Abstract Tephra (volcanic ash) deposits are important isochronous markers for correlating marine sediments or events recorded in marine sediment cores. However, the active tectonics that are commonly associated with volcanic activity at plate tectonicboundaries also drive large-scale deformation, leading to steep and variable local and regional bathymetry (e.g., ridges, basins and canyons systems). This complex bathymetry influences gravity-flow behaviour and paths, which can rework and redeposit tephras, resulting in stratigraphic complexities. Such as, the mis-identification of primary versus reworked tephra deposits, and in turn lead to the development of inaccurate chronostratigraphies. Here we present 36 tephra deposits from 21 shallow marine sediment cores that traverse the length of the southern and central margin of eastern North Island, New Zealand. Using major and trace element geochemical compositions for glass shards from the tephras, we correlate these deposits to three major rhyolitic eruptions from the Taupō Volcanic Zone (TVZ) approximately 200 km west, including; Taupō (1718 cal yrs. BP), Kaharoa (636 cal yrs. BP), and Kawakawa/Oruanui (KOT; 25.4 ka). Based on their morphology, depositional character and inferred emplacement mechanisms, the tephra deposits are grouped into four lithofacies types; (1) primary deposits, (2) volcaniclastic-rich turbidites, (3) blebs/pods of volcaniclastic-rich material, and (4) complex deposits. Primary deposits form syn-eruptively through airfall onto the ocean surface, settling over hours to days through the water column under diffuse vertical gravity currents. Volcaniclastic-rich turbidites are formed through secondary redeposition and entrainment by post-eruptive turbidity currents, while blebs/pods of material are interpreted to have formed by erosion and/or bioturbation. Complex deposits form through the interaction of all these mechanisms producing an overthickened array of primary and redeposited units within a single facies. Herein, we argue that redeposited units of volcaniclastic-rich turbidites or small blebs/pods can be used as tentative chronological markers if the geochemical composition of the glass shards have a homogeneous signature, i.e. a single eruptive source. Where the glass shards in redeposited units have mixed geochemical compositions, and are not stratigraphically associated with a primary deposit source, they cannot be used as chronological marker horizons. This emphasises the need for accurate and rigorous data reduction without overlooking the importance of data points that are statistical outliers. We also show that the highest preservation of tephra deposits is found in semi-confined isolated basin settings, including a wide range of deposit types. Due to erosive sediment flows that bypass through submarine distributary systems, these major sediment dispersal pathways preserve few volcaniclastic deposits. Our findings have important implications not only for identifying primary or redeposited characteristics in marine tephras for building accurate chronostratigraphies, but also as a guide geomorphic sub-environments with the best preservation of tephras in marine sedimentary systems.

中文翻译：

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活跃的 Hikurangi 俯冲边缘海洋沉积物中火山灰的沉积和保存

摘要 Tephra（火山灰）沉积物是关联海洋沉积物或海洋沉积物岩心中记录的事件的重要等时标记。然而，通常与板块构造边界处的火山活动相关的活动构造也会驱动大规模变形，导致陡峭和可变的局部和区域测深（例如，山脊、盆地和峡谷系统）。这种复杂的水深测量会影响重力流行为和路径，从而可以返工和重新沉积火山灰，导致地层复杂。例如，对原始火山灰沉积物与再加工火山灰沉积物的错误识别，进而导致不准确的年代地层学的发展。在这里，我们展示了来自 21 个浅海沉积岩芯的 36 个火山灰沉积物，这些沉积岩穿过北岛东部的南部和中部边缘的长度，新西兰。使用火山灰玻璃碎片的主要和微量元素地球化学成分，我们将这些沉积物与西约 200 公里的陶波火山带 (TVZ) 的三个主要流纹岩喷发相关联，包括：Taupō（1718 cal yrs. BP）、Kaharoa（636 cal yrs. BP）和 Kawakawa/Oruanui（KOT；25.4 ka）。根据其形态、沉积特征和推测的侵位机制，将火山灰矿床分为四种岩相类型；(1) 原生矿床，(2) 富含火山碎屑的浊积岩，(3) 富含火山碎屑物质的气泡/豆荚，以及 (4) 复杂的沉积物。初级沉积物通过空气降落到海洋表面协同喷发形成，在扩散的垂直重力流下通过水柱沉降数小时到数天。富含火山碎屑的浊积岩是通过二次再沉积和喷发后浊流夹带形成的，而物质的气泡/豆荚被解释为由侵蚀和/或生物扰动形成。通过所有这些机制的相互作用，形成了复杂的沉积物，在单一相内产生了过度增厚的原生和再沉积单元阵列。在此，我们认为，如果玻璃碎片的地球化学成分具有均一特征，即单一喷发源，则富含火山碎屑的浊积岩或小气泡/豆荚的再沉积单元可用作暂定年代标记。如果再沉积单元中的玻璃碎片具有混合的地球化学成分，并且在地层上与主要沉积源无关，则它们不能用作年代标记层位。这强调了在不忽视统计异常值数据点的重要性的情况下进行准确和严格的数据缩减的必要性。我们还表明，在半封闭的孤立盆地环境中发现了 tephra 沉积物的最高保存率，包括各种沉积物类型。由于侵蚀性沉积物流绕过海底分流系统，这些主要的沉积物扩散路径保留了很少的火山碎屑沉积物。我们的研究结果不仅对确定海洋火山灰的原始或再沉积特征以构建准确的年代地层具有重要意义，而且还可以作为指导地貌亚环境，在海洋沉积系统中最好地保存火山灰。