Decoding nonlinear growth rates in biogenic environmental archives -: art. no. Q12015

The record of an environmental proxy along a growth axis in biogenic carbonates can reflect changing environmental conditions experienced during the lifetime of the organism. When a chronology of the growth axis is not available, a method based on anchor points is commonly used to introduce a time grid. Between these anchor points a constant growth rate is generally assumed, despite the fact that growth rates change during life in most organisms. Here, we present a method which refines the constant growth rate assumption, at least in situations where the environmental proxy has a periodic component in its signal. A nonlinear growth rate can then be estimated, enabling the construction of a more realistic time base. The anchor point method and the method proposed here are first compared on a synthetic record. It is shown that the choice of the anchor points, which is always a subjective one, is largely influenced by the stochastic noise on the data record. This results in biased growth rate profiles and biased time grids. Next we apply the method on the Mg record in the bivalve Isognomon ephippium, on three samples of the clam Saxidomus giganteus from Washington State and on the Vanuatu coral stable isotope record. The method developed is applicable to a wide range of proxy records, which are measured along an indirect time grid and are complicated by nonlinear accretion rates. Furthermore, this method which takes variable growth rates into account opens perspectives on quantitative modeling of the relationship between environmental proxies recorded by accretive archives and the instrumental record, like temperature or salinity.