Understanding the evolution of lower latitude climate from the most recent glacial periods to post-glacial warmth in the continental tropical regions has been obstructed by a lack of continuous geological records. Here we present results from a lacustrine record from tropical North America. Specifically, we examine sediments from Lake Chalco, located in the Valley of Mexico, central Mexico (19°30’N, 99°W). The basin represents a hydrological closed system surrounded by the Trans-Mexican Volcanic Belt aging from the Oligocene to the present. We used borehole logging data to conduct a cyclostratigraphic analysis of the Lake Chalco sediments. More than 400 m were logged for several geophysical properties including magnetic susceptibility and spectral gamma radiation (SGR).
SGR is a particularly useful tool as it is non-destructive, fast, affordable, and applicable even in cased boreholes. Among the lake deposit of the Chalco sub-basin, 388 total tephra layers (≥1 mm in thickness) were reported from the core description. Tephra layers with specific gamma-ray signatures present a challenge for extracting the primary signals caused by climatic agents. We propose a protocol to identify tephra layers embedded in other sediments using high-resolution SGR.
After extracting the non-volcanic primary signal, we applied a suite of evolutive cyclostratigraphic methods to the Lake Chalco downhole logging data, with a focus on gamma-ray. The high-resolution gamma-ray results suggest that the Lake Chalco sediments contain several rhythmic cycles with a quasi-cyclic pattern comparable with Pleistocene climate evolution, allowing to calculate of a ~500-kyr time span for the Lake Chalco sediment deposition.