The past environment is often reconstructed by measuring a certain proxy (e.g. delta O-18) in an environmental archive, i.e. a biogenic or abiogenic accreting structure which gradually accumulates mass and records the current environment during this mass formation (e.g. corals, shells, trees, ice cores, speleothems, etc.). Proxy analysis usually yields a record along a distance axis. However, to relate the data to environmental variations, the date associated with each data point has to be known too. This transformation from distance to time is not straightforward to solve, since accretion mostly proceeds at a varying and unknown rate. To solve this problem some hypotheses about the growth rate or the time series must be made. Depending on the application, different assumptions may be appropriate, resulting in or requiring a particular method to perform this transformation. The actual method used can hugely influence the final result and hence the interpretation of the data in terms of frequency and timing of events. However, no comparative study has been made so far, and most of the existing methods haven't been thoroughly assessed. Therefore, this paper aims to evaluate and compare the most popular methods. To keep the review manageable the scope was limited to those records where it can be assumed that the time series is periodic. Examples of periods include tidal, seasonal and ENSO (El Nino Southern Oscillation) cycles, and even cycles of thousands of years could be considered, as long as they are resolved in the measured record. Six methods to reconstruct the time base for periodic proxy records are compared in this review. Their performance in the presence of stochastic and systematic errors is tested on simulations and linked to the methods' underlying assumptions. As a final comparison, all methods are applied to a real world example. The goal of this overview is to provide an objective structure and comparison of the methods mostly used, so that the users are aware of the underlying assumptions and their consequences. |
