湖光岩玛珥湖人类世沉积物统一年代标尺的建立

Establishment of unified chronological framework for the Anthropocene sediments in Huguangyan Maar Lake

关键词:  玛珥湖  人类世  年代标尺  多指标研究  磁化率

Key words:  Maar Lake  Anthropocene  chronological framework  multi-proxy research  magnetic susceptibility

摘要:

建立人类世关键标志物和指标体系，需要在全球不同地质生物载体中开展多指标综合研究。为满足多种指标测试，往往需要采集多根平行岩芯。因此，如何经济有效地统一不同岩芯的年代标尺成为一个重要问题。文章以我国人类世研究的重要地点之一广东湖光岩玛珥湖获取的6根平行岩芯为例，对其中1根标准岩芯开展²¹⁰Pb-¹³⁷Cs年代学测试，基于质量深度插值得到岩芯上部0—52 cm的非线性年代模型 （沉积于1870—2020年），同时对所获得的6根岩芯进行经济、快捷且无损的磁化率指标测试，以磁化率曲线中的共性变化特征点为年代控制点，结合质量深度插值或者以文章提出的更为简便的定点缩放法， 统一了其余岩芯过去约150年的年代框架。定点缩放法可理解为，纵向拉伸或压缩标准岩芯，使其年代控制点与目标岩芯的年代控制点重合，无需质量深度数据即可使目标岩芯继承标准岩芯的非线性年代模式。该方法充分考虑了人类世沉积薄、年代精度要求高的特点，比传统几何深度线性插值的方式更适用于人类世沉积物的年代学研究。

Abstract:

Background, aim and scope At present, the primary marker and tracing proxy system of the Anthropocene epoch are under hot debate. Comprehensive multi-proxy research in diverse geological and biological archives is therefore urgently needed. Because the Anthropocene sediments are typically thin, single-core surface sediments rarely yield enough material for all analyses. Multiple cores are usually collected so that different proxies can be measured on separate cores or on homogenised slices of equivalent age. However, unifying the chronologies of multiple cores in an economical and efficient manner remains challenging. In this study, taking Huguangyan Maar Lake (one of the key study sites for the Anthropocene) as an example, we present a protocol that uses ²¹⁰Pb-¹³⁷Cs dating supplemented by cross-comparisons of magnetic susceptibility to solve this problem. Materials and methods We collected six sediment cores from Huguangyan Maar Lake. One core (HGY20-1-3) was selected as the reference and dated by ²¹⁰Pb-¹³⁷Cs.The high-resolution magnetic-susceptibility was measured non-destructively on all cores, and these continuous records were compared to identify readily identifiable, isochronous tie-points that lock the chronologies of the remaining cores to the reference record. Results The ¹³⁷Cs peak at 29.4 cm depth in HGY20-1-3 is assigned to AD 1964. Based on mass depth interpolation, the resulting nonlinear age-depth model indicates deposition from AD 1870 to 2020 for the upper 52 cm, with a mean geometric deposition rate of 0.27 cm/a before AD 1964, consistent with previous studies. The magnetic susceptibility sequences in all six cores vary in parallel within a range from 300×10−8 m3 /kg to 1200×10−8 m3 /kg and show a sharp excursion around the depth of 30 cm. Discussion We choose the depth of 30.4 cm (deposited in AD 1960.4) in HGY20-1-3 as an isochronous tie-point, correlated to the depth of 31.3 cm, 25.3 cm, 34.4 cm, 29.2 cm, and 29.9 cm in HGY20-1-7, HGY20-1-9, HGY20-1-18, HGY20-1-28 and HGY20-1-33. Previous studies often build linear age models directly from geometric depth and several tie-points, but for the thin Anthropocene sediments, which may have very limited identifiable isochronous tie-points, such models would diverge markedly from true ages. We therefore recommend building age models on mass depth; where mass depth is unavailable, we apply a “fixed-point scaling” method, i.e., stretching/compressing the target core until its dated tie-points coincide with those of the reference core (here HGY20-1-3), yielding an adjusted depth scale for final interpolation. The target core can then inherit the non-linear age model of the reference core. Based on the fixed-point scaling method, we unified the age models of all the Huguangyan sediment cores in this study. The results show that the age model of HGY20-1-7 agrees closely with that established by mass depth, thus validating the fixed-point scaling method. Because the 137Cs chronology constrains the upper 52 cm of standard core with high confidence over the past 150 years, the resulting age models for the remaining cores are likewise considered reliable for this interval. The age-magnetic susceptibility sequences of the Huguangyan show that there was a sharp decrease around the 1960s and afterwards the magnetic susceptibility kept stable. We found this variation appears to have been largely caused by human activities. On one hand, the vegetation restoration activities during the period reduced soil erosion and therefore less magnetic susceptibility came into the lake. On the other hand, agriculture and industry activities increased organic matter supply in the lake, which may dilute the magnetic mineral concentration and result in the decrease of the magnetic susceptibility. Conclusions Collecting multiple cores and harmonizing their chronologies before composite sampling remains a bottleneck in highresolution Anthropocene work. Traditional correlation of distinct proxy inflections followed by linear geometricdepth interpolation suffices when multiple tie-points exist, but collapses for thin Anthropocene sequences where very limited tie-points can be recognized. We therefore advocate building mass-depth age models, or where mass depth is unavailable, employing a newly proposed “fixed-point scaling” method that stretches the target core vertically until its dated inflection snaps to that of the reference core. Applied to Huguangyan Maar Lake, one standard core anchored to the AD 1964 137Cs peak and the basin-wide post-AD 1960 magnetic-susceptibility drop synchronized six parallel records, yielding a common 150-yr age model. Moreover, the basin-wide magneticsusceptibility plunge of the 1960s mirrors reduced erosion and rising nutrients, encoding regional human impacts and furnishing available geological evidence for Anthropocene research. Recommendations and perspectives This study provides an important chronological framework for the subsequent multi-proxy analysis in Huguangyan Maar Lake. As magnetic susceptibility testing is economical, fast, and non-destructive, we propose that age dating supplemented by cross-comparisons of magnetic susceptibility is a good way to unify chronological frameworks of different sediment cores, which could be widely applied in multi-proxy research in the Anthropocene.