| 04 Aug 2025
Insights into Lake Baikal Radiocarbon Age Offsets from a Database of AMS Radiocarbon Age Estimates
Abstract. Radiocarbon dates are an essential tool for dating non-varved lake sediments, however their interpretation is hindered by issues such as reservoir age or contamination which culminate in age estimates that can be thousands of years younger or older than the true depositional age of that sediment (we call this an age offset). Often, precise estimators of the radiocarbon age offset are not available, as in the case of Lake Baikal. Linear regression of uncalibrated radiocarbon dates has been used to estimate the age offset, with answers ranging from 0 to 1500 14C yr BP. These have been interpreted to suggest that different regions of Lake Baikal have different age offsets, although some dispute this. Other estimators have returned estimates of approximately 2000 14C yr BP. Despite this, most previous studies have not included any estimates of uncertainties for these age offsets in their proxy analysis, or have included uncertainty of, at most, ± 90 14C yr. Here, we present a complete database of published AMS radiocarbon dates from Lake Baikal sediment cores up to 2023 and, using this, review the use of linear regression on uncalibrated radiocarbon ages as a method for estimating age offsets from the sediments of Lake Baikal. We apply a standardised linear regression age offset method to all cores in our database to better quantify the age offset of Total Organic Carbon (TOC) in the lake’s sediments. We conclude that there is no statistically significant evidence from linear regression methods for a large difference in age offset in different regions of Lake Baikal. Our results return a lake-wide age offset estimate of TOC of 1.56 ± 0.75 14C kyr BP, suggesting previous studies in Lake Baikal have significantly underestimated the temporal uncertainty of radiocarbon ages. Finally, our results are a caution that linear regression-based age offset estimates in lake sediments have a large uncertainty that might only be observable with multiple datasets.
Lake Baikal is one of the most extensively studied lakes in the world. The compilation of published radiocarbon ages from numerous lake sediment cores represents a valuable Earth System Science dataset, and this article provides important context to support it. The dataset will serve future researchers interested in a variety of environmental changes recorded by Lake Baikal sediments. Although this audience may be relatively small, the authors could broaden the significance and novelty of their work by offering a model for reporting radiocarbon ages from sedimentary “total organic carbon” (TOC).
There are different “flavors” of TOC (also called, bulk sediment) depending on how the sediment is pretreated prior to 14C measurement. The choice of procedure can significantly influence the resulting 14C age. Documenting which pretreatment method was used for each sample is necessary not only to compare results across laboratories, as is done in this article, but also to better understand the sources of old carbon underlying the age offsets and thereby to address the authors’ motivation for compiling the dataset– for “insights into radiocarbon age offsets” as stated in the article's title.
The article would be strengthened by extending the context beyond age modeling to recognize that age offsets have been extensively studied in the broader context of watershed carbon cycling in both lake and marine depositional settings. This literature includes reports of downcore changes in age offsets, which reflect shifts in carbon cycling and directly affect the use of downcore trends to estimate long-term TOC age offsets and associated uncertainties, the focus of the data analysis in this article.
For results from different laboratories to be compared with confidence, the specific fraction of organic carbon analyzed should be the same. At first glance, the offset ages appear higher for samples measured in the Nagoya lab than in those from Woods Hole. If so, this difference may reflect pretreatment methods: the former retains all organic carbon (in-capsule acidification), while the latter removes acid-soluble organic carbon as well as organic carbon adsorbed to fine particles (acid washing through a 1 µm filter). While I am not aware of any published studies that quantify the difference in the outcome of these two procedures for 14C, I hypothesize that acid washing and filtration preferentially remove a younger fraction of 14C. I suggest that the authors use their dataset to test the possibility that the age offset depends on how the sediment is pretreated. This could contribute to the goal of using the dataset for "insights into radiocarbon age offsets."
Furthermore, the surface sediment ages reported by Fedotov et al. (2023) are conspicuously old. Although I could not easily determine the protocols used by the Golden Valley AMS lab, I wonder whether the sediments were treated with an alkaline solution to isolate the residual humin fraction, which could explain the anomalously old surface ages. I also note that the dataset lists the “dated material” for these samples as “bulk silty clay” rather than “TOC," which highlights the need for a more precise documentation of what was actually analyzed.
A number of radiocarbon ages in the dataset are cited as “personal communication.” The associated pretreatment procedures are therefore not available. Moreover, including unpublished data seems inconsistent with the authors’ statements that the dataset is derived from published sources. If this article is the first publication to report these ages, then say so; if not, cite the publication.
A full accounting of what constitutes “TOC” should include:
a. in-capsule, which retains acid-soluble organic carbon (fumigation or liquid), or
b. by rinsing, which removes acid-soluble organic carbon.
A dataset designed to provide “insights into radiocarbon age offsets” would also benefit from including, where available, measurements of both carbon and nitrogen abundance in the analyzed material. Alongside δ13C, which is already included, these data can help estimate the proportion of terrestrial versus aquatic carbon sources. Moreover, carbon concentration (C%) is typically inversely related to offset age and can be used to estimate how age offsets vary downcore.
In my view, the text requires extensive revision to adopt a style more appropriate for a journal audience. As it stands, much of the article reads like a first draft. It could be shortened by at least one quarter, perhaps one third, by eliminating repetition, trimming unnecessary words and phrases, using more precise wording, and moving details of specific corrected errors into the dataset’s “comment” field.
Finally, the article would benefit from a concluding figure that clearly illustrates its main findings. Figure 4 provides a useful overview of all ages, but a new figure focusing on the 19 accepted core chronologies is needed. I suggest plotting ages by depth for each of the six regions in Table 5, using distinct symbols for different cores from Buguldeika Saddle and Academician Ridge.
In summary, the manuscript presents a valuable compilation of radiocarbon ages from Lake Baikal sediments. However, the manuscript requires substantial revision before it I would recommend it for publication. In particular, greater attention is needed to (1) clearly documenting what constitutes “TOC” across laboratories by specifying the pretreatment method used for each sample, (2) engaging more fully with the literature on age offsets, including those that specifically investigate temporal changes in age offsets, and (3) improving clarity and conciseness throughout the text.
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