Benthic mineralization and solute exchange on a Celtic Sea sand-bank (Jones Bank)

Benthic carbon mineralization and solute exchange was investigated on a Celtic Sea sandbank during July 2008. The sediment on the top of the bank consisted of consolidated sand, characterized by advective porewater transport and was difficult to sample, this to some extent compromised the investigations at this site. However, intact sediment cores were sampled at 4 stations at the slopes, base of the bank and off the bank (reference site). This sediment was used to assess rates and pathways for benthic diagenesis. Total sediment O2 uptake (TOU) ranged from 5.8 to 9.0 mmol m−2 d−1 and total sediment release rates of dissolved inorganic carbon (DIC) ranged between 8.60 and 13.8 mmol m−2 d−1. Microbial denitrification and sulfate reduction accounted for <2% and 12-28% of the total benthic carbon mineralization, respectively. The remaining mineralization is ascribed to O2 and Fe/Mn respiration, respectively. Activity profiles of unsupported 210Pb from all stations indicated deep mixing, presumably caused by intense trawling activity in the area. Calculation based on satellite tracking of fishing vessels suggest that on average 33% of the sediment is affected by trawling activity every year. This is presumed to facilitate high metal respiration rates by continuously oxidizing reduced Fe that would otherwise accumulate in the sediment. Extracted porewater profiles reflected elevated NO3- levels as compared to microsensor determined NO3- profiles that were measured in parallel. We suggest that this reflects NO3- leakage from meiofauna - and that the intracellular NO3- pool in the top 5 cm of the sediment exceeds the porewater pool up to a factor of 45. However, this intracellular pool is presumably turned over at very slow rates, compared to the porewater pool subject to microbial denitrification.