Eisenhauer, Anton

Fietzke, Jan

OceanRep

Scientific Highlights - 25Strontium (Sr) is used widely to reconstruct the chemical and tem-perature history of the ocean on vari-ous time scales through measurement of the 87Sr/86Sr isotope ratio and the Sr/Ca elemental ratios in marine car-bonates. Although Sr itself is not radi-oactive the isotope 87Sr is supported by the radioactive beta decay of 87Rb (Rb=Rubidium) which has a half-life of 48 billion years. The primary use of this isotope system in the geosciences is for rock and mineral dating. However, because Sr is enriched in the Earth´s mantle whereas Rb is enriched in the Earth´s crust, the 87Sr/86Sr ratio can also be used to distinguish the geo-logical origin of minerals and rocks. Notably, the 87Sr/86Sr ratio of conti-nental rocks and minerals tends to be higher than those originating from the Earth´s mantle. One major application of this element and isotope separa-tion process in the marine sciences is to reconstruct changes in the chemi-cal balance of Sr in the ocean and dis-tinguish the relative contribution of Sr from marine hydrothermal sources to that delivered by continental weather-ing throughout Earth´s history. Such studies shed light on past rates of con-tinental weathering and hence on a key process that controls atmospheric CO2 levels on geological time scales.Throughout decades of Sr isotope research it has been assumed that there are no additional isotope frac-tionation process which alter the 87Sr/86Sr ratio established by the radio-genic ingrowth of 87Sr due to 87Rb beta decay. In particular, this was assumed to hold true for the 88Sr/86Sr ratio, which is used in the calibration pro-cedure for mass spectrometric meas-urement of 87Sr/86Sr ratios. However, recent high precision measurements at IFM-GEOMAR of 88Sr/86Sr (Fietzke and Eisenhauer, 2006) show that this ratio is not stable, but rather is vari-able and temperature dependent. The measurements also showed that the presently accepted δ88/86Sr-value for seawater (δ88/86Srseawater=0) is offset by about 0.4 ‰ from its true value (Fig. 1) which in turn implies that the true87Sr/86Sr isotopic ratio in seawater isalso significantly different (by about0.2 ‰) from its presently-accepted value (δ87/86Sr =  0.7091741(24)). The measurements also revealed, for the first time, that there is a tem-perature-dependent strontium isotope fractionation during calcium carbonate precipitation. This could be shown both for inorganically precipitated aragonite and for coral samples (Fig. 1). These results require further experimental confirmation but may point to a new tool for reconstructing past seawater temperature variations resulting from climate change. The isotopic ratio has the potential to be less sensitive to post depositional diagenetic alteration, water pressure effects and salinity variability than the traditional proxies for temperature reconstruction. Hence the results from these high-precision measurements both open a new door to past climate change but also, as a result of the insight into Sr-isotope fractionation, suggest that the history of continental weathering as deduced from the 87Sr/86Sr-ratio of marine car-bonate has to be reassessed. First, approximate calculations indicate that about 20 % of the Cenozoic 87Sr/86Sr-variations might, in fact, be due to other sources of 88Sr/86Sr-variability (e.g. temperature changes). References:Fietzke, J. and Eisenhauer, A., 2006: Determination of temperature-depend-ent stable strontium isotope (88Sr/86Sr) fractionation via bracketing standard MC-ICP-MS. Geochmistry, Geophys-ics, Geosystems 7(8), doi:10.1029/2006GC001243.Anton Eisenhauer and Jan FietzkeParadigm change: temperature-dependent strontium isotope variationsFigure 1: The δ88/86Sr versus temperature data of inorgani-cally precipitated aragonite and natural coral (Pavona cla-vus) samples. For comparison the δ88/86Sr value of IAPSO seawater standard (0.381 ± 0.010% (2SEM)) is plotted as temperature-independent level. IFM-GEOMAR Report; 2006 (2007) [Annual Report]

Paradigm change: temperature-dependent strontium isotope variations

http://oceanrep.geomar.de/46189/1/Eisenhauer%20Fietzke%20ifm-geomar-report-2006-8-1.pdf

Paradigm change: temperature-dependent strontium isotope variations

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