Calcium Carbonate Preservation Research Articles

Azoic sediments and benthic foraminifera: Environmental quality in a subtropical coastal lagoon in the gulf of California

Marine transitional environments play an important role in human sustainability. Around these ecosystems, coastal lagoons are subject to high anthropogenic pressure from population growth. The increased demand for goods and services is associated with the elevated discharge of untreated and treated wastewater into lagoon systems. The absence of benthic organisms in lagoon environments has been linked to extreme natural conditions and severe anthropogenic impact at both spatial and temporal scales. However, the mechanisms that lead to the presence of azoic sediments in lagoon environments have yet to be studied. This study aimed to determine the vertical variability of textural groups, geochemistry, and benthic foraminiferal fauna to understand how natural and anthropogenic components generate a vertical sediment sequence with low or absent benthic foraminifera in a subtropical coastal lagoon in the southwestern end of the Gulf of California. A 41 cm-long sediment core was collected from La Paz Lagoon at a 1-m depth. The core was sectioned every centimeter, and sediment subsamples were dried and homogenized for grain size, calcium carbonate, elemental and isotopic carbon and nitrogen analyses, and benthic foraminifera quantification. Muds with fine sands towards the core's base characterized the sedimentary sequence. Organic carbon and total nitrogen increased from the base (1.4% and 0.06%, respectively) to the core-top (CT, 3.0% and 0.14%, respectively), significant from the 27 cm interval. Calcium carbonate content was very low (<0.8%). The relationship of δ13C vs. C:N ratio indicated that sedimentary organic carbon was derived from the marine and sewage source mixture. The δ15N of organic matter increased by 3.7‰, starting from the 27 cm interval towards the CT. The nitrogen sewage input source was relatively more significant than nitrogen fixation. The few individuals (<18 ind. in 10 g) and genera (Ammonia and Elphidium), as well as the absence of foraminifera in 19 of 41 intervals in the core, indicated that environmental conditions were unfavorable, even for colonization of environmentally stress-tolerant genera. The frequency of azoic sediments was higher from the 25 cm interval to the CT vs. from the base to the 25 cm interval. Moreover, the AEI revealed severe to moderate hypoxia in the study area. The limited presence of benthic foraminifera and calcium carbonate preservation corroborated that the quality of the lagoon's environment has deteriorated along with population growth, which requires strategic programs to sustain this transitional ecosystem.

Middle to Late Eocene Changes of the Ocean Carbonate Cycle

AbstractSedimentary records show that calcium carbonate (CaCO3) preservation fluctuated during the Eocene. These fluctuations are well documented for the equatorial Pacific. However, data from other basins are sparse. In this study, we report new middle and late Eocene bulk calcium carbonate percentages and accumulation rates from the northwestern Pacific (Ocean Drilling Program—ODP—Site 884) and the Atlantic (ODP Sites 1053, 1090, and 1263) Oceans; in addition, we calculate CaCO3 accumulation rates for sites with published percentage bulk CaCO3 to expand geographic and paleobathymetric coverage. Using these data, we investigate the response of the carbonate cycle to environmental changes (e.g., temperatures, primary productivity, weathering, and ocean circulation) at the beginning of the greenhouse‐icehouse transition (∼43–34 Ma). Our results show that in the middle to late Eocene CaCO3 accumulation rates were highly variable at different paleodepths and ocean basins suggesting that the evolution of carbonate accumulation rates over the Eocene was influenced by different processes in different locations. In particular, our data emphasize the role of surface CaCO3 production and ocean ventilation in driving changes in CaCO3 preservation and burial at the seafloor. Our study also highlights the need for a better understanding of the processes regulating CaCO3 surface production today in order to correctly interpret geological records.

On the sedimentary carbonate accumulation and dissolution in Western Pacific marginal basins

AbstractThe critical role of marginal seas in the global carbon cycle and their response to climate and circulation changes are not well understood. In this study, we used long‐term archives of sedimentary CaCO3 data and a conceptual model to systematically determine the spatial and vertical features of the sediment carbonate system in the marginal seas along the Western Pacific Ocean. Our results show that the northward‐flowing Antarctic Bottom Water and Lower Circumpolar Deep Water produced shallower calcium carbonate preservation depths as they moved north and became more carbonate‐undersaturated. This suggests that the carbonate chemistry in the deep Western Pacific marginal sea basins is mostly governed by the global ocean thermohaline circulation. In contrast, the carbonate system in the deep Japan/East Sea is unique, with limited sedimentary carbonate accumulation due to weak calcification and its internal overturning circulation. Despite differences in the hydrological and ecological settings, the sedimentary calcium carbonate profiles in the Tasman Sea and South Fiji Basin are comparable to those in the Atlantic and Pacific Oceans, reflecting the remarkable influence of open‐ocean carbonate chemistry.

A model for marine sedimentary carbonate diagenesis and paleoclimate proxy signal tracking: IMP v1.0

Abstract. The preservation of calcium carbonate in marine sediments is central to controlling the alkalinity balance of the ocean and, hence, the ocean–atmosphere partitioning of CO2. To successfully address carbon cycle–climate dynamics on geologic (≫1 kyr) timescales, Earth system models then require an appropriate representation of the primary controls on CaCO3 preservation. At the same time, marine sedimentary carbonates represent a major archive of Earth history, as they have the potential to preserve how seawater chemistry, isotopic composition, and even properties of planktic and benthic ecosystems, change with time. However, changes in preservation and even chemical erosion of previously deposited CaCO3, along with the biogenic reworking of upper portions of sediments, whereby sediment particles are translocated both locally and nonlocally between different depths in the sediments, all act to distort the recorded signal. Numerical models can aid in recovering what the “true” environmental changes might have been, but only if they appropriately account for these processes. Building on a classical 1-D reaction-transport framework, we present a new diagenetic model – IMP (Implicit model of Multiple Particles (and diagenesis)) – that simulates biogeochemical transformations in carbonate-hosted proxy signals by allowing for populations of solid carbonate particles to possess different physicochemical characteristics such as isotopic value, solubility and particle size. The model also utilizes a variable transition matrix to implement different styles of bioturbation. We illustrate the utility of the model for deciphering past environmental changes using several hypothesized transitions of seawater proxies obscured by sediment mixing and chemical erosion. To facilitate the use of IMP, we provide the model in Fortran, MATLAB and Python versions. We described IMP with integration into Earth system models in mind, and we present the description of this coupling of IMP with the “cGENIE.muffin” model in a subsequent paper.

The last two millennia: climate, ocean circulation and palaeoproductivity inferred from planktic foraminifera, south-western Svalbard margin

We reconstruct climate and changes in water-mass properties in relation to variations in palaeoproductivity at the south-western Svalbard margin throughout the last 2000 years. Environmental conditions in subsurface (ca. 250–75 m) and near-surface to surface water (75–0 m) were studied on the basis of the distribution patterns and fluxes of planktic foraminiferal faunas. Stable isotopes in three different species were measured, and Mg/Ca- and transfer function-based sea-surface temperatures were calculated. The mean shell weights of planktic foraminiferal species were used to assess changes in calcium carbonate preservation. Modern total planktic foraminiferal distribution patterns from plankton tows and the water column carbonate chemistry were investigated for comparison with the palaeo-data. The results show warm sea-surface conditions and moderate to high surface productivity at ca. 21–400 AD, ca. 900–1400 AD and from about 1850 AD until present, which may be local expressions of the European climatic events known as the Roman Warm Period, the Medieval Climate Anomaly and the Recent Warming. In general, cold near-sea-surface conditions and very low to moderate average productivity occurred at about 400–900 AD and ca. 1400–1850 AD, the latter probably the local expression of the Little Ice Age. The highest and most variable planktic productivity occurred at ca. 1300–1500 AD, ca. 1750–1860 AD and during the last 50 years or so. These periods are linked to the general amelioration of conditions from years with a dense sea-ice cover to years with a rapidly fluctuating summer sea-ice margin.

A new Upper and Middle Pleistocene stratigraphic and dissolution record from the Carnegie Platform, Eastern Equatorial Pacific

Abstract This study presents new stable isotope and dissolution records for a composite “Carnegie Platform” (CP) sediment record composed of KNR195–5 cores CDH-36 and MC-34A (3° 35.85′ S, 83° 57.79′ W, 3225 m) from the Eastern Equatorial Pacific. Renewed interest in this paleoceanographic site prompted a re-coring in 2009 using the CDH giant piston coring system. Giant piston core CDH-36, with a core length of 42.61 m, nearly triples the length of previous cores at this site. Companion multicore MC-34A (0.36 m length) allowed for recovery of intact surface sediments at 0 ka BP. A new age model utilizing Bayesian analysis of radiocarbon dates in MC-34A and the top 3.4 m of CDH-36, and alignment of the CDH 36C. wuellerstorfi δ18O record with the LR04 benthic stack, demonstrates that the CP record extends from 0 to 720 ka (MIS 18). The age model places Eastern Equatorial Pacific volcanic ash layer “L,” which occurs at 15.80–15.88 m in CDH-36, at 229.6–231.7 ka BP in the CP record, consistent with prior age assignments. Coarse fraction percentage (%CF) is utilized to evaluate the impact of dissolution at the CP site;CP %CF primarily records dissolution cycles of ~100 kyr, with enhanced preservation during glacial episodes, but also exhibits variability consistent with local bathymetric and hydrographic effects. The similarity of the CP %CF record to other tropical Pacific dissolution records supports a basin-wide control on calcium carbonate preservation during the middle and Late Pleistocene.

Modelling the abundance of soil calcium carbonate across Australia using geochemical survey data and environmental predictors

Soil calcium carbonate is a key component of the regolith, particularly in arid and semi-arid regions. It influences soil properties, is an important terrestrial carbon store, and is used in mineral exploration. Previous national overviews of soil carbonate distribution have been compiled from regional soil, regolith or geological maps with varying degrees of confidence and consistency. Here we develop a decision tree approach based on a piecewise linear regression model to map soil calcium carbonate abundance at the continental scale. The model is based on relationships established from 1311 field sites sampled across Australia at two depths (0–10 and ~60–80cm) and 49 national environmental predictor datasets reflecting fundamental soil formation processes (e.g., climate, parent material, relief).The resulting map of soil calcium carbonate distribution is based on the median results of twenty model runs generated by randomly selecting 90% training and 10% validation splits of the input data. Results present an average coefficient of determination (R2) of 0.5 on the validation dataset. The predictors used in the modelling are consistent with our understanding of the controls on the sources (inputs), formation, and preservation of soil calcium carbonate across Australian landscapes. The model produces a continuous, internally consistent, quantitative prediction of soil calcium carbonate abundance in surficial regolith at a resolution of 90m with associated uncertainty estimates. This type of analysis provides a framework for better understanding environmental controls on regolith processes and has the potential to also be used to predict other soil geochemical properties. Furthermore, the methodology has utility globally where geochemical survey data are available.

Surface water conditions and calcium carbonate preservation in the Fram Strait during marine isotope stage 2, 28.8-15.4 kyr

[1] We present a high-resolution record of calcium carbonate preservation alongside the distribution pattern of planktic foraminifera from the Fram Strait. The record covers the marine isotope stage (MIS) 2, 28.8 to 15.4 kyr, including the Last Glacial Maximum (LGM) and the early deglaciation in multidecadal temporal resolution. The investigation is based on the distribution patterns of planktic foraminifera, stable isotopes, mean shell weight of Neogloboquadrina pachyderma, the degree of fragmentation of planktic shells, CaCO3 content, and geochemical and sedimentological data. The dissolution proxies indicate long-lasting periods of markedly reduced preservation of calcium carbonate at ~28.8–27.2, ~24.1–23.2, and ~16.6–15.4 kyr in addition to a number of short-lasting periods (< 300 year) during the LGM. These periods are accompanied by a simultaneous and significant reduction in specimens of subpolar planktic foraminifera indicating a southward expansion of Arctic surface water masses. Prolonged periods with high abundances of subpolar foraminiferal species and good preservation of calcium carbonate are attributed to increased influence of Atlantic water masses, which appears to persist throughout most of the MIS 2. Comparison to Holocene (MIS 1) shell weight records shows that the preservation during MIS 2 was overall better, even during the events of dissolution. This was probably a reflection of the low concentration of atmospheric CO2 at the time.

Paleoceanographic changes and calcium carbonate dissolution in the central Fram Strait during the last 20 ka

A deep-sea sediment core covering the last 20ka and located between the Polar and the Arctic fronts in the marginal ice zone (MIZ) of the central Fram Strait has been investigated for changes in paleoceanography and calcium carbonate preservation. The reconstruction is based on the distribution patterns of planktic foraminifera, mean shell weight and the degree of fragmentation of their shells, stable isotopes and other geochemical and sedimentological data. The results show that the planktic foraminifera shells are poorly preserved throughout most of the record. Only the intervals comprising the early Holocene from 10.8 to ~8calka BP and the last 800yr show improved preservation of CaCO3. The dissolution correlated with the extent of Arctic water and the associated marginal ice zone (MIZ) and high organic productivity. Dissolution of planktic foraminifera is generally high during the late deglaciation, mid and late Holocene prior to ~800calyr BP. The abundance of small subpolar species increases in the surface sediments dating from the last century, which could be interpreted as a large and significant surface water warming. However, this apparent high-magnitude warming seems to be overestimated due to preservation changes in the youngest sediments.

Response of deep‐sea CaCO3 sedimentation to Atlantic meridional overturning circulation shutdown

Changes in the distribution of the preservation and burial of calcium carbonate (CaCO3) in deep ocean sediments and associated atmospheric pCO2 response to the shutdown of the Atlantic meridional overturning circulation (MOC) are examined using an Earth system model. We find that shutdown of the Atlantic MOC forced by the freshwater inflow significantly decreases the CaCO3 content in North Atlantic sediments. This is a consequence of a decrease in bottom‐water carbonate ion concentrations and reduction in sea‐surface CaCO3 production. The main sedimentary impacts of these two effects are separated in time, however, with reduced CaCO3 production dominating the decrease in CaCO3 burial during the first 1000 years after the forcing is applied. In the absence of significant overturning circulation in the Atlantic, atmospheric pCO2 increases by 11 ppm, largely due to a decrease in POC export and a weakening biological pump. The change in pCO2 induced by reorganization of CaCO3 burial in deep‐sea sediments is small, only 1 ppm, because increased preservation of CaCO3 in the Pacific largely efficiently buffers decreased preservation in the Atlantic, leaving the global burial and ocean alkalinity minimally changed at equilibrium.

Marine geochemical data assimilation in an efficient Earth System Model of global biogeochemical cycling

Abstract. We have extended the 3-D ocean based "Grid ENabled Integrated Earth system model" (GENIE-1) to help understand the role of ocean biogeochemistry and marine sediments in the long-term (~100 to 100 000 year) regulation of atmospheric CO2, and the importance of feedbacks between CO2 and climate. Here we describe the ocean carbon cycle, which in its first incarnation is based around a simple single nutrient (phosphate) control on biological productivity. The addition of calcium carbonate preservation in deep-sea sediments and its role in regulating atmospheric CO2 is presented elsewhere (Ridgwell and Hargreaves, 2007). We have calibrated the model parameters controlling ocean carbon cycling in GENIE-1 by assimilating 3-D observational datasets of phosphate and alkalinity using an ensemble Kalman filter method. The calibrated (mean) model predicts a global export production of particulate organic carbon (POC) of 8.9 PgC yr−1, and reproduces the main features of dissolved oxygen distributions in the ocean. For estimating biogenic calcium carbonate (CaCO3) production, we have devised a parameterization in which the CaCO3:POC export ratio is related directly to ambient saturation state. Calibrated global CaCO3 export production (1.2 PgC yr-1) is close to recent marine carbonate budget estimates. The GENIE-1 Earth system model is capable of simulating a wide variety of dissolved and isotopic species of relevance to the study of modern global biogeochemical cycles as well as past global environmental changes recorded in paleoceanographic proxies. Importantly, even with 12 active biogeochemical tracers in the ocean and including the calculation of feedbacks between atmospheric CO2 and climate, we achieve better than 1000 years per (2.4 GHz) CPU hour on a desktop PC. The GENIE-1 model thus provides a viable alternative to box and zonally-averaged models for studying global biogeochemical cycling over all but the very longest (&gt;1 000 000 year) time-scales.

Calcium carbonate pump during Quaternary glacial cycles in the South China Sea

The preservation and dissolution of calcium carbonate (namely calcium carbonate pump) controls the pH of seawater in global oceans by its buffer effect, and in turn plays a significant role in global changes in atmospheric CO2 concentration. The results from measured carbonate con- tents over the past 2 Ma at ODP Site 1143 in the South China Sea provide high-resolution records to explore the process of the calcium carbonate pump during Quaternary glacial cy- cles. The results indicate statistically that the highest car- bonate accumulation rate leads the lightest δ δ δ δ 18 Ob y about 3.6

An end to the “rain ratio” reign?

One of the most elegant mechanisms forwarded for late Quaternary atmospheric CO2 variability concerns the sensitivity of calcium carbonate preservation in deep ocean sediments to the relative delivery rates of calcium carbonate and particulate organic carbon (the CaCO3:POC “rain ratio”). It was implicitly assumed that any change in the CaCO3:POC rain ratio of biogenic material produced in the surface ocean will be communicated directly to the sediments. This would allow relatively subtle shifts in ecosystem composition to affect sedimentary CaCO3 preservation (and thus atmospheric CO2). However, recent research into the controls on the transport of POC to depth suggests that the rain ratio “seen” by the sediments may instead be buffered against any perturbation occurring at the surface. This casts doubt on the viability of hypotheses envisaging ecological changes as a means of accounting for the observed glacial‐interglacial CO2 signal.

Late Quaternary variations in calcium carbonate preservation of deep-sea sediments in the northern Cape Basin: results from a multiproxy approach

In this study, we test various parameters in deep-sea sediments (bulk sediment parameters and changes in microfossil abundances and preservation character) which are generally accepted as indicators of calcium carbonate dissolution. We investigate sediment material from station GeoB 1710-3 in the northern Cape Basin (eastern South Atlantic), 280 km away from the Namibian coast, well outside today’s coastal upwelling. As northern Benguela upwelling cells were displaced westward and periodically preceded the core location during the past 245 kyr (Volbers et al., submitted), GeoB 1710-3 sediments reflect these changes in upwelling productivity. Results of the most commonly used calcium carbonate dissolution proxies do not only monitor dissolution within these calcareous sediments but also reflect changes in upwelling intensity. Accordingly, these conventional proxy parameters misrepresent, to some extent, the extent of calcium carbonate dissolution. These results were verified by an independent dissolution proxy, the Globigerina bulloides dissolution index (BDX′) (Volbers and Henrich, submitted). The BDX′ is based on scanning electronic microscope ultrastructural investigation of planktonic foraminiferal tests and indicates persistent good carbonate preservation throughout the past 245 kyr, with the exception of one pronounced dissolution event at early oxygen isotopic stage (OIS) 6. The early OIS 6 is characterized by calcium carbonate contents, sand contents, and planktonic foraminiferal concentrations all at their lowest levels for the last 245 kyr. At the same time, the ratio of radiolarian to planktonic foraminiferal abundances and the ratio of benthic to planktonic foraminiferal tests are strongly increased, as are the rain ratio, the fragmentation index, and the BDX′. The sedimentary calcite lysocline rose above the core position and GeoB 1710-3 sediments were heavily altered, as attested to by the unusual accumulation of pellets, aggregates, sponge spicules, radiolaria, benthic foraminifera, and planktonic foraminiferal assemblages. Solely the early OIS 6 dissolution event altered the coarse fraction intensely, and is therefore reflected by all conventional calcium carbonate preservation proxies and the BDX′. We attribute the more than 1000 m rise of the sedimentary calcite lysocline to the combination of two processes: (a) a prominent change in the deep-water mass distribution within the South Atlantic and (b) intense degradation of organic material within the sediment (preserved as maximum total organic carbon content) creating microenvironments favorable for calcium carbonate dissolution.

R-Selected benthic foraminifera with associated bacterial colonies in Upper Pleistocene sediments of the Ross Sea (Antarctica): implications for calcium carbonate preservation

Investigations on foraminifera from Upper Pleistocene–Holocene sediments cored in the continental slope of the western Ross Sea (2383 m water depth) revealed that the record of calcareous assemblages was restricted to a limited time interval (approximately 6 kyr) and characterized by a dominance of phytodetritus-exploiting taxa, such as Alabaminella weddellensis and Epistominella exigua. Rod-shaped calcified, fossil bacteria infest the partially dissolved calcareous foraminiferal surfaces, either as clusters or as isolated cells, suggesting that significant changes (from under- to oversaturated conditions with respect to calcium carbonate) in the chemistry of the seawater developed before the final burial of the foraminiferal tests. We postulate that bacterial activity on a microenvironmental scale (interface and pore water) in the sea floor could influence pore water conditions in such a way as to preserve carbonate in deep marine regions where environmental conditions usually prevent the establishment of carbonate secreting communities.

A data‐driven model of the global calcite lysocline

Gridded maps of sediment calcium carbonate (calcite) concentration and overlying water saturation state [Archer, 1996] are combined with maps of benthic oxygen fluxes and sediment accumulation rates from Jahnke [1996] and Cweink [1986] to drive a diagenetic model of calcium carbonate preservation in deep‐sea sediments. The only model input for which we cannot draw a detailed map is the rain rate of calcite to the seafloor, so I use the model to calculate the calcite rain rate required to simulate the observed distribution of calcite concentration on the seafloor. The predictive power of the model can be checked by searching for input parameters to which the model is sensitive and comparing the model requirements with independent data. The model is sensitive to the ratio of organic carbon to calcite rain rates (the rain ratio) and, within the constraint of pelagic sediment trap rain ratio data, is unable to reproduce the calcite field without respiratory dissolution, the promotion of calcium carbonate dissolution by the oxic degradation of organic carbon. However, relative to variability in sediment trap data, the required model rain ratio is insensitive to the extent of anoxic respiration, the stoichiometric ratio of O:C during respiration, the bioturbation rate, the dissolution rate constant, the effect of borate chemistry, and small offsets in the saturation state for CaCO3. The model is sensitive to the accumulation rate of non‐CaCO3 material and predicts an Atlantic/Pacific difference in non‐CaCO3 rain rate which is consistent with observations. The model predicts that the dissolution flux of CaCO3 from sediments is 25–40 × 1012 mol yr−1, roughly half of the total deep sea CaCO3 dissolution rate estimated from the water column alkalinity distribution [Broecker and Peng, 1987; Archer and Maier‐Reimer, 1994; Wollast, 1994]. The model also predicts that only 20–30% of the flux of CaCO3 to the seafloor globally escapes dissolution.

Quaternary history of calcium carbonate fluctuations in the western equatorial Indian Ocean (Somali Basin)

A study of the Quaternary history of calcium carbonate fluctuations in the equatorial Indian Ocean, based on the Swedish Deep-Sea Expedition Core 154 demonstrates that the Calcite Compensation Depth (CCD) was shallower during interglacial periods than during glacials. Higher percentages of calcium carbonate, lower percentages of dissolution-resistant planktonic foraminifera and lower values of the Berger dissolution index, suggesting better calcium carbonate preservation, are recorded during glacial periods and/or at glacial/interglacial transitions. In contrast, interglacial periods are marked by stronger calcium carbonate dissolution. The data indicate that dissolution is the main controlling factor of calcium carbonate in the western equatorial Indian Ocean. Higher dissolution during interglacial periods and enriched δ 13C are due to higher noncarbonate productivity and enhanced Antarctic Bottom Water (AABW) ventilation in the equatorial Indian Ocean.

The upwelling record in the sediments of the western continental margin of India

Analysis of 63 sediment samples from the western continental margin of India for planktonic foraminifera, organic carbon, biogenic silica and calcium carbonate content showed higher frequency (30–40%) of Globigerina bulloides and high organic carbon (>3%) coinciding with higher primary productivity (>1 g C m −2 day −1) in the south compared to lesser (20–30%) abundance of G. bulloides and organic carbon (1–2%) and moderate productivity (0.5–0.75 g C m −2 day −1) in the north. This is suggestive of relatively intense upwelling in the south compared to the north. The gradual decrease in abundance of G. bulloides and organic carbon content from upper slope to lower slope similarly reflects a greater intensity of upwelling on the upper slope. The positive correlation between organic carbon and G. bulloides suggests that G. bulloides can be used as an indicator of productivity in the areas of coastal upwelling. Rich occurrences of most sensitive species for dissolution ( Globigerinoides ruber, Orbulina universa, Globigerinella aequilateralis, Globigerinoides sacculifer and Globigerinita glutinata) and less biogenic silica content in the sediments along with more diatoms (538.10–672.50) × 10 3 1 −1 in the water column indicate the preservation of calcium carbonate and dissolution of siliceous constituents.

Quantitative assessment of carbonate dissolution in marine sediments from foraminifer linings vs. shell ratios: Davis Strait, northwest North Atlantic

A new method to evaluate quantitatively the degree of calcium carbonate preservation and dissolution in high-latitude marine sediments is proposed on the basis of relative abundance of CaCO 3 shells and organic linings of benthic foraminifers. This method was applied to a late Quaternary sequence from Davis Strait in the northwest North Atlantic and shows that CaCO 3 dissolution in sediments has increased since the last glacial maximum (ca. 18,000 B.P.) and peaked when subpolar interglacial conditions were established in surface waters. The dissolution in Davis Strait sediments appears to be closely related to organic biogenic production in surface waters, with a regional pattern of bottom-water formation and circulation.

Calcium carbonate preservation in the ocean

Recent microelectrode pH and O2measurements across the sediment-water interface suggest CaCO3dissolution kinetics substantially slower than laboratory values and support a dissolution response to organic matter degradation near the sediment surface. We report a modelling exercise, motivated by these results, that indicates the pattern of calcium carbonate preservation in the equatorial Atlantic and Indian oceans is most readily reproduced using a dissolution rate constant 10 to 100 times smaller than those measured in the laboratory. The model suggests that 40% or more of the particulate CaCO3rain dissolves at the calcite saturation horizon in response to organic matter degradation within the sediments, and that this effect is insensitive to the choice of dissolution rate constant in ranges measured. Implications are that the dissolution flux from the sediments is greater than previously thought, and that calcium carbonate preservation in the deep sea should be strongly dependent on the particulate organic carbon to calcium carbonate rain ratio.