A processing paradigm for loose varved sediments in Anthropocene strata research: An example from the Sihailongwan maar Lake

Abstract

Abstract 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.