Understanding lower temperature natural systems using stable isotopes

地球的空气、水、生物群、岩石和沉积物主要由氧、氢、碳和氮组成。每种元素都以两到三种稳定的同位素形式存在，例如氧-18、氧-17和氧-16。它们在低温矿物和动植物组织中的丰度是由环境条件决定的。我和我的研究小组将根据过去10万年代用记录中保存的气候和生态条件的同位素特征来“回到未来”。我们将在这些记录中寻找可以作为未来风向标的模式。北美有两个地区特别令人感兴趣，大湖地区和育空地区失去的白令更新世生态系统。现代五大湖地区在季节、年代际和更长的时间尺度上对气团的移动很敏感。淡水中含有这些变化的同位素记录。大约20年来，我们收集了该地区的这些数据。这个记录的长度现在可以用来测试气候趋势，也可以作为古代条件的基线。现代植物也保留了生态系统的同位素和分子记录，我们将使用这些记录来解释古代植被的数据。利用氧-17，我们还将量化大气硝酸盐对该地区氮沉积威胁的贡献。安大略早期石油繁荣带来的基岩井泄漏也对饮用水含水层构成威胁。我们将定义特定基岩盐水的同位素和化学指纹，查明污染源，从而帮助经济有效地封堵泄漏。再回到更早的时候，劳伦泰德冰盖消退时冰川融水的爆发可能通过改变海洋环流模式引发了全球变冷。通往大西洋的一条通道是穿过五大湖。我们将利用五大湖沉积物中包含的贝壳动物群和有机生物标志物（正构烷烃）来追踪约15,000年前的融水运动。这些代用物还记录了五大湖流域冰期后的生态系统变化。我们特别感兴趣的是气候因素的同位素信号与其他过程的对比，这些过程导致一些五大湖的水位大幅下降，甚至水力关闭。巨型动物组织（骨骼、牙齿）也是环境变化的极好同位素代用品，记录了气候和饲料（植被、猎物）的季节性变化。我们将利用这些数据来研究北美更新世巨型动物灭绝的可能原因（气候、过度狩猎）。通过对猛犸象、乳齿象和巨海狸的分析，我们将了解更新世末期大湖地区陆地生态系统的特征。育空的白令陆桥也有一个迷人的故事。它的物种多样性（如猛犸象、马、野牛、骆驼、狮子、短面熊）远远高于现代北极。为什么它的冻土带草原如此多产，尽管气候寒冷干燥？猛犸象体内异常的氮-15富集是一个特殊的谜团。这一简单的观察可能表明，白令陆桥的氮循环与今天大不相同。为了验证这一想法，我们将在白令陆桥黄土土壤的白令陆桥气候条件下种植白令陆桥草，分析从永久冻土中提取的约9万至2.5万年前的地松鼠筑巢中的古白令陆桥植被，并将同位素结果（包括单个氨基酸）与该地区的现代植被和土壤进行比较。简而言之，我们将确定在最后一个冰期结束时，两种不同的景观是如何对气候变化做出反应的。为什么有些哺乳动物适应了环境而另一些却灭绝了，这对于预测现代生态系统对环境变化的反应具有重要意义。

Earth’s air, water, biota, rocks and sediments are dominated by oxygen, hydrogen, carbon and nitrogen. Each element occurs in two or three stable forms called isotopes, for example oxygen-18, oxygen-17 and oxygen-16. Their abundances in low-temperature minerals, and in plant and animal tissues, are determined by environmental conditions. My research team and I will go “Back to the Future” based on the isotopic signatures of climatic and ecological conditions preserved in such proxy records over the last 100,000 years. We will search for patterns in these records that can serve as bellwethers for the future. Two areas in North America are of special interest, the Great Lakes region and the lost Beringian Pleistocene ecosystem in the Yukon. The modern Great Lakes region is sensitive to shifting air masses on seasonal, decadal and longer time-scales. Freshwater contains an isotopic record of these variations. For ~20 years, we have collected these data for the region. This record’s length will now allow testing for climatic trends, and can also serve a baseline for ancient conditions. Modern plants also retain ecosystem isotopic and molecular records, which we will use to interpret data for ancient vegetation. Using oxygen-17, we will also quantify the contribution of atmospheric nitrate to the threat posed by nitrogen deposition in the area. Leakage from bedrock wells from Ontario’s early petroleum boom also is a threat to drinking-water aquifers. We will define the isotopic and chemical fingerprints of specific bedrock brines, pinpoint contamination sources, and thus aid cost-effective plugging of the leaks. Further back in time, glacial meltwater outbursts from the retreating Laurentide Ice Sheet may have triggered global cooling by changing oceanic circulation patterns. One pathway to the Atlantic Ocean was through the Great Lakes. We will use shelly fauna and organic biomarkers (n-alkanes) contained in Great Lake sediments to track meltwater movement since ~15,000 years ago. These proxies also record post-glacial ecosystem changes in Great Lakes watersheds. We are especially interested in isotopic signals of climatic factors versus other processes that caused substantial drops in water level – and even hydraulic closure – of some Great Lakes. Megafauna tissues (bone, tooth) are also superb isotopic proxies for environmental change, recording seasonal variations in climate and forage (vegetation, prey). We will use such data to examine possible causes (climate, over-hunting) of Pleistocene megafauna extinctions in North America. By analyzing mammoth, mastodon and giant beaver, we will learn about the terrestrial ecosystem characteristics of the Great Lakes region at the end of the Pleistocene. Yukon’s Beringia also has a fascinating story to tell. Its species diversity (e.g., mammoth, horse, bison, camel, lion, short-faced bear) is much higher than the modern Arctic. Why were its tundra-steppe grasslands so productive, despite the cold and dry climate? Anomalous nitrogen-15 enrichment in mammoths is a special puzzle. This simple observation may signal that the nitrogen cycle in Beringia was quite different from today. To test this idea, we will grow Beringian grasses under Beringian climatic conditions in Beringian loess soils, analyze ancient Beringian vegetation from ~90,000 to 25,000 year-old ground squirrel nests extracted from permafrost, and compare the isotopic results – including individual amino acids – with the region’s modern vegetation and soil. In short, we will determine how two different landscapes responded to climate change at the end of the last glacial period. Why some mammals adapted while others became extinct holds important lessons for anticipating modern ecosystem responses to environmental change.