Hutchinson Scott J, Hamilton Paul B, Patterson R Timothy, Galloway Jennifer M, Nasser Nawaf A, Spence Christopher, Falck Hendrik
Ottawa-Carleton Geoscience Center and Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada.
Research Division, Canadian Museum of Nature, Ottawa, Ontario, Canada.
PeerJ. 2019 Jan 25;7:e6269. doi: 10.7717/peerj.6269. eCollection 2019.
A <5 mm thick volcanic ashfall layer associated with the White River Ash (east lobe [WRAe]) originating from the eruption of Mount Churchill, Alaska (833-850 CE; 1,117-1,100 cal BP) was observed in two freeze cores obtained from Pocket Lake (62.5090°N, -114.3719°W), a small subarctic lake located within the city limits of Yellowknife, Northwest Territories, Canada. Here we analyze changes in diatom assemblages to assess impact of tephra deposition on the aquatic biota of a subarctic lake. In a well-dated core constrained by 8 radiocarbon dates, diatom counts were carried out at 1-mm intervals through an interval spanning 1 cm above and below the tephra layer with each 1 mm sub-sample represented about 2 years of deposition. Non-metric Multidimensional Scaling (NMDS) and Stratigraphically Constrained Incremental Sum of Squares (CONISS) analyses were carried out and three distinct diatom assemblages were identified throughout the interval. The lowermost "Pre-WRAe Assemblage (Pre-WRAeA)" was indicative of slightly acidic and eutrophic lacustrine conditions. Winter deposition of the tephra layer drove a subsequent diatom flora shift to the "WRAe Assemblage (WRAeA)" the following spring. The WRAeA contained elevated abundances of taxa associated with oligotrophic, nutrient depleted and slightly more alkaline lake waters. These changes were only apparent in samples within the WRAe containing interval indicating that they were short lived and only sustained for a single year of deposition. Immediately above the WRAe horizon, a third, "Post-WRAe Assemblage (Post-WRAeA)" was observed. This assemblage was initially similar to that of the Pre-WRAeA but gradually became more distinct upwards, likely due to climatic patterns independent of the WRAe event. These results suggest that lacustrine environments are sensitive to perturbations such as deposition of ash fall, but that ecological communities in subarctic systems can also have high resilience and can recover rapidly. If subsampling of the freeze cores was carried out at a more standard resolution (0.5-1 cm) these subtle diatom ecological responses to perturbation associated with the WRAe depositional event would not have been observed. This research illustrates the importance of high-resolution subsampling when studying the environmental impact of geologically "near instantaneous" events such as episodic deposition of ashfalls.
在从加拿大西北地区耶洛奈夫市范围内的一个小型亚北极湖泊——袖珍湖（北纬62.5090°，西经114.3719°）获取的两个冻芯中，观测到了一层厚度小于5毫米的火山灰层，它与源自阿拉斯加丘吉尔山喷发（公元833 - 850年；距今1117 - 1100年）的白河火山灰（东叶 [WRAe]）有关。在此，我们分析硅藻组合的变化，以评估火山灰沉积对亚北极湖泊水生生物群的影响。在一个由8个放射性碳年代测定数据约束的年代测定良好的岩芯中，在火山灰层上下各1厘米的间隔内，以1毫米的间隔进行硅藻计数，每个1毫米的子样本代表约2年的沉积。进行了非度量多维标度分析（NMDS）和地层约束增量平方和分析（CONISS），并在整个间隔内识别出三种不同的硅藻组合。最底层的“WRAe前组合（Pre - WRAeA）”表明湖泊环境呈微酸性且富营养。火山灰层的冬季沉积导致次年春季硅藻群落随后转变为“WRAe组合（WRAeA）”。WRAeA中与贫营养、营养耗尽且碱性略强的湖水相关的分类群丰度升高。这些变化仅在包含WRAe的区间内的样本中明显，表明它们持续时间短，仅持续了一年的沉积。就在WRAe层位之上，观测到了第三种“WRAe后组合（Post - WRAeA）”。这个组合最初与Pre - WRAeA相似，但向上逐渐变得更加不同，这可能是由于与WRAe事件无关的气候模式所致。这些结果表明，湖泊环境对诸如火山灰沉积等扰动很敏感，但亚北极系统中的生态群落也具有高恢复力，能够迅速恢复。如果以更标准的分辨率（0.5 - 1厘米）对冻芯进行子采样，就不会观察到这些与WRAe沉积事件相关的硅藻对扰动的微妙生态响应。这项研究说明了在研究诸如火山灰间歇性沉积等地质上“近乎瞬间”事件的环境影响时，高分辨率子采样的重要性。
