Posts Tagged 'biogeochemistry'

Ocean acidification and hypoxia alter organic carbon fluxes in marine soft sediments

Anthropogenic stressors can alter the structure and functioning of infaunal communities, which are key drivers of the carbon cycle in marine soft sediments. Nonetheless, the compounded effects of anthropogenic stressors on carbon fluxes in soft benthic systems remain largely unknown. Here, we investigated the cumulative effects of ocean acidification (OA) and hypoxia on the organic carbon fate in marine sediments, through a mesocosm experiment. Isotopically labelled macroalgal detritus (13C) was used as a tracer to assess carbon incorporation in faunal tissue and in sediments under different experimental conditions. In addition, labelled macroalgae (13C), previously exposed to elevated CO2, were also used to assess the organic carbon uptake by fauna and sediments, when both sources and consumers were exposed to elevated CO2. At elevated CO2, infauna increased the uptake of carbon, likely as compensatory response to the higher energetic costs faced under adverse environmental conditions. By contrast, there was no increase in carbon uptake by fauna exposed to both stressors in combination, indicating that even a short‐term hypoxic event may weaken the ability of marine invertebrates to withstand elevated CO2 conditions. In addition, both hypoxia and elevated CO2 increased organic carbon burial in the sediment, potentially affecting sediment biogeochemical processes. Since hypoxia and OA are predicted to increase in the face of climate change, our results suggest that local reduction of hypoxic events may mitigate the impacts of global climate change on marine soft‐sediment systems.

Continue reading ‘Ocean acidification and hypoxia alter organic carbon fluxes in marine soft sediments’

Trends of ocean acidification and pCO2 in the northern North Sea, 2003‐2015

For continental shelf regions, the long‐term trend in sea surface carbon dioxide (CO2) partial pressure (pCO2) and rates of ocean acidification are not accurately known. Here, we investigate the decadal trend of observed wintertime pCO2 as well as computed wintertime pH and aragonite saturation state (Ωar) in the northern North Sea, using the first decade long monthly underway data from a Voluntary Observing Ship (VOS) covering the period 2004‐2015. We also evaluate how seawater CO2 chemistry, in response to physical and biological processes, drives variations in the above parameters on seasonal and interannual timescales. In the northern North Sea, pCO2, pH, and Ωar are subject to strong seasonal variations with mean wintertime values of 375±11 μatm, 8.17±0.01, and 1.96±0.05. Dissolved Inorganic Carbon (DIC) is found to be the primary driver of both seasonal and interannual changes while total alkalinity (TA) and sea surface temperature (SST) have secondary effects that reduce the changes produced by DIC. Average interannual variations during winter are around 3%, 0.1% and 2% for pCO2, pH, and Ωar, respectively, and slightly larger in the eastern part of the study area (Skagerrak region) than in the western part (North Atlantic Water (NAW) region). Statistically significant long‐term trends were found only in the NAW region with mean annual rates of 2.39±0.58 μatm yr‐1, ‐0.0024±0.001 yr‐1, and ‐0.010±0.003 yr‐1for pCO2, pH and Ωar, respectively. The drivers of the observed trends as well as reasons for the lack of statistically significant trends in the Skagerrak region are discussed.

Continue reading ‘Trends of ocean acidification and pCO2 in the northern North Sea, 2003‐2015’

Biogeochemical anomalies at two southern California Current System moorings during the 2014‐16 Warm Anomaly‐El Niño sequence

We analyzed impacts of the 2014‐15 Pacific Warm Anomaly and 2015‐16 El Niño on physical and biogeochemical variables at two southern California Current System moorings (CCE2, nearshore upwelling off Point Conception; CCE1, offshore California Current). Nitrate and Chl‐a fluorescence were < 1 μM and < 1 Standardized Fluorescence Unit, respectively, at CCE2 for the entire durations of the Warm Anomaly and El Niño, the two longest periods of such low values in our timeseries. Negative nitrate and Chl‐a anomalies at CCE2 were interrupted briefly by upwelling conditions in spring 2015. Near‐surface temperature anomalies appeared simultaneously at both moorings in spring 2014, indicating region‐wide onset of Warm Anomaly temperatures, although sustained negative nitrate and Chl‐a anomalies only occurred offshore at CCE1 during El Niño (summer 2015‐spring 2016). Warm Anomaly temperature changes were expressed more strongly in near‐surface (< 40 m) than subsurface (75 m) waters at both moorings, while El Niño produced comparable temperature anomalies at near‐surface and subsurface depths. Nearshore Ωaragonite at 76 m showed notably fewer undersaturation events during both warm periods, suggesting an environment more conducive to calcifying organisms. Planktonic calcifying molluscs (pteropods and heteropods) increased markedly in springs 2014 and 2016 and remained modestly elevated in spring 2015. Moorings provide high‐frequency measurements essential for resolving the onset timing of anomalous conditions and frequency and duration of short‐term (days‐to‐weeks) perturbations (reduced nitrate, aragonite undersaturation events) that can affect marine organisms.

Continue reading ‘Biogeochemical anomalies at two southern California Current System moorings during the 2014‐16 Warm Anomaly‐El Niño sequence’

Carbonate characteristics of the Gulf of Anadyr waters

The first field data describing the dynamics of the carbonate system, the aragonite saturation state, and CO2 fluxes between the ocean and the atmosphere in the Gulf of Anadyr in the late autumn season are presented. It was established that during this period the gulf waters absorbed carbon dioxide from the atmosphere at a rate of –22.5 mmol m–2 day–1, which determined the “classical” mechanism of seawater acidification due to uptake of excess atmospheric CO2. In general, surface waters of the gulf were supersaturated with respect to aragonite. The exception was the highly dynamic region of Anadyr Strait, where the vertical distribution of the investigated parameters was homogeneous, the surface waters were close to equilibrium with respect to aragonite, and the CO2 flux was directed to the atmosphere. The bottom waters of the gulf, in contrast, were characterized by significant seasonal corrosivity due to remineralization of organic matter. It was shown that, during the late fall, relatively salty and acidic, quasi-equilibrium with respect to aragonite, and oxygen-depleted waters with high concentrations of nutrients and CO2 enter into the Chirikov Basin and further to the Arctic Ocean with Navarin Current.

Continue reading ‘Carbonate characteristics of the Gulf of Anadyr waters’

The role of river runoff in the Kara Sea surface layer acidification and carbonate system changes

This study aims to perform the results of the investigation of the Kara Sea carbonate system changes and the factors that determine it. The important feature of the Kara Sea water structure is strong stratification caused mainly by the Ob’ and Yenisey rivers discharge which is estimated as 81% of the total continental runoff to the sea. Occurring climate changes, as an increase in the total volume of the Arctic Ocean water (due to melting of glaciers, sea ice decline and river runoff increase), air temperature and CO2 concentration growth should affect greatly the Kara Sea carbonate system. However, riverine water influence seems to be the main driver of future acidification of the Kara Sea water due to permafrost thawing as it stores a great amount of buried carbon. An increase of carbon (mainly inorganic) flow to the sea will lead to carbonate equilibrium shift, oxidation of organic matter and release of CO2 that ultimately leads to a decrease in pH and therefore acidification. The area of the riverine plume depends on the amount of freshwater flowing into the sea and the conditions of the wind forcing. According to the data from Shirshov Institute cruises within the plume area aragonite saturation is below 1 that shows its state as acidified. Prevalence of pCO2 values in the freshened surface layer over the atmospheric shows that atmospheric carbon dioxide, apparently, cannot serve as the main driver for the acidification of the surface waters of the Kara Sea. At the shallow shelf to the north of the Ob’ Inlet mouth we observe acidification of the whole water column from surface to the bottom layer due to elevated riverine discharge and increase of flowing terrestrial carbon.

Continue reading ‘The role of river runoff in the Kara Sea surface layer acidification and carbonate system changes’

Net community production and carbon exchange from winter to summer in the Atlantic Water inflow to the Arctic Ocean

The eastern Fram Strait and area north of Svalbard, are influenced by the inflow of warm Atlantic water, which is high in nutrients and CO2, influencing the carbon flux into the Arctic Ocean. However, these estimates are mainly based on summer data and there is still doubt on the size of the net ocean Arctic CO2 sink. We use data on carbonate chemistry and nutrients from three cruises in 2014 in the CarbonBridge project (January, May, and August) and one in Fram Strait (August). We describe the seasonal variability and the major drivers explaining the inorganic carbon change (CDIC) in the upper 50 m, such as photosynthesis (CBIO), and air-sea CO2 exchange (CEXCH). Remotely sensed data describes the evolution of the bloom and net community production. The focus area encompasses the meltwater-influenced domain (MWD) along the ice edge, the Atlantic water inflow (AWD), and the West Spitsbergen shelf (SD). The CBIO total was 2.2 mol C m–2 in the MWD derived from the nitrate consumption between January and May. Between January and August, the CBIO was 3.0 mol C m–2 in the AWD, thus CBIO between May and August was 0.8 mol C m–2. The ocean in our study area mainly acted as a CO2 sink throughout the period. The mean CO2 sink varied between 0.1 and 2.1 mol C m–2 in the AWD in August. By the end of August, the AWD acted as a CO2 source of 0.7 mol C m–2, attributed to vertical mixing of CO2-rich waters and contribution from respiratory CO2 as net community production declined. The oceanic CO2 uptake (CEXCH) from the atmosphere had an impact on CDIC between 5 and 36%, which is of similar magnitude as the impact of the calcium carbonate (CaCO3, CCALC) dissolution of 6–18%. CCALC was attributed to be caused by a combination of the sea-ice ikaite dissolution and dissolution of advected CaCO3 shells from the south. Indications of denitrification were observed, associated with sea-ice meltwater and bottom shelf processes. CBIO played a major role (48–89%) for the impact on CDIC.

Continue reading ‘Net community production and carbon exchange from winter to summer in the Atlantic Water inflow to the Arctic Ocean’

Geochemical characterization of highly diverse hydrothermal fluids from volcanic vent systems of the Kermadec intraoceanic arc

During the R/V Sonne cruise SO253 in 2016/2017, hydrothermal vent sites along the Kermadec intraoceanic arc were sampled for hydrothermal fluids at four active volcanoes: Macauley, Haungaroa, Brothers and Rumble III, respectively. Water depths ranged between 290 m and 1700 m. A new vent field was discovered at Haungaroa. The samples were taken from diffuse-flow sites as well as from white and black smokers – rich in metals and gases – with discharge temperatures as high as 311 °C. Their fluid composition is very variable but basically divides into two types: one that indicates distinct magmatic input and another that shows evidence for intense water-rock interaction under hot, acidic conditions.

Fluid samples from Macauley, the shallowest sampling site (~300 m), had Fe concentrations as high as 1.7 mM, Al concentrations up to 122 μM and H2S up to 10 mM at a pH of only 1.2. At Brothers, the deepest sampling site (down to 1600 m), we identified two different fluid types: 1) A magmatically-influenced type at the Upper and Lower Cone with highest temperatures of 115 °C, up to 95.6 mM Mg (the highest Mg concentration measured in fluids from intraoceanic arc systems so far), elevated SO42− (76.9 mM), high H2S (5.0 mM), but Fe concentrations of only 15 μM and 2) A fluid with low Mg (5.4 mM), low H2S (1.1 mM), temperatures reaching 311 °C and high Fe contents (12.4 mM) at the Upper Caldera and NW Caldera Wall, typical of a black smoker fluid. Chloride concentrations in all fluids were similar, or highly enriched when compared to seawater (e.g. up to 787 mM, brine fluids), with also one low-chlorinity vapor-phase fluid sample recovered, indicating that phase separation is occurring at Brothers. Unusual highly elevated Mg concentrations in fluids from the Brothers Lower Cone (95.6 mM, compared to 53.2 mM in ambient seawater) combined with highly elevated concentrations of SO42− (76.9 mM, compared to 29.0 mM in ambient seawater) indicate dissolution of Mg- and SO42−-bearing minerals in the subsurface, such as caminite.

Our data show how highly diverse and variable island arc systems can be with respect to their fluid chemistry, both spatially and temporally. It adds to the still limited data set of arc systems compared to mid-ocean ridges and supplies an important contribution towards a better understanding of geochemical processes along arc volcanoes.

The higher range in fluid chemistry together with shallower water depth implies that the fluids from intraoceanic arcs may contribute a significant fraction of dissolved metals not only to the global oceanic biogeochemical cycle but also into the photic zone, the area of highest bioproductivity.

Continue reading ‘Geochemical characterization of highly diverse hydrothermal fluids from volcanic vent systems of the Kermadec intraoceanic arc’

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Ocean acidification in the IPCC AR5 WG II

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