Kotaro Hirose

Human activities in coastal areas have intensified over the last 200 years, impacting also high-latitude regions such as the Baltic Sea. Benthic foraminifera, protists often with calcite shells (tests), are typically well preserved in marine sediments and known to record past bottom-water conditions. Morphological analyses of marine shells acquired by microcomputed tomography (µCT) have made significant progress toward a better understanding of recent environmental changes. However, limited access to data processing and a lack of guidelines persist when using open-source software adaptable to different microfossil shapes. This study provides a post-data routine to analyze the entire test parameters: average thickness, calcite volume, calcite surface area, number of pores, pore density, and calcite surface area/volume ratio. A case study was used to illustrate this method: 3D time series (i.e., 4D) of Elphidium clavatum specimens recording environmental conditions in the Baltic Sea entrance from the period early industrial (the 1800s) to present-day (the 2010 s). Long-term morphological trends in the foraminiferal record revealed that modern specimens have ∼28% thinner tests and ∼91% more pores than their historic counterparts. However, morphological variability between specimens and the BFAR (specimens cm⁻² yr⁻¹) in E. clavatum were not always synchronous. While the BFAR remained unchanged, morphological variability was linked to natural environmental fluctuations in the early industrial period and the consequences of anthropogenic climate change in the 21st century. During the period 1940–2000 s, the variations in BFAR were synchronous with morphological variability, revealing both the effects of the increase in human activities and major hydrographic changes. Finally, our interpretations, based on E. clavatum morphological variations, highlight environmental changes in the Baltic Sea area, supporting those documented by the foraminiferal assemblages.

For assessment of the potential of the Beppu Bay sediments as a Global Boundaries Stratotype Section and Point (GSSP) candidate for the Anthropocene, we have integrated datasets of 99 proxies. The datasets for the sequences date back 100 years for most proxy records and 1300 years for several records. The cumulative number of occurrences of the anthropogenic fingerprint reveal unprecedented increases above the base of the 1953 flood layer at 64.6 cm (1953 CE), which coincides with an initial increase in global fallout of Pu-239+Pu-240. The onset of the proliferation of anthropogenic fingerprints was followed by diverse human-associated events, including a rapid increase in percent modern C-14 in anchovy scales, changes in nitrogen and carbon cycling as recorded by anchovy delta N-15 and delta C-13, elevated pollution of heavy metals, increased depositions of novel materials (spheroidal carbonaceous particles, microplastics, polychlorinated biphenyls), the occurrence of hypoxia (Re/Mo ratio) and eutrophication (biogenic opal, TOC, TN, diatoms, chlorophyll a), unprecedented microplankton community changes (compositions of carotenoids, diatoms, dinoflagellates), abnormally high spring air temperatures as inferred from diatom fossils, and lithological changes. These lines of evidence indicate that the base of the 1953 layer is the best GSSP level candidate in the stratigraphy at this site.

The timing and magnitude of coastal marine degradation in East Asia during a possible transition from the Holocene to the proposed Anthropocene have still not been quantified with well-dated proxy records over centennial-to millennial timescales. This study uses multi-proxy sedimentary records to address this issue and to document biological and environmental changes in an anoxic coastal marine basin, Beppu Bay, Japan. Analysis of sedimentary diatom and pigment records revealed a notable change in diatom and phytoplankton communities and an abrupt increase in their productivity beginning 1960 s CE. Biogeochemical indices, including total organic carbon, total nitrogen, biogenic opal, bromine, and nickel as well as total sulfur, showed a notable increase in values, reflecting the enhanced primary productivity in the water column due to eutrophication, and reduced oxygen-levels in bottom conditions. Biological changes seen in diatom concentrations and communities were unprecedented over the last 1300 and-670 years, respectively. The eutrophication-associated proxy records demonstrate that anthropogenic degradation of the coastal marine environment occurred in the 1960 s, a time which was associated with the exponential, world-wide spread of coastal marine degradation during the Anthropocene, proposed to commence in the mid-20th century. Beppu Bay sediments have a wealth of excellent anthropogenic proxy records and, therefore, could be a representative archive of coastal Holocene-Anthropocene transitions in East Asia.

秋季シンポジウム「プランクトンと古環境・古生態系復元」の開催報告

Multiproxy analyses were conducted using sediment cores in a low-polluted coastal site (Hiuchi-nada) in the Seto Inland Sea (SIS), Japan. Heavy metal and organic pollution peaked in the 1960s and the bottom environments have ameliorated since the 1980s due to several environmental regulations. First ecological shifts in meiobenthic ostracodes and diatoms occurred in the 1960s due to the initiation of eutrophication. Then, a second ecological shift occurred in the 1980s due to the amelioration of the water and the bottom quality. A compilation of similar analytical results in the coastal seas of the SIS reveals three types of ecological and environmental history since the 20th century. The environmental improvement since the 1980s affects the ecosystems, in particular, in a low-polluted bay. However, ecological compositions are different from those prior to the 1960s, suggesting that the ecosystem was not recovered but changed into the next stage in the SIS.

Suborbital-scale climate variations, possibly caused by solar activity, are observed in the Holocene and last-glacial climates. Recently published bicentennial-resolution paleoceanic environmental records reveal millennial-scale high-amplitude oscillations postdating the last geomagnetic reversal in the Marine Isotope Stage (MIS) 19 interglacial. These oscillations, together with decoupling of post-reversal warming from maximum sea-level highstand in mid-latitudes, are key features for understanding the climate system of MIS 19 and the following Middle Pleistocene. It is unclear whether the oscillations are synchronous, or have the same driver as Holocene cycles. Here we present a high resolution record of western North Pacific submarine anoxia and sea surface bioproductivity from the Chiba Section, central Japan. The record reveals many oxic events in MIS 19, coincident with cold intervals, or with combined cold and sea-level fall events. This allows detailed correlations with paleoceanic records from the midlatitude North Atlantic and Osaka Bay, southwest Japan. We find that the millennial-scale oscillations are synchronous between East and West hemispheres. In addition, during the two warmest intervals, bioproductivity follows the same pattern of change modulated by bicentennial cycles that are possibly related to solar activity.

<p>Vertical variations in <sup>210</sup>Pb, <sup>134</sup>Cs, and <sup>137</sup>Cs content (Bq/kg) were measured in the uppermost 2 m of the INW2012 sediment core, which was taken from Lake Inawashiro-ko. Analyses of <sup>210</sup>Pb<sub>(ex)</sub> suggest that the top 39 cm were deposited within the last 90 years, while <sup>134</sup>Cs and <sup>137</sup>Cs results indicate that a depth of 29 cm corresponds to the beginning of the 1950s. Combined with lithostratigraphic observations, these dates suggest that the massive sandy silt layer observed in the top 13.5 cm of the core was deposited as a result of the 2011 earthquake in the Pacific Ocean off the coast of Tohoku. In addition, the gray-brown clay layer between 42 and 37.5 cm depth is associated with the Mt. Bandai eruption in AD 1888. On the basis of these event horizons and their ages, sedimentation rates are estimated to be 1.0 and 2.0 mm/yr at depths of 200-42 cm and 37.5-13.5 cm, respectively.</p>

Detailed sea-level variation was investigated for Marine Isotope Stage (MIS) 19, based on diatom and grain size analyses of a marine sequence in a core of the Osaka Group. Diatom sea-level proxies represent precession-related signals correlated with highstands MIS 19.3 and 19.1, and lowstand MIS 19.2. Astronomical tuning shows the marine sequence has a uniform accumulation rate of about 60cm ka(-1). A rapid sea-level fall event was found in the earliest MIS 19, demonstrated by several independent sea-level proxies of diatom and grain size. This event began with a rapid sea-level drop, followed by a gradual recovery, at about 783-782 ka. A maximum abundance of pelagic diatom taxa at a core depth of 402.20m evidently shows the highest sea-level peak in MIS 19, supported by many other proxies. Based on the diatom data, sea-level change across MIS 19.1 is characterized by centennial to millennial fluctuations. The sea-level fall event began just after the onset of a cooling event previously reported from the same core. Observations of a comparable sea-level fall signal in many deep-sea core records suggest the event is global. Copyright (C) 2016 John Wiley & Sons, Ltd.

Morphological characteristics of a planktonic diatom Chaetoceros minimus were described from a brackish-water lake, Lake Nakaumi, Shimane Prefecture, Japan. This taxon is small in size with a single plastid, and has weakly silicified valves. The cell body is almost cylindrical in shape; 4.5–12 µm long in pervalvar axis and 2.0–4.0 µm in diameter. The valves of the frustule are similar, asymmetrically conical in shape with tops located axisymmetrically or asymmetrically to the pervalvar axis. A single long seta (0.5–50 µm) arises from the top of the valve face margin, and runs almost parallel (weakly bent) to the pervalvar axis. Ellipsoidal resting spores were observed, which are 3.5–7.5 µm long in pervalvar axis and 2.2–3.0 µm in diameter. Two types of cell sizes of Chaetoceros minimus are distinguishable (large and small),which suggests a relationship between cell size and water quality (salinity and/or water temperature). In this study, cells from Lake Nakaumi belong to the small type. Moreover, thin and thick cell-walled types are recognized from Lake Nakaumi. It is presumed that this dimorphism in shape corresponds to water temperature and/or cell cycle (vegetative or resting cell).

We verified the significance of a simple and easy treatment using ultrasonic cleaner, which was designed to minimize possible modification of original diatom assemblages preserved in sediments. Our ultrasonic treatment demonstrated that diatom valves incurred little damage ; in addition, this treatment was effective to disaggregate the consolidate structure mainly of fecal pellets. Comparing the diatom assemblages in the samples treated by hydrogen peroxide (an established method for diatom slide preparation) with those processed by ultrasonication, we concluded that the ultrasonic treatment provides a better result in preserving micro and fragile valves that otherwise easily flow out while captured in supernatant during the dacantation process of the hydrogen peroxide method. Furthermore, the investigation of the difference between diatom assemblages in aggregate structures (mainly of fecal pellets) and those in sediment matrix suggests the possibility that diatom analysis of community structures in fecal pellets tends to reconstruct the population dynamics of consumers whose fossils are known to be rarely preserved in underwater sediment.

<SUP>210</SUP>Pb and <SUP>137</SUP>Cs age dating, total organic carbon (TOC) and total nitrogen (TN) contents analysis, and diatom analysis were conducted in sediment samples from two short cores collected from Sakoshi Bay (core HNA) and off Aioi City (core HNB) , northern Harima-Nada, southwest Japan. The results of <SUP>210</SUP>Pb and <SUP>137</SUP>Cs dating shows that ages at the bottom of core HNA and that of core HNB were estimated at 1883AD and 1786AD, respectively. Spatio-temporal changes of diatom assemblages and TOC and TN contents were as follows : In coastal areas of northern Harima-Nada, eutrophication occurred earlier than the initiation of the high economic growth period, and diatoms flourished during the 1940s-1960s. Deterioration of marine environments peaked in the 1960s-1970s. In the 1980s, Sakoshi Bay and a site off Aioi City underwent different paleoenvironmental changes. Since the 1990s, water and bottom environments in Sakoshi Bay were deteriorated by oyster farming, while water environments off Aioi City were influenced by the improvement of water quality in northern Harima-Nada.

We investigated the recent diatom assemblages in 30 surface sediment samples from Osaka Bay, Japan, with the identified diatoms divided into five categories [halobious system ; Oligoeuryhaline Polyhalobous (OPH), Euryhaline Polyhalobous (EPH), Mesohalobous (MEH), Euryhaline Oligohalobous (EOH) and Stenohaline Oligohalobous (SOH)] based on their tolerance for salinity. The horizontal distribution pattern of each category suggested that the recent diatom distribution pattern of the bay is mainly controlled by the transportation of brackish/freshwater species from the mouth of the Yodo River through the river-water discharge into the bay. In other words, the distance from the river mouth controls the relative abundance of brackish/freshwater species at each site, which is a major part of the total floral composition and could be a useful paleoecological indicator for river-water discharge. The distribution pattern of <i>Chaetoceros</i> resting spores suggests they have potential as a paleoecological indicator for anthropogenic environmental degradation in exploited embayments. Taphonomic processes of recent diatoms are also discussed.