Ryoshi Ishiwatari

Lacustrine organic geochemistry--an overview of indicators of organic matter sources and diagenesis in lake sediments

The factors affecting the amounts and types of organic matter in lacustrine sediments are summarized in this review, and synthesis, of published studies. Biota hving in the lake and in its watershed are the sources of the organic compounds initially contributed to the lake system. Microbial reworking of these materials during sinking and early sedimentation markedly diminishes the total amount of organic matter while replacing many of the primary compounds with secondary ones. Much of the organic matter content of sediments is the product of this microbial reprocessing. Various organic matter components of lake sediments nonetheless retain source information and thereby contribute to the paleohmnological record. Carbon/nitrogen ratios of total organic matter reflect original proportions of algal and land- derived material. Carbon isotopic compositions indicate the history of lake productivity and carbon recycling. Biomarker compounds provide important information about contributions from different biota. Sterol compositions and chainlength distributions of n-alkanes, n-alkanoic acids, and n-alkanols help distinguish different algal and watershed sources and also record diagenetic alterations. Stabilization of functional-group-containing biomarkers by conversion into saturated or aromatic hydrocarbons or by incorporation into bound forms improves their preservation and hence record of source information. Lignin components provide important evidence of watershed plant cover, and pigments reflect algal assemblages. The interplay of the factors influencing the organic matter content of lake sediments is illustrated by overviews of sedimentary records of four lake systems--Lake Biwa (Japan), Lake Greifen (Switzerland), Lake Washington (Pacific Northwest), and the Great Lakes (American Midwest).

Latest Paleocene benthic foraminiferal extinction and environmental changes at Tawanui, New Zealand

A major extinction of intermediate-water (500–1000 m) benthic foraminiferal species coincided with a major decrease in δ13C (2.8‰) of terrestrial organic matter (n-C29 alkane) and δ34S (20‰) of whole rock sulfide in a continuous siltstone sequence in the Tawanui Section (46°S paleolatitude) along the Akitio River, southeastern North Island, New Zealand, in the middle part of the uppermost Paleocene nannofossil zone (CP8). The benthic extinction (25% of species) occurred over ∼3 kyr at ∼55.5 Ma. Increases in kaolinite/illite and kaolinite/smectite ratios and in terrestrial organic carbon percentages started ∼3 kyr before the major benthic extinctions, lasted over ∼40 kyr, and probably reflect warmer climate and increased rainfall. The productivity of planktonic foraminifera and calcareous nannoplankton decreased ∼3 kyr prior to the major extinctions and recovered at the time of benthic extinctions. These events that started ∼3 kyr before the extinction can be best explained by warming, increased rainfall, reduced salinity of surface waters, and increased influence of warm saline deep water (WSDW). Benthic foraminiferal oxygen indices indicate a strong decrease in dissolved oxygen levels within the intermediate water from low oxic (1.5–3.0 mL/L O2) to suboxic (0.3–1.5 mL/L O2) conditions coinciding with the benthic extinctions. Increases in total organic carbon (TOC) and in the hydrocarbon-generating potential of kerogen (measured as the hydrogen index (HI)) agree with the interpretation of decreased dissolved oxygen levels of the intermediate water. The lowest oxygen conditions lasted ∼40 kyr and coincided with a decrease in calcareous benthic foraminiferal productivity, highest TOC levels, and lowest δ13C of terrestrial organic carbon. Dominant formation of WSDW or sluggish intermediate-water circulation caused by warming and high rainfall in high-latitude areas most likely led to the ∼3-kyr time lag between events on land and in surface waters preceeding the extinction and the development of dysaerobia in the sea, coinciding with the major benthic extinction and decrease in δ13C and δ34S in New Zealand. Global warming of deep and intermediate waters may have caused decomposition of methane hydrate in sediments, resulting in a strongly decreased δ13C of marine carbonates, promoting dysaerobia in the ocean, and warming global climate by increased methane concentrations in the atmosphere. Upwelling of WSDW, occurring soon after it became dominant in high-latitude areas, is likely responsible for the recovery of normal salinity and the concomitant recovery of planktonic foraminifera and calcareous nannoplankton productivity in high-latitude surface waters. Minor benthic foraminiferal extinctions (9% of species) occurred ∼40 kyr after the major extinctions, lasted ≤ ∼6 kyr, and coincided with the initiation of environmental recovery.

The Early Diagenesis of Organic Matter in Lacustrine Sediments

Lakes are diverse in their size, biological communities, watershed types, bottom morphology, and other important limnological factors which impact the accumulation and composition of their sediments. The diversity of lacustrine depositional systems is useful to organic geochemical studies. Different aspects of the early, as well as the more advanced, stages of diagenesis can be investigated, using lakes that have sedimentary characteristics which highlight the aspects of interest. Although sediment deposition in an individual lake is usually unique, such studies have led to generalizations about the processes affecting the organic matter contents of lacustrine sediments.

Source identification of Malaysian atmospheric polycyclic aromatic hydrocarbons nearby forest fires using molecular and isotopic compositions

We report measurements of molecular and carbon isotopic compositions of Malaysian atmospheric polycyclic aromatic hydrocarbons (PAHs) in smoke haze from the 1997 Indonesian forest fire. Comparison of the carbon isotopic compositions (δ13C) of individual PAHs from the smoke haze, with those from other PAHs sources (soot collected from gasoline and diesel vehicle muffler, woodburning smoke), enables us to discriminate among the diverse sources of atmospheric PAHs. Soot PAHs extracted from gasoline and diesel vehicles show heavy isotopic signatures with a large inter-species δ13C variation from −12.9‰ to −26.6‰, compared to soot PAHs extracted from woodburning smoke which are isotopically light, and have a small inter-species δ13C variation from −26.8‰ to −31.6‰. Values from −17.7‰ to −27.9‰ were obtained for the corresponding PAHs extracted from the smoke haze, indicating that they are derived mainly from automotive exhaust. Molecular and isotopic compositions of PAHs extracted from smoke haze were similar to those extracted from non-haze aerosol. Quantitative estimation shows that woodburning contribution to Malaysian atmospheric PAHs ranges from 25% to 35% with no relation to haze intensity, while automotive contribution ranges from 65% to 75%. These results suggest that the major contributor of PAHs in Malaysian air is automotive exhaust whether smoke haze is observed or not.

Diagenetic changes of lignin compounds in a more than 0.6 million-year-old lacustrine sediment (Lake Biwa, Japan)

A vertical profile of lignin in the upper 700 m layer of a 1400 m sediment core of Lake Biwa, an oligotrophic freshwater lake in Japan, was determined using a CuO oxidative degradation method. The results indicated that lignin is found throughout the core, demonstrating lignin to be very stable for over 0.6 million years. Moreover, the upper 250 m (approximately 0.6 million years old) segment of the sediment core was investigated to determine the apparent long term degradation rate of lignin. A downward lignin concentration decrease is observed over the upper 250 m of the core which corresponds to a calculated half life of at least approximately 40 × 104 years, assuming that lignin decrease is due to its in situ degradation (diagenesis).

Organic geochemistry of a lacustrine sediment (Lake Haruna, Japan)

Abstract Total organic matter, lipids, humic compounds, pheopigments, aliphatic and aromatic hydrocarbons, aliphatic mono- and dicarboxylic acids and sterols were analyzed for a 133-cm core sediment sample taken from a mesotrophic freshwater lake, Lake Haruna, Japan. Vertical distributions of major organic compounds can be interpreted in terms of the variation of their possible precursor materials (autochthonous or allochthonous) rather than their post-depositional chemical changes. Variations in the net production of the lake in the past 2500 years was estimated, indicating that a drastic change of the eutrophic level of the lake from oligotrophic to mesotrophic occurred approximately 1000 years ago.

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.

Spike of pyrosynthetic polycyclic aromatic hydrocarbons associated with an abrupt decrease in δ13C of a terrestrial biomarker at the Cretaceous-Tertiary boundary at Caravaca, Spain

The first vertical high-resolution record of polycyclic aromatic hydrocarbons (PAHs) of pyrosynthetic origin and the corresponding δ 13 C profile of a terrestrial biomarker across the Cretaceous-Tertiary (K-T) boundary at Caravaca, Spain, reveals the following. In comparison with adjacent Cretaceous marlstones, the first thin horizon (0 to +0.5 cm; 0 = the K-T boundary) of the boundary-clay layer is (1) enriched as much as 112 to 154 fold in typical pyrosynthetic PAHs such as coronene, benzo(g,h,i)perylene, and benzo(e)pyrene and (2) shows an abrupt δ 13 C decrease of 1.4‰–1.8‰ in terrestrial higher plant-derived n-C 29 alkane. The spike of pyrosynthetic PAHs associated with an abrupt decrease in δ 13 C value of a terrestrial biomarker is interpreted to reflect the prevalence of extensive fires with subsequent δ 13 C decrease in atmospheric CO 2 . It is estimated that the geologically instantaneous combustion of ∼18%–24% of the terrestrial above-ground biomass would be necessary to account for the measured negative isotopic shift at the K-T boundary, on the basis of carbon mass balance between terrestrial above-ground biomass and atmosphere.

Macromolecular material (humic substance) in the water column and sediments

Abstract The progress during the past 10–15 years in the studies of organic macromolecules (humic substances, HS) in seawater and sediments is reviewed. The review describes the advances in the characterization of seawater and sedimentary HS, HS formation models, studies on the geochemical role of marine HS, and future research needs. A macroreticular polystyrene resin (e.g. Amberlite XAD-2) has been generally used to isolate seawater HS. The HS isolated from seawater or sediments have been characterized by the analysis of lignin-phenols from CuO degradation, 1H and 13C NMR, stable carbon isotope analysis, pyrolysis-GCMS etc. The major conclusions of these analyses are: (1) seawater HS are essentially aliphatic and thought to be of phytoplankton origin and (2) sedimentary HS also have an aliphatic structure. The formation model of marine (both seawater and sedimentary) HS may be categorized into two groups: small molecular polymerization model which assumes that biopolymers are first degraded to small molecules and then re-polymerize to produce HS; and biopolymer degradation and polymerization model which assumes that HS are mainly or partly composed of refractory biopolymers and reaction products among refractory biopolymers, modified biopolymers, regenerated molecules, and the refractory biopolymers and their modified ones are the basic building blocks of humin (insoluble fraction of HS). However, analytical data are still insufficient to draw the conclusion. The effort to isolate a large amount of seawater HS and characterize both seawater HS and sedimentary HS by chemical and physical methods, which should be done with the active participation of biochemists, is strongly needed to draw the picture of chemical structure of marine HS and to confirm the HS formation models.

Generation of unsaturated and aromatic hydrocarbons by thermal alteration of young kerogen

Abstract Monounsaturated, monoaromatic and polyaromatic hydrocarbons generated by artificial thermal alteration of young marine (Tanner basin, offshore California) kerogen were studied by computerized GC-MS. Their relative amounts changed with temperature (150–410°C) and time (5–120 hr) of heating as follows: Monounsaturates → Monoaromatics → Polyaromatics. Distribution of alkyl homologs of phenanthrene also changed with increasing degree of thermal alteration. These results are in agreement with those observed for crude oils and shales.