Marine sediment porewaters are commonly depleted in K+ relative to conservative mixing trends, but the mineralogical sink for K+ in the sediment has not been well characterized. Results are presented from a geochemical study of surface waters and porewaters and associated muddy sediments in the Mississippi Delta plain estuary spanning the salinity gradient from 0-12‰. Evidence of K+ depletion in sediment porewaters is integrated with scanning transmission and analytical electron microscopy (STEM/AEM) analyses of clay components to determine possible mineralogical sinks for K+ in the sediments.
Conservative mixing between the freshwater influx from the Atchafalaya/Mississippi river systems and Gulf of Mexico seawater controls the surface-water major-element composition. Porewaters from the most saline site, however, are depleted in K+ by up to 33% relative to overlying water, implying uptake of K+ by the sediment. Textural characterization of the sediment at the same location by TEM indicates that it is dominated by aggregates of highly disordered smectite-rich clay material, which is mixed with a small component of more-ordered illite-rich material. AEM compositions obtained on the most smectite-like component in these aggregates indicates that the average K+ contents increase by up to 1.0 wt % K2O relative to similar smectite-rich clay in freshwater sediments. Ca2+ concentrations in smectite-rich clay decrease concomitantly, whereas Na+ concentrations are similar between the sites. X-ray diffraction (XRD) analyses confirm that smectite-rich clay from the freshwater site has a larger interlayer spacing than smectite-rich clay from the brackish-water site, consistent with exchange of smaller hydrated interlayer K+ (± Na+) for larger Ca2+ (± Mg2+). AEM data indicate that the discrete illite-rich component also takes up K+.
The integrated porewater and sediment analytical data imply that uptake of K+ by smectite-rich clay is a rapid process (decadal) occurring at the earliest stages of diagenesis in estuarine environments upon exposure to brackish water. Our study shows that K+ uptake in marine sediments may not require the formation of authigenic minerals. Instead, exchange of K+ for other interlayer cations in smectite-rich clay and weathered illite may provide a substantial sink for K+ in the marine environment.