Fig 1.
Bathymetric map of marine Lake Grevelingen.
Cores were taken in the Scharendijke basin (51.742°N, 3.849°E; red circle) between 2012 and 2015. The red rectangle indicates the location of Lake Grevelingen in The Netherlands (NL).
Fig 2.
Pore water profiles of key components, as well as isotopic composition of pore water CH4 and measured SO42- reduction rates (SRR) in sediments of the Scharendijke basin.
δ13C-CH4 values are given in ‰ vs. VPDB (Vienna Pee Dee Belemnite) and δD-CH4 values are given in ‰ vs. V-SMOW (Vienna Standard Mean Ocean Water). Note that bubble formation and degassing of CH4 was observed during coring, indicating that actual CH4 concentrations likely are higher.
Fig 3.
Vertical profiles of sedimentary Mo, total solid phase S and Corg.
Grey bars indicate sediments that are enriched in Mo relative to the background, which is assumed to reflect periods of seasonal hypoxia in Lake Grevelingen.
Fig 4.
Fe extraction results including total sedimentary Fe (Fetot), acid volatile sulfide (FeS), chromium reducible sulfur (FeS2), carbonate associated Fe (Fecarb), easily reducible (amorphous) oxides (Feox1), reducible (crystalline) oxides (Feox2), recalcitrant oxides (mostly magnetite, Femagn) and total sedimentary Fe oxides (sum of Feox1, Feox2 and Femagn).
Note that the total amount of Fe oxides may be overestimated due to dissolution of FeS in the hydroxylamine-HCl step (Feox1) [36]. Also note the different scales on the x axes. Grey bars correspond to sediments with elevated concentrations of solid phase Mo, indicative for anoxic bottom waters in the Scharendijke basin during summer.
Fig 5.
Profiles of selected pore water constituents and Corg, as well as rates of SO42- reduction (SRR), methanogenesis and SO4-AOM derived from the reactive transport model.
Note that extensive bubble formation and degassing of CH4 was observed during coring, explaining the offset between modeled and measured CH4 concentrations at depth. Also note that the modeled CH4 concentrations exceed the CH4 solubility concentrations of ~ 10 mM, indicating the presence of free CH4 gas in the sediments of the Scharendijke basin
Fig 6.
A comparison between the measured sedimentary Mo peak concentrations and the reported area of hypoxia for the years 2007 to 2010 [39] reveals a linear correlation (y = 0.0475x + 0.121).
Table 1.
Reported CH4 effluxes from brackish and marine sediments in mol m-2 yr-1.