Is seawater geochemical composition recorded in marine carbonate? Evidence from iron and manganese contents in Late Devonian carbonate rocks

Iron and manganese are the important redoxsensitive elements in the ocean. Previous studies have established a series of paleo-depositional redox proxies based on the form and content of iron in sedimentary rocks(e.g., degree of pyritization, FeHR/FeT, Fe/Al). These proxies were developed and applied on siliciclastic-rich marine sediments. Although marine carbonate rocks are generally considered to preserve the geochemical signals of ancient seawater, neither Fe nor Mn content in marine carbonate rocks(Fecarb, Mncarb) has been independently used as a proxy to quantify environmental cues in paleo-oceans. Both Fe and Mn are insoluble in oxic conditions(Fe_2O_3, Fe(OH)_3,MnO_2), while their reduced forms(Fe^(2+) and Mn^(2+)) are soluble. Therefore, oxic seawater should have low concentrations of dissolved Fe^(2+)and Mn^(2+), and accordingly carbonate rocks precipitated from oxic seawater should have low Fecarband Mncarb, and vice versa. To evaluate whether Fecarband Mncarbcan be used to quantify oxygen fugacity in seawater, we measured Fecarband Mncarbof Upper Devonian marine carbonate rocks collected from nine sections in South China. Fecarbof intraplatform basin samples wassignificantly higher than that of shelf samples, while shelf and basin samples had comparable Mncarb. The modeling result indicates that the dramatic difference in Fecarbcannot be explained by variation in oxygen fugacity between the shelf and basin seawater. Instead, both Fecarband Mncarb appear to be more sensitive to benthic flux from sediment porewater that is enriched in Fe^(2+)and Mn^(2+). Porewater Fe^(2+)and Mn^(2+)derive from bacterial iron and manganese reduction; flux was controlled by sedimentation rate and the depth of the Fe(Mn) reduction zone in sediments, the latter of which is determined by oxygen fugacity at the water–sediment interface. Thus, high Fecarbof the basin samples might be attributed to low sedimentation rate and/or low oxygen fugacity at the seafloor. However, invariant Mncarbof the shelf and basin samples might be the consequence of complete reduction of Mn in sediments. Our study indicates that marine carbonate rocks may not necessarily record seawater composition, particularly for benthic carbonate rocks. The influence of benthic flux might cause carbonate rocks' geochemical signals to deviate significantly from seawater values. Our study suggests that interpretation of geochemical data from carbonate rocks, including carbonate carbon isotopes, should consider the process of carbonate formation.

Effect of rifampicin pre-and post-treatment on rotenone-induced dopaminergic neuronal apoptosis and alpha-synuclein expression

BACKGROUND： Rifampicin inhibits the formation of a-synuclein multimer and protects against 1-methyl-4-phenyl-1,2, 3, 6-tetrahydropyritine （MPTP）-induced PC12 cell apoptosis. OBJECTIVE： To compare the effect of rifampicin pre- and post-treatment on tyrosine hydroxylase and α-synuclein expression in substantia nigra pars compacta in a rat model of Parkinson＇s disease. DESIGN, TIME AND SE＇B＇ING： A randomized, controlled experiment was performed at the Experimental Animal Center of Sun Yat-sen University North Campus （China） from November 2006 to October 2008. MATERIALS： Rifampicin was purchased from MD, USA; rotenone was purchased from Sigma, USA; mouse anti-rat α-synuclein monoclonal antibody was purchased from B＆D, USA; and rabbit anti-rat tyrosine hydroxylase monoclonal antibody was purchased from Chemicon, USA. METHODS： A total of 72 male, Sprague Dawley rats, aged 8 weeks, were randomly assigned to 5 groups： blank control （n = 12）, rifampicin （n = 12）, rotenone （n = 16）, rifampicin pre-treatment （n = 16）, and rifampicin post-treatment （n = 16）. Parkinson＇s disease model rats were established via a subcutaneous injection of rotenone （1.5 mg/kg per day） in the three treatment groups, once a day for 3 successive weeks. Rifampicin （30 mg/kg per day） was intragastrically administered in the rifampicin pre-treatment group 3 days prior to rotenone induction and in the rifampicin post-treatment group 7 days after rotenone induction. Rats were treated with a subcutaneous injection of 1 mL/kg per day sunflower oil in the blank control group and an intragastric injection of 30 mg/kg per day rifampicin in the rifampicin group, once a day for 3 successive weeks in total. MAIN OUTCOME MEASURES： Prior to treatment and in the end of the 3^rd week after treatment, the rats were evaluated using the modified neurological severity score. The substantia nigra from the rats was extracted for hematoxylin-eosin staining. Western blot analysis was performed to determine tyrosine hydroxylase and α-synuclein expression. RESULTS： Hematoxylin-eosin staining revealed a significant reduction in the number of substantia nigral neurons in the rotenone group, in addition to neurodegradation, hypopigmentation, and pyknosis. In the rifampicin pre-treatment and post-treatment groups, the number of dopaminergic neurons was significantly increased compared with the rotenone group （P 〈 0.01）, with slight neuronal damage. Compared with the rotenone group, substantia nigral tyrosine hydroxylase expression was significantly increased in the rifampicin pre-treatment and post-treatment groups （P 〈 0.01）, but α-synuclein expression and modified neurological severity scores were significantly decreased （P 〈 0.01）. In addition, the effect of rifampicin in the pre-treatment group was superior to the post-treatment group. There was no significant difference in tyrosine hydroxylase and α-synuclein expression, or in the modified neurological severity scores, between the blank control and rifampicin groups （P 〉 0.05）. CONCLUSION： Rifampicin significantly attenuated neuropathological and behavioral motor deficits induced by rotenone. Moreover, rifampicin enhanced tyrosine hydroxylase expression, but inhibited α-synuclein expression. The effect of rifampicin pre-treatment was superior to rifampicin post-treatment.

Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater.Mg isotopes are significantly(up to 5%)fractionated during carbonate precipitation by preferential incorporation of ^(24)Mg,while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of^( 26)Mg.Thus,the marine Mg cycle could be reflected by seawater Mg isotopic composition(δ^(26)Mg_(sw)),which might be recorded in marine carbonate.However,carbonates are both texturally and compositionally heterogeneous,and it is unclear which carbonate component is the most reliable for reconstructing δ^(26)Mg_(sw).In this study,we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China.Based on petrographic studies,four carbonate components were recognized:micrite,marine cement,brachiopod shell,and mixture.The four components had distinct δ^(26)Mg:(1)micrite samples ranged from -2.86% to -2.97%;(2)pure marine cements varied from -3.40% to -3.54%,while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range(-3.27% to-3.75%);(3)values for the mixture component were-3.17% and -3.49%;and (4)brachiopod shells ranged from -2.20% to -3.07%,with the thickened hinge area enriched in ^( 24)Mg.Due to having multiple carbonate sources,neither the micrite nor the mixture component could be used to reconstruct δ^(26)Mg_(sw).In addition,the marine cement was homogenous in Mg isotopes,but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ^(26)Mg_(sw).Furthermore,brachiopod shells had heterogeneous C and Mg isotopes,suggesting a significant vital effect during growth.Overall,the heterogeneous δ^(26)Mg of the Huangjin limestone makes it difficult to reconstruct δ^(26)Mg_(sw)using bulk carbonate/calcareous sediments.Finally,δ^(26)Mg_(sw)was only slightly affected by the faunal composition of carbonate-secreting organisms,even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range(0.2%–4.8%)of fractionation during biogenic carbonate formation.

Validating the deep time carbonate carbon isotope records: effect of benthic flux on seafloor carbonate

It is a consensus that marine carbonate archives the isotopic composition of seawater dissolved inorganic carbon(DIC,δ13Csw),the largest active C reservoir in the hydrosphere.Carbonate carbon isotope(δ13Ccarb)excursions have been used to reflect perturbations of the global carbon cycle and related environmental change.However,the deep timeδ13Ccarb records indicate faster and more pronounced perturbations of the carbon cycle compared to the present day.Here,we reportδ13Ccarb and elemental compositions of Late Paleozoic carbonate sections from South China,showing negative correlations betweenδ13-Ccarb and Fe^(2+)content of carbonate(Fecarb).We suggest that,because Late Paleozoic carbonate was mainly produced by benthic carbonate-secreting organisms,δ13Ccarbrecorded the isotopic composition near the seafloor,where benthic flux derived from anaerobic organic matter degradation delivers both Fe2+and 13C-depleted DIC from porewater.The binary mixing between seawater and benthic flux would result in the deviation ofδ13Ccarb fromδ13Csw.The negative correlation implies thatδ13Ccarb is influenced by benthic flux and is affected by the seafloor redox and sedimentation rate.The deep time spatially heterogeneous and temporally oscillatoryδ13Ccarb records in the basin-scale could be alternatively attributed to the variations of local environmental factors rather than aδ13Csw depth-gradient.Thus,the seafloor carbonate precipitation is continuously affected by diagenetic reactions in sediments,suggesting thatδ13Ccarb recording the seawater DIC composition is conditional.Our study urges that the interpretation ofδ13Ccarb should also consider the sedimentary process and depositional environment of marine carbonate.

Revisiting the Mg isotopic systematics of siliciclastic components of sediments and sedimentary rocks:A new geochemical proxy of continental weathering in Earth's history

The Mg isotopic compositions of siliciclastic components(δ^(26)Mg_(sili))of sediments and sedimentary rocks have been commonly used to constrain the intensity of continental weathering,based on observations of,(1)an upward enrichment of26Mg in modern weathering profiles,(2)preferential uptake of26Mg in soil clays,and(3)general positive correlations between weathering intensities andδ^(26)Mg_(sili)values.However,not all weathering profiles display an upward increase ofδ^(26)Mg_(sili),and not all soil clays enrich in26Mg,complicating the interpretations ofδ^(26)Mg_(sili)data of sediments and sedimentary rocks.To further explore the controls ofδ^(26)Mg_(sili)of sediments and sedimentary rocks,here we measuredδ^(26)Mg_(sili),mineralogical compositions and major element compositions of deep sea sediments from South China Sea(SCS)and carbonate rock samples from two late Paleozoic sections in South China.Carbonate samples show decreasing trends of both normalized Mg concentration(Mgnorm)andδ^(26)Mg_(sili)values with the increase of weathering intensity,while SCS sediments samples do not show any correlation between Mgnormorδ^(26)Mg_(sili)values and weathering intensity.A three-stages weathering model is developed to explain the observed Mgnorm andδ^(26)Mg_(sili)data.In the new model,weathering can be divided into three stages,(1)the preliminary weathering stage characterized by dissolution of primary minerals and formation of secondary clay minerals in saprolite,(2)the transitional stage with massive vermiculite and chlorite formation on the bottom of soil,and(3)the advanced stage showing dissolution of vermiculite and chlorite in the top of soil.The new model explains anδ^(26)Mg_(sili)increase and Mgnormdecrease with an increase weathering intensity in preliminary weathering stage and decrease of Mgnormandδ^(26)Mg_(sili)with increase weathering intensity in advanced weathering stage.The revised weathering model warrants the application of Mg isotope in the study of continental weathering in deep time.