Improved spectral estimates of climate oscillations in the Quaternary and Neogene
Creators: Borowiecki, Ryan
Publisher: name: University of Alberta Library
Year: 2023

<p>Centennial, millennial and multi-millennial scale climate cycles have been observed in paleoclimate proxies distributed globally. Examples of these cyclical climate cycles include oscillations with periods of ~2.3, ~1.5, ~1, and ≤0.5 thousand years (kyr) in the Quaternary. In the Neogene, longer record length and decreased resolution limit observations to longer climate oscillations driven by variations in orbital eccentricity (~400 kyr and ~100 kyr), obliquity (~40 kyr) and precession (~20 kyr). These oscillations have been widely observed in high-resolution marine and continental deposits. Understanding forced climate oscillations related to solar fluctuations and orbital cyclicity is critical in understanding the warming trends currently observed in modern climate records. <br>
For the Holocene, countless studies have related cycles with periods of 2.3, 1 and ≤0.5 kyr to fluctuations in solar intensity received at Earth&#39;s surface. For other oscillations such as the 1.5 kyr cycle observed in Northern Hemisphere proxies, there is still some degree of ambiguity on the forcing mechanism behind the cyclical warming/cooling. This is primarily due to difficulty distinguishing cycles from one another; distinguishing this 1.5 kyr cycle from surrounding cycles and determining the onset and cessation of this cycle is crucial to the evaluation of the mechanism driving this cycle. Commonly methods for paleoclimate spectral analysis, such as the Continuous Wavelet Transform and Short-Time Fourier Transform, cannot clearly reveal the onset or cessation of sporadic climatic cycles if neighboring spectral peaks are present within an iii octave due to spectral smearing and leakage. Adapting spectral analysis methods such as the Synchrosqueezing Transform (SST), which is commonly used in other disciplines such as speech analysis to paleoclimate proxies, has numerous advantages for Quaternary and Neogene climate research. Chiefly, the improved time-frequency localization can provide more clarity on the onset and cessation as well as the exact period of oscillations. In addition, when the SST is inverted, it can provide a more precise reconstruction of individual climate oscillations, which allows a direct comparison of individual cycles to the proposed forcing mechanism for that cycle. A few real data examples demonstrate the advantages of the SST method. <br>
In the Miocene/Pliocene, the climate oscillations that can be sufficiently resolved are limited to orbital fluctuations; these fluctuations are commonly observed in marine deposits, but less commonly in continental settings. The chief difficulty in observing these cycles in continental deposits is a relatively slow sediment accumulation rate, thus requiring samples collected at a high resolution to observe. High resolution samples collected from a red clay deposit in Northern China coupled with spectral analysis show that these orbital fluctuations have a tremendous impact on continental climate. Orbital obliquity is observed for the first time in the red clay deposits, and it has a particularly strong impact on the sedimentation processes taking place in aeolian settings like the deposit sampled. <br>
Observation of centennial and millennial scale cycles is not just limited to the Holocene, the same solar fluctuations driving Holocene climate change can be observed in late Pleistocene records as well. Several proxies with sufficient resolution of the shorter millennial and centennial scale oscillations extrapolate the solar influence on iv climate ahead of the Holocene. Spectral analysis on high resolution pollen concentration records from Lake Kotokel in southern Siberia, Russia, during the Last Glacial Maximum (LGM) reveals many of the same oscillations observed in the Holocene. Demonstrating that climate fluctuations in the LGM and Holocene are similar in the time-frequency domain suggests that linkages between climate proxies and solar activity at the centennial time scale in the Holocene can be extended to the LGM.</p>