Abstract
<title>Abstract</title> <p>The marine organic carbon cycle stabilises Earth’s climate by capturing CO2 through ocean productivity and long-term carbon burial. Although conceptual frameworks often explore fluxes and abundances of organic matter, the biological source of marine organic matter itself remains largely untested over millions of years (Myr). Here, we determine the biological source of marine organic matter using a global compilation of 1,274 δ¹³C measurements of phytane and pristane, compounds derived from phytoplankton, archaea, and/or land plants where each potential source has distinct isotopic values. The δ¹³C values of phytane, pristane, and bulk marine organic matter move in lockstep across the past 500 Myr, indicating that phytoplankton have dominated marine production and organic carbon burial. We then explored drivers for δ¹³C variability. Although the evolution of biological carbon acquisition in phytoplankton could theoretically cause notable shifts in δ¹³C values through time, we find that the observed range of isotopic fractionation (12–25‰, typically 17–22‰) matches modern phytoplankton and does not relate to major evolutionary events, indicating that biological functionality remained constant despite phytoplankton evolution. Instead, we find that isotopic fractionation covaries with CO₂, implying that environmental forcing – not evolution – drives variability in phytoplankton δ¹³C values. Together, these results reveal a strikingly stable structure from production to burial in the marine organic carbon cycle over the past half billion years, despite profound changes in climate and life.</p>