Palaeoenvironmental and diagenetic reconstruction of a closed-lacustrine carbonate system - the challenging marginal setting of the Miocene Ries Crater Lake (Germany)

Chemostratigraphic studies on lacustrine sedimentary sequences provide essential insights on past cyclic climatic events, on their repetition and prediction through time. Diagenetic overprint of primary features often hinders the use of such studies for palaeoenvironmental reconstruction. Here the potential of integrated geochemical and petrographic methods is evaluated to record freshwater to saline oscillations within the ancient marginal lacustrine carbonates of the Miocene Ries Crater Lake (Germany). This area is critical because it represents the transition from shoreline to proximal domains of a hydrologically closed system, affected by recurrent emergent events, representing the boundaries of successive sedimentary cycles. Chemostratigraphy targets shifts related to subaerial exposure and/or climatic fluctuations. Methods combine facies changes with δ13C–δ18O chemostratigraphy from matrix carbonates across five closely spaced, temporally equivalent stratigraphic sections. Isotope composition of ostracod shells, gastropods and cements is provided for comparison. Cathodoluminescence and back‐scatter electron microscopy were performed to discriminate primary (syn‐)depositional, from secondary diagenetic features. Meteoric diagenesis is expressed by substantial early dissolution and dark blue luminescent sparry cements carrying negative δ13C and δ18O. Sedimentary cycles are not correlated by isotope chemostratigraphy. Both matrix δ13C and δ18O range from ca −7·5 to +4·0‰ and show clear positive covariance (R = 0·97) whose nature differs from that of previous basin‐oriented studies on the lake: negative values are here unconnected to original freshwater lacustrine conditions but reflect extensive meteoric diagenesis, while positive values probably represent primary saline lake water chemistry. Noisy geochemical curves relate to heterogeneities in (primary) porosity, resulting in selective carbonate diagenesis. This study exemplifies that ancient lacustrine carbonates, despite extensive meteoric weathering, are able to retain key information for both palaeoenvironmental reconstruction and the understanding of diagenetic processes in relation to those primary conditions. Also, it emphasizes the limitation of chemostratigraphy in fossil carbonates, and specifically in settings that are sensitive for the preservation of primary environmental signals, such as lake margins prone to meteoric diagenesis

Ca isotope constraints on chemical weathering processes: evidence from headwater in the Changjiang River, China

This study aims to clarify the relationship between chemical weathering of rocks and the carbon budget of rivers and better understand the weathering mechanisms of plateau watersheds. We chose to study the Jinsha River, which originates from the Tibetan Plateau and also is in the upper reaches of the Changjiang River. Analysis of hydrochemistry, radiogenic strontium isotope and stable calcium isotopes were conducted of the Jinsha River water samples, which were collected along its mainstream and main tributaries in the summer. The results show that the water chemistry of the mainstream waters is dominated by evaporite weathering, which have low 87Sr/86Sr values (0.7098–0.7108) and wide range of Sr contents (2.70–9.35 μmol/L). In contrast, tributaries of the Jinsha River have higher 87Sr/86Sr (0.7090–0.7157) and lower Sr contents (∼1 μmol/L). Moreover, the Ca isotopic compositions in the mainstream (0.87–1.11‰) are heavier than the tributaries (0.68–0.88‰) and could not be fully explained by the conventional mixing of different sources. We suggest that secondary carbonate precipitation fractionates Ca isotopes in the Jinsha River, and fractionation factors are between 0.99935 and 0.99963. At least 66% of Ca was removed in the mainstream of the Jinsha River through secondary mineral precipitation, and the average value is ∼35% in the tributaries. The results highlight that evaporite weathering results in more carbonate precipitation influencing Ca transportation and cycling in the riverine system constrained by stable Ca isotopic compositions and water chemistry

My Childhood Archive

<p>This is a small project containing images of items from my childhood that carry an important meaning for me,</p&gt

Crystallization of ikaite and its pseudomorphic transformation into calcite: Raman spectroscopy evidence

Ikaite (CaCO3 6H2O) is a metastable phase that crystallizes in nature from alkaline waters with high phosphate concentrations at temperatures close to 0 C. This mineral transforms into anhydrous calcium carbonate polymorphs when temperatures rise or when exposed to atmospheric conditions. During the transformation in some cases the shape of the original ikaite crystal is preserved as a pseudomorph. Pseudomorphs after ikaite are considered as a valuable paleoclimatic indicator. In this work we conducted ikaite crystal growth experiments at near-freezing temperatures using the single diffusion silica gel technique, prepared with a natural aqueous solution from the polymictic lake Laguna Potrok Aike (51 570 S, 70 230 W) in Patagonia, Argentina. The ikaite crystals were recovered from the gels and the transformation reactions were monitored by in situ Raman spectroscopy at two different temperatures. The first spectra collected showed the characteristic features of ikaite. In successive spectra new bands at 1072, 1081 and 1086 cm1 and changes in the intensity of bands corresponding to the OH modes were observed. These changes in the Raman spectra were interpreted as corresponding to intermediate stages of the transformation of ikaite into calcite and/or vaterite. After a few hours, the characteristics of the Raman spectrum were consistent with those of calcite. While ikaite directly transforms into calcite at 10 C in contact with air, at 20 C this transformation involves the formation of intermediate, metastable vaterite. During the whole process the external shape of ikaite crystals was preserved. Therefore, this transformation showed the typical characteristics of a pseudomorphic mineral replacement, involving the generation of a large amount of porosity to account for the large difference in molar volumes between ikaite and calcite. A mechanism involving the coupled dissolution of ikaite and crystallization of calcite/vaterite is proposed for this replacement

Lateglacial and Holocene climatic changes in south-eastern Patagonia inferred from carbonate isotope records of Laguna Potrok Aike (Argentina)

First results of strontium, calcium, carbon and oxygen isotope analyses of bulk carbonates from a 106 m long sediment record of Laguna Potrok Aike, located in southern Patagonia are presented. Morphological and isotopic investigations of μm-sized carbonate crystals in the sediment reveal an endogenic origin for the entire Holocene. During this time period the calcium carbonate record of Laguna Potrok Aike turned out to be most likely ikaite-derived. As ikaite precipitation in nature has only been observed in a narrow temperature window between 0 and 7 °C, the respective carbonate oxygen isotope ratios serve as a proxy of hydrological variations rather than of palaeotemperatures. We suggest that oxygen isotope ratios are sensitive to changes of the lake water balance induced by intensity variations of the Southern Hemisphere Westerlies and discuss the role of this wind belt as a driver for climate change in southern South America. In combination with other proxy records the evolution of westerly wind intensities is reconstructed. Our data suggest that weak SHW prevailed during the Lateglacial and the early Holocene, interrupted by an interval with strengthened Westerlies between 13.4 and 11.3 ka cal BP. Wind strength increased at 9.2 ka cal BP and significantly intensified until 7.0 ka cal BP. Subsequently, the wind intensity diminished and stabilised to conditions similar to present day after a period of reduced evaporation during the “Little Ice Age”. Strontium isotopes (87Sr/86Sr ratio) were identified as a potential lake-level indicator and point to a lowering from overflow conditions during the Glacial (∼17 ka cal BP) to lowest lake levels around 8 ka cal BP. Thereafter the strontium isotope curve resembles the lake-level curve which is stepwise rising until the “Little Ice Age”. The variability of the Ca isotope composition of the sediment reflects changes in the Ca budget of the lake, indicating higher degrees of Ca utilisation during the period with lowest lake level

Late Holocene hydrology inferred from lacustrine sediments of Laguna Cháltel (southern Argentina)

Hydrological changes that occurred during the last 4700 years have been reconstructed using multi-proxy analyses of sediment cores from the volcanic crater lake of Laguna Cháltel (50°S, 71°W). The chronology is based on AMS 14C age modeling constrained by paleomagnetic secular variations. Chemical and physical properties of the lake water together with results of lake-water surface-temperature monitoring, as well as sediment characteristics reveal conspicuous features archived as different sedimentary carbonate phases and morphologies which are attributed to lake-level changes.Sedimentological, geochemical and biological proxies together suggest the development from an initial playa lake-phase towards a system with progressively rising lake level. In detail, proxies indicate the existence of an ephemeral lake since 4700 cal BP until a glauberite-bearing carbonate crust formed around 4040 cal BP which probably is associated to the globally recognized 4.2 ka event. This crust is interpreted as a desiccation event terminating the ephemeral lake phase. Following this desiccation a shift towards conditions with a positive hydrological balance of Laguna Cháltel occurs, which leads to the development of a saline lake with ooid formation between 4040 and 3200 cal BP. Further lake-level increase with initially high minerogenic input until 2700 cal BP resulted in a lake freshening which allowed the preservation of diatoms. Sigmoidal and star shaped carbonate crystals occurred until 1720 cal BP indicating a syn- or post-depositional formation of ikaite. Anoxic conditions and increased deposition of clay and sand through fluvial and eolian input are interpreted as a further lake-level rise and/or a prolonged winter ice cover culminating during the Little Ice Age. The highest lake level was probably reached at that time and since then dropped to its present day height.Previous studies have shown that the southern hemisphere westerly winds (SWW) exert an oppositional control on hydrological regimes at the eastern and the western sides of the Patagonian Andes. At Laguna Cháltel SWW forcing is changing evaporation rates by varying wind intensities, air temperatures and lake ice coverages as well as by precipitation rates (easterly vs. westerly sources of moisture). Our data suggests that the lake-level history of Laguna Cháltel reflects changes in the SWW during the last 4.7 ka on the eastern side of the Andes. However, the elevated location of Laguna Cháltel on an 800 m high plateau at the leeward side of the Andes potentially leads to a local overprint of the SWW influence on the hydrological balance