Terrestrial records of the early Albian Ocean Anoxic Event:Evidence from the Fuxin lacustrine basin,NE China

The early Albian Oceanic Anoxic Event 1 b(OAE 1 b) is well documented in the Tethys, Pacific and North Atlantic, but few studies have evaluated whether or not terrestrial records of OAE 1 b exist. In order to identify terrestrial records of the early Albian OAE 1 b and to infer possible driving mechanisms, an integrated multi-proxy study from the late Aptian to Albian in the Fuxin lacustrine basin was conducted,including thick, organic-rich black mudstones, total organic carbon(TOC), organic carbon isotopes(d13 Corg), mercury concentration(Hg) and results from pyrolysis analyses(S2, Tmaxand HI). Results show three distinct short-term negative d13 Corgexcursions corresponding with relatively high TOC values,which could be counterparts of the Kilian, Paquier and Leenhardt sub-events of the early Albian OAE1 b. Atmospheric CO2 concentration(p CO2) recovered from C3 plant d13 Corgcompositions indicates an increasing trend in Unit C during the early Albian, and there are three short-term increases of p CO2 corresponding to the three sub-events of OAE 1 b at this time interval. We infer that a trend of increasing p CO2 during the Kilian sub-event in the study area is closely related to volcanism. Continental weathering calculated using chemical weathering indices(CIA, WIP and MIA(O)) show an increasing trend during the OAE 1 b interval, likely resulting from warmer and more humid conditions. Mixed sources of terrestrial plants and lacustrine plankton demonstrated by pyrolysis analyses(HI vs. Tmaxand S2 vs. TOC), indicate a terrestrial contribution to the organic-rich sediments of the Kilian, Paquier and Leenhardt sub-events of OAE 1 b. We suggest that a CO2-forced greenhouse effect during the early Albian might have triggered the relatively warm and humid palaeoclimatic conditions, and intensified chemical weathering that combined to create high nutrient and organic matter levels that were flushed into lakes contributing to eutrophication and anoxia in lacustrine and in contemporaneous oceanic systems.

Organic Geochemistry of the Early Jurassic Oil Shale from the Shuanghu Area in Northern Tibet and the Early Toarcian Oceanic Anoxic Event

This paper presents new geological and geochemical data from the Shuanghu area in northern Tibet, which recorded the Early Toarcian Oceanic Anoxic Event. The stratigraphic succession in the Shuanghu area consists mostly of grey to dark-colored alternating oil shales, marls and mudstones. Ammonite beds are found at the top of the Shuanghu oil shale section, which are principally of early Toarcian age, roughly within the Harplocearas falciferrum Zone. Therefore, the oil shale strata at Shuanghu can be correlated with early Toarcian black shales distributing extensively in the European epicontinental seas that contain the records of an Oceanic Anoxic Event. Sedimentary organic matter of laminated shale anomalously rich in organic carbon across the Shuanghu area is characterized by high organic carbon contents, ranging from 1.8% to 26.1%. The carbon isotope curve displays the δ 13C values of the kerogen (δ 13Ckerogen) fluctuating from –26.22 to –23.53‰ PDB with a positive excursion close to 2.17‰, which, albeit significantly smaller, may also have been associated with other Early Toarcian Oceanic Anoxic Events (OAEs) in Europe. The organic atomic C/N ratios range between 6 and 43, and the curve of C/N ratios is consistent with that of the δ 13Ckerogen values. The biological assemblage, characterized by scarcity of benthic organisms and bloom of calcareous nannofossils (coccoliths), reveals high biological productivity in the surface water and an unfavorable environment for the benthic fauna in the bottom water during the Oceanic Anoxic Event. On the basis of organic geochemistry and characteristics of the biological assemblage, this study suggests that the carbon-isotope excursion is caused by the changes of sea level and productivity, and that the black shale deposition, especially oil shales, is related to the bloom and high productivity of coccoliths.

A Toarcian Ocean Anoxic Event record from an open-ocean setting in the eastern Tethys: Implications for global climatic change and regional environmental perturbation

The Early Toarcian“Oceanic Anoxic Event”(T-OAE)is recorded by marked disruption to both the climate system and marine ecosystems.Here,we present intergraded high-resolution carbon-isotope data(δ^(13)C),bulk geochemistry,mineral characterization from an open-ocean setting in the eastern Tethys.With these data,we(1)construct the high-resolution record of the T-OAE from an open-ocean setting in the eastern Tethys;(2)show that the T-OAE in the Sewa succession was marked by coarser-grained deposits associated with high-energy conditions within the otherwise low-energy claystone deposits that likely linked to a globally increased supply of clastic sediments into marginal and deeper marine basin;(3)propose that the low C_(org):P_(total) ratios,in combination with bioturbated structure and depletion or slight enrichment in redox-sensitive trace elements of V,Mo,and U suggest a long-term oxygenation event throughout the T-OAE interval at the Sewa succession,and hence,anoxia may not play a fundamental role during the Toarcian negative CIE in this setting;(4)exhibit that a warming and more humid climate began at the start of the T-OAE,and many episodic changes in sediment provenance throughout the T-OAE interval occurred at this location;and(5)suggest that accumulation of organic-matter sediments during the T-OAE is generally controlled by global climatic changes,but a regional environmental perturbation also might influence the preservation of organic matter.

Continental chemical weathering during the Early Cretaceous Oceanic Anoxic Event(OAE1b): a case study from the Fuxin fluvio-lacustrine basin, Liaoning Province,NE China

This study focuses on Early Cretaceous mudstones from the Shahai and Fuxin formations in the Fuxin continental basin. We analyse chemical weathering, land surface temperatures and palaeoclimates based on chemical weathering indices, and emphasize the implications of continental chemical weathering on nutrient fluxes into lakes and oceans.According to Cr and Ni abundance, Al2O3-TiO2, La/Sc-Th/Co and V-Ni-Th×10 plots, as well as rare earth element(REE)analysis, mudstone samples from the Shahai and Fuxin formations were derived from the same type of provenance comprising mainly felsic igneous rocks. Chemical weathering trends reflected by the Chemical Index of Alteration(CIA),Weathering Index of Parker(WIP) and the Mafic Index of Alteration for Oxidative weathering environments(MIA(O)) are consistent with each other and allow the geological succession to be divided into four stages. Land surface temperatures of the Shahai and Fuxin formations are estimated based on the linear relationship of CIA to temperature,and also can be divided into four stages consistent with those determined from chemical weathering trends. During Stage A(early part of the late Aptian) chemical weathering and land surface temperatures were relatively low and showed characteristic high fluctuations, while Stage B(latest Aptian) represented a transitional period where weathering rates and temperatures increased, and high amplitude fluctuations continued. Conditions changed markedly in Stage C(early Albian) with very high and stable weathering, and warm, humid climates, while in Stage D(middle and late Albian) conditions returned to low chemical weathering and land surface temperatures. These stages of chemical weathering and land surface temperature fluctuations represent responses to global climate fluctuations during the Early Cretaceous, with the early Albian high weathering intensities and warm, humid climates combining to create high nutrient levels that would have flushed through rivers into lakes and ultimately oceans. This correlates stratigraphically with the development of Early Cretaceous black shales during Ocean Anoxic Event 1 b, showing the importance of continental weathering regimes as a causal mechanism for lake and ocean anoxia.

Applications of mercury stable isotopes for tracing volcanism in the geologic record

Mercury(Hg)enrichments in ancient sediments have been used as a proxy of volcanism,especially large igneous province(LIP)eruptions.However,considering the existence of other potential Hg sources besides volcanoes and the diverse factors(e.g.,organic matters,clay minerals,sulfide minerals and Fe oxides)that can affect Hg sequestration,there are considerable uncertainties to simply regard sedimentary Hg anomalies as signatures of volcanic activities.Mercury stable isotopes,a promising tool for tracing the origins and transformations of Hg,have been increasingly used for determining the causes of Hg spikes and understanding the geochemical behavior of Hg in the geologic record.To date,lots of researches have applied Hg concentrations and Hg isotopes to identify LIP volcanisms linking with significant geological events such as mass extinctions,ocean anoxic events and other environmental perturbations that mainly occurred in the Phanerozoic.However,the results in previous studies clearly show that not all Hg enrichments are derived from volcanic inputs,which emphasize the need for more caution in using Hg as a fingerprint of volcanism.With a better understanding of Hg isotopes in the future,there will be important implications for Hg isotopes to reconstruct volcanic activities in the rock records and their impacts on biological evolution.

Characterization of the negative carbon isotope shift in segment C2,its global implications as a harbinger of OAEla

Lower Cretaceous C-isotope records show intermittent negative/positive spikes,and consistent patterns of coeval chemostratigraphic curves thus document shifts that signal simultaneous responses of temporal changes in the global carbon reservoir.The standard pattern registered by theδ^(13)C_(org)andδ^(13)C_(carb)in Lower Aptian sediments includes distinct isotope segments C1 to C8(Menegatti et al.,1998).In the El Pui section,Organya Basin,Spain,C-isotope segment C2 is the longest interval preceding segments C3-C6 associated with oceanic anoxic event la(OAE la),and reveals a distinct negative shift of^1.8‰to^2.23‰defining the C-isotope pattern within that interval.Total inorganic carbon(TIC),total organic carbon(TOC),δ^(13)C_(org),microfacies,n-alkanes show no difference before,during,or after the negative inflection.The biomarkers indicate that organic matter(OM)mainly originates from algal/microbial sources because short-chain length homologues(≤nC_(19))dominate.nC_(20)through nC_(25)indicate some contribution from aquatic vegetation,but little from higher plants(>nC_(25)),as also suggested by the terrestrial/aquatic ratio of n-alkanes or(TAR)=[(nC_(27)+nC_(29)+nC_(31))/(nC_(15)+nC_(17)+nC_(19))](averages 0.085).We suggest that conjoint pulses of contemporaneous LIPs(Ontong Java)and massive explosive volcanism in northeast Asia,the Songliao Basin(SB-V),best conform to plausible causes of the negative intra-C2 carbon isotopic excursion(CIE)at that time.Because of its apparent common occurrence the intra-C2 inflection could be a useful marker harbinger to the more pronounced CIE C3,the hallmark of OAE1a.