Lake Zapovednoe's Molten Fragments of Tunguska Airburst in 1908

The Tunguska airburst occurred on June 30, 1908 and it was most likely caused by the impact of the Tunguska cosmic body (TCB). It is not clear what the origin of the TCB was as no impact craters or possible body remains have been found to date. We studied the possible molten fragments of the TCB found in lacustine sediments of Zapovednoe Lake, a water body which is located ~60 km west from the airburst epicentre. Lake sediment cores which were retrieved from the lake contained an event layer dated to 1908–1910 CE. This layer included microscopical molten fragments and anomalous composition.

 

Three short cores (ZP1, ZP2, ZP3) were extracted in the central part of Zapovednoe Lake using a Kajak gravity corer. We used an X-ray fluorescence spectroscopy (XRF) and Scanning Electron Microscopy (SEM) for lake sediment characterization. Magnetic spherules (MSPs) and other magnetic grains were extracted from ZP1 by standard magnetic separation technique and all MSPs were identified with SEM and characterized using elemental microanalysis. We performed XRF analyses of 2 or 5 mm thick slices of the sediment cores and evaluated the concentrations and ratios of individual elements. Sediment samples of ZP2 were used for core dating, using gamma spectrometry for the specific activity of ²¹⁰Pb, ¹³⁷Cs, and ²²⁶Ra isotopes similar to the record from nearby Suzdalevo Lake.

 

Radioisotope activities revealed the age consistent with the year 1908 CE. The gamma spectrometry results were in good agreement with the XRF measurements, where the event layer had increased concentrations of lithogenic elements, such as Mg, Al, Si, S, K, Ca, Ti, Fe, Cu and Mn. The SEM analysis revealed that molten fragments were indeed found among the potential MSPs extracted from the event layer and adjacent layers. These spherical melts were rich in iron and most of them were found at depth corresponding to the event layer. Only a small portion of MSPs was found in the adjacent layers.

 

Our results revealed the presence of the TCB airburst event layer. The anomalous event layer resulted from increased erosion in the Zapovednoe Lake catchment. However, massive tree falls and subsequent wildfires from the airburst likely contributed to the anomalous elemental composition of the lake sediment as well. We found for the first time in lake sediments preserved MSPs which come from the melts produced by the TCB airburst and may contain an extraterrestrial material.