Santa Monica Basin Research Articles

Dissolved sulfide distributions in the water column and sediment pore waters of the Santa Barbara Basin

Dissolved sulfide concentrations in the water column and in sediment pore waters were measured by square-wave voltammetry (nanomolar detection limit) during three cruises to the Santa Barbara Basin in February 1995, November–December 1995, and April 1997. In the water column, sulfide concentrations measured outside the basin averaged 3 ± 1 nM (n = 28) in the 0 to 600 m depth range. Inside the basin, dissolved sulfides increased to reach values of up to 15 nM at depths >400 m. A suite of box cores and multicores collected at four sites along the northeastern flank of the basin showed considerable range in surficial (<0.5 cm) pore-water sulfide concentrations: <0.008, 0.01, 0.02, to as much as 0.4 μM at the 340, 430, 550, and 590 m sites, respectively. At a core depth of 10 cm, however, pore–water sulfides exhibited an even wider range: 0.005, 0.05, 0.1, and 100 μM at the same sites, respectively. The sulfide flux into the deep basin, estimated from water-column profiles during three cruises, suggests a fairly consistent input of 100–300 nmole m−2 h−1. In contrast, sulfide fluxes estimated from pore-water sulfide gradients at the sediment water interface were much more variable (−4 to 13,000 nmole m−2 h−1). Dissolved silicate profiles show clear indications of irrigation at shallow sites (340 and 430 m) in comparison to deeper basin sites (550 and 590 m) with low (<10 μM) bottom-water dissolved-oxygen concentrations. Pore-water profiles indicate ammonia generation at all sites, but particularly at the deep-basin 590 m site with concentrations increasing with sediment depth to >400 μM at 10 cm. Decreases in water-column nitrate below the sill depth indicate nitrate consumption (−55 to −137 μmole m−2 h−1) similar to nearby Santa Monica Basin. Peaks in pore-water iron concentrations were generally observed between 2 and 5 cm depth with shallowest peaks at the 590 m site. These observations, including observations of the benthic microfauna, suggest that the extent to which the sulfide flux, sustained by elevated pore-water concentrations, reaches the water column may be modulated by the abundance of sulfide-oxidizing bacteria in addition to iron redox and precipitation reactions.

Outcrop‐scale acoustic facies analysis and latest Quaternary development of Hueneme and Dume submarine fans, offshore California

The uppermost Quaternary deposits of the Hueneme and Dume submarine fans in the Santa Monica Basin have been investigated using a closed‐spaced grid of boomer seismic‐reflection profiles, which give vertical resolution of a few tens of centimetres with acoustic penetration to 50 m. Acoustic facies integrated with geometry define six architectural elements, some with discrete subelements that are of a scale that can be recognized in outcrops of ancient turbidite systems. In the Santa Monica Basin, the relationship of these elements to fan morphology, stratigraphy and sediment source is precisely known.The width of upper Hueneme fan valley has been reduced from 5 km since the last glacial maximum to 1 km at present by construction of laterally confined sandy levees within the main valley. The middle fan comprises three main subelements: thick sand deposits at the termination of the fan valley, low‐gradient sandy lobes typically 5 km long and &lt; 10 m thick, and scoured lobes formed of alternating sand and mud beds with many erosional depressions. The site of thickest lobe sediment accumulation shifts through time, with each sand bed deposited in a previous bathymetric low (i.e. compensation cycles). The lower fan and basin plain consists of sheet‐like alternations of sand and mud with shallow channels and lenses.Variations in the rate of late Quaternary sea level rise initiated changes in sediment facies distribution. At lowstand, and during the approximately 11 ka stillstand in sea level, the Hueneme Fan was fed largely by hyperpycnal flow from the Santa Clara River delta, depositing high sediment waves on the right hand levee and thick sandy lobes on the middle fan. At highstand of sea level, most turbidity currents were generated by failure of silty prodelta muds. In contrast, the smaller Dume Fan was apparently always fed from littoral drift of sand through a single‐canyon point source.

Evidence for differential degradation of alkenones under contrasting bottom water oxygen conditions: implication for paleotemperature reconstruction

The successful reconstruction of sea surface temperatures using alkenone paleothermometry (U 37 k′) has relied on the premise that there is no significant differential degradation of alkenones with different states of unsaturation during diagenetic processes. To test this assumption, we conducted a comparative study of contemporary sediments in oxic and anoxic bottom waters from the Santa Monica Basin, offshore California. Long-chain alkenones were quantified and sea surface temperature were calculated using the calibrated U 37 k′–T relationship of Prahl et al. (1988). Our results show that temperature record from the oxic sediments is higher by as much as 4°C compared to those from time-equivalent anoxic sediments as a result of differential degradation of long-chain unsaturated alkenones and bioturbation mixing in the oxic sediments. The differential degradation of C 37:3 vs. C 37:2 alone could account for up to 2.5°C difference between these two records. This finding has significant implication in the interpretation of paleo–sea surface temperature data using alkenone paleothermometry.

Outstanding student paper award

The Ocean Sciences Section presented the outstanding student paper award to Ann Pearson at the 1998 AGU Spring Meeting in Boston, Mass., last May.Ann presented a paper titled “Molecular‐level Radiocarbon Analysis as a Tracer of Organic Matter Source: Results for Lipid Biomarkers from Santa Monica Basin Surface Sediment.” Ann completed a B.A. in chemistry from Oberlin College in 1992. She served in the U.S. Peace Corps in Ecuador in 1993–1994, and began graduate study in the MIT/Woods Hole Oceanographic Institution Joint Program in 1994. She is currently working towards a Ph.D. with Timothy Eglinton and Ann McNichol, both of WHOI. Her interests are broadly based in light isotope biogeochemistry, and she increasingly finds herself drawn to projects investigating microbial pathways of carbon utilization and their impact on the carbon cycle. For her thesis work, Ann is conducting the first systematic investigation of the uses of molecular‐level radiocarbon analysis as both chronometer and biogeochemical tracer.

Sea level controls on the textural characteristics and depositional architecture of the Hueneme and associated submarine fan systems, Santa Monica Basin, California

Hueneme and Dume submarine fans in Santa Monica Basin consist of sandy channel and muddy levee facies on the upper fan, lenticular sand sheets on the middle fan, and thinly bedded turbidite and hemipelagic facies elsewhere. Fifteen widely correlatable key seismic reflections in high‐resolution airgun and deep‐towed boomer profiles subdivide the fan and basin deposits into time‐slices that show different thickness and seismic‐facies distributions, inferred to result from changes in Quaternary sea level and sediment supply. At times of low sea level, highly efficient turbidity currents generated by hyperpycnal flows or sediment failures at river deltas carry sand well out onto the middle‐fan area. Thick, muddy flows formed rapidly prograding high levees mainly on the western (right‐hand) side of three valleys that fed Hueneme fan at different times; the most recently active of the lowstand fan valleys, Hueneme fan valley, now heads in Hueneme Canyon. At times of high sea level, fans receive sand from submarine canyons that intercept littoral‐drift cells and mixed sediment from earthquake‐triggered slumps. Turbidity currents are confined to ‘underfit’ talweg channels in fan valleys and to steep, small, basin‐margin fans like Dume fan. Mud is effectively separated from sand at high sea level and moves basinward across the shelf in plumes and in storm‐generated lutite flows, contributing to a basin‐floor blanket that is locally thicker than contemporary fan deposits and that onlaps older fans at the basin margin. The infilling of Santa Monica Basin has involved both fan and basin‐floor aggradation accompanied by landward and basinward facies shifts. Progradation was restricted to the downslope growth of high muddy levees and the periodic basinward advance of the toe of the steeper and sandier Dume fan. Although the region is tectonically active, major sedimentation changes can be related to eustatic sea‐level changes. The primary controls on facies shifts and fan growth appear to be an interplay of texture of source sediment, the efficiency with which turbidity currents transport sand, and the effects of delta distributary switching, all of which reflect sea‐level changes.

Abstract: Depositional Architecture and Outcrop-scale Acoustic Facies Analysis for Hueneme and Dume Fans, Santa Monica Basin, California

Abstract: Depositional Architecture and Outcrop-scale Acoustic Facies Analysis for Hueneme and Dume Fans, Santa Monica Basin, California

The effects of source lithology, transport, deposition and sampling scale on the composition of southern California sand

ABSTRACTThe Transverse Ranges of southern California represent an uplifted and variably dissected Mesozoic magmatic arc, and Mesozoic to Holocene sedimentary and volcanic strata deposited in convergent and transform tectonic settings. Modern sand within part of the Western Transverse Ranges represents: first‐order sampling scale of the Santa Monica and the San Gabriel Mountains; second‐order sampling scale of the Santa Clara River draining both mountain ranges; and third‐order sampling scale of the beach system between the mouth of the Santa Clara River and the eastern Santa Monica Mountains, and turbidite sand of the Hueneme‐Mugu submarine fan.Source lithology includes plutonic and metamorphic rocks of the San Gabriel Mountains, and sedimentary and volcanic rocks of the Santa Monica Mountains. First‐order sands have large compositional variability. Sand from local coastal drainage of the Santa Monica Mountains ranges from basaltic feldspatholithic to quartzofeldspathic. Sand of the San Gabriel Mountains local drainages has three distinct petrofacies, ranging from metamorphiclastic feldspatholithic to mixed metamorphi/plutoniclastic and plutoniclastic quartzofeldspathic. Second‐order sand is represented by the main channel of the Santa Clara River; the sand has an abrupt downstream compositional change, from feldspathic to quartzofeldspathic. Third‐order sand (beaches and deep‐sea turbidite samples) of the Santa Monica Basin is quartzofeldspathic. Beach sand is more quartz‐rich than is Santa Clara river sand, whereas turbidite sand is more feldspar‐rich than is beach sand. Deep‐sea sand has intermediate composition with respect to second‐order samples of the Santa Clara River and third‐order samples of the beach system, suggesting that (1) the Santa Clara River is the main source of sediments to the marine environment; and (2) local entry points from canyons located near local drainages may generate turbidity currents during exceptional flood conditions. Petrologic data of modern sand of the study area are highly variable at first‐ and second‐order scale, whereas third‐order sand is homogenized. The homogenized composition of deep‐marine sand is similar to the composition of most ancient sandstone derived primarily from the Mesozoic dissected magmatic arc of southern California. This study of the Western Transverse Ranges illustrates the effects of source lithology, transport, depositional environment, and sampling scale on sand composition of a complex system, which provides insights regarding actualistic petrofacies models.

Consistent fractionation of 13C in nature and in the laboratory: growth-rate effects in some haptophyte algae.

The carbon isotopic fractionation accompanying formation of biomass by alkenone-producing algae in natural marine environments varies systematically with the concentration of dissolved phosphate. Specifically, if the fractionation is expressed by epsilon p approximately delta e - delta p, where delta e and delta p are the delta 13C values for dissolved CO2 and for algal biomass (determined by isotopic analysis of C37 alkadienones), respectively, and if Ce is the concentration of dissolved CO2, micromole kg-1, then b = 38 + 160*[PO4], where [PO4] is the concentration of dissolved phosphate, microM, and b = (25 - epsilon p)Ce. The correlation found between b and [PO4] is due to effects linking nutrient levels to growth rates and cellular carbon budgets for alkenone-containing algae, most likely by trace-metal limitations on algal growth. The relationship reported here is characteristic of 39 samples (r2 = 0.95) from the Santa Monica Basin (six different times during the annual cycle), the equatorial Pacific (boreal spring and fall cruises as well as during an iron-enrichment experiment), and the Peru upwelling zone. Points representative of samples from the Sargasso Sea ([PO4] < or = 0.1 microM) fall above the b = f[PO4] line. Analysis of correlations expected between mu (growth rate), epsilon p, and Ce shows that, for our entire data set, most variations in epsilon p result from variations in mu rather than Ce. Accordingly, before concentrations of dissolved CO2 can be estimated from isotopic fractionations, some means of accounting for variations in growth rate must be found, perhaps by drawing on relationships between [PO4] and Cd/Ca ratios in shells of planktonic foraminifera.

Differential contribution of bacteria to sedimentary organic matter in oxic and anoxic environments, Santa Monica Basin, California

Two sediment box-cores in the Santa Monica Basin (SMB), one from the depocenter (900 m water depth) with anoxic bottom water and one from the periphery (840 m water depth) with oxic bottom water, were comparatively studied to assess the role of bottom water oxygen conditions to organic matter preservation and the differential contribution of bacteria to sedimentary organic matter accumulation and composition. Fatty acids from the sediment extracts were analyzed for their abundance and carbon isotopic compositions. Molecular and isotopic evidence indicates that most of the short-chain FAs from the SMB sediment extracts are sourced by bacteria as a result of intense microbial recycling and resyntheses of organic matter, especially in the aerobic environment at the periphery site. Long-chain fatty acids are probably derived from terrestrial sources and partially sourced by microbial processes. Very depleted δ 13C values (−30 to −45‰) of bishomohopanoic acids and triglycerides suggest the presence of chemoautotrophic bacteria in the anoxic water column and in the sediments. There is a much greater contribution from anaerobic microbial biomass and chemoautotrophic bacteria in the depocenter sediments relative to the periphery sediments. In addition, organic matter exhibits bulk and molecular characteristics of enhanced preservation under the condition of anaerobic decomposition at the depocenter site, especially at the sediment/water interface and the first couple of centimeters. Together, enhanced preservation with the addition of microbial biomass could explain the higher organic accumulation rates and H/C ratios in the basin depocenter and the periphery sediments and the trend of decreasing δ 13C values of bulk organic matter in the depocenter sediments. This study has implications for understanding the role of anoxia and associated bacterial processes in controlling the geochemical characteristics ancient organic carbon-rich sediments deposited under oxic and anoxic conditions.

Inter‐decadal signals during the last millennium (AD 1117–1992) in the Varve record of Santa Barbara Basin, California

Annual varve thickness (AD 1117–1992) from Santa Barbara basin has been decomposed into orthogonal components using singular spectrum analysis (SSA) to identify and retrieve inter‐decadal oscillations. After removing all variability with periods greater than 150 years, leading SSA eigenfunctions (EOFs) identify four oscillatory pairs with periods of ∼100, ∼58, ∼25, and ∼12 years respectively. Based on 2500 simulated series and on two‐sided confidence intervals, EOFs 1–7 are significant at the 99% level and EOFs 8–9 are significant at the 90% level. Oscillatory signals retrieved from the marine varves show an abrupt change in frequency and amplitude near AD 1600. The largest contribution to this environmental shift is given by the interdecadal components, especially the ∼25 and the ∼12‐year oscillation. The near‐AD 1600 change may be related to multi‐annual events reported in the stratigraphy of the nearby Santa Monica Basin and in dendrochronological records of the American Southwest.

Depositional events in Santa Monica Basin, California Borderland, over the past five centuries

The southern California coastal climate is characterized by decadal and generational floods that discharge one to three orders of magnitude more sediment than the average annual contribution. These can produce turbidity currents directly or load the canyons and slopes with sediments that then may fail and form turbidity flows. Seismic activity is high on this transform margin and can trigger slope failures where slope sedimentation rates are high and sediments are unstable. These events produce slope failures, and slides and slumps in canyon heads, which evolve into turbidity currents of sufficient volume to reach the adjacent margin basin floors. Such events occur at approximately century frequency. This paper describes six basin-floor turbidites that have been mapped in box cores sampling roughly the top 40–50 cm from the basin floor or Santa Monica Basin in the California Borderland. These deposits were laid down during the past 500 + years of record. Their depositional areas have been conservatively estimated to range from 500 to almost 1500 km 2, and contain masses of sediment ranging from 10,000,000 to 100,000,000 tons. This compares with a long-term average annual terrigenous contribution rate of about 4,000,000 tons/yr from the adjacent major streams, although annual rates vary from 0 to 60,000,000 tons over the last century of record. Three of the six events have been related to two major floods (1884-6, 1969) and to one major seismic event (the Mission Earthquakes of 1822-1812). The older turbidites were deposited before European entry to the area and cannot be assigned to an historic event but, from their size, were probably seismically triggered and are the largest of the six events. It is likely that other smaller events are masked by the more recent turbidites, particularly in the area where box cores are dominantly composed of turbidites. These features exhibit average ‘outcrop’ thickness of 2 to 7 cm with maxima of the order of 10 cm thickness, yet represent major events and significant volumes of sediment. They also document that the two major sources for such events, floods and seismic events, produce deposits of very different mass and can probably be separated in the record on that basis. Thus the frequency of major floods and of major earthquakes in the region could be defined from study of such deposits in longer cores. These deposits also provide examples of continued turbidity current activity to the basin floors at a time of high sea level. This is contrary to the passive models for sequence stratigraphy depositional patterns.

Contemporary channel-levee systems in active borderland basin plains, California Continental Borderland

Long-range large-scale side-scan (GLORIA) information, other seismic reflection profiling studies, and data from cores in the California Continental Borderland, have defined active levee-channel systems extending basinward from the lower fan of Hueneme-Mugu Submarine Fan, Redondo Submarine Fan, and Santa Cruz Canyon Fan in Santa Monica, San Pedro and Santa Cruz Basins, respectively. The Holocene distributaries have been created by a series of turbidity current events. These distributaries range in length from 10–25 km, and are wide (2–5 km) and low-relief (1–10 m) in their distal parts. They are also active conduits for nepheloid flows. Distributions of sedimentological parameters typically mimic the pattern of levee-channel systems. Organic carbon and biogenic carbonate content roughly outline the systems. Channels are incised in the upper to middle fan areas, and become constructional leveed channels in the lower fan and basin plain as the channel gradient adjusts to maintain a graded profile. Thus sediment gravity flows are generally confined to channels in the upper fan zones, but deposit both channelized and over-bank deposits on the lower fan and basin floor. The deposits show that the canyon-fan activity has continued during a rising sea level phase. It is evident that canyon headward erosion rates have been equal to or greater than the transgression rate, and that the canyon-fan systems have remained linked with their sediment sources. Frequency of events was probably higher, and volumes of the events were often larger, during the glacially lowered sea level episodes. However, turbidity currents of sufficient volume to reach the basin floors continue to occur at century or multi-century intervals. As one progresses headward in each system, the number of flows per length of core increases. Small flows that do not pass beyond the distributaries are much more frequent, and may be decadal in frequency, or even more frequent in the Santa Monica Basin system. These California borderland basins are probably typical of narrow active margins where rate of lateral sea level transgression is less than or equal to the rate of canyon headward erosion. The canyons maintain connections with sediment sources during sea level rise, and the systems therefore are active during the entire sea level cycle. Thus sediment supply is not a simple function of eustacy. This contrasts with the simplified sequence model developed on passive margins where canyons turn off as sea level rises.

DDTs and dumpsite in the Santa Monica Basin, California

Sediment core samples from the Santa Monica (SM) Basin generally contain three times as much DDEs (o,p′ and p,p′ as DDTs. Total DDTs (Σ o,p′ -DDE, -DDD and -DDT and p,p′ -DDE, -DDD and -DDT) are relatively higher in some cores than in others in nearsurface horizons. Further, the ratios of o,p′ -DDT [1,1,1-trichloro-2(o-chlorophenyl)-2(p-chlorophenyl) ethane] to p,p′ -DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane] in some slope samples (0.43–0.87) and that of a tar cake (0.90) from Dumpsite II, San Pedro (SP) Basin are significantly higher than that observed in caustic waste from a DDT factory discharged into the southern California sewers or technical grade DDT (0.2–0.3). This observation and the fact that o,p′ -DDT metabolizes more readily than the p,p′ -isomer, suggest that at least part of the DDTs, in the sediments could not have originated from the above two sources. The unique DDT signature of the samples is probably characteristic of the acid wastes of the DDT factory discharged along with refinery wastes into the Dumpsites I and II. The discharged wastes could have entered SM and SP Basins from the dumpsites. Or, more likely, in the absence of precise records before 1961, industrial wastes could have been ‘short-dumped’ in nearshore regions. This could explain the unique DDT profiles from the slope sediments. In contrast, the central basin cores document the history of wastewater input. Thus o,p′p,p′ DDT ratio could potentially be diagnostic of their sources. Contaminants have apparently migrated in the sediment column by both upward and downward diffusion. The presence of the refractory metabolites in sediment trap particles suggests upward diffusion and resuspension.

DDTs and dumpsite in the Santa Monica Basin, California

Sediment core samples from the Santa Monica (SM) Basin generally contain three times as much DDEs ( o, p′ and p, p′ as DDTs. Total DDTs (Σ o, p′ -DDE, -DDD and -DDT and p, p′ -DDE, -DDD and -DDT) are relatively higher in some cores than in others in nearsurface horizons. Further, the ratios of o, p′ -DDT [1,1,1-trichloro-2( o-chlorophenyl)-2( p-chlorophenyl) ethane] to p, p′ -DDT [1,1,1-trichloro-2,2-bis( p-chlorophenyl) ethane] in some slope samples (0.43–0.87) and that of a tar cake (0.90) from Dumpsite II, San Pedro (SP) Basin are significantly higher than that observed in caustic waste from a DDT factory discharged into the southern California sewers or technical grade DDT (0.2–0.3). This observation and the fact that o, p′ -DDT metabolizes more readily than the p,p′ -isomer, suggest that at least part of the DDTs, in the sediments could not have originated from the above two sources. The unique DDT signature of the samples is probably characteristic of the acid wastes of the DDT factory discharged along with refinery wastes into the Dumpsites I and II. The discharged wastes could have entered SM and SP Basins from the dumpsites. Or, more likely, in the absence of precise records before 1961, industrial wastes could have been ‘short-dumped’ in nearshore regions. This could explain the unique DDT profiles from the slope sediments. In contrast, the central basin cores document the history of wastewater input. Thus o,p′ p,p′ DDT ratio could potentially be diagnostic of their sources. Contaminants have apparently migrated in the sediment column by both upward and downward diffusion. The presence of the refractory metabolites in sediment trap particles suggests upward diffusion and resuspension.

Sand-rich Submarine Fans, Mio-Pliocene of Santa Monica Basin Offshore California: Untapped Exploration Targets: ABSTRACT

Santa Monica Basin lies directly west of Los Angeles Basin, one of the world`s most prolific oil provinces. Published literature suggests that Santa Monica Basin was starved of coarse clastics during the late Miocene through Pliocene. However, seismic sequence stratigraphy indicates that deposition of sand-rich fans alternated with mixed-load systems throughout the Delmontian and Repettian stages. Seismic sequences and facies are calibrated to seismic and well data from Beta Oil Field, in San Pedro Basin to the south. Eustasy evidently played a dominant role in controlling sedimentation. Variations in tan lithology, thickness, and basinward extent correspond to worldwide changes in sea level. Regional erosion surfaces apparently signify drops in sea level. Overlying thick seismic packages display hummocky to chaotic seismic facies separated by high- to low-amplitude continuous reflections. These configurations are interpreted as inner- to mid-fan channels separated by overbank deposits in sand-rich lowstand fans. High-amplitude basinwide reflections bound the tops of the sand-rich intervals, and likely represent condensed sections formed during sea-level rises. Thin seismic intervals above the condensed sections display downlap, and are interpreted as interbedded sandstones and shales of prograding highstand fans. Sediment input to Santa Monica Basin, based on seismic-facies and isochron patterns, was predominantly frommore » the (present-day) north, with subordinate input from the east. High-amplitude eustatic variations dominated deposition and sequence development even in this tectonically active basin Previously unidentified sand-rich fans are present, and have not been drilled.« less

A survey of occurrences of Holocene laminated sediments in California Borderland Basins: products of a variety of depositional processes

Abstract Laminated sediments are found in several contemporary depositional settings in basins of the California Continental Borderland. These primary structures are preserved either as a result of anoxia (or near anoxia) in the bottom water when oxygen demand is greater than the oxygenation rate, or as a result of sedimentation rates in excess of bioturbation rates. The controlling processes in the anoxic settings include cyclic seasonal variation in the composition of hemipelagic deposition (hemipelagic laminae), and/or formation and destruction of bacterial mats as a result of periodic bottom water flushing of normally low-oxygen bottom waters (cyclic organic-rich laminae). Non-bioturbated hemipelagic laminations occur in San Pedro, Santa Monica and Santa Barbara Basins. Low-oxygen bottom waters and exclusion of macrobenthos have been characteristic of San Pedro Basin for a few decades. These conditions have existed in Santa Monica Basin for a few centuries and in Santa Barbara Basin for most of the Holocene. Physical sedimentation processes include rapid deposition from turbidity currents. Fine grained contemporary distal turbidites occur in Santa Monica, San Pedro, San Diego, Santa Cruz and San Nicolas Basins. Turbidity current flows into the deep basin floor occur at century scale in Santa Monica Basin and at somewhat longer frequencies in San Pedro Basin. Frequency of such flows is at the kilo-year level in San Diego Trough, Santa Cruz Basin and San Nicolas Basin. Such events are of interest because they show that submarine canyons will remain active through a high sea-level period. In narrow-shelf, active margins, canyons may not be shut off as generally occurs on passive margins when sea level rises rapidly, as is typical of climatically driven sea level fluctuations.

Laminated sediments of Santa Monica Basin, California continental borderland

Abstract Laminated sediments from Santa Monica Basin differ from other laminated sediments of the California Continental Borderland (such as varved sediments of Santa Barbara Basin) because lamina couplets represent a 3–6 year time interval and are affected by distinct local oceanographic and climatic processes. Unfortunately, mechanisms which control the temporal pattern of laminations in this basin are poorly understood. In addition, this laminated sediment record has been underutilized in paleoceanographic and paleoclimatic studies. In an attempt to identify mechanisms which control lamination formation and in order to obtain paleoclimatic and paleoceanographic records from this region, a sediment core from this basin was analysed using a suite of geochemical, sedimentary, and micropaleontologic techniques. This box-core (core DOE 26) was collected from the centre of the basin and was vertically sub-sampled at one millimetre increments, providing a temporal resolution of 1–3 years per sample. A variety of paleoceanographic and paleoclimatic records were constructed via analysis of these samples and include proxy records of: (a) regional rainfall — as expressed by gross variations in downcore density profiles; (b) relative contribution of terrestrial and marine organic carbon to the basin — as expressed by downcore variations in the isotopic composition of total organic carbon (TOC); (c) Primary productivity — as expressed in downcore variations in the carbon isotopic composition of planktonic foraminifera; (d) changes in sea surface temperatures —reflected in downcore variations in the oxygen isotopic composition of N. pachyderma (dextral); (e) large-scale changes in bottom-water oxygen conditions — as expressed by downcore changes in abundance of oxygen- sensitive benthic foraminifera. To extract additional information from these proxy records, spectral analysis was also employed in this study. Although ENSO-length signals are present in several records, decadal-length signals appear to dominate records from this basin. This dominance suggests the presence of a longer-term forcing function (other than the ENSO phenomena) which strongly influences this region. Considered together, preliminary results suggest that the flux of organic carbon is a dominant factor affecting lamination variability in Santa Monica Basin. Organic carbon fluxes in this basin are mainly affected by primary productivity, and to a lesser extent, by terrestrial input. Variations in bottom oxygen conditions may not control lamination variability; rather, long-term depression of oxygen levels appears to allow preservation of this organic matter as laminated sediments.

Evidence for enhanced boundary mixing in the Santa Monica Basin

Transients in the heat content in Santa Monica and San Pedro Basins imply a basin‐wide diapycnal eddy diffusivity greater than 1 cm2/s. This is significantly larger than the value for SF6 of 0.25±0.08 cm2/s determined for the interior of Santa Monica Basin for September 1985 to February 1986 by Ledwell and Watson (1991). However, the exodus of SF6 from Santa Monica Basin after February 1986, by which time the tracer had mixed to the boundaries of the basin, was fast enough to be consistent with a greatly enhanced vertical flux. Since the kinetic energy in the basin had not changed significantly, it is unlikely that a temporal increase in forcing resulted in enhanced fluxes in the interior of the basin. The most likely interpretation is that diapycnal fluxes in the basin are dominated by processes in the boundary regions. Temperature and SF6 profiles from near the edges of the basin do not give conclusive evidence either for or against such enhanced mixing.

A tracer study of mixing in the Santa Cruz Basin

A tracer release experiment to study diapycnal and isopycnal mixing was performed in Santa Cruz Basin in 1988–1989. Approximately 1450 g of sulfur hexafluoride were injected close to an isopycnal surface at 1500 m depth in May 1988. This depth is midway between the bottom of the basin at 1920 m and the sill at 1084 m. The diapycnal dispersion measured during the first few months, while the tracer was still mixing to the walls of the basin, yielded a diapycnal eddy diffusivity of 1.0 cm2 s−1, within a factor of 1.5. This is approximately 4 times larger than the diffusivity found by Ledwell and Watson (1991) in the interior of Santa Monica Basin, where the buoyancy frequency was 5 times greater. The tracer was well mixed to the walls 6 months after the injection, and at that time the diapycnal spread of the tracer was greater near the walls than in the interior. The rate of diapycnal dispersion of the tracer increased dramatically after the tracer had mixed to the walls of the basin, indicating enhanced boundary mixing. An effective basin‐wide diapycnal diffusivity of 10 cm2 s−1 would be required to support the fluxes of tracer observed. A one‐dimensional model for tracer and heat diffusion does not simulate the results very well, however, probably because of the finite mixing time between the boundary region and interior and because of the complexity of the processes in the boundary region.

Geochemical and sedimentologic variations in inter-annually laminated sediments from Santa Monica Basin

Laminated sediments were recently discovered in the center of Santa Monica Basin, California Continental Borderland and extend from the present day to at least the Little Ice Age (ca. 1600 A.D.). Unlike the annual varves observed in Santa Barbara Basin, the light-dark lamination couplets observed in 210Pb-dated cores from the center of Santa Monica Basin occur at 3 to 6 year intervals, very similar in length to the El Niño Southern Oscillation (ENSO) phenomena. The mechanisms that control the temporal patterns of laminations in this basin are unknown. Potential causal mechanisms of lamination formation in Santa Monica Basin include: (1) changes in the flux of terrigenous vs. marine organic carbon and detritus; (2) surface water processes which affect marine productivity and organic carbon flux to the sediments; and (3) deep water processes which affect oxygen renewal and preservation of organic matter within the basin. A box core from the central portion of Santa Monica Basin was analyzed using a suite of geochemical, sedimentary, and micropaleontologic techniques in order to: (a) investigate which, if any, of these mechanisms has contributed to the occurrence of laminations; and (b) reconstruct proxy records of climate change. Samples were extracted from this core at 1 mm increments and provide a temporal resolution of 1–3 years per sample, a resolution which is comparable in length to the thickness of individual laminae within the core. Geochemical and micropaleontologic analyses of core samples provide paleoclimatic and paleoceanographic records which are comparable to, and extend beyond, historical regional and global climate records. In addition to direct comparisons between these records, spectral analysis was used to investigate whether there were similar patterns of variability that would implicate dominant forcing functions in the historic and proxy records. Herein we present preliminary results of records from ca. 1860 to the present. Our results suggest that organic carbon fluxes are the dominant factors that affect lamination variability. Input of organic carbon is primarily a function of primary productivity, and, to a lesser extent, a function of percentage of terrestrial input. Thus, (1) and (2) are both mechanisms of lamination formation. Stability of (3) only allows preservation of the record of these variations. Variations in Southern California rainfall appear to be linked to decadal-scale variations in downcore density profiles, and thus cause variations in the thickness and character of the laminations in the deepest portions of this basin. Thus, filtered density records can be used to reconstruct general paleo-rainfall trends which compare well with historic rainfall records. Variations in the isotopic composition of total organic carbon (TOC) are compared to historic ENSO and climate records and primarily reflect changes in the relative contribution of marine organic carbon to the basin. Variations in the carbon isotopic composition of N. pachyderma mirror changes in the sedimentary δ 13C TOC record from Santa Monica Basin, and suggest that primary productivity is the primary control on the flux of organic carbon to the sediments and exerts a strong influence on the interannual variability in lamination composition. Variations in paleo-sea surface temperatures generated from the oxygen isotopic composition of N. pachyderma correlate well with historical SST records and suggest potential usage as a proxy for general paleoSST patterns. Oxygen-sensitive assemblages of benthic foraminifera ( Bolivina argentea and B. spissa) have been used to identify potential large-scale flushing events and construct a historical record of variations in deep water oxygen levels. Spectral analysis of regional historic records indicate dominance of short-term (ENSO-length) and long-term (decadal) patterns of climate variability. The short-term ENSO signal is not dominant in the Santa Monica proxy records. However, strong decadal-length signals are identified in all of the proxy records and suggest that the dominant forcing mechanisms in Santa Monica Basin may be decadal in length.