-
ers, together with C. Smart and G. Price (University
of Plymouth) for help in obtaining and processing
sediment cores and M. Rózańska for help in collecting ice samples. NOW and WEI sediment material
was provided by the Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada. We thank
an anonymous reviewer for helpful comments
regarding the importance of considering sedimentary processes for future interpretations of IP25 data.
Associate Editor—S. Schouten
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