Gernot Nehrke

Cellular calcium pathways and isotope fractionation in Emiliania huxleyi

The marine calcifying algae Emiliania huxleyi (coccolithophores) was grown in laboratory cultures under varying conditions with respect to the environmental parameters of temperature and carbonate ion concentration [CO32-] concentration. The Ca isotope composition of E. huxleyis coccoliths reveals new insights into fractionation processes during biomineralization. The temperature-dependent Ca isotope fractionation resembles previous calibrations of inorganic and biogenic calcite and aragonite. Unlike inorganically precipitated calcite, the [CO32-] concentration of the medium has no significant effect on the Ca isotope composition of the coccoliths. These results indicate a decoupling of the chemical properties of the bulk medium and the calcifying vesicle. Cellular Ca pathways of E. huxleyi indicate that fractionation cannot occur at the crystal surface, as occurs during inorganic precipitation. The dominant processes leading to the observed Ca isotope fractionation pattern in E. huxleyi are most likely the dehydration of the Ca aquocomplex at the plasma membrane and the attachment of dissolved Ca to proteins of Ca channels. The independence of Ca isotope fractionation from [CO32-] and the small temperature dependence of E. huxleyi are also important for defining the isotopic signature of the oceanic Ca sink. Since coccolithophores contribute to about half the global CaCO3 production, a relatively uniform isotopic composition of the oceanic Ca sink is further supported.

Calcium isotope fractionation during coccolith formation in Emiliania huxleyi: Independence of growth and calcification rate

[1] Recently, calcium isotope fractionation in the coccolithophore Emiliania huxleyi was shown to exhibit a significant temperature dependency. An important subsequent question in this context is whether the observed fractionation patterns are caused by temperature itself or related growth rate changes. In order to separate growth and calcification rate effects from direct temperature effects, batch culture experiments with the coccolithophore E. huxleyi were conducted under varying light intensities. Despite large changes in cellular growth and calcification rates, calcium isotope fractionation remained constant. Independence of calcium isotope fractionation on growth and calcification was also obtained in two additional sets of experiments in which growth rates changed in response to varying calcium concentration and seawater salinity. These experiments also showed no direct effects of calcium concentration and salinity on calcium isotope fractionation. Values for calcium isotope fractionation of E. huxleyi coccoliths fell within a range of −1.0 to −1.6 (1000 lnα), confirming earlier results. This range is similar to that observed in several foraminiferal species and coccolith oozes, suggesting a rather homogeneous calcium isotopic composition in marine biogenic calcite. Our data further show that the calcium isotope fractionation does not change with changing isotopic composition of seawater. This is a basic requirement for reconstructing the calcium isotopic composition of the ocean over time.

Coexistence of three calcium carbonate polymorphs in the shell of the Antarctic clam Laternula elliptica

We analyzed shell cuts of the Antarctic bivalve Laternula elliptica collected at King George Island by means of Confocal Raman Microscopy (CRM) as well as Electron Microprobe (EMP). Large area CRM scans (5 mm x 2 mm) reveal that three polymorphs of calcium carbonate ‐ aragonite, calcite, vaterite ‐ are present in the umbo region, the connection of both shell valves. Until now the shell of L. elliptica was believed to be exclusively composed of aragonite. Annual shell growth layers continued through aragonite and vaterite, suggesting simultaneous mineralization of both polymorphs. Spatially congruent EMP scans showed that the calcium carbonate polymorph affects the distribution of magnesium and strontium within the umbo. Mg was distinctly enriched in the vaterite layers deposited during winter. To the contrary Sr was enriched in the aragonite layers, also deposited during winter. This is, to our knowledge, the first report of the coexistence of three calcium carbonate polymorphs within the mineralized structures of a marine calcifying organism, including foraminifera, bivalves, and corals. Particularly the significant amounts of vaterite are quite unusual. The strong effect of the calcium carbonate polymorph on trace element fractionation restricts the suitability of Mg and Sr based proxies in shells of Laternula elliptica. Further analyses will show whether this is a unique finding typical for L. elliptica, or if this also applies to other bivalve species.

Incorporation of uranium in benthic foraminiferal calcite reflects seawater carbonate ion concentration

The chemical and isotopic composition of foraminiferal shells (so-called proxies) reflects the physico-chemical properties of the seawater. In current day paleoclimate research, the reconstruction of past seawater carbonate system to infer atmospheric CO2 concentrations is one of the most pressing challenges and a variety of proxies have been investigated, such as foraminiferal U/Ca. Since in natural seawater and traditional CO2 perturbation experiments, the carbonate system parameters co-vary, it is not possible to determine the parameter of the carbonate system causing e.g. changes in U/Ca, complicating the use of the latter as a carbonate system proxy. We overcome this problem, by culturing the benthic foraminifer Ammonia sp. at a range of carbonate chemistry manipulation treatments. Shell U/Ca values were determined to test sensitivity of U incorporation to various parameters of the carbonate system. We argue that [CO32-] is the parameter affecting the U/Ca ratio and consequently, the partitioning coefficient for U in Ammonia sp DU. We can confirm the strong potential of foraminiferal U/Ca as a [CO32-] proxy.

The morphological response of Emiliania huxleyi to seawater carbonate chemistry changes: an inter-strain comparison

Four strains of the coccolithophore Emiliania huxleyi (RCC1212, RCC1216, RCC1238, RCC1256) were grown in dilute batch culture at four CO 2 levels ranging from ~200 µatm to ~1200 µatm. Coccolith morphology was analyzed based on scanning electron micrographs. Three of the four strains did not exhibit a change in morphol- ogy over the CO 2 range tested. One strain (RCC1256) displayed an increase in the percentage of malformed coccoliths with increasing CO 2 concentration. We conclude that the sensitivity of the coccolith-shaping machinery to carbonate chemistry changes is strain-specific. Although it has been shown before that carbonate chemistry related changes in growth- and calcification rate are strain-specific, there seems to be no consistent correlation between coccolith mor - phology and growth or calcification rate. We did not observe an increase in the percentage of incomplete coccoliths in RCC1256, indicating that the coccolith-shaping machinery per se is affected by acidification and not the signalling pathway that produces the stop-signal for coccolith growth.

Characterization of ikaite (CaCO3•6H2O) crystals in first year Arctic sea ice north of Svalbard

Abstract We identified ikaite crystals (CaCO3 ·6H2O) and examined their shape and size distribution in first-year Arctic pack ice, overlying snow and slush layers during the spring melt onset north of Svalbard. Additional measurements of total alkalinity (TA) were made for melted snow and sea-ice samples. Ikaite crystals were mainly found in the bottom of the snowpack, in slush and the surface layers of the sea ice where the temperature was generally lower and salinity higher than in the ice below. Image analysis showed that ikaite crystals were characterized by a roughly elliptical shape and a maximum caliper diameter of 201.0±115.9 μm (n = 918). Since the ice-melting season had already started, ikaite crystals may already have begun to dissolve, which might explain the lack of a relationship between ikaite crystal size and sea-ice parameters (temperature, salinity, and thickness of snow and ice). Comparisons of salinity and TA profiles for melted ice samples suggest that the precipitation/dissolution of ikaite crystals occurred at the top of the sea ice and the bottom of the snowpack during ice formation/melting processes.

Effect of glacial drainage water on the CO2 system and ocean acidification state in an Arctic tidewater‐glacier fjord during two contrasting years

In order to investigate the effect of glacial water on the CO2 system in the fjord, we studied the variability of the total alkalinity (AT), total dissolved inorganic carbon (CT), dissolved inorganic nutrients, oxygen isotopic ratio (d18O), and freshwater fractions from the glacier front to the outer Tempelfjorden on Spitsbergen in winter 2012 (January, March, and April) and 2013 (April) and summer/fall 2013 (September). The two contrasting years clearly showed that the influence of freshwater, mixing, and haline convection affected the chemical and physical characteristics of the fjord. The seasonal variability showed the lowest calcium carbonate saturation state (X) and pH values in March 2012 coinciding with the highest freshwater fractions. The highest X and pH were found in September 2013, mostly due to CO2 uptake during primary production. Overall, we found that increased freshwater supply decreased X, pH, and AT. On the other hand, we observed higher AT relative to salinity in the freshwater end-member in the mild and rainy winter of 2012 (1142 lmol kg21) compared to AT in 2013 (526 lmol kg21). Observations of calcite and dolomite crystals in the glacial ice suggested supply of carbonate-rich glacial drainage water to the fjord. This implies that winters with a large amount of glacial drainage water partly provide a lessening of further ocean acidification, which will also affect the air-sea CO2 exchange.

Elevated CO2 Levels do not Affect the Shell Structure of the Bivalve Arctica islandica from the Western Baltic

Shells of the bivalve Arctica islandica are used to reconstruct paleo-environmental conditions (e.g. temperature) via biogeochemical proxies, i.e. biogenic components that are related closely to environmental parameters at the time of shell formation. Several studies have shown that proxies like element and isotope-ratios can be affected by shell growth and microstructure. Thus it is essential to evaluate the impact of changing environmental parameters such as high pCO2 and consequent changes in carbonate chemistry on shell properties to validate these biogeochemical proxies for a wider range of environmental conditions. Growth experiments with Arctica islandica from the Western Baltic Sea kept under different pCO2 levels (from 380 to 1120 µatm) indicate no affect of elevated pCO2 on shell growth or crystal microstructure, indicating that A. islandica shows an adaptation to a wider range of pCO2 levels than reported for other species. Accordingly, proxy information derived from A. islandica shells of this region contains no pCO2 related bias.

Impact of sample pretreatment on the measured element concentrations in the bivalve Arctica islandica

Correlating metal to calcium (Me/Ca) ratios of marine biogenic carbonates, such as bivalve shells, to environmental parameters has led to contradictory results. Biogenic carbonates represent complex composites of organic and inorganic phases. Some elements are incorporated preferentially into organic phases, and others are incorporated into inorganic phases. Chemical sample pretreatment to remove the organic matrix prior to trace element analysis may increase the applicability of the investigated proxy relationship, though its efficiency and side effects remain questionable. We treated inorganic calcium carbonate and bivalve shell powder (Arctica islandica) with eight different chemical treatments including H2O2, NaOH, NaOCl, and acetone and analyzed the effects on (1) Me/Ca ratios (Sr/Ca, Mg/Ca, Ba/Ca, and Mn/Ca), (2) organic matter (≈N) content, and (3) mineralogical composition of the calcium carbonate. The different treatments (1) cause element and treatment specific changes of Me/Ca ratios, (2) vary in their efficiency to remove organic matter, and (3) can even alter the phase composition of the calcium carbonate (e.g., formation of Ca(OH)2 during NaOH treatment). Among all examined treatments there were none without any side effects. In addition, certain Me/Ca changes we observed upon chemical treatment contradict our expectations that lattice-bound elements (Sr and Ba) should not be affected, whereas non-lattice-bound elements (Mg and Mn) should decrease upon removal of the organic matrix. For instance, we observe that NaOCl treatment did not alter Sr/Ca ratios but caused unexpected changes of the Mg/Ca ratios. The latter demonstrates that the buildup of complex biogenic composites like the shell of Arctica islandica are still poorly understood.

Limpet Shells from the Aterian Level 8 of El Harhoura 2 Cave (Témara, Morocco): Preservation State of Crossed-Foliated Layers

The exploitation of mollusks by the first anatomically modern humans is a central question for archaeologists. This paper focuses on level 8 (dated around ∼ 100 ka BP) of El Harhoura 2 Cave, located along the coastline in the Rabat-Témara region (Morocco). The large quantity of Patella sp. shells found in this level highlights questions regarding their origin and preservation. This study presents an estimation of the preservation status of these shells. We focus here on the diagenetic evolution of both the microstructural patterns and organic components of crossed-foliated shell layers, in order to assess the viability of further investigations based on shell layer minor elements, isotopic or biochemical compositions. The results show that the shells seem to be well conserved, with microstructural patterns preserved down to sub-micrometric scales, and that some organic components are still present in situ. But faint taphonomic degradations affecting both mineral and organic components are nonetheless evidenced, such as the disappearance of organic envelopes surrounding crossed-foliated lamellae, combined with a partial recrystallization of the lamellae. Our results provide a solid case-study of the early stages of the diagenetic evolution of crossed-foliated shell layers. Moreover, they highlight the fact that extreme caution must be taken before using fossil shells for palaeoenvironmental or geochronological reconstructions. Without thorough investigation, the alteration patterns illustrated here would easily have gone unnoticed. However, these degradations are liable to bias any proxy based on the elemental, isotopic or biochemical composition of the shells. This study also provides significant data concerning human subsistence behavior: the presence of notches and the good preservation state of limpet shells (no dissolution/recrystallization, no bioerosion and no abrasion/fragmentation aspects) would attest that limpets were gathered alive with tools by Middle Palaeolithic (Aterian) populations in North Africa for consumption.