Figure 2. LA-ICP-MS line scan data of Baltic Sea sediments from
(a) HIHTM and (b) HIMCA. (left)
High-resolution geochemical profiles of sediments from site F80.
Horizontal gray bars indicate the subdivision of HTMHIand MCAHI into numbered hypoxic events, as given in
Jilbert and Slomp (2013). Upper panels = calibrated LA-ICP-MS profiles
of Mo/Al. Lower panels = 20‒100 year bandpass-filtered profiles of
Mo/Al, Br/P and Fe/Al (detrended and normalized to unit variance prior
to filtering). (center) Blackman-Tukey spectral analysis of Mo/Al, Br/P
and Fe/Al data for the entire time intervals shown on the left. Gray
field indicates the period 60‒100 years. (right) Coherence and phase
analysis of Mo/Al and Br/P (solid lines) and Mo/Al and Fe/Al (dashed
lines) for the entire time intervals shown on the left. Phase in radians
(0 = in phase, П = antiphase).
3.2 Evidence for oscillations in past Fe shuttling
In the intervals where the oscillations in Mo/Al and Br/P are most
pronounced (e.g., 1.6‒1.2 ky BP; 5.3‒4.9 ky BP), the LA-ICP-MS data also
show similarly-paced variability in Fe/Al (Fig. 2, left and Supporting
Information Figs. S2 and S3). Accordingly, a similar peak in the 60‒100
year band is observed in the power spectrum of the Fe/Al data (Fig. 2,
center), as well as high-coherence and close-to-zero phase relation
between Mo/Al and Fe/Al (Fig. 2, right). This indicates that
shelf-to-basin shuttling of Fe in the Baltic Sea was also sensitive to
multidecadal variability in hypoxia. Namely, during periods of low
deep-water oxygen, more Fe was transported laterally downslope into the
deep basins via cycles of dissolution and reprecipitation (Lyons &
Severmann, 2006).
3.3 Oscillations in box model simulations
Model simulations with constant forcing confirm that multidecadal
oscillations in hypoxia and phosphorus regeneration may have been an
intrinsic feature of biogeochemical cycles in the Baltic Sea under the
forcing conditions of the HTMHI and
MCAHI (Fig. 3). Steady-state solutions of the model are
unstable when parameters including basin geometry and external loading
of P are set to constant realistic values for these intervals,
indicating the presence of unforced oscillations. The periodicity of
these unforced oscillations in the model is typically 130‒170 years,
slightly longer than observed in the sediment records (Fig. 3). Within
each oscillation, during the period of low deep water oxygen, the
sediment Fe-P inventory is at a minimum, whereas deep water phosphorus
(P ), and sediment organic carbon (Corg )
and phosphorus (Porg ), show maximum values (Fig.
3). Conversely when deep water oxygen is high, the opposite trends are
observed.
3.4 Mechanism and frequency of the oscillations
The presence of relatively large amounts of Fe-P in Baltic Sea sediments
under low P loading conditions is a prerequisite for the observed
instability. The quantitative representation of the sigmoid function
used in the model (Supporting Information Fig. S5) shows that the
sensitivity of the Fe-P inventory to the oxygen supply-demand ratio is
high at intermediate values (Fe-P = 50 ‒ 150 mmol
m-2). During the recharge phase (Fig. 1b),
storage of Fe-P in sediments provides a reinforcing feedback towards a
higher supply-demand ratio. Conversely the discharge phase is
characterized by release of Fe-P and a reinforcing feedback towards
lower supply-demand ratio. However when oxygen is plentiful and
productivity low (high values of the supply-demand ratio), the sediment
Fe-P pool becomes increasingly saturated (close to 200 mmol
m-2 Fe-P), leading to a leveling-off in the Fe-P
inventory. Similarly, when oxygen is scarce and productivity high, the
Fe-P concentration levels off due to the approaching exhaustion of the
sedimentary Fe-P pool. This insensitivity at high and low values makes
the system vulnerable to a switch in directionality.