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Rivers discharge significant quantities of dissolved organic carbon (DOC) to the ocean, yet biomarker and isotope studies suggest that terrigenous DOC makes up only a small amount DOC in the ocean. One of the removal pathways proposed for riverine DOC is sorption to marine sediments. This process is chemically selective, but whether sorption alters the isotopic composition of riverine DOC is unknown. Because there is isotopic variability across different organic compound classes, sorptive removal of DOC could also alter the isotopic composition of DOC. As a first step in addressing this question, we examined phase partitioning and isotopic composition of a riverine DOC standard in the presence of marine sediment particles. In a series of controlled experiments, the standard was mixed with marine sediment in 35‰ NaCl solution, then separated into particulate and dissolved phases for analyses of mass, δ¹³C, and ∆¹⁴C of organic carbon (OC). Across a range of sediment OC to DOC mass ratios (from < 0.1 to ~ 3), we found that: (1) sediment sorbed 0.8 μg OC per mg of sediment; and (2) DOC compounds with higher ∆¹⁴C and lower δ¹³C values relative to the bulk DOC was preferentially removed from solution. In effect, mixing a riverine DOC standard with marine sediment resulted in increased ∆¹⁴C and decreased δ¹³C of the DOC that remained in solution. These results show that sorption of DOC to sediment can alter the isotopic content of riverine DOC.

 

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 bomb¹⁴C 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 the¹⁴C 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.

 

Abstract The removal mechanism of refractory deep-ocean dissolved organic carbon (deep-DOC) is poorly understood. The Amundsen Sea Polynya (ASP) serves as a natural test basin for assessing the fate of deep-DOC when it is supplied with a large amount of fresh-DOC and exposed to strong solar radiation during the polynya opening in austral summer. We measured the radiocarbon content of DOC in the water column on the western Amundsen shelf. The radiocarbon content of DOC in the surface water of the ASP reflected higher primary production than in the region covered by sea ice. The radiocarbon measurements of DOC, taken two years apart in the ASP, were different, suggesting rapid cycling of DOC. The increase in DOC concentration was less than expected from the observed increase in radiocarbon content from those at the greatest depths. Based on a radiocarbon mass balance, we show that deep-DOC is consumed along with fresh-DOC in the ASP. Our observations imply that water circulation through the surface layer, where fresh-DOC is produced, may play an important role in global DOC cycling. 

Radiocarbon (∆¹⁴C) measurements suggest the deep ocean stores marine dissolved organic carbon (DOC) on millennial timescales. The mechanisms that mediate this residence time remain unconstrained. Solid‐phase extraction (SPE) has emerged as a widely used technique to isolate DOC for subsequent analyses. We present SPE‐DOC concentrations and ∆¹⁴C values for three GO‐SHIP Repeat Hydrography transects, spanning the Pacific, Southern and Indian Oceans. Comparisons of SPE‐DOC with total DOC ∆¹⁴C values are used with an isotopic mass‐balance to estimate the size of the refractory DOC (RDOC) reservoir and changes in RDOC relative abundance in the global ocean. Estimated RDOC abundance is similar across the deep Pacific and Indian Oceans (average = 93 ± 5%, 35 ± 6 μM), whereas RDOC in the surface ocean varies as a function of total DOC concentration. Our results fill in spatial SPE‐DOC ∆¹⁴C sampling gaps for the global ocean, and our mass‐balance RDOC estimates are consistent with previous observations.

 

We have generated a high‐resolution coral Δ¹⁴C record from the leeward side of the Big Island of Hawai’i in the subtropical North Pacific. The record spans 1947–1992, when the coral was collected, and includes a brief prebomb interval as well as the postbomb era. Mean prebomb (1947–1954) values average −55‰ (±1, SE of the mean) with a clear seasonal cycle. Values are less positive during winter when vertical exchange mixes surface and lower‐¹⁴C subsurface waters. The postbomb annual maximum occurs in 1971 (+160‰) and decreases in a series of shifts to +105‰ in 1991, the end of our coral‐based reconstruction. The decrease is not monotonic and has inflection points during the La Niña years of 1973, 1977, and 1984. Imbedded in the Δ¹⁴C record is interannual variability in the El Nino‐Southern Oscillation band which is interpreted to reflect the lateral advection of low latitude surface waters as part of the oceanic Hadley Cell driven by Sverdrup dynamics.

 

ABSTRACT Radiocarbon ( 14 C) in dissolved inorganic carbon (DIC) was measured for water samples collected from six deep stations in the Kuroshio Extension (KE) region in the northwestern North Pacific in April–May 2015. Vertical profiles of Δ 14 C-DIC indicate that bomb-produced 14 C was present from the surface to ~1500 m water depth. Large variations in Δ 14 C-DIC values (300‰) were observed at 500 m water depth among the stations and the differences were likely controlled by transport and mixing dynamics of different water masses in the region. The major Pacific western boundary currents, such as Kuroshio and Oyashio and regional mesoscale eddies, could play important roles affecting the observed Δ 14 C-DIC variability. The depth profiles of both Δ 14 C-DIC and DIC concentrations can be predicted by the solution mixing model and can be used as conservative tracers of water mass movement and water parcel homogenization in the ocean. 

Large volumes of cool water are drawn up to the surface in the tropical oceans. A companion paper shows that the cool water reaches the surface in or near the upwelling zones off northern and southern Africa and Peru. The cool water has a subantarctic origin and spreads extensively across the Atlantic and Pacific basins after it reaches the surface. Here, we look at the spreading in two low‐resolution ocean general circulation models and find that the spreading in the models is much less extensive than observed. The problem seems to be the way the upwelling and the spreading are connected (or not connected) to the ocean's large‐scale overturning. As proposed here, the cool upwelling develops when warm buoyant water in the western tropics is drawn away to become deep water in the North Atlantic. The “drawing away” shoals the tropical thermocline in a way that allows cool subantarctic water to be drawn up to the surface along the eastern margins. The amounts of upwelling produced this way exceed the amounts generated by the winds in the upwelling zones by as much as 4 times. Flow restrictions make it difficult for the warm buoyant water in our models to be drawn away.

 

We report marine dissolved organic carbon (DOC) concentrations, and DOC Δ¹⁴C and δ¹³C values in seawater collected from the Southern Ocean and eastern Pacific GOSHIP cruise P18 in 2016/2017. The aging of¹⁴C in DOC in circumpolar deep water northward from 69°S to 20°N was similar to that measured in dissolved inorganic carbon in the same samples, indicating that the transport of deep waters northward is the primary control of¹⁴C in DIC and DOC. Low DOC ∆¹⁴C and δ¹³C measurements between 1,200 and 3,400 m depth may be evidence of a source of DOC produced in nearby hydrothermal ridge systems (East Pacific Rise).

 

We report marine dissolved organic carbon (DOC) concentrations, and DOC ∆¹⁴C and δ¹³C values in seawater collected from the central Pacific. Surface ∆¹⁴C values are low in equatorial and polar regions where upwelling occurs and high in subtropical regions dominated by downwelling. A core feature of these data is that¹⁴C aging of DOC (682 ± 86¹⁴C years) and dissolved inorganic carbon (643 ± 40¹⁴C years) in Antarctic Bottom Water between 54.0°S and 53.5°N are similar. These estimates of aging are minimum values due to mixing with deep waters. We also observe minimum ∆¹⁴C values (−550‰ to −570‰) between the depths of 2,000 and 3,500 m in the North Pacific, though the source of the low values cannot be determined at this time.