CHARACTERISTICS OF AND CORRECTIONS FOR CORE SHORTENING IN UNCONSOLIDATED SEDIMENTS

Thinning, bypassing, and compaction of shallow unconsolidated sediment s during manual coring or vibracoring operations probably cause more s ediment deformation and greater stratigraphic displacement than is com monly reported in the wetland literature. We measured core shortening in open-barrel cores from fluvial wetlands, lagoonal flats, and marshe s to document the magnitude and characteristics of shortening where se diments may be stiff and require extra mechanical effort to recover a sufficient length of sample for analysis. Results of those measurement s indicate that thinning or non-recovery of discrete sediment interval s can range from 0 to 67 percent and cumulative core shortening can be as much as 30 percent even for cores less than one meter long. Detail ed open-barrel measurements also show that core shortening is not unif ormly distributed throughout the depth of penetration as is often assu med. Analytical data derived from shortened cores can only be properly interpreted if patterns of shortening are established and incorporate d into the analysis. Minor artificial displacement of sediment depths can alter plots of physico-chemical parameters and can significantly i nfluence calculated rates of sedimentation and other depth-dependent s tatistical relationships. This study (1) demonstrates how plots of int erval shortening and cumulative shortening can be used to characterize the distribution of shortening at depth and (2) presents a simple equ ation for stratigraphic restoration so that core observations and anal yses are corrected to their original depths.