Posts Tagged 'chemistry'

The influence of CO2 seeps to coastal environments of Shikine Island in Japan as indicated by geochemistry analyses of seafloor sediments

Recently, two shallow CO2 seeps were described in Ashitsuki and Mikama Bay (Shikine Island, Japan). These sites were deemed to have potentials for studying the impacts of ocean acidification. Here, we report geochemistry analyses of seawater and seafloor sediments collected from the shallow coasts on and around the two CO2 seeps. Seawater analyses indicated that shallow waters in the area share similar acidic characteristics (e.g. Avg. pH = ca. 7.1), supporting the result of a previous study. Next, the sediments from all sampling loci also share similar properties (Avg. Fe: Si = 0.043; Avg. organic content = 1.26%; Avg. relative Si content = 75.25%). However, sediments from Matsugashitamiyabi hot spring, which is located near the Ashitsuki seep, showed high Fe: Si ratio (1.250) when compared to other loci. This is most likely a local phenomenon, where iron accumulates in the sediment by the precipitation of rust produced through the mixing of FeS from the hot spring and carbonated seawater of the nearby CO2 seeps. We also compared seawater (e.g. Avg. pH = 8.3) and sediments (Avg. Fe: Si = 0.126; Avg. organic content = 2.06%; Avg. Si = 69.06%) of Hidaka Port in central Wakayama (as a standard sample of coastal surface water environment), to the Shikine Island samples excluding the Matsugashitamiyabi hot spring samples. The differences in characteristics (i.e. lower seawater pH and lower Avg. Fe: Si ratio of the latter) were probably caused by CO2 seep influence, and indicate that the influence of the hot spring water to the sediment of both CO2 seeps was minimal, or probably none. Accordingly, these seep sites are useful for future studies on the effects of ocean acidification on sea floor sediment composition, and its implication to biodiversity and the ecosystem.

Continue reading ‘The influence of CO2 seeps to coastal environments of Shikine Island in Japan as indicated by geochemistry analyses of seafloor sediments’

Variations in the summer oceanic pCO2 and carbon sink in Prydz Bay using the self-organizing map analysis approach

This study applies a neural network technique to produce maps of oceanic surface pCO2 in Prydz Bay in the Southern Ocean on a weekly 0.1∘ longitude × 0.1∘ latitude grid based on in situ measurements obtained during the 31st CHINARE cruise from February to early March 2015. This study area was divided into three regions, namely, the “open-ocean” region, “sea-ice” region and “shelf” region. The distribution of oceanic pCO2 was mainly affected by physical processes in the open-ocean region, where mixing and upwelling were the main controls. In the sea-ice region, oceanic pCO2 changed sharply due to the strong change in seasonal ice. In the shelf region, biological factors were the main control. The weekly oceanic pCO2 was estimated using a self-organizing map (SOM) with four proxy parameters (sea surface temperature, chlorophyll a concentration, mixed Layer Depth and sea surface salinity) to overcome the complex relationship between the biogeochemical and physical conditions in the Prydz Bay region. The reconstructed oceanic pCO2 data coincide well with the in situ pCO2 data from SOCAT, with a root mean square error of 22.14 µatm. Prydz Bay was mainly a strong CO2 sink in February 2015, with a monthly averaged uptake of 23.57±6.36 TgC. The oceanic CO2 sink is pronounced in the shelf region due to its low oceanic pCO2 values and peak biological production.

Continue reading ‘Variations in the summer oceanic pCO2 and carbon sink in Prydz Bay using the self-organizing map analysis approach’

Fates of vent CO2 and its impact on carbonate chemistry in the shallow-water hydrothermal field offshore Kueishantao Islet, NE Taiwan

Highlights

• CO2 dissolution and fluid entrainment shape carbonate chemistry in vertical plumes.
• Fluids in the near-vent area have a short flushing time (tens of minutes).
• Mixing of vent fluids with seawater acts to retain vent carbon in ocean.

Abstract

Increasing public awareness of anthropogenic CO2 emissions and consequent global change has stimulated the development of pragmatic approaches for the study of shallow-water CO2 vents and seeps as natural laboratories of CO2 perturbations. How CO2 propagates from the emission sites into surrounding environments (ocean and atmosphere), and its effects on seawater carbonate chemistry, have never been studied from a mechanistic perspective. Here, we combine experimental and modeling approaches to investigate the carbonate chemistry of a shallow-water hydrothermal field offshore Kueishantao Islet, NE Taiwan. A simple Si-based mixing model is used to trace hydrothermal fluid mixing with seawater along convection pathways. The estimated vent fluid component in the near-vent region is generally <1%. We further employed a modified bubble-plume model to examine gas bubble-aqueous phase interaction. We explain the dissolved inorganic carbon characteristics of the vertical plume as a synergistic interaction between CO2 gas dissolution and fluid entrainment. The bubble-plume model provides a conservative estimate of the flushing time (tens of minutes) for water in the near-vent region. The acidic, dissolved inorganic carbon-rich water in the lateral buoyant plume readily releases CO2, but mixing with seawater rapidly quenches its degassing potential, so that hydrothermal carbon is retained in the ocean. Ebullition, governed by initial bubble size distribution, is the key mechanism for vent CO2 to exit the seawater carbonate system.

Continue reading ‘Fates of vent CO2 and its impact on carbonate chemistry in the shallow-water hydrothermal field offshore Kueishantao Islet, NE Taiwan’

The sea-air CO2 net fluxes in the South Atlantic Ocean and the role played by Agulhas eddies

Highlights

• A mean FCO2 of −3.76 mmol m−2 d−1 was obtained in the FORSA cruise track.
• An Agulhas eddy can uptake up to −3.16 kg CO2 d−1, leading to −2.5 t CO2 lifetime−1.
• The seawater temperature is the main driver of the CO2 variability in the SAO.

Abstract

The South Atlantic Ocean is vitally important to the global overturning circulation, which is influenced by heat, salt and other properties carried by Agulhas eddies. However, this influence is not yet fully understood, mainly in the context of the biogeochemistry changes on the CO2 system. This study uses in situ data obtained during the Following Ocean Rings in the South Atlantic cruise, which occurred between Cape Town, South Africa and Arraial do Cabo, Brazil in July 2015 when six eddies and the surrounding waters were sampled. The seawater and atmospheric CO2 molar fraction, surface temperature and salinity were continuously measured to calculate the oceanic and atmospheric CO2 partial pressures (pCO2sw and pCO2atm, respectively). This study investigated the role played by the Agulhas eddies in the sea-air CO2 net flux (FCO2) and modeled the seawater CO2 as a function of environmental parameters. The mean pCO2sw and pCO2atm for the entire region were 351.5 and 390.6 μatm, respectively. The mean difference (ΔpCO2) was −39.1 μatm. The CO2 uptake was dominated by temperature (r = 0.88) during the period analyzed. The mean FCO2 was −3.76 and −3.62 mmol m−2 d−1 using two different KT-models. We show that an Agulhas eddy can contribute to an ocean uptake of −3.16 kg CO2 d−1, leading to the capture of approximately 2.52 t CO2 lifetime−1. Thus, providing evidence that the Agulhas eddies propagation can likely play a key role on the rapid seawater acidification of the South Atlantic Central Water. A multiple linear regression model was developed that could reliably reconstruct the cruise survey with better results than previously published.

Continue reading ‘The sea-air CO2 net fluxes in the South Atlantic Ocean and the role played by Agulhas eddies’

Environmental and physiochemical controls on coral calcification along a latitudinal temperature gradient in Western Australia

The processes that occur at the micro‐scale site of calcification are fundamental to understanding the response of coral growth in a changing world. However, our mechanistic understanding of chemical processes driving calcification is still evolving. Here, we report the results of a long‐term in situ study of coral calcification rates, photo‐physiology, and calcifying fluid (cf) carbonate chemistry (using boron isotopes, elemental systematics, and Raman spectroscopy) for seven species (four genera) of symbiotic corals growing in their natural environments at tropical, subtropical, and temperate locations in Western Australia (latitudinal range of ~11°). We find that changes in net coral calcification rates are primarily driven by pHcf and carbonate ion concentration []cf in conjunction with temperature and DICcf. Coral pHcf varies with latitudinal and seasonal changes in temperature and works together with the seasonally varying DICcf to optimize []cf at species‐dependent levels. Our results indicate that corals shift their pHcf to adapt and/or acclimatize to their localized thermal regimes. This biological response is likely to have critical implications for predicting the future of coral reefs under CO2‐driven warming and acidification.

Continue reading ‘Environmental and physiochemical controls on coral calcification along a latitudinal temperature gradient in Western Australia’

Benthic alkalinity and DIC fluxes in the Rhône River prodelta generated by decoupled aerobic and anaerobic processes

Estuarine regions are generally considered a net source of atmospheric CO2 as a result of the high organic carbon (OC) mineralization rates in the water column and their sediments. Yet, the intensity of anaerobic respiration processes in the sediments tempered by the reoxidation of reduced metabolites controls the net production of alkalinity from sediments that may partially buffer the metabolic CO2 generated by OC respiration. In this study, a benthic chamber was deployed in the Rhône River prodelta and the adjacent continental shelf (Gulf of Lions, NW Mediterranean) to assess the fluxes of total alkalinity (TA) and dissolved inorganic carbon (DIC) from the sediment. Concurrently, in situ O2 and pH microprofiles, electrochemical profiles, pore water and solid composition were measured in surface sediments to identify the main biogeochemical processes controlling the net production of alkalinity in these sediments. The benthic fluxes of TA and DIC, ranging between 14 and 74mmolm−2d−1 and 18 and 78mmolm−2d−1, respectively, were up to 8 times higher than the DOU fluxes (10.4±0.9mmolm−2d−1) close to the river mouth, but their intensity decreased offshore, as a result of the decline in OC inputs. Low nitrate concentrations and strong pore water sulfate gradients indicated that the majority of the TA and DIC was produced by sulfate and iron reduction. Despite the complete removal of sulfate from the pore waters, dissolved sulfide concentrations were low due to the precipitation and burial of iron sulfide minerals (12.5mmolm−2d−1 near the river mouth), while soluble organic-Fe(III) complexes were concurrently found throughout the sediment column. The presence of organic-Fe(III) complexes together with low sulfide concentrations and high sulfate consumption suggests a dynamic system driven by the variability of the organic and inorganic particulate input originating from the river. By preventing reduced substances from being reoxidized, the precipitation and burial of iron sulfide decouples the iron and sulfur cycles from oxygen, therefore allowing a flux of alkalinity out of the sediments. In these conditions, the sediment provides a source of alkalinity to the bottom waters which mitigates the effect of the benthic DIC flux on the carbonate chemistry of coastal waters.

Continue reading ‘Benthic alkalinity and DIC fluxes in the Rhône River prodelta generated by decoupled aerobic and anaerobic processes’

Evaluation of autonomously measured alkalinity, pH, and pCO2 variability on a coral reef

Currently, our understanding of alkalinity (AT) variability in highly dynamic
environments such as coral reefs is limited by the dearth of AT measurements. In order to better characterize these environments, high temporal resolution AT data are needed. This work employed the newly developed Submersible Autonomous Moored Instrument for Alkalinity (SAMI-alk), a fully autonomous in situ AT analyzer, to study seawater AT variability. The main goals of this research were to evaluate the utility of combining the SAMI-alk data with currently available in situ measurements of pH and partial pressure of carbon dioxide (pCO2) to characterize the inorganic carbon cycle, and to measure AT variability and determine what drives it on a coral reef. Autonomous AT and pH sensors (SAMI-alk and SAMI-pH) were deployed along with existing pCO2 (MAPCO2) and pH (SeaFET) sensors in Kanoehe Bay, HI from June 4 – 21, 2013. The results show that the pH – AT combination can provide important information about autonomously measured in situ data quality, and that it can be used to fully characterize the inorganic CO2 system in seawater. The SAMI-alk data were also used to examine AT variability and thereby calcification rates on coral reefs in Kaneohe Bay. AT varied by more than 100 µmol kg-1 on a diel basis due to CaCO3 production and dissolution. Dissolved inorganic carbon (DIC), calculated from the pH – AT sensor pair, varied by more than 200 µmol kg-1 , due primarily to biological metabolism on the reef. Reef calcification and metabolism dramatically alter the seawater chemistry from the open ocean source water and drive the large diel changes in all measured inorganic carbon parameters (i.e. aragonite saturation state (Ωarag), pH, pCO2, AT, DIC). This data set demonstrates the value of a high-quality in situ AT analyzer in a coral reef environment; making it possible to determine combined CO2 system variability with unprecedented temporal resolution. These data show that NEC can be consistently sustained (net CaCO3 production) until a threshold level of net respiration (NEP) is reached, around -50 (mmol m-2 h-1), which corresponds to an AT : DIC ratio of about 1:1.

Continue reading ‘Evaluation of autonomously measured alkalinity, pH, and pCO2 variability on a coral reef’


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Ocean acidification in the IPCC AR5 WG II

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