1. Research has focused on the toxic effects of heavy metals such as lead, cadmium, mercury, and aluminum; however, even copper and zinc, which are essential elements, can become toxic in high concentrations.

译文：研究把重点放在重金属的毒性作用，如铅、镉、汞、铝；然而即使是铜和锌这样的必需元素，浓度过高也会产生毒性。

结构分析：such as举例信号词，对heavy metal具体化；however表示前后两句转折；后一句的主语被which引导的定语从句修饰，可理解为插入语，将主谓分开。

2. Mineral deficiencies can often be detected by specific symptoms such as chlorosis (loss of chlorophyll resulting in yellow or white leaf issue), necrosis (isolated dead patches), anthocyanin formation (development of deep red pigmentation of leaves or stem), stunted growth, and development of woody tissue in an herbaceous plant.

译文：矿物质缺乏通常可通过特定的症状检测出来，如褪绿（叶绿素损失导致黄叶或白叶的现象）、坏疽（孤立的坏死斑）、花青素的形成（形成深红色叶片和茎色素沉积）、发育不良，以及草本植物木质组织的生长。

结构分析：主干比较简单Mineral deficiencies can often be detected by specific symptoms；such as举例信号词，后5个专业词汇/短语并列，对specific symptoms具体化。

3. The explosion is also calculated to have produced vast quantities of nitric acid and melted rock that sprayed out over much of Earth, starting widespread fires that must have consumed most terrestrial forests and grassland.

译文：爆炸被计算出产生了大量的硝酸和经过溶解并喷出地球外的岩浆，造成了大面积火灾，大部分的森林和草原被燃尽。

结构分析：主干信息为The explosion is also calculated to have produced vast quantities of nitric acid and melted rock，被动结构；that引导的定语从句修饰melted rock；starting….fires动名词做结果状语，对前面整句话进行修饰； that引导的定语从句修饰fires。

4. While there are a dozen or more mass extinctions in the geological record, the Cretaceous mass extinction has always intrigued paleontologists because it marks the end of the age of the dinosaurs.

译文：虽然地质史料记载，地球上有十几次甚至更多大型的物种灭绝，但是白垩纪大灭绝引起了古生物学家的兴趣，因为它标志着恐龙时代的结束。

结构分析：该句的结构比较清晰；while引导让步状语从句；句尾because引导的原因状语从句。

5. When the well reaches a pool, oil usually rises up the well because of its density difference with water beneath it or because of the pressure of expanding gas trapped above it.

译文：当井探至油床时，因为石油的密度与在下层的水不同，或者因为石油上面的气体扩张形成的压力，石油通常会上升至井内。

结构分析：when引导时间状语从句；or连接两个because of引导的原因状语；trapped……，v+ed短语为后置定语修饰gas。

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