HIGHLY REACTIVE FE LIMITATION DURING OCEANIC ANOXIC EVENT 2

Iron is a key element in understanding marine paleo-redox. The ratios of highly reactive iron to total iron (Fe_HR/Fe_T) as well as total iron to aluminum (Fe_T/Al) have been used to distinguish deposition beneath oxic versus anoxic and sulfidic (euxinic) water columns. The degree of Fe_HR enrichment in a euxinic basin is strongly correlated to the shelf source to basin sink ratio. It has been postulated that during the Cenomanian-Turonian Oceanic Anoxic Event (OAE2), when euxinic conditions likely persisted for >10⁵ years on a regional to global scale, enrichment of Fe_HR in sediments may have been limited due to a large basinal sink. Here, we present Fe speciation and S-isotope results (as pyrite) from the Western Interior Seaway (WIS) and ODP Site 1258 at Demerara Rise. Fe_T/Al values in the WIS range from ~0.25 before OAE2 to as high as 1.6 during OAE2. Fe_HR/Fe_T values are generally high prior to OAE2, decrease to 0.37 at the onset of the anoxic event and rise to as high as 0.92 during and after OAE2. The proportion of Fe_T/Al at Demerara Rise does not vary significantly from average shale values (0.5) either before or after OAE2 and only rises to as high as 0.88 during the event. Fe_HR/Fe_T values are notably low prior to OAE2 (~0.3) and only rise to as high as 0.55 during and after OAE2. These data support a hypothesis of Fe_HR limitation during OAE2 due to global Fe sequestration (pyrite burial) under euxinic conditions.