Geological Forcing Sensitivity Analysis.

The original geologic forcing scenario over the Late Ordovician–Silurian–Early Devonian (460–400 Ma) has a generally high level of degassing (D) ∼1.5 (with a slight rise and then fall) relative to today, consistent with it being an interval of high levels of volcanic activity and formation of subduction zones. The original uplift forcing (U) is more variable, dropping from U ∼1.08 at 470 Ma, to a minimum U = 0.84 at 440 Ma, increasing to a maximum U = 1.04 at 410 Ma, then dropping again to U ∼0.8 at 390–380 Ma. This two-peak structure can be related to the main phases of mountain building of the Taconic orogeny and the Caledonian (Acadian) orogeny, separated by a more quiescent interval in the Early Silurian. However, the imposed drop in uplift U in the Middle to Late Ordovician was originally inferred from a significant drop in the ⁸⁷Sr/⁸⁶Sr composition of seawater, which could also have been due to extensive weathering of volcanic rocks with low ⁸⁷Sr/⁸⁶Sr composition, at a time of significant ongoing uplift associated with the Taconic orogeny.

To explore the consequences of uncertainty in the geologic (degassing and uplift) forcing of the model, we undertake a sensitivity analysis based on that conducted by Royer et al. (91) using the GEOCARBSULF model, but with an expanded uncertainty range; their uncertainty range on spreading rate over time, which is equivalent to degassing, is ±17.5%, whereas for dependence of weathering on relief, which is equivalent to uplift, it is only ±8% (because the GEOCARBSULF model tends to crash under wider ranges in input parameters). We opt for a consistent ±20% uncertainty range on both parameters, D and U, either side of the best-guess trajectories for them from ref. 91. The default degassing trajectory is similar to the original COPSE degassing forcing, whereas the default uplift trajectory is much smoother and only captures very long timescale variation with the supercontinent cycle. Nevertheless, a ±20% uncertainty range produces an envelope which encompasses the more temporally variable original COPSE uplift forcing (Fig. S4 A and B).

With the central estimates for degassing and uplift forcing from Royer et al. (91), the baseline run of the model is subtly altered during the focal interval 480–380 Ma (Fig. S4 C–F), due to the smoothing out of the uplift forcing, but the effects on organic carbon burial δ¹³C (<0.1‰) and atmospheric O₂ (∼0.02 PAL) are small. Results of the sensitivity analysis for varying geological forcing are summarized in Table S4, where all results are given relative to the original baseline (including the effects of the new baseline geological forcing relative to the original baseline). Again, changes in total organic carbon burial flux (F_Org-C), δ¹³C, and atmospheric CO₂ are given at 445 Ma, whereas changes in atmospheric O₂ are given at 410 Ma, which is typically close to the time of maximum change in O₂.