Bottom Water Oxygen Concentrations Research Articles

Mid-depth respired carbon storage and oxygenation of the eastern equatorial Pacific over the last 25,000 years

A growing body of evidence suggests that respired carbon was stored in mid-depth waters (∼1–3 km) during the last glacial maximum (LGM) and released to the atmosphere from upwelling regions during deglaciation. Decreased ventilation, enhanced productivity, and enhanced carbonate dissolution are among the mechanisms that have been cited as possible drivers of glacial CO2 drawdown. However, the relative importance of each of these mechanisms is poorly understood. New approaches to quantitatively constrain bottom water carbonate chemistry and oxygenation provide methods for estimating historic changes in respired carbon storage. While increased CO2 drawdown during the LGM should have resulted in decreased oxygenation and a shift in dissolved inorganic carbon (DIC) speciation towards lower carbonate ion concentrations, this is complicated by the interplay of carbonate compensation, export productivity, and circulation. To disentangle these processes, we use a multiproxy approach that includes boron to calcium (B/Ca) ratios of the benthic foraminifera Cibicidoides wuellerstorfi to reconstruct deep-water carbonate ion concentrations ([CO32−]) and the uranium to calcium (U/Ca) ratio of foraminiferal coatings in combination with benthic foraminiferal carbon isotopes to reconstruct changes in bottom water oxygen concentrations ([O2]) and organic carbon export. Our records indicate that LGM [CO32−] and [O2] was reduced at mid water depths of the eastern equatorial Pacific (EEP), consistent with increased respired carbon storage. Furthermore, our results suggest enhanced mixing of lower Circumpolar Deep Water (LCDW) to EEP mid water depths and provide evidence for the importance of circulation for oceanic-atmospheric CO2 exchange.

Seasonal Variability of Magnetotactic Bacteria in a Freshwater Pond

AbstractMagnetotactic bacteria (MTB) synthesize ferrimagnetic crystals that contribute to the remanent magnetization in sediments, yet knowledge of how MTB populations vary in natural environments over time remains limited. We report abundances of three MTB morphotypes from nine sites collected and measured every month over a two‐year period from a natural pond near Munich, Germany. Morphotype populations underwent coherent temporal trends among the nine sites—especially at proximal sites with similar water depths. MTB populations varied independently of bottom water oxygen concentrations or temperature over the two‐year period, except for spirilla, which flourished during the summer at some sites. Magnetic properties of the sediments did not reflect living MTB abundances, but instead varied with water depth. Deeper sites, which were also lower in organic carbon, nitrogen, and oxygen concentrations than shallower sites, had higher saturation magnetizations and were richer in single‐domain particles.

Importance of nitrate reduction in benthic carbon mineralization in two eutrophic estuaries: Modeling, observations and laboratory experiments

Estuaries are important nutrient filters between rivers and coastal zones. However, the quantification of the nutrient mitigation capacity related to benthic diagenesis is still poorly quantified. In this paper, we investigated carbon mineralization and the contribution of benthic nitrate reduction in two macrotidal eutrophic estuaries (Elorn and Aulne, Brittany, France) during winter and spring. These anthropized estuaries exhibited large variations of bottom water nitrate concentrations from very high values in upstream waters (up to 500 μM) to low values downstream (andlt; 10 μM). Bottom water oxygen concentrations presented small gradients compared to nitrate concentrations gradients resulting in large variation in nitrate to oxygen ratios NOx −/O2 between downstream and upstream (0.03–1.6). We combined the use of diagenesis modeling with field data (porosity, organic carbon and nitrogen, pore water profiles of dissolved oxygen, nitrogen, iron, manganese and sulfide concentrations, published in Khalil et al., 2013) and experimentally-determined nitrate reduction rates, in order to investigate the different organic carbon mineralization pathways in these estuarine sediments including denitrification and the contribution of benthic nitrate reduction to the estuarine N budget. Overall a good agreement between pore water data (organic carbon, oxygen, nitrate and ammonium) and model simulations was observed. The modeled organic matter mineralization rates were high in the upstream estuary and low in the saline estuary for the two estuaries Aulne and Elorn. This decrease may be related to the dilution and the trapping of allochtonous organic matter in estuarine sediments and its subsequent recycling in the upper estuary. Organic carbon mineralization rates were higher in the Elorn than in the Aulne estuary, which is most likely related to the labile character of the organic matter from urban origin exported from the Elorn watershed. The contribution of nitrate reduction to the total mineralization was generally high in upstream sediments (15–35%) of both Aulne and Elorn estuaries and decreased consistently downstream to 5–10%. The relative large contribution of nitrate reduction to organic matter degradation was to a large extent related to high bottom water nitrate concentrations that fueled 37–78% of total nitrate reduction in the upstream part of the estuaries. Overall, the reduction of bottom water nitrate by denitrification to N2 represented 3–13% of the river flux of nitrate in these estuaries in winter and spring, and could reach ~ 50% during summer. These results highlight that nitrate reduction in the sediment attenuate the high river nitrate flux, despite the huge quantities of anthropogenic nitrate discharged by the two rivers to the estuaries.

Dissolved oxygen dynamic across the new-born interface of bottom water and sediment after dredging contaminated sediment

借助微氧电极测试技术对太湖贡湖湾试验区疏浚后的新生界面溶解氧动态进行一年的跟踪调研，分析溶解氧在新生微米级界面的分布特征、扩散通量以及界面附近有机质矿化速率.结果表明疏浚后半年内，溶解氧在表层沉积物的侵蚀深度增大，氧化层明显加厚.氧气在新生界面表层沉积物中呈指数下降，但衰减相对较缓.在连续一年的跟踪调查中发现，仅秋季新生界面附近溶解氧浓度明显高于对照，而在其他月份无差异.污染底泥疏浚后一个月内氧扩散通量及有机碳矿化速率下降最为明显，仅为疏浚前的13%，其他月份沉积物-水界面氧的扩散通量、氧气的消耗速率、有机碳的降解速率均有不同程度下降，疏浚后新生界面氧气交换速率下降以及由此导致的有机碳矿化过程变缓可能深刻影响界面生源要素的迁移过程.;Oxygen dynamic had been conducted a yearlong investigation to obtain oxygen distribution, oxygen diffusion flux and mineralization rate of organic matter across the new-born interface of bottom water and sediments due to dredge. The results showed that oxygen penetration depth increased with thickening oxidation layer in new-born sediments. Oxygen concentrations decreased rapidly with depth in the sediments, but relatively moderate. Below the penetration depth, the O₂ concentration inside the sediment was under the detection limit (approximation for 0). Oxygen concentration in bottom water in autumn was significantly higher in dredge treatment than those in control, however, there was not statistically significant among the three sampling during the investigation. The diffusive O₂ flux (F_O₂) and the oxygen consumption rate (R_O₂) were observed based on O₂ microprofiles, otherwise, organic carbon degradation rates (R_org) were obtained from the oxygen consumption rate based on the modified Redfield ratio (O₂/C_org=170/117) and the values also obviously decreased. Therefore, thickening oxidation layer and slowdown organic carbon degradation rates could significantly influence nutrients biogeochemical cycle after dredge.

Cardiac responses of the bay scallop Argopecten irradians to diel-cycling hypoxia

Bottom water oxygen concentrations in coastal environments can oscillate between fully oxygenated and hypoxic conditions on a daily basis. How benthic organisms respond to such drastic changes in oxygen availability is not well understood. Specifically, we do not know the magnitude, duration, and frequency at which diel-cycling hypoxic conditions become stressful. Here we have used non-invasive, infrared sensors to measure the cardiac activity of the Atlantic bay scallop, Argopecten irradians, in response to diel-cycling hypoxia in-situ over one-month periods as well as in controlled laboratory incubations using animals conditioned to contrasting field conditions. In the field, heartbeat rates at a well‑oxygenated site were 23.0±1.8 beats minute−1 with 12.7±2.1% variance while heartbeat rates at sites with pronounced diel-cycling hypoxia were higher and more variable (site 1: 34.5±3.1 beats minute−1 with 20.8±3.2% variance; site 2: 48.4 and 45.8 beats minute−1 with 16% variance). Maximal heartbeat rates were commonly recorded around dawn when oxygen concentrations fell below 5mg O2 L−1 suggesting this was a threshold concentration or critical PO2 (Pc) that induced a switch to oxyconformity and onset of anaerobic metabolic pathways. In-situ cardiac activity at locations with diel-cycling hypoxia indicate that A. irradians spent nearly 40% of each day in sub-optimal conditions during which metabolic activity was reduced and/or at least partially sustained by anaerobic metabolism. During laboratory experiments, an increase in heartbeat rate in response to initial declines below 5mg O2 L−1 from fully oxygenated conditions suggests a regulatory response in which cardiac activity was enhanced to maintain oxygen supply. At DO below 2mg O2 L−1, however, heartbeat rates declined reaching a state of bradycardia and acardia during anoxia, suggesting a conformer response to severe hypoxia. Heartbeat frequency was a suitable proxy for respiration under normoxia, but heartbeat and respiration rates decoupled during severe hypoxia (<2.0mg O2 L−1). A. irradians were able to survive anoxic periods between 12 and 14h and cardiac activity rapidly returned to basal rates once full oxygen saturation was re-established. Mortality occurred after 23–32h in anoxia regardless of prior conditioning. We speculate that repetitive exposure to periods of DO oscillations with exposure below 5mg O2 L−1 in the field can cause sub-lethal effects to A. irradians affecting fitness, growth, and reproductive success.

Benthic Dinitrogen Fixation Traversing the Oxygen Minimum Zone Off Mauritania (NW Africa)

Despite its potential to provide new nitrogen (N) to the environment, knowledge on benthic dinitrogen (N2) fixation remains relatively sparse and its contribution to the marine N budget, is regarded as minor. Benthic N2 fixation is often observed in organic-rich sediments coupled to heterotrophic metabolisms, such as sulfate reduction. In the present study, benthic N2 fixation together with sulfate reduction and other heterotrophic metabolisms were investigated at six station between 47 and 1108 m water depth along the 18°N transect traversing the highly productive upwelling region known as Mauritanian oxygen minimum zone (OMZ). Bottom water oxygen concentrations ranged between 30 and 138 µM. Benthic N2 fixation determined by the acetylene reduction assay was detected at all stations with highest rates (0.15 mmol m-2 d-1) on the shelf (47 and 90 m water depth) and lowest rates (0.08 mmol m-2 d-1) below 412 m water depth. The biogeochemical data suggest that part of the N2 fixation could be linked to sulfate- and iron-reducing bacteria. Molecular analysis of the key functional marker gene for N2 fixation, nifH, confirmed the presence of sulfate- and iron-reducing diazotrophs. High N2 fixation further coincided with bioirrigation activity caused by burrowing macrofauna, both of which showed high rates at the shelf sites and low rates in deeper waters. However, statistical analyses proved that none of these processes and environmental variables were significantly correlated with benthic diazotrophy, which lead to the conclusion that either the key parameter controlling benthic N2 fixation in Mauritanian sediments remains unidentified or that a more complex interaction of control mechanisms exists. N2 fixation rates in Mauritanian sediments were 2.7 times lower than those from the anoxic Peruvian OMZ.

Climate and Anthropogenic Controls of Coastal Deoxygenation on Interannual to Centennial Timescales

AbstractUnderstanding dissolved oxygen variability in the ocean is limited by the short duration of direct measurements; however, sedimentary oxidation‐reduction reactions can provide context for modern observations. Here we use bulk sediment redox‐sensitive metal enrichment factors (MoEF, ReEF, and UEF) and scanning X‐ray fluorescence records to examine annual‐scale sedimentary oxygen concentrations in the Santa Barbara Basin from the Industrial Revolution (Common Era ~1850) to present. Enrichments are linked to measured bottom water oxygen concentrations after 1986. We reveal gradual intensification of the coastal oxygen minimum zone (OMZ) on the southern California margin coinciding with the twentieth century anthropogenic warming trend that leads to reduced oxygen solubility and greater stratification. High‐frequency interannual oscillations become more prominent over the last three decades. These are attributed to local “flushing events” triggered by the transition from El Niño to La Niña conditions, which further amplify changes in the extratropical southern Californian OMZ.

Assessing oxygen depletion in the Northeastern Pacific Ocean during the last deglaciation using I/Ca ratios from multiple benthic foraminiferal species

AbstractPaleo‐redox proxies are crucial for reconstructing past bottom water oxygen concentration changes brought about by ocean circulation and marine productivity shifts in response to climate forcing. Carbonate I/Ca ratios of multiple benthic foraminifera species from Ocean Drilling Program Hole 1017E—a core drilled within the Californian oxygen minimum zone (OMZ), on the continental slope—are employed to reexamine the transition from the well‐oxygenated last glacial into poorly oxygenated modern conditions. The redox and export productivity history of this site is constrained by numerous proxies used to assess sensitivity of I/Ca ratios of benthic foraminifera to changes in bottom and pore water O2 concentrations. Reconstructed iodate (IO3−) availability is from the I/Ca ratio of epifaunal (Cibicidoides sp.), shallow infaunal (Uvigerina peregrina), and deep infaunal (Bolivina spissa) foraminifera. The reconstructed IO3− availability profile is used to determine the contribution of bottom water O2 relative to oxidant demand on pore water O2 concentrations. These results suggest that high export productivity on the California Margin drove low pore water O2 concentrations during the Bølling. In contrast, low bottom water O2 concentrations at 950 m water depth only contributed to reduced sediments during the Allerød. Increased contribution of modified North Pacific Intermediate Water to the California Current System ventilated the California OMZ during the late glacial and the Younger Dryas such that water overlying the site was oxygenated. These results highlight the promising potential of this new proxy for understanding the relative influence of bottom water O2 concentration and pore water oxidant demand on OMZs.

Sediment phosphorus speciation and mobility under dynamic redox conditions

Abstract. Anthropogenic nutrient enrichment has caused phosphorus (P) accumulation in many freshwater sediments, raising concerns that internal loading from legacy P may delay the recovery of aquatic ecosystems suffering from eutrophication. Benthic recycling of P strongly depends on the redox regime within surficial sediment. In many shallow environments, redox conditions tend to be highly dynamic as a result of, among others, bioturbation by macrofauna, root activity, sediment resuspension and seasonal variations in bottom-water oxygen (O2) concentrations. To gain insight into the mobility and biogeochemistry of P under fluctuating redox conditions, a suspension of sediment from a hypereutrophic freshwater marsh was exposed to alternating 7-day periods of purging with air and nitrogen gas (N2), for a total duration of 74 days, in a bioreactor system. We present comprehensive data time series of bulk aqueous- and solid-phase chemistry, solid-phase phosphorus speciation and hydrolytic enzyme activities demonstrating the mass balanced redistribution of P in sediment during redox cycling. Aqueous phosphate concentrations remained low ( ∼ 2.5 µM) under oxic conditions due to sorption to iron(III) oxyhydroxides. During anoxic periods, once nitrate was depleted, the reductive dissolution of iron(III) oxyhydroxides released P. However, only 4.5 % of the released P accumulated in solution while the rest was redistributed between the MgCl2 and NaHCO3 extractable fractions of the solid phase. Thus, under the short redox fluctuations imposed in the experiments, P remobilization to the aqueous phase remained relatively limited. Orthophosphate predominated at all times during the experiment in both the solid and aqueous phase. Combined P monoesters and diesters accounted for between 9 and 16 % of sediment particulate P. Phosphatase activities up to 2.4 mmol h−1 kg−1 indicated the potential for rapid mineralization of organic P (Po), in particular during periods of aeration when the activity of phosphomonoesterases was 37 % higher than under N2 sparging. The results emphasize that the magnitude and timing of internal P loading during periods of anoxia are dependent on both P redistribution within sediments and bottom-water nitrate concentrations.

Dynamics of the Physical and Biogeochemical Processes during Hypoxia in Jinhae Bay, South Korea

ABSTRACT Lee, J.; Kim, S.-g., and An, S., 2017. Dynamics of the physical and biogeochemical processes during hypoxia in Jinhae Bay, South Korea. Bottom-water hypoxia occurs during the summer in Jinhae Bay, a semienclosed embayment in South Korea, and can have detrimental effects on marine ecosystems around the region. Measurements of the oxygen demand in the bottom water and sediments and time-series measurements of hydrographic characteristics were conducted in Jinhae Bay in 2015. Variations in dissolved oxygen (DO) can be divided into three distinct time periods. During period I (late March to late May), bottom DO decreased linearly with thermocline development. During period II (late May to late August), hypoxia persisted. During period III (after late August), bottom DO increased as the thermal stratification began to break down. The seasonal bottom-water oxygen concentration was significantly correlated with the temperature difference between surface and bottom water, indicating that hypoxia developm...

Holocene Refreshening and Reoxygenation of a Bothnian Sea Estuary Led to Enhanced Phosphorus Burial

Salinity variations in restricted basins like the Baltic Sea can alter their vulnerability to hypoxia (i.e., bottom water oxygen concentrations <2 mg/l) and can affect the burial of phosphorus (P), a key nutrient for marine organisms. We combine porewater and solid-phase geochemistry, micro-analysis of sieved sediments (including XRD and synchrotron-based XAS), and foraminiferal δ18O and δ13C analyses to reconstruct the bottom water salinity, redox conditions, and P burial in the Ångermanälven estuary, Bothnian Sea. Our sediment records were retrieved during the Integrated Ocean Drilling Program (IODP) Baltic Sea Paleoenvironment Expedition 347 in 2013. We demonstrate that bottom waters in the Ångermanälven estuary became anoxic upon the intrusion of seawater in the early Holocene, like in the central Bothnian Sea. The subsequent refreshening and reoxygenation, which was caused by gradual isostatic uplift, promoted P burial in the sediment in the form of Mn-rich vivianite. Vivianite authigenesis in the surface sediments of the more isolated part of the estuary ultimately ceased, likely due to continued refreshening and an associated decline in productivity and P supply to the sediment. The observed shifts in environmental conditions also created conditions for post-depositional formation of authigenic vivianite, and possibly apatite formation, at ∼8 m composite depth. These salinity-related changes in redox conditions and P burial are highly relevant in light of current climate change. The results specifically highlight that increased freshwater input linked to global warming may enhance coastal P retention, thereby contributing to oligotrophication in both coastal and adjacent open waters.

The influence of dissolved oxygen on dinoflagellate cyst distribution across Sluice Pond, a meromictic lake in NE Massachusetts, USA

ABSTRACTAssemblages of dinoflagellate cysts in lakebed sediments across the deep, meromictic basin of Sluice Pond (Lynn, Massachusetts) were compared with measurements of water quality and sediment geochemistry and with testate amoeba and tintinnid assemblages. Lakebed sediments below oxygenated waters (DO >2 mg/L) contain diverse testate amoebae, but sparse Peridinium willei-dominated dinoflagellate cyst assemblages. Sediments collected under hypoxic conditions (DO < 2 mg/L) showed higher preservation of organic carbon and nitrogen, and these deep basin sediments are characterised by diverse dinoflagellate cyst assemblages, but sieved microfossil assemblages are typically dominated by the planktonic Codonella cratera and the pseudo-planktonic Cucurbitella tricuspis. Strong inverse relationships of cysts of Peridinium willei (R2 = 0.81) and Difflugia oblonga tests (R2 = 0.7) with water depth are attributed to variations in bottom water oxygen concentrations. Both ecological and taphonomic factors must be considered when reconstructing palaeoenvironments from microfossil assemblages.

The roles of resuspension, diffusion and biogeochemical processes on oxygen dynamics offshore of the Rhône River, France: a numerical modeling study

Abstract. Observations indicate that resuspension and associated fluxes of organic material and porewater between the seabed and overlying water can alter biogeochemical dynamics in some environments, but measuring the role of sediment processes on oxygen and nutrient dynamics is challenging. A modeling approach offers a means of quantifying these fluxes for a range of conditions, but models have typically relied on simplifying assumptions regarding seabed–water-column interactions. Thus, to evaluate the role of resuspension on biogeochemical dynamics, we developed a coupled hydrodynamic, sediment transport, and biogeochemical model (HydroBioSed) within the Regional Ocean Modeling System (ROMS). This coupled model accounts for processes including the storage of particulate organic matter (POM) and dissolved nutrients within the seabed; fluxes of this material between the seabed and the water column via erosion, deposition, and diffusion at the sediment–water interface; and biogeochemical reactions within the seabed. A one-dimensional version of HydroBioSed was then implemented for the Rhône subaqueous delta in France. To isolate the role of resuspension on biogeochemical dynamics, this model implementation was run for a 2-month period that included three resuspension events; also, the supply of organic matter, oxygen, and nutrients to the model was held constant in time. Consistent with time series observations from the Rhône Delta, model results showed that erosion increased the diffusive flux of oxygen into the seabed by increasing the vertical gradient of oxygen at the seabed–water interface. This enhanced supply of oxygen to the seabed, as well as resuspension-induced increases in ammonium availability in surficial sediments, allowed seabed oxygen consumption to increase via nitrification. This increase in nitrification compensated for the decrease in seabed oxygen consumption due to aerobic remineralization that occurred as organic matter was entrained into the water column. Additionally, entrainment of POM into the water column during resuspension events, and the associated increase in remineralization there, also increased oxygen consumption in the region of the water column below the pycnocline. During these resuspension events, modeled rates of oxygen consumption increased by factors of up to ∼ 2 and ∼ 8 in the seabed and below the pycnocline, respectively. When averaged over 2 months, the intermittent cycles of erosion and deposition led to a ∼ 16 % increase of oxygen consumption in the seabed, as well as a larger increase of ∼ 140 % below the pycnocline. These results imply that observations collected during quiescent periods, and biogeochemical models that neglect resuspension or use typical parameterizations for resuspension, may underestimate net oxygen consumption at sites like the Rhône Delta. Local resuspension likely has the most pronounced effect on oxygen dynamics at study sites with a high oxygen concentration in bottom waters, only a thin seabed oxic layer, and abundant labile organic matter.

Penultimate and last glacial oceanographic variations in the Bering Sea on millennial timescales: Links to North Atlantic climate

We present high-resolution multi-proxy records from a marine sediment core (SO201-2-85KL) from the western Bering Sea to assess orbital- and millennial-scale paleoceanographic conditions during two last glacial intervals, including both terminations. Based on changes in foraminiferal assemblages, grain-size content and previously published TOC and δ13C records, we reconstruct variations in sea-surface biological productivity, intermediate-water oxygenation and sea-ice conditions during the last 180 kyr. Our data demonstrate remarkable differences between the penultimate (MIS 6) and last (MIS 4-2) glacial. Relatively high sea surface bioproductivity and reduced sea-ice cover are reconstructed for the penultimate glacial interval, whereas low bioproductivity and expanded sea-ice cover appear to be typical for the last glacial. Millennial-scale changes in intermediate water ventilation are inferred from faunal records for the middle part of the penultimate glacial. High-amplitude environmental variability during the penultimate glacial time in the Bering Sea resembles the well-known Dansgaard-Oeschger oscillations, and roughly corresponds to similar rapid climatic fluctuations found in North Atlantic records. The Termination II and I intervals display a similar succession of high-bioproductivity events, being more pronounced during the penultimate glacial-interglacial transition, probably due to the different orbital configuration. During the late phase of Termination II, two short intervals, characterized by high sea surface bioproductivity and low oxygen content of bottom waters, resemble the Bølling and Allerød warmings, whereas an episode with low bioproductivity occurs in between, similar to the Older Dryas. Our results provide support for a close circumpolar coupling between high-latitude environments on millennial timescales at least since the penultimate glacial.

Organically bound iodine as a bottom-water redox proxy: Preliminary validation and application

Carbonate-associated iodine (I/Ca) has been used as a proxy of local, upper-ocean redox conditions, and has successfully demonstrated highly dynamic spatial and temporal patterns across different time scales of Earth history. To further explore the utility of iodine as a paleo-environmental proxy, we present here a new method of extracting organically bound iodine (Iorg) from shale using volumes of samples on the order of tens of milligrams, thus offering the potential for high-resolution work across thin shale beds. The ratio of Iorg to total organic carbon (I/TOC) in modern surface and subsurface sediments decreases with decreasing bottom-water oxygen, which may be used to reconstruct paleo-redox changes.As a proof of concept, we evaluate the I/TOC proxy in Holocene sediments from the Baltic Sea, Landsort Deep (IODP 347) and discuss those data within a framework of additional independent redox proxies, e.g., iron speciation and [Mo]. The results imply that I/TOC may be sensitive to hypoxic–suboxic conditions, complementary to proxies sensitive to more reducing, anoxic–euxinic conditions. Then, we test the usage of I/TOC in sediments deposited during Late Cretaceous, Cenomanian–Turonian Oceanic Anoxic Event (OAE) 2 from ~94millionyears ago (Ma). We generated I/TOC and Iorg records from six OAE 2 sections: Tarfaya (Morocco), Furlo (central Italy), Demerara Rise (western equatorial Atlantic), Cape Verde Basin (eastern equatorial Atlantic), South Ferriby (UK), and Kerguelen Plateau (southern Indian Ocean), which provide a broad spatial coverage. Generally, I/TOC decreases over the interval recorded by the positive carbon-isotope excursion, the global signature of OAE 2, suggesting an expansion of more reducing bottom-water conditions and consistent with independent constraints from iron speciation and redox-sensitive trace-metals (e.g., Mo). Relatively higher I/TOC values (thus more oxic conditions) are recorded at two high latitude sites for OAE 2, supporting previous model simulations (cGENIE) that indicated higher bottom water oxygen concentrations in these regions. Our results also indicate that organic-rich and oxygenated seafloors are likely a major sink of iodine and correspondingly influence its global seawater inventory.

Patchiness of phytoplankton and primary production in Liaodong Bay, China.

A comprehensive study of water quality, phytoplankton biomass, and photosynthetic rates in Liaodong Bay, China, during June and July of 2013 revealed two large patches of high biomass and production with dimensions on the order of 10 km. Nutrient concentrations were above growth-rate-saturating concentrations throughout the bay, with the possible exception of phosphate at some stations. The presence of the patches therefore appeared to reflect the distribution of water temperature and variation of light penetration restricted by water turbidity. There was no patch of high phytoplankton biomass or production in a third, linear patch of water with characteristics suitable for rapid phytoplankton growth; the absence of a bloom in that patch likely reflected the fact that the width of the patch was less than the critical size required to overcome losses of phytoplankton to turbulent diffusion. The bottom waters of virtually all of the eastern half of the bay were below the depth of the mixed layer, and the lowest bottom water oxygen concentrations, 3–5 mg L–1, were found in that part of the bay. The water column in much of the remainder of the bay was within the mixed layer, and oxygen concentrations in both surface and bottom waters exceeded 5 mg L–1.

Strong and Dynamic Benthic-Pelagic Coupling and Feedbacks in a Coastal Upwelling System (Peruvian Shelf)

Monthly time-series data (1998–2009) of bottom water oxygen, nitrate and nitrite concentrations from the outer shelf (150 m water depth) in the oxygen minimum zone offshore Peru were coupled to a layered biogeochemical sediment model to investigate benthic-pelagic coupling over multi-annual time scales. The model includes the mineralization of four reactive pools of particulate organic carbon (POC) with lifetimes of 0.13, 1.3, 20 and 1700 yr that were constrained using empirical data. Total POC rain rates to the seafloor were derived from satellite based estimates of primary production. Solute fluxes and concentrations in sediment porewater showed highly dynamic behavior over the course of a typical year. Conversion of fixed N to N2 by denitrification varied from 0.9 mmol m-2 d-1 of N in winter to 1.6 mmol m-2 d-1 of N in summer with a long term mean N loss for the shelf of 1.3 mmol m-2 d-1 of N. Fixed N loss across the whole time-series agreed very well with a previously-published vertically-integrated sediment model for coupling the benthic and pelagic N cycle in regional and global models. Dissimilatory nitrate reduction to ammonium (DNRA) emerges as a major process in the benthic N cycle, producing on average 2.0 mmol m-2 d-1 of ammonium: more than twice the rate of ammonification of organic nitrogen. The model predicts sulfide emissions from the sediment of up to 1 mmol m-2 d-1 when POC rain rate exceeds 20 mmol m-2 d-1, in agreement with past observations of benthic sulfide fluxes and sulfide plume distributions in the water column. This study demonstrates that sediments on the Peruvian shelf are not static repositories that are independent of changes taking place in the water column. Our results strongly suggest the shelf sediments must exert an important feedback on biogeochemical processes in the overlying waters, and should be considered in regional model studies.

Influence of Natural Oxygenation of Baltic Proper Deep Water on Benthic Recycling and Removal of Phosphorus, Nitrogen, Silicon and Carbon

At the end of 2014, a Major Baltic Inflow (MBI) brought oxygenated, salty water into the Baltic proper and reached the long-term anoxic Eastern Gotland Basin (EGB) by March 2015. In July 2015, we measured benthic fluxes of phosphorus (P), nitrogen (N) and silicon (Si) nutrients and dissolved inorganic carbon (DIC) in situ using an autonomous benthic lander at deep sites (170-210 m) in the EGB, where the bottom water oxygen concentration was 30-45 μM. The same in situ methodology was used to measure benthic fluxes at the same sites in 2008-2010, but then under anoxic conditions. The high efflux of phosphate under anoxic conditions became lower upon oxygenation, and turned into an influx in about 50% of the flux measurements. The C:P and N:P ratios of the benthic solute flux changed from clearly below the Redfield ratio (on average about 70 and 3-4, respectively) under anoxia to approaching or being well above the Redfield ratio upon oxygenation. These observations demonstrate retention of P in newly oxygenated sediments. We found no significant effect of oxygenation on the benthic ammonium, silicate and DIC flux. We also measured benthic denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) rates at the same sites using isotope-pairing techniques. The bottom water of the long-term anoxic EGB contained less than 0.5 μM nitrate in 2008-2010, but the oxygenation event created bottom water nitrate concentrations of about 10 μM in July 2015 and the benthic flux of nitrate was consistently directed into the sediment. Nitrate reduction to both dinitrogen gas (denitrification) and ammonium (DNRA) was initiated in the newly oxygenated sediments, while anammox activity was negligible. We estimated the influence of this oxygenation event on the magnitudes of the integrated benthic P flux (the internal P load) and the fixed N removal through benthic and pelagic denitrification by comparing with a hypothetical scenario without the MBI. Our calculations suggest that the oxygenation triggered by the MBI in July 2015, extrapolated to the basin-wide scale of the Baltic proper, decreased the internal P load by 23% and increased the total (benthic plus pelagic) denitrification by 18%.

Carbon and Phosphorus Cycling in Arabian Sea Sediments across the Oxygen Minimum Zone

Several studies have focused on carbon, oxygen, and phosphorus dynamics across the modern oxygen minimum zone (OMZ) to constrain how signals of modern systems get “locked in” upon burial. In this study, a sequential phosphorus fractionation technique was applied to surficial and sub-surface sediments from stations at depths spanning the OMZ on the Pakistan margin of the Arabian Sea in order to test the oxygen-carbon-phosphorus connection in modern marine sediments. Some early diagenetic loss of phosphorus compared to organic carbon was observed, but a significant portion of the released phosphorus was retained by uptake on oxyhydroxides and by the formation of an authigenic phosphorus-bearing phase. This process is unaffected by station location relative to the OMZ, and results in an effective organic carbon-to-reactive-phosphorus sediment ratio that is close to the average observed for open-ocean sediments, regardless of bottom water oxygen content.

High resolution I/Ca ratios of benthic foraminifera from the Peruvian oxygen-minimum-zone: A SIMS derived assessment of a potential redox proxy

Oceanic oxygen decline due to anthropogenic climate change is a matter of growing concern. Tropical oxygen minimum zones (OMZs) are the most important areas of oxygen depletion in the modern oceans. A quantitative oxygen proxy in OMZs is highly desirable in order to identify and monitor recent dynamics as well as to reconstruct pre-Anthropocene changes in amplitude and extension of oxygen depletion.A previous study revealed that there are significant correlations between I/Ca ratios of foraminiferal bulk samples for different benthic foraminiferal species from the Peruvian OMZ. Nevertheless, species for which less specimens were available showed a higher variability between I/Ca ratios in different badges. To test if this might be related to intra- or inter-shell heterogeneity we focused on microanalyses of I/Ca ratios within these species in our present study. We developed a method for measuring benthic foraminiferal I/Ca ratios, a potential proxy for the reconstruction of marine oxygen concentrations. We applied 92 spot analyses in individual foraminiferal specimens from the Peruvian OMZ using secondary ion mass-spectrometry (SIMS). The I/Ca ratios on 8 of 11 cleaned Uvigerina striata and Planulina limbata specimens determined with SIMS showed no significant difference to previous ICP-MS measurements on bulk samples from the same species. This indicates that both techniques are suited to the analysis and that the applied cleaning protocols efficiently removed the strong iodine contaminations.Nevertheless, despite the highly significant correlation between bulk ICP-MS I/Ca ratios and bottom water oxygen concentrations for U. striata, no significant correlation was observed for the SIMS derived individual I/Ca ratios. This indicates that ICP-MS bulk analyses on pooled bulk samples might be more suitable for reliable oxygen reconstructions using I/Ca ratios. On the contrary, the strong intra-test (e.g. -shell) variations could be induced by the oxygen variability in the habitats of foraminifera. Therefore, the high resolution findings provide the perspective for tracking relative short term oxygen fluctuations by measuring ontogenetic changes in I/Ca ratios within individual foraminiferal tests.Measurements on cross-sections of uncleaned U. striata specimens revealed a strong contaminant iodine phase within the massive centre of the foraminiferal test walls which usually would be considered to be free of contamination. The contaminant iodine is probably associated to organic matter and located inside a microporous framework within the foraminiferal calcite. This might be related to microtubular structures which have been revealed in previous studies during early dissolution states of foraminiferal test walls.