Biogeochemistry of Dissolved Carbohydrates in the Arctic

Abstract

A detailed characterization of dissolved carbohydrates in the Siberian rivers and shelf areas, the Kara and Laptev Seas, the central Arctic Ocean, Fram Strait, and Greenland Sea is presented. Highest concentrations of total carbohydrates (25 mM C on average) were measured in the samples of the Russian rivers. A strong decrease of the concentrations occurred during the transition into the marine environment (5.3 mM C on average). In deep waters, the total carbohydrate concentrations were below 2 mM C. The main component of dissolved carbohydrates was glucose, which was well suited for the characterization of the diagenetic state of DOM. Terrigenous and diagenetic old marine DOM had very low glucose proportions between 20 and 28% of total carbohydrates. In contrast, freshly produced DOM was characterized by high glucose proportions of up to 86%. Indicators for diagenetic modifications were also fucose, rhamnose, and arabinose, whose proportions increased with increasing diagenetic degree to approx. 40%. The composition of refractory carbohydrates was determined by extraction of recalcitrant organic matter from the seawater using a combination of XAD-2 and -4 adsorption resins. With increasing age of DOM, the proportion of the humic substances of the total DOC and of the total carbohydrates increased from 45% in surface samples to 67% in the deep sea. Part of refractory carbohydrates may be composed of chemically modified carbohydrates, which are not accessible with the chromatographic method. More than the half of the XAD-extractable sugars of surface samples and more than 70% of deep sea samples were found in the unpolar fraction. It was concluded that the majority of refractory carbohydrates were bound or associated to unpolar compounds of small molecular size. Using the ratio of the sum of the molar proportions of the deoxysugars and the sum of arabinose and xylose, the extracted humic substances were characterized with respect to their diagenetic degree.