PYRITIZATION OF IRON AND TRACE METALS DURING EARLY DIAGENESIS OF ORGANIC-RICH SULFIDIC MARSH SEDIMENTS IN THE URBAN HACKENSACK MEADOWLANDS ESTUARY, NJ

The Hackensack Meadowlands estuary is impacted by multiple contaminant sources resulting in elevated concentrations of trace metals (TM) in sediment (e.g., Ag, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn). To better understand the geochemical cycling of TM upon deposition in the salt marshes, intertidal mud-rich sediment and porewaters were sampled via box cores at 2-cm intervals to a depth of 26 cm. Analyses were obtained on various sediment digests to determine mineral phase associations: 1) TM content of “reactive FeS” (acid volatile sulfides (AVS)) and Fe-Mn-oxyhydroxides (Fe-Mn-oxy) (cool 1N-HCl), 2) “total” TM content of AVS, Fe-Mn-oxy, and pyrite, and less soluble TM- sulfides (concentrated hot HNO₃), 3) S-content of Fe- and TM-AVS (3N HCl), and (4) S-content of pyrite (Cr(II)-reducible).

Rapid depletion of porewater SO₄^2- and enrichment of DIC and dissolved sulfide indicates that sediment redox conditions are mediated by organic matter oxidation by sulfate-reducing bacteria. 210-Pb analyses indicate several box cores penetrated only young sediment (<25 yrs), whereas “old” sediment (>150 yrs), (pre-industrial deposition), was sampled at an excavated site undergoing restoration and from below ~10 cm at other sites. Young sediment contains 5-10 wt% C (C/S=3-5), whereas old sediment contains 3-4 wt% C (C/S=1.5-2.7). AVS-S content is 5-27% of total S in young sediment but only 0.05-1.5% in old sediment and inversely correlates with the degree of Fe-pyritization (DOP) (50-75% in young and 80-93% in old). TM can be grouped according to their degree of pyritization (DTMP) relative to Fe (DOP). Cd, Pb, Zn, and Mn undergo less DTMP than Fe. Cd, Pb, and Zn also correlate with AVS-S, consistent with precipitation as discrete MeS phases or sorption onto “FeS”. Mn likely occurs as discrete oxyhydroxides. Co, Cr, and Ni undergo similar DTMP as Fe suggesting sorption or coprecipitation with pyrite. Ag shows greater DTMP than Fe consistent with occurrence as a highly insoluble MeS₂ phase. Cu undergoes greater DTMP than Fe in young sediment, but lesser DTMP in old sediment indicating complicated phase associations. These data suggest that the relative mobility of individual TM will be different under changing Eh or pH conditions and controlled by the relative stability of host minerals.