Food for thought: Mathematical approaches for the conversion of high-resolution sclerochronological oxygen isotope records into sub-annually resolved time series
Abstract Oxygen isotope (δ 18 O) records of incrementally banded marine calcifiers provide high-resolution information about modern and past environmental conditions. However, given their biological origin, these records are often distorted by ontogenetic and seasonal growth pattern. To evaluate their reliability, many studies correlate skeletal δ 18 O records with instrumentally measured time series of environmental conditions using a variety of methods to recalculate their temporal framework. Lacking consistency and a limited traceability of many temporal alignment methods underline the need for standardized and flexible method for the temporal alignment of sclerochronological records. Here, two methods for the temporal alignment of sclerochronological data and their application on measured oxygen isotope signatures of a fish otolith and a marine bivalve shell are presented and compared. The first method is a flexible approach using a linear interpolation between carefully selected temporal reference points for recalculating ontogenetic growth effects with an optional extension accounting for seasonal growth oscillations. The second approach uses the seasonally-oscillating Von Bertalanffy growth function parameterized based on the oxygen isotope record to reconstruct the underlying growth model and reconstruct thereby its temporal framework. Using these methods, we re-align δ 18 O-based water temperature records of a Carlarius heudelotii otolith and a Venus crebrisulca shell with measured sea surface temperature time series and compare the resulting correlation as well as the different underlying growth models. Our results show that both methods represent useful tools for the temporal alignment of sclerochronological records. However, underlying growth models strongly differ depending on the temporal alignment method and the parameters used for the reconstruction of the temporal framework. This underlines the importance of a careful selection of a particular alignment method depending on the complexity of distortion due to seasonal and ontogenetic growth trajectories. Moreover, we show that both organisms represent reliable paleoenvironmental archives providing sub-seasonally resolved water temperature records.