OS32A-01 INVITED
Submarine Groundwater Discharge Revealed by 228Ra Distribution in the Upper Atlantic Ocean
Submarine groundwater discharge (SGD) is defined as any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force. The flux of SGD provides an important pathway for enriching coastal waters in nutrients, metals, and carbon. Determining the total flux of SGD to the ocean is a daunting task. Here I use the decay of 228Ra (half life = 5.7 years) in the upper Atlantic Ocean to infer the total flux of 228Ra to the upper Atlantic. The fact that radioactive decay, a highly predictable process, controls the inventory of 228Ra places a strong constraint on its flux. About 12% of the 228Ra inventory disappears each year. To maintain steady-state, there must be an equivalent flux from the continents. No other isotope, element, or compound shares these attributes of widespread distribution throughout the upper ocean, a removal term that is constrained to within a few percent, and a supply term that is due almost entirely to continental input. Inputs from dust, rivers, and coastal sediments explain less than half of the total 228Ra loss. The remainder must derive from SGD. Using estimates of the concentration of 228Ra in SGD from sites throughout the Atlantic yields a total SGD flux of (0.8-2.4) x 1016 L/yr. The calculated SGD flux is 33-100% of the river flux to the Atlantic. Because concentrations of nutrients, metals, and carbon in SGD are typically much higher than in rivers, the fluxes of these materials due to SGD are likely much greater than their riverine fluxes.
OS32A-02
Oceanic volcanic 3He: where is it going?
Various estimates that have been made of the flux of volcanic 3He from the solid earth suggest that of order 1000 moles per year are introduced into the abyssal ocean, and that a significant fraction of this occurs within the Pacific. Away from the points of injection, this 3He serves as a valuable conservative tracer of deep water flow, especially as part of the global planetary scale overturning circulation. In the southeastern South Pacific in particular, the deep 3He distribution highlights the return of Pacific Deep Water to the Antarctic Circumpolar Current (ACC) region. Using data acquired during the World Ocean Circulation Experiment and other cruises, we calculate the flux of deep 3He in the Antarctic Circumpolar Current at three critical points: south of Africa, south of Australia, and in the Drake Passage. The average zonal flux of 3He in the ACC is about 1300 mol y-1, roughly comparable in magnitude to the estimated global volcanic flux. This average transport must represent a balance between the entrainment of 3He -tagged deep waters and losses due to other mechanisms. The truly remarkable result is that the flux divergence between Australia and the Drake Passage is barely significantly greater than zero, and can at most be ~ 100 mol y-1. The implication of this small number is either that the (1) volcanic flux is in fact much smaller than previously estimated, (2) the returning Pacific Deep Water is upwelled and in large part returned northward at intermediate depths in the Pacific without significant entrainment in the ACC, or (3) upwelling and degassing of 3He in the ACC is sufficiently strong to closely compensate for the 3He entrained during its passage through the Pacific sector. We address these basic questions by consideration of the data available, along with presenting calculations of the observed flux of 3He from the surface ocean to the atmosphere during the WOCE surveys. The patterns and quantities thus observed have interesting implications for both the topology of the global conveyor and the nature of upwelling and outgassing in the southern hemispheric oceans. Moreover, the correlation between abyssal 3He and nutrients allows us to constrain the contribution of this upwelling to new production in the southern ocean.
OS32A-03
Using noble gases to constrain gas exchange and biological productivity
The five noble gases (He, Ne, Ar, Kr, and Xe) are biologically and chemically inert, making them useful oceanographic tracers. Moreover, the noble gases have a wide range of solubilities and diffusivities, and thus respond differently to physical forcing. We present here a one year time-series of the five noble gases and the isotope 3He, measured in the upper 400 m of the Sargasso Sea with monthly resolution at the Bermuda Atlantic Time-series Site (BATS). Two profiles of the noble gases in the entire water column down to 4200 m are presented as well. We combine the upper ocean noble gas time-series data, nutrient, oxygen, and hydrographic data from BATS, and a one-dimensional vertical mixed layer model (a modified Price-Weller-Pinkel model) in order to quantify air-sea gas exchange processes. We use inverse modeling to quantify the magnitude of both diffusive gas exchange and air injection processes. The estimates obtained constrain the seasonal time-scale gas exchange rate to a precision of 6% and the bubble injection fluxes to 15%, valid for wind speeds up to 15 m/sec. The overall results suggest that the Wanninkhof quadratic formulation needs to be adjusted downward by approximately 20%. Additionally, 3He is used as a tracer of upwelling nutrients in order to constrain new production. Nutrients in the upper thermocline are well correlated with 3He, and thus 3He and nitrate measurements, combined with estimates of gas exchange, are used to quantify the input of new nutrients into the mixed layer. 3He measurement are also used in conjunction with tritium and oxygen data in order to calculate apparent oxygen utilization rates (AOUR) and thus to estimate export production.
OS32A-04
Osmium Isotope investigation of hydrothermal fluids from Mid-Atlantic Ridge
It is generally believed that the variations in the osmium isotope composition of seawater over time, principally reflects a change in the balance of inputs from continental weathering and from hydrothermal exchange with oceanic crust (through submarine alteration and hydrothermal fluids). Some 20 percent of osmium in present-day seawater is thought to be of mantle/cosmic origin with 187Os/188Os composition of 0.13. However, existing Os data from high temperature hydrothermal fluids suggests that hydrothermal alteration does not provide significant unradiogenic Os to the oceans. This study present new Os isotope data for axial high temperature hydrothermal solutions from three localities along the Mid Atlantic Ridge (Snake Pit, TAG, Broken Spur). Os concentrations range from 0.5 to 5.2 picomol/kg and show large enrichment in osmium concentration over seawater by factor of 10 to 100. These concentrations are also one to two orders of magnitude higher than those reported previously. The Os isotope composition varies from 0.112 to 0.220. Most remarkably, the Os rich solutions are extremely unradiogenic relative to MORB or PUM (~0.13). All the data lie on a single mixing line using Os isotope composition versus inverse concentration, between radiogenic seawater (~1.06) and an unradiogenic component with 187Os/188Os composition of 0.11. This unradiogenic component yields a model age of 2.5 Ga and is the oldest yet observed for the sub-oceanic mantle. This observation indicates the existence of ancient depleted mantle accessed by the fluids at the studied localities. All the fluids analysed here show a linear covariation in 187Os/188Os versus Mg/Os or Sr/Os indicating that mixing between hydrothermal fluids and seawater is conservative. These results serve to aid the interpretation of variations in seawater Os over time, and highlight the similarity of sources of Os and Sr to the oceans
OS32A-05
Sediment Flux from Source to Sink in the Brazos-Trinity Depositional System
During the Late Pleistocene a series of intra-slope basins offshore Texas in the Western Gulf of Mexico, received a high influx of clastic sediments derived primarily from the Brazos, Trinity, and Mississippi rivers. Sediment failures initiated at shelf edge deltas resulted in mass flows that negotiate a complex slope and basin topography caused by salt tectonics. Sediment locally fill ponded basins eventually spilling into subsequent basins downstream. Interaction between these flows and slope topography leads to a complex partitioning of sediment over time and space that can only be unraveled with high-resolution data. The availability of system-wide coverage with conventional 3d seismic surveys, a dense grid of high-resolution 2d seismic lines and cored wells from two of the four linked intraslope basins, makes this locale an ideal area to investigate the transfer of sediment across the continental margin, from river sources to the ultimate sink within an enclosed intraslope basin. Data from IODP Expedition 308 and industry wells, combined with data from previous studies on the shelf constrain an integrated seismic stratigraphic framework for the depositional system. Numerous radiocarbon age dates coupled with multiple stratigraphic tools (seismic-, bio-, and tephra correlations and oxygen isotope measurements) provide an unprecedented high-resolution chronology that allow for detailed estimation of sedimentation rates in this turbidite system and calculation of sediment volumes in each of the basins over time intervals of a few millennia during the late Pleistocene. We find that rates of sedimentation exceed 10 m/kyr during some periods of ultra-fast turbidite accumulation. Rates of channel incision and tectonic subsidence can also be calculated and are comparable to the rapid accumulation rates measured in the basin fill. Our observations indicate that while sealevel changes exert a first order control on delivery of sediment to the basins, the sedimentary record suggests that delta dynamics, basin tectonics and the interaction between gravity flows and basin topography are equally important in determining the distribution of sediments in time and space along this depositional system.
OS32A-06
Deconvolving tectonic and climatic effects on Sedimentation Patterns in Deep-Marine Clastics, Eocene, Spanish Pyrenees
The Eocene Ainsa basin, Spanish Pyrenees, contains ~4 km of deep-marine deposits that accumulated in ~10 M.yr., as two principal angular-unconformity-bound tectono-stratigraphic units, in which the younger unit is structurally less deformed and shows a WSW shift in depositional axis, showing a first-order tectonic control on accommodation and deposition. The basin evolved with mainly non-marine and marginal marine environments in the eastern sectors, whilst farther west there was an overall change from fluvio-deltaic to deep-marine systems. The deep-marine deposits are overlain by ~0.5 km of fluvio-deltaic and related sediments fed mainly from the south and east. Micropaleontology suggests water depths in the Ainsa basin were in the range 400-800 m. The two "tectono-stratigraphic" units contain 8 coarse clastic systems, each in the order of 100-200 m thick, and vertically separated by up to several tens of meters of mainly marls with lesser amounts of thin- to very thin- bedded sandstone turbidites. Each system typically contains 2-7 individual sandbodies (amounting to at least 25 throughout the basin), from 30-100 m thick, separated by 10s m of mainly thin- and very thin-bedded sandstones with subordinate marls. Within each sandbody, submarine channels show a consistent WSW offset stacking pattern away from the deformation front that acted as one of the confining basin lateral-margins. In contrast to the longer-term 4-5 M.yr. tectonic driver, the number of deep-marine sandbodies in the Ainsa basin (~25) that accumulated in ~10 M.yr, suggests the operation of a faster, non-tectonic driver, possibly related to the 400 k.yr. long-period eccentricity. Spectral analysis of bioturbation intensity in thin-bedded, fine-grained sediments in one of the cored intervals shows a likely 41 k.yr. and ~100 k.yr. Milankovitch cyclicity. The age model for the Ainsa basin yields an average sediment accumulation rate of ~40 cm/k.yr., that is consistent with that inferred from the spectral analysis (~30 cm/k.yr.) for fine-grained sedimentation. A detailed analysis of the Ainsa basin sediments has permitted the deconvolution of tectonic and climatic signals at several time scales within a tectonically active basin.
OS32A-07 INVITED
Controls on Coarse-Grained Sediment Delivery and Distribution in the Holocene Santa Monica Basin, California: Implications for Evaluating Source-to-Sink Flux at Millennial Time Scales in a Deep-Marine Basin
Accumulations of terrigenous sediment in deep-marine basins commonly represent the terminal position for source-to-sink sediment flux across a continental margin. The sedimentary succession in the sink records the interactions of external, or allogenic, controls (e.g., eustasy, climatic conditions, tectonic activity) and intrinsic, or autogenic, dynamics (e.g., sediment gravity flow processes and development of depositional relief). Analyzing terrigenous sediment from a sink to determine the relative contributions and thus, the history of external controls has been difficult owing to limited knowledge of event timing. In this study, six new radiocarbon (14C) dates are integrated with five previously published, but recalibrated, dates from a 12.5 meter-thick turbidite section from ODP Site 1015 in Santa Monica Basin, offshore southern California (33°42.925"N; 118°49.185"W; water depth = 900 m). This borehole is tied to high-resolution seismic-reflection profiles that cover a 1,000 km2 area of the middle and lower Hueneme submarine fan and most of the basin plain. This regional stratigraphic framework provides the highest temporal resolution to date for a thick-bedded Holocene turbidite succession, permitting an evaluation of source-to-sink controls at millennial (103 yr) scales. The depositional history from 7 ka to present indicates that the recurrence interval for large turbidity current events is relatively constant (300-360 yrs), but the volume of sediment deposited on the fan and in the basin plain has increased by a factor of two during this period. Moreover, the amount of sand per event (i.e., thickness of turbidite bed) on the basin plain during the same interval increased by a factor of six. Maps of sediment distribution derived from correlation of seismic-reflection profiles indicate that this trend cannot be attributed exclusively to autogenic processes (e.g., lobe progradation). The observed variability in sediment accumulation rates is thus mainly controlled by allogenic factors, including: (1) increased discharge of Santa Clara River as a result of increased magnitude and frequency of ENSO events from approximately 2 ka to present; (2) decreasing rates of sea-level rise (i.e., sea level reaches present stand approximately 7 ka); and (3) an apparent change in routing of coarse-grained sediment within the staging area at approximately 2-3 ka (i.e., from direct river input to indirect, littoral cell input into Hueneme submarine canyon). The Holocene history of the Santa Clara River-Santa Monica Basin source-to-sink system demonstrates how the interaction of varying sediment flux and changes in dispersal pathways affects the basinal stratigraphic record.
OS32A-08
Relative Influence of Extrinsic and Intrinsic Controls at Different Time and Length Scales in Deep-Water Channel-Levee Systems
The evolution of deep-water depositional systems is influenced by a variety of extrinsic and intrinsic controls acting over a broad range of time and length scales. Recognition of the relative influence of these various controls is essential for proper characterization of deep-water depositional systems. At longer time and length scales, extrinsic forcing mechanisms dominate, forming relatively extensive surface-bounded units of genetically related sediments. Conversely, at shorter time and length scales, the primary forcing mechanisms tend to be intrinsic and exert local influence. These intrinsic controls are dominated by the interaction of the sediment-gravity flow with the bed surface and result in the formation of morphodynamic features. Interpretation of deep-water channel-levee systems from regional to bed-set scale reveals discreet stratigraphic units with predictable vertical variations in grain-size and stratigraphic architecture. Larger units are bounded by regional to sub-regional, chronostratigraphically significant surfaces indicating controls acting over large areas for relatively long periods of time. Within these larger stratigraphic units, a variety of morphodynamic features are observed which are formed over relatively short length and time scales. Effective interpretation and characterization of stratigraphic architecture requires proper definition of a chronostratigraphic framework, as well as an understanding of the morphodynamic processes resulting in the final, preserved deposit. Definition of a stratigraphic framework relies on the well established seismic stratigraphic methodology to identify and map key surfaces with regional to sub-regional significance. Within the context of the stratigraphic framework, morphodynamic principles and depositional element interpretation provide improved reservoir characterization. The combination of these two methodologies enables improved understanding of the depositional system by integrating the full range of controls and processes acting on the system.