Abiotic and biotic constraints on Earth's ancient colonisers in the Proterozoic

The occurrence of stromatolites in the rock record dates from 3.485 Ga, displaying varying diversity and abundance between 2575 and 500 Ma. This paper investigates, for the first time, the role of temperature and bioactive trace elements (including micronutrients) to explain the trend in abundance of stromatolites, using random forests analysis. We modelled the proportion of stromatolite bearing geological units between 2.575 Ga and 540 Ma by using predictors consisting of trace elements (TE) and temperature (T). The variation in TE was obtained using marine pyrite TE content in black shales through time and multiple measurements of ocean T were obtained from geological and biological assays. Analyses were calculated across the whole period (2575-540 Ma) and for specific periods (540-1000, 1000-1300, 1300-2000, 2000-2575.5 Ma), and included TE and T as predictors, in combination and separately. Assessment of model fit was summarised using r2 statistics and with relative mean square errors. A combination of TE and T for the four periods produced the best fit. The predictors were further assessed using importance scores along with their assocaited P values. The most important predictor was T across all periods and within each period - warranting much needed attention to physical properties of marine systems. The most important and significant TEs were Ag, As, Cd, Co, Cu, Pb, Sb, Tl with two (Pb and As) achieving significance in multiple periods and the rest in single periods. Only Pb was important in the oldest period (2575-2000 Ma) and the others were restricted to the most recent periods (1300-1000 and 540-1000 Ma). The predicted variation of stromatolites with individual TEs were consistent with biotic influences from metazoa or eukaryotes, and that of two TEs (Pb and As) in particular suggested the occurrence of ecological jumps. These striking observations prompt a need to investigate the consequences of TE bioavailability alongside T, on the various forms of life (prokaryotes, eukaryotes including early metazoans) and their interactions. This paper highlights the role of abiotic factors and discusses biotic consequences (such as ecological pressures), to provide a nuanced understanding of variation in stromatolite abundance through the Proterozoic.