Test of an equal taxon–weight modification of Middeldorp's pollen density dating on data from varved sediments of Lake Gościąż, Poland

Abstract

The cores from the varved sediments of Lake Gościąż enable testing of methods, based on pollen concentration, to reconstruct the time-basis of a palynological data set. The sediments' varves or annual laminae were counted and their thicknesses measured, and the pollen content was recorded at known intervals with regard to depth and time. In the present study, we have treated this pollen record as if the sediments were not annually laminated and as if, consequently, the time-basis of the pollen record was unknown. We reconstructed several different time-bases by means of two methods using pollen concentration, namely pollen density dating (PDD) according to Middeldorp and a modification of that method, which gives all included arboreal pollen-taxa the same weight: equal taxon–weight modification (ETW). After reconstructing these time-bases we compared them with the varve time-basis. We conclude that both PDD and ETW result in good time-bases. Our modification improves the performance of Middeldorp's method, largely because of the use of spline interpolation instead of a polynomial regression of varve age on pollen variables. An advantage of ETW over PDD is that ETW using a spline produces a good approximation of arboreal influx, preserving both trends and short-term variability, whereas PDD results in an arboreal influx being an unnaturally smooth function of time.