Gwenyth Gordon

Iron isotope and trace metal records of iron cycling in the proto‐North Atlantic during the Cenomanian‐Turonian oceanic anoxic event (OAE‐2)

[1] The global carbon cycle during the mid-Cretaceous (� 125–88 million years ago, Ma) experienced numerous major perturbations linked to increased organic carbon burial under widespread, possibly basin-scale oxygen deficiency and episodes of euxinia (anoxic and H2S-containing). The largest of these episodes, the Cenomanian-Turonian boundary event (ca. 93.5 Ma), or oceanic anoxic event (OAE) 2, was marked by pervasive deposition of organic-rich, laminated black shales in deep waters and in some cases across continental shelves. This deposition is recorded in a pronounced positive carbon isotope excursion seen ubiquitously in carbonates and organic matter. Enrichments of redox-sensitive, often bioessential trace metals, including Fe and Mo, indicate major shifts in their biogeochemical cycles under reducing conditions that may be linked to changes in primary production. Iron enrichments and bulk Fe isotope compositions track the sources and sinks of Fe in the proto-North Atlantic at seven localities marked by diverse depositional conditions. Included are an ancestral mid-ocean ridge and euxinic, intermittently euxinic, and oxic settings across varying paleodepths throughout the basin. These data yield evidence for a reactive Fe shuttle that likely delivered Fe from the shallow shelf to the deep ocean basin, as well as (1) hydrothermal sources enhanced by accelerated seafloor spreading or emplacement of large igneous province(s) and (2) local-scale Fe remobilization within the sediment column. This study, the first to explore Fe cycling and enrichment patterns on an ocean scale using iron isotope data, demonstrates the complex processes operating on this scale that can mask simple source-sink relationships. The data imply that the proto-North Atlantic received elevated Fe inputs from several sources (e.g., hydrothermal, shuttle and detrital inputs) and that the redox state of the basin was not exclusively euxinic, suggesting previously unknown heterogeneity in depositional conditions and biogeochemical cycling within those settings during OAE-2.

Extreme change in sulfide concentrations in the Black Sea during the Little Ice Age reconstructed using molybdenum isotopes

The Black Sea is the largest and most studied anoxic basin in the modern world. Much of this research has focused on the redox structure of the water column, specifically on the driving forces behind variations in the position, stability, and structure of the oxic-anoxic interface (chemocline). However, none of these studies has been able to quantify the historical sulfide concentrations associated with the changes in chemocline depth. Using the isotopic composition of molybdenum in sediments as a proxy, we show for the first time that varying concentrations of dissolved sulfide can be fingerprinted in historical systems. Our molybdenum isotope data indicate that in the region of the Bosporus inlet, the chemocline rose more than 65 m, reaching concentrations over 100 μM sulfide in the bottom water ca. 300 yr B.P. This historical shoaling of the chemocline and extreme change in bottom-water sulfide concentration exceeds the modern changes that have been observed directly and attributed to anthropogenic influences on the Black Sea chemistry/hydrology. The first cold interval of the Little Ice Age, when temperature and circulation changes occurred in the Black Sea basins, may have provided the natural trigger for this extreme rise in bottom-water sulfide concentrations.