A CRITICAL LOOK AT IRON PALEOREDOX PROXIES: NEW INSIGHTS FROM MODERN EUXINIC MARINE BASINS

Abstract

Enrichments in reactive iron occur under euxinic marine conditions, that is, where dissolved sulfide is present in the water column. These enrichments result primarily from the export of remobilized iron from the oxic shelf, which is scavenged from the euxinic water column during syngenetic pyrite formation and deposited in the underlying sediments. Strongly elevated ratios of highly reactive iron to total iron (FeHR/FeT) and total iron to aluminum (FeT/Al) and high degrees of pyritization (DOP) are each products of this enrichment process. These paleoredox proxies are among the most faithful recorders of ancient euxinia. Contrary to previous arguments, iron enrichment is decoupled from biogenic sediment inputs, but it does appear to be a uniquely euxinic phenomenon. In other words, we can rule out a major contribution from preferential physical transport of FeHR-rich detrital sediment to the deep basin, which could also operate under oxic conditions. Furthermore, enrichment via the shuttling of iron remobilized from oxic shelves appears to be limited by inefficient transport and trapping processes in deep oxic basins. Elevated FeT/Al ratios in the euxinic sediments also cannot be a product of internal enhancement of the reactivity of the detrital iron pool without net FeHR addition. These conclusions are supported by observations in the modern Black Sea, Orca Basin, and Effingham Inlet. FeT/Al ratios are unambiguous recorders of paleoredox even in sediments that have experienced high degrees of metamorphic alteration. However, this study suggests that high siliciclastic accumulation rates can swamp the enrichment mechanism, resulting in only intermediate DOP values for euxinic sediments and FeT/Al ratios that mimic the oxic shelf. Such dilution effects are well expressed in Black Sea basinal turbidites and rapidly accumulating muds on euxinic basin margins. Under conditions of persistent euxinia, varying extents of FeHR enrichment can illuminate spatial and temporal gradients in siliciclastic sedimentation. The magnitude of enrichment is a function of the source (shelf) to sink (ocean basin) areal ratio, suggesting that iron proxies can also record ocean-scale paleoenvironmental properties through muted enrichments at times of very widespread euxinia. For the first time, manganese data are interpreted in light of the redox shuttle model. As for the iron data, the Black Sea, Orca Basin, and Effingham Inlet show enrichments in total manganese in the deep euxinic basin, suggesting export from the suboxic porewaters of the oxic shelf and scavenging and burial in the basin. The Black Sea data reveal iron and manganese enrichment across the broad, deep euxinic basin, suggesting efficient lateral transport and deep-water mixing tied to the physical properties of the water column. © 2006 Elsevier Inc. All rights reserved.