Diatom Biostratigraphy Research Articles

Stratigraphic framework of the late Miocene Pisco Formation at Cerro Los Quesos (Ica Desert, Peru)

The enormous concentration of marine vertebrates documented within the Pisco Formation is unique for Peru and South America and places this unit among the prime fossil Lagerstätten for Miocene to Pliocene marine mammals worldwide. In order to provide a robust stratigraphic framework for the fossil-bearing locality of Cerro Los Quesos, this study presents a 1:10,000 scale geological map covering an area of about 21 km2, a detailed measured section spanning 290 m of strata, and a refined chronostratigraphy for the studied succession well constrained by diatom biostratigraphy and high-resolution 40Ar/39Ar isotopic dating of three interbedded ash layers. Within the apparently monotonous, diatomite-dominated sedimentary section, the Pisco Formation has been subdivided into six local members, with stratigraphic control over the different outcrops facilitated by the establishment of a detailed marker bed stratigraphy based on 15 readily distinguishable sediment layers of different nature.

Pliocene and Pleistocene paleoceanography in the western subarctic Pacific based on diatom analyses of ODP Leg 145 Hole 884B and IODP Expedition 323 Holes U1341B and U1343E

Late Pliocene–Pleistocene fossil diatom assemblages from Ocean Drilling Program (ODP) Leg 145 Hole 884B in the western subarctic Pacific were investigated to improve the diatom biostratigraphy. Moreover, the paleoceanographic records at Hole 884B were compared with those in Integrated Ocean Drilling Program (IODP) Expedition 323 Holes U1341B and U1343E in the Bering Sea for the time interval of 2.5–0Ma in order to clarify the differences in the diatom assemblage fluctuations and the history of past ocean circulations around the subarctic Pacific. In Hole 884B, five diatom zones from the Neodenticula koizumii–Neodenticula kamtschatica Zone to the Neodenticula seminae Zone were identified. The decreases in the relative abundance of temperate-water species were recognized at ca. 1.9Ma in Hole U1343E, ca. 2.1Ma in Holes 884B and U1341B. The decreases of the temperate-water species at these sites indicate that a widespread cooling event must have occurred over the subarctic Pacific at ~2Ma. In addition, the continuous low occurrences of the sea-ice related species from around 2Ma onward, which indicate the presence of sea-ice, were observed at Sites 884 and U1341. We also point out that the age differences in decrease of temperate-water species at the three sites (i.e., delay in response of the temperate-water species at Site U1343) may be related to changes of seawater inflow from the North Pacific to Sites U1341 and U1343 and of the East Kamchatka Current from the Bering Sea to Site 884.

Stratigraphic framework of the late Miocene to Pliocene Pisco Formation at Cerro Colorado (Ica Desert, Peru)

This paper describes a ∼200 m-thick section of the Pisco Formation exposed at Cerro Colorado, an important fossiliferous site in the Ica desert. In order to properly place the fauna in its correct relative position, this study establishes the stratigraphic framework within which the different fossil-bearing intervals of this site can be compared and may prove invaluable in future high-resolution studies on the faunal change. Most of the Pisco Formation deposits exposed at Cerro Colorado consist of gently dipping fine-grained sandstones, diatomaceous siltstones and diatomites with minor ash layers and dolomites deposited within nearshore and offshore settings. To facilitate detailed stratigraphic correlations within the Pisco strata for a 30 km2 area, eight marker beds have been defined and large-scale (1:10,000 scale) geological mapping conducted to determine fault positions, styles and offsets. The geological map shows that there are two important angular unconformities in the study area. The first one is the interformational basal unconformity of the Pisco Formation against folded, faulted, and planated Oligo-Miocene rocks of the Chilcatay Formation. The second is a low-angle intraformational erosional discontinuity of up to 4° angular discordance that allows the subdivision of the Pisco stratigraphy exposed in the study area into two informal allomembers. Dating of the exposed succession by diatom biostratigraphy suggests that the age of the lower allomember is late Miocene, whereas the upper allomember is late Miocene or younger.

Diatom records in the Quaternary marine sequences around the Japanese Islands

Understanding the Quaternary is a key to estimating what the Earth's climate will be like in the future. Such studies demand high-resolution analyses based on paleoclimatic proxy records of changing Earth's orbital forcing and solar insolation that affect the climate system. Quaternary diatom biostratigraphy and paleoceanography have been well established based on the Quaternary marine sequences obtained by piston coring and deep-sea drilling around the Japanese Islands. This paper firstly reviews the Quaternary diatom datum levels that are directly tied to magnetic polarity, and then the late Pleistocene and Holocene rhythmic fluctuations in Td'-derived SSTs (°C) which correlate with the Earth's orbital parameters, and finally reveals the large and abrupt climatic changes that have occurred around the Japanese Islands on centennial to millennial time scales.The main aim is to provide the results of Td'-SSTs (°C) based on the late Pliocene to Pleistocene sequences from three holes obtained by deep-sea drilling (DSDP-ODP). The main difference between Td' and Twt is that Twt gives 0.5 values to Xt (warm transitional taxa). Thalassiosira oestrupii is grouped with Xt in Twt but with warm-water species in Td'. Differences between the Twt and Td'-SSTs (°C) curves at Hole 436 are unremarkable. The remarkable variations of paleo-temperatures based on Td'-SSTs (°C) show four conspicuous episodes which correspond to the double precession cycle of MIS 11. In the Japan Sea, the Twt ratio remarkably decreases at 2.6 Ma due to a large increase in cold-water taxa, and indicates the beginning of the glacial age defined as the Pliocene/Pleistocene boundary. Both Twt and Td'-SSTs (°C) increase at 2.60–2.0 Ma, and coincide with the lithologic change from diatom-bearing clay with few dark-colored layers to fine-grained sediments with distinct dark–light colored cycles. Wavelet analysis of Td'-SSTs (C) at Site 798 indicates a reversed saw-tooth pattern of 48 to 24-ky periods during 1.2–0.7 Ma, and 24 to 12-ky periods during 0.7–0 Ma, resulting in a change from longer to shorter cycles. These fluctuations correlate with the Earth's orbital parameters and climatic changes on a millennial time scale.

日本海最上部第四系コアの珪藻化石層序および珪藻起源の黒色粒子とその古海洋学的意義

日本海最上部第四系コアの珪藻化石層序および珪藻起源の黒色粒子とその古海洋学的意義

Revised Neogene diatom biostratigraphy ODP Site 887, the Gulf of Alaska in Northeast Pacific

Revised Neogene diatom biostratigraphy ODP Site 887, the Gulf of Alaska in Northeast Pacific

Diatom, silicoflagellate and ebridian biostratigraphy and paleoceanography in IODP 323 Hole U1343E at the Bering slope site

Integrated Ocean Drilling Program (IODP) Expedition 323 was dedicated to reconstruct the details of the Pliocene–Pleistocene paleoenvironmental conditions of the Bering Sea. In this study, fossil diatom assemblages from Hole U1343E were investigated to define diatom biostratigraphy and reveal paleoceanography of the Bering slope region throughout Pleistocene. As the results, four diatom zones with Neogene North Pacific Diatom zone codes (NPD) were determined from the Neodenticula seminae Zone (NPD 12) to the N. koizumii Zone (NPD 9). Diatom biohorizons defined in this study closely correspond with onboard data, but the distinct difference is recognized at the top of the N. koizumii. Additional four biostratigraphic zones estimated by three silicoflagellate and one ebridian datums suggest the core sediment age younger than 2.5Ma. The fluctuations of several paleoenvironmental diatom indicators show that cold conditions with sea-ice existed throughout the duration of the interval studied as well as sea ice expansion occurred at ca. 1.9Ma and ca. 0.9Ma. Moreover, increases of neritic diatom species reveal a drop in sea level. Neodenticula seminae is the diatom tracer of the Aleutian Current and hence the decrease of this taxon suggests decrease in inflow of the North Pacific water mass. Although the sea levels must have been dropped during the cold time interval, the consistent occurrences of N. seminae indicate that the surface water circulation in the Bering Sea was significantly influenced by the Alaskan Stream at ca. 1.9Ma. On the other hand, during the other cooling event at ca. 0.9Ma, which corresponds to the mid-Pleistocene transition (MPT) event, an increase of neritic diatom species and decrease of N. seminae were observed. This suggests that coastal environmental conditions accompanied the sea level drop and that the inflow from the North Pacific was weakened. The increases of sea-ice and neritic diatom species suggest that the sea ice and coastal conditions were developed after the cooling at 0.9Ma. The ages at remarkable changes of silicoflagellate assemblages correspond to the ages of global climate shifts at ca. 1.4Ma, 1.25Ma, and 0.8–0.9Ma. The unusual alternate dominances of Distephanus medianoctisol in glacial and Dictyocha species in interglacial periods during the 1.25–0.8Ma interval suggest the north–south migration of water masses between the Bering Sea and the Subarctic North Pacific.

Refinement of late-Early and Middle Miocene diatom biostratigraphy for the east coast of the United States

Integrated Ocean Drilling Program (IODP) Expedition 313 continuously cored Lower to Middle Miocene sequences at three continental shelf sites off New Jersey, USA. The most seaward of these, Site M29, contains a well-preserved Early and Middle Miocene succession of planktonic diatoms that have been independently correlated with the geomagnetic polarity time scale derived in studies from the equatorial and North Pacific. Shallow water diatoms (species of Delphineis , Rhaphoneis , and Sceptroneis ) dominate in onshore sequences in Maryland and Virginia, forming the basis for the East Coast Diatom Zones (ECDZ). Integrated study of both planktonic and shallow water diatoms in Hole M29A as well as in onshore sequences in Maryland (the Baltimore Gas and Electric Company well) and Delaware (the Ocean Drilling Program Bethany Beach corehole) allows the refinement of ECDZ zones into a high-resolution biochronology that can be successfully applied in both onshore and offshore regions of the East Coast of the United States. Strontium isotope stratigraphy supports the diatom biochronology, although for much of the Middle Miocene it suggests ages that are on average 0.4 m.y. older. The ECDZ zonal definitions are updated to include evolutionary events of Delphineis species, and regional occurrences of important planktonic diatom marker taxa are included. Updated taxonomy, reference to published figures, and photographic images are provided that will aid in the application of this diatom biostratigraphy.

Miocene diatom biostratigraphy of Gharandal Group, west-central Sinai, Egypt

Detailed diatom analysis has been carried out on one hundred forty-two samples collected from the Lower to Middle Miocene stratigraphic succession located in the Abu Qada basin, west-central Sinai. The succession is composed only of two lithostratigraphic units belonging to Gharandal Group; they are Rudies Formation followed directly by Kareem Formation. The diatom analysis shows interesting diverse and abundant different diatom assemblages of both marine and non-marine origin.Atotal of 84 diatom species and varieties belonging to 32 genera were identified. The preservation of the recorded taxa varies from poorly preserved in some samples to generally moderately to well preserve in other samples. Several intervals of pronounced diatom dissolution detected throughout the studied section. A biostratigraphic zonal framework consisting of three planktonic diatom zones is proposed and the Early /Middle Miocene boundary is determined accurately by means of diatom taxa.

Neogene tectonic and climatic evolution of the Western Ross Sea, Antarctica — Chronology of events from the AND-1B drill hole

Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 1284.87m sedimentary succession from beneath the sea floor. Key age data for the core include magnetic polarity stratigraphy for the entire succession, diatom biostratigraphy for the upper 600m and 40Ar/39Ar ages for in-situ volcanic deposits as well as reworked volcanic clasts. A vertical seismic profile for the drill hole allows correlation between the drill hole and a regional seismic network and inference of age constraint by correlation with well‐dated regional volcanic events through direct recognition of interlayered volcanic deposits as well as by inference from flexural loading of pre‐existing strata. The combined age model implies relatively rapid (1m/2–5ky) accumulation of sediment punctuated by hiatuses, which account for approximately 50% of the record. Three of the longer hiatuses coincide with basin‐wide seismic reflectors and, along with two thick volcanic intervals, they subdivide the succession into seven chronostratigraphic intervals with characteristic facies:1.The base of the cored succession (1275–1220mbsf) comprises middle Miocene volcaniclastic sandstone dated at approx 13.5Ma by several reworked volcanic clasts;2.A late-Miocene sub-polar orbitally controlled glacial–interglacial succession (1220–760mbsf) bounded by two unconformities correlated with basin‐wide reflectors associated with early development of the terror rift;3.A late Miocene volcanigenic succession (760–596mbsf) terminating with a ~1my hiatus at 596.35mbsf which spans the Miocene–Pliocene boundary and is not recognised in regional seismic data;4.An early Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (590–440mbsf), separated from;5.A late Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (440–150mbsf) by a 750ky unconformity interpreted to represent a major sequence boundary at other locations;6.An early Pleistocene interbedded volcanic, diamictite and diatomite succession (150–80mbsf), and;7.A late Pleistocene glacigene succession (80–0mbsf) comprising diamictite dominated sedimentary cycles deposited in a polar environment.

The 2011 recipient of the Brady Medal: Dr John A. Barron

![][1] Dr John Barron is an internationally renowned expert in marine diatom taxonomy, biostratigraphy, palaeoceanography and palaeoclimatology. He graduated from the University of California, Los Angeles (UCLA) in 1969 with a BS Geology degree and completed his doctoral research in Geology at UCLA, as a student of Helen N. Tappan, in 1974. John took up a position as a research geologist/micropalaeontologist for the United States Geological Survey (USGS) in Menlo Park, California in 1974 where he remains today as a Supervisory Research Geologist within the Volcano Science Center. Notably, he is the Survey’s internationally recognized expert on the climate of the past 15 000 years in the North Pacific area and on the climatic and glacial history of Antarctica during the past 65 million years. In 1984, John was elected a Fellow of the Geological Society of America and, in 1986, he was presented the Charles Schuchert Award by the Paleontological Society for excellence and promise in palaeontology. In 1994 he was awarded a US Department of Interior Meritorious Service Award by the USGS and, in 2005, was elected a Fellow of the California Academy of Sciences. John is the leading world authority on Cenozoic marine diatom biostratigraphy and evolution, having established and revised many biostratigraphic zonations during his 40+ year career, and described numerous new diatom species, such as Actinocyclus fryxellae , Thalassiosira gersondei and T. mahoodii (Baldauf & Barron, 1991). He has repeatedly demonstrated the value of marine diatoms for solving a range of important geological problems for the entire diatom fossil record (Cretaceous to Holocene), particularly in areas such as the Southern Ocean where other microfossil groups are rare. A creative thinker, from the time of his first publication on Neogene ocean palaeotemperatures (Barron, 1973) John was, in many respects, a leader in the field … [1]: /embed/graphic-1.gif

Oligocene Age of the Classic Belén Fruit and Seed Assemblage of North Coastal Peru based on Diatom Biostratigraphy

The Belén flora, in north coastal Peru, is the most diverse fruit and seed assemblage yet known from the Paleogene of South America. Little original paleobotanical work has been performed on this assemblage since the pioneering treatments published by E. W. Berry in the 1920s, and the precise age has not been determined. Nevertheless, the flora has been regarded as a focal point in understanding the vegetational, orogenic, and climatic history of northern South America, and in recent literature it has been assumed to be early Eocene. In order to tighten this age assignment, which has varied from early Eocene to early Oligocene in the opinions of different authors, we revisited the Belén site, measured the stratigraphic section, and processed the fruit- and seed-containing sediment for age-diagnostic microfossils. Although pollen and foraminifera were not recovered, the sediment is rich in diatoms. The diatom assemblage includes Lisitzinia ornata and Rocella vigilans, among others, indicating a latest early Oligocene age (∼30–28.5 Ma) for these deeper marine sediments, which we infer to have been subsequently reworked into the Belén environment. We also reevaluate the botanical identifications, which are based on the original museum specimens supplemented by more recently collected specimens. The Belén flora provides a window into extinct forests in South America that were present before the rising of the Andes in western Peru.

Pre-Neogene non-marine diatom biochronology with new data from the late Eocene Florissant Formation of Colorado, USA

Studies of the non-marine diatom assemblages from Eocene deposits in North America extend the fossil record of a number of extant freshwater diatom lineages. These and other accounts show that the earliest documented appearances of non-marine diatoms consisted of 7 genera from late Cretaceous deposits, followed by 36 more from the Eocene, and 4 additional genera from Oligocene to early Miocene localities. A total of 47 diatom genera, 45 of which are extant, therefore, are known from pre-Neogene (>23.8 Ma) non-marine deposits. The chronological extension of these first-occurrences of genera has implications for terrestrial diatom biostratigraphy, paleoclimate, and diatom phylogenetic studies. An overview of the pre-Neogene non-marine diatom biochronology from published records and original work provides the geologic and geographic context for the more current findings. Two geologic range charts synthesize the data from the pre-Neogene non-marine (fresh and brackish water) diatom floras from the 9 published localities, supplemented by additional findings from a recent floristics study of the Florissant site in Colorado. These charts allow easy comparison of the timing, floral composition, and taxon-richness at the genus level and higher for the 9 sites. Among the pre-Neogene sites discussed here are early and middle Eocene deposits in Canada and middle Eocene deposits in Idaho andWyoming that provide evidence of an early pulse in the post-Cretaceous radiation of freshwater diatoms. The survey of the fossil diatoms from lake sediments of the Florissant Formation in Colorado showed that this deposit contains the most diverse (genus-rich) early freshwater diatom flora on record. This late Eocene flora suggests a later pulse in freshwater diatom radiation during pre-Neogene time that contributed to the rich taxonomic diversity of today. The Florissant fossil diatom investigations yield a total of 33 freshwater diatom genera. This number exceeds by 20 genera any other single known non-marine diatom-bearing Eocene and older deposit. The Florissant diatom flora consists of taxa with decidedly modern affinities, has superior diversity (taxon-richness), and yields the first-reported occurrence in the geologic record of 14 freshwater genera. From the 9 featured pre-Neogene sites, genera of centric, araphid, biraphid, and monoraphid morphologic groups are represented. While the findings of these studies are not inconsistent with the previously predicted timing of the first appearance of the major morphological diatom groups, the actual known fossil record post-dates the presumed timing of such morphological differentiation. Although the first occurrences in these deposits must be treated as apparent first occurrences due to the innate incompleteness of the fossil record, such findings confirm that species of many of the modern non-marine diatom genera had evolved prior to the end of the Eocene. It is anticipated that this synthesis of the pre-Neogene non-marine diatom floras will provide necessary paleontological evidence for interpreting the true geologic ranges of several diatom lineages. This work could facilitate future application of diatoms in biostratigraphic studies of fossil lake beds in hydrologic and climate change studies and in hydrocarbon exploration. Perhaps such data will generate insights to diatom phylogenetics that may lead to a greater understanding of their evolutionary histories. Further research could potentially relate changes in diatom forms to global events that might include climate shifts.

Tetracyclus radius (Bacillariophyta) a new fossil species from Miocene freshwater sediments in the Japan Sea

A new fossil diatom species, Tetracyclus radius sp. nov, is described from freshwater sediments under the Yamato Bank in the Japan Sea. Its valve outline is circular with radiating costae and has a single rimoportula. There is no distinct sternum but a hyaline central area is present. The freshwater sediments are deposited before the Early Pliocene by diatom biostratigraphy of the overlying marine sediments.

Diatom biostratigraphy of the lower Miocene Yunagaya Group, Joban area, Japan

Diatom biostratigraphy of the lower Miocene Yunagaya Group, Joban area, Japan

Diatom biostratigraphy of the middle Miocene Oidawara Formation in the Mizunami area, Gifu Prefecture, central Japan

Diatom biostratigraphy of the middle Miocene Oidawara Formation in the Mizunami area, Gifu Prefecture, central Japan

Taxonomic composition, paleoecology and biostratigraphy of Late Cretaceous diatoms from Devon Island, Nunavut, Canadian High Arctic

Upper Cretaceous sediments of the Kanguk Formation exposed in Eidsbotn and Viks Fiord grabens on Devon Island, Nunavut, Canadian High Arctic, yielded 91 fossil marine diatom species and varieties (including indeterminate taxa), representing 41 genera. Excellent preservation of the assemblages was aided by shallow burial, protection in downfaulted linear grabens, and the presence of abundant volcanic material. Planktonic species and resting spores comprise nearly 70% of the diatom assemblage, and provided abundant food resources for the Late Cretaceous Arctic ecosystem. Deposition of the approximately 225 m-thick stratigraphic sequence was predominantly in a shallow marine neritic setting, with an upward progression to interbedded terrestrial deposits of the Expedition Fiord Formation, reflecting a regression and eventual persistence of terrestrial facies into the Early Cenozoic. The Kanguk Formation is widespread across the Canadian Arctic, and diatom biostratigraphy indicates a Santonian–Campanian age for the sequences reported herein, based on the presence of Gladius antiquus in the lowermost strata and occurrence of Costopyxis antiqua throughout the succession. However, Amblypyrgus sp. A and Archepyrgus sp. aff. A. melosiroides, encountered in the lower part of the succession, are known exclusively from the Lower Cretaceous. This may suggest a slightly older age. New information on shallow shelf diatom assemblages from this study is compared to reports on two other Late Cretacous Arctic diatom assemblages. These three sites represent an environmental transect from shallow to distal shelf settings and into the oceanic realm.

Pliocene–Pleistocene diatom biostratigraphy of nearshore Antarctica from the AND-1B drillcore, McMurdo Sound

The near-shore open-marine diatom record recovered in the ANtarctic geological DRILLing (ANDRILL) McMurdo Ice Shelf Project (MIS) AND-1B drillcore, McMurdo Sound, Antarctica, advances our understanding of the marine conditions present in the southern Ross Sea during the Pliocene and early Pleistocene. This diatom history is recorded within alternating diamictite and diatomite that reflect alternating glacial activity and high marine primary productivity. The diatomite units were deposited in a continental shelf open-marine setting during periods of reduced ice cover in West Antarctica. A new diatom biostratigraphic scheme spanning the last ca. 5Ma is proposed for the Antarctic near-shore area, based on prior work from high latitude drillcores. Four new zones are proposed for the Pliocene/Pleistocene, with eight in total for the new zonal scheme, utilizing Actinocylus fasciculatus, Actinocyclus maccollumii, Fragilariopsis bohatyii, Rouxia antarctica, and Thalassiosira fasciculata as new zonal markers. The early Pliocene shares the most assemblage commonality with that of the Southern Ocean with greater numbers of endemic species observed in the late Pliocene and early Pleistocene; a group of related Fragilaripsis species characterizes much of this later part of the time column. Two new species are proposed, Fragilariopsis tigris sp. nov. Riesselman and Thalassiosira teres sp. nov. Winter; a formal name is also proposed for another species, Rhizosolenia harwoodii sp. nov. Winter. The new zonation is tied to a robust chronology utilizing diatom biostratigraphy, volcanic 40Ar/39Ar ages and magnetostratigraphy.

Diatom-based correlation of early to mid-Pliocene drillcores from the southwestern Ross Sea, Antarctica

The ANDRILL AND-1B drillcore in southern Victoria Land Basin, Antarctica recovered an upper Miocene to early Pleistocene continental shelf stratigraphic section, including a spectacular Pliocene and Pleistocene sequence of alternating diamictite and diatom-rich sediments. This punctuated sequence reflects variation between glacial sediments deposited by ice advance over the site and open-marine diatom productivity and sedimentation. An early Pliocene age for the base of diatom-rich sediment is constrained through integrated diatom biostratigraphy and magnetostratigraphy. The presence of marine diatom Shionodiscus tetraoestrupii from the top of the core to 583.64mbsf indicates the lowest diatom-bearing sediments are younger than 5Ma. This drillcore provides important new chronostratigraphic control for paleoenvironmental changes also recorded in three drillcores from the western coast of the McMurdo Sound area, DVDP-10 and -11 in Taylor Valley and CIROS-2 from the seaward edge of Ferrar Fjord. The refined continental shelf biostratigraphy developed from AND-1B provides a framework for regional correlation to understand better the timing and character of large paleoenvironmental changes in the western Ross Sea that involve the history of the West Antarctic Ice Sheet (WAIS). Multivariate analysis, along with traditional biostratigraphic approaches, enables the correlation and comparison of coeval intervals in these drillcores. A composite stratigraphic sequence from these four cores suggests the early and mid-Pliocene Ross Sea experienced extended intervals of open-marine conditions with minimal sea-ice cover and high diatom production and sedimentation. This new information provides important constraints on Antarctic paleoclimate and ice sheet history during an important interval when global climate was warmer than today. The history preserved in these four drillcores will be an important tool to guide and test future ice sheet and climate models.

Revised Diatom Biostratigraphy of DSDP Leg 19 Drill Cores and Dredged Samples from the Subarctic Pacific and Bering Sea

Revising the Miocene through Pleistocene diatom biostratigraphy on the drill cores recovered in DSDP Leg 19 results in the dating of Neogene sequences especially for Sites 183 and 192 in the far North Pacific Ocean. Ages for the basal part of the lithologic unit A (opal-A), diatom ooze to diatomaceous mud, overlying the unit B (opal-CT), mudstone with no diatom, are older than 10 Ma at 210 mbsf in Site 183, about 9.5 Ma at 718 mbsf in Site 192, and about 4.6 Ma at 582 mbsf in Site 188, at 370 mbsf in Site 187, at 587 mbsf in Site 185 and at 599 mbsf in Site 184, and about 3.2 Ma at 521 mbsf in Site 191, at 618 mbsf in Site 190, at 541 mbsf in Site 189 and at 926 mbsf in Site 186. The diagenetic boundary between opal-A and opal-CT distinctively forms a diachronic plane in the subarctic Pacific. Fossil diatoms in the dredged samples from the Bering Sea indicate stratigraphically major increasing times of diatom mass accumulation rates in the North Pacific: 10-9 Ma, 7-6 Ma, and 3-2 Ma.