Santa Barbara Basin Research Articles

Seawater and porewater fluorescence in the Santa Barbara Basin

The fluorescence of filtered seawater and porewaters from the Santa Barbara Basin has been measured. Porewater fluorescence exceeded that in the overlying seawater by a factor of 10–500. The water column profile of fluorescence can be explained by photochemical degradation of fluorescent components in surface waters and diffusion of “marine humic substances” from the sediments into basin bottom waters. Porewater fluorescence increases linearly with depth to at least 2 m and correlates well with titration alkalinity and ammonium ion concentration. The sources and the type of molecules responsible for fluorescence remain unknown.

Habitat characterization and nutritional strategies of the endosymbiont-bearing bivalve Lucinoma aequizonata

A population of the lucinid bivalve Lucinorna aequizonata, with sulfur-oxidizing endosymbiotic bacteria in the gills, is restricted to a narrow depth range (500 -f10m) on the slope of the Santa Barbara Basin, California, USA. In this zone, the seawater just above the substratum is sub-oxic ([02] < 2 0 FM). The organically rich sediments in which these clams live are well mixed by bioturbation, which appears to maintain a redox condition Limiting the extensive accumulation of hydrogen sulfide. However, thiol levels in the blood of the clams indicate an exposure to significant amounts of sulfide and/or thiosulfate apparently from randomly &spersed short-lived pockets of sulfidic mud that can be reached by the clam's burrowing vermiform foot. When the bivalves are incubated in the presence of hydrogen sulfide, thiosulfate is concentrated in the blood and apparently utilized by the bacteria for metabohc energy and the production of intracellular elemental sulfur. Laboratory growth experiments demonstrated that sulfide concentrations greater than 10 FM were detrimental to the host, even though the bacteria continued to accumulate elemental sulfur. The utilization of thiosulfate under near anaerobic conditions and the accumulation of intracellular elemental sulfur by the endosymbiotic bacteria coupled with the high availability of environmental nitrate and low molecular oxygen suggests a metabolic strategy analogous to the free-living sulfur oxidizer Thlobacillus denjtrificans. The 6 ' 3 ~ values of the purified endosymbiont bacteria (-34.0 5 0.8%) were significantly lighter than those of the host tissue (-29.0 + 0.7 %) suggesting that in addition to the nutrition provided by the bacteria, over 25 % of the host carbon may be attributed to exogenous dissolved carbon which is in high concentrations in its natural habitat.

Accumulation Rates of Biogenous Silica and Terrigenous Detritus in Late Miocene-Early Pliocene Santa Maria Basin, California: ABSTRACT

The upper-Miocene-lower Pliocene laminated to massive diatomaceous strata of the Santa Maria basin encompass portions of the Monterey and overlying conformable to nonconformable Sisquoc Formations and record the continued accumulation of admixed siliceous (mostly diatoms) and terrigenous (silt and clay) components in a Neogene borderland basin. Lithologies, which vary from diatomite to diatomaceous mudstone, are the result of fluctuations in slope-accumulating silica and terrigenous debris. Bulk accumulation rates, as well as silica and terrigenous component accumulation rates, were calculated for four measured sections using accurate stratal thicknesses, absolute ages, rock compositions, and bulk densities. Bulk accumulation rates range from less than 20 to greater than 70 mg/cm/sup 2//year and are comparable to bulk accumulation rates in the present-day Santa Barbara basin and Gulf o f California. Silica accumulation rates (20-30 mg/cm/sup 2//year) suggest that sedimentation took place beneath productive waters and they exceed those encountered in other high-productivity oceanic areas, such as the Bering Sea and Peruvian coastal waters. Relatively high terrigenous accumulation rates (18-35 mg/cm/sup 2//year) reflect the dilution of these high-productivity waters by continentally derived detritus. Comparison of calculated accumulation rates with rates determined for similarly aged strata of the Santa Barbara basin reveals that silica and terrigenousmore » debris were accumulating at higher rates within the Santa Maria basin. These differences are due to the complex interplay of tectonics and oceanographics and to the proximity of the basins relative to the strandline.« less

Phosphatic Hardgrounds and Hiatus Concretions in Neogene Marine Sequences of California Coastal Ranges: ABSTRACT

Distinctive phosphatic intervals mark hiatuses of varying durations at several localities along the central California coast. At Capitola in the La Honda basin, a complex series of carbonate-cemented hardgrounds containing authigenic phosphatic nodules and marine vertebrate bones occurs within 50 cm of the upper Purisima Formation (Pliocene). Paleomagnetic measurements and diatom stratigraphy indicate that this 50-cm interval represents a hiatus of about 1 m.y. (4.5-3.5 Ma). At Naples Beach in the Santa Barbara basin, a severely condensed section of about 7 m of interbedded nodular phosphatic hardgrounds, phosphatic marlstones, and dolomites appears to represent as much as 5 m.y. (14.5-9.5 Ma). In the Lompoc Quarry of the Santa Maria basin, a thin phosphatic hardground at the contact between the Monterey and the Sisquoc Formations represents about 0.5-0.7 m.y. (between 7.0 and 6.0 Ma). At Mussel Rock in the Santa Maria basin, a nodular phosphatic conglomerate occurs at the boundary between the Monterey and Sisquoc Formations; the age of the conglomerate determined from diatom biostratigraphy is approximately 6.2 Ma (Messinian), but no time gap can be recognized given the resolution of diatom age dating. The phosphatic hardgrounds of Capitola, Lompoc Quarry, and Naples Beach appear to correspond to high sea levelmore » stands, whereas the conglomerate at Mussel Rock coincides with a sea level lowstand. During high sea level stands, depocenters shifted landward, leading to clastic-sediment starvation possibly accompanied by current-induced erosion and winnowing, which in term led to condensation and early diagenetic carbonate and phosphatic cementation.« less

El Niño in Santa Barbara Basin: Diatom, radiolarian and foraminiferan responses to the “1983 El Niño” event

Santa Barbara Basin box cores collected in August of 1985 were sub-sampled and processed in search of microfossil evidence of the “1983 El Niño”. Microplankton and microbenthos indicate the presence of that El Niño bounded by non-El Niño conditions. Specifically the event is recognized by: an increase in diatom diversity, and the presence of several warm water taxa (e.g., Coscinodiscus plicatoides, Hemidiscus cuneiformis, Nitzschia kolaczekii, Pseudoeunotia doliolus, Thalassiosira lineata, Thalassiosira nodulolineata); an increase in polycystine radiolarian flux, and the presence of the eastern tropical Pacific species Spongaster pentas; an increase in the diversity of planktonic foraminifera, and the occurrence of the warm water species Globoturborotalita rubescens. There is also evidence for an oxygenation of the basin during the 1983 El Niño as suggested by the increase in the abundance of the benthic foraminifera Bolivina vaughani and by the decrease in the preservation of phaeodarian radiolarian spines which are best preserved under anoxic conditions. A new onboard procedure for sampling of box cores and freezing of subcores is described.

Neogene Sedimentation of Continental Borderland Basins, Offshore Southern California: ABSTRACT

Recent seismic stratigraphic analysis shows that the present borderland basins in offshore southern California did not begin to take shape until the late Miocene. The distribution of early Miocene and older strata is not affected by the basin configuration, and only the late Miocene and younger sediments are ponded in the basins. The late Miocene strata are characterized by the widespread Monterey-type diatomaceous sediments that form drape-like sequences covering all the topographic irregularities. The Pliocene-Quaternary sediments occur as flat-lying clastic sequences filling in the lows of the basins. Seismic facies analysis permits delineation of facies patterns of the Pliocene-Quaternary basin-fill sequences of each basin. In the offshore basins such as Patton, Tanner, Santa Cruz, and San Nicolas, fine-grained mass-flow and hemipelagic sediments derived from the local insular sources dominate the basin-fill sequences. Catalina basin is a transitional basin filled with fine-grained turbiditic and hemipelagic sediments derived from both the continent and local insular sources. The nearshore basins, including Santa Barbara, Santa Monica, and San Pedro basins and San Diego trough, are filled with thick sequences of continent-derived sediments that mostly form prominent deep-sea fans. Approximate estimations of accumulation rates show that sediment accumulation in the offshore basins as well asmore » San Diego trough has remained fairly constant since the Pliocene, while the nearshore basins exhibit an increasing trend that might be related to the progradation of continent-derived sediments. Stratigraphic characteristics of borderland basin deposits indicate that the borderland was dominated by biogenic pelagic sedimentation in the late Miocene and by mass-flow deposition in the Pliocene and Quaternary.« less

Magnetotactic bacteria and single-domain magnetite in hemipelagic sediments

Remanent magnetism in marine sediments has been used extensively over the past twenty years to calibrate the geological time-scale, study geomagnetic reversals and secular variation, and measure the rates of seafloor spreading. Although these sediments may contain different magnetic minerals, magnetite is the most commonly observed and magnetically stable phase, and its size, shape and post-depositional fate affect the magnetic remanence of the sediments. Biogenic magnetites are single-domain, with a high natural magnetic remanence (NRM), and have been suggested as a significant source of magnetic remanence in marine sediments. We have studied surface sediments from the Santa Barbara Basin and report the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra-fine-grained, single-domain magnetite. Using a novel extraction technique, transmission electron microscopy and SQUID magnetometry, we show that these bacteria and the magnetite they produce are the major source of stable remanent magnetism in these sediments.

Origin of mass flow sedimentary structures in hemipelagic basin deposits: Santa Barbara Basin, California Borderland

Holocene mass flow processes produce distinctive sedimentary structures in this silled, anoxic basin characterized by high sedimentation rates of fine-grained fill. Mud flow deposits possess a 0899 1444 V 3 diagnostic vertical sequence of structures on the lower slope. Basin floor turbidites possess a variety of bed set combinations of: 1) finely laminated layers, 2) graded silt and clay layers, and 3) ungraded, massive silts and clays with floating sand grains. Massive, ungraded flood-derived suspensate deposits can be distinguished from turbidites. Turbidites and suspensate deposits constitute 30% of the sampled sediment column. Varve counting of the remaining 70% provides frequencies of turbidite and flood suspensate layer deposition.

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).

Paleoceanographic and tectonic controls on deposition of the Monterey formation and related siliceous rocks in California

The timing of paleoceanographic and tectonic events that shaped the deposition of the Monterey Formation of California and related siliceous rocks has been determined by application of a refined biochronology. The base of the Monterey at 17.5 Ma coincides with rising global sea level and a switch in biogenous silica deposition from the Caribbean and low-latitude North Atlantic to the North Pacific. Major polar cooling, which began at 15 Ma, postdates the base of the Monterey by more than 2 Ma and cannot be invoked to cause the deposition of diatomaceous sediments occurring in the lowermost Monterey. Later polar cooling in the early late Miocene, however, apparently caused increased upwelling and deposition of purer diatomites in the upper Monterey. The top of the Monterey at about 6 Ma coincides with a major sea level drop and is commonly marked by an unconformity. Equivalent unconformities are widespread around the rim of the North Pacific and typically separate more pelagic sediments from overlying sediments with a greater terrigenous component. Above the Monterey, diatoms persist in California sediments to 4.5–4.0 m.y., where their decline coincides with increased deposition of diatoms in the Antarctic. Carbon isotope records in the Pacific and Indian Oceans record storage of 12C in the Monterey Formation and equivalent organic-rich sediments around the rim of the North Pacific. A +1.0‰ excursion in δ 13C beginning at 17.5 Ma coincides with rising sea level and probably reflects storage of organic material in Monterey-like marginal reservoirs. A reverse −1.0‰ shift at 6.2 Ma closely approximates the top of the Monterey and may represent erosion of these marginal reservoirs and reintroduction of stored organic carbon into the ocean—atmosphere system. Initiation of transform faulting and extension in the California margin in the latest Oligocene and early Miocene caused the subsidence of basins which later received Monterey sediments. A major tectonic event centered at 10–9 Ma, which included rotation of the Santa Barbara Basin and a change in Pacific plate motion, is recorded by an unconformity and/or interval of compressed sediments in many Monterey sections. Another change in plate motion at about 6 Ma may be expressed by the angular discordance observed in many sections across the unconformity at the top of the Monterey.

A note on the relationships between organic matter and some geotechnical properties of a marine sediment

Abstract An analysis of the relationship between organic matter and liquid and plastic limits, and grain‐specific gravity of a marine sediment was accomplished by making a stepwise adjustment in the organic content of that sediment. The sample used was from Santa Barbara Basin (off southern California) and is typical of fine‐grained marine sediments: it is a clayey silt with a common suite of minerals and other constituents. During the experiment, texture and composition (except organic content) were constant; only the quantity of natural, indigenous organic matter was changed. A strong linear relationship exists between the independent variable, the amount of organic carbon present in the sample, and the dependent variables. Liquid limit, plastic limit, and plasticity index all increased with increasing organic content over the range studied (0.57–3.20% organic carbon). Grain‐specific gravity decreased. All had linear correlation coefficients (r) greater than |0.90| and r 2 values greater than 90%, excep...

Morphology, Acoustic Characteristics, and Late Quaternary Growth of Conception Fan, Santa Barbara Basin, California: ABSTRACT

A radial borderland-basin fan in the western half of the Santa Barbara basin, the Conception Fan, shows characteristics of a debris slope. More than 3000 mi of closely spaced (3.5 kHz) high-resolution profiles, 270 gravity cores, and 8 borings were used to map channel and fan morphology, and channel, levee, and lobe acoustic facies. Two major unconformities are recognized on the seismic profiles. The upper unconformity represents the 10-k.y.B.P. horizon. The lower unconformity is the erosional surface of the late Wisconsinan lowstand of sea level, 18-26 k.y.B.P. Eustasy and tectonism produced two pulses of deposition, each from a different point source, during the Flandrian transgression. Prior to the late Pleistocene, the Conception Fan was fed by one major canyon/channel system, above the western part of the fan. During the late Pleistocene, two small submarine canyons were cut into the slope 7 mi east. Four major channels, smaller than the western channel system, were incised into the fan surface, indicating the eustatic decrease in sediment input. The fault-controlled western canyon (Sacate) fed all but the eastern channel. Faulting and slumping on the slope cut the eastern canyon (Gaviota) and formed the eastern channel. Numerous slope gullies influenced eastern canyon and channelmore » development. Holocene currents rounding Point Conception have winnowed fine sediments in the western channel region, resulting in hummocky topography and the scoured appearance of the channel. Hemipelagic deposition dominates the lower-middle and lower fan of the eastern part of the fan. The western part of the fan seems to be receiving slope-like deposits over the relict fan surface.« less

Radiolarian Indicators of El Nino and Anti-El Nino Events in Holocene Sediments of Santa Barbara Basin: ABSTRACT

Radiolarian Indicators of El Nino and Anti-El Nino Events in Holocene Sediments of Santa Barbara Basin: ABSTRACT

Turnover time of waters in a deep basin in the Aegean Sea

The Western Gulf is a basin in the Saronikos Gulf. Below about 100 m the water is almost stagnant, essentially isothermal (around 14°C) and it has practically uniform salinity. The level and the mechanism of enrichment in nutrients is discussed. The age of the water estimated from nutrient budget calculations is between one and eight years. The replacement time of water is similar to that found in the Santa Barbara basin, but different from those of the Cariaco Trench and the Black Sea.

Fine-grained slope and basin deposits, California continental borderland: Facies, depositional mechanisms and geotechnical properties

Surficial sediments from slopes and slope-aprons in Santa Barbara, Santa Cruz and San Nicolas basins (California borderland) are dominantly silts and clays. Facies are defined and interpreted on the basis of sedimentary structures as follows: (1) disorganized beds — mudflow or debris flow deposits; (2) beds with deformation features, micro-faults or bowl-shaped dislocations — slump deposits; (3) beds with fluid escape structures — partly fluidized or liquefied beds; (4) graded beds with Bouma sequences — turbidities; (5) ripple cross-bedded silts with mud interlaminae — traction current deposits; (6) horizontally laminated silty clay — suspensate or nepheloid flow deposits; and (7) bioturbated beds. Clay fabrics reflect mode of deposition or later deformation of some of the facies. Facies 1 shows random or swirled fabrics, while facies 2 has horizontal domains which alternate with random zones. Where fluidization or liquefaction has occurred (facies 3) clay fabrics consist of vertically aligned domains near fluid escape tubes, or are random. Other facies (4, 6, 7) show mostly random fabric patterns. Plasticity indices and water contents vary with texture; both are higher with increasing clay content and organic carbon content. Individual facies are also an influencing factor. For example, beds which were sufficiently consolidated to develop microfaults have lower values than the other facies. Facies distributions correlate with sedimentation rates. Proximal basins (> 50 mg cm −2 yr −1) have more mudflow, debris flow and slump deposits while distal basins (<10 mg cm −2 yr −1) have more turbidites and laminated deposits. Similarly within individual basins, facies correlate to local seafloor gradients, with debris flow and slump deposits more common in steeper areas and turbidites and laminated “varves” more common in flatter areas. Mudflow deposits are more common in intermediate slope areas, especially in those areas undergoing creep and syn-sedimentary gravity-faulting in Santa Barbara Basin.

Particulate iron and manganese in the Santa Barbara Basin, California

Particulate Fe and Mn may be important trace metal scavengers in the water column as well as being probable indicators of biologically mediated redox processes. A study has been made of suspended particulate composition in the Santa Barbara Basin, a shallow near-shore basin off southern California with sub-oxic conditions below sill depth. Observations have revealed several interesting phenomena relating to the geochemistry of Fe and Mn. Most striking is a profound enrichment of particulate Fe in samples from the bottom two hundred meters. These particulates have a constant Fe/P mole ratio of about three and may originate at the sediment-water interface or may be transported to the basin from local marshes. For particulate Mn, enrichments are observed both in the sub-sill waters and near the base of the euphotic zone. A consideration of particle removal rates suggests that the sub-photic zone enrichment has a biogenic origin. In the sub-sill waters, enrichment in Mn is apparently due to the precipitation of dissolved Mn diffusing from the anoxic basin sediments. A simple mass balance suggests that most of the Mn lost from the sediments is transported from the Santa Barbara Basin in dissolved form.

Abundance versus rates of accumulation in fine-grained strata of the Miocene Santa Barbara Basin, California

Although best known as a deposit rich in silica and organic matter, the Miocene Monterey Formation actually had long-term rates of silica and organic Matter accumulation equal to or lower than values in the underlying and overlying detritus-rich formations in the Santa Barbara coastal area. The factor Mainly influencing the composition of the Monterey Formation in this area was the slow accumulation of terrigenous debris. In contrast, with greater than average silica abundance within the Monterey Formation probably resulted from rapid short-term silica accumulation (of hundreds to tens of thousands years duration) rather than from especially slow accumulation of diluting debris.

Depositional environments in the Monterey Formation, Cuyama Basin, California

The Monterey Formation of the Cuyama Basin accumulated in an inboard basin within a Miocene borderland analogous to the Recent continental borderland off southern California. Analysis of benthic foraminiferal biofacies, lithology, and sedimentary structures permits recognition of several depositional environments within the Monterey Formation in this; basin. The lower member of the Monterey Formation, the Saltos Shale (early Miocene), contains basin-plain, slope, and various submarine-fan subenvironments. These predominantly terrigenous rocks are interbedded with impure carbonates composed primarily of foraminifera and calcareous nannofossils. The upper member, the Whiterock Bluff Shale (middle Miocene), contains highly biogenous deposits representing slope and basin-plain environments. Both siliceous and calcareous biogenous rocks are prominent in this unit. The complex: array of lithofacies within the Monterey Formation is the product of the interplay between terrigenous and biogenous sedimentation. Various factors control this interplay, including global sea level, climate, paleo-oceanographic changes, local tectonics, and paleogeography. The Monterey Formation in the Cuyama Basin is more terrigenous than coeval sections in more outboard basins of the Miocene borderland (as represented by the Santa Barbara Basin). These latter areas contain many lithofacies indicative of low terrigenous sedimentation rates: highly siliceous and calcareous rocks, phosphatic rocks, and highly organic rocks. The inboard Cuyama Basin acted as a sediment trap during much of the Miocene and is partly responsible for these lithofacies patterns. Both inboard and outboard basins contain variations in bulk-accumulation rates that correlate to variations in relative sea level. This study demonstrates the importance of depositional patterns, paleogeography, and multidisciplinary analysis for understanding the complex stratigraphic units found along active continental margins.

Elemental sulfur in the gills of three species of clams containing chemoautotrophic symbiotic bacteria: a possible inorganic energy storage compound

Sulfur content and fine structure were studied for tissues of three species of clams, Lucinoma annulata, Calyptogena elongata and Lucina floridana, which inhabit sulfide-rich environments and whose gills harbor symbiotic sulfur bacteria. Lucinoma annulata and C. elongata were dredged from the Santa Barbara basin, California, USA, at a depth of 480 to 490 m, and Lucina floridana were dug from below the roots of seagrasses in Saint Joseph Bay, Florida, at a depth of 0.25 to 2m. Foot tissue of Lucinoma annulata, without symbionts, had a total sulfur content of 1.4±0.1 (SD) mg 100 mg-1 dry weight of tissue (%DW). The symbiont-containing gill tissue of different individuals of L. annulata varied in color from dark red to pale yellow, and the total sulfur content was 2.5±0.4% DW in red gills and was 5.6±3.3 % DW in the yellowest gills. Maintenance of L. annulata in the laboratory for 21 d in the absence of sulfide resulted in the loss from the gill of yellow deposits which were elemental sulfur in the form of liquid-crystalline sulfur globules rather than solid orthorhombic sulfur crystals. The foot tissue did not contain elemental sulfur. When examined by freeze-etch microscopy, sulfur globules were found only within bacteria and not in the animal host cytoplasm. Sulfur globules were confined to the periplasmic space of the bacteria. C. elongata and Lucina floridana resembled Lucinoma annulata in the physical form and distribution of elemental sulfur. The absence of elemental sulfur in the animal cytoplasm suggests that its formation from sulfide is not a detoxification scheme to protect animal tissue from sulfide toxicity. The sulfur deposits probably represent inorganic energy reserves that permit the symbiotic bacteria to function even during the temporary absence of external sulfide.

Hemipelagic deposits in a Miocene basin, California: toward a model of lithologic variation and sequence

Summary Deep marine basins began to form along the continental margin of central California in late Oligocene time as a result of the initiation of a translational plate boundary. Deposition of transgressive shallow-marine sand (Vaqueros Formation) was followed by deposition of a thick (&gt; 500 m) massive clay unit (Rincon Shale). Rapid subsidence (&gt; 2000 m) coincident with rising sea-level created sediment-starved basins by late early Miocene time ( c. 18 Ma). Typical of many of these borderland basins was the Santa Barbara basin, located at least 50 km from the Miocene strandline. In this basin, between late early and latest Miocene time (18–5.5 Ma), was deposited a heterogeneous sequence of highly biogenous sediment (Monterey Formation) consisting of mixed diatomaceous, foraminiferal-coccolithic, and terrigenous debris generally rich in organic matter (mean 8%). Overlying latest Miocene and Pliocene sediments (Sisquoc Formation) contain increasingly abundant clay debris. The Miocene biogenous sequence reflects hemipelagic sedimentation from highly productive coastal waters during an extended period of generally low terrigenous influx. Accumulation of terrigenous debris decreased markedly in the early Miocene (from 11 to 1 g/cm 2 ·10 3 yr) and was low throughout the middle Miocene, increasing somewhat at about 8 Ma and markedly after 5.5 Ma; this pattern is partly related to global sea-level changes but also resulted from high rates of subsidence and tectonic events. Biogenous silica accumulation peaked (mean 2.5 g/cm 2 ·10 3 yr) in the late early Miocene (18–15 Ma) and again in the late Miocene and early Pliocene (8–3.5 Ma) and was 5 to 25 times slower in the interval 15–11 Ma; these variations are thought to reflect primarily variations in the intensity of upwelling and associated production of diatoms. Accumulation rates of biogenous calcite gradually declined from a late early Miocene peak through the early late Miocene and virtually ceased in latest Miocene (8–5.5 Ma), probably due to dissolution. Massive stratification indicates that bottom waters were oxygenated until latest early Miocene time ( c. 16 Ma), when expansion of the oxygen-minimum zone or tectonic formation of a sill within the oxygen-minimum zone produced marked lowering of oxygen levels resulting in the dominance of laminated stratification between 16 and 5.5 Ma. Abundance of organic matter is closely associated with the abundance of clay and of calcite but is inversely correlated with silica content. Organic matter is also about twice as abundant in massive or discontinuously laminated beds (indicating good to moderate oxygenation) than in associated beds with varve-like lamination (indicating minimal oxygenation). These relations indicate that grain-size outweighed the influence of low-oxygen bottom waters in preserving organic matter.