CosmoChron: A versatile age-depth modeling approach using cosmogenic nuclides and direct age constraints

Abstract

We introduce a novel age-depth modeling approach called CosmoChron that integrates both cosmogenic nuclide concentrations and other age constraints, such as radiocarbon and OSL ages, from different depths in a sedimentary sequence. Based on probabilistic inverse modeling, CosmoChron constrains the age-depth relationship of a sedimentary sequence along with associated uncertainties. Knowledge about the sample origins and the accumulation process is incorporated in the prior model. The ²⁶Al/¹⁰Be ratio is computed at different depths in the forward model by accounting for different pre-burial scenarios, radioactive decay and post-burial production of ²⁶Al-¹⁰Be, which is directly tied to the age-depth relation itself. Synthetic test cases demonstrate the method's ability to construct accurate age-depth relationships given by the posterior distribution, even for complex scenarios that include slow and varying accumulation rates, complex pre-burial histories, hiatuses, and unconformities. Based on observed unconformities, users have the option to manually input hiatuses into the model at specific depths, which allows estimation of their durations. Application of CosmoChron to real ²⁶Al/¹⁰Be data from the Laujunmiao section in China yields ages that are similar to those obtained with conventional burial dating methods for specific stratigraphic layers. However, the associated uncertainties are significantly reduced with CosmoChron (by ∼47 % on average) because it exploits the vertical coupling of data combined with knowledge of the relative age of the samples, which must become younger towards the top of the profile. Additionally, the age-depth model reflets the duration of three hiatuses inferred from unconformities observed in the field. When CosmoChron is applied to OSL-derived ages from Jingbian section A on the Chinese Loess Plateau, covering the last ∼140 ka, the method produces results that are almost identical to those obtained with the well-established Bacon age-depth modeling approach. CosmoChron consequently offers a new, versatile and reliable tool to construct age-depth models for Quaternary sediment sequences.