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The Almahata Sitta (AhS) meteorite is a unique polymict ureilite. Recently, carbonaceous chondritic lithologies were identified in AhS. Organic matter (OM) is ubiquitously found in primitive carbonaceous chondrites. The molecular and isotopic characteristics of this OM reflect its origin and parent body processes, and are particularly sensitive to heating. The C1 lithologies AhS 671 and AhS 91A were investigated, focusing mainly on the OM. We found that the OM in these lithologies is unique and contains primitive isotopic signatures, but experienced slight heating possibly by short-term heating event(s). These characteristics support the idea that one or more carbonaceous chondritic bodies were incorporated into the ureilitic parent body. The uniqueness of the OM in the AhS samples implies that there were large variations in primitive carbonaceous chondritic materials in the solar system other than known primitive carbonaceous chondrite groups such as CI, CM, and CR chondrites.

The Aptian–Albian boundary is marked by one of the major oceanic perturbations during the Cretaceous, called Oceanic Anoxic Event (OAE) 1b. Extensive volcanic episodes at the Southern Kerguelen Plateau has been suggested as the trigger of OAE1b, but compelling evidence remains lacking. Here, we reconstructed the temporal variations of marine Os isotopic ratios across the Aptian–Albian boundary in the Tethyan and Pacific pelagic sedimentary records to elucidate the causal links between OAE1b, the biotic turnover, and volcanic episodes. Our new Os isotopic records show two negative spikes that correlate with a period of planktonic foraminiferal turnover across the Aptian–Albian boundary during OAE1b and suggest multiple submarine volcanic events. By comparing our Os isotopic profile with carbon isotopic compositions of carbonate, CaCO3 content, and the relative abundances of agglutinated foraminifera, we conclude that ocean acidification caused by the massive release of CO2 through extensive volcanic episodes could have promoted the major planktonic foraminiferal turnover during OAE1b.

We present the first data describing molecular evidence of planktonic coenzyme factor 430 (hereafter referred to as F430), a key signature of methanogenic archaea, obtained from a water column sample in a freshwater lake in Japan. We report concentrations of native F430 ranging from 6.8–35 × 102 femto mol g-wet−1 and 8.5–86 × 102 femto mol g-wet−1 in the water column and core-top sediment, respectively. Among these profiles, the epimer F430 of the deactivated chemical formula was very low overall in both planktonic and benthic environments. Small subunit (SSU) rRNA gene sequencing revealed that the planktonic microbial community was clearly different from that of the benthic sediment. Planktonic cyanobacteria (e.g., Microcystis) were predominant in the water column, whereas a wide variety of bacteria and archaea, including methanogens (e.g., Methanobacterium, Methanoregula, Methanothrix, Methanosarcina, and Methanocella), were observed in the core-top sediment. These results suggest that the planktonic cyanobacterial community may be rapidly degraded at the water–sediment interface driven by activated benthic microbial heterotrophic processes. Graphic abstract

The southwestern Ryukyu Trench represents the ultimate sink of sediments shed from Taiwan into the Philippine Sea, which are mainly transported to the trench by turbidity currents via submarine canyons. Here, we present trench turbidites intercalated with hemipelagites in a gravity pilot core and a piston core acquired on the Ryukyu Trench floor at 6147 m water depth. We performed X-ray fluorescence core scans (ITRAX profiles), magnetic measurements, and organic geochemistry analyses to discriminate turbidites from hemipelagites. We identified 36 turbidites (0.9–4.2 cm thick) based on visual core descriptions and Ca/Fe ratios in the ITRAX profiles. Three of these turbidites show magnetic signatures indicating the presence of pyrrhotite and peaks in the magnetic susceptibility profile, suggesting that Taiwan-sourced sediments are transported to the Ryukyu Trench by long-runout turbidity currents. Pyrrhotite is also present in hemipelagites of the upper part of the retrieved cores, indicating a dominant sediment source in Taiwan over the last several thousand years. Ca/Fe and Zr/Rb ratios in the ITRAX profiles mark distal turbidites (about 1–3 cm thick), and Zr/Rb peaks mainly reflect grain size changes. Detailed analyses of a representative turbidite show good correlation between Ca/Fe and Zr/Rb peaks with upward-coarsening and upward-fining trends that delimit the turbidite. Sedimentary organic matter in hemipelagites is characterized by higher total organic carbon and total nitrogen contents and higher δ13C values than that in turbidites. Our multi-proxy approach employing high-resolution XRF core scans to differentiate turbidites from hemipelagites contributes to establishing a comprehensive view of modern trench sedimentation from Taiwan to the southwestern Ryukyu Trench.

Today, upwelling along the Oman margin in the Arabian Sea is governed by the South Asian Monsoon winds. The Oman upwelling results in the formation of an oxygen minimum zone (OMZ) spanning across the Arabian Sea and large parts of the Indian Ocean. While these conditions are recorded as early as the middle Miocene (~15 Ma), the long‐term dynamics of upwelling in the Arabian Sea are as of yet poorly constrained during the middle to late Miocene. Here, we use organic and inorganic proxies combined with sedimentary and paleontological records to constrain the evolution of upwelling at Ocean Drilling Program Site 722B between ~15 and ~8.7 Ma. Our record shows that Mn depletion occurred at ~14.5 Ma, likely due to regionally confined OMZ formation at that time. Biogenic silica accumulation intensified between ~12.5 and ~11 Ma. The δ15N values (>6‰) provide evidence for the onset of at least intermittent denitrification between ~11 and ~9.5 Ma during the apex of the global “carbonate crash.” Our data demonstrate that upwelling and OMZ intensity in the Arabian Sea were linked to the reorganization of the Indian Ocean circulation system and South Asian Monsoon during the Miocene. The initiation of these systems occurred once the regional tectonic configuration (i.e., the uplift of the Tibetan Plateau ~25 Ma and the closure of the Tethyan seaway ~20 Ma) was in place. The subsequent development of monsoonal upwelling after 14 Ma responded to latitudinal shifts in climatic belts following the progressive Miocene glaciation of Antarctica.

The abyssal plain covers more than half the Earth’s surface. The main food source to abyssal ecosystems is phytodetritus, which originates from phytoplankton in the surface ocean, and thus its variability to the seafloor is a major driver of abyssal ecosystem biomass and functioning. In this study, we conducted a comparative survey on organic matter (OM) quality and quantity in abyssal plain sediments and examined the distributions of megafauna, macrofauna, meiofauna, prokaryotes, and viruses in eutrophic (39°N), oligotrophic (1°N), and ultra-oligotrophic (12°N) areas of the western Pacific. We also analyzed stable carbon and nitrogen isotopic compositions of organisms at 39°N and 1°N to assess differences in benthic abyssal food-web structures with contrasting trophic states. Sediments collected at 39°N presented highest concentrations of total organic carbon (TOC) and labile OM, and high diffusive oxygen uptake rates. By contrast, the lowest values were found at 12°N. Vertical distributions of sediment macrofauna, meiofauna, and prokaryotes matched with labile OM profiles. There were prominent differences in abundances of macro- and megafauna among stations with different OM fluxes, whereas the abundance of meiofauna and prokaryotes showed smaller differences among stations. Such differences could be explained by higher turnover rates of smaller organisms. Food-web structures of abyssal plains are likely influenced by both the type and size of primary producers in surface ocean. Our results underscore the crucial importance of OM fluxes and their compositions to the abundances and vertical profiles of labile OM and benthic biota in abyssal ecosystems.

We examined the distributions of tetrapyrrole pigments (i.e. intact chlorophylls and bacteriochlorophylls, pheopigments) as well as their compound-specific carbon and nitrogen isotopic compositions in the sediments of three Swiss lakes (Lakes Rotsee, Cadagno and Zurich) and the Black Sea to investigate the biogeochemical cycling of carbon and nitrogen mediated by phototrophic eukaryotes (algae) and bacteria. The factors controlling chlorin isotope variations are discussed and the feasibility to use chlorins as indicators for reconstructions of surface water environments is evaluated. Chlorophyll a and its derivatives including pheophytin a, a pheophytin a epimer, pyropheophytin a, 132,173-cyclopheophorbide-a-enol, chlorophyllone a as well as steryl and carotenol chlorin esters were detected in all sediments. The presence of bacteriochlorophylls e and their derivatives confirmed the presence of brown strains of green phototrophic sulfur bacteria (Chlorobiaceae; GSB) in all three lakes. In the shallower Lakes Rotsee and Cadagno, purple sulfur bacteria (Chromatiaceae; PSB) were also present as confirmed by bacteriochlorophyll a derivatives. Despite the different degrees of water column hypoxia at the studied sites, the chlorins in all sediments were attributed to rapid transformation of intact tetrapyrroles and the formation of related pheopigments. The scatter of compound-specific carbon isotopic compositions of Chl a and its derivatives resulted from different timing of pheopigment formation, likely due to the interaction of blooms of various phytoplankton communities at different times of the year and the variable degrees of carbon limitation and/or different contributions of recycled organic matter (OM). The nitrogen isotopic composition of the chloropigments mainly derived from nitrate assimilation in Lake Zurich and the Black Sea, whereas ammonium and nitrate assimilation were predominant in Lake Rotsee. In the epilimnion of the meromictic Lake Cadagno, dissolved organic nitrogen (DON) supplied to the surface water from ammonium assimilation in the chemocline may be the main nitrogen source. Phototrophic sulfur bacteria in Lakes Rotsee and Cadagno thrived mainly under dissolved organic carbon depleted conditions within the chemocline and in the hypolimnion. GSB may use predominantly ammonium and at least in Lake Cadagno also perform N2 fixation. In contrast, the nitrogen source of PSB could not be reconstructed with δ15N values of bacteriochlorins, because nitrogen isotopic fractionation during BChl a synthesis seems to be almost independent of the assimilated substrate.

To reconstruct the impact of eutrophication on phototrophic communities and the biogeochemical cycling of carbon and nitrogen in the surface water, we investigated the distributions and carbon and nitrogen isotopic compositions (δ13C and δ15N values) of chlorins in the sediments of Lake Zurich. The chlorin distributions were dominated by chlorophyll a (Chl a) and its derivatives, which reflect rapid degradation to the pheopigments in the water column and sediments of the lake. The δ13C values of these sedimentary chlorins followed the historical trends of eutrophication and reoligotrophication, except in the surface sediments, which were characterised by higher relative contributions of aged, redeposited organic matter (OM). The δ13C values of the sedimentary chlorins together with bulk sediment δ13C values and C/N ratios indicate that the phototrophic communities in the lake used a 13C-depleted carbon source, which is mainly of aquatic origin. The δ15N values of chlorins reflect the predominance of nitrate assimilating phototrophs, especially the non-N2-fixing cyanobacterium Planktothrix rubescens prevalent during sediment deposition. Shifts in δ15N values of Chl a followed mostly the trends in eutrophication and reoligotrophication, but were also affected by community assemblage shifts to diatoms and/or other cyanobacteria at the end of the 19th century and during the eutrophication maximum in the 1970s. The lower δ15NChl-a values in the surface sediments coincide with increasing nitrogen to phosphorus ratios and reduced water column mixing that characterise the recent reoligotrophication period and may explain the predominance of P. rubescens in Lake Zurich. In contrast, the higher contributions of laterally transported OM explains the large offset of δ15N values of the pheopigments relative to Chl a, which is supported by the high radiocarbon age of the surface sediments.

The Messinian salinity crisis is a dramatic hydrological and biological crisis that occurred in the Mediterranean basin at 5.97–5.33 Ma. The interpretation of the facies and stratigraphic associations of the Messinian salt deposits is still the object of active research because of the absence of modern depositional analogues of comparable scale. In this study, the spatial distributions of Na, Mg, S, O, Si, and Al in a potassic-magnesian salt and a halite layers of Messinian evaporites from the Realmonte mine on Sicily were determined using synchrotron based micro-X-ray fluorescence. The dominant molecular host site of Mg and S obtained by X-ray absorption near edge structure (XANES) is applied to specify the hydrochemistry of hypersaline brines and the presence of diagenetic minerals, thus shedding light on evaporative concentration processes in the Caltanissetta Basin of Sicily. Mg and S K-edge XANES spectra revealed the presence of highly soluble Mg-bearing sulfates. The massive halite layer “unit C,” contains less soluble minerals, thus did not exceed the stage of halite crystallization. We infer that as evaporative concentration increased, the density of the brine at the shallow margin of the basin increased as salinity increased to concentrations over 70 times the starting values, creating brines that were oversaturated with Mg-sulfate. Density stratification of the deep basin caused heavy brines to sink to the bottom and become overlain by more dilute brines. We propose lateral advection of dense Mg-sulfate brines that certainly affected marine biota.

Although the climate in the Arabian Sea is controlled primarily by the southwest monsoon, the southwest monsoon has little influence in the Gulf of Aden. To examine the different responses to monsoons between the Gulf of Aden and areas outside the gulf, a comprehensive data set of bulk organic and inorganic geochemistry, sea surface temperature, and δ15N of pheopigments was obtained from deep-sea sediment cores recovered from inner and outer regions of the gulf. The results indicated that during the past 220 kyr, the influence of the southwest monsoon was stronger in the outer region of the gulf than in the inner region, which implies that the southwest monsoon trajectory has not changed substantially during that time period. Furthermore, influxes of O2-depleted water from the Southern Ocean and the lateral advection of upwelled seawater also had limited influence in the inner region. In contrast, concentrations of lithogenic materials transported by the southwest monsoon were similar in the two regions. δ15N of pheopigments indicated that during the last glacial maximum, the southwest monsoon was weaker and the northeast monsoon was stronger than at present. A stronger southwest monsoon during interglacials enhanced primary productivity and may have caused anoxic conditions to develop in the Arabian Sea, as indicated by redox proxies in the outer region. Anoxic conditions also formed during MIS 3, but no increase in the primary productivity is indicated; therefore, another mechanism, such as an influx of O2-depleted water from the Southern Ocean, may have been the cause.

Article history: The Paleoproterozoic (2.5-2.0 Ga) is one of the most important periods in Earth's history, and was characterized by a rise in atmospheric oxygen levels and repeated (at least three) severe glaciations (the Huronian glaciations). In this study, we investigate redox conditions in the atmosphere and in shallow- marine environments immediately after the first Huronian glaciation based on the isotopic composition of Os, and the abundance of redox-sensitive elements (Os, Re, and Mo) in sedimentary rocks from the Huronian Supergroup, Canada. We found no significant authigenic enrichment of Os in the sedimentary rocks deposited during the first Huronian deglaciation. The initial isotopic composition of Os in the sediments was close to that of chondrite at the time of deposition ( 187 Os/

A novel method has been developed for compound-specific nitrogen isotope compositions with an achiral column which was previously shown to offer high precision for nitrogen isotopic analysis. We applied the method to determine the amino acid contents and stable nitrogen isotopic compositions of individual amino acids from the thermally metamorphosed (above 500 °C) Antarctic carbonaceous chondrites Ivuna-like (CI)1 (or CI-like) Yamato (Y) 980115 and Ornans-like (CO)3.5 Allan Hills (ALH) A77003 with the use of gas chromatography/combustion/isotope ratio mass spectrometry. ALHA77003 was deprived of amino acids due to its extended thermal alteration history. Amino acids were unambiguously identified in Y-980115, and the δ15N values of selected amino acids (glycine +144.8 ‰; α-alanine +121.2 ‰) are clearly extraterrestrial. Y-980115 has experienced an extended period of aqueous alteration as indicated by the presence of hydrous mineral phases. It has also been exposed to at least one post-hydration short-lived thermal metamorphism. Glycine and alanine were possibly produced shortly after the accretion event of the asteroid parent body during the course of an extensive aqueous alteration event and have abstained from the short-term post-aqueous alteration heating due to the heterogeneity of the parent body composition and porosity. These carbonaceous chondrite samples are good analogs that offer important insights into the target asteroid Ryugu of the Hayabusa-2 mission, which is a C-type asteroid likely composed of heterogeneous materials including hydrated and dehydrated minerals.

After the severest mass extinction event in the Phanerozoic, biotic recovery from the extinction at the Permian–Triassic boundary required approximately 5 my, which covers the entire Early Triassic. It is important to obtain information on the superocean Panthalassa, which occupied most of the world ocean, to explore paleoenvironmental changes during the Early Triassic at the global scale. In order to establish the continuous lithostratigraphy of pelagic sediments in Panthalassa during the Early Triassic, high-resolution reconstruction of the Lower Triassic pelagic sequence in Japan was conducted for the first time based on detailed field mapping and lithostratigraphic correlation in the Inuyama area, central Japan. The reconstructed Early Triassic sequence is approximately 9.5 m thick, consists of five rock types, and is divided into eight lithological units. For the reconstructed continuous sequence, measurement of carbon isotopic composition of sedimentary organic matter (δ13Corg) was carried out. Stratigraphic variation of the δ13Corg value shows large-amplitude fluctuations between −34.4 and −21.0‰ throughout the sequence. In order to establish a higher resolution age model for the reconstructed Lower Triassic pelagic sequence, we correlated δ13Corg records in the Inuyama area with high-resolution isotopic profiles of carbonate carbon (δ13Ccarb) from shallow-marine carbonate sequences in southern China based on the similarity in general variation patterns with age constraints by radiolarian and conodont biostratigraphy. The result provides a high-resolution time scale for the pelagic sequence of Panthalassa during the Smithian and Spathian. The age model suggests a drastic increase in sedimentation rate during the late Smithian, which should have been caused by the increase in terrigenous input to this site.

We present initial isotopic ratios of lead for Early Cretaceous (Barremian–Aptian) sections from Shatsky Rise (Pacific) and Gorgo a Cerbara (Italy). Our Pb isotopic data track an interval representing Oceanic Anoxic Event (OAE)-1a, which is characterized by quasi-global deposition of organic carbon-rich black shale. Pb isotopic compositions of sediments from Shatsky Rise decrease at the end of Barremian time, from radiogenic continental values to unradiogenic values, and subsequently remained less radiogenic until the end of early Aptian time. We explain the isotopic shift by a significant increase in supply rate of unradiogenic Pb, most likely due to massive volcanism. In contrast, the Pb isotopic compositions from the Italian section, which was situated at the western end of Tethys, are mostly identical to those of upper continental crust, showing no significant change in supply rate of unradiogenic Pb. The discrepancy between two sites is attributed to quiescent deep-submarine eruptions of Pacific large igneous provinces (LIPs) such as the Ontong Java Plateau (OJP), which severely limited dispersion of Pb-carrying particles out of the Pacific Ocean. Published Os isotopic data from the Italian section indicate two episodes of massive eruptions of OJP or contemporaneous Manihiki and Hikurangi plateaus starting from earliest Aptian time, slightly later than that indicated by the sedimentary Pb isotopic record from Shatsky Rise. Differences in isotopic variations between Pb and Os likely reflect differences in their chemical behaviors in the oceans, i.e., Pb isotopic compositions would have varied in response to local or regional changes in sediment provenances, whereas large-scale changes in Os inputs are required to explain variations in seawater Os isotopic compositions. Our Pb isotopic data, together with the published Os isotopic record, provide new evidence for the eruptive history of OJP together with contemporaneous Pacific plateaus and its environmental consequences, starting from end-Barremian time and extending through early Aptian time.

The early diagenetic effects of organic matter (OM) decomposition in carbonaceous sediments overlain by an anoxic water body were investigated using a sediment core (KAI4) from the saline meromictic Lake Kai-ike, in southwest Japan. Lake Kai-ike, a current model for past anoxic ocean environments, is characterized by development of photic zone anoxia and a “bacterial plate,” which create permanent density stratification in the water. Over the last ~ 40 years, rapid decreases in organic carbon (Corg; ~ 55%), nitrogen (~ 60%), and reactive phosphorus (ΔP; ~ 85%) contents accompanied by ~ 1‰ increase (positive shift) in the stable isotope compositions of Corg (δ13Corg), which is attributed to a preferential loss of low-δ13Corg components during early diagenesis, are reflected with increasing depth of the sediments. The molar Corg/ΔP and N/ΔP ratios increase with depth, from initial values that are very close to those of the Redfield ratio. During the early diagenesis of OM degradation, the decomposition rate of P is much faster than that of Corg and N. In contrast, the molar Corg/N ratios are rather insensitive to various biogeochemical reactions, confirming their utility as a proxy for source OM. Although the stable isotope compositions of nitrogen (δ15N) in sedimentary OM are useful indicators of OM sources, correlations to the above-mentioned parameters for the KAI4 core remain unclear. We emphasize that the OM present in ancient carbonaceous sediments or sedimentary rocks was most likely subject to very rapid (

Oceanic Anoxic Events (OAEs) are geological time intervals characterized by extremely high burial rates of organic carbon that led to deposition of organic-rich “black shales” in the global ocean. It has been proposed that oceanic anoxic events are ultimately caused by massive volcanism associated with formation of large igneous provinces (LIPs) because of chronological similarities, but no general consensus has developed yet. To investigate the possibility of LIP volcanism instigating OAEs, we measured stable isotopic compositions of bulk organic carbon (δ13Corg) and lead (Pb) isotopic compositions in the silicate sediment fraction across the Bonarelli black shale (Italy), a type stratigraphic section for the end-Cenomanian OAE (OAE-2; 94 Ma). Ultra-high-resolution δ13Corg records determined every 1.5 mm capture a 3‰ sharp negative shift at the base of the Bonarelli. At the same stratigraphic level, Pb isotopic compositions in the silicate sediment fraction exhibit significant shifts toward characteristic values of volcanic rocks from contemporaneous LIPs (Caribbean and Madagascar flood basalts). These data suggest a rapid, substantial increase in the relative supply of silicate minerals from the two LIPs. Massive subaerial volcanism associated with LIP formation provides a simple explanation for these two isotopic geochemical signals via release of a huge amount of carbon dioxide (∼ 105 Gt CO2) and particulate materials into the atmosphere, which resulted in a rapid negative shift of δ13C in sea water and changes in Pb isotopic compositions in the silicate sediment fraction, respectively. We interpret that massive volcanism triggered significant climatic changes, inducing biotic crises and oceanic anoxia.

In Livello Bonarelli black shale deposited during Cretaceous Oceanic Anoxic Event 2 (OAE-2, ca.94 Ma), nitrogen isotopic compositions of bulk sediments are mostly in a narrow range from –2.7 to –0.7‰. We also determined molecular distribution and nitrogen isotopic compositions of geoporphyrins extracted from the black shale. The nitrogen isotopic compositions of C32 Ni deoxophylloerythroetioporphyrin (DPEP) and total Ni porphyrins are –3.5 and –3.3‰, respectively, leading us to the estimation that the mean nitrogen isotopic composition of photoautotrophic cells were around +1‰ during the formation of Bonarelli black shale. This value is suggestive of N2-fixation, a dominant process for these photoautotrophs when assimilating nitrogen. Furthermore, Ni-chelated C32 DPEP, derived mainly from chlorophyll a had the highest concentration. Based on this evidence, we conclude that diazotrophic cyanobacteria were major primary producers during that time. Cyanobacteria may be key photoautotrophs during the formation of black shale type sediments intermittently observed throughout the later half of the Earth's history, and hence may have played a crucial role in the evolution of geochemical cycles even in the later half of the Earth's history.

To demonstrate the utility of coral skeletons as a recorder of nitrate dynamics in the surface ocean, we collected coral skeletons of Porites lobata and determined their nitrogen isotope composition (δ15N_[coral]) from 2002 to 2006. Skeletons were collected at Okinotori Island in southwestern Japan, far from any sources of terrestrial nitrogen. Nitrogen isotope compositions along the growth direction were determined at 800 μm intervals (∼1 month resolution) and compared against the skeletal carbon isotope composition (δ13C_[coral-carb]), barium/calcium ratio (Ba/Ca), and Chlorophyll-a concentration (Chl-a). From 2002 to 2004, ratios of the δ15N_[coral] varied between +0.8 and +8.3‰ with inverse variation to SST (r = -0.53). Ba/Ca ratios and Chl-a concentrations were also observed to be high during seasons with low SST. These results suggested that the vertical mixing that occurs during periods of low SST carries nutrients from deeper water (δ15N_[DIN]; +5∼+6‰) to the sea surface. In 2005 onward, δ15N_[coral] and Ba/Ca ratios also had positive peaks even in high SST during periods of transient upwelling caused by frequent large typhoons (maximum wind speed 30 m/s). In addition, low δ15N_[coral] (+0.8∼+2.0‰) four months after the last typhoon implied nitrogen fixation because of the lack of typhoon upwelling through the four years record of δ15N_[coral]. Variations in the δ13C_[coral-carb] and δ15N_[coral] were synchronized, suggesting that nitrate concentration could control zooxanthellae photosynthesis. Our results suggested that δ15N_[coral] holds promise as a proxy for reconstructing the transport dynamics of marine nitrate and thus also a tool for estimating nitrate origins in the tropical and subtropical oceans.

Early Palaeoproterozoic (2.5-2.0 billion years ago) was a critical phase in Earth's history, characterized by multiple severe glaciations and a rise in atmospheric O(2) (the Great Oxidation Event). Although glaciations occurred at the time of O(2) increase, the relationship between climatic and atmospheric transitions remains poorly understood. Here we report high concentrations of the redox-sensitive element Os with high initial (187)Os/(188)Os values in a sandstone-siltstone interval that spans the transition from glacial diamictite to overlying carbonate in the Huronian Supergroup, Canada. Together with the results of Re, Mo and S analyses of the sediments, we suggest that immediately after the second Palaeoproterozoic glaciation, atmospheric O(2) levels became sufficiently high to deliver radiogenic continental Os to shallow-marine environments, indicating the synchronicity of an episode of increasing O(2) and deglaciation. This result supports the hypothesis that climatic recovery from the glaciations acted to accelerate the Great Oxidation Event.

Joint carbon and nitrogen isotope measurements were made from chlorins (chlorophyll a, phaeophytin a and pyrophaeophytin a) extracted from the last glacial-interglacial transition sediments of Lake Suigetsu, central Japan. These data highlight both the potential and difficulty of using chlorin-specific isotopes to track aquatic change from lake sediments. δ13C and δ15N of the three chlorins show coherent patterns with time, supporting the theory that phaeophytin a and pyrophaeophytin a are early diagenetic products of chlorophyll a and that despite this transition, their isotopic signatures remain intact. However, our data suggest that the isotopic composition of phaeophytin a and pyrophaeophytin a can be imprecise proxies for the isotope composition of chlorophyll a, possibly owing to the complex array of factors which affect the synthesis, transformation and sedimentation of these phaeopigments in nature. The total accumulation of organic matter in Lake Suigetsu appears to be controlled by the balance of allocthonous and authocthonous material as reflected by the C/N ratio. However, both bulk organic and chlorin-specific δ13C show similar changes, suggesting that the first order variability in bulk organic δ13C reflects aquatic change. By contrast, there is no similarity between chlorin and bulk δ15N, suggesting that interpretation of bulk δ15N in this setting is compromised by diagenetic alteration. The isotopic composition of chlorins are interpreted to reflect the response of aquatic primary productivity to post-glacial environmental change. However, further research into the synthesis and transformation of chlorins in the modern environment is required in order to facilitate a more rigorous approach to interpreting isotope ratios in chlorins extracted from sediments.

Early Paleoproterozoic time (2.5–2.0 Ga) spanned a critical phase in Earth's history, characterized by repeated glaciations and an increase in atmospheric oxygen (the Great Oxidation Event (GOE)). Following the last and most intense glaciation of this period, marine carbonates record a large positive excursion of δ^(13)C value (termed the “Lomagundi event”) between about 2.2 and 2.1 Ga coinciding with the global appearances of red beds and sulfates, which suggest an accumulation of high levels of atmospheric oxygen. Here we report the discovery of large negative excursions of δ^(13)C in organic matter (down to −55‰) from quartzose sandstones (of the Marquette Range and the Huronian Supergroups, North America) intermediate in age between the last Paleoproterozoic glaciation and the possible onset of the Lomagundi event. The negative excursion is concomitant with the appearance of intensely weathered quartzose sandstones, which may represent hot and humid conditions. There are some interpretations that potentially explain the negative excursions: (1) redeposition of older ^(13)C-depleted kerogen, (2) later post-depositional infiltration of oil, (3) active methane productions by methanogens in shallow-marine environments, or (4) dissociation of methane hydrate. If the latter two were the case, they would provide clues for understanding the environmental change connecting the intense glaciation and an increase in oxygen.

We determined the mean crustal uplifting rate during the late Holocene along the Soya Coast, Lützow-Holm Bay, East Antarctica, by dating a marine-lacustrine transition recorded in lake sediments. We focused on temporal variations in the chemical composition of sediments recovered from Lake Skallen Oike at Skallen and Lake Oyako at Skarvsnes. Both sets of lake sediments record environmental changes associated with a transition from marine to lacustrine (fresh water) settings, as indicated by analyses of sedimentary facies for carbon and nitrogen contents, nitrogen isotopic compositions (15N/14N), and major element concentrations. Changes in the dominant primary producers during the marine-lacustrine transition were also clearly revealed by biogenic Opal-A, diatom assemblages, and gradient gel electrophoresis (DGGE) with 16S rRNA gene analysis. Geochronology based on radiocarbon dating of acid-insoluble organic carbon suggested that the environmental transition from saline to fresh water occurred at 2940±100 cal yr BP at L. Skallen and 1060±90 cal yr BP at L. Oyako. Based on these data and a linear approximation model, we estimated a mean crustal uplifting rate of 3.6 mm yr−1 for the period since the marine-lacustrine transition via brackish condition; this uplift is attributed to glacial-isostatic rebound along the Soya Coast. The geological setting was the primary factor in controlling the emergence event and the occurrence of simultaneous changes in sedimentary and biological facies along the zone of crustal uplift.

The paleolimnology of two lakes which were isolated as a result of the crustal uplift during the late Holocene along the Soya Coast, Lutzow-Holm Bay, East Antarctica were studied. The focus was on temporal variations in the biogeochemical composition of sediment cores recovered from Lake Skallen at Skallen and Lake Oyako at Skarvsnes. Both sets of lake sediments record environmental changes associated with a transition from marine to lacustrine settings, as indicated by analyses of C and N contents, nitrogen isotopic compositions (δ 15 N), and major element concentrations. Changes in the dominant primary producers during the marine–lacustrine transition (marine diatom to cyanobacteria) at L. Skallen was clearly revealed by biogenic opal-A, diatom assemblages, and molecular signature from denaturing gradient gel electrophoresis (DGGE) with 16S ribosomal RNA (rRNA) gene analysis. Radiocarbon dating of acid-insoluble organic C suggested that the environmental transition from marine to fresh water occurred at 2940 ± 100 cal yr BP at L. Skallen and 1060 ± 90 cal yr BP at L. Oyako. Based on these data, a mean crustal uplift rate of 3.2 mm yr −1 is inferred for the history of marine–lacustrine transition via brackish conditions. The geological setting causing glacio-isostatic uplift was the primary factor in controlling the transition event in sedimentary and biological facies.