Presentation Information
[O12-P18]Growth rate of Porites coral fossils on Holocene coral reef terrace III of Kikai Island
*Kento Inage1,2 (1. Kikai High School, Kagoshima, 2. KIKAI institute for Coral Reef Sciences)
Keywords:
Porites annual bands,Holocene,Kikaijima Island
Kikai Island is located within the Amami Islands. The island is covered by fossil corals that formed during the past 100 thousand years. During this time the island has been tectonicly uplifting. Among the corals, the skeletons of the genus Porites feature tree-like growth rings. The wider the growth rings, the faster the growth rate. Research on the Holocene coral reef terraces around the coast of Kikai Island has been conducted since the 1970s (Ota et al. 1978, etc). Based on the age of the fossils, four Holocene aged terraces have been identified (Ota et al. 1978). According to (Garas et al. 2022), a cold period occurred in Kikai Island approximately 4,000 years ago. Terrace III, formed between approximately 3,000 and 4,000 years ago. allows for the measurement of the coral growth rates from corals from this terrace allows us to analyse conditions when sea temperatures were lower than present.
Since Kikai Island is dominated by seasonal winds from the south in the summer and from the northwest in the winter, samples were collected from Holocene coral reef terraces in different parts of the Islands to account for the varying effects of wind and waves.
Core samples for analysis were collected from Terrace III at Tekutsuku (south), Shitoke (northeast), Nakama (central-west), and Keraji (central-south). The equipment used to collect samples consisted of an electric drill fitted with a 1.5-inch corer. For sampling, we selected the genus Porites, a reef-building coral known for its longevity, large skeleton, and clearly visible annual rings compared to other species. The collected core samples were imaged using a CT scanner at the Kikai Institute For Coral Reef Science to measure the growth rings. We analyzed the width of the annual rings to determine the variation in growth rate (mm/year), as well as the mean, maximum, and minimum values. Sea surface temperatures at that time were calculated using a formula from a previous study (Goodkin et al., 2011). To compare with current growth rates, we compared our results with data from Garas et al.(2022).
The results are as follows Core samples were collected from four locations: 63 years’ worth at
Tekutsuku: 9.2 mm ±2.2/year, Age: 63 years
Shitoke: 6.4 mm ±1.7/year, Age:68 years
Nakama: 7.1 mm ±1.7/year, Age: 68 years
Keraji: 7.4 mm ±1.6/year, Age: 43 years
The Shitoke core sample was radiocarbon dated in another study and the age was determined to be 3,382–4,401 years ago.
At each site, there were years in which the annual extension rate exceeded the standard deviation and showed notably high growth. This suggests that the sampling site on Surface III may have been subject to relatively high seawater temperatures during the summer in the past. In this presentation, we plan to compare the present coral skeletal extension rates with those of Terrace III to infer the environmental conditions of the coral reef during that period. In addition, we aim to review previous studies on the paleoenvironment of Kikaijima and examine the oceanographic conditions that may have contributed to these patterns.
Since Kikai Island is dominated by seasonal winds from the south in the summer and from the northwest in the winter, samples were collected from Holocene coral reef terraces in different parts of the Islands to account for the varying effects of wind and waves.
Core samples for analysis were collected from Terrace III at Tekutsuku (south), Shitoke (northeast), Nakama (central-west), and Keraji (central-south). The equipment used to collect samples consisted of an electric drill fitted with a 1.5-inch corer. For sampling, we selected the genus Porites, a reef-building coral known for its longevity, large skeleton, and clearly visible annual rings compared to other species. The collected core samples were imaged using a CT scanner at the Kikai Institute For Coral Reef Science to measure the growth rings. We analyzed the width of the annual rings to determine the variation in growth rate (mm/year), as well as the mean, maximum, and minimum values. Sea surface temperatures at that time were calculated using a formula from a previous study (Goodkin et al., 2011). To compare with current growth rates, we compared our results with data from Garas et al.(2022).
The results are as follows Core samples were collected from four locations: 63 years’ worth at
Tekutsuku: 9.2 mm ±2.2/year, Age: 63 years
Shitoke: 6.4 mm ±1.7/year, Age:68 years
Nakama: 7.1 mm ±1.7/year, Age: 68 years
Keraji: 7.4 mm ±1.6/year, Age: 43 years
The Shitoke core sample was radiocarbon dated in another study and the age was determined to be 3,382–4,401 years ago.
At each site, there were years in which the annual extension rate exceeded the standard deviation and showed notably high growth. This suggests that the sampling site on Surface III may have been subject to relatively high seawater temperatures during the summer in the past. In this presentation, we plan to compare the present coral skeletal extension rates with those of Terrace III to infer the environmental conditions of the coral reef during that period. In addition, we aim to review previous studies on the paleoenvironment of Kikaijima and examine the oceanographic conditions that may have contributed to these patterns.