USING LEAST-SQUARES SPECTRAL ANALYSIS TO TEST THE PROPOSED CORRELATION BETWEEN WHOLE-ROCK DELTA13C EXCURSIONS AND LITHOLOGIC CYCLES DURING THE LATE PALEOZOIC ICE AGE: ELY-BIRD SPRING BASIN, NEVADA, USA

This study tests whether patterns in carbonate-carbon isotope record correlate with glacioeustatic coarsening-upward lithologic cycles from three sections in the Pennsylvanian Ely-Bird Spring Basin (EBSB) of Nevada. During the Late Paleozoic Ice Age, the cyclic retreat and advance of glaciers at high latitudes were recorded in low-latitude shallow-marine basins as cyclic lithologic changes. Glacioeustatic cyclicity also was reflected by changes in marine geochemistry, including the δ¹³C isotope signal. The EBSB contains carbonates that were deposited on a shallow equatorial shelf. Basin strata are characterized by meter- to decameter-scale coarsening-upward lithologic cycles.

The three sampled sections are in the northern (Grindstone, n=243, 478 m), central (Illipah, n=227, 426 m), and southern (Mountain Springs, n=183, 331 m) EBSB. The sections have 27-35 lithologic cycles, with 4-10 analyses per cycle. In order to detect lithologic cycle-linked periodicity in δ¹³C values, we utilized a Lomb-Scargle periodogram, or least-squares spectral analysis. This method is qualitatively similar to Fourier analysis, and was selected as it is designed to analyze data sampled in unequal time intervals. Our hypothesis was that there was cyclicity in δ¹³C values with each coarsening upward sequence, with initial search parameters looking for cyclicity between 0.9 and 1.1 cycle lengths, with the expectation that a cyclicity near 1.0 would support a link between global glacioeustatic processes and low-latitude isotope values.

Of the three sections, Grindstone showed significant cyclicity at approximately 0.98 (0.7‰) and 1.09 (0.5‰) cycles, supporting δ¹³C linked to glacioeustatic coarsening-upward cycles. We then perform a new search over a broader period window of 0.5 to 5 cycles. Under these parameters, Mountain Springs contained a significant peak at a period of 0.527 cycles and Illipah contained significant peaks at 1.18 and 4.24 cycles. These peaks were confirmed by multiple resampling attempts and are strong evidence of cyclical patterns in δ¹³C occurring in the backdrop of a complicated system of global change. The differences in peak periods may be due to a combination of position within the basin, water circulation, local diagenetic alteration, misidentification of cycles, and sampling interval.