Undersaturated Bottom Water Research Articles

Seafloor alkalinity enhancement as a carbon dioxide removal strategy in the Baltic Sea

Carbon dioxide removal from the atmosphere and storage over long times scales in terrestrial and marine reservoirs is urgently needed to limit global warming and for sustainable management of the global carbon cycle. Ocean alkalinity enhancement by the artificial addition of carbonate minerals to the seafloor has been proposed as a method to sequester atmospheric CO2 and store it in the ocean as dissolved bicarbonate. Here, a reaction-transport model is used to scrutinize the efficacy of calcite addition and dissolution at a well-studied site in the southwestern Baltic Sea – a brackish coastal water body in northern Europe. We find that most calcite is simply buried without dissolution under moderate addition rates. Applying the model to other sites in the Baltic Sea suggests that dissolution rates and efficiencies are higher in areas with low salinity and undersaturated bottom waters. A simple box model predicts a tentative net CO2 uptake rate from the atmosphere of 3.2 megatonnes of carbon dioxide per year for the wider Baltic Sea after continually adding calcite to muddy sediments for 10 years. More robust estimates now require validation by field studies.

Biostratigraphy and paleoenvironments of the Late Oligocene in the north-western Transylvanian Basin revealed by the foraminifera assemblages

The paleoenvironmental evolution and biostratigraphy of the Oligocene Vima Formation from the north-western Transylvanian Basin (Maramureș county, Romania) is presented based on planktonic and benthic foraminiferal assemblages. The deposits were assigned to late Rupelian–early Chattian ages, more precisely the Globigerina angulisuturalis – Chiloguembelina cubensis CRZ (O4) and Paragloborotalia opima HOZ (O5) Biozones. The ecological preference of benthic foraminifera and statistical treatment of the raw data set were used to identify relationships between assemblages and reconstruct the ancient environments. Paleobathymetrical estimates indicate a deposition of the sediments in outer shelf to upper bathyal settings. The shelf is characterized by alternating well oxygenated bottom waters and poor oxygen content indicated by low-oxygen tolerant taxa. The upper bathyal assemblages dominated by tubular agglutinated foraminifera suggest oligotrophic environments periodically disturbed by turbidite currents, while the whole agglutinated communities suggest undersaturated bottom waters in respect to calcium-carbonate and possible depletion in oxygen content due to water column stratification. Paleoclimatic reconstructions based on planktonic foraminifera suggest cool to temperate surface water temperatures with boreal influences. The younger deposits of the Fântânele section indicate a warming trend at the surface waters. The aggradational to progradational stacking pattern of the identified high-frequency sequences suggest a high stand systems tract under the control of the regional tectonics.

A new perspective on coastal hypoxia: The role of saline groundwater

Processes leading to the development of coastal hypoxia are usually attributed to anthropogenic nutrient loading. We present new evidence documenting a natural process capable of producing large-scale water masses undersaturated with respect to dissolved oxygen, independent of human-caused nutrient loading and subsequent nutrient-enhanced productivity. Along the heavily developed coastline of Myrtle Beach, South Carolina, the offshore discharge of saline, anoxic groundwater generates undersaturated bottom water masses which can be advected into nearshore waters. Highly elevated radium isotope activities measured in low-oxygen bottom waters cannot be supported by any known terrestrial source. Rather, discharge of cold, anoxic, high-radium water from offshore carbonate aquifers is the likely source. Density stratification inhibits mixing of these discharged waters during inshore transport. During a low-oxygen event in August 2012, this restricted mixing is likely the mechanism for the observed oxygen deficit in bottom waters. This discovery suggests benthic layers undersaturated with respect to dissolved oxygen may be considerably more prevalent than currently recognized, particularly around the carbonate-platform of the US East Coast.

Global calcite cycling constrained by sediment preservation controls

We assess the global balance of calcite export through the water column and burial in sediments as it varies regionally. We first drive a comprehensive 1‐D model for sediment calcite preservation with globally gridded field observations and satellite‐based syntheses. We then reformulate this model into a simpler five‐parameter box model, and combine it with algorithms for surface calcite export and water column dissolution for a single expression for the vertical calcite balance. The resulting metamodel is optimized to fit the observed distributions of calcite burial flux. We quantify the degree to which calcite export, saturation state, organic carbon respiration, and lithogenic sedimentation modulate the burial of calcite. We find that 46% of burial and 88% of dissolution occurs in sediments overlain by undersaturated bottom water with sediment calcite burial strongly modulated by surface export. Relative to organic carbon export, we find surface calcite export skewed geographically toward relatively warm, oligotrophic areas dominated by small, prokaryotic phytoplankton. We assess century‐scale projected impacts of warming and acidification on calcite export, finding high sensitive to inferred saturation state controls. With respect to long‐term glacial cycling, our analysis supports the hypothesis that strong glacial abyssal stratification drives the lysocline toward much closer correspondence with the saturation horizon. Our analysis suggests that, over the transition from interglacial to glacial ocean, a resulting ∼0.029 PgC a−1decrease in deep Atlantic, Indian and Southern Ocean calcite burial leads to slow increase in ocean alkalinity until Pacific mid‐depth calcite burial increases to compensate.

Living benthic foraminifera of the Okhotsk Sea: Faunal composition, standing stocks and microhabitats

Live (Rose Bengal stained) benthic foraminifera were investigated in surface sediment samples from the Okhotsk Sea to reveal the relationship between faunal characteristics and environmental parameters. Live benthic foraminifera were quantified in the size fraction > 125 µm in the upper 8 cm of replicate sediment cores, recovered with a multicorer at five stations along the Sakhalin margin, and at three stations on the southwestern Kamchatka slope. The stations are from water depths between 625 to 1752 m, located close or within the present Okhotsk Sea oxygen minimum zone, with oxygen levels between 0.3 and 1.5 ml l - 1 . At the high-productivity and ice-free Kamchatka stations, live benthic foraminifera are characterized by maximal standing stocks (about 1700-3700 individuals per 50 cm 2), strong dominance of calcareous species (up to 87-91% of total live faunas), and maximal habitat depths (down to 5.2-6.7 cm depth). Vertical distributions of total faunal abundances exhibit a clear subsurface maximum in sediments. At the Sakhalin stations, which are seasonally ice-covered and less productive, live benthic foraminifera show lower standing stocks (about 200-1100 individuals per 50 cm 2), lower abundance of calcareous species (10-64% of total live faunas), and shallower habitat depths (down to 2.5-5.4 cm depth). Faunal vertical distributions are characterized by maximum in the uppermost surface sediments. It is suggested that 1) lower and strongly seasonal organic matter flux, caused by the seasonal sea ice cover and seasonal upwelling, 2) lower bottom water oxygenation (0.3-1.1 ml l - 1 ), and 3) more pronounced influence of carbonate undersaturated bottom water along the Sakhalin margin are the main factors responsible for the observed faunal differences. According to species downcore distributions and average living depths, common calcareous species were identified as preferentially shallow, intermediate and deep infaunal. Foraminiferal microhabitat occupation correlates with the organic matter flux and the depth of the oxygenated layer in sediments.

Precipitation and Dissolution of Calcite in a Swiss High Alpine Lake

Water chemistry and sediment trap data from the 18.9-m-deep, high-altitude, hardwater lake Hagelseewli (2339 m asl.) indicates that biogenic induced calcite precipitation occurs in a water depth of 6 to 9 m at a temperature of 4°C. Our data indicates that calcite precipitation takes place as a short pulse that lasts less than 14 d in response to photosynthetic CO2 uptake in late summer probably by cyanobacterial picoplankton. In up to 8.4 times supersaturated water 30 μm large calcite crystals precipitate that show surface features typical for eutrophic lakes (rough surface and enhanced columnar growth). Subsequently, during the long period of ice coverage calcite is completely dissolved in strongly undersaturated bottom waters. Sediment calcite concentrations therefore are below 0.3%, although the comparison of trap fluxes and sediment accumulation rates indicates that they could be as high as 25%. The theoretical annual accumulation of calcite in Hagelseewli is similar in magnitude to a single spring precipitation event in a low-altitude temperate, hardwater lake. In the most recent sediments the occurrence of higher amounts of organic carbon and sulfur indicates increasing bottom water anoxia during the last 30 to 40 yr.

Precipitation and Dissolution of Calcite in a Swiss High Alpine Lake

Water chemistry and sediment trap data from the 18.9-m-deep, high-altitude, hardwater lake Hagelseewli (2339 m asl.) indicates that biogenic induced calcite precipitation occurs in a water depth of 6 to 9 m at a temperature of 4°C. Our data indicates that calcite precipitation takes place as a short pulse that lasts less than 14 d in response to photosynthetic CO2 uptake in late summer probably by cyanobacterial picoplankton. In up to 8.4 times supersaturated water 30 μm large calcite crystals precipitate that show surface features typical for eutrophic lakes (rough surface and enhanced columnar growth). Subsequently, during the long period of ice coverage calcite is completely dissolved in strongly undersaturated bottom waters. Sediment calcite concentrations therefore are below 0.3%, although the comparison of trap fluxes and sediment accumulation rates indicates that they could be as high as 25%. The theoretical annual accumulation of calcite in Hagelseewli is similar in magnitude to a single spring precipitation event in a low-altitude temperate, hardwater lake. In the most recent sediments the occurrence of higher amounts of organic carbon and sulfur indicates increasing bottom water anoxia during the last 30 to 40 yr.

Recycling of manganese from anoxic sediments in stagnant basins by seawater inflow: a study of surface sediments from the Gotland Basin, Baltic Sea

Abstract The occurrence of Mn carbonates in sapropelic sediments has been proposed to indicate that the host sediment originally accumulated under oxygenated bottomwaters. In the central Baltic Sea, Ca-rich rhodochrosite layers in sapropelic sediments have been related to inflows of oxic seawater from the North Sea into the brackish, predominantly anoxic, deeps of the central Baltic. This study attempts to verify the model of authigenic Mn carbonate formation by comparing oceanographic records directly with the composition of surficial sediments of the Gotland Basin. Surface sediments of the Gotland Deep sampled in 1997 displayed significant Ca-rich rhodochrosite enrichments at 6.5, 11 and 15 cm sediment depth which reflect periods of intense seawater inflows in 1969–76, 1948–56, and 1931–39, respectively. However, the latest major seawater inflow detected in 1993 was not reflected in the surface sediment. The topmost 6 cm of the sediment was totally depleted in Mn and 210Pb data implied a disturbed top layer down to 5 cm. Calculations of saturation indices indicate that the porewaters were undersaturated with respect to Mn and Ca carbonate phases in the uppermost 8 cm in 1997. At greater depths, the porewaters were close to equilibrium with respect to calcite and rhodochrosite. The Ca-rhodochrosite layers corresponded well to a sediment core sampled at the same location in 1994, but that core displayed a further enrichment of Ca-rich rhodochrosite close to the sediment surface, which can be related to a major inflow of North Sea water in 1993. The significant decrease of about 2.3 mol/m2 Mn in the uppermost 6 cm of a disturbed sediment surface from 1994 to 1997 corresponds to an enrichment of about 20 μmol/l of dissolved Mn within the 90 m thick anoxic water column in 1997. We conclude that several inflows of dense seawater into the Gotland Deep, monitored between 1994 and 1997, led to a resuspension of the unstable, fluffy surface sediment layer. Authigenic Ca-rich rhodochrosite which had formed immediately after the major inflow of oxic seawater in 1993 was therefore redissolved in undersaturated bottomwaters and Mn was recycled from surficial sediments into the water column. This in turn means that, in contrast to the established model of Ca-rhodochrosite formation as a result of seawater inflow, the occurrence of mixed, Mn-depleted layers does not prove that seawater inflow has not occurred during the deposition of these sediments.

The radiocarbon age of calcite dissolving at the sea floor: estimates from pore water data

We measured the radiocarbon content and stable isotopic composition of pore water and bottom water ΣCO 2, sedimentary organic carbon, and CaCO 3 at two sites on the Ceara Rise, one underlying bottom water that is supersaturated with respect to calcite (Site B), the other underlying undersaturated bottom water (Site G). The results were combined with pore water O 2, ΣCO 2, and Ca 2+ profiles (Martin and Sayles, 1996) to estimate the radiocarbon content of the CaCO 3 that is dissolving in the sediment mixed layer. At Site G, the CaCO 3 that is dissolving in the upper 2 cm of the sediments is clearly younger (richer in 14C) than the bulk sedimentary CaCO 3, indicating that nonhomogeneous CaCO 3 dissolution occurs there. The case for nonhomogeneous dissolution is much weaker at the site underlying supersaturated bottom water. The results indicate that nonhomogeneous dissolution occurs in sediments underlying undersaturated bottom water, that the dissolution is rapid relative to the rate of homogenization of the CaCO 3 in the mixed layer by bioturbation, and that the dissolution rate of CaCO 3 decreases as it ages in the sediment mixed layer. The results support the hypothesis, based on solid phase analyses, that the preferential dissolution of young (i.e., radiocarbon-rich) CaCO 3 leads to a pattern of increasing radiocarbon age of mixed-layer CaCO 3 as the degree of undersaturation of bottom water increases (Keir, 1984; Broecker et al., 1991).

Mobility and Immobility of Redox-Sensitive Elements in Deep-Sea Turbidites During Shallow Burial

During the first few tens of thousands of years after an organic-rich turbidite is emplaced on an abyssal plain, surficial oxidation of the unit occurs by downwards diffusion of bottom water O2. From work on piston cores, it is known that a suite of redox-sensitive elements form characteristic relocation peaks in such units around the deepest reaches (upper few dm) achieved by oxidation (fossil oxic/post-oxic boundaries). Four individual turbidites now buried at depths of 130-230 m below seafloor (mbsf), obtained by ODP Leg 157 drilling on the Madeira Abyssal Plain (MAP), have been investigated to determine the effects of burial diagenesis on these geochemical signals over 3-7 My. Porewater data indicate that post-oxic conditions persist to around 130 mbsf on the MAP, below which sulphate reduction predominates. All four units have retained high total organic carbon (Corg) contents (∼1.4%) in their lower reaches, despite the fact that bottom waters have previously oxidised this same organic matter in the turbidite tops to ∼0.2%. The apparent inertness of the turbidite Corg contrasts with the reactivity of organic matter observed in hemipelagic sediment from other ODP investigations. In all four cases, low initial CaCO3 contents have been quantitatively removed along with Corg oxidation, with additional dissolution of carbonate by undersaturated bottom waters. Diagenetic fractions of the elements Se, Cd, Sb, Tl, and V are present in highly-localised peaks and show no evidence of migration. In different combinations, the elements Cu, Co, Ni, and Zn appear to have migrated over short (<5 cm) distances in thin (mm) dark-coloured sulphide bands, probably sourced by reduction of a Mn-oxyhydroxide host phase during early (70 ky) burial. Two examples of local diagenetic pyritisation (up to 20 wt% FeS2) are observed in the bodies of the two turbidites buried to ∼130 mbsf, confirming that sulphate reduction is active at this level. This diagenetic pyrite is associated with high concentrations of As, and lesser enrichments of Ni, Zn, Co, Se, Sb, and possibly Mo.

Respiration and dissolution in the sediments of the western North Atlantic: estimates from models of in situ microelectrode measurements of porewater oxygen and pH

We present in situ microelectrode measurements of sediment formation factor and porewater oxygen and pH from six stations in the North Atlantic varying in depth from 2159 to 5380 m. A numerical model of the oxygen data indicates that fluxes of oxygen to the sediments are as much as an order of magnitude higher than benthic chamber flux measurements previously reported in the same area. Model results require dissolution driven by metabolic CO 2 production within the sediments to explain the pH data; even at the station with the most undersaturated bottom waters >60% of the calcite dissolution occurs in response to metabolic CO 2. Aragonite dissolution alone cannot provide the observed buffering of porewater pH, even at the shallowest station. A sensitivity test of the model that accounts for uncertainties in the bottom water saturation state and the stoichiometry between oxygen consumption and CO 2 production during respiration constrains the dissolution rate constant for calcite to between 3 and 30% day −1, in agreement with earlier in situ determinations of the rate constant. Model results predict that over 35% of the calcium carbonate rain to these sediments dissolves at all stations, confirmed by sediment trap and CaCO 3 accumulation data.

In situ measurements of calcium carbonate dissolution rates in deep-sea sediments

Benthic fluxes of alkalinity, carbon dioxide, and oxygen have been measured using an in situ incubation chamber at three sites (MANOP Sites C and S and PACFLUX Site SC) in the central equatorial north Pacific. At two carbonate-rich sites (C and SC), a budget for oxygen, alkalinity, and TCO 2 fluxes indicate a net CaCO 3 dissolution rate of approximately 0.4 mmol m −2 day −1. This rate is only 20% of previous estimates but is consistent with a dissolution rate constant predicted from laboratory experiments with deep-sea sediments and derived from in situ flux measurements in sediments of the southern California borderland. Organic matter oxidation in the sediment column provides the acid for 60–100% of the calcium carbonate dissolution occurring at these sites with the remainder derived from undersaturated bottom water. At a low-carbonate site (S), no carbonate dissolution in the sediment or on the sea floor is apparent, although sediment traps indicate a rain of CaCO 3 through 3400 m. The rates of remineralization relative to resuspension or erosion at this site must differ for organic carbon and calcium carbonate, so that carbonate grains reaching the sea floor are physically removed by erosion or resuspension before they dissolve, while organic carbon is largely oxidized before it can be physically removed.

The rate of dissolution of calcium carbonate from the surface of deep-ocean turbidite sediments

It is known that past periods of high atmospheric carbon dioxide concentration are associated with poor carbonate preservation in the deep-ocean sedimentary record. Bottom water can become more aggressive towards carbonate sediments during such periods. To interpret the sedimentary record more exactly, and to predict future atmospheric carbon dioxide levels, it is necessary to know the rate of solution of carbonate for a given degree of bottom-water undersaturation. In parts of the Atlantic Ocean, turbidite sedimentation mechanisms have emplaced carbonate-rich material in contact with undersaturated bottom water. The time of the emplacement event can be determined from natural radionuclide distributions, and the degree of carbonate dissolution in this time can be measured. This provides a direct measurement of dissolution rate from a natural sediment surface at a known degree of undersaturation. The range of applicability of the method is explored with a mathematical model, and field data from a 5430 m depth Atlantic site are presented.

Calcareous nannofossil biostratigraphy and assemblages of the Cenomanian‐Turonian boundary interval: Implications for the origin and timing of oceanic anoxia

A detailed nannofossil zonation has been derived for the Cenomanian‐Turonian boundary interval consisting of three zones, two subzones, and nine additional biohorizons. This biostratigraphy, based on the investigation of numerous sections from Europe, North America, and Africa, allows accurate correlation of sections from shelf and deep‐sea environments. The increased resolution helps solve several pertinent paleoceanographic and sedimentologic problems in this time period. Facies patterns are variable in this interval for numerous reasons. A widespread hiatus occurred in the latest Cenomanian and earliest Turonian in much of northwest Europe and may have been an indirect result of peak marine transgression. In parts of the deep Tethys, dissolution of carbonate took place in undersaturated bottom waters or during early diagenesis. Only a few boundary sections, mostly within the Western Interior, United States, are characterized by continuous carbonate deposition. In many sections a ∥13C excursion (which has been associated with the burial of large amounts of marine organic matter) coincides with a facies change, lower ratios of coccolith carbonate to micrite, and poorer nannofossil preservation. This excursion therefore may be strongly overprinted by diagenesis. Numerous nannofossil events indicate that the carbon excursion is slightly diachronous between sequences of the Western Interior, and that the shift occurred distinctly later in this region than in northwest Germany. The data obtained do not invalidate the concept of a Cenomanian‐Turonian ocean‐wide anoxic event but show how local tectonic, climatic, and oceanographic perturbations have masked the global scenario. In particular, they indicate that carbon shifts may be basinal and not always oceanic phenomena, a conclusion which can be rationalized with overall sluggish middle Cretaceous circulation. Nannofossil assemblages in almost all sequences observed contain exceptionally high abundances of two dissolution‐resistant taxa in particular samples: Eprolithus floralis and Broinsonia sp.. However, the overall composition of assemblages does not change dramatically in the boundary interval, in contrast to the macrofossils and foraminifera, which suffered widespread extinctions.

Biogenic Silica Accumulation in the Central Equatorial Pacific and its Implications for Cenozoic Paleoceanography

Research Article| September 01, 1979 Biogenic Silica Accumulation in the Central Equatorial Pacific and its Implications for Cenozoic Paleoceanography Margaret Leinen Margaret Leinen 1Graduate School of Oceanography, University of Rhode Island. Kingston, Rhode Island 02883 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1979) 90 (9_Part_II): 1310–1376. https://doi.org/10.1130/GSAB-P2-90-1310 Article history received: 26 Dec 1978 accepted: 25 Apr 1979 rev-recd: 02 May 1979 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Margaret Leinen; Biogenic Silica Accumulation in the Central Equatorial Pacific and its Implications for Cenozoic Paleoceanography. GSA Bulletin 1979;; 90 (9_Part_II): 1310–1376. doi: https://doi.org/10.1130/GSAB-P2-90-1310 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Since the Deep Sea Drilling Project (DSDP) has made available long cores through Cenozoic deep-sea sediments, geologists have begun to study the history of sedimentation in the world ocean and to infer the oceanographic conditions which have influenced deposition of the sediment. During this time, the central equatorial Pacific Ocean has become one of the most intensively studied geologic provinces in the world ocean. This area lies beneath the biologically productive equatorial current system which supplies great quantities of calcium carbonate and amorphous silica (opal) to the sediment in the form of skeletal tests. This biogenic debris, together with terrigenous, authigenic, and hydrothermal sediment, has produced an unusually thick and complete marine record which was cored by four DSDP Legs (5, 8, 9, and 16). Because the productivity of the equatorial Pacific is a direct consequence of large nutrient concentrations associated with upwelling in the complex equatorial current system, the sedimentary section can be used to interpret the paleoceanography as well as the history of productivity in the equatorial Pacific.The history of carbonate sedimentation in the central equatorial Pacific is well known from the Initial Reports of DSDP Legs 8, 9, and 16 (Tracey and others, 1971; Hays and others, 1972; van Andel and Heath, 1973), as well as from a wealth of published information (Hays and others, 1969; McManus and others, 1972; Berger, 1973; Winterer, 1973; van Andel and others, 1975). There are limitations, however, to the use of carbonate sediments for interpreting the paleoceanography and productivity of surface waters. The patterns of distribution and accumulation of calcareous sediments in the deep ocean are modified by the dissolution of carbonate in undersaturated bottom Waters at rates which vary with depth (Peterson, 1966; Berger, 1973) and with time (van Andel and others, 1975). Because we are, at present, unable to determine past rates of carbonate dissolution in bottom waters unambiguously (van Andel and others, 1975; Heath and others, 1977), we are also unable to assess accurately the rate of supply of carbonate to an area in the past. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Biogenic silica accumulation in the central equatorial Pacific and its implications for Cenozoic paleoceanography: Summary

Since the Deep Sea Drilling Project (DSDP) has made available long cores through Cenozoic deep-sea sediments, geologists have begun to study the history of sedimentation in the world ocean and to infer the oceanographic conditions which have influenced deposition of the sediment. During this time, the central equatorial Pacific Ocean has become one of the most intensively studied geologic provinces in the world ocean. This area lies beneath the biologically productive equatorial current system which supplies great quantities of calcium carbonate and amorphous silica (opal) to the sediment in the form of skeletal tests. This biogenic debris, together with terrigenous, authigenic, and hydrothermal sediment, has produced an unusually thick and complete marine record which was cored by four DSDP Legs (5, 8, 9, and 16). Because the productivity of the equatorial Pacific is a direct consequence of large nutrient concentrations associated with upwelling in the complex equatorial current system, the sedimentary section can be used to interpret the paleoceanography as well as the history of productivity in the equatorial Pacific. The history of carbonate sedimentation in the central equatorial Pacific is well known from the Initial Reports of DSDP Legs 8, 9, and 16 (Tracey and others, 1971; Hays and others, 1972; van Andel and Heath, 1973), as well as from a wealth of published information (Hays and others, 1969; McManus and others, 1972; Berger, 1973; Winterer, 1973; van Andel and others, 1975). There are limitations, however, to the use of carbonate sediments for interpreting the paleoceanography and productivity of surface waters. The patterns of distribution and accumulation of calcareous sediments in the deep ocean are modified by the dissolution of carbonate in undersaturated bottom Waters at rates which vary with depth (Peterson, 1966; Berger, 1973) and with time (van Andel and others, 1975). Because we are, at present, unable to determine past rates of carbonate dissolution in bottom waters unambiguously (van Andel and others, 1975; Heath and others, 1977), we are also unable to assess accurately the rate of supply of carbonate to an area in the past.