A lithium-isotope perspective on the evolution of carbon and silicon cycles

The evolution of the global carbon and silicon cycles is thought to have contributed to
the long-term stability of Earth’s climate1–3. Many questions remain, however,
regarding the feedback mechanisms at play, and there are limited quantitative
constraints on the sources and sinks of these elements in Earth’s surface
environments4–12. Here we argue that the lithium-isotope record can be used to track
the processes controlling the long-term carbon and silicon cycles. By analysing more
than 600 shallow-water marine carbonate samples from more than 100 stratigraphic
units, we construct a new carbonate-based lithium-isotope record spanning the past
3 billion years. The data suggest an increase in the carbonate lithium-isotope values
over time, which we propose was driven by long-term changes in the lithium-isotopic
conditions of sea water, rather than by changes in the sedimentary alterations of older
samples. Using a mass-balance modelling approach, we propose that the observed
trend in lithium-isotope values reflects a transition from Precambrian carbon and
silicon cycles to those characteristic of the modern. We speculate that this transition
was linked to a gradual shift to a biologically controlled marine silicon cycle and the
evolutionary radiation of land plants13,14.