Shifts in genetic composition. Sediments less than 1200 years old (upper grey band) are genetically distinct from sediments over 1700 years old (lower grey band). This trend holds in comparisons limited to Ackerman and Tower Lakes, with samples spanning both time periods.
Anderson-Carpenter et al. BMC Evolutionary Biology 2011 11:30 doi:10.1186/1471-2148-11-30