Stable isotope and trace element composition of foraminiferal calcite [Elektronische Ressource] : from incorporation to dissolution / vorgelegt von Bärbel Hönisch

Elmar Haake

Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2002
• Nombre de lectures: 27
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

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







Dissertation
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
- Dr. rer. nat. -
im Fachbereich 2 (Biologie/Chemie)
der Universität Bremen











vorgelegt von

Bärbel Hönisch


Bremen, 2002











Tag des öffentlichen Kolloquiums:


14. November 2002













Gutachter der Dissertation:


Prof. Dieter Wolf-Gladrow

PD Dr. Ralph Schneider
Abstract

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
11(elevated photosynthetic rates). As boron isotope values trace seawater-pH, the lower δ B
translates to a reduction in recorded pH of approximately 0.2 units. Data obtained for field-
11grown, symbiont-barren Globigerina bulloides record significantly lighter δ B 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 photosynthesis- and respiration-driven offsets are constant over a wide
11pH-range and do not compromise the reliability of δ B 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
-1100 µmol kg the distribution coefficient D (= Ba/Ca / Ba/Ca ) decreased by Ba shell seawater
0.004. This change is well within the error of D determined to date and the weak influence Ba
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 D in this species is the same as D in O. universa. In line with Ba Ba
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
18 13 11Mg/Ca, δ O and δ C, significant effects on Sr/Ca and δ B 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
18 13process is not yet fully understood. While δ O, δ C, Mg/Ca and maybe Sr/Ca can be
explained by preferential dissolution of ontogenetic calcite and a shift of the bulk shell
11chemistry to calcite secreted at greater depth (gametogenic and/or crust calcite), δ B and
44δ Ca 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
2-size range has been proposed to reflect bottom water carbonate ion concentration ([CO ]). A 3
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
2-for the pressure impact on the [CO ] at calcite saturation is overestimated and should be 16 3
-1 -1 -1 -1 2-µmol kg km instead of 20 µmol kg km . (2) The offset in [CO ] between bottom and 3
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
2-geological timescales. Growth conditions such as [CO ], light and temperature affect 3
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.
2-Correcting the published estimate of glacial bottom water [CO ] for the various effects and 3
uncertainties discussed in Publication IV considerably reduces the published estimate of the
2- -1 -1Atlantic glacial upper deep water [CO ]-increase from +14 mol kg to only +4 µmol kg . 3



Zusammenfassung


Die Rekonstruktion vergangener Klima- und Umweltbedingungen anhand von
marinen Sedimenten wird maßgeblich durch die Verläßlichkeit der Proxyindikatoren
bestimmt, die man aus Schalen planktischer und benthischer Foraminiferen gewinnen kann.
Das Ziel dieser Arbeit ist deshalb die Verbesserung des Verständnisses und Vertrauens in
Proxies aus planktischen Foraminiferen als Indikatoren mariner Karbonatchemie und der
Stabilität von Proxies in angelösten Schalen. Um mögliche Einflüsse zu untersuchen, wurden
Laborexperimente mit lebenden Foraminiferen und leeren Schalen aus Sedimenten
durchgeführt.
In Kulturexperimenten mit der lebenden planktischen Foraminifere Orbulina universa
wurde der Einfluß der Symbiontenaktivität auf die Borisotopenzusammensetzung der
Kalkschale untersucht (Publikation I). Im Vergleich zu Individuen, die unter hohem
Lichtangebot gewachsen sind (hohe Photosyntheseraten), ist die
Borisotopenzusammensetzung von Schalen, die unter geringem Lichtangebot (geringe
Photosyntheseraten) gebildet wurden, etwa 1.5‰ leichter. Da die Borisotopie ein Anzeiger
11für den marinen pH-Wert ist, entspricht der geringere δ B-Wert einer Verminderung des pH-
Werts um etwa 0,2 Einheiten. Die Borisotopenzusammensetzung der Symbionten-freien
Foraminifere Globigerina bulloides ist deutlich leichter als die der Symbionten-tragenden O.
universa und unterstützt damit die Hypothese, daß Respiration und Photosynthese die
physiologischen Schlüsselparameter für artspezifische Vitaleffekte darstellen. Obwohl dieses
Experiment nahelegen könnte, daß Symbionten-freie Foraminiferen die Meerwasserchemie
genauer aufzeichnen als Symbionten-tragende Arten, deuten Modellergebnisse daraufhin, daß
die durch Photosynthese und Respiration bedingten Unterschiede über weite pH-Bereiche
11konstant sind und die Zuverlässigkeit von δ B aus Schalen Symbionten-tragender
Foraminiferen als paläo-pH Indikator nicht beeintr