Kinetics of organic carbon mineralization and methane formation in marine sediments (Aarhus Bay, Denmark)

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Abstract

Sediments were sampled at nine stations on a transect across a 7–10 m thick Holocene mud layer in Aarhus Bay, Denmark, to investigate the linkages between CH4 dynamics and the rate and depth distribution of organic matter degradation. High-resolution sulfate reduction rates determined by tracer experiments (35S-SRR) decreased by several orders of magnitude down through the mud layer. The rates showed a power law dependency on sediment age: SRR (nmol cm−3 d−1) = 106.18 × Age−2.17. The rate data were used to independently quantify enhanced SO42− transport by bioirrigation. Field data (SO42–, TCO2, T13CO2, NH4+ and CH4 concentrations) could be simulated with a reaction-transport model using the derived bioirrigation rates and assuming that the power law was continuous into the methanogenic sediments below the sulfate-methane transition zone (SMTZ). The model predicted an increase in anaerobic organic carbon mineralization rates across the transect from 2410 to 3540 nmol C cm−2 d−1 caused by an increase in the sediment accumulation rate. Although methanogenesis accounted for only ∼1% of carbon mineralization, a large relative increase in methanogenesis along the transect led to a considerable shallowing of the SMTZ from 428 to 257 cm. Methane gas bubbles appeared once a threshold in the sedimentation accumulation rate was surpassed.

The 35S-measured SRR data indicated active sulfate reduction throughout the SO42− zone whereas quasi-linear SO42− gradients over the same zone indicated insignificant sulfate reduction. This apparent inconsistency, observed at all stations, was reconciled by considering the transport of SO42− into the sediment by bioirrigation, which accounted for 94 ± 2% of the total SO42− flux across the sediment-water interface. The SRR determined from the quasi-linear SO42− gradients were two orders of magnitude lower than measured rates. We conclude that models solely based on SO42− concentration gradients will not capture high SRRs at the top of the sulfate reduction zone if they do not properly account for (i) SO42− influx by bioirrigation, and/or (ii) the continuity of organic matter reactivity with sediment depth or age.

Introduction

The degradation and preservation of organic matter in seafloor sediments links the short-term carbon cycle on time-scales of ocean mixing to the long-term geological sequestration of carbon (Berner, 1990). The rate and depth in the sediment where organic carbon is mineralized defines the vertical biogeochemical structure of sediments and the fluxes of redox sensitive elements to and from the seabed (Stolpovsky et al., 2015). The rate of organic matter degradation generally decreases with sediment depth and age, as the more reactive fractions are degraded closer to the sediment surface leaving more recalcitrant fractions to be buried to deeper sediment layers. This behaviour can be described mathematically as the exponential decay of one or more discrete fractions of organic matter (Berner, 1964, Jørgensen, 1978) or by using continuum models that simulate the degradation of an infinite number of such carbon pools. In this latter category, the power law model (Middelburg, 1989) and the reactive continuum model (Boudreau and Ruddick, 1991) have been shown to reproduce the degradation of bulk organic matter in freshwater, brackish and marine sediments (e.g. Jørgensen, 1978, Wallmann et al., 2006, Arndt et al., 2009, Jørgensen and Parkes, 2010, Mogollón et al., 2012, Katsev and Crowe, 2015, Stolpovsky et al., 2015).

Organic carbon turnover has been investigated along a sampling transect in the fine-grained muddy sediments of Aarhus Bay, Denmark (Fig. 1, Flury et al., 2016). Surface sediments there are organic-rich and reworked by animals that physically mix the surface centimeters of sediment (bioturbation) and by tube-dwelling animals that enhance the exchange of seawater with porewater by their pumping activity (bioirrigation) (Chen et al., 2017). Concentrations of sulfate (SO42−) decrease quasi-linearly below the mixed layer to the sulfate-methane transition zone (SMTZ) where the anaerobic oxidation of methane (AOM) takes place, with methane (CH4) increasing below and eventually leading to methane gas formation. The rate of carbon degradation determined from sulfate reduction rate measurements (SRR) by the 35S-tracer method decreased from the surface mixed layer down to the SMTZ following a power law distribution with sediment depth or age. By extrapolating the power law into the methanogenic sediments below the SMTZ, Flury et al. (2016) calculated that organic matter mineralized by methanogenesis was equivalent to only 1 % of that respired by sulfate reduction. This contradicts the quasi-linear, presumably diffusion-driven SO42− concentration gradients, which imply an approximate balance between SO42− reduction and CH4 production (Borowski et al., 1996, Berelson et al., 2005, Burdige et al., 2016b). Similar trends in SO42− and CH4 profiles have been observed at many sites around the world (Egger et al., 2018).

In the present study we analyze further the cycling of organic carbon, SO42− and CH4 at nine sites in Aarhus Bay investigated by Flury et al. (2016) with additional rate data and δ13C values of dissolved inorganic carbon (TCO2). The data are analysed using a model that couples organic matter degradation to the transport of dissolved species in sediment porewaters. Our motivation was to understand methane cycling in coastal sediments by exploring the controls on free CH4 gas formation and the linkages with the rate and depth distribution of organic matter degradation. We provide additional experimental evidence supporting a continuous degradation of POC through and below the SMTZ that can be described using a power law. We further explore how the cycling of CH4 in the SMTZ impacts δ13C-TCO2 dynamics. Finally, we show that modelled SRR may be grossly underestimated if (i) the kinetics of organic carbon degradation and (ii) SO42− transport into the sediment by bioirrigation, are not carefully evaluated.

Section snippets

Study site

The Baltic region has experienced a dynamic history since the last deglaciation ca. 11 kyr ago when the Baltic Sea Basin was a freshwater body, the Baltic Ice Lake (Andrén et al., 2000). The shifting balance between sea level rise and isostatic rebound resulted in a marine transgression and the formation of the brackish Yolida Sea (10–9.5 kyr ago) which gradually freshened to the Ancylus Lake stage (9.5–8 kyr ago). The sediments from this time are late glacial tills and freshwater clays.

Results

The main result compares simulated and measured profiles of SO42−, CH4, NH4+, TCO2, and δ13C-TCO2 (Fig. 4). To keep the model as objective as possible, we endeavored to use the same parameter values at all stations. Given the short horizontal distance of the sediment sampling transect (260 m from M24 to M30), it seems reasonable that most of the parameters listed in Table 3 do not vary much from site-to-site. The main exceptions are ω, which is known at each station, the bioirrigation

Discussion

The SO42– and CH4 data in Aarhus Bay show the classical spatial features of a SR zone where SO42– concentrations decrease from the surface to the SMTZ and CH4 concentrations increase below the SMTZ (Barnes and Goldberg, 1976, Iversen and Jørgensen, 1985). The general trends of these data and diffusive fluxes of solutes have been discussed by Flury et al. (2016). Here we focus on CH4(g) formation, δ13C-TCO2 dynamics, and POC mineralization kinetics.

Conclusions

Sediments in Aarhus Bay have been analyzed to investigate the kinetics of organic matter degradation in a Holocene mud layer that varied in thickness from ca. 7 to 10 m. High-resolution 35S measurements showed that SRR decreased by 4–5 orders of magnitude from the surface to the SMTZ and could be described using a power law dependency of SRR on sediment age or depth. The high rates of SO42– consumption were not apparent from the measured SO42– concentrations that presented a quasi-linear

Acknowledgements

The research was funded by the Danish National Research Foundation (DNRF grant no. 104), the project BALTIC GAS (EU 7th FP program BONUS, grant no. 217246), the Swiss National Science Foundation (Grant no. PBEZP2-129527), The Danish Council for Strategic Research (ECO-CLIM), the European Research Council (ERC) Advanced Grant, MICROENERGY grant no. 294200), and the Danish Center for Marine Research. PR received funding from the VERIFY project from the European Union’s Horizon 2020 research and

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