Application of sediment core modelling to understanding climates of the past: An example from glacial-interglacial changes in Southern Ocean silica cycling

International audiencePaleoceanographic evidence from the Southern Ocean reveals an apparent stark meridional divide in biogeochemical dynamics associated with the glacial-interglacial cycles of the late Neogene. South of the present-day position of the Antarctic Polar Front biogenic opal is generally much more abundant in sediments during interglacials compared to glacials. To the north, an anti-phased relationship is observed, with maximum opal abundance instead occurring during glacials. This antagonistic response of sedimentary properties is an important model validation target for testing hypotheses of glacial-interglacial change, particularly with respect to understanding the causes of the variability in atmospheric CO2. Here, I illustrate a time-dependent modelling approach to helping understand past climatic change by means of the generation of synthetic sediment core records. I find a close match between model-predicted and observed down-core changes in sedimentary opal content is achieved when changes in seasonal sea-ice extent is imposed, suggesting that the cryosphere is probably the primary driver of the striking features exhibited by the paleoceanographic record of this region

Controls on the spatial distribution of oceanic <i>δ</i>¹³C_DIC

We describe the design and evaluation of a large ensemble of coupled climate–carbon cycle simulations with the Earth system model of intermediate complexity GENIE. This ensemble has been designed for application to a range of carbon cycle questions, including the causes of late- Quaternary fluctuations in atmospheric CO2. Here we evaluate the ensemble by applying it to a transient experiment over the recent industrial era (1858 to 2008 AD). We employ singular vector decomposition and principal component emulation to investigate the spatial modes of ensemble variability of oceanic dissolved inorganic carbon (DIC) δ13C, considering both the spun-up pre-industrial state and the transient change. These analyses allow us to separate the natural (preindustrial) and anthropogenic controls on the δ13CDIC distribution. We apply the same dimensionally reduced emulation techniques to consider the drivers of the spatial uncertainty in anthropogenic DIC. We show that the sources of uncertainty related to the uptake of anthropogenic δ13CDIC and DIC are quite distinct. Uncertainty in anthropogenic δ13C uptake is controlled by air–sea gas exchange, which explains 63% of modelled variance. This mode of variability is largely absent from the ensemble variability in CO2 uptake, which is rather driven by uncertainties in thermocline ventilation rates. Although the need to account for air–sea gas exchange is well known, these results suggest that, to leading order, uncertainties in the ocean uptake of anthropogenic 13C and CO2 are governed by very different processes. This illustrates the difficulties in reconstructing one from the other, and furthermore highlights the need for careful targeting of both δ13CDIC and DIC observations to better constrain the ocean sink of anthropogenic CO2

Calibration of key temperature-dependent ocean microbial processes in the cGENIE.muffin Earth system model

Temperature is a master parameter in the marine carbon cycle, exerting a critical control on the rate of biological transformation of a variety of solid and dissolved reactants and substrates. Although in the construction of numerical models of marine carbon cycling, temperature has been long-recognised as a key parameter in the production and export of organic matter at the ocean surface, it is much less commonly taken into account in the ocean interior. There, bacteria (primarily) transform sinking particulate organic matter into its dissolved constituents and thereby consume dissolved oxygen (and/or other electron acceptors such as sulphate) and release nutrients, which are then available for transport back to the surface. Here we present and calibrate a more complete temperature-dependent representation of marine carbon cycling in the cGENIE.muffin Earth system model, intended for both past and future climate applications. In this, we combine a temperature-dependent remineralisation scheme for sinking organic matter with a biological export production scheme that also includes a temperature-dependent limitation on nutrient uptake in surface waters (and hence phytoplankton growth). Via a parameter ensemble, we jointly calibrate the two parameterisations by statistically contrasting model projected fields of nutrients, oxygen, and the stable carbon isotopic signature (δ13C) of dissolved inorganic carbon in the ocean, with modern observations. We find that for the present-day, the temperature-dependent version shows as-good-as or better fit to data than the existing tuned non-temperature dependent version of the cGENIE.muffin. The main impact of adding temperature-dependent remineralisation is in driving higher rates of remineralisation in warmer waters and hence a more rapid return of nutrients to the surface there – stimulating organic matter production. As a result, more organic matter is exported below 80 m in mid and low latitude warmer waters as compared to the standard model. Conversely, at higher latitudes, colder water temperature reduces the rate of nutrient supply to the surface as a result of slower in-situ rates of remineralisation. We also assess the implications of including a more complete set of temperature-dependent parameterisations by analysing a series of historical transient experiments. We find that between the pre-industrial and the present day, in response to a simulated air temperature increase of 0.9 °C and ocean warming of 0.12 °C (0.6 °C in surface waters and 0.02 °C in deep waters), a reduction in POC export at 80 m of just 0.3 % occurs. In contrast, with no assumed temperature-dependent biological processes, global POC export at 80 m falls by 2.9 % between the pre-industrial and present day as a consequence of ocean stratification and reduced nutrient supply to the surface. This suggests that increased nutrient recycling in warmer conditions offsets some of the stratification-induced surface nutrient limitation in a warmer world, and that less carbon (and nutrients) then reaches the inner and deep ocean. This extension to the cGENIE.muffin Earth system model provides it with additional capabilities in addressing marine carbon cycling in warmer past and future worlds

Ocean warming, not acidification, controlled coccolithophore response during past greenhouse climate change

Current carbon dioxide emissions are an assumed threat to oceanic calcifying plankton (coccolithophores) not just due to rising sea-surface temperatures, but also because of ocean acidification (OA). This assessment is based on single species culture experiments that are now revealing complex, synergistic, and adaptive responses to such environmental change. Despite this complexity, there is still a widespread perception that coccolithophore calcification will be inhibited by OA. These plankton have an excellent fossil record, and so we can test for the impact of OA during geological carbon cycle events, providing the added advantages of exploring entire communities across real-world major climate perturbation and recovery. Here we target fossil coccolithophore groups (holococcoliths and braarudosphaerids) expected to exhibit greatest sensitivity to acidification because of their reliance on extracellular calcification. Across the Paleocene-Eocene Thermal Maximum (56 Ma) rapid warming event, the biogeography and abundance of these extracellular calcifiers shifted dramatically, disappearing entirely from low latitudes to become limited to cooler, lower saturation-state areas. By comparing these range shift data with the environmental parameters from an Earth system model, we show that the principal control on these range retractions was temperature, with survival maintained in high-latitude refugia, despite more adverse ocean chemistry conditions. Deleterious effects of OA were only evidenced when twinned with elevated temperatures

Diversity decoupled from ecosystem function and resilience during mass extinction recovery

The Chicxulub bolide impact 66 million years ago drove the near-instantaneous collapse of ocean ecosystems. The devastating loss of diversity at the base of ocean food webs probably triggered cascading extinctions across all trophic levels and caused severe disruption of the biogeochemical functions of the ocean, and especially disrupted the cycling of carbon between the surface and deep sea. The absence of sufficiently detailed biotic data that span the post-extinction interval has limited our understanding of how ecosystem resilience and biochemical function was restored; estimates of ecosystem ‘recovery’ vary from less than 100 years to 10 million years. Here, using a 13-million-year-long nannoplankton time series, we show that post-extinction communities exhibited 1.8 million years of exceptional volatility before a more stable equilibrium-state community emerged that displayed hallmarks of resilience. The transition to this new equilibrium-state community with a broader spectrum of cell sizes coincides with indicators of carbon-cycle restoration and a fully functioning biological pump. These findings suggest a fundamental link between ecosystem recovery and biogeochemical cycling over timescales that are longer than those suggested by proxies of export production, but far shorter than the return of taxonomic richness. The fact that species richness remained low as both community stability and biological pump efficiency re-emerged suggests that ecological functions rather than the number of species are more important to community resilience and biochemical functions

Data-constrained assessment of ocean circulation changes since the middle Miocene in an Earth system model

Since the middle Miocene (15 Ma, million years ago), the Earth's climate has undergone a long-term cooling trend, characterised by a reduction in ocean temperatures of up to 7–8 ∘C. The causes of this cooling are primarily thought to be due to tectonic plate movements driving changes in large-scale ocean circulation patterns, and hence heat redistribution, in conjunction with a drop in atmospheric greenhouse gas forcing (and attendant ice-sheet growth and feedback). In this study, we assess the potential to constrain the evolving patterns of global ocean circulation and cooling over the last 15 Ma by assimilating a variety of marine sediment proxy data in an Earth system model. We do this by first compiling surface and benthic ocean temperature and benthic carbon-13 (δ13C) data in a series of seven time slices spaced at approximately 2.5 Myr intervals. We then pair this with a corresponding series of tectonic and climate boundary condition reconstructions in the cGENIE (“muffin” release) Earth system model, including alternative possibilities for an open vs. closed Central American Seaway (CAS) from 10 Ma onwards. In the cGENIE model, we explore uncertainty in greenhouse gas forcing and the magnitude of North Pacific to North Atlantic salinity flux adjustment required in the model to create an Atlantic Meridional Overturning Circulation (AMOC) of a specific strength, via a series of 12 (one for each tectonic reconstruction) 2D parameter ensembles. Each ensemble member is then tested against the observed global temperature and benthic δ13C patterns. We identify that a relatively high CO2 equivalent forcing of 1120 ppm is required at 15 Ma in cGENIE to reproduce proxy temperature estimates in the model, noting that this CO2 forcing is dependent on the cGENIE model's climate sensitivity and that it incorporates the effects of all greenhouse gases. We find that reproducing the observed long-term cooling trend requires a progressively declining greenhouse gas forcing in the model. In parallel to this, the strength of the AMOC increases with time despite a reduction in the salinity of the surface North Atlantic over the cooling period, attributable to falling intensity of the hydrological cycle and to lowering polar temperatures, both caused by CO2-driven global cooling. We also find that a closed CAS from 10 Ma to present shows better agreement between benthic δ13C patterns and our particular series of model configurations and data. A final outcome of our analysis is a pronounced ca. 1.5 ‰ decline occurring in atmospheric (and ca. 1 ‰ ocean surface) δ13C that could be used to inform future δ13C-based proxy reconstructions.</p

The influence of the biological pump on ocean chemistry:Implications for long-term trends in marine redox chemistry, the global carbon cycle, and marine animal ecosystems

The net export of organic matter from the surface ocean and its respiration at depth create vertical gradients in nutrient and oxygen availability that play a primary role in structuring marine ecosystems. Changes in the properties of this ‘biological pump’ have been hypothesized to account for important shifts in marine ecosystem structure, including the Cambrian explosion. However, the influence of variation in the behavior of the biological pump on ocean biogeochemistry remains poorly quantified, preventing any detailed exploration of how changes in the biological pump over geological time may have shaped long‐term shifts in ocean chemistry, biogeochemical cycling, and ecosystem structure. Here, we use a 3‐dimensional Earth system model of intermediate complexity to quantitatively explore the effects of the biological pump on marine chemistry. We find that when respiration of sinking organic matter is efficient, due to slower sinking or higher respiration rates, anoxia tends to be more prevalent and to occur in shallower waters. Consequently, the Phanerozoic trend toward less bottom‐water anoxia in continental shelf settings can potentially be explained by a change in the spatial dynamics of nutrient cycling rather than by any change in the ocean phosphate inventory. The model results further suggest that the Phanerozoic decline in the prevalence ocean anoxia is, in part, a consequence of the evolution of larger phytoplankton, many of which produce mineralized tests. We hypothesize that the Phanerozoic trend toward greater animal abundance and metabolic demand was driven more by increased oxygen concentrations in shelf environments than by greater food (nutrient) availability. In fact, a lower‐than‐modern ocean phosphate inventory in our closed system model is unable to account for the Paleozoic prevalence of bottom‐water anoxia. Overall, these model simulations suggest that the changing spatial distribution of photosynthesis and respiration in the oceans has exerted a first‐order control on Earth system evolution across Phanerozoic time

Mitigation of Extreme Ocean Anoxic Event Conditions by Organic Matter Sulfurization

Past occurrences of widespread and severe anoxia in the ocean have frequently been associated with abundant geological evidence for free hydrogen sulfide (H 2 S) in the water column, so-called euxinic conditions. Free H 2 S may react with, and modify, the chemical structure of organic matter settling through the water column and in marine sediments, with hypothesized implications for carbon sequestration. Here, taking the example of Ocean Anoxic Event 2, we explore the potential impact of organic matter sulfurization on marine carbon and oxygen cycling by means of Earth system modeling. Our model experiments demonstrate that rapid sulfurization (k sulf ≥ = 10 5  M −1  year −1 ) of organic matter in the water column can drive a more than 30% enhancement of organic carbon preservation and burial in marine sediments and hence help accelerate climate cooling and Ocean Anoxic Event 2 recovery. As a consequence of organic matter sulfurization, we also find that H 2 S can be rapidly scavenged and the euxinic ocean volume reduced by up to 80%—helping reoxygenate the ocean as well as reducing toxic H 2 S emissions to the atmosphere, with potential implications for the kill mechanism at the end-Permian. Finally, we find that the addition of organic matter sulfurization induces a series of additional feedbacks, including further atmospheric CO 2 drawdown and ocean reoxygenation by the creation of a previously unrecognized net source of alkalinity to the ocean as H 2 S is scavenged and buried.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

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, one-dimensional 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, OMEN-SED 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.</p