Molybdenum dynamics in sediments of a seasonally-hypoxic coastal marine basin

Peer reviewe

Impact of biogeochemical processes on pH dynamics in marine systems

Uptake of anthropogenic carbon dioxide (CO2) from the atmosphere has resulted in a range of changes in ocean chemistry, including the lowering of pH, collectively referred to as ocean acidification. Rates of coastal-zone acidification exceed those of the open ocean since coastal-ocean pH is influenced by many other processes than absorption of CO­2 alone. These processes do not only play a role in long-term acidification but also impact pH on seasonal timescales. Examples are enhanced atmospheric sulphur and nitrogen deposition, as well as eutrophication, the latter which can additionally result in the development of low-oxygen waters. The degree to which these processes induce a change in pH depends both on their rates and the extent to which the water can buffer acid production or consumption. This acid-base buffering capacity has been shown to decrease substantially in hypoxic waters, suggesting that low-oxygen conditions exacerbate ocean acidification. In this dissertation the key factors controlling the seasonal pH variability and longer-term pH changes in both the coastal and open ocean were examined, showing that buffering mechanisms play a crucial role in the impact of any biogeochemical or physical process on pH. Monthly or seasonal water-column chemistry and process-rate measurements in a transiently hypoxic coastal marine basin indicate that, despite generally higher process rates in the surface water of the basin, the amplitude of pH variability as mainly governed by the balance between primary production and respiration is greater in the seasonally-hypoxic bottom water, due to a considerable reduction of its acid-base buffering capacity in summer. A proton budget, based on these measurements and set up for each season, shows that the net change in pH is much smaller than the flux of protons induced by each of the individual processes. The interplay between absorption of atmospheric CO2 and atmospheric sulphur and nitrogen deposition in the coastal ocean was found to depend on the water-column concentration of CO2 relative to the atmosphere. If the atmospheric concentration surpasses that of the surface water, then this part of the coastal ocean is most sensitive to CO2-induced acidification, but least affected by additional acidification resulting from atmospheric acid deposition. Although coastal seas will become up to a factor 4 more sensitive to atmospheric deposition-induced acidification between the present-day and 2100, the annual change in proton concentration will only increase by 28% at most. Finally, a set of general expressions describing the sensitivity of pH to a change in ocean chemistry was derived. These expressions, which can include as many acid-base systems as relevant and are thus generally applicable, were tested on several long-term open ocean data sets. For each of these sites, pH can be properly predicted if seasonal cycles of temperature, salinity, total alkalinity (TA) and the total concentrations of acid-base species are known. By the end of the 21st century a change in most acid-base parameters will induce a comparably greater pH excursion. This increased vulnerability is driven by enhanced CO2 concentrations and slightly moderated by the projected global warming

Circulation and Reaction Hotspots in an Intertidal Salt Marsh: a Modeling Study

Proceedings of the 2011 Georgia Water Resources Conference, April 11, 12, and 13, 2011, Athens, Georgia.Intertidal salt marshes are highly productive, dynamic ecosystems at the interface between the land and the ocean that can play a significant role in reducing nutrient loading to the coastal ocean. To assess the spatio‐temporal patterns in salt marsh biogeochemistry, a reactive transport model describing tidally‐driven flow as well as solute dynamics across a marsh cross‐section was developed. Porewater residence times were computed to identify zones of rapid fluid exchange. Model simulations suggest the presence of circulation hotspots at the creek bank and the upland‐marsh transition zone, whose intensity varies over a tidal cycle. The location and magnitude of these regions of rapid fluid exchange depend on the tidal amplitude, and on the presence or absence of terrestrial groundwater input from the upland. The introduction of oxygenated creek water to the marsh subsurface also promotes biogeochemical reactions and hence may be important for regulating the marsh’s filter function. Reaction hotspots are located at the interface between chemically distinct water bodies such as upland‐derived groundwater and the intruding tidal creek water. As a result, these hotspots develop at the fringes of circulation hotspots, but are not identical to the locations of highest infiltration. The relative importance of reaction hotspots varies substantially with tidal amplitude and their presence has important implications for the placement of monitoring wells in field studies.Sponsored by: Georgia Environmental Protection Division U.S. Geological Survey, Georgia Water Science Center U.S. Department of Agriculture, Natural Resources Conservation Service Georgia Institute of Technology, Georgia Water Resources Institute The University of Georgia, Water Resources FacultyThis book was published by Warnell School of Forestry and Natural Resources, The University of Georgia, Athens, Georgia 30602-2152. The views and statements advanced in this publication are solely those of the authors and do not represent official views or policies of The University of Georgia, the U.S. Geological Survey, the Georgia Water Research Institute as authorized by the Water Research Institutes Authorization Act of 1990 (P.L. 101-307) or the other conference sponsors

Understanding alkalinity to quantify ocean buffering

Ocean alkalinity plays a major role in ocean’s carbon uptake, in buffering, and in calcium carbonate production and dissolution, and it impacts and is affected by various biogeochemical processes

Ocean alkalinity, buffering and biogeochemical processes

Alkalinity, the excess of proton acceptors over donors, plays a major role in ocean chemistry, in buffering and in calcium carbonate precipitation and dissolution. Understanding alkalinity dynamics is pivotal to quantify ocean carbon dioxide uptake during times of global change. Here we review ocean alkalinity and its role in ocean buffering as well as the biogeochemical processes governing alkalinity and pH in the ocean. We show that it is important to distinguish between measurable titration alkalinity and charge balance alkalinity that is used to quantify calcification and carbonate dissolution and needed to understand the impact of biogeochemical processes on components of the carbon dioxide system. A general treatment of ocean buffering and quantification via sensitivity factors is presented and used to link existing buffer and sensitivity factors. The impact of individual biogeochemical processes on ocean alkalinity and pH is discussed and quantified using these sensitivity factors. Processes governing ocean alkalinity on longer time scales such as carbonate compensation, (reversed) silicate weathering, and anaerobic mineralization are discussed and used to derive a close‐to‐balance ocean alkalinity budget for the modern ocean.<br/

Current estimates of K1* and K2* appear inconsistent with measured CO2 system parameters in cold oceanic regions

Seawater absorption of anthropogenic atmospheric carbon dioxide (CO2) has led to a range of changes in carbonate chemistry, collectively referred to as ocean acidification. Stoichiometric dissociation constants used to convert measured carbonate system variables (pH, pCO2, dissolved inorganic carbon, total alkalinity) into globally comparable parameters are crucial for accurately quantifying these changes. The temperature and salinity coefficients of these constants have generally been experimentally derived under controlled laboratory conditions. Here, we use field measurements of carbonate system variables taken from the Global Ocean Data Analysis Project version 2 and the Surface Ocean CO2 Atlas data products to evaluate the temperature dependence of the carbonic acid stoichiometric dissociation constants. By applying a novel iterative procedure to a large dataset of 948 surface-water, quality-controlled samples where four carbonate system variables were independently measured, we show that the set of equations published by Lueker et al. (2000), currently preferred by the ocean acidification community, overestimates the stoichiometric dissociation constants at temperatures below about 8 ∘C. We apply these newly derived temperature coefficients to high-latitude Argo float and cruise data to quantify the effects on surface-water pCO2 and calcite saturation states. These findings highlight the critical implications of uncertainty in stoichiometric dissociation constants for future projections of ocean acidification in polar regions and the need to improve knowledge of what causes the CO2 system inconsistencies in cold waters

Ocean Alkalinity, Buffering and Biogeochemical Processes: Review Article

Alkalinity, the excess of proton acceptors over donors, plays a major role in ocean chemistry, in buffering and in calcium carbonate precipitation and dissolution. Understanding alkalinity dynamics is pivotal to quantify ocean carbon dioxide uptake during times of global change. Here we review ocean alkalinity and its role in ocean buffering as well as the biogeochemical processes governing alkalinity and pH in the ocean. We show that it is important to distinguish between measurable titration alkalinity and charge balance alkalinity that is used to quantify calcification and carbonate dissolution and needed to understand the impact of biogeochemical processes on components of the carbon dioxide system. A general treatment of ocean buffering and quantification via sensitivity factors is presented and used to link existing buffer and sensitivity factors. The impact of individual biogeochemical processes on ocean alkalinity and pH is discussed and quantified using these sensitivity factors. Processes governing ocean alkalinity on longer time scales such as carbonate compensation, (reversed) silicate weathering, and anaerobic mineralization are discussed and used to derive a close‐to‐balance ocean alkalinity budget for the modern ocean

Pore water and solid-phase measurements on sediment core 347-M0065 from the Bornholm Basin

Phosphorus (P) concentrations in sediments are frequently used to reconstruct past environmental conditions in freshwater and marine systems, with high values thought to be indicative of a high biological productivity. Recent studies suggest that the post-depositional formation of vivianite, an iron(II)-phosphate mineral, might significantly alter trends in P with sediment depth. To assess its importance, we investigate a sediment record from the Bornholm Basin that was retrieved during the Integrated Ocean Drilling Program (IODP) Baltic Sea Paleoenvironment Expedition 347 in 2013, consisting of lake sediments overlain by brackish-marine deposits. Combining bulk sediment geochemistry with microanalysis using scanning electron microscope energy dispersive spectroscopy (SEM-EDS) and synchrotron-based X-ray absorption spectroscopy (XAS), we demonstrate that vivianite-type minerals rich in manganese and magnesium are present in the lake deposits just below the transition to the brackish-marine sediments (at 11.5 to 12 m sediment depth). In this depth interval, phosphate that diffuses down from the organic-rich, brackish-marine sediments meets porewaters rich in dissolved iron in the lake sediments, resulting in the precipitation of iron(II) phosphate. Results from a reactive transport model suggest that the peak in iron(II) phosphate originally occurred at the lake-marine transition (9 to 10 m) and moved downwards due to changes in the depth of a sulfidization front. However, its current position relative to the lake-marine transition is stable as the vivianite-type minerals and active sulfidization fronts have been spatially separated over time. Experiments in which vivianite was subjected to sulfidic conditions demonstrate that incorporation of manganese or magnesium in vivianite does not affect its susceptibility to sulfide-induced dissolution. Our work highlights that post-depositional formation of iron(II) phosphates such as vivianite has the potential to strongly alter sedimentary P records particularly in systems that are subject to environmental perturbation, such as a change in primary productivity, which can be associated with a lake-marine transition