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

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 mu m to similar to 1 nm) for the detailed characterization of DOC chemical composition and radiocarbon (Delta C-14) signatures. However, while total DOC Delta(14)Cvalues are generally thought to be quite similar in the world ocean, previous studies have reported widely different Delta C-14 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 Delta C-14 values? In this study we evaluate the effects of varying proportions of HMW vs. low molecular weight (LMW) DOC on UDOC Delta C-14 values. Using concentration factor (CF) as a proxy for MW distributions, we modeled the retention of both OC and Delta C-14 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 Delta C-14 UF permeation coefficients generally increase with depth, consistent with mass balance trends, indicating very significant permeation of LMW, C-14-depleted DOC at depth, and higher recoveries of Delta C-14-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 Delta C-14 values. Together these results suggest that previously reported disparities in UDOC Delta C-14 values are reconciled by linked trends of Delta C-14 content vs. MW. At low CFs, UDOC samples have similar Delta C-14 values to total DOC. In contrast, UDOC samples collected at extremely high CFs (and after diafiltration) have more positive Delta C-14 values. We demonstrate that the observed relationships between UDOC Delta C-14 and CF derived from our data can directly explain offsets in all previously published UDOC Delta C-14 values for the NPSG. While CF is not traditionally considered in UF studies, our results indicate it can substantially influence the interpretation of UDOC C-14 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 Delta C-14 analyses, presents a new tool for studying relationships between molecular size, age, and labile DOC distributions in the ocean. (C) 2011 Elsevier Ltd. All rights reserved.