Description: Ocean environmental conditions can be inferred from the chemical composition of bamboo coral skeletons. The high magnesium calcite internodes of these long-living octocorals may therefore represent a potential archive for seawater properties such as salinity or temperature where instrumental time series are absent. To extend these time series into the past using a natural archive the principles of temperature and salinity signal incorporation into cold-water coral skeletal material need to be investigated. Since skeletal Na and S concentrations have been proposed as environmental proxies, we mapped the spatial distribution and concentration of these elements in two Atlantic specimens of Keratoisis grayi (family Isididae). These measurements were conducted with an electron microprobe applying a spatial resolution of 4 μm. The mean apparent distribution coefficient of Na/Ca for the two samples was within 2.5 and 2.8*10−4, while that of S shows a similar depletion relative to seawater with 3.8 and 3.6*10−3. The two elements show an inverse correlation in bamboo coral skeletons. The mean apparent distribution coefficient of Na is similar to that of abiotic calcites. This similarity can be interpreted as the absence of significant vital effects for skeletal Na/Ca. Hence it corroborates the idea that the average skeletal composition of bamboo corals holds the potential to record past seawater conditions. In contrast, it appears unlikely that the spatial variations of the element distribution of seemingly simultaneously precipitated material along growth rings are exclusively controlled by environmental factors. We further exclude Rayleigh fractionation, ion-specific pumping, and Ca/proton exchange as the driver of Na and S distribution in bamboo corals. Instead, we adapt a calcification model originally proposed for scleractinians to bamboo corals. This model can explain the observed distribution of Na and S in the skeleton by a combination of Ca/proton pumping, bicarbonate active transport, and the formation of an organic skeletal matrix. The adapted model can further be used to predict the theoretical behaviour of other elements and disentangle vital effects from external factors influencing compositional features. It is therefore a useful tool for future studies on the potential of bamboo corals as environmental archives.

Description: During Earth’s history, precipitation of calcium carbonate by heterotrophic microbes has substantially contributed to the genesis of copious amounts of carbonate sediment and its subsequent lithification. Previous work identified the microbial sulfur and nitrogen cycle as principal pathways involved in the formation of marine calcium carbonate deposits. While substantial knowledge exists for the importance of the sulfur cycle, specifically sulfate reduction, with regard to carbonate formation, information about carbonate genesis connected to the microbial nitrogen cycle is dissatisfactory. In addition to the established pathways for carbonate mineral formation, also the potential of microbial carbonic anhydrase, a carbonate-relevant, zinc-containing enzyme, is receiving currently increased attention. However, also in this field knowledge is scarce and fragmentary. Here we demonstrate microbial carbonate precipitation as a direct result of the interplay between the microbial nitrogen cycle and a microbially produced enzyme. Using Alcanivorax borkumensis as a model organism, our experiments depict precipitation of a peloidal carbonate matrix within days to weeks, induced by simultaneous ammonification and extracellular carbonic anhydrase activity. The precipitates show similar morphology, mineralogy, δ44/40Ca, and δ88/86Sr to analogs of modern carbonate peloids. The obtained Sr/Ca partition coefficient DSr showed no clear deviation from inorganic carbonate phases, indicating that microbially mediated carbonate precipitation, indeed, follows the principles of physico-chemical precipitation. The observed relative enrichment of the precipitates in zinc might help to constrain zinc variations in natural carbonate archives. Our study demonstrates that ammonification, due to intense microbial organic matter degradation, and carbonic anhydrase may play a substantial role for calcium carbonate precipitation in paleo- and recent shallow marine environments.

Description: The geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of carbonate rocks. As a result, calcium (Ca) isotope ratios (44Ca/40Ca, expressed as δ44/40Ca) can help to constrain ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. However, the δ44/40Ca of carbonate rocks is influenced not only by the δ44/40Ca of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonate δ44/40Ca in Upper Permian to Middle Triassic limestones (ca. 253 to 244 Ma) from south China and Turkey. This time interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawater δ44/40Ca may be expected based on several large carbon isotope (δ13C) excursions ranging from − 2 to + 8‰. Parallel negative δ13C and δ44/40Ca excursions were previously identified across the end-Permian extinction horizon. Here, we find a second negative excursion in δ44/40Ca of ~ 0.2‰ within Lower Triassic strata in both south China and Turkey; however, this excursion is not synchronous between regions and thus cannot be interpreted to reflect secular change in the δ44/40Ca of global seawater. Additionally, δ44/40Ca values from Turkey are consistently 0.3‰ lower than contemporaneous samples from south China, providing further support for local or regional influences. By measuring δ44/40Ca and Sr concentrations ([Sr]) in two stratigraphic sections located at opposite margins of the Paleo-Tethys Ocean, we can determine whether the data represent global conditions (e.g., secular variations in the δ44/40Ca of seawater) versus local controls (e.g., original mineralogy or diagenetic alteration). The [Sr] and δ44/40Ca data from this study are best described statistically by a log-linear correlation that also exists in many previously published datasets of various geological ages. Using a model of early marine diagenetic water-rock interaction, we illustrate that this general correlation can be explained by the chemical evolution of bulk carbonate sediment samples with different initial mineralogical compositions that subsequently underwent recrystallization. Although early diagenetic resetting and carbonate mineralogy strongly influence the carbonate δ44/40Ca values, the relationship between [Sr] and δ44/40Ca holds potential for reconstructing first-order secular changes in seawater δ44/40Ca composition.

Description: Coccoliths comprise a major fraction of the global carbonate sink. Therefore, changes in coccolithophores' Ca isotopic fractionation could affect seawater Ca isotopic composition, affecting interpretations of the global Ca cycle and related changes in seawater chemistry and climate. Despite this, a quantitative interpretation of coccolith Ca isotopic fractionation and a clear understanding of the mechanisms driving it are not yet available. Here, we address this gap in knowledge by developing a simple model (CaSri–Co) to track coccolith Ca isotopic fractionation during cellular Ca uptake and allocation to calcification. We then apply it to published and new δ44/40Ca and Sr/Ca data of cultured coccolithophores of the species Emiliania huxleyi and Gephyrocapsa oceanica. We identify changes in calcification rates, Ca retention efficiency and solvation–desolvation rates as major drivers of the Ca isotopic fractionation and Sr/Ca variations observed in cultures. Higher calcification rates, higher Ca retention efficiencies and lower solvation–desolvation rates increase both coccolith Ca isotopic fractionation and Sr/Ca. Coccolith Ca isotopic fractionation is most sensitive to changes in solvation–desolvation rates. Changes in Ca retention efficiency may be a major driver of coccolith Sr/Ca variations in cultures. We suggest that substantial changes in the water structure strength caused by past changes in temperature could have induced significant changes in coccolithophores' Ca isotopic fractionation, potentially having some influence on seawater Ca isotopic composition. We also suggest a potential effect on Ca isotopic fractionation via modification of the solvation environment through cellular exudates, a hypothesis that remains to be tested.

Description: Highlights • First systematic description of Pleistocene facies of the Maldives reveals shallow-water deposits • Only U-series ages from Pleistocene deposits of the Maldives (MIS 5e) • Geochronology and paleo-bathymetric analyses allow estimation of late Quaternary subsidence of this major carbonate platform location to 0.09 - 0.16 m/kyr To date, there is hardly any knowledge of facies and age of Pleistocene reef limestone in the Maldives. Likewise, there are no robust estimates of Quaternary subsidence in this major shallow-water carbonate platform and reef area. In a core recovered on the windward margin of Rasdhoo Atoll in the central part of the archipelago, Pleistocene coralgal grainstone facies belonging to marine isotope stage (MIS) 5e were recovered underlying a Holocene reef succession, 14.5 m below modern sea level. Based on the occurrence of shallow-water stony corals such as Isopora palifera and possibly Acropora gr. robusta, high-energy coralline algae including Porolithon onkodes, in part associated with vermetids, and grain-supported limestone texture, the paleoenvironment is interpreted as a shallow back reef area with a paleo-waterdepth of 〈10 m. Based on a reliable U-series age from a Pleistocene acroporid coral of 136.9 kyr BP and assuming a + 7.5 m higher-than-present peak sea level during MIS 5e, late Quaternary subsidence is estimated to 0.09 m/kyr (minimum)–0.16 m/kyr (maximum value). A sea level of +2.5 m during the early MIS 5e would reduce the rates to 0.05 m/kyr (minimum)–0.12 m/kyr (maximum). These numbers are significant for reconstructions of depositional environments of this major carbonate platform area in the Quaternary. The subsidence estimates are not as crucial for historical reconstruction of relative sea level and for predictions of the near future in this low-lying archipelago, because they will add only a minor portion to the predicted rates of 21st century sea-level rise.

Description: Fossil carbonate skeletons of marine organisms are archives for understanding the development and evolution of palaeo-environments. However, the correct assessment of past environment dynamics is only possible when pristine skeletons and their biogenic characteristics are unequivocally distinguishable from diagenetically-altered skeletal elements and non-biogenic features. In this study, we extend our work on diagenesis of biogenic aragonite (Casella et al. 2017) to the investigation of biogenic low-Mg calcite using brachiopod shells. We examined and compared microstructural characteristics induced by laboratory-based alteration to structural features derived from diagenetic alteration in natural environments. We used four screening methods: cathodoluminescence (CL), cryogenic and conventional field emission-scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and electron backscatter diffraction (EBSD). We base our assessments of diagenetic alteration and overprint on measurements of, a) images of optical overprint signals, b) changes in calcite crystal orientation patterns, and c) crystal co-orientation statistics. According to the screening process, altered and overprinted samples define two groups. In Group 1 the entire shell is diagenetically overprinted, whereas in Group 2 the shell contains pristine as well as overprinted parts. In the case of Group 2 shells, alteration occurred either along the periphery of the shell including the primary layer or at the interior-facing surface of the fibrous/columnar layer. In addition, we observed an important mode of the overprinting process, namely the migration of diagenetic fluids through the endopunctae corroborated by mineral formation and overprinting in their immediate vicinity, while leaving shell parts between endopunctae in pristine condition. Luminescence (CL) and microstructural imaging (FE-SEM) screening give first-order observations of the degree of overprint as they cover macro-to micron scale alteration features. For a comprehensive assessment of diagenetic overprint these screening methods should be complemented by screening techniques such as EBSD and AFM. They visualise diagenetic changes at submicron and nanoscale levels depicting the replacement of pristine nanocomposite mesocrystal biocarbonate (NMB) by inorganic rhombohedral calcite (IRC). The integration of screening methods allows for the unequivocal identification of highly-detailed alteration features as well as an assessment of the degree of diagenetic alteration.

Description: Takapoto Atoll (northern Tuamotu Islands, French Polynesia, Central Pacific) was selected as a test area to clarify the conditions of atoll island accretion in relation to mid- to late-Holocene sea-level changes. Surveys were conducted along two distinct cross-island profiles, on the windward coast of the atoll. In addition, the stratigraphy of an ocean-facing islet was described from an excavation in order to reconstruct the successive island accretionary stages. At both sites, the basement of the atoll-rims consists of conglomerate pavements on which lie shingle ridges, reaching 4 m in elevation. Stratigraphic analysis of the excavated ridge reveals alternation of gravelly sand-supported to gravel-dominated sediments. The chronology of island accretion is based on dating of 41 U/Th surface and excavated coral specimens. Ridge initiation occurred from about 1000 yr BP when sea level was close to its present position, shingle deposits progressively prograded from the lagoon margins oceanwards and were partially cemented at their bases. Cementation may have increased the resistance of the islets to erosion. As a result, some island lands accumulated and have persisted over the last millennium. The modern gross island morphology was acquired during the last 500 years. This model can be considered to be of regional value for the northern Tuamotu islands, adjusted for local thermal subsidence, hydroisostasy and/or lithospheric flexure. Compared to some other Indo-Pacific reef islands, island initiation at Takapoto appears to be have been delayed by 2 to 4 millennia, probably in response to retardation in the reef catching-up with mid-Holocene sea level. Dating of individual coarse-grained coral clasts allowed the major wave-surge events that have hit Takapoto to be identified for the last millennium. The use of gravels results in the identification of a greater number of medium-energy surge impacts, when compared with megaclast-based records. The frequency of storm events identified is consistent with that derived from historical observations; severe storms have a very low frequency of occurrence – one to two events per century on average.

Description: This study used the Sea of Galilee (Lake Kinneret, northern Israel) as a “natural laboratory” to investigate the fractionation of the stable Sr isotope ratio (88Sr/86Sr) during precipitation of inorganic (primary) calcite from the lake's water. It was found that the absolute value of the 88Sr/86Sr fractionation factor, Δ88/86Sr, increases as a function of calcite accumulation rate (Δ88/86Sr [‰] = −0.05 to 0.042·log(R) [μmol·m−2·d−1], where R is the accumulation rate). Furthermore, the 87Sr/86Sr and 88Sr/86Sr ratios in the freshwater and brines that enter the lake were used to calculate the contributions of these sources to the lake Sr budget. The 87Sr/86Sr and 88Sr/86Sr ratios were measured in primary calcite, aragonite shells of live Melanopsis, lake water and various water sources to the lake. While the lake's 87Sr/86Sr ratios are determined by the mixture of freshwater of the Jordan River and saline springs, the 88Sr/86Sr ratios of the lake reflect a more complex mass balance that includes the effect of isotopic fractionation associated with the precipitation of primary calcite. Data analysis suggests that long-term accumulation of inorganic calcite depleted in the heavy isotope 88Sr, results in an increase of the δ88/86Sr value of the lake water by 0.05‰. In contrast to the primary inorganic calcite, biogenic aragonite of the Melanopsis shells show a rather constant 88Sr/86Sr water-CaCO3 fractionation of Δ88/86Sr = −0.21‰. Similar Δ88/86Sr values were reported for the precipitation of coralline and inorganic aragonite from seawater and the precipitation of inorganic calcite from various continental waters. The Δ88/86Sr value of inorganic calcite is modulated by the rate of carbonate precipitation, as noted above and shown by precipitation experiments. Massive precipitation of primary calcite with a wide spread of accumulation rates occurs during the spring phytoplankton bloom in Lake Kinneret. The bloom dictates the degree of calcite saturation in the lake and hence the stable Sr isotopic composition of the precipitating calcite. The correlation between Δ88Sr/86Sr and the calcite accumulation rate can be therefore used as a tool to reconstruct environmental and ecological variations in the historical lake by analyzing the 88Sr/86Sr ratio in the primary CaCO3 recovered from the lake's sedimentary archives.

Description: In this study we examine the behavior of stable Sr isotopes between strontianite [SrCO3] and reactive fluid during mineral dissolution, precipitation, and at chemical equilibrium. Experiments were performed in batch reactors at 25 °C in 0.01 M NaCl solutions wherein the pH was adjusted by bubbling of a water saturated gas phase of pure CO2 or atmospheric air. The equilibrium Sr isotope fractionation between strontianite and fluid after dissolution of the solid under 1 atm CO2 atmosphere was estimated as Δ88/86SrSrCO3-fluid = δ88/86Sr SrCO3 − δ88/86Srfluid = −0.05 ± 0.01‰. On the other hand, during strontianite precipitation, an enrichment of the fluid phase in 88Sr, the heavy isotopomer, was observed. The evolution of the δ88/86Srfluid during strontianite precipitation can be modeled using a Rayleigh distillation approach and the estimated, kinetically driven, fractionation factor αSrCO3-fluid between solid and fluid is calculated to be 0.99985 ± 0.00003 corresponding to Δ88/86SrSrCO3-fluid = −0.15‰. The obtained results further support that under chemical equilibrium conditions between solid and fluid a continuous exchange of isotopes occurs until the system approaches isotopic equilibrium. This isotopic exchange is not limited to the outer surface layer of the strontianite crystal, but extends to ∼7–8 unit cells below the crystal surface. The behavior of Sr isotopes in this study is in excellent agreement with the concept of dynamic equilibrium and it suggests that the time needed for achievement of chemical equilibrium is generally shorter compared to that for isotopic equilibrium. Thus it is suggested that in natural Sr-bearing carbonates an isotopic change may still occur close to thermodynamic equilibrium, despite no observable change in aqueous elemental concentrations. As such, a secondary and ongoing change of Sr isotope signals in carbonate minerals caused by isotopic re-equilibration with fluids has to be considered in order to use Sr isotopes as environmental proxies in aquatic environments.

Description: The larger area of and around the Early Iron Age fortress Heuneburg is focus of decades of archaeological excavations and observations. Additionally, to deciduous oak (Quercus sp.) known as the main timber during this period (Middle to Late Hallstatt c. 750–450 BCE), silver fir (Abies alba) was found recently on the Heuneburg plateau itself and its surroundings. Silver fir is now recognized being a significant source of timber during the Iron Age, in a region where its nowadays natural status is debated and its occurrence is rare. The aim of this study was to determine the possible source of the used timber, which might have been taken from the some 80 km distant Abies-rich Black Forest on the primary bed rock or from stands developed on younger geological formations nearer to the site. For this approach, radiogenic strontium isotope (87Sr/86Sr) analyses were performed on waterlogged wood and on charcoal remains buried in four archaeological contexts from Late Hallstatt period around the princely seat Heuneburg. The 87Sr/86Sr ratios were compared to those of living trees, soils and sediments with respect to the diverse geological background. The geological ground of the archaeological wood was narrowed down to molasse for three structures and to loess, moraine or mass chalk sites for one. The isotopic ratio thus points to growing sites more in the surrounding of the concerned archaeological sites and a more distant Black Forest source can be ruled out. Local evidence of Abies was additionally supported by palynological data from four archaeological sites and by anthracological data from a funeral context near the Heuneburg.