Posts Tagged 'chemistry'

Porewater carbonate chemistry dynamics in a temperate and a subtropical seagrass system

Seagrass systems are integral components of both local and global carbon cycles and can substantially modify seawater biogeochemistry, which has ecological ramifications. However, the influence of seagrass on porewater biogeochemistry has not been fully described, and the exact role of this marine macrophyte and associated microbial communities in the modification of porewater chemistry remains equivocal. In the present study, carbonate chemistry in the water column and porewater was investigated over diel timescales in contrasting, tidally influenced seagrass systems in Southern California and Bermuda, including vegetated (Zostera marina) and unvegetated biomes (0–16 cm) in Mission Bay, San Diego, USA and a vegetated system (Thallasia testudinium) in Mangrove Bay, Ferry Reach, Bermuda. In Mission Bay, dissolved inorganic carbon (DIC) and total alkalinity (TA) exhibited strong increasing gradients with sediment depth. Vertical porewater profiles differed between the sites, with almost twice as high concentrations of DIC and TA observed in the vegetated compared to the unvegetated sediments. In Mangrove Bay, both the range and vertical profiles of porewater carbonate parameters such as DIC and TA were much lower and, in contrast to Mission Bay where no distinct temporal signal was observed, biogeochemical parameters followed the semi-diurnal tidal signal in the water column. The observed differences between the study sites most likely reflect a differential influence of biological (biomass, detritus and infauna) and physical processes (e.g., sediment permeability, residence time and mixing) on porewater carbonate chemistry in the different settings.

Continue reading ‘Porewater carbonate chemistry dynamics in a temperate and a subtropical seagrass system’

Pacific-wide pH snapshots reveal that high coral cover correlates with low, but variable pH

Ocean acidification (OA) is impairing the construction of coral reefs while simultaneously accelerating their breakdown. The metabolism of different reef organism assemblages alters seawater pH in different ways, possibly buffering or exacerbating OA impacts. In spite of this, field data relating benthic community structure and seawater pH are sparse. We collected pH time-series data snapshots at 10 m depth from 28 different reefs (n = 13 lagoon, n = 15 fore reef) across 22 Pacific islands, spanning 31° latitude and 90° longitude. Coincident with all deployments, we measured percent cover of the benthic community. On fore reefs, high coral cover (CC) negatively correlated with mean and minimum pH, but positively correlated with pH variability. Conversely, pH minima were positively correlated to coverage of coralline and turf algae. Benthic cover did not correlate with pH in lagoonal reefs. From 0%–100% CC, mean pH and aragonite saturation state (Ωarag ) declined −0.081 and −0.51, respectively, while declines in minimum values were greater (Δmin pH = −0.164, Δmin Ωarag = −0.96). Based upon previously published relationships, the mean pH decline from 0%–100% CC would depress coral calcification 7.7%–18.0% and increase biologically-mediated dissolution 13.5%–27.9%, with pH minima depressing dark coral calcification 14.4%–35.2% and increasing biologically-mediated dissolution 31.0%–62.2%. This spatially expansive dataset provides evidence that coral reefs with the highest coral cover may experience the lowest and most extreme pH values with OA.

Continue reading ‘Pacific-wide pH snapshots reveal that high coral cover correlates with low, but variable pH’

Seasonal variability of the CO2 system in a large coastal plain estuary

The Chesapeake Bay, a large coastal plain estuary, has been studied extensively in terms of its water quality, and yet, comparatively less is known about its carbonate system. Here we present discrete observations of dissolved inorganic carbon (DIC) and total alkalinity from four seasonal cruises in 2016–2017. These new observations are used to characterize the regional CO2 system and to construct a DIC budget of the mainstem. In all seasons, elevated DIC concentrations were observed at the mouth of the bay associated with inflowing Atlantic Ocean waters, while minimum concentrations of DIC were associated with fresher waters at the head of the bay. Significant spatial variability of the partial pressure of CO2 was observed throughout the mainstem, with net uptake of atmospheric CO2 during each season in the upper mainstem and weak seasonal outgassing of CO2 near the outflow to the Atlantic Ocean. During the time frame of this study, the Chesapeake Bay mainstem was (1) net autotrophic in the mixed layer (net community production of 0.31‐mol C m−2 ·year−1 ) and net heterotrophic throughout the water column (net community production of −0.48‐mol C m−2 ·year−1), (2) a sink of 0.38‐mol C m−2 ·year−1 for atmospheric CO2, and (3) significantly seasonally and spatially variable with respect to biologically driven changes in DIC.

Continue reading ‘Seasonal variability of the CO2 system in a large coastal plain estuary’

Towards an understanding of the Ca isotopic signal related to ocean acidification and alkalinity overshoots in the rock record

In this contribution, we explore the idea that the Ca isotope proxy has utility as an indicator of carbonate authigenesis (i.e., post-depositional precipitation of CaCO3 within the sedimentary package). Given the strong contrast in isotopic fractionation factor between the formational and diagenetic environments, Ca isotopes have the potential to fingerprint carbonate authigenesis when it occurs close to the seawater-sediment interface. We demonstrate that Ca isotopes are particularly applicable to exploring ocean acidification events, and potentially ocean anoxic events, and focus our attention on ocean acidification related to the Paleocene-Eocene Thermal Maximum (PETM). We present three scenarios that vary in magnitude and duration of carbon fluxes simulated using an Earth System model of intermediate complexity (cGENIE) and use the cGENIE output to constrain the upper boundary conditions of 1-D reactive transport models of authigenesis and recrystallization in the sedimentary section. Along with simple mixing calculations, the models inform our exploration of the hypothesis that authigenic carbonate induced by a saturation state overshoot during the PETM explains recently published Ca isotope records, and perhaps bulk carbonate records over Ocean Anoxic Event (OAE) 2. Our simulations suggest that fractionation factor variability does not explain the PETM δ44Ca records, and we propose a δ44Ca-CaCO3 space framework to assist with the elucidation of authigenic additions over time scales that are short relative to the residence time of Ca in the ocean (~1 Ma). Ultimately, we find that the ‘authigenic zone’ generated in the sedimentary column may be influenced by alkalinity overshoots or redox state; the CaCO3 produced in this zone can overprint temporal signals with depth-dependent signals that reflect lithology and sedimentation rate and need not be spatially uniform, even when driven by a global event. Ultimately, we demonstrate the utility of Ca isotopes for exploring short time scale climatic events and a quantitative framework to guide interpretations.

Continue reading ‘Towards an understanding of the Ca isotopic signal related to ocean acidification and alkalinity overshoots in the rock record’

Mid-infrared sensor system based on tunable laser absorption spectroscopy for dissolved carbon dioxide analysis in the South China Sea: system-level integration and deployment

System-level integration of a mid-infrared carbon dioxide (CO2) sensor system based on tunable laser absorption spectroscopy (TLAS) was realized for the analysis of dissolved CO2 in seawater employing an interband cascade laser (ICL) centered at 4319 nm and a multi-pass cell (MPC) with an optical path length of 29.8 m. At low measurement pressure of 30 Torr, three absorption lines of 12CO2 were selected to realize different measurement ranges, and a 13CO2 absorption line was targeted for simultaneous isotopic abundance analysis of δ13CO2. The sensor system was compactly integrated into a standalone system with automatic operation for underwater field deployment, and the working process was controlled by a specially-designed electrical system. A gas-liquid separator system was developed for CO2 extraction from water, and a pressure-control mechanism with two operation modes (i.e. static and dynamic mode) was proposed to make the sensor system applicable under deep-sea environment. Series of experiments were carried out in laboratory for performance assessment of the developed sensor system employed for the analysis of dissolved CO2 in water. The sensor was deployed for a field test for natural gas hydrates exploration at an underwater depth of 0−2000 m in the South China Sea, with the sensor operating normally during the deployment.

Continue reading ‘Mid-infrared sensor system based on tunable laser absorption spectroscopy for dissolved carbon dioxide analysis in the South China Sea: system-level integration and deployment’

Variability and stability of anthropogenic CO2 in Antarctic Bottom Waters observed in the Indian sector of the Southern Ocean, 1978-2018

Antarctic bottom waters (AABWs) are known as a long term sink for anthropogenic CO2 (Cant) but is hardly quantified because of the scarcity of the observations, specifically at an interannual scale. We present in this manuscript an original dataset combining 40 years of carbonate system observations in the Indian sector of the Southern Ocean (Enderby Basin) to evaluate and interpret the interannual variability of Cant in the AABW. This investigation is based on regular observations collected at the same location (63° E/56.5° S) in the frame of the French observatory OISO from 1998 to 2018 extended by GEOSECS and INDIGO observations (1978, 1985 and 1987).

At this location the main sources of AABW sampled is the fresh and younger Cape Darnley bottom water (CDBW) and the Weddell Sea deep water (WSDW). Our calculations reveal that Cant concentrations increased significantly in AABW, from about +7 µ in 1978-1987 to +13 µ 18 in 2010-2018. This is comparable to previous estimates in other SO basins, with the exception of bottom waters close to their formation sites where Cant concentrations are about twice as large. Our analysis shows that the CT and Cant increasing rates in AABW are about the same over the period 1978-2018, and we conclude that the long-term change in CT is mainly due to the uptake of anthropogenic CO2 in the different formation regions. This is however modulated by significant interannual to pluriannual variability associated with variations in hydrological (ϴ, S) and biogeochemical (CT, AT, O2) properties. A surprising result is the apparent stability of Cant concentrations in recent years despite the increase in CT and the gradual acceleration of atmospheric CO2.

The Cant sequestration by AABWs is more variable than expected and depends on a complex combination of physical, chemical and biological processes at the formation sites and during the transit of the different AABWs. The interannual variability at play in AABW needs to be carefully considered on the extrapolated estimation of Cant sequestration based on sparse observations over several years.

Continue reading ‘Variability and stability of anthropogenic CO2 in Antarctic Bottom Waters observed in the Indian sector of the Southern Ocean, 1978-2018’

Characterizing biogeochemical fluctuations in a world of extremes: A synthesis for temperate intertidal habitats in the face of global change

Coastal and intertidal habitats are at the forefront of anthropogenic influence and environmental change. The species occupying these habitats are adapted to a world of extremes, which may render them robust to the changing climate or more vulnerable if they are at their physiological limits. We characterized the diurnal, seasonal and interannual patterns of flux in biogeochemistry across an intertidal gradient on a temperate sandstone platform in eastern Australia over 6 years (2009–2015) and present a synthesis of our current understanding of this habitat in context with global change. We used rock pools as natural mesocosms to determine biogeochemistry dynamics and patterns of eco‐stress experienced by resident biota. In situ measurements and discrete water samples were collected night and day during neap low tide events to capture diurnal biogeochemistry cycles. Calculation of pHT using total alkalinity (TA) and dissolved inorganic carbon (DIC) revealed that the mid‐intertidal habitat exhibited the greatest flux over the years (pHT 7.52–8.87), and over a single tidal cycle (1.11 pHT units), while the low‐intertidal (pHT 7.82–8.30) and subtidal (pHT 7.87–8.30) were less variable. Temperature flux was also greatest in the mid‐intertidal (8.0–34.5°C) and over a single tidal event (14°C range), as typical of temperate rocky shores. Mean TA and DIC increased at night and decreased during the day, with the most extreme conditions measured in the mid‐intertidal owing to prolonged emersion periods. Temporal sampling revealed that net ecosystem calcification and production were highest during the day and lowest at night, particularly in the mid‐intertidal. Characterization of biogeochemical fluctuations in a world of extremes demonstrates the variable conditions that intertidal biota routinely experience and highlight potential microhabitat‐specific vulnerabilities and climate change refugia.

Continue reading ‘Characterizing biogeochemical fluctuations in a world of extremes: A synthesis for temperate intertidal habitats in the face of global change’

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Ocean acidification in the IPCC AR5 WG II

OUP book