Bottom-water Oxygen Levels Research Articles

Birth and death of the Late Cretaceous “La Luna Sea”, and origin of the Tres Esquinas phosphorites

Deposition of organic carbon-rich intervals of the La Luna and Navay formations of northwestern Venezuela was governed by the development of key paleobathymetric barriers (Santa Marta and Santander massifs, Paraguana Block, and ancestral Mérida Andes). These enhanced the development of anoxia in the “La Luna Sea” by causing poor circulation and limited ventilation. Anoxia was also promoted by high evaporation and low precipitation rates (high salinity bottom water), and high levels of marine algal productivity (high organic matter flux). Nutrient supply was augmented by infrequent fluvial sources. Bottom water oxygen levels increased from the Late Santonian through the end of the Cretaceous. Ventilation of anoxic bottom waters may have been enhanced by more frequent or intense seasonal upwelling (caused by higher wind stress) and catastrophic overturn, as well as the removal of a key paleobathymetric barrier. Common byproducts of overturn events were massive phytoplankton blooms, which produced red tides. Fish and marine reptile bone beds within the Tres Esquinas Member (La Luna Formation) are attributed to massive mortality during these events, and are correlative with similar Campanian units in eastern Colombia. During the Maastrichtian, increasing ventilation, combined with siliciclastic dilution, ultimately produced sediments with lower total organic carbon (TOC) content.

Geochemical characterization of oceanographic and climatic changes recorded in upper Albian to lower Maastrichtian strata, western Venezuela

Organic and inorganic geochemical data (Rock-Eval, TOC, biomarker GC/MS, and mineral, major and trace element analyses) were used to characterize the palaeoceanographic and palaeoclimatic conditions that influenced the deposition of upper Albian to Maastrichtian rocks of western Venezuela. These data show that the late Albian to early Santonian was characterized by the accumulation and preservation of hydrogen-rich marine algal and foraminiferal organic matter, and was modulated, in part, by siliciclastic dilution (via eolian and fluvial/turbidite processes). The contemporaneous development of palaeobathymetric barriers surrounding the Maracaibo and Barinas/Apure Basins resulted in stagnation and poor circulation, and along with high evaporation rates, produced salinity stratification and entrapment of anoxic bottom waters. Bottom water oxygen levels increased from the late Santonian through the end of the Cretaceous as seasonal upwelling intensified, and stratified and entrapped anoxic bottom waters underwent frequent overturn and ventilation. These major changes in depositional patterns can now be identified and linked to regional or global oceanographic and climatic events. The depositional events that best demonstrate this link include: (1) late Albian to early Cenomanian drowning of the Maraca Formation carbonate platform (linked to the global mid-Cretaceous platform drowning event); (2) Cenomanian-Turonian drowning of the Guayacan Member (Capacho Formation) carbonate platform (southern Maracaibo Basin) and deposition of organic carbon-rich sediments (linked to the global Cenomanian-Turonian oceanic anoxic event); (3) depositions of Tres Esquinas Member (La Luna Formation) phosphates and glauconites in the Santonian to Campanian (linked to a regional increase in fluvially derived sediments); and (4) onset of delta progradation from Colombia into western Venezuela during the Campanian through Maastrichtian (linked to regional tectonic activity and cooling global climate).

The use of magnetic susceptibility and trace element geochemistry for the correlation of fine-grained siliciclastic sequences: a Late Llandovery example from northwest England

High-resolution lithological and magnetic susceptibility logs were made of two sections of the Upper Llandovery Browgill Formation in northwest England: Stockdale Beck, the type section of the Browgill Formation, and Spengill. The Browgill Formation is composed of fine-grained deep marine siliciclastics, which can be divided broadly into two facies: a homogeneous grey mudstone, deposited under oxygenated bottom-water conditions, and subordinate beds of laminated, graptolite-bearing black mudstone deposited under low bottom-water oxygen levels. The latter facies is often partially or fully diagenetically altered to chlorite nodules, occasionally with manganese carbonate nucleii. Magnetic susceptibility logs are shown to reflect variations in the illite–chlorite ratio of the clay mineralogy. Chlorite is a paramagnetic mineral, so the bands of chlorite nodules produce magnetic susceptibility highs. Correlation demonstrates that diagenetically altered beds of laminated black mudstone are continuous between the two sections, now situated 32 km apart. This lateral continuity would favour pelagic fallout in preference to gravity flow as a depositional model for both facies. A correlated sequence within the turriculatus Biozone is 3.8 times thicker at Spengill than at Stockdale Beck, probably reflecting variations in sediment accumulation rates. Correlation also identifies significant non-sequences in both sections: at least 47% of the turriculatus Biozone (including maximus Sub-biozone) is missing at Stockdale Beck and at least 77% of the crispus Biozone is missing at Spengill. The identification of non-sequences may be of value for refining graptolite biostratigraphy.

A mid-late Holocene sapropelic sediment unit from the southern Marmara sea shelf and its palaeoceanographic significance

A sapropelic layer, having an age of ca 4750 and 3500 14C y BP, was discovered at 0.90–2.35 m below the sea floor (mbsf) in gravity cores from the southern shelf of the Marmara Sea. It is a 15–50 cm thick, phosphorescent green to grey, plastic, clayey hemipelagic mud horizon, containing 1.5–2.9% organic carbon (C org). The increase in C org and biogenic carbonate, together with a rich planktonic foraminiferal fauna, indicate increased organic productivity and warm surface waters during the deposition of the sapropelic layer. The down-core profiles of Mn, Fe, Cu, Zn, Pb, Cr, Ni and Co suggest that the sapropelic layer was deposited through an essentially oxic water column. The benthonic foraminiferal fauna indicates reduced oxygen levels in bottomwaters. The sapropelic unit was deposited during a high stand of global sea level. Its deposition was initiated by a large input of terrestrial organic matter and nutrient-rich fresh waters under a relatively warm and wet climate. The fresh water supply caused a strong water stratification, which, in turn, together with high organic matter input, resulted in reduced oxygen levels in the bottomwaters.

Consequences of the 1993 Mississippi River flood in the Gulf of Mexico

Seasonally severe hypoxia (≤2 mg O2 l−1) occurs in waters below the pycnocline on the northern Gulf of Mexico inner continental shelf in May through September over extensive areas (up to 18 000 km2). Spatial and temporal variability in the distribution of hypoxic water masses is related, in part, to the amplitude and phasing of freshwater discharge from the Mississippi and Atchafalaya Rivers, circulation patterns, nutrient flux and a close coupling with net productivity. The Mississippi River flood in 1993 and sustained freshwater inputs to the Gulf of Mexico occurred during mid-summer through early autumn when long-term mean flows (1930–1995) are normally lowest. Long-term studies of the Louisiana shelf hypoxic zone provided a natural experiment to examine the effects of extreme high river flow on the adjacent continental shelf. Oxygen levels in bottom waters were severely reduced in July, August and September compared to long-term averages (1985–1992). Also, the areal extent of the bottom-water hypoxia in mid-summer 1993 was approximately twice as large as the average area mapped in the previous 8 years during mid-summer shelfwide surveys. Contributing to increased severity and areal extent of hypoxia in 1993 were reduced surface water salinities, increased strength of the pycnocline, five to ten times higher nutrient concentrations, greater phytoplankton biomass, an order of magnitude greater abundance of phytoplankton, mostly small, coccoid cyanobacteria, and a shift in diatom community dynamics. An equally extensive hypoxic zone in mid-summer of 1994, when riverine fluxes of freshwater and nutrients were ‘normal’, suggests some residual effects of the 1993 summer flooding. © 1998 John Wiley & Sons, Ltd.

Influence of water column anoxia and sediment supply on the burial and preservation of organic carbon in marine shales

Previous work has suggested that the laminated, organic-rich and bioturbated, organic-poor shales of the Camp Run Member of the Late Devonian-Early Mississippian New Albany Shale formed under anoxic and oxygenated bottomwater conditions, respectively, and that the interbedding of the two faciès was due to the vertical oscillation of a water-column anoxic/oxic boundary where it impinged on the basin margin. We have extended this analysis by examining the chemical and mineralogical differences between the two shale facies in a single borehole core, by seeking evidence for deposition of the laminated shales under bottom-water oxia or anoxia, and by determining whether the laminated shales formed when the carbon supply to the sea floor was higher. The results of this study show that the laminated and bioturbated shales are mineralogically and chemically distinct; relative to Al, an index of the aluminosilicate content, Si, Ti, Fe, P, Na, Ba, Co, Cr, Cu, Mo, Ni, V, Zn, and Zr are all higher, whereas Mn, Ca, Mg, and Sr are lower in the laminated compared with the bioturbated shales. The differences are due to a higher quartz, feldspar, titanite/ilmenite, and zircon content in the laminated shales, probably indicating a coarser grain-size, and the greater abundance of manganoan calcite in the bioturbated shales. Dissolved oxygen was present in bottom waters during the deposition of some of the laminated shale intervals because of the presence of manganoan calcite, a phase that can only form in sediments with an oxic surface. In addition, the organic matter preserved in the two shale types is isotopically different; δ 13C organic values are 1.9z.permil; lighter on average in the laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ 15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ 15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, suggesting that nutrient drawdown was less during the deposition of the organic-rich, laminated shales. The chemical, mineralogical, and isotopic contrasts between the two shale facies of the Camp Run Member indicate that the conditions of sedimentation were different during their deposition. The difference was possibly related to variations in sea level, which would have caused the Camp Run shoreline to move closer to and farther from the core site, causing, in turn, the deposition of coarser and finer grained sediments that contained different mixtures of marine and terrestrial organic matter. Bottomwater conditions were anoxic during deposition of most laminated intervals. Bottom-water anoxia or dysoxia led to decreased burial and preservation of the essential nutrient phosphorus in the laminated, organic-rich shales relative to the rocks deposited beneath better oxygenated bottomwaters. Increased availability of phosphorus in the water column on long timescales leads to increased productivity and a higher settling flux of organic matter, causing bottom-water oxygen levels to fall. This is consistent with the nitrogen isotope evidence suggesting that production was probably higher during the deposition of the organic-rich shales. Thus, production variations coupled with enhanced sedimentary regeneration of phosphorus from sediments related to low oxygen bottom-water concentrations provide a general mechanism for the formation of the alternating facies of this member of the New Albany Shale.

Sampling the oxygen minimum zone off Pakistan: glacial-interglacial variations of anoxia and productivity (preliminary results, sonne 90 cruise)

During cruise sonne 90 the well developed oxygen minimum zone (OMZ) in the northeastern Arabian Sea was sampled in great detail to study surface ocean productivity and organic carbon accumulation/preservation during the last hundred to hundred thousands of years. Nearly anoxic conditions on and below the seafloor are indicated by the lack of macrobenthos and nekton, absence of bioturbation, high marine organic carbon accumulation and preservation of laminated sediments between about 300 and 1000 m water depth on the steep continental slope. This pattern approximately coincides with the zone of extremely low O 2 concentrations presently observed in the water column. Accumulation of organic material on the deeper slope near the lower boundary of the OMZ is enhanced by downslope transport. Downcore we observe a complex alternation of dark-colored, laminated, and light-colored, homogeneous facies. In general, laminated, organic carbon-rich intervals were deposited under suboxic bottom water and enhanced productivity conditions, mainly during warm stages (e.g., Bölling/Alleröd and Holocene from 8000 yrs B.P. to Recent). The homogeneous, bioturbated intervals indicate normal bottom water oxygen levels and reduced productivity during or just after cold stages (e.g. Terminations IA and B, and Younger Dryas). The striking facies similarities of the sedimentary record in cores collected from different areas and water depths suggest a unique pattern of late Pleistocene C org accumulation in the northeastern Arabian Sea which depends mainly on regional surface ocean productivity and intermediate water oxygenation. However, the sedimentary record is also influenced by variations in sediment supply from the Makran and Karachi shelf and from Indus River discharge during rapid sea-level changes.

The ecology of living (stained) deep‐sea benthic foraminifera from the Sulu Sea

The distribution of living (rose bengal stained) deep‐sea benthic foraminifera was determined in the upper 20 cm of sediments of eight Soutar box cores taken from two depth transects (510‐4515 m) in the thermospheric (> 10°C) Sulu Sea. Despite the uniformity of bottom water temperatures, salinities, and dissolved oxygen levels below 1000 m, significant faunal differences exist at different depths in the low‐oxygen (∼1.25 mL/L below 1000 m) basin. The shallowest site (510 m) is dominated (> 10% of the calcareous fauna) by Cibicidoides, Uvigerina, (> 150 µm) and Bolivina (> 63 µm), while Siphonina is codominant with Cibicidoides and Uvigerina in the 1005‐m core. The 2000‐m cores are dominated by Cibicidoides, Gyroidinoides, and Oridorsalis, while Cibicidoides bradyi and Oridorsalis umbonatus dominate the 3000‐ and 4000‐m cores. Infaunal assemblages of Valvulineria mexicana are found in the sediments of the 4515‐m core. Relatively low bottom water oxygen values do not necessarily yield "typical low‐oxygen taxa" such as Bolivina, Uvigerina, Chilostomella, Bulimina, and Globobulimina. Changes in the abundances of these taxa in fossil assemblages have been used as indicators of changes in ancient bottom water oxygen levels but may instead reflect organic carbon contents of the sediments. An examination of the vertical distributions of foraminiferal assemblages from the > 63‐µm and > 150‐µm fractions reveals that taxa have microhabitat preferences similar to those observed in other regions. Taxa found in the upper 0‐ to 1‐cm interval (epifaunal) include Cibicidoides wuellerstorfi and Hoeglundina elegans, while taxa such as Chilostomella and Globobulimina reach maximum abundances in subsurface sediments and have infaunal microhabitat preferences. Cibicidoides bradyi and O. umbonatus live in sediment depths from 0‐ to 4‐cm and have transitional preferences with both epifaunal and infaunal occurrences. Intrageneric differences in test morphologies, including pore distribution, rounded peripheries, and variable spire height, are observed in Cibicidoides and Gyroidinoides and are suggested to be related to microhabitat preferences. Vertical distributions of a number of taxa found in both the 63‐ to 150‐µm and > 150‐µm fractions are similar, suggesting that juveniles and adults live under similar microhabitat conditions. Ontogenetic changes in microhabitat preferences of most species are not observed in this study and therefore would not be expected to account for isotopic vital effects reported for some taxa in previous studies.

Geochemistry of the surface sediments of the Sulu and South China Seas

The chemical and mineralogical composition of the surface sediments of the Sulu Sea, a deep, hydrographically-restricted basin in the Indonesian archipelago, and the neighbouring area of the South China Sea have been examined. The aim of the study was to characterize the geochemistry of the sediments of the isolated, oxygen-deficient basin and to compare them with sediments at equivalent water depths outside the basin, as a prelude to a further study of the response of the composition of the sediments to oceanographic changes, including possible anoxia, during the last glacial maximum. The concentrations of most of the chemical constituents in the surface sediments along four transects change markedly with water depth in the Sulu Sea (SS) and the South China Sea (SCS). Thus, CaCO 3 decreases from around 60% by weight in the shallowest samples (800–1000 m) to around a few percent in the deepest sample at 4203 m in the SCS, where the CCD lies at around 4000 m. In the SS, however, the CCD lies much deeper, so that the CaCO 3 contents are still approximately 40% at 4500 m depth. Organic C also decreases with depth in both basins, from values of around 1–2% in the shallowest samples to less than 1% in the deepest samples. The ratios of Si, Mg, Na, K, Co, Cr, Cu, Ni, Pb, Rb, Y, Zr and Zn to Al all decrease with increasing water depth because of the change in mineralogy and sediment texture with water depth. The P Al is higher in shallower water in both areas because of the higher abundance of detrital phosphorite or phosphatized limestone in these sediments. The Sr Al ratio is also higher in shallower water, reflecting the supply of aragonitic shell debris from the shallow banks and shelves around the basin. The Ba Al ratio has a maximum at 2000 m depth in the SCS, due to higher primary production in overlying surface waters, whereas it is uniformly low in the SS. Manganese is also enriched at intermediate water depths in both the SS and the SCS because of the presence of oxyhydroxides, and this also produces a slight enrichment in Mo in the same samples because of their association in this authigenic phase. The concentration of I and the I C organic ratio decreases with water depth in both areas due to the higher abundance of fresh, marine organic matter at the shallow water sites, which scavenges proportionally more I from sea water. Finally, the δ 13C organic values tend to decrease irregularly with increasing water depth probably reflecting a higher contribution of terrestrial organic matter in deeper water. A discriminant analysis of the full data set reveals that, compared with the SCS samples, the oxygen-deficient SS sediments have higher CaCO 3 contents, lower N and S contents and lower I C organic , Ba Al , Fe Al , K Al , Pb Al , Rb Al , Ti Al , V Al , Y Al and Zn Al ratios. These contrasts are brought about by the different depths of the CCD, differences in the nature of the organic matter and different types of aluminosilicate detritus in the two areas. Although the I C organic ratios are slightly but significantly lower in the SS, which may be due to the low dissolved oxygen contents of the waters of this basin, the Mn and Mo contents and other minor elements that respond significantly to ambient bottom water oxygen levels are not significantly different in the SS and the SCS. Moreover, the organic C contents are also indistinguishable in the two areas; hence, the low bottom water oxygen levels in the SS do not lead to higher organic carbon contents in the surface sediments. The composition of sediments in contact with waters having even lower oxygen concentrations should be evaluated in order to conduct a more definitive test of the effect of low oxygen conditions on carbon preservation. Comparison of glacial-interglacial sediment sections from this area must take particular care to separate the effects of source variations on those potentially caused by low oxygen levels.

Sediment mixing and accumulation rates in the Sulu and South China Seas: Implications for organic carbon preservation in deep-sea environments

Recent rates of carbon burial in two deep-sea basins, the Sulu and South China Seas, are contrasted to determine factors influencing organic carbon preservation in deep-sea sediments. The Sulu Sea is an oxygen-deficient, warm-water basin, whereas the adjacent South China Sea is more typical of open ocean conditions. High-resolution sea-bed profiles of 14C reveal that accumulation rates for the Sulu Sea are 2–3 times higher than the South China Sea at any given water depth (e.g., 1.3 g cm −2 kyr −1 versus 4.3 g cm −1 kyr −1, respectively, at 3000 m). Higher accumulation rates for the Sulu Sea are attributed to higher terrigenous input and better carbonate preservation in the warmer bottom waters. Benthic mixing rates determined using 210Pb geochronology generally are lower in the Sulu Sea (0.004–0.254 cm 2 yr −1) as compared with the South China Sea (0.17–1.57 cm 2 yr −1), especially at the 3000 m and 4000 m sites in the Sulu Sea where benthic mixing is negligible. Despite the higher bottom-water oxygen levels in the South China Sea, organic carbon concentration profiles are strikingly similar for the two basins at any given water depth. With similar concentration profiles, higher accumulation rates in the Sulu Sea result in equivalently higher carbon preservation rates. As primary production is thought to be similar for both basins (if anything slightly higher for the South China Sea), the production rate can not explain the observed differences in carbon preservation. In addition, benthic mixing rates apparently do not play a major role in carbon preservation as extremely low rates at the deeper Sulu Sea would predict relatively low carbon preservation, contrary to our observations. We conclude that sediment accumulation rate is the dominant control on carbon preservation in this area of the western equatorial Pacific.

Lack of evidence for enhanced preservation of sedimentary organic matter in the oxygen minimum of the Gulf of California: Comment and Reply

The role of bottom-water oxygenation in the degradation and preservation of marine sedimentary organic matter is the subject of controversy. Two opposing theories have been proposed to explain the typically higher organic carbon concentrations found in laminated, fully anoxic sediments and sedimentary rocks, when compared to their bioturbated, more oxygenated counterparts. One theory holds that preservation of organic matter is enhanced in laminated sediments because of slower net rates of organic carbon decomposition under fully anoxic conditions (e.g., Cranfield, 1989, 1992). Alternatively, it has been suggested that organic-rich laminated sediments reflect high levels of primary productivity in the surface waters and, consequently, increased proportions of organic matter in the particulate flux deposited at the water-sediment interface, rather than enhanced preservation in the sediments (e.g., Pedersen and Calvert, 1990). Calvert et al (1992) compared data on modern surface sediments of the Guaymas Basin accumulating insides as well as outside the zone where the oxygen minimum impinges on the sea floor. The choice of sediments deposited in close proximity minimizes the effects related to variations in the nature and mass flux or organic matter reaching the water-sediment interface. The data were collected on the first 2-5 cm of gravity cores. With themore » high sediment-accumulation rates in the Guaymas Basin (see Calvert et al., 1992), organic matter in the sediments analyzed has been subjected to early diagenetic processing for only short periods of time, at most 10-20 years. Calvert et al. reported the absence of systematic variations of the total-organic-carbon concentrations and Rock-Eval hydrogen and oxygen indices with bottom-water oxygen concentrations. This led them to conclude that bottom-water oxygen levels exert no major influence on the concentration and composition (in terms of the H/C and O/C ratios) of organic matter in the sediments. 10 refs.« less

Benthic community analysis and the prediction of organic carbon content in Mesozoic black shale facies

The burial and preservation of significant quantities of marine organic matter in epicontinental basins results from low bottom water oxygen levels, and moderate to low sedimentation rates of fine-grained siliciclastic sediment under at least moderately productive waters. Because benthic biotas are extremely sensitive to such factors as bottom water oxygen, grain size, substrate consistency and sedimentation rate, there is a potentially predictive relationship between biofacies and Corg potential of marine strata. High-resolution biological, sedimentological and geochemical studies of Mesozoic organic-rich black shale facies, comprising major hydrocarbon source rocks, allow characterization of this relationship: a) A series of recurrent, bivalve-dominated benthic communities are defined based on trends in diversity, abundance, equitability and trophic specialization. They comprise biofacies which reflect oxygen and substrate gradients in time and space, and thus the Corg preservation potential of marine sediments/strata. The biotas are surprisingly robust in terms of population size and inferred biomass, and demonstrably colonized the benthic zone (as opposed to pseudoplanktic origins). They include both resident communities of taxa specifically adapted to low oxygen (dysoxic) conditions through physiological and anatomical means (e.g., expanded oxygen adsorptive surfaces), or possibly through bacterial chemosymbiosis, and event communities of less tolerant taxa reflecting short-term improvements in benthic conditions; b) many organic-rich sequences contain common event biofacies, indicating a dynamic benthic environment with frequent short-term fluctuations in oxygen and substrate. These are attributable to mixing during large storms, productivity blooms, rapid deposition from mass flows, or reduced sedimentation during condensation/bypass events. However, increasing frequency of events is not necessarily correlated with decrease in Corg levels; c) comparison of trends in the quantity and quality of Corg to biofacies patterns indicate that the most favorable Corg preservation is associated with “benthic boundary biofacies.” These biofacies represent a redox boundary at or near the sediment-water interface and a predominantly dysoxic to episodically oxic water column. The “benthic boundary” may have been associated with the presence of microbial (mainly sulphur bacterial) mats on the substrate surface, which could have regulated its position and contributed to the pool of sedimented Corg; d) mapping of benthic boundary biofacies in time and space within a basin makes possible correlation of Corg preservation potential to paleogeography and sea level history. Although the results of such analyses from the Greenhorn Formation, Western Interior basin, suggest highest Corg burial and preservation in fine-grained shale facies of distal offshore environments during sea level rise, high Corg is not necessarily associated with an anoxic water column. This suggests the need for reappraisal of stagnant basin scenarios for source rock formation. The most important factors in Corg accumulation and preservation may be the development of benthic boundaries and consequent maintenance of anoxia below the sediment-water interface in fine-grained, organic-rich substrates, and the contribution of bacteria to the flux of Corg into the sediment.

Accumulation and Preservation of Organic Carbon in Marine Sediments: The Roles of Anoxia vs. Production: ABSTRACT

Organic carbon enrichments in marine sediments and sedimentary rocks commonly are explained by the preferential preservation of the deposited organic matter under anoxic conditions; the role of primary organic (plankton) production is seldom considered. A review of the available information shows that modern marine sediment accumulating in oxic and anoxic basins in similar topographic and sedimentary settings have very similar carbon contents. On continental slopes, carbon maxima are apparently produced by the complex interplay between the supply of carbon to the sea floor, the texture of the sediment, the dilution of carbon by other sediment components, and the decreasing settling flux of carbon in the deeper waters of the open ocean. Contrary to contemporary thought, there is no causal relationship between such maxima and the position of the oxygen minimum. The degradation of sedimentary organic matter by aerobes and by sulfate reducers is very similar where the supply of fresh organic matter to the sea floor is similar. Hence, there is no evidence for the preferential preservation of organic matter under anoxic conditions. Terrestrial organic matter, however, appears to be degraded to a lesser extent by sulfate reducers. The burial of carbon below the surficial, oxygenated horizons of a sedimentmore » removes the easily oxidized fractions leaving material that may be less susceptible to attack by sulfate reducers. Sedimentary carbon maxima in Pleistocene glacial horizons are due to the increased settling flux of organic matter brought about by climatically induced increases in upwelling in the equatorial and marginal areas of the ocean. Changes in bottom water oxygen levels during these periods plays a minor role in producing these signals. Previous work that claimed that anoxic bottom waters were prevalent during the accumulation of organic-rich black shales in the geological record should be reevaluated.« less

The response of benthic foraminifera to productivity cycles in the eastern equatorial Pacific: Faunal and geochemical constraints on glacial bottom water oxygen levels

The Stage 2 glacial maximum in eastern equatorial Pacific Ocean sediments is characterized by a pronounced increase in the organic carbon content which has been attributed either to increased upwelling and higher production along the equatorial divergence during the last glacial period or to enhanced preservation as a consequence of a lower concentration of dissolved O2 in glacial bottom water. Several lines of evidence are presented, based on data from two gravity cores collected from the central Panama Basin, which support the former interpretation. First, both the abundance and the size of benthic foraminifera of the genus Uvigerina increase in Stage 2, coincident with the increase in organic carbon concentration, most probably because of the greater availability of organic detritus during the glacial period. The increase in foraminiferal size and associated greater requirement of oxygen for respiration argues against the probability that oxygen was substantially depleted in the upper centimeter where Uvigerina spp. are thought to live. Second, the number of meiofaunal faecal pellets increases markedly during the Stage 2 maximum, indicating a more populous and active infauna, which again argues against oxygen depletion. Third, there is no significant decrease in the iodine:organic carbon (I:Corg) ratio in the Stage 2 sediments. Because iodine is depleted relative to carbon in anoxic basins but enriched in association with organic matter in oxic environments, the absence of a depletion in the I:Corg ratio in Stage 2 is evidence that bottom water remained oxygen‐ replete during the last glacial. Fourth, molybdenum is not enriched in sediments deposited in Stage 2; the presence of anoxic or dysaerobic bottom water or shallow pore waters during that time would have been recorded by the addition of Mo to the sediments via coprecipitation with authigenic iron sulphides. The combined faunal and geochemical data are consistent with the notion that Panama Basin deep waters remained oxygen‐replete during the last glacial maximum. It is concluded that the late Stage 2 carbon enrichment in the eastern equatorial Pacific more probably reflects higher productivity rather than enhanced preservation.

Sources and effects of oxygen deficiencies in the south-eastern North Sea

Abstract Oxygen concentrations, oxygen comsumption and its possible sources were investigated in the southeastern North Sea (east of 6°N20′E, south of 56°50′N) in 1983–1985. 1983 was a year with extremely low, 1984 with high oxygen concentrations in the bottom water, but even in 1984 only 40–70 % oxygen saturation was found in some areas in the subthermocline waters. BOD values suggest that the oxygen content in the deep water could be used up within 20–30 days, given a calm weather period. This oxygen consumption, extrapolated from the BOD measurements, was fastest well north of areas with highest organic seston stocks. This supports the assumption of a northward transport of seston clouds with the residual currents. The theoretical oxygen demand for the degradation of measured organic seston stocks of the water column was as high as about half of the saturation oxygen content of the subthermocline water. Thus, even in years with average plankton and other organic seston stocks the oxygen balance of the bottom water would be endangered quickly, should a prolonged vertical density stratification develop. Mean oxygen consumption for macrobenthos inhabited sediment was 18 mg · m−2 · h−1. Average benthic community oxygen consumption amounted to 14 % of total biological oxygen demand of subthermocline water. Possible impact of oxygen depletion on benthic communities was investigated. Cluster analysis, diversity index and evenness index showed that benthic community structures were different in years with and without oxygen deficiency in the bottom water, suggesting effects of low bottom water oxygen levels on benthos.

Some effects of eutrophication and the removal of aquatic plants by grass carp(Ctenopharyngodon idella)on rainbow trout(Salmo gairdnerii) in Lake Parkinson, New Zealand

Abstract Temporary removal of aquatic plants in Lake Parkinson, a small, eutrophic dune lake, resulted in a number of changes to the population of stocked rainbow trout (Salmo gairdnerii). During each summer the lake stratified and low oxygen levels limited the distribution of trout to shallow (0–4 m) surface waters. In the first summer following weed removal the numbers of black shags (Phallacrocorax carbo) counted at the lake increased, and their predation resulted in a decline in trout density. However, the growth rate and condition of the trout population then exceeded that of trout present before weed removal. During the second summer after weed removal a cladoceran bloom was followed by low phytoplankton levels and high ammonia concentrations. A prolonged calm compounded this situation with the result that oxygen levels in bottom and surface waters dropped below 2 ppm. These low oxygen levels eliminated the trout population, but other fish species present survived. Elimination of aquatic plants affected the population dynamics of other fish species in the lake with potential implications for the trout. The experiment demonstrated the importance of weed beds in maintaining a stable fish community in lakes such as Lake Parkinson.

The thermal, oxygen and light regimes in isolated columns of water in a turbid impoundment

SUMMARY. Temperature, oxygen and secchi disc transparency were measured in 5‐m diameter isolation columns positioned in water approximately 4 m deep. In general the difference in thermal conditions between the adjacent open water and columns was slight. Oxygen levels in bottom waters in the enriched columns were often slightly lower than in the open water but never dropped below 50% saturation. It seems unlikely that sufficient depletion occurred to markedly increase release of phosphorus from the sediment. Light penetration in the columns was more varied than in the open water but was seldom sufficient to permit the development of large stands of benthic algae.