CO2 Outgassing Research Articles

Carbon Cycling in Floodplain Ecosystems: Out-Gassing and Photosynthesis Transmit Soil δ13C Gradient Through Stream Food Webs

Natural braided river floodplains typically possess high groundwater–surface water exchange, which is vital to the overall function and structure of these complex ecosystems. Spring-fed streams on the floodplain are also hotspots of benthic invertebrate diversity and productivity. The sources of carbon that drive these productive spring-fed systems are not well-known. We conducted field assessments and a manipulation, modeling, and a laboratory experiment to address this issue. Initially δ13C values of both dissolved inorganic carbon (DIC) and food-web components of five springs were used to assess the sources of carbon to spring food webs. Partial pressures of CO2 in upwelling water ranged from 2 to 7 times atmospheric pressure, but rapidly approached equilibrium with the atmosphere downstream commensurate with 13C enrichment of DIC. Speciation modeling and a laboratory out-gassing experiment suggested that downstream changes in pH could be explained solely by CO2 out-gassing. However, field results indicated that both out-gassing and photosynthetic drawdown by aquatic plants controlled the net flux of CO2. A whole stream manipulation indicated out-gassing was the primary effect at the spring source, which was confirmed by invariant diel pH. At 1296 m downstream from the spring source a large diel shift in pH indicated a plant effect on CO2 concentration which would contribute to the overall downstream gradient in δ13C DIC. Within the first 1296 m the gradient in δ13 DIC was transmitted through three trophic levels of the spring food web. These findings indicate dependency on groundwater inorganic carbon by spring stream food webs and strong hydrologically mediated linkages connecting terrestrial, subsurface, and aquatic components of the floodplain.

Discovery of a natural CO2seep in the German North Sea: Implications for shallow dissolved gas and seep detection

A natural carbon dioxide (CO2) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO2 levels of ∼10–20 times above background were detected in seawater above a natural salt dome ∼30 km north of the East-Frisian Island Juist. A single elevated value 53 times higher than background was measured, indicating a possible CO2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO2 concentration. These results are presented in the context of CO2 seepage detection, in light of proposed subsurface CO2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO2 bubble and plume seepage and potential flux paths to the atmosphere. Shallow bubble release experiments conducted in a lake combined with discrete-bubble modeling suggest that shallow CO2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble-plume modeling further shows that a CO2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO2-enriched water tends to sink toward the seabed. Results suggest that released CO2 will tend to stay near the bottom in shallow systems (<200 m) and will vent to the atmosphere only during deep water convection (water column turnover). While isotope signatures point to a biogenic source, the exact origin is inconclusive because of dilution. This site could serve as a natural laboratory to further study the effects of carbon sequestration below the seafloor.

Gaseous and fluvial carbon export from an Amazon forest watershed

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO2 supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH4) flux, direct CO2 and CH4 fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO2 concentrations ranged from 6,491 to 14,976 μatm and directly-measured stream CO2 outgassing flux was 5,994 ± 677 g C m−2 y−1 of stream surface. Stream pCH4 concentrations ranged from 291 to 438 μatm and measured stream CH4 outgassing flux was 987 ± 221 g C m−2 y−1. Despite high flux rates from the stream surface, the small area of stream itself (970 m2, or 0.007% of watershed area) led to small directly-measured annual fluxes of CO2 (0.44 ± 0.05 g C m2 y−1) and CH4 (0.07 ± 0.02 g C m2 y−1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m−2 y−1), DOC (0.16 ± 0.03 g C m−2 y−1) and coarse plus fine particulate C (0.001 ± 0.001 g C m−2 y−1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m−2 y−1 as CO2 outgassing, 11.29 g C m−2 y−1 as fluvial DIC and 0.64 g C m−2 y−1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40–50 g C m−2 y−1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m−2 y−1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m−2 (±1.5 Mg C ha−1) that is exchanged annually across Amazon tropical forest canopies.

Upper ocean response to Typhoon Choi-Wan as measured by the Kuroshio Extension Observatory mooring

[1] The Kuroshio Extension Observatory (KEO) is a highly instrumented moored reference station located at 32.3°N, 144.5°E in the recirculation gyre south of the Kuroshio Extension. On 19 September 2009, the eye of Typhoon Choi-Wan (International designation: 0914) passed ∼40 km to the southeast of the KEO surface mooring. Hourly meteorological and physical oceanographic measurements together with 3 hourly air-sea carbon dioxide observations telemetered from KEO in near real time show the evolution of the upper ocean and its associated air-sea fluxes during the passage of this storm and its aftermath. During the approach of the storm, the mixed layer freshened because of intense rainfall. This was followed by a large outgassing of CO2, rapid cooling, and an increase in salinity. Although these changes in mixed layer properties imply substantial entrainment, they were accompanied by upwelling and ultimately a temporary ∼20 m shoaling of the mixed layer. This upwelling, which was observed at all depths, including the deepest sensor near 500 m, was coincident with the onset of near-inertial oscillations in the mixed layer currents. After the typhoon passed, inertial pumping caused ∼15–20 m amplitude vertical displacements throughout the top 500 m that continued for at least 6 days. A large oceanic response was observed in this case even though the eye of Choi-Wan passed to the right of KEO, resulting in winds rotating cyclonically with time, in opposition to the anticyclonic-rotating near-inertial currents.

Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere

Streams, rivers, lakes, and other inland waters are important agents in the coupling of biogeochemical cycles between continents, atmosphere, and oceans. The depiction of these roles in global‐scale assessments of carbon (C) and other bioactive elements remains limited, yet recent findings suggest that C discharged to the oceans is only a fraction of that entering rivers from terrestrial ecosystems via soil respiration, leaching, chemical weathering, and physical erosion. Most of this C influx is returned to the atmosphere from inland waters as carbon dioxide (CO2) or buried in sedimentary deposits within impoundments, lakes, floodplains, and other wetlands. Carbon and mineral cycles are coupled by both erosion–deposition processes and chemical weathering, with the latter producing dissolved inorganic C and carbonate buffering capacity that strongly modulate downstream pH, biological production of calcium‐carbonate shells, and CO2 outgassing in rivers, estuaries, and coastal zones. Human activities substantially affect all of these processes.

Biogeochemistry of Carbon in the Amazonian Floodplains over a 2000-km Reach: Insights from a Process-Based Model

Abstract The influence of Amazonian floodplains on the hydrological, sedimentary, and biogeochemical river budget was investigated over a 2000-km reach. A process-based model relying on the closure of chemical fluxes and isotopic signals was implemented. On average for the whole studied reach, the overall fluxes of carbon associated with mineralization and aquatic photosynthesis were estimated to 35.7 and 15.3 Tg C yr−1, respectively. Almost 57% of the carbon sequestrated by photosynthesis comes from aerial sources (flooded forest); the remaining 43% resulted from aquatic sources (várzea grasses and phytoplankton). The process rates substantially fluctuate over the annual cycle, depending particularly on the extension of flooded area and on the river–floodplain connectivity. As the river level declines, the drastic decrease of turbidity and the lower supply of carbon substrates promote autotrophy to the detriment of heterotrophy, leading to substantial changes of pH and gaseous equilibria in the river water. The main consequences are (i) the side-chain oxidation of dissolved organic matter leading to the concomitant rises of the carbon to nitrogen atomic ratio and nitrate contents and (ii) the sorption of hydrophobic humic acids, which fractionate 13C and thus lead to 13C-depleted particulate organic matter (fine fraction) compared to remaining dissolved organic matter. As the river flow rises, the heterotrophy prevails over autotrophy and this tends to attenuate the chemical signature imprinted by the latter. The significant contribution of aerial autochthonous sources to the budget of carbon indicates that the fluxes of mineralization are sustained by the net primary production of river corridors. The variable extension of submerged areas defines the proportions of CO2 exported by the river and released to the atmosphere. The rate of CO2 outgassing on the studied reach (18.8 Tg C yr−1) represents about 50% of the incoming dissolved inorganic carbon flux. The rate of methane emission is estimated as 2.2 Tg C yr−1 and that of denitrification is estimated as 0.87 Tg N yr−1, representing 1.5 times the flux of dissolved inorganic nitrogen (DIN) exported by the Amazon River at the station of Óbidos (0.64 Tg N yr−1).

Aluminum polymers formed following alum treatment of lake water

Alum (aluminum sulfate) is increasingly being used in lake management to control internal recycling of phosphorus from bottom sediments. Alum added to water undergoes rapid hydrolysis reactions, forming an amorphous Al(OH) 3 floc with a high capacity for sorption of phosphorus. While it is known that the Al(OH) 3 floc transforms over time to more ordered microcrystalline and crystalline gibbsite phases, there remains an incomplete understanding of the forms of Al present immediately following alum addition to lake water. A laboratory study was thus undertaken to evaluate the forms of Al present following alum addition using ferron (8-hydroxy-7-iodo-5-quinolinesulfonic acid) timed-colorimetric and 27Al-NMR measurements. A polymeric Al species with moderate reactivity with ferron (Al b2) was initially formed, although it rapidly transformed to a less ferron-reactive colloidal form (Al c) and also decomposed at low alum doses to monomeric Al (Al a) in response to pH increases associated with outgassing of CO 2. The Al a fraction in these solutions could be adequately estimated based upon measured pH assuming Al solubility was controlled by an amorphous Al(OH) 3 phase. Al 13 was inferred from ferron measurements to be present, but only at quite low concentrations in the alum-treated waters.

Controls on the origin and cycling of riverine dissolved inorganic carbon in the Brazos River, Texas

Rivers draining watersheds that include carbonate bedrock or organic matter (OM)-rich sedimentary rocks frequently have 14C-depleted dissolved inorganic carbon (DIC) relative to rivers draining carbonate- and OM-free watersheds, due to dissolution of carbonate and/or decomposition of ancient OM. However, our results from a subtropical river, the Brazos River in Texas, USA, show that in this watershed human activities appear to dominate basin lithology in controlling the origin and metabolism of DIC. The middle Brazos flows through limestone and coal-bearing bedrock, but DIC isotope data suggest no limestone dissolution or respiration of ancient OM, and instead reflect efficient air–water CO2 exchange, degradation of relatively young OM and photosynthesis in the river as a result of river damming and urban treated wastewater input. The lower Brazos drains only small areas of carbonate and coal-bearing bedrock, but DIC isotope data suggest the strong influence of carbonate dissolution, with a potentially minor contribution from decomposition of old soil organic matter (SOM). Oyster shells and crushed carbonate minerals used in road construction are likely sources of carbonate in the lower Brazos, in addition to natural marl and pedogenic carbonate. Additionally, the generally low pCO2 and high DIC concentration in the Brazos River lead to a low CO2 outgassing:DIC export ratio, distinguishing the Brazos River from other rivers.

Calcareous Nannoplankton Response to Surface-Water Acidification Around Oceanic Anoxic Event 1a

Ocean acidification induced by atmospheric CO2 may be a major threat to marine ecosystems, particularly to calcareous nannoplankton. We show that, during the Aptian (approximately 120 million years ago) Oceanic Anoxic Event 1a, which resulted from a massive addition of volcanic CO2, the morphological features of calcareous nannofossils traced the biological response to acidified surface waters. We observe the demise of heavily calcified nannoconids and reduced calcite paleofluxes at the beginning of a pre-anoxia calcification crisis. Ephemeral coccolith dwarfism and malformation represent species-specific adjustments to survive lower pH, whereas later, abundance peaks indicate intermittent alkalinity recovery. Deepwater acidification occurred with a delay of 25,000 to 30,000 years. After the dissolution climax, nannoplankton and carbonate recovery developed over approximately 160,000 years under persisting global dysoxia-anoxia.

Organic and inorganic carbon concentrations and fluxes from managed and unmanaged boreal first-order catchments

Seasonal and between stream variation (catchment dependent variation) in losses of organic and inorganic carbon via downstream transport and outgassing of CO 2 into the atmosphere were studied in 11 small boreal catchments situated in close proximity to each other. Of these catchments four were undrained peatland rich catchments, four drained peatland rich catchments and three managed mineral soil-dominated catchments. Downstream export of total inorganic carbon (TIC) varied between 870 and 1400 kg km − 2 a − 1 and was rather consistent between the catchments, except in the case of the mineral soil-dominated catchment Kangaslampi, where export was only 420 kg km − 2 a − 1 . The export of total organic carbon (TOC) varied between 2300 and 14,800 kg km − 2 a − 1 and was highest in peatland rich catchments. Peatland drainage decreased TIC and TOC concentrations in the long term, but did not affect lateral carbon export due to increased runoff from the catchments. Partial pressure of CO 2 in streams was the highest in undrained peatland rich catchments, but the outgassing of CO 2 into the atmosphere was also high from drained peatlands due to the higher discharge rate and long ditch networks. In mineral soil-dominated catchments both downstream export of carbon and emission into the atmosphere were low. TOC exports were compared in two climatically different years (2003 and 2007). The results indicate that climate change might alter the timing of the TOC export from the catchments, the importance of the spring ice melt diminishing and both snow cover and snow free period export increasing.

Earth’s CO₂ degassing in Italy

Earth’s CO2 emission in Italy includes both volcanic and non-volcanic degassing, with measured CO2 fluxes of about 35 60 Mt/y. Zones of non-volcanic CO2 emission include Tuscany, Latium, Campania, the Apennines, Sicily, and Sardinia. Volcanic emissions are particularly abundant at Mt. Etna in Sicily, but also at Vesuvio, Campi Flegrei, Ischia, Vulcano, and Stromboli, in Central-Southern Italy. The anomalous CO2 emission in Italy is related to the complex geodynamic evolution of this area, in which upper crustal rocks, including carbonate sediments, have been introduced into the upper mantle by Oligocene to present subduction processes. Integrated petrological, geochemical and geophysical data allow us to work out a model for the generation of anomalously high lithospheric CO2 fluxes. Melting of sediments and/or continental crust of the subducted Adriatic-Ionian (African) lithosphere at pressure greater than 4 GPa (130 km) is proposed to represent an efficient mean for deep carbon cycling into the upper mantle and into the exosphere in the Western Mediterranean area. Melting of carbonated lithologies, induced by the progressive rise of mantle temperatures behind the eastward retreating Adriatic-Ionian subducting plate formed a carbonated partially molten CO2-rich mantle in the depth range from 130 km to 70 km. Further upwelling of carbonate-rich melts induces massive outgassing of CO2. Buoyancy forces, probably favored by fluid overpressures, are able to allow migration of CO2 from the mantle to the surface, through deep lithospheric faults, and its accumulation beneath the Moho, and within the lower crust. Citation: 2010. Earth’s CO2 degassing in Italy. In: (Eds.) Marco Beltrando, Angelo Peccerillo, Massimo Mattei, Sandro Conticelli, and Carlo Doglioni, Journal of the Virtual Explorer, volume 36, paper 21, doi: 10.3809/jvirtex. 2010.00227 Journal of the Virtual Explorer, 2010 Volume 36

Cardiac Volume Overload Induces Mitochondrial Dysfunction in Murine Cardiomyocytes

The last several years have seen a very rapid increase in the development and availability of unmanned aerial vehicles/systems (UAV or UAS), more commonly called “drones.” UAS are remotely-operated aerial vehicles that can be fixed-wing planes, helicopters, or aerial systems with multiple propellers. Small versions of UAS, here called “sUAS,” in particular, have become so prevalent that they can be easily purchased at general retail stores. This paper seeks to review and summarize the use of sUAS in volcanic research. Given their size, low cost, and relative durability, sUAS provide a light-weight tool platform that is easy to transport to field locations; quick to set-up; and easy to launch, and control. They require very little launch and recovery space. Because of these characteristics, sUAS are useful in collecting immediate and real-time aerial data, especially in remote, inaccessible, dynamic, and/or hazardous, volcanic environments. In volcanic areas, sUAS have been used for mapping, sample collection, thermal imaging, magnetic surveys, slope stability studies, and as platforms for sensors to measure outgassing of CO2 and SO2. They are also becoming invaluable for real-time hazard assessment during and after an eruption. They are, however, limited by their flight time, which is greatly affected by wind speeds. Since they are predominately made of plastic, they are also impacted by the high thermal temperatures found in active volcanic areas, which can degrade their structure and performance. With continued technological improvement however, sUAS have the potential to dramatically improve our ability to collect field data. Because of additional natural, technological, and legal challenges related to their use, it is critical that users be aware of, and adhere to, all national and local laws associated with sUAS.

DETECTING PLANETARY GEOCHEMICAL CYCLES ON EXOPLANETS: ATMOSPHERIC SIGNATURES AND THE CASE OF SO2

We study the spectrum of a planetary atmosphere to derive detectable features in low resolution of different global geochemical cycles on exoplanets - using the sulphur cycle as our example. We derive low resolution detectable features for first generation space- and ground- based telescopes as a first step in comparative planetology. We assume that the surfaces and atmospheres of terrestrial exoplanets (Earth-like and super-Earths) will most often be dominated by a specific geochemical cycle. Here we concentrate on the sulphur cycle driven by outgassing of SO2 and H2S followed by their transformation to other sulphur-bearing species which is clearly distinguishable from the carbon cycle which is driven by outgassing of CO2. Due to increased volcanism, the sulphur cycle is potentially the dominant global geochemical cycle on dry super-Earths with active tectonics. We calculate planetary emission, reflection and transmission spectrum from 0.4 to 40 micrometer with high and low resolution to assess detectable features using current and Archean Earth models with varying SO2 and H2S concentrations to explore reducing and oxidizing habitable environments on rocky planets. We find specific spectral signatures that are observable with low resolution in a planetary atmosphere with high SO2 and H2S concentration. Therefore first generation space and ground based telescopes can test our understanding of geochemical cycles on rocky planets and potentially distinguish planetary environments dominated by the carbon and sulphur cycle.

Cave air ventilation and CO2 outgassing by radon-222 modeling: How fast do caves breathe?

In general, the rate and timing of calcite precipitation is in part affected by variations in cave air CO2 concentrations. Knowledge of cave ventilation processes is required to quantify the effect variations in CO2 concentrations have on speleothem deposition rates and thus paleoclimate records. In this study we use radon-222 (222Rn) as a proxy of ventilation to estimate CO2 outgassing from the cave to the atmosphere, which can be used to infer relative speleothem deposition rates. Hollow Ridge Cave, a wild cave preserve in Marianna, Florida, is instrumented inside and out with multiple micro-meteorological sensor stations that record continuous physical and air chemistry time-series data. Our time series datasets indicate diurnal and seasonal variations in cave air 222Rn and CO2 concentrations, punctuated by events that provide clues to ventilation and drip water degassing mechanisms. Average cave air 222Rn and CO2 concentrations vary seasonally between winter (222Rn = 50 dpm L− 1, where 1 dpm L− 1 = 60 Bq m− 3; CO2 = 360 ppmv) and summer (222Rn = 1400 dpm L− 1; CO2 = 3900 ppmv). Large amplitude diurnal variations are observed during late summer and autumn (222Rn = 6 to 581 dpm L− 1; CO2 = 360 to 2500 ppmv). We employ a simple first-order 222Rn mass balance model to estimate cave air exchange rates with the outside atmosphere. Ventilation occurs via density driven flow and by winds across the entrances which create a ‘venturi’ effect. The most rapid ventilation occurs 25 m inside the cave near the entrance: 45 h− 1 (1.33 min turnover time). Farther inside (175 m) exchange is slower and maximum ventilation rates are 3 h− 1 (22 min turnover time). We estimate net CO2 flux from the epikarst to the cave atmosphere using a CO2 mass balance model tuned with the 222Rn model. Net CO2 flux from the epikarst is highest in summer (72 mmol m− 2 day− 1) and lowest in late autumn and winter (12 mmol m− 2 day− 1). Modeled ventilation and net CO2 fluxes are used to estimate net CO2 outgassing from the cave to the atmosphere. Average net CO2 outgassing is positive (net loss from the cave) and is highest in late summer and early autumn (about 4 mol h− 1) and lowest in winter (about 0.5 mol h− 1). Modeling of ventilation, net CO2 flux from the epikarst, and CO2 outgassing to the atmosphere from cave monitoring time-series can help better constrain paleoclimatic interpretations of speleothem geochemical records.

Speleothem preservation and diagenesis in South African hominin sites implications for paleoenvironments and geochronology

AbstractPlio‐Pleistocene speleothems from australopithecine‐bearing caves of South Africa have the potential to yield paleoenvironmental and geochronological information using isotope geochemistry. Prior to such studies it is important to assess the preservation of geochemical signals within the calcitic and aragonitic speleothems, given the tendency of aragonitic speleothems to recrystallize to calcite. This study documents the geochemical suitability of speleothems from the principal hominin‐bearing deposits of South Africa. We use petrography, together with stable isotope and trace element analysis, to identify the occurrence of primary aragonite, primary calcite, and secondary calcite. This study highlights the presence of diagenetic alteration at many of the sites, often observed as interbedded primary and secondary fabrics. Trace element and stable isotopic values distinguish primary calcite from secondary calcite and offer insights into geochemical aspects of the past cave environment. δ13C values of the primary and secondary calcites range from +6 to −9‰ and δ18O values range from −4 to −6‰. The data are thus typical of meteoric calcites with highly variable δ13C and relatively invariant δ18O. High carbon isotope values in these deposits are associated with the effects of recrystallization and rapid outgassing of CO2 during precipitation. Mg/Ca and Sr/Ca ratios differ between primary and secondary calcite speleothems, aiding their identification. Carbon and oxygen isotope values in primary calcite reflect the proportion of C3 and C4 vegetation in the local environment and the oxygen isotope composition of rainfall. Primary calcite speleothems preserve the pristine geochemical signals vital for ongoing paleoenvironmental and geochronological research. © 2009 Wiley Periodicals, Inc.

Projected Changes to the Southern Hemisphere Ocean and Sea Ice in the IPCC AR4 Climate Models

Abstract Fidelity and projected changes in the climate models, used for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), are assessed with regard to the Southern Hemisphere extratropical ocean and sea ice systems. While individual models span different physical parameterizations and resolutions, a major component of intermodel variability results from surface wind differences. Projected changes to the surface wind field are also central in modifying future extratropical circulation and internal properties. A robust southward shift of the circumpolar current and subtropical gyres is projected, with a strong spinup of the Atlantic gyre. An associated increase in the core strength of the circumpolar circulation is evident; however, this does not translate into robust increases in Drake Passage transport. While an overarching oceanic warming is projected, the circulation-driven poleward shift of the temperature field explains much of the midlatitude warming pattern. The effect of this shift is less clear for salinity, where, instead, surface freshwater forcing dominates. Surface warming and high-latitude freshwater increases drive intensified stratification, and a shoaling and southward shift of the deep mixed layers. Despite large intermodel differences, there is also a robust weakening in bottom water formation and its northward outflow. At the same time the wind intensification invigorates the upwelling of deep water, transporting warm, salty water southward and upward, with major implications for sequestration and outgassing of CO2. A robust decrease is projected for both the sea ice concentration and the seasonal cycling of ice volume, potentially altering the salt and heat budget at high latitudes.

Factors controlling present-day tufa dynamics in the Monasterio de Piedra Natural Park (Iberian Range, Spain): depositional environmental settings, sedimentation rates and hydrochemistry

The tufa record and hydrochemical characteristics of the River Piedra in the Monasterio de Piedra Natural Park (NE Spain) were studied for 6 years. The mean discharge of this river was 1.22 m3/s. The water was supersaturated with calcium carbonate. The HCO3 −, Ca2+ and TDIC concentrations decreased along the 0.5-km-long studied stretch, whereas the calcite SI showed no systematic downstream or seasonal variation over the same stretch. Several sedimentary subenvironments exist in which four broad types of tufa facies form: (1) Dense laminated tufa (stromatolites), (2) Dense to porous, massive tufa, (3) Porous, coarsely laminated tufa with bryophytes and algae, and (4) Dense, hard, laminated deposits in caves. The half-yearly period thickness and weight of sediment accumulated on 14 tablets installed in several subenvironments showed that the deposition rate was greater in fast flowing river areas and in stepped waterfalls, and lower in slow flowing or standing river areas and in spray and splash areas. Mechanical CO2 outgassing is the main factor controlling calcite precipitation on the river bed and in waterfalls, but this process does not explain the seasonal changes in depositional rates. The deposition rates showed a half-yearly period pattern recorded in all fluvial subenvironments persistent over time (5.26 mm, 0.86 g/cm2 in warm periods; 2.26 mm, 0.13 g/cm2 in cool periods). Mass balance calculations showed higher calcite mass values in warm (21.58 mg/L) than in cool (13.68 mg/L) periods. This biannual variation is mainly attributed to the seasonal differences in temperature that caused changes in inorganic calcite precipitation rate and in biomass and the correlative photosynthetic activity. Tufa sedimentation was therefore controlled by both physicochemical and biological processes. The results of this study may help test depositional rates and their environmental controls and thus assess the climatic and hydrological significance of ancient tufas in semi-arid conditions, in particular in the Quaternary.

Impact of enhanced vertical mixing on marine biogeochemistry: lessons for geo-engineering and natural variability

Abstract. Artificially enhanced vertical mixing has been suggested as a means by which to fertilize the biological pump with subsurface nutrients and thus increase the oceanic CO2 sink. We use an ocean general circulation and biogeochemistry model (OGCBM) to examine the impact of artificially enhanced vertical mixing on biological productivity and atmospheric CO2, as well as the climatically significant gases nitrous oxide (N2O) and dimethyl sulphide (DMS) during simulations between 2000 and 2020. Overall, we find a large increase in the amount of organic carbon exported from surface waters, but an overall increase in atmospheric CO2 concentrations by 2020. We quantified the individual effect of changes in dissolved inorganic carbon (DIC), alkalinity and biological production on the change in pCO2 at characteristic sites and found the increased vertical supply of carbon rich subsurface water to be primarily responsible for the enhanced CO2 outgassing, although increased alkalinity and, to a lesser degree, biological production can compensate in some regions. While ocean-atmosphere fluxes of DMS do increase slightly, which might reduce radiative forcing, the oceanic N2O source also expands. Our study has implications for understanding how natural variability in vertical mixing in different ocean regions (such as that observed recently in the Southern Ocean) can impact the ocean CO2 sink via changes in DIC, alkalinity and carbon export.

Changes in thermal and oxygen stratification pattern coupled to CO2 outgassing persistence in two oligotrophic shallow lakes of the Atlantic Tropical Forest, Southeast Brazil

Diel variations of temperature, O2 and CO2 profiles were measured in two oligotrophic shallow lakes situated next to a large preserved area of the Atlantic Tropical Forest (Brazil) in three sampling periods between the rainy season (spring and summer) and the dry winter of 2005. Our hypothesis was that lakes with high dissolved organic carbon (DOC) concentrations by terrestrial inputs might show the persistence of CO2 emissions to the atmosphere over the course of the year, despite changes in the water stratification pattern. In both lakes, temperature, O2 and CO2 showed a significant stratification in the beginning and end of the rainy season, and destratification in the dry winter. The beginning of the rainy season showed DOC concentrations and CO2 saturation that were significantly higher, but a persistence of CO2 emissions to the atmosphere was observed in all sampling periods. In conclusion, tropical shallow oligotrophic lakes might show events of thermal, O2 or CO2 stratification and destratification coupled to persistence of CO2 outgassing, possibly subsiding by terrestrial influence.

Carbonate metasomatism and CO 2 lithosphere–asthenosphere degassing beneath the Western Mediterranean: An integrated model arising from petrological and geophysical data

We present an integrated petrological, geochemical, and geophysical model that offers an explanation for the present-day anomalously high non-volcanic deep (mantle derived) CO 2 emission in the Tyrrhenian region. We investigate how decarbonation or melting of carbonate-rich lithologies from a subducted lithosphere may affect the efficiency of carbon release in the lithosphere–asthenosphere system. We propose that melting of sediments and/or continental crust of the subducted Adriatic–Ionian (African) lithosphere at pressure greater than 4 GPa (130 km) may represent an efficient mean for carbon cycling into the upper mantle and into the exosphere in the Western Mediterranean area. Melting of carbonated lithologies, induced by the progressive rise of mantle temperatures behind the eastward retreating Adriatic–Ionian subducting plate, generates low fractions of carbonate-rich (hydrous-silicate) melts. Due to their low density and viscosity, such melts can migrate upward through the mantle, forming a carbonated partially molten CO 2-rich mantle recorded by tomographic images in the depth range from 130 to 60 km. Upwelling in the mantle of carbonate-rich melts to depths less than 60–70 km, induces massive outgassing of CO 2. Buoyancy forces, probably favored by fluid overpressures, are able to allow migration of CO 2 from the mantle to the surface, through deep lithospheric faults, and its accumulation beneath the Moho and within the lower crust. The present model may also explain CO 2 enrichment of the Etna active volcano. Deep CO 2 cycling is tentatively quantified in terms of conservative carbon mantle flux in the investigated area.