Well-preserved Diatoms Research Articles

Palaeohydrological changes recorded from a small Moroccan Middle Atlas pond during the last 6000 cal. yr BP: a multi-proxy study

The perennial and seasonal wetland diversity of the Moroccan Middle Atlas region provides a valuable “test-bed” for understanding the response of different hydrosystems to climatic variations. A multiproxy study, based on sedimentological descriptions, together with mineralogy, carbonate content, XRF core scanning and biological proxies supported by AMS 14C dates, were applied to the 3-m-long core extracted from “Flowers Marsh”, a small Middle Atlas pond. This approach provides evidence for a continuous paleohydrological and paleoenvironmental record during the Mid- to Late Holocene. The investigated aquatic system evolved from a dry or very shallow waterbody towards a system with a progressively rising water level. The dominance of the detrital fraction with poor preservation of bioindicators and eroded pollen, indicate the existence of an ephemeral waterbody from 6000 cal. yr BP until a transitional phase characterized by new sedimentological facies and the appearance of ostracods around 2300 cal. yr BP. This transition, ending at 2000 cal. yr BP, is interpreted as a flooding phase leading to an ephemeral lake. It is certainly fed by the excess water from the nearby Aguelmam Azigza Lake during its high level period. Afterwards, enhanced organic matter deposition and the appearance of well-preserved diatoms until 1400 cal. yr BP corroborate a high water-level trend. Endogenic carbonate to detrital fraction ratios indicate fluctuating, but generally shallow, water levels from 1400 cal. yr BP until 650 cal. yr BP when a relatively rapid rise in water level occurred. Flowers Marsh data are, generally, consistent with most of the existing regional records. The highstand period recorded between 2000 and 1400 cal. yr is a common feature extending to more distant sites from the northern Mediterranean. It corresponds to the wetter Iberian-Roman period. Fluctuating shallow water levels recorded since 1400 cal. yr BP to now could be linked to drier/wetter phases associated with the Medieval Climate Anomaly and the Little Ice Age (650– 150 cal. yr BP) respectively, in the western Mediterranean realm. The present study demonstrates the ability of Flowers Marsh to record valuable palaeohydrological changes since the Mid-Holocene and confirms the high sensitivity of Middle Atlas hydrosystems to climatic changes.

Middle Pleistocene to recent diatoms and stratigraphy of the Magadi Basin, south Kenya Rift

Two cores were recovered from the Lake Magadi and Nasikie Engida Basins in the south Kenya Rift. Core MAG14-2A (194 m) contains a middle Pleistocene to Holocene record, whereas core NAS15/19 (4.36 m) covers only the late Holocene. Surficial sediments from springs and shallow-water sites were sampled in both basins. MAG14-2A rests on trachyte dated at 1.08 Ma. Diatoms are rare in the oldest sediments, but well preserved after about 545 ka, documenting a trend from less to more saline water. Core MAG14-2A contains fifteen facies, five of which are diatomaceous. In contrast, NAS15/19 is dominated by two facies, each containing well-preserved diatoms. Both sequences are distinct from others of similar age in the Kenya Rift in lacking pedogenic horizons, reflecting the location of Lake Magadi and Nasikie Engida in a tectonic sump where aquatic environments were maintained by geothermal and meteoric springs. Canonical Correspondence Analysis distinguishes three assemblages in the modern surface muds of Lake Magadi and Nasikie Engida, but with no pre-Holocene counterparts. Eleven diatom zones are recognised in MAG14-2A: Zones D2 to D10 contain rare to common diatoms dominated by Aulacoseira granulata and its varieties, Aulacoseira agassizii, Thalassiosira faurii, Thalassiosira rudolfi and Cyclotella meneghiniana. Individual samples commonly include a mixture of benthic and planktonic taxa and saline and freshwater species. These assemblages indicate waters that ranged between pH 7.4 and 11.5 with conductivities of ~ 300 to > 25,000 μS cm−1. Correlations with the neighbouring Olorgesailie and Koora Basins indicate four major environmental phases that affected the south Kenya Rift during the last million years with fresh to moderately saline water, or land surfaces, developing during Phase I (1000 to 750 ka). These environments gave way to generally wetter conditions and freshwater lakes in all basins during Phase II (~ 750 to 500 ka). Phase III (~ 500 to 325 ka) was characterised by drier conditions with paleoenvironments becoming more variable and dry during Phase IV (325 ka to present).

Microfossil assemblages (diatoms, calcareous nannofossils, and silicoflagellates), paleoenvironment, and hydrocarbon source rock potential of the Oligocene Ruslar Formation at Karadere, Bulgaria

The Oligocene Ruslar Formation, an equivalent of the Maykop Suite, is a potential hydrocarbon source rock in the western Black Sea Basin. In contrast to the offshore areas, the depositional environment and hydrocarbon source rock potential of onshore Bulgaria sediments are largely unknown. Hence, a 14-m-thick section of the Ruslar Formation, exposed near Karadere (Black Cape) along the Black Sea coast, provides an excellent opportunity to study the upper part of the Ruslar Formation. Here, laminated diatomrich mudstones with frequent thin sandstone beds and a prominent concretion horizon are exposed. Furthermore, the fossil diatom assemblages provide a key component to understand the paleoenvironment. Overall, Paleogene diatoms are understudied in the Black Sea Basin and therefore only a genus-level study is undertaken here. The studied Ruslar Formation contain remarkably diverse and wellpreserved diatom assemblage with 23 different genera. The most frequent genera are Paralia, Distephanosira, and Stephanopyxis. Common genera include Coscinodiscus, Hemiaulus, Pseudopodosira, Rouxia, and Xanthiopyxis. Rare taxa include Actinoptychus, Asterolampra, Azpeitia, Delphineis, Distephanosira, Diploneis, Eunotogramma, Eurossia, Lyrella, Liradiscus, Plagiogramma, Radialiplicata, Rutilaria, Saeptifera, and Triceratium. The diatom assemblages together with calcareous nannoplankton, silicoflagellates, and the presence of rare foraminifera indicate a fully marine neritic environment without major salinity variations. The calcareous nannoplankton investigated can be assigned to biozone NP23 (Early Oligocene). The exposed fragment of the Ruslar Formation was deposited after the low salinity Solenovian event, which represents the maximum isolation of the Paratethys present in the lower part of the NP23. Bulk geochemical parameters from 35 samples (avg. TOC: 1.80% wt.; avg. HI: 226 mg HC/g TOC) show that the exposed part of the Ruslar Formation contains type II-III kerogen and a fair to good hydrocarbon potential. The Ruslar Formation is immature (avg. Tmax: 424 °C), but may generate about 0.5 tons of hydrocarbons per square meter if mature. Biomarker proxies support the low maturity and are characterized by diatom-related biomarkers (24-norcholestane; C25HBI alkanes and thiophenes). Land-plant-derived biomarkers suggest a significant input of angiosperms. Based on biomarker ratios, the depositional environment was oxygen-depleted but probably not strictly anoxic. Reworking of biomass by chemoautotrophic bacteria is suggested by the presence of 28,30-bisnorhopane.

Hydrocarbon source rock potential of Miocene diatomaceous sequences in Szurdokpüspöki (Hungary) and Parisdorf/Limberg (Austria)

Abstract Diatomaceous sediments are often prolific hydrocarbon source rocks. In the Paratethys area, diatomaceous rocks are widespread in the Oligo-Miocene strata. Diatomites from three locations, Szurdokpüspöki (Hungary) and Limberg and Parisdorf (Austria), were selected for this study, together with core materials from rocks underlying diatomites in the Limberg area. Bulk geochemical parameters (total organic carbon [TOC], carbonate and sulphur contents and hydrogen index [HI]) were determined for a total of 44 samples in order to study their petroleum potential. Additionally, 24 samples were prepared to investigate diatom assemblages. The middle Miocene diatomite from Szurdokpüspöki (Pannonian Basin) formed in a restricted basin near a volcanic silica source. The diatom-rich succession is separated by a rhyolitic tuff into a lower non-marine and an upper marine layer. An approximately 12-m thick interval in the lower part has been investigated. It contains carbonate-rich diatomaceous rocks with a fair to good oil potential (average TOC: 1.28% wt.; HI: 178 to 723 mg HC/g TOC) in its lower part and carbonate-free sediments without oil potential in its upper part (average TOC: 0.14% wt.). The composition of the well-preserved diatom flora supports a near-shore brackish environment. The studied succession is thermally immature. If mature, the carbonate-rich part of the succession may generate about 0.25 tons of hydrocarbons per square meter. The diatomaceous Limberg Member of the lower Miocene Zellerndorf Formation reflects upwelling along the northern margin of the Alpine-Carpathian Foreland. TOC contents are very low (average TOC: 0.13% wt.) and demonstrate that the Limberg Member is a very poor source rock. The same is true for the underlying and over-lying rocks of the Zellerndorf Formation (average TOC: 0.78% wt.). Diatom preservation was found to differ considerably between the study sites. The Szurdokpüspöki section is characterised by excellent diatom preservation, while the diatom valves from Parisdorf/Limberg are highly broken. One reason for this contrast could be the different depositional environments. Volcanic input is also likely to have contributed to the excellent diatom preservation in Szurdokpüspöki. In contrast, high-energy upwelling currents and wave action may have contributed to the poor diatom preservation in Parisdorf. The hydrocarbon potential of diatomaceous rocks of Oligocene (Chert Member; Western Carpathians) and Miocene ages (Groisenbach Member, Aflenz Basin; Kozakhurian sediments, Kaliakra canyon of the western Black Sea) has been studied previously. The comparison shows that diatomaceous rocks deposited in similar depositional settings may hold largely varying petroleum potential and that the petroleum potential is mainly controlled by local factors. For example, both the Kozakhurian sediments and the Limberg Member accumulated in upwelling environments but differ greatly in source rock potential. Moreover, the petroleum potential of the Szurdokpüspöki diatomite, the Chert Member and the Groisenbach Member differs greatly, although all units are deposited in silled basins.

Petrology and industrial application of main diatomite deposits in the Transuralian region (Russian Federation)

In the middle of the last centuary, large resources of Paleocene–Eocene diatomites were discovered in the Transuralian region. By now, more than 100 near-surface diatomite deposits and potential sites have been identified within the Serov–Shadrinsk lithofacial zone. Despite the large diatomite resources, relatively simple mining–engineering conditions, proximity of some well-known outcrops of diatomite to transportation facilities and large industrial towns, the matter of effective practical application of diatomites from the Transuralian region is still debatable. This work presents results of studying the material composition and textural features of Eocene diatomites of the deposits at Agirish, Irbit, and Kamyshlov, Eocene diatomaceous clays of the Shadrinsk deposit and, for the first time, Paleocene diatomites of the Brusyana deposit. Diatomites of these Transuralian deposits are similar in SiO2 to other large diatomite deposits of the Russian Federation. They are characterized by thin horizontal lamination and a large microporosity, as well as similar textural peculiarities. Diatomites are mostly composed of well-preserved diatoms (85–95%); the SiO2 content in some fossils is as high as 98–100%. According to chemical and mineralogical composition, Paleocene diatomites from the Brusyana outcrop are considered as the most “pure” varieties among all Transuralian diatomites. Diatomaceous clays of the Transuralian region are characterized by lower SiO2 contents, a higher proportion of clay and a low preservation degree of siliceous fossils. The results of analytical studies suggest that Transuralian diatomites can be used as both raw material for production of construction materials and potential raw material for filter materials, catalysts, and sorbents.

北海道東部浦幌町の古第三系縫別層から産出する保存の良い珪藻化石

To provide insight into the biostratigraphy and evolutionary radiation of marine diatoms, Paleogene fossil records are essential because modern floral assemblages are thought to have originated in the late Oligocene. However, Paleogene diatoms are rarely reported from the Northwest Pacific Ocean because of their poor preservation and limited occurrence. In this paper, we report well-preserved diatoms from methane-seepage-induced limestone of the lower Oligocene (Rupelian) Nuibetsu Formation in the Urahoro area, eastern Hokkaido. The diatoms are dominated by Stephanopyxis spp., Odontella sawamurae, Hemiaulus spp., and resting spores. The assemblage is remarkable because it lacks late Oligocene to early Miocene index species, reflecting floral turnover during the Oligocene.

Diatom, phytolith, and pollen records from a 10Be/9Be dated lacustrine succession in the Chad basin: Insight on the Miocene–Pliocene paleoenvironmental changes in Central Africa

Abstract A discontinuous 200 m-long borehole drilled in the Bol Archipelago (13°N, Lake Chad) provided 25 samples, which were dated using the 10 Be/ 9 Be method and analyzed for their micro-biological content. The dating provided ages ranging from 6.3 ± 0.1 to 2.6 ± 0.1 Ma, a period contemporaneous with the Pliocene fossil localities located in the current Djurab desert of Chad (16–17°N). Well-preserved diatom assemblages first occurred at 4.7 ± 0.1 Ma and were dominated by the freshwater planktonic genera Aulacoseira and Stephanodiscus until the end of the Pliocene. This supports the recurrence of lacustrine conditions at Bol during all the Pliocene. The presence of pelite and argillaceous deposits in the core before 4.7 ± 0.1 Ma, however, suggests that the lake settled earlier, at least since 6.3 ± 0.1 Ma. The abundance of Afromontane pollen taxa at 4.2 ± 0.1 Ma and the occurrence of trapeziform polylobate phytoliths throughout the sequence suggest significant vegetation inputs from the southern highlands, while the importance of kaolinite in the clay sediments indicates a water supply predominantly from the south during the Pliocene. Phytolith assemblages are all dominated by lobate grass silica short cells and by blocky and elongate types, which attest to the presence of herbaceous-dominated vegetation around Bol and/or in the southern drainage basin during the Pliocene. This result is also supported by the pollen assemblage described at 4.2 ± 0.1 Ma, which shows highest affinity for the savanna biome. Moreover, low values for the Xerophytic grass phytolith index indicate the presence of humid-loving (mesophytic and aquatic) grass communities in this vegetation. At last, significant variations in the abundance of blocky and elongate phytoliths are indicative of local alternations of fully lacustrine and marshy conditions at Bol. Particularly between 3.6 and 2.7 Ma, the abundance of silicified bulliform cells combined with the absence of diatoms support a significant lacustrine reduction at Bol favoring the increasing of local marshy vegetation.

Fragilariopsis tigris sp. nov., a new late Pliocene Antarctic continental shelf diatom with biostratigraphic promise

A new species within the genus Fragilariopsis, F. tigris, is described and illustrated using light microscopy and scanning electron microscopy. This species is restricted to a single 8-meter-thick diatom unit within the 585-meter-long section of alternating diatomites and diamictites recovered in the upper portion of the ANtarctic geological DRILLing (ANDRILL) McMurdo Ice Shelf Project (MIS) AND-1B marine sediment core. This new taxon from a diverse, well-preserved diatom assemblage is inferred to be the youngest member of the well-documented, biostratigraphically useful F. praeinterfrigidaria – F. interfrigidaria – F. weaveri lineage and may represent a near-shore corollary to the open-ocean species F. weaveri. Based on available chronostratigraphic data from AND-1B, F. tigris appears to be restricted to the earliest late Pliocene (first occurrence datum ~3.2 Ma) and is extinct before 3.0 Ma.

Limnological and Climatic Environments at Upper Klamath Lake, Oregon during the past 45 000 years

Upper Klamath Lake, in south-central Oregon, contains long sediment records with well-preserved diatoms and lithological variations that reflect climate-induced limnological changes. These sediment archives complement and extend high resolution terrestrial records along a north–south transect that includes areas influenced by the Aleutian Low and Subtropical High, which control both marine and continental climates in the western United States. The longest and oldest core collected in this study came from the southwest margin of the lake at Caledonia Marsh, and was dated by radiocarbon and tephrochronology to an age of about 45 ka. Paleolimnological interpretations of this core, based upon geochemical and diatom analyses, have been augmented by data from a short core collected from open water environments at nearby Howards Bay and from a 9-m core extending to 15 ka raised from the center of the northwestern part of Upper Klamath Lake. Pre- and full-glacial intervals of the Caledonia Marsh core are characterized and dominated by lithic detrital material. Planktic diatom taxa characteristic of cold-water habitats (Aulacoseira subarctica and A. islandica) alternate with warm-water planktic diatoms (A. ambigua) between 45 and 23 ka, documenting climate changes at millennial scales during oxygen isotope stage (OIS) 3. The full-glacial interval contains mostly cold-water planktic, benthic, and reworked Pliocene lacustrine diatoms (from the surrounding Yonna Formation) that document shallow water conditions in a cold, windy environment. After 15 ka, diatom productivity increased. Organic carbon and biogenic silica became significant sediment components and diatoms that live in the lake today, indicative of warm, eutrophic water, became prominent. Lake levels fell during the mid-Holocene and marsh environments extended over the core site. This interval is characterized by high levels of organic carbon from emergent aquatic vegetation (Scirpus) and by the Mazama ash (7.55 ka), generated by the eruption that created nearby Crater Lake. For a brief time the ash increased the salinity of Upper Klamath Lake. High concentrations of molybdenum, arsenic, and vanadium indicate that Caledonia Marsh was anoxic from about 7 to 5 ka. After the mid-Holocene, shallow, but open-water environments returned to the core site. The sediments became dominated (>80%) by biogenic silica. The open-water cores show analogous but less extreme limnological and climatic changes more typical of mid-lake environments. Millennial-scale lake and climate changes during OIS 3 at Upper Klamath Lake contrast with a similar record of variation at Owens Lake, about 750 km south. When Upper Klamath Lake experienced cold-climate episodes during OIS 3, Owens Lake had warm but wet episodes; the reverse occurred during warmer intervals at Upper Klamath Lake. Such climatic alternations apparently reflect the variable position and strength of the Aleutian Low during the mid-Wisconsin.

Palaeoclimatic significance of late Quaternary diatom assemblages from southern Windmill Islands, East Antarctica

The late Quaternary palaeoenvironmental history of the southern Windmill Islands, East Antarctica, has been reconstructed using diatom assemblages from two long, well-dated sediment cores taken in two marine bays. The diatom assemblage of the lowest sediment layers suggests a warm climate with mostly open water conditions during the late Pleistocene. During the following glacial, the Windmill Islands were covered by grounded ice preventing any in situ bioproductivity. Following deglaciation, a sapropel with a well-preserved diatom assemblage was deposited from ∼10 500 cal yr BP. Between ∼10 500 and ∼4000 cal yr BP, total organic carbon (C org) and total diatom valve concentrations as well as the diatom species composition suggest relatively cool summer temperatures. Hydrological conditions in coastal bays were characterised by combined winter sea-ice and open water conditions. This extensive period of glacial retreat was followed by the Holocene optimum (∼4000 to ∼1000 cal yr BP), which occurred later in the southern Windmill Islands than in most other Antarctic coastal regions. Diatom assemblages in this period suggest ice-free conditions and meltwater-stratified waters in the marine bays during summer, which is also reflected in high proportions of freshwater diatoms in the sediments. The diatom assemblage in the upper sediments of both cores indicates Neoglacial cooling from ∼1000 cal yr BP, which again led to seasonally persistent sea-ice on the bays. The Holocene optimum and cooling trends in the Windmill Islands did not occur contemporaneously with other Antarctic coastal regions, showing that the here presented record reflects partly local environmental conditions rather than global climatic trends.

Sublacustrine precipitation of hydrothermal silica in rift lakes: evidence from Lake Baringo, central Kenya Rift Valley

Abstract Many lakes in volcanic regions are fed by hot springs that, in some basins, can contribute a large percentage of the annual recharge, especially during times of aridity. It is important to recognize any contemporary hydrothermal contribution in paleoenvironmental reconstruction of lake basins because recharge from thermal waters can potentially confuse paleoclimatic signals preserved in the lacustrine sedimentary record. Hot spring deposits (travertine, sinter) provide the most tangible evidence for thermal recharge to lakes. Although subaerial spring deposits have been widely studied, lacustrine thermal spring deposits, especially sublacustrine siliceous sinters, remain poorly known. Detailed field, petrographic and scanning electron microscope (SEM) studies have been made of fossil sublacustrine sinter exposed at Soro hot springs along the northeastern shoreline of Ol Kokwe, a volcanic island in Lake Baringo, Kenya. Modern hot springs at Soro, which discharge Na–HCO3–Cl waters from a deep reservoir (∼180 °C ), have thin (1–10 mm), friable microbial silica crusts around their subaerial vents, but thicker (>1 cm) sinter deposits are not forming. The fossil sinter, which is present as intergranular cements and crusts in littoral conglomerates and sandstones, is composed mainly of opaline silica (opal-A). Three types of fossil sinter are recognized: (1) massive structureless silica, which fills intergranular pores and forms crusts up to 5 cm thick; (2) pore-lining silica, some of which is isopachous, and (3) laminated silica crusts, which formed mainly on the upper surfaces of detrital particles. All three types contain well-preserved diatoms including lacustrine planktonic forms. Microbial remains, mainly filamentous and coccoid bacteria (including cyanobacteria) and extracellular polymeric gels, are locally abundant in the opaline silica, together with detrital clays and thin laminae composed of authigenic chlorite (?). Most of the hydrothermal silica precipitated when the thermal springs were submerged by fresh lake water. Silica precipitated upon rapid cooling of thermal (∼90 °C ) waters at and just below their interface with the overlying cooler (∼25 °C ) lake waters. Microbial mats locally acted as a filter that limited mixing and rapid dilution of the thermal fluids. Some of the silica originally may have been soft and partly gelatinous. Planktonic diatoms and detrital clay rained down, then became incorporated in the amorphous silica. Following a fall in lake level, the opal-A lithified and partly altered to cristobalite (opal-C) and chalcedony. The lowest fossil sinters were later encrusted by calcite stromatolites, with calcite and quartz forming late pore-filling cements. The age of the sublacustrine sinters is unknown, but some of the deposits could date back to the late Pleistocene. Similar conglomerates cemented by hydrothermal silica are present along fault lines at neighbouring Lake Bogoria. Such rocks may provide evidence for deep, hot fluid recharge to lakes when encountered in the geological record.

The last occurrence of Proboscia curvirostris in the North Atlantic marine isotope stages 9-8

Well-preserved diatoms are present in high sedimentation rate Pleistocene cores retrieved on Ocean Drilling Program (ODP) Legs 151, 152, 162 and IMAGES cruises of R/V Marion Dufresne from the North Atlantic. Investigation of the stratigraphic occurrence of diatom species shows that the youngest diatom event observed in the area is the last occurrence (LO) of Proboscia curvirostris (Jousé) Jordan and Priddle. P. curvirostris is a robust species that can easily be identified in the sediments, and therefore can be a practical biostratigraphic tool. We have mapped its areal distribution, and found that it stretches from 40°N to 80°N in the North Atlantic. Further, we have correlated the LO P. curvirostris to the oxygen isotope records of six cores to refine the age of this biostratigraphic event. The extinction of P. curvirostris is latitudinally diachronous through Marine Isotope Stages (MIS) 9 to 8 within the North Atlantic. This is closely related to the paleoceanography of the area. P. curvirostris first disappeared within interglacial MIS 9 (324 ka) from the northern areas that are most sensitive to climatic forcing, like the East Greenland current and the sea–ice margin. It survived in mid-North Atlantic until the conditions of the MIS 8 (glaciation) became too severe (260 ka). In the North Pacific at ODP Site 883 the LO P. curvirostris falls within MIS 8. The observed overlap in age between the North Atlantic and the North Pacific strongly suggests that the extinction of P. curvirostris is synchronous between these oceans.

Upper Oligocene diatomaceous deposits in the northern North Sea - silica diagenesis and paleogeographic implications

Relatively well-preserved diatoms, often cemented together as silica aggregates and mainly composed of opal-A, are present in Upper Oligocene sediments at burial depths down to about 1500 m in the ...

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.

Lower Cretaceous Organic-Rich Sediments Drilled on Antarctic Continental Margin During ODP Leg 113: ABSTRACT

Lower Cretaceous organic-rich sediments were recovered below a major unconformity at two sites on the eastern Weddell Sea margin during ODP Leg 113. No age overlap exists between the two sites. A more continuous Cretaceous section was previously recovered at DSDP Site 511 on the Falkland plateau. Site 692 (2880 m water depth) is located on a mid-slope bench in Wegener Canyon. Early Cretaceous-age (pre-Albian) sediments extend from 53-98 m below sea floor (mbsf) and are dominated by organic-rich nannofossil claystone. Macrofossils (e.g., belemites and ammonites), thin carbonate lenses (<1 cm), and water escape structures are abundant. Thin beds of devitrified ash, bioturbation, and graded bedding are present. Site 693 (2360 m water depth), 30 km west of Site 692 on the canyon's outer rim, recovered Albian-age organic-rich claystones and mudstones from 416-483 mbsf. Site 693 sediments have lower organic contents than those at Site 692. Glauconite is common in the upper part of the unit. Well-preserved diatoms and diatomite layers suggest high productivity. These sediments differ from Albian-age sediments at Site 511, which consist of open-marine nannofossil ooze. The sediments were deposited in an anoxic or hypoxic upper bathyal (500-1000 mbsl) marine environment. Oxygenation occurred later along the Antarcticmore » margin than the Falkland plateau, possibly as the inflow of Atlantic water proceeded south from an opening near the Falkland plateau.« less

Origin of Diatom Ooze Belt in the Southern Ocean: Implications for Late Quaternary Paleoceanography

Diatom preservation patterns in surface sediments of the Pacific sector of the Southern Ocean were examined. A belt of well-preserved diatoms is bounded on the north and south by sediments containing poorly preserved diatoms. The northern boundary correlates closely with the Subantarctic Front and appears to result from reduced phytoplankton productivity caused by temperature effects on metabolic processes. The southern boundary is caused by the damping effect that late winter/early spring sea-ice has on diatom productivity in the water column. Our evidence suggests that the southern boundary represents the spring sea-ice position rather than summer position. When these data are applied to late Quaternary sections in the Pacific sector it appears that there was only a modest northward expansion of winter sea-ice cover during glacial maxima.

Biostratigraphy and Paleoecology of Naples Bluff Coastal Section Determined from Diatoms and Benthic Foraminifers: ABSTRACT

An integrated biostratigraphic and paleoecologic study of the Naples Bluff coastal section used diatoms and benthic foraminifers. This section, composed of a thick sequence of Tertiary marine rocks, represents one of the most completely exposed Neogene sections along the central coast of California. The interval examined in this study is 1800 ft thick, and represents early to late Miocene rocks and the overlying early Pliocene sediments. This sequence contains well-preserved diatom floras and benthic foraminiferal faunas that enabled us to integrate diatom biochronology with the provincial benthic foraminifer stages and to construct a sediment accumulation curve. Although generally abundant in this section, the benthic faunas are dominated by an inner marginal bathyal assemblage. However, rare deeper water assemblages occur in some samples. Five faunal events are recognized that indicate response to paleo-oceanographic and tectonic influences. The diatom assemblages in this section are relatively abundant and range from the Actinocyclus ingens zone to the Thalassiosira oestrupii zone of J.A. Barron. The diversity and abundance of these assemblages made precise age control for this section possible. A major unconformity or severely compressed interval developed between 15 and 9 Ma.

Geology and Diagenesis of Belridge Diatomite and Brown Shale, San Joaquin Valley, California: ABSTRACT

End_Page 988------------------------------The late Miocene to Pliocene argillaceous and siliceous shales of the upper Monterey Formation (the Brown Shale and overlying Belridge Diatomite) represent laterally continuous deposits of nearshore, diatom-rich muds and open marine, argillaceous, diatom-bearing muds on a developing shelf and growing anticline which flanked the San Joaquin basin. These units are laterally equivalent to submarine fan and turbidite sedimentary rocks deposited into the basin from the west, south, and east. The lithology of the Brown Shale and Belridge Diatomite is controlled by original composition and subsequent burial diagenesis. The muds underwent diagenetic alteration with the development of silica phases in the sequence: (1) unaltered diatom frustules (biogenic amorphous silica or opal-A); (2) diagenetic amorphous silica (opal-A); and (3) diagenetic crystalline silica (opal-CT). In the oil field, the Belridge Diatomite grades downward from a massive or faintly laminated siliceous mudstone with diatom frustules and opal-A^prime, to a laminated and massive siliceous mudstone with opal-A^prime and some well-preserved diatoms. The Brown Shale grades from a massive or laminated siliceous mudstone with opal-A^prime and some diatoms to a well-laminated argillaceous, diatom-bearing mudston with opal-A^prime and some opal-CT. There is no distinct break in lithology between the Belridge Diatomite and Brown Shale reservoirs, but recrystallization and detrital clay increase downward across the boundary. Porosity in the best diatomite is about 62%, and permeability is about 1 md. End_of_Article - Last_Page 989------------

Non-marine diatoms from late Wisconsin perched deltas in Taylor Valley, Antarctica

Late Wisconsin perched deltas in Taylor Valley, Antarctica, and environs contain numerous, well-preserved diatoms characteristic of antarctic inland waters. Twenty-seven non-marine species and fragments of the marine species were encountered. Of the non-marine species, six ( Achnanthes taylorensis, Navicula airdevronsixii, Navicula deltaica Navicula papilio, Navicula quaternaria, and Nitzschia westii) are new. In various combinations, the 37 species form six floral assemblages. One assemblage is restricted to deltas in nearby Ferrar Valley, three are related to delta age or elevation within Taylor Valley, and one contains poorly preserved fragments of marine species. The significance of the sixth assemblage is not clear because it occurs in deltas of widely varying age, elevation, and geographic location. Many of the diatoms found in the deltas occur today in lakes that are characterized by highly variable ionic concentration and composition (from nearly fresh to super-saline). These saline lakes are not synonymous with a marine environment. For this reason, the species indicate non-marine rather than fresh-water or marine paleoenvironments. Glacial Lake Washburn, in which the Taylor Valley deltas were deposited, was non-marine. Occasional fragments of marine diatoms are attributed to reworking of older Ross Sea drift. The ice dam that blocked Lake Washburn in Taylor Valley between about 21 200 B.P. and about 8340 B.P. was apparently grounded, consistent with the contention that a former grounded ice sheet filled McMurdo Sound and the Ross Sea. The non-marine nature of the Taylor Valley deltas refutes the hypothesis (Miagkov et al., 1976) that sea level in the Ross Sea region was about 308 m higher than it is today at 17 000–21 200 B.P. We suggest that similar deltas in nearby Salmon Valley were deposited in small non-marine lakes dammed by ice-cored moraines deposited by the last Ross Sea glaciation. This interpretation is in contrast to the hypothesis (Miagkov et al., 1976) that the Salmon Valley deltas, which contain many of the same non-marine diatom species found in Taylor Valley deltas, formed in sea water along the ice-sheet edge during a general lowering of sea level caused by tectonism.

Neogene coastal upwelling history off northwest and southwest Africa

In Neogene sediments from DSDP Legs 14, 40 and 41 indications of the influence of coastal upwelling on sediment composition have been found in the Upper Miocene (SW Africa) and in the Lower and Middle Miocene (NW Africa). Parameters pointing to upwelling influence include well-preserved diatom and radiolarian assemblages, low values for the ratio of planktic to benthic foraminifers, increased amounts of fish debris and the presence of phosphorite grains. First indications of strong coastal upwelling at Site 369A, today situated at the northern boundary of the trade-wind belt, are probably associated with the Early Miocene location of that site within the trade-wind belt.