PRF/M: Age of the Amazon Rainforest: New Evidence Using Stable Isotopes and Fossil Mammals

碳-13等稳定同位素可以用来分析第四纪晚期居住在亚马逊河流域的哺乳动物的饮食，以及当地的栖息地和区域气候。先前对哺乳动物化石和古土壤碳酸盐的研究表明，亚马逊在过去的几到几万年里经历了重大变化。这一论断可以通过评估古代生态群落食草动物食用的植物类型来验证，因为：(a)光合碳进入动物硬组织（骨、牙本质、牙釉质）只有两个来源——C3（卡尔文循环）植物和C4（哈奇-松弛循环）植物，每种类型植物的比例为当时流行植被的组成提供了线索；(b)二氧化碳中碳的同位素含量可以用质谱法用粉碎的化石或碳酸盐样品来测量。古代土壤和骨骼化石中沉淀的碳酸盐分析，其中一些原来是羟基磷灰石ıCa10- (PO4)6-(OH)2！，孔隙度高，易发生成岩作用（原始化学成分的次生蚀变）。应该有来自几个目的超过25种哺乳动物，包括齿齿目（如齿齿树懒、巨齿兽、犰狳、雕齿兽）、吻齿目（如Haplomastodon）、尾足目（貘）、偶蹄目和无齿目。研究结果应该有助于深入了解现在的亚马逊雨林以前是否至少在某种程度上被草原覆盖的问题。这些发现可能反过来揭示全球变化在多大程度上是有方向性的或在很长一段时间内是周期性的。

Stable isotopes such as carbon-13 can be used to analyze diets of mammals that inhabited the Amazon River basin during the late Quaternary period, as well as local habitats and regional climates. Prior studies of fossil mammals and paleosol carbonates indicate the Amazon has undergone major change within the last few to several tens of thousands of years. This assertion can be tested by assessing the types of plants eaten by herbivores of ancient ecological communities because: (a) incorporation into animal hard tissues (bone, dentine, enamel) of photosynthetic carbon is from only two sources - - C3 (Calvin cycle) plants or C4 (Hatch-Slack cycle) plants, and the ratio of each type plant gives a clue as to the composition of the prevalent vegetation at the time; (b) isotopic content of carbon from carbon dioxide can be measured by mass spectroscopy using pulverized fossil or carbonate samples. Analysis of carbonates precipitated in ancient soils and fossilized bone, some of which was originally hydroxyapatite ıCa10- (PO4)6-(OH)2!, highly porous and subject to diagenesis (secondary alteration of the original chemical composition). More than 25 species of mammals from several Orders should be available, including Edentata (e.g., mylodont sloths, Megalonyx, armadillos, glyptodonts), Proboscidea such as Haplomastodon, Perissodactyla (Tapirus), Artiodactyla, and Notoungulata. %%% Results should yield insight into the question of whether the present Amazon rainforests were formerly covered at least to some extent by grasslands. These findings may in turn reveal to what extent global changes have been directional or cyclic over extended time periods.