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doi:10.2204/iodp.proc.337.201.2015

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 337 aims to understand the relationship between a deep-seated (~1950 meters below seafloor [mbsf]) hydrocarbon reservoir off Shimokita peninsula (northeast Japan) and the microbial community at such depth. Understanding the sources and pathways of fluids is one of the primary objectives of the expedition (see the “Expedition 337 summary” chapter [Expedition 337 Scientists, 2013a]). Migration of fluid supports the deep microbial community as well as the shallow-dwelling organisms through delivery of critical nutrients and elements. To this aim, we use the strontium (Sr) isotope ratios from pore water to identify the sources and migration pathway of the deep fluid.

Radiogenic Sr isotope ratio, ⁸⁷Sr/⁸⁶Sr, is known to be free from biological fractionation (Mook, 2001), which makes it a valuable and conservative tracer for fluid from different sources (Torres et al., 2004; Teichert et al., 2005; Joseph et al., 2012, 2013). Possible sources include modern seawater (⁸⁷Sr/⁸⁶Sr ≅ 0.7092; McArthur et al., 2012), continental felsic and basaltic rocks (<0.7180; Faure and Powell, 1972), biogenic calcite (~0.7075–0.7092), volcanic material (~0.7065), and oceanic crust (~0.703; Veizer, 1989). Whereas some of the dissolved constituents measured in the pore water samples show significant contamination by drilling fluid, Sr is considered to be relatively free from such contamination due to the low Sr concentration in drilling fluid (<30 μM) (see the “Methods” chapter [Expedition 337 Scientists, 2013b]).

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