Experiments and Observations Based on the Finding that Scleractinian Corals Secrete Calcite Instead of Aragonite in Cretaceous Seawater

摘要在过去的5亿年里，随着深海火山活动速率的变化，海水的化学成分发生了变化。拟议的研究特别涉及由热液活动将钙从地幔释放到海洋并从海水中消耗镁的速率变化引起的海水镁/钙比率的变化。这些变化与重要造礁生物和石灰岩形成生物的矿物学之间的相关性表明，海洋的镁/钙比例对海洋生物的生物矿化作用产生了强烈影响。当镁钙比低于1时，石灰石的主要成分碳酸钙以低镁方解石的形式无机沉淀。当比值在1和2之间时，会析出高镁方解石，当比值大于2时，会析出高镁方解石和/或文石（一种具有不同晶体结构的矿物）。目前海底火山作用较弱，使得海水镁钙比较高（5.2）。可以假设，今天珊瑚作为珊瑚礁建设者而蓬勃发展，因为它们分泌文石，这受到高镁/钙环境比例的青睐。在白垩纪晚期，大约1 - 6500万年前，珊瑚取代了原始双壳类的主要礁石建造者角色，后者分泌的贝壳主要由方解石组成。此时海水的镁钙比已降至1附近，有利于方解石的沉淀。直到渐新世，当镁/钙比值再次上升到文石域时，珊瑚才恢复其主导的造礁作用。现在提出的研究将通过实验室实验测试镁/钙比例对珊瑚骨骼生长的影响。四种珊瑚将在7种不同镁钙比例的海水中培育，包括现代和晚白垩纪海域的海水。将比较不同治疗方法的骨骼生长速率和骨骼矿物学。“白垩纪”海水中相对较低的生长率表明，晚白垩纪海洋的低镁钙比是珊瑚在这些海洋中弱造礁作用的原因。如果“白垩纪”海水部分或完全转变为方解石而不是文石的沉淀，则表明海水化学对生物矿化的影响更大。

ABSTRACTThe chemistry of seawater has changed over the past half billion years in response to changes in rates of volcanism in the deep sea. The proposed research relates in particular to changes in the magnesium/calcium ratio of seawater induced by changes in the rate at which hydrothermal activity releases calcium from Earth's mantle into the ocean and consumes magnesium from seawater. Correlations between these changes and the mineralogy of important reef-building and limestone-forming organisms suggest that the magnesium/calcium ratio of the ocean has exerted a strong influence over biomineralization by marine life.At magnesium/calcium ratios below 1, calcium carbonate, the primary constituent of limestone, precipitates inorganically in the form of the mineral low-magnesium calcite. At ratios between 1 and 2, high-magnesium calcite precipitates instead, and at ratios above 2, high-magnesium calcite and/or aragonite (a mineral with a different crystal structure) precipitate. At the present time submarine volcanism is relatively weak, so that the magnesium/calcium ratio of seawater is high (5.2). It can be hypothesized that corals flourish as reef builders today because they secrete aragonite, which is favored by the high ambient magnesium/calcium ratio. During Late Cretaceous time, about 100-65 million years ago, corals yielded their role as dominant reef builders to rudist bivalves, which secreted shells consisting largely of calcite. At that time, the magnesium/calcium ratio of seawater had dropped to the vicinity of 1, favoring calcite precipitation. Corals did not resume their dominant reef-building role until Oligocene time, when the magnesium/calcium ratio was again rising into the aragonite domain.The research now proposed will test the effects of the magnesium/calcium ratio on coral skeletal growth through laboratory experiments. Four species of corals will be cultured in seawaters having 7 different magnesium/calcium ratios, including those of modern and Late Cretaceous seas. Skeletal growth rates and skeletal mineralogies will be compared for the various treatments. Relatively low growth rates in "Cretaceous" seawater would suggest that the low magnesium/calcium ratio of Late Cretaceous seas accounted for the weak reef-building role of corals in those seas. A partial or complete shift in "Cretaceous" seawater to precipitation of calcite instead of aragonite would demonstrate an even stronger influence of seawater chemistry on biomineralization.