Ryuji Tada

Land-ocean linkages over orbital and millennial timescales recorded in Late Quaternary sediments of the Japan Sea

Late Quaternary sediments of the Japan Sea are characterized by centimeter- to meter-scale alternations of dark and light layers which are synchronous basinwide. High-resolution analyses of the sediments from Ocean Drilling Program site 797 reveal that deposition of the meter-scale alternations reflect variations in paleoceanographic conditions which were closely associated with glacio-eustatic sea level changes through the modulation of the volume and character of the influx to the sea through the Tsushima Strait. The centimeter- to decimeter-scale alternations reflect millennial-scale variations which are possibly associated with Dansgaard-Oeschger (D-O) cycles, with each dark layer appearing to correspond to an interstadial. This variability is attributed to the development of a humid climate in central to eastern Asia and the consequent increase in discharge from the Huanghe and Changjiang Rivers during interstadials. This caused expansion of the East China Sea coastal water (ECSCW), which penetrated more strongly into the Japan Sea. The increased influence of the lower-salinity, nutrient-enriched ECSCW reduced deep water ventilation and enhanced the surface productivity, leading to the development of anoxic bottom waters and deposition of the dark layers. Thus the centimeter- to decimeter-scale alternations of the dark and light layers record wet and dry cycles in central to eastern Asia possibly associated with D-O cycles.

A complete terrestrial radiocarbon record for 11.2 to 52.8 kyr B.P

Dating Carbon Radiocarbon dating is the best way to determine the age of samples that contain carbon and that are younger than ∼50,000 years, the limit of precision for the method. There are several factors that complicate such age determinations, however, some of the most important of which include variability of the 14C production in the atmosphere (which affects organic samples whose radiocarbon inventories are derived from atmospheric CO2), surface ocean reservoir effects (which affect marine samples that acquire their radiocarbon signatures from seawater), and variable dead carbon fraction effects (which affect speleothems that derive their carbon from groundwaters). Bronk Ramsey et al. (p. 370; see the Perspective by Reimer) avoid the need to make such assumptions, reporting the 14C results of sediments from Lake Suigetsu, Japan. Analysis of terrestrial plant macrofossils in annually layered datable sediments yielded a direct record of atmospheric radiocarbon for the entire measurable interval up to 52.8 thousand years ago. Radiocarbon measurements of samples from Lake Suigetsu, Japan, extend the 14C time scale back to more than 50,000 years ago. Radiocarbon (14C) provides a way to date material that contains carbon with an age up to ~50,000 years and is also an important tracer of the global carbon cycle. However, the lack of a comprehensive record reflecting atmospheric 14C prior to 12.5 thousand years before the present (kyr B.P.) has limited the application of radiocarbon dating of samples from the Last Glacial period. Here, we report 14C results from Lake Suigetsu, Japan (35°35′N, 135°53′E), which provide a comprehensive record of terrestrial radiocarbon to the present limit of the 14C method. The time scale we present in this work allows direct comparison of Lake Suigetsu paleoclimatic data with other terrestrial climatic records and gives information on the connection between global atmospheric and regional marine radiocarbon levels.

Tracing the provenance of fine-grained dust deposited on the central Chinese Loess Plateau

Eolian dust deposits in north China provide an excellent means of determining past variations in continental paleoclimate and atmospheric circulation. However, debate still exists on which deserts in east Asia are the dominant sources of Chinese loess and whether the dust provenance has shifted significantly at different time scales. Here we present new constraints on the provenance of fine-grained dust deposited on the central Chinese Loess Plateau (CLP) by combining electron spin resonance signal intensity and crystallinity index of fine-grained quartz contained in samples from two loess-paleosol sequences. Our results show that the fine-grained dust deposits on the CLP originate mainly from the Gobi desert in southern Mongolia and the sandy deserts in northern China (primarily the Badain Juran and Tengger deserts), rather than from the Taklimakan desert in western China, at least during the last climatic cycle. The dominant source of fine-grained dust varied significantly, from southern Mongolia during cold periods, to northern China during warm periods. The glacial-interglacial provenance fluctuations are strongly coupled with changes in the intensity of the near-surface northwesterly winter monsoon.

Pre-Miocene birth of the Yangtze River

The development of fluvial systems in East Asia is closely linked to the evolving topography following India–Eurasia collision. Despite this, the age of the Yangtze River system has been strongly debated, with estimates ranging from 40 to 45 Ma, to a more recent initiation around 2 Ma. Here, we present 40Ar/39Ar ages from basalts interbedded with fluvial sediments from the lower reaches of the Yangtze together with detrital zircon U–Pb ages from sand grains within these sediments. We show that a river containing sediments indistinguishable from the modern river was established before ∼23 Ma. We argue that the connection through the Three Gorges must postdate 36.5 Ma because of evaporite and lacustrine sedimentation in the Jianghan Basin before that time. We propose that the present Yangtze River system formed in response to regional extension throughout eastern China, synchronous with the start of strike–slip tectonism and surface uplift in eastern Tibet and fed by strengthened rains caused by the newly intensified summer monsoon.

A new mechanism for pressure solution in porous quartzose sandstone

Abstract The mechanism of pressure solution, a source of controversy for years, must be understood before we can evaluate the effectiveness of pressure solution during geological processes. The water film diffusion (WFD) mechanism proposed by Weyl (1959) and Rutter (1976, 1983) is believed by many to be the primary mechanism responsible for intergranular pressure solution (IPS) in non-porous metamorphic rocks as well as porous sedimentary rocks. Tada and Siever (1986), experimenting with halite single crystals, suggested the new plastic deformation plus free-face pressure solution (PD + FFPS) mechanism. The effectiveness of PD + FFPS as an IPS mechanism is theoretically evaluated for porous quartzose sandstone and compared with WFD. The result suggests that, though the driving force of the reaction (relative activity increase) is 4 to 5 orders of magnitude larger in WFD, the ease of diffusion (diffusion path width times the diffusion coefficient) is 7 to 9 orders of magnitude larger in PD + FFPS. Consequently. PD + FFPS yields diffusion rates 2 to 5 orders of magnitude faster than WFD. In WFD, diffusion is always the rate-controlling process, whereas either dissolution at IPS contacts or precipitation on free grain surfaces may be the rate-controlling process in PD + FFPS, when temperatures are low and/or grain sizes are small. The dissolution or precipitation rate of PD + FFPS is faster than the diffusion rate of WFD except when the total free grain surface area is very small. In final stages of compaction, when the total free grain surface area has become very small, WFD replaces PD + FFPS.

Continent‐ocean interactions within the East Asian Marginal seas

Interactions between continents and oceans are a frontier area for the Earth sciences in the 21st century. An AGU Chapman Conference, Continent-Ocean Interactions within the East Asian Marginal Seas, examined the nature of these interactions in the marginal seas of east Asia. The objective was to highlight both recent advances, and especially the contributions made by the Ocean Drilling Program (ODP)semi; as well as to identify key future science goals. The types of continent-ocean interactions discussed were wide-ranging, including climate-tectonic interactions, continental-oceanic climate linkages, and the material flux from the rivers of Asia to the ocean, as well as how continental tectonic evolution since the India-Asia collision has influenced the tectonics of the western Pacific and vice-versa. The marginal seas of east Asia form the transition between the worlds largest continent and its largest ocean, and are major repositories of information on the interaction between the two.

High-resolution reconstruction of variation in aeolian dust (Kosa) deposition at ODP Site 797, the Japan Sea, during the last 200 ka

Abstract In order to reconstruct past variations in the east Asian monsoon and to establish a direct link between terrestrial and marine climatic records, we evaluated the temporal variation in aeolian dust (Kosa or yellow sand) flux to the Japan Sea using the late Quaternary hemipelagic sediments in the sea. Contributions of four detrital subcomponents, identified by Irino and Tada [Geochem. J. 34(1) (2000) 59] as Kosa derived from “typical” loess, Kosa from “weathered” loess and fine and coarse arc-derived detritus, have varied in millennial scale, as well as in glacial–interglacial scale, during the last 200 ka. Millennial scale variability of Kosa from “typical” loess suggests the change in dust availability controlled by high-frequency variation in summer monsoon precipitation in the central to east Asia during the last 200 ka, whereas variation in Kosa from “weathered” loess suggests the change in transport pathway of dust which was probably affected by the winter monsoon variation.

Late Oligocene–early Miocene birth of the Taklimakan Desert

Significance The formation of the Taklimakan Desert marked a major geological event in central Asia during the Cenozoic, with far-reaching impacts. Deposition of both eolian sand dunes in the basin center and the genetically equivalent loessite along the basin margins provide evidence for the birth of the Taklimakan Desert. This paper resolves a long-standing debate concerning the age of the Taklimakan Desert, specifically whether it dates to ∼3.4–7 Ma, currently the dominant view. Our result shows that the desert came into existence during late Oligocene–early Miocene, between ∼26.7 Ma and 22.6 Ma, as a result of widespread regional aridification and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan–Pamir Plateau and Tian Shan. As the world’s second largest sand sea and one of the most important dust sources to the global aerosol system, the formation of the Taklimakan Desert marks a major environmental event in central Asia during the Cenozoic. Determining when and how the desert formed holds the key to better understanding the tectonic–climatic linkage in this critical region. However, the age of the Taklimakan remains controversial, with the dominant view being from ∼3.4 Ma to ∼7 Ma based on magnetostratigraphy of sedimentary sequences within and along the margins of the desert. In this study, we applied radioisotopic methods to precisely date a volcanic tuff preserved in the stratigraphy. We constrained the initial desertification to be late Oligocene to early Miocene, between ∼26.7 Ma and 22.6 Ma. We suggest that the Taklimakan Desert was formed as a response to a combination of widespread regional aridification and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan–Pamir Plateau and Tian Shan, which had reached a climatically sensitive threshold at this time.

Petrology and Diagenetic Changes of Neogene Siliceous Rocks in Northern Japan

ABSTRACT Diagenetic changes in textures and mass properties of Neogene marine noncalcareous siliceous rocks corresponding to silica-phase transformations (opal-A opal-CT quartz) were studied in subsurface and surface sections of northern Japan. In opal-A diatomite, siliceous tests are well preserved, no cementation is observable, and pores exist chiefly as inter- and intragranular micropores (2 10µ). During the opal-A to opal-CT transformation, siliceous tests are extensively dissolved to form fine aggregates of opal-CT in matrix and cement of opaline porcelanite. An abrupt decrease of as much as 15% in porosity occurs between opal-A and opal-CT zones in subsurface sections, a decrease which may be due to the destruction of intragranular pores in siliceous tests by dissolution. Ultramicropores (10 µ) as molds and chambers of siliceous tests in quartzose porcelanite. Porosity of Neogene noncalcareous siliceous rocks in subsurface sections of northern Japan gradually decreases with the increase of burial depth, and silica-phase transformations have a relatively small effect on it. This gradual decrease in porosity, which corresponds well to that of Neogene mudstone and clean, fine-grained sandstone, indicates that little additional silica cementation occurs during burial (Iijima and Tada, 1981). At a depth of 4.5 km and a temperature of 130° C, quartzose porcelanite holds porosity of 10-15%. Burial depth of around 10 km might be required to form dense quartzose chert without porosity by burial diagenesis of siliceous rocks if extrapolating from the burial depth-porosity relation. This depth is too large. Some additional mechanisms are required ther than mechanical compaction, silica-phase transformations and clay alteration. Pressure solution-reprecipitation of quartz is the most probable one.

Complex tsunami waves suggested by the Cretaceous-Tertiary boundary deposit at the Moncada section, western Cuba

The Moncada Formation in western Cuba is an 2-m-thick weakly metamorphosed complex characterized by repetition of calcareous sandstone units that show overall upward fining and thinning. The Moncada Formation contains abundant shocked quartz, altered vesicular impact-melt fragments, and altered and deformed greenish grains of possible impact glass origin. In addition, a high iridium (0.8 ppb)