PP43B-01
Mid to Late Holocene Sediment Flux in annually laminated ocean sediments in Saanich Inlet, BC using X-ray Fluorescence and Scanning Electron Microscope
We present high-resolution geochemical data at 200 micrometer using ITRAX core scanning X-ray fluorescence, and physical properties obtained by scanning electron microscope images of the sediment thin sections in order to reveal the depositional history during the Holocene. The core MD02-2490 has been taken from Saanich Inlet, BC, a silled fjord on southeastern Vancouver Island, as part of the IMAGES VIII (International Marine Past Global Changes Study) program. Annual laminations in Saanich Inlet are mainly couplets made of a biogenic layer and a terrigenous layer. Lamina thickness measured from 2,000 to 7,000 yr BP reveals an abrupt increase in biogenic input around 6,500 yr BP, while the annual accumulation rate is likely to be constant. During this period, the relative abundances of major and minor elements also change significantly. The spectral analysis performed on the thickness of both biogenic and terrigenous layers shows that there are strong 2-3 year and 16 year cycles around 2,000 years BP, although 2-3 year and 64 year cycles becomes stronger around 5,000 yr BP. We argue that these changes are due to changes in the climate patterns during the Holocene because they easily affect the seasonal variability of the water-column stratification at the site. We developed a high-resolution age model based on varve counts and previous work made on this core and the nearby ODP169S hole 1033 core. The differences between modeled ages and varve years range from 7 to 20 % of the varve years. Our high-resolution age-model is then used to verify if the terrigenous sediment delivery was continuous or episodic during the Holocene, and to assess the impact of abrupt climate change on depositional history in Saanich Inlet.
PP43B-02
Extreme Erosional Events Recorded by Flood Deposits in the Late Quaternary Santa Barbara Basin, Coastal Southern California
Terrestrial runoff from major storm and flood events in southern California is captured in the Santa Barbara Basin (SBB) as distinct gray layers of well-sorted clay to silt that punctuate typical olive hemipelagic mud. Thirty-two partially overlapping piston cores recovered from a breached anticline provide ultra-high resolution windows into an approx. 700,000-year climate record. We document the magnitude, frequency, mass accumulation rate, grain size and mineralogy of these deposits to gain insight into climatically related variations and trends. In addition to investigating chronological trends, we also compare glacial, interglacial and intermediate climate regimes. Bulk and clay mineralogy (determined by X-ray diffraction) of the gray layers has remained remarkably constant throughout the entire record indicating a consistent source for terrigenous input despite fluctuating sea level. Relative abundance of smectite:illite may reflect changes in weathering, vegetation and/or humidity. Mean grain size shows little variation (10.43-14.01 micrometers) through time, with no apparent relationship to the thickness of event deposits. Therefore, gray layer thickness likely reflects the intensity and transport efficiency of the erosional event rather than a change in proximity or energy of depositional currents. Gray layers often occur in amalgamated beds or clusters, possibly representing pulses of runoff during storm fronts or reworking of major deposits over extended time. Bulk mass accumulation rates (MARs) were calculated for each core using varve-counted time and bulk density. MARs show little variation (0.06-0.12 gcm-2yr-1), lacking a definite climatic trend. Average MARs are slightly higher during interglacial (0.097 gcm-2yr-1) than glacial (0.089gcm-2yr-1) intervals and show significant variation within similar climatic regimes. Where not bioturbated, gray layers account for a significant percentage of overall sedimentation (8-17%). Mass percent gray is slightly greater (11.4%) during interglacial intervals than during glacial (10.5%) or intermediate (8.5%) climatic intervals. Gray-layer contribution to overall sedimentation peaked near the OIS 8/9 transition (16%) and the Holocene (17%). Small events (<0.3 gcm-2) are by far the most frequent. Based on comparison of frequency and magnitude of historic precipitation events, the majority of events we measured occurs less frequently and match or exceed the magnitude of moderate to strong El Niño events. Large events (>0.6 gcm-2) are most frequent during the intermediate climate regimes (avg. 470 years) and least frequent during glacial MIS 8 (approx. 1000 yrs) indicating either smaller storms during this glacial or more vegetation to restrict erosion. Glacial MIS 12, MIS 6 and the Holocene have the greatest frequency of large events (approx. 250 yr). Large events occur every 450-550 years during interglacials and 500-650 years during intermediate intervals. The gray layers recorded in these cores are up to ten times larger by mass than the largest historic flood deposit in the SBB. The SBB provides a record of major precipitation and erosional events, changes in local climate and an opportunity to link coastal southern California climate to global and nonmarine climate archives.
PP43B-03
Last Century Patterns of Sea Surface Temperature and Diatom Variability in the Southern California Current
A time series reconstruction for the last 100 years of the variability of diatoms from laminated sediments off the southern Baja California margin shows the sensitivity of this phytoplankton group to oceanographic variability in the California Current on different timescales. Here we show that cool (warm) years are associated with higher (lower) diatom accumulation values, a proxy for their export production from the water column to the seafloor. There is a clear direction in the changes of the larger opaline flora community structure throughout the past century that parallels the observed trend towards higher SSTs in the southern part of the California Current. The diatom succession is reflected in three types of assemblages that show a clear progression from cool spring to early summer bloom species in the earlier part of the century towards alternating cool California Current and warm subtropical waters during the mid-century, to subtropical and tropical assemblages reflecting more stratified oligotrophic conditions during the latter part of the last century. This association between opaline floral succession and the SST anomaly trend implies a long term change that cannot be explained by the same mechanisms that are commonly associated with the typical interannual or interdecadal oceanographic variability. A small group of diatom species appears to follow the periodic swings of the large scale Pacific Decadal Oscilation index. The Coscinodiscus argus/radiatus and Coscinodiscus decrescens/marginatus complexes, commonly found in the California Current waters, closely follow the inter-decadal variability patterns over the Pacific, although with a decreasing amplitude after the mid 1970s shift.
PP43B-04
Hypoxic cyclicity in sediments of Soledad Basin, Baja Mexico: A record of high-frequency climate fluctuations?
The sedimentary record in Soledad Basin, 45 km west of Baja, Mexico, shows high-frequency oscillations in hypoxia, which can be linked to fluctuations in climate. Soledad Basin, a semi-enclosed basin with a sill depth of 290m, has been shown to exhibit variable levels of hypoxia throughout the geologic past. Located at the intersection of the California Current and California Undercurrent, Soledad Basin is highly responsive to changes in current strength and upwelling, the combination of which creates fluctuations in hypoxia. During climatic cool periods, the California Current is weakened decreasing upwelling and biologic productivity along the Baja Borderland. This causes increased hypoxia in Soledad Basin. The California Undercurrent is also weakened during these cooler periods and brings less nutrients and oxygen to the basin further increasing hypoxia. Since Soledad Basin sediments are undisturbed and have accumulated rapidly, this is a prime location to study high frequency variations in hypoxia in the sedimentary record. The objective of this study was to examine how and to what extent hypoxic events have been recorded in the sedimentary record of Soledad Basin, and gain insight into what controls these events. Surface sediment samples and a single 1.1m gravity core were collected aboard the S.S.V. Robert C. Seamans on a SEA Semester cruise in October 2005. The core was taken at a depth of 490 m near the deepest point of the basin. The core contained laminated sediments consisting of >95% mud. Using 210Pb analysis, a sedimentation rate of 15 cm over the past 100 years was determined, which is consistent with previous research. Trace metal analyses were performed at the cm-scale on selected intervals between 0.34-0.44m and 0.78-0.92m. These intervals correspond to dark organic-rich (>15% organic content) laminations alternating with lighter layers containing less organic material (<15% organic content). All sediments were found to be enriched in Molybdenum, Rhenium, Vanadium, Cadmium, and Uranium, all of which are indicative of hypoxic conditions. The highest concentrations of these trace metals correlate with the darker, organic-rich laminations, suggesting there was less oxygen present during deposition of these sediments. The frequency and intensity of these hypoxic events may be ultimately controlled by climatic cooler periods such as ENSO events, the Pacific Decadal Oscillation and the Little Ice Age. Correlations exist between the darker laminations representing higher levels of hypoxia to past ENSO events and the Little Ice Age. Higher concentrations of trace metals in surficial sediments indicate recently increasing levels of hypoxia. Also, laminations are more pronounced in these surficial layers, which may indicate that climatic fluctuations are becoming more frequent and intense.
PP43B-05
Composition and Origin of Non-Annual Laminae From the Gulf of California: Its Possible Relation with ENSO Events
Alfonso Basin sediments spanning the last 8000yr are characterized by laminated structure. In contrast to central Gulf basins, laminae are non-annual. Sediment rate is about 0.3 mm/yr, whith average 11.2yr for the dark-light laminae. Microfossils, X-ray dispersive energy, and magnetic mineral analyses have been conducted on 13 light and 13 dark laminae to investigate its genesis. Radiolarian abundance and diversity in light laminae are higher than in dark. Paleo-productivity inferred on radiolarians per/gram is low, in comparison with basins in the Gulf. Radiolarian taxa in light laminae (Acrosphaera murrayana, Euchitonia triangulum and Phorticium pylonium (group)) suggest origin related with local upwelling conditions, promoting high productivity events. These conditions could be induced by wind stress mainly during windy seasons. Dark laminae contain high relative abundance of tropical radiolarian assemblage dominated by Tetrapyle octacantha and Stylochlamidium venustum. T. octacantha and S. venustum are associated with the subtropical plankton assemblage that is important during ENSO. S. venustum have showed an extraordinary contribution (60-95%) of total radiolarians during El Nino events. Dark laminae contain more terrigenous material, dominated by silicoaluminates. Almost all elements (Si, Al, Fe, Mg, K, S) analyzed, except Ca, are always higher in the dark laminae. Ca is higher in the light laminae and mostly represents biogenic input. Si correlate with typical terrigenous elements, which can be related to the siliceous tuffs. Magnetic hysteresis loops, IRM acquisition and back-field IRM demagnetization curves were measured in dark and light laminae, to investigate magnetic mineralogy, domain state and relative contents. In dark laminae hysteresis loops show saturation at low fields and high saturation magnetization. Contribution of SP material, also suggested from frequency-dependent susceptibility, is apparent from data-point distribution above mixing SD-MD curves. Changes in radiolarian taxa, chemical composition and, magnetic minerals reflect penetration of warm equatorial waters and terrigenous input, we suggest that El Nino events are reflected in the dark laminae.
PP43B-06
Climate and ocean variability recorded in laminated sediments from Carmen Basin, Gulf of California
We analyze a sedimentary laminated sequence; kasten core K58 (215 cm length), which was recovered in the western part of Carmen Basin at 720 m depth collected aboard of the UNAM R/V "El Puma". Core contains a high resolution record of paleoclimatic and paleoceanographic changes during the Holocene. Carmen Basin is one of a series of deep tectonically active basins developed in the Gulf of California, which are characterized by contrasting climatic, oceanographic and geologic conditions from the gulf mouth to the north. In order to provide a climate and ocean scenario during the late Holocene, radiolarian assemblages trace elements and magnetic properties are used as proxies of oceanographic and climatic variability. The record provides a tentative scenario for the last 4000 years. Analysis of the radiolarian polycystine were completed on samples spaced every 10 cm, whereas geochemical and magnetic properties analysis were completed in 30 light and dark laminae through out the core. Bulk ICP-MS geochemical analysis was performed on alternate laminae. The following elements were analyzed Al, V, Cl, Fe, Co, Ni, Cu, Zn, Ag, Cd, Ba, Pb and Hg and illustrate their variations in the sediment sequence. Radiolarians occurred through the entire core; they are diverse and show good preservation. Radiolarian assemblages were determined using Q-mode factor analysis of 22 samples of the core. For mineral magnetic analyses, low-field magnetic susceptibility was measured in 174 samples at low and high frequencies. Susceptibility displays its higher values at the top of the core, and a characteristic pattern of variations with depth. The remanent magnetization shows normal polarity, with downward inclinations around 60o. The magnetic hysteresis loops and IRM acquisition and back-field IRM demagnetization curves were measured in samples from the dark and light laminae, to investigate any differences in magnetic mineralogy, domain state and relative contents. Plots of hysteresis ratio parameters indicate that all samples fall within the PSD domain field, suggesting a mixture of SD and MD grains. Contribution of fine-grained SP material, which is also suggested from the frequency-dependent susceptibility measurements, is apparent from the data-point distribution above the mixing SD-MD curves.
PP43B-07
The Coastal upwellings of the Baja California Peninsula during the 1997-2003 ENSO conditions, through remote sensing
Upwelling interannual variation study was conducted in Baja California peninsula based on several parameters. Wind stress (upwelling index), Sea Surface Temperature (SST), Area Sea Surface Temperature (15°C), Sea Surface High (SSH) and, Chlorophyll-a (Chl-a) data were analyzed in order to compare ENSO conditions between 1997 and 2003. Our result indicate that SST and SSH were expected to ENSO conditions. Upwellings index was high during weak El Nino 2003 and less evident during La Nina 1998-2001 events. On the other hand, Chl-a biomass was 50% lower during El Nino 1997-98 than records of La Nina 1998-2001 and showed its maximum value during transition conditions in 2002. Chl-a biomass was similar in El Nino 2003 and La Nina 1998-2001. Our data do not match with the current proposed hypothesis that El Nino depress primary productivity (PP), while La Niña promotes high PP. These conditions are according only with the strong El Nino 1997-1998 in the southern California Current System; however during weak-to moderate events the conditions area variable in the region and inter-events. This supports the idea that southern CCS is also modulated by different inter-annual oscillations, as the North Pacific Decadal Oscillation.