Rates of diagenetic changes of humic substances in Baltic surface sediments

Stratified bottom sediments were collected from the Bornholm Deep, the Southern Baltic by means of a Niemistö type corer. On board, they were cut into 10 mm thick slices and kept deep frozen until analyses on shore. Humic substances isolated quantitatively from the subsamples were divided into fulvic acids and humic acids and analysed for elemental composition, absorption spectra (UV, VIS, IR, 1H NMR), and molecular weight distribution. Moreover, the contents of humic and fulvic acids, moisture and organic carbon were measured in the sediments. The age of the sediment slices was established by means of the 210Pb method and validated with 137Cs distribution in the core. It was established that, with increasing age of sediments, molecular weight of extracted humic acids increases, carbon to hydrogen ratios decrease, and the aromaticity increases, while humic to fulvic acids ratios show no regular trend. These changes were accompanied by decreasing organic matter content and increasing humification of organic matter in the investigated sediments.