Plate-Driving Forces

Figure 1. Carbon isotopic values for organic matter isolated from samples taken from the United Clay Mine in Maryland. Pollen samples indicate that the isotopic excursion at about 4 m (gray arrow) occurred during about 1 million years in the early Aptian.

Figure 2. Plate motions (black arrows) and subduction rates (colored arrows) for time periods (a) before the Early Aptian isotopic excursion and (b) after it. The change in Farallon plate motion at this time resulted in significant slowing of subduction rates, as quantified in (c). Assuming a model for locking of the plate boundary during the early Aptian (Fig. 3), we estimated the total uplift during a 1 Myr time period (d), and accompanying release of methane hydrates from the continental margin. Symbols in (d) indicate geological evidence for Aptian uplift (see paper below for details).

Figure 3. Model for plate methane hydrate release at subduction zones. Initially, a subducting plate slides smoothly into the mantle beneath an overriding continental plate, as in (a). An increase in frictional interaction along the plate boundary , as in (b), impedes motion of the subducting plate into the mantle. Instead, horizontal convergence associated with subduction is converted into horizontal shortening of the continental margin. This results in uplift, which releases methane hydrates stored along the continental margin.

Subduction Zone Uplift and Methane Hydrates

A.H. Jahren, C.P. Conrad, N.C. Arens, G. Mora, and C. Lithgow-Bertelloni, "A plate tectonic mechanism for methane hydrate release along subduction zones," Earth and Planetary Science Letters, 236, 691-704, 2005. [abstract] [online version] [reprint]

A significant reservoir of carbon resides on the ocean floor near continental margins in the form of methane hydrates. Because the stability of methane hydrates is sensitive to pressure and temperature, this reservoir is vulnerable to perturbations that can cause part of it to become unstable, resulting in a significant release of carbon into the atmosphere. We have identified one such event in the early Aptian (119 million years ago) and have proposed a plate-tectonic mechanism for the disruption of the continental margin.

A negative carbon isotope excursion identified from rock samples taken from the Arundel Clay Formation in Maryland (Fig. 1) shows a significant negative excursion at about 119 million years ago. A negative excursion of this magnitude is most easily explained by the release of 1137 Gt of carbon from methane hydrates into the atmosphere and ocean. The duration of this event was about 1 million years (see paper below for details).

The early Aptian was a time of significant tectonic change in the Pacific basin. Prior to the early Aptian, the Farallon plate was subducting northward beneath subduction zones in Japan, Alaska, and California (Fig. 2a). During the early Aptian, a change in Farallon plate motion (Fig. 2b) caused subduction beneath Northeast Asia, Alaska, and the Northwest North America slowed dramatically (Fig. 2c). This change in Farallon plate motions was coincident with evidence of uplift along these plate margins during the Aptian (Fig. 2d).

We propose that the change in Farallon plate motion was caused by increased friction along the northern boundaries of the Pacific basin (Fig. 3). Increased friction along a subducting plate boundary typically causes "seismic coupling" of that boundary. If an efficiently subducting Farallon plate (Fig. 3a) experienced onset of seismic coupling during the early Aptian, several geological consequences would result. First, the compressional interaction might weaken the slab, diminishing the slab pull force (see great earthquakes, slab weakening, and slab pull), slowing the trenchward motion of the Farallon plate. Second, seismic coupling is generally associated with compression in the back-arc. Thus, the onset of this compression would initiate uplift (Fig. 3b). Third, this uplift would elevate any methane hydrates stored along the subducting plate margin out of the methane hydrate stability field, releasing this carbon into the atmosphere and ocean.

We estimate (see paper below for details) that about 900 Gt of methane hydrate carbon could have been released by this mechanism during the early Aptian. This value, while dependent on our choice of uplift model parameters, is comparable to the amount estimated isotopically. This highlights the potential importance of subduction zone tectonics in the determining the stability of the methane hydrates in particular, and to the carbon cycle and climate in general.