Changes in Trace Metal Cycling Driven by Sediment Redox Conditions Book uri icon

abstract

  • The cycling and accumulation of trace metals in sediments is often strongly dependent on the redox conditions within the sediments. Understanding the relationship between redox chemistry, metal chemistry and other diagenetic processes, such as bioturbation, is essential in order to use trace metals as tracers of paleo-redox conditions and for predicting the long term fate of metal-contaminated marine sediments. In this study the behavior of the metals, silver, copper and lead, under seasonally varying redox conditions has been investigated at two contrasting sites in coastal Massachusetts. The first site is in Boston Harbor and has high rates of organic carbon oxidation and active sulfate reduction, with porewater sulfide detected below 5 cm. The second site is further offshore in Massachusetts Bay and also has an oxygen penetration depth of less than 1 cm, but no sulfide is detected in the porewaters down to a depth of 30 cm. The variations in redox conditions throughout the year have a strong impact on the metal behavior and the metal profiles in the sediments. At both sites the trace metals are scavenged by iron oxyhydroxides in the surface oxic zone and are released to the porewaters when these oxides are reduced. At the more reducing Boston Harbor site, as the rates of organic matter degradation increase over summer, the sulfide interface moves upwards and the trace metals are transferred from the oxide phases to sulfides phases. At the offshore site, due to the lack of sulfide, the metals are focused into the surface oxide layer, giving a solid phase enrichment that is not observed at the Harbor site. Silver porewater data from the coastal zone are further compared with porewater profiles from an oceanic transect heading offshore from the Oregon coast. The transect covers a range of organic carbon fluxes and bottom water oxygen levels, extending our investigation of metal behavior across a spectrum of sediment redox conditions, from strongly reducing coastal environments to oxic oceanic sites.

publication date

  • October 2007