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Fig. 1 Model/proxy comparison of temperature signal anomalies (9 ka BP vs. CTRL) during
summer (JAS). The left plot displays SSTs comparison (standard method), the right plot displays a
comparison of the T 0 30 signal (integrated temperatures over the depth interval between 0 and
30 m). The new method consisted in recalculating reconstructions for the temperature signal
including the subsurface to consider the living depth of the foraminifera used as proxy. This
calculation was based on the existing SST reconstructions. Both former SSTs reconstruction (dots
on the left plot) and new T 0 30 reconstruction (triangle on the right plot) are compared to model
results (background color) for the HIM (9 ka BP) versus CTRL (Adloff et al. 2011 )
conditions, thus questioning the long-standing
hypothesis. Our
results show that the differential degrees of organic matter diagenesis controlled the
spatial and temporal variations in
15 N instead of increased nitrogen availability and
primary productivity (M
bius et al. 2010 ). This implies that the enhanced accu-
mulation of organic carbon and observed
15 N depletion in sapropels is caused by
the absence of oxygen in deep waters. This scenario is in accordance with the
establishment of deep-sea faunas dominated by epibenthic taxa during the earliest
Holocene, a few centuries before the onset of sapropel S1, indicating the estab-
lishment of oligotrophic conditions. Our results suggest that present-day organic
carbon burial
fluxes, as estimated from sediment trap studies, could be suf
cient to
create a sapropel layer under anoxic deep-water conditions (Fig. 2a ).
Records of the foraminifer-based oxygen index from the EMS imply an expo-
nential decrease of average oxygen levels with increasing water depth across the S1
interval, suggesting a basin-wide shallowing of vertical convection. Our results
reveal rapid regime shifts in bathyal and abyssal benthic ecosystems from a high-
diverse state to a low-diverse or even azoic state almost synchronously with onset
of S1 deposition (Fig. 2b ). This suggests a rapid vertical and horizontal propagation
of dysoxic to anoxic conditions, supporting previous geochemical evidence (De
Lange et al. 2008 ). This result also suggests the ecological importance of oxygen
thresholds. The recovery of bathyal deep-sea benthic ecosystems of the EMS starts
around 8 ka BP at upper bathyal depths and is controlled by subsequently deeper
convection and re-ventilation over a period of approximately 1,500 years (Fig. 2b )
(Schmiedl et al. 2010 ).
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