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The high-altitude lakes of the Altiplano–Puna Plateau in the Central Andes commonly have large radiocarbon reservoir effects. This, combined with the general scarcity of terrestrial organic matter, makes obtaining a reliable and accurate chronological model based on radiocarbon ages a challenge. As a result, age–depth models based on radiocarbon dating are often constructed by correcting for the modern reservoir effect, but commonly without consideration of spatial and possible temporal variations of reservoir ages within the lake and across the basin. In order to get a better constraint on the spatial variability of the radiocarbon reservoir effects, we analyse 14 C ages of modern terrestrial and aquatic plants from the El Peinado basin in the southern Puna Plateau, which hosts Laguna del Peinado fed by hydrothermal springs. The oldest 14 C ages of modern samples (> 18 000 and > 26 000 BP) were found in hot springs discharging into the lake, likely resulting from the input of 14 C-depleted carbon from old groundwater and 14 C-free magmatic CO 2. In the littoral and central part of Laguna del Peinado, 14 C ages of modern samples were several thousand years younger (> 13 000 and > 12 000 BP) compared to the inflowing waters as a result of CO 2 exchange with the atmosphere. Altogether, our findings reveal a spatial variability of up to 14 000 14 C years of the modern reservoir effect between the hot springs and the northern part of the Peinado lake basin. Temporal changes of reservoir effects in sediment records are more difficult to quantify, but 14 C ages from a short core from Laguna del Peinado may suggest temporal reservoir age variations of a few thousand years. This study has implications for accurate 14 C-based chronologies for palaeoclimate studies in the Altiplano–Puna Plateau and similar settings. Our results highlight the need to consider spatial and likely also temporal variations in the reservoir effects when constructing age–depth models. [ABSTRACT FROM AUTHOR]

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 are 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 ~ 183.73 + 0.35/− 0.50 Ma for the Pliensbachian–Toarcian boundary, and 182.77 + 0.11/− 0.15 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. [ABSTRACT FROM AUTHOR]

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