Iron and sulfur cycling in the cGENIE.muffin Earth system model (v0.9.21)

Abstract

Abstract. The coupled biogeochemical cycles of iron and sulfur are central to the long-term biogeochemical evolution of Earth's oceans. For instance, before the development of a persistently oxygenated deep ocean, the ocean interior likely alternated between states buffered by reduced sulfur (“euxinic”) and buffered by reduced iron (“ferruginous”), with important implications for the cycles and hence bioavailability of dissolved iron (and phosphate). Even after atmospheric oxygen concentrations rose to modern-like values, the ocean episodically continued to develop regions of euxinic or ferruginous conditions, such as those associated with past key intervals of organic carbon deposition (e.g. during the Cretaceous) and extinction events (e.g. at the Permian–Triassic boundary). A better understanding of the cycling of iron and sulfur in an anoxic ocean, how geochemical patterns in the ocean relate to the available spatially heterogeneous geological observations, and quantification of the feedback strengths between nutrient cycling, biological productivity, and ocean redox requires a spatially resolved representation of ocean circulation together with an extended set of (bio)geochemical reactions. Here, we extend the “muffin” release of the intermediate-complexity Earth system model cGENIE to now include an anoxic iron and sulfur cycle (expanding the existing oxic iron and sulfur cycles), enabling the model to simulate ferruginous and euxinic redox states as well as the precipitation of reduced iron and sulfur minerals (pyrite, siderite, greenalite) and attendant iron and sulfur isotope signatures, which we describe in full. Because tests against present-day (oxic) ocean iron cycling exercises only a small part of the new code, we use an idealized ocean configuration to explore model sensitivity across a selection of key parameters. We also present the spatial patterns of concentrations and δ56Fe and δ34S isotope signatures of both dissolved and solid-phase Fe and S species in an anoxic ocean as an example application. Our sensitivity analyses show that the first-order results of the model are relatively robust against the choice of kinetic parameter values within the Fe–S system and that simulated concentrations and reaction rates are comparable to those observed in process analogues for ancient oceans (i.e. anoxic lakes). Future model developments will address sedimentary recycling and benthic iron fluxes back to the water column, together with the coupling of nutrient (in particular phosphate) cycling to the iron cycle.

Inclusion of a suite of weathering tracers in the cGENIE Earth system model – muffin release v.0.9.23

“…Similarly, the observed fractionation factor for stable Sr isotopes during biogenic CaCO 3 formation is too large to equilibrate the model at the observed low δ 88/86 Sr if we do not assume that all Sr sources only provide mantle-like light Sr (δ 88/86 Sr = 0.256 ‰, TUNED simulation). Corroborating the conclusion of, e.g., Vance et al (2009), this suggests that the isotopic composition of marine dissolved Sr is currently not at equilibrium. The simulations capture the homogeneous vertical profiles of Sr concentrations observed at most sites (see Figs.…”

“…Assuming rivers to be the only supply of continental Sr to the oceans and that the marine Sr reservoir size is in equilibrium, Hodell et al (1989) estimate the marine residence time of Sr to be 1.9-3.45 Myr. Considering the potentially appreciable Sr influx from groundwater, the actual residence time might be at or below the lower end of this estimate, and it may be even more different if the marine Sr reservoir is not currently in equilibrium with Sr inputs, as suggested by, e.g., Vance et al (2009). Strontium has four stable isotopes (0.56 % 84 Sr, 9.87 % 86 Sr, 7.04 % 87 Sr, 82.53 % 88 Sr) (Veizer, 1989), 87 Sr being the product of β decay of 87 Rb (half-life 4.88 × 10 10 years, Faure and Mensing, 2005).…”

Bioturbation and the δ56Fe signature of dissolved iron fluxes from marine sediments

“…This choice mainly affects the estimated global flux, and does not greatly alter our conclusions on the relative impact of bioturbation on sedimentary Fe release and isotope dynamics (as these are independent of the FeOOH influx; see below). This issue could be addressed in the future by coupling the proposed benthic Fe flux equations to a pelagic Fe model (such as cGEnIE.muffin, PISCES or UVic; [ 32 , 70 , 71 ]), which would greatly improve global benthic Fe flux predictions.…”

Oxygenation of the Earth aided by mineral–organic carbon preservation

“…Our model is developed from a series of well-established conceptual models of global marine biogeochemistry [21][22][23][24][25][26] . The model tracks the global cycles of organic and inorganic carbon, sulfate and sulfide anions, Fe, O 2 and P in a five-box ocean system representing the shallow shelf, deep shelf, surface open ocean, surface high latitude and deep ocean and has explicit air-sea exchange with a single well-mixed atmosphere box (Fig.…”

“…Dissolved Fe is free to redistribute around the ocean via circulation and mixing between ocean boxes. The reaction pathways for the Fe cycle in the ocean boxes are taken to be Fe reduction, pyrite formation, siderite precipitation, aerobic Fe oxidation and Fe(iii) scavenging by particles (the formation of nanoparticulate Fe (oxyhydr) oxides, which aggrade with other particles and sink to the sediments), following recent works by Wallmann et al 24 and van de Velde et al 26 . Another flux of Fe (oxyhydr)oxides to the sediments is the weathering and transport of particulate Fe directly from land (through both river and dust; Fig.…”