CHROMOPHORIC DISSOLVED ORGANIC MATTER (CDOM) IN THE OPEN OCEAN: OPTICAL AND CHEMICAL PROPERTIES AND THEIR RELATION TO CDOM STRUCTURE AND SOURCES.

Abstract

The carbon contained as dissolved organic matter (DOM) in the Earth’s oceans is an important factor in the global carbon cycle, but studying and tracking DOM in the aquatic environment can be challenging. However, the light-absorbing and emitting subcomponents of DOM, called chromophoric dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) can be directly probed using absorption and fluorescence spectroscopy, respectively.

Detailed studies on CDOM from the open oceans are limited with many of the existing studies having very limited data sets (only select wavelengths or indices). To address this, the optical properties of CDOM from a variety of geographic locations (North Pacific Ocean: NPO, Equatorial Atlantic Ocean: EAO, Middle Atlantic Bight: MAB, Delaware River and Delaware Bay) were compared, and chemical tests performed (sodium borohydride (NaBH4) reductions and pH titrations). The responses to the chemical tests along with similarities and differences in the optical properties were examined to compare the structures present in terrestrial, coastal and open ocean samples.

A long-pathlength capillary waveguide spectrometer was used to characterize open ocean CDOM samples, with the need for a calibration and validated protocol addressed prior to use. The optical properties of the NPO samples did not vary significantly at depths from ~300-4500 meters with only the surface samples showing significant differences. Solid phase extraction of the natural waters did remove unique absorbing and emitting bands in the UV region that could be marine in origin, while enriching the “humic-like” fraction.

The open ocean samples showed similarities to the coastal and riverine samples including: 1) monotonically decreasing and unstructured absorbance with increasing wavelength; 2) loss of absorption upon NaBH4 reduction at all wavelength, with the largest percent loss in the visible; 3) enhanced absorption with increasing pH with spectral changes that occurred over the same pH ranges as the pKas of carboxylic acids and phenols; 4) attenuation of absorption enhancement with increasing pH following reduction at most wavelengths. These similarities not only suggest that there are structural similarities throughout all samples, but also indicate that there may be a terrestrial source of CDOM in the open ocean.