To reconstruct environmental changes in the late Quaternary, accurate dating is the most important. The framework of the late Quaternary chronology is based on magnetostratigraphy due to the reversal of geomagnetic polarity and oxygen isotope stratigraphy of foraminifera in marine sediments, which has been considered to be driven by astronomical forcing of solar energy reaching to the Earth, that is, the Milancovitch cycle. For dating during the past several tens of thouthands of years including the Last Glacial Period and the Holocene, radiocarbon dating is the most useful method. Recent developement of accelerator mass spectrometry (AMS) has realized precise dating by using mg amounts of carbon and AMS systems have been introduced widely. Basic concepts and methods of radiocarbon dating, especially on AMS radiocarbon dating, are reviewed and problems relating calibration of measured radiocarbon ages to calender ages are discussed.
It has been thought that about 9Oppm decrease of atmospheric CO2 in the last glacial maximum was attributed to the change in oceanic processes. In order to explain this decrease quantitatively, change in three processes (solubility pump, biological pump, alkalinity pump) and these effect on atmospheric CO2 in the last glacial period should be clarified. Although various kinds of paleoceanographical analysis on sea floor sediments and numerical simulation based on these results with some assumptions have been carried out, quantitative conclusion has not been suggested yet. In this review paper, biological pump and, especially, alkalinity pump in the last glacial period are focused on and update of interpretation with respect to the effect of oceanic change on atmospheric CO2 is introduced, taking into account for ocean carbonate chemistry.
We present analytical results of organic compounds in the deep sea sediments recovered from the Central Pacific and the western tropical Pacific and describe recent progresses in sedimentary biomolecules as tools for reconstructing paleoenviroments. Biomolecules derived from terrestrial higher plants such as C25-C35n-alkanes, C24-C28 fatty alcohols, or C23-C34 fatty acids in the surface sediments from 175° E transect show high concentrations in the higher latitudes in the Northern Hemisphere. Relative abundances of these terrestrial biomarkers are markedly different between low (15°N-15°S) and high (48°N-19°N) latitudes. Such a difference could be interpreted in terms of different wind regimes between the two areas. Unsaturation degree of long-chain alkenones (Uk37) derived from haptophyte algae suggests that sea surface temperature in the western tropical Pacific during the last glacial maximum was nearly the same as that of the present. We also found that the alkenone abundances over the last 20 kyr are anti-correlated with sedimentary nitrogen isotopic ratios, suggesting that a production rate of alkenones by haptophyte algae is controlled by the nutritional state of the algae.
Recently stable carbon and nitrogen isotopic ratios (δ13C and δ15N) of the sedimentary organic matter have become to be applied to reconstruct past changes of the biogeochemical processes in surface waters, although they were used only as the mixing tracers of terrestrial organic matter into coastal sediments during 1970's and 80's. While both of the δ13C and δ15N values of the organic matter produced in the surface water change due to the isotopic fractionation processes during the uptake of inorganic substance (CO2 and NO3- etc.) by phytoplankton, the reconstructed paleoceanograhic factors are completely different between the δ13C and δ15N, such as the [CO2] aq and the phytoplankton growth rate for the δ13C and the nitrate availability for δ15N, reflecting the difference in Σ CO2 and nitrate budgets in the sea surface water. Because the magnitude of carbon isotopic fractionation during the photosynthesis of phytoplankton are influenced by [CO2] aq in the surface water, many recent studies of the organic δ13C in the sediment cores discuss about past changes in pCO2 in the surface water. However, the phytoplankton growth rate also governs the δ13C values of phytoplankton, andtherefore, it is actually very difficult to extract purely the [CO2] aq data from the organic δ13C values. On the other hand, δ15N of sediment is now believed to reflect the past changes in nitrate utilization rates in the surface water (i.e. biological pumping efficiency). Because the isotope fractionation during the nitrate uptake by phytoplankton makes theδ15N values of the remaining nitrate much higher in the surface water body, the relativelylow δ15N values of organic matter can be used to trace a upwelling center of the surface water. The δ13C and δ15N values in the particulate organic matter change largely during their sinking and sedimentation processes. Both of the two isotopic values in the sinking particledecrease downward irrespective of their sample sites, probably due to the selective decomposition of amino acids which have relatively higher δ13C and δ15N values than the bulk organic matter. Contrary to this, the δ13C and δ15N values increase again on the sediment surface. Because the changes in δ13C and δ15N values of organic matter in the sinking and sedimentation processes are universal and somewhat constant processes, it seems reasonable to apply the downcore δ13C and δ15N variations to reconstruct the past changes in the surface water processes.
Since the ice core records from central Greenland revealed the presence and significance of millennial-scale large and abrupt climatic changes, widely known as Dansgaard-Oeschger [D-O] Cycles, it becomes the major objective of paleoclimatological researches to clarify their extent, nature, propagation mechanism, and driving force. Although the ultimate driving force is not yet understood, results of recent studies suggest 1) D-O Cycles are global phenomena, 2) they involve complicated interactions and feedback processes among the subsystems including atmosphere, cryosphere, hydrosphere, and biosphere, and 3) they seem tohave initiated from changes in atmospheric circulation. Catastrophic surges of Laurentide Ice Sheet called Heinrich Events are closely associated with D-O Cycles. Although Heinrich Events are likely to have been caused by free oscillationsof ice sheet growth and decay, they were probably not a cause of D-O Cycles but the events seem to have been phase-locked by D-O Cycles. Results of numerical modelling suggest the presence of multi-modes for global deepwater circulation. Switching among the modes is most likely caused by slight variation in hydrogical cycles which change the fresh water balance between Atlantic and Pacific.
Marginal seas, such are located as Bering Sea, Okhotsk Sea, Japan Sea, East China Sea, South China Sea and Indonesian seas, locate along the western rim of the Pacific. They have only a small area but are regions of high biological productivity, and thus expected to play an important role on global environment. Each of them has an unique characteristics on bathymetry, water exchange between the adjacent marginal seas and/or open ocean, primary productivity and volumes of fresh water input and terrigeneous material supply. Because they connect each other and to the open ocean through the shallow straits, characteristics of water masses flowed into the marginal seas were highly influenced by global sea level fluctuations. Furthermore, climatic change affected the volume of terrigenous supply to the seas. Therefore, paleoenvironments of the marginal seas largely changed with long-term sea level changes and short-term climatic changes. Recent studies on paleoceanography of the Japan Sea and the South China Sea suggest the importance of Asian monsoon to control the oceanic environments of the marginal seas. Hemipelagic sediments with higher sedimentation rates in the marginal seas recorded paleoceanographic changes in high resolution. Multidisciplinary studies for large-diameter long piston cores from the East Asian marginal seas may provide good information for global climate changes in the late Quaternary.
To analyze oceanic paleoenvironmental histories, paleoceanography has used various biological proxies such as characteristic species, molecular bio-markers, chemical components and others. Much of paleoceanographic information originates from skeletal remains and/or chemical compounds of marine organisms. Accordingly, we are always faced with the biological problem of so-called “vital effects”. Experimental paleontology is a research method to determine relationships between organisms and their biotic and / or abiotic environments through well-controlled culture experiments. This is one of the best ways to shed light into the “vital effect” black box. In this article, I review previous studies which have tried to solve paleontological problems through culture experiments using foraminifera. There are three different scales of experimental methods. First is a culture in a petri dish. This method is advantageous to observe the relation between individuals and environmental factors. Second is micro-and mesocosm experiments which tried to reconstruct a part of the marine ecosystem in laboratory. With this method one can examine interactions between organisms and biotic and/or abiotic factors. Third is in situ experiment in the sea using submersibles or benthic landers. These methods play the role of finding clues about, or proving the nature of, currentbiological proxies of paleoceanography. I strongly invite young scientists to work with experimental cultures for the better understanding biological proxies in paleoceanography. Several suggestions for future studies are also proposed in the text.
Bottom sediments in the open sea mainly originate from sinking particles. Therefore it is very important to investigate chemical properties, seasonality and interannual variability of sinking particles in order to reconstruct paleoenvironments in more detail. Data sets on total, carbonate, organic matter, and biogenic opal fluxes which were measured for more than 5 months are compiled from various areas of the world ocean. Mean total fluxes vary from 0.1 to 16.7 mg Cm-2 day-1 They reflect primary productivity. The enhanced values are observed in arctic, subarctic and equatorial upwelling regions. In general they decrease from coastal region to open sea. Although mean carbonate and biogenic fluxes generally reflect primary productivity, biogenic opal fluxes fluctuated more than carbonate fluxes. Carbonate and biogenic opal are dissolved in the deep sea, therefore it needs much efforts to reconstruct original assemblages of biogenic skeletons. Particle fluxes are affected by river input and upwelling induced by Asian monsoon, upwelling in coastal and equatorial regions, sea ice formation, and volcanic eruption. Long term sediment trap experiments evaluate the surface to the deep sea and/or seafloor, which is one of the most important subjects for understanding paleoenvironments.
Seismic reflection methods have been developed through several innovative steps in the history. They are : CDP (or CMP), geophone grouping, deconvolution and seismic migration methods evolved from 1960s through 1980s. Although their assumption was horizontally stratified layers, these techniques have been taken into standard acquisition and processing chain to image invisible subsurface structures such as petroleum reservoirs even for relatively complex structure where the assumption may break up. Due to the recent tendency to have their target structures in smaller scale or more complex than it used to be, new tendency is about to be brought into the existing processing chain. We may summarize the tendency as follows : a) true amplitude acquisition/processing to detect seismic signals with non-grouped geophones, i.e., mono-geophone acquisition, b) 4-C marine data acquisition (tri-component geophone and pressure gauge), c) deconvolution for deep water with vertical or deep-tow cable, d) multichannel signal processing to form seismic beams to enhance S/N, e) inclusion of anisotropy and attenuation to help quantitative processing like AVO (Amplitude Versus Offset), f) multi-resolutional imaging using measurements with different frequency signals, and g) GLI (Generalized Linear Inverse) type imaging.