Fumiko Watanabe Nara

Temporal changes in the phytoplankton community of the southern basin of Lake Baikal over the last 24,000 years recorded by photosynthetic pigments in a sediment core

Photosynthetic pigments and other indicators of phytoplankton were analyzed in a dated undisturbed sediment core obtained from the southern basin of Lake Baikal to reveal temporal changes in the phytoplankton community in the lake through the last glacial/post-glacial transition. The sedimentation age of the core spans the last 24 14 C ka. Chlorophyll a, its derivatives, carotenoids and total organic carbon (TOC) started to increase after 15 14 C ka, and the onset of biogenic silica occurred at 10 14 C ka. This indicated that the post-glacial growth of diatoms was preceded by that of other phytoplankton groups. In the record of the pigments and TOC, a temporary decrease was observed in the period 11.5-10.5 14 C ka, corresponding to the Younger Dryas cold period. The similarity found between the depth profiles of pyropheophytin a and steryl chlorin esters formed through predation of phytoplankton by zooplankton and that of TOC suggested the important contribution of fecal pellets to sedimentary organic matter in the lake.

Fast and precise method for HPLC-size exclusion chromatography with UV and TOC (NDIR) detection: importance of multiple detectors to evaluate the characteristics of dissolved organic matter.

A new type of high-performance liquid chromatography (HPLC)-size exclusion chromatography (SEC) system with ultraviolet (UV) absorbance detection and non-dispersive infrared (NDIR) detection of total organic carbon is described. The introduction of an online degassing tube and a low-volume HPLC column helped to reduce the analytical time and increase the sensitivity of the SEC system. This study is the first in which linear calibration curves (R(2)>0.99) were obtained for both UV absorbance and NDIR data for polystyrene sulfonate standards, which are the most suitable standards for molecular size analysis of aquatic humic substances as well as dissolved organic matter (DOM). Using the calibration curves, the molecular size distribution of DOM in water collected from Lake Kasumigaura and in pore water from lake sediments was estimated. Most of the DOM had a molecular weight less than 4000 Daltons (Da), and the amount of low-molecular-weight DOM (∼ 2000 Da) with low UV absorbance increased with depth in the sediment pore water. This result shows the importance of combining quantitative analysis by NDIR detection with qualitative analysis by UV detection to determine the chemical and physical properties of DOM. The possible sources and reactivity of DOM in Lake Kasumigaura and its sediment pore water are also discussed.

Crystal Growth and Luminescence Properties of Tm:BaF2 Single Crystals

BaF2 is an intensively studied scintillator for the detection of gamma radiation due to its relatively high stopping power, radiation hardness and extremely fast response. BaF2 possesses the fast cross-luminescence component at 195 and 220 nm with a lifetime of several hundred picoseconds. However, this component coexists with slow one at 310 nm related to the self-trapped exciton (STE). In our present study, we focused on BaF2 host material and introduced the Tm3+ (activator) 5d–4f vacuum ultraviolet (VUV) luminescence center to suppress the STE luminescence. Tm:BaF2 single crystals were successfully grown by the micro-pulling-down (µ-PD) method with up to 10 mol % of Tm. The STE emission intensity decreased with increasing Tm concentration. Tm3+ 5d–4f emission was observed while cross-luminescence was decreased at higher Tm concentrations.

Seasonal Variation in Sources of Dissolved Organic Carbon in a Lacustrine Environment Revealed by Paired Isotopic Measurements (Δ14C and δ13C)

To investigate the sources and cycling of dissolved organic carbon (DOC) in a lacustrine environment, isotopic measurements of 14C and 13C in DOC were carried out for Lake Kasumigaurawhich is famous as a very eutrophic and shallow (mean depth 4.0 m) lake in central Japanand its tributary rivers. Lake and river samples were collected in the spring and autumn (May and September) of 2003. The ∆14C measurements of DOC were performed using the accelerator mass spectrometer at the National Institute for Environmental Studies (NIES-TERRA), Japan. In September, the 14C values of DOC were light (around -200‰) and did not differ significantly between lake and river water samples, indicating that DOC in Lake Kasumigaura and its tributary rivers yields older 14C ages than the age expected from the lake-water residence time (average 200 days). This result suggests that terrestrial sources are important contributors to DOC in Lake Kasumigaura. Nevertheless, δ13C values indicated that during spring, DOC in the lake is mainly autochthonous. Thus, sources and cycling of DOC in Lake Kasumigaura may vary seasonally.

Residual Photosynthetic Pigments in the Sediment of Lake Baikal as Indicators of Phytoplankton History

Phytoplankton photosynthetic pigments preserved in lake sediments provide important information about the past environment of the lake as it affected the distribution of phytoplankton. This review chapter demonstrates that the pigments can represent the changes in the algal community in Lake Baikal during both millennial-scale and mega-annum-scale periods.

Effects of soil erosion and anoxic-euxinic ocean in the Permian-Triassic marine crisis.

The largest mass extinction of biota in the Earth’s history occurred during the Permian–Triassic transition and included two extinctions, one each at the latest Permian (first phase) and earliest Triassic (second phase). High seawater temperature in the surface water accompanied by euxinic deep-intermediate water, intrusion of the euxinic water to the surface water, a decrease in pH, and hypercapnia have been proposed as direct causes of the marine crisis. For the first-phase extinction, we here add a causal mechanism beginning from massive soil and rock erosion and leading to algal blooms, release of toxic components, asphyxiation, and oxygen-depleted nearshore bottom water that created environmental stress for nearshore marine animals. For the second-phase extinction, we show that a soil and rock erosion/algal bloom event did not occur, but culmination of anoxia–euxinia in intermediate waters did occur, spanning the second-phase extinction. We investigated sedimentary organic molecules, and the results indicated a peak of a massive soil erosion proxy followed by peaks of marine productivity proxy. Anoxic proxies of surface sediments and water occurred in the shallow nearshore sea at the eastern and western margins of the Paleotethys at the first-phase extinction horizon, but not at the second-phase extinction horizon. Our reconstruction of ocean redox structure at low latitudes indicates that a gradual increase in temperature spanning the two extinctions could have induced a gradual change from a well-mixed oxic to a stratified euxinic ocean beginning immediately prior to the first-phase extinction, followed by culmination of anoxia in nearshore surface waters and of anoxia and euxinia in the shallow-intermediate waters at the second-phase extinction over a period of approximately one million years or more. Enhanced global warming, ocean acidification, and hypercapnia could have caused the second-phase extinction approximately 60 kyr after the first-phase extinction. The causes of the first-phase extinction were not only those environmental stresses but also environmental stresses caused by the soil and rock erosion/algal bloom event.

A New 14C Data Set of the PY608W-PC Sediment Core from Lake Pumoyum Co (Southeastern Tibetan Plateau) over the Last 19 kyr

A new continuous sediment core (PY608W-PC; 3.8 m length) for reconstruction of climatic and environmental changes in the southeastern Tibetan Plateau was taken from the eastern part of Lake Pumoyum Co in August 2006. Sediment layers of the lower part of PY608W-PC (380-300 cm depth) were composed mainly of relatively large plant residues (up to ~3 cm in length) with an admixture of fine sand and sandy silt. The large plant residues disappeared at ~300-290 cm depth in core PY608W-PC and were replaced by silt-silty clay. The large plant residues from the lower part of PY608W-PC could be aquatic, because the plant residues were extremely enriched in 13C (up to -3.0‰, -5.6 ± 2.3‰ on average). On the other hand, the plant residue concentrates (PRC fractions) from the upper part of the core (290-0 cm in depth) could be terrestrial C3 plants (?13C = -21.8 ± 1.7‰ on average). Radiocarbon dating was performed on the large plant residues and PRC fractions from the PY608W-PC sediment core, which represented the chronology from ~19,000 cal BP to present.

High Contribution of Recalcitrant Organic Matter to DOC in a Japanese Oligotrophic Lake Revealed by 14C Measurements

Carbon isotopes (14C and 13C) of dissolved organic carbon (DOC) in a Japanese oligotrophic lake (Lake Towada) were measured to study the origin and cycling of dissolved organic matter (DOM) in Lake Towada. Lake water samples were collected at 3 depths (0, 30, and 80 or 85 m) during 4 months (April, June, August, and October) in 2006. 14C measurements of DOC were performed by accelerator mass spectrometry (AMS) at the National Institute for Environmental Studies (NIES-TERRA) in Japan. ∆14C and δ13C values of DOC in Lake Towada showed light carbon isotopic values ranging from -750 to -514‰ and -29.0 to -27.8‰, respectively. These values are similar to those of humic substances reported. The very low carbon isotopic values of DOC in Lake Towada suggest a very small contribution of DOC derived from fresh phytoplankton to the lake DOC. There is an extremely high linear relationship between the ∆14C and δ13C of DOC in Lake Towada when all data points are plotted (r2 = 0.818, p < 0.01), suggesting that the DOC in Lake Towada has 2 specific sources contributing heavy and light carbon isotopes. Although the freshly produced DOC of phytoplankton origin can be decomposed easily, the variation in the autochthonous DOC should influence the carbon isotopic values of DOC in Lake Towada.

RADIOCARBON AND STABLE CARBON ISOTOPE RATIO DATA FROM A 4.7-M-LONG SEDIMENT CORE OF LAKE BAIKAL (SOUTHERN SIBERIA, RUSSIA)

A sediment core (VER99G12; core length, 4.66 m) was taken from the Buguldeika Saddle of Lake Baikal in 1999. Radiocarbon measurements of total organic carbon (TOC) and pollen concentrate fractions from the VER99G12 core were performed by a Tandetron accelerator mass spectrometry (AMS) system (Model-4130, HVEE) at Nagoya University. The AMS 14C ages showed that the VER99G12 core spans the past ~30 cal ka BP (from the MIS 3 to present), and the average sedimentation rate of this core was calculated to be 13.6 cm/kyr based on the calibrated ages. This means that the time resolution of VER99G12 sediment samples in this study is better than ~70-80 yr/cm. Stable carbon isotope ratios of TOC (δ13CTOC) in the VER99G12 core varied widely from about 26.6‰ to 31.3‰ during the last glacial/post-glacial transition period (about 17?12 cal ka BP). Therefore, a rapid change in the carbon sources in Lake Baikal occurred in the last glacial/post-glacial transition period is concluded.

14C Dating of Holocene Soils from an Island in Lake Pumoyum Co (Southeastern Tibetan Plateau)

Soil samples from an 85-cm-long continuous section (PY608ES) were collected from an island in Lake Pumoyum Co (southeastern Tibetan Plateau, ~5020 m asl) in August 2006. To estimate past environmental conditions of Lake Pumoyum Co during the Holocene, we analyzed radiocarbon ages, stable carbon isotope compositions, and total organic carbon/total nitrogen (TOC/TN) atomic ratios of the soil samples. The 14C measurements were performed with the Tandetron accelerator mass spectrometry system at the Center for Chronological Research, Nagoya University. The 14C concentration in the surface layer (101 pMC; 5-10 cm soil depth) was nearly modern. A 14C chronology of the sequence indicated that continuous soil development began on the island in Lake Pumoyum Co at ~5800 cal BP (at 63 cm soil depth, the top of a gravel layer). These results may reflect a decrease in the lake level in the middle Holocene. The age of the obvious lithologic boundary (~5800 cal BP) corresponds to the end of Holocene climate optimum.