Fluid conduits formed along burrows of giant bivalves at a cold seep site, Sounthern Taiwan

At a Pliocene methane seep site in Taiwan, an about half meter-long dolomitic concretion hosting a lucinid bivalve Anodontia goliath, ∼11 cm in diameter reveals the impact of chemosymbiotic bivalves on fluid migration through the shallow sediment. The concretion consists of a central channel filled with sparite and varying amounts of brecciated material derived from the channel walls. The studied bivalve is preserved with the hinge upward. When restored to life position, the central channel of the upper, ∼10 cm-long concretion segment is connected to the shell above the anterior side of the umbos. For the lower, ∼30 cm-long concretion segment, the central channel is connected to the shell underneath the posterior sector of the bivalve. The channels are interpreted as anterior tube for supply of respiration water and posterior tube, mining sulphide generated in the sulphate-methane transition zone of the sediment, respectively. The dolomitic cortex surrounding the central channel is characterized by δ¹³C values as low as -29‰ VPDB, indicating that the rate of anaerobic oxidation of methane was enhanced around the conduit. At the base of the central channel, upward soft sediment deformation structures likely resulted from the dragging effect when fluids migrated out of the sediment and entered the channel. Brecciation of the channel walls indicates pulsating seepage during the incipient stages of cementation. The burrow, thus, acted as a highly permeable conduit channeling upstreaming methane-charged fluid for a substantial period of time. This example illustrates that burrows are efficient in pre-determining the formation of preferential fluid migration conduits at seep sites, most likely with an important impact on fluid mixing and diagenetic reaction rates in the sediment, and ultimately the rate of methane release to the ocean.