Regression trees for modelling geochemical data- an application to Late Jurassic carbonates (Ammonitico Rosso)

Coimbra, R., Rodriguez-Galiano, V.F., Oloriz, F. and Chica-Olmo, M. (2014) Regression trees for modelling geochemical data- an application to Late Jurassic carbonates (Ammonitico Rosso). Computers & Geosciences, 73, 198-207. (doi:10.1016/j.cageo.2014.09.007).

Abstract

Research based on ancient carbonate geochemical records is often assisted by multivariate statistical analysis, among others, used for data mining. This contribution reports a complementary approach that can be applied to paleoenvironmental research. The choice to use a machine learning method, here regression trees (RT), relied in the ability to learn complex patterns, integrating multiple types of data with different statistical distributions to obtain a knowledge model of geochemical behaviour along a paleo-platform.

The Late Jurassic epioceanic deposits under scope are represented by six stratigraphic sections located in SE Spain and on the Majorca Island. The used database comprises a total of 1960 data points corresponding to eight variables (stable C and O isotopes, the elements Ca, Mg, Sr, Fe, Mn and skeletal content). This study uses RT models in which the predictive variables are the geochemical proxies, whilst skeletal content is used as a target variable. The resulting model is data driven, explaining variations in the target variable and providing additional information on the relative importance of each variable to each prediction, as well as its corresponding threshold values.

The obtained RT revealed a structured distribution of samples, organized either by stratigraphic section or sets of nearby sections. Averaged estimated skeletal abundance confirmed the initial observations of higher skeletal content for the most distal sections with estimated values from 18 to 27%. In contrast, lower skeletal abundance from 5 to 15% is proposed for the remaining sections. The geochemical variable that best discriminates this major trend is δ¹⁸O, at a threshold value of -0.2‰, interpreted as evidence for separation of water-mass properties across the studied areas. Other four variables were considered relevant by the obtained decision tree: C isotopes, Ca, Sr and Mn, providing new insights for further differentiation between sets of samples.

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