Dissolved Organic Radiocarbon in the Central Pacific Ocean

Abstract

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 that 14C aging of DOC (682 ± 86 14C years) and dissolved inorganic carbon (643 ± 40 14C 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.

Radiocarbon in Dissolved Organic and Inorganic Carbon of the South China Sea

“…No significant aging of the DOC can be seen for deep water in the SCS basin, suggesting that DOC has been transported with the inflow from the NWP as it traveled across the Luzon Strait and well mixed into the deep water. Additionally, δ 13 C‐DOC values in the SCS ranged between −20.5‰ and −23.5‰, consistent with the values reported recently for DOC in the ocean (Druffel et al, ; Zigah et al, ), suggesting that DOC cycling in the deep ocean could derive from mixed marine, terrestrial, and hydrothermal sources (Follett et al, ; Zigah et al, ). The surface circulation gyres affected by the strong monsoon have effectively isolated the SCS basin from the influence of land runoff in a wide oligotrophic area, such as the interior of the major ocean basins (Gong et al, ; Hu et al, ; Liu et al, ).…”

“…Furthermore, there was a significant correlation ( R 2 = 0.93, p < 0.001) between the Δ 14 C‐DOC and Δ 14 C‐DIC values obtained from water in the same Niskin bottles at the four stations in the SCS (Figure c). The parallel profiles and significant correlation suggest that DIC and DOC Δ 14 C are tightly coupled throughout the entire water column, and a common process would influence both carbon pools, as shown previously using data from the central NP (Beaupré & Druffel, ; Druffel et al, ).…”

“…Note that the depth below 2,500 m is on a different scale. The following Δ 14 C‐DIC and Δ 14 C‐DOC values are also shown for comparison with the results in this paper: (a) the Northwestern Pacific stations P10‐74 (28.5°N 149.3°E) in 1993, P10N‐73 (29.2°N 149.3°E) and P10N‐64 (22.5°N 149.4°E) in 2005 (Fukasawa et al, ; Key, ; Key et al, ); and (c) the central North Pacific stations Eve‐1 (31°N 159°W) in 1987, P16‐106 (19°N 152°W) and P16‐130 (31°N 152°W) in 2015 (Druffel et al, , ).…”

“…Comparing the SCS Δ 14 C‐DOC with the values reported previously in the central NP (Figure c) showed lower Δ 14 C‐DOC values (−322‰ to −258‰) in the SCS water shallower than 250 m because of intense vertical mixing of the deep waters that upwelled to the surface in this marginal sea, bringing lower Δ 14 C waters to the surface. In contrast, higher Δ 14 C‐DOC values (greater than −250‰) were found on the surface of the subtropical Pacific, where downwelling is dominant (Druffel et al, ). The relatively constant Δ 14 C‐DOC values for samples below 1,500 m in the SCS were similar to those from the central NP, with averages that were nearly equal within an acceptable error range (−525 ± 15‰ and −533 ± 17‰, respectively) (Figure c).…”

Low Molecular Weight Dissolved Organic Carbon: Aging, Compositional Changes, and Selective Utilization During Global Ocean Circulation

“…Global Biogeochemical Cycles shown that the Δ C value of DOC tracks the Δ 14 C value of DIC and is consistent with the time scales of water mass transit times (e.g., Bercovici, McNichol, et al, 2018;Druffel et al, 2019). Within our separate fractions we hypothesize that changes in Δ 14 C that differ from that of total DOC can be interpreted as changes in the age distribution of the pool caused by the removal or addition of DOC.…”