Keita Yamada

Molecular-isotopic stratigraphy of long-chain n-alkanes in Lake Baikal Holocene and glacial age sediments

The molecular distribution and the carbon-isotopic composition (d 13 C) of n-alkanes extracted from a Lake Baikal core spanning the last 20 kyr of sediment accumulation have been investigated. A terrestrial origin has been inferred for the odd carbon-numbered long-chain (>C27) n-alkanes, on the basis of the observed high CPI27-33 values (range: 8.7‐ 10.8) typical of n-alkanes derived from leaf waxes of higher plants. A shift in the abundance of n-C27 alkane relative to n-C31 homologue is observed across the late Pleistocene glacial‐Holocene interglacial climate change, perhaps indicative of the climate-induced vegetational change previously deduced from palynological analyses. Compound-specific isotope analyses indicate remarkably uniform d 13 C values in the range ofˇ31.0 toˇ33.5% for the leaf-wax C27‐C33 nalkanes in the entire cored sequence. Such an isotopic compositional range is characteristic for n-alkanes biosynthesized by plants utilizing the C3 photosynthetic pathway. Our data suggest that the observed 13 C-enrichment in the bulk organic matter in the glacial age sediments, relative to d 13 C values of total organic carbon in the Holocene section, is therefore unlikely to be attributed to an expansion of C4-type vegetation in the Baikal watershed during the late Pleistocene glacial interval. # 2000 Elsevier Science Ltd. All rights reserved.

Organic molecular and carbon isotopic records of the Japan Sea over the past 30 kyr

The organic and isotopic geochemical study of two sediment cores (KH-79-3, L-3, and KH-79-3, C-3) from the Oki Ridge in the Japan Sea has revealed that total organic carbon (TOC) mass accumulation rates are extremely high in the 12–11 ka (calendar age) interval and TOC in the sections in the 24–17 ka interval is depleted in 13C by 3.5‰ relative to Holocene sediments. Alkenone sea surface temperature (SST) shows a decrease from 18° to 14°C from 17.5 to 11.6 ka and a sharp increase from 14° to 19°C from 11.6 to 11.1 ka. The SST changes are associated with the inflow of cold seawater with the vertical water mixing and the inflowof warm Tsushima Current into the Japan Sea. The δ13C values for both 24-methylcholesta-5,22-dien-3β-ol (diatom marker) and dinosterol (dinoflagellate marker), are at their minimum from 24 to 17 ka, while those for long-chain alkenones are not. The theoretical considerations on δ13C for biomarkers suggest low photosynthetic carbon demand of diatoms and dinoflagellates from 24 to 17 ka.

Carbon isotopic compositions of long-chain n-alkanes in the Japan Sea sediments: implications for paleoenvironmental changes over the past 85 kyr

Abstract Carbon isotopic compositions of long-chain n-alkanes in cores KH-79-3 (C-3 and L-3) taken from the Oki Ridge (37°04′N, 134°42′E, water depth 935 m) in the Japan Sea were measured by gas chromatography/isotope ratio mass spectrometry. Fluctuations in the carbon isotopic composition of terrestrially derived material, as determined from δ 13 C n-alkane (weighted average of δ 13 C values for higher plant-derived n-C29 and n-C31 alkanes), range from −29.1 to −32.1‰ for the past 85 kyr in core KH-79-3, C-3. δ 13 C n-alkane values in colder climates ( 18 O stages 2 and 4) are depleted in 13 C by approximately 1‰ relative to those in warmer climates (stages 1, 3 and 5.1). This suggests that the temporal variations of the δ 13 C values for higher plant-derived n-alkanes exhibit a close relationship with glacial–interglacial cycles. The results over the past 30 kyr ( 14 C age) reveal that the δ 13 C n-alkane values shifted abruptly from −31.1±0.5‰ to −29.9±0.4‰ at around 10 kyr B.P., with the isotopic values remaining relatively constant thereafter. This positive (1.2‰) shift in δ 13 C n-alkane values can be attributed to one or both of the following factors: (i) enhancement of primary productivity on land; and (ii) change in vegetational types (e.g. C4/C3 plants ratios and/or changes in the species of C3 plants).

Organic geochemistry of the Japan Sea sediments-1: Bulk organic matter and hydrocarbon analyses of Core KH-79-3, C-3 from the Oki Ridge for paleoenvironment assessments

Organic geochemical study of bulk organic matter (OM), hopanoid hydrocarbon and normal hydrocarbon (C23∼C35) was conducted for a 936-cm-long sediment core sample from the Oki Ridge of the Japan Sea (Core KH-79-3, C-3; 37°03.5′ N, 134°42.6′E, water depth 935 m). Stable carbon isotopic ratios were also measured for both bulk OM and individual hydrocarbons. The following results were obtained: (1) The weight ratios of total organic carbon to total nitrogen range from 6.2 to 9.4 in the core. The δ13C values of bulk OM range from −25.1∼−20.7%.. The δ13C values of OM in the sections of 140∼190 cm are lower (−25∼−24‰) than those in the other sections (−23∼−21‰). This result indicates that OM in the core except for the 140∼190 cm sections is essentially of marine origin. (2) The δ13C value of diploptene (a hopanoid hydrocarbon) in the last glacial maximum (LGM), is −66.3‰ (vs. PDB), which indicates it originating in methanotrophic bacteria. This result provides evidence to support for the previous ideas (Oba et al., 1980, 1984; Masuzawa and Kitano, 1984) that the bottom waters in the Japan Sea were anoxic in LGM. (3) Long chain (C23∼C35) n-alkanes of higher-plant wax origin were found throughout the core. Their concentration is high in 140∼190 cm in depth, suggesting that eolian dust load was high in LGM. (4) The n-alkane/TOC ratio increases with decreasing δ13C values of bulk OM. This result indicates that the load of terrestrial (probably eolian dust-derived) OM to the Japan Sea became higher in colder climates. (5) The CPI values of long-chain n-alkanes are different in different δ13O stages of paleoclimate, probably reflecting variations in species of terrestrial higher plants as a result of climatological adaptations.

Carbon isotope composition of individual n-alkanes in recent sediments

Carbon isotope composition of individual long-chain n-alkanes from Tokyo Bay sediments (a 3 m-core sample and surficial sediment samples) was determined by using gas chromatography/isotope ratio mass spectrometry (GC-IRMS). δ 13C values for C27–C33n-alkanes in shallow (0–40 cm in depth, present—1965 AD) sections range from −31.5 to −28.2‰, showing even carbon number n-alkanes being isotopically heavier than the co-occurring odd n-alkanes. C27–C33n-alkanes in the deep sections (70–220 cm in depth) have the δ 13C values ranging from −29.6 to −32.9‰ (average value: 31.2 ± 0.7‰, with no isotopic differences between odd and even numbered n-alkanes. The results for n-alkanes in shallow sediments are satisfactorily explained by a two end-member mixing model of indigenous higher plant wax n-alkanes from oil pollution. The onset of significant levels of pollution in Tokyo Bay was between 1960 and 1965.

Sources of 24-ethylcholest-5-en-3β-ol in Japan Sea sediments over the past 30,000 years inferred from its carbon isotopic composition

Abstract Carbon isotopic analyses of 24-ethylcholest-5-en-3β-ol in a marine sediment core (KH-79-3, C-3) covering the last 30,000 years, using gas chromatography/isotope ratio mass spectrometry (GC/IRMS), are presented. The source of 24-ethylcholest-5-en-3β-ol is unclear from the abundances for marine and terrestrial biomarkers. However, the carbon isotope results for this sterol exhibit a clear indication of the sources. The carbon isotopic composition (δ 13 C) of 24-ethylcholest-5-en-3β-ol in the C-3 core ranges from −23.3 to −32.0‰ over the past 30,000 years. After the last glacial period (10–73 cm section; 2–9 ka), 24-ethylcholest-5-en-3β-ol is derived from marine algae (−24.2±1.1‰ on average). However, in the last glacial stage (140–274 cm section; 15–29 ka), it is likely that the value indicates a terrestrial higher plant origin (−30.5±0.9‰ on average). The trend in the isotopic composition of 24-ethylcholest-5-en-3β-ol is nearly consistent with paleoclimatic changes around the Japan Sea over the past 30,000 years.

Recent sedimentary hopanoids in the northwestern Pacific alongside the Japanese Islands — their concentrations and carbon isotopic compositions

Two hopanoids, 17b(H),21b(H)-hop-22(29)-ene (diploptene) and 17b(H),21b(H)-bishomohopanoic acid (bbC32HA), are the most abundant among the pentacyclic triterpenoids found in Recent sediments of the Pacific Ocean alongside the Japanese Islands. The concentration of diploptene normalized to organic carbon content is higher in sediments where C37 polyunsaturated alkene (a biomarker of Haptophytes) is in high concentration, suggesting that the diploptene may be associated with the accumulation of marine organic matter. In open marine settings, d 13 C values of bbC32-HA range fromˇ23.8 toˇ19.4% (relative to PDB), being enriched in 13 C relative to diploptene (ˇ31.6 to ˇ26.3%) by 5‐9%. The isotopic diAerence indicates the presence of at least partially diAerent sources for the two hopanoids. While diploptene is derived from cyanobacteria and chemotrophic bacteria in the water column or sediment, bishomohopanoic acid may be produced mainly by heterotrophs in the sediment using marine organic matter. In contrast, the d 13 C values of the two hopanoids from river and bay sediments are similar (ˇ31 toˇ29%), indicating a common source derived from soil components (terrestrial plants or bacteria in soils). # 2000 Elsevier Science Ltd. All rights reserved.

Source of Organic Matter in Sinking Particles in the Japan Trench: Molecular Composition and Carbon Isotopic Analyses

Molecular and stable carbon isotopic compositions were obtained for the organic matter (OM) in sediment trap samples from the Japan Trench at 4500 and 8500–8800 m in depth. The analytical results show that OM in sinking particles are composed of both freshly-deposited and resuspended OM. The relative abundance of resuspended OM in the total OM in shallower sediment trap samples is generally lower than that in deeper trap samples. However, no marked differences both in py-GCMS characteristics and stable carbon isotopic compositions of TOC and biomarkers (n-alkanes, fatty acids) are recognized. All sinking particles examined indicate that the major source of OM is plankton-derived. Terrestrial OM contribution is minor.