Anaerobic oxidation of methane and sulfate reduction along the Chilean continental margin

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Abstract

Anaerobic oxidation of methane (AOM) and sulfate reduction (SR) were investigated in sediments of the Chilean upwelling region at three stations between 800 and 3000 m water depth. Major goals of this study were to quantify and evaluate rates of AOM and SR in a coastal marine upwelling system with high organic input, to analyze the impact of AOM on the methane budget, and to determine the contribution of AOM to SR within the sulfate-methane transition zone (SMT). Furthermore, we investigated the formation of authigenic carbonates correlated with AOM. We determined the vertical distribution of AOM and SR activity, methane, sulfate, sulfide, pH, total chlorins, and a variety of other geochemical parameters. Depth-integrated rates of AOM within the SMT were between 7 and 1124 mmol m−2 a−1, effectively removing methane below the sediment-water interface. Single measurements revealed AOM peaks of 2 to 51 nmol cm−3 d−1, with highest rates at the shallowest station (800 m). The methane turnover was higher than in other diffusive systems of similar ocean depth. This higher turnover was most likely due to elevated organic matter input in this upwelling region offering significant amounts of substrates for methanogenesis. SR within the SMT was mostly fuelled by methane. AOM led to the formation of isotopically light DIC (δ13C: −24.6‰ VPDB) and of distinct layers of authigenic carbonates (δ13C: −14.6‰ VPDB).

Introduction

The microbial process of anaerobic oxidation of methane (AOM) effectively removes methane from marine sediments before it reaches the sediment-water interface (Hinrichs and Boetius, 2002, and references therein). AOM thereby plays a significant role in the regulation of the global methane budget and the emission of methane into the atmosphere, where it acts as a strong greenhouse gas.

During AOM, methane is oxidized with concurrent sulfate reduction (SR) according to the following net equation (Eqn. 1): CH4+SO42HCO3+HS+H2O Since sulfate is the electron acceptor, AOM is limited to the zone where sulfate penetrates and overlaps with methane. In diffusive systems, the activity of AOM leads to a typical concave-up profile of methane concentration (Iversen and Jørgensen, 1985). The peak in AOM profiles coincides with the sulfate-methane transition (SMT), where both substrates are exhausted. At methane seeps, high AOM activity was also often found to result in the formation of authigenic carbonates (e.g., Bohrmann et al 1998, Peckmann et al 2001) due to an increase in alkalinity.

Experimental measurements of AOM have been conducted mainly in shelf sediments (water depth <200 m), e.g., the Baltic Sea, Cape Lookout Bight, and Scan Bay (Reeburgh 1980, Iversen and Blackburn 1981, Alperin and Reeburgh 1985, Iversen and Jørgensen 1985, Hoehler et al 1994, Hansen et al 1998, Bussmann et al 1999). Determinations of AOM rates in sediments from more than 200 m water depth are primarily based on modeling (e.g., Reeburgh 1976, Borowski et al 2000, Jørgensen et al 2001). Several recent investigations of AOM focus on extreme environments such as methane seeps and gas hydrate sites revealing high advective methane fluxes and AOM rates (Boetius et al 2000, Orphan et al 2001, Michaelis et al 2002, Treude et al 2003, Joye et al 2004). However, since methane is a general product of organic matter degradation in marine sediments where considerable amounts of organic matter reach the ocean floor (Claypool and Kaplan 1974, Reeburgh 1996), rates of AOM need to be investigated throughout the productive marine regions worldwide, from littoral to bathyal depths.

Very little is known about AOM in coastal upwelling systems, which are some of the most productive regions of the ocean. In sediments off the Namibian coast, estimates of AOM rates were made by modeling the methane and sulfate profiles (Niewöhner et al 1998, Fossing et al 2000). Methane produced in these organic-rich sediments is completely consumed within the anoxic sediment pointing to the important role of AOM in such systems. In the present study we investigated AOM in sediments of another major upwelling system located off the Chilean coast. The euphotic zone along the Peruvian and Chilean shelf is the world’s largest high-productivity area (up to 0.11 mol C m−3 d−1 off central Chile; Peterson et al., 1988) among the eastern boundary current systems (Berger et al., 1987). The ocean floor, especially of shallow water depths, receives a continuous deposition of organic material produced by phytoplankton (Arntz and Fahrbach, 1991). We can expect that despite degradation processes in the water column the sediments even of greater water depths still receive sufficient amounts of degradable material to result in significant methane production—and consequential methane turnover. In the present study we sampled three stations along the Chilean continental margin at water depths between 800 and 3000 m (1) to investigate whether organic matter input to the sediments is sufficient to support significant microbial degradation processes including methane production, and therewith AOM supported by in situ methane production; (2) to investigate whether organoclastic SR, i.e., SR based directly on the degradation of organic matter, at the sediment surface affects the distribution of sulfate and methane in the sediment; (3) to determine the ratio between AOM and SR within the SMT; (4) to investigate the formation of authigenic carbonates related to AOM; (5) to evaluate AOM rates in an upwelling system in comparison to other marine systems.

Section snippets

Sampling Sites

Sediments were sampled with gravity and multiple corers at three stations along the Chilean continental margin (Fig.1, Table 1) during R/V Sonne cruise SO-156/3 in April/May 2001. Station GeoB (Geowissenschaften Bremen) 7155 and 7165 were located off Central Chile at 34°35 and 36°32S, respectively. Station GeoB 7186 was located further to the south at 44°09S. A detailed core description for each station is given by Hebbeln et al. (2001). The main sediment characteristics can be summarized as

Temperature Increase during Presampling

At the beginning of the presampling the temperatures of all cores were around 5°C. Highest core temperatures were recorded towards the end of the presampling procedure (after approx. 2 hrs) due to a gradual warming from in situ (4°C, Hebbeln et al., 2000) towards the ambient temperature (approx. 20°C). Highest temperatures at the periphery were 20.7°C, 13.7°C, and 15.9°C for core 7165, 7186, and 7155, respectively. Highest temperatures in the center were 15.0°C, 9.2°C, and 10.0°C, respectively.

Porosity and Density

Organic Matter Input and Consequences for Microbial Degradation Processes

Organic matter input from the water column is the main carbon and energy source for microbial decomposition processes in the sediment. The TOC concentrations reported in this study were higher than those in hemipelagic slope sediments from nonupwelling regions, that typically contain 0.3%–1% TOC, and deep sea sediments with a medium value of 0.1% TOC (Rullkötter, 2000, and references therein). High chlorin concentrations and low Chlorin Indices indicate an input of fresh phytodetritus (Schubert

Conclusion

Following the aims of the present study, we conclude the following.

  • 1

    High input and fresh organic matter deposited in the Chilean upwelling region fueled microbial degradation processes near the sediment surface (e.g., organoclastic SR) as well as methanogenesis deeper in the sediment.

  • 2

    Sulfate penetration depths were insensitive to organoclastic SR activity near the sediment surface. High diffusion distances strengthened the effect of methanotrophic SR activity at depth, and thus only methane and

Acknowledgments

We thank the officers, crew, and shipboard scientific party of the R/V Sonne for excellent support during expedition SO-156. We particularly thank D. Hebbeln for logistic support, F. Schewe and S. Stregel for excellent coring, J. Wulf, P. Boening and L. Toffin for technical help on board as well as M. Hartmann, A. Rohwedder and S. Knipp for technical help in the home laboratory. We would also like to thank M. Böttcher for inspiring discussions about biogeochemical parameters in the sediment as

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