Sediments from the northern end of Juan de Fuca Ridge are LateQuaternary in age and contain widely correlatable cycles of turbidity current and hemipelagic sedimentation. Sediments from the Ridge were examined for their mineralogy, structure, components of the sand fraction, rates of sedimentation and grain size distribution to establish processes of sedimentation, stratigraphy, correlation and local hydrothermal relationships. Ten gravity and Phleger core sites along two profiles of the Ridge were examined in detail, one section was perpendicular to West Valley, the main spreading centre, and one section was within and parallel to West Valley. Sediment from Cascadia Basin was compared to the results of the Ridge study. Changes in sedimentation defined by core X-radiograph structure, components of the sand fraction and grain size distribution, indicated cycles of relatively coarse sediment overlain by finer bioturbated sediment with a repeated stratigraphic relationship in all but one Juan de Fuca Ridge core. Changes in sediment composition are attributed to brief, episodic, continent derived turbidity current deposition followed by lengthy periods of hemipelagic sedimentation for each cycle. Differences in composition exist between sediment of ridges and valleys, with a greater winnowed foraminiferal-hemipelagic and a lesser turbidity current influence in the former area. Radiocarbon dated foraminiferal-rich intervals from ridge sediments were exclusively Late Pleistocene with Middle Ridge sediment having an inferred 9000-9500 B.P. Late Pleistocene-Holocene boundary. Similar sedimentation cycles between Middle Ridge and valley localities enabled correlation of ridge and valley stratigraphy and the Late Pleistocene-Holocene boundary. A stratigraphic relationship based on the episodic deposition of continent derived turbidites exists between the northern end of Juan de Fuca Ridge and the continental Pacific Northwest. Pulses of turbidity current sedimentation coincide with initial interglacial warming trends during the Late Pleistocene. Holocene sedimentation for Juan de Fuca Ridge is of hemipelagic origin with rare local turbidity current deposition. Hydrothermal minerals were not detected.
Contributors:Ivanochko, Tara S.
Short-term fluctuations in the bottom water oxygen content of Santa Barbara Basin have been previously recognized from variations in a sedimentary bioturbation index (Behl and Kennett, 1996). A correlation between such anoxic events in the basin and Dansgaard-Oeschger interstadials, as measured from δ180ice in Greenland ice cores, was then used by the same authors to relate variations in bottom water oxygenation to high-frequency changes in the ventilation of the Santa Barbara Basin, driven by pan-hemispheric changes in climate transmitted through the atmosphere. An additional control on the oxygen content at depth is the local settling flux of metabolizable organic matter. Trace metal measurements from closely-spaced sediment samples are used here to distinguish oxygen depletion resulting from local increases in export production from oxygen depletion introduced by the importation of O2-poor water. Molybdenum, Re, U, Cd, and Ag and interelement ratios are used in conjunction with organic carbon concentrations, opal fluxes, and δ15N measurements to deduce into past redox conditions of the basin, variations in the depth of the redox boundary and the flux of organic material to the basin floor. Comparisons between ODP Holes 893A (Santa Barbara Basin), 1019 and 1017 (both California margin) allows one to distinguish of regional signals dominated by ventilation changes from local signals dominated by vertical organic flux. During the Holocene, variations in productivity appear indeed to have impacted the oxygen content of the Santa Barbara Basin bottom waters. However, anoxic events concurrent with the Bolling-Allerod and during the last glacial interval are regional events associated with intermediate water mass characteristics.
Contributors:Ganeshram, Raja S.
Glacial-interglacial variability in upwelling on the NW Mexican margin is assessed by reconstructing the history of organic carbon and biogenic opal deposition and measuring the Ba/Al ratio in three piston cores that span the upper to the lower continental slope. Rates of accumulation of organic carbon, opal and to some degree biogenic barite are higher in interglacial intervals, indicating that upwelling-induced productivity was higher during the warm periods over the last 140,000 years. Despite cyclic changes in organic carbon accumulation, matrix-corrected HI values in the mid- and lower- slope cores are invariant and are similar to values in the laminated intervals from the oxygen-minimum site. This suggests that changes in organic carbon content are controlled by productivity variations and are not due to differential preservation induced by variations in bottom water oxygen concentrations. The lowest HI values in Mexican Margin sediments occur concurrently with large increases in grain size. Thus, increased degradation resulting from winnowing is offered as the leading explanation for the hydrocarbon impoverishments in the bioturbated upper slope deposits. LateQuaternary records of denitrification in the oxygen-deficient subsurface water masses of the Eastern Tropical North Pacific (ETNP) are constructed using ¹⁵N/¹⁴N ratios measured on bulk sediments. The profiles show a synchronous decrease in denitrification during the glacial periods over the last 140 kyrs. It is suggested that, because nitrate is a limiting nutrient in the modern ocean, a consequent increase in the oceanic nitrate inventory could have contributed to the observed decrease in glacial atmospheric pCO₂ by enhancing the fertility of the ocean. The glacial decreases in denitrification in the ETNP are attributed to large reductions in upwelling-induced fluxes of organic detritus on the margin in response to glacial shifts in the wind field off NW Mexico associated with the growth of Laurentide ice on northern North America, the establishment of a resident high pressure cell over the ice sheet, and the bifurcation of the Jet Stream.
Contributors:Schiefer, Erik Karl
Lacustrine sediments have provided important information on historical sediment transfer processes and past environmental conditions for lake catchment systems over a wide range of temporal scales. In this study, contemporary patterns of lacustrine sedimentation are described and interpreted for Green Lake, a geomorphically active and morphologically complex lake catchment system in the southern Coast Mountains of British Columbia. Emphasis is placed on detailing spatial variability and characterizing stratigraphic signatures associated with major sediment delivery events. Identified trends in the sedimentary record are related to internal and external lake catchment process domains and physical controls. The research program has included a combination of intensive lake sedimentcoring, laboratory-based sediment analysis, detailed reconstructions of lacustrine sediment deposition, and case studies of the most significant sediment delivery events of the last 70 years. All bulk physical properties investigated exhibit a dominant monotonic pattern of variation from the principal lake inflow to more distal lake settings. An influence of water depth is observed only in shallow, near-shore environments. Highest variability occurs near lake inflows, in shallow water settings, and at sites disturbed by land use activities. There is an overall non-linear decrease in sedimentation rates with increasing distance from the lake inflows; however, this pattern is disrupted in deep water sites of intervening lake sub-basins where locally higher accumulation rates are recorded. This trend of increased sedimentation with water depth is most pronounced in the proximal basin and becomes less significant in more distal sub-basin settings. These relations are quantitatively described by an empirically derived sedimentation model. Discernible patterns of spatial variability exceed resolved temporal variability by an order of magnitude. Major sediment delivery events of the last 70 years, discriminated at intra-annual to annual scales, include rapid glacial recession of the early 20lh century, extreme late-summer and autumn rainstorm floods, and unusual snowmelt conditions. Some years of anomalous sedimentation can be related to the occurrence of multiple sediment delivery events and other associated geomorphic processes, including Quaternary valley fill landslides and major channel destabilization effects. Stratigraphic characteristics and spatial sedimentation patterns vary between different types of moderate and extreme sediment delivery events in relation to the defined average-regime deposition model. Total sediment yield is calculated to average 205 Mg/km²/yr, with suspended load, bedload, and dissolved load components accounting for similar proportions of the total yield when integrated over the contemporary period. At inter-annual time scales, temporal variability primarily reflects sediment delivery processes associated with rapid glacial recession observed during the 1930's and early 1940's and extreme autumn rainstorm effects occurring in the early 1990's. Sediment yield responses between events and transfer components differ considerably with respect to magnitude, lag time, and duration.