Sulfate-methane seeps

Sulfur cycling in seafloor methane seeps

Methane is a greenhouse gas 20 times more effective than CO₂ and seafloor methane seeps represent an estimated 8% of global methane production.¹ Methane often seeps from the ground at the edges of the continental shelves, at either passive or active plate margins, where tectonic activity causes deeper groundwaters to vent petroleum and natural gas.¹ Seeps are easily recognized because of the abundance of unique organisms colonizing the sediment surface. The community of macrofauna (tubeworms, clams, mussels, crabs, shrimp) found near methane seeps is different from the communities found in other regions of venting, such as hydrothermal vents.²

For this reason, methane seeps interest scientists from many disciplines, including geologists, chemical oceanographers, and biologists and many interdisciplinary studies have been conducted at these sites.

The microbial communities supporting the macrofauna at methane seeps are optimized for this environment: one that is rich in organic carbon, seawater sulfate, and methane.³ A subset of this microbial community is a symbiotic consortium made of a mixed group of microbes that together perform anaerobic (without oxygen) methane oxidation:^{4, 5}

SO₄^2- + CH_4(aq) HCO₃^– + HS^– + H₂O

The activity of this microbial consortium results in carbonate formation in sediments at methane seeps:⁶

Ca²⁺ + HCO₃^– CaCO_3(s) + H⁺

Anaerobic methane oxidation also provides sulfide-rich water that supports sulfide oxidizers in the microbial mats blanketing the sediments near seeps.⁷, Carbonate minerals and sulfide minerals formed in modern methane seeps preserve a record of environmental conditions in Earth’s history.⁶ Understanding how these minerals store information about the environmental conditions in modern systems helps us interpret past records and understand Earth’s changing environment throughout time:

“The present is the key to the past.” –Charles Lyell

Comparing methane seeps in different settings: Santa Monica Basin vs. Costa Rica continental margin

Santa Monica Basin is a rift basin created by localized faulting along the passive continental margin of California. The basin sits within the Southern California Countercurrent, an eddy in the larger California Current. In the center of the basin, bottom waters are anoxic (very low oxygen) and cannot support decomposition, resulting in organic-rich sediments. The subsurface environment is relatively stable because the sediments are rarely disturbed by bioturbation (mixing of sediments caused by burrowing animals).⁸ Within the basin are methane seeps, sourced from natural gas deposits deep within the sediments, associated with methane-derived carbonates⁹ and supporting microbial mats on the sediment surface.

In contrast, the methane seeps in Costa Rica are along the continental slope, not in a basin. Bottom waters are oxygenated, and methane seeps are identified by distinct macrofauna. The active margin faulting in this area is caused by tectonic plate collision. The dome feature of the methane seep is a mud volcano that releases warmer, fresher water influenced by hydrothermal venting. At different times, the zone of anaerobic methane oxidation (AOM) and sulfide production fluctuates or migrates vertically within the sediment. When the flow rate of water in the seep is high, the AOM is nearer the sediment surface and the new carbonate formed from the AOM begins to seal the fractures in existing carbonates. When the fluid flow rate is lower, the AOM is deeper and the carbonate begins to form a crust within the sediment.⁷

What does the rock record truly represent?

Long-term preservation of environmental change in the geologic record requires mineral formation. In the Stable Isotope Biogeochemistry lab at Washington University in St. Louis, we study geochemical signatures of sulfur cycling to understand how environmental conditions are preserved. We use a high resolution film capture technique to acquire snapshots of sulfur chemistry in the water in the sediments that we analyze for patterns over small distances and short time spans. We also use a variety of techniques to analyze the mineral record (accumulated over long time periods of centuries) on different scales to develop a more accurate picture. Does the mineral record represent long-term or short-term processes in the environment? Local or global processes? By studying modern environments (such as methane seeps), we can look for causal relationships between the instantaneous record captured from the water in the sediments and the more permanent archive that is the mineral record.

1 Tavormina, P. et al. (2008) Planktonic and Sediment-Associated Aerobic Methanotrophs in Two Seep Systems along the North American Margin. Applied and Environmental Microbiology, 74:13. 3985-3995.

2 Levin, L. et al. (2012) A hydrothermal seep on the Costa Rica margin: middle ground in a continuum of reducing ecosystems. Proceedings of the Royal Society B. doi:10.1098/rspb.2012.0205

3 Bohrmann, G. et al. (2002) Widespread fluid expulsion along the seafloor of the Costa Rica convergent margin. Terra Nova, 14:2. 69-79.

4 Valentine, D. and W. Reeburgh (2000) New perspectives on anaerobic methane oxidation. Environmental Microbiology, 2:5. 477-484.

5 Orphan, V. et al. (2009) Patterns of 15N assimilation and growth of methanotrophic ANME-2 archaea and sulfate-reducing bacteria within structured syntrophic consortia revealed by FISH-SIMS. Environmental Microbiology, 11:7. 1777-1791.

6 Han, X. et al. (2004) Fluid venting activity on the Costa Rica margin: new results from authigenic carbonates. International Journal of Earth Science, 93. 596-611.

7 Mau, S. et al. (2006) Estimates of methane output from mud extrusions at the erosive convergent margin off Costa Rica. Marine Geology, 225. 129-144.

8 Gong, C. and D. Hollander (1997) Differential contribution of bacteria to sedimentary organic matter in oxic and anoxic environments, Santa Monica Basin, California. Organic Geochemistry, 26:9. 545-563.

9 Heintz, M. et. al. (2012) Physical control on methanotrophic potential in waters of the Santa Monica Basin, Southern California. Limnology and Oceangraphy, 57:2. 420-432.