Oxygen isotopic composition δ18O of Carboniferous seawater: from a phylloid algal reef in South China
石炭纪海水氧同位素组成（δ18O）：根据华南叶状藻礁的计算结果
Hong-Xia Jiang (姜红霞)1, Lu Zhang (张禄)2 Ya-Sheng Wu (吴亚生)3,4*
1 Hebei
International Joint Research Center for Paleoanthropology, College of Earth
Science, Hebei GEO University, Shijiazhuang 050000, China.
2 China University of Geosciences, Beijing 100083, China.
3 University of Chinese Academy of Sciences, Beijing 100049, China.
4 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China.
*Corresponding author: Email: wys@mail.igcas.ac.cn
Abstract
Calculating the temperatures of the seawater in geological times using the oxygen isotopic compositions of carbonate or phosphatic mineral such as skeletons of brachiopods or conodonts is one of the main contents in research on evolutionary history of Earth’s system. In order to do it, the oxygen isotopic composition, δ18O of the ancient seawater in which the minerals formed must be determined in advance. Because there is no good way to determine the oxygen isotopes of ancient seawaters, they were generally artificially assumed. Since the assumed δ18O is not necessarily equal to the actual values, the calculated paleotemperature is not necessarily equal to the real temperature.
To overcome this shortcoming, it is necessary to develop a new method to calculate the oxygen isotope composition of deep-time seawater. Udotea flabellum, a extant marine chlorophyta, needs to live in water with a temperature range of 26-28°C, or it will die. The best water temperature is 27 °C to it. Here a Carboniferous phylloid algal reef in Guizhou, China is studied. According to previous research, the phylloid alga in the reef belongs to the same group of organisms as today's Udotea flabellum. So, the optimum water temperature for the phylloid alga is 27°C.
On the condition that the temperature of the ancient seawater and the oxygen isotope of the carbonate minerals formed in the seawater are known, the δ18O value of the seawater can be calculated according to Craig’s formula, T =16.9 - 4.2 (δ18Oc-δ18Ow) +0.13 (δ18Oc-δ18Ow)2. We collected many fossil brachiopod shells from the thalloid algal reef, and measured the oxygen isotopic compositions of two of them, which are -2.39‰ and 3.27‰ (PDB). Givan that the temperature of the seawater on the reef at that time is 27 °C and the oxygen isotope of the brachiopod shell is -2.39‰ (PDB), the oxygen isotope composition of the seawater on this Carboniferous reef is calculated to be -0.14‰ (PDB).
Key words: Oxygen isotope of seawater, δ18O, phylloid algal reef, palaeotemperature
根据海相地层中的碳酸盐矿物，如腕足动物的壳，或磷酸盐矿物，如牙形刺化石，计算形成海相地层的海水的古温度是研究地球环境演化历史的一个重要内容。其实现需要以当时海水的氧同位素组成（δ18O）为已知这一前提。由于无法知道古代海水的氧同位素组成，以往的这类研究假设海水的氧同位素组成为某一值。这种假设无论如何都是人为的，不一定就是当时海水的真实的氧同位素组成。这导致古海水温度的计算值有较大的人为性。为了克服这个缺点，需要探索建立计算地质时期海水氧同位素组成的方法。现代海洋的绿藻门的扇形钙扇藻（Udotea flabellum）生活的水温必须是26-28°C，否则会死亡。我们在贵州发现了石炭纪叶状藻生物礁。根据研究，这种叶状藻与现今的钙扇藻属于同一类生物。所以这种叶状藻生物礁形成时的海水温度可以认为是26-28°C。在知道古海水温度、古海水中形成的碳酸盐矿物的氧同位素值的前提下，可以根据Craig的古水温计算公式计算当时古海水的δ18O值。我们自该叶状藻生物礁中采集了大量的腕足动物化石壳，并且测试了其氧同位素组成，获得的数值为-2.39‰（PDB）。根据公式计算，得到石炭纪华南板块古海水的氧同位素组成为-0.14‰。
关键词：δ18O, 古海水氧同位素, 生物礁, 古水温，全球变化