Clumped isotope composition of modern cold-water corals

Abstract: The carbonate clumped isotope thermometer is a promising tool for determining past ocean temperatures. It is based on the temperature dependence of rare isotopes ‘clumping’ into the same carbonate ion group in the carbonate mineral lattice. The extent of this clumping effect is independent of the isotope composition of the water from which carbonate precipitates, providing unique advantages over many other paleotemperature proxies. Existing calibrations of this thermometer in cold- water and warm-water corals suggest clumped isotope ‘vital effects’ are negligible in cold-water corals but may be significant in warm-water corals. Here, we test the calibration of the carbonate clumped isotope thermometer in cold-water corals with a recently collected and well characterised sample set spanning a range of coral genera (Balanophyllia, Caryophyllia, Dasmosmilia, Desmophyllum, Enallopsammia and Javania). The clumped isotope compositions (D47) of these corals exhibit systematic dependences on their growth temperatures, confirming the basis of the carbonate clumped isotope thermometer. However, some cold-water coral genera show D47 values that are higher than the expected equilibrium values by up to 0.05‰ (equivalent to underestimating temperature by 9°C) similar to previous findings for some warm-water corals. This finding suggests that the vital effects affecting corals Δ47 are common to both warm- and cold-water corals. By comparison with models of the coral calcification process we suggest that the clumped isotope offsets in these genera are related to the kinetic isotope effects associated with CO2 hydration/hydroxylation reactions in the corals’ calcifying fluid. Our findings complicate the use of the car- bonate clumped isotope thermometer in corals, but suggest that species- or genus-specific calibrations could be useful for the future application of this paleotemperature proxy.

Other Description: Spooner, Peter T.; Guo, Weifu; Robinson, Laura F.; Thiagarajan, Nivedita; Hendry, Katherine R.; Rosenheim, Brad E.; Leng, Melanie J. (2016) "Clumped isotope composition of cold-water corals: A role for vital effects?", Geochimica et Cosmochimica Acta, 79, 123-141. doi:10.1016/j.gca.2016.01.023

摘要：碳酸盐团簇同位素温度计（carbonate clumped isotope thermometer）是一种极具应用前景的古海洋温度重建工具。其原理基于碳酸盐矿物晶格中，稀有同位素“团簇”结合至同一碳酸根离子基团的温度依赖性。该团簇效应的程度与碳酸盐沉淀所处水体的同位素组成无关，相较诸多其他古温度代用指标（paleotemperature proxy）具备独特优势。
目前针对冷水珊瑚与暖水珊瑚的该温度计校准研究表明，团簇同位素“生命效应”在冷水珊瑚中可忽略不计，但在暖水珊瑚中或较为显著。本文依托新近采集且表征完善的样本集——涵盖多个珊瑚属（Balanophyllia、Caryophyllia、Dasmosmilia、Desmophyllum、Enallopsammia及Javania）——对冷水珊瑚中的碳酸盐团簇同位素温度计校准方案开展验证。
这些珊瑚的团簇同位素组成（D47）呈现出与生长温度的系统性关联，证实了碳酸盐团簇同位素温度计的理论基础。然而部分冷水珊瑚属的D47值较理论平衡值高出至多0.05‰（相当于将温度低估9℃），这与此前部分暖水珊瑚的研究结果类似。该发现表明，影响珊瑚Δ47的生命效应在暖水与冷水珊瑚中普遍存在。
通过与珊瑚钙化过程模型对比，本文提出上述类群的团簇同位素偏移与珊瑚钙化液中CO₂水合/羟化反应相关的动力学同位素效应（kinetic isotope effects）存在关联。本研究结果使碳酸盐团簇同位素温度计在珊瑚中的应用复杂化，但同时指出，针对物种或属的专属校准方案，可为该古温度代用指标的后续应用提供可行路径。

其他说明：Spooner, Peter T.; Guo, Weifu; Robinson, Laura F.; Thiagarajan, Nivedita; Hendry, Katherine R.; Rosenheim, Brad E.; Leng, Melanie J. (2016) "Clumped isotope composition of cold-water corals: A role for vital effects?", 《地球化学与宇宙化学学报》, 79, 123-141. doi:10.1016/j.gca.2016.01.023