Eastern Tropical North Pacific Research Articles

Glacial‐interglacial modulation of eastern tropical North Pacific denitrification over the last 1.8‐Myr

A 1.8 Myr‐long δ15N record from the California margin provides the first continuous record of denitrification in the Eastern Tropical North Pacific (ETNP) over the entire Pleistocene epoch. Comparison of the nitrogen isotopic time series with proxies for ice volume, sea surface temperature (SST), and biological productivity place variations in the intensity of the denitrification zone in the context of global and regional climatic changes. Throughout the Pleistocene, ETNP denitrification remained linked to glacial‐interglacial climate cycles. The pacing of denitrification cycles switched from 41‐kyr in the mid‐ to early Pleistocene to 100‐kyr in the late Pleistocene, in tandem with the well‐known frequency shift identified from ice volume records. It therefore appears that denitrification remained an integral component of the marine feedbacks to orbital forcing, and through its effect on the nitrate inventory of the ocean might have played a role in driving atmospheric CO2 cycles throughout the Pleistocene.

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

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.

Contributions of denitrification and mixing on the distribution of nitrous oxide in the North Pacific

We analyzed N2O isotopomer ratios (distribution of isotopes within N2O molecules) in the eastern tropical North Pacific. The N2O isotopomer ratios indicate the contribution of denitrification in the oxygen minimum zone (OMZ, ∼600 m in depth) in the western North Pacific, which is not consistent with the widely accepted nitrification hypothesis. Our models indicate that the N2O yield per mole O2 consumed (dN2O/−dO2) is 0.008 (0–0.015) nmol/μmol during remineralization and nitrification in the western North Pacific. Nitrification in aerobic deep waters is a minor source of oceanic N2O, whereas the N2O production in the OMZ is the dominant factor for the oceanic N2O distribution. The denitrification in the OMZ is consistent with the correlation between ΔN2O (level above atmospheric equilibrium) and AOU (apparent oxygen utilization), and the parallel 18O‐enrichment of N2O and O2 in the OMZ, which have been believed to support the nitrification hypothesis.

Methane along the western Mexican margin

We investigated the processes controlling the water‐column and sediment methane distributions at 14 stations along the western Mexican margin, in and around the Gulf of California. Stations were grouped into two categories: coastal basins and open margins. Diffusive methane fluxes from the sediment at all sites, as estimated from sediment methane gradients, were 0.24–5.5 µmol m−2 d−1, with the highest fluxes observed on the Pacific margin of Baja California at both basin and open‐margin sites. These high rates occur despite the lack of significant terrestrial input to these sediments, reflecting the importance of upwelling‐induced productivity. Methane concentrations in the upper water column were supersaturated with respect to the present atmosphere at all sites, with sea‐air fluxes of methane of 0.5–5.9 µmol m−2 d−1. Four of the open‐margin sites had seafloor depths extending below the oxygen minimum zone (~400–800 m) and contained low methane concentrations below the subsurface methane maximum. The remaining margin site was shallow (593 m), with a seafloor that intersected the oxygen minimum zone, and had elevated methane concentrations throughout the water column; this indicates that such sediments may be a significant source of methane to the eastern tropical North Pacific (ETNP). The seawater within silled basin sites also had supersaturated methane concentrations, reflecting the anoxic conditions within the basins. However, methane levels were low at the sill depth, indicating that the silled basins were unlikely to be significant sources of methane to the ETNP. We observed an inverse relationship between methane concentration and δ13C‐CH4 value in the basin waters, consistent with biological aerobic oxidation of methane being released from the sediment; an apparent kinetic isotopic fractionation factor of 1.0100–0.0038 was calculated for this process. Isotopically heavy methane resulting from similar oxidation of seafloor‐derived methane may be the source of large pools of heavy methane previously observed offshore in the ETNP.

Glacial‐interglacial variability in the eastern tropical North Pacific oxygen minimum zone recorded by redox‐sensitive trace metals

Changes in the intensity of the oxygen minimum zone (OMZ) in the eastern tropical North Pacific over the past 140 kyr are recorded as enrichments and depletions of redox‐sensitive metals in sediments of two piston cores, one within and one below the modern OMZ, from the continental margin off Mazatlán, Mexico (22°41′N, 106°28′W). Concentrations of Al (7.7 ± 0.6%), Ti (0.37 ± 0.03%), Fe (3.1 ± 0.25%), Mn (320 ± 31 ppm), and Ba (560 ± 82 ppm) in core NH15P (within the OMZ at 420 m water depth) were relatively constant over the last 110 kyr. In contrast, concentrations of Cd (4.8 ± 2 ppm), Cu (29.8 ± 9.1 ppm), U (9.1 ± 3.7 ppm), Mo (12.1 ± 5.0 ppm), V (138.2 ± 51.9 ppm), and Re (45.8 ± 25.5 ppb) were all at least 30% higher in interglacial stages compared to glacial stages. Concentrations of Al (7.4 ± 0.3%), Ti (0.35 ± 0.03%), Fe (3.5 ± 0.4%), and Mn (385 ± 77 ppm) in core NH22P (below the OMZ at 2025 m water depth) were comparable to those in core NH15P, while concentrations of Ba (1662 ± 292 ppm) were about a factor of three higher. In contrast, concentrations of Cd (0.9 ± 0.3 ppm), U (6.6 ± 0.9 ppm), Mo (3.2 ± 1.2 ppm), V (81.8 ± 10.1 ppm), and Re (25.4 ± 12 ppb) were lower in sediments of comparable age than the concentrations of these same metals in the OMZ core, and the differences in their concentrations over glacial‐interglacial cycles were less pronounced than those in shallower core. Comparison of the nonlithogenic fraction of metals in the sediments with their estimated contribution from plankton suggests that organic matter is probably the major source of Cu, Ba, and perhaps Cd to the sediment, whereas the indirect effects of organic carbon and low bottom water oxygen concentrations on sediment redox state appear to be more important controls on the distributions U, Re, Mo, and V. Changes in the depth at which Re and Mo precipitated in the sediments and in the Re/Mo ratio suggest that the redox state of the surface sediment and overlying water at both core depths varied over time. Re and Mo removal depths were shallower and Re/Mo ratios were lower at the OMZ site than in deeper water, suggesting that a more reducing environment prevailed over time at the shallower site. Although the redox state of the waters and sediment at both sites varied over glacial‐interglacial cycles, the OMZ likely was never anoxic in the last 140 kyr. This variability in redox state could be attributed to changes in regional export productivity, changes in ocean circulation, or a combination of both processes. A paleoproductivity reconstruction from biogenic Ba data suggests that glacial productivity was considerable lower than it was during interglacial stages. Model results suggest that the oxygen penetration depth, an indicator of sediment redox state, changed less than 1 cm as a result of the change in productivity. Changes in oxygen penetration depth estimated from Re and U removal depths are significantly larger, suggesting that changes in ventilation are a more important control on sediment redox state. Overall, trace metal results confirm the tight coupling between ocean circulation, marine productivity, and global change.

Stable nitrogen isotope dynamics of dissolved nitrate in a transect from the north Pacific subtropical gyre to the eastern tropical north Pacific

The stable nitrogen isotopic composition of nitrate, concentrations of inorganic nitrogen and phosphorus, dissolved oxygen and nitrification rates were determined at six stations ranging from the oligotrophic North Pacific Subtropical Gyre (NPSG) to the more productive Eastern Tropical North Pacific (ETNP). Nitrification rates increased along the transect from a maximum rate of 1 nmol L −1 d −1 at station ALOHA to 23.7 nmol L −1 d −1 at station 6. In oxic surface waters, nitrate isotopically enriched in 15N (maximum δ 15N-NO 3 − value of 12.5‰) was most likely the result of assimilatory nitrate reduction. In contrast, high δ 15N-NO 3 − values (maximum of 12.3‰) in association with high nitrate deficits and anoxic conditions supported the interpretation of isotopic fractionation due to denitrification. A one-dimensional vertical advection and diffusion model was used to estimate the fractionation factor for denitrification at two stations in the ETNP. A comparison of modeled to observed δ 15N-NO 3 − data indicated an isotopic enrichment factor (ε) of 30‰ at station 4 and 30 to 35‰ at station 5. Isotopically light nitrate (1.1 and 3.2‰) was observed in the upper 200 m of the water column at stations in the ETNP. Tracer studies of 15NH 4 and biogeochemical indicators of nitrogen fixation supported the interpretation of nitrification as the most plausible explanation for low δ 15N-NO 3 − values observed in water column samples. Our results are consistent with the occurrence of nitrification within the euphotic zone and for the first time provide corroborating stable nitrogen isotopic evidence for this process.

Glacial‐Holocene δ15N record from the Gulf of Tehuantepec, Mexico: Implications for denitrification in the eastern equatorial Pacific and changes in atmospheric N2O

The intensity of denitrification in the Gulf of Tehuantepec, along the Mexican margin of the eastern tropical North Pacific (ETNP), was reconstructed for the last 23,000 years using the nitrogen isotopic composition of sediment organic matter (δ15Norg) in conjunction with other geochemical tracers of water column productivity. During the last glacial period, δ15Norg values ranged from 6.5 to 8.6‰ with values increasing between 23 and 17 ka. Deglaciation was marked by an abrupt increase in δ15Norg with maximum values (9.9‰) occurring during the Bølling/Allerød period and the earliest Holocene. A decline in δ15Norg values occurred over the last 10,000 years, with late Holocene values being similar to those of the glacial period. Our data suggest that denitrification in this region was minimal during the last glacial and is consistent with the deposition of bioturbated sediments at the study site. The intensity of denitrification increased dramatically during deglaciation and the early Holocene, signaling the presence of a strong oxygen minimum zone. This period is also marked by the deposition of laminated sediments in the region. The δ15Norg records display similar temporal trends to those seen in the Greenland ice core record of atmospheric N2O concentrations. At present, denitrification within the eastern tropical Pacific and Arabian Sea accounts for a major portion of the global marine loss of fixed nitrogen, and a significant fraction of the ocean‐atmosphere N2O flux originates from these areas. Large changes in N2O emissions from these regions in the past would have directly impacted global greenhouse gas concentrations.

Nitrogen isotope and chlorin paleoproductivity records from the Northern South China Sea: remote vs. local forcing of millennial- and orbital-scale variability

Variations in nitrogen isotopic composition (δ 15N) and total chlorin accumulation rate (AR) are employed as proxies to reconstruct oceanic nitrate inventory, the balance between denitrification and N fixation, and paleoproductivity in a rapidly accumulating sediment drift deposit beneath the Western Pacific Warm Pool for the last 145 Kyr. Subsurface and deep waters of the northern South China Sea (SCS) are sourced from the shallow Kuroshio Current (KC) and Pacific Intermediate Water, respectively. Their relative importance in determining sedimentary δ 15N and paleoproductivity have been altered by changes in equatorial circulation, summer- and winter-monsoon intensity and relative sea level. The location and basin configuration of the marginal SCS renders it especially sensitive to such changes. Assuming complete annual nitrate utilization, low δ 15N values during glacial stages are interpreted as a reflection of reduced remote denitrification in the Eastern Tropical North Pacific (ETNP) source waters, while much of marine isotope stage (MIS) 3 and the last interglacial were characterized by high denitrification. However, intervals of anomalously low δ 15N values are interpreted as reflecting the contribution from regional N fixation in West Pacific surface waters, transmitted to the site by the shallow KC. Unusually, the Holocene is characterized by declining δ 15N values and an inverse correlation with organic matter content since ca. 8.2 Ka. Millennial-scale variations during MIS 3 indicate higher frequency variations in both ETNP denitrification and local N fixation, which may be coherent with a hemispheric response to Dansgaard–Oeschger events recorded at high latitudes. For much of the last 145 Kyr, paleoproductivity was decoupled from δ 15N, and instead seems to reflect the extent of the global nitrate inventory stimulated by elevated dust fertilization, and regional mixed-layer deepening associated with the relative intensity of the SE Asian winter monsoon. Despite evaluation of possible conflicting influences on the record of each proxy, we interpret our data as clear evidence of glacial/interglacial changes in marine nutrient inventory across the whole of the North Pacific, with a corresponding biogeochemical response and important implications for global CO 2 drawdown via an invigorated biological pump. The relative importance of local, regional and global contributions to our records appears to be strongly modulated by relative sea level, controlling trans- and extra-basinal circulation in the SCS.

Distinguishing between water column and sedimentary denitrification in the Santa Barbara Basin using the stable isotopes of nitrate

Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O2] ∼ 3 μM), with only brief periods of ventilation. Associated with development of suboxia, the concentration of nitrate decreases with depth into the basin without an associated decrease in phosphate, indicating that a substantial fraction of the nitrate supplied to the basin is removed by denitrification. Coincident with the decrease in nitrate concentration across the “redoxcline” (the interface between oxic and suboxic waters) within the SBB, there is an increase in the 15N/14N of that nitrate, as would be anticipated from the isotopic fractionation associated with denitrification. However, the increase in 15N/14N of nitrate is much smaller than occurs in the open eastern tropical North Pacific (ETNP) for a comparable amount of nitrate loss. Both the concentrations of N species within the basin and measurements of nitrate 18O/16O suggest that the lower‐than‐expected 15N enrichment in the suboxic SBB involves denitrification, rather than being due to some unknown source of low‐15N/14N N to the deep SBB. Calculations with a range of models of nitrate supply and consumption indicate that the degree of nitrate consumption in the basin is too small for differences in water circulation to explain the isotopic differences between the Santa Barbara Basin and the open ETNP. Previous studies indicate that the isotope effect of sedimentary denitrification is negligible due to nitrate diffusion in sediment pore waters. Thus we infer that the small magnitude of the isotopic enrichment of SBB water column nitrate is due to the importance of sedimentary denitrification within the basin. Assuming that water column and sedimentary denitrification have isotope effects of 25 and 1.5 per mil, respectively, our results suggest that sedimentary denitrification accounts for more than 75% of the nitrate loss within the suboxic SBB.

Nitrogen isotope and productivity variations along the northeast Pacific margin over the last 120 kyr: Surface and subsurface paleoceanography

Glacial‐interglacial changes in sedimentary δ15N over the last 120 kyr display a remarkably similar pattern in timing and amplitude in core records extending from the denitrification zone in the eastern tropical North Pacific (ETNP), where subsurface denitrification is active, to the Oregon margin, where no denitrification occurs today. Low δ15N values (4–6‰) generally characterize glacial stages 2 and 4, and higher δ15N values (7–10‰) are representative of the Holocene, millennial‐scale periods within stage 3, and stage 5. The inferred synchroneity of δ15N variations along the entire margin implies that the nitrate isotopic signal produced in the oxygen‐poor subsurface waters in the ETNP is rapidly advected northward and recorded at sites far beyond the boundaries of the modern denitrification zone. Similar to δ15N, primary production indicators (percent Corg, Ba/Al, and percent opal) show glacial‐interglacial as well as millennial‐scale variations along the NE Pacific margin, with higher primary production during warm periods. However, the relative phasing between δ15N and paleoproduction tracers within individual records changes latitudinally. Whereas δ15N and primary production vary approximately synchronously in the midlatitudes, production lags δ15N in the ETNP by several kiloyears. This lag calls for a new understanding of the processes driving denitrification in the ETNP. We suggest that oxygen input by the Equatorial Undercurrent as well as local organic matter flux controls denitrification rates in the ETNP. Moreover, the differences in relative timing point to a time‐transgressive development of upwelling‐favorable winds along the NE Pacific margin after the last glaciation, with those in the north developing several kiloyears earlier.

Suboxic trace metal geochemistry in the Eastern Tropical North Pacific

We analyzed Al, Ti, Fe, Mn, Cu, Ba, Cd, U, Mo, V, and Re in water column, settling particulate, and sediment (0 to 22 cm) samples from the intense oxygen minimum zone (OMZ) of the eastern tropical North Pacific near Mazatlán, Mexico. The goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique “suboxic” geochemical signature.The water column was characterized by a Mn maximum, reaching ∼8 nmol kg−1 at 400 m. Concentrations of Cu, Ba, Cd, Mo, Re, U, and V were unaffected by the low O2 conditions and were comparable to those of the open ocean. Sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin. Al, Ti, and Fe were mostly (at least 79%) lithogenic. About 75% of the Mn was nonlithogenic. Significant amounts (at least 58%) of Cu, Ba, Cd, and Mo were nonlithogenic.Sediment geochemistry varied across the continental shelf and slope. Cadmium, U, and Re have prominent maxima centered at 310 m, with 12.3 ppm, 10.9 ppm, and 68.3 ppb, respectively, at the core top. High values of Mo (averaging 6.8 ppm) and V (averaging 90 ppm) are seen in OMZ surface sediment. Additional down-core enrichment occurs for all redox-sensitive elements in the top 10 cm. For U, Mo, V, and Re, surface sediments are a poor indicator of metal enrichment. Comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition, especially for Cd, U, and Mo.We evaluated Re/Mo and Cd/U ratios as tracers for redox environments. Rhenium and Mo concentrations and Re/Mo ratios do not lead to consistent conclusions. Concurrent enrichments of Re and Mo are an indicator of an anoxic depositional environment. In contrast, high Re/Mo ratios are an indicator of suboxic conditions. Cadmium is enriched in surface sediments, while U has considerable down-core enrichment. The concentrations of Cd and U and the Cd/U ratio do not follow patterns predicted from thermodynamics. Though the water column is suboxic, these four redox-sensitive elements indicate that the sediments are anoxic. The implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column.

Impact of suboxia on sinking particulate organic carbon: Enhanced carbon flux and preferential degradation of amino acids via denitrification

Fluxes of particulate organic carbon (POC) through the oxygen deficient waters in the eastern tropical North Pacific were found to be relatively less attenuated with depth than elsewhere in the eastern North Pacific. The attenuation coefficient (b) for the flux was found to be 0.40 versus the composite value of 0.86 determined by Martin et al. (1987). To examine this further, sinking POC was collected using sediment traps and allowed to degrade in oxic and suboxic experiments. Using a kinetic model, it was found that degradation proceeded at similar rates (roughly 0.8 day−1) under oxic and suboxic conditions, but a greater fraction of bulk POC was resistant to degradation in the suboxic experiments (61% vs. 23%). Amino acids accounted for 37% of POC collected at 75m, but following degradation the value dropped to 17% and 16% in the oxic and suboxic experiments respectively. POC collected from 500m was 10% amino acids. The non-AA component of POC collected at 75m was not degraded under suboxic conditions, while under oxic conditions it was. These results suggest that microbes degrading OC under suboxic conditions via denitrification preferentially utilize nitrogen-rich amino acids. This preferential degradation of amino acids suggests that 9% more nitrogen may be lost via water column denitrification than is accounted for when a more “Redfieldian” stoichiometry for POC is assumed.

Bacteriochlorophyll a in the ocean: Is anoxygenic bacterial photosynthesis important?

Some groups of marine bacteria within the Α‐proteobacteria are capable of anoxygenic photosynthesis in oxic environments. Their primary photosynthetic pigment is bacteriochlorophyll a (BacChl a). The impact of these bacteria on flows of energy and carbon in the ocean has been difficult to ascertain in the past. Recently, however, Kolber et al. (2001) reported that such bacteria are abundant in the upper ocean and that these might contribute significantly to photosynthetically driven electron transport since measured and inferred ratios of BacChl a and chlorophyll a (Chl a) were about 0.8% in coastal environments and as high as 10% in the Eastern Tropical North Pacific. The authors suggested that the globally averaged BacChl a/Chl a ratio could be as high as 5 to 10%. To determine whether such high values are representative of other marine environments, concentrations of BacChl a were measured in samples collected in eutrophic nearshore and mesotrophic and oligotrophic offshore environments off Southern California. The average BacChl a/Chl a ratio was 1.1% in nearshore and 0.5% in mesotrophic shelf and oligotrophic offshore environments. Assuming that rates of photosynthesis scale with concentrations of photosynthetic pigments, these data suggest that the contribution of BacChl a driven anoxygenic bacterial photosynthesis to energy production in coastal eutrophic and offshore oligotrophic areas of the California current system is small, substantially smaller than the suggested global average of 5 to 10%.

Highly elevated methane in the eastern tropical North Pacific and associated isotopically enriched fluxes to the atmosphere

During the May ‐ June, 2000 Eastern Pacific Redox Experiment (EPREX) we examined the dynamics of methane (CH4) in the eastern tropical North Pacific (ETNP), a large region of high surface‐ocean productivity fueled by coastal upwelling. We discovered that (1) the ETNP contains by far the largest pool of CH4 yet discovered in the open ocean; (2) CH4 production in the upper half of this subsurface pool is associated with the decomposition of locally produced sinking particulate matter; (3) the deeper half of this pool is from a coastal source; (4) advection and oxidation of the upper pool leads to the heavy CH4 isotopic values seen at midwater in the North Pacific subtropical gyre; and (5) the ETNP is a source of isotopically enriched CH4 to the atmosphere. Our results suggest that other oceanic areas of upwelling‐induced anoxia may be sites of significant atmospheric input of isotopically heavy CH4.

Role of the California Undercurrent in the export of denitrified waters from the eastern tropical North Pacific

A transect off Point Sur, California (36°20'N), was occupied bimonthly from April 1988 to May 1990 to resolve spatial and temporal patterns in the physics and chemistry of the California Current system. Alongshore velocity fields and the chemical parameter N* [Gruber and Sarmiento, 1997] show coherence. Subarctic waters, conveyed by the California Current (CC), were characterized by high N* values, while in the region of poleward flow of the California Undercurrent (CU) a minimum of N* (N*< −5 μmol kg−1) was observed. Low N* is a signature of denitrification, and its possible sources are considered. The distribution of N* on isopycnal surfaces, using data from the World Ocean Circulation Experiment Hydrographic Programme, suggests that northward advection from the eastern tropical North Pacific by the CU is the primary source of low N*. Local benthic and water column denitrification rates cannot account for the observed values. From nitrate deficits and velocity fields an annual nitrate transport deficit off Point Sur of 4.6 Tg N yr−1 was estimated for the study period. Maximum offshore extent of the low N* signal was observed during the upwelling period, probably associated with an expansion of the CU and mixing due to mesoscale eddies detached from it. This mixing propagates the denitrification signal into the CC and the subtropical gyre. When the loss from the poleward flow due to horizontal mixing is calculated, an export out of the eastern tropical North Pacific of 8.3 Tg N yr−1 by the CU is estimated. The bulk of the denitrified waters off Point Sur were below 400 m and did not affect primary production in the central California upwelling ecosystem. However, the export of nitrate‐deficient waters into the subtropical gyre may play a role in the biogeochemical cycling of this region.

Millennial‐scale oscillation of denitrification intensity in the Arabian Sea during the Late Quaternary and its potential influence on atmospheric N2O and global climate

The intensity of denitrification in the Arabian Sea during the last 65 kyr is reconstructed using high‐resolution δ15N records of three sediment cores in conjunction with other geochemical tracers for water column oxygenation and productivity. The results reveal a close link to the Greenland ice core record with low or absent water column denitrification during the Last Glacial Maximum, the stadials, and at the time of the Heinrich Events including the Younger Dryas. In contrast, denitrification was high during the Holocene and the interstadials. The intensification of denitrification is related to stronger SW monsoonal upwelling, which enhances organic matter flux and degradation, resulting in a strengthening of the midwater oxygen deficiency. Such a combination of enhanced upwelling and denitrification has also been implied for the Eastern Tropical North Pacific (ETNP), where these events occur during the Holocene and to some extent during the interstadials, too. Today, the Arabian Sea and the ETNP together contribute substantially to the global marine water column denitrification, and a significant fraction of the ocean‐atmosphere N2O flux originates from these areas. Changes in N2O emissions from these areas could thus have effected the recently described stadial/interstadial variations in the atmospheric concentration of this greenhouse gas as deduced from ice cores. Moreover, denitrification is the major sink for oceanic nitrate and provides a primary control for the oceanic nutrient inventory, which in turn influences global primary productivity and CO2 sequestration by the biological pump. Short‐term switches between a nondenitrification mode and a denitrification mode in these marine regions therefore have an impact on global climate.

Nitrogen isotope patterns in the oxygen-deficient waters of the Eastern Tropical North Pacific Ocean

We analyzed the stable nitrogen isotope composition of an extensive set of samples of particulate matter (PM) and seawater nitrate collected during October/November 1997 along the Mexican coastline from 24° to 11.5°N. At the northern and southern end of our study area, the δ 15N of PM ranged between 5 and 7‰ in the upper 200 m of the water column with higher values at intermediate depths. These data are very similar to those reported from other parts of the open ocean. In the oxygen minimum zone (OMZ), we found significantly higher δ 15N values for suspended particles. Furthermore, the δ 15N of nitrate (NO 3) was elevated within the OMZ and we found a strong relationship between the oxygen concentration, nitrate deficit and the 15N content of the nitrate. The core of the OMZ between 22°N, 105°W and 15°N, 110°W coincided with higher nitrate deficits and δ 15N values relative to the stations near the boundaries. The δ 15N of nitrate was highest, with values up to 18.7‰, where oxygen concentrations were below 1–2 μmol/l. This pattern is consistent with an overall nitrogen isotopic enrichment factor of 22.5‰ for denitrification in the core of the OMZ using the Rayleigh equation (closed-system approach). Results from a diffusion model (open-system approach), however, gave a fractionation factor of 30±7.5‰, implying that the Rayleigh formula only gives a lower estimate of the fractionation factor ε. The vertical flux of particles collected in short-term deployments (ca. 35 h) of a drifting sediment trap was not significantly correlated with the water column nitrate deficit. The isotopic signature of the nitrate within the gradient is very similar to the δ 15N value of sedimenting particles, suggesting that there might be a strong link between the production and sedimentation of particles. Upward flux of nitrate across the thermocline can account for less than half of the particle flux leaving the mixed layer. Mixing and transport of nitrate across the lower boundary of the OMZ can lead to significant enrichment in the 15N content of deep waters, and our isotopic data imply that at least 14% of the nitrate in the waters below the OMZ originates from this source.

Nitrogen isotope variations in Santa Barbara Basin sediments: Implications for denitrification in the eastern tropical North Pacific during the last 50,000 years

Nitrogen isotope variations preserved in Santa Barbara Basin sediments are used to evaluate changes in denitrification in the eastern tropical North Pacific (ETNP) during the last 50,000 years. A significant component of the subsurface waters (∼100–400 m) that presently fill the Santa Barbara Basin is derived from the low‐oxygen, denitrifying zone in the ETNP, and the nitrate in these waters has a δ15N value of 8–9‰. During the last glacial, the δ15N values of Santa Barbara Basin sediments were typically 6–7‰, indicating decreased denitrification in the ETNP and a better oxygenated intermediate water mass in the Santa Barbara Basin at this time. This reduced denitrification during the last glacial would have increased the pool of fixed nitrogen and may have contributed to the higher productivity previously reported for various regions of the global ocean during this period. At the onset of deglaciation, sediment δ15N values increase by more than 2‰, indicating increased denitrification in the ETNP. During Younger Dryas time, δ15N values decreased by 3‰ and record a brief return to better ventilated conditions in the subsurface waters of the ETNP. This is followed by an increase in δ15N to over 9‰ at ∼10,000 years ago, indicating intense denitrification at the beginning of the Holocene.

A novel niche for Prochlorococcus sp. in low-light suboxic environments in the Arabian Sea and the Eastern Tropical North Pacific

Prochlorococcus marinus is present in all tropical and subtropical oceans, where it is often found throughout the euphotic zone, contributing significantly to phytoplankton biomass and primary production and growing at rates comparable to those of other picoplankters. Clearly, Prochlorococcus and eucaryotic picoplankton share significant niche dimensions in the open ocean. Here we report the discovery of populations of Prochlorococcus in layers below the oxyclines of the oxygen minimum zones of the Arabian Sea and the Eastern Tropical North Pacific off Mexico. The unusual aspects of these populations are that these were at times virtual monoalgal cultures found at a depth of 80 to 140 m, often below the euphotic zone, where irradiance ranged from less than 0.1 to 2% of the surface irradiance ( I 0). The pigment complement of these deep populations was characterized in detail. The previously unidentified Chl-c-like pigment of Prochlorococcus is Mg-3,8-divinylpheoporphyrin a 5 monomethylester. The carotenoid complement of populations in these deep layers was similar to that of cultured Prochlorococcus strains, except for high concentrations of a 7′,8′-dihydro-derivative of zeaxanthin, quite likely parasiloxanthin. Even though cellular concentrations of pigments were very high in these populations, suggesting acclimation to low irradiance, ambient light experienced by these populations in the Arabian Sea, <0.1% I 0, may not have been sufficient to support normal photoautotrophic growth. Off Mexico these deep Prochlorococcus populations were located at 0.2 and 2% I 0 isolumes, an irradiance likely sufficient for slow growth. Environmental conditions in these layers, except for concentrations of oxygen, are similar to those found at and below the subsurface chlorophyll maxima of the subtropical central gyres. As only Prochlorococcus thrives in these layers but Prochlorococcus and eucaryotic picoplankton coexist in and below subsurface chlorophyll maxima, we conclude that the low oxygen concentrations at the deep Prochlorococcus maxima are the determining factor, but we are not able to identify any specific physiological functions that are affected by low oxygen concentrations in eucaryotes but not Prochlorococcus.

Organic matter diagenesis in sediments on the continental shelf and slope of the Eastern Tropical and temperate North Pacific

Carbon and nitrogen mineralization were determined in sediments from about 100 to 1000 m depth on the continental shelf and slope off the Pacific coast of Mexico and Washington State. Vertical down core profiles of carbon mineralization were measured both as sulfate reduction using short-term 35 S incubations and as net TCO 2 production using long-term anaerobic incubations (`jar’ technique). Nitrogen mineralization was measured as net NH + 4 production using only the latter technique. The down core attenuation of jar rates was described adequately by a double exponential decay model. By subtraction of 35 S rates from jar rates in the upper 3 cm of the sediment, oxic plus suboxic respiration were roughly estimated to account for at least 5–28% (Mexico) and 10–19% (Washington) of the 0–30 cm integrated carbon mineralization. The agreement between measured porewater profiles of TCO 2 and NH + 4 and those estimated by diagenetic modelling using rates obtained from jar incubations in the oxygen deficient zone off Mexico indicated no infaunal irrigation. At all other stations the benthic fauna appeared responsible for considerably lower measured porewater concentrations than predicted by a simple diffusion-based diagenetic model. Total carbon mineralization in sediments off Mexico was reduced 40% with water depth from 100–200 m to 1000 m, while the reduction in the same depth interval off Washington was about 80%. Water depth attenuation of total sediment nitrogen mineralization at the two locations, on the other hand, was almost similar with a reduction of 70–80%. The overall molar C : N ratio of mineralization products was generally higher off Washington than in the oxygen deficient zone off Mexico. It is suggested that the difference in sediment mineralization patterns are associated with changes in water column processes related to the different strength of the oxygen minimum at the two locations.