Large-volume ultrafiltration for the study of radiocarbon signatures and size vs. age relationships in marine dissolved organic matter

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In recent decades, tangential-flow ultrafiltration (UF) technology has become a primary tool for isolating large amounts of “ultrafiltered” marine dissolved organic carbon (UDOC; 0.1 μm to ∼1 nm) for the detailed characterization of DOC chemical composition and radiocarbon (Δ 14C) signatures. However, while total DOC Δ 14C values are generally thought to be quite similar in the world ocean, previous studies have reported widely different Δ 14C values for UDOC, even from very similar ocean regions, raising questions about the relative “reactivity” of high molecular weight (HMW) DOC. Specifically, to what degree do variations in DOM molecular weight (MW) vs. composition alter its relative persistence, and therefore HMW DOC Δ 14C values? In this study we evaluate the effects of varying proportions of HMW vs. low molecular weight (LMW) DOC on UDOC Δ 14C values. Using concentration factor (CF) as a proxy for MW distributions, we modeled the retention of both OC and Δ 14C in several very large CF experiments (CF >3000), from three depths (20, 670, and 915 m) in the North Pacific Subtropical Gyre (NPSG). The resulting DOC and Δ 14C UF permeation coefficients generally increase with depth, consistent with mass balance trends, indicating very significant permeation of LMW, 14C-depleted DOC at depth, and higher recoveries of Δ 14C-enriched, HMW DOC in the surface. In addition, changes in CF during sample concentration and ionic strength during sample diafiltration had very large and predictable impacts on UDOC Δ 14C values. Together these results suggest that previously reported disparities in UDOC Δ 14C values are reconciled by linked trends of Δ 14C content vs. MW. At low CFs, UDOC samples have similar Δ 14C values to total DOC. In contrast, UDOC samples collected at extremely high CFs (and after diafiltration) have more positive Δ 14C values. We demonstrate that the observed relationships between UDOC Δ 14C and CF derived from our data can directly explain offsets in all previously published UDOC Δ 14C values for the NPSG. While CF is not traditionally considered in UF studies, our results indicate it can substantially influence the interpretation of UDOC 14C “age”, and thus reactivity, in the marine environment. In addition, our results indicate that CF can in fact be used as a proxy for average MW. We suggest that a variable-CF-UF approach, coupled with molecular-level Δ 14C analyses, presents a new tool for studying relationships between molecular size, age, and “labile” DOC distributions in the ocean.