Calcium Carbonate Flux Research Articles

Millennial‐Scale Carbon Flux Variability in the Subantarctic Pacific During Marine Isotope Stage 3 (57–29 ka)

AbstractAntarctic ice cores reveal a glacial climate state during Marine Isotope Stage 3 (MIS‐3; 57–29 ka) punctuated by millennial‐scale warming events and pulses of CO2. This study further explores how changes in Southern Ocean carbon cycling contributed to these millennial‐scale fluctuations in climate. Evidence from South Atlantic sediment cores suggests that warming events were associated with decreased dust‐borne iron flux, reduced export production, and increased upwelling from the deep Southern Ocean (SO). These processes are considered to have contributed to rising atmospheric CO2 during periods of rapid warming. Here we investigate whether the same processes occurred in the southwest Pacific sector of the SO at TAN1106‐28. We show that reduced New Zealand glaciation and localized iron limitation in the southwest Pacific led to reduced export production during millennial‐scale warming events. Decreases in foraminifera‐bound δ15N during all MIS‐3 warming events may reflect increased nutrient supply by upwelling. Increased calcium carbonate flux during MIS‐3 warming events likely reflects coccolithophore production in response to sea surface temperatures, which, would increase carbonate counter pump strength and reduce CO2 sequestration. Concomitant decreases in bottom water oxygen, inferred from redox‐sensitive U and Mn sediment concentrations, and increases in the 14C age of deep waters, suggest that old, nutrient‐rich waters influenced southwest Pacific middepth waters during warming events. This signature may reflect an expansion of Pacific Deep Water into the SO during warming. Taken together, our multi‐proxy data set reveals that the southwest subantarctic Pacific acted as a source of CO2 during millennial‐scale warming events of MIS‐3.

Exploring the impact of climate change on the global distribution of non-spinose planktonic foraminifera using a trait-based ecosystem model.

Planktonic foraminifera are one of the primary calcifiers in the modern ocean, contributing 23%-56% of total global pelagic carbonate production. However, a mechanistic understanding of how physiology and environmental conditions control their abundance and distribution is lacking, hindering the projection of the impact of future climate change. This understanding is important, not only for ecosystem dynamics, but also for marine carbon cycling because of foraminifera's key role in carbonate production. Here we present and apply a global trait-based ecosystem model of non-spinose planktonic foraminifera ('ForamEcoGEnIE') to assess their ecology and global distribution under future climate change. ForamEcoGEnIE considers the traits of calcium carbonate production, shell size, and foraging. It captures the main characteristic of biogeographical patterns of non-spinose species - with maximum biomass concentrations found in mid- to high-latitude waters and upwelling areas. The model also reproduces the magnitude of global carbonate production relatively well, although the foraminifera standing stock is systematically overestimated. In response to future scenarios of rising atmospheric CO2 (RCP6 and RCP8.5), on a regional scale, the modelled foraminifera biomass and export flux increases in the subpolar regions of the North Atlantic and the Southern Ocean while it decreases everywhere else. In the absence of adaptation, the biomass decline in the low-latitude South Pacific suggests extirpation. The model projects a global average loss in non-spinose foraminifera biomass between 8% (RCP6) and 11% (RCP8.5) by 2050 and between 14% and 18% by 2100 as a response to ocean warming and associated changes in primary production and ecological dynamics. Global calcium carbonate flux associated with non-spinose foraminifera declines by 13%-18% by 2100. That decline can slow down the ocean carbonate pump and create short-term positive feedback on rising atmospheric pCO2 .

Distribution and Abundances of Planktic Foraminifera and Shelled Pteropods During the Polar Night in the Sea-Ice Covered Northern Barents Sea

Planktic foraminfera and shelled pteropods are important calcifying groups of zooplankton in all oceans. Their calcium carbonate shells are sensitive to changes in ocean carbonate chemistry predisposing them as an important indicator of ocean acidification. Moreover, planktic foraminfera and shelled pteropods contribute significantly to food webs and vertical flux of calcium carbonate in polar pelagic ecosystems. Here we provide, for the first time, information on the under-ice planktic foraminifera and shelled pteropod abundance, species composition and vertical distribution along a transect (82°–76°N) covering the Nansen Basin and the northern Barents Sea during the polar night in December 2019. The two groups of calcifiers were examined in different environments in the context of water masses, sea ice cover, and ocean chemistry (nutrients and carbonate system). The average abundance of planktic foraminifera under the sea-ice was low with the highest average abundance (2 ind. m–3) close to the sea-ice margin. The maximum abundances of planktic foraminifera were concentrated at 20–50 m depth (4 and 7 ind. m–3) in the Nansen Basin and at 80–100 m depth (13 ind. m–3) close to the sea-ice margin. The highest average abundance (13 ind. m–3) and the maximum abundance of pteropods (40 ind. m–3) were found in the surface Polar Water at 0–20 m depth with very low temperatures (–1.9 to –1°C), low salinity (<34.4) and relatively low aragonite saturation of 1.43–1.68. The lowest aragonite saturation (<1.3) was observed in the bottom water in the northern Barents Sea. The species distribution of these calcifiers reflected the water mass distribution with subpolar species at locations and depths influenced by warm and saline Atlantic Water, and polar species in very cold and less saline Polar Water. The population of planktic foraminifera was represented by adults and juveniles of the polar speciesNeogloboquadrina pachydermaand the subpolar speciesTurborotalita quinqueloba. The dominating polar pteropod speciesLimacina helicinawas represented by the juvenile and veliger stages. This winter study offers a unique contribution to our understanding of the inter-seasonal variability of planktic foraminfera and shelled pteropods abundance, distribution and population size structure in the Arctic Ocean.

Global Ocean Sediment Composition and Burial Flux in the Deep Sea

AbstractQuantitative knowledge about the burial of sedimentary components at the seafloor has wide‐ranging implications in ocean science, from global climate to continental weathering. The use of 230Th‐normalized fluxes reduces uncertainties that many prior studies faced by accounting for the effects of sediment redistribution by bottom currents and minimizing the impact of age model uncertainty. Here we employ a recently compiled global data set of 230Th‐normalized fluxes with an updated database of seafloor surface sediment composition to derive atlases of the deep‐sea burial flux of calcium carbonate, biogenic opal, total organic carbon (TOC), nonbiogenic material, iron, mercury, and excess barium (Baxs). The spatial patterns of major component burial are mainly consistent with prior work, but the new quantitative estimates allow evaluations of deep‐sea budgets. Our integrated deep‐sea burial fluxes are 136 Tg C/yr CaCO3, 153 Tg Si/yr opal, 20Tg C/yr TOC, 220 Mg Hg/yr, and 2.6 Tg Baxs/yr. This opal flux is roughly a factor of 2 increase over previous estimates, with important implications for the global Si cycle. Sedimentary Fe fluxes reflect a mixture of sources including lithogenic material, hydrothermal inputs and authigenic phases. The fluxes of some commonly used paleo‐productivity proxies (TOC, biogenic opal, and Baxs) are not well‐correlated geographically with satellite‐based productivity estimates. Our new compilation of sedimentary fluxes provides detailed regional and global information, which will help refine the understanding of sediment preservation.

Simulated effects of interactions between ocean acidification, marine organism calcification, and organic carbon export on ocean carbon and oxygen cycles

Ocean acidification caused by oceanic uptake of anthropogenic carbon dioxide (CO2) tends to suppress the calcification of some marine organisms. This reduced calcification then enhances surface ocean alkalinity and increases oceanic CO2 uptake, a process that is termed calcification feedback. On the other hand, decreased calcification also reduces the export flux of calcium carbonate (CaCO3), potentially reducing CaCO3-bound organic carbon export flux and CO2 uptake, a process that is termed ballast feedback. In this study, we incorporate a range of different parameterizations of the links between organic carbon export, calcification, and ocean acidification into an Earth system model, in order to quantify the long-term effects on oceanic CO2 uptake that result from calcification and ballast feedbacks. We utilize an intensive CO2 emission scenario to drive the model in which an estimated fossil fuel resource of 5000 Pg C is burnt out over the course of just a few centuries. Simulated results show that, in the absence of both calcification and ballast feedbacks, by year 3500, accumulated oceanic CO2 uptake is 2041 Pg C . Inclusion of calcification feedback alone increases the simulated uptake by 629 Pg C (31%), while the inclusion of both calcification and ballast feedbacks increase simulated uptake by 449–498 Pg C (22–24%), depending on the parameter values used in the ballast feedback scheme. These results indicate that ballast effect counteracts calcification effect in oceanic CO2 uptake. Ballast effect causes more organic carbon to accumulate and decompose in the upper ocean, which in turn leads to decreased oxygen concentration in the upper ocean and increased oxygen at depths. By year 2600, the inclusion of ballast effect would decrease oxygen concentration by 11% at depth of ca. 200 m in tropics. Our study highlights the potentially critical effects of interactions between ocean acidification, marine organism calcification, and CaCO3-bound organic carbon export on the ocean carbon and oxygen cycles.

Biogeochemical flux and phytoplankton succession: A year-long sediment trap record in the Australian sector of the Subantarctic Zone

The Subantarctic Zone (SAZ) plays a crucial role in global carbon cycling as a significant sink for atmospheric CO2. In the Australian sector, the SAZ exports large quantities of organic carbon from the surface ocean, despite lower algal biomass accumulation in surface waters than other Southern Ocean sectors. We present the first analysis of diatom and coccolithophore assemblages and seasonality, as well as the first annual quantification of bulk organic components of captured material at the base of the mixed layer (500m depth) in the SAZ. Sediment traps were moored in the SAZ southwest of Tasmania as part of the long-term SAZ Project for one year (September 2003 to September 2004). Annual mass flux at 500m and 2000m was composed mainly of calcium carbonate, while biogenic silica made up on average <10% of material captured in the traps. Organic carbon flux was estimated at 1.1gm−2 y−1 at 500m, close to the estimated global mean carbon flux. Low diatom fluxes and high fluxes of coccoliths were consistent with low biogenic silica and high calcium carbonate fluxes, respectively. Diatoms and coccoliths were identified to species level. Diatom and coccolithophore sinking assemblages reflected some seasonal ecological succession. A theoretical scheme of diatom succession in live assemblages is compared to successional patterns presented in sediment traps. This study provides a unique, direct measurement of the biogeochemical fluxes and their main biological carbon vectors just below the winter mixed layer depth at which effective sequestration of carbon occurs. Comparison of these results with previous sediment trap deployments at the same site at deeper depths (i.e. 1000, 2000 and 3800m) documents the changes particle fluxes experience in the lower “twilight zone” where biological processes and remineralisation of carbon reduce the efficiency of carbon sequestration.

Variability in diel and seasonal in situ metabolism of the tropical gastropod Tectus niloticus

Researchers often use metabolic measurements in the field over narrow time periods to estimate an organism's metabolism over large time scales. Here, we measured in situ respira- tion, calcification and excretion rates of the tropical gastropod Tectus niloticus L. through benthic chamber experiments. Our samples spanned a 21 h time frame and were taken during both the warm and cool seasons. We assessed diel and seasonal variability in metabolic rates, as well as the effect of individual size and the contribution of shell epi- and endobionts. Our results show that metabolic rates vary through time at both diel and seasonal scales, as measured fluxes for respira- tion and calcification were significantly higher at night during the warm season. This nocturnal pattern was not significant in the cool season. Size effects were significant with higher respiration and calcification rates for small individuals regardless of the season, although the difference tended to be more pronounced in the warm season. We also found that shell epi- and endobionts made an important contribution to respiration, as 40 and up to 100% of total measured fluxes for night and day, respectively, could be attributed to the shell community. More importantly, the direction of the measured flux was occasionally opposite that of the individual trochus, highlight- ing that the contribution of shell epi- and endobionts must be accounted for in order to achieve an accurate understanding of individual metabolism. Lastly, depending on the time of day and season when measurements are taken, ignoring diel or seasonal variations in metabolic rates could result in important under- or overestimation of the contributions of gastropods to carbon and calcium carbonate fluxes in coastal ecosystems.

Characterization of El Niño, La Niña, and normal conditions through planktonic foraminifera (2006–2007) in the southeastern Pacific

We conducted a study to characterize the foraminiferal association off south-central Chile under positive (January to December 2006), neutral (January to June 2007), and negative (July to September 2007) thermal anomalies, which coincided approximately with El Niño (EN), normal (NC), and La Niña (LN) conditions, respectively, using samples collected in a sediment trap located off Concepción at 2300 m depth. During the three environmental conditions, total species richness was 19, with 18 species present during EN, 14 during NC, and 14 during LN. Moreover, a typical association of subantarctic waters was observed during NC and LN, but not during EN, when we found a mixture of cold and warm water species as well as higher foraminiferal and calcium carbonate fluxes. However, diversity was very similar under the three conditions. We observed significant differences in foraminiferal and calcium carbonate fluxes between NC and EN, with higher values during the warm phase.

Seasonal variations of planktic foraminifera during 2005-2006 off Iquique (20°S) and Concepción (36°S), Chile

We studied for first time the seasonal variations of planktic foraminifera off Iquique (20oS, 70oW) and Concepcion (36oS, 74oW), Chile. The samples were collected by sediment traps located at two depths: 1000 and 2300 m between September and December 2005, under normal conditions (with permanent upwelling and the absence of El Nino) in the northern area and between January and October during the event El Nino 2006, in the southern area (with seasonal upwelling). In this study we determined in total 23 species, 22 which were collected off Iquique and 16 off Concepcion. Furthermore, we determined the following parameters: carbonate and individual fluxes and the diversity in order to define faunal patterns in these areas in different oceanographic conditions. The results show that off Iquique (20°S) under normal conditions, there is a constant individual and carbonate flux, with peaks in September and October 2005 reaffirming the presence of coastal upwelling in this period. In contrast, off Concepcion, under El Nino 2006, the individuals and calcium carbonate fluxes exhibits maxima during the summer upwelling showing however are of shorter duration (January-February) compared to that described in normal conditions for this area. The diversity was higher to a lower depth in the both areas remained constant throughout the period in the northern area, in contrast to the southern area, whose highest values were observed during the summer coinciding with coastal upwelling in this area.

A new look at the alkalinity of the Sea of Japan

The most important feature of the distribution of the alkalinity and calcium in the Sea of Japan—the increase in the potential alkalinity with depth under the conditions when the waters are supersaturated in relation to calcium carbonate—is considered. It is demonstrated that this fact cannot be accounted for by the reaction of the formation-dissolution of calcium carbonate. A new concept explaining the alkalinity distribution in the sea is proposed. According to it, the biological pump is the basic process responsible for the alkalinity transport from the euphotic layer into the interior of the sea. Photosynthesis is the driving force for this process. The role of the active element transporting the alkalinity is not calcium carbonate, as has been claimed elsewhere, but extracellular polysaccharides (EPSs) produced by phytoplankton. EPSs bind to calcium and other cations to form transparent exopolymer particles (TEPs). The proposed conception makes it possible to explain the following: (a) the vertical flux of calcium carbonate that is independent of the super-saturation—undersaturation state of the ambient water regarding calcium carbonate; (b) the existence of the calcium carbonate flux regardless of the nature of the plankton skeletons; (c) the nonstoichiometric ratio between the alkalinity and calcium fluxes.

Phytoplankton community during a coccolithophorid bloom in the Patagonian Shelf: microscopic and high-performance liquid chromatography pigment analyses

We describe the phytoplankton community and biomass during a summer coccolithophorid bloom sampled over the Patagonian shelf (48.5°S–50.5°S). Those phytoplankton species can contribute to the flux of calcium carbonate out of surface waters. Results from both microscope and high-performance liquid chromatography (HPLC) analysis are shown to complement information on the phytoplankton community. From CHEMTAX analysis of HPLC data, the most important organisms and groups identified were the coccolithophoridEmiliania huxleyi, the haptophytePhaeocystis antarctica, dinoflagellates, diatoms, cryptophytes, prasinophytes and cyanobacteria. Phytoplankton microscope counts were converted into phytoplankton group-specific biovolume estimates. Although some microscope-identified taxa could not be determined by CHEMTAX, e.g. the autotrophic ciliateMyrionecta rubra, cluster analyses from both techniques showed similar results for the main groups. BothEmiliania huxleyicell concentration and biomass, and the pigment 19′-hexanoyloxyfucoxanthin were the most important biological features during the sampling period. At surface, nitrate was moderately high (0.2–4.2 µM) in coccolithophorid-dominated samples, whereas phosphate (&lt;0.33 µM) and silicate (&lt;1.35 µM) concentrations were low. Among the environmental factors low Si:N ratios were mainly associated with the dominance ofE. huxleyi. Competition and probably differential grazing could also promote a coccolithophorid outgrowth over other photoautotrophs during the summer season in the Patagonian shelf.

Evaluating effect of ballast mineral on deep‐ocean nutrient concentration by using an ocean general circulation model

The ballast parameterization separates sinking organic flux into two parts: the flux associated with ballast minerals and the flux independent of minerals. The ballast parameterization begins to be incorporated into global ocean biogeochemical models. However, parameters used in this parameterization such as those of calcium carbonate flux are not constrained enough and it has been difficult to evaluate quantitatively how much the ballast‐induced flux affects nutrient concentration in deep ocean. In this study, we obtain optimized parameters of calcium carbonate flux by conducting 64 sensitivity simulations using export production estimated from satellite observation and an ocean circulation field simulated by a state‐of‐the‐art ocean general circulation model. By comparing simulations with and without the ballast‐induced flux with the optimized parameters, 6% of nutrient of the deep Pacific is estimated to be transported by the ballast‐induced flux. Because this accounts for 30% of effects of the total biological pump, it indicates that the ability of the biological pump to carry nutrients to deep ocean significantly depends on the ballast‐induced flux. Results of additional sensitivity simulations suggest that choice of model parameters associated with calcium carbonate flux strongly affects its estimation. Although the implementation of the ballast parameterization has potential ability to improve simulation of nutrient concentration, it is possible only when the model parameters of calcium carbonate flux are set appropriately.

Authigenic calcium carbonate flux in groundwater-controlled lakes: Implications for lacustrine paleoclimate records

Groundwater dominated lakes are an important feature of many landscapes. Their sediments are a particularly valuable source of paleoenvironmental information in semiarid regions where perennial lakes may otherwise be scarce. Where groundwater and lake composition are favorable, carbonate mineral precipitation, evaporative concentration of lake water, and microbial processes can combine to strongly deplete dissolved Ca relative to influent groundwaters. The authigenic carbonate flux (ACF) can then become limited by water column cation availability and thereby be coupled to groundwater inflow rates and aquifer recharge. Here we analyze sedimentary records from two marl-producing, groundwater-controlled lakes and demonstrate a link between one-dimensional ACF and the Palmer Drought Severity Index (PDSI), a measure of land surface wetness. In a restricted outflow lake with high-carbonate alkalinity, ACF is enhanced during historically wet climatic periods in response to increased aquifer recharge rates. ACF in this lake declines during droughts. A neighboring dilute lake with a high rate of groundwater outflow shows comparatively weak coupling between ACF and PDSI history. Ionic chemistry, carbonate mineral equilibria, and δ 13C patterns of dissolved inorganic carbon show that the sensitivity of the ACF signal depends on the degree of evaporative evolution of lake water and the mineral saturation state of the water column under conditions of stratification and ice cover.

Long time series of deep water particle flux in three biogeochemical provinces of the northeast Atlantic

Long-term observations of the deep ocean particle flux from three sites in the northeast Atlantic (33°N, 22°W; 47°N, 20°W; 54°N, 20°W) provide the basis for comparison and characterization of the biogeochemical provinces in terms of sedimentation pattern. Deep ocean particle flux data (2000 m) for fluxes of total mass and the flux composition are presented and compared to published sediment trap data from this area to consider regional-scale variations in the quantity and composition of settling material. The observations show that in the northeast Atlantic gradient of decreasing mass flux from North to South, exists consistent with known changes of biological productivity in surface waters. This gradient is associated with similar trends in opal and particulate organic carbon, whereas calcium carbonate shows trend in the opposite direction. The changes in the composition of the settling material found along the transect are indicating that the calcium carbonate flux is critical in removing organic matter from the upper ocean to the deeper sink. Its role declines from the subtropical ocean (60–80% of the particle flux) towards North (< 40%) reflecting the decreasing importance of coccolithophorid/foraminiferal blooms for particle flux from the subtropical to the subpolar North Atlantic. In contrast, the role of biogenic silica (opal) in regard to the ballasting effect increases towards North. The northern sites have much higher percentage of biogenic silica than the sites in the South, because of the deep winter mixing and the seasonality of phytoplankton dominated by diatom blooms during spring and summer. The comparison of the seasonal pattern of particle flux with the seasonal pattern of surface chlorophyll a concentrations from SeaWiFS together with the similarity of the pattern observed in calcium carbonate and opal leads to the conclusion that the particle flux at two positions (33°N, 22°W; 47°N, 20°W) is fast and directly coupled to the phytoplankton dynamics in the overlying euphotic zone.

A mid-Holocene shift in Arctic sea-ice variability on the East Greenland Shelf

Records of iceberg-rafting and palaeohydrography from two East Greenland shelf cores (JM96-1206/1-GC and JM96-1207/1-GC) are reported. Benthic foraminifera, stable isotopes and IRD fluxes indicate a shift toward colder, lower-salinity ‘polar’ conditions c. 5 cal. ka. A new proxy of iceberg-rafting on the East Greenland Shelf is the flux of calcium carbonate (TIC) thought to be derived from glacial erosion of Cretaceous calcareous mudstones. A change in the regularity and spacing of carbonate flux peaks at c. 4.7 cal. ka in JM96-1207 coincides with the onset of Neoglacial cooling in the Renland ice core δ18O record. We propose that the carbonate flux peaks between 4.7 and 0.4 cal. ka are related to sea-surface coolings associated with increased flux of polar water and sea ice in the East Greenland Current. These peaks are synchronous with sea-surface coolings interpreted from North Atlantic deep-sea cores, but additional peaks centred around 2.4 and 3.8 cal. ka in JM96-1207 suggest that the shelf site captures higher-frequency events. The data indicate that severe Arctic sea-ice events began in the Neoglacial interval, and that earlier-Holocene cool events in deep-sea records are associated with other processes, such as release of meltwater from residual glacier ice and glacial lakes.

Alkenones and particulate fluxes in sediment traps from the central equatorial Pacific

We investigated a year-long (September 1992 to August 1993) time series of total mass, calcium carbonate, organic carbon, opal, and alkenone fluxes in sinking particles collected with sediment traps moored at 1770 and 4220 m in the central equatorial Pacific. The total mass fluxes varied from 14.7 to 68.7 mg/m 2/day at 1770 m, with greater fluxes in October–November and February–April, and from 14.6 to 50.4 mg/m 2/day with peak fluxes during October–November at 4220 m. High flux in the spring season shown at 1770 m was not indicated at 4220 m; instead, a slight increase was shown during a broad period from March to June. The calcium carbonate fluxes varied from 10.8 to 49.1 mg/m 2/day with higher fluxes in October–November and March–April at 1770 m, and from 8.9 to 37.0 mg/m 2/day with a higher flux in October–November at 4220 m. The organic carbon fluxes varied from 0.36 to 5.91 mg/m 2/day, with higher fluxes in October–November and March–April at 1770 m, and from 0.72 to 2.58 mg/m 2/day at 4220 m. The annual mean organic carbon flux was 1.84 and 1.28 mg/m 2/day at 1770 and at 4220 m, respectively. These values were less than half of those reported for the EqPac sediment trap experiment. The opal fluxes varied from 0.55 to 4.4 mg/m 2/day at 1770 m and from 1.23 to 2.95 mg/m 2/day at 4220 m. Alkenone fluxes varied significantly from 0.05 to 0.84 μg/m 2/day, with high values in November, February–March, and June at 1770 m. For the 4220 m trap, these values ranged from 0.05 to 0.25 μg/m 2/day, with slightly higher fluxes in April–May and June–July, which followed periods of high alkenone fluxes observed in February–April and June–July, respectively, at 1770 m depth. These values were remarkably low compared with those reported by the previous studies at other sites. U 37 K′ values were constantly high >0.95 throughout the collection period. However, relatively low U 37 K′ values (0.92 and 0.93) were occasionally observed during February to March. Estimated alkenone temperatures from those U 37 K′ values were about 27–29°C and consistent with the observed temperature of the upper layer at ca.100 m depth. The seasonal change of the U 37 K′ values could be affected by not only water temperature but also the relative amount of ‘warm’ and ‘cold’ types of alkenone producer in the central equatorial Pacific.

The fate of coccoliths at 48°N 21°W, Northeastern Atlantic

As part of the Joint Global Ocean Flux Study Program (JGOFS) North Atlantic Bloom Experiment (NABE), coccolithophore fluxes were recorded over one-year period, from April 1989 to April 1990. Three vertically moored sediment traps, synchronized on a 14-day interval, were deployed at 48° N 21° W at three water depths, at 1, 2 and 3.7 km (sea floor at 4.4 km). We present the results on the vertical variability of coccolith and coccosphere fluxes through the water column. Calcium carbonate was the largest component of the total mass flux, followed by particulate organic matter and opal. Coccolith and coccosphere seasonal fluxes followed the seasonality of the biogeochemical particle fluxes at all three depths, with maxima occurring during the spring particle “bloom”. The decrease in correlation between coccosphere and coccolith fluxes with depth (from r=0.98 at 1 km depth to 0.60 at 3.7 km depth) documents that disaggregation of the coccospheres occurs during settling in the deeper part of the water column between 1 and 3.7 km water depth. Annual coccosphere and coccolith fluxes were 1.5×10 8 and 0.9×10 11 m −2 year −1 respectively, at 1 km depth (or 3.1×10 8 and 1.4×10 11 m −2 year −1 including the spring 1990 episode). For the 3.7 km trap the corresponding values were 0.5×10 8 and 1.1×10 11 (0.6×10 8 and 1.1×10 11 including the partially recorded spring 1990 episode). The slight increase in coccolith numbers with water depth can be explained by coccosphere disintegration and lateral influx. Nevertheless, selective dissolution of delicate coccolith species from 1 to 3.7 km water depth occurs (e.g., relative abundances of Oolithothus fragilis coccoliths decrease from 3% at 1 km to less than 0.5% at 3.7 km water depth). Emiliania huxleyi, Gephyrocapsa muellerae, Calcidiscus leptoporus and Coccolithus pelagicus constitute from 65% (at 1 km depth) to 95% (at 3.7 km depth) of the total coccolith assemblages. The same four coccolith taxa dominate the fossil assemblages in the underlying surface sediments. Selective dissolution continues in the sediments, most notably the abundance of the solution-resistant species C. leptoporus increases from 10 to 15% in the trap samples to 15–40% in the sediments. Annual flux of calcium carbonate in sediment trap samples accounts for about 60% of the total annual mass flux, with the coccolith fraction (<32 μm) being about 45% of the annual carbonate flux (mean value of 41% at 1 km and 50% at 3.7 km water depth). The correlation between the <32 μm CaCO 3 and the calculated coccolithophore and calcisphere CaCO 3 fluxes decreases with depth ( r=0.92 at 1 km and r=0.86 at 3.7 km), indicating that partial dissolution and fragmentation of the delicate, less calcified, coccolithophore and planktic foraminifera species occurs during settling. This fine fragmentation of biogenic CaCO 3 can contribute to the <32 μm fraction causing the decrease of correlation, with the calculated CaCO 3 fluxes being based only on the flux of intact coccoliths and calcispheres.

Bioerosion in the carbonate budget of two Indo-Pacific reefs: La Réunion (Indian Ocean) and Mooréa (Pacific Ocean)

On coral reefs, products of bioerosion through the activity of micro- and macro-boring organisms and grazers are particulate and dissolved carbonate. The con-tribution to bioerosion of the major boring and grazing species was estimated on French Polynesia and the island La Réunion. The influence of bioerosion on the calcification/dissolution budget of reefal waters and on particulate calcium car-bonate fluxes was investigated. Studies on bioerosion were performed in order to assess: (a) eroding activity of grazers through the gut content analysis of echinoids and scarid parrot fish, (b) eroding activity of micro- and macroborers from ex-perimental substrates exposed for one year, and (c) role of sponges in total bioero-sion on reefal and experimental substrates. Calcification/dissolution budget was measured using the alkalinity anomaly technique, and particulate carbonate pro-duction was analysed from trapped sediments.

Return of the coral reef hypothesis: basin to shelf partitioning of CaCO3 and its effect on atmospheric CO2.

Differences in the rate of coral reef carbonate deposition from the Pleistocene to the Holocene may account for the Quaternary variation of atmospheric CO2. Volumes of carbonate associated with Holocene reefs require an average deposition rate of 2.0 x 10(13) mol/yr for the past 5 ka. In light of combined riverine, midocean ridge, and ground-water fluxes of calcium to the oceans of 2.3 x 10(13) mol/yr, the current flux of calcium carbonate to pelagic sediments must be far below the Pleistocene average of 1.2 x 10(13) mol/yr. We suggest that sea-level change shifts the locus of carbonate deposition from the deep sea to the shelves as the normal glacial-interglacial pattern of deposition for Quaternary global carbonates. To assess the impact of these changes on atmospheric CO2, a simple numerical simulation of the global carbon cycle was developed. Atmospheric CO2 as well as calcite saturation depth and sediment responses to these carbonate deposition changes are examined. Atmospheric CO2 changes close to those observed in the Vostok ice core, approximately 80 ppm CO2, for the Quaternary are observed as well as the approximate depth changes in percent carbonate of sediments measured in the Pacific Ocean over the same time interval.

Pelagic Sedimentation of Aragonite: Its Geochemical Significance

The relative importance of the pelagic flux of aragonite, as compared to calcite, to the deep-sea floor has been evaluated by means of a quantitative x-ray diffraction study of samples collected from sediment traps and from an unusually shallow portion of the open Atlantic Ocean (the Rio Grande Rise). The results suggest that the aragonite flux constitutes at least 12 percent of the total flux of calcium carbonate on a worldwide basis. The presence of high-magnesium calcite in several samples suggests that some of the calcareous material falling to the deep-sea floor may be derived from the long-distance transport of debris from shallow-water beenthic organisms as well as from the settling of planktonic remains. This observation supports the contention that 12 percent represents a minimum value. Aragonite and high-magnesium calcite transported laterally from shallow-water regions, upon dissolution during settling into deeper water, may contribute to the neutralization of excess anthropogenic carbon dioxide added to the oceans.