Investigating the sources and structure of chromophoric dissolved organic matter (CDOM) in the North Pacific Ocean (NPO) utilizing optical spectroscopy combined with solid phase extraction and borohydride reduction

Abstract

Prior optical measurements of waters in the Equatorial Atlantic Ocean (EAO) provided evidence of a major terrestrial “humic-like” component of the CDOM that absorbed in the ultraviolet (UV) and visible and emitted across the visible, along with a marine component that primarily absorbed and emitted in the UV. Here we extend these measurements to the North Pacific Ocean (NPO) at Station Aloha (22o 45′ N, 158o 00′ W). Detailed optical measurements of both the natural waters (CDOM) and C18 organic matter extracts of these waters (C18-OM) were acquired before and after sodium borohydride (NaBH4) reduction of samples obtained throughout the water column. Optical properties of the “humic-like” component were relatively uniform with depth below ~600 m [aCDOM(350) ~ 0.08 (m−1), a*CDOM (350) ~ 0.2 (m−1 mg−1C L), SUVA254 ~ 0.55 (m−1 mg−1C L), E2:E3 ~ 10, S300–700 ~ 0.02 (nm−1), S350–400 ~ 0.012 (nm−1), SR ~1.7, F(350/450) ~ 0.009 (QSE), and ϕ360 ~ 0.026], but were significantly different in surface waters, likely due to photobleaching and biological activity [aCDOM(350) ~ 0.026 (m−1), a*CDOM (350) ~ 0.027 (m−1 mg−1C L), SUVA254 ~ 0.36 (m−1 mg−1C L), E2:E3 ~ 45, S300–700 ~ 0.03 (nm−1), S350–400 ~ 0.003 (nm−1), SR ~6.8, F(350/450) ~ 0.003 (QSE), and ϕ350 ~ 0.024]. Optical properties of the short-wavelength components (UV bands) were more variable with depth. Response to solid phase extraction was also relatively uniform with depth, with preferential extraction of the long-wavelength absorbing/emitting “humic-like” component (~30–50% extraction efficiency at λ <300 nm and ~50–80% at λ >400 nm) and virtually no extraction of the the UV absorbing/emitting bands. Response to NaBH4 reduction was also similar down the water column with preferential loss of absorption in the visible region, and enhanced, blue-shifted fluorescence emission. As in the EAO the ‘humic-like” component exhibited very similar, although not identical, optical and chemical properties to those observed for terrestrially-dominated estuarine and coastal environments, providing evidence that this component originates from a terrestrial source. Although this component dominated the absorption, marine contributions (i.e. UV bands) similar to those observed in the EAO were also observed. However, these components were found to absorb and emit primarily in the UV and were not efficiently extracted by the C18 columns, clearly showing that they are structurally distinct from the “humic-like” component.