Geochemical heterogeneity of a coastal mangrove wetland

Abstract

Preliminary geochemical studies of intertidal mangrove sediments from Polaris Point, Guam (13°30'N 144°45'E) showed a composition transition from a terrestrial to a marine environment. Seven mangrove core samples were taken in a transect across the mangrove swamp during the October wet season. Mineralogy and major (Si, Al, Fe, Mn, Mg, Ca, P) and minor elements (Ga, Zn, Ni, Co, Mn, Cr, Pb, Rb, Sr, V, Y, Zr) distribution in the mangrove swamp were determined using techniques of X-ray diffraction (XRD) and X-ray fluorescence (XFR). Element concentrations largely reflect the sediment mineralogy which includes terrestrially-derived volcanic material, marine-derived carbonate and clay minerals. High concentrations of the metals Ni, Co, Mn and Cr are indicative of the basaltic source , although some residual contamination may exist from a previous oil spill. Movement of a subsurface salt-wedge over a 12-hour tidal period is thought to be reflected by a negative correlation between the tidal height (m) and groundwater salinity (ppt), and a positive correlation between groundwater pH, Eh (mV) and Mn concentration (ppm). these results suggest further investigation of the temporal and spatial variations of transition elements in the mangrove swamp, together with the causes of these variations, is warranted.