Reconstructing Sea Ice Conditions in the Arctic and Sub-Arctic Prior to Human Observations - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 27

of sea ice [e.g., Jennings et al. , 2002; Knudsen et al. , 2004].
IRD may also relate to ablation at the outlet of glaciers, lead-
ing to the drift of icebergs that contain detrital particles and
dust eventually released distally into the ocean [e.g., Reeh ,
2004]. Thus, IRD in the open ocean certainly relate to ocean
circulation pattern and temperature but not unequivocally to
sea ice formation and extent.

to be accomplished. The development of modern dinocyst
databases permitted the development of transfer functions
for the quantitative reconstruction of changes in sea ice
cover as expressed in number of months per year with more
than 50% (in areal extent) of sea ice cover [e.g., de Vernal
and Hillaire-Marcel , 2000; de Vernal et al. , 2001, 2005a,
2005b]. Time series of sea ice cover spanning the last glacial
interval and the postglacial are thus available at the scale of
the Arctic and subarctic seas. They are presented below after
a critical examination of the approaches leading to past sea
ice estimates.
In addition to diatoms and dinoflagellate cysts, the occur-
rence of a number of biological indicators, including coccol-
iths and foraminifera, can be used as proxies for sea ice-free
conditions, at least seasonally. Molecular biomarkers such
as alkenones, also referred to as UK 37 , could be used as in-
dicators of ice-free conditions since they rely on biogenic
production of coccoliths [e.g., Rosell-Melé and McClymont ,
2007]. Another biomarker named IP 25 produced by sea ice
diatoms is currently under examination as a direct tracer of
sea ice cover [cf. Belt et al. , 2007]. It seems to be a promis-
ing approach, but calibration still needs to be done prior to
application on long time series.
Finally, another biological indication of past sea ice extent
comes from the recovery of remains from marine mammals
occurring in environments with seasonal sea ice. For exam-
ple, the distribution of bones from bowheads, which follow
the sea ice edge during their life span, was used to assess the
seasonal sea ice opening in the Canadian Arctic Archipelago
and Northwest Passage some 9000 years ago [cf. Dyke et al. ,
1996; Dyke and Savelle , 2001].

2.2. Biogenic Tracers

Most sea ice proxies currently used in paleoceanography
are related to biological activity, especially those related to
planktic production in the photic zone, which is severely
constrained by sea ice. Sea ice is indeed a determinant com-
ponent of the ecosystem inasmuch as it influences the light
penetration and photosynthesis. Thus, close relationships
exist between sea ice cover and phytoplanktic populations.
Sediments accumulating below the permanent multiyear
pack ice are often deprived of biological remains, whereas
sediments deposited below the ice marginal zone may con-
tain abundant microfossils.
Among the microfossils used as sea ice proxy, diatoms
are most useful because they include species specific to the
ice marginal zone. In the circum-Antarctic the distribution
of species and assemblages has been documented in relation
with the extent of sea ice cover [e.g., Armand et al. , 2005;
Crosta et al. , 2005]. On this basis, diatoms have been used
qualitatively and quantitatively for assessing sea ice extent
in the Southern Ocean during the last Glacial Maximum
21,000 years ago [e.g., Crosta et al. , 1998; Gersonde et al. ,
2005] and on longer time ranges [ Crosta et al. , 2004]. In
the Arctic and sub-Arctic, there are many studies using dia-
tom assemblages, which led to qualitative reconstruction of
changes in sea ice cover extent during the late Quaternary, in
particular in the northernmost Pacific [e.g., Sancetta , 1981,
1983], the Nordic seas [ Koç et al. , 1993], or the Russian
Arctic seas [e.g., Bauch and Polyakova , 2000]. However,
because diatoms are composed of opal, which is suscepti-
ble to dissolution in marine waters unsaturated with respect
to dissolved silica, their preservation in Arctic Ocean and
North Atlantic sediments is often very poor. Most studies
dealing with past sea ice cover in the Arctic and subarctic
seas based on diatoms only report on the abundance of dia-
toms and the presence of sea ice, but none provide yet any
reconstruction of seasonal extent of sea ice.
Other proxies of sea ice include cysts of dinoflagellates
(or dinocysts), which yield relatively diversified and abun-
dant organic-walled microfossils in the sea ice marginal
zone. The distribution of dinocysts appears dependent upon
the duration of the ice-free season during which the life cy-
cle, including sexual reproduction and cyst formation, has

2.3. Geochemical Tracers

The ice cores from continental ice caps can provide some
indication on past sea ice because they may contain com-
pounds related to marine brines in relation to sea ice-free
conditions in adjacent basins. For example, sea salt varia-
tions in ice cores from Devon Island and Greenland have
been interpreted as indices of sea ice cover changes in adja-
cent marine environment [e.g., Mayewski and White , 2002].
Such indications are, however, difficult to quantify in terms
of concentration or density of sea ice cover. In a similar fash-
ion, indirect inferences on sea ice conditions in the Arctic
have been proposed based on mineralogical, geochemical
and radiogenic isotope tracers in sediments that label sedi-
ment sources and transport mechanisms [e.g., Darby , 2003;
Haley et al. , 2008]. However, here again, transcription of
such data into sea ice concentration and/or density remains
speculative. The isotopic composition (δ 18 O) of polar fo-
raminifera may also provide indication of sea ice formation