Dissolved Organic Radiocarbon in the West Indian Ocean

We report marine dissolved organic carbon (DOC) concentrations, and DOC Δ14C and δ13C in seawater collected from the West Indian Ocean during the GO‐SHIP I07N cruise in 2018. We find bomb 14C in DOC from the upper 1,000 m of the water column. There is no significant change in ∆14C of DOC in deep water northward, unlike that of dissolved inorganic carbon (DIC), suggesting that transport of deep water northward is not controlling the 14C age of DOC. Variability of DOC ∆14C, including high values in the deep waters, is more pronounced than in other oceans, suggesting that dissolution of surface derived particulate organic carbon is a source of modern carbon to deep DOC in the West Indian Ocean. Low δ13C are present at two of the five stations studied, suggesting a source of low δ13C DOC, or additional microbial utilization of deep DOC.

• 14 C aging of deep dissolved organic carbon (DOC) northward is not similar to that of deep dissolved inorganic carbon, showing that transport of deep water is not the main control of 14 C in DOC • Variability of deep DOC 14 C indicates heterogeneity in the DOC pool, perhaps by dissolution of surface-derived, particulate organic C • Low DOC δ 13 C in deep water from two stations may suggest a source of chemosynthetically produced organic matter

Supporting Information:
Supporting Information may be found in the online version of this article. 10.1029/2023GL104732 2 of 8 The global ocean overturning circulation begins with formation of dense, cold waters at high latitudes that sink to deep depths in the North Atlantic and Southern Oceans.Deep waters flow toward lower latitudes, mixing with warmer, less dense waters above, transforming to warmer, less dense waters (Indian and Pacific deep waters) that return to the surface in the Southern Ocean (Stuiver et al., 1983;Talley, 2013;Toggweiler et al., 1989aToggweiler et al., , 1989b)).
In the deep Indian Ocean, Srinivasan et al. (2000) used corrected DI 14 C concentrations (adjusted for the addition of surface-derived, sinking particles) to estimate the mean upwelling transport of bottom water.They found faster upwelling in the West Indian Ocean than that further east due to the rough bottom topography in the west.Unlike the Pacific and Atlantic oceans, the Indian Ocean contains numerous, isolated basins, whose sills (2,500 m depth) prevent significant horizontal transport of deep water (Figure 1).Bercovici et al. (2018) reported DOC 14 C measurements from the southeast Indian and Southern oceans (from 29°S to 56°S) and concluded that DOC Δ 14 C are similar across water masses.For example, circumpolar deep water (CDW) and North Atlantic Deep Water (NADW) had DOC Δ 14 C of −491 ± 13‰ and −481 ± 8‰, respectively.
The Indian Ocean is unique because of its undersea topography, circulation patterns, and permanent oxygen minimum zone (OMZ) to the North.Despite its importance, the Indian Ocean is understudied relative to other ocean basins.The focus of this work is to use DOC carbon isotopic signatures (Δ 14 C and δ 13 C) to study DOC cycling in the Indian Ocean.We show that DOC Δ 14 C in deep water from the West Indian Ocean ranges from −442‰ to −517‰, with variability that is greater than that observed in the Pacific and Southern oceans (Druffel et al., 2019(Druffel et al., , 2021)).

Collection and Methods
Seawater was collected from two legs of the GO-SHIP I07N cruise along 52°E-57°E from 29°S to 8°N aboard the NOAA Ship Ronald H. Brown (April-June 2018) (Figure 1, Table S1 in Supporting Information S1).Samples collected from five stations (3, 23, 47, 72, and 97) were analyzed for 14 C and 13 C and reported here (Druffel, 2023).Samples collected from two other stations (119 and 123) at 17°N and 18°N were lost due to a failed freezer.DOC samples shallower than 800 m were filtered using pre-combusted (500°C, 2 hr), GF/F (0.7 μm 70 mm diameter) filters into pre-combusted, 1 L Amber Boston Round glass bottles and capped with acid cleaned (10% HCl), polytetrafluorethylene (PTFE)-lined caps and PTFE sheet liners (cleaned in Chromerge).DOC samples were frozen at sea at −20°C.
In the lab, DOC samples were thawed by immersion in hot water and shaken to fully dissolve precipitated salt crystals (Beaupré et al., 2007;B. D. Walker et al., 2019).Samples were diluted with 18.2 MΩ Milli-Q water ([DOC] = 0.8 ± 0.3 μM), acidified with LCMS grade 85% phosphoric acid, and stripped of DIC with He gas (grade 5.0).Samples were UV oxidized to CO 2 for 4 hr in a quartz reactor and collected as described by Beaupré et al. (2007).DOC concentrations ([DOC]) were corrected for CO 2 loss due to breakthrough from the Horibe glass trap cooled with liquid nitrogen during collection (B.D. Walker et al., 2019).Uncertainty of [DOC] values were ±0.6 μM.The mass was quantified by integration using an infrared CO 2 gas analyzer (LI-COR Inc., model LI-6252) (B.D. Walker et al., 2019).
For Δ 14 C measurements, samples were converted from CO 2 to graphite by reduction on iron catalyst using zinc (B.D. Walker & Xu, 2019;Xu et al., 2007).Radiocarbon measurements were made by us at the Keck Carbon Cycle AMS Laboratory at the University of California, Irvine (Santos et al., 2010).Radiocarbon results are reported as Δ 14 C values that are corrected for date of collection according to convention (Stuiver & Polach, 1977).Total uncertainty of the Δ 14 C analyses was ±4‰, the standard deviation of the differences between duplicate Δ 14 C analyses of seawater from the same Niskin collections.There was one set of duplicate analyses run for each station.The δ 13 C of each sample was measured on a split of the CO 2 produced from UV oxidation of the DOC sample using a Gas Bench II and Thermo Finnegan Delta Plus isotope ratio mass spectrometer, with a total uncertainty of ±0.2‰.

DOC Concentrations
The concentrations of DOC were highest in the upper 50 m of the water column (69.2-76.8μM; Figure S2 in Supporting Information S1 and Druffel, 2023).Concentrations decreased markedly between 83 m and 1,000 m

DOC δ 13 C Measurements
The

Discussion
The discussion is presented in four parts.First, the presence of bomb 14 C in DOC in the upper 1,000 m of the Indian Ocean water column is discussed.Second, we compare the 14 C aging of DOC in the deep Indian to that of DIC from previous cruises in CDW flowing northward.Third, we discuss the variability of the DOC Δ 14 C and δ 13 C in the deep Indian and its possible sources.Fourth, the relationship between surface DOC δ 13 C and sea surface temperature (SST) is discussed.

DOC Δ 14 C in Surface and Intermediate Water Masses
Bomb 14 C, produced in the late 1950s and early 1960s by thermonuclear weapons testing, is present in DIC (Stuiver & Ostlund, 1983) in the upper 1,000 m of the water column.Evidence of bomb 14 C in DOC in the upper 1,000 m of the water column has been shown (Cherrier et al., 1999;Williams & Druffel, 1987).The DOC in the North Pacific surface waters (Δ 14 C = −146) has been estimated to contain 44% old deep DOC (−520‰) and 56% surface bomb DOC (+150‰) (Williams & Druffel, 1987).

14 C Aging of DOC and DIC in Circumpolar Deep Water and Input of Surface POC to the Deep Indian
DOC 14 C ages in CDW are calculated from DOC Δ 14 C ( 14 C age = -ln((Δ 14 C + 1,000)/1,000) × 8,033 years) for samples whose densities (sigma4) ranged from 45.75-45.80(∼2,400-3,900 m depth).Flow of NADW travels around South Africa and then northward on both sides of Madagascar (Mantyla & Reid, 1995), though the I07N cruise was north of this flow.The DOC 14 C ages in CDW are plotted versus latitude of collection for water samples (Figure 4a).The Model II geometric mean regression of DOC 14 C ages versus latitude shows an insignificant increase (35 ± 100 14 C years) from 29°S to 8°N.Two DOC 14 C ages for samples from stn 3 (3,826 m 4,740 14 C years) and stn 97 (3,300 m 5,180 14 C years) were anomalous, having 14 C ages much younger than the other deep results and were not included in this analysis (see Section 4.3).
We compare DOC aging to DIC aging using measurements from past cruises that are plotted versus latitude (Figure 4b) (Key & Quay, 2002;Stuiver & Ostlund, 1983).The Model II geometric mean regression displays that an increase of the DIC 14 C age between 29°S (1,580 ± 10 14 C years) and 8°N (1,690 ± 10 14 C years) is 110 ± 20 14 C years (Figure S2b in Supporting Information S1).Results for DIC 14 C ages of results from the I07N cruise in 2018 from NOSAMS (Hansmann & Sonnerup, 2022) (107 ± 40 14 C years) (Figure 4c) are equivalent to those from the earlier DIC results.
Our analysis shows that DOC and DIC aging are dissimilar in CDW that flows northward in the West Indian Ocean.It is noted that the distance tracked is short (29°S to 8°N).Nonetheless, it appears that transport northward is not the primary control of DOC aging between these latitudes.D. C. Smith et al. (1992) reported that hydrolytic enzyme activity on marine aggregates rapidly converts POC to DOC.It is conceivable that hydrolysis of modern POC from the surface produces DOC during transport to the deep Indian, causing deep DOC Δ 14 C to be higher, thus counteracting the aging of DOC as water flows northward.To obtain aging of DOC similar to that of the DIC (110 14 C years), Δ 14 C of DOC at 8°N would need to decrease from −500‰ at 29°S to −507‰ at 8°N.This would require an input of 1% of surface POC with a Δ 14 C of 25‰ (Hansmann & Sonnerup, 2022) to deep DOC with a Δ 14 C of −507‰.Given that the average flux of organic C to the deep Northwest Indian (1,882 m depth) was ∼11 gC/m −2 /yr in 1996 (S.L. Smith, 2001), and a standing stock of DOC in the water column of 1,900 gC (4,000 m depth), this would provide an input of 0.6% of the deep DOC per year ((11 gC/1,900 gC) × 100).This is of the same magnitude as the amount of surface POC required to dissolve to DOC in the deep ocean to counteract the aging of 14 C of deep DOC (1%), and thus offers a possible mechanism for maintaining the relatively high Δ 14 C of DOC in the deep Northwest Indian Ocean.

Possible Source(s) of DOC Δ 14 C and δ 13 C Variability
Most of the deep DOC Δ 14 C between 2,600 and 4,000 m depth are comparable, with an average of −500 ± 3‰ (n = 12).There are four outlier values, from stn 3 (−442‰ 3,826 m), stn 97 (−475‰ 3,300 m, −513‰ 3,000 m), and stn 72 (−517‰ 2,601 m), that were not included in this average.In addition, there are four DOC δ 13 C values from stn 3 (averaged −21.9 ± 0.1‰) that are lower than all other I07N results.Also, there are 5 results from Stn 72 that are lower than most other analyses (average −21.5 ± 0.1‰).The average for all other samples was −21.2 ± 0.3‰ (n = 55).All samples with outlier Δ 14 C or δ 13 C have [DOC] values within error of the average deep [DOC].This suggests that either the cause of the isotopic outliers could be independent of [DOC] and within our analytical precision, or that some internal cycling of DOC is present at these locations (e.g., concomitant addition and removal of DOC while maintaining steady state [DOC]).
Reasons for isotopic variance in the deep DOC include: (a) the addition of DOC from either sinking POC or chemoautotrophic formation of DOC, (b) 14 C-free contamination (e.g., oil), and (c) input of chemosynthetic, low δ 13 C DOC from hydrothermal ridges and flanks.First, all samples deeper than 800 m depth were not filtered prior to collection.The concentration of suspended POC in open ocean water is low, generally <0.2 μmol kg −1 , whereas [DOC] ranges from 39 to 43μmol kg −1 .It is possible that large, sinking particles were collected in the niskin bottles from stns 3 and 97, and were transferred to our sampling bottles; this seems improbable, given that duplicate Δ 14 C and δ 13 C analyses from the same niskin water were ±4‰ and ±0.2‰, respectively (see Section 2).An estimate of the amount of surface POC that could account for increases in Δ 14 C (−442‰ at stn 3, -475‰ at stn 97) are made assuming surface Δ 14 C of POC was 25‰ in 2018 (Hansmann & Sonnerup, 2022) at both locations, and a deep DOC Δ 14 C average of −500‰.We calculate that 5% and 10% modern POC would need to have been dissolved to DOC at stn 97 and stn 3 to increase their Δ 14 C to −475‰ and −446‰, respectively.A 5%-10% shift in DOC concentration is 2-4 μM, which is >2 sigma of the error of our concentration values (±0.6 μM).
Of note is that the Arabian Sea is a significant OMZ in the North Indian, and we pose the question of whether this region could contribute young and/or old DOC to the deep water.Presently, little information is available as to how OMZs affect [DOC] and DOC cycling.Rixen et al. (2010) estimate that the Arabian Sea and the Gulf of Bengal have 21% of the total volume of oxygen depleted ocean waters, though our stations were outside of the limits of the Arabian Sea OMZ (lowest oxygen concentration was 25 μmol/kg at stn 97, 700 m).It remains an open question whether processes that occur in OMZs (annamox, denitrification, chemosynthesis) contribute to changes in the cycling of DOC in the Arabian Sea or the Bay of Bengal.
Second, the presence of a contaminant whose δ 13 C is lower than that of oceanic DOC (e.g., oil) likely did not occur in the four samples from stn 3 (Figure 3), because [DOC] would have been significantly higher in these samples than in those from surrounding seawater.We see no differences between [DOC] in the samples with low Δ 14 C or those with low δ 13 C than those of other samples from similar depths (Druffel, 2023).
Third, input of low δ 13 C DOC to the deep ocean, such as that associated with on and off-axis hydrothermal vent fluids may be occurring in the Indian Ocean, where hydrothermal ridge and flank systems are abundant, particularly near stn 3 and stn 97.Lang et al. (2006) reported chemosynthesis of DOC in ridge and flank basalts, and McCarthy et al. (2011) found low Δ 14 C and low δ 13 C in DOC emanating from ridge-flank and on-axis hydrothermal fluids in the northeastern Pacific.Thus, it is possible that DOC produced in hydrothermal flanks and ridge systems is present and responsible for lowering of DOC Δ 14 C and δ 13 C.

Surface DOC δ 13 C Versus Temperature
Surface ocean DOC δ 13 C (from 3 to 20 m depth) and SST of the water samples from the I07N cruise show similar correlation to those obtained from the Pacific and Southern oceans (Druffel et al., 2019(Druffel et al., , 2021) ) (Figure S3 in  Rau et al. (1989) reported that δ 13 C of surface plankton was correlated with SST, and that the variability of surface plankton δ 13 C was due to carbon isotopic fractionation caused by variable concentration of aqueous CO 2 in seawater.The similarity of plankton δ 13 C with DOC δ 13 C indicates that surface primary producers are a main component of DOC in the surface ocean.The similarity of DOC δ 13 C with plankton δ 13 C provides an important end-member in the two-component mixing model of marine DOC, where surface waters are believed to be a mixture of recently produced DOC and a background of old, refractory carbon (Beaupré et al., 2020).

Implications for the DOC Cycle and Future Work
We show that the aging of DOC and DIC northward in the deep subtropical and equatorial West Indian Ocean are dissimilar, unlike those observed previously for the central and East Pacific (Druffel et al., 2019(Druffel et al., , 2021)).This indicates that there are processes affecting the isotopic signatures of deep DOC in the Indian that are unlike those controlling DIC circulation in the deep global ocean.
A large fraction of the DOC in the deep ocean is refractory (Hansell, 2013), however, in the West Indian Ocean, there may be significant input of young DOC from dissolution of surface-derived POC.The North Indian Ocean contains a large OMZ that is maintained, in part, by large fluxes of POC into the deep ocean.There were only two samples from the Indian that had low Δ 14 C values (−513‰, 517‰), fewer than those found in the deep Pacific that suggested evidence of ancient DOC produced in hydrothermal ridge systems (Druffel et al., 2021).Locations similar to the Indian Ocean, where there are hydrothermal systems present throughout, may be swamped by large amounts of young DOC that is formed, possibly as a result of OMZ systems.

Figure 1 .
Figure 1.Map of stations sampled during the I07N cruise in the West Indian Ocean in April-June 2018.Dark blue regions indicate depths >3,000 m, and ridges and other areas <2,500 m depth are indicated by lighter shades of blue.