Iron‐Phosphorus Feedbacks Drive Multidecadal Oscillations in Baltic Sea Hypoxia. Issue 24 (13th December 2021)

• Record Type: Journal Article
• Authors: Jilbert, Tom; Gustafsson, Bo G.; Veldhuijzen, Simon; Reed, Daniel C.; van Helmond, Niels A. G. M.; Hermans, Martijn; Slomp, Caroline P.
• Is Part Of: Geophysical research letters. Volume 48:Issue 24(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-12-13
• Subjects: Baltic Sea -- biogeochemistry -- box modeling -- deoxygenation -- oscillations -- sediments; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL095908
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Hypoxia has occurred intermittently in the Baltic Sea since the establishment of brackish‐water conditions at ∼8, 000 years B.P., principally as recurrent hypoxic events during the Holocene Thermal Maximum (HTM) and the Medieval Climate Anomaly (MCA). Sedimentary phosphorus release has been implicated as a key driver of these events, but previous paleoenvironmental reconstructions have lacked the sampling resolution to investigate feedbacks in past iron‐phosphorus cycling on short timescales. Here we employ Laser Ablation (LA)‐ICP‐MS scanning of sediment cores to generate ultra‐high resolution geochemical records of past hypoxic events. We show that in‐phase multidecadal oscillations in hypoxia intensity and iron‐phosphorus cycling occurred throughout these events. Using a box model, we demonstrate that such oscillations were likely driven by instabilities in the dynamics of iron‐phosphorus cycling under preindustrial phosphorus loads, and modulated by external climate forcing. Oscillatory behavior could complicate the recovery from hypoxia during future trajectories of external loading reductions. Plain Language Summary: Hypoxia in the coastal ocean is expanding worldwide, and inputs of nutrients from waste water and agriculture are mainly to blame. Nutrients feed plankton blooms, which then consume oxygen as they decay. Because much of this decay takes place at the seafloor, sediments play an important role in deoxygenation, and in the recycling of nutrients in coastal regions. It is known that the amount of iron oxides in sediments has a strong effect on nutrient recycling during algal decay. Iron oxide‐rich sediments in healthy oxygen‐rich areas can trap phosphorus, a key nutrient element from decaying algae. In contrast, iron oxide‐poor sediments in deoxygenated areas release phosphorus back to the water to fuel more algal growth. Our study shows that changes in the distribution of iron oxides between deep and shallow areas of the Baltic Sea led to self‐sustaining variability (oscillations) in oxygen stress on decadal timescales during past intervals in the Sea's 8000‐year history. We use a model to demonstrate that under certain conditions of climate and nutrient pressure, such variability may occur naturally, and therefore may influence the future recovery of the Baltic Sea from its present nutrient‐rich state. Key Points: Past hypoxic intervals in the Baltic Sea were characterized by multidecadal oscillations in oxygen stress Regularly paced internal oscillations caused by feedbacks in coupled iron‐phosphorus dynamics External loading of phosphorus and climate forcing dictate the amplitude of internal oscillatory behavior