North Pacific Oxygen Minimum Zone Research Articles

North Atlantic temperature control on deoxygenation in the northern tropical Pacific

Ocean oxygen content is decreasing with global change. A major challenge for modelling future declines in oxygen concentration is our lack of knowledge of the natural variability associated with marine oxygen inventory on interannual and multidecadal timescales. Here, we present 10 annually resolved 200 year-long records of denitrification, a marker of deoxygenation, from a varved sedimentary archive in the North Pacific oxygen minimum zone covering key periods over the last glacial–interglacial cycle. Spectral analyses on these records reveal strong signals at periodicities typical of today’s Atlantic multidecadal oscillation. Modern subsurface circulation reanalyses regressed on the positive Atlantic and Pacific Climatic Oscillation indices further confirm that North Atlantic temperature patterns are the main control on the subsurface zonal circulation and therefore the most likely dominant driver of oxygen variability in the tropical Pacific. With currently increasing temperatures in the Northern Hemisphere high latitudes and North Atlantic, we suggest deoxygenation will intensify in the region.

Anoxic waters constrain the vertical distribution of fish developmental stages in an oxygen minimum zone

AbstractIn the Eastern Tropical North Pacific Oxygen Minimum Zone (ETNP‐OMZ), fish larvae undergo development amidst highly variable dissolved oxygen environments. As OMZs expand, understanding the implications of low‐oxygen environments on fish development becomes increasingly relevant for fisheries management and ecosystem modeling. Using horizontal zooplankton tows to track five oxygen levels (oxic [200 μmol/kg], hypoxic [100 μmol/kg] suboxic [10 μmol/kg], anoxic [<1 μmol/kg], and deep [10 μmol/kg at ~ 1000 m depth]), this study analyzed the three‐dimensional distribution of fish larvae and adults across the ETNP‐OMZ. Results revealed a wide midwater anoxic core, extending from Costa Rica to Baja California, that was almost devoid of fish larvae (< 1 larvae/1000 m3). Early larval stages primarily inhabited the oxic and hypoxic levels above the core, while postflexion and transformation stages occurred across a wider oxygen gradient, including the deep level below the anoxic core. Epipelagic species (e.g., Auxis sp.) were predominantly found in the surface oxic level, whereas coastal‐demersal species (e.g., Bregmaceros bathymaster, Ophidion spp.) were prevalent in the hypoxic level above the core. Meso‐bathypelagic species (e.g., Diogenichthys laternatus, Cyclothone spp.) were present throughout the study area, including below the anoxic core as transformation larvae and juveniles. These findings indicate that a vertical expansion of anoxic waters in OMZs could further constrain the habitat of epipelagic species, while also affecting the ontogenic vertical movements of meso‐bathypelagic species.

Regional Fluctuations in the Eastern Tropical North Pacific Oxygen Minimum Zone during the Late Holocene

This study presents a high-resolution record of δ15Nsed, which serves as a proxy for water column denitrification and oxygen minimum zone (OMZ) intensity, from the Soledad Basin in the Eastern Tropical North Pacific OMZ. The Soledad Basin δ15Nsed record is compared to the Pescadero Slope and Santa Barbara Basin (SBB) δ15Nsed records to gain insight into regional variations in the ETNP OMZ. During the Medieval Climate Anomaly (MCA; 950–1250 CE), Soledad Basin, Pescadero Slope, and SBB records exhibit coherent trends suggesting that there was general water column oxygenation stability. During the Little Ice Age (LIA; 1350–1850 CE), Soledad Basin and SBB showed a similar decreasing trend in δ15Nsed values while the Pescadero Slope δ15Nsed exhibited an increasing trend until values abruptly declined between 1740 and 1840 CE. We suggest that increased δ15Nsed variability and the different trends at the Pescadero Slope during the LIA are due to the influence of the North American monsoon (NAM), which can suppress upwelling when enhanced and result in OMZ contraction. The decoupling between the Soledad Basin, SBB, and the Pescadero Slope could also be due to the increased influence of enriched 15NO3− subarctic waters in the California Current System. Since each site is influenced by local productivity, basin morphology, and regional atmospheric and ocean circulation patterns, we suggest that assessing OMZ fluctuations from multiple sites provides a more comprehensive view of regional OMZ dynamics in response to climate variations.

Re‐Evaluating Hydrogen Sulfide as a Sink for Cadmium and Zinc in the Oxic to Suboxic Upper Water Column of the Pacific Ocean

AbstractHydrogen sulfide is produced by heterotrophic bacteria in anoxic waters and via carbonyl sulfide hydrolysis and phytoplankton emissions under oxic conditions. Apparent losses of dissolved cadmium (dCd) and zinc (dZn) in oxygen minimum zones (OMZs) of the Atlantic and Pacific Oceans have been attributed to metal‐sulfide precipitation formed via dissimilatory sulfate reduction. It has also been argued that such a removal process could be a globally important sink for dCd and dZn. However, our studies from the North Pacific OMZ show that dissolved and particulate sulfide concentrations are insufficient to support the removal of dCd via precipitation. In contrast, apparent dCd and dZn deficits in the eastern tropical South Pacific OMZ do reside in the oxycline with particulate sulfide maxima, but they also coincide with the secondary fluorescence maxima, suggesting that removal via sulfide precipitation may be due to a combination of dissimilatory and assimilatory sulfate reduction. Notably, dCd loss via precipitation with sulfide from assimilatory reduction was found in upper oxic waters of the North Pacific. While dissimilatory sulfate reduction may explain local dCd and dZn losses in some OMZs, our evaluation of North Pacific OMZs demonstrates that dCd and dZn losses are unlikely to be a globally relevant sink. Nevertheless, metal sulfide losses due to assimilatory sulfate reduction in surface waters should be considered in future biogeochemical models of oceanic Cd (and perhaps Zn) cycling.

All about nitrite: exploring nitrite sources and sinks in the eastern tropical North Pacific oxygen minimum zone

Abstract. Oxygen minimum zones (OMZs), due to their large volumes of perennially deoxygenated waters, are critical regions for understanding how the interplay between anaerobic and aerobic nitrogen (N) cycling microbial pathways affects the marine N budget. Here, we present a suite of measurements of the most significant OMZ N cycling rates, which all involve nitrite (NO2-) as a product, reactant, or intermediate, in the eastern tropical North Pacific (ETNP) OMZ. These measurements and comparisons to data from previously published OMZ cruises present additional evidence that NO3- reduction is the predominant OMZ N flux, followed by NO2- oxidation back to NO3-. The combined rates of both of these N recycling processes were observed to be much greater (up to nearly 200 times) than the combined rates of the N loss processes of anammox and denitrification, especially in waters near the anoxic–oxic interface. We also show that NO2- oxidation can occur when O2 is maintained near 1 nM by a continuous-purge system, NO2- oxidation and O2 measurements that further strengthen the case for truly anaerobic NO2- oxidation. We also evaluate the possibility that NO2- dismutation provides the oxidative power for anaerobic NO2- oxidation. The partitioning of N loss between anammox and denitrification differed widely from stoichiometric predictions of at most 29 % anammox; in fact, N loss rates at many depths were entirely due to anammox. Our new NO3- reduction, NO2- oxidation, dismutation, and N loss data shed light on many open questions in OMZ N cycling research, especially the possibility of truly anaerobic NO2- oxidation.

All about Nitrite: Exploring Nitrite Sources and Sinks in the Eastern Tropical North Pacific Oxygen Minimum Zone

All about Nitrite: Exploring Nitrite Sources and Sinks in the Eastern Tropical North Pacific Oxygen Minimum Zone

How does the phytoplankton–light feedback affect the marine N2O inventory?

Abstract. The phytoplankton–light feedback (PLF) describes the interaction between phytoplankton biomass and the downwelling shortwave radiation entering the ocean. The PLF allows the simulation of differential heating across the ocean water column as a function of phytoplankton concentration. Only one third of the Earth system models contributing to the 6th phase of the Coupled Model Intercomparison Project (CMIP6) include a complete representation of the PLF. In other models, the PLF is either approximated by a prescribed climatology of chlorophyll or not represented at all. Consequences of an incomplete representation of the PLF on the modelled biogeochemical state have not yet been fully assessed and remain a source of multi-model uncertainty in future projection. Here, we evaluate within a coherent modelling framework how representations of the PLF of varying complexity impact ocean physics and ultimately marine production of nitrous oxide (N2O), a major greenhouse gas. We exploit global sensitivity simulations at 1∘ horizontal resolution over the last 2 decades (1999–2018), coupling ocean, sea ice and marine biogeochemistry. The representation of the PLF impacts ocean heat uptake and temperature of the first 300 m of the tropical ocean. Temperature anomalies due to an incomplete PLF representation drive perturbations of ocean stratification, dynamics and oxygen concentration. These perturbations translate into different projection pathways for N2O production depending on the choice of the PLF representation. The oxygen concentration in the North Pacific oxygen-minimum zone is overestimated in model runs with an incomplete representation of the PLF, which results in an underestimation of local N2O production. This leads to important regional differences of sea-to-air N2O fluxes: fluxes are enhanced by up to 24 % in the South Pacific and South Atlantic subtropical gyres but reduced by up to 12 % in oxygen-minimum zones of the Northern Hemisphere. Our results, based on a global ocean–biogeochemical model at CMIP6 state-of-the-art level, shed light on current uncertainties in modelled marine nitrous oxide budgets in climate models.

The Fate of Oxygen in the Ocean and Its Sensitivity to Local Changes in Biological Production

AbstractWe investigate the sensitivity of the oxygen content and true oxygen utilization of key low‐oxygen regions Ω to pointwise changes in biological production. To understand how the combined water and biogenic particle transport controls the sensitivity patterns and the fate of oxygen in the ocean, we develop new relationships that link the steady‐state oxygen content and deficit of Ω to the downstream and upstream oxygen utilization rate (OUR), respectively. We find that the amount of oxygen from Ω that will be lost per unit volume at point r is linked to OUR(r) through the mean oxygen age accumulated in Ω. The geographic sensitivity pattern of the Ω‐integrated oxygen deficit is shaped by where the utilization occurs that causes this deficit. The contribution to the oxygen deficit of Ω from utilization at r is controlled by the mean time that water at r spends in Ω before next ventilation at the surface. We illustrate these relationships and the new transport timescales using a simple steady‐state data‐constrained carbon and oxygen model. We focus on Ω being the global ocean, the Pacific Hypoxic Zone (PHZ, [O2] < 62.5 µM), and the North Pacific oxygen minimum zone. The oxygen deficit of the PHZ is most sensitive where mode and intermediate waters form and where increased organic‐matter production directly increases the PHZ's oxygen demand. The fraction of the local oxygen concentration that will be utilized in respiration is as high as 90% in the PHZ and up to 70% in the water column beneath it.

Diversity and N2O Production Potential of Fungi in an Oceanic Oxygen Minimum Zone.

Fungi in terrestrial environments are known to play a key role in carbon and nitrogen biogeochemistry and exhibit high diversity. In contrast, the diversity and function of fungi in the ocean has remained underexplored and largely neglected. In the eastern tropical North Pacific oxygen minimum zone, we examined the fungal diversity by sequencing the internal transcribed spacer region 2 (ITS2) and mining a metagenome dataset collected from the same region. Additionally, we coupled 15N-tracer experiments with a selective inhibition method to determine the potential contribution of marine fungi to nitrous oxide (N2O) production. Fungal communities evaluated by ITS2 sequencing were dominated by the phyla Basidiomycota and Ascomycota at most depths. However, the metagenome dataset showed that about one third of the fungal community belong to early-diverging phyla. Fungal N2O production rates peaked at the oxic–anoxic interface of the water column, and when integrated from the oxycline to the top of the anoxic depths, fungi accounted for 18–22% of total N2O production. Our findings highlight the limitation of ITS-based methods typically used to investigate terrestrial fungal diversity and indicate that fungi may play an active role in marine nitrogen cycling.

Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations

Abstract. Oxygen-depleted regions of the global ocean are rapidly expanding, with important implications for global biogeochemical cycles. However, our ability to make projections about the future of oxygen in the ocean is limited by a lack of empirical data with which to test and constrain the behavior of global climatic and oceanographic models. We use depth-stratified plankton tows to demonstrate that some species of planktic foraminifera are adapted to life in the heart of the pelagic oxygen minimum zone (OMZ). In particular, we identify two species, Globorotaloides hexagonus and Hastigerina parapelagica, living within the eastern tropical North Pacific OMZ. The tests of the former are preserved in marine sediments and could be used to trace the extent and intensity of low-oxygen pelagic habitats in the fossil record. Additional morphometric analyses of G. hexagonus show that tests found in the lowest oxygen environments are larger, more porous, less dense, and have more chambers in the final whorl. The association of this species with the OMZ and the apparent plasticity of its test in response to ambient oxygenation invites the use of G. hexagonus tests in sediment cores as potential proxies for both the presence and intensity of overlying OMZs.

Expansion of the Southern California oxygen minimum zone during the early-to mid-Holocene due to reduced ventilation of the Northeast Pacific

Oceanic deoxygenation observed since the 1950s has drawn increasing concern as dissolved oxygen (DO) regulates marine biodiversity and biogeochemical cycles. Yet the DO time series from direct instrumental measurements is insufficiently long (several decades) to constrain the centennial to millennial variability. Here we present redox-sensitive metal (Re, U, and Mo) records from Santa Barbara Basin (SBB), California to reconstruct subsurface DO through the Holocene. Significantly higher authigenic metal fluxes and coherent benthic foraminifera faunal responses along a published depth transect reveal vertical expansion and intensification of the Southern California oxygen minimum zone (OMZ) prior to ∼6 ka. Comparison with post-Industrial trace metal records suggests low-oxygen conditions were more extreme during early to mid-Holocene (MH), highlighting the magnitude of OMZ variability in the absence of anthropogenic influences.Inconsistencies between authigenic metal fluxes and local oxygen control indicate a remote weakening of the North Pacific Intermediate Water (NPIW) formation prior to the MH. Reduced ventilation of the mid-depth North Pacific is supported by state-of-the-art Earth system model simulations with realistic MH (6 ka) forcing, highlighting the importance of brine rejection during sea ice formation in changing the buoyancy of NPIW and its basin-wide influences on North Pacific OMZ. Evidence from Holocene marine/lake records and model simulations further suggests a southward Intertropical Convergence Zone (ITCZ) migration and a consequent deepening of the Aleutian Low (AL) towards late Holocene, which favors more sea ice brine rejection. Thus, mid-depth ventilation in the North Pacific was closely associated with atmospheric circulation during the Holocene.

Ocean deoxygenation and zooplankton: Very small oxygen differences matter.

Oxygen minimum zones (OMZs), large midwater regions of very low oxygen, are expected to expand as a result of climate change. While oxygen is known to be important in structuring midwater ecosystems, a precise and mechanistic understanding of the effects of oxygen on zooplankton is lacking. Zooplankton are important components of midwater food webs and biogeochemical cycles. Here, we show that, in the eastern tropical North Pacific OMZ, previously undescribed submesoscale oxygen variability has a direct effect on the distribution of many major zooplankton groups. Despite extraordinary hypoxia tolerance, many zooplankton live near their physiological limits and respond to slight (≤1%) changes in oxygen. Ocean oxygen loss (deoxygenation) may, thus, elicit major unanticipated changes to midwater ecosystem structure and function.

Remineralization of particulate organic carbon in an ocean oxygen minimum zone

Biological oceanic processes, principally the surface production, sinking and interior remineralization of organic particles, keep atmospheric CO2 lower than if the ocean was abiotic. The remineralization length scale (RLS, the vertical distance over which organic particle flux declines by 63%, affected by particle respiration, fragmentation and sinking rates) controls the size of this effect and is anomalously high in oxygen minimum zones (OMZ). Here we show in the Eastern Tropical North Pacific OMZ 70% of POC remineralization is due to microbial respiration, indicating that the high RLS is the result of lower particle fragmentation by zooplankton, likely due to the almost complete absence of zooplankton particle interactions in OMZ waters. Hence, the sensitivity of zooplankton to ocean oxygen concentrations can have direct implications for atmospheric carbon sequestration. Future expansion of OMZs is likely to increase biological ocean carbon storage and act as a negative feedback on climate change.

Origin and fate of methane in the Eastern Tropical North Pacific oxygen minimum zone

Oxygen minimum zones (OMZs) contain the largest pools of oceanic methane but its origin and fate are poorly understood. High-resolution (<15 m) water column profiles revealed a 300 m thick layer of elevated methane (20–105 nM) in the anoxic core of the largest OMZ, the Eastern Tropical North Pacific. Sediment core incubations identified a clear benthic methane source where the OMZ meets the continental shelf, between 350 and 650 m, with the flux reflecting the concentration of methane in the overlying anoxic water. Further incubations characterised a methanogenic potential in the presence of both porewater sulphate and nitrate of up to 88 nmol g−1day−1 in the sediment surface layer. In these methane-producing sediments, the majority (85%) of methyl coenzyme M reductase alpha subunit (mcrA) gene sequences clustered with Methanosarcinaceae (⩾96% similarity to Methanococcoides sp.), a family capable of performing non-competitive methanogenesis. Incubations with 13C-CH4 showed potential for both aerobic and anaerobic methane oxidation in the waters within and above the OMZ. Both aerobic and anaerobic methane oxidation is corroborated by the presence of particulate methane monooxygenase (pmoA) gene sequences, related to type I methanotrophs and the lineage of Candidatus Methylomirabilis oxyfera, known to perform nitrite-dependent anaerobic methane oxidation (N-DAMO), respectively.

Decadal to centennial fluctuations in the intensity of the eastern tropical North Pacific oxygen minimum zone during the last 1200 years

AbstractOxygen minimum zones (OMZs), located below highly productive marine regions, are sites of microbially mediated denitrification and biogeochemical cycling that have global significance. The intensity of OMZs fluctuates naturally; however, the degree of these fluctuations and a comprehensive understanding of the factors that drive these fluctuations on decadal to centennial time scales is lacking. Our high‐resolution (near‐annual) record of δ15Nsed from laminated sediments at the Pescadero Slope in the Gulf of California (eastern tropical North Pacific) fluctuates between maximum values of 10.5‰ and minimum values of 8.0‰ over the past 1200 years. An analysis of the relationship between δ15NO3− and [O2] in the water column suggests that the observed range of δ15Nsed values is equivalent to an approximately 8 µM fluctuation in O2 content and that these changes can occur in less than 25 years. Our findings show that the OMZ typically intensifies quickly and contracts gradually; the average rate of OMZ intensification (−0.24 µM O2/yr) is twice as fast as the rate of OMZ reoxygenation. Spectral analyses of the δ15Nsed record and Br/Cl counts, with the latter are used as a proxy for organic carbon preservation, suggest that the Pacific Decadal Oscillation and the Suess (deVries) solar cycle (solar irradiance) may influence the intensity of the OMZ and carbon production/export during the late Holocene. Coherence between δ15Nsed and weight percent organic carbon also suggests that similar mechanisms influence both OMZ fluctuations and variation in organic carbon production/export.

Factors controlling methane and nitrous-oxide variability in the southern British Columbia coastal upwelling system

Coastal upwelling systems are important marine sources of methane (CH4) and nitrous-oxide (N2O). Current understanding of the controls on CH4 and N2O distributions in these coastal waters is restricted by limited data availability. We present the first multi-year measurements of CH4 and N2O distributions from the seasonally upwelling shelf waters of British Columbia, Canada, a coastal end-member of the north Pacific oxygen minimum zone (OMZ). Our data show significant seasonal differences in CH4 and N2O distributions and fluxes driven predominantly by upwelling. Methane is supplied to the water column primarily from sediments (especially near methane seeps), and is transported to the surface mixed layer by upwelling. A positive correlation between CH4 concentrations and salinity indicates limited inputs from Fraser River estuary waters to the study site. Shelf waters receive N2O from a deep, off-shelf N2O maximum in the OMZ core, and from nitrification in the water column and possibly sediments. Both the physical transport of N2O and its apparent in situ production are enhanced under upwelling conditions. N2O yields from nitrification, estimated from changes in N2O and nitrate+nitrite (NO3−+NO2−) along isopycnals, ranged from 0.04–0.49%, with the highest values observed under low ambient O2 concentrations. Sea–air fluxes ranged from −4.5–21.9μmolm−2day−1 for N2O and 2.5–34.1μmolm−2day−1 for CH4, with the highest surface fluxes observed following summer upwelling over the broad continental shelf of southern Vancouver Island. Our results provide new insight into the factors driving spatial and inter-annual variability in marine CH4 and N2O in high productivity coastal upwelling regions. Continued time-series measurements will be invaluable in understanding the longer-term impacts of climate-driven variability on marine biogeochemical cycles in these dynamic near-shore waters.

North Pacific deglacial hypoxic events linked to abrupt ocean warming.

Marine sediments from the North Pacific document two episodes of expansion and strengthening of the subsurface oxygen minimum zone (OMZ) accompanied by seafloor hypoxia during the last deglacial transition. The mechanisms driving this hypoxia remain under debate. We present a new high-resolution alkenone palaeotemperature reconstruction from the Gulf of Alaska that reveals two abrupt warming events of 4-5 degrees Celsius at the onset of the Bølling and Holocene intervals that coincide with sudden shifts to hypoxia at intermediate depths. The presence of diatomaceous laminations and hypoxia-tolerant benthic foraminiferal species, peaks in redox-sensitive trace metals, and enhanced (15)N/(14)N ratio of organic matter, collectively suggest association with high export production. A decrease in (18)O/(16)O values of benthic foraminifera accompanying the most severe deoxygenation event indicates subsurface warming of up to about 2 degrees Celsius. We infer that abrupt warming triggered expansion of the North Pacific OMZ through reduced oxygen solubility and increased marine productivity via physiological effects; following initiation of hypoxia, remobilization of iron from hypoxic sediments could have provided a positive feedback on ocean deoxygenation through increased nutrient utilization and carbon export. Such a biogeochemical amplification process implies high sensitivity of OMZ expansion to warming.

Meta-omic signatures of microbial metal and nitrogen cycling in marine oxygen minimum zones.

Iron (Fe) and copper (Cu) are essential cofactors for microbial metalloenzymes, but little is known about the metalloenyzme inventory of anaerobic marine microbial communities despite their importance to the nitrogen cycle. We compared dissolved O2, NO, NO, Fe and Cu concentrations with nucleic acid sequences encoding Fe and Cu-binding proteins in 21 metagenomes and 9 metatranscriptomes from Eastern Tropical North and South Pacific oxygen minimum zones and 7 metagenomes from the Bermuda Atlantic Time-series Station. Dissolved Fe concentrations increased sharply at upper oxic-anoxic transition zones, with the highest Fe:Cu molar ratio (1.8) occurring at the anoxic core of the Eastern Tropical North Pacific oxygen minimum zone and matching the predicted maximum ratio based on data from diverse ocean sites. The relative abundance of genes encoding Fe-binding proteins was negatively correlated with O2, driven by significant increases in genes encoding Fe-proteins involved in dissimilatory nitrogen metabolisms under anoxia. Transcripts encoding cytochrome c oxidase, the Fe- and Cu-containing terminal reductase in aerobic respiration, were positively correlated with O2 content. A comparison of the taxonomy of genes encoding Fe- and Cu-binding vs. bulk proteins in OMZs revealed that Planctomycetes represented a higher percentage of Fe genes while Thaumarchaeota represented a higher percentage of Cu genes, particularly at oxyclines. These results are broadly consistent with higher relative abundance of genes encoding Fe-proteins in the genome of a marine planctomycete vs. higher relative abundance of genes encoding Cu-proteins in the genome of a marine thaumarchaeote. These findings highlight the importance of metalloenzymes for microbial processes in oxygen minimum zones and suggest preferential Cu use in oxic habitats with Cu > Fe vs. preferential Fe use in anoxic niches with Fe > Cu.

The cycling of iron, zinc and cadmium in the North East Pacific Ocean – Insights from stable isotopes

Dissolved stable isotope ratios of the transition metals provide useful information, both for understanding the cycling of these bioactive trace elements through the oceans, and tracing their marine sources and sinks. Here, we present seawater dissolved Fe, Zn and Cd concentration and stable isotope ratio (δ56Fe, δ66Zn, and δ114Cd) profiles from two stations in the Pacific Ocean, the SAFe Station (30°N 140°W) in the subtropical North East Pacific from the GEOTRACES IC2 cruise, and the marginal San Pedro Basin (33.8°N 118.4°W) within the South California Bight. These data represent, to our knowledge, the first full-water column profiles for δ66Zn and δ56Fe from the open-ocean North Pacific, and the first observations of dissolved δ66Zn and δ114Cd in a low-oxygen marginal basin. At the SAFe station, δ56Fe is isotopically lighter throughout the water column (−0.6 to +0.1‰, relative to IRRM-014) compared to the North Atlantic, suggesting significant differences in Fe sources or Fe cycling between these two ocean basins. A broad minimum in δ56Fe associated with the North Pacific oxygen minimum zone (OMZ; <75μmolkg−1 dissolved oxygen; ∼550–2000m depth) is consistent with reductive sediments along the California margin being an important source of dissolved Fe to the North Pacific. Other processes which may influence δ56Fe at SAFe include biological cycling in the upper ocean, and input of Fe from hydrothermal vents and oxic sediments below the OMZ. Zn and Cd concentration profiles at both stations broadly match the distribution of the macronutrients silicate and phosphate, respectively. At SAFe, δ114Cd increases towards the surface, reflecting the biological preference for assimilation of lighter Cd isotopes, while negative Cd∗ (−0.12) associated with low oxygen waters supports the recently proposed hypothesis of water-column CdS precipitation. In contrast to δ114Cd, δ66Zn at SAFe decreases towards the surface ocean, perhaps due to scavenging of isotopically heavy Zn, while at intermediate depths δ66Zn provides further evidence of a mid-depth dissolved δ66Zn maximum. We suggest this may be a global feature of Zn biogeochemistry related to either regeneration of heavy adsorbed Zn, or to ZnS formation and removal within the water column. Data from San Pedro shows that anoxic sediments can be a source of isotopically light Zn to the water column (δ66Zn of ∼−0.3‰ relative to JMC Lyon), though evidence of this signal is not observed being transported to SAFe. Within North Pacific Intermediate Water at SAFe (NPIW; ∼500m) elevated Cd∗ and Zn∗ and a focused minimum in δ56Fe suggest possible transport of Fe, Zn, and Cd over thousands of km from subpolar waters, meaning that NPIW may have a strong influence on the subsurface distribution of trace metals throughout the North Pacific.

Transcriptomic evidence for microbial sulfur cycling in the eastern tropical North Pacific oxygen minimum zone

Microbial communities play central roles in ocean biogeochemical cycles, and are particularly important in in oceanic oxygen minimum zones (OMZs). However, the key carbon, nitrogen, and sulfur (S) cycling processes catalyzed by OMZ microbial communities are poorly constrained spatially, temporally, and with regard to the different microbial groups involved. Here we sample across dissolved oxygen (DO) gradients in the oceans’ largest OMZ by volume—the eastern tropical North Pacific ocean, or ETNP—and quantify 16S rRNA and functional gene transcripts to detect and constrain the activity of different S-cycling groups. Based on gene expression profiles, putative dissimilatory sulfite reductase (dsrA) genes are actively expressed within the ETNP OMZ. dsrA expression was limited almost entirely to samples with elevated nitrite concentrations, consistent with previous observations in the Eastern Tropical South Pacific OMZ. dsrA and ‘reverse’ dissimilatory sulfite reductase (rdsrA) genes are related and the associated enzymes are known to operate in either direction—reducing or oxidizing different S compounds. We found that rdsrA genes and soxB genes were expressed in the same samples, suggestive of active S cycling in the ETNP OMZ. These data provide potential thresholds for S cycling in OMZs that closely mimic recent predictions, and indicate that S cycling may be broadly relevant in OMZs.