Pelagic clays as archives of marine iron isotope chemistry

Abstract

Slowly accumulating pelagic clays are enriched in metals that were formerly in seawater, including iron, an important micronutrient. Because the metals are minimally remobilized in oxygenated porewater, pelagic clays may be a potential archive for records of past marine micronutrient cycling. Here, we present a record of changes in hydrogenous iron (Fe) isotopes since the late Cretaceous derived from pelagic clays that we dated with osmium isotope chronostratigraphy. To optimize the separation of the hydrogenous metal (oxy)hydroxides from bulk sediment, we repeatedly leached an oxic pelagic clay sample under variable conditions (HCl molarity, temperature, time) and measured the element concentrations, Fe isotopes, and Os isotopes. The common behavior of elements amidst the permutations of the leach experiment offers insight into which components were dissolved and we defined a range of successful leaches. We applied our optimal leach for Fe and Os isotopes (1 M HCl, for 24 h at 20 °C) to 45 samples at Site U1366 in the South Pacific Gyre. The resulting record suggests a dynamic Fe cycle in the water column overlying Site U1366 over the past 95 million years. Early in the site's history, trends in the Fe isotopes are interpreted as reflecting changes in hydrothermal Fe with distance from the ridge. Contributions from a background Fe source are identified as well as a transition to dust-like source after 50 Ma until present. Constructing similar records at multiple sites will provide a basin-wide perspective on how the marine Fe cycle has changed over million-year timescales.