Abstract. We present a one-dimensional reactive transport model to estimate benthic fluxes of dissolved inorganic carbon (DIC) and alkalinity (AT) from coastal marine sediments. The model incorporates the transport processes of sediment accumulation, molecular diffusion, bioturbation and bioirrigation, while the reactions included are the redox pathways of organic carbon oxidation, re-oxidation of reduced nitrogen, iron and sulfur compounds, pore water acid-base equilibria, and dissolution of particulate inorganic carbon (calcite, aragonite, and Mg-calcite). The coastal zone is divided into four environmental units with different particulate inorganic carbon (PIC) and particulate organic carbon (POC) fluxes: reefs, banks and bays, carbonate shelves and non-carbonate shelves. Model results are analyzed separately for each environment and then scaled up to the whole coastal ocean. The model-derived estimate for the present-day global coastal benthic DIC efflux is 126 Tmol yr−1, based on a global coastal reactive POC depositional flux of 117 Tmol yr−1. The POC decomposition leads to a carbonate dissolution from shallow marine sediments of 7 Tmol yr−1 (on the order of 0.1 Pg C yr−1. Assuming complete re-oxidation of aqueous sulfide released from sediments, the effective net flux of alkalinity to the water column is 29 Teq. yr−1, primarily from PIC dissolution (46%) and ammonification (33%). Because our POC depositional flux falls in the high range of global values given in the literature, the reported DIC and alkalinity fluxes should be viewed as upper-bound estimates. Increasing coastal seawater DIC to what might be expected in year 2100 due to the uptake of anthropogenic CO2 increases PIC dissolution by 2.3 Tmol yr−1and alkalinity efflux by 4.8 Teq. yr−1. Our reactive transport modeling approach not only yields global estimates of benthic DIC, alkalinity and nutrient fluxes under variable scenarios of ocean productivity and chemistry, but also provides insights into the underlying processes.
Abstract. This study applies the Carbon-Generic Estuary Model (C-GEM) modeling platform to simulate the estuarine biogeochemical dynamics – in particular the air–water CO2 exchange – in three idealized tidal estuaries characterized by increasing riverine influence, from a so-called "marine estuary" to a "riverine estuary". An intermediate case called "mixed estuary" is also considered. C-GEM uses a generic biogeochemical reaction network and a unique set of model parameters extracted from a comprehensive literature survey to perform steady-state simulations representing average conditions for temperate estuaries worldwide. Climate and boundary conditions are extracted from published global databases (e.g., World Ocean Atlas, GLORICH) and catchment model outputs (GlobalNEWS2). The whole-system biogeochemical indicators net ecosystem metabolism (NEM), C and N filtering capacities (FCTC and FCTN, respectively) and CO2 gas exchanges (FCO2) are calculated across the three idealized systems and are related to their main hydrodynamic and transport characteristics. A sensitivity analysis, which propagates the parameter uncertainties, is also carried out, followed by projections of changes in the biogeochemical indicators for the year 2050. Results show that the average C filtering capacities for baseline conditions are 40, 30 and 22 % for the marine, mixed and riverine estuary, respectively, while N filtering capacities, calculated in a similar fashion, range from 22 % for the marine estuary to 18 and 15 % for the mixed and the riverine estuaries. Sensitivity analysis performed by varying the rate constants for aerobic degradation, denitrification and nitrification over the range of values reported in the literature significantly widens these ranges for both C and N. Simulations for the year 2050 suggest that all estuaries will remain largely heterotrophic, although a slight improvement of the estuarine trophic status is predicted. In addition, our results suggest that, while the riverine and mixed systems will only marginally be affected by an increase in atmospheric pCO2, the marine estuary is likely to become a significant CO2 sink in its downstream section. In the decades to come, such a change in behavior might strengthen the overall CO2 sink of the estuary–coastal ocean continuum.
Abstract. Over recent decades the highest rates of water column warming and sea ice loss across the Arctic Ocean have been observed in the Barents Sea. These physical changes have resulted in rapid ecosystem adjustments, manifesting as a northward migration of temperate phytoplankton species at the expense of silica-based diatoms. These changes will potentially alter the composition of phytodetritus deposited at the seafloor, which acts as a biogeochemical reactor and is pivotal in the recycling of key nutrients, such as silicon (Si). To appreciate the sensitivity of the Barents Sea benthic system to the observed changes in surface primary production, there is a need to better understand this benthic–pelagic coupling. Stable Si isotopic compositions of sediment pore waters and the solid phase from three stations in the Barents Sea reveal a coupling of the iron (Fe) and Si cycles, the contemporaneous dissolution of lithogenic silicate minerals (LSi) alongside biogenic silica (BSi), and the potential for the reprecipitation of dissolved silicic acid (DSi) as authigenic clay minerals (AuSi). However, as reaction rates cannot be quantified from observational data alone, a mechanistic understanding of which factors control these processes is missing. Here, we employ reaction–transport modelling together with observational data to disentangle the reaction pathways controlling the cycling of Si within the seafloor. Processes such as the dissolution of BSi are active on multiple timescales, ranging from weeks to hundreds of years, which we are able to examine through steady state and transient model runs. Steady state simulations show that 60 % to 98 % of the sediment pore water DSi pool may be sourced from the dissolution of LSi, while the isotopic composition is also strongly influenced by the desorption of Si from metal oxides, most likely Fe (oxyhydr)oxides (FeSi), as they reductively dissolve. Further, our model simulations indicate that between 2.9 % and 37 % of the DSi released into sediment pore waters is subsequently removed by a process that has a fractionation factor of approximately −2 ‰, most likely representing reprecipitation as AuSi. These observations are significant as the dissolution of LSi represents a source of new Si to the ocean DSi pool and precipitation of AuSi an additional sink, which could address imbalances in the current regional ocean Si budget. Lastly, transient modelling suggests that at least one-third of the total annual benthic DSi flux could be sourced from the dissolution of more reactive, diatom-derived BSi deposited after the surface water bloom at the marginal ice zone. This benthic–pelagic coupling will be subject to change with the continued northward migration of Atlantic phytoplankton species, the northward retreat of the marginal ice zone and the observed decline in the DSi inventory of the subpolar North Atlantic Ocean over the last 3 decades.
Predicting the impact of ongoing anthropogenic CO 2 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 CO 2 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.
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.
Abstract. We present a new version of the coupled Earth system model GEOCLIM. The new release, GEOCLIM reloaded, links the existing atmosphere and weathering modules to a novel, temporally and spatially resolved model of the global ocean circulation, which provides a physical framework for a mechanistic description of the marine biogeochemical dynamics of carbon, nitrogen, phosphorus and oxygen. The ocean model is also coupled to a fully formulated, vertically resolved diagenetic model. GEOCLIM reloaded is thus a unique tool to investigate the short- and long-term feedbacks between climatic conditions, continental inputs, ocean biogeochemical dynamics and diagenesis. A complete and detailed description of the resulting Earth system model and its new features is first provided. The performance of GEOCLIM reloaded is then evaluated by comparing steady-state simulation under present-day conditions with a comprehensive set of oceanic data and existing global estimates of bio-element cycling in the pelagic and benthic compartments.
<p>The Arctic Ocean is currently experiencing rapid oceanographic shifts and significant sea-ice loss as a result of regional atmospheric and oceanic warming. The Barents Sea is a notable example of these phenomena, having seen a near 40% decline of its April sea-ice extent since 1979, and a progressive northward expansion of Atlantic Water (i.e., Atlantification). Such changes affect primary productivity and nutrient cycling in ways that remain poorly understood. Longer ice-free periods and the inflow of warmer Atlantic Water are expected to lead to extended bloom seasons on short, near-future timescales and therefore increase nutrient uptake in upper water layers. The benthic recycling of nutrients is believed to play an important part in replenishing nutrient inventories in overlying waters thus maintaining high primary productivity over the continuously expanding growth season. Therefore, it is crucial to increase our understanding of nutrient dynamic controls in changing oceans to make more accurate predictions and decipher the complex feedbacks involved in these evolving environments. However, most efforts to constrain and quantify nutrient fluxes so far have been directed at silicon, nitrogen or iron. This study aims to provide specific insight into phosphorus (P) cycling through its response to OM fluctuations and coupling with iron cycling. An integrated data-model approach was used to investigate the dynamics of P cycling at the sediment-water interface across five locations along the 30&#176;E meridian that were drilled in the framework of the ChAOS project in the Barents Sea. The model approach allowed to explore the sensitivity of P cycling to plausible ranges of reactive iron and OM inputs. Greater inputs of reactive iron were found to decrease benthic phosphate fluxes (J<sub>PO4</sub>) whereas greater inputs of OM increased phosphate return to the water column. The quality of these inputs is equally significant: J<sub>PO4 </sub>decreased when iron hydroxides were made more reactive and increased with more reactive OM. Our findings indicate that variation in climatically sensitive processes, such as burial of terrestrial sediments and iron cycling, could represent powerful feedbacks on J<sub>PO4</sub> through adsorption/desorption mechanisms. Results also reveal significant oceanographic controls on J<sub>PO4</sub>, suggesting Atlantification of the Barents Sea will play into future phosphate availability.</p>
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