R-3 Repository :: Browsing by Author "Dunbar, Robert B."

Browsing by Author "Dunbar, Robert B."

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A study in the hydrodynamics of sediment transport
(1983) Singer, Jill Karen; Anderson, John B.; Casey, Richard E.; Dunbar, Robert B.
Antarctica is a unique environment where sediments are influenced by the interaction of glaciers and marine currents. This results in little or no sorting of the sediment prior to its introduction into the marine environment. The existing data relating current velocity to grain size were not designed to apply to assess the ability of marine currents to winnow and sort texturally homogeneous glacial and glacial marine sediments. However, estimates from flume studies suggest currents with velocities greater than 4 cm/sec are required to erode consolidated sediments. A puzzling observation made in studying modern Antarctic sediments has been the degree to which compacted and cohesive glacial and glacial marine sediments are being reworked and re-sedimented by bottom currents which move at velocities less than 15 cm/sec. This study has focused on the role of biological mixing in the initiation of sediment erosion and entrainment at low current velocities. This more adequately reflects the conditions which occur today on high latitude seafloors. It was found that with simulated bioturbation of the bed material, particles in the silt and clay-size range were transported at current velocities less than 3 cm/sec, with sand-sized material removed from the bed by 15 cm/sec. Without simulated bioturbation, currents up to 2 cm/sec had a negligable effect on the bed. Important applications of the results from this study include understanding sediment dynamics. It does not appear likely that erosion and sorting of overcompacted and cohesive glacial and glacial marine sediments is possible without the action of bioturbation. The results from this flume study are useful in the indirect determination of the long term normalized current velocity using surface samples and physical oceanographic information. Results can also be applied in the estimation of paleocurrent velocity from downcore variations of textural data.
Carbon-13/carbon-12 ratios in surface water and sea ice organic matter-variability and contributions to the sediments of the Ross Sea, Antarctica
(1995) Rogers, Jennifer Carol; Dunbar, Robert B.
Elemental and carbon isotopic analyses of suspended and sea ice particulate organic matter (POC) from Antarctica show enrichments in $\sp{13}$C unrelated to atmospheric CO$\sb2$ variations. Phytoplankton bloom-related POC from the southwestern Ross Sea during 1992 had a range in $\delta\sp{13}$C of 7$\rm\perthous.\ \delta\sp{13}C\sb{POC}$ collected from pack ice had an 11$\rm\perthous$ range, with highest values found in the Bellingshausen and Amundsen Seas and lower values found in Ross Sea pack ice. Organic matter $\delta\sp{13}$C from phytoplankton blooms in the southwestern Ross Sea is strongly correlated with POC concentration, suggesting high rates of bloom-related CO$\sb2$(aqueous) drawdown that may cause disequilibrium as a result of stable stratification of the upper water column and partial isolation of the surface water CO$\sb2$ pool. Isotopic and organic carbon composition of surface sediments is similar to the upper water column, thus primary isotopic signals are preserved. Surface sediment $\delta\sp{13}$C values are 1 to 3$\rm\perthous$ enriched in $\sp{13}$C relative to surface water POC $\delta\sp{13}$C. Highest isotopic values were found in areas that receive large amounts of phytoplankton bloom detritus. Future downcore carbon isotopic analyses may prove useful as tracers of bloom dynamics and sea ice production.
Marine geology of the northwestern Weddell Sea and adjacent coastal fjords and bays: implications for glacial history
(1985) Smith, Michael John; Anderson, John B.; Bally, Albert W.; Dunbar, Robert B.
Sedimentologic, compositional, and cleat shape analyses reveal that piston cores obtained on the northwest Weddell See continental shelf penetrated residual glacial marine sediments adjacent to Seymour Island, compound glacial marine sediments north of 64
Oxygen isotope analysis of corals from the Gulf of California and Gulf of Panama: Application and implications for coral-based paleoclimate reconstruction
(1995) Jones, William Albert, Jr; Dunbar, Robert B.
Oxygen isotope analyses of corals from the Gulf of California and Gulf of Panama are calibrated with environmental parameters. Uncertainty in the isotopic composition of sea water in the Gulf of California prohibits strong correlation between temperature and $\delta\sp{18}$O. The marginal condition for coral growth in the Gulf of California prevents the construction of a long paleoclimate record. The Gulf of Panama record is strongly correlated with salinity (r$\sp2$ = 0.72) and indicates a trend toward drier conditions in Panama since 1950. Although El Nino is recorded in both records through a decrease in the annual maximum isotopic value, this signature is not unique to El Nino thereby minimizing the ability of corals to monitor this system. Strong coherence between the Gulf of Panama record and three other Panamanian records attests to the ability of corals to record regional climate variability.
Productivity, preservation, and cyclic sedimentation within the Mowry Shale depositional sequence, Lower Cretaceous, western interior seaway
(1989) Miskell-Gerhardt, Kimberlee J.; Dunbar, Robert B.
The late Albian Mowry Shale of Wyoming and Montana, and the time equivalent Big River Formation of Saskatchewan, were studied to determine the cause of anoxia within the Mowry Seaway and the presence or absence of small scale (fourth order) cyclicity. The Mowry Shale is an important source rock, characterized by high total organic carbon content (TOC) (0.5-5.0%), abundant fish debris, numerous bentonites, and prolific radiolarians. Minimal bioturbation at distal sites from Wyoming to Canada indicates that the preservational quality of bottom water with respect to organic matter was high throughout the basinal area of the seaway. TOC values were highest at the southern end however, coincident with a high in radiolarian abundance. This correlation suggests an added input of planktonic organic carbon at the southern end of the seaway, due to higher levels of paleoproductivity. Therefore, anoxia within the Mowry Seaway may have resulted from two processes, stagnation in Canada, and elevated planktonic fertility in Wyoming and Montana. Seasonal upwelling has been predicted along the SE margin of the Albian seaway from independent climate models. Burial rates of Si and P in the Mowry Shale indicate paleoproductivity levels comparable to lower values reported from modern coastal upwelling zones. This fertility could have been supported entirely by nutrients derived from a (depleted) seawater source, even with limited renewal rates. Six locations in N. Wyoming were tied into an onshore - offshore resistivity well log transect. TOC, phosphorous content, and lithologic profiles from each section could then be correlated using numbered bentonite markers. Vertical sedimentation trends were divisible into three intervals. Within the context of sequence stratigraphy, these units may be interpreted as the Transgressive Systems Tract (decreasing sand, increasing TOC), the Condensed Interval (minimum sand, peak TOC), and the Highstand Systems Tract (increasing sand, decreasing TOC). Higher order cyclicity is present in the Mowry Shale, as 1-10m coarsening-up cycles near the western margin, and 5-30m fertility cycles in the basin. Correlation of marginal to basinal cycles indicate that during shallowing-up, decreases in TOC on the shelf coincide with minor increases in TOC in the basin.
Radiolarian responses to the 1957-1958 and 1964 El Ninos and 1963 anti-El Nino and a search for similar events in the fossil record
(1985) Weinheimer, Amy; Casey, Richard E.; Anderson, John B.; Dunbar, Robert B.
Radiolarian distributions and physical oceanographic data from the southern California borderlands indicate the following: Strong anti-El Nino periods can be characterized by 1) average radiolarian standing crop, 2) high percentage of transition-central radiolarian fauna, and 3) low percentage and number of warm water radiolarian fauna. This distribution pattern is attributed to strong wind-driven upwelling and reduced northward transport by the California Countercurrent during anti-El Nino periods. Strong El Nino periods were typically 1) high in radiolarian standing crop, 2) low percentage and high number of cold water fauna, and 3) high percentage and number of warm water fauna. This distribution is attributed to reduced wind-driven upwelling, enhanced northward countercurrent transport, and geostrophic doming of the cold water masses in the shear zone between the California Current and California Countercurrent. Preliminary work on a varved sample from the Monterey Formation, Lompoc, California indicates that variations of radiolarian populations appear to represent changes in oceanic circulation similar to those recorded in the Santa Barbara Basin varved sediments.
Radiolarian responses to the 1982-83 California El Nino and their implications
(1985) Carson, Thomas; Casey, Richard E.; Clark, Howard C.; Dunbar, Robert B.
Radiolarian responses to the 1982-83 California El Nino event were unusually high standing crops of both warm and cold water radiolarians, high diversity, and the appearance of distinctly central gyre and eastern tropical Pacific faunas. These and other observations suggest that anomalously strong transport from the west and south occurred through at least November 1953 and that the California Current had shifted eastward over the Southern California Eorderland. A generic group of radiolarians, the spongasterids, was used effectively in monitoring these water mass influences. Radiolarian distributions in 3.-3.5 Ma circuoi-northeastern Pacific sediments indicate a warm North Facific at this time. Marked breaks in distributional trends off southern Baja California suggest an oceanic frontal zone in this area, To the south radiolarian assemblages exhibit a distinctly eastern tropical Pacific character. Spongaster pentas occurs as far north as Deep Sea Prilling Site 173. This suggests periodic breakdowns of this frontal zone (El Ninos?) may have occurred.
Recent biogenic sedimentation on the Antarctic continental margin
(1988) Leventer, Amy Ruth; Dunbar, Robert B.
Floral and geochemical analyses were conducted on sinking and suspended particulate matter, and on sea ice and recent sediment samples from the northwestern Weddell Sea--northern Antarctic Peninsula area, and McMurdo Sound, in the southwestern Ross Sea. Data from McMurdo Sound reveal that although large numbers of diatom frustules dissolve within the upper water column the decrease in silica mass flux is much smaller. At mid-water depths, increases in the absolute flux of Nitzschia curta and Thalassiosira spp. indicate the influence of lateral advection. Within a near-bottom nepheloid layer additional dissolution occurs. Dilution and preferential dissolution is responsible for production of a surface sediment assemblage dominated by Thalassiosira spp. and Nitzschia curta. The surface sediment diatom assemblage in McMurdo Sound documents the seasonal history of sea ice extent and regional circulation patterns. Thalassiosira spp., indicative of water column primary productivity, is most common in eastern and northwestern McMurdo Sound, the result of advection from areas of open water. Nitzschia curta, a member of the sea ice microbial community and of ice edge blooms in the Ross Sea is most abundant in the southwestern Sound where the northward advection of oligotrophic water results in the dominance of the local flora. Maximum downcore Thalassiosira abundances occurred between 1600-1875 A.D., during the Little Ice Age. Warmer atmospheric temperatures or more persistent winds may have been responsible for more prevalent polynyas, suggesting that within the southwestern Ross Sea, the production of High Salinity Shelf Water, and hence Antarctic Bottom Water, may have been greater at that time. Measurements of water and acid soluble and insoluble phosphate performed on samples of suspended particulate matter from the Antarctic Peninsula region show the concentration of water-soluble, loosely bound organic phosphates decreases rapidly within the upper 100 m, the result of rapid phosphorus regeneration. A gradual decrease in insoluble organic phosphate signifies slower recycling of cell wall material. Acid-soluble phosphatic skeletal parts are transported efficiently. Concentration of insoluble phosphate increases in an eastern and southern direction as a function of increased influence of old Warm Deep Water containing higher levels of refractory organic material.
Sedimentation on the Weddell Sea continental margin and Abyssal Plain, Antarctica
(1983) Fisco, Mary Pamela P.; Anderson, John B.; Casey, Richard E.; Dunbar, Robert B.
Abyssal sediments collected in piston cores were examined to determine sediment distribution patterns and active sedimentary processes in the Weddell Sea. Pelagic, hemipelagic, debris flow, and turbidite deposits were identified and distinguished by their individual characteristics of grain-size, sorting, texture, bedding, and sediment source. Sediment distribution patterns show a large turbidite fan complex extending across the northeastern portion of the basin. Recent turbidite deposition in the Weddell Sea has occurred under polar glacial conditions. This has produced sequences distinctly different from those occurring in temperate glacial environments, as in the northern high latitudes. Polar turbidites consist of massive, thick-bedded gravels, sands, silts, and muds, which are significantly depleted in silt. Temperate turbidites are rich in silt, and typically consist of interlaminated, graded, silts and muds. Similarities exist between Weddell Basin and Bellingshausen Basin seismic data. These data suggest that turbidite deposits constitute most of the sediment fill in these basins
Sedimentologic and paleoceanographic implications of terrigenous deposits on the Maurice Ewing Bank, southwest Atlantic Ocean
(1984) MacDonald, Scott Edward; Anderson, John B.; Casey, Richard E.; Dunbar, Robert B.
The Maurice Ewing Bank (MEB) provides a unique environment in which to study deep-sea clastic sedimentation as the observed sedimentary sequences are not readily explained by traditional depositional models. Grain size distributions of sediment samples are utilized to identify styles of sediment transport and current velocities at the benthic boundary layer. These data, along with physical oceanographic information, allow for examination of the relationship between sedimentation on the bank and circumpolar currents. Causal relationships between glacial episodes, current intensification, and bottom current scour were previously postulated. Results indicate that modern sediment distribution is controlled by both bottom and surface currents, and is modified by mass-flow processes. Indirect determinations of current velocities, using a curve relating sediment transport to current velocity, suggest a uniform energy regime of 8-12 cm/sec over the bank. These moderate velocities are supported by geostrophic velocity profiles. The results of this study also demonstrate that misinterpretations regarding current velocities may result when statistical parameters, such as mean grain size and sorting, are used as indicators of fluctuations in bottom current velocities. Similar conclusions can be drawn concerning the use of bottom photographs and nephelometer profiles to infer the relative strength of bottom energy regimes. Current velocities assigned to down-core samples indicate no major fluctuations in circumpolar current intensity since PIio-Pleistocene time. Similar results were obtained from examination of Argentine Basin cores. These results refute the previously inferred causal relationship between current intensification and glacial episodes. The identification of mass-flow deposits provides an alternate explanation for the depositional and erosional history of the MEB.
Sedimentological and geochemical study of the late Eocene to early Oligocene Yumaque formation, east Pisco Basin, Peru
(1993) Frantz, Erika Lee; Dunbar, Robert B.
Yumaque formation biosiliceous sediments resemble modern and Neogene sediments deposited under coastal upwelling conditions. High biogenic silica accumulation rates (up to 69 g$\cdot$cm$\sp{-2}\cdot$yr$\sp{-3}$), preservation of fine sedimentary features, and high original organic carbon, resulted from favorable basin geometries and upwelling of nutrient-rich water. Light biosiliceous-rich and dark detrital-rich alternations occur at scales ranging from millimeters to meters. Mm-scale laminae couplets are interpreted as varves. Spectral analysis of laminae thickness using a varve-calibrated time scale reveals significant variance at periods of 5-6 and 8 years, within the El Nino (ENSO) frequency band, and at 11 years, possibly linked to the 10-12 year solar cycle. Centimeter to meter-scale cyclicity representing between a few hundred to a few thousand years correlates to solar and geomagnetic phenomenon. The Yumaque formation and similar biosiliceous deposits along the eastern Pacific margin may be indicative of widespread late Eocene biosiliceous sedimentation around the Pacific.
Sedimentology of the North Victoria land continental margin, Antartica
(1983) Brake, Christopher F.; Anderson, John B.; Casey, Richard E.; Dunbar, Robert B.
Sediments and sedimentary processes in Lake Houston, Texas
(1984) Matty, Jane Miller; Anderson, John B.; Dunbar, Robert B.; Bedient, Philip B.
Lake Houston is a man-made reservoir located northeast of Houston, Texas. The purpose of this investigation was to determine the nature of sediments and sedimentary processes in the lake. Cores were collected throughout the lake and sediment traps placed in strategic locations to collect suspended sediments. Samples were analyzed for grain-size, organic carbon, and a number of trace elements. The volume of the lake has been reduced by 7.8% during its first 28.5 years, with most of the sediment accumulating near the mouths of inflowing rivers. Sediment input depends primarily on the intensity of rainfall in the watershed. Sediment movement within the lake is strongly influenced by wave activity which resuspends sediments from shallow areas. The increased residence time due to resuspension allows greater decomposition of organic matter and the release of several trace elements. The principal source of both organic material and trace elements appears to be effluent from sewage treatment plants. Fluctuations in current velocities and the subsequent suspension/deposition of particles may explain variations in coliform bacteria, which occasionally pollute Lake Houston.
Stratigraphy and chemical sedimentology of Cenozoic biogenic sediments from the Pisco and Sechura Basins, Peru
(1989) Marty, Richard Charles; Dunbar, Robert B.
Neogene sediments rich in siliceous microfossils, phosphate, and organic carbon formed simultaneously in many basins around the Pacific. This work focuses on these (and similar older) sediments from two basins of western South America (East Pisco and Sechura Basins) and studies the effects of tectonism, climate, and oceanic circulation on their formation. Transgressions inundated the East Pisco Basin during the late Eocene, late early Miocene, and late Miocene. Diatomaceous sediments formed during each transgression. Late Eocene diatomaceous sediments suggest that: upwelling off Peru dates from the late Eocene, a proto-Humboldt current existed during the Eocene, Antarctic cooling began before the latest Eocene, and the terminal Eocene event was the culmination of oceanic-climatic change. The late Eocene diatomites differ from overlying late Miocene sediments. Eocene diatomites are restricted to the western (offshore) margin of the basin, are separated from paralic sediments by a mud blanket, and were deposited well below wave base. Upper Miocene-Pliocene diatomites occur throughout the basin, grade into paralic deposits, and were deposited, in part, above storm wave base. The Sechura Basin experienced four transgressions between the Eocene and late Miocene. These transgressions formed four sedimentary sequences. Diatomaceous sediments are found in each sequence except the second (upper Oligocene-lower Miocene). In the third sequence (lower Zapallal Formation) diatomite-phosphorite became important between 14 and 8.1 Ma, and apparently reflects increased biogenic and decreased (?) terrigenous sedimentation rates. The increased biogenic accumulation rate reflects increased primary productivity or preservation. Sediments of the third sequence are separated from the fourth sequence by an angular unconformity (which correlates with subduction of the Nazca Ridge under the basin). Sediments of the fourth sequence differ from those of the third, and apparently record cooling and strengthened currents during the latest Miocene.
Tertiary geology of the Quebrada Huaricangana area, East Pisco Basin, southern Peru: Late Paleogene to Neogene transgressive sedimentation within a forearc basin
(1990) Stock, Carrie Elaine; Dunbar, Robert B.
Clastic packages studied in the southern East Pisco Basin represent the transgressive Late Eocene to earliest Oligocene Paracas Group, with questionable presence of the latest Oligocene to Early Miocene Chilcatay formation. Syndepositional faulting and rugged topography protected Late Eocene sediment from direct wave attack. Fluvial systems were poorly developed in this arid environment, where sediment gravity flows provided poorly sorted sediment to fan deltas in proximal and inner shelf areas. Shelf facies contain evidence of storm-dominated sand transport and variable rates of bioturbation and suspended sediment input. In conjunction with compositional increases in igneous derived sediment, the preservation of wave induced structures increased between Paleogene and Neogene time. Factors associated with these changes include higher rates of transgression and lower sediment supply during the Late Eocene, less protected depositional areas, arid climates, and a dramatic increase in the supply of sand-sized volcanic sediment in Neogene time.
The use of vertical grain size progressions in establishing the depositional environment of ancient sand bodies
(1982) Wolfteich, Carl Martin; Anderson, John B.; Dunbar, Robert B.; Casey, Richard E.
This study demonstrates the validity of using vertical grain size progressions as an environmental indicator for ancient clastic sequences. Previous methods of grain size analysis are shown to be generally unreliable for environmental determinations. Grain size data is interpreted here in terms of idealized sequences for different sedimentary environments. The environments examined in this study include braided river-alluvial fans, meandering rivers and coastal barriers. Braided river systems are characterized by the random variability of grain Meandering size parameters within a vertical section. rivers and coastal barriers are each characterized by a distinct vertical sequence of sub-facies. These stratigraphic sequences are reflected in the vertical grain size progressions that characterize each of these environments. Vertical progressions in ancient sequences correlate well with lateral progressions in modern analagous environments. With improved coring technology, this method of grain size analysis can be used to establish the depositional environment of subsurface sand bodies. Vertical grain size progressions may prove to be an important exploratory tool for the petroleum industry as well as a reliable grain size method for the field geologist.
Turbidite deposition on the Bellingshausen Abyssal Plain: sedimentologic implications
(1985) Baegi, Mohamed Bashir; Anderson, John B.; Casey, Richard E.; Dunbar, Robert B.
A number of piston cores from the Bellingshausen abyssal plain contain gravel and sand units which are interpreted as turbidites deposited in submarine channels. Middle and outer fan deposits are lacking, although core coverage in the region is sufficient

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