Dissolved organic carbon export and subsequent remineralization in the mesopelagic and bathypelagic realms of the North Atlantic basin

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Abstract

Dissolved organic carbon (DOC) data are presented from three meridional transects conducted in the North Atlantic as part of the US Climate Variability (CLIVAR) Repeat Hydrography program in 2003. The hydrographic sections covered a latitudinal range of 6°S to 63°N along longitudes 20°W (CLIVAR line A16), 52°W (A20) and 66°W (A22). Over 3700 individual measurements reveal unprecedented detail in the DOC distribution and systematic variations in the mesopelagic and bathypelagic zones of the North Atlantic basin. Latitudinal gradients in DOC concentrations combined with published estimates of ventilation rates for the main thermocline and North Atlantic Deep Water (NADW) indicate a net DOC export rate of 0.081 Pg C yr−1 from the epipelagic zone into the mesopelagic and bathypelagic zones. Model II regression and multiple linear regression models applied to pairwise measures of DOC and chlorofluorocarbon (CFC-12) ventilation age, retrieved from major water masses within the main thermocline and NADW, indicate decay rates for exported DOC ranging from 0.13 to 0.94 μmol kg−1 yr−1, with higher DOC concentrations driving higher rates. The contribution of DOC oxidation to oxygen consumption ranged from 5 to 29% while mineralization of sinking biogenic particles drove the balance of the apparent oxygen utilization.

Introduction

Dissolved organic carbon (DOC) is the largest pool of organic matter in the ocean, yet it is only over the past decade that the oceanographic community has developed the analytical skill necessary to begin a reliable description and quantification of its contribution to the marine carbon cycle (Sharp et al., 2002). DOC plays an important role in the biogeochemistry of the ocean carbon cycle and can contribute to the biological pump (Copin-Montégut and Avril, 1993, Carlson et al., 1994, Hansell and Carlson, 2001a, Kähler and Koeve, 2001, Amon et al., 2003, Sohrin and Sempere, 2005). The export of sinking biogenic particles has long been known to drive respiration in the ocean interior and to help maintain the ocean’s strong vertical gradients of inorganic carbon and nutrient concentrations. However, it is only recently that the contribution of DOC to carbon export and the maintenance of vertical gradients on biogeochemical properties have been recognized (Hansell and Carlson, 2001b).

DOC export from the surface ocean is a consequence of its accumulation in the euphotic zone, redistribution to higher latitudes with the wind-driven circulation, and eventual transport to depth with the overturning circulation at high latitudes and subduction in the subtropical gyres (Hansell, 2002). DOC contributes to the long-term sequestration (decades to centuries) of carbon via meridional overturning circulation resulting in ventilation of the deepest ocean layers. While observations show a 29% decrease in the deep-water DOC concentrations as it is transported via abyssal circulation from the North Atlantic to the North Pacific (Hansell and Carlson, 1998, Hansell et al., 2009), a detailed picture of the transport and decay of DOC along its pathways into the deep North Atlantic is still largely missing. Furthermore, the extent of DOC variability, the details of its export (amount and quality), and its rate of decay and contribution to apparent oxygen utilization (AOU) in the interior of the North Atlantic basin have yet to be detailed. These processes set the initial condition for further distribution of the exported DOC into the rest of the deep ocean via the global ocean “conveyor belt”.

This lack of knowledge exists because of the paucity of high-quality DOC observations throughout the world oceans and from the North Atlantic in particular. In 2003, high quality and spatially expansive sampling of DOC was conducted in this basin as part of the Climate Variability (CLIVAR) Repeat Hydrography program (Feely et al., 2005). Here, we present recent DOC and hydrographic observations from three basin-scale meridional transects across the North Atlantic Ocean (i.e. CLIVAR lines A16N, A20, and A22). These data reveal the distribution patterns and dynamics of DOC as it is exported and transformed within the mesopelagic and bathypelagic zones of the North Atlantic basin. The DOC data, in combination with chlorofluorocarbon (CFC) ventilation ages and AOU tracers, were used to: (1) estimate DOC export and its contribution to total export production; (2) estimate the decay rates of exported DOC; and 3) determine the degree to which the oxidation of DOC contributes to AOU within the interior of the North Atlantic basin. The data confirm that DOC export by the Atlantic Ocean’s meridional overturning circulation is an important component of the biological pump.

Section snippets

Study area

Data are presented from three meridional transects conducted in the North Atlantic as part of the US CLIVAR Repeat Hydrography program in 2003. The cruises were conducted aboard the NOAA Ship Ronald H. Brown (A16N; June–July 2003) and the R.V. Knorr (A20 and A22; October–November 2003), covering latitudinal ranges of 6°S to 63°N along longitude 20°–29°W (line A16N), 7°N–43°N along 52°W (line A20), and 11°N–40°N along 66°W (line A22) (Fig. 1). Hydrographic data were collected with a nominal

Meridional trends in mean DOC concentrations within the euphotic, mesopelagic and bathypelagic zones

Characteristics of North Atlantic water masses considered here are presented in Table 2. In general, water masses of higher density had elevated pCFC-12 ages and AOU-Ceq, and lower DOC concentrations. Mean DOC concentrations demonstrate systematic meridional trends within depth horizons characteristic of the epipelagic zone (0–100 m), mesopelagic zone (100–1000 m) and bathypelagic zone (1000–3000 m) (Fig. 2). The mean euphotic zone DOC concentration maxima were observed in the highly stratified

Acknowledgments

This work was conducted as a component of the US CLIVAR repeat hydrography program, supported by funds from NSF OCE-0752972 to D.A.H and C.A.C. and OCE02241614 and NSF OCE02241614, NSF OCE0648541 and NASA NNX09AL09G to N.B.N., D.S. and C.A.C. Support for WMS is from OCE-0223951. We thank the officers, crew, chief scientists and technicians from the R.V. Ron Brown and the R.V. Knorr. Conversations with M. Brzezinski, S. Goldberg, C. Swan and C. Nelson and comments from three anonymous reviewers

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