Since the mid-1980s, our understanding of nutrient limitation of oceanic primary production has radically changed. Mesoscale iron addition experiments (FeAXs) have unequivocally shown that iron supply limits production in one-third of the world ocean, where surface macronutrient concentrations are perennially high. The findings of these 12 FeAXs also reveal that iron supply exerts controls on the dynamics of plankton blooms, which in turn affect the biogeochemical cycles of carbon, nitrogen, silicon, and sulfur and ultimately influence the Earth climate system. However, extrapolation of the key results of FeAXs to regional and seasonal scales in some cases is limited because of differing modes of iron supply in FeAXs and in the modern and paleo-oceans. New research directions include quantification of the coupling of oceanic iron and carbon biogeochemistry.

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Mesoscale iron enrichment experiments 1993-2005 : Synthesis and future directions

Nitrogen fixation is crucial for maintaining biological productivity in the oceans, because it replaces the biologically available nitrogen that is lost through denitrification. But, owing to its temporal and spatial variability, the global distribution of marine nitrogen fixation is difficult to determine from direct shipboard measurements. This uncertainty limits our understanding of the factors that influence nitrogen fixation, which may include iron, nitrogen-to-phosphorus ratios, and physical conditions such as temperature. Here we determine nitrogen fixation rates in the world's oceans through their impact on nitrate and phosphate concentrations in surface waters, using an ocean circulation model. Our results indicate that nitrogen fixation rates are highest in the Pacific Ocean, where water column denitrification rates are high but the rate of atmospheric iron deposition is low. We conclude that oceanic nitrogen fixation is closely tied to the generation of nitrogen-deficient waters in denitrification zones, supporting the view that nitrogen fixation stabilizes the oceanic inventory of fixed nitrogen over time.

Spatial coupling of nitrogen inputs and losses in the ocean

[1] Water column depth profiles along the North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate (NO3−) 15N/14N and 18O/16O associated with denitrification in the oxygen-deficient thermocline waters of the eastern tropical North Pacific. The increase in δ18O is up to 3‰ greater than in δ15N, whereas our experiments with denitrifier cultures in seawater medium indicate a 1:1 increase in NO3− δ18O and δ15N during NO3− consumption. Moreover, the maximum in NO3− δ18O is somewhat shallower than the maximum in NO3− δ15N. These two observations can be summarized as an “anomaly” from the 1:1 δ18O-to-δ15N relationship expected from culture results. Comparison among stations and with other data indicates that this anomaly is generated locally. The anomaly has two plausible interpretations: (1) the addition of low-δ15N NO3− to the shallow thermocline by the remineralization of newly fixed nitrogen, or (2) active cycling between NO3− and NO2− (coupled NO3− reduction and NO2− oxidation) in the suboxic zone.

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Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin

. Measurements of the N2 produced by denitrification, a better understanding of non-canonical pathways for N2 production such as the anammox reaction, better appreciation of the multiple environments in which denitrification can occur (e.g. brine pockets in ice, within particles outside of suboxic water, etc.) suggest that it is unlikely that the oceanic denitrification rate is less than 400 Tg N a−1. Because this sink term far exceeds present estimates for nitrogen fixation, the main source for oceanic fixed-N, there is a large apparent deficit (~200 Tg N a−1) in the oceanic fixed-N budget. The size of the deficit appears to conflict with apparent constraints of the atmospheric carbon dioxide and sedimentary δ15N records that suggest homeostasis during the Holocene. In addition, the oceanic nitrate/phosphate ratio tends to be close to the canonical Redfield biological uptake ratio of 16 (by N and P atoms) which can be interpreted to indicate the existence of a powerful feed-back mechanism that forces the system towards a balance. The main point of this paper is that one cannot solve this conundrum by reducing the oceanic sink term. To do so would violate an avalanche of recent data on oceanic denitrification. A solution to this problem may be as simple as an upwards revision of the oceanic nitrogen fixation rate, and it is noted that most direct estimates for this term have concentrated on nitrogen fixation by autotrophs in the photic zone, even though nitrogen fixing genes are widespread. Another simple explanation may be that we are simply no longer in the Holocene and one might expect to see temporary imbalances in the oceanic fixed-N budget as we transition from the Holocene to the Anthropocene in line with an apparent denitrification maximum during the Glacial-Holocene transition. Other possible full or partial explanations involve plausible changes in the oceanic nitrate/phosphate and N/C ratios, an oceanic phosphorus budget that may also be in deficit, and oscillations in the source and sink terms that are short enough to be averaged out in the atmospheric and geologic records, but which could, perhaps, last long enough to have significant impacts.

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An oceanic fixed nitrogen sink exceeding 400 Tg N a −1 vs the concept of homeostasis in the fixed-nitrogen inventory

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Expanding the role of reactive transport models in critical zone processes

The distribution of nitrate 15N/14N in marine sediments and the impact of benthic nitrogen loss on the isotopic composition of oceanic nitrate

[1] On the basis of the normalization to phosphate, a significant amount of nitrate is missing from the deep Bering Sea (BS). Benthic denitrification has been suggested previously to be the dominant cause for the BS nitrate deficit. We measured water column nitrate 15N/14N and 18O/16O as integrative tracers of microbial denitrification, together with pore water-derived benthic nitrate fluxes in the deep BS basin, in order to gain new constraints on the mechanism of fixed nitrogen loss in the BS. The lack of any nitrate isotope enrichment into the deep part of the BS supports the benthic denitrification hypothesis. On the basis of the nitrate deficit in the water column with respect to the adjacent North Pacific and a radiocarbon-derived ventilation age of ∼50 years, we calculate an average deep BS (>2000 m water depth) sedimentary denitrification rate of ∼230 μmol N m−2 d−1 (or 1.27 Tg N yr−1), more than 3 times higher than high-end estimates of the average global sedimentary denitrification rate for the same depth interval. Pore water-derived estimates of benthic denitrification were variable, and uncertainties in estimates were large. A very high denitrification rate measured from the base of the steep northern slope of the basin suggests that the elevated average sedimentary denitrification rate of the deep Bering calculated from the nitrate deficit is driven by organic matter supply to the base of the continental slope, owing to a combination of high primary productivity in the surface waters along the shelf break and efficient down-slope sediment focusing along the steep continental slopes that characterize the BS.

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Origin of the deep Bering Sea nitrate deficit : Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes

Is there a 230Th deficit in Arctic sediments

In most of the global ocean, the radionuclide thorium-230 is removed from the water column by adsorption onto particles and deposition in seafloor sediments, at a rate approximately in balance with its local production by the decay of uranium dissolved in seawater, allowing its use in assessing rates of marine processes. However, several previous studies have suggested that the flux of 230Th to the sediments of the Central Arctic is far too small to balance its production in the overlying water column. If this is so, 230Th produced in the low particle-flux Central Arctic basins would be deposited elsewhere, either by boundary scavenging at the margins or by export from the Arctic to lower latitudes. In order to evaluate this possibility, we compare the expected 230Th production and measured inventories for five sites in the Western Arctic, combining previously published 230Th data with reported AMS radiocarbon dates, and find no evidence for a substantial deficit of 230Th in these sediments. Instead, we find evidence for near balance in the 230Th budget during both the Holocene and late glacial periods. These intervals are separated by a brief deglacial period of apparently higher sedimentation rates and 230Th deposition. During the Holocene, the average sedimentary inventory of 230Th at these sites is largely within 30% of the water column production, in good agreement with observations and model results from other ocean basins.

Sharon Hoffmann

Papers

The distribution of nitrate 15N/14N in marine sediments and the impact of benthic nitrogen loss on the isotopic composition of oceanic nitrate

Origin of the deep Bering Sea nitrate deficit : Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes

Is there a 230Th deficit in Arctic sediments

Is There a 230Th Deficit in Arctic Sediments

The Nitrogen and Oxygen Isotope Composition of Porewater Nitrate from Bering Sea Sediments