TEM characterization of stacking relations in I/S of expanded shale samples from the Gulf Coast and Michigan Basin was carried out to address the issues of the degree of coherency and the nature of layer stacking sequences in smectite, I/S and illite. The two-dimensional lattice fringe images obtained from this study show that cross fringes are commonly observed to be continuous over at least 3–4 layers for smectite, 6–7 layers for ordered I/S and 9–10 layers for illite-rich I/S. This demonstrates that such sequences are coherent, or at least semi-coherent (in smectite) units (MacEwan crystallites). The observations demonstrate that so-called fundamental particles are fragments of MacEwan crystallites formed primarily as a result of disaggregation along weakly-bonded smectite interlayers. However, both 0k1 and h01 reflections may coexist in selected area electron diffraction (SAED) patterns. The frequency of occurrence of the coexistence in SAED patterns decreases in the order smectite, I/S and illite for Gulf Coast samples. This is consistent with the presence of turbostratically-related interfaces in packets of all of these materials. Therefore, any given layer sequence in smectite, ordered I/S and illite may have both turbostratic and coherent interfaces. The proportion of coherently-related layers increases with increasing proportion of illite-like layers. The concept of fundamental or elementary particles is not related to layer sequences in non-disaggregated, original rocks. Indeed, it implies relations that are not valid.

The lattice fringe images, SAED and optical diffraction patterns demonstrate that where layers are coherently-related, 2M1 is the dominant polytypic sequence in all samples. However, this periodic 2M1 stacking is so frequently interrupted by stacking faults in smectite that it gives rise to apparent lMd polytypism. The degree to which the periodic 2M1 sequences are interrupted by stacking faults decreases with increasing proportion of illite-like layers. The SAED patterns of I/S and illite unmodified since its formation are diffuse parallel to c* and have poorly-defined, non-periodic reflections for indices k ≠ 3N as a measure of local ordering superimposed on poorly-ordered coherent sequences with a turbostratic component. X-ray diffraction (XRD) patterns, as integrated over domains with a range of heterogeneous stacking relations, do not represent simple mixtures of discrete IM and 2M1 polytypes.

The observations of this study imply that dissolution-crystallization is a dominant mechanism for the smectite-to-illite transition. The semi-coherent stacking of smectite-like layers in smectite-rich samples implies that either a dissolution-crystallization process took place subsequent to deposition of detrital smectite or that Gulf Coast smectite is an in-situ alteration product of volcanic ash.

Clay minerals in shales from cores at Site 808, Nankai Trough, have been studied using X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and analytical electron microscopy (AEM) to compare the rates and mechanisms of illitization with those of coeval bentonites, which were described previously. Authigenic K-rich smectite having a high Fe content (∼7 wt. %) was observed to form directly as an alteration product of volcanic glass at a depth of ∼500 meters below seafloor (mbsf) with no intermediate precursor. Smectite is then largely replaced by Reichweite, R, (R = 1) illite-smectite (I-S) and minor illite and chlorite over depths from ∼550 to ∼700 mbsf. No further mineralogical changes occur to the maximum depth cored, ∼1300 m. Most smectite and I-S in shales are derived from alteration of glass, rather than being detrital, as is usually assumed. Discrete layer sequences of smectite, I-S, or illite coexist, indicating discontinuities of the transformation from smectite to (R = 1) I-S to illite. Authigenic Fe-rich chlorite forms concomitantly with I-S and illite, with the source of Fe from reactant smectite.

Smectite forms from glass with an intermediate precursor in coeval bentonites at approximately the same depth as in shales, but the smectite remains largely unchanged, with the exception of exchange of interlayer cations (K → Na → Ca) in response to formation of zeolites, to the bottom of the core. Differences in rates of illitization reflect the metastability of the clays. Temperature, structure-state, and composition of reactant smectite are ruled out as determining factors that increase reaction rates here, whereas differences in water/rock ratio (porosity/permeability), Si and K activities, and organic acid content are likely candidates.

Clay particle aggregation affects a number of environmental processes, such as contaminant sorption/desorption, particle movement/deposition, and sediment structure and stability, yet factors that control clay aggregation are not well understood. This study was designed to investigate how microbial reduction of Fe(III) in clay structure, a common process in soils and sediments, affects clay-particle aggregation. Microbial Fe(III) reduction experiments were conducted with Shewanella putrefaciens CN32 in bicarbonate buffer with structural Fe (III) in nontronite as the sole electron acceptor, lactate as the sole electron donor, and AQDS as an electron shuttle. Four size fractions of nontronite (D5–D95 of 0.12–0.22 µm, 0.41–0.69 µm, 0.73–0.96 µm and 1.42–1.78 µm) were used to evaluate size-dependent aggregation kinetics. The extent of Fe(III) bioreduction and the amount of exopolysaccharide (EPS), a major biopolymer secreted by CN32 cells during Fe(III) bioreduction, were measured with chemical methods. Nontronite particle aggregation was determined by photon correlation spectroscopy and scanning electron microscopy. The maximum extent of Fe(III) bioreduction reached 36% and 24% for the smallest and the largest size fractions, respectively. Within the same time duration, the effective diameter, measured at 95% percentile (D95), increased by a factor of 43.7 and 7.7 for these two fractions, respectively. Because there was production of EPS by CN32 cells during Fe(III) reduction, it was difficult to assess the relative role of Fe(III) bioreduction and EPS bridging in particle aggregation. Thus, additional experiments were performed. Reduction of Fe(III) by dithionite was designed to examine the effect of Fe(III) reduction, and pure EPS isolated from CN32 cells was used to examine the effect of EPS. The data showed that both Fe(III) reduction and EPS were important in promoting clay mineral aggregation. In natural environments, the relative importance of these two factors may be dependent on local conditions. These results have important implications for understanding factors in controlling clay particle aggregation in natural environments.

Fe-reducing micro-organisms can change the oxidation state of structural Fe in clay minerals. The interactions with complex clays and clay minerals in natural materials remain poorly understood, however. The objective of this study was to determine if Fe(III) in loess was available as an electron acceptor and to study subsequent mineralogical changes. The loess samples were collected from St. Louis (Peoria), Missouri, USA, and Huanxia (HX) and Yanchang (YCH), in the Shanxi Province of China. The total Fe concentrations for the three samples was 1.69, 2.76, and 3.29 wt.%, respectively, and Fe(III) content was 0.48, 0.69, and 1.27 wt.%, respectively. All unreduced loess sediments contained Fe (oxyhydr)oxides and phyllosilicates. Bioreduction experiments were performed using Shewanella putrefaciens CN32 with lactate as the sole electron donor and Fe(III) in loess as the sole electron acceptor with and without anthraquinone-2, 6-disulfonate (AQDS) as an electron shuttle. Experiments were performed in non-growth (bicarbonate buffer) and growth (M1) media. The unreduced and bioreduced solids were analyzed by X-ray diffraction, Mössbauer spectroscopy, diffuse reflectance spectroscopy, and scanning electron microscopy/energy dispersive spectroscopy. Despite many similarities among the three loess samples, the extent and rate of Fe(III) reduction varied significantly. In the presence of AQDS the extent of reduction in the non-growth experiment was 25% of total Fe(III) in HX, 34% in Peoria, and 38% in YCH. The extent of reduction in the growth experiment was 72% in HX, 94% in Peoria, and 65% in YCH. The extent of bioreduction was less in the absence of AQDS. Overall, AQDS and the M1 growth medium significantly enhanced the rate and extent of bioreduction. Fe(III) in (oxyhydr)oxides and phyllosilicates was bioreduced. Siderite was absent in control samples, but was identified in bioreduced samples. The present research suggests that Fe(III) in loess sediments is an important potential source of electron acceptors that could support microbial activity under favorable conditions.

Electron energy-loss spectroscopy (EELS), energy-filtered transmission electron microscopy (EFTEM), and high-resolution transmission electron microscopy (HRTEM) have been applied in mineralogy and materials research to determine the oxidation states of various metals at high spatial resolution. Such information is critical in understanding the kinetics and mechanisms of mineral–microbe interactions. To date, the aforementioned techniques have not been applied widely in the study of such interactions. In the present study, the three techniques above were employed to investigate mineral transformations associated with microbial Fe(III) reduction in magnetite. Shewanella putrefaciens strain CN32, a dissimilatory metal-reducing bacterium, was incubated with magnetite as the sole electron acceptor and lactate as the electron donor for 14 days under anoxic conditions in bicarbonate buffer. The extent of bioreduction was determined by wet chemistry and mineral solids were investigated by HRTEM, EFTEM, and EELS. Magnetite was partially reduced and biogenic siderite formed. The elemental maps of Fe, O, and C and red-green-blue (RGB) composite map for residual magnetite and newly formed siderite were contrasted by the EFTEM technique. The HRTEM revealed nm-sized magnetite crystals coating bacterial cells. The Fe oxidation state in residual magnetite and biogenic siderite was determined using the EELS technique (the integral ratio of L3 to L2). The integral ratio of L3 to L2 for magnetite (6.29) and siderite (2.71) corresponded to 71% of Fe(III) in magnetite, and 24% of Fe(III) in siderite, respectively. A chemical shift (~1.9 eV) in the Fe-L3 edge of magnetite and siderite indicated a difference in the oxidation state of Fe between these two minerals. Furthermore, the EELS images of magnetite (709 eV) and siderite (707 eV) were extracted from the electron energy-loss spectra collected, ranging from 675 to 755 eV, displaying different oxidation states of Fe in the magnetite and siderite phases. The results demonstrate that EELS is a powerful technique for studying the Fe oxidation-state change as a result of microbial interaction with Fe-containing minerals.

Reports of scientific investigations published in Clays and Clay Minerals address a wide range of topics covering virtually every physical and natural science and thousands of industrial products and processes worldwide. Even though these nano-sized materials occur naturally throughout the Earth's crust, a large proportion of the studies carried out in the past regarding their origins, nature, and uses, as reported in Clays and Clay Minerals, have arisen predominantly from North America and Europe. In recent years, however, manuscript submissions from China have increased appreciably (Figure 1). In the current issue, a number of those studies were selected for a compilation in order to highlight the contributions being made by Chinese colleagues, which gives a glimpse into the types of clay research under way in that country. The manuscripts selected do not, of course, represent a comprehensive treatise of clay research in China, but rather an opportune emphasis that is intended to call attention to the work being done there.

Shewanella putrefaciens CN32 reduces Fe(III) within two illites which have different properties: the Fithian bulk fraction and the <0.2 µm fraction of Muloorina. The Fithian illite contained 4.6% (w/w) total Fe, 81% of which was Fe(III). It was dominated by illite with some jarosite (∼32% of the total Fe(III)) and goethite (11% of the total Fe(III)). The Muloorina illite was pure and contained 9.2% Fe, 93% of which was Fe(III). Illite suspensions were buffered at pH 7 and were inoculated with CN32 cells with lactate as the electron donor. Select treatments included anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle. Bioproduction of Fe(II) was determined by ferrozine analysis. The unreduced and bioreduced solids were characterized by Mössbauer spectroscopy, X-ray diffraction and transmission electron microscopy. The extent of Fe(III) reduction in the bulk Fithian illite was enhanced by the presence of AQDS (73%) with complete reduction of jarosite and goethite and partial reduction of illite. Mössbauer spectroscopy and chemical extraction determined that 21–25% of illite-associated Fe(III) was bioreduced. The extent of bioreduction was less in the absence of AQDS (63%) and only jarosite was completely reduced with partial reduction of goethite and illite. The XRD and TEM data revealed no significant illite dissolution or biogenic minerals, suggesting that illite was reduced in the solid state and biogenic Fe(II) from jarosite and goethite was either released to aqueous solution or adsorbed onto residual solid surfaces. In contrast, only 1% of the structural Fe(III) in Muloorina illite was bioreduced. The difference in the extent and rate of bioreduction between the two illites was probably due to the difference in layer charge and the total structural Fe content between the Fithian illite (0.56 per formula) and Muloorina illite (0.87). There may be other factors contributing to the observed differences, such as expandability, surface area and the arrangements of Fe in the octahedral sheets. The results of this study have important implications for predicting microbe-induced physical and chemical changes of clay minerals in soils and sediments.

The role of saline lake sediments in preserving organic matter has long been recognized. In order to further understand the preservation mechanisms, the role of clay minerals was studied. Three sediment cores, 25, 57, and 500 cm long, were collected from Qinghai Lake, NW China, and dissected into multiple subsamples. Multiple techniques were employed, including density fractionation, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM), total organic carbon (TOC) and carbon compound analyses, and surface area determination. The sediments were oxic near the water-sediment interface, but became anoxic at depth. The clay mineral content was as much as 36.8%, consisting mostly of illite, chlorite, and halloysite. The TEM observations revealed that organic matter occurred primarily as organic matter-clay mineral aggregates. The TOC and clay mineral abundances are greatest in the mid-density fraction, with a positive correlation between the TOC and mineral surface area. The TOC of the bulk sediments ranges from 1 to 3% with the non-hydrocarbon fraction being predominant, followed by bitumen, saturated hydrocarbon, aromatic hydrocarbons, and chloroform-soluble bitumen. The bimodal distribution of carbon compounds of the saturated hydrocarbon fraction suggests that organic matter in the sediments was derived from two sources: terrestrial plants and microorganisms/algae. Depth-related systematic changes in the distribution patterns of the carbon compounds suggest that the oxidizing conditions and microbial abundance near the water-sediment interface promote degradation of labile organic matter, probably in adsorbed form. The reducing conditions and small microbial biomass deeper in the sediments favor preservation of organic matter, because of the less labile nature of organic matter, probably occurring within clay mineral-organic matter aggregates that are inaccessible to microorganisms. These results have important implications for our understanding of mechanisms of organic matter preservation in saline lake sediments.

This study was undertaken to investigate the changes in flocculation properties of Fe-rich smectite (nontronite, NAu-1) suspensions, including settling velocity, aggregate size and floc architecture associated with microbial Fe(III)-reduction in the smectite structure. The dissimilatory Fe-reducing bacterium Shewanella oneidensis MR-1 was incubated with lactate as the electron donor and structural Fe(III) as the sole electron acceptor for 3, 12, 24 and 48 h in an anaerobic chamber. Two controls were prepared; the first was identical to the experimental treatments except that heat-killed cells were used (non-reduced control), and the second control was the same as the first except that the incubation was carried out in an aerobic environment. The extent of Fe(III) reduction for the 48 h incubation was observed to reach up to 18%. Neither the non-reduced control nor the aerobically inoculated sample showed Fe(III) reduction. Compared with the non-reduced control, there was a 2.7 μm increase in mean aggregate size and a 30-fold increase in average settling velocity in the bioreduced smectite suspensions as measured using a Micromeritics Sedigraph®. The aerobically inoculated smectite showed a similar aggregate-size distribution to that of the non-reduced control. Significant changes in physical properties of smectite suspensions induced by microbial Fe(III) reduction were measured directly using transmission electron microscopy. The floc architecture of bioreduced smectite revealed less open structures compared to those of a non-reduced control. The aspect ratio (thickness/length) of individual smectite particle increased from 0.11 for the non-reduced control to 0.18 on average for the bioreduced smectite suspensions. The effects of pH on the clay flocculation were minimal in this study because the value of pH remained nearly constant at pH = 7.0–7.3 before and after the experiments. We therefore suggest that the increase in net negative charge caused by microbial Fe(III) reduction significantly promoted clay flocculation by increasing the electrochemical attraction in the smectite suspensions.

River-dominated delta areas are primary sites of active biogeochemical cycling, with productivity enhanced by terrestrial inputs of nutrients. Particle aggregation in these areas primarily controls the deposition of suspended particles, yet factors that control particle aggregation and resulting sedimentation in these environments are poorly understood. This study was designed to investigate the role of microbial Fe(III) reduction and solution chemistry in aggregation of suspended particles in the Mississippi Delta. Three representative sites along the salinity gradient were selected and sediments were collected from the sediment-water interface. Based on quantitative mineralogical analyses 88–89 wt.% of all minerals in the sediments are clays, mainly smectite and illite. Consumption of ${\text{SO}}_4^{2-}$\$\end{document} and the formation of H2S and pyrite during microbial Fe(III) reduction of the non-sterile sediments by Shewanella putrefaciens CN32 in artificial pore water (APW) media suggest simultaneous sulfate and Fe(III) reduction activity. The pHPZNPC of the sediments was ⩽3.5 and their zeta potentials at the sediment-water interface pH (6.9–7.3) varied from −35 to −45 mV, suggesting that both edges and faces of clay particles have negative surface charge. Therefore, high concentrations of cations in pore water are expected to be a predominant factor in particle aggregation consistent with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Experiments on aggregation of different types of sediments in the same APW composition revealed that the sediment with low zeta potential had a high rate of aggregation. Similarly, addition of external Fe(II) (i.e. not derived from sediments) was normally found to enhance particle aggregation and deposition in all sediments, probably resulting from a decrease in surface potential of particles due to specific Fe(II) sorption. Scanning and transmission electron microscopy (SEM, TEM) images showed predominant face-to-face clay aggregation in native sediments and composite mixtures of biopolymer, bacteria, and clay minerals in the bioreduced sediments. However, a clear need remains for additional information on the conditions, if any, that favor the development of anoxia in deep- and bottom-water bodies supporting Fe(III) reduction and resulting in particle aggregation and sedimentation.

This study was undertaken to investigate mechanisms of mineral transformations associated with microbial reduction of structural Fe(III) in smectite. Shewanella oneidensis strain MR-1 cells were inoculated with lactate as the electron donor and Fe(III) in smectite as the electron acceptor. The extent of Fe(III) reduction was observed to reach up to 26%. Reduction proceeded via association of live bacterial cells with smectite. At the end of incubation, a large fraction of starting smectite was transformed to euhedral flakes of biogenic smectite with different morphology, structure, and composition. Lattice-fringe images obtained from environmental cell transmission electron microscope displayed a decrease of layer spacing from 1.5±0.1 nm for the unreduced smectite to 1.1±0.1 nm for the reduced smectite. The biogenic smectite contained more abundant interlayer cations, apparently as a result of charge compensation for the reduced oxidation state of Fe in the octahedral site. To capture the dynamics of smectite reduction, a separate experiment was designed. The experiment consisted of several systems, where various combinations of carbon source (lactate) and different concentrations of AQDS, an electron shuttle, were used. Selected area electron diffraction patterns of smectite showed progressive change from single-crystal patterns for the control experiment (oxidized, unaltered smectite), to diffuse ring patterns for the no-carbon experiment (oxidized, but altered smectite), to well-ordered single crystal pattern for the experiment amended with 1 mM AQDS (well crystalline, biogenic smectite). Large crystals of vivianite and finegrained silica of biogenic origin were also detected in the bioreduced sample. These data collectively demonstrate that microbial reduction of Fe(III) in smectite was achieved via dissolution of smectite and formation of biogenic minerals. The microbially mediated mineral dissolution-precipitation mechanism has important implications for mineral reactions in natural environments, where the reaction rates may be substantially enhanced by the presence of bacteria.

The normal prograde diagenetic and low-grade metamorphic sequence of dioctahedral clay minerals including illite-rich I-S and illite, as observed by TEM, proceeds from a partially disordered 1Md stacking sequence to 2M1; i.e. 1M does not normally occur as an intermediate polytype. Examples of 1M illite stacking sequences have been studied, however, from the Golden Cross gold deposit, New Zealand, the Broadlands-Ohaaki geothermal system, New Zealand, the Potsdam Sandstone, New York, and the Silverton Caldera, Colorado. Specific clay-mineral packets identified by TEM techniques as 1M illite were found to have anomalously high Mg contents. The Broadlands illite provides the most definitive data, as separate packets of 1M and 2M1 illite coexist. Average compositions for 1M and 2M1 illite are (K1.66Ca0.04)Σ1.70(Al3.32Fe0.31Mg0.57Mn0.06)Σ4.26(Si6.43Al1.57)Σ8O20(OH)4 and (K1.57Na0.31Ca0.03)Σ1.91(Al3.58Fe0.05Mg0.29Mn0.01)Σ3.93(Si6.70Al1.30)Σ8O20(OH)4, respectively. In addition, 1Mdillite, which is the polytype occurring in the common 1Mdto 2M1 prograde sequence, is relatively Mg poor, but coexists with Mg-rich illite in the Silverton Caldera sample.

These data confirm that 1M stacking is caused by compositional anomalies, and thus explain the lack of the 1M stacking sequence in normal diagenetic sequences in pelitic rocks, as most illite in such environments has a relatively small phengitic component. The parameter Δz, a measure of the corrugation of the oxygen sheets, may be the key parameter reflecting the polytypic state of dioctahedral and trioctahedral micaceous minerals. Such composition-determined relations may be related to the occurrence of 1M polytypism in glauconite and celadonite, both dioctahedral 2:1 clay minerals having large Mg or Fe octahedral-cation components, and in trioctahedral micas. Insofar as the 1M stacking sequence does not have the same composition as 2M1 material, these data confirm that the different varieties of illite are not polytypes, sensu stricto.

The formation of illite through the smectite-to-illite (S-I) reaction is considered to be one of the most important mineral reactions occurring during diagenesis. In biologically catalyzed systems, however, this transformation has been suggested to be rapid and to bypass the high temperature and long time requirements. To understand the factors that promote the S-I reaction, the present study focused on the effects of pH, temperature, solution chemistry, and aging on the S-I reaction in microbially mediated systems. Fe(III)-reduction experiments were performed in both growth and non-growth media with two types of bacteria: mesophilic (Shewanella putrefaciens CN32) and thermophilic (Thermus scotoductus SA-01). Reductive dissolution of NAu-2 was observed and the formation of illite in treatment with thermophilic SA-01 was indicated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). A basic pH (8.4) and high temperature (65°C) were the most favorable conditions forthe formation of illite. A long incubation time was also found to enhance the formation of illite. K-nontronite (non-permanent fixation of K) was also detected and differentiated from the discrete illite in the XRD profiles. These results collectively suggested that the formation of illite associated with the biologically catalyzed smectite-to-illite reaction pathway may bypass the prolonged time and high temperature required for the S-I reaction in the absence of microbial activity.

Clay minerals impart important chemical properties to soils, in part, by virtue of changes in the redox state of Fe in their crystal structures. Therefore, measurement of Fe(III)/Fe(II) and partitioning of Fe(II) in different reactive sites in clay minerals (during biological and chemical Fe(III) reduction) is essential to understand their role and their relative reactivity in terms of reduction and immobilization of heavy metal contaminants such as technetium. This study had three objectives: (1) to understand the degree of dissolution of nontronite (Fe-rich smectite) as a result of chemical and biological reduction of Fe(III) in the structure; (2) to quantify partitioning of chemically and biologically produced Fe(II) into different reactive sites in reduced nontronite, including aqueous Fe2+, ammonium chloride-extractable Fe(II) (mainly from the ion-exchangeable sites, denoted as $\text{Fe}{\left(\text{II}\right)}_{{\text{NH}}_4\text{Cl}}$${\rm{Fe}}{\left( {{\rm{II}}} \right)_{{\rm{N}}{{\rm{H}}_4}{\rm{Cl}}}}$), sodium acetate-extractable Fe(II) (mainly from the surface complexation sites, denoted as Fe(II)acetate), and structural Fe(II) (denoted as Fe(II)str); and (3) to evaluate the reactivity of these Fe(II) species in terms of Tc(VII) reduction. Chemical and biological reduction of Fe(III) in nontronite (NAu-2) was performed, and reduced nontronite samples with different extents of Fe(III) reduction (1.2–71%) were prepared. The extent of reductive dissolution was measured as a function of the extent of Fe(III) reduction. Our results demonstrated that chemically and biologically produced Fe(II) in NAu-2 may be accommodated in the NAu-2 structure if the extent of Fe(III) reduction is small (< ∼30%). When the extent of reduction was >∼30%, dissolution of nontronite occurred with a corresponding decrease in crystallinity of residual nontronite. The Fe(II) produced was available for partitioning into four species: ${\text{Fe}}_{\left(\text{ab}\right)}^{2+}$${\rm{Fe}}_{\left( {{\rm{ab}}} \right)}^{2 + }$, Fe(II)acetate, $\text{Fe}{\left(\text{II}\right)}_{{\text{NH}}_4\text{Cl}}$${\rm{Fe}}{\left( {{\rm{II}}} \right)_{{\rm{N}}{{\rm{H}}_4}{\rm{Cl}}}}$, and Fe(II)str. The increase in Fe(II)acetate during the early stages of Fe(III) reduction indicated that the Fe(II) released had the greatest affinity for the surface-complexation sites, but this site had a limited capacity (∼60 µmol of Fe(II)/g of NAu-2). The subsequent increase in $\text{Fe}{\left(\text{II}\right)}_{{\text{NH}}_4\text{Cl}}$${\rm{Fe}}{\left( {{\rm{II}}} \right)_{{\rm{N}}{{\rm{H}}_4}{\rm{Cl}}}}$ indicated that the released Fe(II) partitioned into the exchangeable sites once the amount of Fe at the surface-complexation sites reached half of its maximum site capacity. The fraction of Fe(II)str decreased concomitantly, as a result of Fe(II) release from the NAu-2 structure, from 100% when the extent of Fe(III) reduction was <30% to nearly 65% when the extent of Fe(III) reduction reached 71%. The Fe(II)acetate and Fe(II)str exhibited greater reactivity in terms of Tc(VII) reduction than the $\text{Fe}{\left(\text{II}\right)}_{{\text{NH}}_4\text{Cl}}$${\rm{Fe}}{\left( {{\rm{II}}} \right)_{{\rm{N}}{{\rm{H}}_4}{\rm{Cl}}}}$. Clearly, the surface-complexed and structural Fe(II) are the desirable species when reduced clay minerals are used to reduce and immobilize soluble heavy metals in contaminated groundwater and soils. These results have important implications for understanding microbe—clay mineral interactions and heavy metal immobilization in clay-rich natural environments.

We investigate the distribution of archaeal lipids in a 5.8-m-long sedimentary core recovered from Lake Qinghai to extract regional hydroclimate and temperature signals since the last deglaciation for this important region. The paleohydrology was reconstructed from the relative abundance of thaumarchaeol (%thaum) and the archaeol and caldarchaeol ecometric (ACE) index. The %thaum-inferred lake-level record was extended to deglaciation, showing three periods (11.9–13.0, 14.1–14.7 and 15.1–17.2 cal ka BP) with relatively higher lake levels than those during the early Holocene. The ACE record demonstrates three periods (10.6–11.2, 13.2–13.4 and 17.4–17.6 cal ka BP) of elevated salinity when the lake was shallow. Filtered TEX86 record based on archaeal lipid distributions corresponded to relatively higher lake levels, implying that a certain lake size is required for using the TEX86 paleothermometer. At 1–4 cal ka BP, the reconstructed temperature fluctuated significantly and correlated negatively with inferred lake level, indicating that lake temperature and hydrological change might be coupled during this period. We attribute this co-variance to the importance of summer temperature in controlling evaporation for this arid/semi-arid region. Moreover, our results indicate that archaeal lipids have potential in reconstructing paleoclimate patterns from lacustrine sedimentary cores, but the data should be interpreted with care.