Geoscience Reference
In-Depth Information
Studies of biomarkers, microfossils, stable and radiogenic isotopes, as well as
sedimentological proxies on core MSM5/5-712 (western Svalbard margin) allowed
to reconstruct the variability of Atlantic Water advection (Fig.
1
) to the Arctic and
its effect on the regional ice coverage (Werner et al.
2011
,
2013
;M
ller et al.
2012
).
IP
25
accumulation rates are mostly low in late Early Holocene sediments and
medium high at 7 to 3 ka, while phytoplankton biomarker contents decrease from 9
to 5 ka (Fig.
1
). Proxy data and NAOSIM reconstructions (Fig.
2
) suggest a general
cooling trend and a successive southeastward shift of the sea-ice margin towards
Svalbard since the Early Holocene (M
ü
ller et al.
2011
,
2012
), responding to the
postglacial sea-level rise, a related onset of modern sea-ice production on the
Siberian shelves (Werner et al.
2013
), and to insolation-induced sea-ice and cir-
culation changes. After 3 ka, biomarker accumulation rate peaks, highest contents
of ice-rafted detritus, and a change in Nd isotope ratios of sediment leachates are
evidence of a sea-ice margin rapidly advancing to and retreating from the core site
(M
ü
ller et al.
2012
; Werner et al.
2014
). This interpretation is corroborated by
microfossil and planktic isotope data. Generally high relative abundances of sub-
polar foraminifers until 5 ka indicate a strong in
ü
uence of Atlantic Water as a near-
surface water mass off western Svalbard, but low amounts thereafter demonstrate a
drastic change to colder conditions (Werner et al.
2011
,
2013
). Low carbon and
high oxygen isotope values of the polar planktic foraminifer species during the last
3 ka suggest a strengthened stratification due to a low-saline surface water layer.
The drastic increase of subpolar species percentages in the last
100 years indi-
cates an equally strong increase of Atlantic Water temperatures and advection to the
Arctic. Modern temperatures are unprecedented for the last 5 ka, probably as a
response to global warming, and pose a severe threat to the Arctic sea-ice cover
(Spielhagen et al.
2011
).
Transient simulations with ECHO-G were analyzed to investigate the history of
circum-Arctic river run-off and elucidate its possible role in the environmental
changes observed in the Fram Strait (Wagner et al.
2011
). The discharges, as
calculated by the model, are driven by the difference between precipitation and
evaporation and show a strong variability on annual to multi-centennial timescales.
Results reveal that the discharge from Eurasian rivers increased slightly (2.1
*
0.6
%) between 7 ka and 1800 CE while that of North American rivers decreased by
4.6
±
0.6 %. In the Holocene, the total discharge remained at a constant level, while
atmospheric temperatures decreased. This may have supported a stable halocline, as
indicated by proxy data, and favored sea-ice formation. The last 100 years have
seen a strong run-off increase from both continents (7.6 % for the total Arctic
Ocean), in line with 20th century observations, however, accompanied by a rapid
warming of the Arctic. These
±
findings, together with the results from high-reso-
lution sediment cores, clearly reveal extremely strong and rapid environmental
changes during the Industrial Period in the Arctic which are unprecedented for
several millennia before.
Search WWH ::
Custom Search