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.