The oceans contain the largest proportion of carbon atoms that participate in biogeochemical cycles on Earth's surface over timescales ranging from less than a second to thousands of years. Radiocarbon (14C) measurements suggests that most (~95 %) of the carbon atoms bound within organic molecules in seawater can persists for thousands of years. The abundances of these atoms at each time and place in the ocean are complete records of their histories according to the laws of conservation of mass, thermodynamics, and kinetics. However, marine organic matter is a complex mixture of compounds that are frequently too scarce or unreactive to decode this information with conventional chemical methods. Therefore, the production and loss mechanisms that control this massive reservoir of carbon remain either unconstrainted or undiscovered. Fortunately, the natural carbon isotopes (12C, 13C, 14C) within these compounds also depend upon their histories. In this talk, I will discuss my approach of coupling measurements of bulk kinetics and carbon isotope abundances to constrain the reactivity of similar populations of carbon found in the ocean. Much of this work has been enabled by advances in our ability to measure 14C abundances in small samples (<10 μg C) by accelerator mass spectrometry (AMS) and 13C abundances in bacteria by stable isotope probing of single cell resonance Raman spectroscopy (SIP-SCRR). These methods have made carbon isotopes readily observable stowaways aboard organic molecules that provide otherwise unobtainable constraints on the biogeochemistry of marine organic matter.