Sandra Arndt

Potential methane reservoirs beneath Antarctica

Once thought to be devoid of life, the ice-covered parts of Antarctica are now known to be a reservoir of metabolically active microbial cells and organic carbon. The potential for methanogenic archaea to support the degradation of organic carbon to methane beneath the ice, however, has not yet been evaluated. Large sedimentary basins containing marine sequences up to 14 kilometres thick and an estimated 21,000 petagrams (1 Pg equals 1015 g) of organic carbon are buried beneath the Antarctic Ice Sheet. No data exist for rates of methanogenesis in sub-Antarctic marine sediments. Here we present experimental data from other subglacial environments that demonstrate the potential for overridden organic matter beneath glacial systems to produce methane. We also numerically simulate the accumulation of methane in Antarctic sedimentary basins using an established one-dimensional hydrate model and show that pressure/temperature conditions favour methane hydrate formation down to sediment depths of about 300 metres in West Antarctica and 700 metres in East Antarctica. Our results demonstrate the potential for methane hydrate accumulation in Antarctic sedimentary basins, where the total inventory depends on rates of organic carbon degradation and conditions at the ice-sheet bed. We calculate that the sub-Antarctic hydrate inventory could be of the same order of magnitude as that of recent estimates made for Arctic permafrost. Our findings suggest that the Antarctic Ice Sheet may be a neglected but important component of the global methane budget, with the potential to act as a positive feedback on climate warming during ice-sheet wastage.

Diatom growth response to physical forcing in a macrotidal estuary: Coupling hydrodynamics, sediment transport, and biogeochemistry

[1] A two-dimensional, nested grid, hydrodynamic, and reactive-transport model of the macrotidal Scheldt estuary (B/NL) and its tributaries has been developed to identify the driving forces controlling the temporal and spatial dynamics of primary production during a summer diatom bloom. The hydrodynamic model indicates that energy dissipation reaches its maximum 90 km upstream from the mouth, closely followed by a minimum farther upstream. Suspended particulate matter (SPM) dynamics is simulated to provide the transient light conditions in the water column. Results show that the spatial distribution of SPM mirrors closely the profile of energy dissipation. The temporal SPM dynamics is highly sensitive to fluctuations in river discharge, whose influence decreases downstream. Peaks in SPM are triggered by high discharges and can be recorded as far as 50 km seaward of the upstream model boundary. Results from the phytoplankton model demonstrate the fast response of diatom growth to changes in the physical environment, especially those due to daily variations in river discharge which continuously modify the SPM concentrations and residence times. Episodes of persistent low flow conditions lead to a progressive depletion of dissolved silica. Simulated diatom growth becomes increasingly controlled by silica availability, until primary production collapses. The spatiotemporal evolution of primary production is explored over the entire domain of forcing conditions. The distribution of the daily maximum of net primary production and its location reveal that four different system states can be identified in the forcing planes. The transition from one state to the other characterizes the diatom growth response in the estuary.

Surviving rapid climate change in the deep sea during the Paleogene hyperthermals

Predicting the impact of ongoing anthropogenic CO2 emissions on calcifying marine organisms is complex, owing to the synergy between direct changes (acidification) and indirect changes through climate change (e.g., warming, changes in ocean circulation, and deoxygenation). Laboratory experiments, particularly on longer-lived organisms, tend to be too short to reveal the potential of organisms to acclimatize, adapt, or evolve and usually do not incorporate multiple stressors. We studied two examples of rapid carbon release in the geological record, Eocene Thermal Maximum 2 (∼53.2 Ma) and the Paleocene Eocene Thermal Maximum (PETM, ∼55.5 Ma), the best analogs over the last 65 Ma for future ocean acidification related to high atmospheric CO2 levels. We use benthic foraminifers, which suffered severe extinction during the PETM, as a model group. Using synchrotron radiation X-ray tomographic microscopy, we reconstruct the calcification response of survivor species and find, contrary to expectations, that calcification significantly increased during the PETM. In contrast, there was no significant response to the smaller Eocene Thermal Maximum 2, which was associated with a minor change in diversity only. These observations suggest that there is a response threshold for extinction and calcification response, while highlighting the utility of the geological record in helping constrain the sensitivity of biotic response to environmental change.

Modelling Estuarine Biogeochemical Dynamics: From the Local to the Global Scale

Abstract Estuaries act as strong carbon and nutrient filters and are relevant contributors to the atmospheric CO2 budget. They thus play an important, yet poorly constrained, role for global biogeochemical cycles and climate. This manuscript reviews recent developments in the modelling of estuarine biogeochemical dynamics. The first part provides an overview of the dominant physical and biogeochemical processes that control the transformations and fluxes of carbon and nutrients along the estuarine gradient. It highlights the tight links between estuarine geometry, hydrodynamics and scalar transport, as well as the role of transient and nonlinear dynamics. The most important biogeochemical processes are then discussed in the context of key biogeochemical indicators such as the net ecosystem metabolism (NEM), air–water CO2 fluxes, nutrient-filtering capacities and element budgets. In the second part of the paper, we illustrate, on the basis of local estuarine modelling studies, the power of reaction-transport models (RTMs) in understanding and quantifying estuarine biogeochemical dynamics. We show how a combination of RTM and high-resolution data can help disentangle the complex process interplay, which underlies the estuarine NEM, carbon and nutrient fluxes, and how such approaches can provide integrated assessments of the air–water CO2 fluxes along river–estuary–coastal zone continua. In addition, trends in estuarine biogeochemical dynamics across estuarine geometries and environmental scenario are explored, and the results are discussed in the context of improving the modelling of estuarine carbon and CO2 dynamics at regional and global scales.

Why Dissolved Organics Matter: DOC in Ancient Oceans and Past Climate Change

Abstract Earth history is punctuated by a huge variety of transitions and perturbations in climate and global biogeochemical cycles. Some of these events exhibit evidence for greenhouse warming and CO2 release and hence potentially provide clues regarding future changes. Negative spikes and transients in the isotopic composition of carbon in sedimentary rocks bear the testimony of these carbon cycle perturbations and are generally taken to indicate the introduction of carbon with a relatively depleted isotopic signature into the ocean and atmosphere. One reservoir of isotopically depleted carbon is the dissolved organic carbon (DOC) present in the ocean. This has led to the idea that both the cycling of DOC and its reservoir size could have been fundamentally different in the past and that change in the oceanic DOC cycle may be mechanistically linked to major events in the geological record. This chapter provides an overview of how the proposed link between DOC and major global carbon cycle perturbation events in the geological record arises. We start by presenting a summary of the traditional view of how the marine carbon cycle operates. We then introduce how the geological record is interpreted, focusing on the ways in which the carbon isotopic signature of sedimentary rocks reflects past changes in global carbon cycling. We go on to critically assess recent thinking on the potential role of DOC as a driver for extreme climate events before touching on the future implications of a DOC cycle active on geological timescales.

Bridging the divide: a model-data approach to Polar and Alpine microbiology.

Advances in microbial ecology in the cryosphere continue to be driven by empirical approaches including field sampling and laboratory-based analyses. Although mathematical models are commonly used to investigate the physical dynamics of Polar and Alpine regions, they are rarely applied in microbial studies. Yet integrating modelling approaches with ongoing observational and laboratory-based work is ideally suited to Polar and Alpine microbial ecosystems given their harsh environmental and biogeochemical characteristics, simple trophic structures, distinct seasonality, often difficult accessibility, geographical expansiveness and susceptibility to accelerated climate changes. In this opinion paper, we explain how mathematical modelling ideally complements field and laboratory-based analyses. We thus argue that mathematical modelling is a powerful tool for the investigation of these extreme environments and that fully integrated, interdisciplinary model-data approaches could help the Polar and Alpine microbiology community address some of the great research challenges of the 21st century (e.g. assessing global significance and response to climate change). However, a better integration of field and laboratory work with model design and calibration/validation, as well as a stronger focus on quantitative information is required to advance models that can be used to make predictions and upscale processes and fluxes beyond what can be captured by observations alone.

Modeling radiocarbon constraints on the dilution of dissolved organic carbon in the deep ocean

The recalcitrance of dissolved organic carbon (DOC) that leads to its accumulation in the deep ocean is typically considered a function of its reactivity. Yet, recent experimental evidence has shown that DOC from the deep ocean, if concentrated, can support significant microbial growth. This supports an alternative hypothesis that [DOC] may become too dilute to support microbial growth. The radiocarbon signature of DOC is a key constraint on the DOC cycling that allows testing of the plausibility of this hypothesis. Here, we use a box model of diluted DOC in the deep ocean and its radiocarbon signature that is constrained on the basis of the new experimental evidence, as well as current knowledge of deep ocean DOC cycling to quantitatively test the dilution hypothesis. We explore the uncertainty in model results across a range of plausible dilution thresholds, additional processes, and fluxes of DOC to the deep ocean. Results show that the model is able to predict the observed radiocarbon signature for a dilution threshold close to the observed deep ocean [DOC] and for fluxes close to published estimates. Sensitivity analysis shows that this result is highly sensitive to variations in the dilution threshold, and the assumption that diluted DOC is able to survive ocean overturning. The experimental findings can be alternatively reconciled over a large range of different conditions assuming a small pool of diluted DOC with a modern radiocarbon signature, consistent with recent observations, and offering a parsimonious interpretation of dilution with existing hypotheses on DOC recalcitrance.

Temperature and volume of global marine sediments

Marine sediments contribute significantly to global element cycles on multiple time scales. This is due in large part to microbial activity in the shallower layers and abiotic reactions resulting from increasing temperatures and pressures at greater depths. Quantifying the rates of these diagenetic changes requires a three-dimensional description of the physiochemical properties of marine sediments. In a step toward reaching this goal, we have combined global data sets describing bathymetry, heat conduction, bottom-water temperatures, and sediment thickness to quantify the three-dimensional distribution of temperature in marine sediments. This model has revealed that ∼35% of sediments are above 60 °C, conditions that are suitable for petroleum generation. Furthermore, significant microbial activity could be inhibited in ∼25% of marine sediments, if 80 °C is taken as a major thermal barrier for subsurface life. In addition to a temperature model, we have calculated new values for the total volume (3.01 × 108 km3) and average thickness (721 m) of marine sediments, and provide the only known determination of the volume of marine-sediment pore water (8.46 × 107 km3), equivalent to ∼6.3% of the volume of the ocean. The results presented here can be used to help quantify the rates of mineral transformations, lithification, catagenesis, and the extent of life in the subsurface on a global scale.

OMEN-SED 0.9: A novel, numerically efficient organic matter sediment diagenesis module for coupling to Earth system models

1. Comment: Model formulation The model assumes no overlap of mineralization reactions with different terminal electron acceptors, and assumes that secondary redox reactions can be collapsed onto the interfaces between different mineralization zones (p.9). This is probably ok in environments typically encountered at greater water depth, but there is ample evidence of ’overlapping’ mineralization pathways in surficial sediments, in particular in permeable or bioturbated settings.

The impact of alkenone degradation on paleothermometry: A model‐derived assessment

The U37K′ proxy for past sea surface temperature (SST) is based on the unsaturation ratio of C37 alkenones. It is considered a diagenetically robust proxy, but biases have been invoked because the index can be altered by preferential degradation of the C37:3 alkenone, resulting in higher reconstructed SST. However, alkenone degradation rate constants are poorly constrained, making it difficult to evaluate the plausibility of such a bias. Therefore, we quantitatively assessed the effect of: (1) different alkenone degradation rate constants; (2) differential degradation factors between di- and tri-unsaturated C37 alkenones; (3) and initial U37K′ values on the U37K′ paleothermometer for two depositional environments (shelf and upper-slope), by means of a Reaction-Transport Model (RTM). RTM results reveal that preferential degradation of C37:3 can potentially alter the original signal of the U37K′ paleothermometer, but SST biases (ΔSST) are largely within U37K′ calibration error (ΔSST   1.5 °C are associated with marked downcore decreases in alkenone concentration. Consequently, we caution against the interpretation of U37K′ indices when extensive degradation results in very low alkenone concentrations (<5 ng g−1).