Posts Tagged 'biogeochemistry'

Towards an intensified summer CO2 sink behaviour in the Southern Ocean coastal regions

Highlights

• We analysed the FCO2 and CO2 system in an important region of the Southern Ocean.

• The Gerlache Strait acts as a stronger CO2 sink than nearby open ocean areas during the austral summer.

• We identified both strong and near-equilibrium sink scenarios for FCO2.

• The pattern of variability of FCO2 has changed since 2012 to a higher frequency of years with a strong CO2 sink.

Abstract

The Southern Ocean is a globally important carbon sink region. However, the austral coastal zones are usually not considered in global estimations due to their general undersampling and large regional dynamics. Thus, estimations of carbon uptake in the Southern Ocean may differ considerably from current values, i.e., without accounting for coastal regions. Here, we conducted a case study in the Gerlache Strait, an ecologically important Antarctic coastal zone. We show that the net sea-air CO2 flux (FCO2) in the strait may reach the same or greater magnitudes than those in large open sea regions around Antarctica during summer, despite having a much smaller area. A large mean FCO2 of –31 ± 19 mmol m–2 d–1 was observed in the strong CO2 sink years (i.e., FCO2 < –12 mmol m–2 d–1), in contrast to –1 ± 7 mmol m–2 d–1 in CO2 near-equilibrium conditions (i.e., CO2 sea–air difference ≈ 0). This variability is mainly modulated by phytoplankton activity and likely upwelling processes. We also identified two cycles of variability with 2-year and 4-year periodicities from 1999 to 2017. The 2-year periodicity becomes stronger after 2012, intensifying the strong CO2 sink scenario in the Gerlache Strait. Our findings reinforce the importance of polar coastal zones as CO2 sinks during the austral summer and the need to broaden our understanding of the role of these regions at other time scales.

Continue reading ‘Towards an intensified summer CO2 sink behaviour in the Southern Ocean coastal regions’

Coastal acidification induced by biogeochemical processes driven by sea-ice melt in the western Arctic Ocean

To better understand the extent of acidification in the Arctic Ocean, we present pH measurements collected along a shelf-slope-basin transect from the Chukchi Sea shelf to the Chukchi Abyssal Plain (CAP) in the western Arctic Ocean during the summer 2010 Chinese Arctic National Research Expedition (CHINARE) cruise. We observed low pH values in the Chukchi Sea shelf bottom waters (∼30 m-bottom) and CAP upper haloline layer (UHL) (100-200 m). In the shelf bottom waters, the pH values were 7.66-8.13, about 0.07-0.68 pH units lower than the surface values of 8.20-8.24. In the CAP subsurface waters, the pH values were 7.85-7.98, about 0.08-0.31 pH units lower than the surface values of 8.20-8.24. Biogeochemical model simulations suggest that remineralized CO2 driven by sea-ice loss is primarily responsible for the low pH values in the bottom waters of the Chukchi Sea (shelf) and the UHL waters of the CAP (basin). Recent sea-ice melt enhanced organic matter production in surface waters and subsequent supported the increased microbial respiration of organic matter in bottom waters. Moreover, low pH bottom waters were flushed into the UHL during winter to sustain the low pH characteristics in the subsurface basin layers. In addition, our simplified model suggests that the thermodynamic effect of pH on is small. However, increasing temperature significantly increased aragonite saturation (Ωarag) which slowed down the speed of acidification.

Continue reading ‘Coastal acidification induced by biogeochemical processes driven by sea-ice melt in the western Arctic Ocean’

Hydrologic controls on CO2 chemistry and flux in subtropical lagoonal estuaries of the northwestern Gulf of Mexico

Estuaries are generally considered a source of CO2 to the atmosphere, although with significant uncertainties in magnitude and controlling factors between and within estuaries. We studied four northwestern Gulf of Mexico estuaries that experience extreme hydrologic conditions between April 2014 and February 2017 to determine the role of dry/wet cycle on estuarine CO2 system. Annual air–water CO2 flux ranged from 2.7 to 35.9 mol·C·m−2·yr−1; CO2 flux declined by approximately an order of magnitude along with declining river discharge. Episodic flooding made CO2 flux differ between dry (−0.7 to 20.9 mmol·C·m−2·d−1) and wet (11.6–170.0 mmol·C·m–2·d–1) conditions. During wet condition, increases in dissolved inorganic carbon (DIC) and total alkalinity (TA) significantly elevated CO2 degassing. Furthermore, ventilation of river‐borne CO2 strengthened degassing when estuaries became overwhelmingly river‐dominated. During flood relaxation, all estuaries experienced heightened productivity, evidenced by DIC and TA consumption in the mid‐salinity range (10–30). When prolonged drought led to hypersalinity (>36.5), biogeochemical and evaporative effects enhanced DIC and TA consumption and CO2 degassing. Due to flooding and high wind speeds, these estuaries were a strong CO2 source during spring and summer. Then they transitioned to a weak CO2 source or sink during the fall. Low temperatures further depressed CO2 efflux during winter except when a pulse of freshwater input occurred. This study demonstrates that changes in the hydrologic condition of estuaries, such as dry/wet cycle and river discharge gradient, will greatly alter air–water CO2 flux and estuarine contribution to the global carbon budget.

Continue reading ‘Hydrologic controls on CO2 chemistry and flux in subtropical lagoonal estuaries of the northwestern Gulf of Mexico’

Dense Mytilus beds along freshwater-influenced Greenland shores: resistance to corrosive waters under high food supply

Arctic calcifiers are believed to be particularly vulnerable to ocean acidification as the Arctic already experiences low carbonate saturations states due to low temperature and high inputs of freshwater. Here, we report the finding of dense beds of Mytilus growing in tidal lagoons and river mouths, where the availability of carbonate ions is remarkably low Ωarag < 0.5. Although these Mytilus grow in the intertidal zone, and therefore are covered by seawater during high tide, δ18O isotopes of shell carbonate were low − 2.48 ± 0.05‰, confirming that their shells were deposited under low salinity conditions, i.e., reflecting a contribution from 18O-depleted freshwater. δ18O isotopes of shell carbonate became heavier with increasing salinity, with mean values of − 0.74 ± 0.96‰ for Mytilus growing in tidal pools. We calculated, based on δ18O isotopic composition standardized to a common temperature, that freshwater accounted for 7% of the carbonate oxygen in the shells of Mytilus at the habitats with near full-strength seawater salinity compared with 25% in shells collected at sites temporarily exposed to freshwater. The composition of the periostracum revealed a trend for shells from river mouths and brackish tidal lagoons to be more depleted in polysaccharides than shells exposed to higher salinity. We conclude that the high food supply associated with riverine discharge allows Mytilus to cope with the low saturation states by using energy to calcify and modify their periostracum to protect the shells from dissolution. These findings suggest that Arctic Mytilus are highly resistant to low saturation states of carbon minerals if supplied with sufficient food.

Continue reading ‘Dense Mytilus beds along freshwater-influenced Greenland shores: resistance to corrosive waters under high food supply’

A meta-analysis of microcosm experiments shows that dimethyl sulfide (DMS) production in polar waters is insensitive to ocean acidification

Emissions of dimethylsulfide (DMS) from the polar oceans play a key role in atmospheric processes and climate. Therefore, it is important to increase our understanding of how DMS production in these regions may respond to climate change. The polar oceans are particularly vulnerable to ocean acidification (OA). However, our understanding of the polar DMS response is limited to two studies conducted in Arctic waters, where in both cases DMS concentrations decreased with increasing acidity. Here, we report on our findings from seven summertime shipboard microcosm experiments undertaken in a variety of locations in the Arctic Ocean and Southern Ocean. These experiments reveal no significant effects of short-term OA on the net production of DMS by planktonic communities. This is in contrast to similar experiments from temperate north-western European shelf waters where surface ocean communities responded to OA with significant increases in dissolved DMS concentrations. A meta-analysis of the findings from both temperate and polar waters (n=18 experiments) reveals clear regional differences in the DMS response to OA. Based on our findings, we hypothesize that the differences in DMS response between temperate and polar waters reflect the natural variability in carbonate chemistry to which the respective communities of each region may already be adapted. If so, future temperate oceans could be more sensitive to OA, resulting in an increase in DMS emissions to the atmosphere, whilst perhaps surprisingly DMS emissions from the polar oceans may remain relatively unchanged. By demonstrating that DMS emissions from geographically distinct regions may vary in their response to OA, our results may facilitate a better understanding of Earth’s future climate. Our study suggests that the way in which processes that generate DMS respond to OA may be regionally distinct, and this should be taken into account in predicting future DMS emissions and their influence on Earth’s climate.

Continue reading ‘A meta-analysis of microcosm experiments shows that dimethyl sulfide (DMS) production in polar waters is insensitive to ocean acidification’

The northern European shelf as increasing net sink for CO2

We developed a simple method to refine existing open ocean maps towards different coastal seas. Using a multi linear regression we produced monthly maps of surface ocean fCO2 in the northern European coastal seas (North Sea, Baltic Sea, Norwegian Coast and in the Barents Sea) covering a time period from 1998 to 2016. A comparison with gridded SOCAT v5 data revealed standard deviations of the residuals 0 ± 26 μatm in the North Sea, 0 ± 16 μatm along the Norwegian Coast, 0 ± 19 μatm in the Barents Sea, and 2 ± 42 μatm in the Baltic Sea.We used these maps as basis to investigate trends in fCO2, pH and air-sea CO2 flux. The surface ocean fCO2 trends are smaller than the atmospheric trend in most of the studied region. Only the western part of the North Sea is showing an increase in fCO2 close to 2 μatm yr−1, which is similar to the atmospheric trend. The Baltic Sea does not show a significant trend. Here, the variability was much larger than possibly observable trends. Consistently, the pH trends were smaller than expected for an increase of fCO2 in pace with the rise of atmospheric CO2 levels. The calculated air-sea CO2 fluxes revealed that most regions were net sinks for CO2. Only the southern North Sea and the Baltic Sea emitted CO2 to the atmosphere. Especially in the northern regions the sink strength increased during the studied period.

Continue reading ‘The northern European shelf as increasing net sink for CO2’

Submarine groundwater and vent discharge in a volcanic area associated with coastal acidification

We investigated submarine groundwater discharge (SGD) in a volcanic coastal area that hosts the world’s most biodiverse reefs. Measurements of 222Rn activity in coastal seawater, a tracer for groundwater, indicated prevalent SGD. In areas where seawater 222Rn activity was generally higher, we discovered hydrothermal springs emitting acidic waters (pH~5.4‐6.0) and venting magmatic CO2 which brought local pCO2 levels up to 95,000 ppm. The collection of vents raised CO2 and lowered pH over 1‐2 km of coastline. The hydrogen and oxygen isotope composition of water and chloride concentration revealed that the springs discharge recirculated seawater mixed variably with terrestrial groundwater. Shallower springs and porewater have a higher proportion of terrestrial groundwater than deeper springs, which emit mostly recirculated seawater. This suggests that different SGD mechanisms are present. The SGD could be contributing to the evolution and function of the biodiverse ecosystem but it also represents myriad pathways for contamination.

Continue reading ‘Submarine groundwater and vent discharge in a volcanic area associated with coastal acidification’


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Ocean acidification in the IPCC AR5 WG II

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