The role of dispersed ash in orbital-scale time-series studies of explosive arc volcanism: insights from IODP Hole U1437B, Northwest Pacific Ocean

Tephra fallout beds in marine sediments provide chronologically precise and highly resolved records of volcanism at time scales relevant to Quaternary climate cycles. While the record of discrete (visible) thin tephra beds is readily accessible, the significance of the dispersed (invisible) tephra record remains unclear. Here we evaluate the role of dispersed tephra for orbital-scale volcanic time variations in the Quaternary (<1.2 Ma) carbonate mud of IODP Hole U1437B (Northwest Pacific). The carbonate mud contains cyclic series of discrete fallout tephra beds from the oceanic Izu Bonin (~85% of tephra beds) and the continental Japan (~15%) volcanic arcs, respectively. Our results show the inorganic aluminosilicate (lithogenic) fraction is a mixture of dispersed Izu Bonin and Japan ash, and Asian dust. The time distribution of the Izu Bonin ash with its distinct composition appears to confirm and enhance the cyclic time variation of the discrete ash beds at Hole U1437B. Dispersed Japan ash resembles Asian dust in trace elements and is only distinguishable in Sr-Nd isotope space. Collectively, our results confirm the existence of periodic, orbital-scale fluctuations of arc volcanic frequency. Orbital-scale time variations of marine ash may be best established by series of discrete marine ash beds, yet the concomitant dispersed ash flux must also be recorded in order to understand the total flux of arc volcanic ash into the ocean basins and thus the role of the volcanism-climate link.

海洋沉积物中的火山灰沉降层，可提供与第四纪气候旋回时间尺度相匹配的、年代学精准且分辨率极高的火山活动记录。尽管离散（可见）薄层火山灰沉积的记录易于获取，但分散（不可见）火山灰记录的研究意义仍不明朗。本研究针对西北太平洋海域综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）U1437B钻孔的第四纪（<1.2 Ma）碳酸盐泥质沉积物，探讨分散型火山灰在轨道尺度火山活动时间变化中的作用。该碳酸盐泥质沉积物中，分别赋存来自伊豆-小笠原洋弧（约占总火山灰床的85%）与日本大陆弧（约15%）的周期性系列离散火山灰沉降层。研究结果显示，无机铝硅酸盐（岩石成因）组分是分散型伊豆-小笠原火山灰、日本火山灰与亚洲风尘的混合物。成分特征显著的伊豆-小笠原火山灰的时间分布，似乎验证并强化了U1437B钻孔离散火山灰床的周期性时间变化规律。分散型日本火山灰的微量元素组成与亚洲风尘相似，仅可通过锶-钕同位素组成加以区分。综合来看，本研究结果证实了弧火山活动频率存在周期性的轨道尺度波动。尽管通过一系列离散海洋火山灰床，或可最佳地确立海洋火山灰的轨道尺度时间变化规律，但要全面理解弧火山灰向海盆的总输入通量，进而阐明火山活动与气候之间的关联机制，同步记录伴随产生的分散型火山灰通量亦是必不可少的。