Linked changes in marine dissolved organic carbon molecular size and radiocarbon age

Marine dissolved organic carbon (DOC) is a major global carbon reservoir, yet its cycling remains poorly understood. Previous work suggests that DOC molecular size and chemical composition can significantly affect its bioavailability. Thus, DOC size and composition may control DOC cycling and radiocarbon age (via Δ¹⁴C). Here we show that DOC molecular size is correlated to DOC Δ¹⁴C in the Pacific Ocean. Our results, based on a series of increasing molecular size fractions from three depths in the Pacific, show increasing DOC Δ¹⁴C with increasing molecular size. We use a size-age distribution model to predict the DOC and Δ¹⁴C of ultrafiltered DOC. The model predicts both large and small surface DOC with high Δ¹⁴C and a narrow range (200–500 Da) of low Δ¹⁴C DOC. Deep model offsets suggest different size distributions and/or Δ¹⁴C sources at 670–915 m. Our results suggest that molecular size and composition are linked to DOC reactivity and storage in the ocean.