Mid-water Oxygen Minimum Zone Research Articles

Deglacial restructuring of the Eastern equatorial Pacific oxygen minimum zone

Oxygenation in the Eastern Equatorial Pacific is responsive to ongoing climate change in the modern ocean, although whether the region saw a deglacial change in extent or position of the Oxygen Minimum Zone remains poorly constrained. Here, stable isotopes from the shells of an Oxygen Minimum Zone-dwelling planktic foraminifer are used to reassess the position of the mid-water Oxygen Minimum Zone relative to both the thermocline and benthos. Oxygen isotopes record a rapid shoaling of the Oxygen Minimum Zone towards the thermocline associated with Heinrich Stadial 1 and persisting through the deglaciation. Meanwhile, carbon isotope similarities between Oxygen Minimum Zone-dwelling Globorotaloides hexagonus and benthic Cibicidoides wuellerstorfi suggest a shared source water through the deglaciation. Results support a direct role for the Eastern Equatorial Pacific in venting carbon to the atmosphere through the deglaciation, a deglacial expansion of the Oxygen Minimum Zone, and a restructuring of mid-water oxygen and carbon dynamics from the glacial to Holocene intervals.

Shallow ocean oxygen decline during the end-Triassic mass extinction

The end-Triassic mass extinction (ETME) was associated with intensified deep-water anoxia in epicontinental seas and mid-depth waters, yet the absolute oxygenation state in the shallow ocean is uncharacterized. Here we report carbonate-associated iodine data from the peritidal Mount Sparagio section (Southern Italy) that documents the ETME (~ 200 Ma) in the western Tethys. We find a sharp drop in carbonate I/(Ca + Mg) ratios across the extinction horizon and persisting into the Early Jurassic. This records local dissolved oxygen and iodate decline in the near-surface ocean of low-latitude Tethys due to the development of depleted oxygen concentrations. Consequently, during the ETME even shallow-water animals, such as the megalodonts seen at Mount Sparagio, were likely the victims of oxygen-poor conditions. The shallow ocean deoxygenation coincides with the synchronous spread of deeper anoxic waters and widespread anoxic deposition on continental shelves and slopes. An upwards expansion of the mid-water oxygen minimum zone in the latest Triassic shoaled the oxycline and triggered a major marine crisis.

In situ observations show vertical community structure of pelagic fauna in the eastern tropical North Atlantic off Cape Verde

Distribution patterns of fragile gelatinous fauna in the open ocean remain scarcely documented. Using epi-and mesopelagic video transects in the eastern tropical North Atlantic, which features a mild but intensifying midwater oxygen minimum zone (OMZ), we established one of the first regional observations of diversity and abundance of large gelatinous zooplankton. We quantified the day and night vertical distribution of 46 taxa in relation to environmental conditions. While distribution may be driven by multiple factors, abundance peaks of individual taxa were observed in the OMZ core, both above and below the OMZ, only above, or only below the OMZ whereas some taxa did not have an obvious distribution pattern. In the eastern eropical North Atlantic, OMZ expansion in the course of global climate change may detrimentally impact taxa that avoid low oxygen concentrations (Beroe, doliolids), but favour taxa that occur in the OMZ (Lilyopsis, phaeodarians, Cydippida, Colobonema, Haliscera conica and Halitrephes) as their habitat volume might increase. While future efforts need to focus on physiology and taxonomy of pelagic fauna in the study region, our study presents biodiversity and distribution data for the regional epi- and mesopelagic zones of Cape Verde providing a regional baseline to monitor how climate change may impact the largest habitat on the planet, the deep pelagic realm.

Editorial: Zooplankton and Nekton: Gatekeepers of the Biological Pump

Editorial on the Research Topic Zooplankton and Nekton: Gatekeepers of the Biological Pump Zooplankton and nekton organisms create and destroy particles in manifold ways. They feed on the diverse components of the plankton community and on detrital matter. They disaggregate these components, but also repackage them into fecal pellets. Zooplankton and nekton thereby contributes to the attenuation, but also to the export of vertically settling particles. Many zooplankton and nekton organisms also ascend to the surface layer of the ocean at dusk to feed during the dark hours, and return to midwater at the break of dawn. This diurnal vertical migration (DVM) shuttles organic matter from the surface ocean to deeper layers, where it is metabolized and excreted. This active flux (as opposed to the passive flux of sinking particles) can contribute substantially to the biological pump, the downward export of carbon and nutrients into the oceans interior. Due to their multiple roles in oceanic particle dynamics, zooplankton and nekton organisms can actually be considered the gatekeepers of the biological pump. Several articles in this Research Topic deal with the contribution of zooplankton and nekton-mediated active flux to the total export of organic matter. Using biomass and enzyme transport system (ETS) assessments of respiratory flux for both mesozooplankton and micronekton communities, Hernandez-Leon et al. estimated the total active transport of carbon (respiration, excretion, mortality, and egestion) along a transect in the Atlantic from the Canary Islands to Brazil. They found that active flux by these communities ranged from 25 to 80% of the total particulate organic carbon export at 150 m depth and that the importance of active flux increased with increasing surface productivity. Kwong et al. compared biomass, diel vertical migration, and active flux of mesozooplankton and micronekton across a range of mesoscale eddy structures along the east-coast of Australia during winter and spring. They found that although all eddy regimes had similar integrated biomass of mesozooplakton and micronekton, the organisms in the individual eddies had different migratory behavior, which resulted in contrasting importance of active flux. Kiko et al. assessed the impact of mesozooplankton DVM on elemental cycling at three stations in the Eastern Tropical North Atlantic. They found that approximately 31 to 41% of the total nitrogen loss from the upper 200 m of the water column was attributable to DVM mediated fluxes. They also suggest that gut flux-the flux created by migrators when they evacuate their gut at DVM-depth-and migrator mortality at DVM-depth contribute to an Intermediate Particle Maximum. In their study conducted in the Peruvian upwelling system (which features a severe midwater oxygen minimum zone), Kiko and Hauss concluded that the metabolic suppression

The geochemistry and formation of ferromanganese oxides on the eastern flank of the Gagua Ridge

Ferromanganese oxides on the eastern flank of the Gagua Ridge were collected from a water depth of 4071 m by TV-grab in 2016 and analyzed for their mineralogical and chemical compositions. These oxide fragments were broken from a large poly-core crust and can be divided into three parts: a dense inner nodule, a radial middle crust and a loose outer crust. The close proximity of the nodule and crust is similar to that observed in the amalgamated deposits from the Shatsky Rise, NW Pacific. All samples are mainly composed of Fe-rich vernadite, amorphous FeOOH and associated quartz, plagioclase, pyroxene and Mg-titanomagnetite. The mineralogy, Mn/Fe ratios, rare earth element patterns, and slow growth rates (with a mean value of 6.2 mm/Myr) of the samples reflect their predominantly hydrogenetic origin. Co, Ni, Cu, REY and other critical metals are most enriched in the Mn-rich inner nodule samples, which also have the highest Fe contents. In contrast, the Si and Al contents in the samples of the middle and outer crusts are very high, up to 19 wt% and 6.29 wt%, respectively. Correlation analysis indicates that Li, Sc, Cr, Rb, and Cs tend to concentrate in detrital silica-aluminosilicate phases. The Gagua Ridge samples generally have lower metal contents than the hydrogenetic crusts and nodules from other open ocean regions, which can be attributed to their dilution by detritus and the poor development of a mid-water oxygen minimum zone. However, the incorporation of detritus has little effect on the generation of Fe-Mn oxides in the Gagua Ridge. Based on their mineralogy, textures and chemical variations, the accretion of the hydrogenetic Fe-Mn oxides was initiated in the late Miocene, and the formation of the amalgamated Fe-Mn deposits comprised three stages.

Organic carbon production, mineralisation and preservation on the Peruvian margin

Abstract. Carbon cycling in Peruvian margin sediments (11 and 12° S) was examined at 16 stations, from 74 m water depth on the middle shelf down to 1024 m, using a combination of in situ flux measurements, sedimentary geochemistry and modelling. Bottom water oxygen was below detection limit down to ca. 400 m and increased to 53 μM at the deepest station. Sediment accumulation rates decreased sharply seaward of the middle shelf and subsequently increased at the deep stations. The organic carbon burial efficiency (CBE) was unusually low on the middle shelf (<20%) when compared to an existing global database, for reasons which may be linked to episodic ventilation of the bottom waters by oceanographic anomalies. Deposition of reworked, degraded material originating from sites higher up on the slope is proposed to explain unusually high sedimentation rates and CBE (>60%) at the deep oxygenated sites. In line with other studies, CBE was elevated under oxygen-deficient waters in the mid-water oxygen minimum zone. Organic carbon rain rates calculated from the benthic fluxes alluded to efficient mineralisation of organic matter in the water column compared to other oxygen-deficient environments. The observations at the Peruvian margin suggest that a lack of oxygen does not greatly affect the degradation of organic matter in the water column but promotes the preservation of organic matter in sediments.

Are Early Cretaceous environmental changes recorded in deposits of the Western part of the Silesian Nappe? A geochemical approach

The organic matter (OM)-rich, relatively monotonous sequence of upper Barremian–Albian deposits in the western part of the Silesian Nappe in the Polish Outer Carpathians, was investigated, using geochemical proxies to show environmental changes. The amounts of organic carbon (0.4–4.32wt.% of TOC) co-vary with the concentration of S and/or redox sensitive trace elements (RSTE: Mo, U, Co, V, As). Kerogen is present as Type III and mixed Type II/III. Maceral assemblages consist of vitrinite, inertinite and liptinite. The carbon isotopic composition of the organic matter (δ13Corg) varies between −25.38 and −21.31‰ with an outlier at −15.14‰. During the Early Cretaceous the Proto-Silesian Basin was the site of OM accumulation in a detrital oceanic anoxic event as a result of increased sedimentation rates of terrestrial organic matter. Runoff of terrestrial organic matter was accompanied by the production of plankton on an occasional basis. The marine matter contribution is inferred from occurrence of siliceous microfauna, the domination of short-chain n-alkanes, the distribution of sterane and prevalence of cholestane, and enrichment in nutrient elements (P2O5, Ba, Ag, Cd). The bottom waters could have been dysoxic to anoxic, with a mid-water oxygen minimum zone (OMZ) developed owing to the oxidation of organic matter. Anoxia is inferred from the lamination, pyritization patterns, RSTE concentration and low Pr/Ph and C23t/C30 H ratios. Intervals enriched in TOC and RSTE in the Veřovice Formation record pulses of marine productivity, preceded by volcanic (teshenite) and hydrothermal activity. They are referable to the following episodes: the Taxy (Barremian–Aptian), early Aptian Selli, Fallot and Paquier during the late Aptian and at the Aptian–Albian boundary. The initiation of turbidite deposition during the Albian led to bottom ventilation. The Lhoty Formation records the late Albian Toolebuc Episode, marked by a TOC peak, which resulted from the detrital flux. Acidic solution was responsible for reducing conditions in the sediment.

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

Abstract. The Arabian Sea plays an important role in the marine nitrogen cycle because of its pronounced mid-water oxygen minimum zone (OMZ) in which bio-available nitrate (NO3−) is reduced to dinitrogen gas (N2). As the nitrogen cycle can respond fast to climate-induced changes in productivity and circulation, the Arabian Sea sediments are an important palaeoclimatic archive. In order to understand seasonal and interannual variations in the nitrogen cycle, nutrient data were obtained from the literature published prior to 1993, evaluated, and compared with data measured during five expeditions carried out in the framework of the Joint Global Ocean Flux Study (JGOFS) in the Arabian Sea in 1995 and during a research cruise of RV Meteor in 2007. The data comparison showed that the area characterized by a pronounced secondary nitrite maximum (SNM) was by 63% larger in 1995 than a similarly determined estimate based on pre-JGOFS data. This area, referred to as the core of the denitrifying zone, showed strong seasonal and interannual variations driven by the monsoon. During the SW monsoon, the SNM retreated eastward due to the inflow of oxygen-enriched Indian Ocean Central Water (ICW). During the NE monsoon, the SNM expanded westward because of the reversal of the current regime. On an interannual timescale, a weaker SW monsoon decreased the inflow of ICW from the equatorial Indian Ocean and increased the accumulation of denitrification tracers by extending the residence time of water in the SNM. This is supported by palaeoclimatic studies showing an enhanced preservation of accumulative denitrification tracers in marine sediments in conjunction with a weakening of the SW monsoon during the late Holocene.

Locomotion and behavior of Humboldt squid,Dosidicus gigas, in relation to natural hypoxia in the Gulf of California, Mexico

We studied the locomotion and behavior of Dosidicus gigas using pop-up archival transmitting (PAT) tags to record environmental parameters (depth, temperature and light) and an animal-borne video package (AVP) to log these parameters plus acceleration along three axes and record forward-directed video under natural lighting. A basic cycle of locomotor behavior in D. gigas involves an active climb of a few meters followed by a passive (with respect to jetting) downward glide carried out in a fins-first direction. Temporal summation of such climb-and-glide events underlies a rich assortment of vertical movements that can reach vertical velocities of 3 m s(-1). In contrast to such rapid movements, D. gigas spends more than 80% of total time gliding at a vertical velocity of essentially zero (53% at 0±0.05 m s(-1)) or sinking very slowly (28% at -0.05 to -0.15 m s(-1)). The vertical distribution of squid was compared with physical features of the local water column (temperature, oxygen and light). Oxygen concentrations of ≤20 μmol kg(-1), characteristic of the midwater oxygen minimum zone (OMZ), can influence the daytime depth of squid, but this depends on location and season, and squid can 'decouple' from this environmental feature. Light is also an important factor in determining daytime depth, and temperature can limit nighttime depth. Vertical velocities were compared over specific depth ranges characterized by large differences in dissolved oxygen. Velocities were generally reduced under OMZ conditions, with faster jetting being most strongly affected. These data are discussed in terms of increased efficiency of climb-and-glide swimming and the potential for foraging at hypoxic depths.

Impact of monsoon-driven surface ocean processes on a coral off Port Blair on the Andaman Islands and their link to North Atlantic climate variations

North Atlantic climate variations are reflected in sedimentary records from the northern Indian Ocean in which two basins, the Arabian Sea and the Bay of Bengal, are strongly affected by the monsoon. Contrary to the Bay of Bengal the Arabian Sea plays an important role in the global marine nitrogen cycle. In its mid-water oxygen minimum zone (OMZ) bioavailable fixed nitrogen is reduced to nitrogen gas (NO 3 − - > N 2), whereas oxygen concentrations are slightly above the threshold of nitrate reduction in the OMZ of the Bay of Bengal. A coral colony ( Porites lutea) growing south of Port Blair on the Andaman Islands in the Bay of Bengal was studied for its response to changes in the monsoon system and its link to temperature changes in the North Atlantic Ocean, between 1975 and 2006. Its linear extension rates, δ 13C and δ 18O values measured within the coral skeleton reveal a strong seasonality, which seems to be caused by the monsoon-driven reversal of the surface ocean circulation. The sampling site appears to be influenced by low salinity Bay of Bengal Water during the NE monsoon (boreal winter) and by the high salinity Arabian Sea Water during the SW monsoon in summer. The high salinity Arabian Sea Water circulates along with the Summer Monsoon Current (S-MC) from the Arabia Sea into the Bay of Bengal. Decreasing δ 18O and reconstructed salinity values correlate to the increasing SSTs in the North Atlantic Ocean indicating a reduced influence of the S-MC at the sampling site in the course of northern hemispheric warming. During such periods oxygen depletion became stronger in the OMZ of the Arabian Sea as indicated by the sedimentary records. A reduced propagation of oxygen-depleted high salinity Arabian Sea Water into the Bay of Bengal could be a mechanism maintaining oxygen concentration above the threshold of nitrate reduction in the OMZ of the Bay of Bengal in times of global warming.

Faunal responses to oxygen gradients on the Pakistan margin: A comparison of foraminiferans, macrofauna and megafauna

The Pakistan Margin is characterised by a strong mid-water oxygen minimum zone (OMZ) that intercepts the seabed at bathyal depths (150–1300 m). We investigated whether faunal abundance and diversity trends were similar among protists (foraminiferans and gromiids), metazoan macrofauna and megafauna along a transect (140–1850 m water depth) across the OMZ during the 2003 intermonsoon (March–May) and late/post-monsoon (August–October) seasons. All groups exhibited some drop in abundance in the OMZ core (250–500 m water depth; O2: 0.10–0.13 mL L?1=4.46–5.80 ?M) but to differing degrees. Densities of foraminiferans >63 ?m were slightly depressed at 300 m, peaked at 738 m, and were much lower at deeper stations. Foraminiferans >300 ?m were the overwhelmingly dominant macrofaunal organisms in the OMZ core. Macrofaunal metazoans reached maximum densities at 140 m depth, with additional peaks at 850, 940 and 1850 m where foraminiferans were less abundant. The polychaete Linopherus sp. was responsible for a macrofaunal biomass peak at 950 m. Apart from large swimming animals (fish and natant decapods), metazoan megafauna were absent between 300 and 900 m (O2 0.2 mL L?1=8.92 ?M). The progressively deeper abundance peaks for foraminiferans (>63 ?m), Linopherus sp. and ophiuroids probably represent lower OMZ boundary edge effects and suggest a link between body size and tolerance of hypoxia. Macro- and megafaunal organisms collected between 800 and 1100 m were dominated by a succession of different taxa, indicating that the lower part of the OMZ is also a region of rapid faunal change. Species diversity was depressed in all groups in the OMZ core, but this was much more pronounced for macrofauna and megafauna than for foraminiferans. Oxygen levels strongly influenced the taxonomic composition of all faunal groups. Calcareous foraminiferans dominated the seasonally and permanently hypoxic sites (136–300 m); agglutinated foraminiferans were relatively more abundant at deeper stations where oxygen concentrations were >0.13 mL L?1(=5.80 ?M). Polychaetes were the main macrofaunal taxon within the OMZ; calcareous macrofauna and megafauna (molluscs and echinoderms) were rare or absent where oxygen levels were lowest. The rarity of larger animals between 300 and 700 m on the Pakistan Margin, compared with the abundant macrofauna in the OMZ core off Oman, is the most notable contrast between the two sides of the Arabian Sea. This difference probably reflects the slightly higher oxygen levels and better food quality on the western side.

Carbon and nitrogen elemental and stable isotopic compositions of surficial sediments from the Pakistan margin of the Arabian Sea

Abstract The factors controlling sedimentary organic matter distributions are the subject of ongoing debate. We report here the carbon and nitrogen contents and stable isotopic compositions for surface sediments and downcore profiles from sites ranging in water depth from 140 to 3200 m, located across the Indus margin of the Arabian Sea. The sites spanned the intense midwater oxygen minimum zone (OMZ) and showed dramatic differences in bottom-water redox conditions, benthic communities, and degrees of bioturbation. Five primary stations were studied, at 140, 300, 940, 1200 and 1850 m water depth, with sampling in March–May (“intermonsoon”) and August–October (“late-to-postmonsoon”) 2003. High-resolution sampling was also conducted across the upper (140–275 m) and lower OMZ (600–1200 m) boundaries. The sampling strategy was selected to help clarify the roles of oxygen availability and other potential environmental factors in determining sediment organic matter content and composition, the extent of organic matter cycling within the sediments under contrasting depositional conditions, and the fate of organic matter deposited during the southwest monsoon. As previously noted on this and other OMZ margins, there was a roughly inverse spatial relationship between bottom-water oxygen and sedimentary organic matter concentrations. However, variability in organic carbon content within the OMZ that was unrelated to oxygen, and the fact that maximal organic carbon contents occurred under relatively oxidizing conditions towards the lower OMZ boundary, rather than at the OMZ core, indicate that factors other than oxygen contribute significantly as controls on carbon distributions. Despite a 3-fold range in organic carbon content, carbon-to-nitrogen ratios showed no systematic trends across the OMZ, with values reflecting uniform (predominantly marine) organic matter inputs. Nitrogen isotopic signatures (δ15N values) in the range of 6.5–8.5‰ also did not show clear cross-OMZ trends, and are attributed to a 14N-depleted nitrate pool created by pelagic denitrification. There was little downcore change in organic matter content at any site, suggesting either that organic matter decay occurs largely in the water column, or that decay within the sediments (at oxygenated sites) is masked by bioturbation. Seasonal variability in organic matter content and composition was restricted to the most surficial sediments, and changes were ephemeral, suggesting rapid turnover of organic matter deposited during the southwest monsoon, on timescales of weeks. Stable C isotopic compositions trended towards more negative values in surficial sediments at hypoxic sites within the OMZ core. This is attributed to a chemosynthetic bacterial imprint.

Bacterial numbers and growth in surficial deep-sea sediments and phytodetritus in the NE Atlantic: Relationships with particulate organic carbon and total nitrogen

Bacteria in deep-sea sediments constitute the largest global fraction of total benthic bacteria, and play a major role in most biogeochemical cycles. However, as yet the relationship between bacterial production and substrate availability (particulate organic carbon and nitrogen) in deep-sea sediments is not well understood. Therefore the aim of this study was to explore the relationships between bacterial numbers and DNA synthesis (a measure of bacterial growth) and the concentration of bulk POC and total nitrogen in surficial deep-sea sediments and recently deposited phytodetritus, at three sites in the NE Atlantic. We found statistically significant positive exponential relationships between bacterial numbers and [ 3H]-thymidine incorporation rates (DNA synthesis) and sediment and phytodetritus % particulate organic carbon (POC) and % total nitrogen (%TN) from samples collected in the deep NE Atlantic at three different times at 3 contrasting sites. Mean bacterial numbers were in the range 5.6–53.2×10 10 cell l −1 in surficial sediments and 9.2–12.9×10 10 cells l −1 in the phytodetritus; [ 3H]-thymidine incorporation rates were 14.8–593.7 pmol l −1 h −1 in the sediment and 395.8–491.4 pmol l −1 h −1 in the phytodetritus. POC concentrations were 0.22–0.61% in the sediment and 0.92–0.99% in the phytodetritus; TN was 0.04–0.09% in the sediment and 0.13–0.14% in the phytodetritus. The C:N ratio was 6.7–9.0 in sediment and 7.6–7.9 in phytodetritus. In addition, a positive exponential relationship was still evident with the inclusion of other data from the NE Atlantic and Solomon and Coral Sea. The logarithmic regression for this combined relationship was not statistically different from the one derived from the current data alone. However, the relationship failed with the inclusion of data from the highly eutrophic Arabian Sea, which has a mid-water oxygen minimum zone that may inhibit heterotrophic bacterial production. This implies that such a relationship is not generic to all ocean systems, and additional research is required to further test the relationship between bacterial [ 3H]-thymidine incorporation rates and % POC from other oceanic deep-sea surficial sediments.

Laminated sediments from the oxygen-minimum zone of the northeastern Arabian Sea

Abstract Holocene and late Pleistocene hemipelagic muds with fine, mm-scale laminations were discovered during the International Indian Ocean Expedition (IIOE) 30 years ago in the northeastern Arabian Sea, where an expanded mid-water oxygen-minimum zone (OMZ) impinges on the continental slope and prevents bioturbation. In 1993, R.V. Sonne revisited Pakistani waters in order to study in more detail the complex interactions between surface and deep water hydrography, productivity and terrigenous input which form and preserve organic-rich sediments. Laminated sediments frequently occur along the Indian-Pakistani continental margin between 300 and 900 m water depth. The zone of high organic carbon contents roughly coincides with the depth of the present OMZ (200–1200 m water depth). However, resedimentation processes influence the character of sediment laminae and the distribution of organic carbon in the slope sediments. Microscopic and SEM studies suggest that the lamination may be due to the seasonal changes of surface productivity and to the lateral supply of fine-grained sediment. We infer that the OMZ must have varied considerably in space and time. This is documented by the distinct, possibly cyclic alternations of dark-coloured, organic-carbon-rich, laminated intervals with light-coloured, carbonate-rich, bioturbated sedimentary sequences, reflecting variations between anoxic to suboxic and oxic bottom water conditions.

Foraminiferal palaeoenvironmental constraints in the Sirt Shale (late Campanian-early Maastrichtian) transgressive succession of the Hameimat Basin) northern Libya

The Sirt Shale consists of a transgressive succession ranging in age from late Campanian to early Maastrichtian. The investigated area includes the deeper section of the Hameimat Trough (eastern Sirt Basin) flanked by higher relief to the north and east. Planktonic and benthonic foraminifera are used as a tool in determining biostratigraphy and palaeoecologic setting. Spatial and stratigraphic distributions suggest a depth stratification of the planktonic taxa, with intermediate (epi- to mesopelagic) water exchange with the main Sirt Basin from the latest Campanian. Limits on biostratigraphic resolution are indicated by the late introduction into the Hameimat Basin of the single-keeled globotroncanids. Dominance of low-oxygen tolerant taxa characterizes the benthonic foraminiferal faunal composition. A striking reduction in species diversity at the top of the succession in the deep basin has been interpreted as the establishment of a mid-water oxygen-minimum zone.

The Cenomanian-Turonian Oceanic Anoxic Event in SE Poland

Stable isotopes ( 13C, 18O), CaCO 3 contents, carbon content insoluble in HCl (C org) and in the carbonates (C carb), and foraminifers have been investigated in the Cenomanian-Turonian boundary interval (uppermost Rotalipora cushmani Zone and lower Whiteinella archaeocretacea Zone) in SE Poland. The stage boundary is defined by the appearance of the coccolith species Quadrum gartneri Prins & Perch-Nielsen. The higher content of C org in the uppermost Rotalipora cushmani Zone and lowermost Whiteinella archaeocretacea Zone, a δ 13C and δ 18O stable isotope anomaly in the uppermost Cenomanian, and substantial changes in the foraminiferal assemblages have been recorded. A major carbon stable isotope excursion with a shift of up to +2.0‰ δ 13C (PDB) is located a short distance above the Rotalipora cushmani-Whiteinella archaeocretacea boundary within marly limestones. The peak value of δ 13C corresponds to the minimum P/B ratio, and minima in the diversity and abundance of foraminiferal assemblages. A late Cenomanian expanding mid-water oxygen minimum zone is thought to be responsible for changes in foraminiferal assemblages in the uppermost Rotalipora cushmani and lower Whiteinella archaeocretacea Zones.

Cyclic fluctuations of anoxia during Cretaceous time in the South Atlantic Ocean

The holes of the DSDP-IPOD program in the South Atlantic Ocean document two major anoxic events during Oxfordian to middle Albian times and secondly from late Cenomanian to Santonian times. The black shales formed during these two anoxic events differ in their rhythmicity and origin. During Lower Cretaceous time, the anoxic conditions resulted from the confined, euxinic nature of the basins. The rhythmicity of these black shales probably does not result from a global phenomenon (climatic or tectono-eustatic), but from local conditions resulting from the slender dimensions of the young ocean basin(s). The diversity and the diachroneity of the deposits from the south to the north precisely reflect the dynamics of the oceanic spreading. During Upper Cretaceous time, the anoxic conditions fluctuated in relation to a mid-water oxygen-minimum zone. The rhythmicity of black shale deposition seems to result from a global phenomenon, because of the widespread occurrence of the event. In the South Atlantic ocean, the cyclic fluctuations of anoxia were due to cyclic variations in the depth of the mid-water oxygen-minimum zone. There is no simple process to explain such rhythmicity. It probably results from the interplay of the three main variables which characterize the oceans at the time of the Cenomanian-Turonian boundary: the increased rate of sea floor spreading, high sea-levels and low water-circulation.

Middle Cretaceous calcareous nannofossil biogeography and preservation in the Atlantic and Indian oceans: Implications for paleoceanography

The quantitative study of calcareous nannofossil preservation and biogeographic distribution puts constraints on paleoceanographic models for the Atlantic and Indian oceans during the middle Cretaceous. We establish a dissolution ranking of middle Cretaceous calcareous nannofossils based on susceptibility to dissolution. A positive correlation of organic carbon content of the sediments and the relative abundance of Watznaueria indicates that much of the carbonate dissolution was caused by the release of carbon dioxide during oxidation of organic matter. The depth distribution of the most dissolution resistant species, Watznaueria barnesae, shows relative-abundance maxima at the deepest sites and a maximum where intermediate waters impinge on slopes; this reflects stagnant, oxygen-poor bottom waters and possibly a mid-water oxygen minimum zone with increased deposition and catabolic breakdown of organic matter. Factor analyses are performed on various data sets for three different time slices ranging in age from late Aptian to early Albian, middle Albian and late Albian to Cenomanian. The resulting factors can be related to paleoceanographic conditions such as colder water at high latitudes, high fertility (upwelling conditions) and neritic conditions for all three time intervals. A wide tropical to subtropical zone was flanked by boreal and austral zones. Upwelling conditions probably existed along the North African margin and over the Falkland Plateau. Zonal upwelling is tentatively postulated for the northwestern and northeastern margin of the North Atlantic in the middle Albian and for the eastern margin of the North Atlantic off Western Europe for the late Albian to Cenomanian. We speculate on surface water current patterns, location of upwelling regions and haline-driven deep circulation for the middle Cretaceous.

Geologic evolution of Hess Rise, central North Pacific Ocean

Cores from four Deep Sea Drilling Project (DSDP) sites (310, 464, 465, and 466) and seismic-reflection profiles provide data that are used to interpret the geological evolution and paleoenvironments of Hess Rise, a prominent oceanic plateau in the central North Pacific Ocean. Hess Rise apparently formed in the Southern Hemisphere along the western flank of the Pacific-Farallon Ridge 110 to 100 m.y. B.P. Core stratigraphies and lithologies show the response of sedimentation to subsidence and northward movement of Hess Rise on the Pacific plate. Oceanic islands, which crowned Hess Rise during its early evolution, were eroded and subsequently subsided below sea level. Major structural trends include three northwest-trending (∼327°) arms, or ridges, and an east-northeast-trending southern Hess Rise that parallels the Mendocino Fracture Zone (060°). Normal faults offset basement as much as 3,000 m along the southern edge and 1,500 m on the western flank of Hess Rise. Many faults were active during sedimentation. Tholeiitic basalt from the base of Hole 464, trachyte from the base of Hole 465A, and alkalic basalt clasts within sediment of Hole 466 show the diversity of rock types that constitute the igneous basement. A major rock unit is middle Cretaceous limestone, chalk, and minor chert that form the basal sedimentary unit. Some limestone samples, rich in organic carbon, reflect accumulation above the carbonate compensation depth (CCD) within a mid-water oxygen minimum zone. The organic-carbon-rich sediments probably were deposited on the submarine slopes of islands and banks that were at upper bathyal depths as Hess Rise crossed the wide equatorial divergence where increased upwelling and biogenic productivity contributed to high accumulation rates. The source of organic matter was mostly lipid-rich, autochthonous, marine organic matter. Analyses of sediment samples from across the Cretaceous-Tertiary boundary at Site 465 show that there was a significant decrease in surface water temperature and biological productivity. An abrupt increase in transition metals and iridium suggests that an outside source, perhaps extraterrestrial, was the cause for many of the sudden oceanographic, geochemical, and biological changes at the boundary.