Keywords
0404 Anoxic and hypoxic environments (4802, 4834)
0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
0473 Paleoclimatology and paleoceanography (3344, 4900)
4243 Marginal and semi-enclosed seas
4845 Nutrients and nutrient cycling (0470, 1050)
1 Introduction
Marginal marine basins such as the Baltic Sea, Black Sea and many estuaries are naturally susceptible to hypoxia due to stratification and isolation of deep water masses (Conley et al., 2009; Lefort et al., 2012; Eckert et al., 2013). Hypoxia has been further exacerbated in modern coastal systems by anthropogenic nutrient inputs, leading to an expansion of marine ‘dead zones’ worldwide (Rabalais et al., 2010). Positive feedbacks in the phosphorus (P) cycle play a key role in intensifying hypoxic conditions in aquatic systems. Phosphorus release from sediments accelerates under low-oxygen conditions, due to the dissolution of iron oxide-bound P (Fe-P; Mortimer, 1941) and the preferential regeneration of P from organic matter (Ingall et al., 1993). Enhanced sedimentary P release, in turn, sustains high productivity and oxygen demand (Van Cappellen & Ingall, 1994; Vahtera et al., 2007).
An important caveat to the positive feedback between P cycling and hypoxia is that in spatially finite systems, the release of Fe-P from sediments cannot proceed indefinitely. Low-oxygen conditions at any given location may rapidly exhaust the local sedimentary inventory of Fe-P (Jilbert et al., 2011; Reed et al., 2011), while the maximum area of potential hypoxia may be limited by thermohaline stratification (Carstensen et al., 2014). Theoretically, if external P loads are insufficient to maintain high productivity after the exhaustion of the Fe-P pool, a reverse of the positive feedback may be induced, leading to sequestration of P and abatement of hypoxia. This dual directionality of the Fe-P feedback has been proposed to explain the rapid transitions at the onset and termination of centennial-timescale hypoxic events in the Baltic Sea (Jilbert & Slomp, 2013).
The hypoxic events defined in Jilbert and Slomp (2013) are subdivisions of longer hypoxic intervals (HI) in the sedimentary record of the Baltic Sea (Zillén et al., 2008), which occurred during the Holocene Thermal Maximum (HTMHI, approx. 8000–4000 cal. yr. B.P.), the Medieval Climate Anomaly (MCAHI, 1200–750 cal. yr. B.P.) and the modern period (Mod.HI., ~1980–present). Hypoxic conditions during these intervals are thought to have been induced by a combination of physical oceanographic forcing and external nutrient inputs, and sustained by P cycle feedbacks. Due to constraints on the sampling resolution of sediment cores, investigation of multiannual to multidecadal variability in past hypoxia during these intervals is challenging. However, such variability is essential to study, because of the relevance for managing the recovery from modern hypoxia, one major goal of the Baltic Sea Action Plan (HELCOM, 2007). Here we present new, ultra-high resolution records of past Baltic Sea hypoxia and Fe-P dynamics using LA-ICP-MS line scanning of resin-embedded sediments. These records cover the entire HTMHI and MCAHI intervals and thus significantly extend the initial LA-ICP-MS data presented in Jilbert and Slomp (2013). Using time-dependent box-model, we show that multidecadal oscillations observed in the sediment proxies could plausibly have been sustained by instabilities in the feedbacks between Fe-P dynamics and hypoxia on these timescales.
2 Materials and Methods
2.1 Sediment sampling
A gravity core (0‒406 cm) was collected from site F80 (58°00.00N, 19°53.81E, water depth 191m) in the Fårö Deep of the Baltic Sea during the HYPER/COMBINE cruise of R/V Aranda (May/June 2009), and sectioned in a nitrogen-filled glovebox at 0.5–2.0 cm intervals. Undisturbed U-channel subsamples were taken from laminated intervals and embedded with Spurr’s epoxy resin by the fluid displacive method under nitrogen (Jilbert et al., 2008).
2.2 Sample preparation and geochemical analysis
Discrete sediment samples were freeze-dried and ground under nitrogen. Aliquots of 0.125 g dry sediment were digested in 2.5 ml HF (40%) and 2.5 ml of HClO4 (70%)/HNO3 (65%) mixture at 90°C overnight, before evaporation at 160°C and redissolution in 1M HNO3. Solutions were analyzed by ICP-OES for total Al, Fe and Mo and P (precision and accuracy <10 %).
Resin-embedded sediment trays were cut to expose the internal surface of the sediment, and sectioned into shorter blocks to fit inside the Laser Ablation (LA) ICP-MS sampling chamber. LA-ICP-MS line scanning was performed at 0.04 mm s-1 using an Excimer laser (193 nm, spot size 120 µm, repetition rate 10 Hz, fluence 8 J cm-2) coupled to an Element 2 ICP-MS (Hennekam et al., 2015). Count intensities of the following isotopes were measured for use in this study: 27Al, 31P,57Fe, 79Br and98Mo. Calibration of the LA-ICP-MS data was performed according to a modified version of the two-step procedure described in the supplement of Jilbert and Slomp (2013). Full details of the calibration procedure are given in the Supporting Information (Text S1 and S2).
2.3 Rationale of the geochemical proxies
Sedimentary Mo/Al and Fe/Al are established proxies for hypoxia intensity and Fe shuttling, respectively (Jilbert & Slomp, 2013; Lenz et al., 2015). The ratio of bromine to phosphorus (Br/P) is here used as a surrogate for C/P, an indicator of the intensity of preferential P regeneration (Jilbert & Slomp, 2013). The rationale for using Br/P is that C cannot be measured in the epoxy-embedded samples by LA-ICP-MS, but Br and C are strongly correlated in sediments due to the bromination of organic matter by seawater bromide during degradation (Ziegler et al., 2008), hence LA-ICP-MS-derived Br provides a surrogate for C. Absolute Br contents were also determined on selected samples by repeated digestion in H2O2 followed by ICP-MS analysis (Ziegler et al., 2008). These values were used to generate discrete sample Br/Al ratios for the final step of the calibration of the LA-ICP-MS data.
2.4 Sediment dating and time series analysis
The age-depth model used for the sediments in this study is described in the supplement of Jilbert and Slomp (2013). Briefly, the discrete-sample Corg profile was tuned to the Loss on Ignition (LOI) profile of reference core 372740-3 from the Gotland Deep (Lougheed et al., 2012). 372740-3 is dated using known paleomagnetic secular variation (PSV) features and lead (Pb) isochrones (12 features total in the 406 cm core covering ~7000 years, giving an average spacing of ~600 years). Linear sedimentation rate is assumed between dating points. In the early part of the record, this age-depth model differs by up to 500 years from a recently published model for Baltic Sea sediments based on benthic foraminiferal14C dating (Warden et al., 2017). However, this offset is not expected to impact on the conclusions of the present study, which focuses on frequencies of variability rather than absolute ages of events.
Time series analysis of the LA-ICP-MS Mo/Al, Fe/Al and Br/P profiles was performed in Analyseries 1.1.1 (Paillard et al., 1996). Raw elemental ratios were detrended and normalized to unit variance, resampled to 1 year resolution and processed by Fast Fourier Transform (FFT, Blackman-Tukey method with Bartlett window) to yield a power spectrum of cyclic components in each profile. Cross-spectral analysis (same settings as above) was also applied to investigate coherence and phase relations between the elemental ratio data. A 20‒100 year bandpass filter was subsequently applied to the detrended, normalized data for visual comparison of the time series.
2.5 Box modeling
A simple box model was created to represent the coupled cycling of P and Fe in the Baltic Sea. The basic formulation is the same for the HTMHI, MCAHI, and Mod.HI(Fig. 1a), with six state variables related by a system of equations. A full model description, including calibration and validation against present day data, is provided in the Supporting Information (Text S3). Production of organic matter in surface waters is limited by the phosphorus concentration P . Organic matter sinks vertically and accumulates in the sediments, where remineralization occurs. Phosphorus may be regenerated from sediments due to remineralization or release from sedimentary Fe-P, or buried. Oxygen is constant in the surface layer but allowed to vary in the deep-water box, simulating the observed oxygen depletion in the sub-halocline water column of the Baltic Sea (Carstensen et al., 2014). A key feature of the model is a sigmoid function describing the relationship between the sedimentary Fe-P inventory and the oxygen supply-demand ratio , the dimensionless ratio between oxygen flux to sediments and oxygen demand by organic matter mineralization (Fig. 1b). The oxygen flux to sediments is directly proportional to oxygen concentration. Advective exchange of dissolved constituents is permitted, both vertically between the two layers and laterally between these layers and a hypothetical adjacent sea (simulating the real exchange between the Baltic and the North Sea). External loading of P is prescribed for each time interval based on literature estimates. The sensitivity and stability of steady state solutions to changes in external loads and vertical exchange were assessed. Simulations with variable frequency and amplitude of vertical mixing were conducted to demonstrate the potential of variable climate forcing to trigger and control oscillatory behavior. Spectral analysis was performed on 10000 year long annual oxygen time-series using FFT with Hamming window of the autocovariance limited to 512 time steps.