Atmospheric pCO2 Levels Research Articles

Sources and sinks of CO2 in the west coast of Bay of Bengal

Observations at high spatial resolution (100×50 km2) in the western continental shelf of Bay of Bengal during southwest monsoon, when peak discharge occurs into the Bay through major rivers of the Indian subcontinent, revealed that freshwater discharge exerts dominant control on the inorganic carbon components in surface waters. Lower than present atmospheric pCO2 levels were found in the northwestern (NW) than southwestern (SW) coastal Bay of Bengal. The pCO2 levels in the peninsular rivers were an order of magnitude higher (5000–17000 µatm) than that of atmospheric levels and glacial river Ganges (∼500 µatm). The discharge from the peninsular rivers has a stronger influence in the SW region, whereas the Ganges river discharge has a stronger influences in the NW region. Source or sink of CO2 in the shelf region depends on the discharged river characteristics and the East India Coastal Current that distributes discharged water along the coast. Although during northeast monsoon, the situation is briefly reversed and the region acts as a sink, and on annual scale, the western Bay of Bengal acts as a source for atmospheric CO2 than hitherto hypothesised.

Ground‐truthing Late Ordovician climate models using the paleobiogeography of graptolites

The δ18O records obtained from conodonts suggest that during the Mid‐Ordovician, equatorial temperatures stabilized at close to the present day, a hypothesis broadly supported by published climate models. However, the degree to which equatorial temperatures represent global climate state and varied between different climatic modes (greenhouse/icehouse, interglacial/glacial) and to what extent Ordovician δ18O and climate models truly reflect the global climate remain to be tested. Here we present paleobiogeographical data for the planktonic graptolites, from the gracilis time slice (i.e., early Sandbian Stage) that just postdates the postulated onset of climate stabilization. TWINSPAN analysis and constrained seriation provide robust ecological groupings in paleobiogeographical presence/absence data. The highest‐level groups reflect tropical‐subtropical assemblages and a high–southerly latitude assemblage. Constrained seriation defines latitude‐distinct biotopes that are considered equivalent to modern zooplanktonic provinces. The distribution pattern of graptolite biotopes in the gracilis time slice show (1) that models explaining local graptolite ecological assemblages using lateral differentiation (e.g., upwelling) are to be favored above those using depth stratification and (2) a steep faunal gradient from equator to pole, which is typical of a cool, nongreenhouse world and comparable to the modern situation. We therefore broadly support the climate stabilization hypothesis. Paradoxically the climate of the early Late Ordovician appears similar to that of the present day despite the higher atmospheric pCO2 levels. Graptolite species were indicative of temperature‐controlled ocean water masses, in much the same way as the modern zooplankton.

The Eocene Arctic Azolla bloom: environmental conditions, productivity and carbon drawdown

Enormous quantities of the free-floating freshwater fern Azolla grew and reproduced in situ in the Arctic Ocean during the middle Eocene, as was demonstrated by microscopic analysis of microlaminated sediments recovered from the Lomonosov Ridge during Integrated Ocean Drilling Program (IODP) Expedition 302. The timing of the Azolla phase (approximately 48.5 Ma) coincides with the earliest signs of onset of the transition from a greenhouse towards the modern icehouse Earth. The sustained growth of Azolla, currently ranking among the fastest growing plants on Earth, in a major anoxic oceanic basin may have contributed to decreasing atmospheric pCO2 levels via burial of Azolla-derived organic matter. The consequences of these enormous Azolla blooms for regional and global nutrient and carbon cycles are still largely unknown. Cultivation experiments have been set up to investigate the influence of elevated pCO2 on Azolla growth, showing a marked increase in Azolla productivity under elevated (760 and 1910 ppm) pCO2 conditions. The combined results of organic carbon, sulphur, nitrogen content and 15N and 13C measurements of sediments from the Azolla interval illustrate the potential contribution of nitrogen fixation in a euxinic stratified Eocene Arctic. Flux calculations were used to quantitatively reconstruct the potential storage of carbon (0.9-3.5 10(18) gC) in the Arctic during the Azolla interval. It is estimated that storing 0.9 10(18) to 3.5 10(18) g carbon would result in a 55 to 470 ppm drawdown of pCO2 under Eocene conditions, indicating that the Arctic Azolla blooms may have had a significant effect on global atmospheric pCO2 levels through enhanced burial of organic matter.

Mid‐Miocene paleoproductivity in the Atlantic Ocean and implications for the global carbon cycle

A prominent, middle Miocene (17.5–13.5 Ma) carbon isotope excursion ubiquitously recorded in carbonate sediments has been attributed to enhanced marine productivity and sequestration of 13C depleted organic carbon in marine sediments or enhanced carbon burial in peat/lignite deposits on land. Here we test the hypothesis that the marine δ13C record reflects a change in productivity with proxy records from three Atlantic Ocean sites (Deep Sea Drilling Program Site 608 and Ocean Drilling Program Sites 925 and 1265). Our multiproxy approach is based on benthic foraminiferal accumulation rates, elemental ratios (Ba/Al and P/Al), the δ13C of bulk sedimentary organic matter, and dissolution indices. We compare these proxies to benthic foraminiferal δ13C values measured on the same samples. Our results indicate that marine paleoproductivity in the Atlantic Ocean is not related to the benthic foraminiferal δ13C excursion. A numerical box model confirms that marine productivity cannot account for the δ13C maximum. The model shows that sequestration of 1.5 × 1018 mol C in the terrestrial realm over a period of 3 Ma leads to a 0.9‰ δ13C increase in the deep ocean, which is near the observed records. Therefore, an increase in continental organic carbon sequestration is the most plausible way to enrich the ocean's carbon pool with 13C, which is consistent with coeval lignite deposits worldwide. The δ13C values of bulk sedimentary organic matter parallel the δ13C of dissolved inorganic carbon as reflected by benthic foraminiferal δ13C values suggesting no significant change in atmospheric pCO2 levels over the investigated period.

A Late Quaternary Climate Record Based on Multi‐Proxies Analysis from the Jiaochang Loess Section in the Eastern Tibetan Plateau, China

Abstract: We compared the stable carbon isotopic records from a loess transect of the Jiaochang in the eastern Tibetan Plateau, spanning the last ∼21,000 years, with multiproxy data for pedogenesis, including magnetic susceptibility, clay fraction, Fed/Fet ratio, carbonate and total organic carbon content, in order to probe the mechanisms of δ13C values of organic matter and Late Quaternary climate variations in the eastern Tibetan Plateau. Our results indicate that there is no simple relationship between δ13C of organic matter and summer monsoon variations. The change in δ13C values of organic matter (in accordance with the ratios of C3 to C4 plants) results from the interaction among temperature, aridity and atmospheric pCO2 level. Drier climate and lower atmospheric pCO2 level contribute to positive carbon isotopic excursion, while negative carbon isotopic excursion is the result of lower temperature and increased atmospheric pCO2 level. Additionally, our results imply that the Tibetan monsoon may play an important role in climate system in the eastern Tibet Plateau, which specifically reflects frequently changing climate in that area. The results provide new insights into the forcing mechanisms on both the δ13C values of organic matter and the local climate system.

A possible sequence of events for the generalized glacial‐interglacial cycle

There is not yet widespread agreement as to the underlying cause of the 80–100 ppmv roughly 100‐kyr‐duration glacial‐interglacial cycles in atmospheric pCO2. Most of the mechanisms which have been proposed to account for the observed pCO2 variations appear to in some way violate interpretations of paleo proxy data. The inability of a single mechanism to explain the observed cycles in atmospheric CO2 (which show amazing similarity over the past 430,000 years) is perplexing, and leads us to consider whether a combination of mechanisms might be consistent with available evidence. Consistent with previous work, we find that physical changes (ocean circulation, temperature, mixing) can only explain part of the observed atmospheric pCO2 variability; changes in ocean chemistry are invoked to explain the remainder. In order to account for the initial pCO2 drawdown (from “interglacial” to “intermediate” levels), we invoke physical changes in the ocean (mixing, temperature). The transition from intermediate atmospheric pCO2 levels to full glacial conditions involves a small increase in mean ocean nutrient levels and mean ocean alkalinity, accomplished by falling sea level and subsequent erosion of organic‐rich shelf sediments. The first part of the transition out of full glacial conditions is achieved through increased temperature and increased mixing in the Southern Ocean. The final part of the atmospheric pCO2 rise up to full interglacial conditions is accomplished through rising sea level and the subsequent change in mean ocean alkalinity and phosphate, and a rise in the Northern Hemisphere temperature and ocean mixing. The proposed sequence of events is consistent with most existing proxy evidence for paleo‐nutrient levels and changes in export production over the last glacial‐interglacial cycle. Furthermore, it is consistent with evidence for a whole‐ocean shift in δ13C toward significantly more negative values in the late glacial. The proposed scenario is also consistent with ice core‐based timing constraints, as summarized by Broecker and Henderson (1998). We show that we are able to explain the full magnitude of the glacial‐interglacial cycle in atmospheric pCO2 without the need to invoke iron‐fertilization in the Southern Ocean.

Relationships between bacterial-algal proliferating and mass extinction in the Late Devonian Frasnian-Famennian transition: Enlightening from carbon isotopes and molecular fossils

Studies show positive shifts of inorganic and organic carbon isotope values ( δ 13 C carb and δ 13 C kerogen ) from +0.43 (‰ V-PDB) to +3.54 (‰ V-PDB) and from -29.38 (‰ V-PDB) to -24.14 (‰ V-PDB), respectively, B* (Ba* = Ba/ (Al 2 O 3 X 15%)) values from 0.015 to 0.144, TOC values from 0.02% to 0.21%, V/Cr values from 0.3 to 2.0, Sr/Ba values from 3.20 to 49.50 in the Late Devonian Frasnian Upper rhenana zone to the top linguiformis zone of the Yangdi section deposited in carbonate slope facies of Guilin, Guangxi, South China, which indicates that bio-mass, productivity, organic carbon burial and salinity increase and that oxygenation near the boundary between sediments and waters decreases from the Late Devonian Frasnian Upper rhenana zone to the top linguiformis zone. Abundance of molecular fossils in-creases and normal alkanes, isoprenoid hydrocarbon, terpanes and steranes are dominated from the Late Devonian Frasnian to the bottom of Famennian, which shows that the predecessors of molecular fossils of the Frasnian-Famennian (F-F) transition are dominated by marine phyto-plankton, zooplankton and benthic bacteria with no photosynthesis. Therefore, it is considered that the F-F transitional mass extinction with a multistage, selection and global synchronizing was caused by bacterial-algal proliferating, continuing deterioration of the shallow marine ecoenvi-ronment of the middle-lower latitudes. A simple cause and effect chain can be expressed as: appearance of seed plants and multi-storied forests → enhanced chemical and biochemical weathering and pedogenesis → wide development of soils → increasing riverine nutrient fluxes in epicontinental sea → from superoligotrophic to eutrophic in epicontinental sea → proliferating of marine phytoplankton and zooplankton → frequent red tide and anoxia → mass extinction of shallow marine organisms in the middle-lower latitudes. It is worth notice that the factor draw-down of atmospheric Pco2, climatic cooling and sea level falling caused by eutrophication, anoxia and organic carbon burial increasing may be important for the mass extinction.

Altered carbon cycling and coupled changes in Early Cretaceous weathering patterns: Evidence from integrated carbon isotope and sandstone records of the western Tethys

In this study we investigate if a major perturbation of the Early Cretaceous carbon cycle was accompanied by altered weathering and erosion rates. The large Aptian carbon isotope anomaly records the response of the biosphere to widespread volcanic activity and probably resulting changes in atmospheric pCO2 levels. Elevated pCO2 levels should also result in an accelerated hydrological cycle and increased silicate weathering, creating a negative feedback loop removing CO2 from the atmosphere. We propose to interpret the widespread occurrence of quartz sandstones in the Tethys–Atlantic seaway as a result of altered weathering and erosion rates in the wake of the Aptian carbon cycle excursion. We challenge the traditional notion that these are ‘flysch’ deposits associated with Early Cretaceous orogenic movements in the western Tethys. We propose that these sandstones were most likely part of a large conveyor belt system, acting along the Iberian and European margin of the Tethys seaway. Using chemostratigraphic correlations, we show that the activity of this system was only short-lived and coeval with changes in coastal ecology and the Aptian carbon cycle perturbations. We tentatively relate the existence of this system to a transient climate regime, characterized by fluctuating pCO2 levels.

Obliquity forcing with 8–12 times preindustrial levels of atmospheric pCO2 during the Late Ordovician glaciation

Results from coupled ice-sheet and atmospheric general circulation models show that the waxing and waning of ice sheets during the Late Ordovician were very sensitive to changes in atmospheric p CO 2 and orbital forcing at the obliquity time scale (30–40 k.y.). Without orbital forcing, ice sheets can grow with p CO 2 level as high as 10 times preindustrial atmospheric level (PAL). However, with orbital forcing, ice sheets can grow only with p CO 2 levels of 8 times PAL or lower. These results indicate that the threshold of p CO 2 for the initiation of glaciation is on the lower end of previously published estimates of 8–20 times PAL. The ice-sheet model results further indicate that during exceptionally long periods of low summer insolation and low p CO 2 levels (8–10 times PAL), large ice sheets could have formed that were able to sustain permanent glaciation under subsequently higher p CO 2 values. This finding suggests that in order to end the Late Ordovician glaciation with a rise in p CO 2 , atmospheric p CO 2 must have risen to at least 12 times PAL. Ice sheets therefore introduce nonlinearities and hysteresis effects to the Ordovician climate system. These nonlinearities might have also played a role in the initiation and termination of other glaciations in Earth history.

Global sea–air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects

Based on about 940,000 measurements of surface-water pCO2 obtained since the International Geophysical Year of 1956–59, the climatological, monthly distribution of pCO2 in the global surface waters representing mean non-El Nino conditions has been obtained with a spatial resolution of 4°×5° for a reference year 1995. The monthly and annual net sea–air CO2 flux has been computed using the NCEP/NCAR 41-year mean monthly wind speeds. An annual net uptake flux of CO2 by the global oceans has been estimated to be 2.2 (+22% or ?19%) Pg C yr?1 using the (wind speed)2 dependence of the CO2 gas transfer velocity of Wanninkhof (J. Geophys. Res. 97 (1992) 7373). The errors associated with the wind-speed variation have been estimated using one standard deviation (about±2 m s?1) from the mean monthly wind speed observed over each 4°×5° pixel area of the global oceans. The new global uptake flux obtained with the Wanninkhof (wind speed)2 dependence is compared with those obtained previously using a smaller number of measurements, about 250,000 and 550,000, respectively, and are found to be consistent within±0.2 Pg C yr?1. This estimate for the global ocean uptake flux is consistent with the values of 2.0±0.6 Pg C yr?1 estimated on the basis of the observed changes in the atmospheric CO2 and oxygen concentrations during the 1990s (Nature 381 (1996) 218; Science 287 (2000) 2467). However, if the (wind speed)3 dependence of Wanninkhof and McGillis (Res. Lett. 26 (1999) 1889) is used instead, the annual ocean uptake as well as the sensitivity to wind-speed variability is increased by about 70%. A zone between 40° and 60° latitudes in both the northern and southern hemispheres is found to be a major sink for atmospheric CO2. In these areas, poleward-flowing warm waters meet and mix with the cold subpolar waters rich in nutrients. The pCO2 in the surface water is decreased by the cooling effect on warm waters and by the biological drawdown of pCO2 in subpolar waters. High wind speeds over these low pCO2 waters increase the CO2 uptake rate by the ocean waters. The pCO2 in surface waters of the global oceans varies seasonally over a wide range of about 60% above and below the current atmospheric pCO2 level of about 360 ?atm. A global map showing the seasonal amplitude of surface-water pCO2 is presented. The effect of biological utilization of CO2 is differentiated from that of seasonal temperature changes using seasonal temperature data. The seasonal amplitude of surface-water pCO2 in high-latitude waters located poleward of about 40° latitude and in the equatorial zone is dominated by the biology effect, whereas that in the temperate gyre regions is dominated by the temperature effect. These effects are about 6 months out of phase. Accordingly, along the boundaries between these two regimes, they tend to cancel each other, forming a zone of small pCO2 amplitude. In the oligotrophic waters of the northern and southern temperate gyres, the biology effect is about 35 ?atm on average. This is consistent with the biological export flux estimated by Laws et al. (Glob. Biogeochem. Cycles 14 (2000) 1231). Small areas such as the northwestern Arabian Sea and the eastern equatorial Pacific, where seasonal upwelling occurs, exhibit intense seasonal changes in pCO2 due to the biological drawdown of CO2.

A chemical model for evaporites on early Mars: Possible sedimentary tracers of the early climate and implications for exploration

Martian geomorphology seems to indicate extensive hydrological activity during the Noachian era. Liquid water at the surface would require a large greenhouse effect that is widely hypothesized to have been caused by a high partial pressure of atmospheric carbon dioxide,PCO2. A sedimentation model driven by sequential evaporation is used to calculate the evaporite mineral sequence in a closed basin lake subject to highPCO2. The initial fluid is derived from weathered igneous rock similar to Martian meteorite basalts. Siderite (FeCO3) is always the first major carbonate to precipitate. Thus siderite is predicted to be an important fades component in ancient Martian sediments along with silica, which is also an early precipitate. These would form varves in lakes that undergo cycles of evaporation and water recharge. After silica and siderite, the sequence is magnesian calcite, nearly pure hydromagnesite, and gypsum, followed by highly soluble salts like NaCl. The presence of siderite sediments generally requires an atmosphericPCO2level in excess of ∼0.1 bar, otherwise iron silicates (such as greenalite) would form. This may be used in exploration as an observational test on the past atmospheric composition of Mars, subject to consideration of the depositional environment. AtPCO2of approximately several bar, gypsum precipitation could occur before calcite upon evaporation if the initial SO42−:Ca2+ratio is high and there is no water recharge. The predicted carbonate sequence upon evaporation is generally consistent with recent hypotheses suggesting traces of evaporite carbonates in some Martian meteorites. Several mechanisms destroy or obscure carbonates at the Martian surface. Consequently, in situ analysis of the interior of ejecta from recent impact craters lying within sedimentary basins may offer the most practical approach to future exploration of ancient carbonate sediments.

Strong CO2 emissions from the Arabian Sea during south‐west monsoon

The partial pressure of CO2 (pCO2) was measured during the 1995 South‐West Monsoon in the Arabian Sea. The Arabian Sea was characterized throughout by a moderate supersaturation of 12–30 µatm. The stable atmospheric pCO2 level was around 345 µatm. An extreme supersaturation was found in areas of coastal upwelling off the Omani coast with pCO2 peak values in surface waters of 750 µatm. Such two‐fold saturation (218%) is rarely found elsewhere in open ocean environments. We also encountered cold upwelled water 300 nm off the Omani coast in the region of Ekman pumping, which was also characterized by a strongly elevated seawater pCO2 of up to 525 µatm. Due to the strong monsoonal wind forcing the Arabian Sea as a whole and the areas of upwelling in particular represent a significant source of atmospheric CO2 with flux densities from around 2 mmol m−2 d−1 in the open ocean to 119 mmol m−2 d−1 in coastal upwelling. Local air masses passing the area of coastal upwelling showed increasing CO2 concentrations, which are consistent with such strong emissions.

CO2 cycling in the coastal ocean. I – A numerical analysis of the southeastern Bering Sea with applications to the Chukchi Sea and the northern Gulf of Mexico

A quasi-two dimensional model of the carbon and nitrogen cycling above the 70m isobath of the southeastern Bering Sea at 57°N replicates the observed seasonal cycles of nitrate, ammonium, ΣCO2, pCO2, light penetration, chlorophyll, phytoplankton growth rate, and primary production, as constrained by changes in wind, incident radiation, temperature, ice cover, vertical and lateral mixing, grazing stress, benthic processing of phytodetritus and zooplankton fecal pellets, and the pelagic microbial loop of DOC, bacteria, and their predators. About half of the seasonal resupply of nitrate stocks to their initial winter conditions is derived from in situ nitrification, with the rest obtained from deep-sea influxes. Under the present conditions of atmospheric forcing, shelf-break exchange, and food web structure, this shelf ecosystem serves as a sink for atmospheric CO2, with storage in the forms of exported DOC, DIC, and unutilized POC (phytoplankton, bacteria, and fecal pellets).As a consequence of just the rising levels of atmospheric pCO2 since the the Industrial Revolution, however, the biophysical CO2 status of the Southeastern Bering Sea shelf may have switched over the last 250 years, from a prior source to the present sink, since this relatively pristine ecosystem has unergone little eutrophication. Such fluctuations of CO2 status may thus be reversed by the physical processes of : (1) reduction of atmospheric pCO2, (2) increased on welling of deep-sea ΣCO2, and (3) warming of shelf waters. Based on our application of this model to the Chukchi Sea and the Gulf of Mexico, about 1.0–1.2 gigatons C y-1 of atmospheric CO2 may now be sequestered by temperate and polar shelf ecosystems. When tropical systems are included, however, a positive net sink of only 0.6–0.8. × 1015g C y−1 may prevail over all shelves.

Late Pleistocene Variability of the Carbon Isotopic Composition of Organic Matter in the Eastern Mediterranean: Monitor of Changes in Carbon Sources and Atmospheric CO2 Concentrations

The organic carbon isotopic record of the sapropels (S1 and S3–S10) and intercalated marl oozes has been determined in a 12‐m piston core from the eastern Mediterranean. The δ13Corganic values are systematically lighter (mean=−21.0±0.82 ‰) in all sapropels and heavier (mean=−18.8±1.07‰) in the marl oozes. These differences are not due to variable marine and terrestrial organic matter mixtures because all values are heavier than modern plankton in the Mediterranean, there is no relationship between the Corganic/N ratios and the isotopic values, and published information on the abundance and distribution of organic biomarkers shows that terrestrial material constitutes a minor fraction of the total organic matter. Temperature effects on isotope fractionation are also discounted because the change in δ13Corganic values between glacial and interglacial horizons is in the opposite sense. Diagenesis, which can produce relatively small changes in the carbon isotopic composition of sedimentary organic matter under certain circumstances, is unlikely to have caused the observed differences because this mechanism would cause an enrichment in 12C, implying that all values were even heavier originally, and there is no secular trend in the δ13Corganic record. The observed differences in δ13Corganic between the two lithologies are probably produced by changes in the isotopic composition and the concentration of dissolved CO2. First, freshwater flooding during the formation of the sapropels caused the isotopic composition of the dissolved inorganic carbon in the surface waters of the Mediterranean to become lighter because of the 13C deficiency in fresh waters. Hence photosynthesis would have produced isotopically lighter organic material. Second, changes in atmospheric pCO2 between glacial and interglacial periods, as shown by the Vostok ice core, caused marked changes in the concentration of free dissolved CO2 in the mixed layer; lower values during glacial maxima caused a smaller fractionation of the carbon isotopes by phytoplankton, whereas levels were less limiting during the interglacials. Concentrations of dissolved CO2 could also have been much higher during the deposition of the sapropels because of the supply of regenerated CO2 to the mixed layer by upwelling, and this could have further lightened the δ13Corganic values in the sapropels themselves. Carbon isotope records may provide an alternative method for estimating atmospheric pCO2 levels over longer time periods than can be obtained from ice cores.

The δ18O of atmospheric O2 from air inclusions in the Vostok Ice Core: Timing of CO2 and ice volume changes during the penultimate deglaciation

One important question about the CO2‐climate connection is how increasing atmospheric pCO2 levels affect climate and thereby the mass balance of continental ice sheets. A record of atmospheric CO2 variations over the last 160,000 years has recently been constructed by analyzing the trapped gas in the Vostok ice core [Barnola et al., 1987]. The relationship between changes in atmospheric CO2 and the size of the continental ice sheets has been difficult to ascertain because the CO2 record is obtained from ice cores while the ice volume record has been constructed from the stable isotopic composition of biogenic CaCO3 in deep‐sea sediment cores. In order to compare these two records in a more precise manner, we present a record of the isotopic composition (δ18O) of atmospheric O2 trapped in the Vostok ice core, and propose that it may be considered a proxy for the δ18O of seawater and hence ice volume. Having a record of atmospheric CO2 along with a continental ice volume proxy in trapped air in the same ice core allows us to compare the timing of changes in these two parameters with little uncertainty in the relative ages of important events. Our results suggest that, during the penultimate glacial termination, atmospheric CO2 began to increase at least 3 kyr before the initial introduction of meltwater to the oceans. Possible errors in the relative age‐depth curve of the Vostok ice core and uncertainties in the influence of biological and hydrologic processes on the δ18O of atmospheric O2 introduce some uncertainty into our conclusions. However, our results are in general agreement with the observed phase relationship between atmospheric CO2 and ice volume [Imbrie et al., 1984; Imbrie et al., 1989] (inferred from changes in the δ13C difference between contemporaneous planktonic and benthic forams deposited in deep‐sea cores and the SPECMAP ice volume record).

The δ13C of equatorial Atlantic surface waters: Implications for Ice Age pCO2 levels

Carbon isotopic records of nutrient‐depleted surface water place constraints on the past fertility of the oceans and on past atmospheric pCO2 levels. The best records of nutrient‐depleted δ13C are obtained from planktonic foraminifera living in the thick mixed layers of the western equatorial and tropical Atlantic Ocean. We have produced a composite, stacked Globigerinoides sacculifer δ13C record from the equatorial Atlantic, which exhibits significant spectral power at the 100,000‐ and 41,000‐year Milankovitch periods, but no power at the 23,000‐year period. Similar to the record presented by Shackleton and Pisias [1985], surface‐deep ocean Δδ13C produced with the G. sacculifer record leads the δ18O ice volume record. However, the glacial‐interglacial amplitudes of Δδ13C differ between our record and Shackleton and Pisias [1985] record. Although large changes in Δδ13C occur in the equatorial Atlantic during early stages of the last three glacial cycles, surface‐deep Δδ13C at glacial maxima (18O stage 2, late stage 6, and late stage 8) was only about 0.2‰ greater than during the subsequent interglacial. Our results imply that nutrient‐driven pCO2 changes account for about one third of the pCO2 decrease observed in ice cores, and consequently, Δδ13C should not be used as a proxy pCO2 index. Enough variance in the ice core pCO2 records remains to be explained that conclusions about pCO2 and ice volume phase relationships should also be reexamined. As much as 40 ppm pCO2 change still has not been accounted for by models of past physics and chemistry of the ocean.