Abstract
The maximum depth of sediment biomixing is directly related to the vertical extent of post-depositional environmental alteration in the sediment; consequently, it is important to determine the maximum burrowing depth. This study examined the maximum depth of bioturbation in a natural marine environment in Funakoshi Bay, northeastern Japan, using observations of bioturbation structures developed in an event layer (tsunami deposits of the 2011 Tohoku-Oki earthquake) and measurements of the radioactive cesium concentrations in this layer. The observations revealed that the depth of bioturbation (i.e., the thickness of the biomixing layer) ranged between 11 and 22 cm, and varied among the sampling sites. In contrast, the radioactive cesium concentrations showed that the processing of radioactive cesium in coastal environments may include other pathways in addition to bioturbation. The data also revealed the nature of the bioturbation by the heart urchin Echinocardium cordatum (Echinoidea: Loveniidae), which is one of the important ecosystem engineers in seafloor environments. The maximum burrowing depth of E. cordatum in Funakoshi Bay was 22 cm from the seafloor surface.
Highlights
The mixing of seafloor sediments by benthic organisms, or bioturbation, is important in marine environments because it influences the biogeochemistry of seafloor deposits [1,2,3] and disturbs the stratigraphic order [4,5,6]
We investigated the maximum depth of bioturbation in a natural marine environment by observing bioturbation structures developed in an event layer and by measuring the radioactive cesium concentrations of marine sediments in Funakoshi Bay, northeastern Japan (Fig 2)
Layer 2, which represents the sediments between the lower part of Layer 1 and the base of the coarse-grained bed, was not bioturbated and contained well-defined physical sedimentary structures such as parallel laminations
Summary
The mixing of seafloor sediments by benthic organisms, or bioturbation, is important in marine environments because it influences the biogeochemistry of seafloor deposits [1,2,3] and disturbs the stratigraphic order [4,5,6]. The examination of sediment cores enables detailed observations of the physical and biogenic sedimentary structures preserved within deposits, thereby providing important information on the vertical distribution of these structures [5, 6, 8, 9]. Sedimentary structures do not necessarily reveal current burrowing activity, as we cannot distinguish current bioturbation structures from those produced in the past (i.e., pre-existing burrows). Records of event sedimentation such as volcanic ash layers, tempestites (storm deposits), and turbidites may provide information on the nature of bioturbation.
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