1:45 PM - 3:15 PM
[O08-P71] Comparison of the erosion process between Porites sp. and Dipsastraea sp. coral skeleton
Keywords:coral skeleton, erosion process, skeletal structure
Coral reef terraces along the coast of Kikai Island, Japan is an ideal field to estimate calcium carbonate production (CCP) through the Holocene. However, the preservation of fossil coral skeletons might be different due to the species of corals, or their skeletal structures, which could potentially affect the estimation of CCP. In this study, the potential differences in the responses to chemical erosion and physical erosion across distinct coral species, were clarified.
In this study, skeletons of modern Porites sp. and Dipsastraea sp. collected in Kikai Island, were compared in the responses to chemical erosion and physical erosion. For the chemical erosion experiment, the skeletons were immersed in hydrochloric acid (2.5% and 5.0%) for a certain period of time (~24 hours). The dry weights of the skeletons were measured to compare the changes of the remaining portions of the skeleton. The amount of carbon dioxide gas produced by the chemical reaction during the erosion process was visually checked. For the physical erosion experiment, 1.5 inch holes were made in coral blocks (approximately 20 cm square) of the Porites sp. and the Dipsastraea sp. to reproduce potholes. The skeletons were placed in a box filled with flowing water in the hole using a milk former for 1.5 hours. Also, their skeletal microstructures were observed by optical microscopy.
In the chemical erosion experiment, the Dipsastraea finally dissolved faster than the Porites sp. under the both cases of 2.5% and 5.0% hydrochloric acid. Skeleton of the Dipsastraea sp. dissolved faster in 2.5% hydrochloric acid, while in 5.0% hydrochloric acid the Porites sp. dissolved faster at the beginning and the Dipsastraea sp. dissolved faster in the end. The production of carbon dioxide gas during the erosion was more rapid for the Porites under both the 5.0% and 2.5% hydrochloric acid, and gradually increased for the Dipsastraea sp. The denser skeletal parts of the skeleton of the Dipsastraea sp. remained after the chemical erosion process (Figure 1). In the physical erosion experiment, the amount of powder that came out of the skeleton during the erosion and the speed of the surrounding water being turbid was greater and faster in the Porites sp. case than in the Dipsastraea sp. case, suggesting that the Porites were more susceptible to physical erosion than the Dipsastraea sp. The visual analysis using optical microscopy revealed that the skeletal microstructures of Porites sp.were fine (thinner and smaller) than those of Dipsastraea sp (Table 1).
The faster reaction with acid in the Dipsastraea sp. than the Porites sp. should be related to the fact that skeletal density of the Porites sp. is greater than that of the Dipsastraea sp. (Indo-Pacific Carbonate Production v 1.3, University of Exeter). The result that the denser parts of the skeleton of the Dipsastraea sp. remained after the erosion process (Figure 1), suggesting that higher skeletal density is more likely to be chemically eroded. The reason why the Porites sp. were more susceptible to physical erosion than the Dipsastraea sp., should be related to the differences in the skeletal microstructures. Previous study (Matsubara et al., 2012) suggested that cracks that enter coral blocks with more pores, are inhibited by pores. It is suggested that for Porites sp. with more fine skeletal microstructure than Dipsastraea sp. should have more holes per unit area, the physical erosion could be accelerated by the increased number of the subsequent cracks.
In this study it was indicated that Dipsastraea sp. with lower skeletal density is more susceptible to chemical erosion, while Porites sp. with more fine microstructure is more susceptible to physical erosion. For future perspectives, it is necessary to increase the number of samples using other coral species, and to investigate more about the difference in erosion process between the different microstructures in one coral sample.
In this study, skeletons of modern Porites sp. and Dipsastraea sp. collected in Kikai Island, were compared in the responses to chemical erosion and physical erosion. For the chemical erosion experiment, the skeletons were immersed in hydrochloric acid (2.5% and 5.0%) for a certain period of time (~24 hours). The dry weights of the skeletons were measured to compare the changes of the remaining portions of the skeleton. The amount of carbon dioxide gas produced by the chemical reaction during the erosion process was visually checked. For the physical erosion experiment, 1.5 inch holes were made in coral blocks (approximately 20 cm square) of the Porites sp. and the Dipsastraea sp. to reproduce potholes. The skeletons were placed in a box filled with flowing water in the hole using a milk former for 1.5 hours. Also, their skeletal microstructures were observed by optical microscopy.
In the chemical erosion experiment, the Dipsastraea finally dissolved faster than the Porites sp. under the both cases of 2.5% and 5.0% hydrochloric acid. Skeleton of the Dipsastraea sp. dissolved faster in 2.5% hydrochloric acid, while in 5.0% hydrochloric acid the Porites sp. dissolved faster at the beginning and the Dipsastraea sp. dissolved faster in the end. The production of carbon dioxide gas during the erosion was more rapid for the Porites under both the 5.0% and 2.5% hydrochloric acid, and gradually increased for the Dipsastraea sp. The denser skeletal parts of the skeleton of the Dipsastraea sp. remained after the chemical erosion process (Figure 1). In the physical erosion experiment, the amount of powder that came out of the skeleton during the erosion and the speed of the surrounding water being turbid was greater and faster in the Porites sp. case than in the Dipsastraea sp. case, suggesting that the Porites were more susceptible to physical erosion than the Dipsastraea sp. The visual analysis using optical microscopy revealed that the skeletal microstructures of Porites sp.were fine (thinner and smaller) than those of Dipsastraea sp (Table 1).
The faster reaction with acid in the Dipsastraea sp. than the Porites sp. should be related to the fact that skeletal density of the Porites sp. is greater than that of the Dipsastraea sp. (Indo-Pacific Carbonate Production v 1.3, University of Exeter). The result that the denser parts of the skeleton of the Dipsastraea sp. remained after the erosion process (Figure 1), suggesting that higher skeletal density is more likely to be chemically eroded. The reason why the Porites sp. were more susceptible to physical erosion than the Dipsastraea sp., should be related to the differences in the skeletal microstructures. Previous study (Matsubara et al., 2012) suggested that cracks that enter coral blocks with more pores, are inhibited by pores. It is suggested that for Porites sp. with more fine skeletal microstructure than Dipsastraea sp. should have more holes per unit area, the physical erosion could be accelerated by the increased number of the subsequent cracks.
In this study it was indicated that Dipsastraea sp. with lower skeletal density is more susceptible to chemical erosion, while Porites sp. with more fine microstructure is more susceptible to physical erosion. For future perspectives, it is necessary to increase the number of samples using other coral species, and to investigate more about the difference in erosion process between the different microstructures in one coral sample.