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The variations in carbon isotope (δ13C) values of the Aptian marine and terrestrial strata are globally comparable and regarded as an important tool for the stratigraphic correlation of the Aptian successions. The most remarkable feature of the δ13C curve of the Aptian is an abruptly negative excursion, which characterizes the onset of the early Aptian Oceanic Anoxic Event (OAE 1a, ~121 Ma). However, the identification of OAE 1a equivalent level in the Tibetan Himalaya remains debatable. Based on previously well-established foraminiferal biostratigraphy, we provide a lower Aptian high-resolution δ13C curve of bulk organic carbon from an expanded shale-dominated section, Chaqiela Section in the Tibetan Himalaya. The TOC/TN ratios of the studied section vary from 3 to 10, indicating that the organic matter is mainly sourced from marine plankton. Thus, we suggest that our results present the secular changes of δ13C in the dissolve carbon pool of the shallow sea on the south margin of eastern Tethys. The lower Aptian δ13C curve of the Chaqiela section is divided into eight segments, which can be well correlated to the representative sections in the western Tethys area. In the lower part of the section, an interval of sharp negative carbon isotope excursion (CIE) of ~1 ‰ is observed, which is followed by recovery of the δ13C values by a magnitude of ~2 ‰. This short-term carbon perturbation is superimposed on the long-term decrease in δ13C values of the early Aptian. The δ13C profile at this interval perfectly records the diagnostic characteristics of the OAE 1a: negative (C3), positive (C4), steady (C5), and positive (C6) segments. Lacking covariations of δ13C values with TOC/TN and TOC, and δ13C and Al in this interval indicate that the δ13C variations are not attributed to source changes of the organic matter. No obvious enrichments of the redox-sensitive major and trace elements indicate an oxic to sbuoxic condition during the sedimentation, it is in good agreement with the low Corg:Ptot ratios, but the slightly increase of Corg:Ptot ratios during the OAE 1a interval may imply a relatively oxygen-reducing trend and sustains till the end of OAE 1a. The paleoclimatic proxies imply a relatively warm and humid paleo-environment in mid-high latitudes (~50°S) of Tibetan Himalaya during this time interval, it is also testified by the moderate chemical weathering condition revealed by the A-CN-K ternary diagram.

Paleoclimate during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, 94.5–93.9 Ma) was characterized by elevated atmospheric CO2 concentrations and peak global temperatures. In this study, we employ δ66Zn measured on samples spanning OAE 2 in an expanded hemipelagic section in Tibet to trace changes in the major fluxes that influence biogeochemical cycles. The prominent feature of the δ66Zn record in the studied section is a continuous decrease from ∼1‰ at the onset of OAE 2 to a minimum of ∼0.2‰ within the Plenus Cold Event (ca. 94.3 Ma), followed by a stepwise recovery through the upper part of OAE 2. The negative shift in δ66Zn corresponds with higher terrigenous inputs, as revealed by previously published detrital index and TOC/TN (total organic carbon to total nitrogen) ratio records, and covaries with a notable decreasing trend recorded in compiled pCO2 data of different basins. We propose that influx of isotopically light Zn from weathered volcanic rocks associated with submarine large igneous provinces and/or (sub)tropic Indian continental volcanics is likely responsible for the δ66Zn decrease. We infer that the recovery of δ66Zn was caused by continued high primary production and an inevitable decline in the flux of light Zn as volcanic terrains were progressively weathered. The ultimate cessation of OAE 2 may have been a consequence of the same effect, with the nutrient supply from weathering reaching a minimum threshold to maintain productivity-anoxia feedback.

Whether ooids are formed through biotic or abiotic progress seems to become increasingly controversial in recent decades. New studies indicate that the microbial effect could make important contributions to the construction, destruction and modification of ooids. Previous studies mainly focused on the organomineralization of carbonate minerals in ooids. This paper investigates the formation process of iron minerals in red oolitic grainstones of the Lower Jurassic Nieniexiongla Formation in the Tethyan Himalaya of southern Tibet. Petrographic studies and electron probe microanalysis revealed that the colorful minerals present in red ooids are hematites. Sulfur was detected in the hematites, which indicates that they were transformed from precursor pyrites. These hematites retain euhedral or framboid shapes of the pyrites. Hematite pseudomorphs distribute randomly in ooids and could not form a whole concentric layer of the ooids. Euhedral crystals usually have larger diameters than amorphous spheres and are scattered mostly in nuclei. Based on the distribution and morphology of hematites, we speculate that precursor pyrites were formed through metabolism of sulfate-reducing bacteria. Moreover, the timing of the pyrite formation was after the deposition but before the lithification. The decomposition of the microorganisms within the ooids by bacterial sulfate-reducing provides the HS-, the degradation of organic matters would produce spaces for the intrusion of pore water with sufficient Fe2+ to facilitate the pyrite deposition. In nuclei, the framboid aggregates of pyrites were transferred to euhedral crystals through continuous growth of the constituent microcrystals. Our study demonstrated organomineralization of sulfate-reducing bacteria mediated pyrites in carbonate ooids.

The mid-Cretaceous greenhouse climate was induced by volcanic outgassing and the release of biogenic or thermogenic methane into the ocean-atmosphere system. During this period, major episodes of oceanic anoxic conditions enabled the large scale deposition of marine black shales rich in organic carbon, serving as a source for methane production. Studies on the anaerobic oxidation of methane, the key biogeochemical process at sites where methane is transported toward the seafloor, and its associated authigenic carbonates from mid-Cretaceous strata can contribute to elucidate such extreme climatic conditions. A total of nine layers of authigenic carbonates were recognized in the sequence of mid-Cretaceous inner shelf sedimentary rocks of the Qiangdong section, Gamba area, southern Tibet. In this study, we decipher the mode and causes of carbonate authigenesis by combining field observation, thin section petrography, mineralogy, and carbon and oxygen stable isotope geochemistry. The lowest obtained δ13Ccarb value of −30.5‰ is not low enough to exclude oil compounds as a carbon source, but since other evidence of oil seepage is lacking, methane is the most likely carbon source that was admixed to dissolved inorganic carbon to form authigenic carbonate. Typical seep carbonate phases including 13C depleted matrix micrite, clotted micrite, and banded and botryoidal cement as well as three types of authigenic pyrite comprising framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals indicate the occurrence of methane seepage in the mid-Cretaceous depositional environments. In situ brecciation and subvertical tubular carbonates – the former reflecting rupture caused by local fluid overpressure and the latter interpreted as part of a plumping system – agree with widespread and vigorous seepage. Expanded shallow and deep-water anoxia in combination with low seawater sulfate concentration prior to the oceanic anoxic event 2 (OAE 2) event apparently caused an increased flux of organic matter to the methanogenic zone, stimulating methanogenesis and enabling a higher flux of methane toward the seafloor. In the water column, aerobic oxidation of methane would have enhanced oxygen depletion, thereby contributing to black shale deposition and providing a positive feedback to organic matter preservation and burial. Based on correlation with coeval authigenic carbonate deposits from the Tethyan continental margin, we conclude that organic matter accumulation and sea-level change were the main factors controlling carbonate authigenesis in the mid-Cretaceous.

During the Albian and Cenomanian, the Earth underwent profound climatic and oceanographic changes that were recorded in sedimentary successions on a global scale. Carbon isotope records spanning this time interval have been established in the western Tethys, eastern Pacific and North Atlantic Oceans, but not yet in the eastern Tethys Ocean. In this paper, we present biostratigraphic, chemostratigraphic and cyclostratigraphic characteristics of the uppermost Albian–lowermost Cenomanian in an eastern Tethyan section (Youxia, southern Tibet). Based on calcareous nannofossil biozones and the bulk rock δ13C curve, the Albian-Cenomanian boundary interval (ACBI) was identified and correlated to the western Tethys and Atlantic Oceans. In the Youxia section, δ13C values range from approximately 0‰ to +1.3‰ (−0.03‰ to +1.31‰). Four subevents (a, b, c and d) were distinguished in the ACBI carbon isotope curve via correlation with other sections. Based on a spectral analysis of the carbonate content, we recognized Milankovitch short eccentricity (~100 kyr) and precession (22.2 kyr) cycles, suggesting that orbital variations modulated depositional processes. The duration of the ACBI was estimated at ~311 kyr, while OAE 1d lasted for ~233 kyr in the eastern Tethys Ocean, consistent with the duration calculated from Atlantic Ocean records.

The role of microbial vs. abiotic processes in ooid formation has been a controversial topic for over a century. Recent sedimentological, geochemical and biomarker data indicate that microbes make an important contribution to the construction, destruction and modification of ooids. Here, we present a detailed petrographic and chemical investigation of well-preserved red oolites from the Lower–Middle Jurassic Nieniexiongla Formation in the Tethyan Himalaya of southern Tibet. Petrographic studies and electron probe microanalysis indicate that the red coloration is due to the presence of hematite minerals. The hematites show euhedral and amorphous spheroidal morphology and residual elemental sulfur was detected within minerals. The δ34S values in the leachate (δ34SNaCl) and in carbonate associated sulfate (δ34SCAS) of red oolites are lower than gray oolites and micritic limestones. Moreover, δ34SNaCl and δ34SCAS values are lowest in samples with the highest content of hematitic ooids. Together, these observations suggest that the hematites are transformed from pyrite. The Fe2+ (and possibly SO42−) provided by pore-water combined with degradation of syn-depositional organic matter within ooids produced anoxic microenvironments that facilitated local pyrite precipitation. Some spheroidal pyrites were subsequently transformed to euhedral crystals through continuous growth of the constituent microcrystals. This study demonstrates that organomineralization mediated by sulfate-reducing bacteria can produce pyrite grains within carbonate ooids. The results emphasize the important role that microbial communities play in the deposition and modifications of ooids.

The early Aptian oceanic anoxic event (OAE 1a) is well known in the western Tethys, the North Atlantic and the Pacific Ocean, but has not been reported in the eastern Tethys to date. In this paper, we present bulk organic carbon isotope data and ammonite biostratigraphy of a lower Aptian succession from the Gucuo area (southern Tibet). These findings document the occurrence of the OAE 1a for the first time from the eastern Tethys. The studied sequence can be attributed to the D. forbesi and D. deshayesi ammonite zones of the lower Aptian. The δ 13 C org data can be correlated with published early Aptian carbon isotope records from the western Tethys and the Pacific. A distinctive negative carbon isotope excursion of 2.4‰ in the upper part of the section corresponds to segment C3 of the OAE 1a, and the following positive excursion correlates to segment C4. The absolute values of the carbon isotope ratio in the Gucuo area are higher than those of known sections in the western Tethys and equatorial Pacific. We suggest that diagenetic alteration is the major cause of the higher absolute values in the Gucuo area.