P2C2: Late Holocene Changes in the SPCZ from Algal Lipid D/H in Lakes Across Oceania

热带水循环影响全球气候。在过去的30年里，它经历了显著的变化。在热带地区，降雨量增加了，而在干燥地区，降雨量减少了。这是伴随着在两个半球的热带地区向极地扩张，导致沙漠地区在大陆上扩张。虽然热带降雨在许多气候模型中表现不佳，但简单的物理论据和广泛的模型表明，在全球变暖情景下，这些趋势可能会继续下去。目前尚不清楚热带降雨量的变化是否超出了工业化前气候的自然变化范围。这是因为主要来自卫星的广阔热带海洋的降雨数据仅限于过去35年。如此短暂的天气记录并不足以确定最近的降雨量变化是否异常。通过将热带南太平洋的降雨量记录从目前的35年延长到2000年，该项目将产生必要的数据，以评估过去35年的降雨量变化是否异常，如果异常，是否可归因于大气中温室气体的积累。它们还将提供必要的数据，以确定哪些气候模式能够准确地再现热带降雨模式，从而哪些模式可能提供最可靠的预报。该项目将对整个南太平洋辐合带（SPCZ）过去2,000年的降雨量进行定量重建。将在现代SPCZ区域8个岛屿的沉积物岩心中测量（3#946;，4#945;，5#945;，22 E）-4，23-二甲基麦角甾-22-烯-3-醇（dinosterol）的氢同位素比（2 H/1H，或D/H）。甲藻甾醇几乎完全由甲藻微藻的某些属合成。由于热带地区降雨中D/H比值与降雨率呈反比关系，即所谓的“数量效应”，我们发现整个SPCZ地区21个表层沉积物中的甾醇（#948;Ddino）的D/H比值与降雨率高度相关，关系式为#948;Ddino=-16.6（±2.45）*P-180（±15.3）[R2=0.71，N=21]，其中P是以mm/天为单位的降雨率。误差的蒙特卡罗分析表明，降雨率估计与0.3毫米/天的不确定性是可能的，使用这种技术。已经收集了来自所罗门群岛、瓦努阿图、沃利斯富图纳群岛和斐济（代表5个研究区域中的4个）的沉积物岩心。作为该项目的一部分，将收集萨摩亚和美属萨摩亚湖泊的新岩心。初步结果表明，在从1,000年前的中世纪暖期过渡到600年前的小冰河期期间，位于SPCZ西侧的所罗门群岛的降雨量每天增加2.6毫米，位于SPCZ东侧的沃利斯和富图纳群岛的降雨量每天减少1.1毫米。

Non-Technical AbstractThe tropical water cycle influences the global climate. Over the last 30 years it has undergone remarkable changes. Rainfall has increased in the regions of the tropics that normally receive lots of rain, and decreased in the drier regions. This was accompanied by a poleward expansion of the tropics in both hemispheres that has led to an expansion of desert regions over continents. Although tropical rainfall is poorly represented in many climate models, simple physical arguments and a wide range of models suggest these trends are likely to continue under global warming scenarios. It is unknown at this point whether the current changes in tropical rainfall are outside the range of natural variations of the pre-industrial climate. This is because rainfall data over the vast tropical oceans, which is derived primarily from satellites, is limited to the last 35 years. Such a brief weather record is insufficient to establish whether the recent changes in rainfall are anomalous. By extending the record of rainfall in the tropical South Pacific from the current 35 years to 2000 years, this project will produce the data necessary to assess whether rainfall changes over the last 35 years are unusual, and if so, whether they can be attributed to the buildup of greenhouse gases in the atmosphere. They will also provide the data necessary to determine which climate models are able to accurately reproduce tropical rainfall patterns, and therefore which models are likely to provide the most reliable forecasts.Technical AbstractIn order to place recent changes in rainfall over the tropical South Pacific Ocean into a longer-term context, this project will produce quantitative reconstructions of rainfall throughout the South Pacific Convergence Zone (SPCZ) that span the last 2,000 years. The hydrogen isotope ratio (2H/1H, or D/H) of (3β,4α,5α,22E)-4,23- dimethylergost-22-en-3-ol, dinosterol, will be measured in sediment cores from 8 islands across the modern SPCZ region. Dinosterol is synthesized almost exclusively by certain genera of dinoflagellate microalgae. Owing to the inverse relationship between D/H ratios in rain and rainfall rates in the tropics, the so-called "amount effect," we find that D/H ratios of dinosterol (δDdino) in 21 surface sediments throughout the SPCZ region are highly correlated with rainfall rates according to the relationship δDdino=-16.6(±2.45)*P-180(±15.3) [R2=0.71, N=21], where P is the rainfall rate in mm/day. A Monte Carlo analysis of errors indicates that rainfall rate estimates with 0.3 mm/day uncertainty are possible using this technique. Sediment cores from the Solomon Islands, Vanuatu, Wallis & Futuna, and Fiji, representing 4 of the 5 study regions have already been collected. New cores from lakes in Samoa and American Samoa will be collected as part of this project. Initial results indicate a 2.6 mm/day increase in rainfall in the Solomon Islands, on the western side of the SPCZ, and a 1.1 mm/day decrease in Wallis and Futuna, on the eastern side of the SPCZ, during the transition from the Medieval Warm Period 1,000 years ago to the Little Ice Age 600 years ago.