Late Quaternary Climate Variations Reflected in Baltic Sea Sediments - The Baltic Sea Basin - page 126

the brackish Baltic sediments does not represent varves. The data do reflect system
shifts (Hagen and Feistel 2005 ) between different modes on the decadal to centen-
nial time scale recording the invariant (average) component of the facies. These data
do not reflect relatively high frequently changing oceanographic and meteorological
conditions that have been analysed by the authors mentioned above.
In conclusion, we assume that for the last millennium, on average, periods on the
centennial time scale of predominant positive NAO (maritime mode) are linked with
saline bottom water in the Eastern Gotland Basin, oxygen deficiency, and the for-
mation of laminated sediments, whereas predominant negative (continental) NAO
is linked with fresher oxygenated bottom water and bioturbated sediments. Due to
the similarity between the uppermost sediments representing the last millennium
and the whole sequence of the brackish Holocene sediments at the master station
in the Eastern Gotland Basin we extrapolate the model of the last millennium to
the whole of series of B zones. Consequently, B1, B3, and B5 zones represent, in
our interpretation, periods of a maritime NAO mode whereas B2, B4, and B6 stand
for a continental NAO mode. This assumption holds as a rule, but exceptions may
occur. Exceptionally, even during negative NAO mode strong westerlies may occur
due to an expanded sea-ice cover in the Greenland Sea (Dawson et al. 2002 ) . Such
situations might be the reason for the diatom record pointing at brackish-marine
conditions at the base of the (continental) B4 zone.
Consequently, we search for the driving force of the changing depositional envi-
ronment and try to find some hints in the results of the periodicity analysis of
sediment proxy data (Fig. 5.14 ) . There are two periodicities indicated in both of
the proxy variables investigated by time series analysis: the dominant 900-years
period and the 1500-years period. The 900-years period identified also in the grey-
scale time series at the master station of the Eastern Gotland Basin correlates well
with a 900-years component of the oxygen isotope records from the Greenland site
GISP2 (Kotov and Harff 2006 ) . Schulz and Paul ( 2002 ) have noted the significant
correlation of the Greenland oxygen isotope records with the 900-year signal com-
ponent in summer insolation at 65 ◦ N in the time span 3.5-8 k years BP. Loutre
et al. ( 1992 ) referred this cyclicity to an orbital (eccentricity-linked) period modu-
lating incoming solar radiation. Sarnthein et al. ( 2003 ) found a very similar cycle
(885 years) in sediments of the western Barents shelf. The authors reported about
cyclic injections of coarser layers into the marine sediment succession over the
whole Holocene which are interpreted as a result of storm-induced erosion along
the northern coast of the Kola Peninsula with a periodicity due to solar forcing.
This effect might also be seen in the basin sediments of the central Baltic inves-
tigated here. The second cycle, less dominant, but clearly visualized by the time
series analysis, seems to reflect a global climate signal. Bond et al. ( 1997 , 2001 )
called attention to this cyclicity as a Holocene climate phenomenon, which was
known before for the Pleistocene ocean dynamics as Heinrich/Bond cycles with its
Dansgaard Oeschger events (Rahmstorf, 2002 ) . These cycles have now been found
in many marine Holocene sediment sequences (for instance, Bianchi and McCave
1999 , Andresen et al. 2005 , Moros et al. 2009 ) indicating general periodical changes
in ocean dynamics even after the deglaciation of the continents. The 1470 cycle in