Total hydrolysable amino acid mineralisation in sediments across the northeastern Atlantic continental slope (Goban Spur)

Total hydrolysable amino acids (THAA), individual amino acid distributions, total organic carbon (TOC) and total nitrogen (TN) were measured in sediments across the Goban Spur continental slope at water depths of 651, 1296 and 3650 m. Objectives were to examine (1) differences in organic matter (OM) degradation state in surface sediments across the slope from sedimentary amino acid compositions, and (2) whether these differences are related to particle size distributions. Application of a ‘reaction–diffusion’ model to the sediment concentration profiles showed that TOC and THAA degradation rate constants decreased with increasing water depth. Ratios of degradation rate constants of THAA over TOC indicated that THAA turn over faster than TOC at 651 and 1296 m water depth only. From estimates of degradation rate constants of individual amino acids, it was concluded that with increasing water depth fewer amino acids contribute to overall THAA degradation. The contribution of THAA to TOC mineralisation decreased from the upper to the lower slope. Since at all three sampling stations the amino acids with the highest relative contribution to THAA had a higher abundance in sediments with reduced THAA mineralisation rates, we conclude that the overall amino acid reactivity decreases with increasing water column depth. A principal component analyses, carried out on normalised amino acid mole percentages, established significant shifts in amino acid compositions and confirmed that (1) OM degradation state increased from 651 to 3650 m and (2) that OM in the finest fraction at the shallowest station appeared to be considerably less degraded than in the coarser fractions or any size fractions at the deeper stations. Therefore, we conclude that downslope transport, sorting and accumulation of fine particles with continuous mineralisation of OM attached to the particles during vertical and lateral transport results in an increasing organic matter degradation state from the upper slope to the abyssal plain.