Radiocarbon dating of organic-rich deposits: Difficulties of paleogeographical interpretations in highlands of Russian Altai

The high mountainous southeastern part of Russian Altai is characterized by complicated sedimentation history. As a result of tectonic movements, Paleogene, Neogene, and even more old Carboniferous and Jurassic organicrich deposits had been partly uplifted and exhumed on the ridge’s slopes, where during the Pleistocene, they were affected by various exogenous processes including glaciation, glacio-fluvial erosion, winnowing activity of ice-dammed lakes, sliding during lake-draining events, followed by further intensive Holocene erosion, pedogenesis, and permafrost formation/degradation.Remobilized ancient organic matter had been involved into geomorphic and pedogenesis processes and affected the results of radiocarbon dating. Numerous radiocarbon ages obtained revealed several typical problems in interpretation of dating results, which was confirmed by multidisciplinary investigations of associated sediments in a wider regional context.This article presents a discussion on obtained apparent radiocarbon dates of organic material from ten sections of the SE Altai. In addition to radiocarbon analysis, in each case multidisciplinary study was carried out in order to properly interpret obtained dates, as well as to explain the inability of directly using apparent 14C ages as a geochronological basis for paleogeographical reconstruction. The analysis presented is of vital importance for establishing the chronology of formation of large ice-dammed lakes and their cataclysmic draining; revealing chronology and paleoenvironmental conditions of pedogenesis in the highlands of the SE Altai; and estimating the range and magnitude of the tectonically driven topography rebuilding in the post-Neogene time.

Stacked megafans of the Kalahari Basin as archives of paleogeography, river capture, and Cenozoic paleoclimate of southwestern Africa

ABSTRACT The Cenozoic Kalahari Basin covers large parts of southern Africa. A continuous 400 m core was obtained in northern Namibia and analyzed in detail. Here, we present sedimentological, geochemical, mineralogical, granulometric, and hydraulic data, which were used to derive the sedimentation history and the Cenozoic paleoclimate and paleogeography of SW Africa. The first absolute ages for the Kalahari Basin were obtained by dating of calcretes, which showed that the core covers almost the entire Cenozoic. Two megafans could be distinguished. The older, buried Olukonda Megafan stems from a mafic source rock, potentially the Kunene Intrusive Complex, and was deposited by a paleo–Kunene River towards the southeast and east, under a semiarid climate. The younger Cubango Megafan (Andoni Formation) has a completely different provenance, namely felsic metamorphic and granitoid rocks, transported from the north by the Cubango River. The capture of the Kunene towards the Atlantic during the Eocene resulted in this change in provenance. Despite the distinct differences between the formations, the temporal hiatus between them must have been short. The results are a showcase of the potential of megafans for hosting major deep freshwater aquifers.

Impact of sedimentation history for As distribution in Late Pleistocene-Holocene sediments in the Hetao Basin, China

Purpose To understand the impact of geochemical sedimentation history for arsenic (As) distribution in the sediment profiles of the Hetao Basin, we (1) evaluated sediments provenance and variations of weathering intensities, (2) attempted to reconstruct the depositional environments, and (3) explored the As and Fe speciation in the sediments. Combining the information above, different sedimentation facies were distinguished in the vertical profiles. Methods Two sediments cores were drilled up to 80 m depth. Major and trace element compositions, including rare earth elements (REE), were analyzed. Carbon isotope ratios (δ13Corg) of embedded organic matter in the sediments were analyzed by isotope ratio mass spectrometry (IR-MS). Arsenic and Fe speciation of the sediments were determined by sequential extractions. Results and discussion The similar REE geochemistry of rocks from the Lang Mountains and sediments in the Hetao Basin indicated that the sediments originated from the Lang Mountains. The C/N ratio (~ 4 to ~ 10) in combination with δ13Corg (− 27‰ to −2 4‰) suggested that sediments were mainly deposited in aquatic environments. The unconfined aquifer equaled the lacustrine deposit with less intensive weathering during last glacial maximum (LGM). Here, the As content (average, 5.4 mg kg−1) was higher than in the aquifer sediments below (average, 3.6 mg kg−1). Conclusion Higher content of releasable As in combination with paleolake-derived organic matter aquifer sediments probably contributes to higher groundwater As concentration in the unconfined aquifer. This study provides the first insight into the impact of sedimentation history on As distributions in sediment profiles in the Hetao Basin.