Itsuki Suto

The Cenozoic palaeoenvironment of the Arctic Ocean

The history of the Arctic Ocean during the Cenozoic era (0–65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm ‘greenhouse’ world, during the late Palaeocene and early Eocene epochs, to a colder ‘icehouse’ world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1–2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.

A siliceous microfossil view of middle Eocene Arctic paleoenvironments: A window of biosilica production and preservation

Integrated Ocean Drilling Program (IODP) Expedition 302, “The Arctic Coring Expedition” (ACEX), unearthed the most significant find of Paleogene siliceous microfossils in nearly 2 decades. 100 m of early middle Eocene, organic-rich, finely laminated sediments contain abundant marine and freshwater siliceous microfossils allowing intriguing insights into central Arctic paleoenvironments during the start of Cenozoic cooling. Largely endemic assemblages of marine diatoms and ebridians are preserved along with very high abundances of chrysophyte cysts, the endogenously formed resting stage of freshwater algae. An overall brackish environment is invoked, but variations in group dominance suggest episodic changes in salinity, stratification, and trophic status. With the backing of inorganic geochemistry we synthesize the sediment characteristics by hypothesizing an environmental model for the cooccurrence of these diverse siliceous microfossil groups. We also report on initial insights into the composition of some of the laminations, which may help explain the formation of this rich sediment archive.

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska.

Significance In coastal Alaska and the St. Elias orogen, over the past 1.2 million years, mass flux leaving the mountains due to glacial erosion exceeds the plate tectonic input. This finding underscores the power of climate in driving erosion rates, potential feedback mechanisms linking climate, erosion, and tectonics, and the complex nature of climate−tectonic coupling in transient responses toward longer-term dynamic equilibration of landscapes with ever-changing environments. Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8–1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2–0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50–80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

TAXONOMY OF THE MARINE DIATOM RESTING SPORE GENERA DICLADIA EHRENBERG, MONOCLADIA GEN. NOV. AND SYNDENDRIUM EHRENBERG AND THEIR STRATIGRAPHIC SIGNIFICANCE IN MIOCENE STRATA

The morphology and taxonomy of the fossil diatom resting spore genera Dicladia Ehrenberg, Monocladia Suto gen. nov. and Syndendrium Ehrenberg in samples of Neogene age from DSDP Hole 438 from the northwest Pacific Ocean were examined. Dicladia is characterized by an epivalve with two conical elevations, and includes three species: D. capreola Ehrenberg, D. mitra Bailey and D. japonica Pantocsek. The new genus Monocladia is established here to receive two new species: M. humilis Suto and M. alta Suto. Monocladia differs from Dicladia by having only one elevation and a single hyaline, branching process on the epivalve. Syndendrium includes two species: S. diadema Ehrenberg and the new species S. akibae Suto. This genus is characterised by two or more hyaline, branching processes that arise from the central portion of the epivalve. Monocladia humilis occurs sporadically in the interval from the lower Denticulopsis lauta Zone (NPD 4A) through the upper Thalassiosira yabei Zone (NPD 5C). Monocladia alta and S. akibae are both characterised by a short-ranging occurrence from the lower Denticulopsis praedimorpha Zone (NPD 5B) through the upper Denticulopsis dimorpha Zone (NPD 5D). Monocladia humilis, M. alta and S. akibae are potentially useful biostrati-graphic markers to improve resolution of Neogene diatom biostratigraphy of the North Pacific Ocean.

Fossil marine diatom resting spore morpho-genus Xanthiopyxis Ehrenberg in the North Pacific and Norwegian Sea

Abstract Fossil marine diatom resting spore species in the morpho-genus Xanthiopyxis Ehrenberg are described using samples from DSDP Site 338 in the Norwegian Sea, Sites 436 and 438 in the northwest Pacific and from the onland section at Newport Beach, California. Xanthiopyxis is characterized by numerous knobs, spines and bristles covering the entire valve face. In this paper eleven species, of which seven are new species, are described and their stratigraphic ranges are presented: X. polaris Gran, X. norwegica Suto, sp. nov., X. brevispinosa Suto, sp. nov., X. teneropunctata Suto, sp. nov., X. lanceolatus Suto, sp. nov., X. circulatus Suto, sp. nov., X. reticulata Suto, sp. nov., X. obesa Suto, sp. nov., X. hirsuta Hanna and Grant, X. oblonga Ehrenberg and X. globosa Ehrenberg. In addition, resting spores which lack sufficient characteristics to identify easily are assigned to three informal species: Xanthiopyxis type A (knobbly type), X. type B (short spiny type) and X. type C (long spiny type).

Fossil marine diatom resting spore morpho-genus Gemellodiscus gen. nov. in the North Pacific and Norwegian Sea

Abstract A new fossil marine diatom resting spore morpho-genus Gemellodiscus Suto gen. nov. is described using samples from DSDP Site 338 in the Norwegian Sea, Sites 436 and 438 in the northwest Pacific and the onland Newport Beach Section, California. Gemellodiscus is characterized by possessing a valve with setae of several types: bifurcated seta, fused seta and crossed seta. Eleven taxa are described and their stratigraphic ranges are presented: G. incurvus (Bailey) Suto comb. nov., G. pliocenus (Brun) Suto comb. nov., G. cingulus Suto var. cingulus sp. nov., G. cingulus var. longus Suto var. nov., G. bifurcus Suto sp. nov., G. hirtus Suto sp. nov., G. caveatus Suto sp. nov., G. micronodosus Suto sp. nov., G. dicollinus Suto sp. nov., G. geminus Suto sp. nov. and G. dimontanus Suto sp. nov.

TAXONOMY AND BIOSTRATIGRAPHY OF THE FOSSIL MARINE DIATOM RESTING SPORE GENERA DICLADIA EHRENBERG, MONOCLADIA SUTO AND SYNDENDRIUM EHRENBERG IN THE NORTH PACIFIC AND NORWEGIAN SEA

The taxonomy and biostratigraphy of the fossil marine diatom resting spore genera Dicladia Ehrenberg, Monocladia Suto and Syndendrium Ehrenberg are discussed. The three genera are probably the fossil resting spores of Chaetoceros and Chaetoceros-related genera, and are characterized by dichotomous branching processes on their valves. They were studied by examining samples from DSDP Sites 436, 438 (northwest Pacific) and 338 (Norwegian Sea) and the Newport Beach Section (California). The genus Dicladia is characterized by having an epivalve with two conical elevations, and includes three species: D. capreola Ehrenberg, D. mitra Bailey and D. japonica Pantocsek. The genus occurs in the Neogene. The genus Monocladia possesses four species including two new species: M. humilis Suto, M alta Suto, M. perizoma sp. nov., and M. norvegica sp. nov. The occurrence of this genus is restricted to the Miocene. Monocladia differs from Dicladia by having only one elevation and a single hyaline, branching process on the epivalve. The genus Syndendrium holds seven species including five new species: S. akibae Suto, S. diadema Ehrenberg, S. medusae sp. nov., S. scarabaeum sp. nov., S. rugosum sp. nov., S. altantemna sp. nov., and S. humiliantemna sp. nov. This genus occurs from the early Oligecene to the Recent. This genus is characterized by two or more hyaline, branching processes that arise from the central portion of the epivalve. Seven new species M. perizoma, M. norvegica, S. medusae, S. scarabaeum, S. rugosum, S. altantemna and S. humiliantenma are described in this paper. Some species have short-ranging occurrences and specific characteristics allowing for easy identification, and they may be biostratigraphically useful.

Vallodiscus gen. nov., a new fossil resting spore morpho‐genus related to the marine diatom genus Chaetoceros (Bacillariophyceae)

A new fossil marine diatom resting spore morphogenus, Vallodiscus Suto gen. nov., is described using samples from Deep Sea Drilling Project Site 338 in the Norwegian Sea, Sites 436 and 438 in the north‐west Pacific Ocean and the onland Newport Beach Section, California. Vallodiscus is characterized by a single ring of veins along the epivalve margin and a hypovalve covered with circular depressions of several sizes with gentle elevation. The morpho‐genus bears three new species and one new combination: Vallodiscus simplexus Suto sp. nov., Vallodiscus complexus Suto sp. nov., Vallodiscus lanceolatus Suto sp. nov. and Vallodiscus chinchae (Mereschkowsky) Suto comb. nov.

TAXONOMY OF THE FOSSIL MARINE DIATOM RESTING SPORE GENUS GONIOTHECIUM EHRENBERG AND ITS ALLIED SPECIES

Taxonomic notes are provided on the genus Goniothecium and its allied groups from Arctic core materials, in addition to a synonymy list, microscope observations and several key references for each species. Moreover, the type material of Goniothecium from the Ehrenberg Collection (no. 1651a and 1651b) in the Museum für Naturkunde, Humboldt Universität, Berlin, was obtained in order to examine the structure of G. rogersii. The extinct genus is characterized by a subrectangular frustule and forming paired frustules connected by two epivalves, which possess interlocking linking spines. The genus presently includes 3 species: G. rogersii Ehrenberg, G. danicum Grunow and G. decoratum Bran. A fourth species, Goniothecium loricatum Fenner is here transferred to Hemiaulus loricatus (Fenner in Schrader et Fenner) Suto comb, nov., based on the valve similarity of genus Hemiaulus. Goniothecium has a long stratigraphic history from the Cretaceous to the Pliocene. Goniothecium rogersii and G. decoratum appeared in the Southern Hemisphere in the late Cretaceous and in the middle Eocene, respectively. From the Oligocene, they spread to the Northern Hemisphere. On the other hand, G. danicum appeared in the North Atlantic and/or Arctic Ocean and spread to the Southern Hemisphere in the Oligocene. The spread (or migration) of Goniothecium species from North to South and vice versa might have been influenced by changes in the thermohaline circulation and ocean current structure across the Eocene/Oligocene boundary. Since diatoms live in the euphotic zone, transportation of living cells over long distances most likely occurs by surface (or intermediate) water currents, however, more resistant resting spores may be laterally advected by bottom water currents.

Taxonomy of the fossil marine diatom resting spore morpho-genera Xanthioisthmus Suto gen. nov. and Quadrocistella Suto gen. nov. in the North Pacific and Norwegian Sea

The morphology and taxonomy of the fossil diatom resting spore morpho-genera Xanthioisthmus Suto gen. nov. and Quadrocistella Suto gen. nov. are described. The two new genera are probably fossil resting spores of the marine diatom genus Chaetoceros. They were studied by examining samples from DSDP Sites 436, 438 (northwest Pacific) and 338 (Norwegian Sea), and the Newport Beach Section (California). The genus Xanthioisthmus is characterized by an elongate valve composed of two flat circles joined together by a hyaline broad isthmus and includes five species: X. biscoctiformis (Forti) Suto comb. nov., X. specticularis (Hanna) Suto comb. nov., X. panduraeformis (Pantocsek) Suto comb. nov., X. praemaculata sp. nov. and X. maculata (Hanna) Suto comb. nov. The genus Quadrocistella differs from Xanthioisthmus by its elongate and rectangular valve and bears five new species: Q. rectagonuma sp. nov., Q. tubera sp. nov., Q. paliesa sp. nov., Q. montana sp. nov. and Q. palmesa sp. nov.