Southern California Continental Borderland Research Articles

Strong SH‐to‐Love Wave Scattering off the Southern California Continental Borderland

AbstractSeismic scattering is commonly observed and results from wave propagation in heterogeneous medium. Yet deterministic characterization of scatterers associated with lateral heterogeneities remains challenging. In this study, we analyze broadband waveforms recorded by the Southern California Seismic Network and observe strongly scattered Love waves following the arrival of teleseismic SH wave. These scattered Love waves travel approximately in the same (azimuthal) direction as the incident SH wave at a dominant period of ~10 s but at an apparent velocity of ~3.6 km/s as compared to the ~11 km/s for the SH wave. Back projection suggests that this strong scattering is associated with pronounced bathymetric relief in the Southern California Continental Borderland, in particular the Patton Escarpment. Finite‐difference simulations using a simplified 2‐D bathymetric and crustal model are able to predict the arrival times and amplitudes of major scatterers. The modeling suggests a relatively low shear wave velocity in the Continental Borderland.

Two new species of the genusMesocletodes(Copepoda, Harpacticoida, Argestidae) from the Pacific Ocean, San Diego Trough

Abstract Two new species of Mesocletodes were identified and described from the samples collected for an ecological study of benthic meiofauna in the bathyal zone of San Diego Trough (Southern California Continental Borderland, Pacific margin). Mesocletodes tetrasetosus appears to be closely related to M. langi and M. ameliae by sharing five setae in the endopod of mandibular palp. However, M. tetrasetosus differs from both congeners in the following features: (1) the second segment of antenna with seven setae, (2) the exopod of mandibular palp with two setae, (3) the terminal margin of praecoxal arthrite of maxilla with eight elements, (4) the basis of maxilla with a total of six setae, (5) the syncoxa of maxilliped with a strong or robust bipinnate seta, (6) the P1 enp-1 with an inner seta, (7) the P1 enp-2 innermost distal seta and distal spine are translocated, (6) the P3–P4 endopod with total of 4:3 elements, (7) the P5 endopod outermost seta three times as long as in M. ameliae, (8) the P5 endopod m...

Earth Science in the Urban Ocean: The Southern California Continental Borderland

This 481-page collection, published in 2009, contains 26 papers, primarily by U.S. Geological Survey (USGS) authors but including a few papers from non-USGS agencies, such as the Southern California Coastal Water Research Project and various university authors. The book is divided into six sections: (1) Surficial Seafloor Mapping and Characterization, (2) Source to Sink Sedimentation, (3) Southern California Physical Oceanography and Sediment Transport, (4) Regional Tectonic Structure: Earthquake and Tsunami Hazards, (5) Coastal Aquifers of Southern California, and (6) Contaminant and Biological Implications of Earth Science Studies in the Southern California Bight. Each section contains an introduction that summarizes the papers that follow. The study area comprises the Southern California Continental Borderland and the Southern California Bight, physiographic features resulting from the unique tectonic evolution of the region. The Bight consists of the coast from Point Conception south past Santa Monica and San Pedro Bays to the United States–Mexico border, as well as the Channel Islands. The Bight is isolated from the major currents of the eastern Pacific and is characterized by the presence of islands, submarine canyons, and active faults. Filled basins, now above sea level, are home to a population of over 20 million people. A comprehensive map of this area near the beginning of the book would better orient the reader; however, an adequate figure is found on page 233. Perhaps the most striking aspect of the book is the abundance of detailed illustrations, including seafloor maps produced using multi-beam, high-resolution echosounders (MBES) and seismic reflection profiles produced by “deep-tow boomers.” The seafloor maps depict the various bathymetric features, such as …

Barite-forming environments along a rifted continental margin, Southern California Borderland

The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites in the SCCB were analyzed for mineralogy, chemical (54 elements) and isotopic (S, Sr) compositions, and petrography. Barite from Palos Verdes (PV) Peninsula sea-cliff outcrops is hosted by the Miocene Monterey Formation and underlying basalt; carbonate rocks from those outcrops were analyzed for C, O, and Sr isotopes and the basalt for S isotopes. Cold-seep barite from Monterey Bay, California was analyzed for comparison. SCCB offshore samples occur at water depths from about 500 to 1800 m. Those barites vary significantly in texture and occurrence, from friable, highly porous actively growing seafloor mounds to dense, brecciated, vein barite. This latter type of barite contrasts with cold-seep barite in being much more coarse grained, forms thick veins in places, and completely replaced rock clasts in breccia. The barite samples range from 94 to 99 wt% BaSO4, with low trace-element contents, except for high Sr, Zr, Br, U, and Hg concentrations compared to their crustal abundances. δ34S for SCCB offshore barites range from 21.6‰ to 67.4‰, and for PV barite from 62‰ to 70‰. Pyrite from PV sea-cliff basalt and sedimentary rocks that host the barites averages 7.8‰ and 2.2‰, respectively. Two offshore barite samples have δ34S values (21.6‰, 22.1‰) close to that of modern seawater sulfate, whereas all other samples are enriched to strongly enriched in 34S. 87Sr/86Sr ratios for the barites vary over a narrow range of 0.70830–0.70856 and are much lower than that of modern seawater and also lower than the middle Miocene seawater ratio, the time of deposition of the host rocks for the PV samples. This indicates that the mineralizing fluids were not unaltered seawater. We develop a model in which the barites precipitated both below the sediment–water interface and at the seafloor from low-temperature fluids that circulated along faults. The isotopic, chemical, and textural data indicate that the barites formed by several processes. Mesozoic and Cenozoic basement rocks (crystalline and overlying sedimentary rocks), Quaternary basin fill, turbidite fans, and seawater provided various elements for the barites in different environments. The fluids had a deep-seated source and were hydrothermal in the deeper parts of the system for all the barite types, including the seafloor cold-seep deposit, based on Sr isotopes and regional geothermal gradients. These deep-seated fluids mixed with other fluids as they ascended, including seawater at and near the seafloor. The high δ34S values may have resulted from extreme Rayleigh fractionation during bacterially mediated (semi)closed-system sulfate reduction, probably driven by the flux of methane- and hydrocarbon-bearing fluids through basement rocks and basin sediments. Early diagenetic dolomite and pyrite in the host Monterey Formation in the PV Headland also formed in a zone of sulfate reduction, but within sediment buried only centimeters to a few meters and with a predominantly seawater source for the sulfur. Dolomite occurring with vein barite in the PV Headland formed at temperatures in the range of 40–90 °C. The cold-seep barites have δ34S values near that of modern seawater, although still somewhat fractionated. The barites that precipitated below the sediment–water interface have higher δ34S values, suggesting that the fluids were relatively reduced with molar dissolved barium in excess of dissolved sulfate. Those samples were exposed at the seafloor by uplift along faults and are composed predominantly of massive, brecciated, and vein barite.

Deep‐towed gravity surveys in the southern California Continental Borderland

We surveyed two sites in the southern California continental borderland with a newly developed instrument, a towed deep ocean gravimeter—a gravity sensor that can be towed a few tens of meters above the sea floor. During its development phase, we used the instrument to survey two regions off the coast of southern California. The first was along two tracks in the San Diego Trough. The second was over a seamount‐like feature named Emery Knoll. Results of the trough survey reveal a basin with a geometry consistent with seismic data. We observed no significant density contrast across the San Diego Trough fault in the near‐surface sediments. The survey of Emery Knoll shed light on the question of the origin of this structure; modeling the knoll to determine its bulk density suggested a nearly uniform structure surrounded by sedimentary basins with a more massive central intrusive body. Derived densities of 2850 kg/m3for the knoll and 3050 kg/m3for the central intrusion assume that no deep unmodeled sources exist directly underneath the knoll. The gravity data favor a model of metamorphic basement rock uplifted and containing igneous intrusives.

Physiography of an active transpressive margin basin: high-resolution bathymetry of the Santa Barbara basin, Southern California continental borderland

High-resolution swath bathymetry and backscatter data from the Santa Barbara basin reveal a distinct assemblage of morphologic and sedimentologic features characteristic of the tectonically active setting of the basin. Such characteristics include the north–south asymmetry of the basin, the presence of an intra-basinal structural and morphologic high, extensive mass wasting, and fields of mounds and pockmarks. The north–south asymmetry of the basin and the increased abundance of slope failure along the northern basin slope likely reflect higher rates of tectonic shortening and sediment accumulation along the northern basin margin. High sedimentation rates lead to high pore water content of the sediment which, in combination with seismic shaking, may explain the abundance of mass wasting features even on shallowly inclined lower basin slopes. Mounds and pockmarks are the surface expression of locally extensive fluid seepage which we interpret to be driven by a combination of high sedimentation rates, a high organic content of sediment, and anoxic bottom water conditions during extended periods in Neogene and Quaternary times. During ROV dives we observed active fluid seepage to have occurred preferentially on the shelf and on a structural and topographic high, presumably due to structural focusing of migrating formation fluids. The preferred alignment of some pockmarks along faults is consistent with structurally controlled expulsion of formation fluids from underlying Neogene units. Despite the high tectonic activity of the basin, fault scarps are rare and only observed in areas of low sediment accumulation on the shelf and on topographic highs. Mass balance considerations suggest that sedimentation rates averaged over interseismic time scales are too high and scarp growth rates too low for scarps to be characteristic features of the basin floor. Long-term sediment accumulation rates that account for the effect of sediment compaction are low enough, however, to lead to long-wavelength surface expressions of the most active faults.

Upper crustal structure and Neogene tectonic development of the California continental borderland

Multichannel seismic-reflection data, sonobuoy seismic-refraction data, and regional geology are used to define the upper crustal structure of the southern California continental borderland and to delineate the characteristics of the main lithotectonic belts of the region. The Catalina Schist belt is separated on its west side from the gently deformed Nicolas forearc belt by faults that have steep west dips and pronounced normal separations. On its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the Peninsular Ranges belt at the coastline near Oceanside. The Catalina Schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early Miocene time in conjunction with about 10° of clockwise rotation of the western Transverse Ranges belt. Part of the uplift of the Catalina Schist could have occurred on the detachment fault, but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt. A large amount of uplift is required, and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western Transverse Ranges and Nicolas forearc belts. Extension, accompanied by probable large amounts of right slip, continued in the borderland region during and after middle Miocene time. The later stage of extension was accompanied by rapid clockwise rotation of the western Transverse Ranges of at least 90°. Most of the borderland, including the belt of schist that was uplifted in early Miocene time, was further deformed into numerous basins and ridges during this stage of oblique extension. The primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the Pacific plate after it had become coupled to the Farallon plate system, which had previously been subducted beneath the borderland.

Cretaceous tectonic evolution of coastal southern California: Insights from the Catalina Schist

The relatively high‐T, high‐P/T, subduction‐related basement in the Southern California Continental Borderland preserves important information regarding the late Mesozoic tectonic evolution of coastal southern California. Integrated petrologic, geochemical, and geochronological study of the subduction‐related Catalina Schist exposed on Santa Catalina Island documents the formation of lawsonite‐blueschist through amphibolite facies rocks during progressive underplating in a progressively cooling, newly initiated subduction zone. Petrologic and 40Ar/39Ar thermochronologic data demonstrate similar, perhaps shared, retrograde PT histories for all but the lawsonite‐albite facies rocks under pumpellyite‐actinolite facies conditions as early as ∼95–100 Ma. The 40Ar/39Ar data, together with the abundance of pumpellyite‐actinolite facies vein assemblages and lack of higher‐P/T (epidote‐ or lawsonite‐blueschist) over‐printing in high‐grade units, are consistent with the rapid emplacement of the Catalina Schist into the shallow‐ to mid‐crust by ∼ 95 Ma by combined underplating and tectonic/erosional removal of overlying rocks in the accretionary complex (cf. Platt, 1986). The Catalina Schist and the adjacent Peninsular Ranges Batholith potentially represent paired metamorphic belts formed along southwest North America at 120 to 90 Ma. Combining the evidence for the evolution of the Catalina Schist with the previously advanced hypotheses regarding tectonic evolution of the batholith, suturing of a fringing island arc to the continent may have triggered an oceanward trench stepout and the formation of a new subduction zone, at ∼ 120–115 Ma, in which the Catalina Schist was accreted and metamorphosed.

Miocene Tectono-Stratigraphic History of La Mision Basin, Northwestern Baja California: Implications for Early Tectonic Development of Southern California Continental Borderland: ABSTRACT

The middle Miocene La Mision basin in northwestern Baja California, Mexico, provides a rare opportunity to study an onshore portion of the southern California continental borderland. Stratigraphy, geometry of dispersal, and a variety of lithotypes within the volcanic and volcaniclastic sediments of the Rosarito Beach Formation provide clues to the nature of early tectonic evolution of this area during the Miocene. The elongated, trough-shaped La Mision basin formed in response to peninsular basement uplifts and the formation of volcanic highlands west of the present coastline. Lithologies and depositional environments represented within the basin sediments include: subaerial basalt flows and airfall tuffs, submarine muddy- and sandy-matrix mudflow breccias, lapilli tuffs, crystal tuffs, tuffaceous sandstones,d diatomites, and conglomerates. The environments of deposition range from fluvatile to intertidal to shallow marine. Early basin infilling is characterized by sediments and basalts, with a western source terrane, that were deposited against the faulted seacliffs. progressive infilling against the seacliff resulted in the formation of an extensive eastward-sloping basaltic platform extending eastward to the foothill coastal belt of the Peninsular Ranges. Marine transgression and subsequent regression are recorded by diverse marine volcaniclastic lithologies. Abundant fossils, K-Ar dates, and paleomagnetic data obtained from the La Mision basinmore » allow precise correlation with other areas in the continental borderland and provide conclusive evidence that this block of the borderland was formed and in its present position by 16-14 Ma.« less

Radiolarian responses to recent and neogene California El Niño and anti-El Niño events

Plankton tows from 1983 and 1964 California El Niños suggest that high standing crops of both warm and cold radiolarian faunas are characteristic of these conditions over the southern California continental borderland. High standing crops of both warm and cold radiolarians are suggested to occur during California El Niños because the California Current diminishes in intesity allowing warm faunas to invade from the west (central-gyre waters) and south (gyre and eastern tropical Pacific waters); and, a strongly developed California Countercurrent aids in creating a geostrophic dome that allows cold faunas to invade from the north. These same radiolarian faunas characteristic of California El Niños are found in the Holocene varved sediments of the Santa Barbara Basin and can be correlated to the known California El Niño events of 1957–1958 and 1964. El Niño like events determined via the same radiolarian criteria are found at 6 and 8 Ma in Monterey laminated (presumably varved) sedimentary rocks. It is suggested that warm and cold periods in Monterey may be related to the frequency and magnitude of El Niño like events. It is also suggested that during Neogene warm periods (such as at 8 and 10 Ma) the paleo-California Current was narrower, closer to the coast, and did not extend as far south as during anti-El Niño periods (such as at 5 Ma).

Radiolarian Indices of Physical and Chemical Oceanographic Phenomena in Recent Sediments of Southern California Continental Borderland: ABSTRACT

The California Current, the eastern limb of the North Pacific gyre, exhibits the following characteristics common to eastern boundary currents: wide, shallow, slow, diffuse boundaries; common upwelling; great seasonal variation; invasions of water masses from outside the system; and cold, low-salinity waters. Studies on plankton tows and Holocene sediments have correlated components of the siliceous microplankton (radiolarians and some diatoms) with several characteristics, such as the main directions of movement of the invading waters, the provenance of these waters, the presence and degree of upwelling, and seasonality and its impact on the underlying sediments. Certain types of radiolarians are potentially useful in determining fossil anoxic and oxic conditions as well as paleodepth. In this study, the authors analyzed box-core sediment and plankton tow samples from the southern California continental borderland, as well as radiolarian density, diversity, taxonomic makeup, and other features related to oceanographic and environmental conditions. Depositional environments were defined for the sediment samples, and radiolarian indicators for paleoenvironmental interpretation were determined. Several borderland environments were identified, and the anoxic nearshore basin was found to have the best preservational qualities for radiolarians and thus the most representative radiolarian biocoenosis.

Modern California Current System and Radiolarian Responses to Normal"(Anti-El Nino) Conditions: ABSTRACT"

The modern California Current is a relatively wide, slow, southward flow of cold, low-salinity water subject to considerable seasonal upwelling and other seasonal and supraseasonal perturbations. The radiolarian fauna contained within these waters reflects the parameters and perturbations common to eastern boundary currents. Radiolarian faunas characteristic of the California Current (subarctic and transitional waters), the offshore gyre (North Pacific anticyclonic subtropical gyre), the eastern tropical Pacific, and underlying intermediate and deep waters have been documented and characterized. During normal (anti-El Nino) conditions within the California Current system, the following physical oceanographic changes (and their characteristic radiolarian responses) occur. Spring and summer are dominated by the strongest southerly flow of the California Current - with high-standing crops of subarctic and transitional radiolarians in the core of that current - whose core is seaward of the southern California continental borderland. Spring and summer are also periods of strongest upwelling, with deeper radiolarians appearing at or near the surface. During fall and into winter, the California Current slows and a coastal countercurrent, the Davidson Current, develops. Radiolarians indicate that a much reduced core of the California Current swings in over the southern California continental borderland, and that faunas from the south are broughtmore » northward near the shore.« less

Phaeodarian Radiolarians as Indicators of Recent and Past (Monterey) Anoxic Environments: ABSTRACT

Tests of phaeodarian radiolarians are composed largely of organic matter, containing only a small percentage of opaline silica in their matrix. Because of their highly organic nature, these tests are generally dissolved prior to deposition at the sediment-water interfaces of oxic environments. Anoxic environments, however, offer settings of little bioturbation and low oxygenation and may enhance phaeodarian preservation in the sediment records of anoxic locations. Recent southern California continental borderland surface sediment samples are studied in this research to provide a quantitative index of anoxia using phaeodarian surface area as a criterion. Californian Miocene and Pliocene samples have been compared to verify the index in the fossil record.

Oil and Gas Production from Submarine Fans of Los Angeles Basin: ABSTRACT

The Los Angeles basin is a small but relatively deep Neogene basin located in the most northeast portion of the southern California continental borderland. It was formed along a transform margin during the early to middle Miocene, as were numerous basins within the continental borderland south of the Transverse Ranges. A series of prograding submarine fans filled most of the basin during the middle to upper Miocene, Pliocene, and Pleistocene. The morphology of these fans appears to fit most basin-floor fan models but these models were greatly modified by paleobathymetry. The sediment transport mechanism was primarily turbidity flows from submarine canyons but other mass sediment transport such as debris flows, fluidized sediment flow, grain flow, etc was also significant. Three primary, often coalescing, submarine fans have been recognized: the Tarzana fan in the northwestern Los Angeles basin, the San Gabriel fan in the central and southern parts, and the Santa Ana fan in the eastern part of the basin. Most of the oil produced in the Los Angeles basin comes from submarine fan sandstones. Much of the future oil to be found will be from the same fans or heretofore unrecognized submarine fans. Structural geologic concepts will be of equal importance when exploring for these subtle traps. Total cumulative production to 1982 has been 7.3 billion bbl of oil and 5.8 tcf of gas. Per unit volume of sediment, the Los Angeles basin is one of the richest in the world. It was a silled basin within the oxygen-minimum oceanographic zone during the upper Miocene and Pliocene. A combination of rich biogenic sedimentation along with rapid burial by coarse to fine clastics and relatively high paleoheat flow provided almost perfect conditions for the generation and migration of oil and gas. Structural deformation was intermittent throughout the Neogene but was most intense and culminated in the late Pleistocene to Recent. The multistoried oil sands with different crude oil types suggest most of the oil is indigenous to the formation in which it is found. End_of_Article - Last_Page 520------------

Seismicity, Faulting and Tectonics of Inner Continental Borderland Offshore Northern Baja California, Mexico: ABSTRACT

Using recently collected high resolution seismic reflection data and existing bathymetric, geomagnetic, and seismological data, we find that the inner continental borderland of northern Baja California, Mexico, is extensively deformed and tectonically active. The region is crossed by three major wrench fault zones typified by one or more relatively continuous main fault(s), numerous smaller, subparallel, en echelon and oblique conjugate faults, and transversely oriented folds. These three fault zones are the southward continuations of the Santa Cruz-San Clemente-San Isidro, San Pedro-San Diego Trough-Maximinos, and Palos Verdes Hills-Coronado Bank-Agua Blanca fault zones, mapped in the southern California continental borderland. Each of the fault zones shows evidence of Q aternary activity, such as sea-floor displacement and faulted Quaternary sediments. Earthquake epicenters roughly delineate the major fault zones, with the most significant activity occurring along the Santa Cruz-San Clemente-San Isidro fault zone. Strike-slip is suggested for the main faults by offset submarine canyons, and sea-floor scarps that reverse along strike. Large scarps and vertical separations also suggest significant dip-slip in some areas. Earthquake focal mechanisms show that present-day movements along the major fault zones is predominantly dextral strike-slip, with a significant component of dip-slip in some cases, consistent with the motion of the San Andreas fault system and Pacific-North American plate boundary tectonics. Complexity in the regional tectonics is demons rated by earthquakes in the San Clemente Island-Fortymile Bank area, which are observed to have focal mechanisms opposite to those predicted by the rigid plate theory. End_of_Article - Last_Page 974------------

Diatoms and Silicoflagellates From Holocene Sediments, Southern California Continental Borderland: ABSTRACT

Samples from five basins on the southern California continental borderland were examined for diatoms and silicoflagellates. These basins, Santa Monica, San Pedro, Santa Catalina, San Nicolas, and Tanner, represent a transect of approximately 85 nmi in a southwesterly direction across the continental shelf. The two nearshore basins, Santa Monica and San Pedro, contain the greatest species diversity and the smallest number of reworked specimens. Tanner basin, however, has the greatest abundance of diatoms and silicoflagellates in the study area, and the highest percentage of reworked specimens. The reworked diatoms are mid-Miocene to late Miocene in age. San Nicolas and Santa Catalina basin samples contained both lower species diversity and abundance totals than the other t ree basins. These two basins also contained a significant number of reworked diatoms. As might be expected benthic species were common in Santa Monica and San Pedro Basins, however, a large number of benthic species were in the Tanner Basin samples, which was not expected. There was also a disproportionately low number of smaller planktonic diatoms in the sediments when compared to the very large numbers found in the phytoplankton standing crop in this area. End_of_Article - Last_Page 438------------

Palynomorphs From Holocene Sediments of Basins in Southern California Continental Borderland: ABSTRACT

A suite of 17 box core samples taken along an onshore-offshore transect across the southern California continental borderland representing Tanner, Santa Monica, San Pedro, Santa Catalina, and San Nicolas basins were analyzed for palynomorph content. Dinoflagellate cysts, acritarchs (organic-walled phytoplankton), and spores and pollen of terrestrial plants are present in all samples. Dinoflagellate cysts and acritarchs are more common in samples from Santa Monica and San Pedro basins, and show a general decrease in abundance with increasing distance from shore. Cyst genera include Leptodinium, Lingulodinium, Nematosphaeropsis, and Spiniferites (referrable to the thecate genus Gonyaulax) and species of Peridinium. Lingulodinium machaerophorum Deflandre and Cookson (= Gonya lax polyedra Stein) is the dominant species in all samples. Spores, pollen, and other plant tissue fragments provide an indication of the terrestrial component of these sediments. Evidence of reworking of older material into recent sediments is suggested by differential stain uptake observed among individual specimens of phytoplankton and pollen. End_of_Article - Last_Page 440------------

Spatial autocorrelation with RUM (Remote Underwater Manipulator): vertical and horizontal structure of a bathyal benthic community

Abstract The RUM/ORB unmanned submersible system (Marine Physical Laboratory, Scripps Institution of Oceanography) was used at 1220-m depth to collect precisely located samples separated by distances from 0.1 to 500 m in the San Diego Trough, Southern California continental borderland. Newly developed spatial autocorrelation procedures from the field of quantitative geography were used on this irregularly spaced network of samples to test the statistical significance of the calculated autocorrelations. Scales less than 200 m showed little spatial autocorrelation in total numbers of macrofaunal individuals per sample. Density of Polyophthalamus sp. (Polychaeta, Opheliidae), however, displayed a complex autocorrelative pattern; ‘patches’ of individuals of similar size were observed. Ceratocephale pacifica (Polychaeta, Nereidae), on the other hand, showed negative spatial autocorrelation at the smallest inter-sample distance, perhaps indicative of territoriality. Vertical segregation among confamilial and congeneric species was observed in vertically sectioned subsamples. The majority (82%) of polychaetes, however, appear regularly to inhabit the uppermost centimeter of sediments, suggesting a refinement of current bioturbation models.

Genetic variation in deep-sea invertebrate populations: The bathyal gastropod Bathybembix bairdii

Five populations of the bathyal trochid gastropod Bathybembix bairdii (Dall) taken from depths of 579 to 1156 m in the Southern California Continental Borderland were surveyed for levels of genetic variability at 18 presumptive gene loci, using techniques of starch gel electrophoresis. All the populations were highly similar genetically; none of the populations possessed unique alleles. Four of 5 polymorphic loci scored in all the populations displayed statistically significant heterogeneity of allelic frequencies among stations. Only the diallelic fumarase (FUM) locus displayed a trend with depth. The geographic and depth patterns of the genetic variability in these populations do not support the hypothesis that the high levels of genetic variability observed in bathyal populations are an artifact of gene flow between populations differentiated with depth. The 16.2% of the loci heterozygous per individual of B. bairdii is of the same order as the average heterozygosities reported for other deep-sea invertebrate taxa. These reports of high genetic variability in a physically constant and seasonless environment are reviewed and discussed in the context of the trophic-stability hypothesis of Ayala and Valentine and the time, size and divergence hypothesis of Soule. Evidence is presented that despite the observed correlation of average heterozygosities in benthic marine invertebrates with seasonality of the trophic regime, this is not apparent for an enzyme system (leucine amino peptidase) which might be expected to display this trend if it reflected a genetic strategy to cope with trophic seasonality. The time, size and divergence hypothesis, focusing on population size, accounts for the general features of levels of genetic variability in deep-sea invertebrates.

Environmental grain and polychaete species' diversity in a bathyal benthic community

Patterns of polychaete species' dispersion in the San Diego Trough, Southern California Continental Borderland, North Pacific Ocean, at a depth of 1230 m are analyzed. Samples consist of 4 replicate 0.25-m2 box cores, each partitioned into twenty-five 0.01-m2 subcores, and of 1 unpartitioned core. The sampler is biased but, when this bias is taken into account, few species show strong aggregation either between or within cores. If all species are considered together as replicates, intraspecific aggregation is detected between cores, but uniform dispersion dominates within cores. Patchiness or habitat partitioning on the assayed scales can thus account for little of the extremely high species' diversity observed. Apparently due to an environmental structure having a “grain” smaller than 0.01-m2, the diversity of species assumed to be sedentary is higher than the diversity of species assumed to be mobile. One evidence of such structure is a negative covariance between paraonid abundance and density of “mudballs” constructed by a cirratulid. Consideration of these results suggests that either grain specialization or simultaneous, biogenic disequilibrium or both act on spatial scales smaller than 0.01-m2 to maintain high polychaete species' diversity in the San Diego Trough.