Distribution of authigenic and allogenic fractions in recent lake sediment: Isotopic and chemical compositions
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Recent sediments in Lake Kozjak, in the Plitvice Lakes National Park, Croatia, were used to study the impact of the material delivered to the lake sediment via Rječica Brook. Sediment cores, top 40 cm, were taken near the mouth of the Rječica Brook at three locations in the lake at different distances from the shore, water depth 2-9 m and analyzed using different isotopic and chemical methods. Sediment depth profiles of the following parameters were obtained: organic and carbonate fraction, C/N analyses of organic fraction, 14C activity (a14C) and d13C values of carbonate (d13Ccarb) and organic fraction (d13Corg) and d15N of total nitrogen. The results were compared with sediment core collected in the middle of Lake Kozjak, water depth 21 m. Distribution of organic matter, C/N values, d13Corg and d13Ccarb in sediment profiles showed strong influence of terrestrial organic and carbonate fraction run-off via Rječica Brook which decreased rapidly in the direction of the open lake. The sedimentation rate as well as the ratio of allogenic and authigenic fraction in sediments was estimated. Lower d13Corg values in surface sediments might be a consequence of the increase in algal productivity but might also be a contribution of fossil fuel combustion. The anthropogenicaly induced 14C in the atmosphere in 1963/64 is visible though shadowed in sediments.Keywords:
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ABSTRACT Carbonate concretions and host shales from the Cretaceous Northern Shimanto Belt in southwestern Japan were studied. There are three types of carbonate concretions: spherical micritic, lenticular micritic, and lenticular crystalline. They contain calcite, clay minerals, framboidal pyrite, and detrital quartz and feldspar. The concretions have carbon isotopic compositions between -17.3 and -6.6 (PDB), reflecting the contribution of light carbon derived from organic matter. Distribution patterns of elements in the histogram of Al2O3/TiO2, and in the diagrams of Zr-TiO2, SiO2/TiO2-Al2O3/TiO2, and Zr-K2O-Rb suggest that the concretions contain more acidic tuff than do the host shales. Supply of acidic tuff to the site of deposition is also suggested by the presence of arcuate or cuspate detrital quartz grains in the samples. The formation of authigenic carbonates was probably related to an increase in influx of acidic tuff. In particular, the formation of the lenticular micritic concretions is a good representative.
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碳酸盐是湖泊沉积物的组成部分,其碳、氧同位素组成是恢复湖区古气候/古环境的有效代用指标.沉积物碳酸盐包括物源区带来的外源碳酸盐和湖泊内生沉淀产生的自生碳酸盐,其中只有自生碳酸盐才具有古气候指示意义.故沉积物碳酸盐来源辨识是开展碳酸盐古环境记录研究的基础和前提.通过多种方法的综合判别,证明了程海沉积物碳酸盐主要是自生碳酸盐,为开展碳酸盐记录研究提供了可靠依据.程海是开展碳酸盐碳氧同位素与古气候研究的理想场所,尤其值得深入研究.;The research of stable carbon and oxygen isotope of Carbonate of lake sediments, is one of the most effective indices to reconstruct paleoclimatic and paleoenvironmental information. Sedimentary carbonate consists mainly of allochthonous carbonates produced by surface runoff and authigenic carbonates produced by inorganic precipitation. However, the authigenic carbonate has the paleoclimatic and paleoenvironmental meaning. Consequently, distinguishing the source of carbonate is the basis and premise for the research of sedimentary carbonates. This paper utilized various methods to prove that the carbonate in Lake Chenghai,Yunnan Province was authigenic, which provided the basis for the research of sedimentary carbonates. Lake Chenghai provides an ideal lacustrine setting for research of stable carbon and oxygen isotope to reconstruct paleoclimate, and the future research will be deserved.
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Abstract The accumulation of authigenic carbonate in marine sediments has been regarded as a significant contributor to the carbon cycle over Earth's history. However, accurate quantification of the extent of authigenic carbonate formation in both modern and ancient oceans has been challenging due to limited approaches. Here, we analyzed calcium, magnesium, dissolved inorganic carbon, and alkalinity pore‐water profiles of 153 sites influenced by methane diffusion from the northern South China Sea. By combining these data with published data of diffusive sulfate flux, we found a significant positive correlation between carbonate precipitation rate and sulfate flux in the subseafloor. This correlation can be used to quantify the regional extent of authigenic carbonate formation, representing an advantage over a spatial interpolation approach. We therefore propose that the observed correlation can serve as a new approach for quantifying authigenic carbonate formation in shallow sediments along continental margins for both modern and ancient oceans.
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Abstract Authigenic carbonate was recently invoked as a third major global carbon sink in addition to primary marine carbonate and organic carbon. Distinguishing the two carbonate sinks is fundamental to our understanding of Earth’s carbon cycle and its role in regulating the evolution of atmospheric oxygen. Here, using microscale geochemical measurements of carbonates in Early Triassic strata, we show that the growth of authigenic carbonate follows a different trajectory from primary marine carbonate in a cross-plot of uranium concentration and carbon isotope composition. Thus, a combination of the two geochemical variables is able to distinguish between the two carbonate sinks. The temporal distribution of authigenic carbonates in the Early Triassic strata suggests that the increase in the extent of carbonate authigenesis acted as a negative feedback to the elevated atmospheric CO 2 concentration.
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