Washington Continental Shelf Research Articles

Generation and Propagation of Nonlinear Internal Waves in Sheared Currents Over the Washington Continental Shelf

AbstractThe generation, propagation, and dissipation of nonlinear internal waves (NLIW) in sheared background currents is examined using 7 days of shipboard microstructure surveys and two moorings on the continental shelf offshore of Washington state. Surveys near the hypothesized generation region show semi‐diurnal (D2) energy flux is onshore and that the ratio of energy flux to group speed times energy ( ) increases sharply at the shelf break, suggesting that the incident D2 internal tide is partially reflected and partially transmitted. NLIW appear at an inshore mooring at the leading edge of the onshore phase of the baroclinic tide, consistent with nonlinear transformation of the shoaling internal tide as their generation mechanism. Of the D2 energy flux observed at the eastern extent of the generation region (133 ± 18 Wm−1), approximately 30% goes into the NLIW observed inshore (36 ± 11 Wm−1). Inshore of the moorings, 7 waves are tracked into shallow (30‐40 m) water, where a vertically sheared, southward current becomes strong. As train‐like waves propagate onshore, wave amplitudes of 25‐30 m and energies of 5 MJ decrease to 12 m and 10 kJ, respectively. The observed direction of propagation rotates from N of E to S of E in the strongly sheared region. Linear ray tracing using the Taylor‐Goldstein equation to incorporate parallel shear effects accounts for only a small portion of the observed rotation, suggesting that three‐dimensionality of the wave crests and the background currents is important here.

Instabilities in nonlinear internal waves on the Washington continental shelf

Previous studies have identified two primary mechanisms (shear instability and convective instability) by which nonlinear internal waves (NLIWs) induce mixing on continental shelves. To determine the relative importance of these and their dependence on background flow conditions, we examine a much longer (6 month) data set from a moored ADCP/thermistor chain with 2 m vertical spacing in which over 600 NLIWs are detected. Turbulent properties of the 318 waves with detectable overturning instabilities are documented using Thorpe scales. The 130 waves detected while an ADCP was functioning are classified based on a Froude number criterion (Fr = uc, where u is velocity in the wave propagation direction, c is the wave phase speed). Of these, 108 waves are identified as shear-instability (Type I; Fr 1). Composites are constructed by averaging in a wave coordinate frame over all waves in each category, showing the mean spatial structure of dissipation and other wave quantities. Turbulence is highest at the sheared interface for Type I waves and throughout the wave core for Type II waves. No relationship between wave instability mechanisms and wave/background parameters such as wave steepness, stratification, or mean flow is found, except that unstable waves tend to be more energetic, demonstrating a need to better understand wave propagation and breaking in complex and variable coastal oceanographic background flows.

Characteristics, generation and mass transport of nonlinear internal waves on theWashington continental shelf

AbstractAs a step toward better understanding the generation of nonlinear internal waves (NLIWs) on continental shelves and the factors determining their morphology, amplitude and propagation, we analyze more than 1500 NLIWs detected on the Washington (WA) continental shelf using four summer/fall time series of temperature and velocity measurements from a surface mooring deployed in 100 m of water. Propagating onshore toward the northeast, these NLIWs take a variety of forms, including internal solitary waves, solitary wave trains and bores. Nearly all are mode‐1 depression waves that arrive semidiurnally along with the internal tide. The NLIW energy flux is correlated with the internal tide energy flux but not the local barotropic forcing, implying that the observed NLIWs arise primarily from shoaling remotely generated internal tides rather than local generation. Estimated onshore transport by the waves can equal or exceed offshore Ekman transport, suggesting the waves may play an important role in the mass balance on the continental shelf.

Processes influencing seasonal hypoxia in the northern California Current System

This paper delineates the role of physical and biological processes contributing to hypoxia, dissolved oxygen (DO) < 1.4 mL/L, over the continental shelf of Washington State in the northern portion of the California Current System (CCS). In the historical record (1950-1986) during the summer upwelling season, hypoxia is more prevalent and severe off Washington than further south off northern Oregon. Recent data (2003-2005) show that hypoxia over the Washington shelf occurred at levels previously observed in the historical data. 2006 was an exception, with hypoxia covering ~5000 km(2) of the Washington continental shelf and DO concentrations below 0.5 mL/L at the inner shelf, lower than any known previous observations at that location. In the four years studied, upwelling of low DO water and changes in source water contribute to interannual variability, but cannot account for seasonal decreases below hypoxic concentrations. Deficits of DO along salinity surfaces, indicating biochemical consumption of DO, vary significantly between surveys, accounting for additional decreases of 0.5-2.5 mL/L by late summer. DO consumption is associated with denitrification, an indicator of biochemical sediment processes. Mass balances of DO and nitrate show that biochemical processes in the water column and sediments each contribute ~50% to the total consumption of DO in near-bottom water. At shorter than seasonal time scales on the inner shelf, along-shelf advection of hypoxic patches and cross-shelf advection of seasonal gradients are both shown to be important, changing DO concentrations by 1.5 mL/L or more over five days.

FORAMINIFERS AS HARD SUBSTRATES: AN EXAMPLE FROM THE WASHINGTON (USA) CONTINENTAL SHELF OF SMALLER FORAMINIFERS ATTACHED TO LARGER, AGGLUTINATE FORAMINIFERS

Research Article| January 01, 2008 FORAMINIFERS AS HARD SUBSTRATES: AN EXAMPLE FROM THE WASHINGTON (USA) CONTINENTAL SHELF OF SMALLER FORAMINIFERS ATTACHED TO LARGER, AGGLUTINATE FORAMINIFERS Ruth Martin Ruth Martin Department of Earth and Space Sciences and the Burke Museum, University of Washington, P.O. Box 353010, Seattle, WA 98195-3010., E-mail: ruthm2@u.washington.edu Search for other works by this author on: GSW Google Scholar Author and Article Information Ruth Martin Department of Earth and Space Sciences and the Burke Museum, University of Washington, P.O. Box 353010, Seattle, WA 98195-3010., E-mail: ruthm2@u.washington.edu Publisher: Cushman Foundation for Foraminiferal Research Received: 02 Jan 2007 Accepted: 05 Aug 2007 First Online: 03 Mar 2017 Online ISSN: 1943-264X Print ISSN: 0096-1191 © 2008 Journal of Foraminiferal Research Journal of Foraminiferal Research (2008) 38 (1): 3–10. https://doi.org/10.2113/gsjfr.38.1.3 Article history Received: 02 Jan 2007 Accepted: 05 Aug 2007 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ruth Martin; FORAMINIFERS AS HARD SUBSTRATES: AN EXAMPLE FROM THE WASHINGTON (USA) CONTINENTAL SHELF OF SMALLER FORAMINIFERS ATTACHED TO LARGER, AGGLUTINATE FORAMINIFERS. Journal of Foraminiferal Research 2008;; 38 (1): 3–10. doi: https://doi.org/10.2113/gsjfr.38.1.3 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Foraminiferal Research Search Advanced Search Abstract This study from the continental shelf offshore from Washington, USA, describes an occurrence of attached foraminifers using other, larger foraminifers as their hard substrate. Isolated instances of such associations have been illustrated in a few publications; however, the example documented here is of an extensive relationship between smaller attached foraminifers and their substrate species. Host foraminifers were unusually large (4 mm to 5 cm) specimens of the agglutinate species Ammodiscus pacificus, Haplophragmoides canariensis, Rhabdammina linearis and Bathysiphon filiformis. The attached foraminifers were Cibicides pseudoungerianus, Tritaxis fusca and Cibicides refulgens. In addition to the attached foraminiferal tests, several attachment scars were evident, some of which can be attributed to epibiont foraminifers that became detached from the host.There are several possible reasons these smaller foraminifers colonized the larger ones in this area. Most likely, the host foraminifers provided a stable platform for feeding. In addition, if the host was living, its motility could have prevented the smaller foraminifers from being smothered in this high sediment accumulation setting. Thus, these larger foraminifers act as convenient “benthic islands” (Taylor and Wilson, 2003) for their epibionts. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Sedimentary organic matter preservation; a test for selective degradation under oxic conditions

The authors report here a test of the hypothesis that the extent of organic matter preservation in continental margin sediments is controlled by the average period accumulating particles reside in oxic porewater immediately beneath the water/sediment interface. Oxygen penetration depths, organic element compositions, and mineral surface areas were determined for 16 sediment cores collected along an offshore transect across the Washington continental shelf, slope, and adjacent Cascadia Basin. Individual amino acid, sugar, and pollen distributions were analyzed for a 11 to 12 cm horizon from each core, and {sup 14}C-based sediment accumulation rates and stable carbon isotope compositions were determined from depth profiles within a subset of six cores from representative sites. Sediment accumulation rates decreased, and dissolved O{sub 2} penetration depths increased offshore along the sampling transect. As a result, oxygen exposure times (OET) increased seaward from decades (mid-shelf and upper slope) to more than a thousand years (outer Cascadia Basin). Organic contents and compositions were essentially constant within individual sediment cores but varied consistently with location. In particular, organic carbon/surface area ratios decreased progressively offshore and with increasing OET. Three independent compositional parameters demonstrated that the remnant organic matter in farther offshore sediments is more degraded. Both concentrationmore » and compositional patterns indicated that sedimentary organic matter exhibits a distinct and reproducible oxic effect. OET helps integrate and explain organic matter preservation in accumulating continental margin sediments and hence provides a useful tool for assessing transfer of organic matter from the biosphere to the geosphere.« less

Listric normal faulting on the Cascadia continental margin

Analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern Oregon and Washington continental shelf and upper slope, suggesting E‐W extension in this region. Fault activity began in the late Miocene and, in some cases, has continued into the Holocene. Most listric faults sole out into a subhorizontal décollement coincident with the upper contact of an Eocene to middle Miocene mélange and broken formation (MBF), known as the Hoh rock assemblage onshore, whereas other faults penetrate and offset the top of the MBF. The areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the MBF. Evidence onshore and on the continental shelf suggests that the MBF is overpressured and mobile. For listric faults which become subhorizontal at depth, these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low‐angle (0.1°–2.5°) detachment coincident with the upper MBF contact. Mobilization, extension, and unconstrained westward movement of the MBF may also contribute to brittle extension of the overlying sediments. No Pliocene or Quaternary extensional faults have been identified off the central Oregon or northernmost Washington coast, where the shelf is underlain by the rigid basaltic basement of the Siletzia terrane. Quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope, suggesting that the shelf and upper slope are decoupled from subduction‐related compression.

Assessment of Postmortem Transportation of Modern Benthic Foraminifera of the Washington Continental Shelf

Assessment of Postmortem Transportation of Modern Benthic Foraminifera of the Washington Continental Shelf

Sediment transport on the Washington continental shelf: Estimates of dispersal rates from Mount St. Helens ash

Following the 18 May, 1980 eruption of Mount St. Helens, samples from 10 stations on the Washington continental shelf were collected during cruises in October 1980, January 1981, October 1981, and January 1982, in an attempt to trace the movement of ash which was introduced to the Pacific Ocean by the Columbia River. Prior to October 1980, marine ash dispersal was restricted to points west and southwest of the Columbia River mouth due to summer circulation patterns. Five winter storms were probably responsible for most of the north-northwesterly transport of ash from October 1980 to October 1981. The size of this ash ranged from 7 to 22 μm (equivalent spherical diameter; reference density = 2.65 Mg m −3) at distances as far as 84 km from the river mouth. Only fine ash (7–12 μm) was recovered 84–125 km from the river mouth. The calculated suspensate transport rate for 16–22 μm ash is 80 km y −1 (16 km storm −1). Finer ash (7–12 μm) was transported at estimated rates of 73–190 km y −1 (14–38 km storm −1). These calculated transport rates are similar to rates previously estimated by Smith and Hopkins (1972 ; 80 km y −1) and Sternberg and McManus (1972 ; 220 km y −1), but are less than those predicted by Kachel (1980 ; 25–330 km storm −1), even though the ash was finer than the average sediment upon which these previous estimates were based.

Has man altered the cycling of nutrients and organic C on the Washington continental shelf and slope?

Several sets of water column and sediment data from the Washington coastal zone reveal that man has not detectably increased fluxes of planktonic C to continental slow sediments in recent decades by either increasing nutrient fertilization of the waters or by overharvesting higher tropic level organisms. Washington slope sediments are not as important depocenters for storage of anthropogenic CO 2 as the model of Walsh et al. (1985, Deep-Sea Research, 32, 853–883) predicts for slope sediments.

Benthic nutrient regeneration and denitrification on the Washington continental shelf

Benthic nutrient regeneration on the Washington continental shelf was investigated using vertical profiles of pore-water nutrient concentrations and whole sediment sulfate reduction rates. In August, carbon oxidation rates by sulfate reduction frequently exceeded those calculated from previously reported oxygen consumption rates. Total carbon oxidation rates averaged 18.9 pmol CO 2 cm −2 s −1, and using the 106 C: 16 N atomic ratio, 2.86 pmol N cm −2 s −1 would have been regenerated. Nutrient fluxes were calculated from pore-water profiles using diagenetic equations containing terms for vertical molecular diffusion, sedimentation, adsorption and macrobenthic irrigation which was estimated by modeling published radon distributions. In August, the calculated nitrate influx was 0.81 pmol N cm −2 s −1. The outward ammonium flux of 0.53 pmol N cm −2 s −1, calculated from pore-water ammonium profiles, was only 19% of the N regeneration rate. The missing ammonium could be accounted for by coupled nitrification and denitrification (1.16 pmol N cm −2 s −1) and by consumption of organic matter with high C:N ratios (9.8 by atoms). A comparison of annual regeneration and burial rates indicated that 84% of the deposited carbon is regenerated. For nitrogen, the annual outward ammonium flux was about half of the sum of the organic N burial rate and nitrate influx suggesting that these shelf sediments were a net sink for nitrogen. If the annual nitrate influx (0.57 pmol NO 3 − cm −2 s −1) to this shelf approximated that in other shelves, the global denitrification rate in continental shelf sediments wound be 69 Tg N y −1, a value similar to the oceanic water-column denitrifications rate.

Transport and accumulation of river-derived sediment on the Washington continental shelf, USA

The combined results of many physical oceanographic, geochemical, and geological investigations provide a coherent picture of the sedimentology of the Washington continental shelf. The major source of modern sediment is the Columbia River. Sediment on the shelf is transported by bottom currents resulting from surface wind stress and gravity waves which occur as individual storm or wave events lasting one to several days, primarily during the winter months. Field measurements of the sediment response to wind- and wave-generated bottom currents suggest that the modern sediment is transported as suspended load and disperses primarily northward parallel to the isobaths and is associated with a well-developed mid-shelf silt deposit. Smaller quantities of sediment are transported seaward over the shelf edge and into the numerous submarine canyons that incise the shelf. Analysis of available wave data suggests that thresholds of sediment motion were exceeded on the order of 22%, 16%, and 1.5% of the time annually on the inner, central, and outer shelf respectively. Bottom current measurements suggest that threshold conditions were exceeded 13% and 6% of the time annually on the inner and central shelf respectively. Measurements of accumulation rates using 210 Pb geochronology show that about 67% of the annual sediment discharge from the Columbia River is being deposited in the mid-shelf silt deposit; 6% is being deposited over the shelf edge on the open slope, and 11% is accumulating in the four major submarine canyons.

Accumulation of modern sediment in Quinault submarine canyon

Twenty box cores have been studied from Quinault submarine canyon, a major pathway between the Washington continental shelf and the deep sea for silt and clay originating from the Columbia River. Sediment accumulation rates (on a 100-yr time scale) are calculated from profiles of excess 210Pb and of 137Cs. Accumulation rates decrease with increasing distance downcanyon: from 14 × 10 −2 g cm −2 yr −1 in the upper canyon (lower head) to 3 × 10 −2 g cm −2 yr −1 in the lower canyon. In the upper canyon (lower head), channel-floor areas have a higher mean accumulation rate ( 12 × 10 −2 g cm −2 yr −1 ) than channel-wall areas ( 5 × 10 −2 g cm −2 yr −1 ). The difference in accumulation rate between channel-floor and channel-wall areas indicates preferential accumulation (i.e., focusing) in topographic lows. The decrease in accumulation rate (from high rates in the channel floors of the upper canyon to low rates in the lower canyon) may be caused by progressive depletion of a nepheloid-layer source and/or may be an artifact of sediment focusing in the upper canyon. The silty clay and clayey silt accumulating in the canyon show no significant trend in grain size or clay mineralogy with depth in cores or with distance downcanyon. The lack of sorting probably results from the transport of sediment in aggregate form, with a nearly random distribution of size and mineralogy within aggregates. Quinault Canyon is accumulating approximately 5.5 × 10 5 MT of sediment yr −1. The lithogenic component (64%) accounts for 1–4% of the Columbia River discharge. Similarity of sediment-trap deposition rates (including storm events) and 210Pb accumulation rates indicates that the vertical flux is a major input of sediment to Quinault Canyon and that storms are important in the accumulation of sediment on a 100-yr time scale.

Contemporary sedimentation processes in and around an active West Coast submarine canyon

Contemporary processes controlling the off-shelf transport of suspended particles in the vicinity of Quinault submarine canyon on the Washington continental shelf were identified using CTD/transmissometer surveys and a three-month deployment of current meter/transmissometer/sequentially-sampling sediment trap arrays. Resuspension events on the shelf create a shelf-break-depth nepheloid layer that transports sediment off the shelf. Fluctuations of the particle concentration in this layer were positively and significantly correlated with the short-term (9.5 days) temporal variation of trap deposition rates in the canyon and on the adjacent open slope. Contributions to the deposition rate from downslope transport along the canyon floor, from local resuspension of canyon sediments, and from particles in the ambient bottom nepheloid layer were negligible. These observations indicate that deposition on the slope is controlled by fast-sinking (100–200 m day −1) amorphous aggregates that scavenge fine-grained particles from the shelf-break-depth nepheloid layer. Areal variations in the deposition rate result from interaction between the regional flow pattern and the canyon geometry. Rates were high (∼60 g m −2 day −1) in the canyon upper head because it indents the shelf and thus underlies the shelf-break-depth nepheloid layer created by across-isobath advective transport of resuspended mid-shelf silty sediments. Rates were low (3–6 g m −2 day −1) elsewhere in the canyon and on the open slope because the transport is much less rapid and direct, occurring primarily by horizontal diffusion rather than across-isobath advection of particles, and because the availability of erodable fine-grained particles is reduced in the relatively coarser outer-shelf sediments. Trap deposition rate, extrapolated to an annual rate based on a historical seasonal pattern of shelf storms, was consistent with published radiometric bottom sediment accumulation rates throughout the study area.

Chemical evidence of differential particle dispersal in the southern washington coastal environment

Aliphatic hydrocarbons, cupric oxide oxidation products of lignin and polycyclic aromatic hydrocarbons (PAH) were analyzed by capillary gas chromatography in sediments from the southern Washington continental shelf and slope. The concentration of diploptene relative to plantwax n- alkanes increased systematically in surface sediments with distance offshore along east-west transects of the study area and remained constant in surface sediments along the midshelf silt deposit. Analogous trends were also observed for the concentration of cinnamyl phenols relative to vanillyl phenols and total methylphenanthrenes relative to phenanthrene. These changes in sedimentary composition are evidence that diploptene from some terrestrial source, lignin characteristic of non-woody vascular plant tissue and a fossil organic material contained within weathered rock debris disperse across the Washington continental shelf and slope in geographic patterns distinct from that for other river-derived, chemically related materials. The compositional variations are explained by the particulate associations of the land-derived chemicals and differential hydraulic dispersion of their respective carrier particles after discharge at the mouth of the Columbia River.

Sources of sedimentary humic substances: vascular plant debris

A modern Washington continental shelf sediment was fractionated densimetrically using either an organic solvent, CBrCl 3, or aqueous ZnCl 2. The resulting low density materials (<2.06 g/ml) account for only 1% of the sediment mass but contain 25% of the sedimentary organic carbon and 53% of the lignin. The C/N ratios (30–40) and lignin phenol yields ( Λ = 8) and compositions indicate that the low density materials are essentially pure vascular plant debris which is slightly enriched in woody ( versus nonwoody) tissues compared to the bulk sediment. The low density materials yield approximately one-third of their organic carbon as humic substances and contribute 23% and 14% of the total sedimentary humic and fulvic acids, respectively. Assuming that the lignin remaining in the sedimentary fraction is also contained in plant fragments that yield similar levels of humic substances, then 50% and 30% of the total humic and fulvic acids, respectively, arise directly from plant debris. Base-extraction of fresh and naturally degraded vascular plant materials reveals that significant levels of humic and fulvic acids are obtained using classical extraction techniques. Approximately 1–2% of the carbon from fresh woods and 10–25% from leaves and bark were isolated as humic acids and 2–4 times those levels as fulvic acids. A highly degraded hardwood yielded up to 44% of its carbon as humic and fulvic acids. The humic acids from fresh plants are generally enriched in lignin components relative to carbohydrates and recognizable biochemicals account for up to 50% of the total carbon. Humic and fulvic acids extracted directly from sedimentary plant debris could be responsible for a major fraction of the biochemical component of humic substances.

Biological enhancement of solute exchange between sediments and bottom water on the Washington continental shelf

Solute exchange between the interstitial waters and overlying waters on the Washington continental shelf was investigated based on measurements of the pore-water sulfate distribution and sulfate reduction rates as well as through models describing the distribution of sulfate in anaerobic pore waters. The depth-integrated sulfate reduction rate greatly exceeded the influx of sulfate attributable to molecular diffusion and sediment accumulation acting on the measured vertical sulfate gradients, and indicated that additional transport mechanisms must have been operating. Sediment mixing was probably not the primary mechanism since high eddy diffusivities would be required to depths of 30 cm to maintain the observed sulfate distribution, whereas previously measured 210Pb distributions indicated sediment mixing is primarily restricted to depths <7 cm. Irrigation of bottom water through animal burrows was the most likely mechanism. To describe this process, a general diffusive irrigation coefficient, B, was formulated. Vertical profiles of B showed the main irrigation zone occurred between 2 and 10 cm with reduced irrigation rates occurring below this. These coefficients calculated from the sulfate distribution were similar to ones calculated from previously published radon measurements at the same stations, indicating that this formulation of irrigation exchange may be useful in modelling the exchange of dissolved solutes between the pore water and the bottom water.

The effect of sediment mixing on Pb-210 accumulation rates for the Washington continental shelf

Nine cores from the Washington continental shelf were examined by radiochemical techniques in order to evaluate the effect of mixing on the calculation of sediment accumulation rates from Pb-210 profiles. Th-234 profiles indicate mixing coefficients of 140 cm 2 yr −1 for the seabed offshore from the Columbia River and 47 cm 2 yr −1 for the seabed of the Mid-Shelf Silt Deposit (75 km north of the Columbia River). These large mixing coefficients demonstrate that particles can penetrate to the base of the intensely mixed surface layer (∼ 10 cm) within one year after emplacement at the seabed surface. Observed depths of Cs-137 penetration within the seabed are compared with depths predicted from the surface mixed-layer thickness and the Pb-210 accumulation rate. The observed and predicted depths agree well at all but one station. At this station a combination of Th-234, Co-60, Pb-210 and Cs-137 profiles suggests that active mixing occurs below the intensely mixed surface layer. The agreement of Pb-210 and Cs-137 data at the other stations, however, indicates a general absence of deep mixing on the Washington shelf. Thus, the apparent accumulation rates calculated from Pb-210 profiles (below the intensely mixed surface layer) on the Washington shelf generally reflect the true rates of sediment accumulation.

Sources and distributions of sedimentary organic matter in the Columbia River drainage basin, Washington and Oregon1

Nonwoody angiosperm tissues and gymnosperm woods are the major types of vascular plant debris in bottom sediments from 16 sites within the Columbia River and its tributaries. In tributary sediments relative abundances of these types of tissue vary regionally and are consistent with drainage basin vegetation. Within the extensively dammed main river, vascular plant debris mixtures are more uniform, indicating downstream mixing of sedimentary material from reservoir to reservoir. Vascular plant tissues account for an average of a third of the total sedimentary organic matter at the 16 river sites. The remaining organic material has an atomic C:N of 10–15 and a stable carbon isotope composition similar to “average” land plants. Organic matter in sediments of the Columbia River system is generally enriched in nonwoody angiosperm tissues and lignin‐poor organic matter in comparison to Columbia River‐derived organic matter in modern marine sediments from the southern Washington continental shelf.

Polycyclic aromatic hydrocarbon (PAH)-phase associations in Washington coastal sediment

Polycyclic aromatic (PAH) and aliphatic hydrocarbon compositions, organic carbon, nitrogen and lignin contents were determined in whole, unfractionated sediment from the Washington continental shelf and in discrete sediment fractions separated by particle size and density. At least 20 to 25% of perylene and PAH derived from pyrolytic processes and 50% of the retene measured in whole sediment are contained within organic C- and lignin-rich panicles of density ≤ 1.9 g/cc. These particles, which include primarily vascular plant remains and bits of charcoal, comprise less than 1% of the total sediment weight. In contrast, a series of methylated phenanthrene homologs, possibly of fossil origin, are concentrated in some component of the more dense, lithic matrix of the sediment. Equilibrium models of PAH sorption/desorption from aqueous phase onto small particles of high surface area do not appear applicable to the behavior of the major PAH types identified in this aquatic environment.