Marine dissolved organic carbon (DOC) is a large (660 Pg C) reactive carbon reservoir that mediates the oceanic microbial food web and interacts with climate on both short and long timescales. Carbon isotopic content provides information on the DOC source via δ 13 C and age via Δ 14 C. Bulk isotope measurements suggest a microbially sourced DOC reservoir with two distinct components of differing radiocarbon age. However, such measurements cannot determine internal dynamics and fluxes. Here we analyze serial oxidation experiments to quantify the isotopic diversity of DOC at an oligotrophic site in the central Pacific Ocean. Our results show diversity in both stable and radio isotopes at all depths, confirming DOC cycling hidden within bulk analyses. We confirm the presence of isotopically enriched, modern DOC cocycling with an isotopically depleted older fraction in the upper ocean. However, our results show that up to 30% of the deep DOC reservoir is modern and supported by a 1 Pg/y carbon flux, which is 10 times higher than inferred from bulk isotope measurements. Isotopically depleted material turns over at an apparent time scale of 30,000 y, which is far slower than indicated by bulk isotope measurements. These results are consistent with global DOC measurements and explain both the fluctuations in deep DOC concentration and the anomalous radiocarbon values of DOC in the Southern Ocean. Collectively these results provide an unprecedented view of the ways in which DOC moves through the marine carbon cycle.carbon cycle | carbon isotopes | dissolved organic carbon | radiocarbon | oceanography R adiocarbon is a natural tracer of carbon flow through dissolved organic carbon (DOC) (1). As plankton grow and are consumed by grazers, organic matter with a modern radiocarbon value ðΔ 14 C > − 50‰Þ is released into surface waters where it accumulates as semilabile DOC (2-4). Semilabile DOC undergoes net remineralization below the euphotic zone and gradually diminishes in concentration with depth to approximately 1,000 m, below which it appears to vanish. Oceanic profiles of total DOC and DOC radiocarbon (DOCΔ 14 C) are therefore characterized by high values (60-80 μM carbon; −200‰ to −400‰) in surface waters and lower values at depth (35-40 μM; −400‰ to −550‰) (2). The −200‰ to −300‰ depletion in DOCΔ 14 C values in surface seawater relative to semilabile DOCΔ 14 C indicates the presence of a second refractory DOC fraction with an old radiocarbon age. The inverse proportionality between DOC concentration and radiocarbon value in depth profiles suggests that refractory DOC is well mixed throughout the entire water column (2, 3, 5, 6). The origin of the refractory DOC fraction is obscure, but stable isotopes ðδ 13 CÞ show little change with depth δ 13 C = −21:7‰ ð−23:2‰ to − 20:2‰Þ (2, 7-9), indicating a common, autochthonous planktonic source for both fractions (2).DOC and DOCΔ 14 C profiles can be reproduced in a simple two-component model (TCM) that includes a variable amount of semilabile DOC cycling in the upper ocea...

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We present concentration and isotopic profiles of total, size, and polarity fractionated dissolved organic carbon (DOC) from Station ALOHA (A Long‐term Oligotrophic Habitat Assessment), an oligotrophic site in the North Pacific Ocean. The data show that, between the surface and 3500 m, low molecular weight (LMW) hydrophilic DOC, LMW hydrophobic DOC, and high molecular weight (HMW) DOC constitute 22–33%, 45–52%, and 23–35% of DOC, respectively. LMW hydrophilic DOC is more isotopically depleted (δ13C of −23.9‰ to −31.5‰ and Δ14C of −304‰ to −795‰; mean age of 2850 to 15000 years) than the LMW hydrophobic DOC (δ13C of −22‰ to −23‰ and Δ14C of −270‰ to −568‰; 2470 to 6680 years) and HMW DOC (δ13C of ~−21‰ and Δ14C of −24‰ to −294‰; 135–2700 years). Our analyses suggest that a large fraction of DOC may be derived from allochthonous sources such as terrestrial and hydrothermal DOC and cycle on much longer time scales of >10000 years or enter the ocean as preaged carbon.

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