Search for: All records

Ocean circulation supplies the surface ocean with the nutrients that fuel global ocean productivity. However, the mechanisms and rates of water and nutrient transport from the deep ocean to the upper ocean are poorly known. Here, we use the nitrogen isotopic composition of nitrate to place observational constraints on nutrient transport from the Southern Ocean surface into the global pycnocline (roughly the upper 1.2 km), as opposed to directly from the deep ocean. We estimate that 62 ± 5% of the pycnocline nitrate and phosphate originate from the Southern Ocean. Mixing, as opposed to advection, accounts for most of the gross nutrient input to the pycnocline. However, in net, mixing carries nutrients away from the pycnocline. Despite the quantitative dominance of mixing in the gross nutrient transport, the nutrient richness of the pycnocline relies on the large-scale advective flow, through which nutrient-rich water is converted to nutrient-poor surface water that eventually flows to the North Atlantic.

Bulk sediment δ¹⁵N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ¹⁵N, then a deglacial δ¹⁵N maximum, followed by a gradual decline to a late Holocene δ¹⁵N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ¹⁵N of the LGM has been interpreted to reflect an ice age reduction in water column denitrification. We report foraminifera shell‐bound nitrogen isotope (FB‐δ¹⁵N) measurements for the two speciesNeogloboquadrina dutertreiandNeogloboquadrina incomptaover the last 35 ka in two sediment cores from the eastern equatorial Pacific (EEP), both of which have the typical LGM‐to‐Holocene increase in bulk sediment δ¹⁵N. FB‐δ¹⁵N contrasts with bulk sediment δ¹⁵N by not indicating a lower δ¹⁵N during the LGM. Instead, the FB‐δ¹⁵N records are dominated by a deglacial δ¹⁵N maximum, with comparable LGM and Holocene values. The lower LGM δ¹⁵N of the bulk sediment records may be an artifact, possibly related to greater exogenous N inputs and/or weaker sedimentary diagenesis during the LGM. The new data raise the possibility that the previously inferred glacial reduction in ETP water column denitrification was incorrect. A review of reconstructed ice age conditions and geochemical box model output provides mechanistic support for this possibility. However, equatorial ocean circulation and nitrate‐rich surface water overlying both core sites allow for other possible interpretations, calling for replication at non‐equatorial ETP sites.

Dissolved organic nitrogen (DON) is the dominant form of fixed nitrogen in most low and middle latitude ocean surface waters. Here, we report measurements of DON isotopic composition (δ¹⁵N) from the west South China Sea (SCS), with the goal of providing new insight into DON cycling. The concentration of DON in the surface ocean is correlated (r = 0.70,p < 0.0001) with chlorophyllaconcentration, indicating DON production in these surface waters. The concentration and δ¹⁵N of DON fall in a relatively narrow range in the surface ocean (4.6 ± 0.1 μM and 4.3 ± 0.2‰ vs. air, respectively; ±SD), similar to other ocean regions. The mean DON δ¹⁵N above 50 m (4.5 ± 0.3‰) is similar to the δ¹⁵N of nitrate in the “shallow subsurface” (i.e., immediately below the euphotic zone; 4.6 ± 0.2‰) but is higher than the δ¹⁵N of suspended particles in the surface ocean (~2.3‰). This set of isotopic relationships has been observed previously (e.g., in the oligotrophic North Atlantic and North Pacific) and can be explained by the cycling of N between particulate organic nitrogen (PON), DON, and ammonium, in which an isotope effect associated with DON degradation preferentially concentrates¹⁵N in DON. Consistent with this view, a negative correlation (r = 0.70) between the concentration and the δ¹⁵N of DON is observed in the upper 75 m, suggesting an isotope effect of ~4.9 ± 0.4‰ for DON degradation. Comparing the DON δ¹⁵N data from the SCS with other regions, we find that the δ¹⁵N difference between euphotic zone DON and shallow subsurface nitrate δ¹⁵N (Δδ¹⁵N_(DON‐NO3)) rises from ocean regions of inferred net DON production to regions of net DON consumption, with the SCS representing an intermediate case.