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，海水的化学成分随着深海火山活动速率的变化而发生了变化。 拟议的研究特别涉及热液活动将钙从地幔释放到海洋并消耗海水中的镁的速率变化引起的海水镁/钙比率的变化。这些变化与重要的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/钙比率对海洋生物的生物矿化作用产生了强烈的影响，当镁/钙比率低于1时，石灰岩的主要成分碳酸钙以矿物低镁方解石的形式无机沉淀。 在1和2之间的比率下，高镁方解石沉淀，而在2以上的比率下，高镁方解石和/或文石（具有不同晶体结构的矿物）沉淀。 目前海底火山活动相对较弱，因此海水镁/钙比值较高（5.2）。 可以假设，珊瑚之所以能在今天成为珊瑚礁的建造者，是因为它们分泌文石，而文石是环境中高镁/钙比例所青睐的。 在白垩纪晚期，大约1亿至6500万年前，珊瑚将其作为主要造礁者的角色让给了双壳贝类，双壳贝类分泌的外壳主要由方解石组成。当时，海水的镁/钙比已降至1附近，有利于方解石沉淀。 直至渐新世，镁/钙比例再次上升至文石范围，珊瑚才恢复其主要造礁角色。现时建议的研究将透过实验室实验，测试镁/钙比例对珊瑚骨骼生长的影响。 四种珊瑚将在具有7种不同镁/钙比例的海水中养殖，包括现代和晚白垩世海水。 将比较各种处理的骨骼生长速率和骨骼矿物学。 “白垩纪”海水中相对较低的增长率表明，晚白垩纪海洋中镁/钙比率低是这些海洋中珊瑚礁形成作用弱的原因。 部分或全部转移到“白垩纪”海水沉淀方解石，而不是文石，将证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.