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1.
Benthic foraminiferal oxygen and carbon isotopic records from Southern Ocean sediment cores show that during the last glacial period, the South Atlantic sector of the deep Southern Ocean filled to roughly 2500 m with water uniformly low in δ13C, resulting in the appearance of a strong mid-depth nutricline similar to those observed in glacial northern oceans. Concomitantly, deep water isotopic gradients developed between the Pacific and Atlantic sectors of the Southern Ocean; the δ13C of benthic foraminifera in Pacific sediments remained significantly higher than those in the Atlantic during the glacial episode. These two observations help to define the extent of what has become known as the ‘Southern Ocean low δ13C problem’. One explanation for this glacial distribution of δ13C calls upon surface productivity overprints or changes in the microhabitat of benthic foraminifera to lower glacial age δ13C values. We show here, however, that glacial-interglacial δ13C shifts are similarly large everywhere in the deep South Atlantic, regardless of productivity regime or sedimentary environment. Furthermore, the degree of isotopic decoupling between the Atlantic and Pacific basins is proportional to the magnitude of δ13C change in the Atlantic on all time scales. Thus, we conclude that the profoundly altered distribution of δ13C in the glacial Southern Ocean is most likely the result of deep ocean circulation changes. While the characteristics of the Southern Ocean δ13C records clearly point to reduced North Atlantic Deep Water input during glacial periods, the basinal differences suggest that the mode of Southern Ocean deep water formation must have been altered as well.  相似文献   

2.
Ocean Drilling Program Leg 199 Site 1220 provides a continuous sedimentary section across the Paleocene/Eocene (P/E) transition in the carbonate‐bearing sediments on 56–57 Ma oceanic crust. The large negative δ13C shift in seawater is likely due to the disintegration of methane hydrate, which is expected to be rapidly changed to carbon dioxide in the atmosphere and well‐oxygenated seawater, leading to a reduction in deep‐sea pH. A pH decrease was very likely responsible for the emergence of agglutinated foraminiferal fauna as calcareous fauna was eliminated by acidification at the P/E transition at Site 1220. The absence of the more resistant calcareous benthic foraminifera and the presence of the planktonic foraminifera at Site 1220 is interesting and unique, which indicates that calcareous benthic foraminifera suffered greatly from living on the seafloor. Box model calculation demonstrates that, assuming the same mean alkalinity as today, pCO2 must increase from 280 ppm to about 410 ppm for the calcite undersaturation in the deep ocean and for the oversaturation in the surface ocean during the P/E transition. The calculated increased pCO2 coincides with paleo‐botanical evidence. The current global emission rate (～7.3 peta (1015) gC/y) of anthropogenic carbon input is approximately 30 times of the estimate at the P/E transition. The results at the P/E transition give an implication that the deep sea benthic fauna will be threatened in future in combination with ocean acidification, increased sea surface temperature and more stratified surface water.  相似文献   

3.
While ocean circulation is driven by the formation of deep water in the North Atlantic and the Circum-Antarctic, the role of southern-sourced deep water formation in climate change is poorly understood. Here we address the balance of northern- and southern-sourced waters in the South Atlantic through the last glacial period using neodymium isotope ratios of authigenic ferromanganese oxides in thirteen deep sea cores from throughout the South Atlantic. The data indicate that northern-sourced water did not reach the Southern Ocean during the late glacial, and was replaced by southern-derived intermediate and deep waters. The high-resolution neodymium isotope record (~ 300 yr sample spacing) from two spliced deep Cape Basin sites indicates that over the last glacial period northern-sourced water mass export to the Southern Ocean was stronger during the major Greenland millennial warming intervals (and Southern Hemisphere cool periods), and particularly during the major interstadials 8, 12, and 14. Northern-sourced water mass export was weaker during Greenland stadials and reached minima during Heinrich Events. The benthic foraminiferal carbon isotopes in the same Cape Basin core reflect a partial control by Southern Hemisphere climate changes and indicate that deep water formation and ventilation occurred in the Southern Ocean during major Greenland cooling intervals (stadials). Together, neodymium isotopes and benthic carbon isotopes provide new information about water mass sourcing and circulation in deep Southern Ocean waters during rapid glacial climate changes. Combining carbon and neodymium isotopes can be used to monitor the relative proportion of northern- and southern-sourced waters in the Cape Basin to gain insight into the processes which control the carbon isotopic composition of deep waters. In this study we show that deep water formation and circulation was more important than biological productivity and nutrient regeneration changes for controlling the carbon isotope chemistry of Antarctic Bottom Water during millennial-scale glacial climate cycles. This observation also lends support to the hypothesis that ocean circulation is linked to interhemispheric climate changes on short timescales, and that ventilation in the glacial ocean rapidly switched between the northern and Southern Hemisphere on millennial timescales.  相似文献   

4.
13C and ΣCO2 data from the North and South Atlantic, the Antarctic, and the North and South Pacific are given. The δ13C of the ΣCO2 in the deep water (～3000m) decreases from 1.7‰ in the North Atlantic to ?0.10‰ in the North Pacific. This change is attributed to the addition of about 158 μmoles of CO2 per kg of seawater. The in-situ oxidation of organic matter accounts for 83% of this increase in ΣCO2, while the remainder is attributed to dissolution of calcium carbonate.The δ13C of the dissolved CO2 in mid-latitude surface water samples is controlled by a quasi-steady-state equilibrium with atmospheric CO2 at a mean temperature of 16°C. The δ13C and ΣCO2 values of Antarctic surface water samples suggest that these waters are derived from a mixture of North Atlantic deep water and equilibrated surface water.  相似文献   

5.
Records of the past neodymium (Nd) isotope composition of the deep ocean can resolve ambiguities in the interpretation of other tracers. We present the first Nd isotope data for sedimentary benthic foraminifera. Comparison of the ?Nd of core-top foraminifera from a depth transect on the Cape Basin side of the Walvis Ridge to published seawater data, and to the modern dissolved SiO2–?Nd trend of the deep Atlantic, suggests that benthic foraminifera represent a reliable archive of the deep water Nd isotope composition. Neodymium isotope values of benthic foraminifera from ODP Site 1264A (Angola Basin side of the Walvis Ridge) from the last 8 Ma agree with Fe–Mn oxide coatings from the same samples and are also broadly consistent with existing fish teeth data for the deep South Atlantic, yielding confidence in the preservation of the marine Nd isotope signal in all these archives. The marine origin of the Nd in the coatings is confirmed by their marine Sr isotope values. These important results allow application of the technique to down-core samples.The new ?Nd datasets, along with ancillary Cd/Ca and Nd/Ca ratios from the same foraminiferal samples, are interpreted in the context of debates on the Neogene history of North Atlantic Deep Water (NADW) export to the South Atlantic. In general, the ?Nd and δ13C records are closely correlated over the past 4.5 Ma. The Nd isotope data suggest strong NADW export from 8 to 5 Ma, consistent with one interpretation of published δ13C gradients. Where the ?Nd record differs from the nutrient-based records, changes in the pre-formed δ13C or Cd/Ca of southern-derived deep water might account for the difference. Maximum NADW-export for the entire record is suggested by all proxies at 3.5–4 Ma. Chemical conditions from 3 to 1 Ma are totally different, showing, on average, the lowest NADW export of the record. Modern-day values again imply NADW export that is about as strong as at any stage over the past 8 Ma.  相似文献   

6.
How would typhoon activity over the western North Pacific change for various scenarios of future global warming?Using the model projections of the Coupled Model Intercomparison Project phase 3(CMIP 3)under the SRES A1B scenario,we generated summer(September)ice-free Arctic conditions,also referred to as Blue Arctic conditions,and then used the corresponding monthly sea surface temperature(SST)and a set of CO2concentrations to drive an AGCM model to simulate the resulting changes in background conditions affecting typhoon activity over the western North Pacific.Our results show that,during typhoon season(June to October),atmospheric and ocean circulations over the western North Pacific would be significantly different from the present circulations.Changes in the vertical shear of zonal wind and outgoing longwave radiation(OLR)in the western North Pacific are favorable for westward and northward shift,respectively,of the location of typhoon genesis.Moreover,changes in the above fields over the key area may be conducive to less frequent typhoons.In addition,the tropical cyclone genesis potential index(GPI)over the western North Pacific would decrease(increase)east(west)of 150°E(140°E).  相似文献   

7.
Oxygen isotope fractionation in dissolved oxygen in the deep sea   总被引：1，自引：0，他引：1  
18O variations in dissolved oxygen have been measured at five stations from the eastern equatorial Pacific, at the GEOSECS-I and -II intercalibration stations in the North Pacific and North Atlantic, and along an Antarctic-South Pacific section from MONSOON expedition. Relative to atmospheric oxygen, dissolved oxygen in the ocean is enriched in18O up to a maximum of 14‰, the extreme enrichments occurring in the oxygen-minimum region of the eastern Pacific. The vertical diffusion-advection model has been used to determine the isotopic fractionation of deep-water in-situ oxygen consumption ascribed to bacterial metabolism. The single-stage enrichment, ε, in Pacific Deep Water below 1 km is 10‰ (α = 0.99,16O consumed preferentially). The model calculations show that the isotopic data cannot be fit without the introduction of a fractionation factor, just as the dissolved oxygen data cannot be fit without an in-situ consumption parameter. The consistency of the positive sign for ε and the negative source term for O2, observed in all deep Pacific profiles analyzed to date, provide strong evidence that vertical transport and in-situ consumption terms dominate the horizontal tracer flux terms, and indicate the presence of a significant “deep metabolism” in abyssal ocean waters.  相似文献   

8.
A detailed oxygen and carbon isotope record has been obtained from benthic Foraminifera in core V19-30 from the Carnegie Ridge on the south side of the Panama Basin. Expressing these records and the oxygen and carbon isotope records previously published for Atlantic core M-12392 on a common timescale, it is apparent that the oxygen isotope records are very similar but that the carbon isotope records are quite different. By obtaining the carbon isotope gradient between the two sites as a function of time we show that the production of North Atlantic Deep Water has varied over a wide range during the late Pleistocene, and that the pattern of variation is not simply related to the well known oxygen isotope record. Although the two oxygen isotope records are very similar, changes in the interoceanic gradient are detectable and support the hypothesis that in the glacial mode the North Atlantic was colder, and less oxygenated, than it is today.Shackleton's [1] 1977 interpretation whereby the carbon isotope record from the Atlantic core reflects changes in the terrestrial biomass, is an over-simplification. However, the record from the Pacific core V19-30 probably can be explained in these terms since it probably approximates the carbon isotope record of global mean oceanic dissolved CO2.  相似文献   

9.
The Milankovi theory stresses that the summer insolation in the high northern latitudes that is dominated by the precession cycle controls the glacial/interglacial cycles in global climate change.If the climate system responds linearly to the external insolation forcing,the precession cycle of 23 or 19 ka should dominate the variations in the climatic proxy records.I performed spectral and evolutive cross spectral analyses on the high resolution benthic 18O and 13C records from the South China Sea and the North Atlantic,the proxies of global ice volume and ocean carbon reservoir respectively.I found that the obliquity instead of the eccentricity or the precession is the most marked cycle in the global ice volume and ocean carbon reservoir variations over the past 5 Ma.The analysis further reveals that only at the obliquity band instead of the eccentricity or the precession band does the global ice volume and ocean carbon reservoir display consistently high coherency and stable phase relationship over the past 5 Ma.The consistently positive or near-zero phases of the benthic 18O relative to the benthic13C at the obliquity band suggest that the global carbon cycle is involved in the polar ice sheet growth as an important internal feedback,not a determinative driving factor.The obliquity instead of the precession or the eccentricity takes the dominant role of driving the global climate change during the Pliocene and Pleistocene.  相似文献   

10.
By modelling the observed distribution of210Pb and210Po in surface waters of the Pacific, residence times relative to particulate removal are determined. For the center of the North Pacific gyre these are τPo = 0.6years andτPb = 1.7years. The surface ocean τPb is determined by particulate transport rather than plankton settling. The fact that it is about two orders of magnitude smaller than τPb for the deep ocean implies a sharp change in the adsorptive quality of particles during descent through the water column.  相似文献   

11.
Extensive CO2 vents have been discovered in the Wagner Basin, northern Gulf of California, where they create large areas with lowered seawater pH. Such areas are suitable for investigations of long-term biological effects of ocean acidification and effects of CO2 leakage from subsea carbon capture storage. Here, we show responses of benthic foraminifera to seawater pH gradients at 74–207 m water depth. Living (rose Bengal stained) benthic foraminifera included Nonionella basispinata, Epistominella bradyana and Bulimina marginata. Studies on foraminifera at CO2 vents in the Mediterranean and off Papua New Guinea have shown dramatic long-term effects of acidified seawater. We found living calcareous benthic foraminifera in low pH conditions in the northern Gulf of California, although there was an impoverished species assemblage and evidence of post-mortem test dissolution.  相似文献   

12.
The importance of ocean temperature to global biogeochemistry   总被引：1，自引：0，他引：1  
Variations in the mean temperature of the ocean, on time scales from millennial to millions of years, in the past and projected for the future, are large enough to impact the geochemistry of the carbon, oxygen, and methane geochemical systems. In each system, the time scale of the temperature perturbation is key. On time frames of 1-100 ky, atmospheric CO2 is controlled by the ocean. CO2 temperature-dependent solubility and greenhouse forcing combine to create an amplifying feedback with ocean temperature; the CaCO3 cycle increases this effect somewhat on time scales longer than ∼5-10 ky. The CO2/T feedback can be seen in the climate record from Vostok, and a model including the temperature feedback predicts that 10% of the fossil fuel CO2 will reside in the atmosphere for longer than 100 ky. Timing is important for oxygen, as well; the atmosphere controls the ocean on short time scales, but ocean anoxia controls atmospheric pO2 on million-year time scales and longer. Warming the ocean to Cretaceous temperatures might eventually increase pO2 by approximately 25%, in the absence of other perturbations. The response of methane clathrate to climate change in the coming century will probably be small, but on longer time scales of 1-10 ky, there may be a positive feedback with ocean temperature, amplifying the long-term climate impact of anthropogenic CO2 release.  相似文献   

13.
Organic carbon in estuarine sediments can have many different sources. Terrestrial, riverine, estuarine and marine C pools may all contribute to and influence the organic C (Corg) inventory of the estuarine sediments and the differing stable isotope signatures of the sources are reflected in the sediment's overall 13C content. Ecological interpretations of sedimentary isotope data may, however, be limited by the fact the total Corg inventory of a sediment may not be an accurate representation of the fraction that is labile and being actively turned over by the sedimentary community. To gain a better understanding of sedimentary Corg dynamics in estuaries and the relationship between the sedimentary C pool and the Corg undergoing mineralisation, we studied three components of an estuarine system: (1) the sedimentary Corg inventory on a transect from the mouth to the upper end of the estuary, (2) temporal changes of sedimentary Corg at one station throughout a year, and (3) the δ13C of respired CO2 compared to the δ13C of available source material and sedimentary Corg in a novel application of methods developed for soil science. Our experiments demonstrated that material of marine origin dominated the studied estuary. At the time-series station, material of marine origin dominated the sedimentary Corg throughout the 1-yr study period. δ13C values of CO2 released from the sediment differed significantly from the sedimentary Corg inventory at all study sites, but also clearly reflected differences between the main sections of the estuary. These results suggest that δ13C measurements of respired CO2 are promising as a tool to advance our understanding of C cycling in estuaries, and highlight that the sedimentary Corg pool alone may not be a satisfactory indicator of OM utilisation in estuarine sediments.  相似文献   

14.
High resolution benthic foraminiferal stable isotopes (δ18O, δ13C) and molecular biomarkers in the sediments are used here to infer rapid climatic changes for the last 8200 years in the Ría de Muros (NW Iberian Margin). Benthic foraminiferal δ18O and δ13C potentially register migrations in the position of the hydrographic front formed between two different intermediate water masses: Eastern North Atlantic Central Water of subpolar origin (ENACWsp) and subtropical origin (ENACWst). The molecular biomarkers in the sediment show a strong coupling between continental organic matter inputs and negative δ13C values in benthic foraminifera. The rapid centennial and millennial events registered in these records have been compared with two well known North Atlantic Holocene records from the subtropical Atlantic sea surface temperatures (SST) anomalies off Cape Blanc, NW Africa and the subpolar Atlantic (Hematite Stained Grains percentage, subpolar North Atlantic). Comparison supports a strong link between high- and low-latitude climatic perturbations at centennial–millennial time scales during the Holocene. Spectral analyses also points to a pole-to-equator propagation of the so-called 1500 yr cycles. Our results demonstrate that during the Holocene, the NW Iberian Margin has undergone a series of rapid events which are likely triggered at high latitudes in the North Atlantic and are rapidly propagated towards lower latitudes. Conceivably, the propagation of these rapid climatic changes involves a shift in atmospheric and oceanic circulatory systems.  相似文献   

15.
Solar geoengineering has been proposed as a potential mechanism to counteract global warming. Here we use the University of Victoria Earth System Model (UVic) to simulate the effect of idealized sunshade geoengineering on the global carbon cycle. We conduct two simulations. The first is the A2 simulation, where the model is driven by prescribed emission scenario based on the SRES A2 CO2 emission pathway. The second is the solar geoengineering simulation in which the model is driven by the A2 CO2 emission scenario combined with sunshade solar geoengineering. In the model, solar geoengineering is represented by a spatially uniform reduction in solar insolation that is implemented at year 2020 to offset CO2-induced global mean surface temperature change. Our results show that solar geoengineering increases global carbon uptake relative to A2, in particular CO2 uptake by the terrestrial biosphere. The increase in land carbon uptake is mainly associated with increased net primary production (NPP) in the tropics in the geoengineering simulation, which prevents excess warming in tropics. By year 2100, solar geoengineering decreases A2-simulated atmospheric CO2 by 110 ppm (12%) and causes a 60% (251 Pg C) increase in land carbon accumulation compared to A2. Solar geoengineering also prevents the reduction in ocean oxygen concentration caused by increased ocean temperatures and decreased ocean ventilation, but reduces global ocean NPP. Our results suggest that to fully access the climate effect of solar geoengineering, the response of the global carbon cycle should be taken into account.  相似文献   

16.
The environmental and biogeochemical information extracted from the sediments collected from the northern shelf of the South China Sea shows that terrigenous inputs of phosphorus into the sea remained relatively constant, and the variation of phosphorus contents at different depths was caused by climatic and environmental changes. The findings also suggest that the vertical variation of phosphorus content was opposite to those of calcium carbonate and cadmium, and the functional correlation between CO2 and PO 4 3− in seawater was given by calculating the chemical equilibrium, indicating that the accumulation of marine sedimentary phosphorus may have something to do with the variation of atmospheric CO2. The decreased phosphorus accumulation as well as the correspondingly-increased calcium carbonate content might be one of key factors causing glacial atmospheric CO2 decline.  相似文献   

17.
Seven Miocene Pacific Ocean Deep Sea Drilling Project sites from four different water masses (planktonic foraminiferal biogeographic regions) have been correlated using 18 prominent carbon isotopic events defined in the benthic foraminiferal δ13C records in DSDP Site 289. The correlations are based on the assumption that there are global or at least Pacific-wide controls on the δ13C of deep-water HCO3?. Each of the individual δ13C records is correlated to Site 289 based on the shape of the curves in a manner analogous to that used to correlate sea-floor magnetic anomaly patterns.The results of this correlation experiment confirm that planktonic foraminiferal biostratigraphy and carbon isotopic stratigraphy are consistent within the tropical surface water mass and precise to ±100,000 years. Correlations between surface water masses suggest that the precision of foraminiferal biostratigraphy is on the average less than ±200,000 years due to the lack of cosmopolitan marker species and diachronism of species occurrences. Carbon isotope stratigraphy used in conjunction with biostratigraphy has the potential to provide an easily utilized, globally applicable, correlation tool (with an interregional precision of ±100,000 years or better) as more continuous and undisturbed deep-sea sections become available as a result of the Hydraulic Piston Coring Program.  相似文献   

18.
Weiwei Fu 《Ocean Dynamics》2018,68(10):1311-1319
Ocean heat content (OHC) plays an important role in ocean carbon uptake (OCU). However, the changes of OHC and OCU are model-dependent and have large bias compared with observations. This makes it difficult to quantify their relationship. Here, we propose a new metric to measure the uncertainty of the relationship between OHC and OCU. The new metric can link the uncertainty with different OCU processes and allow direct comparison of the impact of OHC on the OCU in different simulations. The metric is illustrated in different simulations of the Coupled Model Intercomparison Project phase 5 (CMIP5) in which atmospheric CO2 is increased by 1%/year. Results show that OHC in 0–500 m plays a dominant role in the OCU for the radiatively coupled (RAD) experiment because warming intensifies the carbon loss in the upper ocean. Relatively, OHC in the intermediate waters (500–2000 m) are crucial for the fully coupled and biogeochemically coupled experiment because this layer largely regulates the OCU. For different ocean basins, the intermediate Southern Ocean and deep North Atlantic are more important for the OCU in the RAD simulation. The metric also suggests the importance of global overturning circulation and the Southern Ocean in the OCU.  相似文献   

19.
New data from three Tasman Sea cores support Keigwin's [1] observation that the δ13C of Pacific benthic foraminifera (and by inference bottom-water TCO2) decreased by 0.7‰ at about 6.5 Myr B.P. Simple box models are developed and used to test several hypotheses about the cause of the δ13C decrease. We favor the idea that the δ13C shift was due to a rapid change in TCO2 cycling within the oceans (such as would result from either a decrease in upwelling rate, or an increase in the fraction of PO43? reaching the deep oceans in particulate organic matter and a corresponding drop in the preformed PO43? concentration). The δ13C decrease across the shift might reflect either a global decrease in upwelling rate, or a different abyssal circulation pattern before the shift.  相似文献   

20.
To constrain short-term changes of climate and oceanography in the northern South China Sea(SCS)over interglacial marine isotope stage(MIS)5.5,we studied planktic and benthic 18O records of seven marine sediment cores with a time resolution of70–700 yr.Using 6–8 tie points the planktic records were tuned to the U/Th chronology of speleothem 18O records in China and Europe.The last occurrence of pink Globigerinoides ruber marks the top of Heinrich stadial 11(HS-11)near 128.4 ka.HS-11 matches a 2300-yr long positive 18O excursion by 1.5/0.8‰both in planktic and benthic 18O records.Hence half of the planktic 18O signal was linked to increased upwelling of18O-and12C-enriched deep waters in the southwestern SCS.The increase was possibly linked to a strengthened inflow of Pacific deep waters through the Bashi Strait,that form a boundary current along the northern slope of the SCS,building a major sediment drift.At its lower margin near 2300–2400 m water depth(w.d.)Parasound records reveal a belt of modern erosion.At the end of glacial termination 2,stratigraphic gaps deleted HS-11 in core MD05-2904 and subsequent peak MIS 5.5 at ODP Site 1144.Likewise hiatuses probably earmarked all preceding glacial terminations at Site 1144 back to 650 ka.Accordingly,boundary current erosion then shifted~300 m upslope to~2040–2060 m w.d.These vertical shifts imply a rise in boundary current buoyancy,that in turn may be linked to transient events of North Pacific deepwater formation similar to that traced in SCS and North Pacific paleoceanographic records over glacial termination 1.  相似文献   

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