New Chronostratigraphic Constraints on the Lower Jurassic Pliensbachian–Toarcian Boundary at Chacay Melehue (Neuquén Basin, Argentina)

The Pliensbachian–Toarcian boundary interval is characterized by a ~3‰ negative carbon-isotope excursion (CIE) in organic and inorganic marine and terrestrial archives from sections in Europe, such as Peniche (Portugal) and Hawsker Bottoms, Yorkshire (UK). A new high-resolution organic-carbon isotope record, illustrating the same chemostratigraphic feature, is presented from the Southern Hemisphere Arroyo Chacay Melehue section, Chos Malal, Argentina, corroborating the global significance of this disturbance to the carbon cycle. The negative carbon-isotope excursion, mercury and organic-matter enrichment is accompanied by high-resolution ammonite and nannofossil biostratigraphy together with U-Pb CA-ID-TIMS geochronology derived from intercalated volcanic ash beds. A new age of ~183.71 ± 0.40/-0.51 Ma for the Pliensbachian–Toarcian boundary, and 182.77 +0.11/-0.21 for the tenuicostatum–serpentinum zonal boundary, is assigned based on high-precision U-Pb zircon geochronology and a Bayesian Markov chain Monte Carlo (MCMC) stratigraphic age model.

A new record of the Toarcian oceanic anoxic event from Scotland (UK) and environmental responses

<p>The early Toarcian oceanic anoxic event (T-OAE, ~183 Ma) was characterized by a prominent environmental perturbation, likely associated with a large amount of <sup>12</sup>C-enriched carbon released into the global ocean-atmosphere system. This effusion caused a marked disruption to the global carbon cycle and propagated a series of remarkable changes in ocean chemistry and climate. Although the T-OAE has been recognized worldwide, clear geographic differences in the character of the event and its environmental effects have been recognized. Here, we present new geochemical data from a lower Toarcian succession on the Isle of Raasay, NE Scotland (Hebrides Basin, Northwest European Shelf). Organic carbon isotope data through the Raasay section reveal a pronounced negative excursion, similar to that recognised globally. The excursion interval is enriched in organic matter, and redox sensitive element data suggest that suboxic bottom water conditions contemporaneously occurred, likely interspersed with anoxic episodes. Our findings contrast with evidence of more pervasive anoxia/euxinia in nearby basins, and emphasize how deoxygenation was spatially variable within the T-OAE. Inorganic geochemical data and sedimentological observations suggest a significant enhancement in chemical weathering and coarse-grained detrital flux during the T-OAE on Raasay. These findings support evidence from other localities for a strengthening of hydrological cycling in response to global warming during the T-OAE.</p>

Early to Mid-Holocene Palaeoenvironment Change and Sedimentary Evolution in the Xianghu Area, Zhejiang

A 2.5 m long sediment core (XH-2) obtained from Xianghu area, near the Kuahuqiao site, were analyzed for grain size, diatom index, and geochemistry of organic carbon. The results of the total organic carbon (TOC) and stable organic carbon isotope (δ13C) in sediment samples from core XH-2 in the Xianghu area in Zhejiang Province have revealed the evolution history of sedimentary environmental and climatic changes during the breeding–prosperity–decline period of the Kuahuqiao culture. During 9300–8200 cal a BP, TOC contents were relatively high and stable, whereas δ13C values tended to be negative. This condition indicated that the climate was humid, and the sedimentary environment in the Xianghu area was stable. During 8200–7500 cal a BP, TOC contents presented a fluctuating declining trend, and δ13C values were significantly high, implying that the climate was arid, and the Xianghu area was gradually reduced to land. Thus, conducive conditions were provided for the development of the Kuohuqiao culture (7700–7400 cal a BP). From 7500 cal a BP, TOC contents obviously declined, and δ13C values were partially low, suggesting strengthened hydrodynamic force and wet conditions in the Xianghu area. This condition was related to the rise in sea level at approximately 7400 cal a BP, and the Kuahuqiao site became obsolete due to the transgression event. The TOC contents in core XH-2 were remarkably influenced by grain size, whereas no significant correlation existed between the δ13C variability and grain size. Sedimentary environment changes in the Xianghu area from 9300 to 6600 cal a BP, which was reflected by the TOC and δ13C records in core XH-2, accorded with the diatom results in this core and those in the Baima Lake area.