Masao Minagawa

Stable carbon and nitrogen isotopic characterization of organic matter in a mangrove ecosystem on the southwestern coast of Thailand

Organic matter in a tropical mangrove ecosystem was characterized by stable carbon and nitrogen isotopic analyze, conducted on various organic samples, including land and mangrove plants, soils, particulate organic matter (POM), and sea and river sediments along the southwestern coast of Thailand. The δ13C values of land plants and POM in river water can be explained in terms of a greater influence of C3 plants than C4 plants in this area. The POM and sediments from the Trang River and Ko Talibong area showed systematically higher δ15N values than those from Ko Muk and other coastal areas. Organic matter in the Trang River might be influenced by nitrogen released from agricultural or human waste, which could affect the isotopic composition of POM and sediments in the Trang River estuary and along the coast near the river mouth. We used a stochastic method to estimate the contributions of four organic end-members, identifiable by their δ13C and δ15N values. The results implied that seagrasses were a major source of sedimentary organic matter, contributing 42 ± 5% in the Ko Muk area and 36 ± 5% in the Ko Talibong area. The contribution of coastal POM to sediments was estimated to be only 13% in Ko Muk and 19% in Ko Talibong. Mangrove plants contributed approximately 23% in both areas. It was concluded that seagrasses are an important source of sedimentary organic matter in this coastal region of southwestern Thailand.

Stable isotopes (13C/12C and 15N/14N) insettling organic matter of the northwestern Mediterranean Sea: biogeochemical implications

Within the framework of the High Frequency Flux (HFF) experiment (MATER programme), time-series sediment traps have been deployed for two months on the continental slope off Marseilles to measure downward particle fluxes at a high frequency sampling rate (two and six days). Combined isotopic analyses of carbon and nitrogen have been performed on selected samples. Both isotopic tracers have been used for the first time on organic material of the Mediterranean Sea (Gulf of Lions) to determine the main biological sources and to address the biogeochemical processes that affected this material. Settling particles were characterised by very low values of delta N-15 (near 0 parts per thousand) and delta C-13 (near -24.5 parts per thousand) that indicate the existence of a mixed material with two sources that differ according to the considered element (C or N). The terrestrial source slightly dominates the carbon pool because of its higher C:N ratio, whereas the nitrogen pool may mostly originate from N-2-fixing cyanobacteria (delta N-15 = 0 parts per thousand, low C:N ratio). These preliminary data suggest that dissolved atmospheric N, may act as a significant new nitrogen source in the Mediterranean Sea.

Removal of234Th from a coastal sea: Funka Bay, Japan

Abstract In Funka Bay of Hokkaido, Japan, seawater, suspended matter and settling matter were collected once every month in the summer of 1974. These samples were analyzed for234Th, a short-lived daughter of dissolved238U. A pronounced disequilibrium between234Th and238U, and a highly variable concentration of234Th were found. Positive correlation, however, exist among the deficiency of234Th relative to238U in seawater, the concentration of particulate234Th, the fraction of particulate234Th to total234Th in seawater, the total dry weight of suspended matter, and the primary productivity during the month previous to sampling. The specific activity of234Th for the settling particles (620 ± 170 dpm/g) was nearly equal to that for suspended particles (720 ± 600 dpm/g) but much greater than that for plankton (47 ± 24 dpm/g). These facts suggest that suspended particles are somehow closely related to the removal of heavy metals from seawater, in spite of the negligibly small settling flux of suspended matter. The residence time of thorium in Funka Bay (mean depth: 60 m) is found to be about 60 days, which is nearly equal to those of210Pb and210Po.

Behavior of heavy metals and particulate matters in seawater expected from that of radioactive nuclides

Vertical profiles of210Pb and230Th in the deep water were analyzed by using a simple one-dimensional model. Both nuclides are considered to settle down with the velocity of 1×10−4 cm/sec. The diameter of particle corresponding to the velocity is calculated to be 5μ, while only about 10 % of the nuclides can be collected on filter paper with a pore size of 0.5μ. It is supposed that the nuclides exist in particulate materials which is changeable in size. This suggestion is supported from the following evidences. (1) The directly observed behavior of marine snow and the size distribution of particles observed with a coulter counter. (2) The existence of many chemical elements of which residence time is about 150 years. (3) Their possible existence as eutectic solid phases in the seawater. (4) The consistency of the observed accumulation rate of pelagic sediments with that estimated from the settling velocity. (5) The consistency of the decomposition rate of organic matter in the deep water estimated from the oxygen comsumption with that from the settling velocity.

Sedimentary record of marine and terrigenous organic matter delivery to the Shatsky Rise, western North Pacific, over the last 130 kyr

Abstract Biomarker and carbon isotopic analyses of piston core samples were undertaken to reconstruct changes over the last 130 kyr in the delivery of organic matter from marine plankton and terrestrial sources to sediments on the Shatsky Rise in the western North Pacific. Marine and terrigenous organic matter delivery was reconstructed from alkenone and n-alkane concentrations using multiple regression analysis, which gave results consistent with a δ13C mass balance model. Marine organic carbon comprises more than 86% of the total organic carbon throughout the sediment record. Fluxes of marine organic carbon were higher in glacial times, indicating that marine production increased in these periods. We conclude that in addition to the southward shift of the Subarctic front, eolian dust supply at this location played an essential role in enhancing paleoproduction during the last glacial maximum.

The rise and fall of wild boar in a northern environment: Evidence from stable isotopes and subfossil finds

Here we use long-term data from Holocene archaeological bone assemblages in order to investigate how changes in climate, vegetation and human land use might have influenced the distribution of wild boar (Sus scrofa) in Norway during the Holocene. We combine four lines of evidence: an analysis of stable isotopes of carbon and nitrogen in a time series of subfossil teeth, distribution data of bone finds from 31 archaeological sites, changes in the relative abundance of wild ungulates from four of these sites and historical documents. A significant change in Sus diet, indicated by δ15N levels, was found. This coincided with the spread of agriculture and indicates a change from wild boar to domestic pigs. Based on this finding the distributional data show that the Norwegian wild boar was limited to the coastal broadleaved forests even during the most optimal climatic period. A reduction in the relative abundance of wild boar, and later extinction, seems to coincide with a reduction in such habitat, caused partly by climatic change but more pronouncedly by human habitat alterations, and competition with domestic animals. The data indicate that the primary factor limiting the northern distribution of wild boar is the availability of food resources.

δ15N of PON and Nitrate as a Clue to the Origin and Transformation of Nitrogen in the Subarctic North Pacific and Its Marginal Sea

Nitrogen isotope compositions of particulate organic matter and nitrate were analyzed for seawater sampled at five stations at the Alaskan Gyre, Western Subarctic Gyre and East China Sea, focusing on the samples from the surface to 5000 m water to characterize the nitrogen cycling in the subarctic North Pacific Ocean and its marginal sea. The δ15N of particulate organic matter showed little agreement with a conceptual closed model that interprets isotopic variation as being caused by isotope discrimination on nitrate utilization. The δ15N and δ13C of particulate organic matter varied with the water depth. A correlation between isotope compositions and C/N elemental ratio was found generally at all stations, although some irregular data were also found in deep layers. We developed a hypothetical nitrogen balance model based on N2 fixation and denitrification in seawater and attempted to apply it to distinguish nutrient cycling using both δ15N-NO3− and N* variation in seawater. This model was applied to the observed data set of δ15N-NO3− and N* in the North Pacific water and estimated the δ15N-NO3− of primordial nitrate in the North Pacific deep water as 4.8‰. The North Pacific intermediate water for all stations showed similar δ15N-NO3− and N* values of 6‰ and −3 µmol/kg, respectively, suggesting a similar nitrogen biogeochemistry. In the East China Sea, analysis showed evidence of water exchange with the North Pacific intermediate water but a significant influence of nitrogen from the river runoff was found in depths shallower than 400 m.

Preservation in 70% ethanol solution does not affect δ 13 C and δ 15 N values of reindeer blood samples – relevance for stable isotope studies of diet

We compared duplicate samples of whole blood samples from 18 reindeer that were preserved either by immediate freezing or by immersion in 70 % ethanol. All samples were dried at 60 °C, powdered, treated with 1:1 chloroform: methanol, and dried again before isotope analysis. There were no differences in the values of δ 13 C and δ 15 N between the methods of preservation. Isotopic differences were absolutely small (δ 13 C = 0.1±0.10/00; δ 15 N=0.2±0.20/00), random in direction, and within the limits of analytical precision for the mass spectrometer. Preservation in ethanol thus appears to be an effective and efficient method for preserving blood samples for stable isotope analysis under field conditions. Abstract in Norwegian / Sammendrag: Konservering av blodprover fra rein i 70% etanollosning pavirker ikke verdiene av δ 13 C and δ 15 N–verdiene og er en fullgod metode for analyse av stabile isotoper Vi sammenlignet to og to prover av blodprover fra 18 reinsdyr. Provene var enten konservert ved umiddelbar frysing eller ved bruk av 70% etanol. Alle prover ble torket ved 60 °C, pulverisert og behandlet med kloroform:metanol i forholdet 1:1. Til slutt ble de torket pa nytt for gjennomforing av isotopanalysen. Vi fant ingen forskjell i verdiene av δ 13 C and δ 15 N mellom de to konserveringsmetodene. I absolutte verdier var isotopforskjellene sma (δ 13 C = 0.1±0.1 0/00; δ 15 N=0.2±0.2 0/00). Forskjellene var tilfeldige og innenfor grensene for massespektrometerets presisjon. Bruk av etanol framstar som en effektiv og fullgod metode til konservering av blodprover for analyse av stabile isotoper under feltforhold.

Assessment of denitrification process in lower Ishikari river system, Japan

Sediment denitrification rate and its role in removal of dissolved nitrate load in lower Ishikari river system were examined. Denitrification rate were measured using acetylene inhibition technique on the sediment samples collected during August 2009-July 2010. The denitrification rate varied from 0.001 to 1.9 μg Ng(-1) DM h(-1) with an average value of 0.21 μg Ng(-1) DM h(-1) in lower Ishikari river system. Denitrification rate showed positive correlation with dissolved nitrate concentration in the river basin, indicating overlying water column supplied nitrate for the sediment denitrification processes. Nutrient enrichment experiments result showed that denitrification rate increased significantly with addition of nitrate in case of samples collected from Barato Lake however no such increase was observed in the samples collected from Ishikari river main channel and its major tributaries indicating that factors other than substrate concentration such as population of denitrifier and hydrological properties of stream channel including channel depth and flow velocity may affects the denitrification rate in lower Ishikari river system. Denitrification rate showed no significant increase with the addition of labile carbon (glucose), indicating that sediment samples had sufficient organic matter to sustain denitrification activity. The result of nutrient spiraling model indicates that in- stream denitrification process removes on an average 5%d(-1) of dissolve nitrate load in Ishikari river. This study was carried out to fill the gap present in the availability of riverine denitrification rate measurement and its role in nitrogen budget from Japanese rivers characterize by small river length and high flow rate.