Pliocene Diatom Research Articles

Upper Pliocene Diatom Complexes and its Significance for Establishing the Lower Boundary of Quarter (South of the Far East)

Upper Pliocene Diatom Complexes and its Significance for Establishing the Lower Boundary of Quarter (South of the Far East)

Occurrence of two Krsticiella species in Beringia suggests this genus is a Cenozoic relict

The genus Krsticiella Levkov was previously known only from the basins of Lakes Ohrid and Prespa, and from Lake Baikal. Examination of type materials of diatoms originally described by Niels Foged from the North Slope of Alaska revealed that Navicula lenis Foged and N. meadeensis Foged belong to Krsticiella, to which they are formally transferred here. One of these species, K. lenis, is also found in thermokarst tundra lakes of the Kolyma Lowland in Northeastern Siberia. Several illustrations in previously published floras of Pliocene and Pleistocene diatoms of Eurasia indicate wider past distributions of Krsticiella species. The current restriction of this genus to non-glaciated regions of the Arctic and large ancient lakes in temperate zone indicates that it is a Cenozoic relict.

Windblown Pliocene diatoms and East Antarctic Ice Sheet retreat

Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise.

Pliocene diatom and sponge spicule oxygen isotope ratios from the Bering Sea: isotopic offsets and future directions

Abstract. Oxygen isotope analyses of different size fractions of Pliocene diatoms (δ18Odiatom) from the Bering Sea show no evidence of an isotope offset and support the use of bulk diatom species samples for palaeoceanographic reconstructions. Additional samples containing concentrations of sponge spicules produce δ18O values several per mille (‰) lower than δ18Odiatom with a calculated mean offset of 3.9‰ ± 1.5. This difference is significantly greater than modern-day variations in water δ18O through the regional water column. Despite the potential for oxygen isotope disequilibrium within δ18Osponge, there appears to be some similarity between δ18Osponge and a global stacked benthic δ18Oforam record. This highlights the potential for δ18Osponge in palaeoenvironmental research at sites where carbonates are not readily preserved.

Resolving views on Antarctic Neogene glacial history – the Sirius debate

ABSTRACTThe discovery of marine Pliocene diatoms in warm-based glacial deposits (now termed the Sirius Group) high in the Transantarctic Mountains in the 1980s began a three-decade-long controversy over the stability of the East Antarctic Ice Sheet. Their presence implied that this ice sheet had collapsed as recently as three million years ago to allow their deposition in shallow interior seas, followed by transport and deposition from an expanded over-riding ice sheet. Though the glacial deposits included clasts with older diatoms, no evidence of clasts with Pliocene diatoms was published, but the hypothesis gained wide acceptance. Increasing knowledge of ice sheet behaviour and the antiquity and stability of the Transantarctic Mountains, along with new techniques for dating age and denudation rates for landscapes, has led to a more likely alternative hypothesis – that the high-level Sirius Group deposits pre-date Transantarctic Mountains uplift and their Pliocene diatoms are atmospheric contaminants. Surveys have shown that marine diatoms from the Antarctic margin and the Southern Ocean are indeed reaching the surface of the ice sheet and blowing through the mountains, with permafrost processes providing opportunities for contamination. Modelling and geological evidence is now consistent with a stable East Antarctic Ice Sheet in the interior for the last 14 Ma, with some retreat around the margins and periodic collapse of the West Antarctic ice sheet in Pliocene times.

Pliocene diatoms from freshwater diatomite in Yamaga Town, Kyushu, Japan

Pliocene diatoms from freshwater diatomite in Yamaga Town, Kyushu, Japan

Pliocaenicus tanimurae, a new Pliocene diatom species from Aguni Island, Okinawa Prefecture, southwestern Japan

Pliocaenicus tanimurae sp. nov., from Early Pliocene sediment of the Fudenzaki Formation, Aguni Island, Okinawa Prefecture, southwestern Japan is described here. The new species is round to elliptical with a slightly tangential undulated valve face. Valve face of the elliptical valves are more elliptical than the valves themselves. Several valve face fultoportulae in the valve center form a circle. Costae alternate thicker (sometimes slightly recessed) and thinner with mantle fultoportulae located on slightly recessed thicker costae. Both valve face and mantle fultoportulae have three satellite pores. Alveoli have centrifugal ‘roofing over’. One rimoportula is located near the edge of the marginal area of the valve face at the base of a costa. The new species is shown in light microscopy and scanning electron microscopy photographs and compared with similar taxa of P. costatus (Loginova, Lupikina & Khursevich) Flower, Ozornina & A.I.Kuzmina, P. seczkinae Stachura-Suchoples, Genkal & Khursevich and P. omarensis (Kuptsova) Stachura-Suchoples & Khursevich.

Early to mid-Pliocene environmentally constrained diatom assemblages from the AND-1B drillcore, McMurdo Sound, Antarctica

In extended lower to mid-Pliocene stratigraphic record of near-shore environmental variation in the southwestern Ross Sea, Antarctica is preserved in ANDRILL’s McMurdo Ice Shelf Project AND-1B drillcore.A94meter-thick sequence of three diatomite and diatomaceous mudstone intervals represents the highest latitude record (78° S) of persistent open-marine conditions on the Antarctic continental shelf, in response to a protracted period of Pliocene warmth. This core provides new chronostratigraphic control for Antarctic paleoclimatic events, and describes a sequence of distinct diatom assemblages comprised of both extant and extinct species that are unknown on the Antarctic shelf today. Common taxa include those with modern distributions close to the Polar Frontal Zone (e.g. Shionodiscus tetraoestrupii, Stellarima stellaris, Thalassionema nitzschioides), suggesting marine temperatures in the southwestern Ross Sea warmer than at present. Representatives of the modern sea-ice assemblage (e.g. Fragilariopsis curta, Stellarima microtrias) and cold open-marine species associated with the retreating sea-ice front (e.g. Eucampia antarctica var. recta) are also present, but in much lower abundance than in the Ross Sea today as suggested by core top and surface sample data. The co-occurrence of species from these three distinct modern environments in Pliocene sediments, in close proximity to the continent, suggests that modern ecological zonal concepts cannot be applied directly in Pliocene paleoenvironmental analyses. Species present in the 4 m-thick lowest diatomite unit (~4.5 Ma) indicate a cold openmarine environment,with the period of deposition likely comprising the length of one interglacial period. The middle diatomite unit (~4.1-4.3 Ma; 14m) records warmer and perhaps seasonally mixed conditions with often abundant Thalassionema nitzschioides and Shionodiscus tetraoestrupii and little evidence of sea-ice associated species. Documentation of changes in diatom assemblages through the upper 75m-thick diatomite interval (~3.3-3.6Ma) offers the opportunity to resolve environmental limits of Pliocene diatom species thatwill serve as a guide to diatom-based paleoenvironmental interpretations for other Pliocene deposits. This upper unit records deposition during several glacial/interglacial cycles indicating an extended interval with open marine productivity during the mid-Pliocene. In addition, environmental associations for extinct species are inferred by their co-occurrence with extant taxa. Persistent intervals of pure diatomite suggest high productivity in an open-marine setting and minimal to no influence of sea ice. Commonly occurring species can be placed in five ecological cateories; Neritic/Stratified, Warm, Cold/Sea Ice, Heavy/Wind Mixed and Mixed. Comparison of this long diatom-bearing interval with coeval Antarctic deposits presents a regional and continental picture of Pliocene warmth, suggesting East Antarctic Ice Sheet was smaller with sub-polar marginal conditions.

PLANKTONIC MARINE DIATOM RECORD OF THE PAST 18 M.Y.: APPEARANCES AND EXTINCTIONS IN THE PACIFIC AND SOUTHERN OCEANS

Recently published diatom biochronologies provide accurate (to 0.1 m.y.) determination of the ages of appearances and disappearances of planktonic diatoms during the past 18 m.y. in the equatorial Pacific, North Pacific, and Southern Ocean. Comparisons of these records reveal the age of evolutionary appearance and extinction of species and their region of origin. Extinct planktonic diatom species have a mean longevity of 3.4 ± 2.8 m.y. (SD, n = 53) in the equatorial Pacific, 2.5 ± 2.1 m.y. (n = 52) in the North Pacific, and 2.9 ± 2.3 m.y. (n = 38) in the Southern Ocean. The relatively large standard deviations are likely due to the inclusion of taxa that probably could be subdivided into two or more species. In the equatorial Pacific, evolutionary turnover of diatom species was relatively high between 18.0 and 6.0 Ma compared with the period after 6.0 Ma, presumably reflecting changing oceanic circulation and evolving water masses. In the North Pacific, evolutionary turnover speaked between 10.0 and 4.5 Ma, with increasing high-latitude cooling and enhanced provincialism. In the Southern Ocean, evolutionary turnover of endemic diatoms was greatest between 5.0 and 1.6 Ma, which provides evidence for the strong provincial character of Pliocene diatom assemblages. Taken as a whole, oceanic diatom assemblages became increasingly provincial in character during the late Miocene and Pliocene, as pole-to-equator thermal gradients increased and oceanic frontal systems were strengthened.

The early Matuyama Diatom Maximum off SW Africa: a conceptual model

An important discovery during Ocean Drilling Program Leg 175, when investigating the record of upwelling off Namibia, was the finding of a distinct Late Pliocene diatom maximum spanning the lower half of the Matuyama reversed polarity chron (MDM, Matuyama Diatom Maximum) and centered around 2.6–2.0 Ma. This maximum was observed at all sites off southwestern Africa between 20°S and 30°S, and is most strongly represented in sediments of Site 1084, off Lüderitz, Namibia. The MDM is characterized by high biogenic opal content, high numbers of diatom valves, and a diatom flora rich in Southern Ocean representatives (with Thalassiothrix antarctica forming diatom mats) as well as coastal upwelling components. Before MDM time, diatoms are rare until ca. 3.6 Ma. After the MDM, in the Pleistocene, the composition of the diatom flora points to increased importance of coastal upwelling toward the present, but is accompanied by a general decrease in opal and diatom deposition. Here we present a simple conceptual model as a first step in formalizing a possible forcing mechanism responsible for the record of opal deposition in the upwelling system off Namibia. The model takes into account Southern Ocean oceanography, and a link with deepwater circulation and deepwater nutrient chemistry which, in turn, are coupled to the evolution of North Atlantic Deep Water (NADW). The model proposes that between the MDM and the Mid-Pleistocene climate revolution, opal deposition off Namibia is not directly tied to glacial–interglacial fluctuations (as seen in the global δ 18O record), but that, instead, a strong deepwater link exists with increased NADW production (as seen in the deepwater δ 13C record) accounting for higher supply of silicate to the thermocline waters that feed the upwelling process. The opal record of Site 1084 shows affinity to eccentricity on the 400-kyr scale but not for the 100-kyr scale. This points toward long-term geologic processes for delivery of silica to the ocean.

A late Pliocene diatom Ge/Si record from the Southeast Atlantic

As a result of both culture and sediment core studies, the ratio of germanium (Ge) to silicon (Si) in diatom shells has been proposed as a proxy for monitoring whole-ocean changes in seawater Ge/Si, a ratio affected by changes in continental weathering. However, because of the difficulties of extracting and cleaning diatom frustules from deep-sea sediments, only samples from highly pure diatom oozes in the Antarctic region have been previously analyzed. Here we present data on diatom Ge/Si ratios, (Ge/Si) opal, for the time interval between 3.1 and 1.9 Ma from a mid-latitude, coastal upwelling area where significant terrigenous sediment input complicated the sample processing and analyses. In general, our (Ge/Si) opal values show the same decreasing trend after 2.6 Ma than previously measured in Antarctic sediments (Shemesh et al., 1989. Paleoceanography 4, 221–231), but with a noisier background that may reflect the local imprint of proximal continental input superimposed upon global changes in the ocean reservoir. The time of initiation of large-scale North Hemisphere glaciation at ∼2.6 Ma is characterized by a declining pattern of diatom Ge/Si ratios, which could have resulted from a global increase in the input of riverine Si due to enhanced silica weathering and/or equatorward (northward) intrusions of subantarctic waters enriched in silica. High (Ge/Si) opal ratios are associated with high opal contents from the same sediment samples and with warm climate as indicated by depleted benthic foraminiferal δ 18O values from the North and Equatorial Atlantic. Cold periods signified by enriched benthic δ 18O values, on the contrary, are associated with lower (Ge/Si) opal ratios. We interpret diatom Ge/Si values to reflect the prevailing weathering state on the continents, with greater chemical weathering during warm and wet periods of the Pliocene and less during cooler and drier intervals.

Late Quaternary sediment facies in Prydz Bay, East Antarctica and their relationship to glacial advance onto the continental shelf

A marine survey in Prydz Bay, provides an unparalleled view of glacigenic and marine sedimentation across Prydz Channel and Amery Depression during the Late Quaternary. Gravity cores and a suite of eight radiocarbon dates indicate that the Late Wisconsin Glacial Maximum (LGM) was associated with grounding of a palaeo-ice shelf along the periphery of Prydz Channel. Deposition in front of the grounding line was dominated by ice-rafting. A granulated facies, containing angular clay and diamicton clasts, was producd by a combination of regelation freezing, near to the grounding line, and remelting of this basal debris in the sub-ice shelf setting. Beneath these LGM marine deposits lie two key beds of diatom ooze that are distinct in size sorting and Pliocene diatoms. These “interstadial” units can be traced across most of the Prydz Channel, and are underlain by additional glacial marine units. Debris related to the Lambert Deep is distinct from detritus from eastern Prydz Bay and deposition of these two sources within the channel oscillated during the LGM. We suggest that coastal drainage systems contributed to a limited glaciation of the shelf during the LGM, rather than direct outflow via the Lambert/Amery system. It is proposed that shelf-wide glaciation is related to the duration of glacial sea level lowstands rather than the absolute magnitude of eustatic fall during such episodes.

Late Neogene changes in diatom sedimentation in the North Pacific

During the late Neogene, North Pacific diatom sedimentation underwent major changes in response to high-latitude cooling and changes in surface and deep water circulation. At 9 Ma diatom mass accumulation rates (MARs) increased in the NW Pacific and off northeast Japan, possibly due to shoaling of the Isthmus of Panama, which lead to an enrichment of nutrients in North Pacific deep waters. During the latest Miocene, diatom MARs increased progressively off southem California (6.5 Ma), at high latitudes of the North Pacific (6.2 Ma), and off northeastern Japan (5.5 Ma), presumably in response to high latitude cooling. At about 4.5 Ma diatom sedimentation abruptly increased in the NW Pacific but declined off Japan and California, coincident with the onset of a prolonged period of high-latitude warmth. Enhanced upwelling of nutrient-rich deep waters in the NW Pacific probably stimulated diatom production there. A major step in high latitude cooling at 2.7 Ma caused a reversal of these mid Pliocene diatom sedimentation patterns. Upwelling of deep, nutrient-rich waters waned at higher latitudes, leading to a decline in diatom productivity; while wind-driven, coastal upwelling increased off southern California and stimulated diatom growth.

Late Pliocene Diatoms in a Diatomite from Prydz Bay, East Antarctica

Very well-preserved Pliocene diatoms from a diatomite unit interbedded within glacial sediments at Ocean Drilling Program Site 742 in Prydz Bay, Antarctica are documented and illustrated. The presence of Thalassiosira kolbei, T torokina, Actinocyclus actinochilus, A. karstenii and the absence of Nitzschia interfrigidaria. T insigna and T vulnifica in Sample 119-742A-15R-4, 44-46cm constrain its age to ca. 2.2-1.8 Ma (late Pliocene). Diatoms associated with sea ice constitute 35% of the Pliocene diatom assemblage, compared with 71% of the modem sediment assemblage at the site, suggesting that sea ice was present during the late Pliocene period of deposition of the sample, although it probably was not the significant feature it is today. Thalassiosira ellitipora (Donahue) Fenner is described and illustrated in detail and is validly published. An expanded description and numerous illustrations are also presented for T. torokina Brady. INTRODUCTION During Ocean Drilling Program Leg 119 on the continental shelf in Prydz Bay, Antarctica, a 60-cm-thick unit of distinctive greenish gray to gray diatomite (lithologic Subunit IIIB) was recovered within a thick (300+m) sequence of coarse-grained glacial diamictite in Core 15R of Hole 742A at 67?32.98'S, 75?24.27'E and a water depth of 426.2m (text-fig. 1) (Shipboard Scientific Party 1989). The abundant Pliocene diatoms found in this diatomite were apparently deposited during a period of enhanced biologic productivity in a distal glaciomarine setting, possibly during an interglacial period warmer than today (Shipboard Scientific Party 1989). The presence of well-preserved Pliocene diatoms within otherwise glacial sediments that typify the Antarctic shelf is relatively rare and offers an opportunity to study a nearshore environment of East Antarctica during an interval of the late Pliocene. A study of the diatoms of this Pliocene diatomite may provide evidence that bears on the hypothesis of Webb et al. (1984) and Webb and Harwood (1991) that calls for large scale deglaciation of East Antarctica during parts of the early and late Pliocene. In addition, the well-preserved nature of the diatoms in this deposit allows their study under the scanning electron microscope(SEM) and may contribute to solving some of the numerous problems of taxonomic confusion that currently plagues the study of Pliocene Antarctic diatoms (Harwood and Maruyama 1992). MATERIALS AND METHODS Sample 119-742A-15R-4, 44-46cm was collected from the 60cm-thick diatomite unit. The sample was processed for light microscopy (LM) and SEM by the Van der Werff (1955) hydrogen peroxide method. Strew slides were mounted in Hyrax for LM with individual species ringed for identification and photographic reference. Examination of taxa was made using a Leitz Ortholux II and an Olympus IMT at magnifications of x500 and x1250. SEM examination and photography was made using a Cambridge 250 mark II SEM at the U.S. Geological Survey (Menlo Park) and a Hitachi S520 at the California Academy of Sciences. Diatom terminology follows that of Ross et al. (1979). Sample material (CAS number 613278, ODP 119-742A-15R-4, 44-46cm) and slides (CAS numbers 217016-217018, 217022, 217026, 217027, 217029) are deposited at the California Academy of Sciences. TEXT-FIGURE 1 Location of ODP Site 742 in Prydz Bay, East Antarctica, and other sites cores by ODP Leg 119. Contours in meters. micropaleontology, vol. 42, no. 3, pp. 285-302, text-figure 1, plates 1-8, table 1, 1996 285 This content downloaded from 157.55.39.138 on Tue, 19 Apr 2016 06:51:08 UTC All use subject to http://about.jstor.org/terms Albert D. Mahood and John A. Barron: Late Pliocene Diatoms in a Diatomite from Prydz Bay, East Antarctica TABLE 1 Tabulation of diatom taxa in Pliocene Sample ODP-119-742A-15R-4, 44-46cm and their relative percentage compared with the relative percentage of taxa recorded in surface sediment at the site by Stockwell et al. (1991). * = extinct; ( ) = possible decendent.

Biostratigraphy and palaeoecology of early Pliocene diatom assemblages from the Larsemann Hills, eastern Antarctica

Considerable controversy surrounds the biostratigraphic and palaeoclimatic interpretation of Antarctic marine Pliocene sequences (Webb & Harwood 1991). The object of this note is to document and reassess a marine Pliocene diatom assemblage from the Larsemann Hills and to discuss its significance with respect to other Antarctic Neogene deposits.

Late-glacial to holocene changes in winds, upwelling, and seasonal production of the northern California current system

A core 120 km off the coast of southern Oregon was examined for changes in lithology, diatoms, and pollen over the past 30,000 yr. Primary production during the late Pleistocene was about half that of the Holocene. Evidence from diatoms and pollen indicates that summer upwelling was much weaker, implying an absence of strong northerly winds. Early Pliocene diatoms found throughout the late Pleistocene section were probably derived from diatomites east of the Cascades and provide evidence for strong easterly winds over a dry continental interior. The findings verify predictions of a climate model based on glacial maximum conditions. There is no compelling evidence for a climatic reversal corresponding to the European Younger Dryas chron. During the early Holocene (9000−7000 yr B.P.) there may have been years when winds were insufficiently strong to support upwelling, so that warm stratified waters lay closer to the coast.

Reproducibility in the Random Settling Method for Quantitative Diatom Analysis

Seven lower Pliocene sediment samples from Ocean Drilling Program (ODP) Hole 642B on the outer V0ring Plateau were analyzed in order to test the reproducibility of Moore's random settling method in quantitative diatom analysis. The results indicate that the absolute abundance of diatom valves per gram dry sediment are obtained within an error of ?5% at the 95% confidence level after a count of 400 valves. The results also indicate that relative abundance percentages for abundant species (approximately 20% of the total assemblage) are determined with a maximum error of ?3.5%, whereas relative abundance percentages for less abundant species are determined with a maximum error of ?1.5%. Species having relative abundances of approximately two percent or less may be overlooked. This study demonstrates that absolute abundance data are required in order to understand the cause of change in the relative abundance. INTRODUCTION The usefulness of quantitative methods in diatom biostratigraphy and paleoceanography has been demonstrated by, for example, Burckle and Trainer (1979). They were using relative abundances to define Pliocene diatom datums not only by first and last appearances, but also by recurrences and by changes in dominance of two closely related species (see also Baldauf 1985; Barron 1985). Counts of total diatom abundance changes in marine sediments provide valuable paleoceanographic information. For example, Stabell (1986) analyzed absolute abundances of diatoms in Quaternary sediments for recognition of high productivity events that were caused by movements of the oceanic polar front (see also Pokras and Ruddiman 1989; Stabell 1989). Among described methods that are used to calculate diatom abundances per weight or volume of unit sediment are the evaporation technique (Batterbee 1973), the pipetting technique (Schrader and Gersonde 1978), and the use of marker grains (Kaland and Stabell 1981). The random settling method described by Moore (1973) was developed for studies of radiolarian abundances. Laws (1983) tested the accuracy of the method by using a known mixture of microspheres, and showed that this method produced accurate strewn mounts within the limits of error relative to the original sample. In contrast, pipetted mounts were biased toward larger particles (i.e. larger particles were obtained at frequencies higher than the occurrence within the original sample). Moore's method can also be adopted to estimate absolute abundances of diatoms. The major purpose of this study is to test empirically the reproducibility of Moore's method for use in quantitative diatom analysis, using both sample splitting and settling containers. Another purpose is to evaluate how absolute abundance changes are reflected in a relative abundance analysis. In order to achieve these goals, both absolute and relative abundances were analyzed in seven samples, and counts were performed on multiple slides from each sample. This allows comparison between absolute and relative abundance changes, and the reliability of the obtained values can be evaluated.

Diatoms in Antarctic ice cores: Some implications for the glacial history of Antarctica

The presence of reworked Pliocene and older marine microfossils in glacial tills of the Sirius Group suggests that these sediments were emplaced during the Pliocene or later by glaciers riding up the eastern (East Antarctic) slope of the Transantarctic Mountains. This interpretation implies that there was considerable fluctuation in the size of the East Antarctic ice sheet as recently as ca. 3 Ma. Here we test an alternate scenario (eolian emplacement of the Pliocene microfossils) by comparing diatoms in the Sirius Group with diatoms in Holocene and last glacial maximum ice from the Dome C core from the central Antarctic Plateau. Since the diatoms in Dome C ice are known to be eolian in origin, we hypothesize that if the Sirius diatoms are also eolian, then the two Antarctic sites (Dome C and the Sirius Group), equally remote from the Southern Ocean, will have the same exotic flora (i.e., windblown diatoms from either a fresh-water or marine source). However, whereas the Dome C diatoms are predominantly fresh water, those of the Sirius are predominantly marine. We conclude from these data that the Sirius and Dome C diatoms each reflect a different mode of emplacement and that the diatoms in the Sirius till were not emplaced via wind transport. This conclusion does not prove that the Pliocene diatoms in the Sirius till record a deglaciation and Pliocene reglaciation of East Antarctica but, rather, it removes an alternate mechanism (eolian transport) raised by those who oppose this scenario.

Diatom Stratigraphy of the Lower Pliocene Part of the Sisquoc Formation, Harris Grade Section, California

Lower Pliocene diatoms were studied from the Sisquoc Formation and lowermost Foxen Mudstone, exposed along Harris Grade north of Lompoc, California, to refine the diatom biostratigraphy of post-Monterey Formation sediments in California. Sixty-seven diatom taxa were identified in the 25 samples examined from the 790-m thick (2950-ft) section. The diatoms are assignable to the uppermost Nitzschia reinholdii Zone and Thalassiosira oestrupii Zone of Barron (1981), and five tentative subzones for local correlation are proposed. Regional correlations and taxon occurrence are discussed, and the base of the Nitzschia reinholdii Zone is redefined as at the last occurrence of Thalassionema schraderi.

Middle and Late Pliocene Diatom Datum Levels from the Central Pacific

Sixteen diatom datum levels from the Middle and Late Pliocene of the central Pacific are ranked and related to the paleomagnetic reversal record and the oxygen isotope record. The datum levels and their positions relative to the paleomagnetic record are as follows: (1) first appearance of Thalassiosira convexa in the upper part of the Gilbert; (2) disappearance of T. convexa in the lower part of the Gauss; (3) recurrence of T. convexa in the Mammoth Event of the Gauss; (4) first appearance of Rhizosolenia praebergonii in the normal event between the Mammoth and Kaena Events; (5) last appearance of Nitzschia jouseae in the upper part of the Gauss; (6) first appearance of R. praebergonii var. robusta in the lower part of the Matuyama; (7) recurrence of Coscinodiscus nodulifer var. cyclopus in the lower part of the Matuyama; (8 and 9) last appearance of T. convexa and T. convexa var. aspinosa in the lower part of the Matuyama; (10) last appearance of C. nodulifer var. cyclopus; (11 ) change in dominance between R. praebergonii and R. praebergonii var. robusta below the Olduvai Event; (1 2) recurrence of C. nodulifer var. cyclopus just below the Olduvai Event; (13) final occurrence of C. nodulifer var. cyclopus just below the Olduvai Event; (14) final occurrence of R. praebergonii in the lower part of the Olduvai; (15) first occurrence of Pseudoeunotia doliolus in the mid-part of the Olduvai Event; (16) final occurrence of R. praebergonii var. robusta just above the Olduvai Event. When these data are related to the oxygen isotope record it becomes apparent that climatic fluctuations are an important arbiter of diatom first and last appearances. A new variety, Rhizosolenia praebergonii var. robusta, is named. Middle and Late Pliocene diatom datum levels from the central Pacific