Portable X-Ray fluorescence (PXRF) instruments have become more and more common tools in the last few years for chemical analysis (major, minor and some traces elements) in carbonates and sedimentary rocks. PXRF is a relatively cheap and fast technique, non-destructive and prevents tedious sample preparation. Manufacturers calibrations are usually made for a very large range of rock types or materials but specific empirical calibrations are essential to improve the quantification of multi-elemental concentrations.

We present a general workflow for building an empirical calibration model for a PXRF spectrometer from choosing reference materials to validating calibration’s efficiency and quantifying its limit of quantification. We also compare the quantified results on a sedimentary rock suite obtained from sixteen calibrations that were built using several software available on the market, including a simple homemade spreadsheet, the Bruker calibration software named EasyCal, a free and open-source calibration app named CloudCal, and the built-in GeoMining and GeoExplorer calibrations provided by Bruker manufacturer on the Tracer 5 PXRF. This decoupled approach allows (i) to explore the pros and cons for each calibration software solutions, (ii) to investigate and weight the effect of each correction parameters (i.e. regression line geometry, peaks interferences, spectra normalization, influence coefficients, multi-phases excitation conditions) on the quantified results and (iii) to propose an original and empirical approach to estimate the lower limit of quantification (LOQ) of a PXRF instrument and its calibration.

For quality check, each of these newly built calibrations was then validated on carbonate samples of known composition, corresponding to a series of Devonian limestones (Belgium) showing a large CaCO3 content ranging between 20 and 99 wt% and for which powder press pellets were formerly measured on a laboratory WD-XRF. These matrix-match empirical calibrations are systematically bringing improvement of the quantification efficiency, compared to the manufacturer calibrations.

How to cite: Da Silva, A.-C., triantafyllou, A., and Delmelle, N.: Portable X-Ray Fluorescence calibration – Workflow to Optimize the Analysis of Carbonates., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3270, https://doi.org/10.5194/egusphere-egu23-3270, 2023.

Reef islands accumulate as shallow landforms on or adjacent to reef complexes and are mainly composed of reef-derived carbonate sediment. Due to their unconsolidated and low-lying nature, reef islands are exposed to hydrodynamic processes, whereby they naturally experience erosion and accumulation. In order to compensate deficits of sediments lost during erosion, a sufficient supply of suitable carbonate sediment is required for accumulation. Under changing environmental conditions, the reefs are exposed to multiple stress factors that, among other things, can alter carbonate production and thus also threaten the stability of the reef islands. Hence, it is of importance to understand how past changes in the production of carbonate sediment have affected the evolution of reef islands systems, in order to assess potential future scenarios. Here we reconstruct the Holocene and recent sedimentological dynamics from one inhabited Indonesian reef island on the outer shelf of the Spermonde Archipelago, directly bordering the Strait of Makassar. We investigate the carbonate facies from sediment obtained from deep push cores up to 8 meters below surface and from shallow, hand-drilled boreholes. Targeted radiocarbon dating helps to reveal the temporal understanding of facies evolution and formation dynamics. Our data show that the island complex started to form in the mid-Holocene, around 6,400 years cal BP. The carbonate sediment from this time is almost exclusively composed of sand-sized coral fragments. Over time, the sediment composition becomes more diverse, with increased abundances of mollusks and algae, likely reflecting the ecological evolution of the reef. The shallow samples close to mean relative sea-level show radiocarbon ages of 4,000 to 3,000 years cal BP, suggesting that the initial island formed around this time and accreted in the following millennia. Of note, the proportions of the green algae Halimeda is even further increased in the youngest sediments. Overall, this suggests that the island has been able to continue growing despite changes in the ecosystems that provide the sediment, and that the increased production of green algae may have even promoted island accretion in general. Our study provides valuable insights into the dynamics of reef island development in the context of evolving reef systems and extends the understanding of Holocene island formation in the study area.

How to cite: Kappelmann, Y., Westphal, H., Kneer, D., and Mann, T.: From coral to Halimeda: Green algae as an important factor for Holocene reef island formation in Southeast Asia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14502, https://doi.org/10.5194/egusphere-egu23-14502, 2023.

Marine calcite cement's crystal habit is often considered a function of the fluid Mg/Ca ratio. In contrast, marine aragonite fabrics are commonly described as acicular (needle) cement with pointed terminations and width-to-length ratios in the order of 1:10. Similarly, botryoidal or spherulitic aragonite cements are well-known from Mesozoic (and older) reefal and slope depositional environments but are less common in Recent depositional environments. Here, we explore a wide range of abiogenic aragonite cement habits (morphologies) in caves and coastal marine depositional environments. We propose that the cement habit represents a novel (and underexplored) archive of diagenetic environment, fluid chemistry and precipitation kinetics. Based on SEM imaging, we find the often-described acicular and fibrous fabrics but also a plethora of less well-known morphotypes such as columnar (often pseudo-hexagonal prisms) and lath, tabular and sheet-like (prismatic single crystals) forms. Twinning of (flat) needle aragonite is observed and might point to precursor phases. Based on the data available, the common needle-type aragonite cement typifies normal marine diagenetic (porewater) environments. Increasingly complex habits (polysynthetic twins, flat needles, pseudo-prismatic and prismatic sheet-like) aragonite crystals) are present under increasing levels of restriction and precipitate from high-salinity porewaters. Intriguingly, pseudo-prismatic aragonite crystals are also present in meteoric, gravity-defying cave carbonates, specifically helictites. Aragonite cement habits are documented and placed into context with their depositional and diagenetic environment, and preliminary interpretations are presented.

How to cite: Immenhauser, A., Zhong, Y., Lokier, S. W., Hennhoefer, D., Pederson, C. L., and Mueller, M.: Abiogenic aragonite crystal habit – Novel archive of precipitation environment?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1149, https://doi.org/10.5194/egusphere-egu23-1149, 2023.

Oncoids and thrombolites are microbial carbonates and important indicators for paleogeography. We firstly found lacustrine oncoids and thrombolitic clots mixed with terrigenous clasts in the Lower Cretaceous in the Junggar Basin in Central Asia and aimed at their mixing process and the paleogeographic significance. We conducted a comprehensive petrological study including optical, cathodoluminescence, fluorescence, quantitative evaluation of minerals by scanning electron microscopy, Raman spectroscopy, carbon and oxygen isotopes. Intraclasts, calcite spherulites, botryoidal grains, cortoids, and ooids also mixed with the terrigenous clasts. The cortices of oncoids and ooids are composed of layers of organic carbon-rich micrites, sparry calcites, and apatites. Some oncoids exhibit alternating dark and light laminae with unequal thicknesses and binding structures of microbiota. Thrombolitic clots grew with silt-sized terrigenous clasts and ooids. Grains were cemented by fibrous, dog-tooth, and interstitial granular calcite cement, and some grains are enveloped by micrites. Compared to published data in the Tianshan Region and the Junggar Basin, positive δ¹³C of the mixed siliciclastic-carbonate sediments may indicate the ¹²CO2 absorption of microbiota. Based on the geomorphology and sedimentary distribution, we concluded that mixed siliciclastic-carbonate sediments were developed on a gentle slope with a relatively inadequate sediment supply during highstand periods. The sparry calcite-cemented mixed siliciclastic-carbonate sediments experienced rapid in-situ mixing process in an agitated lakeshore environment. Sparry carbonate cortices peeled off from the oncoids were densely packed in a shallow lake environment. These mixed siliciclastic-carbonate sediments were sedimentary response to extensive lake transgression and humidification in the Early Cretaceous following coarse-grained alluvial sediments and aridification in the Late Jurassic. 

How to cite: Guan, X. and Wu, C.: Lacustrine oncoids and thrombolitic clots mixed with terrigenous clasts in the Lower Cretaceous Junggar Basin, Central Asia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12673, https://doi.org/10.5194/egusphere-egu23-12673, 2023.

Despite the numerous studies dedicated to understanding distally steepened carbonate ramps, the origin of these carbonate systems and controls over their development remain an ongoing discussion. Previous studies have shown the possible impact of climate, tectonics, inherited slope, and differential sedimentation on distally steepened ramp development. However, how these processes create steep slopes in carbonate ramps has yet to be fully understood. To test the role of different processes such as sea-level fluctuation, paleosurface inclination, differential sedimentation, and wave propagation on the geometry of carbonate ramps, we developed a highly constrained stratigraphic forward model referenced to the well-studied Upper Miocene Menorca ramp (Spain). Based on this forward model, several sensitivity analyses were performed and revealed that a complex interaction of changes in accommodation, carbonate production and sediment transport controls steep slope development in carbonate ramps. The results also show that protracted sea level low stand followed by high-frequency sea-level fluctuations with amplitude between 30 m and 40 m favour the initiation of steepened slopes in carbonate ramps. In contrast, models developed with low-frequency and higher amplitude sea-level fluctuations of about 115 m showed no significant slope development. Testing with different inclinations of paleosurfaces shows that flat-to-subhorizontal paleosurfaces result in ramps that mirror the antecedent slope. In contrast, steeper paleosurfaces tend to result in ramps with well-defined slopes. Steeper paleosurfaces also enhance facies differentiation and steep slope development than flat paleosurfaces. Our models, thus, show that the ramp profile becomes more influenced by the depth constraints on the carbonate sediment producers than by the geometry of the underlying topography as the inclination of the paleosurface increases. The models also show that shallow carbonate production tends towards steep slopes due to the low-transport characteristic due to seagrass trapping. This steepness could, however, be altered by the introduction of high-transport sediment grains from deeper carbonate producers, which fill the slopes and more distal sections of the ramp profile.

How to cite: Tella, T., Winterleitner, G., Morsilli, M., and Mutti, M.: Investigating the controls on the development of distally steepened carbonate ramps: A stratigraphic forward modelling of the Upper Miocene distally steepened Menorca ramp, Spain., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5993, https://doi.org/10.5194/egusphere-egu23-5993, 2023.

The detailed reconstruction of reef geometry, composition and evolution during the mid-late Holocene brings valuable information regarding coral reef dynamics and coastal processes during periods of higher sea level and wave energy regimes.

This study provides a high-resolution reconstruction of the reef growth history in French Polynesia and their response to low-amplitude, high-frequency relative sea-level (RSL) changes and associated environmental changes over the past 6,000 years. After the stabilisation of sea level at its present position, the newdevelopment of reef systems was initiated by the creation of accommodation space due to a glacio-eustatic sea-level rise from 6.0 to 4.1 kyr BP, and controlled by the antecedent topography of the islands. A single,short-lived sea-level highstand of less than one metre between 4.1 and 3.4 kyr BP is documented preceding a fall in sea level between 3.6 and 1.2 kyr BP. The reported RSL changes are characterized by slow rates ranging from a few tens of millimetres per year up to 2.5 mm/yr and by significant sea-level stability (stillstands) lasting more than a century and up to 250 years, defining a step-like pattern.

Regionally, the persistence of stable and optimal depositional environments over the last 6,000 years is demonstrated by the constant overall composition and diversity of reef communities and the almost continuouswidespread development of microatoll fields and reef flat units. The facies distribution as well as the lateral extension and shift of facies belts have been governed by variations in accommodation space, which are controlled by RSL changes and antecedent topography. The recurrent sea-level stillstands that punctuated the mid-late Holocene sea-level history have played a significant role in the development of dense reef frameworks.

The widespread development of mid-late Holocene reef deposits in coastal areas, especially reef flat units and storm conglomerate deposits, suggests that they have played a prominent role in the formation and shaping of modern islands and islets, creating significant topographic features that formed the foundations of these islands.

How to cite: Hallmann, N., Camoin, G., Eisenhauer, A., Samankassou, E., Vella, C., Botella, A., Milne, G., Humblet, M., Braga, J. C., Fietzke, J., and Goepfert, T.: Dynamics and facies analysis of Mid-Late Holocene reef systems in French Polynesia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4075, https://doi.org/10.5194/egusphere-egu23-4075, 2023.

Dolomite, CaMg(CO₃)₂ and related Mg/Ca carbonates (disordered calcian dolomite, high Mg calcite, and similar) are common rock-forming minerals that occur widely in numerous diagenetic settings. They represent pore-filling precipitates (cements), diagenetic replacement phases of precursor carbonates or hydrothermal/metamorphic phases. Dolomites crystallize in the space group R-3 with trigonal symmetry and quite typical rhombohedral growth habit. A unique subtype of dolomites are saddle-dolomites which are characterized by curved (warped) surfaces. These show sweeping extinction under cross-polarized light which points to internal lattice strain. Saddle dolomites are of significance because, based on empirical evidence, they are considered to be one of the key indicators of warm-to-hot hydrothermal fluids in the burial diagenetic realm. However, saddle dolomites are not present in all natural hydrothermal deposits, and where they occur, they may coexist with Mg/Ca carbonates that often (but not always) lack warped crystal surfaces. Thus, the physicochemical conditions that lead to their formation remain unclear. We have undertaken an experimental study to explore the growth of saddle dolomites systematically as a function of various parameters (temperature, crystal size of reactants, fluid salinity, and Mg/Ca ratio of solutions). It was found that newly formed dolomites that grow epitaxially on rhombohedral or saddle dolomites exhibit warped surfaces whereas Mg-rich calcite crystals that grow on calcite seeds form flat surfaces. High Mg-calcite that forms at high temperatures (~ 220 °C) from calcium-rich fluids (Mg/Ca = 0.43) also have curved surfaces. In all cases, crystals with curved surfaces are characterized by excess calcium (Mg/Ca ~ 0.31 – 0.35). These results suggest that curvature of high Mg calcites and dolomites are related to excess Ca in the structure that causes lattice strain. Availability of dolomite precursors on which the crystals can grow, environments that provide excess Ca (e.g. highly saline ones) and rapid growth kinetics at high temperatures seem to facilitate the formation of crystals with curved surfaces. Thus this study identifies some factors that control the formation of high Mg-carbonates with curved surfaces and will aid to anticipate settings where they may be expected to occur in nature and understand why dolomites with curved as well as flat surfaces may coexist.

How to cite: Zhang, Y., Mueller, M., Hoffmann, R., Riechelmann, S., Chakraborty, S., Beyer, C., and Immenhauser, A.: Mg/Ca carbonates with curved crystal surfaces: an experimental study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5711, https://doi.org/10.5194/egusphere-egu23-5711, 2023.

So-called coarse-grained cryogenic cave carbonates (CCC) have gained increasing attention in recent years as indicators of past glaciation episodes in caves. Unlike the fine-grained variety, which forms seasonally by freezing water films, the coarse-grained type has never been observed in statu nascendi. This limits the significance of geochronological and geochemical data obtained from these speleothems.

The patchy distribution of coarse-grained CCC in currently ice-free caves and recent observations of CCC associated with ice in a few modern ice caves suggest that coarse-grained CCC form by very slow freezing of pools in ice created by drip water. It is assumed that an ice cap is formed on such pools by freezing and that CCC precipitation occurs subaqueously in a semi-closed system, limiting degassing of carbon dioxide, and producing a characteristic depletion in O and an enrichment in C isotopic values. This hypothesis has never been tested.

During winter 2021/22 we conducted freezing experiments in the inner and microclimatically fairly stable part of a large Alpine ice cave (Eisriesenwelt). We filled artificial holes in floor ice with water and monitored the freezing process. Freezing was completed within a few weeks and resulted in an updoming of the ice cap, formation of radial cracks, and episodic pulses of small amounts of residual water escaping through these cracks. The newly formed ice contained abundant pressurised gas inclusions which led this ice to break easily, sometimes associated with a distinct bursting noise.

The H and O isotope data of the newly formed ice define a slope lower than the meteoric water line, consistent with theoretical and experimental freezing studies. The H and O isotope values increase from top down and from the margin to the centre of the frozen pool, consistent with a progressive centripetal advance of the freezing front.

A few tenths of gram of CCC per litre of melted ice were obtained from the inner part of the former pools. The CCC range in size from a few hundredths to several tenths of a millimetre and would thus classify as the fine-grained type. Only calcite was identified by X-ray diffraction. A variety of morphologies were found including thin plates, branched aggregates, dumbbell- and rice-shaped forms. Despite their small particle size the CCC show depleted O and enriched C isotopic compositions, similar to coarse-grained CCC of Pleistocene age from this cave, and consistent with precipitation due to progressive freezing in a semi-closed system.

Our experiments provide a first field-based validation of the conceptual model of coarse-grained CCC formation in freezing water pools. The small crystal size suggests that the freezing rate at our experimental site was likely significantly faster than in settings where mm- to even cm-size aggregates formed during climatically cold periods.

Our data also show that grain size is not a reliable indicator of the mode of CCC formation, which calls for a revision of the currently used CCC terminology (coarse vs. fine grained).

How to cite: Spötl, C., Koltai, G., Dublyansky, Y., and Németh, P.: Formation of cryogenic carbonates: experimental validation in a modern ice cave setting, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9506, https://doi.org/10.5194/egusphere-egu23-9506, 2023.

Cryogenic cave carbonates (CCCs) and, more generally, cryogenic cave minerals, form by solute rejection mechanisms when water enters and freezes in caves, or parts of caves, at temperatures below the freezing point. Negative temperatures may occur in caves in conjunction with climatic factors ranging from seasonal (e.g., sag-type cave entrances or ventilated parts of caves) to millennial (development of permafrost in karst massifs) timescales. When using CCCs for paleoclimate reconstructions, it is important to understand the environment in which they were formed.

Conventionally, two ‘end-members’ of cave microclimate settings are distinguished, in which water freezes either rapidly or slowly. These settings are responsible for the formation of fine-grained (typical sizes < 1 mm) vs. coarse-grained CCC (> 1 mm; the largest individual CCC encountered so far weighs 123 g). Besides difference in size, these two types of CCC show distinct stable isotope (O and C) patterns, currently considered as the most reliable (if not the only) feature that allows discriminating between the two types. The slowly formed CCCs that are coarse grained and show characteristic trends of ¹⁸O depletion and ¹³C enrichment (relative to non-cryogenic carbonates from the same cave) are thought to represent markers of past permafrost.   

Although both defining features, size and stable isotopic properties of CCCs, are controlled by the rate of freezing, the control may have different characteristic times. The rate of freezing is controlled by: (1) the overcooling of the site; (2) the amount of water entering the cave; and (3) the regime of water inflow (semi-continuous or pulse-like). Large amounts of water (even of low temperature) entering a cave entrain large amounts of heat that need to be dissipated before freezing commences. Further, the release of latent heat maintains the temperature of the freezing water body at 0 °C for the duration of the freezing process. Slow freezing leads to progressive depletion of the residual water in ¹⁸O and, as a result, CCCs are also increasingly depleted in ¹⁸O. We observed depletions of several permil in the time span of a few days in laboratory experiments.  

In caves located in relatively cold environments (e.g., Alpine or high-latitude caves) seasonal freezing conditions can be created by low-intensity influx of outside cold air into sections of caves located in already relatively cold rock. In such zones a small overcooling may be maintained for a sufficiently long time, producing CCCs with isotope properties mimicking (partly or entirely) those of CCCs forming in static settings of “true” permafrost.

How to cite: Dublyansky, Y., Spötl, C., and Koltai, G.: Settings of cryogenic cave carbonate formation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14557, https://doi.org/10.5194/egusphere-egu23-14557, 2023.