Controls on Cyclic Formation of Quaternary Early Diagenetic Dolomite

The origin of sedimentary dolomite and the factors that control its formation within the geological record remain speculative. In most models, dolomite formation is linked to evaporative conditions, high water temperature, increasing Mg/Ca ratio, increasing alkalinity, and high amounts of biomass. Here we challenge these archetypal views, by documenting a case example of Quaternary dolomite which formed in Lake Van at constantly low temperature (<4°C) and without direct control of the latter conditions. Dolomite occurs within highstand sediments related to suborbital climate variability (Dansgaard‐Oeschger cycles). We propose that dolomite precipitation is a product of a microbially influenced process, triggered by ecological stress, resulting from reventilation of the water‐sediment interface. Independently from the validity of this hypothesis, our results call for a reevaluation of the paleoenvironmental conditions often invoked for early diagenetic dolomite‐rich intervals within sedimentary sequences and for caution when interpreting time series of subrecent lacustrine carbonates

Refining the interpretation of lacustrine carbonate isotope records: Implications of a mineralogy-specific Lake Van case study

Oxygen and carbon isotope (δ18O and δ13C) data from bulk carbonates are widely applied proxies for temperature, the precipitation/evaporation ratio and productivity in lacustrine palaeohydrology and palaeoclimatology. In case of the terminal and alkaline Lake Van, however, previous studies have shown that bulk oxygen isotope compositions are in disagreement with other proxies when interpreted in a conventional manner. Similarly, the reports on the nature and the timing and site of carbonate precipitation in Lake Van are inconsistent. This study provides evidence on the mineralogy (X-ray powder diffraction analysis, scanning electron microscope imaging, confocal Raman microscopy, electron microprobe analysis) and isotope composition (δ18O and δ13C) of non-skeletal carbonate minerals in a Lake Van sedimentary profile spanning the last ca. 150 kyr. Carbonate phases present in the sediment include aragonite, low-Mg calcite, and calcian dolomite. Dolomite forms as an early diagenetic phase and occurs episodically in high concentrations driving the bulk isotope record towards the higher dolomite δ18O and varying δ13C values. Aragonite and low-Mg calcite precipitate in the surface water and are present in the sediments in varying amounts (relative aragonite to calcite content for dolomite-poor samples Ar/(Ar + Cc) of 93 to 41 wt%). In an attempt to explain this variation, we revised a precipitation model based on annually laminated sediments containing both aragonite and calcite spatially separated in light and dark coloured laminae, respectively. According to our model, spring calcite precipitation, under close-to-freshwater conditions, is followed by evapoconcentration-driven aragonite precipitation in late summer. The precipitation of these carbonate polymorphs from chemically differing surface waters (i.e. freshwater-influenced and evapoconcentrated) leads to distinctly different oxygen and carbon isotope signatures between sedimentary penecontemporaneous aragonite and calcite. The δ18O and δ13C values of aragonite relative to calcite are significantly higher by several per mille than inferred from aragonite-calcite fractionation factors alone, suggesting that the generalised assumption of sedimentary coeval calcite and aragonite precipitating from water with the same isotopic composition is flawed. The here proposed revised hydrologically-separated carbonate precipitation model is not only taking (i) differences in the isotopic fractionation between carbonate minerals into account, but also (ii) considering the hydrological conditions and the processes favouring the precipitation of a given mineral and ultimately controlling its isotopic composition. If mixed mineralogies are present, this mineralogy-specific approach has the potential of refining environmental reconstructions and reconciling apparently equivocal interpretations of different proxy records

Ventilation and cave air PCO2 in the Bunker-Emst Cave System (NW Germany): implications for speleothem proxy data

Cave air pCO2 (carbon dioxide partial pressure) is, along with drip rate, one of the most important factors controlling speleothem carbonate precipitation. As a consequence, pCO2 has an indirect but important control on speleothem proxy data (e.g., elemental concentrations, isotopic values). The CO2 concentration of cave air depends on CO2 source(s) and productivity, CO2 transport through the epikarst and karst zone, and cave air ventilation. To assess ventilation patterns in the Bunker-Emst Cave (BEC) System, we monitored the pCO2 value approximately 100 m from the lower entrance (Bunker Cave) at bi-hourly resolution between April 2012 and February 2014. The two entrances of the BEC system were artificially opened between 1860?1863 (Emst Cave) and 1926 (Bunker Cave). Near-atmospheric minimum pCO2dynamics of 408 ppmv are measured in winter, and up to 811 ppmv are recorded in summer. Outside air contributes the highest proportion to cave air CO2, while soil, and possibly also ground air, provide a far smaller proportion throughout the whole year. Cave air pCO2 correlates positively with the temperature difference between surface and cave air during summer and negatively in winter, with no clear pattern for spring and autumn. Dynamic ventilation is driven by temperature and resulting density differences between cave and surface air. In summer, warm atmospheric air is entrained through the upper cave entrance where it cools. With increasing density, the cooled air flows toward the lower entrance. In winter, this pattern is reversed, due to cold, atmospheric air entering the cave via the lower entrance, while relatively warm cave air rises and exits the cave via the upper entrance. The situation is further modulated by preferential south-southwestern winds that point directly on both cave entrances. Thus, cave ventilation is frequently disturbed, especially during periods with higher wind speed. Modern ventilation of the BEC system-induced by artificially openings-is not a direct analogue for pre-1860 ventilation conditions. The artificial change of ventilation resulted in a strong increase of ?13Cspeleothem values. Prior to the cave opening in 1860, Holocene ?13Cspeleothem values were significantly lower, probably related to limited ventilation due to the lack of significant connections between the surface and cave. Reduced ventilation led to significantly higher pCO2 values, minimal CO2 degassing from drip water and low kinetic isotope fractionation. Both modern and fossil speleothem precipitation rates are driven by water supply and carbonate saturation, and not by cave air pCO2. Today, pCO2 variability is too small to affect carbonate precipitation rates and the same is likely true for pCO2 variability prior to artificial opening of the cave. Thus, fossil speleothems from BEC System are likely more sensitive to temperature and infiltration dynamics. The Bunker-Emst Cave System, therefore, represents different ventilation patterns and their influence on speleothem proxy data in an exemplary manner, and it may serve as a template for other cave systems

Marine bivalve geochemistry and shell ultrastructure from modern low pH environments

Abstract. Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels Mytilus galloprovincialis (from the Mediterranean) and M. edulis (from the Wadden Sea) combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pHT 7.3 and mean pHT 8.1 near CO2 vents on the east coast of the island of Ischia. The shells of transplanted mussels were compared with M. edulis collected at pH ~8.2 from Sylt (German Wadden Sea). Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive.</jats:p

Bunker Cave stalagmites: an archive for central European Holocene climate variability

Holocene climate was characterised by variability on multi-centennial to multi-decadal time scales. In central Europe, these fluctuations were most pronounced during winter. Here we present a record of past winter climate variability for the last 10.8 ka based on four speleothems from Bunker Cave, western Germany. Due to its central European location, the cave site is particularly well suited to record changes in precipitation and temperature in response to changes in the North Atlantic realm. We present high-resolution records of δ18O, δ13C values and Mg/Ca ratios. Changes in the Mg/Ca ratio are attributed to past meteoric precipitation variability. The stable C isotope composition of the speleothems most likely reflects changes in vegetation and precipitation, and variations in the δ18O signal are interpreted as variations in meteoric precipitation and temperature. We found cold and dry periods between 8 and 7 ka, 6.5 and 5.5 ka, 4 and 3 ka as well as between 0.7 and 0.2 ka. The proxy signals in the Bunker Cave stalagmites compare well with other isotope records and, thus, seem representative for central European Holocene climate variability. The prominent 8.2 ka event and the Little Ice Age cold events are both recorded in the Bunker Cave record. However, these events show a contrasting relationship between climate and δ18O, which is explained by different causes underlying the two climate anomalies. Whereas the Little Ice Age is attributed to a pronounced negative phase of the North Atlantic Oscillation, the 8.2 ka event was triggered by cooler conditions in the North Atlantic due to a slowdown of the thermohaline circulation

Late Palaeozoic to Neogene Geodynamic Evolution of the north-eastern Oman Margin.

When the highlands of Arabia were still covered with an ice shield in the latest Carboniferous/Early Permian period, separation of Gondwana started. This led to the creation of the Batain basin (part of the early Indian Ocean), off the northeastern margin of Oman. The rifting reactivated an Infra-Cambrian rift shoulder along the northeastern Oman margin and detritus from this high was shed into the interior Oman basin. Whereas carbonate platform deposits became widespread along the margin of the Neo-Tethys (northern rim of Oman), drifting and oceanization of the Batain basin started only in Late Jurassic/Early Cretaceous time. Extensional tectonics was followed in the Late Cretaceous by contraction caused by the northward drift of Greater India and Afro-Arabia. This resulted in the collision of Afro-Arabia with an intra-oceanic trench and obduction of the Semail ophiolite and the Hawasina nappes south to southwestward onto the northern Oman margin ~80 m.y. ago. During the middle Cretaceous, the oceanic lithosphere (including the future eastern ophiolites of Oman) drifted northwards as part of the Indian plate. At the Cretaceous-Palaeogene transition (~65 Ma), oblique convergence between Greater India and Afro-Arabia caused fragments of the early Indian Ocean to be thrust onto the Batain basin. Subsequently, the Lower Permian to uppermost Maastrichtian sediments and volcanic rocks of the Batain basin, along with fragments of Indian Ocean floor (eastern ophiolites), were obducted northwestward onto the northeastern margin of Oman. Palaeogene neo-autochtonous sedimentary rocks subsequently covered the nappe pile. Tertiary extensional tectonics related to Red Sea rifting in the Late Eocene was followed by Miocene shortening, associated with the collision of Arabia and Eurasia and the formation of the Oman Mountains

Sensitivity of Bunker Cave to climatic forcings highlighted through multi-annual monitoring of rain-, soil-, and dripwaters

The last two decades have seen a considerable increase in studies using speleothems as archives of past climate variability. Caves under study are now monitored for a wide range of environmental parameters and results placed in context with speleothem data. The present study investigates trends from a seven year long monitoring of Bunker Cave, northwestern Germany, in order to assess the hydraulic response and transfer time of meteoric water from the surface to the cave. Rain-, soil-, and dripwater were collected from August 2006 to August 2013 at a monthly to bimonthly resolution and their oxygen and hydrogen isotope composition was measured. Furthermore, drip rates were quantified. Due to different drip characteristics, annual mean values were calculated for the drip rates of each drip site. Correlations of the annual mean drip rate of each site with precipitation and infiltration demonstrate that the annual infiltration, and thus the annual precipitation control the inter-annual drip-rate variability for all except one site. The hydraulic response is not delayed on an annual basis. All drip sites display identical long-term trends, which suggests a draining of a common karst reservoir over these seven years of monitoring. Correlations of soil- and dripwater monthly δ18O and δD values with atmospheric temperature data reveal water transfer times of 3 months to reach a depth of 40 cm (soilwater at site BW 2) and 4 months for 70 cm depth (soilwater at site BW 1). Finally, the water reaches the cave chambers (15 to 30 m below land surface) after ca. 2.5 years. Consequently, a temporal offset of 29 to 31 months (ca. 2.5 years) between the hydraulic response time (no time lag on annual basis) and the water transfer time (time lag of 29 to 31 months) was found, which is negligible with regard to Bunker Cave speleothems because of their slow growth rates. Here, proxies recording precipitation/infiltration and temperature are registered on a decadal scale. Variations in drip rate and thus precipitation and infiltration are recorded by δ13C and Mg/Ca ratios in speleothem calcite. Speleothem δ18O values reflect both temperature and precipitation signals due to drip rate-related fractionation processes. We document that long-term patterns in temperature and precipitation are recorded in dripwater patterns of Bunker Cave and that these are linked to the North Atlantic Oscillation (NAO)

Cryogenic and non-cryogenic pool calcites indicating permafrost and non-permafrost periods: a case study from the Herbstlabyrinth-Advent Cave system (Germany)

Weichselian cryogenic calcites collected in what is referred to as the Rätselhalle of the Herbstlabyrinth-Advent Cave system are structurally classified as rhombohedral crystals and spherulitic aggregates. The carbon and oxygen isotopic composition of these precipitates (&lt;i&gt;&amp;delta;&lt;/i&gt;&lt;sup&gt;13&lt;/sup&gt;C = +0.6 to −7.3&amp;permil; &lt;i&gt;&amp;delta;&lt;/i&gt;&lt;sup&gt;18&lt;/sup&gt;O = −6.9 to −18.0&amp;permil;) corresponds to those of known slowly precipitated cryogenic cave calcites under conditions of isotopic equilibrium between water and ice of Central European caves. The carbon and oxygen isotopic composition varies between different caves which is attributed to the effects of cave air ventilation before the freezing started. &lt;br&gt;&lt;br&gt; By petrographic and geochemical comparisons of Weichselian cryogenic calcite with recent to sub-recent precipitates as well as Weichselian non-cryogenic calcites of the same locality, a model for the precipitation of these calcites is proposed. While the recent and sub-recent pool-calcites isotopically match the composition of interglacial speleothems (stalagmites, etc.), isotope ratios of Weichselian non-cryogenic pool-calcites reflect cooler conditions. Weichselian cryogenic calcites show a trend towards low &lt;i&gt;&amp;delta;&lt;/i&gt;&lt;sup&gt;18&lt;/sup&gt;O values with higher carbon isotope ratios reflecting slow freezing of the precipitating solution. In essence, the isotope geochemistry of the Weichselian calcites reflects the climate history changing from overall initial permafrost conditions to permafrost-free and subsequently to renewed permafrost conditions. Judging from the data compiled here, the last permafrost stage in the Rätselhalle is followed by a warm period (interstadial and/or Holocene). During this warmer period, the cave ice melted and cryogenic and non-cryogenic Weichselian calcite precipitates were deposited on the cave ground or on fallen blocks, respectively

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