Ocean ventilation and deoxygenation in a warming world: introduction and overview.

Changes of ocean ventilation rates and deoxygenation are two of the less obvious but important indirect impacts expected as a result of climate change on the oceans. They are expected to occur because of (i) the effects of increased stratification on ocean circulation and hence its ventilation, due to reduced upwelling, deep-water formation and turbulent mixing, (ii) reduced oxygenation through decreased oxygen solubility at higher surface temperature, and (iii) the effects of warming on biological production, respiration and remineralization. The potential socio-economic consequences of reduced oxygen levels on fisheries and ecosystems may be far-reaching and significant. At a Royal Society Discussion Meeting convened to discuss these matters, 12 oral presentations and 23 posters were presented, covering a wide range of the physical, chemical and biological aspects of the issue. Overall, it appears that there are still considerable discrepancies between the observations and model simulations of the relevant processes. Our current understanding of both the causes and consequences of reduced oxygen in the ocean, and our ability to represent them in models are therefore inadequate, and the reasons for this remain unclear. It is too early to say whether or not the socio-economic consequences are likely to be serious. However, the consequences are ecologically, biogeochemically and climatically potentially very significant, and further research on these indirect impacts of climate change via reduced ventilation and oxygenation of the oceans should be accorded a high priority.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

Depth perception: the need to report ocean biogeochemical rates as functions of temperature, not depth.

For over 50 years, ocean scientists have oddly represented ocean oxygen consumption rates as a function of depth but not temperature in most biogeochemical models. This unique tradition or tactic inhibits useful discussion of climate change impacts, where specific and fundamental temperature-dependent terms are required. Tracer-based determinations of oxygen consumption rates in the deep sea are nearly universally reported as a function of depth in spite of their well-known microbial basis. In recent work, we have shown that a carefully determined profile of oxygen consumption rates in the Sargasso Sea can be well represented by a classical Arrhenius function with an activation energy of 86.5 kJ mol-1, leading to a Q10 of 3.63. This indicates that for 2°C warming, we will have a 29% increase in ocean oxygen consumption rates, and for 3°C warming, a 47% increase, potentially leading to large-scale ocean hypoxia should a sufficient amount of organic matter be available to microbes. Here, we show that the same principles apply to a worldwide collation of tracer-based oxygen consumption rate data and that some 95% of ocean oxygen consumption is driven by temperature, not depth, and thus will have a strong climate dependence. The Arrhenius/Eyring equations are no simple panacea and they require a non-equilibrium steady state to exist. Where transient events are in progress, this stricture is not obeyed and we show one such possible example.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

Use of a free ocean CO2 enrichment (FOCE) system to evaluate the effects of ocean acidification on the foraging behavior of a deep-sea urchin.

The influence of ocean acidification in deep-sea ecosystems is poorly understood but is expected to be large because of the presumed low tolerance of deep-sea taxa to environmental change. We used a newly developed deep-sea free ocean CO2 enrichment (dp-FOCE) system to evaluate the potential consequences of future ocean acidification on the feeding behavior of a deep-sea echinoid, the sea urchin, Strongylocentrotus fragilis. The dp-FOCE system simulated future ocean acidification inside an experimental enclosure where observations of feeding behavior were performed. We measured the average movement (speed) of urchins as well as the time required (foraging time) for S. fragilis to approach its preferred food (giant kelp) in the dp-FOCE chamber (-0.46 pH units) and a control chamber (ambient pH). Measurements were performed during each of 4 trials (days -2, 2, 24, 27 after CO2 injection) during the month-long period when groups of urchins were continuously exposed to low pH or control conditions. Although urchin speed did not vary significantly in relation to pH or time exposed, foraging time was significantly longer for urchins in the low-pH treatment. This first deep-sea FOCE experiment demonstrated the utility of the FOCE system approach and suggests that the chemosensory behavior of a deep-sea urchin may be impaired by ocean acidification.

A review of advances in deep-ocean Raman spectroscopy.

We review the rapid progress made in the applications of Raman spectroscopy to deep-ocean science. This is made possible by deployment of instrumentation on remotely operated vehicles used for providing power and data flow and for precise positioning on targets of interest. Early prototype systems have now been replaced by compact and robust units that have been deployed well over 100 times on an expeditionary basis over a very wide range of ocean depths without failure. Real-time access to the spectra obtained in the vehicle control room allows for expedition decision making. Quantification of some of the solutes in seawater or pore waters observed in the spectra is made possible by self-referencing to the ubiquitous ν(2) water bending peak. The applications include detection of the structure and composition of complex thermogenic gas hydrates both occurring naturally on the sea floor and in controlled sea floor experiments designed to simulate the growth of such natural systems. New developments in the ability to probe the chemistry of sediment pore waters in situ, long thought impossible candidates for Raman study due to fluorescence observed in recovered samples, have occurred. This permits accurate measurement of the abundance of dissolved methane and sulfide in sediment pore waters. In areas where a high gas flux is observed coming out of the sediments a difference of about ×30 between in situ Raman measurement and the quantity observed in recovered cores has been found. New applications under development include the ability to address deep-sea biological processes and the ability to survey the sea floor chemical conditions associated with potential sub-sea geologic CO(2) disposal in abandoned oil and gas fields.

Geochemistry of chemical weapon breakdown products on the seafloor: 1,4-thioxane in seawater.

The long-term fate of chemical weapon debris disposed of in the ocean some 50 years ago, now sinking into marine sediments and leaking into the ocean environment, is poorly known. Direct evidence exists showing chemical weapon agents actively being released on the sea floor with detrimental effects including harm to marine life. Thus there is strong interest in determining the fate and lifetime of these materials, their decomposition products, and the affected zones around these sites. Here we study the geochemical properties of a mustard gas breakdown product, 1,4-thioxane (TO), using Raman spectroscopy. We show that TO forms a hydrate with a help-gas (a second guest added to stabilize the hydrate), such as methane or hydrogen sulfide, with the hydrate stability regime some 10 degrees C above pure methane hydrate. The temperature, pressure, and reducing conditions required for hydrate formation commonly occur at known disposal sites. The TO solubility was measured in seawater and found to vary from 0.65 to 0.63 mol/kg water between 4.5 and 25.0 degrees C. Similar to other hydrate systems, the TO solubility decreased in the presence of hydrate. A low solubility in water coupled with its ability to form a hydrate within marine sediments can greatly decrease molecular mobility and increase its lifetime. These results demonstrate how unanticipated reactions with marine sediments can occur, and how little is known of the processes controlling the environmental science of these materials.

Clathrate hydrates in nature.

Scientific knowledge of natural clathrate hydrates has grown enormously over the past decade, with spectacular new findings of large exposures of complex hydrates on the sea floor, the development of new tools for examining the solid phase in situ, significant progress in modeling natural hydrate systems, and the discovery of exotic hydrates associated with sea floor venting of liquid CO2. Major unresolved questions remain about the role of hydrates in response to climate change today, and correlations between the hydrate reservoir of Earth and the stable isotopic evidence of massive hydrate dissociation in the geologic past. The examination of hydrates as a possible energy resource is proceeding apace for the subpermafrost accumulations in the Arctic, but serious questions remain about the viability of marine hydrates as an economic resource. New and energetic explorations by nations such as India and China are quickly uncovering large hydrate findings on their continental shelves.

Field studies on the formation of sinking CO2 particles for ocean carbon sequestration: effects of injector geometry on particle density and dissolution rate and model simulation of plume behavior.

We have carried out the second phase of field studies to determine the effectiveness of a coflow injector which mixes liquid CO2 and ambient seawater to produce a hydrate slurry as a possible CO2 delivery method for ocean carbon sequestration. The experiments were carried out at ocean depths of 1000-1300 m in Monterey Bay, CA, using a larger injector than that initially employed under remotely operated vehicle control and imaging of the product. Solidlike composite particles comprised of water, solid CO2 hydrate, and liquid CO2 were produced in both studies. In the recent injections, the particles consistently sank at rates of approximately 5 cm s(-1). The density of the sinking particles suggested that approximately 40% of the injected CO2 was converted to hydrate, while image analysis of the particle shrinking rate indicated a CO2 dissolution rate of 0.76-1.29 micromol cm(-2) s(-1). Plume modeling of the hydrate composite particles suggests that while discrete particles may sink 10-70 m, injections with CO2 mass fluxes of 1-1000 kg s(-1) would result in sinking plumes 120-1000 m belowthe injection point.

Seeing a deep ocean CO2 enrichment experiment in a new light: laser raman detection of dissolved CO2 in seawater.

We used a newly developed in situ laser Raman spectrometer (LRS) for detection of elevated levels of dissolved CO2 in seawater. The experiment was carried out at 500 m depth, 6 degrees C, to examine new protocols for detection of CO2-enriched seawater emanating from a liquid CO2 source in the ocean, and to determine current detection limits under field conditions. A system of two interconnected 5 L chambers was built, with flow between them controlled by a valve and pump system, and this unit was mounted on an ROV. The first chamber was fitted with a pH electrode and the optical probe of the LRS. In the second chamber approximately 580 mL of liquid CO2 was introduced. Dissolution of CO2 across the CO2-seawater interface then occurred, the valves were opened, and a fixed volume of low-pH/CO2-enriched seawater was transferred to the first chamber for combined pH/Raman sensing, where we estimate a mean dissolution rate of approximately 0.5 (micromol/cm2)/s. This sequence was repeated, resulting in measurement of a progressively CO2 enriched seawater sample. The rapid in-growth of CO2 was readily detected as the Fermi dyad of the dissolved state with a detection limit of approximately 10 mM with spectral acquisition times of 150 s. The detection of background levels of CO2 species in seawater (approximately 2.2 mM, dominantly HCO3-) will require an improvement in instrument sensitivity by a factor of 5-10, which could be obtained by the use of a liquid core waveguide.

Experimental determination of the fate of rising CO2 droplets in seawater.

Direct oceanic disposal of fossil fuel CO2 is being considered as a possible means to moderate the growth rate of CO2 in the atmosphere. We have measured the rise rate and dissolution rate of freely released CO2 droplets in the open ocean to provide fundamental data for carbon sequestration options. A small amount of liquid CO2 was released at 800 m, at 4.4 degrees C, and the rising droplet stream was imaged with a HDTV camera carried on a remotely operated vehicle. The initial rise rate for 0.9-cm diameter droplets was 10 cm/s at 800 m, and the dissolution rate was 3.0 micromol cm(-2) s(-1). While visual contact was maintained for 1 h and over a 400 m ascent, 90% of the mass loss occurred within 30 min over a 200 m ascent above the release point. Images of droplets crossing the liquid-gas-phase boundary showed formation of a gas head, pinching off of a liquid tail, and rapid gas bubble separation and dissolution.