Cover Image Credits

Carbon isotopes have played a crucial role in shaping our understanding of Earth surface evolution, with carbon isotope mass balance representing a foundational tool for tracking carbon burial and atmospheric CO₂ through Earth's history. Further, the presence of a baseline carbonate carbon isotope value—near 0%—over the past 3.8 billon years has been touted as evidence that the ratio of organic to carbonate carbon burial has not changed throughout Earth's history. This constancy has become a cornerstone in our understanding of Earth's evolving carbon, oxygen, and nutrient cycles. In this issue, Planavsky and others (in an article entitled “On carbon burial and net primary production through Earth's history”) (p. 413–460) revisit this record by coupling a new weathering model to global carbon isotope mass balance. In doing so, they provide a new perspective on the coupled carbon and oxygen cycles and provide additional evidence that the isotopic composition of the carbon input term to the ocean-atmosphere system varies as a function of atmospheric oxygen. In this light, the ratio of organic to carbonate carbon burial has changed dramatically through Earth's history, and carbonate carbon isotope values have been constant simply because of a mechanistic link between the oxygen cycle and carbon isotope mass balance. This view follows from relatively simple principles but is in strong contrast to the traditional view of the long-term carbon cycle.

Explanation of cover figure: This figure shows the most recently compiled longterm carbonate carbon isotope record, illustrating a baseline carbonate δ¹³C_carb value of ~0% throughout Earth's history.