Deep-water carbonate concentrations in the southwest Pacific

We have compiled carbonate chemistry and sedimentary CaCO₃% data for the deep-waters (>1500 m water depth) of the southwest (SW) Pacific region. The complex topography in the SW Pacific influences the deep-water circulation and affects the carbonate ion concentration ([CO₃2−]), and the associated calcite saturation horizon (CSH, where ??_calcite=1). The Tasman Basin and the southeast (SE) New Zealand region have the deepest CSH at ∼3100 m, primarily influenced by middle and lower Circumpolar Deep Waters (m or lCPDW), while to the northeast of New Zealand the CSH is ∼2800 m, due to the corrosive influence of the old North Pacific deep waters (NPDW) on the upper CPDW (uCPDW). The carbonate compensation depth (CCD; defined by a sedimentary CaCO₃ content of <20%), also varies between the basins in the SW Pacific. The CCD is ∼4600 m to the SE New Zealand, but only ∼4000 m to the NE New Zealand. The CaCO₃ content of the sediment, however, can be influenced by a number of different factors other than dissolution; therefore, we suggest using the water chemistry to estimate the CCD. The depth difference between the CSH and CCD (??Z_CSH−CCD), however, varies considerably in this region and globally. The global ??Z_CSH−CCD appears to expand with increase in age of the deep-water, resulting from a shoaling of the CSH. In contrast the depth of the chemical lysocline (??_calcite=0.8) is less variable globally and is relatively similar, or close, to the CCD determined from the sedimentary CaCO₃%. Geochemical definitions of the CCD, however, cannot be used to determine changes in the paleo-CCD. For the given range of factors that influence the sedimentary CaCO₃%, an independent dissolution proxy, such as the foraminifera fragmentation % (>40%=foraminiferal lysocline) is required to define a depth where significant CaCO₃ dissolution has occurred back through time. The current foraminiferal lysocline for the SW Pacific region ranges from 3100-3500 m, which is predictably just slightly deeper than the CSH. This compilation of sediment and water chemistry data provides a CaCO₃ dataset for the present SW Pacific for comparison with glacial/interglacial CaCO₃ variations in deep-water sediment cores, and to monitor future changes in [CO₃2−] and dissolution of sedimentary CaCO₃ resulting from increasing anthropogenic CO₂.     

Coulometric total carbon dioxide analysis for marine studies: maximizing the performance of an automated gas extraction system and coulometric detector

A global survey of the distribution of dissolved CO₂ taking advantage of sampling opportunities provided by the World Ocean Circulation Experiment: World Hydrographic Program (WOCE-WHP) is being carried out through 1995. Goals include the measurement of oceanic inorganic carbon transport and the development of a data base from which future fossil-fuel CO₂ build-up can be monitored. The analytical method selected for total carbon dioxide (C_T) is gas extraction of acidified seawater with coulometric titration of the acid formed by the resultant carbon dioxide and monoethanolamine. To combine high accuracy and precision (± 1.5 μmol/kg for C_T ≥ 2000 μmol/kg) with a high rate of analysis, we have modified an automated single-parameter system. Following prototype development between 1987 and 1990, an instrument emerged with the acronym Somma standing for single-operator multiparameter metabolic analyzer. Improved functional and operating procedures have integrated electronic calibration, CO₂ gas calibration, and sample analysis with automated pressure, temperature, and conductivity (salinity) sensing into a single convenient transportable package.     

铀、钍在碳酸钙-海水界面的附着行为研究

运用实验模拟的手段,对铀(U)、钍(Th)在碳酸钙-海水界面的附着行为进行了初步研究,通过测定相关的分异系数(D)定量评估了文石和方解石两种碳酸钙矿物对U和Th的捕集作用。实验结果显示,不仅U和Th之间存在显著差异,而且两种碳酸钙矿物之间也存在明显不同。U介于碳酸钙和海水之间的分异系数(DU)分别在1.5~3.5(文石)和0.04~0.19(方解石)区间内变化,分异行为非常微弱但其分异系数随着碳酸钙沉淀速率的增加而逐渐上升。这表明U元素主要是以共沉淀的方式进入到文石或方解石沉淀中,但由于其在海水溶液中主要以2 22 3UO(CO)nn的形式存在,因而只能以占据晶格缺陷的方式进入碳酸钙。与之相反的是,文石和方解石都对Th表现出极强的捕集能力,所测定的分异系数(DTh)分别在240~6 330(文石)和430~6 160(方解石)范围内变化,但DTh与碳酸钙沉淀速率之间无明显关联。Th在碳酸钙表面的附着行为可界定为单纯的吸附作用,这一行为应与Th主要以Th(OH)4的形式赋存于溶液中有关。     

Coulometric TCO2 analyses for marine studies; an introduction

Progress in the introduction of coulometry for the analysis of total carbon dioxide (TCO₂) in marine waters is described. An extractor—stripper removes CO₂ that is measured coulometrically by the quantity of electricity (coulombs) used to electrogenerate OH^? ions for the titration of the acid formed by the reaction of CO₂ and ethanolamine. The equivalence point is detected photometrically with thymolphthalein as the indicator, and Faraday's Law relates coulombs to equivalents of titrant generated and CO₂ determined so that there are no standard curves needed or titrants to standardize or store. Accuracy was determined by adding gelatin capsules containing 100–1500 μg C of pure CaCO₃ into the stripper, and accuracies of better than ± 1 μg C were achieved. The best precision for natural seawater (± 1 standard error) of ± 0.5 μmol l^?1 was found for 17 samples of Bermuda coastal waters having a mean TCO₂ of 2007.2 μmol l^?1 (0.05% CV). Sources of error and precautions are discussed. This method, which has been used successfully at sea, can be used to study a variety of marine phenomena involving TCO₂.     

Total inorganic carbon in sea ice

The method proposed for determining the total inorganic carbon (TC) concentrations in sea ice (Arctic region, North Pole-35 expedition) based on the measurement of the total alkalinity (TA) and the pH in the melt waters without the CO₂ exchange with the atmosphere is considered. It is shown that the TC/Sal and TA/TC values through the entire ice section remain similar to these parameters in the subice water. The surface snow and the uppermost ice layers are characterized by elevated TA/TC values, which indicate the reaction Ca²⁺ + 2HCO₃⁻ = ↓CaCO₃ + ↑CO₂ + H₂O. The release of CO₂ to the atmosphere due to the decomposition of calcium hydrocarbonate is as high as ∼20 mmol/m². The meltwater of the examined ice is undersaturated with CO₂, which may result in a sink of atmospheric CO₂ (∼30 mmol/m²).     

Reef building and carbonate production modes in the west-central Tethys during the Cenozoic

Changing components, rock textures, lithofacies, platform types and architecture throughout time are unique characteristics of carbonate rocks. Characterizing these attributes has been approached by 1) building reference models for specific Phanerozoic intervals, 2) recognizing the climatic impact in modulating carbonate production, and 3) analyzing the influence of changing bio-geochemical conditions. The reference-model approach is mostly based on biological evolution, the climatic-impact approach emphasizes temperature, and the bio-geochemical approach considers the changes in Mg/Ca ratios and Ca⁺⁺ concentrations in the oceans. To date, however, an analysis integrating all of these factors is still missing. The analysis presented here includes all these factors but also CO₂, which is fundamental for both photosynthesis and CaCO₃ precipitation.Here we analyze the waxing and waning of Cenozoic reef limestones from the central Tethys region through several steps: 1) on the basis of rock volume, rock textures, associated sediments and light-dependent skeletal components, as records of light penetration and wave energy (depth); 2) on global environmental conditions (δ¹³C, δ¹⁸O, pCO₂, temperature); and 3) on the basis of functionality, nutritional requirements and available resources.Through the Cenozoic, water motion, whether induced by surface or internal waves or by currents, increased as the thermal gradients strengthened, both with depth and with latitude. Active water motion is essential for plankton catchers such as corals, but less so for many larger benthic foraminifers (LBF). Pycnoclines in the meso-oligophotic zone would then favor the benthic plankton catchers such as corals, but would be detrimental for many LBF. Warm temperatures favored LBF. The Eocene LBF families predominated during lowering of atmospheric pCO₂ by using respiratory CO₂ to enhance the symbiont production of photosynthates under oligotrophic conditions and limited turbulence, whereas the Miocene families had to adapt to a progressive increase in turbulence. The eurythermal coralline red algae, however, became preponderant producers in the mesophotic zone during times when the δ¹³C was relatively high. This explains two apparent paradoxes: 1) corals thrive best when the Earth's high latitudes cool, and 2) the dominance of corals and LBF is inversely correlated, despite they both require tropical conditions and have similar trophic strategies (mixotrophy).     

Seasonal variation of Mn adsorption on to calcareous surfaces in the English Channel, and its implication on the manganese distribution coefficient

Dissolved and particulate manganese in seawater samples derived from the English Channel has been analyzed using graphite furnace atomic absorption (GFAAS), and inductively coupled plasma atomic emission (ICP—AES) spectroscopies. Because of the high contents of carbonate minerals found in the suspended matter samples, the English Channel constitutes an ideal field area for the study of the Mn²⁺/Ca²⁺/CaCO₃ system. Owing to the chemical speciation of particulate manganese and the combined use of the X-ray diffraction and electron spin resonance spectroscopy we have shown the importance of the carbonate phase in the stabilization of manganese (II). This has been confirmed by p-pH measurements in the field. All these studies have also indicated that: (1) manganese is associated with calcite in the form of a solid solution, Mn_xCa_1−xCO₃; and (2) significant increases in the concentrations of particulate manganese, especially in offshore waters, occur in summer. This seasonal phenomenon has been attributed to the proliferation of coccolithophorids, which are known to be covered with calcified skeletons at high specific surface areas. To appraise the implication of the coccolithophorid-blooms phenomenon on the Mn²⁺/Ca²⁺/CaCO₃ system, we have used the manganese distribution coefficient, D_i, between the liquid phase and CaCO₃ particles. Overall we have shown that: (1) D_i in summer (i.e. when coccoliths considered as very fine-grained calcite are abundant) is much higher than that obtained in winter; and (2) in the vicinity of the French coast, D_i does not vary significantly even in summer. This is because of the high content of chalk-derived particles found in the near-shore waters.     

GEOCHEMISTRY OF Si, Al, Fe AND CaCO3 IN SEDIMENTS FROM THE CENTRAL PACIFIC

In sediments of the Central Pacific, water depth is the controlling factor of the distribution of Si, Al, Fe and CaCO, .In the shallow and subabysmal areas, CaCO3 content is much greater than that in the abysmal areas where Si, Al and Fe contents are much lower than those in shallow and subabysmal areas. The contents of Si, Al, and Fe increase and that of CaCO3 decrease with the variation of grain size of sediments from coarse to fine. The contents of Al, Si, and Fe are positively related to each other and the content of CaCO3 is negatively related to that of the three elements. The evident regularities of the distribution of Si, Al, Fe and CaCO, in the sediments have revealed to a certain extent that they may be used as the environmental indicators in some cases.     

Theoretical considerations about the reactions of calcification in sea water

We examine the concentration variations of the different parameters X of the carbonate system in seawater when calcium carbonate precipitation occurs. Variations are expressed as ∂[X]/∂[Ca²⁺]. Four different cases are considered: spontaneous chemical precipitation; calcification combined with photosynthetic activity under a constant ΔCT/Δ[Ca²⁺] ratio; precipitation under constant p_CO2 and precipitation under constant [Ca²⁺]·[CO₃²⁻] ionic concentration product. The last condition should be maintained by an ecosystem which, thanks to the regulation of its calcifying and photosynthetic activity, would absorb 1 mol of carbon for organic tissue each time 1 mol of CaCO₃ is formed. This stoichiometric ratio would allow the activity of these biological communities to go on in practically closed systems during periods compatible with their growth or development cycles.     

Rates of calcium carbonate dissolution and organic carbon decomposition in the North Pacific ocean

Recent carbonate data collected in the North Pacific were combined with the data in the literature in order to understand more clearly the carbonate chemistry in the North Pacific. Our analyses show that inorganic CaCO₃ dissolution contributes about 26% of the total inorganic CO₂ increase of deep water, after leaving the Southern Ocean. The calcium and alkalinity data indicate a CaCO₃ dissolution rate of 0.060±0.010 and 0.053±0.005 µ mol kg^–1 yr^–1 respectively, for waters deeper than 2,000 m in reference to the Weddell Sea Deep Water. The organic carbon decomposition rate is 0.107±0.012 µ mol kg^–1 yr^–1 while the oxygen consumption rate is 0.13±0.002 µ mol kg^–1 yr^–1. These results which are based on the direct comparison of two water masses agree well with other estimates which are based on methods such as the one-dimensional-diffusion-advection model. Comparison of data along the two sections at 165°E and 150°W shows no significant difference in the ratio of the CaCO₃ dissolution rate and the organic carbon decomposition rate. The eastern section, however, has a higher TCO₂ input than the western section because of the older age of the deep water along the eastern section.     

Benthic remineralization and burial of biogenic SiO2, CaCO3, organic carbon, and detrital material in the Southern Ocean along a transect at 170° West

We investigated the composition, recycling, and mass accumulation rates of sediments along a transect in the Southern Ocean located from 66°S to 57°S at 170°W. This transect also corresponds to the location of a sediment trap mooring line. The sediments at the seven sites studied range from largely terrigenous material to nearly pure (>90%) biogenic silica. CaCO₃ is a minor but persistent component at most sites. Mass accumulation rates have been determined on the basis of excess ²³⁰Th in the sediments, i.e., ²³⁰Th-normalized accumulation rates. The influence of redistribution of sediments on the sea floor has been estimated from ¹⁴C analyses. The recycling of material delivered to the sediments has been characterized on the basis of pore water studies that make extensive use of both in situ sampling and shipboard extractions. The influence of the highly variable rates of input of particulate matter that characterize much of the Southern Ocean upon pore water gradients and fluxes across the sediment water interface has been considered.We find only poor correspondence between BSiO₂ burial fraction (=burial/particulate flux), a quantifiable measure of preservation efficiency, and BSiO₂ particulate rain along the transect. However, preservation does appear to be closely linked to a combination of sedimentation rate and particulate rain.The burial fraction of BSiO₂ is small relative to benthic rain (5–19%). Despite the small fraction buried, burial flux normalized to (sedimentation rate)^1/2 appears to provide a very consistent means of predicting benthic particulate rain over a large range of rain rates, including data from a number of different studies and environments. At sites with BSiO₂ rain 250 mmol m⁻² yr⁻¹ the average difference between predicted and observed rain is 25–30%. Such rain rates occur in many marine areas, particularly the Southern Ocean, with the result that this relationship potentially provides a means of estimating BSiO₂ benthic rain over prolonged periods in the past on the basis of readily measured sediment parameters.At the southern-most deep ocean station, the particulate flux was characterized by an extremely high C_org/CaCO₃ ratio (>10), but this high ratio does not appear to have a substantial influence on CaCO₃ burial. CaCO₃ is preserved in the sediments at this site despite a particulate flux with a 10-fold excess of C_org above that required for complete dissolution in the sediments. The unexpectedly high preservation of CaCO₃ is due largely to the very steep C_org oxidation rate profile at this site. As a result, a large fraction of the organic matter oxidized in the sediments does so in close proximity to the sediment–water interface where most of the metabolic CO₂ is neutralized by CO₃²⁻ from the overlying water, rather than by the dissolution of sedimentary CaCO₃.Diagenetic modeling indicates that at several of the stations, the remineralization fluxes of carbonate species across the sediment–water interface may not have been at steady state as a result of the highly pulsed nature of particulate rain in this environment. We estimate that at the time of our sampling it is possible that near-interface fluxes could have been a factor of 1.6–2 times the annual average.At every site on the transect, the burial fluxes of detrital material are substantially greater than the detrital particulate rain measured in the sediment traps, by as much as a factor of 40. Detrital burial is bimodal, being greatest at the southern and northern extremes of the transect. We postulate that the excess of burial over particulate rain in the south reflects the contribution of ice rafted debris at these high latitudes. Increases in the supply at the northern stations must have a different source. We believe that the excess at these stations is material eroded from the sea floor to the west, possibly on the Campbell Plateau, and advected by currents to the northern portion of the transect at depths below the shallow traps.     

Legal challenges of carbon capture and storage in the South China Sea region

Carbon capture and storage (CCS) is one method to reduce CO₂ emissions to the atmosphere. As CCS has the greatest potential for greenhouse gas (GHG) emission reduction, it gains a wide currency in the developed countries. However, the technology for CCS does not spread at the international level especially to the developing countries like South China Sea states. As the CO₂ storage period is quite long and potential environmental risks involved, fully deployed CCS projects need not only the technological support but the appropriate legal and regulatory regime to safeguard CCS operations. This article offers a survey of CCS projects in the South China Sea region and discusses the legal challenges associated with CCS activities in state practice.     

Uptake of atmospheric carbon dioxide in Arctic shelf seas: evaluation of the relative importance of processes that influence pCO2 in water transported over the Bering–Chukchi Sea shelf

The uptake of atmospheric carbon dioxide in the water transported over the Bering–Chukchi shelves has been assessed from the change in carbon-related chemical constituents. The calculated uptake of atmospheric CO₂ from the time that the water enters the Bering Sea shelf until it reaches the northern Chukchi Sea shelf slope (1 year) was estimated to be 86±22 g C m⁻² in the upper 100 m. Combining the average uptake per m³ with a volume flow of 0.83×10⁶ m³ s⁻¹ through the Bering Strait yields a flux of 22×10¹² g C year⁻¹. We have also estimated the relative contribution from cooling, biology, freshening, CaCO₃ dissolution, and denitrification for the modification of the seawater pCO₂ over the shelf. The latter three had negligible impact on pCO₂ compared to biology and cooling. Biology was found to be almost twice as important as cooling for lowering the pCO₂ in the water on the Bering–Chukchi shelves. Those results were compared with earlier surveys made in the Barents Sea, where the uptake of atmospheric CO₂ was about half that estimated in the Bering–Chukchi Seas. Cooling and biology were of nearly equal significance in the Barents Sea in driving the flux of CO₂ into the ocean. The differences between the two regions are discussed. The loss of inorganic carbon due to primary production was estimated from the change in phosphate concentration in the water column. A larger loss of nitrate relative to phosphate compared to the classical ΔN/ΔP ratio of 16 was found. This excess loss was about 30% of the initial nitrate concentration and could possibly be explained by denitrification in the sediment of the Bering and Chukchi Seas.     

Phase stability and kinetics of methane hydrate formation in presence of calcium and magnesium carbonate

The huge amount of methane hydrate deposits identified in deep marine sediments is considered as the new resource for future energy. Since carbonates are one of the major components of marine sediments, in the present study, an investigation has been made to study methane hydrate stability and kinetics in the presence of CaCO₃ and MgCO₃. Effect of the presence of carbonates on the solubility of methane in the system has also been examined as it directly affects the hydrate formation process. It has been observed that in presence of CaCO₃ and MgCO₃, the hydrate formation is inhibited. Comparative studies have also been done in the presence of artificial seawater to consider the effect of presence of different salts. Mole consumption of methane gas during hydrate formation in different carbonate samples was measured using real gas equation and found to be minimum in CaCO₃ in seawater sample due to the combined effect of the presence of CaCO₃ and different salts of seawater. An increase in nucleation and induction time was also observed demonstrating the inhibition of hydrate formation in the presence of these components. Further, the decrease in hydrate formation rate also confirmed the inhibition effect of CaCO₃ and MgCO₃ on hydrate formation.     

Physiological Responses of the Mediterranean Subtidal Alga <Emphasis Type="Italic">Peyssonnelia squamaria</Emphasis> to Elevated CO<Subscript>2</Subscript>

The ecological consequences of ocean acidification are unclear due to varying physiological properties of macroalgae and species-specific responses. Therefore, in the present study, we used a laboratory culture experiment to analyse the eco-physiological responses of the Mediterranean subtidal red alga Peyssonnelia squamaria to CO₂-induced lower pH. Our results showed an increase in the photosynthetic performance and growth rate of P. squamaria, despite the reduction in CaCO₃ content in the low pH treatment. According to our results, we believe that samples exposed to elevated CO₂ could be regulated own nitrogen metabolism to support increased growth rate and it may be down-regulated nitrate uptake. As a result, we hypothesize that P. squamaria may benefit from ocean acidification.     

On the variation in calcium content of the waters of Laccadives (Arabian Sea)

The concentration of calcium was determined in the samples collected from four stations in the Laccadive Sea and from two lagoons of Kavaratti and Minicoy atolls. The calcium/chlorinity ratio for the open ocean samples was found to be 0.02168 ± 0.000015 with an average calcium concentration of 424.9 mg kg^?1. A maximum in this ratio was observed at about 200 m depth, below the salinity maximum corresponding to Arabian Sea Surface Water mass. No increase in calcium concentration or in calcium/chlorinity ratio was observed down to 1500 m depth, thereby ruling out the possibility of any calcium carbonate dissolution at these depths. Samples from Kavaratti and Minicoy lagoons gave much lower values of the Ca/Cl ratio (0.02145 ± 0.000036 and 0.02142 ± 0.000046, respectively). These low values are apparently the result of calcium utilization by the coral reefs. Using the reduction in the calcium concentration inside the lagoon, in the absence of any chemical precipitation, the annual CaCO₃ production by reef flat and lagoon on Kavaratti Atoll has been estimated as 1 · 10⁷ kg. This gives an average gross production of 1.4 kg CaCO₃ per m² per yr.     

Perspectives on Biological Research for CO<Subscript>2</Subscript> Ocean Sequestration

Feasibility studies recently suggest that sequestration of anthropogenic CO₂ in the deep ocean could help reduce the atmospheric CO₂ concentration. However, implementation of this strategy could have a significant environmental impact on marine organisms. This has highlighted the urgent need of further studies concerning the biological impact of CO₂ ocean sequestration. In this paper we summarize the recent literature reporting on the biological impact of CO₂ and discuss the research work required for the future. Although fundamental research of the effect of CO₂ on marine organisms before the practical consideration of CO₂ ocean sequestration was limited, laboratory and field studies concerning biological impacts have been increasing after the first international workshop in 1991 discussing CO₂ ocean sequestration. Acute impacts of CO₂ ocean sequestration could be determined by laboratory and field experiments and assessed by simulation models as described by the following papers in this section. On the other hand, chronic effects of CO₂ ocean sequestration, those directly related to the marine ecosystem, would be difficult to verify by means of experiments and to assess using ecosystem models. One of the practical solutions for this issue implies field experiments starting with controlled small scale and eventually to a large scale of CO₂ injection intended to determine ecosystem alteration. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbonate-related parameters of subsurface waters in the West Philippine, South China and Sulu Seas

Like most other deep basins in Southeast Asia, the deep Sulu Sea (SS) basin is isolated from the neighboring seas by surrounding topography. While the near-surface circulation is mainly governed by the seasonally reversing monsoon winds, below the warm and fresh surface layer, the core of the incoming Subtropical Lower Water from the West Philippine Sea (WPS), by way of the South China Sea (SCS), can be seen, at a depth of around 200 m, to have a distinct salinity maximum. It lies well above the sill depth (420 m) in the Mindoro Strait and thus, its spreading is not hampered by topography. The deep circulation is forced by an inflow of upper North Pacific Intermediate Water (NPIW) from the SCS through the Mindoro Str. Below 1000 m, the physico-chemical properties are remarkably homogeneous. The higher temperature, but lower salinity, oxygen and nutrients, of the deep SS waters, compared to those of the SCS, is indicative of the intrusion of NPIW above the sill depth. The excess, anthropogenic CO₂ penetrates the entire water column, because of the over-spill of the excess CO₂-laden water from the SCS.It has been reported that the bottom of the SS is CaCO₃ rich, relative to the SCS. Previous investigators attribute this to the higher θ in the SS. Indeed, the aragonite does not become undersaturated in the SS until below 1400 m, compared to 600 m in both the WPS and SCS; and the calcite does not become undersaturated until below 3800 m in the SS, compared to 2500 m in the SCS and around 1600 m in the WPS. However, the temperature effect is relatively small. These large differences are, in fact, largely a result of higher CO₃²⁻ concentrations in the SS, relative to the WPS and SCS. The higher CO₃²⁻ concentration in the SS, in turn, is mainly caused by the smaller amounts of organic carbon decomposition.     

Public acceptance of the oceanic carbon sequestration

CO₂ ocean sequestration may be an effective option for mitigating global warming. There are risks associated with this process, particularly the local impact on deep-sea environments. Public acceptance is required for the implementation of this technology, even though the impacts have been proven to be trivial. In this study, a questionnaire survey was conducted to find the correlation between public acceptance of CO₂ sequestration and influential factors by covariance structure analysis. In addition, risk communication via the Internet was carried out. These analyses revealed that careful investigation of the target oceanic site and field experiments are important in gaining public acceptance of CO₂ sequestration.