Oxygen-depleted Bottom Waters Research Articles

Rare earth elements in phosphorite and associated sediment from the Namibian and South African continental shelves

Rare earth elements (REE) and major elements have been determined in twenty-nine phosphorite and associated sediment samples from the southwestern coast of Africa, including thirteen samples of unconsolidated sediment collected during recent cruises to phosphate-rich areas on the Namibian shelf. The concentrations of REE in various phosphorite types from southwestern Africa differ by more than two orders-of-magnitude. The samples mostly exhibit REE patterns which are similar to that of shale, having no Ce anomaly. The Ce content appears to have been controlled by the palaeoredox conditions of the localised environment of deposition or diagenesis and not to have been influenced by marine upwelling, or by the general palaeoredox conditions of the water column. The environments in which REE were incorporated into the francolite appear to have been anoxic, or sub-oxic, in the case of both Namibian unconsolidated authigenic phosphorites, and the predominantly ‘diagenetic’ phosphorites off the South African coast. The ultimate REE source in both cases appears to have been (mainly eolic) continental detritus. Pelletal phosphorites are significantly enriched in REE relative to non-pelletal varieties. If sequestration of REE dissolved in the water column contributed to the high concentrations of REE in pelletal phosphorites, then the main carrier phase appears to have been inorganic detritus or biogenic silica equilibrated with oxygen-depleted bottom waters but not fish debris. The contention that onland phosphorites are enriched in REE relative to their offshore counterparts is, however, refuted in the case of southwestern Africa, and the effects of groundwater REE enrichment are not obvious in the majority of onland phosphorites analysed in this study.

Revised Ages for Laminated Sediment and a Holocene-Marker Diatom from the Northern California Continental Slope

Conventional and accelerator mass spectrometry 14C ages indicate that laminated sediment in three cores from the northern California continental slope near 38°N and 39°N were deposited between 42,000 and 25,000 yr B.P. This revises and refines our previous estimates that laminated sediment accumulated during the late Pleistocene to early Holocene (J. V. Gardner and E. Hemphill-Haley, 1986, Geology 14, 691-694). Preservation of laminated sediment on the upper slope in this area suggests a period of intense coastal upwelling, high primary productivity, and resultant depletion of oxygen in bottomwaters preceding the onset of global glacial conditions. The transition from Pleistocene to Holocene conditions, and the establishment of a modern climatic regime driven by the California Current, included the incursion of the subtropical diatom, Pseudoeunotia doliola. P. doliola is common in sediment younger than about 10,000 yr and thus is a reliable marker species for identifying Holocene deposits off northern California.

Nutrient cycling and productivity in a desert saline lake: observations from a dry, low-productivity year

Seasonal variability of nutrients and productivity were examined in Pyramid Lake, a hyposaline, N-deficient, terminal desert lake, during a dry period. River inflow and N-fixation during 1990 were minimal allowing internal nutrient cycling to be more closely studied. Nutrient cycling was strongly affected by seasonal thermal stratification that was typical for a warm monomictic lake. Concentrations of nitrate, phosphate, and silicate in surface waters were highest during winter mixing and decreased rapidly in the spring due to a diatom bloom. Maximum average chlorophyll concentration in surface waters was 2.7 ± 1.2 µg 1−1 and occurred in April while surface nutrients were being depleted. In contrast to chlorophyll, maximum particulate carbon in surface waters occurred in July–August when areal productivity was highest (367–398 mg C m−2 day−1). Concurrent with spring nutrient depletion in surface waters was increasing N-deficiency in the plankton. After the spring bloom dissipated in May, particulate matter (POM) became increasingly N-deficient reaching maximum elemental C : N of > 18 during summer-fall. Profiles of the C : N ratio of POM were nearly constant with depth for individual sampling dates suggesting that the residence time of POM in the water column was short (< 1 month). While surface waters were nutrient depleted during summer stratification, nutrient concentrations of bottom waters progressively increased, presumably through the oxidation of POM sinking to the bottom (103 m). Converting the rate of oxygen depletion in bottom waters to carbon equivalents of POM suggests that 42 % of mean annual phytoplankton production in overlying waters during 1990 was mineralized in bottom waters.

Crystallographic and chemical variations during pyritization in the upper Barremian and lower Aptian dark claystones from the Lower Saxonian Basin (NW Germany)

ABSTRACTIn the Lower Saxonian Basin, cores from three drill holes provide a cross‐section from the basin centre towards the margin through the upper Barremian/lowermost Aptian pyritic claystones and marls. Microscopic analyses distinguish six types of pyrite (I: ‘cones and tubes’, II: ‘tubes’, III: ‘isometric crystal aggregates’, IV: ‘pseudomorphs of fossils’, V: ‘star‐like concretions’, and VI: ‘filiform pyrite‐marcasite intergrowths’) which are associated with subordinate amounts of Fe carbonate, chalcopyrite, sphalerite and wurtzite. The crystal morphology of these sulphides shows a clear‐cut diagenetic sequence from pyrite crystals dominated by the octahedron to those dominated by the cube.Among these early to late diagenetic pyrites, the conversion of crystal habits is accompanied by a striking variation in trace element contents, some of which have extremely low values (e.g. Au, Se, Te, Tl, Co). In this context, classification of crystal habits and analyses of As and Ni establishes a sequence of pyritization which may help define a basin zonation for these argillaceous sedimentary rocks, categorized as ‘normal facies’—oxygenated bottom waters—interrupted by short episodes of bottom water oxygen depletion (‘bituminous facies’). Substitution of the cube for the octahedron in the Fe bisulphide aggregates led to a continuous removal of As and Ni from the pyrite, attesting to a steady decrease of those elements in the pore fluids with time. By contrast, within each type of pyrite, As and Ni contents increase from the margin towards the centre of the basin, owing to pre‐concentration of both elements in the basinal sediments, which are more abundant in organic matter. Some other elements, however, such as Mn, Cu, Sb and Ag, do not follow this trend and are unrelated to the lattice transformation of the Fe bisulphides. Their sometimes anomalously high quantities in Fe bisulphides are controlled by the host rock chemistry (presence of volcaniclastic material) and accessory minerals intergrown with pyrite (e.g. chalcopyrite).Pyrite I through III formed in tubular hollows of burrowing organisms, and type IV in tests of fossils. Types V and VI did not result from replacement or void filling but were generated by diffusion of sulphate along inhomogeneties, such as bedding planes, shrinkage cracks or water escape structures.

Organic geochemistry of an anoxic cycle: A case history from the Oligocene section, Hungary

Total organic carbon, extractable organic matter, sulfur content, kerogen type and saturated hydrocarbons have been determined in 25 core samples from an approximately 100 m thick section of the Oligocene Tard Clay, near Budapest. The Tard Clay records a five million year long anoxic cycle initiated by isolation of the sea, development of a positive water balance and resulting density stratification, and terminated by the re-opening of access to the sea. The lower, non- and moderately laminated shales record the build-up of anoxia, they contain 0.5–5.0% of organic carbon and Type III kerogen. Their saturated hydrocarbon fractions show high abundance of phyllocladanes, biomarkers characterising Coniferae-derived organic matter. The overlying moderately and strongly laminated sediments containing 1.5–5.0% organic carbon, a predominance of hopanes over the n-alkanes and Type II kerogen show the imprint of increased planktonic productivity. The nutrients needed for this high productivity were liberated by alteration processes of contemporaneous volcanic glass. The upper part of the strongly laminated shales containing a maximum of 2% organic carbon and Type III kerogen, deposited after the termination of volcanic ash falls, records a drop of planktonic productivity. At the top of the Tard Clay the weak lamination and the low content (<1%) of organic carbon signifies the end of the bottom water oxygen depletion due to the destruction of density stratification of the water column.

The Cenomanian-Turonian Oceanic Anoxic Event, II. Palaeoceanographic controls on organic-matter production and preservation

Summary Correlation of the δ 13 C spike with the well dated occurrences of strata rich in organic carbon detailed in Schlanger et al. (this volume), indicates that a global episode of intense organic carbon ( org C) burial took place during the latest Cenomanian-earliest Turonian ‘Oceanic Anoxic Event’ (OAE) ( A. plenus through I. labiatus macrofossil zones and upper R. cushmani TRZ through W. archecretacea PRZ foraminiferal zones) over a period of no more than 1 million years (m.y.). The shape of the δ 13 C curve indicates that rates of org C burial gradually increased in the early part of the late Cenomanian, increased more rapidly in the later Cenomanian, and levelled off at peak values in latest Cenomanian-early Turonian time during the maximum rate of org C burial. The δ 13 C values decreased nearly to pre-late Cenomanian levels in the early to middle Turonian. The decrease in δ 13 C reflects decreasing rates of org C burial following the Cenomanian-Turonian ‘oceanic anoxic event’ as well as the probable oxidation and return of significant amounts of org C to the oceans following regression and re-oxygenation of much of the deeper water masses in contact with the seafloor. The Cenomanian-Turonian OAE coincided with a maximum sea level highstand. We suggest that sea level, which may be responding to some volcano-tectonic event, is the common link and ultimately the driving force for org C deposition in globally distributed basins under different climatic and ocean circulation regimes. The rate of production of warm, saline deep water may have been proportional to the area of shelf flooding such that the maximum occurred near the Cenomanian-Turonian boundary. As rates of deep-water formation increased, rates of upwelling of deeper oceanic water masses must also have increased thereby increasing sea-surface fertility and productivity. In somewhat restricted higher latitude basins, such as the Cretaceous Interior Seaway of North America, periodic high rates of freshwater runoff coupled with deepening seas during the transgression created periodic salinity stratification, oxygen depletion in bottom waters, and resultant enhanced org C preservation. The disappearance of some types of keeled planktonic formainifers and ammonites at the Cenomanian-Turonian boundary is probably due to the rather sudden but short-term disappearance of suitable shallow midwater habitats because of widespread severe oxygen depletion in these levels. This interpretation is strengthened by the occurrence of benthic-free zones or depauperate benthic faunas near the Cenomanian-Turonian boundary in many localities.

Foraminiferal stratigraphy and environment of Late Weichselian deposits on the continental shelf off Troms, northern Norway

Based on the analysis of 8 selected gravity cores from the upper sediment layers of Andfjorden and Malangsdjupet off northern Norway six Late Weichselian foraminiferal assemblage-zones are defined. The two oldest assemblage zones (8 and 7) are found in basal tills and contain mixed faunas, reworked and deposited by glaciers which reached the shelf edge during the early Late Weichselian. During the earliest phase of the deglaciation of the continental shelf, a sparse fauna characterized by Buccella frigida (zone 6) was deposited. Zone 5 is dominated by Cassidulina laevigata and a fairly high content of planktic foraminifera indicating periods of ice-free normal-marine surface waters. During a glacial advance between 14,000 and 13,000 B.P. the fauna (zone 4) was dominated by Elphidium excavatum. This fauna reflects near-glacial conditions characterized by unstable bottom water conditions. The last c. 500 years of this period suffered from oxygen-depleted bottom waters. Zone 3 formed between 13,000 and 10,000 B.P. comprises several species, e.g. Cassidulina reniforme, Nonion barleeanum, N. labradoricum and Cibicides lobatulus. During the first 2000 years of this period the fauna reflects a climatic amelioration, e.g. an increase in temperature, salinity and current activity, probably due to intrusion of Atlantic water. During the Younger Dryas, zone 3 reflects a climatic deterioration.

Zones of coastal hypoxia revealed by satellite scanning have implications for strategic fishing

Hypoxic (oxygen concentrations <2.5mg l−1) and occasionally anoxic bottom waters occur along the inner continental shelf of Texas and Louisiana1–4. Little is known, however, about the spatial and temporal scales of these hypoxic areas. Boesch5 concludes the nearshore continental shelf west of the Mississippi River Delta has characteristics potentially favourable for large-scale bottom water oxygen depletion. These characteristics include influences from river and marsh discharges of freshwaters, rich in nutrients and sediments, The river effluent flows over the higher salinity shelf waters, creating two-layer vertical density stratification. When surface waters are heated during the summer, vertical stability increases, especially during calm wind periods, and vertical eddy diffusion is limited. Large-scale phytoplankton blooms stimulated by nutrients from the freshwater runoff can then produce large amounts of organic matter which sink and, through respiration and decomposition, deplete oxygen concentrations in the bottom layer. The hypoxia is normally limited to several metres of the water column next to the bottom6 in water depths usua <50m. Because hypoxia appears to be related to surface chlorophyll and temperature, which can both be measured with the Coastal Zone Color Scanner (CZCS) on board the Nimbus 7 spacecraft7,8, we have tried to determine if conditions favourable for formation of hypoxia could be detected and monitored from space. A linear discriminant function successfully identified areas of bottom water hypoxia detected by research vessels up to 10 days after satellite overpass. The discriminant function also successfully predicted hypoxic areas in June 1983 without resort to research vessel data. Shrimp and finfish were absent in the hypoxic zones; hence, the mapping from space of conditions favourable for hypoxia development may have significant marine resource implications for both strategic fishing and management.

Influence of Paleoenvironmental Factors on Preservation of Organic Matter in Middle Cretaceous Greenhorn Formation, Pueblo, Colorado

Sedimentological and geochemical studies of the Upper Cretaceous Hartland Shale and overlying Bridge Creek Limestone Members of the Greenhorn Formation reveal close associations between abundance of current-induced sedimentary structures, extent of bioturbation, and type of preserved organic matter. Abundant hydrogen-rich organic matter (4-5 wt. %) and low sulfur to organic carbon ratios are characteristic of the laminated facies, which lack current-induced sedimentary structures. Sparse hydrogen-poor organic matter (0.1-0.5 wt. %) and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies, which contain numerous sedimentary structures indicative of currents. The concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively (by currents) rather than by the rate of oxygen consumption (by decomposition). Thus, paleoclimatic and paleo-oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units. It is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen-depleted bottom water and high preservation of organic matter, thus accounting for the intermittent widespread deposition of organic-rich strata during maximum transgression and depth of the Greenhorn sea. The lateral association of voluminous deltaic deposits with dee -water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the Hartland Shale and Bridge Creek Limestone Members.

Sedimentation of organic-rich limestones in the late Precambrian of Southern Norway

Abstract Organic-rich laminated limestones are a major facies of the Late Precambrian Biri Formation, deposited around the margins of the Sparagmite basin of southern Norway. The laminated limestones consist of an alternation of calcitic laminae and clay laminae. Trace element and stable isotope analyses confirm a marine origin for the carbonate and suggest that the original precipitate was aragonite. The organic matter reaches 3% (weight loss on ignition) and apparently is finely dispersed within the sediment. Both cm-thick limestone beds of probable storm origin, and thin, disrupted and folded horizons of slump origin occur within the laminated facies. The organic-rich laminated limestones occur at several horizons and are interbedded with carbonate grainstones of shallow-water origin. Basinwards, they pass into light to dark grey to green, organic-deficient mudrocks with resedimented and slumped units. The facies context of the organic-rich laminated limestones suggests deposition in an outer shelf setting at times of higher sea-level stands. The transgressive events, possibly induced by changes in glacial ice cover of near or distant landmasses, led to increased organic productivity through seasonal blooms of phytoplankton. Preservation of the organic matter in the sediments resulted from the development of anoxic or at least oxygen-depleted bottom waters. Some physical constriction of water circulation in the Sparagmite basin may have contributed towards the anoxic events. The development of Precambrian organic-rich ‘black shales’ in the Sparagmite basin of southern Norway does show that potential hydrocarbon source rocks were formed in the pre-Phanerozoic, in situations comparable to some Mesozoic and Tertiary oil source rocks.

Aspects of the physics, chemistry, and phytoplankton biology of Lake Taupo

Lake Taupo was sampled monthly from August 1974 to March 1976 at three sites where the depth was about 100 m. The lake stratifies in summer and mixes freely in winter, when water temperatures drop to 10–11°C. Oxygen depletion in bottom waters towards the end of the stratification period amounts to about 4 g.m‐3. Water transparency, as measured by Secchi disc, ranges from 11 m to 21 m. The pH is typically between 7 and 8; extreme values are 6.8 and 8.5. Major ion concentrations total about 2.3 eq.m‐3. Anion concentrations follow the pattern HCO3 ‐> Cl‐> SO4 2‐, and cations Na+> Ca2+> Mg2+>K+. The major ions varied little seasonally, spatially, or with depth, except bicarbonate, for which bottom concentrations were 1–2 g.m‐3 higher than at the surface. Reactive silicon concentrations were 9.2–10.3 g.m‐3; there was no apparent relationship with diatom populations. Total dissolved iron concentrations ranged from 2 mg.m‐3 to 8 mg.m‐3, and manganous‐manganese from 5 mg.m‐3 to 10 mg.m‐3, with no discernible patt...