Deep-Sea Research II 52 (2005) 1947–1964 Dissolved organic carbon (DOC) fluxes of deep-sea sediments from the Arabian Sea and NE Atlantic Niko Lahajnar a,Ã , Tim Rixen b , Birgit Gaye-Haake a , Petra Scha¨fer a , Venugopalan Ittekkot b a Institut fu¨r Biogeochemie und Meereschemie, Universita¨t Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany b Zentrum fu¨r Marine Tropeno¨kologie, Fahrenheitstrasse 6, D-28359 Bremen, Germany Received 25 March 2004; accepted 21 May 2005 Available online 15 August 2005 Abstract Diffusivefluxofdissolvedorganiccarbon(DOC)fromdeep-seasedimentsisapoorlyconstrainedcomponentofthe global marine carbon cycle, but may play an important role for DOC concentrations in the oceans. During six cruises between the years 1995 and 2000 we measured DOC concentrations in pore-waters of surface sediments and the ambientbottomwatersatfivestationsinthedeepArabianSea(waterdepths:3190–4420m)andattwositesintheNE Atlantic (4500–4800m). Pore-water DOC concentrations were usually one order of magnitude greater than in bottom waters, with maximum DOC contents between 509 and 1526 mmol/L in the uppermost sediment interval (0–0.5cm). DOCvaluesgenerallydecreasetowards 3cmcoredepthwithslightlyincreasingconcentrationsindeeperlayersunder sub-oxic to anoxic conditions. Calculated benthic DOC effluxes into the bottom water caused by concentration gradients ranged between 59.8 and 223.9 mmolm 2 d 1 in the Arabian Sea and 47.2 and 121.2 mmolm 2 d 1 in the NE Atlantic. DOC efflux rates were highlycorrelatedwiththeannualverticalparticlefluxratesatthesitesandthusappeartobeafunctionoftheregional productivity.TakinganaverageglobaloceanicDOCeffluxratebetween50.5and78.1 mmolm 2 d 1 ,wecalculatedthat the DOC release is in the order of 70–103  10 12 gCyr 1 from sediments below 2000m water depth. Thus, deep-sea sediments have the potential to represent a significant source of DOC in the oceans. r 2005 Elsevier Ltd. All rights reserved. 1. Introduction Dissolved organic carbon (DOC) is by far the largest organic carbon pool in the oceans (0.6  10 18 g DOC). It holds almost as much carbon as the atmosphere (0.75  10 18 g) and even more than the terrestrial biosphere (0.56  10 18 g) ARTICLE IN PRESS www.elsevier.com/locate/dsr2 0967-0645/$-see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2005.05.006 Ã Corresponding author. Tel.: +4940428387087; fax:+4940428386347. E-mail address: lahajnar@geowiss.uni-hamburg.de (N. Lahajnar).