Beschreibung: Culture experiments were carried out with the planktonic foraminifer Orbulina universa under high and low light levels in order to determine the influence of symbiont photosynthetic activity on the boron isotopic composition of shell calcite. 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). In terms of inferred pH, the lower boron isotope values correspond to a reduction in pH of approximately 0.2 units. The boron isotopic composition of Orbulina universa from plankton tows is similar to that of shells grown under low light conditions in the laboratory. These data are consistent with reduced symbiont concentrations in recently secreted shells. In addition to laboratory and field grown O. universa, we present the first data for a symbiont-barren foraminifer, Globigerina bulloides. Data obtained for G. bulloides fall ∼1.4‰ below those of the field grown O. universa. Although the plankton tow results are preliminary, they support the hypothesis that respiration and photosynthesis are the key physiological parameters responsible for species-specific vital effects.

Beschreibung: We critically evaluate the applicability of Ca-isotope ratios in planktonic foraminifers as proxy for past sea surface temperatures (SST) and isotope composition of paleo-seawater (δ44Casw) reconstructions. Previous studies have shown discrepancies regarding the temperature sensitivity of Ca isotope fractionation in foraminifers of more than one order of magnitude. We present new data from the planktonic foraminifer species Orbulina universa, Globigerinoides sacculifer and Neogloboquadrina pachyderma (sinistral) from culture experiments, multinet deployments and coretop samples. Specimens of G. sacculifer cultured at low salinities (33–34.5) show predominantly no major temperature dependent Ca isotope fractionation, in contrast to previous individuals cultured at higher salinities of 34.5–36. The new data of O. universa are consistent with previously published results, revealing a small but significant temperature sensitivity. Calcium isotope fractionation in tests of N. pachyderma shows a significant variation with temperature, which is not uniform over the total investigated temperature range (−1.6 °C to +10 °C), possibly reflecting the influence of additional controlling factors besides temperature. Controlled dissolution experiments in the laboratory indicate that the Ca-isotope composition of G. sacculifer and N. pachyderma is relatively insensitive to partial dissolution of their tests. Calcium isotope ratios in the planktonic foraminifers G. sacculifer and N. pachyderma (s) reveal a complex Ca isotope fractionation behaviour, which is not yet fully understood. Additional validation studies are crucial to enhance the basic understanding of the calcium isotope systematics in planktic foraminifer shells, and the potential for applying Ca-isotope ratios as proxies for seawater temperature and the oceanic Ca budget.

Beschreibung: Culture experiments were carried out with the planktonic foraminifer Orbulina universa under high and low light levels in order to determine the influence of symbiont photosynthetic activity on the boron isotopic composition of shell calcite. Under low light (reduced photosynthetic rates) the boron isotopic composition of the tests is 1.5 per mil lower compared to shells grown under high light (elevated photosynthetic rates). In terms of inferred pH, the lower boron isotope values correspond to a reduction in pH of approximately 0.2 units. The boron isotopic composition of Orbulina universa from plankton tows is similar to shells grown under low light conditions in the laboratory. These data are consistent with reduced symbiont concentrations in recently secreted shells. In addition to laboratory and field grown O. universa, we present the first data for a symbiont-barren foraminifer, Globigerina bulloides. Data obtained for G. bulloides fall ~1.4 per mil below the field grown O. universa. Although the plankton tow results are preliminary, they support the hypothesis that respiration and photosynthesis are the key physiological parameters responsible for species-specific vital effects.

Beschreibung: The stable boron isotope composition (d11B) of fossil foraminiferalshells is used as a paleo-pH recorder and is therefore one of themost promising paleo-carbonate chemistry proxies ('paleoacidimetry').One crucial question regarding this proxy is whether foraminiferarecord the pHof the bulk seawater or the pH of the microenvironment (diffusiveboundary layer, ~500mum) which is strongly influenced bylife processes. Here we present a novel, theoretical approach toaddress this question by using a diffusion-reaction model. Model resultsindicate that the d11B in planktonic foraminifera is primarilycontrolled by the pH of the microenvironment. We therefore predictthat the d11B of different species (e.g. symbiont-bearing vs.symbiont-barren) or of foraminifera grown in the dark andin the light should be offset from the d11B of inorganic calcite.This theoretical prediction was experimentally confirmed while thispaper was written (Hoenisch et al., in revision).Most importantly, the model predicts that this offset is constantover a wide pH range. Thus, the use of d11B as a paleo-pHindicator is not compromised through vital effects as modeled here.

Beschreibung: Broecker and Clark (2002) use the "size normalized weight" of planktic foraminifera to estimate the carbonate ion concentration ([CO3=]) of Atlantic glacial upper deep water. This method was introduced by Lohmann (1995) and is based on the fact that, within a defined size fraction, dissolution decreases shell weight in proportion to the degree of undersaturation with respect to CaCO3. Based on the following assumptions,1) the thickness of the foraminiferal shell wall does not depend on growth conditions,2) the saturation [CO3=] for calcite, [CO3=]sat., increases by 20 µmol kg-1 km-1 (Broecker and Clark, 2001),3) the weight loss slope is universal and ca. 0.3 µg (µmol kg-1)-1 (Broecker and Clark, 2001),4) the offset between the bottom water and pore water [CO3=] was the same during glacial time as during the Holocene,Broecker and Clark (2002) calculate that the [CO3=] of Atlantic glacial upper deep water was 14 µmol kg-1 higher than during the Holocene. Although they recognize that some of their assumptions are not strictly valid, they do not assess the impact of those assumptions on their [CO3=] estimate. Here we comment on several of those assumptions and attempt to quantify their impact on Broecker and Clark's (2002) calculations.