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the Baltic area. These authors claim that under positive NAO, increasing inflow of
freshwater due to intensified precipitation cause a decrease in the salinity of the sur-
face and bottom water, the latter by increased vertical mixing. The results are based
mainly on modelling of processes of the Baltic Sea and statistical analysis of hydro-
graphic and meteorological data for the last century, which may not be relevant for
longer time scales. Zorita and Laine ( 2000 ) mention that saline water entering the
Baltic is distributed on a monthly up to an annual time scale. Meteorological pro-
cesses vary even on shorter time scales than the hydrography of the Baltic Sea. This
might be the reason why due to findings of Mariotti and Arkin ( 2007 ) a general
and direct correlation between positive NAO and a high precipitation to the Baltic
catchment area (freshwater inflow) is questionable. The latter authors found by an
analysis of global meteorological and oceanographic databases that the North Sea
and the Baltic precipitation is positively correlated to the NAO only for December to
February. Even during these months zones of positive correlation do regionally not
cover whole Scandinavia and the Baltic Sea basin. During spring and the fall months
the correlation is not specific, and even negative during June, July, and August.
Erikssen et al. ( 2007a ) did not find any statistically significant trend in the annual
river runoff to the Baltic Sea during the last half millennium. Erikssen et al. ( 2007b )
claim regarding the analysis of hydrographic data of the Baltic Sea the “statistical
methods by themselves are incomplete to identify physical mechanisms for the cen-
tennial variation”. When the oxygenation is included into the analysis the processes
even become more complex (Zillén et al. 2008 ) . The oxygen consumption in the
deep water is mainly caused by degradation of organic matter, annually produced
in the euphotic zone and sinking down to the sea floor. The formation of long-term
anoxic bottom water thus depends on the presence of a density (pycno-) cline and
the “competition” between oxygen consumption and (lateral) oxygen supply. After
more recent studies (Matthäus 2006 , Matthäus et al. 2008 ) we know that the oxy-
gen decline in, e.g. the Gotland deep after an inflow event (1993) is faster (a few
months or a year only) than the sequence of (new) inflows (years to decade). The
critical region for long-term anoxia is the central Baltic Sea because the northern
Baltic has regular vertical convection and towards the western Baltic Sea, saline
(oxygen) water inflows become more frequent. Thus the Gotland Basin is the typi-
cal region of formation of laminated sediments, representing long-term anoxia. For
this area Pers and Rahm ( 2000 ) have clearly postulated that the deepwater inflow
is the “main supply of oxygen except during periods with stagnant conditions in
which case the diffusive supply from surface waters is dominant”. This dominance
is even intensified by downward convection which ventilates the water column in
particular during the strong north to northeasterly wind when the NAO turns nega-
tive to the continental mode (Hagen and Feistel 2005 ) . On the other side, sediment
data investigated here record processes with a decadal resolution. The sampling and
measuring spacing is between 0.5 cm (XRF scans) and 2 cm (MSCL data). Taking
into account a sedimentation rate of 1 mm/year, the geological record mirrors vari-
ations of hydrographic processes in a resolution of 5-20 years. That means, in this
study the geological data even of laminated sediments do not reflect high process
variability on the monthly or seasonal scale. NB: laminated sediment texture of
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