Chris Mays, Richard V Tyson, Michael T Hren

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.