Diatomaceous Sediments Research Articles

Production and dissolution of biogenic silica in the ocean: Revised global estimates, comparison with regional data and relationship to biogenic sedimentation

We estimate the global rate of biogenic silica production in the ocean to be between 200 and 280 × 1012 mol Si yr−1. The upper limit is derived from information on the primary productivity of the oceans, the relative contribution of diatoms to primary production and diatom Si/C ratios. The lower limit is derived independently using a multi‐compartment model of nutrient transport and biogenic particle flux, and field data on the balance between silica production and dissolution in the upper ocean. Our upper limit is 30–50% lower than several previous estimates, due to new data indicating lower values for both the relative contribution of diatoms to primary productivity and their Si/C ratios. Globally, at least 50% of the silica produced by diatoms in the euphotic zone dissolves in the upper 100 m, resulting in an estimated export of 100–140 × 1012 mol Si yr−l to the deep ocean. Our estimates correspond to a global mean rate of biogenic silica production between 0.6 and 0.8 mol Si m−2 yr−1. Incubation experiments indicate that silica production rates exceed that mean by a factor of 3–12 in coastal areas and are 2–4 times less than the global average in the oligotrophic mid‐ocean gyres. The mean silica production rate in waters overlying diatomaceous sediments (approximately 10–12% of the surface area of the oceans) is 0.7–1.2 mol Si m−2 yr−1. That rate is only slightly higher than the global average, indicating that the silica produced in those regions is only 10–25% of the global total. The estimated production of biogenic silica in surface waters of the mid‐ocean gyres is approximately equal to that for all major areas of opal sediment accumulation combined. Regional comparison of silica production and accumulation rates suggests a strongly bimodal character in the efficiency of opal preservation in the sea. In waters overlying diatom‐rich sediments 15–25% of the silica produced in the surface layer accumulates in the seabed, while virtually none of the silica produced in other areas is preserved. The global burial/production ratio of ˜ 3% is a composite of those two very different systems. The mechanisms leading to more efficient opal preservation in regions of silica accumulation are presently unknown, but they have no simple relationship to primary productivity. Regional differences in opal preservation appear to be controlled by factors such as low surface temperature, selective grazing and aggregate formation, which diminish the rate of silica dissolution in surface waters and/or accelerate its transport to the seafloor.

Origin of freshwater-diatom-rich pyroclastic-debris-flow deposit in a shallow-marine Tertiary forearc basin, NW Oregon

ABSTRACT An unusual freshwater-diatom-bearing pyroclastic-debris-flow deposit is present within the shallow-marine upper Eocene to Oligocene Pittsburg Bluff Formation of northwestern Oregon. The subaerially generated pyroclastic-debris flow rapidly debouched into the shallow-marine environment. The flow formed a wedge-shaped deposit, up to 3.5 m thick, that is mappable over several square kilometers. Thickness and maximum clast size decrease offshore; near the distal margins the debris flow mixed with seawater and became a high-density shelf turbidity current. Where emplaced above storm wave base, the deposit was locally reworked to form a thin lag conglomerate. Thalassinoides burrows are present in the upper 30 cm of the deposit. The rhyodacite chemical and mineral composition, regional geologic setting, and thickness and clast-size variations indicate a nearby highly explosive calcalkaline volcanic source to the northeast (i.e., Western Cascade are). This deposit documents the earliest known Cascade arc explosive event that directly affected sedimentation in the Tertiary forearc basin of western Oregon. A sanidine- and biotite-bearing ash fall tuff 3 m above the debris-flow deposit was derived from a backarc eruptive source (e.g., Oligocene John Day Formation). This tuff yielded an 40Ar/39Ar date of 29.83 Ma. The well-preserved diatom flora within the debris-flow deposit matrix extends the geologic range of some genera (e.g., Gomphonema) and widens the geographic distribution of others (e.g., Gomphopleura). Possible scenarios to explain the presence of exclusively freshwater diatoms in the debris-flow deposit in this shallow-marine section include: (1) a primary pyroclastic debris flow may have passed through a diatom-rich lake between the site of eruption and its entrance into the sea; (2) freshly erupted pyroclastics temporarily blocked a river drainage, creating an alpine lake that produced the debris flow upon dam failure; (3) primary pyroclastic debris may have been vented directly through a caldera or crater lake rich in diatomaceous sediment.

Incipient bitumen generation in Miocene siliceous sedimentary rocks from the Japan Sea

Samples of Quaternary through Miocene diatomaceous sediment and siliceous sedimentary rock from the Japan Sea (ODP Leg 128, Site 799) were examined to determine the origin of bitumen encountered during drilling, whether previously published studies using Rock-Eval pyrolysis have accurately assessed organic matter type and maturity, and if evidence exists to postulate the presence of a Kuroko-type massive sulfide deposit at depth or along depositional strike. Whole-rock total organic carbon:total nitrogen ratios have values ranging from 31 to 4. Elemental atomic H:C and O:C kerogen ratios decrease from ca 1.3 and 0.3, respectively, in Quaternary and Pliocene samples to 1.2 and 0.1, respectively, in samples obtained from the bottom of the hole. The bitumen ratio of these rocks increases from 50 mg bitumen/g OC at 700 mbsf to 150 mg bitumen/g at 1000 mbsf. The kerogen in these samples comprises a mixture of marine and terrigenous organic material that reaches the earliest stages of catagenesis in the bottom part of this hole. The bitumen is dominated by asphaltenes, resins and polar components. Because these compounds have very high boiling points, Rock-Eval pyrolysis does not assess the thermal maturity, or organic matter type accurately in the early stages of catagenesis. The bitumens have a very low thermal maturity and the observed degree of thermal alteration is consistent with the modern geothermal gradient in the Kita-Yamato trough.

無機地球化学組成から見た女川珪藻質根源岩相の堆積物組成と古海洋環境

In order to assess the hydrocarbon potential of the Middle Miocene (10-12Ma) Onnagawa diatomaceous sediments, outcrop samples from seven sites in the Aomori and Akita Prefectures were examined for the distribution of their major and minor elements. Three factors optimal for the good source rock generation, i. e., supply of biogenous materials, small dilution effect by terrigenous detrius, and anoxic bottom water condition, were extracted by multivariate statistical analyses. Spatial distributions of these factors indicate source facies developed in the central deep part of the basin, which was kept stagnant by restricted circulation of sea water. During 10 to 9Ma, paleoceanic condition changed drastically due to the initiation of localized upwelling.

Biofilms, microbial mats and microbe‐particle interactions: electron microscope observations from diatomaceous sediments

ABSTRACTSediments and diatoms from the mudflats of the Bay of Bourgneuf in western France were examined in an electron microscope study of biofilms and microbial mats. The sediments were kept in an aquarium for study and a diatom culture was made of the benthic diatoms. The sediment biofilm was composed of exopolymeric substances (EPS), incorporated clay particles and, rarely, bacteria. This film coated all particles at the sediment‐water interface. Its surface morphology reflected its composition and internal structure. Thin films were smooth, whilst a lumpy structure or incorporated fibrils produced either a mammillated or ropy surface, and clays in the structure gave rise to a flaky morphology. At shallow depths in the sediment column (0.5 cm) the biofilm was already degraded. The biofilm coating degraded diatom frustules in the benthic diatom culture consisted of EPS and bacteria and presented a ragged appearance.Microbial mats occurred on the surface of the fresh littoral sediments as well as those in the aquarium, and on the wall of the aquarium. The mat on the surface of the aquarium sediments had an open structure with webs of fibrils and bacteria in the pore space. It formed in a relatively quiet environment. Pore space was more limited in the mat from the surface of the fresh littoral sediments, in which direct contact between biofilm coated particles was common. In the exposed environment of the aquarium wall there was a thick, resistant coating of EPS.In addition to binding particles together, the presence of mats and biofilms in sediments affects sediment physical properties such as porosity and permeability, the flux of dissolved substances in pore waters and the dissolution of particles and can, therefore, influence early diagenesis. Mats and biofilms seem to be more readily preserved in the geological record than the micro‐organisms, such as bacteria, which produce them. Their identification in the sedimentary record would greatly aid interpretation of sediment genesis and evaluation of the microbial role in sediment formation.

A Guide to the Extraction of Fossil Diatoms from Lithified or Partially Consolidated Sediments

Based on experience gained from processing samples from the Cenozoic basins of south-wesin bt Ecuador, a composite procedure for the processing of fossil diatoms from lithified or partially consolidated sediments is presented. The procedure is divided into four stages: (I) initial sample breakdown. (II) cemented rock breakdown and removal of carbonate component. (III) organic matter removal. (IV) clay removal. These are presented in the form of annotated tables linked by an accompanying flow chart. INTRODUCTION Marine Oligo-Miocene sediments from the Manabi and Progreso basins of south-west Ecuador were examined for diatoms. Lithologies encountered varied from partially consolidated clay-rich bentonites of the Portoviejo Formation, to lithified chocolate-brown shades and siltstones of the Tosagua Formation. These units contained a mixed marine fossil assemblage of foraminifera, (Whittaker 1988), radiolaria, diatoms and silicoflagellates. A biostratigraphy for the area has been erected using foraminifera (Whittaker 1988) and radiolaria (Robinson 1992); diatoms and silicoflagellates have also proved essential for palaeo-ecological interpretation and a refined biostratigraphy (Hinchey 1992). Diatomaceous sediments from Ecuador differ considerably in their lithology and therefore react differently to conventional diatom processing techniques (Lohmann 1972, Schrader 1973). Samples from the bentonitic Portoviejo Formation broke down fairly easily, but problems were encountered in the removal of clays from these sediments. Lithified samples from the clay-rich Tosagua Formation, however, proved very problematic; conventional processing techniques were ineffective in most stages of breakdown and the soaking of samples in petroleum spirit (Abelmann 1988, Boden 1991) also failed to disaggregate the samples satisfactorily. OBJ ECTIVES As standard diatom processing techniques failed to breakdown many of the samples collected from Ecuador, a different approach was clearly required. A literature review was conducted on previous techniques and experienced workers were asked for their comments (see acknowledgments). Techniques suggested were applied to material from Ecuador and the most effective methods noted. The result of this work has produced a composite processing procedure which can be generally applied to most types of diatomaceous sediments, including those which are lithified or partially consolidated. The procedure is presented in the form of a flow chart (text-fig. 1) and a series of annotated tables (1-4), each table representing a particular aspect of processing. The procedure is designed to be user friendly and emphasizes the use of wet mounts between certain stages (e.g. removal of clays and organic matter; text-fig. 1, Tables 3 and 4) to evaluate whether or not further processing is required. The tables are designed to work independently of each other and therefore should allow workers to initially assess the lithology of the sediments they are studying and to then determine which of the tables apply to that particular type of sediment. ADDITIONAL NOTES TO TABLES Table 1 Mechanical breakdown of the sediment should be done with as little damage to frustules as possible. Not all stages listed will be necessary. Physico-chemical breakdown (Table 2 (2a) and Table 3 (la-d)) can be used for more resistant samples. Table 2 Warming of sample + HCI (Ib) is not always necessary. Heat only if very little reaction occurs. Table 3 Addition of a few drops of methanol to a hydrogen peroxide solution (lb) prevents strong foaming in organic rich samples. Table 4 In some samples, all clay cannot be removed. As a consequence the worker must assess the residue (using wet mounts) to evaluate what is an acceptable amount of clay in their particular study. Note on the use of wet mounts. Tables 3 and 4 advocate the use of wet mounts at various stages of the cleaning procedure to check whether further processing is required. It must be noted that the removal of material for wet mount preparation will bias any quantitative examination (P. Boden, pers. comm.). Therefore, if high resolution quantitative studies are to be attempted it is suggested that either 1) wet mounts should not be used, or that 2) two splits of the same sample be processed. One split being used to assess whether further processing is necessary and the other used for quantitative work. The wet mount procedure forms an essential stage in the cleaning of diatomaceous lithified sediments from Ecuador. However, it has proved unnecessary for DSDP deep sea muds from the equatorial Pacific. Therefore the use of wet mounts is wholly dependant on the nature of sediment and the purpose of study. SLIDE PREPARATION The method of slide preparation utilized will depend on the type of study proposed. The pipetting method of Schrader and Gersonde (1978) proves suitable for biostratigraphic studies, while the ranmicropaleontology, vol. 40, no. 4, pp. 368-372, text-figure 1, tables 1-4, 1994 368 This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 06:45:44 UTC All use subject to http://about.jstor.org/terms Micropaleontology, vol. 40, no. 4, 1994 I.INITIAL SAMPLE BREAKDOWN

Diatom paleoecology of the Mar Chiquita lagoon delta, Argentina

A 68 cm thick outcrop of diatomaceous sediments at the head of Mar Chiquita Lagoon on the Argentine coast near Buenos Aires provides evidence of Holocene paleoenvironmental changes related to sea level changes and freshwater input to the lagoon system. Salinity tolerances of extant diatom taxa were used for the reconstruction and multivariate analytical techniques were applied to reduce subjective interpretations of the diatom percentage data.

Miocene intensification of upwelling along the California margin as recorded in siliceous facies of the Monterey Formation and offshore DSDP sites

Abstract Diatomaceous sediments and their diagenetic equivalents in the Monterey Formation record a variable history of upwelling along the California margin. Distrinctive dark opal-CT and quartz chertz found in distal basins of the Monterey Formation are the result of burial diagenesis of pure biosiliceous oozes (biosiliceous oozes without significant admixtures of clay) and are therefore evidence of intensified coastal upwelling during the early middle Miocene. Dating of six sections of the Monterey Formation, largely by diatom biostratigraphy, suggests that at the Point Reyes and Point Año Nuevo sections in north-central California, the age of the earliest chert intervals is between 13.8 and 15.0 Ma, and 14.3 and 14.8 Ma, respectively. In south-central California, ages from the Shell Beach, Mussel Rock, and Lions Head sections imply that the age of the base of the chert intervals is between 12.7 and 13.3 Ma. Both ages correlate to an early middle Miocene high latitude cooling step that resulted in more vigorous surface water circulation, upwelling of nutrient-rich waters, and increased biosiliceous sedimentation in the North Pacific. The north-south difference in age of the base of the chert interval probably reflects a progressive intensification of the California Current from 15.0 to 12.7 Ma. The age of the onset of biosiliceous sedimentation at DSDP sites of the northeastern Pacific is also generally younger at the more southern sites; however, these particular DSDP sites were located some distance from the centres of coastal upwelling and are not as reliable indicators of the intensification of upwelling along the California margin.

Lack of evidence for enhanced preservation of sedimentary organic matter in the oxygen minimum of the Gulf of California

A test of the hypothesis that hydrogen-rich organic matter accumulates preferentially in sediments under oxygen minima has been carried out by examining the composition of laminated and homogeneous diatomaceous sediments of the Guaymas Basin in the Gulf of California. The laminated sediments occur where an intense oxygen minimum intersects the slope around the margins of the basin. Above and below the depth of the oxygen minimum, the sediments are homogenized by a burrowing infauna. The two sediment types have very similar bulk chemical and mineral compositions, and the organic matter is overwhelmingly marine. The total organic carbon contents and Rock-Eval parameters (hydrogen and oxygen indices) of the two sediment types are not significantly different. Whereas a road organic carbon maximum in the homogeneous sediments is centered at roughly 1200 m depth on the slope of the basin, this maximum lies deeper than the core of the oxygen minimum. Thus, both the concentration and the nature of the organic matter in the sediments of this highly productive marginal marine basin are not related to the concentration of dissolved oxygen in the bottom waters.

EARLY NEOGENE FORAMINIFERA AND ASSOCIATED MICROFOSSILS OF THE CERRO TIERRA BLANCA MEMBER (EL CIEN FORMATION), BAJA CALIFORNIA SUR, MEXICO

The foraminifera and associated microfossils from the type section of the Cerro Tierra Blanca Member (El Cien Formation), supposedly of late Oligocene age, indicate these strata to be of latest Oligocene to late Miocene age. The analysis done from the sequence demonstrates severa! stratigraphic discontinuities that, associated with the scarce geological information from the area, are indicative of structural complexities not detected during the establishment of the type section. lt was impossible to determine the stratigraphical relation between the strata studied and the supposed two other members of the formation for the same reasons -at El Cien- as well as with other sequences that crop out at El Cien-San Hilario, La Purísima or San Juan de la Costa. The micropaleontological assemblage suggests initial subsidence and deposition of early Miocene volcanics and shelf marine clastics, followed by deposition of upper-bathyal diatomaceous sediments during earliest late Miocene. The observed microfossils are a mixture of low- and high-latitude species, such as the ones found at present, near the distal end of the California current off Baja California Sur, and indicate periods of low and high fertility. Based on the present results, it is not possible to determine if the El Cien Formation is a valid name for the beds surrounding El Cien, nor for those to the east.

A displaced neritic diatom ( Delphineis karstenii) in pelagic sediments of the southeast Atlantic

This paper documents the occurrence of Delphineis karstenii, a neritic diatom, in pelagic sediments of the northern Cape and southern Angola Basins (southeast Atlantic Ocean). Maximum abundances are found in the southern Angola Basin, although the source area appears to be the diatomaceous sediments which blanket much of the Namibian continental shelf. The observed pattern implies large-scale displacement of D. karstenii valves first northward along the continental margin, and then westward into deep waters. Transport evidently occurs in the surface waters of the Benguela Current, which flow northward along the Namibian coast and then turn westward, away from the coast of Angola and out into the Angola Basin. These observations suggest that Delphineis valves, and perhaps other shallow-water diatom valves, may be displaced downslope on other continental margins. Failure to recognize such displacement could lead to incorrect ecological and paleoecological interpretations.

Global Paleomagnetic Correlation of the Blake Polarity Episode

The authors have studied a high-resolution paleomagnetic record of a magnetic polarity episode recovered from a 20-m section of diatomaceous lacustrine sediments from Pringle Falls near La Pine, Oregon. A total of 982 specimens were collected at about 5-cm intervals from the 20-m section. The age of this record is at present constrained between 0.6 Ma and 18,000 yr based upon a pollen date. The characteristic magnetization of the samples was determined by alternating field (AF) demagnetization to 15 mT. The average normal polarity stable declination and inclination of the nonmagnetic polarity episode sampled section (61{degree}; alpha{sub 95} = 2.6) is statistically equivalent to that expected at this site for a geocentric axial dipole (62{degree}). The record provides a Virtual Geomagnetic Pole (VGP) path that identifies a well-defined clockwise northeast-southwest loop with a radius of 60{degree}, followed by a second younger feature that traces out a counterclockwise loop 15{degree} in radius. Interpretation of the natural remanent magnetization/anhysteretic remanent magnetization (NRM/ARM) ratios for the samples indicates that the field intensity may have fallen and then increased above the average dipole field intensity. The low-intensity interval ends before the onset of the polarity episode as defined by directions and may suggest thatmore » the episode is a result of an unstable regeneration of the field after a significant lowering of overall field intensity. These two characteristic features are similar to the two features present in the Blake geomagnetic polarity episode at 114,000 and 105,000 Ma observed in different parts of the world.« less

Intrastratal Microfractured Zones in Laminated Diatomaceous Sediments from the Monterey Formation, California: A Record of Miocene Paleoseismicity?

Finely laminated diatomaceous sediments from the Miocene Formation possess intrastratal microfractured zones (IMZ), which are nonuniformly distributed through vertical section. These stratally discrete zones of deformation record brittle fractures with minor fault offset and complex, yet consistent, patterns of sediment and fluid redistribution in associated vein arrays. Outcrop, microscopic, and scanning electron microscope observations suggest that IMZ are attributable to a discrete episode of fluid expulsion within shallowly buried sediments. The author credits seismically triggered loss of sediment strength for the formation of IMZ in a single episode of brittle fracture and vein propagation by fluid dilation. The consistent co-occurrence of microfaults with mud-filled veins implies synergistic coupling between the processes of fluid advection and brittle fracture. Constituent faults and veins within IMZ are characteristically sigmoidal and occur in curviplanar-parallel arrays. Their consistent orientation is attributed to a bedding-parallel shear couple, suggesting that these sigmoidal flexures record the spatial orientation of Miocene paleoslopes. Furthermore, the association of IMZ with slump breccias, disharmonically folded strata, and extensively fluidized sediments indicates that IMZ formation may be an important mechanism for initiating sedimentary mass failure in subaqueous slope settings. As a seismically triggered( ) sedimentary structure, the presence or absence of IMZ inmore » varved diatomites may be a useful tool for evaluating seismic history.« less

Late quaternary glacial history of the South Orkney Plateau, Antarctica

Piston cores from the South Orkney Plateau penetrated overcompacted diamictons in water depths of up to 250 m. Detailed textural and petrological analyses of these diamictons indicate that they are basal tills. Seismic records from the plateau show a widespread surface of glacial erosion and provide additional evidence of an ice cap grounded to a depth of 250 m. Piston cores from the slope of the plateau penetrated diatomaceous muds resting directly on poorly sorted muds with very little to no biogenic material. The ice-rafted debris in these glacial-marine sediments is composed almost exclusively of material derived from the South Orkney Islands. This implies deposition beneath an ice shelf as opposed to iceberg rafting. In contrast, diatomaceous muds contain relatively abundant exotic iceberg-rafted material and reflect a glacial-maritime setting similar to that of today. The sharp contact separating diatomaceous surface sediments from basal tills and sub-ice shelf deposits indicates that the ice cap and ice shelf retreated from the plateau rapidly. Radiocarbon dates for diatomaceous muds from a glacial trough on the plateau indicate that the ice cap and ice shelf retreated from the plateau prior to 6000 to 7000 years ago. The homogeneity of surficial diatomaceous sediments suggests that sea ice conditions over the plateau have not changed radically since that time.

Cenozoic marine sedimentation in the Sechura and Pisco basins, Peru

The central and northern Peruvian margin consists of a series of 8 paired forearc basins which may be separated into an inner set of shelf basins and a seaward set of slope basins. We have examined the Cenozoic stratigraphy of the onshore portions of the Sechura Basin (5–7°S) and Pisco Basin (13–16°S), two shelf basins which have accumulated marine sediment discontinuously since the mid to late Eocene. Cenozoic sediments in the Pisco Basin were deposited during at least three major transgressive cycles. Each sequence is preserved as a similar vertical progression of facies including coarse nearshore bioclastic conglomerates and sandstones grading upwards into sandy siltstones and mudstones, and capped by biogenic deposits including diatomites, diatomaceous mudstones, dolomitic horizons, and phosphate deposits. Stratigraphic nomenclature for the Pisco Basin has recently evolved; a stratigraphy presented here includes the Eocene Caballas Fm., upper Eocene Los Choros fm., upper Eocene to lowermost Oligocene Yumaque fm., uppermost Oligocene to middle Miocene Chilcatay fm., and upper Miocene to Pliocene Pisco Fm. Major hiatuses in the Pisco Basin span the Late Cretaceous to middle Eocene, early to late Oligocene, middle Miocene, and late Pliocene/Pleistocene to Recent. Cenozoic sediments of the Sechura Basin were deposited within at least 4 major transgressive cycles with hiatuses during the Paleocene to middle Eocene, Oligocene, early to middle Miocene, and late Miocene. Based on recent biostratigraphic studies, sediments enriched in biogenic components accumulated between about 40-36 Ma, 24-16 Ma, and 11-3 Ma in the Pisco Basin and between 40-37 Ma and 8.5-4.5 Ma in the Sechura Basin. In both basins, the most diatomaceous sediments are restricted to the Late Eocene and Late Miocene through Pliocene. The temporal distribution of biogenic sediments suggests that high productivity conditions linked to coastal upwelling have occurred episodically since at least the Late Eocene. The occurrence of diatomites and phosphorites is diachronous between the Pisco and Sechura Basins and between the Peruvian forearc and other circum-Pacific Monterey Formation analogs, a reflection of the strong influence of local tectonism on sedimentation patterns. The volume of Neogene sediments along the Peruvian forearc is nearly twice that of the Monterey Fm.; despite basin-to-basin facies diachroneity, these deposits very likely contributed to fluctuations of the late Miocene carbon/CO 2 system by acting as large carbon sinks.

Miocene-Early Pliocene Diatomaceous Sedimentation Patterns in the Pacific: A Response to Paleoceanographic and Paleoclimatic Change: ABSTRACT

Biosiliceous sediments greatly expanded their distribution in the North Pacific during the late early to early middle Miocene (18-15 Ma), at least partly due to enhanced basin-basin fractionation of deep waters between the North Atlantic and North Pacific, which began in 17 Ma. Subsequently, in the late middle and late Miocene, diatomaceous sedimentation responded to the following series of successive cooling steps. (1) High-latitude cooling between 14.9 and 12.4 Ma caused an intensification of upwelling in the marginal North Pacific as evidenced by a wider distribution of laminated sediments and a decline in carbonate preservation. (2) Cooling across the middle Miocene-late Miocene boundary (10.3-9.2 Ma and 9.0-8.3 Ma) led to further increases in upwelling and raised biogenic opal accumulation rates in upwelling regions of the eastern equatorial Pacific, California, and Peru. (3) Cooling in the latest Miocene to earliest Pliocene (6.2-4.8 114a) resulted in further regional differentiation of biosiliceous sedimentation patterns in the North Pacific, probably reflecting steeper productivity gradients in upwelling regions. Biogenic opal accumulation rates declined in the eastern equatorial Pacific, but probably increased in coastal upwelling regions off California, Peru, and in the Bering Sea. (4) A decline in diatom sedimentation in waters off southern California duringmore » the earliest Pliocene (4.5-4.0) Ma coincided with a period of warmer paleotemperatures and a decline in opal accumulation rates in the equatorial Pacific. A major increase in biosiliceous sedimentation in the Southern ocean occurred at this same time.« less

Downhole Measurement Data as Paleoclimate Indicators: a Plio-Pleistocene Record of Asian Eolian Dust Deposition to the Sea of Japan: ABSTRACT

Ocean Drilling Program Site 798 (Leg 128) presents a unique opportunity to monitor the late Neogene paleoclimatic evolution of the SE Sea of Japan region because it is positioned on an elevated ridge (the Oki Ridge, 911 mbsl) above the present lysocline and sedimentation rates here are unusually high ({approximately}120 m/Ma). Since the site is located downwind of the westerly Asian dust trajectory, it was anticipated that these diatomaceous sediments would yield a detailed late Neogene record of eolian dust deposition. The authors have collected geophysical and geochemical logs as part of the downhole measurements program at Hole 798B; these data represent continuous in situ measurements of bulk density, porosity, sonic velocity, natural gamma ray activity, and a suite of elemental abundance logs (K, U, Th, Al, Si, Ti, Ca, Gd, Fe, S, H, Cl, and Mg+Na). Based on a paleomagnetic timescale, the natural gamma ray log (which largely reflects variations in gamma-emitting K and Th bound in clay minerals) is dominated by cyclic 41 kyr variations over the interval corresponding to 2.8-1.0 Ma; these cyclic variations are also apparent in other log responses. They interpret these periodic increases in terrigenous sediment content to reflect variations in eolian dust inputmore » from Asian source areas. Studies of Chinese loess deposits have demonstrated that Asian continental aridity has been strongly modulated by variations in Northern Hemisphere glacial ice volume which varied predominantly at the 41 kyr orbital obliquity periodicity during the Matuyama Chron (2.47-0.73 Ma). Cross-spectral analysis of the Hole 798B gamma ray record and the marine oxygen isotopic record over this interval supports this linkage. Work in progress will define more accurately the sedimentologic origin of the various log responses, and hence their fidelity as paleoclimate proxy indicators.« less

Late Cenozoic sedimentation in California Continental Borderland basins as revealed by seismic facies analysis

The California Continental Borderland began to form during late Miocene time. Only the latest Miocene and younger sediments are ponded in the contemporary basins. Upper Miocene strata are characterized by widespread diatomaceous deposits that conformably drape all of the underlying topographic irregularities. The Pliocene-Quaternary sediments occur as flat-lying ponded clastic sequences that fill the contemporary basins. A seismic facies analysis has defined facies patterns in the Pliocene-Quaternary basin-fill sequences of each basin. In the offshore basins (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. Santa Catalina Basin is a transitional basin filled with fine-grained turbidites and hemipelagic sediments derived from both continental and local insular sources. The nearshore basins (Santa Barbara, Santa Monica, and San Pedro Basins and San Diego Trough) are filled by thick sequences of continental sediments that form prominent deep-sea fans. The borderland was dominated by biogenic pelagic sedimentation in the late Miocene and by detrital-sediment-gravity-flow deposition in the Pliocene and Quaternary. Accumulation rates estimated from Pliocene-Quaternary sediment thickness for each basin show that in the offshore basins, as well as in San Diego Trough, rates have remained fairly constant since the Pliocene. In contrast, the nearshore basins exhibit an increasing rate of accumulation versus time that is related to the progradation of continent-derived sediments. Outer- and intermediate-basin accumulation data indicate decreasing contributions with time from offshore highs because of erosion and regional subsidence. Intermediate basins show an increase in mass-wasting deposits (slope facies) with time, which is the result of increasing fine-suspension slope deposition and eventual failure.

Neogene stratigraphy, foraminifera, diatoms, and depositional history of Maria Madre Island, Mexico: Evidence of early Neogene marine conditions in the southern Gulf of California

Foraminifera and diatoms have been analyzed from an upper Miocene through Pleistocene(?) sequence of marine sediments exposed on Maria Madre Island, largest of the Trés Marias Islands off the Pacific coast of Mexico. The Neogene stratigraphic sequence exposed on Maria Madre Island includes a mid-Miocene(?) non-marine and/or shallow marine sandstone unconformably overlain by a lower upper Miocene to uppermost Miocene upper to middle bathyal laminated and massive diatomite, mudstone, and siltstone unit. This unit is unconformably overlain by lower Pliocene middle to lower bathyal sandstones and siltstones which, in turn, are unconformably overlain by upper Pliocene through Pleistocene(?) upper bathyal to upper middle bathyal foraminiferal limestones and siltstones. These beds are unconformably capped by Pleistocene terrace deposits. Basement rocks on the island include Cretaceous granite and granodiorite, and Tertiary(?) andesites and rhyolites. The upper Miocene diatomaceous unit contains a low diversity foraminiferal fauna dominated by species of Bolivina indicating low oxygen conditions in the proto-Gulf Maria Madre basin. The diatomaceous unit grades into a mudstone that contains a latest Miocene upper to middle bathyal biofacies characterized by Baggina californica and Uvigerina hootsi along with displaced neritic taxa. An angular unconformity separates the upper Miocene middle bathyal sediments from overlying lower Pliocene siltstones and mudstones that contain a middle to lower bathyal biofacies and abundant planktonic species including Neogloboquadrina acostaensis and Pulleniatina primalis indicating an early Pliocene age. Significantly, this Pliocene unit contains common occurrences of benthic species restricted to Miocene sediments in California including Bulimina uvigerinaformis. Pliocene to Pleistocene(?) foraminiferal limestones and siltstones characterize submarine bank accumulations formed during uplift of the Trés Marias Island area, and include abundant planktonic foraminifera such as Pulleniatina obliquiloculata and Neogloboquadrina duterteri. Common benthic foraminifera in this unit are indicative of upper bathyal water depths. The Neogene depositional history recorded on Maria Madre Island involves an early late Miocene subsidence event marking formation of the Trés Marias Basin with relatively undiluted diatomaceous sediment deposited in a low oxygen setting. Subsidence and deepening of the basin continued into the early Pliocene along with rapid deposition of terrigenous clastics. Uplift of the basinal sequence began in late Pliocene time accompanied by deposition of upper Pliocene-Pleistocene foraminiferal limestones on a rising submarine bank. Continued episodic uplift of the Neogene deposits brought the island above sea level by late Pleistocene time.

A box corer 30 cm square and 4 m long

To collect long, large-volume cores of diatomaceous sediment on the continental shelf off Namibia, we built a box corer that is 30 cm square and 4 m long. This paper describes the corer and the tools and procedures for sampling the covers. In terms of volume of sediment recovered in a single penetration, the corer may be among the largest ever used. The corer itself consists of a barrel with segments 20 cm long, a release mechanism at top and a thin fiberglass curtain at bottom. To support the large load of sediment without distortion, the curtain follows a semi-circular track, concave upward. During assembly and disassembly, the corer hangs vertically over the side, enabling it to operate from a relatively small ship. To sample the core, an extruding device pushes the sediment from each segment into boxes made of polyurethane foam. Ashore a specially designed jig helps slice these boxes into vertical slabs as thin as 1 cm. In the 6 days at sea that we had to test the corer and collect samples for the project, we took 9 cores, the longest of which was 3 m.