Lithium Isotope Composition of Quarternary and
Tertiary Planktonic Foraminifera: Consequences for the Lithium Budget of the Ocean

Michael Anton Sywall Geochemisches Institut, Universität Göttingen , Goldschmidtstr.1, 37077 Göttingen. Germany

Jochen Hoefs Geochemisches Institut, Universität Göttingen , Goldschmidtstr.1, 37077 Göttingen, Germany;

The lithium isotope composition of 38 Quarternary and Tertiary foraminifera samples from the Atlantic and the Indian Ocean (0 to 40Ma) has been determined by thermionic mass spectrometry (THQ). A mean standard deviation (2s) of 5” was achieved. Recent ocean water was used as external standard and the results are given in the d-notation as d6Li. Accordingly d6Li of recent ocean water is 0”. In contrast mantle-like lithium has an isotope signature of +32” as deduced from the analysis of unaltered gabbros, basalts and carbonatites.

During biogene carbonate production lithium is incorporated into the calcite lattice of foraminifera in the order of 0.5 to 2.5ppm (Ø=1.1ppm). Holocene samples possess a well defined mean lithium isotope composition of +13” relative to ocean water. Tertiary foraminifera show a total variation from -10” to +30”. The lithium-isotope composition of well preserved, diagenetically unaltered foraminifera varies in a more restricted range between -5” and +25”. Smoothing of the data points produces an isotope curve which shows a periodicity of 14 ± 2 Ma. This regular variability of the lithium isotope signature can be interpreted as reflecting shifts in the paleo-ocean water composition.

The lithium supply of the ocean is controlled by the hydrothermal input along the midocean ridges and by the fluvial input, whereas spilitization of basalts and submarine diagenetic clay mineral formation/alteration act as sinks for lithium. These different lithium fluxes can be characterized by their lithium isotope signature. Taking the lithum isotope composition of marine hydrothermal solutions and river waters from Chan and Edmond (1988) and Chan et al. (1993) and own results of marine basalts, shales and porewaters it can be shown, that in a steady state system the hydrothermal input must be smaller than so far estimated. Fluvial and hydrothermal fluxes must be nearly in the same order of magnitude.

Calculations performed with a dynamic, time dependent model show, that in a steady state system variations in the lithium fluxes into the ocean cannot lead to shifts in the d6Li of ocean water exceeding 15”. To match the mean range of variation given by the foraminiferal data (20”) a "non-steady-state" model has to be adopted, in which the hydrothermal flux dominates the lithium isotope composition of the ocean water. A variation of the hydrothermal flux of about the factor of 2 is required to produce the necessary shift in the ocean water d6Li.


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