Abstract Shallow-water reef-building corals have an extensive geological record and many aspects of their evolution, biodiversity, and biogeography are known in great details. In contrast, the adaptive potential and risk of extinction of coral reefs in response to excessive warming and ocean acidification remains largely undocumented. It is well established that anthropogenic CO2 emissions cause global warming and ocean acidification (lowering of pH), which increasingly impede the biomineralization process in many marine calcareous biota. The “light-enhanced” calcification machinery of the shallow-water reef corals is particularly threatened by this development through the combined effect of a lowering of the supersaturation of seawater with CaCO3 (aragonite) and an expulsion of the symbiotic zooxanthellae (bleaching). The bleaching is of prime importance, because it interrupts the supply of DIC and metabolites required for pH upregulation within the calcification fluid. The degree of calcification in scleractinian reef corals may therefore represent a suitable tracer to assess the state of the ocean carbonate system and the photosynthetic performance of the zooxanthellae during past episodes of natural environmental change. This study presents the first comprehensive set of calcification data from reef corals covering the early Miocene to early Pleistocene interval (20.8 to 1.2 million years, Ma). Various screening procedures ensured that the studied coral skeletons are pristine and suited to yield meaningful stable isotope data (δ18O, δ13C) and calcification records. δ18O and δ13C values document growth environments consistent with current tropical and subtropical settings. To assess fossil calcification rates, we use a reference dataset of recent corals from the Indo-Pacific (Porites) and an independent validation dataset from the Western Atlantic-Caribbean (Orbicella). Almost all fossil corals document very low annual rates of upward growth (extension rate) relative to present, and lower skeletal bulk density than predicted by established modern relationships. To allow for a quantitative assessment of coral calcification performance, we use a new approach that we term the calcification anomaly. It is insensitive to sea-surface temperature and well-suited for comparative assessments of calcification performance between reef sites and over time. Based on this approach, the majority of fossil corals in our dataset displays hypo-calcification, while a few show optimal calcification and none display hyper-calcification. Compared to present-day growth conditions, the fossil calcification data show that (1) skeletogenesis responded in a fully compatible way to known environmental stresses (e.g. turbid water, elevated salinity, eutrophy), and that (2) the calcification performance within the reef window (i.e. oligotrophic clear-water settings) remained below that of modern z-corals. Since fossil coral δ13C values are compatible with those of modern reef corals, we infer that the light-enhanced calcification system of symbiotic scleractinian corals was fully established by the beginning of the Neogene and that lower-than-present calcification performance was the likely response to a chronically low pH and/or low carbonate saturation state of the global ocean. If so, the present-day saturation state appears to be rather an exception than the norm and probably not a suitable starting point for predicting future calcification trends. In addition, using trends from the geological past for predicting future developments does not consider anthropogenic side-effects such as eutrophication and pollution.

中文翻译：

---

晚新生代珊瑚礁钙化评估

摘要 浅水造礁珊瑚具​​有广泛的地质记录，其演化、生物多样性和生物地理学的许多方面都非常详细。相比之下，珊瑚礁因过度变暖和海洋酸化而灭绝的适应潜力和风险在很大程度上仍未得到记录。众所周知，人为二氧化碳排放会导致全球变暖和海洋酸化（pH 值降低），这越来越阻碍许多海洋钙质生物群的生物矿化过程。浅水珊瑚礁的“光增强”钙化机制尤其受到这种发展的威胁，这是通过降低海水过饱和度与 CaCO3（文石）和驱逐共生虫黄藻（漂白）的共同作用。漂白是最重要的，因为它中断了钙化液中 pH 值上调所需的 DIC 和代谢物的供应。因此，在过去的自然环境变化事件中，石珊瑚礁珊瑚的钙化程度可能是评估海洋碳酸盐系统状态和虫黄藻光合性能的合适示踪剂。这项研究提供了第一组来自珊瑚礁的综合钙化数据，涵盖了早中新世至早更新世间隔（20.8 至 120 万年，Ma）。各种筛选程序确保所研究的珊瑚骨骼是原始的，适合产生有意义的稳定同位素数据（δ18O、δ13C）和钙化记录。δ18O 和 δ13C 值记录了与当前热带和亚热带环境一致的生长环境。为了评估化石钙化率，我们使用了来自印度洋-太平洋 (Portites) 的近期珊瑚的参考数据集和来自西大西洋-加勒比海 (Orbicella) 的独立验证数据集。几乎所有化石珊瑚都记录了相对于现在非常低的年增长率（扩展率），并且骨骼体积密度低于已建立的现代关系所预测的。为了对珊瑚钙化性能进行定量评估，我们使用了一种称为钙化异常的新方法。它对海面温度不敏感，非常适合对珊瑚礁位置之间和随时间推移的钙化性能进行比较评估。基于这种方法，我们数据集中的大多数化石珊瑚显示低钙化，而少数显示最佳钙化，没有显示高钙化。与目前的生长条件相比，化石钙化数据表明 (1) 骨骼生成以完全兼容的方式对已知的环境压力 (例如浑水、盐度升高、富营养化) 做出反应，以及 (2) 珊瑚礁内的钙化性能窗口（即贫营养清水环境）仍然低于现代 z 珊瑚。由于化石珊瑚 δ13C 值与现代珊瑚礁的值一致，我们推断共生石珊瑚的光增强钙化系统在新近纪开始时已经完全建立，低于目前的钙化性能可能是对全球长期低 pH 值和/或低碳酸盐饱和状态的反应。海洋。如果是这样，当前的饱和状态似乎是一个例外而不是常态，并且可能不是预测未来钙化趋势的合适起点。此外，使用地质过去的趋势来预测未来的发展并没有考虑人为的副作用，如富营养化和污染。目前的饱和状态似乎是一个例外而不是常态，可能不是预测未来钙化趋势的合适起点。此外，使用地质过去的趋势来预测未来的发展并没有考虑人为的副作用，如富营养化和污染。目前的饱和状态似乎是一个例外而不是常态，可能不是预测未来钙化趋势的合适起点。此外，使用地质过去的趋势来预测未来的发展并没有考虑人为的副作用，如富营养化和污染。