A Study Of Iron And Sulfur Species And Sulfur Isotope Geochemistry In Marine Sediments From Gas Hydrate-bearing Regions:Implications For Sulfate-methane Transition Zone

In early diagenesis of continental margin marine sediments,the sulfate-methane transition zone(SMTZ)is an important transport and reaction-controlled boundary where sulfate and methane are co-consumed by anaerobic oxidation of methane(AOM)and heavy sulfur isotope(34S)tend to be significantly enriched in iron sulfides(mainly pyrite).Considering that the early-formed iron sulfides can be maintained during long-term burial of late diagenesis for a scarcity of pore water sulfate,the occurrences of isotopically heavy iron sulfides preserved in ancient sediment column can be regarded as a proxy for identifying the locations of the former SMTZ occurred in the geological past,namely"paleo-SMTZ".However,for this criterion,no lower limit to the sulfur isotopic composition of these 34S-enriched iron sulfides has been specified and there is no constraint on the size and form of pyrite.Nevertheless,what is more critical is that,this criterion can only tell us where the paleo-SMTZ had swept but not its depth feature at that time.On the other hand,only very rare studies have reported that authigenic gypsum formed within deep-seawater sediments worldwide and,even more troubling,if the origin of such gypsum is authigenic is still obscure.Furthermore,recent discoveries of authigenic gypsum in cold seep and/or gas hydrate-bearing sediments are fewer and its relation to the methane-gas hydrate system is still not really clear.Two suites of marine core sediments were chosen here from Sites 1245 and 1252 recovered by Ocean Drilling Program(ODP)Leg 204 at south Hydrate Ridge and from four piston cores 14-P,17-P,23-P,and 25-P sampled by R/V Ocean Researcher I during Cruise 860 at Kaoping Slope.To solve the scientific problem above,a series of works were made at first,including revising the experimental and analytical methods for iron and sulfur as well as developing the early diagenetic transport-reaction model.Also,by combining other conventional methods of analysis and measurement,the morphology and geochemistry of iron and sulfur species as well as authigenic pyrite and gypsum from these two study areas have been analyzed and discussed.It mainly includes:(1)Based on the effectiveness of iron extractions for the pure iron-bearing minerals tested in this study,the sequential extraction procedure for iron in marine sediments has been improved that combines the revised extration method for reactive iron and the selective extration method for pyrite iron.Using this improved method,five iron species can be sequentially extracted which contain a range of almost all kinds of iron-bearing minerals in a marine setting,including:(a)buffered sodium acetate solution extractable iron(FeA),including carbonates associated iron(Fecarb)and acid volatile sulfide(AVS)iron(FeAvs);(b)buffered sodium dithionite solution extractable iron(FeD),principally ferric(hydr)oxide iron(Feox1+2);(c)buffered ammonium oxalate solution extractable iron(FeO),principally magnetite iron(Femag);(d)hydrofluoric acid(HF)extractable iron(FeHF),principally from silicates;(e)nitrate acid(HNO3)extractable iron(FeHNO3),equivalent to pyrite iron(Fepy).(2)Using the widely accepted method,four sulfur species can be extracted which contain a range of almost all kinds of sulfur-bearing minerals in a marine setting,including:(a)AVS sulfur(SAvs),including 100%of mackinawite,75%of greigite,and pyrrhotite with unknown recovery;(b)chromium reducible sulfur(SCRS),including pyrite sulfur and elemental sulfur(SES);(c)acid dissoluble sulfate sulfur(SADS),including pore water sulfate sulfur and gypsum sulfur;(d)water dissoluble sulfate sulfur(SWDS),approximately equivalent to SADS.According to the effectiveness of sulfur extractions for the pure sulfur-bearing minerals tested in this study,the water dissolution method and the correction for SWDs by removing the original pore water sulfate fraction(Srevised WDS)can provide greater simplicity,accuracy,and specificity for the determination of gypsum sulfur than the method of acid dissolution.(3)To avoid the time-consuming preparation procedures when using traditional sulfur isotope measurement method in which the aqueous solution extracted as SWDS should be first precipitated as barium sulfate,a new method for direct sulfur isotope measurement by multiple-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)after dilution was tested.During this test,the matrix effects on sulfur isotope measurement of sulfate solution by MC-ICP-MS caused by element calcium as the matrix were studied carefully and thoroughly.(4)A series of comparative analyses on the iron and sulfur species were developed and then the mineral assemblage extracted as each species in the sediments from Hydrate Ridge and Kaoping Slope can be recognized and verified.Among them,it is worth mentioning that,a new diagnostic plot scheme by comparing the extractions of FeD and Srevised WDS has been proposed to evaluate the artificial oxidation loss of iron sulfides on sample sitting and drying,which is also suitable to distinguish the gypsum origin in non-fresh sediments and further provides a fresh insight to reveal authigenic gypsum in deep-seawater sediments.Before that,no convincing solution has yet emerged for this recognition.In addition,only based on morphology,it is not enough to recognize if the origin of gypsum is deep-sea authigenic or artificial.Afterward,according to the geologic background at Hydrate Ridge and Kaoping Slope,the effectiveness and applicability of the content ratio between highly reactive iron and total iron(FeHR/FeT)as a proxy for evaluating redox deposition condition have been discussed and then the reason why this criterion falls at these two study areas has further been explained.(5)Adapted from the method of foraminifera separation in palaeooceanography,the quantitative separation of coarse graded pyrite and gypsum from the Hydrate Ridge and Kaoping Slope sediments and the subsequent analyses of morphology and sulfur isotope geochemistry were developed.By combining the extraction results of SCRS from total pyrite,the framboid sizes(2nd order)and sulfur isotopic compositions between different aggregate forms(1st order)of pyrite were compared.From this,the origin of the pyrite with different sizes and forms during early diagenesis has been suggested,and then a detail discussion of the indicator significance of each pyrite form to the occurrence of paleo-SMTZ has further been given.After the authigenic origin of the gypsum in the Hydrate Ridge sediments was determined,a comparative analysis of the sulfur isotopic compositions was developed among the coarse graded gypsum with different forms,the SCRS extracted from total pyrite,and the SWDS extracted from total sulfate to put forward the origin of the gypsum with different forms.After that,based on the correlation between gas hydrate-bearing horizon and gypsum form,the relationship between the rosette form of gypsum and the crystallization of gas hydrate has been explored.(6)Taking Sites 994 and 995 recovered by ODP Leg 164 at Blake Ridge as an example,an early diagenetic steady-state transport-reaction model was constructed to develop a theory that quantitatively relates the content and sulfur isotopic composition of iron sulfides to the depth of the SMTZ.Based on this correlation,a contour plot of the SMTZ depth(z value)can be expressed as a function of the content and sulfur isotopic composition of iron sulfides(x-and y-axes)and then a new method for reconstructing the depth of the paleo-SMTZ has been proposed for the first time by interpolation technique.After successful verification that the paleo-SMTZ never rose to 15 mbsf in the past 100 ka at Sites 994 and 995,the transport-reaction model was used to simulate the concentrations/contents and sulfur isotopic compositions of sulfate,methane,hydrogen sulfide,and iron sulfide sulfur at Sites 1245 and 1252,ODP Leg 204.Using the site-specific and simulation parameters as the constraint conditions,the method for reconstructing the paleo-SMTZ depth was further applied to the Hydrate Ridge sites whereby how the depth of the paleo-SMTZ evolved has been quantitatively obtained.Combined with the biostratigraphy age,regional tectonic evolution,and marine isotope stages as references,the stability of sedimentary environment and the dynamic evolution of methane-gas hydrate sysytem over the geological past at Hydrate Ridge have further been restored and disscussed.