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Phase-lagged amplitude modulation of hemipelagic cycles: A potential tool for recognition and analysis of sea-level change

Jií Laurin^*,1, Stephen R. Meyers², Bradley B. Sageman³ and Dave Waltham⁴

¹ Institute of Geophysics, Academy of Sciences of the Czech Republic, Boní II/1401, 141 31 Prague, Czech Republic
² Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA
³ Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208, USA
⁴ Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 OEX, UK

Many ancient rhythmic hemipelagic sequences have been interpreted to record orbital variations, but the exact nature of the climatic and depositional transfer functions responsible for this link remains poorly understood. Two-dimensional numerical simulations were used to explore selected aspects of orbital signal distortion in linked siliciclastic and hemipelagic systems. The models suggest that transfer of multiorder (e.g., 20, 100, and 400 k.y.) oscillations in relative sea level into the hemipelagic record produces an inherent amplitude distortion of the shorter-period (e.g., 20 k.y.) cycle. This distortion gives rise to amplitude modulation (AM), which is qualitatively similar to AM of orbitally driven changes in insolation (e.g., eccentricity modulation of precession-driven cycles). However, unlike the orbitally driven AM, synthesized AM is distinctly phase shifted relative to the stratigraphic record of the long-period (e.g., 100 k.y., 400 k.y.) cycle as a result of sea-level–driven changes in the storage capacity of nearshore through alluvial parts of the source siliciclastic system. Hence, multiorder changes in sea level can leave a distinct AM signature in dilution-affected hemipelagic records, thus making hemipelagic rhythms due to eccentricity-forced sea-level changes distinguishable from other types of orbitally driven hemipelagic cyclicity.

Key Words: amplitude distortion • hemipelagic sedimentation • sea level • cycles • modeling • spectral analysis

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