Isotopic Measurements of Solid-Phase Extracted Dissolved Organic Matter in the Eastern Pacific Ocean

Dissolved organic carbon (DOC) in the deep ocean is mostly refractory with a radiocarbon age of 4900-6500 years. Reasons for why DOC persists so long in the deep ocean are still unclear. Solid-phase extraction (SPE) is a method of concentrating organic matter from abundant seawater salts, and approximates some characteristics of bulk organic matter, including ∆14C signatures in deep ocean. We performed SPE on high-volume seawater samples in the surface (0-200 m) and deep ocean (2000-4000 m) along the GO-SHIP P18 2016/2017 transect through the eastern Pacific Ocean (110°W, 20°N to 69°S). We present SPE concentrations, ∆14C, and δ13C signatures at 9 different stations along the transect, including the eastern Equatorial Pacific, the subtropical gyre, and the Southern Ocean. In the deep ocean throughout the majority of the transect, SPE-DOC ∆14C signatures are identical to bulk DOC (-528±7‰), while slightly higher values are found in the Southern Ocean (-489±4‰). Highest deep ocean SPE-DOC concentrations were also found in the Southern Ocean. In parts of the surface ocean, SPE-DOC ∆14C is offset older than bulk DOC similar to previous work; however, throughout P18 these offsets vary with latitude. Stable carbon isotope signatures (δ13C) are consistently lower relative to bulk DOC. These comparisons of SPE and bulk DOC data are important in supporting previous findings that show deep DOC is likely a stable pool of old, low molecular weight, refractory carbon. Characterizing oceanic DOC, SPE-DOC and other fractions of organic matter in this way is an important step in understanding the role of refractory DOC on longer timescales, and finding the mechanism(s) that constrains the age of DOC to 4900-6500 years and 40 µM in the deep oceans.