Holocene Land-Ocean-Atmosphere Interactions on the Eastern Seaboard of North America

北大西洋地区过去陆地对气候-海洋相互作用的响应是一个重要的研究重点，因为它们显示了受未来全球变暖影响的气候关键方面的变化，如冰盖体积和海洋环流，如何转化为具有社会经济重要性的现象，包括大气水平衡和土壤水分可用性。尽管我们开始了解北大西洋环流在过去一万年中变化的性质和幅度，但就时间、幅度和空间格局而言，陆地对这些事件的反应仍然知之甚少。必须纠正这一缺陷，以便产生和检验解释变化过程的假设，了解海洋和陆地气候变化之间关系的强度，并使未来的土壤条件和水资源能够使用计算机模型进行预测。该项目将调查位于北美东海岸横断面上的四个地点，从纽芬兰北部到缅因州。该地区对过去冰盖物质平衡和海洋环流的变化高度敏感。被选中的地点都是由雨养的泥炭沼泽，它们提供了极好的气候档案。从这些沼泽中提取的年代准确的岩心中保存的动植物化石将用于重建过去8500年来沼泽表面湿度（地表水平衡指数）的变化，时间分辨率为10-100年。同样的岩心样本将被用于分析氧和氢的稳定同位素。这些元素的重同位素在降水中的时空分布与气温有关，因此与大气环流有关。在苔藓纤维素组分中捕获的同位素信号与植物用于纤维素合成的降水密切相关。因此，从隆起的泥炭中发现的泥炭化石保存了过去气候和大气环流变化的清晰信号。北美东部沿海地区的现代降水主要来自大西洋。沿着海岸向南流动的拉布拉多洋流的北极水域和离岸更远的墨西哥湾流的温暖水域之间存在着强烈的温度反差。过去沿海地区降水的同位素比率将受到这些洋流和墨西哥湾流热输送变化的强烈影响。通过结合氧和氢同位素记录，我们将估计氘过剩，这是蒸发发生时海面普遍条件的一个指标，包括海冰的范围。稳定同位素数据将与格陵兰岛和加拿大冰盖的冰芯同位素测量值进行比较，并更谨慎地与沉积物分析得出的表层海水和湖泊的估计同位素值进行比较。因此，对同一岩心水平的化石和稳定同位素的分析将使我们能够重建陆地响应的时间、幅度和空间格局，并探索不同原因（如融水排放、大气和海洋环流变化以及太阳变率）对研究区气候的影响。通过这种方式，我们将深入了解过去8500年来驱动气候变化的机制。需要验证的假设：1)8500 ~ 6800年前，研究区域的气候受到北美冰原不断向北释放的冰川融水的强烈影响，导致气候变冷，沼泽湿度增加。2) 6800年前冰盖消失后，冰岛北部海冰范围、洋流和深水地层的周期性变化间接影响了研究区气候。

Past terrestrial responses to climate-ocean interactions in the North Atlantic region are a critical research priority because they show how changes in key aspects of climate that will be affected by future global warming, such as ice sheet volume & ocean circulation, may be translated into phenomena of socio-economic importance, including the atmospheric water balance & soil moisture availability. Although we are beginning to understand the nature & magnitude of changes in the circulation of the North Atlantic over the last 10,000 years, terrestrial responses to these events are still poorly understood in terms of timing, magnitude & spatial pattern. It is imperative to rectify this deficiency in order to generate & test hypotheses to explain the processes of change, to understand the strength of relationships between oceanic & terrestrial climate change, & to enable future soil conditions & water resources to be predicted using computer models. Plan of work This project will investigate four sites located on a transect along the eastern seaboard of North America, from northern Newfoundland to Maine. This region was highly sensitive to past changes in ice sheet mass balance & ocean circulation. The selected sites are all raised (rain-fed) peat bogs, which provide superb climate archives. Plant & animal (testate amoebae) fossils preserved in well-dated cores extracted from these bogs will be used to reconstruct past changes in Bog Surface Wetness (an index of surface water balance) over the last 8500 years, at a time resolution of 10-100 years. The same core samples will be analysed for stable isotopes of oxygen & hydrogen. The spatial & temporal distribution of the heavy isotopes of these elements in precipitation is related to air temperature, & hence to atmospheric circulation. The isotope signal captured in the cellulose fraction of Sphagnum moss closely tracks that of the precipitation used by the plant for cellulose synthesis. Hence, fossil Sphagnum from raised peat preserves a clear signal of past changes in climate & atmospheric circulation. Modern precipitation along the eastern seaboard of North America is derived mainly from the Atlantic Ocean. A strong temperature contrast exists between the Arctic waters of the Labrador Current, which flows southwards along this coast, & the warm waters of the Gulf Stream further offshore. Past isotopic ratios in precipitation falling over coastal areas will have been strongly influenced by changes in these ocean currents & in the heat transport by the Gulf Stream. By combining the oxygen & hydrogen isotope records, we will estimate the deuterium excess, an index of the conditions prevailing at the sea surface when evaporation occurred, including the extent of sea ice. The stable isotope data will be compared with isotope measurements on ice cores from Greenland & Canadian ice caps, and more cautiously, with estimated isotope values for surface seawater & lakes that have been derived from sediment analyses. Hence, analyses of fossils & stable isotopes from the same core levels will allow us to reconstruct the timing, magnitude & spatial pattern of the terrestrial response, as well as exploring the impact of different causal factors such as meltwater discharges, changes in atmospheric & ocean circulation & solar variability on the climate of the study area. In this way, insight will be gained into the mechanisms that have driven climate change over the last 8500 years. Hypotheses to be tested: 1) Between 8500 & 6800 years ago, the climate of the study area was strongly influenced by repeated discharges of glacial meltwater from the decaying North American Ice Sheet to the north, resulting in cooling & increased bog wetness. 2) After the disappearance of the ice sheet, 6800 years ago, the climate of the study area was indirectly influenced by cyclical variations in sea ice extent, ocean currents & deepwater formation north of Iceland.