Phase lag in amplitude modulation of hemipelagic cycles: A new tool for recognition of relative sea level change

Many ancient rhythmic hemipelagic sequences have been interpreted to record orbital variations, but the exact nature of the climatic and depositional transfer functions re- sponsible 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.