Fig 1.
a) Location of the studied site (MEXI-CHA16-1A) at Lake Chalco. b) Location of Mexico City in Central Mexico. c) Location of Lake Chalco relative to the Sierra Nevada and Sierra Chichinautzin mountain ranges in Central Mexico.
Fig 2.
γ-ray emission line spectra of potassium, thorium and uranium.
The relative intensities of isotopic peaks of potassium (40K) and the equilibrium decay series of uranium (238U and 235U and their daughters) and thorium (232Th and its daughters) are plotted. γ-ray emission of a tephra layer (blue line) at the depth of 95 m and a non-tephra layer (pink line) as a lake sediment at the depth of 88 m are illustrated for comparison.
Fig 3.
γ-ray signals pre- and post-tephra removal.
The distribution of the γ-ray signal measured in borehole MEXI-CHA16-1A of Lake Chalco before (solid line) and after tephra removal (dashed line) from the dataset. The asymmetry of γ-ray distribution is measured as skewness (sk).
Fig 4.
Energy spectrum of the borehole MEXI-CHA16-1A.
Depth variation (ordinate) of the energy spectrum of γ-ray photons (abscissa) across the 300 m lacustrine sediment of Lake Chalco. The color bar indicates the level of detected energy. γ-ray signal is additionally illustrated across the depth. Please note that the results of this study focus only on the upper 180 m of Lake Chalco. The results for depths between 180 and 300 m are documented in the S4 File.
Table 1.
Selected tephra layers above 180 m, used in the training dataset.
Table 2.
Selected non-tephra layers above 180 m, used in the training dataset.
Fig 5.
Tephra index distribution for known and non-tephra samples.
Boxplot showing the distribution of the tephra index (TI) for the known tephra and non-tephra samples. The cut-off was made at the lower quartile of the tephra distribution (TI = 0.47). The accuracy of this model for the known sample sets is 85%.
Fig 6.
Depth variation of γ-ray photon energy.
The contour plot shows the depth variation (ordinate) of the energy spectrum of γ-ray photons (abscissa) between 90 m and 105 m in the borehole MEXI-CHA16-1A. The color bar indicates the level of detected energy. Tephra Index (TI) and γ-ray signal are additionally illustrated across the depth. Two tephra layers with the real thickness of 3 cm and 15 cm predicted by the TI respectively at the depth of 92.6 m and 95.3 m of the spectral γ-ray borehole log.
Fig 7.
Tephra layer depth: Core samples vs predicted from γ-ray spectrum.
Depth distribution of tephra layers from core samples [19] compared to tephra layers predicted from spectral γ-ray log. Panels include: (a) total identified tephra layers (black lines) and gap horizons (green lines) across the upper 180 m of Lake Chalco’s deposits from core sampling (gaps refer to those horizons not recovered during coring and therefore represent regions with no sediment record [19], (b), (c) and (d) distributions of the defined tephra layers filtered based by respective thicknesses of less than 1 cm (< 1 cm), between 1 cm and 10 cm () and greater than 10 cm (> 10 cm), (e) the detected tephra layers based on the calculated tephra index, and (f) the γ-ray signal. An arrow in panel a shows a gap horizon at 68 m depth of the core, and an arrow in panel e at 67.36 m indicates the assumed equivalent TI.
Fig 8.
The cyclical components associated with the γ-ray signal before and after tephra removal.
Wavelet analysis (panel “b” after and panel “d” before removal of the tephra layers) of the detrended γ-ray signal (panel “a” after and panel “c” before removal of tephra layers) was recorded across the lacustrine deposits of Lake Chalco. The color code in the wavelet analysis indicates high cyclicity (red) and low cyclicity (blue) for depth (abscissa) and different periodicity (ordinate). White shading highlights parts not fully covered by data.