Print this pageRecovering Missing Paleoclimatic Signals from High-Resolution but Low-Definition Sedimentary Records
A pre-requisite for a high-resolution (1-100 k.y. or meter scale) paleoclimatic record is a complete sedimentary record. In many ancient coastal-to-shallow marine records composed of m-scale sedimentation cycles, the duration of deposition represented by rocks accounts for 10-70% of the cycle duration, whereas erosion and/or non-deposition for 90-30%. As a result, many recovered high-resolution climatic records of individual cycles only reflect climatic variability in a fraction of a cycle’s duration. The missing time also casts doubts on results of time-series analysis used to assess the periodicity of climate changes. Paleoclimatic conditions during deposition may be relatively easily interpreted from indicators recorded during deposition, but difficult from limited information generated during erosional/non-deposition. To decipher paleoenvironmental and paleoclimatic conditions during erosional/non-depositional phase of an m-scale sedimentary cycle, these researches are needed: 1) estimation of the duration of erosion/non-deposition of an incomplete cycle and the number of completely missing cycles. If Milankovitch climatically-driven cycles can be proven, the missing cycles/beats may be recovered by correlating hierarchical Milankovitch cycles with bundled sedimentary cycles. Missing cycles/cyclic intervals may also be patched up by neighboring complete cycles. 2) Identification of processes operating during erosion and non-deposition to recognize products/imprints of erosion and non-deposition, such as meteoric diagenesis, types of paleosols, and geochemical fingerprints of pedogenesis. The fingerprints may be used to infer paleoclimatic conditions during the period of erosion/non-deposition. Moreover, the magnitude and type of fingerprints are commonly time-dependent. Thus, inverse modeling of the processes may provide estimates of the time needed to reproduce observed fingerprints, which approximates the duration of erosion/non-deposition of an incomplete cycle. Understanding the mechanisms of these processes through modern analogs will provide key model parameters. Finally, common graphic mis-representation of temporal climatic variability in the thickness domain of m-scale sedimentary cycles should be avoided.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009