Seawater contamination by coring and pore water sampling of marine sediments

Abstract

Coring of sediments with minimal physical disturbance, and sampling of pore water with minimal contamination from seawater, are critical in marine geochemistry and microbiology. Yet, sediment coring generally causes smearing of sediment down along the inside of the core liner and leaves seawater and fluid mud trapped around the core. This seawater is a source of diffusive or advective exchange of solutes with the core, which affects the chemistry of retrieved pore water. The exchange depends on the time elapsed between coring and the end of pore water extraction. The degree of seawater contamination is difficult to determine and has generally been neglected in the geochemical literature. Yet, it is possible to quantify seawater contamination due to the high millimolar concentration of sulfate in seawater and low micromolar concentration in the subsurface methanic zone, where sulfate is largely depleted. Analysis of 540 sediment cores from a global database showed that sulfate concentration data beneath the sulfate-methane transition were often truncated and stated as “0” or “0,0” mM, thereby preventing an evaluation of seawater contamination. Among the 181 sediment cores for which the mM sulfate concentrations were reported with two or three decimal places, concentrations measured in methanic sediments were up to an order of magnitude higher than the estimated in situ concentrations in gravity cores and two orders of magnitude higher in cores recovered by scientific ocean drilling. A comparison between pore water extraction by Rhizon suction samplers and by high-pressure squeezers in cores from Baltic Sea Expedition 347 of the IODP (International Ocean Discovery Program) showed no systematic difference in the degree of contamination. A comparison with a perfluorocarbon (PFC) tracer injected by IODP coring into the drilling fluid showed a seawater contamination similar to that based on sulfate in the methanic zone. In IODP cores, where the true in situ sulfate concentration is generally unknown, we calculated the theoretical sulfate concentration that corresponded to equilibrium between SO42−, Ba2+ and barite (BaSO4) to show that the equilibrium sulfate concentrations in the methanic zone are much lower than the sulfate concentrations measured in pore water samples, again indicating seawater contamination. We used a time-resolved diffusion model to calculate how contaminating seawater ions penetrate from the external liner fluid into sediment cores before pore water is extracted. We conclude that nearly all published pore water samples retrieved from subsurface sediments are contaminated by seawater to some degree. We also conclude that sensitive sulfate analyses in methanic sediments constitute a unique opportunity to quantify this contamination and thereby provide a better possibility to control it.