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

摘要

At a Pliocene methane seep site in Taiwan characterized by numerous tubular carbonate bodies and bivalves, a half meter-long dolomitic concretion containing a lucinid bivalve Anodontia goliath reveals the impact of chemosymbiotic bivalves on fluid migration through shallow sediments. The concretion consists of a cortex encasing 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 is connected to the anterior side of the bivalve. In the lower longer segment, the channel is connected to the posterior sector of the bivalve. The channels are interpreted as the anterior tube for supply of respiration water and the posterior tube for mining sulphide generated in the sulfate-methane transition zone of the sediment. The dolomitic cortex surrounding the central channel is characterized by delta 13C values as low as - 29 parts per thousand 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 fluid 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. Therefore, burrows can be efficient in pre-determining the formation of fluid conduits at seep sites, having potentially a large impact on fluid mixing and diagenetic reaction rates in sediments, and ultimately the amount of methane released. The reported bivalve with its conserved burrow fossilized as seep carbonate concertion provides direct evidence that fluid conduits form along animal burrows.