3D morphology of crab (Macrophthalmus japonicus) burrows from the Pearl River Delta front, China: The physicochemical factors, with implications for the rock record

Neoichnological characterization of modern depositional settings addresses the response of benthic animals to ecological conditions and their fluctuations, usually on a relatively short time scale. In this way, analogue models for interpretation of sedimentary strata in the geological record can be developed. Macrophthalmus japonicus (De Haan, 1835), a crab, is commonly found in the Pearl River Delta front of southern China. The burrows of M. japonicus are vertical or inclined, I-, U-, Y-, J- and L-shaped tubes, and these are imaged in 7.5-cm-diameter sediment cores. The cores were studied by X-ray radiography, computed tomography, and VG Studio MAX reconstruction to obtain three-dimensional images for elaborate morphological study. This crab's burrows are analogous to the trace fossil Psilonichnus. Several environmental parameters, i.e., sediment grain size and its total organic carbon (TOC) content, and turbidity and salinity of water, have been measured. The results indicated that M. japonicus prefers to live in mesohaline water with turbidity levels of 20–170 NTU, where bioturbation is more abundant. It prefers to occur in mud and silty sand substrates, with TOC content decreasing as particle size increases. The sedimentary environment, grain size and ichnofacies of analogical trace fossils Psilonichnus are discussed which supports the interpretation that Psilonichnus is a good indicator of delta front environments.