Calcite Lysocline Research Articles

Hydrothermal carbon release to the ocean and atmosphere from the eastern equatorial Pacific during the last glacial termination

Arguably among the most globally impactful climate changes in Earth’s past million years are the glacial terminations that punctuated the Pleistocene epoch. With the acquisition and analysis of marine and continental records, including ice cores, it is now clear that the Earth’s climate was responding profoundly to changes in greenhouse gases that accompanied those glacial terminations. But the ultimate forcing responsible for the greenhouse gas variability remains elusive. The oceans must play a central role in any hypothesis that attempt to explain the systematic variations in pCO2 because the Ocean is a giant carbon capacitor, regulating carbon entering and leaving the atmosphere. For a long time, geological processes that regulate fluxes of carbon to and from the oceans were thought to operate too slowly to account for any of the systematic variations in atmospheric pCO2 that accompanied glacial cycles during the Pleistocene. Here we investigate the role that Earth’s hydrothermal systems had in affecting the flux of carbon to the ocean and ultimately, the atmosphere during the last glacial termination. We document late glacial and deglacial intervals of anomalously old 14C reservoir ages, large benthic-planktic foraminifera 14C age differences, and increased deposition of hydrothermal metals in marine sediments from the eastern equatorial Pacific (EEP) that indicate a significant release of hydrothermal fluids entered the ocean at the last glacial termination. The large 14C anomaly was accompanied by a ∼4-fold increase in Zn/Ca in both benthic and planktic foraminifera that reflects an increase in dissolved [Zn] throughout the water column. Foraminiferal B/Ca and Li/Ca results from these sites document deglacial declines in [] throughout the water column; these were accompanied by carbonate dissolution at water depths that today lie well above the calcite lysocline. Taken together, these results are strong evidence for an increased flux of hydrothermally-derived carbon through the EEP upwelling system at the last glacial termination that would have exchanged with the atmosphere and affected both Δ14C and pCO2. These data do not quantify the amount of carbon released to the atmosphere through the EEP upwelling system but indicate that geologic forcing must be incorporated into models that attempt to simulate the cyclic nature of glacial/interglacial climate variability. Importantly, these results underscore the need to put better constraints on the flux of carbon from geologic reservoirs that affect the global carbon budget.

A novel determination of calcite dissolution kinetics in seawater

We present a novel determination of the dissolution kinetics of inorganic calcite in seawater. We dissolved 13C-labeled calcite in unlabeled seawater, and traced the evolving δ13C composition of the fluid over time to establish dissolution rates. This method provides sensitive determinations of dissolution rate, which we couple with tight constraints on both seawater saturation state and surface area of the dissolving minerals. We have determined dissolution rates for two different abiotic calcite materials and three different grain sizes. Near-equilibrium dissolution rates are highly nonlinear, and are well normalized by geometric surface area, giving an empirical dissolution rate dependence on saturation state (Ω) of:Rate(g/cm2/day)=7.2±0.6·10-4(1-Ω)3.9±0.1.This result substantiates the non-linear response of calcite dissolution to undersaturation. The bulk dissolution rate constant calculated here is in excellent agreement with those determined in far from equilibrium and dilute solution experiments. Plots of dissolution versus undersaturation indicates the presence of at least two dissolution mechanisms, implying a criticality in the calcite-seawater system. Finally, our new rate determination has implications for modeling of pelagic and seafloor dissolution. Nonlinear dissolution kinetics in a simple 1-D lysocline model indicate a possible transition from kinetic to diffusive control with increasing water depth, and also confirm the importance of respiration-driven dissolution in setting the shape of the calcite lysocline.

Effects of seafloor and laboratory dissolution on the Mg/Ca composition of Globigerinoides sacculifer and Orbulina universa tests — A laser ablation ICPMS microanalysis perspective

Partial or selective dissolution of planktonic foraminiferal tests on the seafloor has been shown to alter original test Mg/Ca compositions and thus may limit the accuracy of Mg/Ca-based thermometry for reconstructions of past sea surface temperatures. We have employed laser ablation ICPMS to determine the extent of dissolution-caused changes in Mg/Ca distribution across individual chamber walls of the planktonic foraminifera Globigerinoides sacculifer and Orbulina universa. G. sacculifer samples collected from a core-top depth transect in the NE Indian Ocean and laboratory dissolution experiments show little if any evidence of preferential removal of Mg-rich calcite layers by progressive dissolution of the tests. We attribute the absence of selective dissolution to the banded distribution of Mg across the chamber walls of these foraminiferal species and to the minimal presence of calcite crusts with relatively low-Mg composition on the outer surfaces of tests. Mg/Ca microanalyses of G. sacculifer from core-top samples further indicate that for samples collected above the calcite lysocline the effect of postdepositional dissolution on Mg/Ca sample mean values is minimal and within the uncertainty of Mg/Ca thermometry (i.e. ± 0.4 mmol/mol; ± 0.8 °C at ∼ 28 °C). Comparison with previously published results for G. sacculifer supports these observations. Simple modelling of G. sacculifer test dissolution indicates that selective removal of calcite with high-Mg/Ca values from within the final chamber of G. sacculifer test appears insufficient to cause the ∼ 10% decrease in Mg/Ca values observed above calcite lysocline. These changes in test composition might be related to development/removal as a function of Δ[CO 3 2−] of a thin diagenetic surface coating which has a relatively high-Mg/Ca composition (i.e. 20–25 mmol/mol).

A pervasive link between Antarctic ice core and subarctic Pacific sediment records over the past 800 kyrs

Recently developed XRF core-scanning methods permit paleoceanographic reconstructions on timescales similar to those of ice-core records. We have investigated the distribution of biogenic barium (Ba/Al), opal and carbonate (Ca/Al) in a sediment core retrieved from the abyssal subarctic Pacific (ODP 882, 50°N, 167°E, 3244 m) over an interval that spans the full length of the EPICA Dome C (EDC) ice-core record. Ba/Al and biogenic opal show a strong resemblance to the EDC δD and CO 2, with generally high concentrations during interglacials and lower values during ice ages of the past 800 kyrs. The sedimentary Ba/Al and biogenic opal are most easily interpreted as indicating a reduced sinking flux of organic matter from the surface ocean during cold periods. The Ba/Al maxima during peak interglacials are accompanied by transient Ca/Al peaks in these otherwise carbonate-devoid sediments, which are best explained by a deepening of the calcite lysocline, presumably due to reduced storage of respired CO 2 in the deep North Pacific. For most of the “luke-warm” interglacials noted between 420 and 750 ka in EDC, the Ba/Al peaks in ODP 882 are also lower, further strengthening the evidence for a simple physical link between global climate and the biogeochemistry of the subarctic Pacific.

Influence of export rain ratio changes on atmospheric CO2 and sedimentary calcite preservation

The responses of atmospheric pCO 2 and sediment calcite content to changes in the export rain ratio of calcium carbonate to organic carbon are examined using a diffusion-advection ocean biogeochemical model coupled to a one-dimensional sediment geochemistry model. Our model shows that a 25% reduction in rain ratio decreases atmospheric pCO 2 by 59 ppm. This is caused by alkalinity redistribution by a weakened carbonate pump and an alkalinity increase in the whole ocean via carbonate compensation with decreasing calcite burial. The steady state responses of sedimentary calcite content and calcite preservation efficiency are rather insensitive to the deepening of the saturation horizon of 1.9 km. This insensitivity is a result of the reduced deposition flux that decreases calcite burial, counteracting the saturation horizon deepening that increases calcite burial. However, in the first 10,000 years the effect of reduced calcite deposition on the burial change is more prominent; while after 10,000 years, the effect of saturation horizon deepening is more dominant. The lowering of sediment calcite content for the first 10,000 years is effectively decoupled from the 1.9 km downward shift of the saturation horizon. Our results are in part a consequence of the more dominant role that respiration CO 2 plays in sediment calcite dissolution over bottom water chemistry in our control run and support the decoupling of calcite lysocline depth and saturation horizon shifts, as suggested originally by Archer and Maier-Reimer (1994) and Archer et al. (2000).

In situ calibration of Mg/Ca ratio in planktonic foraminiferal shell using time series sediment trap: A case study of intense dissolution artifact in the South China Sea

This study examines Mg/Ca‐temperature equations through the use of planktonic foraminifers collected from continuous time series sediment traps at four water depths in the South China Sea. This deployment provides an opportunity to refine partial dissolution artifacts and to identify responses of shell chemistry to changes in ambient seawater conditions. Paired Mg/Ca and δ18O measurements on shells are strongly correlated with seasonal variability in terms of temperature at specific habitat depths. However, partial dissolution can significantly alter Mg/Ca ratios even at depths well above the calcite lysocline or carbonate compensation depth. Of the three species studied, Neogloboquadrina dutertrei is the most sensitive species to dissolution relative to the other surface‐dwelling species (Globigerinoides ruber and Globigerinoides sacculifer). Differences between the Mg‐derived temperature equation here and those from previously established core top or sediment trap calibrations can be attributed to various degrees of dissolution and lateral advection, thus highlighting the need for in situ empirical calibration for different ocean basins.

Changes in the hydrological cycle, ocean circulation, and carbon/nutrient cycling during the last interglacial and glacial transition

A complex Earth system model has been forced by insolation changes for the last interglacial (LIG) for which geological records evidence a climate cooling that culminated in the last glacial inception. During an early warm period (127–125 ka B.P.) the Arctic/North Atlantic drainage basin north of 30°N receives ∼27,000 m3/s more fresh water mainly imported across the watershed of North America compared to a cooler period centered around 115 ka B.P. Together with a strong surface warming by up to 2 K, this results in a weaker North Atlantic overturning (reduced by ∼20%). Sea ice cover is reduced by 3 × 106 km2 in the Northern Hemisphere, stimulating a stronger productivity in the Barents and Kara seas. In the North Pacific a lowered salinity and a surface warming of more than 3 K increase the stratification. The eastern Pacific becomes nutrient poorer, which leads to a reduction in the export production in areas with vigorous upwelling. The climate cooling in the course of the LIG induces a carbon release by the terrestrial biosphere of 308 Gt, which is counteracted by an oceanic/sedimentary uptake of 290 Gt. The rest remains in the atmosphere increasing the pCO2 by 9 ppm. The large oceanic CO2 uptake corresponds to a rise of the calcite lysocline of about 300 m in the midlatitude Atlantic. The chemically interactive carbonate sediment pool in the Atlantic is reduced by 10%. For nearly the entire North Atlantic, our results are contradictory to the common interpretation of carbonate preservation proxies as a reliable tool for monitoring changes in deep water circulation.

Carbonate preservation patterns at the Ceará Rise – Evidence for the Pliocene super conveyor

Enhanced Atlantic overturning during the Pliocene was first proposed almost 10 yrs ago. Evidence for this Pliocene super conveyor scenario has been collected using a number of proxies (e.g., benthic δ 13C, Nd isotopic composition of manganese crusts). The present study contributes to the existing evidences by using carbonate dissolution and current vigour history of early Pliocene sediments from the Ceará Rise (ODP Sites 927 and 929). In order to reveal carbonate dissolution history, a number of commonly used and newly established proxies were applied, i.e., sand and carbonate contents, foraminifer fragmentation index, Bulloides Dissolution Index and carbonate silt grain-size distributions. Terrigenous silt grain-size distributions were used to unravel variations in relative current strength and sediment input to the two sites. Overall good carbonate preservation at the shallow Site 927 (3314 m water depth) shows that this level was bathed in North Atlantic Deep Water throughout the early Pliocene. The contrastingly poor carbonate preservation record of the deeper Site 929 (4358 m water depth, at present exposed to Antarctic Bottom Water) is frequently interrupted by phases of good carbonate preservation. These results indicate that the depth of the calcite lysocline was mainly tied to present level (∼ 4200 m water depth), and sometimes even dropped to water depths greater than 4360 m due to even more enhanced circulation. Surprisingly the expansion of NADW is not clearly reflected by an increase in current speed as shown by continuously fine terrigenous grain size.

Calcium carbonate corrosiveness in the South Atlantic during the Last Glacial Maximum as inferred from changes in the preservation of Globigerina bulloides: A proxy to determine deep-water circulation patterns?

The modern Atlantic Ocean, dominated by the interactions of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW), plays a key role in redistributing heat from the Southern to the Northern Hemisphere. In order to reconstruct the evolution of the relative importance of these two water masses, the NADW/AABW transition, reflected by the calcite lysocline, was investigated by the Globigerina bulloides dissolution index (BDX′). The depth level of the Late Glacial Maximum (LGM) calcite lysocline was elevated by several hundred metres, indicating a more corrosive water mass present at modern NADW level. Overall, the small range of BDX′ data and the gradual decrease in preservation below the calcite lysocline point to a less stratified Atlantic Ocean during the LGM. Similar preservation patterns in the West and East Atlantic demonstrate that the modern west–east asymmetry did not exist due to an expansion of southern deep waters compensating for the decrease in NADW formation.

Present water mass calcium carbonate corrosiveness in the eastern South Atlantic inferred from ultrastructural breakdown of Globigerina bulloides in surface sediments

The Atlantic is regarded as a huge carbonate depocenter due to an on average deep calcite lysocline. However, calculations and models that attribute the calcite lysocline to the critical undersaturation depth (hydrographic or chemical lysocline) and not to the depth at which significant calcium carbonate dissolution is observed (sedimentary calcite lysocline) strongly overestimate the preservation potential of calcareous deep-sea sediments. Significant calcium carbonate dissolution is expected to begin firstly below 5000 m in the deep Guinea and Angola Basin and below 4400 m in the Cape Basin. Our study that is based on different calcium carbonate dissolution stages of the planktic foraminifera Globigerina bulloides clearly shows that it starts between 400 and 1600 m shallower depending on the different hydrographic settings of the South Atlantic Ocean. In particular, coastal areas are severely affected by increased supply of organic matter and the resultant production of metabolic CO 2 which seems to create microenvironments favorable for dissolution of calcite well above the hydrographic lysocline.

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.

Redfield stoichiometry in Arabian Sea subsurface waters

A linear inverse mixing model is applied to hydrographic, nutrient, and carbon data collected during Joint Global Ocean Flux Study and World Ocean Circulation Experiment cruises in 1995 to estimate the ΔCorg/ΔN/ΔP/ΔSi/‐ΔO2 remineralization ratios within the Arabian Sea between 550 and 4500 m. The observed concentrations are separated into mixing fractions of source water masses and changes caused by remineralization processes, while the effect of denitrification is considered. In contrast to earlier investigations, diapycnal mixing, which plays an important role in dissolved matter fluxes in the Arabian Sea, is accounted for. The ratios are found to be variable with depth, especially in the upper 2000 m of the water column. We suppose that in general nutrients are released faster than carbon dioxide during remineralization. The ΔCorg/ΔCinorg decrease from ∼4 ± 1 at 550 m to 2 ± 0.2 at 2000 m and 1.2 ± 0.3 at 4000 m, suggesting that the dissolution of calcium carbonate above the calcite lysocline is a potentially important process within the Arabian Sea.

Carbonate dissolution in the South Atlantic Ocean: evidence from ultrastructure breakdown in Globigerina bulloides

Ultrastructure dissolution susceptibility of the planktic foraminifer Globigerina bulloides, carbonate ion content of the water column, calcium carbonate content of the sediment surface, and carbonate/carbon weight percentage ratio derived from sediment surface samples were investigated in order to reconstruct the position of the calcite saturation horizon, the sedimentary calcite lysocline, and the calcium carbonate compensation depth (CCD) in the modern South Atlantic Ocean. Carbonate ion data from the water column refer to the GEOSECS locations 48, 103, and 109 and calcium carbonate data come from 19 GeoB sediment surface samples of 4 transects into the Brazil, the Guinea, and the Cape Basins. We present a new (paleo-) oceanographic tool, namely the Globigerina bulloides dissolution index (BDX). Further, we give evidence (a) for progressive G. bulloides ultrastructural breakdown with increasing carbonate dissolution even above the lysocline; (b) for a sharp BDX increase at the sedimentary lysocline; and (c) for the total absence of this species at the CCD. BDX puts us in the position to distinguish the upper open ocean and the upwelling influenced continental margin above from the deep ocean below the sedimentary lysocline. Carbonate ion data from water column samples, calcite weight percentage data from surface sediment samples, and carbonate/carbon weight percentage ratio appear to be good proxies to confirm BDX. As shown by BDX both the calcite saturation horizon (in the water column) and the sedimentary lysocline (at the sediment–water interface) mark the boundary between the carbonate ion undersaturated and highly corrosive Antarctic Bottom Water and the carbonate ion saturated North Atlantic Deep Water (NADW) of the modern South Atlantic.

The carbon dioxide system in the northwestern Indian Ocean during south–west monsoon

Data on the carbonate system of the Northwestern Indian Ocean obtained on a cruise of F.S. Meteor during SW monsoon in July/August 1995 were compared with those of George et al. [George, M.D., Kumar, M.D., Naqvi, S.W.A., Banerjee, S., Narvekar, P.V., de Sousa, S.N., Jayakumar, D.A., 1994. A study of the carbon dioxide system in the northern Indian Ocean during premonsoon. Mar. Chem. 47, 243–254] collected during intermonsoon. In general, deep water values agreed well between the two expeditions. Surface waters, however, showed a substantial increase in dissolved inorganic carbon (CT) in the coastal regions due to strong upwelling in the SW monsoon. This was also accompanied by very high CO2 partial pressures in surface waters. The north–south gradients in vertical profiles of the measured parameters in the Arabian Sea are discussed by comparing profiles from the oligotrophic equatorial region with those from the highly productive central Arabian Sea. The effect of denitrification on regenerated CT and AT is minor, with contributions of <9 and <8 μmol kg−1, respectively, to the total amount regenerated also utilizing oxygen. The dissolution of biogenic carbonates is discussed; different approaches to define the depth, where the dissolution starts (lysocline(s), carbonate critical depth (CCrD)), are compared together with the calculation of saturation depth from carbonate concentrations. It is shown, that small differences in measured CT and AT (found between our data and those measured during GEOSECS) and different calculation approaches to the CO2 system (different dissociation constants for species involved and taking into account phosphate and silicate concentrations) can produce pronounced differences in the calculated saturation depths. However, CT and AT data suggest substantial dissolution of biogenic carbonate in the water column even above the calcite lysocline, irrespective of the procedures followed to calculate this horizon.

Single particle analysis of suspended matter in the Makasar Strait and Flores Sea with particular reference to tin-bearing particles

Suspended matter samples filtered from surface waters and two depth profiles from the Flores Sea and Makasar Strait were investigated by electron probe X-ray microanalysis (EPXMA) and laser microprobe mass analysis (LAMMA). EPXMA yielded discrete morphological and chemical analysis of the major particle types of suspended matter. Cluster analysis revealed that thirteen main particle types described the composition of suspended matter of the Flores Sea and Makasar Strait. Silicates, aluminosilicates and Fe-oxyhydroxides were the predominant particle types. Suspended matter of the basins studied contained high levels of tin-bearing particles. On the basis of their composition, tin particles can be divided into three groups: (1) tin oxide/hydroxides (cassiterite, romarchite, hydroromarchite); (2) iron-oxyhydroxides with adsorbed tin; and (3) mixed oxidation state tin hydroxysulphates. Only ultra-fine cassiterite particles enter the seawater in suspended state. Dissolved tin species entering the sea have three alternatives: (1) to be scavenged by Fe-oxyhydroxides; (2) to precipitate as tin oxide/hydroxides (romarchite, hydroromarchite); (3) to precipitate as tin hydroxysulphates. The conclusion is that dissolved and suspended tin originate from local sources in the land frame of the basins as well as from remote sources in the Indonesian Archipelago. Four different sectors of the waters studied have suspended matter with different composition: (1) the Mahakam River–Delta zone; (2) the open Flores Sea; (3) the landlocked Saleh Bay; (4) the Makasar Strait proper. The depth distribution of suspended particle types is mainly influenced by: (1) the bottom nepheloid layer and calcite lysocline in the Flores Sea; (2) the high bioproduction in the surface water layer and the vertical distribution of organic matter in the Makasar Strait.

The calcite lysocline as a constraint on glacial/interglacial low‐latitude production changes

We investigate the response of the calcite lysocline to changes in the export production of the low‐latitude surface ocean (the combined equatorial, tropical, and subtropical regions). We employ different CaCO3 throughput schemes in a time‐dependent ocean carbon cycle model to separate the CaCO3 production/lysocline balance from the other components of the model response and to estimate the effect of dissolution driven by organic matter decomposition in surface sediments. With this model, we carry out three experiments based on previous hypotheses for the cause of glacial/interglacial atmospheric CO2 variations: (1) a simple increase in low‐latitude production, (2) a decrease in its CaCO3/organic carbon rain ratio, and (3) an increase in the depth of organic carbon regeneration. None of these changes, when taken alone, can lower atmospheric CO2 to glacial levels without violating observations of the glacial lysocline depth. If a low‐latitude production increase were accompanied by a decrease in the CaCO3/organic carbon rain ratio, then these simultaneous changes could lower atmospheric CO2 to observed glacial levels without causing a significant change in the ocean‐average depth of the lysocline. However, even with these simultaneous changes, the required increase in production is 50% or more of the modern rate of production.

Paleoecology and Taphonomy of Faunal Assemblages in Gray "Core" (Offshore) Shales in Midcontinent Pennsylvanian Cyclothems

Faunal composition and diversity of macroinvertebrate assemblages in the gray facies of nineteen core shales of Mid-continent Pennsylvanian cyclothems confirm the offshore marine origin previously established for those shales. Diverse assemblages of only calcitic fossils that show grain corrosion at northern Midcontinent localities (Nebraska/Iowa) indicate deposition under oxic to slightly dysoxic conditions within the calcite lysocline but below the aragonite compensation depth for this particular sea. Southward (Kansas), sparser calcitic faunas indicate that core shales there were deposited farther from the northern shoreline in water of greater depth and less bottom oxygen, closer to or locally below the calcite compensation depth. Farther southward near the Ouachita detrital source (Oklahoma), greater influx of sediment permitted rapid burial of shelled organisms to overprint the effects of lysoclines and compensation depths. Both the originally aragonitic molluscs as well as calcitic organisms are preserved in those shales. Fine detrital grain size, radiolarian-bearing nonskeletal phosphate, and pelagic fauna (ammonoids and conodonts) further substantiate an offshore marine origin for the black core shale facies that is surrounded by the gray facies, and which was deposited in a probably deeper, more continually anoxic environment where most benthos was excluded.

An atlas of the distribution of calcium carbonate in sediments of the deep sea

Historical observations of the concentration of calcium carbonate in global deep sea sediments are compiled and compared with a new gridded field of seawater CO3= concentration to reveal regional variations in the calcite lysocline. The most obvious mode of variability of the calcite lysocline is the thickness of the lysocline (defined here as the difference in overlying water carbonate saturation, ΔCO3=, between high and low calcite sediments) with a thicker lysocline in the Atlantic than in the Pacific. I attribute this variation to differences in the delivery rate of terriginous material. A recent model for the lower glacial atmospheric pCO2 proposed to change the relationship between the depth of the lysocline and the ΔCO3= of the water column by changing the rain rate ratio of organic carbon to calcite production (the “rain ratio model”: Archer and Maier‐Reimer, 1994). I search the data set for analogs to the proposed glacial world, by looking for a link between the regional climate at the sea surface and the depth of the lysocline below. The ΔCO3= at the carbonate compensation depth (CCD) in the tropics appears to be 10–20 μmol kg−1 ΔCO3= more undersaturated than in high latitudes, but this is smaller than the ∼40 μmol kg−1 shift required by the model. In addition, the general resemblance of the glacial lysocline to the present day requires that the proposed shift in ΔCO3= at the CCD be globally uniform rather than locally variable, as climate forcing would probably generate. I conclude that the rain ratio model would probably require some globally uniform perturbation during glacial time, such as a change in ocean Si content, if it is to explain the entire pCO2 decrease observed in the glacial atmosphere. Finally, I grid the sedimentary data to estimate that the inventory CaCO3 which is available to neutralize fossil fuel CO2 is approximately 1600 Gt carbon, a quantity which may be exceeded by fossil fuel release in the next several centuries.

Equatorial Pacific Calcite Preservation Cycles: Production or Dissolution?

In the Equatorial Pacific Ocean, the depth and shape of the calcite lysocline appears to have changed significantly in response to climate forcing (Farrell and Prell, 1989). Taking the Farrell and Prell record at face value, the lysocline apparently became steeper during each of the last eight glacial stages. Also, the accumulation rate of calcite during the last glacial maximum was 2–4 times higher than it was during the previous interglacial (Arrhenius; 1952,1988; Broecker, 1971; Farrell, 1991). A numerical model for calcite dissolution in sediment is used to interpret these observations. The model was validated by comparison with observations from the present‐day ocean; these results are presented elsewhere. If the lysocline fluctuations in the equatorial Pacific are primarily a dissolution record, then changes in the glacial Pacific [CO3] of 20–40 µM can be inferred. Here I offer the alternative explanation that cycles in equatorial production are responsible for the observations. Higher rates of calcite and organic carbon rain to the sediment in the equatorial region would have depressed the calcite lysocline and increased the calcite accumulation rate, as observed. A twofold increase in glacial production appears to be adequate to explain the observations, but a precise determination is prevented by uncertainties in some of the model parameters.

Modeling the calcite lysocline

A numerical model of calcite dissolution in contact with sediment pore water is used to predict the depth and shape of the calcite lysocline in the deep sea. Model results are compared with lysocline data from 13 regions in the Atlantic, Pacific, and Indian Oceans. The model lysocline shape is sensitive to the calcite dissolution rate constant, the calcite, organic carbon, and refractory material rain rates, and the rates of oxic versus anoxic organic carbon degradation in the sediment. The model is able to reproduce the observed lysocline, within the constraints of the sediment trap and calcite accumulation data, using a calcite dissolution rate constant of 30–100%d−1, a molar ratio of organic carbon to calcite rain rates of 0.5–1.0, and an initial CaCO3 fraction of 90% (excluding organic carbon). This rate constant is consistent with microelectrode results presented by Archer et al. [1989]. The model predicts that 30–50% of the calcite rain to the sea floor at the saturation horizon dissolves in response to organic carbon respiration, consistent with previous modeling studies. The range in the best fit value of the organic‐inorganic carbon rain rates arises from model sensitivity to uncertainty in the rate of anoxic carbon degradation in the sediment and in the rain rate of refractory material, rather than from scatter in the data. Lysocline data from the western equatorial Atlantic are anomalous to the rest of the data; this anomaly may be explained by high rates of refractory material sedimentation from the Amazon River.