Lake Cheko, a roughly 300–500 m wide, ~50 m deep funnel-shaped lake about 8 km from the epicentre of the 1908 Tunguska explosion, was proposed in 2007 as the impact crater of a surviving fragment of the Tunguska Cosmic Body (Gasperini et al., Terra Nova 19:245–251). Collins et al. (2008, Terra Nova 20:165–168) countered that the lake lacks a crater's rim, ejecta and meteoritic material, that pre-1908 trees line its rim, and that a crater-forming fragment would need an implausibly high tensile strength; the contesting literature on both sides agrees that an accurate formation age of the lake is the decisive test, since a basin demonstrably predating 30 June 1908 cannot be a Tunguska crater. Two decades of dating attempts have produced directly conflicting results: radiocaesium/lead-210 and pollen stratigraphy of central cores (Gasperini et al. 2009) and tree-ring patterns (Fantucci et al. 2014) were read as consistent with a 1908 formation, while varve-supported radionuclide chronologies (Rogozin et al. 2017) and regional morphology comparisons (Rogozin et al. 2023, 2024) indicate a basin at least several centuries old, and trajectory modelling places the lake outside the most probable fragment strewn field (Carbognani et al. 2024). The unresolved problem is to determine the lake basin's formation age from the complete sedimentary fill — no published core has penetrated the full ~10 m pile to the anomalous central reflector — and to establish whether any impactor material or impact melt exists in or around the lake, thereby settling whether Lake Cheko is or is not the Tunguska crater.

Background

On 30 June 1908 an explosion over the Podkamennaya Tunguska region of central Siberia flattened more than 2000 km² of taiga. The body — called the Tunguska Cosmic Body (TCB), most probably a stony asteroid 50–80 m across — is thought to have been destroyed in an atmospheric airburst, i.e. an explosion of the body before it reached the ground, at an altitude of roughly 5–10 km, releasing energy of order 10–15 megatons. No macroscopic fragment of the body has ever been recovered, and the ground epicentre (the point below the explosion) was reconstructed from the radial pattern of flattened trees. Because the site lies in permafrost terrain, non-impact explanations for bowl-shaped depressions there have long been available: thermokarst lakes form when ice-rich ground thaws and subsides, and the region contains many such basins.

Lake Cheko is a small (~300–500 m across), unusually deep (~50 m) elliptical lake on the alluvial plain of the Kimku river, ~8 km downrange of the inferred epicentre. Gasperini et al. (2007) proposed that it fills a secondary impact crater made by a TCB fragment that survived the airburst and struck swampy, permafrost-ground: its funnel-like bathymetry and an acoustic stratigraphy of ~1 m of finely laminated lacustrine mud over coarser 'chaotic' deposits were read as crater fill. Collins et al. (2008) replied with four counter-arguments: (1) the lake's planform ellipticity (~4/3) would require an impact angle below ~10° to the ground, inconsistent with the reconstructed trajectory, since impact craters become visibly non-circular only at such oblique angles; (2) fresh craters of this size class (e.g. Odessa) show metres of raised rim and a proximal ejecta blanket — excavated debris around the rim — which Cheko lacks, while aerial photographs from 1938 and 1999 show mature trees standing on the rim that predate 1908, which a crater-forming explosion would have destroyed; (3) excavating a ~300 m crater requires a ~10–20 m fragment arriving at several km/s, demanding an intrinsic tensile strength of 10–40 MPa — one to two orders of magnitude above the strengths inferred for stony meteoroids during atmospheric break-up; and (4) known young sub-300 m terrestrial craters (Morasko, Kaalijärvi, Wabar, Henbury, Odessa, Boxhole, Macha) occur in strewn fields — clusters of craters from near-simultaneous fragment impacts — not as isolated structures, and no companion craters or meteoritic material exist at Cheko. Gasperini et al. (2008) defended the hypothesis, and the exchange has continued ever since.

Both sides treat the lake's formation age as the crux, because the outcome is binary in a decisive age range: if the basin demonstrably predates 1908 it cannot be a Tunguska crater. The dating toolkit: 210^{210}Pb (radionuclide with 22.3-year half-life, deposited continuously from the atmosphere) and 137^{137}Cs (a bomb-test fallout isotope peaked in 1963, with a Chernobyl 1986 marker) together date the last ~150 years — exactly the contested interval; varves are annually laminated sediment couplets that can be counted; radiocarbon (14^{14}C) dating reaches older material but aquatic organic carbon can carry an 'old-carbon' reservoir bias and reworked detrital carbon; dendrochronology (tree-ring analysis) records growth disturbance or release after 1908; pollen of aquatic plants marks the existence of a lake body in the sediment column. Applied to Cheko these methods have produced irreconciled contradictions. Gasperini et al. (2009) dated the transition from chaotic to laminated central-basin deposits at approximately 1908 by 210^{210}Pb/137^{137}Cs and found aquatic-plant pollen only above that horizon, concluding the lake formed at the time of the event. Fantucci et al. (2014) found that lakeshore trees germinated after 1908 grew differently from forest trees, 'consistent with' a 1908 formation. Rogozin et al. (2017), using 137^{137}Cs, 210^{210}Pb and varve counts on their own cores, concluded the lake is significantly older than 1908; Rogozin et al. (2023, 2024) added that two nearby funnel lakes of comparable shape and size hold sediments several thousand years old — so the funnel form is not diagnostic of impact — and estimated Cheko's age at no less than ~300 years. Gasperini et al. (2012) had identified a P-wave-velocity and magnetic anomaly ~10 m beneath the lake centre, compatible with a buried stony object, which they maintain supports the impact origin (contested in a 2023 Comment exchange in Doklady Earth Sciences); Carbognani et al. (2024) computed the TCB strewn field — the area where surviving fragments would land — and found Lake Cheko ~3.5 km outside its 3-sigma boundary. A decisive test therefore requires recovering and dating the complete sedimentary fill of the lake down to the acoustic basement and the anomalous reflector — something no published core has achieved — which would jointly settle the age of the fill and the presence or absence of meteoritic material; a demonstrably pre-1908 age of the basin and its sediments would by itself refute the crater hypothesis.

Problem Statement

Determine the formation age of the Lake Cheko basin, with sufficient stratigraphic coverage and chronological control to decide whether the basin could have been created by a fragment of the Tunguska Cosmic Body on 30 June 1908, and establish whether any impactor material exists in or around the lake. Concretely: (i) determine the age of the complete sedimentary fill of the lake — covering the full lacustrine sequence (about 10 m at the lake centre per seismic reflection data), the transition into the underlying deposits, and the anomalous central reflector that has been proposed to be a buried impactor — with an internally consistent chronology that reconciles or supersedes the conflicting published age models of Gasperini et al. (2009) and Rogozin et al. (2017); (ii) state whether the lacustrine sedimentary record and the basin itself demonstrably predate 1908 — which would refute the impact-crater hypothesis — or whether the basin formed at the time of the event, with explicit treatment of reworking, slumping and old-carbon reservoir effects; and (iii) establish whether any meteoritic material or impact melt is present in the fill, the anomalous reflector and the surrounding terrain. These components jointly serve one objective — settling the origin of Lake Cheko — but they are not conjunctively mandatory in every case: a demonstrably pre-1908 age of the basin and its sedimentary record refutes the crater hypothesis on its own, as practiced for Lake Cheko by Rogozin et al. (2017) and for the parallel Suzdalevo Lake case by Kavková et al. (2022), while recovery of a buried impactor at the lake centre would confirm it. If the basin predates 1908, identifying a non-impact formation mechanism is not a condition for settling the crater hypothesis.

References

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