Liebetrau, Volker, Haeckel, Matthias , Eisenhauer, Anton , Scholz, Florian , Hensen, Christian and Reitz, Anja (2011) Paired Sr isotope (⁸⁷Sr/⁸⁶Sr, δ^88/86Sr) systematic of pore water profiles: A new perspective in marine weathering and seepage studies. [Talk] In: Goldschmidt Conference 2011. , 14.-19.08.2011, Prague, Czech Republic .

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Abstract

The simultaneous and independent determination of the
radiogenic (87Sr/86Sr) and the fractionation reflecting stable
(!88/86Sr) Sr isotope ratio on pore waters, sediments and
precipitates (e.g. carbonates and sulfates) opens a new
perspective in the field of submarine weathering and Sr
contribution to the ocean chemistry.
Four initial case studies covering (1.) CO2 seeps of the
Okinawa Trough (OT), (2.) mud volcanoes (MV) and mounds
in the Gulf of Cadiz (GoC) and the (3.) Central American Fore
Arc as well as first results from the (4.) Black Sea are
conducted and reflect a stable Sr perspective on seeps from a
broad range of geological settings.
Referred to NIST-SRM-987, in this study the IAPSO
seawater (SW) standard has a !88/86Sr of 0.39 ‰ (±0.03, 2SD).
As a prominent systematic deviation the OT pore water (PW)
data from a site with CO2 hydrate and liquid CO2 occurence
show values ranging from 0.27 to 0.59 ‰ (286 to 64 cm
sediment depth), accompanied by a weak inversely correlated
trend from 0.2 to 0.15 ‰ for the corresponding bulk sediment
(286 to 36 cm).
In contradiction to a simple fluid/SW-mixing approach as
driving mechanism for the PW stable Sr trend the 87Sr/86Sr
signature stays within analytical uncertainty constant with
depth (0.70980 (1)) and differs significantly from SW
(0.70917 (1)) and the more radiogenic, slightly heterogeneous
sediment (0.71892-0.71731).
Potential explanation for the observed !88/86Sr trend and
PW signatures heavier than SW are (a) strong fractionation
processes enriching light isotopes in secondary precipitates
and remineralisation products and heavier signatures in the
remaining fluid and/or (b) preferential dissolution of heavier
mineral phases.
Examples for the latter kind of sediment component are
determined in a detailed study of the Mercator MV (GoC) by
high !88/86Sr ratios of 0.72 for authigenic and 0.92 ‰ for
potentially extruded gypsum crystals.
Combined with PW data from the other seep settings (0.2
to 0.52 ‰) a broad range of Sr contribution and fractionation
processes becomes evident.

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