Bottom Water Oxygen Concentrations Research Articles

Effects of progressive oxygen depletion on sediment diagenesis and fluxes: A model for the lower St. Lawrence River Estuary

We applied a diagenetic model to examine the effects of a decadal‐timescale progressive decrease in bottomwater oxygen concentration on sediment geochemistry and fluxes across the sediment‐water interface. The model was calibrated using geochemical data acquired over the last 30 yr in the lower St. Lawrence River Estuary (Canada), where the bottom‐water oxygen concentration has been decreasing at an average rate of 1 µmol L−1 yr−1 over the past 70 yr. Benthic fluxes calculated from the model are comparable to fluxes derived from shipboard sediment incubations. By propagating the model forward in time and allowing the oxygen concentration to decrease further at 1 µmol L−1 yr−1, we predict changes in the distributions and fluxes of iron, manganese, phosphorus, nitrogen, and sulfur. As the oxygen concentration decreases, the concentrations and distributions of reactive phases in the sediment change at an accelerating pace. Fluxes of reduced substances out of the sediment increase, reactive iron and manganese oxide phases become depleted, and the sediment becomes progressively enriched in iron sulfides. A notable exception is the efflux of phosphate, which remains invariant. At the study site, these changes are likely to become measurable within the next 20 yr. Model results are sensitive to the effects of decreasing oxygen concentrations on the benthos, the response of which we represent by two different approximations of the bioturbation and bioirrigation formulations: a gradual decrease and a threshold type decrease. The principal obstacle to reliable predictions of how sediments will respond is the lack of knowledge about the response of bioturbation and bioirrigation to decreasing oxygen in bottom waters.

Assessing environmental control on dinoflagellate cyst distribution in surface sediments of the Benguela upwelling region (eastern South Atlantic)

Organic‐walled dinoflagellate cyst assemblages in surface sediments of the Benguela Current Upwelling System (eastern South Atlantic) show geographic patterns that cannot entirely originate from cyst production or transport. Aimed at answering how far these variations are due to taphonomic control, this study investigated a possible correlation with the changes in bottom‐water oxygen concentrations typifying this region. Based on 36 samples, multivariate statistics were used to analyze community variability with respect to bottom‐water oxygen concentration, temperature, salinity, nutrient content, chlorophyll a (Chl a) concentration, the organic carbon content of surface sediments, and a measure of water column stratification. Determined relationships to salinity, nutrient supply, nutrition, and environmental steadiness point out the requirements for dinoflagellate cyst production, while cross‐shelf transport processes could have introduced variability prior to burial of cysts in surface sediments. The offshore decrease in the relative abundance of protoperidinacean cyst types was consistent with their lower preservation potential under oxygenated conditions and coincided with a change in assemblage composition toward oxidation resistant species. On elimination of covariation, bottom‐water oxygenation was significantly related to this pattern and determined together with seasonal salinity, Chl a, and annual phosphate concentration, the parameter combination best explaining community variability. These results suggest that postdepositional degradation of peridinioid dinoflagellate cysts would partly explain the onshore‐offshore gradient in species distributions and could be responsible for more variability in assemblage compositions than is presently acknowledged.

FORAMINIFERS FROM HOLOCENE SEDIMENTS OF TWO INLAND BRACKISH LAKES IN CENTRAL GERMANY

Two species of the genus Ammonia , Ammonia tepida and A. aberdoveyensis , occur in the Holocene sediment successions of the former Salziger See and the Suesser See, two inland brackish-water lakes in central Germany. Thus, this work represents the first non-marine evidence of calcareous foraminifers in central Europe. A. aberdoveyensis permanently colonized the Suesser See from the early Atlantic to the Subboreal interval (∼8500–4500 cal. yr BP), whereas tests of A. tepida were found only during the Atlantic interval (∼8500–5600 cal. yr BP). Except for some redeposited specimens, the foraminiferal record of the former Salziger See is characterized by the monospecific appearance of A. tepida during the late Atlantic interval (∼6500 cal. yr BP). Both a high rate of test aberrancy of the observed Ammonia specimens and their high test porosity can be associated with enhanced environmental stress within the lake ecosystems caused by hyposalinity and/or periodically low oxygen concentration in the lake bottom waters. Because during the Quaternary, the study area has never been connected to the ocean, an airborne colonization of the Mansfeld lakes by migratory birds can be assumed. The low diversity of the investigated foraminiferal fauna is consistent with the remote position of the study area—more than 300 km from the natural habitat of foraminifers in the Baltic and North Seas—and the tolerance of the genus Ammonia to a wide range of environmental conditions.

Distribution of organic-walled dinoflagellate cysts in shelf surface sediments of the Benguela upwelling system in relationship to environmental conditions

To obtain insight in the relationship between the spatial distribution of organic-walled dinoflagellate cysts (dinocysts) and local environmental conditions, fifty-eight surface sediment samples from the coastal shelf off SW Africa were investigated on their dinocyst content with special focus on the two main river systems and the active upwelling that characterise this region. To avoid possible overprint by species-selective preservation, samples have been selected mainly from shelf sites where high sedimentation rates and/or low bottom water oxygen concentrations prevail. Multivariate ordination analyses have been carried out to investigate the relationship between the distribution patterns of individual species to environmental parameters of the upper water column and sediment transport processes. The main oceanographical variables at the surface (temperature, salinity, nutrients chlorophyll- a) in the region show onshore–offshore gradients. This pattern is reflected in the dinocyst associations with high relative abundances of heterotrophic dinocyst species in neritic regions characterised by high chlorophyll- aand low salinity conditions in surface waters. Phototrophic dinocyst species, notably Operculodinium centrocarpum, dominate in the more oceanic area. Differences in the distribution of phototrophic dinocyst species can be related to sea surface salinity and sea surface temperature gradients and to a lesser extent to chlorophyll- a concentrations. Apart from longitudinal gradients the dinocyst distribution clearly reflects regional environmental features. Six groups of species can be distinguished, characteristic for (1) coastal regions (cysts of Polykrikos kofoidii and Selenopemphix quanta), (2) the vicinity of active upwelling ( Brigantedinium spp., Echinidinium aculeatum, Echinidinium spp. and Echinidinium transparantum), (3) river mouths ( Lejeunecysta oliva, cysts of Protoperidinium americanum, Selenopemphix nephroides and Votadinium calvum), (4) slope and open ocean sediments ( Dalella chathamense, Impagidinium patulum and Operculodinium centrocarpum, (5) the southern Benguela region (south of 24°S) ( Spiniferites ramosus) and (6) the northern Benguela region (north of 24°S) ( Nematosphaeropsis labyrinthus and Pyxidinopsis reticulata). No indication of overprint of the palaeo-ecological signal by lateral transport of allochthonous species could be observed.

Diagenetic and sedimentological controls on the composition of organic matter preserved in California Borderland Basin sediments

Compound‐specific radiocarbon (14C) contents, stable carbon isotopes, and abundances of phytoplankton and vascular plant derived lipid biomarkers (alkenones and fatty acids) were obtained from Santa Barbara Basin and Santa Monica Basin sediments, along with radiocarbon contents of planktic foraminifera and total organic carbon. We investigated core‐top and prebomb sediment intervals at sites from the flanks and depocenters of the basins deposited under contrasting bottom water oxygen concentrations. Bulk organic matter generally has the lowest radiocarbon levels of all sediment constituents measured, whereas planktic foraminifera tend to be the most radiocarbon enriched. Alkenones are systematically depleted in radiocarbon with respect to foraminifera. Short‐chain (C14, C16, C18) fatty acids decrease rapidly in absolute abundance and relative to longer‐chain (>C24) homologues from core‐top to prebomb samples. The loss of short‐chain fatty acids with depth is associated with 13C depletion of short‐chain fatty acids, indicating preferential preservation of terrestrially derived fatty acids. Short‐chain fatty acids tend to be more 14C‐enriched relative to alkenones in core‐top sediments, whereas longerchain homologues are generally the most radiocarbon depleted of the lipids studied here. Less refractory compounds (e.g., short‐chain fatty acids) are thus enriched in radiocarbon with respect to more recalcitrant biomarkers (alkenones, long‐chain fatty acids). The lower 14C content of more refractory compounds reflects a larger proportion of laterally supplied, preaged material. Greater preservation of labile organic compounds observed at the depocenters than in flank sediments results in the presence of “younger” biomarkers, underlining the important influence of selective degradation of labile compounds on their radiocarbon ages.

Sedimentary redox conditions in continental margin sediments (N.E. Pacific) — Influence on the accumulation of redox-sensitive trace metals

Redox conditions in near-surface sediments from the continental margin off western Canada were characterized to determine how they influence the accumulation of redox-sensitive trace metals. Despite the development of suboxic conditions within millimeters of the sediment–water interface there is little enrichment of Re, U, Cd or Mo in the upper 10 to 15 cm of the sediment column. This observation is consistent with the low extractible solid Mn concentrations but high I/C org ratios that imply that near-surface sediment redox conditions are poised between Mn and I reduction. In contrast, below ∼ 15 cm Re, and to a lesser extent Cd and U, are enriched (e.g., up to 65 ng/g Re) but only in those cores collected from within and below the oxygen minimum zone. Molybdenum concentrations remain low (< 1.5 μg/g) in all cores indicating that these near-surface sediments never became fully anoxic. A combination of low sedimentation rates on the slope (< 1 to 6 cm/kyr) and/or intense bioturbation at all locations, including on the shelf where the sedimentation rate is substantially higher (40 cm/kyr), has maintained suboxic conditions in these sediments. Suboxic conditions exist despite a high organic carbon flux to the sediment and low bottom water oxygen concentrations, both of which should favour the development of anoxic conditions close to the sediment–water interface. The enrichment of Re, U and Cd that is observed occurs well below the sediment–water interface, overprinting and thus potentially compromising any paleo-signal. Accordingly, caution is necessary when using redox-sensitive trace metal concentrations as paleo-proxies in environments characterized by low sedimentation rates and/or deep bioturbation.

Selective preservation of organic-walled dinoflagellate cysts as a tool to quantify past net primary production and bottom water oxygen concentrations

To understand the role of the ocean within the global carbon cycle, detailed information is required on key-processes within the marine carbon cycle; bio-production in the upper ocean, export of the produced material to the deep ocean and the storage of carbon in oceanic sediments. Quantification of these processes requires the separation of signals of net primary production and the rate of organic matter decay as reflected in fossil sediments. This study examines the large differences in degradation rates of organic-walled dinoflagellate cyst species to separate these degradation and productivity signals. For this, accumulation rates of cyst species known to be resistant (R-cysts) or sensitive (S-cysts) to aerobic degradation of 62 sites are compared to mean annual chlorophyll- a, sea-surface temperature, sea-surface salinity, nitrate and phosphate concentrations of the upper waters and deep-water oxygen concentrations. Furthermore, the degradation of sensitive cysts, as expressed by the degradation constant k and reaction time t, has been related to bottom water [O 2]. The studied sediments were taken from the Arabian Sea, north-western African Margin (North Atlantic), western-equatorial Atlantic Ocean/Caraibic, south-western African margin (South Atlantic) and Southern Ocean (Atlantic sector). Significant relationships are observed between (a) accumulation rates of R-cysts and upper water chlorophyll- a concentrations, (b) accumulation rates of S-cysts and bottom water [O 2] and (c) degradation rates of S-cysts ( kt) and bottom water [O 2]. Relationships that are extremely weak or are clearly insignificant on all confidence intervals are between (1) S-cyst accumulation rates and chlorophyll- a concentrations, sea-surface temperature (SST), sea-surface salinity (SSS), phosphate concentrations (P) and nitrate concentrations (N), (2) between R-cyst accumulation rates and bottom water [O 2], SST, SSS, P and N, and between (3) kt and water depth. Co-variance is present between the parameters N and P, N, P and chlorophyll- a, oxygen and water depth. Correcting for this co-variance does not influence the significance of the relationship given above. The possible applicability of dinoflagellate cyst degradation to estimate past net primary production and deep ocean ventilation is discussed.

Living deep-sea benthic foraminifera from the warm and oxygen-depleted environment of the Sulu Sea

The semi-enclosed, oligotrophic Sulu Sea is characterised by warm (ca. 10 °C) and oxygen-depleted ( 32 μm) from 56 samples (from surface down to 5 cm). The stained foraminiferal assemblages of the Sulu Sea comprised 285 species, including 137 agglutinated and 148 calcareous species. Standing stock values ranged from 595 individuals/10 cm2 on the continental slope to 100 individuals/10 cm2 in the abyssal zone. Generally, in the low-productivity and low-oxygen environment of the Sulu Sea, stained assemblages are characterized by low-standing stock values, moderate dominance and low diversity. The combination of poor rates of C org flux and low dissolved oxygen content in bottom waters are the main factors controlling the deep-sea benthic fauna in the Sulu Sea. The foraminiferal abundances in the upper bathyal zone respond to both food fluxes and oxygen concentrations. Most of the epifaunal, oxyphilic species have their lower limit of occurrence at a water depth of approximately 2000 m (dissolved oxygen <1.25 ml/l) in the Sulu Sea. At constantly dysoxic (<1 ml/l O2) sites from the lower bathyal and abyssal zone, the assemblages comprised low-oxygen tolerant species and are more dependant on C org fluxes. The correlations between benthic foraminiferal faunas and C org flux and dissolved oxygen values are very good, with correlation coefficients of r2=0.956r2=0.956 and 0.88, respectively. The living deep-sea foraminiferal assemblages consisted mainly of agglutinated tests, with dominant species such as Lagenammina difflugiformis, Ammoscalaria tenuimargo, various Reophax species, and small (<70 μm) trochamminaceans like Portatrochammina wiesneri. Stained individuals of the calcareous Parrelloides bradyi (type 2) occurred in low numbers at oxygen-poor sites, down to a depth of 5 cm in the sediment column. The only stained, calcareous, infaunal species found deeper than 4000 m was Valvulineria mexicana (average living depth of 3 cm). The general microhabitat preferences of common species were consistent with those from other studies, although due to the unique combination of poor food supply and dysoxic conditions, most of the infaunal species had shallower average living depths than those reported from other dysoxic locations.

Change of bottom water conditions at intermediate depths of the Oyashio region, NW Pacific over the past 20,000 yrs

The North Pacific Intermediate Water (NPIW), source water (Oyashio Intermediate Water; OYIW) of which originates east of the southern Kuril Islands, plays an important role in the carbon cycle of the Pacific Ocean. The spatio-temporal changes of NPIW, however, have yet to be clarified. Because of OYIW flowing in the Oyashio region, from east of the southern Kuril Islands through south of Hokkaido to east of Honshu, it is important to understand past bottom water changes at intermediate depths of this region for study of the past NPIW. Two sediment cores, GH02-1030 and MD01-2409, were collected from south of eastern and central Hokkaido. Lithology and physical properties of the cores recorded well the surface and bottom water conditions. Higher diatom productivity in the surface waters led to diatomaceous sediments with higher water contents and lower grain densities during the last deglaciation and late Holocene. These lithological changes occurred not only at the two studied cores but also at several other sites along the western margin of the NW Pacific. Benthic foraminiferal assemblages and occurrence of parallel lamination indicated such a dysoxic bottom water condition during the last deglaciation. Absence of lamination in the late Holocene horizon suggested a somewhat higher dissolved oxygen concentration in bottom water than that during the last deglaciation. On the basis of benthic foraminifer assemblages corresponding to strengthening of intermediate water circulation inferred from coarsening of sediment grain size and decreasing of sedimentation rate indicating stronger winnowing effects, it appears that the bottom water oxygen level increased during the Younger Dryas cold period.

The influence of seasonal processes on geochemical profiles and foraminiferal assemblages on the outer shelf of the Bay of Biscay

Benthic foraminifera are widely used tools to reconstruct paleoenvironments. They reflect bottom water oxygen concentrations and organic carbon flux. To increase their accuracy as proxies, seasonal variability of foraminiferal populations from the outer shelf of the Bay of Biscay (150 m water depth) was followed over a 3-year period. The selected area is characterized by seasonal variation in organic matter deposition. A strong interaction between organic detritus input, macrofaunal bioturbational activity, the degree of separation of redox zones and the reproduction, growth and migrational response of benthic foraminifera was found. During periods of high labile organic matter input, as occurs during spring bloom conditions, the redox zonation was not well established. Different organic degradation pathways occurred at the same depth intervals in the sediment. Shallow infaunal species reacted in response to the high nutrient content with rapid growth ( Bolivina subaenariensis) or reproduction ( Epistominella vitrea, Bolivina seminuda, Bolivina dilatata/spathulata). Infaunally living foraminifera, Chilostomella oolina and to lesser extent Nonion scaphum, migrated to the sediment surface in reaction the increase organic matter input to the seafloor. This migration could have been induced by the more intensive overlapping of redox zones that were perhaps better separated vertically in winter. However, this migration could also have been triggered by the occurrence of fresh phytodetritus in the sediment, becoming even available in slightly deeper sediment level due to the increased bioturbation activity.

FORAMINIFERA IN AN ORGANIC-RICH, BRACKISH-WATER LAGOON, LAKE SAROMA, HOKKAIDO, JAPAN

We investigated the distribution of benthic foraminifera in Lake Saroma, a large lagoon on the coast of the Sea of Okhotsk in Hokkaido, Japan, to establish the characteristics of the foraminiferal fauna in these cool, organic-rich, brackish-water environments. Three types of faunas were identified: a Lagoon-bottom fauna dominated by Haynesina sp. A, a Shallow-water fauna dominated by Trochammina cf. japonica , and a River-mouth fauna dominated by Elphidium excavatum . The distribution of species is controlled by the dissolved oxygen content of bottom water in the summer, and a combination of organic enrichment and high mud content in the substrate that is specific to areas off river mouths. Haynesina sp. A opportunistically adapts to organic-rich and oxygen-poor conditions in deeper basins in the lagoon during the summer, whereas Trochammina cf. japonica mostly occurs in shallower areas where summer oxygen levels are high enough to sustain reproduction. Elphidium excavatum probably adapts to organic-rich sediments specific to areas near river mouths.

Paleo‐indicators and water quality change in the Charlotte Harbor Estuary (Florida)

We reconstructed water quality changes for 1800 to 2000 in Charlotte Harbor (Florida), a shallow subtropical estuary, by using a suite of biological and geochemical proxies in dated sediments collected in the region of a present day, midsummer hypoxic zone. The declining freshwater loading into the estuary from 1931 to the 1980s is not the probable causal agent encouraging the appearance or expansion of a hypoxia zone (measuring up to 90 km2 in summer). Rather, the reconstructed trends in nitrogen loading indicate increased phytoplankton production has likely caused a decline in bottom water oxygen concentrations. Sedimentary biogenic silica (BSi), carbon, nitrogen, and phosphorus concentrations increased concurrently with known or inferred changes in nutrient loadings. There were direct relationships between phytoplankton pigments and BSi, heavier δ34S with increased carbon loading, and sequestration of P, Al, and Fe as carbon loading increased. The results from the sediment analyses and the results from mixing models using C:N ratios and δ13C suggest an estuarine system that is responsive to increased carbon loading from the nitrogen‐limited phytoplankton community and whose sediments are becoming increasingly anoxic as a result. The present nitrogen loading is about three times above that prior to the 1800s, suggesting that without management intervention the anticipated doubling of the watershed’s population from 1990 to 2020 will greatly increase the nitrogen loading to this estuary and will lead to much higher amounts of phytoplankton biomass and accumulation and exacerbate hypoxic conditions.

Physico-chemical characterization of the benthic environment of the Golfo San Jorge, Argentina

The benthic system of the Golfo San Jorge was characterized from physico-chemical parameters based on samplings obtained during seasonal research cruises carried out on board the INIDEP vessels from springtime 1999 through wintertime 2000. Spatial and seasonal variations of temperature, salinity, density, oxygen content and chlorophyll-a in bottom water and concentration of total organic matter, total organic carbon, total nitrogen, chlorophyll-a and phaeopigments in sediments were analysed. The origin and nutritional value of the deposited organic matter were also assessed. The behaviour of the physico-chemical characteristics of the benthic habitat, studied applying statistical techniques, defined three sectors with particular characteristics and minimum seasonal variations: sector 1, the largest and deepest one, comprises the central area of the gulf and corresponds to a depositional environment; sector 2 comprises the areas next to the extremes of the gulf and corresponds to a flow or erosive environment; sector 3 includes the coastal area and south-east part of the gulf and belongs to a transitional environment.

Anaerobic diagenesis of silica and carbon in continental margin sediments: Discrete zones of TCO 2 production

Pore water profiles of dissolved Si, Ca 2+, SO 4 2-, CH 4, and TCO 2 (Dissolved Inorganic Carbon; DIC) were determined from multicores and gravity cores collected at nine sites off Southern California, the west coast of Mexico, and within the Gulf of California. These sites were located within the eastern North Pacific oxygen minimum zone at depths of 400 to 900 m and in settings where bottom water oxygen concentrations were <3 μM and sediments were laminated. Pore water profiles were defined at a resolution of millimeters (whole core squeezing), centimeters (sectioning and squeezing) and meters (gravity core sectioning and squeezing), and diffusive fluxes were calculated for different zones within the sediment column. The flux of dissolved silica across the sediment-water interface (SWI) ranged from 0.3 to 3.4 mmol Si m -2d -1, and TCO 2 fluxes ranged from 0.8 to 4.6 mmol C m -2d -1. A positive correlation ( r = 0.74) existed between these fluxes, yet these two constituents exhibited significantly different diagenetic behavior downcore; dissolved Si generally reached a constant concentration (between 450 and 900 μM) in the upper few cm, whereas TCO 2 concentrations increased monotonically with depth. Methane was detected at micromolar levels in sediment intervals between 0 and 60 cm and at five sites, increased to millimolar levels at depths of 80 to 170 cm. At the horizon marking the appearance of millimolar levels of methane, there was a distinct change in slope of the sulfate and TCO 2 gradients. A flux budget for this horizon was determined by using linear fits to pore water profiles; these budgets indicate that the upward TCO 2 flux away from this horizon is 40 to 50% greater than the downward sulfate flux to this horizon. Given that the TCO 2 flux to this horizon from below was quite small, this imbalance suggests that anaerobic oxidation of methane by sulfate is not the only process producing TCO 2 within this horizon. A budget for TCO 2 at this horizon is balanced when 40 to 80% of the sulfate flux is attributed to organic carbon remineralization. Of the DIC that diffuses across the SWI, 20 to 40% is generated by reactions occurring within or below this deep reaction horizon.

Benthic foraminifera and bottom water evolution in the middle-southern Okinawa Trough during the last 18 ka

A piston sediment core E017 from the middle-southern Okinawa Trough was investigated. A preliminary study of the deep-water evolution since 18 cal. ka BP was performed based on the quantitative census data of benthic foraminiferal fauna, together with planktonic foraminiferal oxygen and carbon isotope, AMS(14)C dating, and the previous results achieved in the southern Okinawa Trough. The result shows that the benthic fauna was dominated by Bulimina aculeata (d'Orbigny), Uvigerina peregrina (Cushman), Hispid Uvigerina and Uvigerina dirupta (Todd) during the glaciation-deglaciation before 9.2 cal. ka BP, while Epistominella exigua (Brady), Pullenia bulloides (d'Orbigny), Cibicidoides hyalina (Hofker), Sphaeroidina bulloides (d'Orbigny) and Globocassidulina subglobosa (Brady) predominated the fauna in the post-glacial period after 9.2 cal. ka BP. The benthic foraminifera accumulation rate (BFAR), paleoproductivity estimates and benthic foraminiferal assemblage conformably indicate that surface water paleoproductivity and organic matter flux during the glaciation-deglaciation were higher than those of the post-glacial period in the middle-southern Okinawa Trough, and gradually enhanced from the southern to the central Okinawa Trough during the glaciation-deglaciation, which could be caused by the discrepancy of the terrigenous nutrients supply. High abundances of E exigua, an indicator of pulsed organic matter input, after 9.2 cal. ka. BP may indicate that the intensity of seasonally riverine pulsed flux during the post-glacial period was stronger than that of the glaciation-deglaciation period, and the seasonal influx in the central trough might be stronger than in the south. The temporal distributions of the typical species indicating bottom water oxygen content and ventilation condition show that the ventilation of the bottom water during the post-glacial period is more active than the glaciation-deglaciation, which reflects that the evolution of the intermediate and deep waters of the northwestern Pacific during the last glaciation has no evident influence on the deep-water of the middle-southern Okinawa Trough. Additionally, the variations in agglutinated benthic foraminiferal abundance and other carbonate dissolution proxies indicate that carbonate dissolution gradually increased since the last 18 ka in the Okinawa Trough and rapidly enhanced at 9.2 cal. ka BP. The modern shallow carbonate lysocline could form at 3 cal. ka BP.

The Second Species of Gromia (Protista) from the Deep Sea: its Natural History and Association with the Pakistan Margin Oxygen Minimum Zone

We describe a gromiid protist Gromia pyriformis sp. nov., from bathyal depths on the Pakistan margin (NE Arabian Sea), an area characterised by a well-developed Oxygen Minimum Zone (OMZ). The new species is smaller (length usually <1 mm) than the only other described deep-sea gromiid species (Gromia sphaerica) or the well-known coastal species Gromia oviformis. Its identification as a gromiid is based on the test-wall ultrastructure. This includes (i) an outer wall (165-300 nm thick) limited by an electron-opaque layer and perforated by pore structures which typically extend through its entire thickness, and (ii) inner "honeycomb membrane" structures which form a discontinuous sheet (18-20 nm thick) lying parallel to the outer wall. An outermost glycocalyx (approximately 75 nm thick), not observed in other gromiid species, is also present and imparts a finely granular appearance to the outer test surface, as seen by Scanning Electron Microscopy (SEM). Numerous rod-shaped prokaryotes are attached to the exterior of the glycocalyx. Gromia pyriformis sp. nov. typically occurs above the sediment-water interface, attached to the large arborescent foraminiferan Pelosina sp. It is confined to a very narrow bathymetric zone (approximately 1000 m water depth) in the lower portion of the OMZ, where bottom-water oxygen concentrations are approximately 0.2 ml l(-1).

Macrobenthic animal assemblages of the continental margin off Chile (22° to 42°S)

A quantitative study of macrobenthos was carried out on three transects on the shelf and continental slope off Chile (22° to 42°S; from 100 to 2000 m water depth) within and beneath the oxygen minimum zone (OMZ). Macrobenthos mean densities ranged from 104 to 13 808 ind m−2, with highest values off Concepción (∼36°S), where highest chloroplastic pigment equivalents were also measured. Polychaetes were the numerically dominant group in all transects and depths, followed by peracarid crustaceans. Species richness and diversity correlated with observed changes in bottom-water oxygen concentrations and sediment-bound pigments. Our results show that the shelf macrobenthic communities were negatively affected by low oxygen. The non-metric multidimensional scaling (nMDS) analysis evidenced depth-related station groups which may be related to the different environments provided by the water masses involved. Indeed, the depth ranges of stations groups with their specific species inventories coincide quite well with the boundaries of the three important water masses in the region, the Equatorial Subsurface Water (∼50 m to ∼400 m depth), the Antarctic Intermediate Water (∼400 to ∼1200 m depth), and the Pacific Deep Water (&gt;1200 m depth).

Benthic carbon mineralization on a global scale

In this study we present a global distribution pattern and budget of the minimum flux of particulate organic carbon to the sea floor (JPOCα). The estimations are based on regionally specific correlations between the diffusive oxygen flux across the sediment‐water interface, the total organic carbon content in surface sediments, and the oxygen concentration in bottom waters. For this, we modified the principal equation of Cai and Reimers [1995] as a basic monod reaction rate, applied within 11 regions where in situ measurements of diffusive oxygen uptake exist. By application of the resulting transfer functions to other regions with similar sedimentary conditions and areal interpolation, we calculated a minimum global budget of particulate organic carbon that actually reaches the sea floor of ∼0.5 GtC yr−1 (&gt;1000 m water depth (wd)), whereas approximately 0.002–0.12 GtC yr−1 is buried in the sediments (0.01–0.4% of surface primary production). Despite the fact that our global budget is in good agreement with previous studies, we found conspicuous differences among the distribution patterns of primary production, calculations based on particle trap collections of the POC flux, and JPOCα of this study. These deviations, especially located at the southeastern and southwestern Atlantic Ocean, the Greenland and Norwegian Sea and the entire equatorial Pacific Ocean, strongly indicate a considerable influence of lateral particle transport on the vertical link between surface waters and underlying sediments. This observation is supported by sediment trap data. Furthermore, local differences in the availability and quality of the organic matter as well as different transport mechanisms through the water column are discussed.

Phosphorus burial as a function of paleoproductivity and redox conditions in Arabian Sea sediments

In this study the response of sedimentary phosphorus (P) burial to changes in primary productivity and bottom water oxygen concentrations during the Late Quaternary is investigated, using two sediment cores from the Arabian Sea, one recovered from the continental slope and the other from the deep basin. The average solid-phase P speciation in both cores is similar, authigenic and biogenic (fish debris) apatite make up the bulk of the P inventory (ca. 70%);whereas P adsorbed to iron oxides, organic P, and detrital apatite constitute minor fractions. Postdepositional redistribution has not significantly altered the downcore distribution of total solid-phase P. Phosphorus burial efficiencies are generally lower during periods of increased paleoproductivity. This is caused by (a) partial decoupling of the P export flux, consisting primarily of particulate organic P, and the P burial flux, consisting primarily of biogenic and authigenic apatite; and (b) the lack of increased rates of authigenic CFA formation during periods of higher P deposition. In addition, fluctuations in bottom water oxygen concentrations may have affected P burial in continental slope sediments. The results of this study indicate that higher primary productivity induces more efficient P cycling. On time scales exceeding the oceanic P residence time, this process may induce higher surface water productivity, thus creating a positive feedback loop. In the Arabian Sea, this feedback mechanism may have contributed to changes in sea surface productivity on sub-Milankovitch time scales because P, regenerated on the continental slopes of the Oman and Somalian coastal upwelling zones, is reintroduced into the photic zone relatively fast.

Foraminiferal record and environmental changes during the deposition of the Early–Middle Pleistocene sapropels in southern Italy

A foraminiferal investigation documents the palaeoenvironmental changes across the sapropel units associated with insolation—cycles 90 and 86 in the Early–Middle Pleistocene IM/Fosso 5 Agosto section (Basilicata, southern Italy). Three benthic foraminiferal fossil associations (Associations A, B and C), identified by performing Hierarchical Cluster Analysis on the quantitative data, can be considered diagnostic of the palaeoenvironmental conditions before, during and after the sapropel intervals. They indicate a gradual decrease of bottom water oxygenation prior to the onset of sapropel deposition and a rapid return to stable environmental conditions at the end of the sapropel events. The constant presence of benthic foraminifera throughout the sapropels implies that the bottom environment was not totally anoxic. The high-resolution distributional pattern of planktonic and benthic foraminifera within the sapropel associated with i-cycle 90 documents a surface water cooling with consequent rapid re-oxygenation of bottom waters that subdivides this interval in two phases. Both of them are characterized by an early subphase with warm climate conditions, a decrease of salinity in surficial water layers and dominated by the low oxygen tolerant benthic species Globobulimina affinis. The late subphase shows thermal stratification during summer and a relatively deep and oxygenated mixed layer during winter. In this subphase, the contemporaneous replacement of G. affinis, the “less opportunistic, but more resistant to low oxygen conditions” species with Bolivina alata, Bolivina dilatata and Brizalina spathulata, the “less resistant to low oxygen conditions” species, indicates the relative oxygen improvement in sediment pore water. The frequent changes detected in the sapropel-associated faunal composition underline the unstable nature of bottom water oxygen content.