Deep on the ocean floor some sediments contain gases, more than 99% of which are methane, created by microorganisms. Microbes create the methane gas found in sediments as they breakdown organic material. Scientists can use the carbon isotopes from that methane to observe microbe activity across depths and even time.
As ocean depth increases, there is usually less organic material available for the microbes. Scientists believe organic material isn’t able to biodegrade at all below certain depths, depending on factors like location, the thermal gradient, or the quantity and quality of organic matter. However, if temperatures increase in these deep-sea environments, this unusable organic material becomes reactivated because of the breakdown of organic compounds, and microbial activity can begin. Heating causes the breakdown of the organic molecules and subsequent release of metabolic intermediates that can be used by the microbes.
Riedinger et al. investigated three sample sites along the Nankai Trough, located east of Japan, to determine the source of these temperature increases. The Nankai Trough is located along a tectonically active subduction zone where the Philippine Sea plate slides below southwest. Their leading hypothesis was that frictional heating, caused by tectonic plates rubbing against each other during earthquakes, provided a temporary heat source that reactivated organic materials and made them usable by the microbes.
The carbon signatures at two of their three study sites showed microbial activity at depths that normally wouldn’t support it. The team attributes the microbial activity either to an influx of bioavailable carbon or to an increase in temperature making the previously unusable organic material able to decompose once again. Both of these methods require some sort of tectonic activity in order to take place.
To corroborate this idea the researchers used a mathematical model to predict when such a seismic even could have occurred. The model predicted that an earthquake occurred somewhere between 200 and 400 years ago.
Thus, the scientists also found another way to use the carbon signatures left behind by the microbial decomposers: The carbon signatures can be used to pin down the timing of recent tectonic events. When a tectonic event occurs, it leads to a break down in organic material, and the carbon isotopes released by microbial decomposers are distinctly different from the original carbon isotope signal. Researchers can then apply a mathematical model to assess the change in the carbon isotope signal and date when tectonic events occurred. (Geochemistry, Geophysics, Geosystems, doi:10.1002/2015GC006050, 2015)
—Cody Sullivan, Writer Intern
Citation: Sullivan, C. (2016), Deep-sea microbes can leave records of the past, Eos, 97, doi:10.1029/2016EO045305. Published on 9 February 2016.