Benthic foraminiferal δ~(13)C minimum events in the southeastern Okhotsk Sea over the last 180 ka

A total of six d13C minimum events,i.e.,VI,V,IV,III,II,and I,were observed via a stable carbon and oxygen isotope analysis of infaunal benthic foraminifera Uvigerina spp.in gravity core OS03-1 in the southeastern Okhotsk Sea over the last 180 ka.These events occurred at112–109,102–90,85–76,57–54,44–40,and 17–10 ka BP.The largest negative excursions reached 2.5%in event V and were greater than 1%in the other events.We proposed that all d13C minimum events were caused by the increase in sea surface water productivity,the weakened formation of Okhotsk Sea intermediate water,and the enhancement of the oxygen minimum zone.The negative excursions were unaffected by methane hydrate destabilization and subsequent methane release based on the results obtained by using archaeal lipid markers.     

Origin and nature of cold seep in northeastern Dongsha area, South China Sea: Evidence from chimney-like seep carbonates

The occurrence of seep carbonates is one of the characteristic features for cold seep sites at continental margins.The carbonates documented the venting history of methane-rich fluid.Compared to the chemoherm carbonates and carbonate pavements which formed on the sediment-water interface,chimney-like seep carbonates precipitated around fluid conduits below the sediment-water interface therefore better recording information of the past fluid flow and composition.Here the chimney-like seep carbonate samples from the northeastern Dongsha area of the South China Sea were studied to understand the origin and nature of the venting fluids and their potential relationship with gas hydrate deposits underneath the seafloor.Based on the occurrence,morphology,petrology,mineralogy and C-and O-isotope compositions,combined with present and past bottom water temperatures and the timing of methane release events,the oxygen isotopic fractionation between calcite and water were used to estimate the equilibriumδ18O values of the precipitating fluids.Theδ13C values ranging from 56.33‰to 42.70‰V-PDB and thus clearly show that the studied chimneys were mainly derived from biogenic methane oxidation.The calculated equilibriumδ18O values of the precipitating fluids ranged from 1.9‰0.3‰to 0.6‰0.3‰V-SMOW,with an average of 1.4‰0.3‰V-SMOW which is heavier than those of seawater even at the last glacial maximum.It is considered that the formation of chimney-like carbonates was closely related to methane hydrate dissociation in the area.The methane hydrates contributed as much as45.7%of water to the venting fluids.It is suggested that the climate and environmental changes(e.g.sea-level lowering,down-cutting canyons and mass wasting)are the major mechanisms maybe responsible for the destabilization of methane hydrates in the study area.The extensive occurrence of seep carbonates indicates that a large amount of the methane released from methane hydrate dissociation has been effectively captured and sequestered by microbial anaerobic oxidation of methane(AOM)before it escapes into the water column.     

A metasomatized mantle wedge origin for low-δ 18O olivine in late Cretaceous Junan and Qingdao basalts in the Sulu orogen

The North China Block (NCB) experienced extensive lithospheric thinning and subsequent mantle accretion in the Mesozoic to Cenozoic, but their mechanism remains controversial. This paper reports in situ chemical and oxygen isotope analyses on olivine xenocrysts and porphyroclastic olivines in mantle xenoliths hosted in the late Cretaceous basalts from Junan and Qingdao, the Sulu orogen. The results show that all these olivines have low δ ¹⁸O values relative to normal mantle peridotite. The olivine xenocrysts and porphyroclastic olivines from Junan have Fo=87.5–89.8, δ ¹⁸O=4.1‰–5.2‰ with an average of 4.8‰; the porphyroclastic olivines from Qingdao also have Fo=89.0–89.9, δ ¹⁸O=4.1‰–5.2‰ with an average of 4.8‰. These peridotite xenoliths have petrological and geochemical affinities similar to newly accreted MORB-type mantle, we hence consider such low-δ ¹⁸O features in the olivines to inherit from a mantle wedge that was metasomatized by melts derived from the subducted oceanic crust, which had experienced high-temperature hydrothermal alteration to acquire the low-δ ¹⁸O signatures. Combined the existence of Cenozoic low-δ ¹⁸O basalts and garnet pyroxenite xenoliths (relicts of recycled oceanic crust) hosted in Cenozoic basalts in the NCB, the subducted oceanic crust likely played an important role in the lithospheric evolution of the NCB during the Mesozoic to Cenozoic.     

Paleoproterozoic glaciation: Evidence from carbon isotope record of the Hutuo Group,Wutai Mountain area of Shanxi Province,China

Palaeoproterozoic global glaciation (～2.3 Ga) may have been the first “snowball” Earth event in Earth history. North China has well-exposed sedimentary deposits of this time period with minimal postdepositional alteration. Sedimentary evidence of a glaciation event has been lacking, however. Here we analyzed carbon isotope composition of the Paleoproterozoic Hutuo Group (2.5–2.2 Ga) in the Wutai Mountain area, Shanxi Province, North China. Our data show, in an older to younger chronostratigraphic order, (1) positive δ ¹³C_carb values in the Dashiling Formation of the Doucun Subgroup (3.2‰ to 1.0‰, VPDB); (2) a decrease in δ ¹³C_carb values from the Wenshan Formation to the middle Daguandong Formation (from 2.0‰ to–1.2‰), during which time the occurrence of stromatolites declined; (3) a pronounced negative excursion in the upper Daguandong Formation, at the boundary of the Daguandong and Huaiyincun formations (from 1.4‰ to ?3.3‰), where stromatolites disappeared; and (4) a gradual increase of δ ¹³C_carb values in the Beidaxing and Tianpengnao formations (from ?1.2‰ to 1.4‰), during which time stromatolites returned. We argue that the negative carbon isotope excursion recorded from the Jian’ancun Formation to the middle Daguandong Formation of the Hutuo Group may have been North China’s response to the Palaeoproterozoic global glaciation.     

Carbon and oxygen isotopes analyses for the Sinian carbonates in the Helan Mountain, North China

We conducted detailed studies of the Sinian carbonates from the Helan Mountain, Ningxia by both field investigation and laboratory analyses of carbon and oxygen isotopes. The carbon isotopic compositions of the Sinian Zhengmuguan Formation shows variations with regularity in the five studied sections, the δ ¹³C values all gradually decrease from bottom to top, with a total range from ?4.51‰ to 0.11‰, and the biggest negative excursion reaches up to ?6.88‰. In addition, abundant macro-body fossils were also found from the Tuerkeng Formation in this study, similar to those found from the Dengying Formation in the South China Block. This observation firmly constrained the age for the Zhengmuguan and Tuerkeng formations to the Sinian period. Here we interpreted that the Zhengmuguan Formation formed during a glacial period when the extremely cold climate substantially decreased the amount of burial of organic matter in the ocean and thus led to the general negative δ ¹³C excursion. The increasing δ ¹³C values in the Tuerkeng Formation dolomites imply the ending of the glacial period in the Zhengmuguan epoch, biological recovery and increase of organic matter burial in the ocean occurred during this period. Comparing our obtained carbon and oxygen isotopes of the Zhengmuguan and Tuerkeng formations with those of coeval carbonate successions from other places all over the world, the Zhengmuguang-aged glacial deposits might be deposited after the Gaskiers glaciation. This study provides important information for paleo-ocean, paleogeographic and biological evolution in the North China Block during the Ediacaran period and adds new carbon and oxygen isotopes data for the global Sinian carbon-oxygen isotopes dataset.     

The long-term denudation rate of granitic regolith in Qinhuangdao,North China determined from the in situ depth profile of the cosmogenic nuclides 26Al and 10Be

This study quantifies the surface denudation rate of granitic regolith via the application of the in situ cosmogenic ^26Al and ^10Be depth profile in China. The concentration ranges of ^26Al and ^10Be in the quartz along the -3-m granitic regolith profile in Qinhuangdao are （4.9-23.1） × 10^5 and （2.3-36.6）×10^4 atoms/g, respectively. With the exception of the surface sample, both ^26Al and ^10Be concentrations decrease exponentially with sample depth. The Chi-square best-fitting results revealed a total denudation rate of -9 m/Ma averaged over a 103-105 a timescale, which is lower than the values observed in global granitic outcrops. Compared with global datasets, the flat terrain due to the lack of tectonic activities is most likely the dominant factor that controls the local denudation process. The surface sample offsets from the theoretical cosmogenic nuclide distribution implies that the denudation rate from river basin sediment could be overestimated because of the bioturbation in the surficial soil layer.     

Precise measurement of stable (δ 88/86Sr) and radiogenic (87Sr/86Sr) strontium isotope ratios in geological standard reference materials using MC-ICP-MS

A new method for high-precision measurements of stable (δ88/86Sr) and radiogenic (87Sr/86Sr) Sr isotope ratios was developed on the MC-ICP-MS using a standard-sample-standard bracketing protocol. By carefully monitoring and controlling the accumulation of the on-peak backgrounds (i.e. the carryover memory) on the instrument, our method can provide an external precision of around 0.010‰ to 0.014‰ for 88/86 Sr measurements. Such precision is comparable to that achieved by double-spike TIMS method (DS-TIMS), and 2-3 times better than those of the previously reported results by MC-ICP-MS without spiking. The results of the standard seawater IAPSO are also identical to those by DS-TIMS. However, our method is more advantageous, efficient and convenient to use for routine Sr isotopic analysis than the DS-TIMS method as there is no need to prepare and calibrate the 84Sr-87 Sr double spike. Using this method, we measured the δ88/86Sr values in a series of international geological rock standards, which show large variability, with the lowest value (0.20‰) registered in JG-2, a terrestrial granite, and the highest value (0.539‰) in UB-N, a serpentinite. This may provide an additional means for isotopic characterization of geological processes, adding a new dimension over the traditional use of radiogenic Sr isotopic ratio 87Sr/86Sr.     

Zircon U/Pb dating and Hf-O isotopes of the Zhouan ultramafic intrusion in the northern margin of the Yangtze Block, SW China: Constraints on the nature of mantle source and timing of the supercontinent Rodinia breakup

The Zhouan ultramafic intrusion in the northern margin of the Yangtze Block is mainly composed of lherzolite.Zircon grains selected from lherzolite are irregular in shape with distinct oscillatory and sector zoning and have Th/U ratios ranging from 0.8 to 10.6,indicating a magmatic origin.The weighted average 206 Pb/238 U age is 637±4 Ma (2σ,n=15),which can be considered as the crystallization age of the Zhouan intrusion.Zircon grains have δ18O values ranging from 5.2‰ to 7.0‰,with an averaged value of 5.8±0.4‰ (1σ,n=33),similar to the mantle δ18O value of zircon.Their 176Hf/177Hf(t) ratios range from 0.282410 to 0.282594 with εHf(t) values ranging from 1.3 to 7.6,lower than the corresponding value of the depleted mantle (～15),indicating an enriched mantle source.The enriched mantle source may have generated from a metasomatized lithospheric mantle with subducted slab.A number of ～635 Ma mafic-ultramafic intrusions in the Suizao basin are associated with coeval bimodal volcanics of the Yaolinghe Formation,indicating a continental rift setting.The ～635 Ma magmatic event in this region may represent the product of the last breakup of the Rodinia supercontinent in the northern margin of the Yangtze Block at Neoproterozoic.     

High-resolution stalagmite δ 13C record of soil processes from southwestern China during the early MIS 3

Knowledge of the evolution of pedogenic processes in the carbonate area is important to understand the dynamics of rock desertification.A precisely-dated stalagmite δ13C record between 61.3 and 50.5 ka BP from southwestern China,potentially related to soil biogenic CO2 production,shifts within a narrow range between -9‰ and -7‰,and exhibits a prominent cycle of 350-470 a,independent of the Asian monsoon changes.This indicates that a threshold effect might control the amplitude of calcite δ13C variability,likely associated with the vertical soil loss in the carbonate area.The periodic δ13C changes reflect that the loss/recovery of soil cover can operate on centennial scale.Thus,the rock desertification in southwestern China might at least initiate 60 ka ago and should be of natural origin.     

Zircon U–Pb and Hf isotopic study of Neoproterozoic granitic gneisses from the Alatage area,Xinjiang:constraints on the Precambrian crustal evolution in the Central Tianshan Block

The architecture and growth history of Precambrian crustal basements in the Central Tianshan Block play a key role in understanding the tectonic evolution of the Chinese Tianshan Orogenic Belt.In this study,we present precise LA-ICP-MS zircon U–Pb dating and LAMC-ICPMS zircon Hf isotopic data for two granitic gneisses from Alatage area in the Central Tianshan Block.The magmatic zircons from both samples yield similar protolith ages of 945±6 and 942±6 Ma,indicating that the early Neoproterozoic magmatism is prevailed in the Alatage area.These zircons have crustal Hf model ages of1.82–2.22 and 1.70–2.03 Ga,respectively,which are significantly older than their crystallization ages.It indicates that their parental magmas were derived from the reworking of ancient crust.However,we suggest that these Paleoproterozoic Hf model ages might result from mixing of continental materials with different ages in the Neoproterozoic crust.The inherited(detrital)zircon cores not only yield a wide age range of ca.989–1617 Ma,but also exhibit large Hf-isotope variations with Hf model ages of1.54–2.30 Ga.In particular,some 1.4–1.6 Ga zircons show high initial176Hf/177Hf ratios,consistent with those of depleted mantle,which indicates that the Mesoproterozoic event involved both reworking of older crust and generation of juvenile crust.The Central Tianshan Block has different Precambrian crustal growth history from the Tarim Craton.Therefore,it would not be a fragment of the Precambrian basement of the Tarim Craton.     

In situ stable isotopic constraints on dolomitizing fluids for the hydrothermally-originated saddle dolomites at Keping, Tarim Basin

Detailed characterization of diagenetic fluid behaviour and evolution is commonly hindered by shortage of appropriate and economic methods to carry out in situ analysis in the tiny diagenetic mineral crystals. Using micromill sampling method, this study presents in situ stable isotopic data for the growth zones of saddle dolomite crystals in the hydrothermally-altered dolostones of Upper Cambrian (Furongian) at Keping, Tarim Basin, NW China. These data show minor and large variations in δ13C ( 0.7‰ to 1.5‰ VPDB) and δ18O ( 8.3‰ to 13.8‰ VPDB), respectively, from the cores to outer rims of the dolomite crystals and suggest that saddle dolomites precipitated from dolomitizing fluids with similar carbon sources but oscillatory temperatures during different formation stages. This scenario is confirmed by microthermometry of fluid inclusions within the growth zones of dolomite crystals. This study indicates that in situ isotope analysis could provide more detailed information related to the source and pathway of dolomitizing fluids, facilitating better characterization of dolomitizing fluids and processes.     

Investigating different bio-responses of the upper ocean to Typhoon Haitang using Argo and satellite data

The responses of the upper ocean to Typhoon Haitang in July 2005 are investigated using Argo float and multiplatform satellite data. The results show decreasing sea surface temperature （SST）, a deepening of the mixed layer depth （MLD）, and enhanced Chloro- phyll-a （Chl-a） concentration. Two extreme cool regions are identified. While the magnitude of SST cooling in the two regions is similar, the biological response （Chl- a enhancement） differs. To facilitate comparisons, the region to the northeast of Taiwan is defined as region A and the region east of Taiwan as region B. Ekman pumping and the intrusion of the Kuroshio play an important role in the enhancement of Chl-a in region A. Cold eddies provide the material source for the forma- tion of the cold center in region B, where mixing is dominant. Because of the relatively high translation speed （5 m/s） in region B, Ekman pumping has little influence on the cooling and Chl-a enhancement pro- cesses. Moreover, the MLD is shallower than the nutri- cline, which means that mixing does not result in a marked increase in nutrients in the euphotic layer （where the nutrient concentration is uniformly depleted）. Sea temperatures, in contrast, gradually decrease with depth below the bottom of the mixed layer. In contrast toregion A, region B showed no significant enhancement of Chl-a but strong SST cooling.     

Qinghu zircon: A working reference for microbeam analysis of U-Pb age and Hf and O isotopes

Zircon is the most useful mineral for studies in U-Pb geochronology and Hf and O isotope geochemistry. Matrix effect is a major problem of the microbeam techniques such as SIMS and LA-（MC）-ICPMS. Therefore, external standardization using well-characterized natural zircon standards is fundamental for accurate microbeam measurements. While the isotopic geochro- nology and geochemistry laboratories equipped with microbeam analytical facilities have been increasingly established in China during the past decade, applications of the isotopic microanalysis are still limited due to shortage of available standards. We report here the Qinghu zircon as a potential new working reference for microbeam analysis of zircon U-Pb age and O-Hf isotopes. This zircon was separated from the Qinghu quartz monzonite from the western Nanling Range, Southeast China. It is fairly homoge- neous in U-Pb age and Hf and O isotopes in terms of large amounts of mircobeam measurements by LA-MC-ICPMS and SIMS at the scales of 20-60 ~tm. SIMS measurements yield consistent 2~6pb/238U age within analytical uncertainties with that obtained by ID-TIMS. Precise determinations of O isotopes by IRMS and Hf isotopes by solution MC-ICPMS are in good agreement with the statistical mean of microbeam measurements. We recommend U-Pb age of = 159.5 ± 0.2 Ma （2SE）, δ^18O = 5.4‰ ± 0.2%0 （2SD） and 176Hf/177Hf = 0.283002 ± 0.000004 （2SD） as the best reference values for the Qinghu zircon.     

The effect of DIO additive on performance improvement of polymer solar cells

A series of polymer solar cells （PSCs） based on poly （diketopyrrolopyrrole-terthiophene） （PDPP3T） and [6,6]-phenyl-C71-butyric acid methyl ester （PC71BM） as active layer were fabricated to investigate the effect of 1,8- diiodooctane （DIO） on the performance of PSCs. The power conversion efficiency （PCE） of PSCs was increased from 3.77 % to 4.37 % for the cells with DIO additive. The underlying reason may be attributed to that DIO additive could make PC71BM more dispersive in the active layer, forming a better bicontinuous interpenetrating network for excition dissociation and charge carrier transport. There- fore, the short circuit current density （Jsc） and fill factor （FF） was increased from 8.25 to 9.18 mA/cm2 and from 67.2 % to 70.0 % for the PSCs with DIO additive compared with PSCs without DIO additive.     

Newly discovered Palaeolithic artefacts from loess deposits and their ages in Lantian,central China

Eight new Palaeolithic open-air sites were identified and 770 stone artefacts were collected from 2009 to 2011 in the Lantian area of the Bahe River valley, central China. Because the famous Homo erectus fossils were unearthed at the Gongwangling and Chenjiawo localities, and more than 30 Palaeolithic open-air sites were investigated in the 1960s in this region, the catchment of Bahe River is regarded as one of the most important hominin sites from the late early Pleistocene to the middle Pleistocene. These eight newly discovered open-air sites are located at the second(n = 6), third(n = 1) or higher terraces(n = 1) of the Bahe River. The Diaozhai section on the second terrace was sampled in detail. Two samples were collected for optically stimulated luminescence dating(OSL). The OSL results suggest that a buried lithic artefact layer at the Diaozhai site spans approximately 70–30 ka. The lithic assemblage analysis suggests that the stone artefacts were made of local pebbles/cobbles such as greywacke, quartz, sandstone and igneous rocks. The main percussion techniques that were used were direct hard hammer percussion and bi-polar techniques. The lithic artefacts comprise hammer stones, cores, flakes, retouched tools and flaking debris. Acheulian-type large cutting tools(LCTs) such as hand-axes, picks and cleavers were indentified in the Lantian region as well. This is the first time Acheulian-type LCTs from the late Pleistocene have been identified in this region. This study distinguishes age gaps between Western world and East Asian Acheulian-type tools.     

Investigation of the effects of MoO_3 buffer layer on charge carrier injection and extraction by capacitance–voltage measurement

The effects of MoO3thin buffer layer on charge carrier injection and extraction in inverted configuration ITO/ZnO/MEH-PPV(poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene))/MoO3(0,5 nm)/Ag hybrid solar cells are investigated by capacitance–voltage measurement under dark and light illumination conditions.The efficiency of charge carrier injection and extraction is enhanced by inserting 5 nm MoO3thin layer,resulting in better device performances.Charge carrier transport of the whole device is improved and the interface energy barrier is reduced by inserting 5 nm MoO3thin buffer layer.The device fill factor is increased from 54.1%to 57.5%after modifying 5 nm MoO3.Simulations and experimental results consistently show that in the forward voltage under dark,the device with the 5 nm MoO3thin layer modification generates larger value of capacitance than the device without MoO3layer.While under illumination,the device with the 5 nm MoO3layer generates smaller value of capacitance than the device without the 5 nm MoO3layer in the bias region of reverse and before the peak position of maximum capacitance(VCmax).The underlying mechanism of the MoO3anode buffer layer on device current density–voltage characteristics is discussed.     

Derivation of P–T paths for Middle–Late Triassic very low-grade metapelite from Hongcan Well 1 in the Zoigê Depression and their tectonic implications

Using XRD measurements of illite crystallinity,b0values of K-rich white micas and chemical compositions of chlorites combined with homogenization temperatures of fluid inclusions in calcite,the metamorphic P–T conditions of very low-grade metapelites from different depths in the 7 km Hongcan Well 1 in the Zoige?Depression,Sichuan,China,have been obtained in this paper.Knowledge of the tectonic evolution of the studied basin allows us to derive geothermal curves for Middle and Late Triassic time(the geothermal peak)and the present day(from thermal logging of the borehole).The retrograde P–T tracks from clockwise P–T–t loops have been revealed by the plotting of individual samples on each geotherm,which can be interpreted by the lithospheric thickening due to sedimentation and folding followed by continuous uplift and erosion.A stratigraphic log indicates that the Well penetrates the steep limb of a fold which perturbed the peak geotherm and caused some P–T paths to cross.     

Molecular evidence for genetic mixing of Arctic and Antarctic subpolar populations of planktonic foraminifers

Bipolarity, the presence of a species in the high latitudes separated by a gap in distribution across the tropics, is a well-known pattern of global species distribution. But the question of whether bipolar species have evolved independently at the poles since the establishment of the cold-water provinces 16-8 million years ago, or if genes have been transferred across the tropics since that time, has not been addressed. Here we examine genetic variation in the small subunit ribosomal RNA gene of three bipolar planktonic foraminiferal morphospecies. We identify at least one identical genotype in all three morphospecies in both the Arctic and Antarctic subpolar provinces, indicating that trans-tropical gene flow must have occurred. Our genetic analysis also reveals that foraminiferal morphospecies can consist of a complex of genetic types. Such occurrences of genetically distinct populations within one morphospecies may affect the use of planktonic foraminifers as a palaeoceanographic proxy for climate change and necessitate a reassessment of the species concept for the group.     

Thermochemical sulfate reduction in fossil Ordovician deposits of the Majiang area: Evidence from a molecular-marker investigation

The main reservoirs of Majiang fossil deposits consist of the Silurian Wengxiang group, dominantly sandstones, and the Ordovician Honghuayuan formation, dominantly carbonate rocks, and the Lower Cambrian Niutitang Formation mudstones serve as the major source rocks. Thermochemical sulfate reduction (TSR) might have taken place in the Paleozoic marine carbonate oil pools, as indicated by high concentrations of dibenzothiophenes in the extracts (MDBT=0.27–4.32 μg/g extract, and MDBT/MPH= 0.71–1.38). Hydrocarbons in the Pojiaozhai Ordovician carbonate reservoirs have undergone severe TSR and are characterized by higher quantities of diamondoids and MDBT and heavier isotopic values (δ ¹³C=?28.4‰). The very large amounts of dibenzothiophenes might be products of reactions between biphenyls and sulfur species associated with TSR.     

Dynamics of total culturable bacteria and its relationship with methylmercury in the soils of the water level fluctuation zone of the Three Gorges Reservoir

Field investigations were conducted to study the temporal and spatial distribution characteristics of total culturable bacteria(TCB)and its relationship with methylmercury(MeHg)in the soils of the water level fluctuation zone of the Three Gorges Reservoir.Different altitudes(170–180,170–175,165–170 and 160–165 m)in Zhenxi(Site 1),Shibaozhai(Site 2)and Tujing(Site 3),Chongqing,China were chosen as sampling sites.Results indicated that TCB did not have significant difference in the top(0–10 cm)and sub(10–20 cm)soil of the non-inundated area(175–180 m),but showing a significant difference in the water level fluctuation zone(\175 m,suggesting that water level fluctuation had an important effect on TCB.Moreover,TCB in soils of various altitudes of Site 1 and 2had significant difference,while for Site 3,this difference was not significant.And the difference of TCB in Site 2was much greater than that in Site 1.These results suggested that there were significant differences for TCB in soils of mainstream and tributaries.In addition,TCB in soils of 10–20 cm had significant or highly significant positive correlations with MeHg level(r C 0.762,P B 0.048),thus we assumed that there may be some aerobic microorganisms playing dominant roles in mercury methylation.