OS33A-0980
Consumption of dissolved oxygen in the deep Japan Sea, giving a precise isotopic fractionation factor
Concentrations and stable isotopic compositions (delta-18O) of dissolved O2 in samples collected in May 2005 from the eastern Japan Sea were measured. The O2 consumption rate and the isotopic fractionation factor during dissolved O2 consumption were obtained from the field observations by applying a simple model to the deep water. The in-situ O2 consumption rates were calculated from the apparent O2 utilization and the turnover time of deep water obtained in the previous tracer studies. The rates were 1.2-1.4 micro-mol kg-1 yr-1 in the deep water below 2000 m. The isotopic fractionation factor estimated was 0.9875 applying a Rayleigh distillation equation to the quasi-deep water of 298-3584 m. The estimated isotopic fractionation factor and the turnover time mean that delta-18O of dissolved O2 will increase with a rate of 0.05-0.06 permil yr-1 for the closed Japan Sea deep water mass.
OS33A-0981
Uptake Of CO2 In The Upper Ocean Crust
Carbonates from the upper oceanic crust have been studied in order to determine the size of the CO2 reservoir, and to assess the conditions governing CO2 uptake. Carbonate is most abundant in volcanic sequences, where it generally marks the last phase of low temperature alteration. The CO2 inventory for eight new drill sites from Atlantic (396B, 543A, 556, 562) and Pacific (573, 595B, 597C, 1224F) ocean basins, spanning an age of 6.8 to 140 Ma, was calculated using whole rock CO2 contents, and the abundance of void filling carbonate. Our results, combined with published CO2 data, identify two age populations for the CO2 content of volcanic sequences, from 6.8 to 74 Ma (0.2-0.9 wt%\) and 110 to 140 Ma (1.95 to 4.1 wt%\). In order to explore what controls the uptake of carbonate, we have determined the O, C and Sr isotopic ratios and trace element contents for carbonate from each site. Stable isotopic data for carbonates indicates inorganic precipitation with a variation of d13C (PDB) between -2.321 and 3.412 per mil. d18O (SMOW) varies from 25.5 to 35.4 per mil, yielding temperatures of 1° to 31 °C, considering initial seawater value of d18O = 0 per mil. Preliminary Mg and Sr data, combined with published data, show these elements behave similarly during carbonate precipitation at most sites. Mg and Sr concentrations show a slight increase with decreasing of temperature, indicating that although temperature is an important parameter it does not control Sr and Mg contents. There is no clear relationship between Sr-isotopic values, Mg and Sr contents, and temperature. We will explore the relative importance of crust age, seawater chemistry and environmental conditions in controlling carbonate formation.
OS33A-0982
Hydrochemical and Isotopic Constraints on the Temporal and Spatial Variability of Chemical Weathering and CO2 Fluxes: An Example From the Australian Victorian Alps
Water from a network of 11 pristine rivers draining the Australian Victorian Alps was collected at different locations during (i) high discharge (June 2006) and (ii) low discharge (February 2007) conditions and was analyzed for dissolved major ions, δ2H and δ18O, and δ34S of dissolved sulphate. River water chemistry implies that solutes are largely derived from precipitation and chemical weathering of silicate lithologies. Cl/Br ratios as low as 30 molar suggest that rivers have not dissolved halite, however, higher salinity (≥100 mmol/L) winter samples have intermediate Cl/Br ratios (600 to 2000 molar) that are attributed to minor halite dissolution at the onset of the rainy season. Subsequent mixing of river water homogenizes ratios and evaporation is the dominant process that increases downstream salinities. Oxygen and Hydrogen isotopes also indicate that mixing and evaporation have occurred. Despite the lack of carbonate outcrops in the study area and uniform negative calcite saturation indices, the dissolution of hydrothermal calcite may account for up to 67% of the total dissolved cations, generating up to 92% of all dissolved Ca and Mg. The sulphur isotope data (16 to 20\‰CDT) indicates that the dissolved SO4 is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS. This militates against sulphuric acid weathering in Victorian rivers. Si/(Na* + K*) ratios suggest that silicate weathering is dominated by the transformation of plagioclase (An40) to smectite and, to a lesser extend, the production of kaolinite. In total, chemical weathering consumes 17.6 x 106 (summer) to 71.59 x 106(winter) mol/km2/yr CO2, with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This range is at the upper end of the global scale and shows that the predominance of fresh silicate lithologies exerts the main control on higher CO2 fluxes; temperature and runoff, in turn, are crucial variables for the inter- seasonal variability in this region. Data on discharge and major ion chemistry, measured in regular intervals between 1977 and 1990, support this; however, the timing of absolute maxima of Si/(Na* + K*) and CO2 flux peaks do not coincide. We suggest that the combination of dissolution of diatoms that precipitated under low flow- and high temperature conditions in the tributaries and Na-adsorption by suspended clay particles, that were probably redistributed locally after bushfires and/or duststorms during drought periods in the early 1980's, as mechanisms to spontaneously elevate Si/(Na* + K*) ratios and, when coupled with irregular discharge fluctuations, explain deviations from seasonal CO2 fluxes.
OS33A-0983
Tracing Chemical Weathering Fluxes in the Fly River, Papua New Guinea
The mountainous rivers of Papua New Guinea have the highest basin-area normalized Si and alkalinity fluxes of all major world rivers. Our work seeks to understand the controls on these anomalously high weathering fluxes. In January 2007, with the assistance of Ok Tedi Mining, LTD., we sampled the Fly River system extensively from upland tributaries to the floodplain confluence of the Middle Fly and Strickland Rivers. The highest alkalinities (up to 3400 μeq/L) come from upland streams draining Mesozoic limestones near the drainage divide. The highest Si concentrations (up to 314 μmol/L) are found in major upland tributaries draining Mesozoic clastic sediments outcropping to the south of the high peaks. These observations suggest a decoupling between the production of alkalinity and silicate weathering, which has implications for the system's impact on the long- term carbon cycle. Despite lower overall alkalinity and Si concentrations in the main-stem Middle Fly River, we observed a significant downstream increase in the fluxes of both alkalinity (3.1 fold) and silica (2.3 fold) carried by the Middle Fly as it crosses the lowland floodplain. The increased flux reflects a combination of new water (and solutes) from floodplain tributaries and return flow of floodwaters to the channel. This observation may indicate that weathering reactions in the lowland floodplain contribute significantly to the river's dissolved load. We are currently applying several tracers to help resolve some of the ambiguities that arise from the major solute data. We are using the Sr isotopic contrast between Miocene carbonates (87Sr/86Sr ~ 0.708) and Jurrasic clastic sediments (87Sr/86Sr ~ 0.715) to model the relative contributions of carbonate vs. silicate weathering to the river's alkalinity flux. Sediment fluxes in the Fly River system are profoundly impacted by mining activity at Ok Tedi. The impact on the river's dissolved load is less significant, but detectable. High germanium concentrations in mine sulfides allow streamwater Ge/Si ratios to serve as a tracer. The Ok Tedi signal is observed as anomalously high Ge/Si ratios (10 μmol/mol) in upland tributaries receiving mine influence. This mine-derived signal decreases downstream due to input of silica from pristine upland tributaries and floodplain weathering.
OS33A-0984
Continent – ocean fluxes and isotopic compositions of dissolved silicon contributed by groundwater
The global cycle of silicon (Si) is closely linked to that of carbon (C). The breakdown of silicate rocks provides essential nutrients, such as Si, to the marine system and is thought to regulate global climate on geological time scales. It is thought that nearly 85% of the oceanic Si budget is directly maintained via riverine pathways, with the remainder being derived from atmospheric deposition and low-temperature alteration of sea-floor basalt (ELDERFIELD and SCHULTZ, 1996; TRÉGUER et al., 1995). Although groundwater-derived fluxes add to the global Si balance their magnitude and effect on the Si isotopic composition of the oceans have not been elucidated. We have investigated the Si fluxes and associated isotopic compositions of subsurface flow within the Bengal Basin. We find that the groundwater Si fluxes are high and comparable to riverine Si fluxes. For instance, the groundwater derived Si fluxes, directly provided by submarine groundwater discharge into the Bay of Bengal, reach ~70% of the combined Ganges-Brahmaputra Si flux. Moreover the Si isotope composition evolves to lighter and even negative δ30Si values with age and depth. The deep groundwater carries δ30Si as negative as -0.1‰, clearly distinct from the positive δ30Si values typically found for river water. The Si isotope composition of the Brahmaputra appears to be a mixture of isotopically heavier Si from the upper tributaries and lighter Si derived from shallow groundwater. Our data indicate that groundwater derived Si fluxes are significant for the global Si cycle. Silicon isotopes might represent a new tool to trace the relative proportions of groundwater versus riverine Si fluxes. We propose that the continental Si flux into the ocean is enhanced by groundwater, and that the net isotopic composition of continental Si delivered to the oceans will reflect the relative proportion of surface waters versus groundwater. References: Elderfield H. and Schultz A. (1996) An. Rev. Earth Planet. Sci. 24, 191-224. Tréguer P., Nelson D. M., et al. (1995) Science 268, 375-379.
OS33A-0985
Glacial-interglacial changes in sediment source to the Meiji sediment deposit in the North Pacific Ocean using Ar and Nd isotopic fingerprints: A drift deposit or a displaced Yukon River submarine fan?
A large sediment deposit known as the Meiji Drift lies just outside of the Bering Sea in the northwestern Pacific Ocean. It is thought that the deposit is related to deep-water circulation in (and out of) the Bering Sea, although no notable deep water forms there presently. We have applied bulk sediment 40Ar-39Ar dating and Nd isotopic analyses to the silt-sized fraction from Ocean Drilling Project core site 884 (51°27' N, 168°20' E). Measurements are made over the last 130,000 years to document sediment source(s) to the Meiji Drift and to determine whether provenance changes through time. There is an order-of-magnitude range in bulk sediment 40Ar-39Ar ages and around ten εNd units of change, both varying on glacial-interglacial cycles. During glacials, bulk sediment 40Ar-39Ar plateau ages range between 40-80 Ma while Nd isotopic values range from εNd = -1 to εNd = +2. During interglacials, the downcore samples become considerably younger and more radiogenic, with bulk sediment plateau ages falling between 2-15 Ma and Nd isotopic values ranging between εNd = +5 to εNd = +9. These data illustrate that older source rocks such as those drained by the Yukon River are the dominant origin of sediments during glacials. This is when the Bering Strait is exposed and all terrigenous sediment entering the Bering region is routed to the North Pacific. Conversely, the Kamchatkan and Aleutian Volcanic Arcs straddling the Meiji Drift contribute the majority of sediment during interglacials. Notably, however, the Ar-Nd provenance record from the Meiji Drift suggests that Yukon-derived material continued to be transported to the North Pacific further into the Holocene and shut off around 5,000 years ago. This is coincident with a major reorganization of the Yukon River delta and further illustrates a possible linkage between the Yukon River and the Meiji Drift. The large ranges in bulk Ar and Nd isotopic values illuminate that this approach will reveal a detailed history of sediment transport in, out and through the Bering Sea on climatic timescales. Moreover, because the Yukon River dominates sedimentation and is a major source of freshwater to the Bering/North Pacific region during glacials, a considerable history of terrestrial-ocean climate linkages will also be better understood through Meiji Drift and Bering Sea sediments.
OS33A-0986
Molybdenum Isotopes in Oxic Sources and Sinks
Authigenic enrichment of molybdenum (Mo) in marine sediments has been used for some time as an indicator for reducing conditions during sediment deposition. More recently Mo isotopes in marine sediments and seawater have received attention for their potential to quantify regional or global-scale ocean (paleo-) redox conditions. Dissolved molybdate shows a conservative profile in the water column. Under oxidizing conditions Mo is removed from seawater primarily by adsorption to Mn oxides. In reducing, sulfide-rich settings it is scavenged by formation of particle-reactive oxythiomolybdates. These processes are thought to produce a strong fractionation of Mo isotopes in the modern oceans that depends on the relative proportions of oxidized and sulfide-rich settings: the Mo isotope composition of seawater is heavier than that of ferromanganese crusts and oxidized pelagic sediments by ~ 1 permil/amu. In contrast, black shales from reducing environments have intermediate values that approach that of seawater in some settings. As a consequence of isotope mass balance, the Mo isotopic composition of seawater should vary with changes in the relative proportions of reducing and oxic sedimentation in the oceans through time, and this variation may be recorded in marine oxic and anoxic sediments. However, to make quantitative statements about changes in ocean redox over geological time scales several variables in the Mo isotope budget have to be better understood. A recent study by Archer et al. has shown that the dissolved Mo isotope composition of rivers is heavier than previously assumed. In order to develop quantitative models for paleo-ocean redox it is essential to understand what controls the Mo isotope composition of the riverine input and if this process is constant over time. We are studying Mo concentrations and Mo isotopes in weathering profiles of granite. Preliminary data show that Mo is gradually leached from the source rock thus providing a base for possible isotope fractionation during the process. Another important prerequisite for the use of Mo as paleo-proxy is the availability of archives that preserve the Mo isotope signal. Towards that end we are analyzing Mo isotopes in Fe-Mn crusts during known sequences of changes in ocean chemistry. (Geochim. Cosmochim. Acta, 71, 15, Suppl. 1, A33)
OS33A-0987
Rare Earth Element patterns as indicators of trace metal sources to the Pacific Equatorial Undercurrent
The Pacific Equatorial Undercurrent (EUC) has high Fe concentration and may represent the principle pathway for supply of this limiting micronutrient to the HNLC region of the eastern equatorial Pacific. Understanding this pathway is therefore important to assess past and future changes in the Pacific carbon cycle. The input of Fe to the EUC at its source, close to Papua New Guinea, is unclear and may come from rivers, sediments, or hydrothermal activity. Limited Rare Earth Elements (REE) data has previously suggested the potential of REE patterns to assess the source of metals in the EUC. In this study, we have fully explored this potential by measurement of about 250 REE patterns in the upper 300 m of the central and eastern Pacific, and in the Bismark Basin where the EUC is sourced. Full REE profiles were measured by HR-ICP-MS (Element2) with typical precisions of 2% and are compared with T, S, nutrient, ADCP, Fe, and Al measurements made on the same cruise (Biocomplexity 2005, RV Revelle). The EUC has a characteristic Ce concentration somewhat higher than surrounding Pacific waters and this concentration remains approximately constant with longitude at 2.5 pmol/kg. The high Ce values are observed particularly in the upper layer of the EUC with the core centred around 120 m at 140°W, shoaling to 100 m by 134°W. This excess Ce is possibly sourced by horizontal advection in the New Guinea Coastal Current and New Guinea Coastal Undercurrent (NGCU) which flow from the Coral Sea and form a large part in EUC. Relative to seawater of the Coral Sea, the EUC has higher REE concentrations, indicating the incorporation of metals during transit of these currents around Papua New Guinea. EUC REE patterns also exhibit well- developed MREE enrichment which probably reflects an island weathering signature as previously observed in the Sepik River and its estuary. Increase of MREE enrichment with depth indicates that the probable source is the shelf of Papua New Guinea, supplied in turn by a large fraction of the Sepik River sediment load. Our REE data support the idea that isopycnal transport into NGCU (and other western boundary equator-flowing undercurrents) may provides a source of dissolved and particulate Al, Fe, REE and other lithogenic elements to the EUC, which are ultimately transported east along the equator to the eastern Pacific.
OS33A-0988
Modeling the distribution of Nd isotopes in the oceans using an offline Ocean General Circulation Model
The authigenic (seawater-derived) Nd isotopic composition of marine archives is increasingly used to study changes in ocean circulation on tectonic to millennial time-scales. Such applications for Nd isotopes assume that water masses are "tagged" with distinct Nd isotopic compositions in source regions and mix quasi- conservatively thereafter. However, there are a number of possible sources and sinks of Nd within the ocean that may complicate quasi-conservative behavior, including input from rivers, dissolution of dust, exchange at continental margin boundaries, and fluxes from the ocean-sediment interface. We use an offline ocean general circulation model (OGCM) to model the distribution of Nd isotope variability in seawater. A major obstacle to a thorough understanding of the marine Nd cycle is the lack of a truly global dataset of Nd isotopes in the modern oceans--most data are focused in only a few regions of the ocean. However, even within the constraints of sparse data, a better understanding of sources, sinks, and internal cycling of Nd and its isotopes can be reached through ocean modeling. We take a simple approach, treating the Nd isotopic composition of seawater as a conservative tracer, neglecting the effect of variable Nd concentrations on mixing. Nd isotope data from modern surface waters are used to generate a map of Nd isotope compositions for the entire surface ocean. This map is treated as a fixed boundary condition, and Nd isotope compositions of the surface are transported and mixed according to the flow characteristics of the OGCM until the interior ocean reaches a steady state. This simple approach produces Nd isotope estimates for North Atlantic Deep Water that are consistent with the observations, but produces values lower than observed in the deep Pacific and Southern Oceans. However, by introducing an additional source of Nd in the deep Pacific with higher Nd isotope ratios, the model output agrees well with the data. With the addition of radiogenic Nd to the deep Pacific, more than half of the measured data from the interior ocean fall within one ε-unit of the model output from the nearest OGCM grid point. These results indicate that for the currently available seawater data, the distribution of Nd in the ocean can be largely explained by quasi-conservative behavior of Nd in all ocean basins except for the Pacific, where a significant internal source of radiogenic Nd must exist.
OS33A-0989
Hyperpycnal transport of terrigenous materials from river to ocean: An example of the 2003 Hokkaido Hidaka Flood
Flood event is an event to supply large amount of terrigenous materials to marine environments. Thus, it is very important process for sea bottom environments, material cycles in the ocean, and strata formation. Mode of dispersion and deposition of flood sediments and its controlling factor is not fully clarified yet. In August 2003, hard rain by the typhoon 200310 occurred in Hidaka area, Pacific coast of Hokkaido, northern Japan. Many land slides occurred by this hard rain, and large amount of terrigenous sand and mud supplied to ocean through rivers. Previous study on this flood indicated that the importance of the inner shelf morphology on mode of flood material transport, especially on maintenance the energy (density) of the flow from the river. Collected sediment cores from mid shelf have some characteristics of hyperpycnal flow deposits. Typical sediment sequence changes from well-sorted very fine sand to brown silt. The uppermost silt has a lot of plant debris indicating after- flood deposition. Further long-distance transport by the hyperpycnal flow was recorded in the sediments of the slope. The large amount of terrigenous mud supply changed the sea bottom condition just after the flood. Most of the flood mud deposited on the inner shelf was removed by waves and currents after the flood. However, thickly deposited flood mud in the inner shelf depression still keeps its thickness due to relatively weak wave and current influence in the depression. Therefore, flood influence disappeared in the most of inner shelf, but still remains at the selected part of the inner shelf.
OS33A-0990
Electrical Resistivity and Seismic Characterization of Submarine Groundwater Discharge in Long Bay, SC
Submarine groundwater discharge (SGD) has been identified as a significant contributor of dissolved nutrients and contaminants to near-shore waters. Little is known, however, about geologic controls on the spatial distribution of SGD seeps. Discharge estimates are typically derived from geochemical tracers such as Rn-222. Such estimates of total fluxes over a given area do not consider the potential for spatial variability in discharge rates. Higher fluxes of chemically distinct SGD over smaller areas could have complex effects on localized water masses, ecosystems, and geological features. In an effort to assess the distribution of SGD, electrical resistivity and seismic surveys were conducted along the inner shelf of Long Bay, South Carolina during a series of cruises between October, 2005 and November 2006. In addition, basic bottom water quality parameters including dissolved oxygen, temperature, salinity, and pH were measured. Preliminary submarine groundwater flux estimates for northern Long Bay were also generated from measurements of Rn-222. The resistivity signal is highly variable along shore with several instances of elevated values suggesting presence of relatively fresher pore waters. In some cases, elevated resistivity measurements were spatially co-registered with seismically defined paleochannels extending across the shelf. Other areas of elevated resistivity values correlate with smaller discontinuities in seismic reflectors. A third category of resistivity anomalies does not correlate with seismically defined features. Overall, anomaly frequency and intensity decrease rapidly with increasing distance from shore. At distances > 1 km from shore, the resistivity signal is uniform in space and low in magnitude, implying less of a fresh water contribution. Water quality parameters are variable along shore and may reflect the influence of SGD. Rn-derived fluxes suggest SGD equivalent to as much as 50% of riverine discharge into Long Bay. Ongoing work is focused on sampling pore water in a variety of geologic settings for redox conditions, nutrients and Rn-222. This information will ultimately provide the basis for determination of SGD-related nutrient fluxes to nearshore waters.
OS33A-0991
Associations of major and trace components in time-series settling particle samples from Cuenca Alfonso, SW Gulf of California
Cuenca Alfonso is a natural sedimentation basin in Bahía de La Paz, southern Baja California Peninsula, an arid subtropical region characterized by limited land runoff with a productive surface layer overlying a well-developed Oxygen Minimum Zone. A time-series sediment trap program was initiated in 2002 to help understand the processes controlling variations in the biogeochemistry of this environment. INAA, ICP-MS and ICP-OES analyses, as well as flame AAS were made for major and trace elements in 105 subsamples of material collected at 350 m depth during 2002-2005. Principal component analysis, including data for organic and inorganic carbon, biogenic silica and the lithogenic fraction, yielded four factors. Factor 1 (25% of the total variance) showed high eigenvalues for the classical terrigenous aluminosilicate elements: Al, Fe, Mn Sc. High loadings also occurred for the group of light to medium REEs (except for Eu). The latter are frequently associated with acid source rocks, such the granites and rhyolitic volcanics exposed on the margin of the bay. Factor 2 (18%) grouped mainly REEs, with high loading on the heavier members of the series that are more typical of basic rocks (thick sequences of andesitic volcanics occur to the NW), as well as Ni and Rb. Factor 3 (12%) appears to represent carbonate organism production, with high loadings for Cinorg, Ca and moderate levels for Corg and Cu, while %litho is moderately negative, as is biogenic silica. The fourth factor (9%) is characterized by high negative loadings for Cd, Co, Se and moderately negative values for Corg. Moderate positive loadings occur only for Sr and, at lower levels, for U, Sb and %lithogenic. This suggests that this grouping is related to particles that have been partially transformed within the Oxygen Minimum Zone. The results will be examined for distinctive periods within the time-series, such as the hurricane-influenced months of August-September 2003, the generally very low fluxes during 2005, and unusual peaks during winter 2002. http://www.CATS-baja.org
OS33A-0992
Vertical distributions of 230Th in mid-latitude of the Pacific Ocean
Thorium is one of the least soluble elements in seawater: its dissolved species is considered as Th(OH)n(4-n)+ [Turner et al., 1981]. Th-232 (t1/2 = 1.4 ~ 1010 years) in the oceans is almost entirely contributed by continental weathering, whereas 230Th (t1/2 = 75,200 years) is produced in situ within the water column through the decay of a parent nuclide 234U in the seawater. Vertical distributions of 230Th have been well described by reversible scavenging model in the Pacific Ocean [Nozaki et al., 1981; Bacon and Anderson, 1982; Nozaki, 1983; Nozaki and Nakanishi, 1985; Nozaki et al., 1987]. Scavenging-mixing models have been adopted in regions where horizontal advection is the dominant factor controlling 230Th distribution, such as in the Weddell Sea [Rutgers van der Loeff and Berger, 1993], the Atlantic Ocean [Vogler et al., 1998; Moran et al., 1997; 2001; 2002] and the Andaman Sea [Okubo et al., 2004]. Previous studies on thorium isotopes in seawater have elucidated scavenging processes in water columns in various oceans. Nevertheless, thorium isotope studies have not been performed sufficiently in mid-latitude of the Pacific Ocean. This study clarifies vertical distributions of thorium isotopes, especially 230Th, to study scavenging processes of thorium and trace metals in this region. We investigated the vertical distribution of thorium isotopes in mid-latitudes of the Pacific Ocean especially 230Th as a test case of scavenging of metals, and discuss the control factor of the distribution of 230Th. In comparison with previous studies in the Pacific Ocean [Nozaki et al., 1981; Nozaki and Nakanishi, 1985; Nozaki et al., 1987], the higher latitude stations show lower 230Th concentrations. This tendency corresponds to the primary productivity in surface oceans. The prominent feature is the depletion of 230Th concentrations compared with that estimated by reversible scavenging model calculations in deep water in BO-3 (30o 01fN, 160o 00fW, Depth: 5778 m) and BO-5 (19o 60fN, 175o 00f W, Depth: 5480 m). For these deficiencies, two possibilities were considered 1) lateral transport of re-suspended particles 2) enhanced scavenging effect in deep water.
OS33A-0993
Molecular Characterization of the Refractory Components of Terrestrially-derived DOM in Marine and Freshwaters of Southern New Zealand
The molecular composition of dissolved organic matter (DOM) in samples collected from a freshwater river in Doubtful Sound (167.0167 E, 45.4097 S) and along a transect extending from neritic water (170.7522 E, 45.7687 S) across the Subtropical Convergence (171.0273 E, 45.7818 S) into Subantarctic Surface Water (171.4125 E, 457976 S) have been measured using ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Approx. 600 molecular formulae have been identified which are common to these four very different water masses and attributed to the refractory portion of the DOM. These molecular formulae have been assigned to nine classes of homologous series containing only containing carbon, hydrogen and oxygen. We suggest that these nine compositionally related families represent specific lignin degradation products resulting from side-chain oxidation, demethylation and aromatic ring opening.
OS33A-0994
Conspicuous seismic reflections in Upper Cretaceous sediments as evidence for black shales off South Africa
The late Cretaceous is commonly associated with greenhouse climate, which comes along with worldwide occurring, so called Oceanic Anoxic Events (OAE), and the ongoing break up of the Gondwana super-continent. Especially the opening of the South Atlantic Ocean and the beginning closure of the Tethys initiated strong variations of the ocean's currents flow paths and lead to a rapid climate change. Only little is known about these changing conditions and OAE appearances, in particular south of South Africa, at that time. A set of high resolution seismic reflection data from the submarine Transkei Basin south of South Africa shows various depositional stages for this area since the late Cretaceous. In these seismic sections, a recurrently appearing very high amplitude horizon within rather weak to homogeneous Upper Cretaceous reflections was observed. This reflector could roughly be dated to a time between ~ 80 Ma - ~ 85 Ma, which falls within the last documented big OAE 3 in the Upper Cretaceous. According to the appearance and reflection characteristics of this conspicuous reflector as well as its time/depth information, it could be the first report of black shales in a deep- sea basin within this region. Moreover, associated with the black shales, it could be the first evidence of an OAE south of South Africa.
OS33A-0995 INVITED
Climate and Tectonic Signals Preserved in the Indus Submarine Fan, Arabian Sea
Sediments preserved in the submarine fan fed by the Indus River record the interactions between climate and tectonics in the western Himalaya since the start of India-Asia collision, especially exhumation of the Greater Himalaya and intensification of the Asian monsoon. Seismic stratigraphic mapping has revealed peaks in sedimentation rates during the Middle Miocene and Plio-Pleistocene. These periods correlate with times of rapid exhumation in the Greater Himalaya and with intense chemical weathering driven by a strong monsoon initiating around 22 Ma. Correlation of these maxima with similar events lasting >1 m.y. in East Asia suggests a dominant climatic/monsoonal control to erosion and sediment flux to the deep sea. Coring of Holocene sediment from the Indus delta allows us to assess climatic controls on erosion over millennial timescales. Total sediment flux increased sharply in the Early Holocene coincident with a strengthening monsoon allowing the coast to prograde seawards despite rising sealevels. Bulk sediment Nd isotope analysis reveals less erosion of the Karakoram and more sediment flux from the Himalaya during the early and middle Holocene. Radiometric Ar-Ar dating of muscovite and U-Pb dating of zircon sand grains indicates that the Lesser Himalaya eroded relatively more strongly than the Greater Himalaya as the summer monsoon intensified after 14 ka. This variation is preserved with no apparent lag in sediments from the delta, but not in the deep Arabian Sea, due to sediment buffering on the continental shelf and the failure of the Indus to reconnect to its submarine fan since ca. 11 ka.
OS33A-0996
Numerical Simulation of Recent Turbidity Currents in the Monterey Canyon System, Offshore California
The method of computational fluid dynamics (CFD) has been used, in the form of a 3D numerical model (Flow- 3D®), to perform a full-scale simulation of turbidity currents measured in December 2002 by three moorings in the Soquel and Monterey canyons. The model was verified by simulation of laboratory flows, and was upscaled to the Monterey Canyon system on the basis of high-resolution bathymetric data and flow measurements. The measured velocity profiles were sufficient to assess the flow thickness, initial velocity and duration in the canyon head zone. A computational grid with a highest feasible resolution was used, and both bathymetry and hydrostatic pressure were accounted for. The volumetric sediment concentration and exact grain- size composition of the flows were unknown, and thus a range of values for the initial concentration and bed roughness were assumed and assessed on a trial-and-error basis. The simulations reveal the behavior of a turbidity current along its descent path, including its local hydraulic characteristics (the 3D field of velocity, sediment concentration, shear stress, strain rate, and dynamic viscosity, as well as the magnitude of velocity and turbulent shear). The results confirm that the velocity structure of turbidity current is highly sensitive to variation in seafloor topography. The December 17th flow in the Soquel Canyon appears to have lost capacity by dilution over a relatively short distance and shown significant velocity fluctuations, which is attributed to the rugged topography of the canyon floor. A major loss of momentum occurred when the flow plunged at high angle into the Monterey Canyon, crashing against its bend's southern wall. The December 20th flow in the Monterey Canyon, in contrast, developed a considerably longer body and strongly accelerated towards the canyon's sharp second bend before crashing against its western wall. The mooring data show a down-canyon decline of velocity and suggest gradual waning, but the flow in reality appears to have had a new waxing phase. The CFD simulations allow the potential behavior of future flows to be predicted and, through a longer series of runs, the zones of erosion and deposition in the canyon system to be delineated.
OS33A-0997
Growth Patterns of Deep-sea Fans Revisited: Evolving Fan and Related Turbidite-system Morphology in the Inner Basins of the Southern California Continental Borderland
This study quantifies Quaternary fan and related turbidite-system morphologies in the eastern Gulf of Santa Catalina and northern San Diego Trough south from Dana Point to offshore San Diego. Recently, Deptuck et al. (in press) quantified lobe dimensions on fans east of Corsica; our study focuses on entire fans and related turbidite systems, including channel, overbank, and lobe elements. From north to south, the informally-named San Mateo turbidite system together with the Oceanside and Carlsbad fans are recognized on the present seafloor. An extensive grid of industry multichannel and USGS high-resolution deep-tow seismic-reflection data was used to construct a detailed seismic-stratigraphic framework of Quaternary basin fill. The stratigraphic framework shows that the basin fill includes eight phases of growth: three each for the Oceanside and Carlsbad fans and two for the San Mateo turbidite system. Incorporating more than 20 radiocarbon dates younger than 45 ka with the seismic-reflection-based correlations provides a chronostratigraphic framework suitable for extrapolating timing of individual turbidite-system growth phases since Oxygen Isotope Stage (OIS) 6. This study describes turbidite-system morphologies produced by successive growth phases based on ratios of symmetry and thickness to surface area; e.g., Oceanside fan is relatively symmetric and thick following its first growth phase (OIS 6), whereas it is asymmetric and areally extensive following its final growth phase (OIS 2). Underlying basin accommodation and sediment supply to the turbidite systems during growth phases were quantified for comparison with observed system morphologies. Bathymetric gradients and accommodation surface areas prior to growth were used to characterize accommodation, and gross sediment volumes deposited during growth were used to infer sediment supply. Relatively high-relief bathymetric gradients and small accommodation areas facilitate the development of more asymmetric and thick morphologies, whereas relatively large sediment supply can smooth subtle bathymetry and facilitate the development of symmetric morphologies. Placed in temporal context, basin fill became successively thinner and more areally extensive for each phase of turbidite-system growth as a result of depositional smoothing of the original high-relief, tectonically-created basin accommodation.
OS33A-0998
Constraining the Controls on the Deposition of Deep-Water Conglomerates, Upper Cretaceous Cerro Toro Formation, Magallanes Basin, Chile
The Upper Cretaceous Cerro Toro Formation in the Magallanes foreland basin exposes several deep-water, conglomeratic channel complexes, representing the cyclic influx of coarse-grained sediment into the basin, that are separated by thick successions of thin-bedded, mud-rich turbidites. The primary objective of this study is to better constrain the controls on the cyclicity of coarse sediment deposition in the Magallanes Basin during the Late Cretaceous. The growing Andean fold-and-thrust belt to the west of the Magallanes Basin during Cerro Toro deposition suggests that active tectonics and possibly arc-related magmatism were key controls on sediment supply. To address the relative influence of tectonism, single grain age dating (U-Pb) on detrital zircons using Sensitive High Resolution Ion Micro Probe Reverse Geometry (SHRIMP RG) is coupled with conventional petrographic analysis of the basal sandstone and conglomerate compositions of each channel complex. In the event that major uplift or magmatic activity is responsible for the influx of coarse debris deposited as the distinct channel complexes, new zircon age populations as well as modified sandstone and conglomerate compositions will appear. These may be as a result of unroofing of deeper-level older rocks, emplacement of thrust sheets, or increasingly younger magmatic activity. To provide age constrains on the distinct channel complexes 87Sr/86Sr isotope stratigraphy on Inoceramus shells is combined with the U-Pb dating of volcanic zircons in ash layers. Preliminary strontium isotope results are consistent with the strontium isotope curve for the global oceans during the Upper Cretaceous. The ash layers contain abundant volcanic zircon grains. This study will improve our understanding of the evolution of the Southern Patagonian Andean arc and its relative influence on deep-water sedimentation in the adjacent retroarc foreland basin.
OS33A-0999
Deformation Rates From Climate Cycles in Marine Synorogenic Turbidites, Jaca Basin, Spanish Pyrenees
Synsedimentary structures provide a link between depositional and deformational processes in orogenic belts. Marine growth strata offer great promise in determining precise long-term deformation rates such as uplift, shortening, fault-slip, and folding rates. Magnetostratigraphy and cyclostratigraphy of the Eocene Arguis Fm., a delta slope deposit and the overlying delta plain Belsue-Atares Fm. monitors the varying pace of deformation at Pico del Aguila, a transverse decollement fold in the south Pyrenean foothills. Anhysteretic remanent magnetization (ARM) data show hierarchical cyclicity at all predicted Milankovitch frequencies. ARM is a proxy for fine-grained detrital magnetite concentration. The age distorting effects of pre-lithification compaction on bed thickness and on the rock and paleomagnetic data series were removed using empirical calibration of anisotropy of anhysteretic remanence magnetization to volume loss from laboratory compaction experiments. The decompacted ARM depth domain was converted to time using an improved magnetostratigraphy within the growth section. Tuning filtered ARM data series to the precession index according to the LA2004 orbital model refined the magnetostratigraphic time scale. The precession-tuned growth strata mapped with precision GPS record variable folding rates at 100skyr timescales for 7 Myr and55° of limb tilt. Folding rates accelerate twice to ~30°/Myr and are punctuated by more gradual decelerations to 0-3°/Myr. Submarine folding rates at Pico del Aguila are attributed to episodic thrusting in the fold core along a roof ramp fault and along the basal decollement. Formation-scale deposition in the Paleogene wedge-top basin responded to tectonic forcing, however, clastic facies patterns in the prodeltaic and slope environments reflect regional uplift controlling sediment supply, sea level variations controlling delta front position and climate forcing (e.g. monsoon strength and frequency, pedogenesis) of runoff variability and ecology in intermountain watersheds. Lithologic parameters sensitive to sea level, such as bed thickness and grain size variations in the turbidite section record strong obliquity and eccentricity modulation, whereas, the ARM derived magnetite concentrations record terrestrial watershed sensitive processes such as aridity and windiness operating at precessional time scales.
OS33A-1000
Clastic Sedimentation in Lake Tahoe as a Record of Submarine Landsliding and Seismic Shaking
Episodic turbidites, densites, and underwater debrites found in Lake Tahoe gravity and piston cores comprise more than 50% of the Holocene sediment signal in the lake. These deposits can be correlated throughout the lake basin using AMS 14C dating, visual descriptions, stratigraphy, and other sediment analyses. For a given event, depositional pattern show a continuum of processes varying areally from debris flows to intermediate density flows to turbidity currents. Anomalous lithologies can be quantified by variations in anisotropy of magnetic susceptibility and grainsize measurements. Debrites occur in close proximity to source areas, densites are found in intermediate zones, and turbidites occur distal to the source areas. Turbidites may be further sub-classified into proximal and distal on the basis of whole core magnetic susceptibility values and grain-size within the turbidite base. Whole-core magnetic susceptibility, anisotropy of magnetic susceptibility, %opal (diatoms), and grain size measurements demonstrate that material within these episodic deposits consists of remobilized lacustrine sediment. Multiple source areas for an individual event are associated with basin margins and known areas of underwater landslides. The remobilization of lacustrine sediments, in conjunction with the locations of source areas and the presence of multiple source areas for individual events, eliminates stream input, delta collapse, and large flood events as potential causes of these episodic deposits. Submarine landslides were initiated at different elevations around the lake, suggesting that lake level change is not a causative factor. Traditional paleoseismic trenching and dating have not been done on the major faults within the basin, with the exception of the relatively minor Incline Village fault. Other fault systems outside the basin, such as the Mt. Rose and Genoa fault systems to the east, can also lead to strong shaking and potential landslides in the Tahoe basin. The timing of episodic deposits in Lake Tahoe is consistent with late Holocene records from the faults that have been trenched, which indicates that a seismic source is the driver for these types of deposits. The lacustrine sediment stratigraphy provides a complete Holocene strong shaking record for the region and extends the paleoseismic record far beyond that of traditional onshore methods.
OS33A-1001
Evidence of Nipigon Phase Overflow From Glacial Lake Agassiz in Northwestern Lake Superior.
The Younger Dryas cold reversal is one of the most prominent known abrupt changes in the Earth's recent climate history. This event has been ascribed to a rapid decrease in the production of North Atlantic Deep Water, which resulted when freshwater outflow from Glacial Lake Agassiz was suddenly diverted through the Great Lakes into the North Atlantic about 11,000 14C BP. This inference grew in strength through years of research on Lake Agassiz, the Gulf of Mexico and the North Atlantic. Recently, however, the existence of eastward drainage of Lake Agassiz during this time period has been challenged on a number of fronts, and we interpret the existing evidence as equivocal. Lake Superior is a crucial site for addressing the existence of eastward drainage of Lake Agassiz at the beginning of the Younger Dryas. There is well documented terrestrial evidence of later post-Younger Dryas drainage from easterly outlets through Lake Superior and the Great Lakes to the North Atlantic. We believe that these events (corresponding to the Nipigon Phase of Lake Agassiz) left diagnostic stratigraphic and geomorphic signatures beneath Lake Superior. If so, earlier eastward drainage during the Younger Dryas should have left analogous features. The purported Younger Dryas episode of eastward Lake Agassiz drainage (Morehead phase) was separated from the incontrovertible younger one (Nipigon phase) by the rapid and shortlived Marquette glacial advance (Emerson phase in Lake Agassiz). The thin, fine-grained till of this advance serves as a stratigraphic marker that would separate the younger drainage features from possible older ones. By sequentially examining the seismic stratigraphy of specific discharge locations beneath Lake Superior, and using the Marquette till as a stratigraphic marker, we believe we can perform a rigorous test of whether or not Lake Agassiz drained eastward during the Younger Dryas, and if so, where. We present here results from the recently concluded first field season of the project, which focused on mapping the lake floor expression of the later (Nipigon Phase) overflows. These post-Marquette deposits include subsurface fan deposits and coarse-grained strata that are buried below a thin sequence of Holocene sediments. These overflow deposits are found at the entrance to Nipigon Bay, a drainage pathway from Glacial Lake Agassiz known to be active during the Nipigon Phase of Lake Agassiz, and possibly at other discharge locations. Erosion features suggestive of high discharge scour events also occur in several places in the study area. The seismic stratigraphic model generated from these data will be used to analyze data collected next year off Thunder Bay, where the earlier overflows linked to the Younger Dryas have been postulated.
OS33A-1002
Lobe Shifting in the Gulf of Papua: Internal or External Forcing?
Recently acquired CHIRP data from the Gulf of Papua (GoP) reveals that the modern clinoform is inherently three- dimensional across a variety of scales. Three depositional lobes are identified in the mid-shelf region of the GoP; a central lobe that is downlapped by younger northern and southern lobes. While the depocenter shift and infilling of available accommodation within the central lobe appears to be predominantly controlled by depositional processes and stacking patterns (autocyclic forces), the marked shift in deposition away from the central lobe to the northern and southern lobes (~60-80 km) is difficult to explain by depositional processes alone. The northern and southern lobes downlap onto the central lobe with no evidence of interfingering, which suggests an abrupt shift in the loci of deposition away from the central lobe. Radiocarbon dates from the youngest units in the central lobe suggest this shift occurred after 2 ka and thus it is difficult to invoke eustatic sea level fluctuations to explain this marked shift in deposition. Sediment rerouting or oceanographic changes may account for this dramatic shift in the depositional lobes. High-resolution mapping of the three dimensional architecture of shelf building clinothems has provided new insight into sediment dispersal systems that is the critical first step to determine whether GoP lobe switching is a stochastic (autocyclic) or externally driven (allocyclic) process.