Chemical composition dictates the size and age of high molecular weight dissolved organic matter

High molecular weight dissolved organic matter (HMWDOM) accounts for a quarter to a third of marine DOM and more than 15% of total marine primary productivity. Yet, many of the characteristics of HMWDOM remain poorly constrained. HMWDOM consists of two major classes of compounds: acylpolysaccharides (APS) and high molecular weight humic substances (HS). The relationship of these compounds, and the influence they exert on the attributes of HMWDOM as a whole, are not well understood. Using diffusion-ordered NMR spectroscopy coupled with mixed-mode chromatography we separated these components and examined their molecular weight (MW) distribution and carbon isotopic properties. The MWs of APS and HS fell within distinct, narrow envelopes between 0.9 and 16 kDa and, for HMWDOM samples collected in the North Pacific Subtropical Gyre (NPSG), the MW of both components decreased with depth. Trends in the MW distribution, coupled with the isotopic signature of the APS endmember suggest that APS has a short lifetime in the ocean and that APS <2 kDa is efficiently removed from the water column. In contrast, the MW distribution of HS narrows significantly with depth due to the removal of HMW components. While the average radiocarbon of HMWDOM in NPSG is ‑30‰ and -238‰, at 15 m and 915 m respectively, our work shows that APS has a modern radiocarbon value, and that the Δ¹⁴C of HMWDOM is controlled primarily by the proportion of HS in the sample. From these results, we propose potential pathways for the production and removal of HS and APS.

Presentation Preference: Oral

Primary Presenter: Benjamin Granzow, Scripps Institution of Oceanography (DOCGranzow@gmail.com)

Authors:

Daniel Repeta, Woods Hole Oceanographic Institution  (drepeta@whoi.edu)