Santa Barbara Basin Sediments Research Articles

Hydrogen-isotopic variability in lipids from Santa Barbara Basin sediments

We conducted an extensive survey of hydrogen-isotopic compositions (D/H ratios) of diverse sedimentary lipids from the Santa Barbara Basin (SBB), offshore southern California. The main goal of this survey was to assess the diversity of D/H ratios in lipids from marine sediments, in order to provide a more detailed understanding of relevant biological and geochemical factors impacting lipid isotopic variability. A total of 1182 individual δD values are reported from two stations in SBB, one located in the suboxic basin depocenter and the other on the fully oxic flank of the basin. Sediments collected from the basin depocenter span a depth of ∼2.5 m and reach the methanogenic zone. Lipids that were analyzed include n-alkanes, n-alkanols and alkenols, short- and long-chain fatty acids, linear isoprenoids, steroids, and hopanoids, and exhibit several systematic patterns. First, there are no significant differences in δD values between the two sampling locations, nor with increasing depth for most lipids, indicating that degradation does not influence sedimentary lipid δD values. Second, relatively large differences in δD values among differing molecular structures are observed in all samples. n-Alkyl lipids of probable marine origin have typical δD values between −150 and −200‰, those from terrestrial leaf waxes and aquatic plants range from −80 to −170‰, while petroleum n-alkanes are typically −90 to −150‰. Third, lipids inferred to derive from bacteria (branched fatty acids and hopanols) living at the sediment surface or in the water column tend to be D-enriched relative to similar algal products by 30‰ or more. At the same time, several other lipids have δD values that decrease strongly with depth, presumably as a result of in situ production by anaerobic bacteria. This dichotomy in isotopic compositions of bacterial lipids is inconsistent with a nearly constant D/H fractionation during lipid biosynthesis, and likely reflects significant variations associated with metabolism.

The adsorption of saxitoxin to clays and sediments in fresh and saline waters

The adsorption of saxitoxin to Na- and Ca-montmorillonite, kaolin (crystalline and amorphous), kaolinite, Bread and Butter Creek sediment (an estuarine tidal creek), Gulf of Mexico sediment, and Santa Barbara Basin sediment in deionized water and 32‰ salinity simulated seawater (Instant Ocean™) is reported. Adsorption was partially reversible for all cases and best described using a Freundlich isotherm. The corresponding Freundlich constants ( K F) ranged from 8.83 × 10 3 μmol/kg to 6.76 × 10 4 μmol/kg for freshwater and 4.73 × 10 3 μmol/kg–1.11 × 10 4 μmol/kg for seawater. There is a positive linear correlation seen between the K F values and the cation-exchange capacity of the adsorbents. The release of saxitoxin from previously equilibrated adsorbents was determined in freshwater (0–18%) and seawater (4–53%).

Adsorption of domoic acid to marine sediments and clays

Conditional solid-water distribution coefficients (K(d)) for the adsorption of domoic acid (DA) to a series of complex sediments and clays were determined in artificial seawater. K(d) ranged from 5.11 L g(-1) to 0.97 L g(-1), with a corresponding ranking of: kaolinite > Gulf of Mexico sediment > Santa Barbara Basin sediment > Bread and Butter Creek sediment > poorly crystallized kaolin > Ca-montmorillonite > Na-montmorillonite > well crystallized kaolin > diatomaceous earth. Adsorption correlated with the anion exchange capacity of the clays tested (R(2) = 0.98), but not the more structurally complex sediments. The effect of added transition metals (Fe(iii), Cu(ii), Al(iii)) and terrestrially derived dissolved organic matter (Suwannee River DOM, SRDOM) on DA adsorption to Na-montmorillonite, well crystallized kaolin, and Gulf of Mexico sediment, was also tested. The addition of transition metals led to increased adsorption to all surfaces by a factor of 2-7, presumably by enabling the adsorption of DA-metal complexes. SRDOM enhanced DA adsorption by a factor of approximately 2.5. The release of adsorbed DA from sediments was also examined. Under our conditions, adsorbed DA equilibrated with the overlying aqueous phase within minutes with approximately 50% release.

Resources for Paleoceanographic and Paleoclimatic Analysis: A 6,700-Year Stratigraphy and Regional Radiocarbon Reservoir-Age ( R) Record Based on Varve Counting and 14C-AMS Dating for the Santa Barbara Basin, Offshore California, U.S.A.

Abstract Holocene sediments from the central Santa Barbara Basin (SBB) contain invaluable paleoceanographic information and document paleoclimatic supra-regional teleconnections. The well-constrained dating and stratigraphy of the SBB can serve as a model for the paleoclimatic exploitation of laminated sediments elsewhere. Here we present an improved 6,700-yr master chronology that is based on sequential downcore counting of annual varves and 14C-AMS dating, starting from the June 2004 sediment surface. Diagnostic lamination patterns and the presence of prominent marker layers in an ~ 9 m long sequence of X-radiographic images from various cores (including ODP Hole 893A and piston core 6P) offer convenient dating for other central SBB sediment cores via direct X-radiographic and/or visual correlation against the master chronology presented here. The new sequence of X-radiographs of piston core 6P spans the coring hiatus at 7.04 mbsf in ODP Hole 893A. Varve-count ages agree well with an independent chronology based on calibrated 14C-ages of planktic foraminifers. In contrast, 14C ages of benthic foraminifers and of kerogen from flood layers are more strongly affected by variable 14C reservoir effects, and are therefore of limited value for absolute dating of SBB sediment layers. A late Holocene shift towards larger 14C age differences between planktic and benthic foraminifers after ~ 2,000 yr. B.P. suggests influx of a more 14C-depleted (thus older and oxygen-depleted) source of sub-sill bottom water into the SBB, consistent with the absence of bioturbating benthos and the enhanced preservation of varves observed in the cores. The independent varve chronology allowed us to derive a record of the regional radiocarbon reservoir age (ΔR), by comparing with the 14C dates for the last 6,700 years.

Seasonal and interannual variation in Santa Barbara Basin water temperatures observed in sediment trap foraminiferal Mg/Ca

We have generated a Santa Barbara Basin sediment trap Mg/Ca record to evaluate culture‐based calibration equations for the temperate/subpolar planktonic foraminiferal species Globigerina bulloides and Neogloboquadrina pachyderma (dextral). G. bulloides and N. pachyderma Mg/Ca are significantly correlated with seasonal variations in temperature of the near‐surface and subthermocline depths, respectively. The average deviation between Mg‐based and δ18O ‐based temperature estimates is ±0.6°C for G. bulloides and ±0.8°C for N. pachyderma. Differences between Mg‐derived temperatures and instrumental temperatures may be due to cleaning methods, environmental factors such as seasonal variations in foraminiferal depth habitats, or possible secondary effects on Mg/Ca. Core‐top Mg/Ca of both G. bulloides and N. pachyderma is in excellent agreement with average values from the sediment trap, lending strong support for the interpretation of downcore Mg/Ca from these species as annual average near‐surface and thermoclinal temperatures.

Variability in vertical distributions of planktonic foraminifera in the California Current: Relationships to vertical ocean structure

Geochemical records from planktonic foraminifera are among the few means available to infer past changes in the upper ocean, although interpretations can be confounded by variability in foraminifera habitat depths. The principal environmental factors affecting foraminiferal vertical distributions were examined through 28 vertically stratified plankton tows taken off southern California in winter, spring, and summer. Each species has a dynamic range of preferred habitat depth, from the mixed layer to the thermocline, characterized by a unique relationship with environmental properties. Globigerina bulloides, Neogloboquadrina pachyderma (dextral), and Neogloboquadrina dutertrei can live near the surface but frequently live deeper as stratification increases and a particular isotherm deepens. Globigerinoides ruber and Orbulina universa vary more closely with the depth of the thermocline than the depth of a given isotherm. The shifts in habitat depth are of sufficient magnitude to affect δ18O records. The effects of such shifts are observed for G. bulloides and N. pachyderma in Santa Barbara Basin sediments. Geochemical signatures from multiple species of foraminifera, each with a different predicted response to environmental change, can permit the detection of changes in habitat depth of a given species and thus more accurately indicate past changes in hydrographic structure.

Evidence against clathrate‐derived methane release to Santa Barbara Basin surface waters?

The Clathrate Gun Hypothesis suggests that methane from the marine sedimentary gas hydrate reservoir was episodically released into the atmosphere where it generated rapid atmospheric warming that drove late Quaternary abrupt millennial‐scale climate change. Brief, large negative δ13C excursions recorded by planktonic (up to −3‰) and benthic (up to −5‰) foraminifera from Santa Barbara Basin sediments have been interpreted as evidence of clathrate‐derived methane release to surface waters during or immediately before marine isotope stage (MIS) 3 interstadials. Molecular biomarkers from the δ13C excursion horizons have been interpreted to support this scenario. However, recent work at modern methane seeps show that fossil specimens can be contaminated with authigenic overgrowths or calcite replacement that may be difficult to detect through microscopy. The possibility that planktonic specimens are contaminated, and thus their δ13C values not a primary signal, warrants further inquiry. We generated a high‐resolution continuous total organic carbon (TOC) δ13C record from fortuitously obtained samples across the younger foraminiferal δ13C excursion during Interstadial 8. The record exhibits variations similar in magnitude and timing to those that occurred in the basin over the last 600 years. Furthermore, previously published TOC δ13C values of discreet samples from this and the three other MIS 3 foraminiferal δ13C excursions are also similar to recent values. Although there are factors that could obscure a surface water methane signal in the TOC δ13C record, these results question the interpretation that methane was released to surface waters. An alternative hypothesis to explain the available data is that the foraminiferal δ13C excursions are an artifact of diagenetic processes at or below the sediment‐water interface. This interpretation may be compatible with the molecular biomarker data. Further work is necessary to differentiate between the two hypotheses.

A 900 year stable isotope record of interdecadal and centennial change from the California Current

This paper describes the development of a high‐resolution record of interdecadal and centennial variability of ocean climate of the California Current from continuous 5 year sampling intervals based on varve counts from Santa Barbara Basin sediments. We show the importance of the choice of species and size fraction as well as the practice of making multiple measurements per interval to obtain a clear environmental signal capable of resolving interdecadal change. Changes in temperature and salinity from oceanographic surveys of the past 50 years indicate that thermal variability dominates the δ18O signal. A δ18O record from Neogloboquadrina dutertrei spanning 1030–1905 A.D. contains interdecadal thermal fluctuations within the upper thermocline that are centered around periods of roughly 55, 70, and 90 years. The interdecadal changes comprise the major share of total variability compared to a low‐amplitude mode of underlying multicentennial variability corresponding to the Medieval Warm Period and the Little Ice Age.

An organic tracer for surface ocean radiocarbon

The Δ14C of surface water dissolved inorganic carbon (DIC) in the Southern California Bight was compared to Δ14C as recorded by the sterols in Santa Monica and Santa Barbara Basin sediments. All of the C26, C27, C28, and C29 sterols as well as dinosterol had 14C concentrations equal to surface water DIC, indicating that all of the major sterols were derived from phytoplanktonic production. There is no detectable terrestrial component. Their tracer capability was confirmed by comparing the “bomb 14C”‐derived change in surface water Δ14CDIC with the change in Δ14Csterol. The “prebomb” Δ14CDIC was −82‰, and prebomb sterols averaged −75±19‰. The Δ14C value in 1996 was +71‰. Eighteen measurements representing eight different sterols from the sediment‐water interface of both Santa Monica and Santa Barbara Basins averaged +62±23‰. When three of these values were eliminated because of suspected contamination, the remaining data averaged +71 ±12‰. The entire compound class could serve as an excellent proxy for the 14C concentration of ocean surface waters.

Nitrogen isotope variations in Santa Barbara Basin sediments: Implications for denitrification in the eastern tropical North Pacific during the last 50,000 years

Nitrogen isotope variations preserved in Santa Barbara Basin sediments are used to evaluate changes in denitrification in the eastern tropical North Pacific (ETNP) during the last 50,000 years. A significant component of the subsurface waters (∼100–400 m) that presently fill the Santa Barbara Basin is derived from the low‐oxygen, denitrifying zone in the ETNP, and the nitrate in these waters has a δ15N value of 8–9‰. During the last glacial, the δ15N values of Santa Barbara Basin sediments were typically 6–7‰, indicating decreased denitrification in the ETNP and a better oxygenated intermediate water mass in the Santa Barbara Basin at this time. This reduced denitrification during the last glacial would have increased the pool of fixed nitrogen and may have contributed to the higher productivity previously reported for various regions of the global ocean during this period. At the onset of deglaciation, sediment δ15N values increase by more than 2‰, indicating increased denitrification in the ETNP. During Younger Dryas time, δ15N values decreased by 3‰ and record a brief return to better ventilated conditions in the subsurface waters of the ETNP. This is followed by an increase in δ15N to over 9‰ at ∼10,000 years ago, indicating intense denitrification at the beginning of the Holocene.

A 560-Year Record of Santa Ana Fires Reconstructed from Charcoal Deposited in the Santa Barbara Basin, California

Microscopic charcoal from varved Santa Barbara Basin sediments was used to reconstruct a 560-yr record (A.D. 1425 to 1985) of Santa Ana fires. Comparison of large (>3750 μm2) charcoal with documented fire records in the Santa Barbara Ranger District shows that high accumulations correspond to large fires (>20,000 ha) that occurred during Santa Ana conditions. The charcoal record reconstructed a minimum of 20 large fires in the Santa Barbara region during the study period. The average time between fires shows no distinct change across three different land use periods: the Chumash period, apparently characterized by frequent burning, the Spanish/Early American period with nominal fire control, and the 20th century with active fire suppression. Pollen data support the conclusion that the fire regime has not dramatically changed during the last 500 yr. Comparison of large charcoal particle accumulation rates and precipitation reconstructed from tree rings show a strong relationship between climate and fire history, with large fires consistently occurring at the end of wet periods and the beginning of droughts.

Radiolarian assemblages from Santa Barbara Basin sediments: Recent interdecadal variability

Santa Barbara Basin contains a sedimentary record ideal for high‐resolution paleoclimate studies because of the annual varves and regional‐ to global‐scale climate signals preserved in the sediments [Lange et al., 1990; Kennett and Ingram, 1995], even though it does not lie directly in the path of the California Current. A nearly 100‐year annual time series (1909–1991) of polycystine radiolarian assemblages from Santa Barbara Basin (SBB) sediments was analyzed to the species level. Counts on a replicate SBB core, dated 1870–1987, indicate that results are reproducible and the flux of a few representative species can be extrapolated to estimate fluxes of environmentally sensitive groups. The frequency of species occurrences resembles a lognormal curve and year‐by‐year comparisons of species fluxes revealed only modest changes in the assemblages from 1909–1991, indicating that the assemblages represent a single oceanic province. For paleoceanographic analysis of the radiolarian record, species were combined into groups according to the water mass in which they occur. To test this method, temperature‐sensitive species were identified using t‐ tests. This generated warm and cool classes exhibiting trends in relative flux similar to those of the water mass groups. Both total flux and relative fluxes of water mass groups relate to low‐frequency, decadal‐scale temperature fluctuations, but not strongly to El Niño‐Southern Oscillation events. Generally, fluxes of species from different water masses covary suggesting changing carrying capacities and productivity through time, while the consistent inverse relationship in relative fluxes indicate variability in climate. The subtle, decadal‐scale changes in assemblages, diversity, and increase in percent warm water fauna are consistent with a spin‐down of the California Current System suggested by other records.

A 160-kyr Record of Alkenone-Derived Sea-Surface Temperatures from Santa Barbara Basin Sediments

A 160-kyr Record of Alkenone-Derived Sea-Surface Temperatures from Santa Barbara Basin Sediments

Carbon isotope relationships between sulfide-bound steroids and proposed functionalized lipid precursors in sediments from the Santa Barbara Basin, California

In this study, molecular and isotopic analyses were used to evaluate relationships between sulfide-bound compounds and their proposed lipid precursors in surficial sediments from the Santa Barbara Basin (SBB), California. The hydrocarbons released from the polar total lipid extract fraction from sediments by reaction with Raney nickel were dominated by phytane with a lesser amount of 5α(H) and 5β(H) C 27, C 28 and C 29 steranes. The steroidal hydrocarbons released were compared with extractable steroidal alcohols from the same depth. Wide and systematic variations were discovered in the range of isotopic compositions of extracted steroidal alcohols and their sulfide-bound analogs with the same carbon skeleton. The isotopic data offer no clear delineation of the product/precursor relationship but do demonstrate that concentration alone does not determine the extent to which a steroidal alcohol is sulfurized. Compound-specific isotope analyses of extractable 24-ethylcholesterols and S-bound 24-ethylcholestane suggest that C 29 steroidal alcohols of terrestrial origin are more susceptible to sulfurization than those that are derived from marine organisms. This investigation of the early diagenetic sulfurization process shows that the amount of steranes released by Raney nickel from the SBB sediments accounts for between 0.2 and 3.0% of the available 4-desmethyl steroidal alcohol precursor pool with a similar carbon skeleton over the first 12 cm; also that the C 27 steroidal alcohols were consistently more reactive to sulfurization than the C 28 steroidal alcohol pool. The 4-methyl steroidal alcohols were unreactive toward sulfurization. The preservation of steroidal biomarkers by the sulfurization process, as revealed by Raney nickel treatment of the polar total lipid extract, indicates that sulfur incoporation does occur to a minor extent at this very early stage of diagenesis and is quite variable among different steroidal alcohol groups.

Tales of 1001 varves: a review of Santa Barbara Basin sediment studies

Abstract The varved marine anoxic sediment in the deep centre of the Santa Barbara Basin off California has been studied extensively by sedimentologists, micropalaeontologists and geochemists during the past four decades. The Santa Barbara Basin is the only basin in the California Borderland Province that exhibits laminations for most of the Holocene. These sediments are essential to further our understanding of climate in that they provide a rare oceanic record of climate change with high resolution. In this overview of the complex and multidisciplinary Santa Barbara Basin sediment research, we briefly introduce the reader to the various analytical approaches and present a comprehensive collection of literature references.

Evolutionary changes over the last 1000 years of reduced sulfur phases and organic carbon in varved sediments of the Santa Barbara Basin, California

Sediments from the Santa Barbara Basin (SBB) of the last 1000 years were analyzed for elemental sulfur (S e), mineral sulfide sulfur (S m), lipid-based organically bound sulfur (S x), residual organically bound sulfur (S o), total organic carbon (TOC) and total nitrogen (TN). The approximate annual resolution of the time-series data permitted us to evaluate the influence of oceanic variables on the benthic environment and the accumulating varved sediment in the SBB. A bacterial mat community at or near the sediment surface is influencing sediment porosity and the production and distribution of S e. Early diagenesis dampens the amplitude of environmental signals in the geochemical time series. The concentrations of S m and S o increase with burial depth, at the expense of decreasing concentrations of S x and S e. At depths greater than 5.5 cm, corresponding to an age of 7 years, S m becomes the largest pool of reduced sulfur. It requires approximately 500 years of primarily bacterially mediated diagenesis, equivalent to a burial depth of ~ 1.4 m, for SBB sediments to gradually approach and finally enter the region assumed to represent typical “normal” marine sediments on a plot of weight percent TOC vs. S m.

Historical oceanographic events reflected in13C/12C ratio of total organic carbon in laminated Santa Barbara Basin Sediment

An 1844‐1987 time series of carbon stable isotope ratios from dated sedimentary total organic carbon (TOC) from the center of the Santa Barbara Basin (SBB) is compared with historical climate and oceanographic records. Four isotopically distinct biogeochemical sources of TOC are important: phytoplankton‐derived marine biomass, macroalgal biomass from kelp forests, terrigenous biomass (mainly flushed into the SBB via river discharge), and redeposited fossil organic carbon. The significance of the latter two sources is largely limited to a few unusual flood and oil spill events, whereas the combination of 13C‐depleted phytoplankton and 13C‐enriched macroalgal biomass appears to be responsible for most of the isotopic variance of the marine coastal biomass as recorded in sedimentary TOC. The isotopic response of marine organic carbon in sediments records strong El Niño‐Southern Oscillation (ENSO) and the frequently associated severe storm and wave events in SBB varved sediments. The plausible major isotopic mechanisms are (1) increased physical liberation of 13C‐enriched kelp carbon from locally abundant giant kelp (Macrocystis spp.) forests during times of physical and environmental stress, and (2) decreased productivity of 13C‐depleted phytoplankton during ENSO events.

Climatically controlled marker layers in Santa Barbara Basin sediments and fine-scale core-to-core correlation

Varved sediments with annual layering can provide a detailed history of environmental conditions of marginal ocean basins and of lakes. To reconstruct this history requires fine-scale correlation and correct age-assignment. In the Santa Barbara Basin, a combination of X-radiography, water content profiles, and microfossil stratigraphy yields correlations of the desired resolution. X-radiography permits identification of individual layers (∼1–2 mm thick) that differ in porosity (or pore-water content) and, hence, X-ray absorption. Microfossils and sedimentological parameters provide the clues to the presence for El Nino years (e.g. in a.d. 1984, 1959, and 1942 varves), allowing the tie-in to absolute years within this century. El Nino layers, once identified, are readily recognized on the radiographs and within the porosity profiles. Counting of laminae in X-radiographs and core-to-core correlation via water content then allow establishing age scales for other cores. A prominent shell layer in the first half of the 19th century is dated at a.d. 1838. This event (Macoma I) is of supraregional significance.

Changes of lead and barium with time in California off-shore basin sediments

During ancient times the natural deposition fluxes of lead which can be leached with dilute acid from sediments in Santa Barbara, Santa Monica and San Pedro basins offshore from the Los Angeles Urban complex, were about 0.7, 0.1 and 0.2 μg Pb/cm 2 yr respectively. Since there was little difference in biological productivity in surface waters of these basins, it is proposed that clay is a major transport vehicle for sequestered soluble lead, which then explains why the lead deposition flux within the Santa Barbara basin was so much larger compared to the other basins. The fluxes of silicate mud in the basins in ancient times were about 92, 19 and 30 mg/cm 2 yr in Santa Barbara, Santa Monica and San Pedro basins respectively. Today deposition fluxes of acid soluble lead within these three basins are 3- to 9-fold greater, being about 2.1, 1.1 and 1.8 μg Pb/cm 2 yr respectively, partly in the form of directly deposited large sewage particles, which account for none, 2 3 and 3 4 of the total industrial lead deposition fluxes in the respective basins. Concentrations of leachable lead in varve dated sediment layers increase with time and isotopic compositions of these leads change in accordance with corresponding known changes of isotopic compositions of industrial lead in the Los Angeles atmosphere. Lead remaining in acid leached sediment residues originates from igneous and clay minerals, exhibiting no change in concentration or isotopic composition since pre-industrial times. Deposition fluxes of total barium in sediments among the three basins were proportional to mass deposition fluxes before 1950 in the same manner as for lead. Afterwards, there are barium concentration maxima with time in both Santa Monica and San Pedro Basin sediments which are attributable to industrial sewage rather than to episodic erosion from barium-rich sedimentary evaporite strata exposed locally along the shore. An increase of barium concentrations in present day Santa Barbara basin sediments may reflect dispersal of barium-rich drilling mud from local drilling operations.

Radon-222 as a tracer for mixing in the water column and benthic exchange in the southern California borderland

The concentrations of 222Rn and 226Ra in the water column and in the sediments of Santa Barbara and San Nicolas Basins have been measured semi-annually over the last four years. Approximately one-third of excess radon profiles obtained in the water column in these basins can be adequately fit with a one-dimensional eddy diffusion-decay model. Exponential profiles in the center of San Nicolas Basin yield a vertical eddy diffusivity of 26±16 cm 2/s and 3.4±1.0 cm 2/s for Santa Barbara Basin. The application of a two-dimensional eddy diffusion-decay model to profiles obtained in the center and on the margins of San Nicolas Basin produces a better fit than is found using a one-dimensional vertical eddy diffusivity. The two-dimensional model for San Nicolas Basin predicts a vertical eddy diffusivity of 17 cm 2/s and a horizontal eddy diffusivity of 10 5 cm 2/s. These values are in reasonable agreement with those predicted from the vertical buoyancy gradient and the horizontal length scale. The vertically integrated radon excess (standing crop) in the water column of Santa Barbara Basin averages 53±23 atoms/m 2 s. This is in good agreement with the flux across the sediment-water interface of 60±15 atoms/m 2 s, calculated by measuring radon emanation in the sediments as a function of depth and applying a molecular diffusion-reaction model. Hence, one-dimensional molecular diffusion accurately predicts the flux of radon from the laminated Santa Barbara Basin sediments. In San Nicolas Basin the integrated radon excess in the water column is 376±143 atoms/m 2 s, but the diffusive randon flux from San Nicolas Basin sediments averages only 190±53 atoms/m 2 s. This descrepancy indicates that a non-diffusive process, probably macrofaunal irrigation, supplies much of the flux of radon from San Nicolas Basin sediments.