Measures of deep-time terrestrial net ecosystem productivity and carbon sink function

Indicators of past biological productivity, or ‘palaeoproductivity proxies’, offer ways to indirectly measure Earth’s deep-time ecosystem and carbon cycle functioning. Given that plants have been the principal primary producers on land for hundreds of millions of years, the abundances of fossil plants in the rock record can indicate past changes in net terrestrial ecosystem productivity (NTEP). This is the net carbon uptake or release by a terrestrial ecosystem, and a measure of whether the ecosystem is a carbon sink or source. When applied on a global scale, NTEP represents a major component of Earth’s carbon cycle. Moreover, since plants are particularly sensitive to rapid climatic events, measuring NTEP with fossil plants should indicate how land carbon sinks are impacted by these climatic changes. Herein, we compare and contrast two proxies of NTEP changes in deep time: terrestrial organic microfossil concentrations (ct) and terrestrial organic carbon (TrOC). However, the preservation pathways of terrestrial organic microfossils (hence, ct and TrOC) are complex and poorly understood. In this review, we have: 1, summarized the factors that influence the preservation of land-derived organic carbon in the fossil record; 2, adapted and applied a framework of modern net ecosystem productivity (NEP) to prehistoric settings by incorporating post-burial effects; and 3, explored the conditions under which ct and TrOC may provide valid estimates of relative changes in prehistoric NTEP. Lastly, we produce a roadmap towards refined proxy of deep-time NTEP, which would constrain biogeochemical models since the emergence of large land plants >360 million years ago.