OMEN-SED 1.0:A novel, numerically efficient organic matter sediment diagenesis module for coupling to Earth system models

We present the first version of OMEN-SED (Organic Matter ENabled SEDiment model), a new, onedimensional analytical early diagenetic model resolving organic matter cycling and the associated biogeochemical dynamics in marine sediments designed to be coupled to Earth system models. OMEN-SED explicitly describes organic matter (OM) cycling and the associated dynamics of the most important terminal electron acceptors (i.e. O2, NO3, SO4) and methane (CH4), related reduced substances (NH4, H2S), macronutrients (PO4) and associated pore water quantities (ALK, DIC). Its reaction network accounts for the most important primary and secondary redox reactions, equilibrium reactions, mineral dissolution and precipitation, as well as adsorption and desorption processes associated with OM dynamics that affect the dissolved and solid species explicitly resolved in the model. To represent a redox-dependent sedimentary P cycle we also include a representation of the formation and burial of Fe-bound P and authigenic Ca-P minerals. Thus, OMEN-SED is able to capture the main features of diagenetic dynamics in marine sediments and therefore offers similar predictive abilities as a complex, numerical diagenetic model. Yet, its computational efficiency allows for its coupling to global Earth system models and therefore the investigation of coupled global biogeochemical dynamics over a wide range of climate-relevant timescales. This paper provides a detailed description of the new sediment model, an extensive sensitivity analysis and an evaluation of OMEN-SED's performance through comprehensive comparisons with observations and results from a more complex numerical model. We find that solid-phase and dissolved pore water profiles for different ocean depths are reproduced with good accuracy and simulated terminal electron acceptor fluxes fall well within the range of globally observed fluxes. Finally, we illustrate its application in an Earth system model framework by coupling OMEN-SED to the Earth system model cGENIE and tune the OM degradation rate constants to optimise the fit of simulated benthic OM contents to global observations. We find that the simulated sediment characteristics of the coupled model framework, such as OM degradation rates, oxygen penetration depths and sediment-water interface fluxes, are generally in good agreement with observations and in line with what one would expect on a global scale. Coupled to an Earth system model, OMENSED is thus a powerful tool that will not only help elucidate the role of benthic-pelagic exchange processes in the evolution and the termination of a wide range of climate events, but will also allow for a direct comparison of model output with the sedimentary record - the most important climate archive on Earth.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Assessing the impact of bioturbation on sedimentary isotopic records through numerical models

The disturbance of seafloor sediments by the activities of bottom-dwelling organisms, known as bioturbation, significantly alters the marine paleorecord by redistributing particles in the upper sediment layers. Consequently, ‘proxy’ signals recorded in these sediment particles, such as the size, abundance, or isotopic composition of plankton shells, are distorted by particle mixing. Accordingly, bioturbation can alter the apparent timing, duration, and magnitude of recorded events by smoothing climatic and oceanographic signals. In an extreme scenario, biological mixing can significantly obscure our view of the past by homogenizing the bioturbated layer, destroying sediment layering, and distorting the relative timing and intensity of past climatological events. Here we explore how bioturbation distorts proxy records of environmental events from a modeling perspective. First, we provide an overview and comparison of different numerical models created for simulating the movement and structural alteration of sediment by bioturbation. Next, we use an updated particle resolving model – iTURBO2 – to illustrate how various modes and intensities of bioturbation distort the signature of past climatological events, considering a range of conceptual shapes of vertical proxy profiles. Finally, we demonstrate how sampled proxy records can differ due to the combined effects of particle mixing and differential abundance changes that often concur with environmental transitions. We make the iTURBO2 MATLAB code openly available to facilitate further exploration of proxy biases due to bioturbation to aid the interpretation of the climatological record preserved in marine sediments

Regional and Global Patterns of Apparent Organic Matter Reactivity in Marine Sediments

Organic matter (OM) degradation in marine sediments is fundamental to understanding and constraining global biogeochemical cycling, whereby OM reactivity is at its core. Here, we use benthic diffusive oxygen uptake (DOU) rates as a proxy for OM reactivity. We apply an analytical diagenetic model to inversely determine OM reactivities in marine sediments (i.e., Reactive Continuum Model parameters a and ν) using data sets of global DOU, surface sediment OM contents, and seafloor boundary conditions. Simulated oxygen depth profiles show good agreement with observations, increasing confidence in our reactivity estimates. Inversely determined reactivities vary over orders of magnitude between individual sites ranging from high (k = 0.252 year−1) for sediments in the Polar region to extremely low (k = 7.96 ⋅ 10−5 year−1) in South Pacific. Our findings highlight the heterogeneity of OM reactivities, revealing regional patterns that broadly agree with observations and prior assessments. In general, high benthic reactivity can be linked to limited pelagic OM degradation favored by either a rapid vertical or lateral OM transport to the sediment or environmental factors, such as low oxygen concentrations or low temperature, slowing pelagic OM degradation. Finally, we develop a set of transfer functions that allow estimating OM reactivity as a function of DOU, OM content and water depths, and use one to derive the first global maps of benthic OM reactivity based on two global DOU maps. Despite the inherent observational biases in the data sets, our results provide a good first-order estimate of the apparent benthic OM reactivity agreeing with our current mechanistic understanding and observations.ISSN:0886-6236ISSN:1944-922

Transfer efficiency of organic carbon in marine sediments

Quantifying the organic carbon (OC) sink in marine sediments is crucial for assessing how the marine carbon cycle regulates Earth’s climate. However, burial efficiency (BE) – the commonly-used metric reporting the percentage of OC deposited on the seafloor that becomes buried (beyond an arbitrary and often unspecified reference depth) – is loosely defined, misleading, and inconsistent. Here, we use a global diagenetic model to highlight orders-of-magnitude differences in sediment ages at fixed sub-seafloor depths (and vice-versa), and vastly different BE’s depending on sediment depth or age horizons used to calculate BE. We propose using transfer efficiencies (Teff’s) for quantifying sediment OC burial: Teff is numerically equivalent to BE but requires precise specification of spatial or temporal references, and emphasizes that OC degradation continues beyond these horizons. Ultimately, quantifying OC burial with precise sediment-depth and sediment-age-resolved metrics will enable a more consistent and transferable assessment of OC fluxes through the Earth system.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

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

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.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Understanding the causes and consequences of past marine carbon cycling variability through models

On geological time-scales, the production and degree of recycling of biogenic carbon in the marine realm and ultimately its removal to sediments, exerts a dominant control on atmospheric CO2 and hence variability in climate. This is a highly complex system involving a myriad of inter-connected biological, chemical, and physical processes. For this reason alone, linking observations, often highly abstracted in the form of proxies, to the primary processes involved and ultimately to explanatory hypotheses for specific geological events and transitions, is challenging. The past few decades have seen a progressive improvement in theoretical and process-based understanding of the various components that make up the marine carbon cycle and, hand-in-hand with this, the development of numerical model representations of the complete system. Models have also been designed and/or adapted with paleoclimate questions in mind and applied to quantitatively explore the role of the marine carbon cycle in both perturbations and long-term geologic evolutionary trends in global climate, and possible feedbacks between them. However, we must ask whether paleoclimate models incorporate sufficiently appropriate representations of the dynamics and sensitivities of the marine carbon cycle, and indeed, whether in the geological context, we really know what these dynamics are. Here we provide a comprehensive overview of how marine carbon cycling and the biological carbon pump is treated in available paleoclimate models, with the aim of critically evaluating their ability to help interpret past marine carbon cycle and climate dynamics. To this end, we first provide an overview of commonly used paleoclimate models and some of their associated paleo-applications, drawing from a wide range of global carbon cycle box models and Earth system Models of Intermediate Complexity (EMICs). Secondly, we review and evaluate the three dominant processes involved in the cycling of organic and inorganic carbon in the marine system and how they are represented in models, namely: biological productivity at the ocean surface, remineralisation/dissolution of particulate carbon within the water column, and the benthic-pelagic coupling at the seafloor. We generate and employ illustrative examples using the model GENIE to show how different parameterisations of water-column and sediment processes can lead to significantly different model projections. Our compilation reveals the prevalence of static parametrisations of marine carbon cycling among existing paleoclimate models, which are commonly empirically derived from present-day observations. Although such approaches tend to represent carbon transfer in the modern ocean well, they are potentially compromised in their ability to reflect the true degree of freedom and strength of feedbacks with respect to past climate events, particularly those characterised by environmental boundary conditions that differ fundamentally from today. Finally, we discuss the importance of using models of different complexities and how questions of model uncertainty may start to be addressed

End-Permian marine extinction due to temperature-driven nutrient recycling and euxinia

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