Substantial heterogeneity of carbon and oxygen stable-isotope compositions of single layers or specimens of natural carbonate materials: New evidence from replicate sampling of continental carbonates affirms previous evidence from marine limestones

ABSTRACT The uncertainty of measurements of carbon and oxygen stable-isotope ratios of carbonate materials is commonly assumed to be the analytical uncertainty determined from replicate analyses of single samples, but this ignores the possibility that heterogeneity of the material studied is greater than the analytical uncertainty. To test this question, we took eight samples from each of 13 layers or specimens of various non-marine (“continental”) carbonates and found ranges of δ13C and δ18O of 0.3 to 5.1‰, all exceeding the range of the typical lab-reported analytical uncertainty, ± 0.1‰, placed around single samples. These results are similar to previous replicate sampling of marine limestone layers, which revealed ranges of 0.2 to 2.8‰. Both sets of results, and other published data, demonstrate that analytical uncertainty derived from replicate analysis of a single sample is not a valid estimate of the uncertainty of δ13C or δ18O values characterizing a layer or specimen, and they remind us that we should not place great credence in anomalies or events defined by single samples of layers or specimens, regardless of the replication of analysis of that single sample. Our results indicate that the required layer-level or specimen-level uncertainty can be derived only from replicate sampling at different locations in layers or specimens, and that the layer-level or specimen-level uncertainty is inevitably greater than typical lab-reported analytical uncertainty. Credibility of anomalies or events in time series would be increased by replicate sampling of a random or dispersed subset of layers to estimate the variability of all layers and/or by replicate sampling of layers at and around a potential but unconfirmed event. The significance of the variability discussed above is evident in use of δ18O data to estimate paleotemperatures, where a difference of 1‰ in δ18O implies a difference of 4°C in temperature. Use of a single sample resulting in mischaracterization of the δ18O of an ancient material by 1.5‰ relative to the true mean for that material (which our results suggest is quite possible) would lead to a corresponding misestimation of temperature of 6°C, a significant difference in paleoenvironmental studies.

Tectono-Sedimentary Evolution of the Madrid Basin (Spain) during the Late Miocene: Data from Paleokarst Profiles in Diagenetically-Complex Continental Carbonates

An intra-Vallesian (Upper Miocene) paleokarst developed at the top of the Intermediate Miocene Unit in the continental intracratonic Madrid Basin is recognized. This paleokarst is an early shallow, tabular-shaped karst that shows a marked control by the depositional facies pattern and lithologies. By integrating morphological, petrological, and geochemical data, three hydrogeological zones were established throughout the paleokarstic profiles: (i) a paleo-vadose zone, characterized by vertically elongated caves and vadose cementation; (ii) a 3–7 m thick paleo-epiphreatic zone (paleo-water table fringe), with development of stratiform breccia bodies, the superimposition of both vadose and phreatic features, and the lowest Fe and Mn contents in host-rock carbonates; and (iii) a paleo-phreatic zone characterized by an increase in δ13C values and the predominance of phreatic cementation. The paleogeographic reconstruction for the intra-Vallesian paleokarst using profiles revealed relative topographic highs to the north and topographic lows to the south, drawing the paleokarst landscape. Immediately overlaying the paleokarst surface are fluvio-lacustrine facies belonging to the Miocene Upper Unit (Late Vallesian to Late Turolian). Their lowermost deposits consist of fluvial terrigenous facies deposited by approximately N–S fluvial streams, and pass upward into fluvio-lacustrine fresh-water limestones. This paleokarstic surface represents a major change in the evolution of sedimentary patterns of basin, from endorheic to exorheic conditions, as the result of a change from compressive to extensional conditions in the tectonic regime.

Microfaulting by early diagenesis of micritic continental carbonates - dilatant and compactive shear localization (Montpellier area, France)

<p>Microfaults formed in continental carbonates reveal poorly known mechanisms of shear localization induced by early diagenesis during compaction. These faults are characterized by sinuous shape, bed-controlled, pervasive distribution, no calcite precipitation, and mainly disaggregation processes. Two main sets were described: (1) The first set is composed by normal-sense, high-angle microfaults affecting the top of carbonate beds showing undulating pedogenic bed surface. They show porosity increase and are sometimes organized in polygonal patterns. Their occurrence seems related to overconsolidation of pedogenic surface and density inversion – phreatic loading – fluid expulsion processes in the surficial carbonate bed. (2) The second set is composed by low-angle compactive microfaults with large slickenlines and incipient shear-offset. Their organization within two conjugate systems (normal-sense set and strike-slip set) almost contemporaneous is consistent with a NS extension following the slope induced by the basin subsidence to the south. Their occurrence seems related to vertical loading below few meters depth and occurred by shear-enhanced compaction and incipient pressure-solution process. The presence of such structures gives news information concerning dilatant or compactive shear processes and rheological properties of micritic carbonates during early diagenesis.</p><p> </p>

Continental carbonates growth pathways, fabrics and diagenesis

<p>Continental carbonates are a repository of exceptional climate and environmental changes at scales from sub-annual to decadal to millennial. Their fabrics and chemistry encapsulate information about temperature and rainfall variability, volcanic eruptions, earthquakes, vegetation changes, as well as microbial interaction. Yet, fabric and chemical properties are influenced by the crystallization pathways and, crucially, growth mechanisms and diagenesis of the carbonate crystals. Here we present examples from diverse continental settings and discuss why fabrics are extremely important to determine the accuracy of preservation of a “primary” signal.</p><p>Most continental carbonate formation is driven by degassing. This is the case of cave carbonate deposits (speleothems), which allowed tremendous breakthrough in palaeoclimate science. Speleothems form in the dark, from drip waters poor in nutrients and organic compounds. Their most common fabric consist of columnar crystals. Nanoscale investigation shows that speleothem crystals have diverse growth pathways, including particle attachment (Frisia et al., 2018). The distribution of climate-sensitive trace elements, thus, rather than following crystallographic sector zoning, follows “parallel growth layers” reflecting environmental changes. The critical parameter in growth process is the drip rate. By contrast, subglacial and cryogenic carbonates, which also grow in the dark and consist of columnar crystals, form in micro-phreatic environment where supersaturation is not attained by degassing, but by concentration of elements by slow freezing.  In this situation trace elements are incorporated following crystallographic faces and provide exceptional information of subglacial processes including volcanic eruptions (Frisia et al., 2017).</p><p>Lacustrine, spring and fluvial carbonates grow at Earth’s surface, being exposed to Sun’s light. These carbonates’ precipitation, similarly to speleothems, is promoted by degassing, but also by the presence of photosynthetic organisms and high substances organic interaction. Their fabrics are commonly characterized by micrite, which is rare in caves and in subglacial samples.</p><p>Evaporitic lake (Great Salt Lake, GSL) and spring deposits described in Della Porta (2015) were observed by TEM. One typical microfabric is peloidal micrite. The GSL peloidal micrite consists of calcite nanocrystals, and the peloids are associated with aragonite and filaments. Spring deposits peloidal micrite also consists of nanocrystal aggregates surrounded by filaments. </p><p>Most speleothems and spring/lake carbonates document a phase of growth that involves nanocrystal aggregation, which we did not observe in the phreatic subglacial samples.</p><p>Implications for palaeoenvironmental research: In speleothems, Ostwald ripening likely transforms nanoparticle aggregates into larger crystals. Critically, in speleothems, Ostwald ripening processes result in removal of some tracers, such as Si, associated to first growth phases, and preservation of those that we use to reconstruct palaeo hydrology. In lake and spring deposits it would seem that micrite preserves the original environmental data, because micrite means that the crystals were protected from ripening by the organic part of the deposit. In subglacial carbonates, growth appears to follow a classical ion attachment at growth sites, thus, their fabrics preserve pristine primary signals.   </p><p> </p><p>References:</p><p>Della Porta, G. (2015) Geological Society, London, Special Publications 418, 17-68.</p><p>Frisia, S. et al (2018) Earth-Science reviews 178, 68-91.</p><p>Frisia, S.,  et al. (2017) Nature Communications 8.</p><p> </p>

Neogene-to-Quaternary post-orogenic tectonic evolution and CaCO3-rich fluids from the Tabernas basin (eastern Betics, Spain) reveal control by deep-seated processes in the mantle

<p>Since Miocene times, the crustal thinning in eastern Betics and the Alboran region associated with westward slab retreat led to the formation and exhumation of metamorphic domes and EW-directed narrow basins.</p><p>The Tabernas basin preserves a sedimentary records of the last stages of metamorphic domes exhumation (14 to 8 Ma). Structural constraints from fault patterns and sedimentary archives show evidence in the field for E-W strike-slip faults that developed close to dome-basin contacts. The evolution of strike-slip faulting and extensional basins reveals strain partitioning during the late Miocene that is consistent with the present-day regional NNW-directed compression and WSW-directed/orogen-parallel extension that result from the NW-SE Africa-Europe plate convergence. A regional cross-section further emphasizes the role of crustal-scale strike-slip faulting and slab detachment and delamination under the Alboran domain.</p><p>Calcite veins that developed during the orogen-parallel extension in the metamorphic basement and the Tortonian sedimentary rocks show a wide variety of stable isotopes ratios. Calcite cements have δ<sup>18</sup>O values ranging from -17.23‰ to -5.30‰ for, and from -15.77‰ to -1.6‰ for δ<sup>13</sup>C isotopic ratios. This patterns is interpreted to reflect the increase of freshwater input buffered by the composition of host carbonate rocks.</p><p>Continental carbonates of Quaternary ages are widespread in the Tabernas basin. Travertines show a close structural relationship with N170 and N50 normal faults, implying tectonically-controlled Ca/CO<sub>2</sub> leakages. Their δ<sup>13</sup>C values are compatible with a hydrothermal origin from a deep-seated carbon source (δ<sup>18</sup>O median of -7.5‰, δ<sup>13</sup>C median of 2.1‰). Degassing associated with regional volcanism from the Serravallian until the Tortonian-Messinian ages is likely to be also the main vector of recent CO<sub>2</sub> storages in rocks. The U-Th ages of travertines, ranging from 8ka ± 0.2 to 354ka ±76, further outline interactions with captive aquifer from 350ka and subsequent Ca/CO<sub>2</sub> leakages due to geodynamic changes.</p>