Magnetic properties of maar lake sediments from Otago, New Zealand

Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits at the base of the cored sedimentary sequence yielded two new 40Ar/39Ar dates of 24.51±0.24 Ma and 23.38±0.24 Ma (2sigma). The younger date agrees within error with a previously published 40Ar/39Ar date of 23.17±0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have overlapped with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi-1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo-single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic concentration and lamina thickness indicate a decrease in erosional input and increase in diatom productivity throughout the depositional period, suggesting a long-term (tens of thousands of years) climatic change in New Zealand in the latest Oligocene/earliest Miocene.

Foulden Maar是新西兰南岛一处高分辨率玛珥湖沉积序列，其主体为纹层状硅藻土，其间穿插大量硅藻浊积岩。在取心沉积序列底部的碎屑流沉积中发现的玄武岩砾石，获得了两组新的氩-40/氩-39（40Ar/39Ar）测年数据：24.51±0.24 Ma与23.38±0.24 Ma（2σ置信区间）。较年轻的测年结果与此前发表的、采自该玛珥火山口相邻玄武岩岩墙的23.17±0.19 Ma氩-40/氩-39测年结果在误差范围内一致。研究推断，这套硅藻土沉积形成于渐新世最晚期至中新世最早期，沉积时长约数万年，且可能与南极地区被称为Mi-1事件的快速冰川作用及后续冰消期重合。沉积物磁学性质与扫描电子显微镜（SEM）测试结果显示，该沉积的磁信号主要由假单畴磁黄铁矿主导。碎屑态磁黄铁矿最可能的物源，是形成该玛珥湖的射气岩浆喷发所形成的推测火山碎屑环中的片岩围岩碎屑。磁浓度与纹层厚度的变化特征表明，在整个沉积期内，侵蚀物质输入量逐渐降低，硅藻生产力逐步提升，这反映出新西兰地区在渐新世最晚期至中新世最早期发生了持续数万年的长期气候变化。