(SGER): Experiments on the Effects of the Magnesium-Calcium Ratio of Seawater on Carbonate Skeletal Secretion

现在有强有力的证据表明，海水中镁钙比的变化在显生界期间产生了振荡，在海水中沉积的非骨架碳酸盐由低镁方解石组成，而这些化学沉淀物则由高镁方解石和文石组成。在以前关于地质记录中的模式的工作中，我们表明，在整个显生界时期，生物上简单的碳酸盐分泌生物体，特别是那些高钙化(产生巨大的骨骼或猖獗地产生沉积物)的生物体，一直坚持‘方解石海’/‘文石海’模式。这一对应关系意味着，只有当周围海水的镁/钙比有利于它们的矿物学时，简单的生物体才会发生超钙化。到目前为止，Pi的研究一直是基于地质记录中的模式。他们现在正在着手一种全新的方法：他们计划进行实验室实验，以测试镁/钙比例对生物钙化的影响。具体地说，他们计划进行三种受控水族馆实验，他们预计这些实验将特别容易处理。他们将确定，当在低和极低(二叠纪和白垩纪)镁/钙比的水中生长时，(1)在现代海水中分泌高镁方解石的珊瑚藻样，其骨骼中吸收的镁比在现代海水中生长时要少；(2)文石藻样的生长速度比在现代海水中生长时慢；(3)与在相同条件下培养但环境镁/钙比与现代海水相同的可比种群相比，分泌方解石的钙质超微浮游植物的种群增长更快。(换句话说，当现存的微型浮游生物物种生长在‘白垩纪’海水中时，海水中的镁/钙比接近1，溶解钙的总浓度很大，它们会像它们的祖先在白垩纪的‘白垩海’中那样繁殖吗？)

There is now powerful evidence that changes in the Mg/Ca ratio of seawater have produced the oscillations during Phanerozoic time between intervals when nonskeletal carbonates precipitating from seawater have consisted of low-Mg calcite and intervals when these chemical precipitates have consisted of high-Mg calcite and aragonite. In previous work that focused on patterns in the geologic record, we showed that throughout Phanerozoic time biologically simple carbonate-secreting organisms, especially those that hypercalcify (produce massive skeletons or generate sediment rampantly) have adhered to the 'calcite sea'/'aragonite sea' pattern. This correspondence implies that simple organisms hypercalcify only when the Mg/Ca ratio of ambient seawater favors their mineralogy. Thus far, PI's research has been based on patterns in the geologic record. They are now embarking on an entirely new approach: they plan to conduct laboratory experiments to test the effects of the Mg/Ca ratio on the calcification of living organisms. Specifically, they plan three kinds of controlled aquarium experiments that they anticipate will be particularly tractable. They will determine whether, when grown in water having low and very low ('Permian' and 'Cretaceous') Mg/Ca ratios, (1) specimens of coralline algae, which secrete high-Mg calcite in modern seawater, incorporate less Mg into their skeletons than when they are grown in modern seawater; (2) specimens of aragonitic algae grow at lower rates than when grown in modern seawater; and (3) calcareous nannoplankton, which secrete calcite, experience more rapid population growth than comparable populations cultured under identical conditions but with an ambient Mg/Ca ratio equal to that of modern seawater. (in other words, when extant species of nannoplankton are grown in 'Cretaceous' seawater, in which the Mg/Ca ratio is close to unity and there is a large total concentration of dissolved Ca, will they bloom as their ancestors did in the 'chalk seas' of the Cretaceous?).