Fig. 4: Conceptional framework and outputs of the multi-box biogeochemical model.

A Paleogeographic reconstruction at ~1.4 Ga, with red star indicating studied area. ITCZ: intertropical convergence zone. The ITCZ generally falls within ~15° of the equator in both hemispheres^{59, 60}. L, Laurentia; Ant, Antarctica; SAC, South Australian Craton; NAC, North Australian Craton; S, Siberia. Purple polygon represents the NCC, with transparent outlines representing the approximate uncertainty from the (temporally) surrounding palaeomagnetic data. Green polygon approximates interconnected landmasses during the tenure of the supercontinent Nuna^61,62,63. Longitude is unconstrained, and the two circles on the north pole represent palaeomagnetic data from Laurentia that constrain Nuna to a low-latitude position at this time⁶⁴. B Conceptional framework for the multi-box biogeochemical model with two shelves (Shelf 1 is shown on the left, with a solid arrow indicating a higher input of P, while Shelf 2 is located on the right, with the dashed arrow indicating a lower input of P). Biogeochemical cycling in different boxes responds to various weathering inputs on the two shelves. C A high weathering flux on Shelf 1 is set to oscillate at periods of 15 kyr, while a low weathering flux on Shelf 2 is set to oscillate at periods of 60 kyr, which is based on the assumption of constant sedimentation rates (see Supplementary Information). However, given that a constant sedimentation rate was highly unlikely, these forcings are applied solely to demonstrate the range of potential fluctuations in ensuing biogeochemical characteristics, and the actual periodicity of these fluctuations may have varied considerably. D High and variable organic carbon burial rates (C_org) occur on Shelf 1, while low and relatively stable rates occur on Shelf 2. E High and variable C_org consumption rates occur due to Fe(III) reduction on Shelf 1, with very low (but variable) remineralization rates by Fe(III) reduction on Shelf 2. F High and variable C_org consumption rates occur due to sulfate reduction on Shelf 1, with very low remineralization rates by sulfate reduction on Shelf 2.