Posts Tagged 'chemistry'

Investigating controls on boron isotope ratios in shallow marine carbonates

The boron isotope-pH proxy has been widely used to reconstruct past ocean pH values. In both planktic foraminifera and corals, species-specific calibrations are required in order to reconstruct absolute values of pH, due to the prevalence of so-called vital effects — physiological modification of the primary environmental signals by the calcifying organisms. Shallow marine abiotic carbonate (e.g. ooids and cements) could conceivably avoid any such calibration requirement, and therefore provide a potentially useful archive for reconstructions in deep (pre-Cenozoic) time. However, shallow marine abiotic carbonates could also be affected by local shifts in pH caused by microbial photosynthesis and respiration, something that has up to now not been fully tested. In this study, we present boron isotope measurements from shallow modern marine carbonates, from the Bahama Bank and Belize to investigate the potential of using shallow water carbonates as pH archives, and to explore the role of microbial processes in driving nominally ‘abiogenic’ carbonate deposition. For Bahama bank samples, our boron-based pH estimates derived from a range of carbonate types (i.e. ooids, peloids, hardground cements, carbonate mud, stromatolitic micrite and calcified filament micrite) are higher than the estimated modern mean-annual seawater pH values for this region. Furthermore, the majority (73%) of our marine carbonate-based pH estimates fall out of the range of the estimated pre-industrial seawater pH values for this region. In shallow sediment cores, we did not observe a correlation between measured pore water pH and boron-derived pH estimates, suggesting boron isotope variability is a depositional rather than early diagenetic signal. For Belize reef cements, conversely, the pH estimates are lower than likely in situ seawater pH at the time of cement formation. This study indicates the potential for complications when using shallow marine non-skeletal carbonates as marine pH archives. In addition, variability in δ11B based pH estimates provides additional support for the idea that photosynthetic CO2 uptake plays a significant role in driving carbonate precipitation in a wide range of shallow water carbonates.

Continue reading ‘Investigating controls on boron isotope ratios in shallow marine carbonates’

Major impacts of climate change on deep-sea benthic ecosystems

The deep sea encompasses the largest ecosystems on Earth. Although poorly known, deep seafloor ecosystems provide services that are vitally important to the entire ocean and biosphere. Rising atmospheric greenhouse gases are bringing about significant changes in the environmental properties of the ocean realm in terms of water column oxygenation, temperature, pH and food supply, with concomitant impacts on deep-sea ecosystems. Projections suggest that abyssal (3000–6000 m) ocean temperatures could increase by 1°C over the next 84 years, while abyssal seafloor habitats under areas of deep-water formation may experience reductions in water column oxygen concentrations by as much as 0.03 mL L–1 by 2100. Bathyal depths (200–3000 m) worldwide will undergo the most significant reductions in pH in all oceans by the year 2100 (0.29 to 0.37 pH units). O2 concentrations will also decline in the bathyal NE Pacific and Southern Oceans, with losses up to 3.7% or more, especially at intermediate depths. Another important environmental parameter, the flux of particulate organic matter to the seafloor, is likely to decline significantly in most oceans, most notably in the abyssal and bathyal Indian Ocean where it is predicted to decrease by 40–55% by the end of the century. Unfortunately, how these major changes will affect deep-seafloor ecosystems is, in some cases, very poorly understood. In this paper, we provide a detailed overview of the impacts of these changing environmental parameters on deep-seafloor ecosystems that will most likely be seen by 2100 in continental margin, abyssal and polar settings. We also consider how these changes may combine with other anthropogenic stressors (e.g., fishing, mineral mining, oil and gas extraction) to further impact deep-seafloor ecosystems and discuss the possible societal implications.

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Spatial and temporal controls on the inorganic carbon system of the Western Arctic Ocean

The Arctic Ocean plays a critical role in the global carbon cycle. It is believed to be particularly sensitive to the effects of climate change, is already undergoing dramatic changes, and is therefore important to study in that context. Most studies of the inorganic carbon system in the Western Arctic focus on hydrographic datasets from summer and/or fall (July-October), and do not consider the full response of the system to the timing of ice retreat, organic matter production and remineralization, and ice advance. Here we present the first dataset to investigate the spatial and temporal controls on the inorganic carbon system from early spring (pre-phytoplankton), late spring (initial phytoplankton bloom), summer (post-bloom), and fall in 2014. Our results suggest that the timing of ice retreat has important implications for the length of the phytoplankton growing season, and thus influences the magnitude of biological carbon cycling. We extend our analysis to include high-resolution temporal estimates of air-sea CO2 flux, and estimate a total annual CO2 uptake in the Chukchi Sea of ~7.7 Tg C. This is the first dataset to evaluate the importance of different seasonal observations within one year on the annual uptake of CO2 in the western Arctic Ocean. Our results show that extrapolations from one observational dataset result in large over- or underestimations of annual CO2 flux.

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Experimental evidence of formation of transparent exopolymer particles (TEP) and POC export provoked by dust addition under current and high pCO2 conditions

The evolution of organic carbon export to the deep ocean, under anthropogenic forcing such as ocean warming and acidification, needs to be investigated in order to evaluate potential positive or negative feedbacks on atmospheric CO2 concentrations, and therefore on climate. As such, modifications of aggregation processes driven by transparent exopolymer particles (TEP) formation have the potential to affect carbon export. The objectives of this study were to experimentally assess the dynamics of organic matter, after the simulation of a Saharan dust deposition event, through the measurement over one week of TEP abundance and size, and to evaluate the effects of ocean acidification on TEP formation and carbon export following a dust deposition event. Three experiments were performed in the laboratory using 300 L tanks filled with filtered seawater collected in the Mediterranean Sea, during two ‘no bloom’ periods (spring at the start of the stratification period and autumn at the end of this stratification period) and during the winter bloom period. For each experiment, one of the two tanks was acidified to reach pH conditions slightly below values projected for 2100 (~ 7.6–7.8). In both tanks, a dust deposition event of 10 g m-2 was simulated at the surface. Our results suggest that Saharan dust deposition triggered the abiotic formation of TEP, leading to the formation of organic-mineral aggregates. The amount of particulate organic carbon (POC) exported was proportional to the flux of lithogenic particles to the sediment traps. Depending on the season, the POC flux following artificial dust deposition ranged between 38 and 90 mg m-2 over six experimental days. Such variability is likely linked to the seasonal differences in the quality and quantity of TEP-precursors initially present in seawater. Finally, these export fluxes were not significantly different at the completion of the three experiments between the two pH conditions.

Continue reading ‘Experimental evidence of formation of transparent exopolymer particles (TEP) and POC export provoked by dust addition under current and high pCO2 conditions’

Development and application of foraminiferal carbonate system proxies to quantify ocean acidification in the California Current

The oceanic uptake of anthropogenic carbon has mitigated climate change, but has also resulted in a global average 0.1 decline in surface ocean pH over 20th century known as ocean acidification. The parallel reduction in carbonate ion concentration ([CO32-]) and the saturation state of seawater (Ω) has caused many major calcium carbonate-secreting organisms such as planktonic foraminifera to exhibit impaired calcification. We develop proxy calibrations and down core records that use calcification and geochemical characteristics of planktonic foraminifera as proxies for the marine carbonate system. This study focuses specifically on the surface ocean chemistry of the California Current Ecosystem (CCE), which has been identified as a region of rapidly progressing ocean acidification due to natural upwelling processes and the low buffering capacity of these waters. The calibration portion of this study uses marine sediments collected by the Santa Barbara Basin (SBB), California sediment-trapping program located in the central region of the CCE. We calibrate the relationships of Globigerina bulloides calcification intensity to [CO3 2-] and the B/Ca ratios of G. bulloides, Neogloboquadrina dutertrei and Neogloboquadrina incompta shells to Ω calcite using in situ measurements and model simulations of these independent variables. By applying these proxy methods to down core, our records from the SBB indicate a 20% reduction in foraminiferal calcification since ~1900, translating to a 35% decline in [CO 32-] in the CCE over this period. Our high-resolution calcification record also reveals a substantial interannual to decadal modulation of ocean acidification in the CCE related to the sign of Pacific Decadal Oscillation and El Niño Southern Oscillation. In the future we can expect these climatic modes to both enhance and moderate anthropogenic ocean acidification. Based on our historic record, we predict that if atmospheric CO2 reaches 540 ppm by the year 2100 as predicted by a conservative CO3 pathway, [CO32-] will experience a net reduction of 55%, resulting in at least a 30% reduction in calcification of planktonic foraminifera that will likely be mirrored by other adversely affected marine calcifiers.

Continue reading ‘Development and application of foraminiferal carbonate system proxies to quantify ocean acidification in the California Current’

Sensitivity of sea urchin fertilization to pH varies across a natural pH mosaic

In the coastal ocean, temporal fluctuations in pH vary dramatically across biogeographic ranges. How such spatial differences in pH variability regimes might shape ocean acidification resistance in marine species remains unknown. We assessed the pH sensitivity of the sea urchin Strongylocentrotus purpuratus in the context of ocean pH variability. Using unique male–female pairs, originating from three sites with similar mean pH but different variability and frequency of low pH (pHT ≤ 7.8) exposures, fertilization was tested across a range of pH (pHT 7.61–8.03) and sperm concentrations. High fertilization success was maintained at low pH via a slight right shift in the fertilization function across sperm concentration. This pH effect differed by site. Urchins from the site with the narrowest pH variability regime exhibited the greatest pH sensitivity. At this site, mechanistic fertilization dynamics models support a decrease in sperm–egg interaction rate with decreasing pH. The site differences in pH sensitivity build upon recent evidence of local pH adaptation in S. purpuratus and highlight the need to incorporate environmental variability in the study of global change biology.

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Inorganic carbon and water masses in the Irminger Sea since 1991

The subpolar gyre region in the North Atlantic is a major sink for anthropogenic carbon. While the storage rates show large interannual variability related to atmospheric forcing, less is known about variability in the natural Dissolved Inorganic Carbon (DIC) and the combined impact of variations in the two components on the total DIC inventories. Here, data from 15 cruises in the Irminger Sea covering 1991–2015 were used to determine changes in total DIC and its natural and anthropogenic components in relation to the distribution and evolution of the main water masses. The inventory of DIC increased by 1.43 ± 0.17 mol m−2 yr−1 over the period, mainly driven by the increase in anthropogenic carbon (1.84 ± 0.16 mol m−2 yr −1), but partially offset by a loss of natural DIC (−0.57 ± 0.22 mol m−2 yr−1). Changes in the carbon storage rate can be driven by concentration changes in the water column, for example due to ageing of water masses, or by changes in the distribution of water masses with different concentrations, either by local formation or advection. A decomposition of the trends into their main drivers showed that variations of natural DIC inventories are mainly driven by changes in the layer thickness of the main water masses, while anthropogenic carbon is most affected by concentration changes. The storage rates of anthropogenic carbon are sensitive to data selection, while changes in DIC inventory show a robust signal on short timescales, associated with the strength of convection.

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Ocean acidification in the IPCC AR5 WG II

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