Stable isotope and trace element composition of foraminiferal calcite - from incorporation to dissolution

Long-term reconstruction of climate and environmental parameters from marine sediments relies heavily on the reliability of proxy indicators from planktonic and benthic foraminifera. Therefore, the aim of this dissertation is to improve our understanding and confidence in planktonic foraminiferal proxies as indicators of seawater carbonate chemistry and their stability in response to dissolution. Towards this goal laboratory experiments with living specimens and empty shells collected from sediments were carried out. In culture experiments with the living planktonic foraminifer Orbulina universa the control of symbiont photosynthetic activity on the boron isotopic composition of shell calcite was investigated (Publication I). Under low light (reduced photosynthetic rates) the boron isotopic composition of the tests is 1.5‰ lower compared to shells grown under high light (elevated photosynthetic rates). As boron isotope values trace seawater-pH, the lower δB translates to a reduction in recorded pH of approximately 0.2 units. Data obtained for fieldgrown, symbiont-barren Globigerina bulloides record significantly lighter δB than the symbiont-bearing O. universa and therefore support the hypothesis that respiration and photosynthesis are the key physiological parameters responsible for species-specific vital effects. Although this experiment may indicate that symbiont-barren foraminifera reflect ambient seawater chemistry more accurately than symbiont-bearing foraminifera, model results suggest that photosynthesisand respiration-driven offsets are constant over a wide pH-range and do not compromise the reliability of δB in symbiont-bearing foraminifera as a paleo-pH indicator. The Ba/Ca ratio in foraminiferal shells has been shown to reflect seawater Ba concentrations, which in turn are correlated to alkalinity. To investigate whether alkalinity may directly influence the Ba incorporation into foraminiferal calcite and thereby compromise the reliability of this alkalinity proxy, symbiont-bearing O. universa and symbiont-barren G. bulloides were grown in seawater of constant Ba concentration at five different alkalinities (Publication II). A weak negative correlation between the Ba uptake ratio in O. universa shells and alkalinity was found under high light conditions. For an increase in alkalinity of 100 μmol kg the distribution coefficient DBa (= Ba/Ca shell / Ba/Caseawater) decreased by 0.004. This change is well within the error of DBa determined to date and the weak influence of alkalinity on Ba incorporation into foraminiferal shells is insignificant for paleoreconstructions. Globigerina bulloides has not been calibrated for Ba before and the experiments revealed that DBa in this species is the same as DBa in O. universa. In line with the similar Ba/Ca uptake ratio of symbiont-bearing and symbiont-barren species, varying light levels do not affect the Ba incorporation of O. universa. To investigate the effect of undersaturated seawater on foraminiferal shell chemistry, well preserved shells of the planktonic foraminifera species Globigerinoides sacculifer and Neogloboquadrina pachyderma (sinistral coiling) were partially dissolved under controlled conditions in the laboratory (Publication III). In addition to known dissolution effects on Mg/Ca, δO and δC, significant effects on Sr/Ca and δB could be determined which are in the same order of magnitude as observed for glacial/interglacial changes. Using previous hypotheses to explain and discuss dissolution patterns, it becomes evident that the overall process is not yet fully understood. While δO, δC, Mg/Ca and maybe Sr/Ca can be explained by preferential dissolution of ontogenetic calcite and a shift of the bulk shell chemistry to calcite secreted at greater depth (gametogenic and/or crust calcite), δB and δCa seem to be inconsistent with such an explanation and the dissolution patterns of these elements need to be dominated by other processes. Reduced calcite stability due to higher Mg/Ca was found to be insignificant to control the overall dissolution behavior in foraminiferal shells and increasing Sr/Ca ratios demonstrate that crystal impurities are not necessarily more prone to dissolution. The microstructural breakdown of shell surfaces, i.e. the formation of fissures and crevices, indicates an increase in porosity upon shell corrosion rather than the removal of outer calcite layers. The resulting increase in surface area leads to the exposition of otherwise protected lattice areas and possibly allows certain elements to be leached out. In summary, dissolution effects appear to be species-specific and depend on the physico-chemical gradients encountered by vertically migrating foraminifera at different locations. The dissolution-driven weight loss of planktonic foraminifera shells in a defined narrow size range has been proposed to reflect bottom water carbonate ion concentration ([CO3]). A number of recent studies used this relationship but disregarded a number of complications that may limit the reconstructions. Publication IV combines experimental results on cultured foraminifera and theoretical arguments to evaluate these complications: (1) The value chosen for the pressure impact on the [CO3] at calcite saturation is overestimated and should be 16 μmol kg km instead of 20 μmol kg km. (2) The offset in [CO3] between bottom and pore water depends on the amount of organic matter that is being degraded within the sediment and consequently the assumed constancy of the offset over time and space is highly unlikely. (3) The initial weight of undissolved shells also changes between sites and over geological timescales. Growth conditions such as [CO3], light and temperature affect respiration, symbiont photosynthesis and calcification processes and cause a significant variability in initial shell weight. (4) As the dissolution susceptibility of foraminifera shells varies between species, a single weight loss slope cannot be used for different species. Correcting the published estimate of glacial bottom water [CO3] for the various effects and uncertainties discussed in Publication IV considerably reduces the published estimate of the Atlantic glacial upper deep water [CO3]-increase from +14 mol kg to only +4 μmol kg.