Bottom Water Oxygen Concentrations Research Articles

Late Cainozoic ostracod faunas and paleoenvironmental changes at ODP Site 1148, South China Sea

Ostracod faunas from ODP Site 1148 in the South China Sea have been analyzed quantitatively in order to understand paleoenvironmental changes in the late Cainozoic deep water. More than 2000 core samples were taken from the Oligocene to Quaternary section, of which 725 contained ostracods and yielded at least 100 species. Krithe dominates the faunas throughout the Oligocene–Quaternary sequences, indicating that deep-sea conditions had existed since the earliest Oligocene (ca. 34.5 Ma). This suggests that the spreading of the South China Sea Basin predates the Oligocene. Three distinct ostracod assemblages have been recognized and their respective faunal turnover reflects a change in paleodepth from the upper bathyal (<1500 m) in the early Oligocene (ca. 34.5–27 Ma) and lower bathyal (1500–2500 m) in the late Oligocene to early middle Miocene (26–14 Ma) to a depth similar to the present (>2500 m) since the late middle Miocene (14 Ma to the present). Intervals when ostracods become very rare are at 28.5–27, 20–18.2 and 13–2.8 Ma. The earliest of these intervals was probably related to the high surface productivity and the depletion of oxygen content in bottom water, while intervals were the product of enhanced carbonate dissolution. Measures of evolutionary activity (origination, extinction and diversity) have been applied to detecting the environmental change and their relationship to global events. Five peaks of evolutionary activity occurred in the Oligocene (P18, P21a and P22) and in the earlier mid-Miocene N8 and late Pliocene–Quaternary N21–N22, respectively. The peaks in P18 and P21a are the possible response respectively to the global cooling events Oi-1 and Oi-2, which occurred in the early Oligocene. The marked increase in extinction, origination and diversity was synchronous with the major enrichment in benthic δ 18O in the late Pliocene N21, suggesting a reaction of the ostracod faunas to the rapid cooling of the bottom water and the initiation of the North Hemispheric Glaciation during the period of 3–2.5 Ma.

Migratory responses of deep-sea benthic foraminifera to variable oxygen conditions: laboratory investigations

Field studies of recent benthic foraminifera have shown that the vertical distribution in the sediment is controlled mainly by two parameters: oxygen concentration and food availability. The aim of the present paper is to determine the response of deep-sea foraminifera to changing bottom and pore water oxygen concentrations in the laboratory, under conditions of controlled temperature and salinity. The experiment, using spectrofluorimetry to distinguish living from dead specimens, has shown that all selected species ( Globobulimina affinis, Hoeglundina elegans, Pyrgo murrhina, Uvigerina peregrina, Uvigerina mediterranea) can live in the oxic sediment layer. However, G. affinis was also found living in the anoxic layers and even has its maximum density there. An accidentally obtained inverse profile of oxygen, with anoxic conditions on the surface and oxic conditions at the bottom, resulted in an inverse distribution of foraminifera, with “shallow infaunal” species ( H. elegans, U. peregrina and U. mediterranea) at the bottom and the “deep infaunal” species G. affinis at the sediment surface. Our results show that some deep-sea species can actively migrate along the sediment column to reach their favourite microhabitat. Furthermore, they suggest that changes in oxygen content are mainly responsible for those migrations. Based on a comparison between our experimental and field results, the ecological requirements of some selected species have been refined.

Temporal variability of nitrogen cycling in continental-shelf sediments of the upwelling ecosystem off central Chile

Abstract The continental shelf region off central Chile (∼36 °S), one of the widest and most productive areas of the eastern South Pacific, is an important site of coastal upwelling. In order to understand how seasonal and inter-annual variability in bottom-water physical and chemical conditions affect benthic nutrient regeneration and sediment characteristics in this area, ammonium (NH 4 + ) and nitrate (NO 3 − ) fluxes at the water–sediment interface were experimentally quantified (March 1998–April 2001), along with net NH 4 + production, potential nitrification and denitrification rates (November 1998–August 2000). NH 4 + fluxes to the overlying water up to 10.4 mmol m −2 d −1 , occurred during the upwelling season (i.e. austral spring and summer), while NH 4 + removal from the water column up to −5.7 mmol m −2 d −1 during non-favorable upwelling conditions was observed (i.e., austral winter and the 1997–1998 El Nino condition). The fate of the benthic N regenerated as NH 4 + appears to be controlled by the amount of labile organic carbon (here indexed as chlorophyll- a ) in the surface sediment and, indirectly, by the bottom-water oxygen concentration. The balance between net NH 4 + production and potential nitrification (4.4–34.3 and 0.3–2.9 mmol m −2 d −1 , respectively) does not support the observed NH 4 + fluxes, suggesting the occurrence of other NH 4 + dissamilative (by dissolved metal or anammox) or assimilative consuming processes. Throughout the entire study period, the sediments acted as a large sink for NO 3 − (−3.4±1.4 mmol m −2 d −1 ) and as an important denitrification site (0.6–2.9 mmol m −2 d −1 ) coupled with NO 3 − produced by nitrification (58–97%). Other processes such as NO 3 − ammonification or active NO 3 − uptake by Thioploca mats could account for NO 3 − uptake from the water column.

BENTHIC FORAMINIFERA IN DEEPSEA RESERCH:RETROSPECT AND PROSPECT

The use of benthic foraminifera in paleoenvironmental studies was started as indicators of paleo-water depth or water mass. With application of new technology, two different microhabitats of benthic foraminifera have been distinguished: epifauna vsinfauna, and it was found that even deep-water benthic foraminifera respond to seasonal ephemeral changes caused by surface planktonic bloom for the supply of organic matter to the sea bottom. Benthic foraminifera are now widely used for estimation of surface productivity of the ocean and the oxygen content of the bottom water. This paper reviews the historical development of ecological studies on deep-sea benthic foraminifera and their use in paleoceanography, with emphasis on new approaches and new technology. Since China has joined such deep-sea research programs as IODP, some new directions in benthic foraminiferal studies are proposed to the Chinese community.

Quantifying porewater exchange across the sediment‐water interface in the deep sea with in situ tracer studies

A numerical model of NaBr tracer transport in benthic flux chamber incubations has been developed to simulate temporal changes in chamber water tracer concentrations and applied to four deep sea locations: the California and North Carolina margins, the Ceara rise, and the Cape Verde plateau. Model variables include the chamber volume (which determines the initial tracer concentration), and rates of diffusive and nonlocal exchange across the sediment‐water interface. Chamber volume and solute exchange rate estimates were obtained by optimizing the fit between observations and model results using a χ2 statistical scheme. We observe enhanced solute exchange in regions with high organic carbon rain rates, over a wide range of bottom water oxygen concentrations. Furthermore, our results demonstrate that solute exchange rates cannot be assumed constant, even within relatively short deployments. In six of 22 cases, chamber volumes and corresponding benthic solute fluxes derived from model fits were more than 20% greater than values calculated using linear extrapolations of chamber tracer concentration‐time relationships. To avoid this discrepancy, future benthic chamber studies should seek to minimize the time interval between tracer injection and sampling while maintaining sufficient time to achieve complete homogenization of the tracer in the chamber waters.

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

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

Enhanced marine productivity off western North America during warm climate intervals of the past 52 k.y.

Studies of the Santa Barbara Basin off the coast of California have linked changes in its bottom-water oxygen content to millennial-scale climate changes as recorded by the oxygen isotope composition of Greenland ice. Through the use of detailed records from a sediment core collected off the Magdalena Margin of Baja California, Mexico, we demonstrate that this teleconnection predominantly arose from changes in marine productivity, rather than changes in ventilation of the North Pacific, as was originally proposed.

On the preservation of laminated sediments along the western margin of North America

Piston, gravity, and multicores as well as hydrographic data were collected along the Pacific margin of Baja California to reconstruct past variations in the intensity of the oxygen‐minimum zone (OMZ). Gravity cores collected from within the OMZ north of 24°N did not contain laminated surface sediments even though bottom water oxygen (BWO) concentrations were close to 5 μmol/kg. However, many of the cores collected south of 24°N did contain millimeter‐ to centimeter‐scale, brown to black laminations in Holocene and older sediments but not in sediments deposited during the Last Glacial Maximum. In addition to the dark laminations, Holocene sediments in Soledad Basin, silled at 290 m, also contain white coccolith laminae that probably represent individual blooms. Two open margin cores from 430 and 700 m depth that were selected for detailed radiocarbon dating show distinct transitions from bioturbated glacial sediment to laminated Holocene sediment occurring at 12.9 and 11.5 ka, respectively. The transition is delayed and more gradual (11.3–10.0 ka) in another dated core from Soledad Basin. The observations indicate that bottom‐water oxygen concentrations dropped below a threshold for the preservation of laminations at different times or that a synchronous hydrographic change left an asynchronous sedimentary imprint due to local factors. With the caveat that laminated sections should therefore not be correlated without independent age control, the pattern of older sequences of laminations along the North American western margin reported by this and previous studies suggests that multiple patterns of regional productivity and ventilation prevailed over the past 60 kyr.

Impact of interrelated and interdependent ecological controls on benthic foraminifera: an example from the Gulf of Guinea

Living and dead benthic foraminiferal assemblages from two size fractions, 63–250 μm and >250 μm, were studied at 128 stations from the shelves down to abyssal plains of the Gulf of Guinea in the eastern equatorial Atlantic. The sample size ranged between 300 and 400 cm 2 of the sediment/water interface, with a penetration depth of 1 cm. Environmental parameters were quantified for sediment composition, food webs, bottom-water oxygen concentrations, and benthic oxygen respiration. Seven major station groupings (named G1–G7) are subdivided by factor analysis. The three most abundant species therein are discussed with respect to their ecological requirements, for both size classes and for living and dead counts. For 49 living species, the range of environmental gradients is established, and for five species live observations are reported. Five station groupings can be attributed toward stepwise ranges scaled by bathymetry and flux rates of organic carbon (G4, 27–75 m; G5, 68–269 m; G6, 250–740 m; G2, 674–2007 m; G1, 1475–4970 m). Two geographically restricted station groupings locally interfinger with other groupings in the upper bathyal and abyssal water depths (G7, 82–451 m; G3, 1002–4658 m). G7 shows lowered oxygen concentrations and benthic oxygen respirations, whereas hemipelagic sediments are typical of G3. The interfingering of G7 and G3, however, is not purely a result of the increasing number of species most perfectly adapted to these environments but rather that these groupings are structured by the decrease of species with lower tolerance towards environmental perturbation. In fact, the more common species have an environmental range much broader than the environmental frame of the groupings they dominate. This observation holds for all size classes and for live and dead counts. We conclude that species-specific preferences and thresholds provide a more valuable and more consistent tool in environmental research than the environmental ranges observed for assemblages.

The pattern and influence of low dissolved oxygen in the Patuxent River, a seasonally hypoxic estuary

Increased nutrient loadings have resulted in low dissolved oxygen (DO) concentrations in bottom waters of the Patuxent River, a tributary of Chesapeake Bay. We synthesize existing and newly collected data to examine spatial and temporal variation in bottom DO, the prevalence of hypoxia-induced mortality of fishes, the tolerance of Patuxent River biota to low DO, and the influence of bottom DO on the vertical distributions and spatial overlap of larval fish and fish eggs with their gelatinous predators and zooplankton prey. We use this information, as well as output from watershed-quality and water-quality models, to configure a spatially-explicit individual-based model to predict how chang- ing land use within the Patuxent watershed may affect survival of early life stages of summer breeding fishes through its effect on DO. Bottom waters in much of the mesohaline Patuxent River are below 50% DO saturation during summer. The system is characterized by high spatial and temporal variation in DO concentrations, and the current severity and extent of hypoxia are sufficient to alter distributions of organisms and trophic interactions in the river. Gelatinous zooplankton are among the most tolerant species of hypoxia, while several of the ecologically and economically important finfish are among the most sensitive. This variation in DO tolerances may make the Patuxent River, and similar estuaries, particularly susceptible to hypoxia-induced alterations in food web dynamics. Model simulations consistently predict high mortality of planktonic bay anchovy eggs (Anchoa mitchilli) under current DO, and increasing survival of fish eggs with increasing DO. Changes in land use that reduce nutrient loadings may either increase or decrease predation mortality of larval fish depending on the baseline DO conditions at any point in space and time. A precautionary approach towards fisheries and ecosystem management would recommend reducing nutrients to levels at which low oxygen effects on estuarine habitat are reduced and, where possible, eliminated.

Biogeochemical processes in a small California estuary. 2. Nitrification activity, community structure and role in nitrogen budgets

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 248:27-40 (2003) - doi:10.3354/meps248027 Biogeochemical processes in a small California estuary. 2. Nitrification activity, community structure and role in nitrogen budgets Jane M. Caffrey1,*, Neil Harrington2, Icarus Solem2, Bess B. Ward3 1Center for Environmental Diagnostics and Bioremediation, University of West Florida, 11000 University Parkway, Pensacola, Florida 32514, USA 2Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, California 95064, USA 3Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA *Email: jcaffrey@uwf.edu ABSTRACT: Nitrification rates, bacterial abundance and productivity, and the diversity of ammonia-oxidizing bacteria were measured at 5 sites in Elkhorn Slough, a small estuary in central California, between 1997 and 1999. Of the sampling sites, 2 received high nutrient runoff from agricultural fields and other non-point sources, and 3 received runoff from grasslands and woodlands. The diversity of ammonia-oxidizing bacteria in terms of DNA sequences was investigated at 3 of the sites in August 1998. Both 16S and amoA sequences from sediment samples were more closely related to those of Nitrosomonas marina than to any other cultured nitrifier, but there was considerable diversity within the group, and site-specific patterns were not detected. Potential nitrification was seasonally and spatially variable, with the highest rates occurring at the head of Elkhorn Slough in late winter and fall. Bacterial productivity was highest during the summer and generally higher at a site adjacent to grasslands compared to the 2 agricultural sites. The variability in nitrification rates was not directly correlated with hydrographic and environmental variables. Physical factors may affect nitrification rates indirectly by controlling salinity and bottom-water oxygen concentrations. Potential nitrification rates were positively correlated with pore water NH4+ concentrations under well flushed conditions but were negligible in the presence of high NH4+ concentrations under poorly flushed conditions, due to low oxygen availability, hypersaline conditions or both. Sediment nitrogen budgets for 3 of the sites suggested that denitrification removed about 25% of the mineralized nitrogen at the poorly flushed site, but 50% or more at well flushed sites. Poorly flushed systems appear to be less efficient at removing high nitrogen inputs than well flushed systems because of the low rates of coupled nitrification-denitrification in the former, implying that physical factors such as the flushing regime or residence time can significantly affect nitrogen removal by denitrification. KEY WORDS: Estuary · Nitrification · Nitrifying bacterial diversity · Eutrophication · Sediment Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 248. Online publication date: February 20, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

THE DISTRIBUTION OF BENTHIC FORAMINIFERA IN THE CELTIC SEA: THE SIGNIFICANCE OF SEASONAL STRATIFICATION

Research Article| January 01, 2003 THE DISTRIBUTION OF BENTHIC FORAMINIFERA IN THE CELTIC SEA: THE SIGNIFICANCE OF SEASONAL STRATIFICATION Gillian A. Scott; Gillian A. Scott School of Ocean Sciences, University of Wales (Bangor), Menai Bridge, Anglesey, LL59 5EY, UK Search for other works by this author on: GSW Google Scholar James D. Scourse; James D. Scourse School of Ocean Sciences, University of Wales (Bangor), Menai Bridge, Anglesey, LL59 5EY, UK 2School of Geography and Geosciences, University of St. Andrew’s, St. Andrew’s, Fife, KY16 9ST, Scotland, UK. Search for other works by this author on: GSW Google Scholar William E. N. Austin William E. N. Austin 1 2Corresponding author; e-mail: j.scourse@bangor.ac.uk Search for other works by this author on: GSW Google Scholar Journal of Foraminiferal Research (2003) 33 (1): 32–61. https://doi.org/10.2113/0330032 Article history received: 02 Jun 2000 accepted: 17 Dec 2000 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gillian A. Scott, James D. Scourse, William E. N. Austin; THE DISTRIBUTION OF BENTHIC FORAMINIFERA IN THE CELTIC SEA: THE SIGNIFICANCE OF SEASONAL STRATIFICATION. Journal of Foraminiferal Research 2003;; 33 (1): 32–61. doi: https://doi.org/10.2113/0330032 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Foraminiferal Research Search Advanced Search Abstract Seasonal stratification is an important phenomenon in tidally-stirred shelf seas, influencing biological productivity, sedimentation rates, the organic content of shelf sediments, and the climate of surrounding landmasses. Previous micropaleontological and stable isotopic investigation investigation of a Holocene sequence from the Celtic Sea suggests that benthic foraminiferal distributions are linked to the physical and biological oceanographic characteristics associated with stratification. We have tested this hypothesis by analyzing the living and dead foraminiferal faunas from surface samples collected during across-frontal cruises during the summers of 1995 and 1996. Foraminiferal and environmental data for 56 samples are presented. Live and dead foraminiferal data were analyzed by factor analysis and, along with the environmental data, canonical correspondence analysis (CCA). Four distinct assemblages were identified from factor analysis of the live data: (1) a frontal assemblage characterized by Stainforthia fusiformis, (2) a mixed water assemblage characterized by Cibicides lobatulus, Textularia bockii, Spiroplectammina wrightii, Ammonia batavus and Quinqueloculina seminulum, (3) a stratified assemblage characterized by Bulimina marginata, Hyalinea balthica, Adercotryma wrighti and Nonionella turgida, and (4) an eastern assemblage dominated by Bulimina gibba, Elphidium excavatum and Eggerelloides scaber. Factor analysis of the dead data reproduces all groupings except the frontal assemblage. These data therefore support interpretations based on earlier stratigraphic data, and highlight the significance of benthic foraminifera as faunal indicators of paleostratification in shelf seas. The distributions also support predicted cross-frontal transfer of nutrients and the existence of surface converging circulation cells. Statistical analyses indicate the significance of unmeasured ecological variables which we speculate might be food supply, and oxygen concentration of bottom and sediment pore waters. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Bottom water renewal in the Santa Barbara Basin

We use quarterly full water depth occupations of a station at the center of the Santa Barbara Basin to estimate the timing, frequency, and magnitude of basin renewal events over the period 1986–2000. Of 58 occupations we have identified 11 events (in 11 of the 15 years) that are highly suggestive of recent bottom water renewal, with periods of annual flushing interspersed with relatively stagnant periods. All but one of these events occurred in winter or spring, with the spring events having the largest changes in bottom water properties (cooling and oxygenation). The event recorded in April 1998, during a period of rapid transition from El Niño‐influenced to La Niña‐influenced local conditions, revealed the potential magnitude of a full basin flushing: an increase in bottom water oxygen content of more than 0.4 mL/L and a cooling of nearly half a degree over 2 months. Water property profiles and near‐surface circulation patterns suggest that sufficiently dense water at the western sill, combined with strong flow into the basin from the northwest, may be key prerequisites to renewal. Interannual variability in the strength, duration, and timing of coastal upwelling and in regional circulation and hydrography, forced largely by the local impact of El Niño events, modulates the flushing potential of the basin. The Santa Barbara Basin apparently does not get flushed every year, and renewal events are not restricted to winter‐spring. Since the frequency and intensity of basin renewal events can greatly impact local denitrification rates, benthic community structure, and underlying sedimentary processes, we recommend that paleoceanographic interpretations of the local sediment record take account of this seasonally and interannually varying cycle of bottom water renewal.

Oxygen minimum zone controlled Mn redistribution in Arabian Sea sediments during the late Quaternary

Accumulation of labile Mn is related to bottom water oxygen concentrations. In this study we investigate the relation between oxygen minimum zone (OMZ) intensity and Mn burial in Arabian Sea during the late Quaternary. Mn carbonates in sediments of the deep Arabian Basin indicate that this part of the basin remained oxygenated for at least the last 185 kyr. Sequential leaching showed manganese carbonate enrichments with concentrations up to 3750 ppm in deep basin sediments. These Mn spikes must have formed diagenetically during periods of increased Mn oxide fluxes to the deep basin, associated with vertical extension of the OMZ. This extension caused remobilization of Mn oxides from a previously oxic slope, resulting in increased Mn accumulation in the deep basin. Mn relocation is evidenced by synchronous Mn depletions in organic‐rich intervals in slope sediments. Mn records from the deep basin can thus be used as a qualitative proxy to reconstruct intermediate water oxygenation.

Effects of hypoxia, and the balance between hypoxia and enrichment, on coastal fishes and fisheries

A reduction in dissolved oxygen concentration is one of the most important direct effects of nutrient over-enrichment of coastal waters on fishes. Because hypoxia can cause mortality, reduced growth rates, and altered distributions and behaviors of fishes, as well as changes in the relative importance of organisms and pathways of carbon flow within food webs, hypoxia and anoxia can lead to large reductions in the abundance, diversity, and harvest of fishes within affected waters. Nutrient enrichment, however, typically increases prey abundance in more highly oxygenated surface waters and beyond the boundaries of the hypoxic zone. Because of this mosaic of high and low oxygen areas within a system, not only the actual oxygen concentration of bottom waters, but the spatial arrangement, predictability, and persistence of highly oxygenated, high productivity habitats, and the ability of fishes to locate and use those favorable habitats, will determine the ultimate effect of low oxygen on fish populations. Negative effects of hypoxia on fish, habitat, and food webs potentially make both fish populations and entire systems more susceptible to additional anthropogenic and natural stressors.

Factors influencing the distribution patterns of Recent deep-sea benthic foraminifera, east of New Zealand, Southwest Pacific Ocean

This study investigates which combination of environmental factors most strongly influences the distribution patterns of benthic foraminferal tests (>0.63 μm) in a region bisected by the Subtropical Front, east of New Zealand. Seafloor sample sites extend from outer shelf (90 m) to abyssal (4700 m) depths, across substrates ranging from biogenic/terrigenous gravelly sand to hemipelagic mud, and occur under the influence of Antarctic intermediate water (AAIW) and circumpolar deep waters as well as receiving detritus from both Subtropical and Subantarctic surface water masses. Elevated values of the planktic foraminiferal fragmentation index and reworked small Paleogene planktic foraminifera at outer shelf and bathyal depths coincide with areas of strong bottom currents. Q-mode cluster analysis on the census counts of 398 benthic species clusters the 66 samples into three large groups (shallow, bathyal, abyssal), and at a lower level 10 mappable associations are recognised. A combination of canonical correspondence analysis and a correlation coefficient matrix was used to relate the faunal data to a set of measured environmental proxies. These analyses show that factors that have a relationship with depth are the most significant in determining foraminiferal distribution. The principal environmental factors which appear to most strongly influence this benthic foraminiferal distribution are: dissolved oxygen content in bottom waters; sustainability of organic carbon flux rates; seasonality of food supply; lateral advection of water masses; bottom water carbonate corrosiveness; energetic state at the benthic boundary layer; grain-size composition of substrate; salinity and temperature of the bottom waters. Shallow water associations (90–1250 m), dominated by Cassidulina carinata and Trifarina angulosa, occur within coarse substrates under well-oxygenated, high energy regimes and sustained food input. The occurrence of the bathyal associations (230–2840 m), dominated by C. carinata, Alabaminella weddellensis and Abditodentrix pseudothalmanni, closely mirrors the distribution of AAIW within a region of variable food supply. The sustainability of food supply combined with bottom water type and associated ventilation and dissolution strongly influence the composition of abyssal associations (1200–4700 m), mostly dominated by Epistominella exigua and A. weddellensis.

Oxygen availability effects on early diagenetic calcite dissolution in the Arabian Sea as inferred from calcareous dinoflagellate cysts

In oceanic regions with high primary production, such as the Arabian Sea, the primary signals of proxies are often altered by diagenetic processes. The present study aims at assessing the effects of early diagenesis on calcareous dinoflagellate cysts, which represent a relatively new tool for reconstructing the paleoenvironmental conditions within the photic zone. For this purpose, surface sediment samples from within and below the oxygen minimum zone (OMZ) of the northeastern and southwestern Arabian Sea have been analysed quantitatively for their calcareous dinoflagellate cyst content. The calculated cyst accumulation rates (ARs), the relative abundances and cyst fragmentation values were compared to bottom water oxygen (BWO) content and ARs of organic carbon at the sample positions. Different patterns were found in the northeastern and southwestern part of the Arabian Sea. In the SW, no relationship between cyst ARs and BWO is distinguishable, and the distribution of cyst ARs is thought to largely reflect primary cyst production. In the NE, much higher ARs of all species are found in samples from within the OMZ in comparison to samples from below it. This is interpreted to result from better calcite preservation within the OMZ, presumably due to reduced oxic degradation of organic matter. The differential drop of cyst ARs of the individual species at the lower boundary of the OMZ in the NE Arabian Sea, as well as the species-specific change in relative abundance and fragmentation, indicate different sensitivity to calcite dissolution of the different species. These results show that early diagenetic calcite dissolution can change both relative and absolute abundances of calcareous dinoflagellate cysts, which has to be considered if using them for paleoenvironmental reconstructions. Furthermore, it is shown that considerable calcite dissolution can occur above the carbonate saturation horizon in high productive areas. However, calcite preservation can be substantially increased, as soon as oxygen concentrations are too low for oxic degradation of OM. Under low oxic conditions (within and near the OMZ), the main factor controlling organic matter (OM) preservation appears to be BWO concentrations. Under higher oxygen levels (below ∼1500 m depth in the NE Arabian Sea) there seems to be an increasing influence of bioturbation and sedimentation rate on the preservation of OM by controlling its oxygen exposure time. This study presents an example of a highly productive basin in which differences in early diagenetic processes can lead to the preservation of a signal that is either dominated by primary production (off Somalia) or by secondary alteration (off Pakistan), although in both areas, an oxygen depleted zone is present. For estimating the effects of early diagenetic calcite dissolution in a sediment by metabolic CO 2 (and probably by H 2S oxidation), not only the content of organic carbon but also other geochemical proxies for paleoredox-conditions have to be included for paleoenvironmental reconstructions.

Sedimentary pyrite formation in the Arabian Sea

In this study, pyrite formation in relation to environmental conditions was investigated in sediment records from the northern Arabian Sea. Considering the low bottom-water oxygen concentrations in the oxygen minimum zone (OMZ) and the high rates of primary productivity on the Oman and Pakistan Margins, sedimentary pyritic S concentrations are unexpectedly low (on average 0.4 wt%) compared to organic contents. Sediments located deeper than the OMZ exhibit a good correlation between organic carbon and reduced sulphur. Conversely, the C versus S distribution for OMZ sediments indicates a partial decoupling between organic carbon accumulation and pyrite formation. Early post-depositional sulphidisation resulted in the diagenetic sulphur enrichment of organic-poor intervals. This is evidenced by the presence of pyrite enrichments located below organic-rich intervals. Pyrite formation in the sediments within the Arabian Sea OMZ was limited by the availability of reactive iron oxides. This was caused by the low oxygen concentrations of the OMZ, which resulted in the reduction of iron oxides in the water column or at the sediment–water interface. However, on the basis of the relatively high sedimentary C/S ratios and the presence of bioturbation it is clear that no euxinic conditions developed in the OMZ in the Arabian sea during the last 120 kyr.

Seasonal Changes in Sulfate Reduction in Sediments in the Inner Part of Tokyo Bay.

In highly eutrophicated Tokyo Bay, anoxic, sulfide-containing bottom water is formed in warm stagnant seasons. It seems to develop especially in the northeast area where there are many large holes made by dredging to collect sand in addition to navigation channels. At such dredged sites, the seasonal variation in the rate of sulfate reduction in sediment was examined by a radiotracer method together with various environmental factors for two years and the results were compared with those obtained at a site on the natural sea floor. The rate of sulfate reduction was much higher at the dredged sites than on the natural sea floor. The dissolved oxygen concentration in bottom water and sulfate concentration in the sediment layer were lower, and the density of sulfate-reducing bacteria and acid-volatile sulfides were higher at the dredged sites than the natural floor site. However, there was no significant correlation between the sulfate reduction rate and any of the environmental factors examined (temperature, ignition loss, DO in bottom water and density of sulfate-reducing bacteria). The rate on an area basis seemed to fluctuate in a similar pattern to the change of chlorophyll a in the water column. The addition of seston mainly composed of diatoms to the sediment sample greatly stimulated sulfate reduction in contrast to the addition of lactate. Sulfate reduction at these sites might be controlled by the supply of organic substrates especially those of algal origin.

Chlorin distribution and degradation in sediments within and below the Arabian Sea oxygen minimum zone

The concentration of chlorophyll a degradation products, i.e. chlorins, preserved in deep-sea sediments is a function of the amount of primary production input and the rate at which it is subsequently degraded. Sedimentary chlorins can be used as a proxy for paleoproductivity; however, our understanding of the factors controlling their preservation is limited. To study the effects of changes in export of primary production from the euphotic zone and of differences in depositional conditions on chlorin concentration in marine sediments, chlorins were analyzed by high pressure liquid chromatography from sediments taken within and below the oxygen minimum zone on the Oman margin in the Arabian Sea. Among five stations at water depths between 400 and 1250 m, variation in chlorin concentration in surface sediments (0–0.5 cm) was significantly related to water depth (used here as a proxy for chlorin fluxes to the sediments) and bottom-water oxygen concentration; the more important control on chlorin concentration of surficial sediments measured in this study is the amount of chlorins reaching the sediment. Chlorins decayed exponentially downcore (0–20 cm). The degradation of sedimentary chlorins was better described by a model in which chlorins decayed at different rates within and below the sediment mixed layer. The degradation rates within the mixed layer were 0.0280±0.0385 yr −1 ( t 1/2=73 yr). Below the mixed layer, degradation rates were one to two orders of magnitude less, ranging from 0.0022±0.0025 yr −1 ( t 1/2=680 yr). Many stations had subsurface chlorin concentration peaks between 6 and 10 cm depth. The most likely explanation for these peaks is a period of increased deposition of chlorins in the past. This could result from changes in local depositional environment or a more general increase in surface production resulting in an increased sedimentation of chlorins to the sediments 500–1000 years ago. Chlorins are a useful indicator of the magnitude of chlorin deposition; however their usage as indicators of paleoproductivity is more complicated.