Reading Orbital Signals Distorted by Sedimentation: Models and Examples
- P. L. de Boer2 and
- D. G. Smith3
Published Online: 29 APR 2009
Copyright © 1994 The International Association of Sedimentologists
Orbital Forcing and Cyclic Sequences
How to Cite
Herbert, T. D. (1994) Reading Orbital Signals Distorted by Sedimentation: Models and Examples, in Orbital Forcing and Cyclic Sequences (eds P. L. de Boer and D. G. Smith), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304039.ch29
Utrecht, The Netherlands
- Published Online: 29 APR 2009
- Published Print: 28 JAN 1994
Print ISBN: 9780632037360
Online ISBN: 9781444304039
- reading orbital signals;
- concept-accumulation versus dilution cycles;
- Cretaceous/Tertiary (C/T) boundary;
- step functions;
- ‘local spatial nyquist’
A stratigraphic column never gives a precisely linear representation of fluxes of sediment components over time. Weight percent measures generally bear a non-linear relationship to accumulation rates, and variations in accumulation rates also distort the depth axis. Orbital climatic signals in sediments provide a good template for measuring such patterns of distortion. A simple two-component model, in which both components may be time-variant, is developed to study stratigraphic patterns of cyclic sedimentation. Four types of harmonic distortions are recognized: self modulation, cyclical modulation, linear modulation and step-function modulation. By properly sampling cyclic waveforms, it may be possible to solve for the dynamics of sedimentation at very high resolution.
The highest frequency distortions are generated by changes in sediment accumulation caused by the climatic effects of a particular orbital cycle. In the depth domain, these generate skewed distributions of the measured variables, and in the frequency domain they generate harmonics of the fundamental frequency. Modelling shows that it is possible to determine which component was more time-variant over the course of a climatic cycle by looking at the convexity/concavity of the waveform. The concept of ‘accumulation’ versus ‘dilution’ cycles in carbonate sediments is rigorously defined by critical ratios of model parameters.
Longer term changes in sedimentation rate modulate the stratigraphic frequency of orbital signals. A bewildering splitting of spectral peaks may occur even in the absence of ‘noise’. Models show that the highest frequency sedimentary cycles are preferentially distorted, in accord with geological observations. Distortions require that the usual definition of the Nyquist frequency, the minimum sampling rate, be greatly modified. One approach to measuring changes in sedimentation rates is moving-window spectral analysis, whereby only subsets of the time series are considered for each spectral estimation; another is through bedding thickness analysis.
Effects similar to those simulated are observed in real sedimentary records. Late Pleistocene carbonate cycles from the equatorial Pacific seem to result from out-of-phase variations in carbonate and non-carbonate accumulation. Cyclical trends in sedimentation rate may be caused by the climatic effects of low-frequency orbital cycles. Carbonate bedding cycles in the Albian of Italy show probable eccentricity modulation at periods of about 100 and 400 ka, and 100-ka carbonate cycles show modulation at a period of about 400 ka. Increases in sedimentation rate detected by moving-window spectral analysis correspond to increases in mean carbonate content, suggesting a simple relationship between the two variables. Nearly linear frequency modulations caused by changes in sedimentation rate can be seen in moving-window spectral analysis of physical property data of equatorial Pacific and Atlantic cores over the past 5 Ma. Step-function changes in sedimentation rate may be rare, but occur at the Cretaceous/Tertiary (K/T) boundary, where precessional carbonate cycles record an abrupt twofold decrease in sedimentation rate in the earliest Tertiary.