Passage VI

In a hydrothermal vent, anaerobic oxidation a methane (AOM) occurs in the top 20 cm of ocean-floor sediment (see Figure 1). This reaction begins when hot fluid containing methane (CH₄) rises from a source deep beneath the ocean-floor surface and mixes with seawater containing sulfate (SO₄^2-).

AOM is represented by the following equation:

CH₄ + SO₄^2- HCO₃^- + HS^- + H₂O

One of the 3 direct products of this reaction, HCO₃^-, can then combine with calcium ( Ca²⁺) in the seawater in a second reaction to produce solid calcium carbonate ( CaCO₃). This CaCO₃ exists as calcite crystals and/or aragonite crystals. Over time, the crystals fill in the pores in the sediment.

At each of 10 locations in a hydrothermal vent, 2 vertical cylindrical cores of the top 20 cm of ocean-floor sediment were collected.

Study 1

One of the 2 cores from each of the 10 locations was cut every 0.5 cm along its length. Each piece of core was squeezed to extract all of the pore water (water present in the sediments pores). The 10 pore water samples from the pieces at the same depth interval in each core were combined and then analyzed for SO₄^2- and CH₄. The average SO₄^2- and CH₄ concentrations, in millimoles per liter (mmol/L), from 0 cm to 20 cm depth are shown in Figure 2.

Study 2

The sediment in the other core from each of the 10 locations was sampled at the top of the core (0 cm depth), then every 0.5 cm along the cores length. The 10 samples from the same depth in each core were combined and then analyzed to determine the average percent by mass of calcite and the average percent by mass of aragonite In the sediment from 0 cm to 20 cm depth (see Figure 3).

Figures 2 and 3 adapted from Roger Luff et al., "Simulation of Long-Term Feedbacks from Authigenic Carbonate Crust Formation at Cold Vent Sites." ©2004 by Elsevier B.V.