铁、锰、锌等微量元素是地球表面生物地球化学过程中不可或缺的组成部分。作为微量营养素，它们对各种生物的生长和地球的碳循环起着至关重要的作用。在覆盖了地球陆地表面10%的冰盖之下，这些物质的流动量比先前假设的要大。格陵兰岛和南极洲的新数据表明了这一点，这些数据是由波茨坦GFZ德国地球科学研究中心和佛罗里达州立大学（美国）的Jon Hawkings领导的国际研究小组收集和分析的。它们对冰、融水和岩石边界的未知过程提供了重要的见解。由于冰团受到全球变暖的显著影响，对气候变化对关键生物地球化学过程（包括海洋、湖泊和湿地等周边生态系统）的影响正在出现新的观点。这项研究今天发表在PNAS杂志上。
Under the Earth's ice sheets melt water forms an extensive hidden wetland of rivulets, rivers and lakes. During the last forty years, over 400 subglacial lakes have been discovered in Antarctica alone; some as large as the Great Lakes of North America. At the boundary between ice, water and rock, a complex ensemble of chemical, physical and microbiological forces is at work, breaking up and grinding rock and releasing trace elements into the meltwater which is carried downstream. These chemical elements occur only in very low concentrations, hence the name. Nevertheless they are -- like vitamins -- essential as nutrients for all living things.
How and in which quantities trace elements are released under the Greenland and Antarctic ice sheets and eventually flow into the adjacent ecosystems, and what role they play in these ecosystems and the global carbon cycle at large, has not yet been studied in detail. This is because measurement campaigns in these remote regions of the world are an enormous logistical and technical challenge.
Elaborate sampling
In order to collect samples from the waters under the Greenland and Antarctic ice sheets and analyse them in the laboratory, Jon Hawkings from GFZ collaborated with an international and interdisciplinary research team. Colleagues Mark Skidmore and John Priscu from Montana State University (USA) led a project to drill more than 1000 metres into the Antarctic ice sheet as part of their SALSA project. This enabled them to tap into the nine-kilometre long and 15-metre deep Mercer Subglacial Lake. "There's a science reason for looking at that specific lake, but then there is the context of these lakes being part of this greater hydrological system," Mark Skidmore said. "So, we want to see what's being generated beneath the ice sheet and how that connects to the coastal environments."
Jon Hawkings himself and colleagues under the lead of Jemma Wadham of the University of Bristol (UK) took samples from sub-ice waters emerging from Leverett Glacier in Greenland over a three-month period in the summer melt season.
The samples were analysed in ultra-clean laboratories to avoid contamination. The researchers filtered the meltwater samples to multiple levels to sort the sample concentrations by size, as many of these trace elements can exist as extremely small nanoparticulate minerals. They determined their chemical composition using particularly sensitive mass-spectrometry methods.
Surprisingly high concentration of iron & Co.
Hawkings and his colleagues discovered that significant amounts of trace elements are released in the melt waters below the ice masses. They found these melt water concentrations can exceed those in rivers and the open ocean by many times. For example, the value for dissolved iron in the Antarctic subglacial lake was more than 1000 micrograms per litre and not around five, as would be expected in dilute ice melt.
"For a long time it was assumed that in the icy regions of the earth trace elements are present in such miniscule quantities that they are of little importance for global elemental cycles. On the contrary, our results show that ice sheets may play a key role in regional mobilization of these elements. The impacts of this need to be further monitored and analysed in the context of climate change. We have now laid a baseline for this," says Jon Hawkings.
Insights into weathering processes under the ice
Furthermore, the concentrations of the individual elements as well as their ratios and the size distribution between dissolved and nanoparticulate mineral forms tell the researchers something about the source material, the sub-ice sheet weathering processes and the paths taken by the water before sampling. For example, it is known that the element vanadium occurs primarily in silicate rock minerals rather than carbonate rock minerals. Elevated concentrations found in this study indicate that higher rates of silicate mineral weathering are occurring under ice sheets than previously thought. Importantly, silicate mineral weathering is a sink for carbon dioxide. Iron, on the other hand, is known to oxidise in an oxygen-rich environment, resulting in precipitated "rust." Large quantities of dissolved iron therefore indicate that some of the water may originate from a region with little oxygen. The researchers also found trace elements like aluminium, iron and titanium occurred in higher concentrations in Antarctica than in Greenland. They therefore hypothesise that the meltwater in the southern polar region has much longer residence times under the ice sheet and greater hydrological isolation than in the northern polar region.
Consequences for ideas on iron fertilization
The new findings are particularly relevant for our understanding of nutrient cycling in the Southern Ocean. There the water is considered to be rich in nutrients like nitrogen and phosphorus but depleted in iron. For this reason, phytoplankton, the plants of the ocean, the base of the global food pyramid and an important CO2 sink, do not grow to their maximum potential. This "iron limitation" has been the subject of previous geoengineering projects to sequester carbon dioxide from the atmosphere by seeding the ocean with iron. The results of Hawkings and his colleagues are consistent with observations of higher quantities of iron and phytoplankton in the immediate vicinity of the Antarctic Ice Sheet. Their results suggest that the ice sheet may naturally fertilize the coastal regions of the Southern Ocean by providing a supply of iron for phytoplankton. To what degree and how this might change in the future with climatic warming remain open questions for further research.
On the trail of life's limits
Hawkings and his collaborators investigated 17 different trace elements. "Each of these tells us its own story and we work like detectives, trying to make a coherent overall narrative out of all the data," says the geoscientist. "We are interested in exploring the limits of life on Earth in terms of the availability of energy and nutrients, and this helps tell us part of that story. We are only just beginning to understand the importance of these large ice masses in this context. Hopefully our research also helps in starting to answer many important outstanding scientific questions, which include the influence of climate change: How will these biogeochemical cycles change if more ice melts? Will this release more and more trace elements or will these processes be slowed down? In addition, it is still open what happens to the substances on their way into the oceans and how much ultimately reaches marine organisms."
Collaborator and SALSA project lead John Priscu points out the importance of interdisciplinary work for scientific discoveries: "This paper intersects many disciplines and shows the power of international collaboration. Results in this manuscript have transformed our view of how polar ice sheets influence the Earth System."