ABSTRACT

Palaeomagnetic records from three cores of lake sediment have been merged, producing a new Holocene geomagnetic secular variation record for northern New Zealand. The cores are from Lake Pupuke, a maar lake in Auckland Volcanic Field (AVF). They contain tephra from Rangitoto Volcano (530 ± 10 yr BP), Taupō (1718 ± 10 yr BP), Tūhua/Mayor Island (7637 ± 100 yr BP) and Rotomā tephra from Ōkataina Volcanic Centre (9423 ± 120 yr BP). These tephra dates are supplemented by selected radiocarbon age estimates. The cores were correlated using tephra and fine-scale variations in magnetic susceptibility. The natural remanent magnetisation is strong, stable and carried by fine-grained titanomagnetite from AVF’s basaltic volcanoes. The resulting palaeosecular variation (PSV) record spans from 10,000 to 1500 yr BP. It shows well-defined, millennial-scale swings in direction between 10,000 and 8000 yr BP and between 4000 and 1500 yr BP, while between 8000 and 4000 yr BP variations are of higher frequency and lower amplitude and centre on the geocentric axial dipole field direction. This is in excellent agreement with published PSV records from Mavora Lakes, 1000 km further south, indicating the regional nature of PSV – resulting from broad-scale changes in the circulation of conducting iron-rich fluid of Earth’s outer core.

摘要

来自三个湖泊沉积物岩心的古地磁记录已合并，产生了新西兰北部全新世地磁长期变化记录。岩芯来自奥克兰火山场 (AVF) 的玛珥湖普普克湖。它们包含来自 Rangitoto 火山 (530 ± 10 yr BP)、Taupō (1718 ± 10 yr BP)、Tūhua/Mayor Island (7637 ± 100 yr BP) 和来自 Ōkataina 火山中心 (9423 ± 120 yr BP) 的火山灰。这些火山灰日期由选定的放射性碳年龄估计值补充。使用 tephra 和磁化率的精细尺度变化对核心进行关联。AVF 玄武岩火山中的细粒钛磁铁矿具有强大、稳定的天然剩磁。由此产生的古长期变异 (PSV) 记录跨越 10,000 到 1500 年 BP。它显示定义明确，10,000~8000 yr BP和4000~1500 yr BP之间的千年尺度波动方向，而8000~4000 yr BP之间的变化频率较高，幅度较低，并以地心轴向偶极子场方向为中心。这与向南 1000 公里的马沃拉湖公布的 PSV 记录非常吻合，表明 PSV 的区域性质——由地球外核导电富铁流体循环的大范围变化引起。