ALBERT

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: A late Pleistocene to Holocene submerged and encrusted speleothem exhibits a complex history including a meteoric phase and two marine phases. A combined study using petrography, mineralogy, and inorganic and organic geochemistry, as well as geochronology has shown that phototrophic and heterotrophic biological activity impacted carbonate precipitation during all phases of carbonate accretion. The stalactite formed ca. 30 m below modern sea level at a marginal overhang in the Blue Hole of Lighthouse Reef Atoll. Unlike purely meteoric speleothems, the Belize example consists of a meteoric core, a marine aragonite crust, and a serpulid-micrite-rich outer crust as a result of postglacial flooding of the karst cave. The core of the stalactite has a tufaceous texture, containing algal or microbial remains, and consists entirely of low-magnesium calcite, formed 19.55-10.68 kyr BP. The texture suggests that the stalactite formed at the cave entrance, and, hence, the former cave ceiling had apparently collapsed earlier during the Pleistocene. Oxygen (δ18O) and carbon (δ13C) isotopes across the core suggest a trend towards drier conditions and reduced soil and plant cover after the last glacial maximum. The marine aragonite crust consists of stacked botryoids in which individual crystals up to 700 lm have dark terminations enriched in high-magnesium calcite. This crust accreted from 10.82 to 9.95 kyr BP in warm shallow water during the early Holocene thermal optimum. Carbonate accretion rates were considerable and averaged 125 μm/yr. The crust has a dense, laminated texture on one side and a porous, shrubby texture on the other. The presence of n-C16:1ω5, n-C17:1ω6, and 10Me-C16 fatty acids in the laminated crust suggests that sulfate-reducing bacteria contributed to aragonite formation in an environment that was less open than the formation environment of the porous crust, where these biomarkers are lacking (n-C16:1ω5,n-C17:1ω6) or are less abundant (10Me-C16). Enrichment of 34S and 18O in carbonate-associated sulfate (CAS) relative to seawater sulfate also suggests sulfate reduction during carbonate formation. The greater contribution of heterotrophic processes to aragonite precipitation in the laminated crust is also reflected in δ13C values as low as -1.3%, whereas no such depletion is observed in the aragonite of the porous crust (δ13C values as low as 0.0%). A pronounced isotopic variability and excursions to positive δ13C values as high as +3.5%0 in the inner half of the laminated crust indicate an episodic, local impact of photosynthesis on aragonite precipitation, whereas the lack of such excursions in the porous crust (δ13C values as high as +1.5%0) is again best explained by a more open environment of formation. After a ca. 5 kyr hiatus, from 4.39 kyr BP, a biogenic crust of abundant serpulids and finely crystalline, microbial and detrital carbonate, consisting of high-magnesium calcite and aragonite, accreted on the outer surface of the stalactite. Outermost crust accretion was probably influenced by the inundation of the Lighthouse Reef lagoon that started to shed abundant fine-grained carbonate sediment into the Blue Hole. The stalactite broke off the cave ceiling either before or after the formation of the outermost crust, likely due to seismic movements along the nearby plate boundary. The study demonstrates that like speleothems from the terrestrial realm, submerged stalactites may have had complex histories with great potential as paleoenvironmental archives.

Description: Mayotte fore-slopes exhibit a distinct pattern in overall morphology, starting in the deep with an unlithified sedimentary wedge and slope, followed upwards by a cemented slope, and finally by a steep, almost vertical wall. On top of the wall, drowned reefs occur. Dated corals may reveal the history of sea-level changes indicating pristine reef growth during late isotope stage 3 (at 55–24 ka) at a present-day water depth greater than 80 m. A maximum sea-level drop of 150 m occurred during the last glacial maximum, around 20 ka. This lowering of sea-level is documented by karst features such as small caves and corroded and jagged surfaces. The phase of deglaciation is recorded by two give-up reef levels at 100–90-m water depth and 65–55-m water depth which we may relate to the Bølling (14 ka) and post Younger Dryas (11.5 ka) meltwater pulses, known from the deep-sea record.

Description: In order to study Strontium (Sr) partitioning and isotope fractionation of Sr and Calcium (Ca) in calcite we performed precipitation experiments decoupling temperature and precipitation rate (R∗). Calcite was precipitated at 12.5, 25.0 and 37.5 °C by diffusing NH3 and CO2 gases into aqueous solutions closely following the experimental setup of Lemarchand et al (2004). The precipitation rate (R∗) for every sample was determined applying the initial rate method and from the specific surface area of almost all samples for each reaction. The order of reaction with respect to Ca2+ ions was determined to be one and independent of T. However, the order of reaction with respect to HCO3- changed from three to one as temperature increases from 12.5, 25 °C and 37.5 °C. Strontium incorporated into calcite (expressed as DSr= [Sr/Ca] calcite/ [Sr/Ca] solution) was found to be R∗ and T dependent. As a function of increasing R∗ the Δ88/86Sr-values become more negative and as temperature increases the Δ88/86Sr values also increase at constant R∗. The DSr and Δ88/86Sr-values are correlated to a high degree and depend only on R∗ being independent of temperature, complexation and varying initial ratios. Latter observation may have important implications for the study of diagenesis, the paleo-sciences and the reconstruction of past environmental conditions. Calcium isotope fractionation (Δ44/40Ca) was also found to be R∗ and T dependent. For 12.5 and 25.0 °C we observe a general increase of the Δ44/40Ca values as a function of R∗ (Lemarchand et al type behavior, Lemarchand et al (2004)). Whereas at 37.5 °C a significant decreasing Δ44/40Ca is observed relative to increasing R∗ (Tang et al type behavior, Tang et al. (2008)). In order to reconcile the discrepant observations we suggest that the temperature triggered change from a Ca2+-NH3-aquacomplex covalent controlled bonding to a Ca2+-H2O-aquacomplex van-der-Waals controlled bonding caused the change in sign of the R∗ - Δ44/40Ca slope due to the switch of an equilibrium type of isotope fractionation related to the covalent bonding during lower temperatures to a kinetic type of isotope fractionation at higher temperatures. This is supported by the observation that the Δ44/40Ca ratios are independent from the [Ca]: [DIC] ratio at 12.5 and 25°C but highly dependent at 37.5°C. Our observations imply the chemical fluid composition and temperature dependent complexation controls the amount and direction of Ca isotope fractionation in contrast to the Sr isotopes which do not show any change of its fractionation behaviour as a function of complexation in the liquid phase.

Description: In order to study Strontium (Sr) partitioning and isotope fractionation of Sr and Calcium (Ca) in aragonite we performed precipitation experiments decoupling temperature and precipitation rates (R∗, μmol/m2.h) in the interval of about 2.3 to 4.5 μmol/m2.h. Aragonite is the only pure solid phase precipitated from a stirred solutions exposed to an atmosphere of NH3 and CO2 gases throughout the spontaneous decomposition of (NH4)2CO3. The order of reaction with respect to Ca ions is one and independent of temperature. However, the order of reaction with respect to the dissolved inorganic carbon (DIC) is temperature dependent and decreases from three via two to one as temperature increases from 12.5 and 25.0 to 37.0 °C, respectively. Strontium distribution coefficient (DSr) increases with decreasing temperature. However, R∗ responds differently depending on the initial Sr/Ca concentration and temperature: at 37.5 °C DSr increase as a function of increasing R∗ but decrease for 12.5 and 25 °C. Not seen at 12.5 and 37.5 °C but at 25°C the DSr-R∗ gradient is also changing sign depending on the initial Sr/Ca ratio. Magnesium (Mg) adsorption coefficient between aragonite and aqueous solution (DMg) decreases with temperature but increases with R∗ in the range of 2.4 to 3.8 μmol/m2.h. Strontium isotope fractionation (Δ88/86Sraragonite-aq) follows the kinetic type of fractionation and become increasingly negative as a function of R∗ for all temperatures. In contrast Ca isotope fractionation (Δ44/40Caaragonite-aq) shows a different behavior than the Sr isotopes. At low temperatures (12.5 and 25°C) Ca isotope fractionation (Δ44/40Caaragonite-aq) becomes positive as a function of R∗. In contrast, at 37.5°C and as a function of increasing R∗ the Δ44/40Caaragonite-aq show a Sr type like behavior and becomes increasingly negative. Concerning both the discrepant behavior of DSr as a function of temperature as well as for the Ca isotope fractionation as a function of temperature we infer that the switch of sign in the trace element partitioning as well as in the direction of the Ca isotope fractionation is probably due to the switch of complexation from a Ca2+-NH3 complexation at and below 25 °C to an Ca2+-H2O aquacomplex at 37.5 °C. The DSr - Δ88/86Srcalcite-aq correlation for calcite is independent of temperature in contrast to aragonite. We interpreted the strong DSr-temperature dependency of aragonite, the smaller range of Sr isotope fractionation as well as the shallower Δ88/86Srcalcite-aq-R∗ gradients to be a consequence of the increased aragonite solubility and the “Mg blocking effect”. In contrast to Sr the Ca isotope fractionation values in calcite and aragonite depend both on the complexation in solution and independent on polymorphism.

Description: Coral epithelia control ion fluxes to the calcification site influencing biomineralization and proxy incorporation. However, data on in vivo characteristics of coral tissue such as permeability, selectivity, and active ion transport are scarce but important for calcification and proxy modeling. To investigate ion permeability and ion fluxes across coral tissues in vivo, we developed an electrophysiological approach for the assessment of active and passive epithelial transport properties. Growing Stylophora pistillata corals in a thin layer over permeable filters allowed ion exchange at the site of skeleton formation for reproducible measurements of electrophysiological properties of coral tissues in a modified Ussing chamber. Compared to former applications, electrical measurements on these coral filter units were dominated by tissue characteristics with minimal influence of skeleton or physical stress. Coral tissues were cation selective. Their overall high electrical resistance characterized them as tight epithelia indicating low paracellular permeability for passive ion diffusion. This includes ions relevant for calcification. A small short-circuit current indicates active charge transport across the entire coral tissue. The present approach is applicable to corals laterally overgrowing substrates. It allows the electrophysiological characterization of coral tissue in vivo in response to environmental conditions. This will improve our knowledge on transepithelial transport relevant for biomineralization in corals.

Description: Mid- to late-Holocene sea-level records from low-latitude regions serve as an important baseline of natural variability in sea level and global ice volume prior to the Anthropocene. Here, we reconstruct a high-resolution sea-level curve encompassing the last 6000 years based on a comprehensive study of coral microatolls, which are sensitive low-tide recorders. Our curve is based on microatolls from several islands in a single region and comprises a total of 82 sea-level index points. Assuming thermosteric contributions are negligible on millennial time scales, our results constrain global ice melting to be 1.5–2.5 m (sea-level equivalent) since ~5500 years before present. The reconstructed curve includes isolated rapid events of several decimetres within a few centuries, one of which is most likely related to loss from the Antarctic ice sheet mass around 5000 years before present. In contrast, the occurrence of large and flat microatolls indicates periods of significant sea-level stability lasting up to ~300 years.

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.