碩士 / 國立臺灣師範大學 / 地球科學系 / 100 / A drilling core (TY-05) was choosen from NW Taiwan and anaylzed the stable carbon and oxygen isotope composition and Sr/Ca ratio of the coral Porites inside. And analyzing the oxygen isotope composition of the seawater in the same study area, together this study will reconstruct the Mid-Holocene paleoclimate of NW Taiwan.
Coral samples were examined by XRD to make sure the coral is pristine. X-ray radiographs were taken to identify the main growth axis. Carbonate powders of coral were micro drilled for U-Th dating, stable carbon and oxygen isotope and Sr/Ca ratio analyses using an electric dental drill along the main growth axis.
Based on the U-Th dating result, our sample grew around 5,845±55 yr BP. δ18O data of the sample showed 18 year seasonal cycles. Annual growth rate between the 4th and the 7th years was abnormally low and less than 4mm/yr. Because the kinetic isotope effect may influence both δ13C and δ18O values of coral skeleton precipitated at < 4mm/yr growth rate (McConnaughey, 1989), the 4th -7th year records were eliminated.
The Average of the maximum (winter) and minimum (summer) Sr/Ca ratio values are 9.66mmol/mol and 8.59 mmol/mol (N= 11), respectively. An abrupt change of Sr/Ca ratio was found in the summer of the 7th year. The average of δ18O values winter and summer are-3.64‰ and -5.15‰ (N=13) , respectively. The d13C values of the maximum and minimum are -0.26‰ and -2.82‰ (N=13), respectively.
Sea surface temperature (SST) in 5845 yr BP inferred by Sr/Ca ratio are 20.4℃ and 28.3℃ in winter and summer, respectively. By coupling both SST and coral’s d18O record, the calculated d18Osw in 5845 yr BP, was 0.15‰ and 0.10‰ in winter and summer, respectively. A warmer and stronger evaporation condition was shown in this data in mid-Holocene, which consist with other record in East Asia.
The abrupt change of Sr/Ca ratio in the summer of the 7th year is truly a change in SST, which represent a 3℃ change in SST. The 18Osw also show a +0.46‰ change in summer, synchronously. It’s possible that the transport of Kuroshio Branch Water was increased in Taiwan Strait during summer, and may cause by a stronger East Asian Summer Monsoon.
Mean seasonal cycle of carbon and oxygen isotope values and the insolation at 30°N in 6000 yr BP are compared together. The maximum and minimum values of the oxygen isotope are defined as January and August, respectively. The maximum value of carbon isotope was found in spring, which preceded the seasonal insolation cycle. This result may due to the effect of both higher insolation and greater nutrient concentration. Winter stable carbon isotope show an increase trend was in this 11-year record. The increase trend in winter SST could cause a stronger photosynthesis that will increase the d13C data in coral skeletons.
| Identifer | oai:union.ndltd.org:TW/100NTNU5135016 |
| Date | January 2011 |
| Creators | Chou-Wei Wu, 吳昭緯 |
| Contributors | Horng-Sheng Mii, Shih-Wei Wang, 米泓生, 王士偉 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 75 |
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