He, Ne and Ar isotopic composition of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

The noble gas nuclide abundances and isotopic ratios of the upmost layer of Fe-Mn crusts from the western and central Pacific Ocean have been determined. The results indicate that the He and Ar nuclide abundances and isotopic ratios can be classified into two types: low He-3/He-4 type and high He-3/He-4 type. The low He-3/He-4 type is characterized by high He-4 abundances of 191x10(-9) cm(3.)STP(.)g(-1) on average, with variable He-4, Ne-20 and Ar-40 abundances in the range (42.8-421)x10(-9) cm(3.)STP(.)g(-1), (5.40-141)x10(-9)cm(3.)STP(.)g(-1), and (773-10976)x10(-9) cm(3.)STP(.)g(-1), respectively. The high He-3/He-4 samples are characterized by low He-4 abundances of 11.7x10(-9) cm(3.)STP(.)g(-1) on average, with He-4, Ne-20 and Ar-40 abundances in the range of (7.57-17.4)x10(-9) cm(3.)STP(.)g(-1), (110.4-25.5)x10(-9) cm(3.)STP(.)g(-1) and (5354-9050)x10(-9) cm(3.)STP(.)g(-1), respectively. The low He-3/He-4 samples have He-3/He-4 ratios (with RIRA ratios of 2.04-2.92) which are lower than those of MORB (R/R-A=8 +/- 1) and Ar-40/Ar-36 ratios (447-543) which are higher than those of air (295.5). The high He-3/He-4 samples have He-3/He-4 ratios (with R/R-A ratios of 10.4-12.0) slightly higher than those of MORB (R/R-A=8 +/- 1) and Ar-40/Ar-36 ratios (293-299) very similar to those of air (295.5). The Ne isotopic ratios (Ne-20/Ne-22 and Ne-21/Ne-22 ratios of 10.3-10.9 and 0.02774-0.03039, respectively) and the Ar-38/Ar-36 ratios (0.1886-0.1963) have narrow ranges which are very similar to those of air (the Ne-20/Ne-22, Ne-21/Ne-22, Ar-38/Ar-36 ratios of 9.80, 0.029 and 0.187, respectively), and cannot be differentiated into different groups. The noble gas nuclide abundances and isotopic ratios, together with their regional variability, suggest that the noble gases in the Fe-Mn crusts originate primarily from the lower mantle. The low He-3/He-4 type and high He-3/He-4 type samples have noble gas characteristics similar to those of HIMU (High U/Pb Mantle)- and EM (Enriched Mantle)-type mantle material, respectively. The low He-3/He-4 type samples with HIMU-type noble gas isotopic ratios occur in the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain and the Mid-Pacific Seamounts whereas the high He-3/He-4 type samples with EM-type noble gas isotopic ratios occur in the Line Island Chain. This difference in noble gas characteristics of these crust types implies that the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain, and the Mid-Pacific Seamounts originated from HIMU-type lower mantle material whereas the Line Island Chain originated from EM-type lower mantle material. This finding is consistent with variations in the Pb-isotope and trace element signatures in the seamount lavas. Differences in the mantle surce may therefore be responsible for variations in the noble gas abundances and isotopic ratios in the Fe-Mn crusts. Mantle degassing appears to be the principal factor controlling noble gas isotopic abundances in Fe-Mn crusts. Decay of radioactive isotopes has a negligible influence on the nuclide abundances and isotopic ratios of noble gases in these crusts on the timescale of their formation.

Geochemistry of the lava and its implications in Musicians Seamounts

Chemical and isotopic data of the lava samples dredged in the southern Bach Ridge and the northern Italian Ridge of the Musicians Seamounts province, northeast of Hawaii. Although most of the samples analyzed are generally altered, a few are fresh. The latter exhibits similar geochemical and isotopic characteristics to normal MORB (Mid-Ocean Ridge Basalts). There are systematic geochemical trends from hotspot to mid-ocean ridge in the province. Incompatible element and isotopic variations suggest that the flow field had at least two distinct parental magmas, one with higher and one with lower MgO concentrations. The two parental magmas could be related by a magma mixing model. The major and trace element modeling shows that the two parental magmas could not have been produced by different degrees of melting of a homogeneous mantle source, but they are consistent with melting of a generally depleted mantle containing variable volumes of embedded enriched heterogeneity enriched interbeds.

Stable carbon and nitrogen isotopic compositions of high molecular weight dissolved organic matter from four US estuaries

High molecular weight dissolved organic matter (HMW-DOM) represents an important component of dissolved organic carbon (DOC) in seawater and fresh-waters. In this paper, we report measurements of stable carbon (delta(13)C) isotopic compositions in total lipid, total hydrolyzable amino acid (THAA), total carbohydrate (TCHO) and acid-insoluble "uncharacterized" organic fractions separated from fourteen HMW-DOM samples collected from four U.S. estuaries. In addition, C/N ratio, delta(13) C and stable nitrogen (delta(15)N) isotopic compositions were also measured for the bulk HMW-DOM samples. Our results indicate that TCHO and THAA are the dominant organic compound classes, contributing 33-46% and 13-20% of the organic carbon in HMW-DOM while total lipid accounts for only <2% of the organic carbon in the samples. In all samples. a significant fraction (35-49%) of HMW-DOM was included in the acid-insoluble fraction. Distinct differences in isotopic compositions exist among bulk samples, the compound classes and the acid-insoluble fractions. Values of delta(13)C and delta(15)N measured for bulk HMW-DOM varied from -22.1 to -30.1parts per thousand and 2.8 to 8.9parts per thousand, respectively and varied among the four estuaries studied as well. Among the Compound classes, TCHO was more enriched in C-13 (delta(13)C = -18.5 to -22.8parts per thousand) compared with THAA (delta(13)C = -20.0 to -29.6parts per thousand) and total lipid (delta(13)C = -25.7 to -30.7parts per thousand). The acid-insoluble organic fractions, in general, had depleted C-13 values (delta(13)C = -23.0 to -34.4parts per thousand). Our results indicate that the observed differences in both delta(13)C and delta(15)N were mainly due to the differences in sources of organic matter and nitrogen inputs to these estuaries in addition to the microbial processes responsible for isotopic fractionation among the compound classes. Both terrestrial sources and local sewage inputs contribute significantly to the HMW-DOM pool in the estuaries studied and thus had a strong influence on its isotopic signatures. Copyright (C) 2004 Elsevier Ltd.

Ablation of hydrogen isotopic spherical pellet due to the impact of energetic ions of the respective isotopes

The ablation rate of a hydrogen isotopic spherical pellet G(is) due to the impact of energetic ions of the respective isotopes and its scaling law are obtained using the transsonic neutral-shielding model, where subscript s might refer to either hydrogen or deuterium. Numerical results show that if E0s/E0e2 greater-than-or-equal-to 1.5, G(is)/G(es) greater-than-or-equal-to 20%, where E0s and E0e are the energy of undisturbed ion and electron, respectively, and G(es) is the ablation rate of a pellet due to the impact of electrons. Hence, under the NBI heating, the effect of the impact of energetic ions on the pellet ablation should be taken into consideration. This result also gives an explanation of the observed enhancement of pellet ablation during NBIH.

Diel isotopic fluctuation in surface seston and its physiological and ecological implications

We reported diet fluctuation in isotopic composition of surface seston from two connected lakes in China, oligotrophic Lake Fuxian and eutrophic Lake Xingyun. The decrease in nighttime and the increase in daytime of isotope signatures of seston might be attributed to the light-dependent balance between the photosynthesis and the respiration of phytoplankton and to the changes in the species composition and the relative abundance of phytoplankton functional groups at the water's surface in diel growth. The relatively high isotopic signatures and the large-extent diel fluctuation of phytoplankton in the eutrophic lake could be due to utilization of heavy-isotope-enriched inorganic sources and the high primary productivity. Extent of diel fluctuation in delta C-13 and delta N-15 of phytoplankton were relatively small compared with the isotopic enrichment per trophic transfer and thus might have negligible effect on the source identification and the trophic evaluation of consumers.