Radiocarbon in dissolved organic matter in the central North Pacific Ocean

The origin of dissolved organic carbon (DOC) in the ocean has been long debated. Whereas Mantoura and Woodward1 have used the conservative nature of DOC in a British estuary to conclude that ≥50% of DOC in the oceans could be river-derived, recent lignin results in the equatorial Pacific2 have indicated that ≤10% of the DOC is potentially of terrestrial origin. In addition, the δ13C signature (relative to the PDB standard) of DOC ranges from –20 to –24‰ (refs 3,4), indicating that the primary source of DOC is from marine-derived organic carbon. Here we present the first detailed profile of radiocarbon measured in DOC and dissolved inorganic carbon (DIC) in the oligotrophic gyre of the central North Pacific. δ14C (per mil deviation from the activity of 19th-century wood) of DOC ranged from –150‰ (1,310 yr BP) in surface waters to –540‰ (6,240 yr BP) at 5,710 m, 40 m off the bottom, where these 'apparent ages' or 'residence times' are mean values for the combined constituents of the DOC. The surprising similarity in the shapes of the profiles of δ14C in the DOC and DIC pools suggests that similar processes are controlling the radiocarbon distribution in each of the two reservoirs and that bomb-produced radiocarbon has penetrated the DOC + DIC pools to a depth of ˜900 m. The depletion of the δ14C DOC values by 300‰ with respect to the δ14C DIC values suggests that a certain fraction of the DOC is recycled within the ocean on longer time-scales than DIC.

addition, the δ 13 C signature (relative to the PDB standard) of DOC ranges from -20 to -24‰ (refs 3,4), indicating that the primary source of DOC is from marine-derived organic carbon. Here we present the first detailed profile of radiocarbon measured in DOC and dissolved inorganic carbon (DIC) in the oligotrophic gyre of the central North Pacific. δ 14 C (per mil deviation from the activity of 19th-century wood) of DOC ranged from -150‰ (1,310 yr BP) in surface waters to -540‰ (6,240 yr BP) at 5,710 m, 40 m off the bottom, where these 'apparent ages' or 'residence times' are mean values for the combined constituents of the DOC. The surprising similarity in the shapes of the profiles of δ 14 C in the DOC and DIC pools suggests that similar processes are controlling the radiocarbon distribution in each of the two reservoirs and that bomb-produced radiocarbon has penetrated the DOC + DIC pools to a depth of ˜900 m. The depletion of the δ INTRODUCTION The origin of DOC in ocean water has long been the subject of debate. We have some basic knowledge of the sources and sinks of the carbon to the DOC pool. Inputs of DOC to sea water include intra-and extra-cellular solubles produced by phytoplankton and from sloppy feeding by zooplankton, dissolution of fecal material, plus DOC derived from atmospheric and riverine inputs. Less is known about the source of DOC from subsurface vents, sediments and living bacteria. Sinks for DOC include microbial utilization, coalescence to form particles and adsorption onto existing particles. Quantitative assessments of the importance of each of these processes are not fully evaluated. Overall, it appears that DOC is derived primarily from marine sources, considering the similarity of the 813C in DOC and in marine organisms (Williams & Gordon, 1970) and the low concentration of lignin present in sea water (Meyers-Schulte & Hedges, 1986). 14C was measured nearly two decades ago in DOC from the northeast Pacific to determine its turnover time in the ocean. Williams, Oeschger and Kinney (1969) reported L14C values of -351±27%o at 1880m and -341±23%0 at 1920m collected in 1968 and 1969, respectively, from 30°N, 120°W. Two years later, four samples from 30°N, 140°W were measured (-88±20, -11±18, -176±14, -274±13% o from 1Om, 200m, 500m and 2000m, respectively) (Arhelger et al, 1974;Williams et al, 1978). These 14C analyses were performed on CO2 resulting from UV-oxidation of acidified sea water. L Jeffries (unpub data) used adsorption onto activated charcoal as a means for separating the DOC from sea water. Her measurements on Gulf of Mexico water were -485%o at 690m, and -560%o at 2290m (Bada & Lee, 1977). Gas proportional counting techniques were used to measure 14C in these samples, thus, 500 -1000L of sea water were needed to make an analysis. Based on these early studies, deep-sea DOC has been assigned an apparent age of 3400 yr BP.  Williams and Druffel (1987) reveals that the 014C of UV-oxidizable DOC in the deep Pacific is much lower than previously reported. We present these measurements and concentrations of FAA, THAA and TCHO in dissolved organic matter from a detailed profile in the central North Pacific. From a near high-precision 14C profile measured in DIC (i14CDIc), we examine the penetration of the bomb 14C transient into the main thermocline since the GEOSECS survey of 1973. In addition, L14CDIC was measured in surface water samples collected every 2-3 days during the Alcyone-5 cruise to study short-term variability at a single open ocean site. The effect of a high wind storm on this short-term variability was also observed.

Collection of Samples
The data presented here are from samples collected aboard the RIV Melville during the Alcyone-5 cruise to the central North Pacific Ocean in October and early November 1985. One location was occupied (20km2, 31°N, 159°W) 800km north of Oahu, Hawaii in the predominantly eastward flowing North Pacific Current.
Gerard barrels, constructed of stainless steel with a volume of 270L, were used to obtain sufficient carbon for near high-precision 014C analyses of the DIC fraction. The barrels were scrubbed and rinsed clean with dilute HCI, xylene, acetone, methanol and distilled water to remove all traces of oil and other organic matter, as well as inorganic carbon. Partial disassemblage of the barrels was required so that o-ring grooves could be cleaned and viton o-rings installed. The plastic parts on the barrels were replaced with similar items constructed of brass (ie, water spigot). Barrels were mounted on the trawl wire and lowered to the desired depths. For near surface samples, a minimum of 20 min elapsed before the messenger was sent to close the lids to allow for adequate flushing at the desired depth. Drains and other tanks aboard ship were flushed out away from the immediate collection site. For the deeper samples, reversing thermometers were used to provide depth corrections, and salinity and silicate measurements from piggy-backed 5L Niskin bottles were compared with those from the barrels to check for pre-tripping or partial closure of the barrels.
Water for DOC concentration and 14C analysis was filtered using a prefired (500°C) GFC glass fiber filter in a PVC holder that attached directly onto the spigot at the bottom of the Gerard barrel. One-gal glass bottles, which were previously cleaned with hot chromic acid, rinsed with double distilled water and protected from the atmosphere, were then rinsed three times with the sample water and filled. They were then sealed with teflonlined caps and frozen at -20°C. Four bottles were collected from depths below 450m and two bottles were taken from shallower depths.
Aliquots of the same filtered sea water were collected in cleaned bottles for FAA, THAA and TCHO analyses, and frozen at -20°C. One-L samples from the upper 150m were collected and immediately filtered for chloro hyl-a and phaeophyton analyses. Samples were taken for total CO2 ([DICJ) and alkalinity (250m1).
The remainder of the water was pumped (using a Jabsco pump), unfiltered, into a plastic 220L plastic drum. The water was acidified, heated (50°C) and purged of CO2 using a peristaltic pump. The purged CO2 was absorbed into a solution of SrC12 and concentrated ammonium hydroxide, wherein strontium carbonate was precipitated (Linick, 1975).

Radiochemical and Chemical Analyses
For the dissolved organic 14C (DO'4C) analyses, lgal samples were quickly defrosted in warm water and 5L introduced into an all-glass reactor (Fig 1), acidified to pH 2.5 ± 0.2 with 50% H3P04, and sparged-free of DIC with nitrogen gas (organic and inorganic carbon-free) at 425m1 min-1 for 3 hr. The sea water was then saturated with clean oxygen gas and the reactor vessel closed using 85% H3PO4 to lubricate and seal the glass ball joints. The sample was oxidized for 6 hr using a 1200 watt Hanovia Hg-arc lamp at 70-75°C. The resultant CO2 derived from DOC was passed through the KI trap to collect chlorine gas, and water frozen out in the first Horibe trap cooled with dry-ice isopropanol. The sample was then collected in the second Horibe trap (Fig 1) using liquid N2. This collection was done by sparging the reactor with nitrogen gas at 235m1 min-1 for 2 hr at atmospheric pressure. The trapped CO2 was then pumped free of condensed air using the rough vacuum, and transferred successively from the Horibe trap to the Utube, then to the cold finger below the Pirani gauge. After measuring the CO2 pressure, the CO2 was split into samples for 13C and 14C analyses. The CO2 from the DOC was converted to graphite at the University of Arizona (Dull et al, 1986). 14C was measured using AMS techniques at the University of Arizona TAMS Facility ). Errors of ±4-15%0 were obtained, and were dependent upon sample size and AMS system stability at the time of analysis. Q14C was calculated assuming a S13C of -21%0 for the graphite targets. S13C-DOC measurements reported in Table 1 were performed on CO2 obtained directly from the UV oxidation of DOC in sea water.
For the DI14C analyses, the strontium chloride and ammonium hydroxide solution was decanted off, then heated to dryness, leaving solid strontium carbonate. CO2 was liberated using 4 N HCI, and converted to acetylene gas (Griffin & Druffel, 1985). The acetylene samples were counted for 4-6 2-day periods in quartz gas proportional beta counters at 90.0cm Hg and 21°C. Errors ranged from ±2.4 -3.6% as determined from counting statistics. S13CDIc was measured on CO2 from reburned acetylene gas samples.
The FAA and THAA analyses were done using HPLC fluorimetric determination of the o-pthaldialdehyde derivatives (Lindroth & Mopper, 1979), after acid hydrolysis of the combined amino acids (Robertson, Williams & Bada,1987). The TCHO was measured spectrophotometrically on hydrolyzed samples using a modification of the procedure of Burney and Sieburth (1977) and are reported as glucose equivalents. The conversion factor for amino acids (nM) to carbon is 4.4 times amino acid concentration, and for the carbohydrates (MM), this factor is 6 times the glucose equivalents.

RESULTS AND INTERPRETATION
The 14CDOC and 14CDIc data and results of chemical analyses are listed in Table 1. Figure 2 shows the O14CDOC and z14CDIC results. An initial discussion and a graphic representation of all but two of the Z14CDOC results were presented by Williams and Druffel (1987).
The L14CDOC values in the upper 2000m of the water column are 60-220%0 lower than those from the same depths observed in the earlier studies (Williams, Oeschger & Kinney, 1969;Arhelger et al, 1974, Williams et al, 1978. Though the earlier results were obtained from 1300-2600km east, the differences in L14CDOC are probably not due to location. We believe that the earlier results are due to incomplete oxidation of the lower activity (older) fraction of DOC as the original yields were only of the order of 70% of the total UV-oxidizable fraction.
The O14CDIc results are plotted vs those reported by Ostlund and Stuiver (1980) for the GEOSECS Station 212 (30°N, 160°W) in September 1973 (Fig 2b). These data show that bomb 14C has penetrated an additional 150m in 12yr, from ca 850-1000m. The prebomb D14CDIC profile in the upper ocean is shown for comparison.
Ol4CDOc values are 300% lower than the z14CDIc values at all depths of the water column (Fig 2c). The similarity in the shape of the two profiles     in atoms 2e 14C/L x 10 vs depth at 31°N, 159°R suggests that bomb 14C has penetrated into the DOC pool to at least the same depth as in the DIC pool, that is ca 1000m. The shift toward lower numbers indicates that a major component of the DOC pool is 'old' with respect to 14C, and is recycled on very long time scales (103-104 yr or longer).
In order to observe changes in the concentration of 14C in each of these carbon pools, [D014C1 and [DI'4C] values are calculated using the following equations: where 8 14C is defined by Stuiver and Polach (1977), f is the 14C/C ratio in 95% NBS oxalic acid-1 standard (1.176 x 10-12; Stuiver, 1980), A is Avogadro's number, and [DOCI and [DICI are in moles/L. Figure 2d shows that the surface [D014C] is four times greater than that in the deepsea, whereas surface [DI 14C] is only 1.3 times that in the deep-sea. The large D014C concentration gradient illustrates that large amounts of DOC are recycled within the upper few hundred meters of the water column by (1) remineralization of organic matter fixed in the euphotic zone, and (2) remineralization of organic matter in the upper 1000m of the water column due to microbial and other degradative processes.
Results of vertical profiles of FAA, THAA, TCHO and DOC are listed in Table 1 and shown in Fig 3. The FAA results (Fig 3a) are very low below 450m depth; they indicate that there is little if any contamination by labile carbon of the Gerard barrel samples during collection. The low results also illustrate rapid bacterial utilization of FAA in deep waters. The THAA results (Fig 3b) were ca 100 times higher than FAA and show variations with depth. The increase in THAA from 400-800m may be caused by partial conversion of the surface-derived particulate organic carbon rain to DOC or insitu production of organic matter (Cindy Lee, pers commun, 1988). The TCHO values are about seven times higher than the THAA values, and are essentially invariant with depth below 900m. The sum of THAA and TCHO carbon is ca 11% of the total DOC below 1000m and comprises 19% of the DOC at the surface (Fig 3c).
The seven DOC values denoted by open circles in Figure 3c are 6-14% higher than the other DOC values. Likewise, the 14CDOC of 6 of these 7 samples were higher than the other values (Fig 2a). We suspect that the higher DOC values were the result of a `younger' fraction of DOC that was oxidized only in the first seven samples processed using the initial "more potent" Hg-arc UV lamp (for discussion, see Druffel, Williams & Suzuki, 1989). '4C measured in 11 surface water samples collected over the course of the 33-day cruise are listed in Table 1. Pre-storm z14CIC results are relatively constant, with an average of 151.3 ± 0.8 (SD) %o (n=3) through day-8 of the cruise. Post-storm results, however, were more variable and averaged 146.6 ± 6.2 (SD) %o (n=8). In order to observe changes in DI14C concentration with time [D114C1 values in the surface seawater samples are calculated using Eq 2 and shown in Figure 4a. [D114C1 was also more variable during the post-storm period. An increase was observed in [DI'4C] 4b), due primarily to the higher total CO2 values. Whether the higher values were due to entrainment of atmospheric CO2 into the mixed layer, or to spatial variability induced by storm activity, cannot be determined from the existing data set.

CONCLUSIONS
The study of 14C in DOC in the central North Pacific has revealed that a major portion of the DOC is recycled within the water column on time scales of 103 to 104 yr or longer. Bomb 14C is present to a depth of at least 1000m in the DOC pool, and is probably present in small amounts in the deep sea. Labile constituents of DOC (THAA and TCHO) are present in minor amounts in sea water (11-19% of DOC).
The 14CDIC profile reveals that the bomb 14C transient has penetrated an additional 150-200m since the GEOSECS survey of 1973. These data have important implications for determining the ventilation rate of the main thermocline with respect to excess CO2.