近日，我院同位素前沿科学研究中心在金属同位素研究方面取得新进展，相关研究结果以“Copper and Zinc isotope signatures in scleratinian corals: implications for Cu and Zn cycling in modern and ancient ocean”为题在国际知名期刊Geochimica et Cosmochimica Acta上发表。
铜、锌是海洋浮游生物生命活动所需金属蛋白酶的关键组分，对全球氮、碳循环起着重要调控作用，其含量和同位素变化是国际地学重大研究计划“GEOTRACES”核心研究内容之一。当前海洋碳酸盐岩中的铜、锌同位素已被学者广泛用于指示地质历史时期中的重大气候环境变化。然而，古碳酸盐岩是否如实记录了海水的铜、锌同位素信号，以及铜、锌同位素分馏的机制和影响因素还尚不明确。
造礁珊瑚能在气候环境相对适宜的海洋中连续生长数百年，所生产的碳酸盐占全球海洋碳酸盐积累量的四分之一。开展造礁珊瑚骨骼铜锌同位素组成的研究，可以探究生命有关金属同位素在生物碳酸盐中的分馏机理，为解析海洋关键生物地球化学循环过程奠定基础。基于此，我们在国家自然科学基金项目（41922019、41961144028和41625012）的资助下，系统研究了南海西沙群岛、南沙群岛、海南岛、北部湾和西太平洋吕宋海峡滨珊瑚(Porites)样品的铜锌含量及其同位素组成的年际变化。结果发现珊瑚骨骼相对于表层海水富集轻的铜同位素(Δ65Cu =δ65Cu珊瑚-δ65Cu海水= -0.33 ± 0.33‰, n = 40, 2SD)，珊瑚个体间和个体内铜同位素组成变化范围大，主要受珊瑚钙化过程中的瑞利分馏控制；而珊瑚与表层海水之间存在有限且相对恒定的锌同位素分馏 (Δ66Zn =δ66Zn珊瑚-δ66Zn海水= +0.10 ± 0.05‰ , n = 33, 2SD)，其幅度远低于前人碳酸盐沉淀实验测定的分馏 (Δ66Zn = ~0.60‰，Mavromatis et al, GCA, 2019.)。这可能是由于珊瑚钙化流体的高pH值改变了溶液相中锌的形态(speciation)，以及溶液相中有机物优先络合重的锌同位素所致。由于滨珊瑚与海水的锌同位素分馏接近于0，所以不像铜同位素一样受到钙化过程的影响，因而滨珊瑚能够记录历史时期海水的锌同位素变化。最近的研究表明，海洋中海水锌同位素组成与有机碳通量之间存在密切关联（Weber et al, Science, 2018）。鉴于锌在表层海洋中极度亏损，其浓度比深海低两个数量级以上，表层海水δ66Zn对有机碳通量的变化更为敏感。因而生活在表层海水的造礁珊瑚中的锌同位素记录在示踪海洋碳循环、上升流变化和人为来源锌输入等气候环境领域有巨大潜力。
我院博士研究生张婷为论文第一作者，刘羿教授为通讯作者，刘丛强院士、陈玖斌教授、郑旺教授和孙若愚教授参与指导了研究工作，已毕业硕士研究生陈璐参与了部分样品采集和数据分析工作。
论文链接：https://www.sciencedirect.com/science/article/abs/pii/S0016703721006177
【论文摘要】
Metal stable isotopes in marine carbonates have been widely used as novel proxies for metal cycling through the geological past. Amongst these metals, copper (Cu) and zinc (Zn) have received great attention owing to their vital roles in metabolic processes. However, whether modern marine biogenic carbonates record the seawater isotope signatures of Cu (δ65Cu) and Zn (δ66Zn) and mechanisms and factors controlling isotope fractionation of these metals remain unclear, hindering applications of both isotope systems in paleoceanography. Here we present annually-resolved records of δ65Cu and δ66Zn in seven coral (Porites) cores sampled from different marine settings in the South China Sea and the Luzon Strait, western Pacific. We find that the aragonitic skeletons of corals are enriched in light Cu but heavy Zn isotopes relative to surface seawaters, with δ65Cu and δ66Zn in the range of -0.16 ± 0.06‰ to 0.40 ± 0.05‰ and 0.06 ± 0.04‰ to 0.46 ± 0.08‰ (2SD), respectively. The coral δ65Cu exhibits significant inter- and intra-colony variations, which are most likely controlled by Rayleigh-type fractionation in the calcifying fluids of corals rather than by changes in environmental factors or seawater δ65Cu. We thus suggest that δ65Cu in ancient carbonates may not be a direct record of Cu isotope compositions in coeval seawaters. In contrast, coral δ66Zn shows insignificant temporal variation, and the Zn isotope fractionation between individual corals and seawaters are relatively small and constant (0.10 ± 0.05‰, n = 33, 2SD). This limited Zn isotope fractionation is much lower than that determined during inorganic precipitation of calcite, which is likely due to pH up-regulation by the coral that changes aqueous Zn speciation, and preferential organic complexation of the heavy Zn isotopes in the calcifying fluids. Therefore, Porites corals are promising archives for tracking historical changes of surface seawater δ66Zn. Our new datasets of reef carbonates, particularly the coral δ65Cu values which are measured for the first time, could provide a better constrain on marine Cu and Zn geochemistry and their modern oceanic mass and isotope budgets.
