Sihailongwan Maar Lake, located in Northeast China, is a candidate Global boundary Stratotype Section and Point (GSSP) for demarcation of the Anthropocene. The lake’s varved sediments are formed by alternating allogenic atmospheric inputs and authigenic lake processes and store a record of environmental and human impacts at a continental-global scale. Varve counting and radiometric dating provided a precise annual-resolution sediment chronology for the site. Time series records of radioactive (239,240Pu, 129I and soot 14C), chemical (spheroidal carbonaceous particles, polycyclic aromatic hydrocarbons, soot, heavy metals, δ13C, etc), physical (magnetic susceptibility and grayscale) and biological (environmental DNA) indicators all show rapid changes in the mid-20th century, coincident with clear lithological changes of the sediments. Statistical analyses of these proxies show a tipping point in 1954 CE. 239,240Pu activities follow a typical unimodal globally-distributed profile, and are proposed as the primary marker for the Anthropocene. A rapid increase in 239,240Pu activities at 88 mm depth in core SHLW21-Fr-13 (1953 CE) is synchronous with rapid changes of other anthropogenic proxies and the Great Acceleration, marking the onset of the Anthropocene. The results indicate that Sihailongwan Maar Lake is an ideal site for the Anthropocene GSSP.

The concept of the “Anthropocene” was first proposed in the early 21st century and aroused widespread discussion in the public and geological community; however it still requires formal definition. The Global Boundary Stratotype Section and Point (GSSP) is a key tool in this process. Accurate chronology and high-resolution samples are the material basis for studying the Anthropocene. The varved sediment, deposited in Sihailongwan Maar Lake (42°17′N, 126°36′E) in northeast China, is a GSSP candidate site and a new location for studying the Anthropocene as a new stratigraphic unit. Anthropocene strata are the top few centimeters of sediment and characterized by small thickness, high water content, and lack of compaction and consolidation, which is physically different from older strata and requires special sampling and processing methods. There is currently a lack of standard and normative sampling and sample processing methods. Therefore, we propose a new paradigm specifically for the precise study of the top 20 cm of loose sediments. To avoid disturbing and disrupting the deposition sequence and structure of the sediments, freezing techniques and cryogenics were applied in coring and core processing. A freeze corer was used to collect the varved sediments in situ. To eliminate the effect of high water-content on X-ray fluorescence core scanning, core strips were freeze-dried and impregnated with resin prior to scanning. By cross-varve dating between parallel cores, we established a precise chronology with an annual resolution, and the age uncertainty was nearly eliminated. The top 20-cm long core represents approximately the last 210 years of deposition. At present, the early 1950s is recommended as the onset of the Anthropocene, the corresponding depth in 1950 CE was between 85-95 cm in our parallel cores. To obtain samples with high temporal resolution for proxy analyses, the cores were subsampled along the varve direction at 1 or 2 mm intervals with a scalpel knife in a -10 °C room, which represents an average of ~2 years. For loose Anthropocene sediments, freeze coring, freeze subsampling, and cross-dating are key for ensuring research accuracy.