ENVIRONMENTAL AND STATISTICAL ANALYSES OF GEOCHEMICAL DATA FROM A GULF OF MEXICO CORE

A sediment core from the Coatzacoalcos River, which discharges in the southeast Gulf of Mexico, was studied to interpret the environmental and statistical significance of geochemical data related to early diagenetic processes. Trace metals (tm=Fe, Pb, Co, Cu, Zn, V) were extracted into two operationally-defined fractions (HCl-tm and pyrite-tm) through which the degree of pyritization [DOP=pyrite-Fe/(pyrite-Fe + HCl-Fe)] and the degree of trace metal pyritization [DTMP-tm=pyrite-tm/(pyrite-tm + HCl-tm)] were calculated as a function of depth. These results were then interpreted for their environmental and geochemical significance. In order to test for significant differences between DOP and DTMP mean values, an univariate two tailed t-test was carried out for each of the trace metals. Visual inspection of the breaker points for the DOP and DTMP values of the vertical profiles was carried out in order to group their shallow and deep values separately. The t-test values were significant for for Fe, Pb, Co and Cu in which the null-hypothesis of equal means was rejected. These results suggest that the vertical changes in DOP and DTMP are significant in terms of the biogeochemical processes taking place in the sediments and that the values are not due to random effects in the system. The high values of DOP suggest that Fe is incorporated into the pyrite phase at depth as the most important trace metal due to its kinetic behavior. Furthermore, it was observed that Cu, Co and Pb followed the same DTMP vertical profile pattern despite the fact that Cu and Pb form sulfide minerals faster than Co, which exhibits a kinetic behavior close to that of Fe. This result may suggest that even with different kinetic behaviors, Pb, Co and Cu are incorporated into the pyrite phase with similar patterns at depth. In contrast, Zn tends to form strong soluble complexes, leaving less of this metal available for solid trace metal sulfide formation. V is depleted in the pyrite fraction probably because its incorporation into this mineral phase occur only under very strong reducing conditions, which were not present in our core. Moreover, it can be observed that even though high Fe-DOP levels are reached at depth, the DTMP for most of the rest of the trace metals decreased with depth.