9:00 AM - 9:15 AM
[BPT04-01] Functional morphological analysis of burrow systems using fluid simulation: An example from the trace fossil Paleodictyon
★Invited Papers
Keywords:graphoglyptids, deep marine deposits, ichnology, functional morphology, computational fluid dynamics
The mode of ethology of benthic animals, including feeding and dwelling, depends on seafloor environmental conditions, and their activities can be preserved as trace fossils. Trace fossil analysis has been used to reconstruct paleoenvironmental conditions and assess responses of the benthic animals to geological events. Understanding the ecology of trace fossil producers and the functional morphology of burrow systems is crucial for investigating relationships between the trace fossil assemblages and the paleoenvironmental conditions. However, analysis in functional morphology of burrow systems have technical difficulties, such as measurement of hydraulic conditions in the millimeter-scale burrows.
To adress this challenge, computational fluid dynamics (CFD) and 3D modeling have been applied to analysis of the functional morphology of trace fossil Paleodictyon. This ichnogenus is characterized by a convex relief of a hexagonal mesh on a bottom surface of a marine sandstone bed, and has been known as a representative ichnotaxon indicating deep-marine environments. Observations of the modern Paleodictyon specimens, indicate that it is composed of hexagonal-mesh-like tunnels in a few millimeters below the seafloor (hexagonal mesh), an arrangement of vertical tunnels from the hexagonal mesh opening to the seafloor (vertical shafts), and an elevation of the seafloor as much as 5 mm toward the central part of the arrangement of the vertical shafts (shield-like mound). Although the origin of these structures is controversial, it was proposed that these structures are ventilation systems for water in the tunnels driven by pressure difference between the top and bottom of the shield-like mound owing to velocity difference of bottom currents. However, the mechanism and efficiency of the ventilation have not yet been clarified.
This study constructed 3D models of Paleodictyon and conducted CFD simulations to examine ventilation mechanism and hydraulic conditions. Nine models were created with varying height of the shield-like mounds between 0–8 mm. The CFD simulations were performed using FLOW-3D (Flow Science, Inc.). In these numerical experiments, 3D model of Paleodictyon was placed on the bottom of the virtual flume (30 × 30 × 20 cm in size) filled with water, and then allowed to flow in one direction for 20 seconds to investigate variations in hydraulic conditions inside and around the burrow system. The flow velocity at upstream end was varied between 0.5–8.0 cm/s. The analysis measured flow velocity and direction within the tunnels, pressure distribution, and shear velocity on the surface of the shield-like mound.
As a result of the experiments, outflow from the vertical shafts were observed in the cases of the mound heights of 1 mm or more, suggesting the necessity of the shield-like mound for ventilation. Pressure distribution implied that the increase of dynamic pressure at upstream side of the mound drove the ventilation. The ventilation rate increased with higher velocity of the bottom current and greater height shield-like mound. However, the Shields number calculated from the shear velocity and the critical shear stress on the top of the mound indicated the possibility of the sediment transport and resulting erosion of the shield-like mound. In contrast, no sediment motion occurs in the case of the mound height with 4–5 mm. These suggest that morphology of the modern Paleodictyon was optimized to balance between efficiency of ventilation and physical stability of the burrow system.
Although the producer of Paleodictyon remains unidentified, this study demonstrated that potential role of the burrow system in respiration and/or feeding. Future analyses using CFD simulations applied to other ichnotaxa are expected to clarify the functions of their burrow systems and enhance understanding of their relationship with environmental conditions.
To adress this challenge, computational fluid dynamics (CFD) and 3D modeling have been applied to analysis of the functional morphology of trace fossil Paleodictyon. This ichnogenus is characterized by a convex relief of a hexagonal mesh on a bottom surface of a marine sandstone bed, and has been known as a representative ichnotaxon indicating deep-marine environments. Observations of the modern Paleodictyon specimens, indicate that it is composed of hexagonal-mesh-like tunnels in a few millimeters below the seafloor (hexagonal mesh), an arrangement of vertical tunnels from the hexagonal mesh opening to the seafloor (vertical shafts), and an elevation of the seafloor as much as 5 mm toward the central part of the arrangement of the vertical shafts (shield-like mound). Although the origin of these structures is controversial, it was proposed that these structures are ventilation systems for water in the tunnels driven by pressure difference between the top and bottom of the shield-like mound owing to velocity difference of bottom currents. However, the mechanism and efficiency of the ventilation have not yet been clarified.
This study constructed 3D models of Paleodictyon and conducted CFD simulations to examine ventilation mechanism and hydraulic conditions. Nine models were created with varying height of the shield-like mounds between 0–8 mm. The CFD simulations were performed using FLOW-3D (Flow Science, Inc.). In these numerical experiments, 3D model of Paleodictyon was placed on the bottom of the virtual flume (30 × 30 × 20 cm in size) filled with water, and then allowed to flow in one direction for 20 seconds to investigate variations in hydraulic conditions inside and around the burrow system. The flow velocity at upstream end was varied between 0.5–8.0 cm/s. The analysis measured flow velocity and direction within the tunnels, pressure distribution, and shear velocity on the surface of the shield-like mound.
As a result of the experiments, outflow from the vertical shafts were observed in the cases of the mound heights of 1 mm or more, suggesting the necessity of the shield-like mound for ventilation. Pressure distribution implied that the increase of dynamic pressure at upstream side of the mound drove the ventilation. The ventilation rate increased with higher velocity of the bottom current and greater height shield-like mound. However, the Shields number calculated from the shear velocity and the critical shear stress on the top of the mound indicated the possibility of the sediment transport and resulting erosion of the shield-like mound. In contrast, no sediment motion occurs in the case of the mound height with 4–5 mm. These suggest that morphology of the modern Paleodictyon was optimized to balance between efficiency of ventilation and physical stability of the burrow system.
Although the producer of Paleodictyon remains unidentified, this study demonstrated that potential role of the burrow system in respiration and/or feeding. Future analyses using CFD simulations applied to other ichnotaxa are expected to clarify the functions of their burrow systems and enhance understanding of their relationship with environmental conditions.