Biogenic Productivity Research Articles

Late quaternary changes in Antarctic bottom water velocity inferred from sediment grain size in the northern Weddell Sea

Newly formed bottom water ( θ ≤ −0.7°C) in the northern Weddell Sea flows E or NE at speeds up to 10–15 cm/s, with velocity decreasing towards the centre of the Weddell Gyre (preliminary results from long-term current meter moorings). Upper Quaternary sediments from this area contain a fine-grained terrigenous component (from the nepheloid layer) plus biogenic silica (mostly diatoms) with a small amount of ice-rafted debris. In cores from between 61° and 66°S and from 3300 m to 4700 m water-depth, the proportion of biogenic silica increases northwards (corresponding to increasing seasonal extent of open water vs sea-ice cover), and the proportion of silt and well-sorted fine sand in surface sediments increases with average current velocity. Downcore, diatom-rich and diatom-poor sediments alternate on a scale of 1–3 m, and intervals with more diatoms contain a higher proportion of silt to clay. Preliminary stratigraphy suggests the cyclicity in composition and texture is related to glacial-interglacial cyclicity. During warm periods (indirectly correlated with isotope stages 1, 3, 5 etc.) biogenic silica production takes place during several months of each year, and silt and fine sand are transported by bottom currents. During glacial periods with greater sea-ice cover than at present, biogenic productivity was suppressed and no silica was preserved in the sediments: in addition, a lower proportion of terrigenous silt implies that bottom currents were weaker. At sites with a present-day average velocity of 10 cm/s, a Last Glacial Maximum average velocity of 1 cm/s or less is inferred from grain-size measurements.

Biogeography of planktonic foraminifera and regional carbon isotope variations: productivity and water masses in late Cretaceous Europe

The first appearances of species of Marginotruncana (planktonic foraminifera) are found stratigraphically earlier in Boreal northwestern Germany than in the Tethys. Helvetoglobotruncana helvetica (Bolli) is rare in northwestern Germany and common in the Tethys. Its stratigraphic range is probably comparable between both areas. The biogeographic patterns are discussed with respect to influences of water depths (based on isotope evidence for the depth distribution of species of Hedbergella, Praeglobotruncana, and Rotalipora) and water temperatures. The regional variation of δ 13C values of Cenomanian-Turonian pelagic carbonates was mapped for the European epicontinental basins and the southern European shelf (Helvetic zone of the Alps). The highest δ 13C values (up to 5‰) were found in northwestern Germany and the eastern North Sea and are coincident with an upwelling zone numerically predicted by three independent climate models. Intermediate values in France and England (2–4‰) correspond to predicted weak upwelling. Lowest values occur in the Alpine Helvetic zone and the south German Regensburg area (1–2‰) and coincide with predicted absence of upwelling. The δ 13C values are interpreted as indicators of increased biogenic productivity in the upwelling zone. Semiquantitative nannofossil data suggest abundance patterns influenced by latitude, neritic-oceanic gradients, and productivity. Observations agree with east-to-west wind-driven surface-current systems. Regional differences in nutrient supply and, subsequently, different primary productivity could have been a major factor controlling the evolution and the biogeographic distribution of planktonic foraminifera in Cenomanian-Turonian Europe.

Characteristics of manganese nodules from the Central Indian Basin: Relationship with the sedimentary environment

The hydrogenetic and diagenetic manganese nodules from the CIB show an unequivocal genetic relationship with Neogene sedimentological patterns. Hydrogenetic nodules were observed at the top of the sedimentary column and interstratified in thin sequences of biosiliceous sediments and underlying Miocene(?) red clays. In cores from topographic highs, Early to Late Pliocene radiolarian fauna were found at the base of the biosiliceous deposits. These condensed biosiliceous sediments are less than 4.0 m thick, and periods of non deposition and/or erosion may have occurred. In the central deep area (11–15° S), significant biogenic deposition began during the Early to Late Pliocene as a result of the development of a high biogenic productivity belt to the south, producing the favourable conditions for diagenetic accretionary processes for Mn nodules. This indicates growth of hydrogenetic nodules prior to the Late Pliocene and the subsequent inception of diagenetic accretion. Formation of the most recent diagenetic nodules is associated with higher rates of sedimentation in the deep-sea troughs, where the siliceous sediments are much thicker. This should promote the diagenetic mobilization of Mn and its precipitation under oxic and suboxic conditions. Nevertheless, diagenetic nodules are restricted to the upper 40 cm of sediments younger than 400,000 years, suggesting fast-growing diagenetic nodules and the dissolution of buried nodules below the first 50 cm of siliceous oozes. Hydrogenetic nodules are formed by dense layers of δMnO 2Fe(OH) 3 and amorphous silica, and they show smooth texture and botryoidal shapes. Analysis of bulk and selected portions of layered crust or rind shows very low Mn Fe ratios (< 2) and relatively high Co and Ti contents. In most cases, the cores of hydrogenetic nodules are fragments of altered ash layers from the Indonesian Volcanic Arc, which is located more than 2500 km to the east. The ash layers are also interstratified with Miocene(?) red clays. Diagenetic nodules show a granular texture and comprise concentric layers with dendritic or columnar structures, mainly of 10 Å manganate. Higher Mn, Ni and Cu contents are observed in this phase. Late diagenetic enrichment of these nodules is continuous within the upper 40 cm of Recent (< 0.4 m.y.) biosiliceous sediments, which favours (a) the replacement of the silicate phases (biogenic silica, clay minerals and volcanic glass) by Fe-Mn oxides, (b) a metallic uptake (Cu > Ni) that enhances the stability of early diagenetic 10 Å manganate (buserite), and (c) the formation of todorokite. Some nodules, referred to as mixed in the literature, show hydrogenetic internal layers which grade outward to a diagenetic crust. This transitional character suggests slow changes in the sedimentary environment as growth advances. The nodules of the CIB and the East Equatorial Pacific (EEP) are similar in terms of both metallic composition and accretionary patterns. However, the nodules of the CIB are considerably younger and present a continuous accretionary history. The history of the manganese nodules from the CIB is related to the post-Late Miocene sedimentary and geodynamic evolution of this basin.

Structure and seismic stratigraphy of the western Challenger Plateau

Abstract The Challenger Basin is a newly discovered sedimentary basin beneath the western margin of the Challenger Plateau. Seismic and gravity data near the site of two Deep Sea Drilling Project holes (DSDP 284 and 593) show more than 2 km of sediment in the basin, probably formed in the early stages of Late Cretaceous rifting in the Tasman Sea. Thinning of about 5 km occurred before separation took place to the southwest and stranded this portion of the rift system. Regional subsidence ensued in the early Cenozoic, followed by a major phase of submarine basaltic volcanism about 38 Ma ago. This volcanism may have been related to the onset of spreading southwest of New Zealand. Subsequent sedimentation has been virtually entirely pelagic. The Oligocene unconformity commonly observed in the New Zealand region was not found in the wells, presumably because of either oceanographic changes induced by tectonic events, or redeposition from adjacent highs. Changes in ocean current patterns and biogenic productivity caused by paleogeographic changes such as the separation of Australia and Antarctica and uplift of the Southern Alps produced unconformities identifiable on seismic reflection data.

Geochemical signature of the Cenomanian to Eocene rocks in Israel ‐a palaeoenvironmental indicator

AbstractDistinctive trace‐metal concentrations characterize Cenomanian to Eocene marine carbonates from Israel. The Cenomanian‐Turonian platform carbonates, including clayey formations, exhibit low average values ranging between 2 and 29 ppm for Zn, Cr, V, Ni, Cu, U and Co. The Santonian‐Campanian and Early to Middle Eocene marine chalks show higher average concentrations of these trace‐metals ranging between 3 and 56 ppm. The highest average concentrations of these metals (5–118 ppm) are found in the Maastrichtian and in the Palaeocene marine chalks and marls. The possible relationship between these metal background levels and the lithology, the biogenic productivity, the organic matter content, the iron oxide concentration, the rate of sedimentation of the studied time‐rock units as well as the palaeogeographical changes are discussed. The extent of the exposed palaeo‐landmasses due to tectonics, the intensity of weathering conditions and the detritus supply into the basin, control primarily the iron and trace‐metal content in the studied sediments.

Postglacial paleoceanography of Hudson Bay: stratigraphic, microfaunal, and palynological evidence

Cores containing representative sequences of postglacial sediments in northern and southern Hudson Bay were analyzed for their microfaunal (foraminifers and ostracods) and palynological (dinocysts, pollen, and spores) content in order to reconstruct the evolution of environments since the last glaciation.In southern Hudson Bay, the marine invasion of the Tyrrell Sea at ca. 8000 BP, following the Lake Ojibway episode, was accompanied by the development of an Arctic-type microflora and microfauna indicative of a dense seasonal sea-ice cover and stratified water masses. Shortly after 8000 BP, the establishment of subarctic conditions in surface waters was accompanied by more intense homogenization of water masses. Subarctic conditions have persisted throughout most of the postglacial interval despite a recent surface-water cooling.In northern Hudson Bay, micropaleontological and lithological data reveal a succession of proximal to distal glaciomarine environments characterized by low biogenic productivity, harsh Arctic conditions, and stratified water masses. An increase in dinocyst abundance and diversity, after 6000 BP, indicates the establishment of cool subarctic conditions in surface waters, while foraminifer assemblages suggest intensified mixing of water masses.The micropaleontological records of northern and southern Hudson Bay reveal a strong latitudinal gradient in biogenic productivity and water mass characteristics throughout the postglacial interval. "Interglacial" conditions, established in southern Hudson Bay very shortly after it was invaded by the sea, seem to have occurred much later in northern Hudson Bay.

Potential Consequences Of “Dirty” Arctic Sea Ice

Observations of high particulate loads on Eurasian Basin sea ice in 1987 raise questions of consequence for sediment budgets, ice melting, ice modeling and remote sensing. Biogenic and lithogenic particles were observed in concentrations high enough to color the ice surface brown over large area (greater than 15 × 15 km2) within the Siberian branch of the Transpolar Drift stream. The sediment is most likely incorporated when ice forms on the Siberian shelf seas, and is concentrated at the ice surface after several years of summer surface melting and biological growth within the Arctic basin. Much of the particle-laden multi-year ice appears to leave the Arctic basin via Fram Strait, depositing its sediment load along the axis of the East Greenland Current.To date, variation in sea-ice particle load has not been taken into consideration when modeling ice thickness or distribution for past or future environmental scenarios, with the exception of soot deposited from nuclear war. Naturally elevated surface-particle concentration may occur if there is increased deposition from long-range or coastal transport of aeolian material, increased sediment input into sea ice which is then exposed to surface melting, and/or increased biogenic productivity on the ice surface. Such conditions may have prevailed during the Younger Dryas. If particle loads become high enough to cause extensive sea-ice melting, changes may be expected in sea-ice concentration and distribution, sea-floor sedimentation rates, and oceanic productivity.

Potential Consequences Of “Dirty” Arctic Sea Ice

Observations of high particulate loads on Eurasian Basin sea ice in 1987 raise questions of consequence for sediment budgets, ice melting, ice modeling and remote sensing. Biogenic and lithogenic particles were observed in concentrations high enough to color the ice surface brown over large area (greater than 15 × 15 km2) within the Siberian branch of the Transpolar Drift stream. The sediment is most likely incorporated when ice forms on the Siberian shelf seas, and is concentrated at the ice surface after several years of summer surface melting and biological growth within the Arctic basin. Much of the particle-laden multi-year ice appears to leave the Arctic basin via Fram Strait, depositing its sediment load along the axis of the East Greenland Current. To date, variation in sea-ice particle load has not been taken into consideration when modeling ice thickness or distribution for past or future environmental scenarios, with the exception of soot deposited from nuclear war. Naturally elevated surface-particle concentration may occur if there is increased deposition from long-range or coastal transport of aeolian material, increased sediment input into sea ice which is then exposed to surface melting, and/or increased biogenic productivity on the ice surface. Such conditions may have prevailed during the Younger Dryas. If particle loads become high enough to cause extensive sea-ice melting, changes may be expected in sea-ice concentration and distribution, sea-floor sedimentation rates, and oceanic productivity.

The sedimentology of three Tunisian lagoons

There are three major lagoons indenting the shoreline of Tunisia. The Lake of Bizerte on the north coast of Tunisia is in the Mediterranean climatic setting. The Lake of Tunis, 50 km to the south, is in a semi-arid climatic setting. Bahiret el Bibane, 450 km south of the Lake of Bizerte near the Libyan border, is in an arid climatic setting. The summer salinities of the three lagoons reflect the climatic differences. Man has had an impact on the two northern lagoons. The Lake of Bizerte outlet to the Mediterranean has been deepened by dredging. The Lake of Tunis has been altered through infilling by man (an ongoing process), construction of a diked ship channel, and introduction of large amounts of raw sewage. Bahiret el Bibane is relatively unaffected by the activities of man. The sediments in the Lake of Bizerte are dominated by mud-size clastics from intermittent streams draining farmland; in addition, there is an important biogenic component. The most abundant sediment in the Lake of Tunis is an organic-rich terrigenous mud derived from the adjacent drainage basin, but biogenic productivity is high and forms a locally dominant sediment component. Calcareous worm reefs flourish because of the high nutrient content. Summer eutrophication is frequent. The sediments of the semi-restricted Bahiret el Bibane are dominated by biogenic fragments (mostly mollusks) but with an important wind-transported detrital component. The north to south changes in the character of the clastic sediments of the Lake of Bizerte and Bahiret el Bibane are mainly related to the source of the sediment which is largely controlled by climate. The influence of man dominates sedimentation in the Lake of Tunis.

Stability of an artificially nourished beach, Balaena Bay, Wellington Harbour, New Zealand

Abstract Balaena Bay, Wellington Harbour, New Zealand, has a small pocket beach that was covered originally by pebbles and cobbles. In February and October 1982, the beach was nourished with sandy granular gravel, the stability of which was monitored until February 1984. Although isolated from oceanic swell, the new beach readily responded to locally generated wind waves which induced both northwards and southwards longshore drift. The net effect was erosion of the southern beach, aggradation over the central beach, and minor fluctuations at the northern end. Yet despite this mobility nearly all the nourishment sediment was retained in the littoral zone. Beach volumes, calculated for each survey, varied little and sediment distribution patterns revealed negligable transport of nourishment sediment to adjacent beaches and offshore areas. Stability is further confirmed by compositional data which record no preferential loss of the sandstone, argillite, and quartz components. The only compositional changes were the incorporation into the new beach of small (< 10%) quantities of sediment derived from the old beach surface and from biogenic productivity.

Paleomagnetism, Paleolatitudes, and Magnetic Overprinting on the North Slope, Alaska: ABSTRACT

Several paleomagnetic studies have been made in Arctic Alaska, by industry, by the U.S. Geological Survey, and by the University of Alaska. In general, the results available to the public have been disappointing--most samples of pre-Cretaceous rocks give very steep magnetic inclinations with respect to present horizontal. This has been generally interpreted in terms of a Cretaceous overprinting event. A study of the paleomagnetism of Cretaceous rocks from the North Slope shows that although the Cretaceous field was steeply inclined, it was not as steep as conventional paleogeographic reconstructions would indicate, and not as steep as the bulk of the apparently remagnetized older rocks. This finding leaves open the possibility that the steeper directions recorded by the older rocks are the result of regional tilt, or the result of a paleogeography that allowed an earlier, steeper remagnetizing field. The shallower inclinations seen in the Cretaceous sediments of the Nanushuk Group (Albian-Cenomanian based on the fossil record with one K-Ar age of 100 Ma from an ash parting) give paleolatitudes of about 75°N. The predicted paleolatitude based on North American paleogeographic reconstructions is 80-85°N. Circumstantial evidence that the paleolatitude was shallower than 80-85°N comes from the enormous biogenic productivity needed to form the extensive coal deposits of the Nanushuk Group. Lower paleolatitudes also may be needed to explain the apparent existence of broad-leaved evergreens and the recently reported dinosaur tracks and skin imprints in the Nanushuk Group. End_of_Article - Last_Page 680------------

Paleo‐oceanography: Global ocean evolution

Paleoceanography, one of the youngest branches of earth sciences, is concerned with the development of oceanic circulation through geologic time. This is a multifaceted and integrated endeavor since none of the subdisciplines is easily separated from the others. The field is holistic in character, and includes the history and effects of bottom, intermediate and surface circulation patterns, planktonic and benthonic biogeographic development and rates of organic evolution, the history of biogenic productivity and dissolution and their effect on sediment distribution and the consequences of sea level change.Paleoceanography was largely born of the Deep Sea Drilling Project (DSDP) and continues to be nourished by it. Because problems are of global scope, the appetite for new sediment core material is large, and is being supplied by numerous expeditions of DSDP. This article is a brief overview of a number of advances that have occurred in paleoceanography and related fields during the period from 1979 to 1982. Because of page limitations, all publications cannot be cited which is not meant to indicate any lack of importance.

Late Quaternary marine stratigraphy southeast of New Zealand

Research Article| June 01, 1983 Late Quaternary marine stratigraphy southeast of New Zealand GARY B. GRIGGS; GARY B. GRIGGS 1Department of Earth Sciences and Center for Coastal Marine Studies, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar LIONEL CARTER; LIONEL CARTER 2New Zealand Oceanographic Institute, P.O. Box 12-346, Wellington, New Zealand Search for other works by this author on: GSW Google Scholar JAMES P. KENNETT; JAMES P. KENNETT 3Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02881 Search for other works by this author on: GSW Google Scholar ROBERT V. CARTER ROBERT V. CARTER 4Department of Geology, James Cook University, Queensland 4811, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information GARY B. GRIGGS 1Department of Earth Sciences and Center for Coastal Marine Studies, University of California, Santa Cruz, California 95064 LIONEL CARTER 2New Zealand Oceanographic Institute, P.O. Box 12-346, Wellington, New Zealand JAMES P. KENNETT 3Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02881 ROBERT V. CARTER 4Department of Geology, James Cook University, Queensland 4811, Australia Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1983) 94 (6): 791–797. https://doi.org/10.1130/0016-7606(1983)94<791:LQMSSO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation GARY B. GRIGGS, LIONEL CARTER, JAMES P. KENNETT, ROBERT V. CARTER; Late Quaternary marine stratigraphy southeast of New Zealand. GSA Bulletin 1983;; 94 (6): 791–797. doi: https://doi.org/10.1130/0016-7606(1983)94<791:LQMSSO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Sediment cores of late Quaternary age from the continental margin and deep sea (Bounty Trough) southeast of New Zealand reveal an alternating sequence of glacial and interglacial sediments. During glacial episodes of lowered sea level, glaciation in the Southern Alps was at its peak, and east coast rivers delivered enormous volumes of terrigenous sediments to the shelf edge. At these times, sediments of the deep adjacent basins were dominated by micaceous hemipelagic deposits with a low biogenic component consisting mostly of siliceous remains (radiolaria and sponge spicules). Planktonic foraminifera, although present, were much reduced in abundance because of increased dissolution, as in deeper water subantarctic cores farther to the south. Cool-water forms dominated.During interglacial episodes, terrigenous sediment supply to the deep basins was reduced in response to higher sea levels and as glaciers retreated and deposited much of their loads in newly formed glacial lakes and on the plains. Terrigenous sediment dilution of the biogenic portion was thus much reduced. Calcium carbonate dissolution also was reduced. These processes, in combination, led to the deposition of foraminifera-rich hemipelagic sediments. Siliceous biogenic productivity decreased.Thus, the late Quaternary marine sediment record in the area adjacent to southeast New Zealand is dominated by paleoclimatic influences that control terrigenous input, calcium carbonate dissolution, biogenic productivity, and the migration of planktonic assemblages. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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.

Shales--Their Sedimentology and Geology: ABSTRACT

Shale and mud form at least 60% of the world's sediments and have been deposited throughout geologic history. They occur in every major depositional basin. They are major source beds for hydrocarbons, hosts for metallic minerals, sources of ceramic materials, cause unstable foundation conditions, and produce soils for our food. Yet the study of shales has lagged far behind that of other sedimentary rocks. However, the following general observations can be made about these important rocks. More studies have been made of recent muds than of ancient shales. Clay mineralogy, geochemistry, and paleontology of muds and shales are better understood than their stratigraphy, petrology, and sedimentology. Most shales occur in marine sequences and are associated with deltaic depocenters. They are more commonly geosynclinal than cratonic, and are most commonly deposited in deep water as distal turbidites and pelagic muds. Cratonic shales occur as widespread, thin sheets distal from their source land. Sedimentology Most muds are transported in suspension and are commonly deposited as aggregates of floccule or fecal pellet origin. Floccule formation is sensitive to water chemistry--salinity, organic compounds, and turbulence. Mud aggregates are commonly deposited under higher hydraulic energy conditions than previously assumed for muds. Size analyses for mud and shales are generally useless for interpretation of depositional hydraulics. Vertical variations in bedding properties, texture, color, organic content, mineral composition, fossil content, and bioturbation are the most useful parameters for the deduction of depositional environments of shales and muds. These, integrated with basin geometry, hold the most promise for future studies. Stratigraphy Shales almost always have an internal stratigraphy that is well-expressed on wire-line geophysical well logs. Thin beds of high or low density, different lithologies and organic content, and fossil zones or concretions within shale sequences are commonly widespread and are good markers for internal stratigraphy. Most shales have a clinoform internal stratigraphy that can be related to basin geometry and can be observed in seismic profiles. Diagenesis Muds compact more than other sediments and hence expel more fluids. Pore water chemistry, mineralogy, thickness, and density all change as pore water is expelled. Water expulsion concentrates hydrocarbons and metals in interbedded porous reservoirs. Seismic response, heat flow, and our perception of original shape of shale bodies all change with burial compaction. Tectonics The concept of lutokenesis--mud makes its own tectonics--applies. Where deposition has been rapid, buried shales are overpressured and undercompacted. Undercompacted shales can form diapiric structures. Shales thin over rising structures and thicken into synclines and subsiding basins. In large-scale overthrusts, bedding-plane faults form fold trains; shear and flow occur mostly in shale units. Source Beds Shales are major source beds of hydrocarbons. Total organic content is sensitive to the original circulation in the basin, biogenic productivity, and influx of fine terrigenous and carbonate muds--a dilution factor. Preservation of organic matter is best in muddy basins because density stratification inhibits vertical mixing and oxidation--rapid burial inhibits biodegradation. Best indicators of thermal history of shaly basins are kerogen, conodonts, vitrain, and clay minerals. Relating Sedimentology of Shales to Resources Establish internal stratigraphy and facies distribution. Relate every resource variable to internal stratigraphy, and make an isopach map and relate facies distribution of basin. For gas, relate primary and secondary porosity (fracturing and permeability) to maturation, facies, and internal stratigraphy. For oil, correlate kerogen content to internal stratigraphy and maturation. For uranium, correlate kerogen to internal stratigraphy. End_of_Article - Last_Page 1682------------

Controls on Diatomaceous Lithofacies in Obliquely Rifted Marginal Basin: Gulf of California: ABSTRACT

DSDP Leg 64 dissected Quaternary sedimentation patterns in the Guaymas Basin which confirm many similarities but underline some differences with other Neogene circum-Pacific diatomite basins. Tectonic setting in this morphologically complex basin includes broad hemipelagic slopes, fault-controlled outer slope basins and highs, and relatively small transform-bounded, obliquely rifted deeper basins with complex ocean crust. Frequent mass flows are triggered from either muddy delta foreslopes or hemipelagic diatom ooze drape. These accumulate as mud turbidites in the narrow rift zones at rates exceeding 2,000 m/m.y. Interaction of climatic and oceanographic parameters control the intensity of biogenic productivity (ergo, the oxygen budget) producing alternating sequences of aminated and homogenous diatomaceous ooze, generally confined to slope regions (400 m/m.y.). Laminated diatom-ooze also accumulated in deeper basins which were deprived of turbidity flows during limited periods. Sediments in slope areas contain a uniform 4% carbon but CaCO3 (mostly foraminifera) ranges episodically from 2 to 25%. Phosphate occurs as fish-debris-rich laminae or rare, soft, centimeter-size pellets. Diagenetic dissolution of silica is recorded at Site 479 on the slope where finely laminated hard muds occurring below an unconformity at 380 m subbottom are now devoid of frustules, except those cemented in dolomite beds. Paradoxically, porcellanites were not encountered, although traces of clinoptilolite suggest that some silica reactions are presently active. Chert only occurs in proximity to basaltic intrusions. Dolomite precipitation occurs at shallow subbottom depths in zones of high alkalinity and methanogenesis, gradually forming decimeter-thick hard layers by slow vertical accretion. These layers commonly preserve primary fabrics. Petrologic and heavy carbon isotope evidence suggest that ions for dolomite precipitation are mainly derived from interstitial waters. End_of_Article - Last_Page 945------------

Paleoceanographic and biogeographic evolution of the Southern Ocean during the Cenozoic, and Cenozoic microfossil datums

The paleoclimatic history of Antarctica is intimately linked with the evolution of the Circum-Antarctic paleocirculation system and hence with Cenozoic plate tectonism. The development of the Circum-Antarctic circulation system occurred as southern land masses moved away, creating unrestricted latitudinal flow. Changing boundary conditions in this region included the opening of the Australian—Antarctic seaway as Australia moved northwards, the opening of Drake Passage and the development of the Kerguelen Plateau. The formation, and later development, of the Circum-Antarctic Current thermally isolated Antarctica by decoupling the warmer subtropical gyres from the colder Subantarctic—Antarctic gyres. The climatic history of the Southern Ocean is, in general, one of decreasing temperatures, including the development of increased antarctic glaciation and later ice sheet formation, a climatic regime which itself had a profound effect on environmental evolution, and thus the biogeography at high southern latitudes. Environmental characteristics resulting from the development of this climatic regime included extensive seasonal sea-ice production, the cooling of waters surrounding the continent, and meteorologically forced upwelling of nutrient-rich intermediate waters which increase biogenic productivity in the Southern Ocean. The thermal barriers in high southern latitudes represented by the Antarctic Convergence and the Subtropical Convergence also became major biogeographic barriers strongly affecting the distribution of planktonic organisms. The development of the Circum-Antarctic Current also removed biogeographic provinciality previously occurring within different sectors of the Southern Ocean as a result of previous land barriers. As a result, cool-water planktonic assemblages of the Southern Hemisphere show little interoceanic difference. During the Cenozoic, major changes have occurred in planktonic microfossil biogeography in the Southern Ocean as reflected in the biostratigraphic sequences. Datum levels representing the first or last appearances of microfossil species (planktonic foraminifera, calcareous nannofossils, radiolaria, diatoms and silicoflagellates) have been compiled for Antarctic and Subantarctic sections. This compilation provides a series of integrated datums and hence a historical sequence of microfossil events through the Cenozoic. The datum levels of Quaternary and Pliocene age, their nature, relative position and age are now well known mainly because of integration with magnetostratigraphy because of superior sediment records. In contrast, the relative position of Miocene and older datum levels is much less firmly established. There are several reasons for this, including poorer stratigraphic control because of a lack of magnetostratigraphic framework; few section that contain siliceous and calcareous microfossil assemblages in association to establish detailed relation; and poor stratigraphic representation especially in the Early Cenozoic. In general, most datum levels do not extend over both the Antarctic and Subantarctic regions. Instead, microfossil events, if they occur in both regions, can be highly diachronous between the two water-mass regimes. This is most strongly developed in the calcareous Cenozoic microfossil groups which, as in the present-day, show major differences across the water-mass boundaries.

The development of planktonic biogeography in the Southern Ocean during the Cenozoic

The development of planktonic biogeography in the Southern Ocean during the Cenozoic

Cenozoic evolution of Antarctic glaciation, the circum-Antarctic Ocean, and their impact on global paleoceanography

Deep-sea drilling in the Antarctic region (Deep-Sea Drilling Project legs 28, 29, 35, and 36) has provided many new data about the development of circum-Antarctic circulation and the closely related glacial evolution of Antarctica. The Antarctic continent has been in a high-latitude position since the middle to late Mesozoic. Glaciation commenced much later, in the middle Tertiary, demonstrating that near-polar position is not sufficient for glacial development. Instead, continental glaciation developed as the present-day Southern Ocean circulation system became established when obstructing land masses moved aside. During the Paleocene (t = ∼65 to 55 m.y. ago), Australia and Antarctica were joined. In the early Eocene (t = ∼55 m.y. ago), Australia began to drift northward from Antarctica, forming an ocean, although circum-Antarctic flow was blocked by the continental South Tasman Rise and Tasmania. During the Eocene (t = 55 to 38 m.y. ago) the Southern Ocean was relatively warm and the continent largely nonglaciated. Cool temperate vegetation existed in some regions. By the late Eocene (t = ∼39 m.y. ago) a shallow water connection had developed between the southern Indian and Pacific oceans over the South Tasman Rise. The first major climatic-glacial threshold was crossed 38 m.y. ago near the Eocene-Oligocene boundary, when substantial Antarctic sea ice began to form. This resulted in a rapid temperature drop in bottom waters of about 5°C and a major crisis in deep-sea faunas. Thermohaline oceanic circulation was initiated at this time much like that of the present day. The resulting change in climatic regime increased bottom water activity over wide areas of the deep ocean basins, creating much sediment erosion, especially in western parts of oceans. A major (∼2000 m) and apparently rapid deepening also occurred in the calcium carbonate compensation depth (CCD). This climatic threshold was crossed as a result of the gradual isolation of Antarctica from Australia and perhaps the opening of the Drake Passage. During the Oligocene (t = 38 to 22 m.y. ago), widespread glaciation probably occurred throughout Antarctica, although no ice cap existed. By the middle to late Oligocene (t = ∼30 to 25 m.y. ago), deep-seated circum-Antarctic flow had developed south of the South Tasman Rise, as this had separated sufficiently from Victoria Land, Antarctica. Major reorganization resulted in southern hemisphere deep-sea sediment distribution patterns. The next principal climatic threshold was crossed during the middle Miocene (t = 14 to 11 m.y. ago) when the Antarctic ice cap formed. This occurred at about the time of closure of the Australian-Indonesian deep-sea passage. During the early Miocene, calcareous biogenic sediments began to be displaced northward by siliceous biogenic sediments with higher rates of sedimentation reflecting the beginning of circulation related to the development of the Antarctic Convergence. Since the middle Miocene the East Antarctic ice cap has remained a semipermanent feature exhibiting some changes in volume. The most important of these occurred during the latest Miocene (t = ∼5 m.y. ago) when ice volumes increased beyond those of the present day. This event was related to global climatic cooling, a rapid northward movement of about 300 km of the Antarctic Convergence, and a eustatic sea level drop that may have been partly responsible for the isolation of the Mediterranean basin. Northern hemisphere ice sheet development began about 2.5–3 m.y. ago, representing the next major global climatic threshold, and was followed by the well-known major oscillations in northern ice sheets. In the Southern Ocean the Quaternary marks a peak in activity of oceanic circulation as reflected by widespread deep-sea erosion, very high biogenic productivity at the Antarctic Convergence and resulting high rates of biogenic sedimentation, and maximum northward distribution of ice-rafted debris.

Suspended particulate loads and transports in the nepheloid layer of the abyssal Atlantic Ocean

Vertical profiles of light scattering from over 1000 L-DGO nephelometer stations in the Atlantic Ocean have been used to calculate mass concentrations of suspended particles based on a calibration from the western North American Basin. From these data are plotted the distributions of particulate concentrations at clear water and in the more turbid near-bottom water. Clear water is the broad minimum in concentration and light scattering that occurs at varying mid-depths in the water column. Concentrations at clear water are as much as one-to-two orders of magnitude lower than those in surface water but still reflect a similar geographic distribution: relatively higher concentrations at ocean margins, especially underneath upwelling areas, and the lowest concentrations underneath central gyre areas. These distributions within the clear water reflect surface-water biogenic productivity, lateral injection of particles from shelf areas and surface circulation patterns and require that the combination of downward vertical and horizontal transport processes of particles retain this pattern throughout the upper water column. Below clear water, the distribution of standing crops of suspended particulate concentrations in the lower water column are presented. The integration of mass of all particles per unit area (gross particulate standing crop) reflects a relative distribution similar to that at the surface and at clear water levels, superimposed on which is the strong imprint of boundary currents along the western margins of the Atlantic. Reducing the gross particulate standing crop by the integral of the concentration of clear water yields a net particulate standing crop. The distribution of this reflects primarily the interaction of circulating abyssal waters with the ocean bottom, i.e. a strong nepheloid layer which is coincident with western boundary currents and which diminishes in intensity equatorward. The resuspended particulate loads in the nepheloid layer of the basins west of the Mid-Atlantic Ridge, resulting from interaction of abyssal currents with the bottom, range from ∼ 2 · 10 6 tons in the equatorial Guyana Basin to ∼ 50 · 10 6 tons in the North American Basin. The total resuspended particulate load in the western basins (111 · 10 6 tons) is almost an order of magnitude greater than that in the basins east of the Mid-Atlantic Ridge (13 · 10 6 tons). The net northward flux of resuspended particles carried in the AABW drops from ∼ 8 · 10 6 tons/year between the southern and northern ends of the Brazil Basin and remains ∼ 1 · 10 6 tons/year across the Guyana Basin.