Fig. 4: COPSE global biogeochemical model with C_org:P_reac imposed based on our geochemical data.

a C_org/P_reac; b C_org burial; c Atmospheric CO₂; d Global average surface temperature and ice cap latitude. Model background run from ref. ⁵⁴ shown as a black dashed line. Red lines show the effects of a pulse of P weathering¹⁸, and purple lines show the combined effect of the same P weathering pulse with additional P recycling, as imposed using the sedimentary C_org:P_reac ratios from our empirical data (shown as the average and lower limit in panel a). The combined model output is shown as a range representing the average and lower limit (where the purple shading represents the average and lower limits, and the purple line represents the mid-point of these two values), in order to provide a conservative estimate for the impact of P recycling. The model predicts substantial increases in P recycling and organic C burial across the interval sampled (b). This also draws down atmospheric CO₂ (c), reducing surface temperature (d). Ice cap latitude is shown in panel d as a blue shaded area, following ref. ³⁶. As discussed in the main text and Supplementary Information, the model results imply a more rapid increase in organic C burial and consequent decrease in CO₂ and temperature across Horizons A and B (shown as a dashed purple line) than suggested by the geologic record. This is attributed to a global overestimation of phosphate bioavailability based on our data from shelf settings, due to subsequent drawdown of recycled phosphate in deeper ocean settings that were poised at a less reducing state than during horizons C–F.