Daily variability of dissolved inorganic radiocarbon in Sargasso Sea surface water

Surface water samples were collected daily in June 2000 at a site in the Sargasso Sea to observe variability of Δ 14 C values in dissolved inorganic carbon (DIC). Temperature, salinity, DIC concentration, alkalinity, and δ 13 C and Δ 14 C values of DIC were measured in the samples. Ten Δ 14 C measurements averaged 81±8 ‰ and had a range of 24 ‰ over the sixteen-day cruise. Δ 14 C values were more variable during the latter half of the cruise. Salinity and temperature measurements in the mixed layer throughout the cruise indicate that there were changes in water mass that occurred at our site. We conclude that the daily range of DIC Δ 14 C values in the surface ocean at our site is several times greater than the annual change in surface waters in the Sargasso Sea during the last two decades of the 20th century. This points to the importance of obtaining multiple measurements of the surface ocean to adequately define the true variability of DIC Δ 14 C measurements.


Introduction
Natural 14 C/ 12 C ratios (b AD 1880) in DIC in the nonpolar surface ocean were approximately 4-7% lower than that in the atmosphere (Bien et al., 1963;Broecker et al., 1960;Linick, 1980). This is known as the reservoir age of the surface ocean (330-580 14 C years) (Stuiver et al., 1986). The depletion of Δ 14 C values in surface seawater with respect to that in the atmosphere is caused primarily by mixing between surface and subsurface waters that are depleted in 14 C, and in small part to the long equilibration time of 14 CO 2 between air and sea (∼10 years). Knowing the short-term variability of surface ocean DIC Δ 14 C is important for establishing the uncertainty in the reservoir age of DIC at a given location. To date, this variability has not been adequately investigated.
An understanding of the natural variability of DIC Δ 14 C in surface waters is necessary for interpretation of Δ 14 C results obtained as a part of carbon cycling studies. Dissolved inorganic carbon is the precursor of all organic material produced in the surface ocean by photosynthesis. Because organic matter is transported to the deep sea in the form of fecal pellets and marine snow, knowledge of the variability of Δ 14 C values in surface DIC is important for interpreting the Δ 14 C measurements of organic matter at depth in the ocean.
An earlier study of surface DIC Δ 14 C results of samples taken every 2-4 days in the central North Pacific Ocean in October 1985 revealed that the Δ 14 C Marine Chemistry 106 (2007) 510 -515 www.elsevier.com/locate/marchem values were more variable during and after a storm . They suggested that high winds caused increased variability of surface Δ 14 C and higher DIC concentrations. This study reports isotopic and chemical measurements of daily surface water samples collected from a site in the Sargasso Sea during June 2000. Our results indicate that variability of DIC Δ 14 C values observed during the cruise was likely associated with changes in the water mass at our site.

Collection site and methods
Samples were collected daily from a single site in the Sargasso Sea, 100 km southeast of Bermuda (31°50′, 63°30′W, bottom depth 4380 m), from 14 to 29 June 2000. This site had previously been occupied from 29 May to 20 June 1989 and DIC Δ 14 C results of a depth profile were reported earlier (Druffel et al., 1992).
Using a plastic bucket and rope deployed over the side of the R/V Knorr, seawater samples were collected from a depth of 0-0.5 m for DIC Δ 14 C, δ 13 C, concentration ([DIC]), alkalinity and salinity measurements. Sea surface temperature (SST) measurements were made using a mercury thermometer (± 0.2°C). Samples were taken daily between 1200 and 1300 h local time. Seawater samples for isotopic, [DIC] and alkalinity analyses were poisoned with saturated HgCl 2 solution to prevent biological remineralization of organic matter.
The DIC Δ 14 C samples were processed for carbon isotopic analysis according to standard methods (McNichol et al., 1994). Six of the sixteen samples were lost prior to analysis. Briefly, a 500-ml sample was acidified with 85% phosphoric acid while nitrogen gas was bubbled through the solution to strip it of CO 2 ; the CO 2 was collected cryogenically in a liquid nitrogen-cooled trap. The CO 2 was converted to graphite with hydrogen gas on cobalt catalyst at 550°C (Vogel et al., 1987) and measured at the Keck Carbon Cycle Accelerator Mass Spectrometry (AMS) Laboratory at UC Irvine (Southon et al., 2004).
Radiocarbon measurements are reported as Δ 14 C in per mil for geochemical samples with age correction from time of collection to time of AMS analysis (Stuiver and Polach, 1977). Statistical uncertainties for the individual AMS Δ 14 C measurements was ±2.5-3.0‰; the total uncertainty determined from replicate seawater analyses was ±3.9‰. Stable carbon isotope results (δ 13 C) were performed on a Finnigan Delta Plus isotope ratio mass spectrometer at UCI on splits of CO 2 from the processed 14 C sample and the total uncertainty was ±0.06‰. Alkalinity and [DIC] measurements were obtained by closed vessel titration of large volume (∼100 ml) samples using an automated titration system (Bradshaw et al., 1981;Brewer et al., 1986) in the laboratory of D. McCorkle (Woods Hole Oceanographic Institution). Alkalinity and [DIC] were determined using a nonlinear curve fitting approach (DOE, 1994) and standardized using certified reference materials (Dickson et al., 2003). The standard deviation of pairs of replicate analyses of culture water was 6 μmol/kg for [DIC]. Alkalinity measurements of samples from this cruise were ∼20 μeq/kg higher than values for samples obtained from a previous occupation to this site in 1989 (Druffel et al., 1992), likely due to the extended period between collection and analysis (1 year). Thus, we assign a larger uncertainty to the alkalinity values (±20 μeq/kg).

Results
The Δ 14 C measurements of daily surface samples ranged from 68‰ (day 10) to 92‰ (day 8) (Fig. 1a and Table 1). The Δ 14 C values for days 3-8 averaged 87 ± 3‰, whereas the following two samples (days 10 and 11) had a significantly lower average Δ 14 C value (69 ± 1‰). The average Δ 14 C value of the last six samples (76 ± 8‰, days 10-16) is 11‰ lower than the average of the four initial samples, however this difference is not statistically significant.
Wind speeds measured during the cruise ranged from 0-6.0 m/s (Fig. 2c). Higher wind speeds (N4.0 m/s) were recorded on days 1, 2, 8, 9, and 14. Data from multiple casts of a Seabird CTD revealed salinity and temperature measurements every 2 m at our site throughout the cruise. Fig. 3a shows large shifts in salinity in the upper 25 m of the water column on days 7, 11, 14 and 16. Temperatures near the surface increased throughout the cruise, and values below 20 m were 2-3°C higher after day 7 (Fig. 3b).

Discussion
Data from the isotopic, chemical and physical time series each displayed unique trends. The Δ 14 C values were significantly lower during days 10-11 and 14 than those during days 3-8. If the lower Δ 14 C values during days 10-11 were the result of increased vertical mixing between surface and subsurface waters caused by increased wind during days 8-9, then a mass balance calculation can be used to assess the mixing necessary to achieve the low Δ 14 C values. The average of DIC Δ 14 C values obtained during days 1 and 2 on this cruise from 20 and 100 m depth (63 ± 8‰; (Druffel et al., in preparation) was 24 ±11‰ lower than the average Δ 14 C value (87 ± 3‰) of the surface samples from days 3-8 (Table 1). Calculations reveal that 75 ± 20% of the surface waters would have been replaced by subsurface waters from day 8 to day 10 to account for the decrease of the average Δ 14 C value observed on days 10 and 11  (69 ± 1‰). Because the highest winds (5.5-6 m/s) of the cruise occurred on day 8, it is feasible that lower Δ 14 C values in the surface samples could have been caused by increased mixing induced by the wind event. δ 13 C values in subsurface waters are equal to or slightly lower than surface values (by 0. 2‰ Druffel et al.,in preparation) because of remineralization of organic matter. Thus, the slight increase of δ 13 C values we observe Nday 11 (Fig. 1b) does not support increased vertical mixing during the second half of the cruise. Instead, lateral change in water mass is the more likely explanation for the variability in isotopic, chemical and physical (SST) parameters we observed during the cruise. The large range of salinity values in the upper 25 m (Fig. 3a) indicates that the water mass was changing with time. At times, salinity values exceeded 37.0‰, indicating the presence of salinity maximum water that originates further south during excessive evaporation (Blanke et al., 2002;Worthington, 1976). It appears that high salinity water moved into the sampling site after day 2 (Fig. 3a). Also, temperature profiles show an increase of 2-3°C between days 7 and 11 ( Fig.  3b), which could not have been caused by insolaration of a single water mass. Instead, the water mass at our site was continually changing, and with it, came waters with different characteristics, including variable Δ 14 C signatures. The Alkalinity and [DIC] values also indicate a different water mass was present during the last day of the cruise because it is unlikely that these changes reflect a biological process in a single water mass.
The trend in Δ 14 C values for the SarC cruise shows a range of 24‰ within a short time period (days). This is compared to the annual decrease of Δ 14 C in Bermuda corals  and surface seawater DIC from 1980-2000 of 3-4‰ per year (Fig. 4). This illustrates that the present, daily variability of Δ 14 C at this site in the Sargasso Sea is several times greater than the annual change in this region. This is important for parameterization of ocean circulation and atmosphere/ocean coupled models that utilize seawater Δ 14 C measurements e.g. Geosecs and WOCE, as a regulator of carbon transfer and cycling between atmosphere and ocean. The true variability of surface ocean Δ 14 C measurements (± 12‰) is actually much larger than the reported uncertainty (±4‰).
In 2000, Δ 14 C of atmospheric CO 2 was 94‰ (Levin et al., 2003), only 13‰ higher than the average value in the surface ocean of the Sargasso Sea (81 ± 8‰) (Fig. 4). This difference is smaller than that observed during steady state conditions bA.D. 1880 (∼ 40 to 50‰ Druffel, 1997). This indicates that by the year 2000, the Sargasso Sea surface ocean was no longer a net sink for bomb 14 C, as had been the case during the 1960s-1990s when the air-sea 14 C gradient exceeded 50‰. Similar findings have been reported for soil organic matter, where pore-space CO 2 is higher in Δ 14 C compared to atmospheric CO 2 because of remineralization of a bomb 14 C-enriched organic matter fraction (Trumbore, 2006).

Conclusions
1. The range of daily surface ocean DIC Δ 14 C values (24‰) is several times greater than the annual change in surface waters in the mid-gyre region (3-4‰). 2. The observed changes in surface ocean DIC Δ 14 C were likely the result of water mass changes that occurred at our sampling location throughout the cruise. 3. We show that single seawater measurements may not reflect the true Δ 14 C value for the seawater in the Sargasso Sea and that numerous measurements taken over a several day period may be necessary to define the true range of values for that location.  ) (small filled circles), and average DIC Δ 14 C values (large filled circles) from the SarC cruise (this work) and the Hydros cruise in June 1989 (Druffel, in preparation;Druffel et al., 1992) (ticks mark 1 January). Δ 14 C values of atmospheric CO 2 are also shown (|) (Levin et al., 2003). Error bars indicate the total uncertainty (Bermuda corals and air CO 2 ) and range of daily values (SS seawater DIC).