Contribution of non-carbonate inorganic and organic alkalinity to total measured alkalinity in pore waters in marine sediments (Gulf of Gdansk, S-E Baltic Sea)

Abstract

Various microbiological, chemical and physical processes during early diagenesis in marine sediments make the chemical composition of pore water significantly different from that of the sea water. The aim of this study was (a) to compare concentrations of total alkalinity measured using titration method (TAmeas.) with carbonate alkalinity calculated (TAcalc.) from dissolved inorganic carbon and pH, and (b) to determine the proportion of non-carbonate inorganic bases (N-CIBs) and alkalinity from organic compounds (OA) in TAmeas. in pore water in the Gulf of Gdansk. In the study, TAmeas. as well as concentrations of dissolved inorganic and organic carbon (DIC and DOC), hydrogen sulphide, sulphate, ammonium, phosphates and borate in near-bottom and pore water were measured at three sampling sites in October 2013. At all the sites, TAmeas. was 1632–14,460μmolkg−1 and increased with depth into the sediment, as did the concentrations of other parameters (excluding sulphate). Inorganic and organic compounds produced during organic matter (OM) decomposition in sediments had a considerable contribution to TAmeas. in pore water. As a result TAmeas. was up to 38% higher than TAcalc. The observed excess alkalinity (TAexcess) was from 56 to 3972μmolkg−1 and increased gradually going deeper into the sediment. TAexcess was correlated with concentration of N-CIBs (mainly hydrogen sulphide, ammonia and phosphates) and DOC present in pore water. The main source of N-CIBs in pore water was bacterial sulphate reduction during OM mineralization. The concentration of N-CIBs in pore water ranged from 85 to 1053μmoll−1 and the percentage share of N-CIBs in TAmeas. was 4–35%. The highest proportion of N-CIBs in TAmeas. was observed in the upper (0–12cm) layers of the sediment. Anoxic decomposition of OM has led to the production of OA in pore water which constituted up to 26% of TAmeas. The concentration of OA varied from 2 to 2953μmoll−1and at all sites increased with depth into the sediment. This is due to the accumulation of products of decomposition of OM in pore water, i.e. organic acids and fulvic and humic acids.