Feldspar size measurements of tephra layers from ODP Leg 165 sites@en

Crystal size measurements have been carried out on tephra fall layers of Miocene to recent age from Sites 998, 999, and 1000 in the western Caribbean Sea. Maximum crystal size is used as a proxy for the grain size characteristics of the layers and an index of atmospheric dispersal from source eruptions. Crystal sizes range from 50 to 650 µm with the majority falling between 200 and 300 µm. All three sites exhibit a coarsening in the grain size of tephra layers with increasing age to the early Miocene that broadly correlates with an increase in the frequency of layers. Analysis of the present lower and upper level atmospheric circulation in the western Caribbean suggests that the layers were derived from source eruptions to the west of the sites somewhere in the Central American region. Minimum distances to these sources are of the order of 700 km. Crystal sizes in tephra layers at these distances are consistent with their derivation from energetic pyroclastic flow-forming eruptions that injected tephra to stratospheric levels by large-scale co-ignimbrite and plinian-style plumes. Coarsening of the layers during the Miocene peak of explosive volcanism cannot be attributed to any major change in paleowind intensity and is taken to represent the occurrence of more energetic eruptions that were able to disperse tephra over larger areas.本数据集针对加勒比海西部998、999与1000号钻井位点的中新世至近代火山碎屑降落层（tephra fall layers）开展了晶体尺寸测量工作。研究以最大晶体尺寸作为该类地层的粒度特征代用指标，同时作为火山源喷发引发的大气扩散效应的量化表征参数。本次测量的晶体尺寸分布区间为50至650微米，其中绝大多数样本的晶体尺寸集中于200至300微米区间。三个钻井位点的火山碎屑层粒度均随年代回溯至中新世早期呈现粗化趋势，且该粒度粗化趋势与地层出现频率的整体升高呈显著相关性。对加勒比海西部现今低层与高层大气环流的分析表明，上述火山碎屑层的物源喷发点位于采样位点西侧的中美洲区域内。该类物源与采样位点的最小距离约为700公里量级。在此距离条件下，火山碎屑层内的晶体尺寸特征，与形成火山碎屑流的高能喷发事件所产生的火山碎屑物特征一致——这类高能喷发通过大规模火成碎屑柱与普林尼式喷发柱（plinian-style plumes），将火山碎屑物质注入至平流层高度。中新世火山爆发活动峰值期内的地层粒度粗化现象，无法归因于古风速的显著变化，该现象被认为代表了更具能量的喷发事件的发生，这类喷发可将火山碎屑物质扩散至更大范围区域。