Pyritization of iron in sediments from the continental slope of the Northern Gulf of Mexico

In sediments from the continental slope of the Northern Gulf of Mexico, generally, the degree of iron pyritization (DOP) is low (<0.1) and dissolved sulfide is below detection limits ( 5 μM), whereas dissolved Fe is typically about 50 to 100 μM. Therefore, the dissolution of kinetically reactive iron minerals generally dominates over the rate of sulfide production in sediments throughout this region. However, in sediments where hydrocarbons have been added via seepage from the subsurface, dissolved-Fe is undetectable, DOP can approach 1, and high concentrations of dissolved sulfide (up to ∼11 mM) are commonly present. Even though these sediments have high total reduced sulfide (TRS) concentrations (typically 150 to 370 μmol gdw(-1)), their average C/S ratio is about 4 times that of "normal" marine sediments reflecting the major input of hydrocarbons. DOP is significantly (∼20%) higher when calculated using reactive-Fe extracted by citrate dithionite than by cold 1N HCl. This difference is primarily due to the greater extraction efficiency of the cold HCl method for silicate-Fe. TRS tends to rise to a maximum, and remains close to constant even at high (mM) dissolved sulfide concentrations. These TRS concentrations, therefore, represent the size of the "kinetically" reactive-Fe pool during early diagenesis.