摘要:

Ice core work on Greenland has produced dramatic evidence for an instability of the climate system in the North Atlantic sector. In this paper, we provide climate modeling results indicating another possible example of a multiple equilibrium climate state, where such behavior might apply to the ice sheet itself. Baseline sensitivity experiments, in which the Greenland Ice Sheet was removed, simulate summer temperatures on Greenland of about 10°C. This number varied between about 6 and 14°C depending on specification of vegetation type, elevation of Greenland, and orbital forcing. The implied hysteresis indicates that the present geographic configuration may have been sufficient to maintain an ice‐free state prior to formation of the ice cap. Explanations for formation of the ice sheet therefore present a dilemma. Four possibilities involve undocumented Miocene‐Pliocene CO2excursions to values lower than present, a high sensitivity of the climate system, a variable sensitivity of the climate system, or significant problems with the climate model. A variable sensitivity is consistent with our results indicating a multiple steady state climate for Greenland. Although most CO2scenarios do not predict a collapse of the Greenland Ice Sheet in the future, our results suggest that if it did, present boundary conditions may be sufficient to maintain Greenland in an ice‐free environment even after the greenhouse effect has dis

ISSN:0883-8305    

DOI:10.1029/95PA00662

年代:1995

数据来源: WILEY

摘要:

Have the ice ages secularly altered the obliquity (axial tilt) of the Earth over geologic time? The waxing and waning of ice caps in response to obliquity oscillations plus mantle adjustment to the weight of the caps alter the Earth's dynamical flattening. This affects the lunar and solar torques on the Earth so as to secularly change the Earth's axial tilt. This mechanism is dubbed “climate friction,” since analogous to tidal friction, it arises from lags in the Earth's response to cyclic external forcing. But the existence of two processes, ice cap waxing and waning and mantle viscous flow, can lead to either an increase or decrease in the obliquity. Evidence indicates that the growth and decay of the ice caps greatly lag the orbital forcing; this causes the axial tilt to increase with time. But the ice cap effects are partly canceled by viscous compensation in the mantle. Low mantle viscosities (about 1021Pa·s) lead to rapid compensation and have only a slight effect on obliquity. High viscosities (about 1022Pa·s) slow the compensation enough so that there could be significant secular change in the obliquity over geologic time, perhaps explaining all of the present tilt of 23.5°. However, in addition to the current uncertainty as to the effective viscosity of the mantle, knowledge of past ice ages is incomplete, so that the amount of obliquity increase presently remains u

ISSN:0883-8305    

DOI:10.1029/95PA00578

年代:1995

数据来源: WILEY

摘要:

Benthic foraminiferal δ13C data from site 502 in the Caribbean Sea (sill depth ∼1800 m) indicate that throughout the past 2.6 m.y., glacial δ13C values in the middepth Atlantic were higher during glaciations than interglaciations. This is interpreted as indicating a greater proportion of Upper North Atlantic Deep Water (UNADW) relative to southern source waters during glaciations. The contribution of UNADW during interglaciations to the middepth Atlantic remained approximately constant, and the contribution during glaciations may have been as much as 10 % higher in the late Pleistocene than in the late Pliocene. This small increase is in striking contrast to the much larger decrease in glacial Lower North Atlantic Deep Water (LNADW) contribution relative to southern sources, from about 80% to about 20%, that occurred over the past 2.6 m.y. Glacial intensification over the past 2.6 m.y. was probably coupled with a decrease in northward heat transport by the upper limb of the North Atlantic circulation cell, as was previously suggested on the basis of a LNADW record alone. Late Pleistocene (1 Ma‐present) δ13C values in the Caribbean Sea were approximately 0.2‰ higher than they were from 2.6 to 2.0 Ma. The δ13C rise is not due to an increase in the mean ocean δ13C value, nor can it be entirely attributed to an increase in the proportion of high‐δ13C source waters. An increase in the δ13C value of the surface source waters must have contributed to

ISSN:0883-8305    

DOI:10.1029/95PA00332

年代:1995

数据来源: WILEY

摘要:

We examine the possibility that glacial increase in the areal extent of reducing sediments might have changed the oceanic Cd inventory, thereby decoupling Cd from PO4. We suggest that the precipitation of Cd‐sulfide in suboxic sediments is the single largest sink in the oceanic Cd budget and that the accumulation of authigenic Cd and U is tightly coupled to the organic carbon flux into the seafloor. Sediments from the Subantarctic Ocean and the Cape Basin (South Atlantic), where oxic conditions currently prevail, show high accumulation rates of authigenic Cd and U during glacial intervals associated with increased accumulation of organic carbon. These elemental enrichments attest to more reducing conditions in glacial sediments in response to an increased flux of organic carbon. A third core, overlain by Circumpolar Deep Water (CPDW) as are the other two cores but located south of the Antarctic Polar Front, shows an approximately inverse pattern to the Subantarctic record. The contrasting patterns to the north and south of the Antarctic Polar Front suggest that higher accumulation rates of Cd and U in Subantarctic sediments were driven primarily by increased productivity. This proposal is consistent with the hypothesis of glacial stage northward migration of the Antarctic Polar Front and its associated belt of high siliceous productivity. However, the increase in authigenic Cd and U glacial accumulation rates is higher than expected simply from a northward shift of the polar fronts, suggesting greater partitioning of organic carbon into the sediments during glacial intervals. Lower oxygen content of CPDW and higher organic carbon to biogenic silica rain rate ratio during glacial stages are possible causes. Higher glacial productivity in the Cape Basin record very likely reflects enhanced coastal up welling in response to increased wind speeds. We suggest that higher productivity might have doubled the areal extent of suboxic sediments during the last glacial maximum. However, our calculations suggest low sensitivity of seawater Cd concentrations to glacial doubling of the extent of reducing sediments. The model suggests that during the last 250 kyr seawater Cd concentrations fluctuated only slightly, between high levels (about 0.66 nmol kg−1) on glacial initiations and reaching lowest values (about 0.57 nmol kg−1) during glacial maxima. The estimated 5% lower Cd content at the last glacial maximum relative to modern levels (0.60 nmol kg−1) cannot explain the discordance between Cd and δ13C, such as observed in the Southern Ocean. This low sensitivity is consistent with foraminiferal data, suggesting minimal change in the glacial Cd mean oceanic

ISSN:0883-8305    

DOI:10.1029/95PA00310

年代:1995

数据来源: WILEY

摘要:

Short‐term imbalances in the global cycle of shallow water calcium carbonate deposition and dissolution may be responsible for much of the observed Pleistocene change in atmospheric carbon dioxide content. However, any proposed changes in the alkalinity balance of the ocean must be reconciled with the sedimentary record of deep‐sea carbonates. The possible magnitude of the effect of shallow water carbonate deposition on the dissolution of pelagic carbonate can be tested using numerical simulations of the global carbon cycle. Boundary conditions can be defined by using extant shallow water carbonate accumulation data and pelagic carbonate deposition/dissolution data. On timescales of thousands of years carbonate deposition versus dissolution is rarely out of equilibrium by more than 1.5 × 1013mole yr−1. Results indicate that the carbonate chemistry of the ocean is rarely at equilibrium on timescales less than 10 ka. This disequilibrium is probably due to sea level‐induced changes in shallow water calcium carbonate deposition/dissolution, an interpretation that does not conflict with pelagic sedimentary data from the central

ISSN:0883-8305    

DOI:10.1029/94PA02963

年代:1995

数据来源: WILEY

摘要:

We have generated a 49‐year‐long, high‐resolution, stable isotope record from a fossil coral,Solenastrea bournoni, to evaluate regional sea surface conditions during the middle Pliocene (3.0 ± 0.5 Ma) in southwest Florida. Continuous routing of the coral slab at an interval of 0.76 mm produced ∼5 to 6 samples per year. The annual cycle is well defined in δ18O and δ13C records confirming the average yearly growth rate estimated from density banding of 4 to 5 mm/yr. High‐density bands are synchronous with the highest δ18O and δ13C values for 79% and 45% of the annual cycles recorded, respectively. The annual range in δ18O is 0.70 to 2.27‰ and in δ13C it is 0.31 to 2.21‰. Partitioning of the coral δ18O signal into sea surface temperature (SST) changes and δ18Owaterchanges is difficult to estimate in the ancient. However, isotopic mass‐balance calculations, using modern climate data, suggest a partitioning of the δ18O signal into 8 to 12% salinity and 92 to 88% temperature at the sea surface. If 100% of the δ18O variation is attributed to temperature, the observed mean annual range in δ18O (mean = 1.54 ± 0.37‰) corresponds to a seasonal SST range of ∼7.0 ± 1.7°C (0.22‰/1°C), which is a mean of 3.5°C less than that of the present day. Isotope data are consistent with the hypothesis that δ13C values covary with the number of sunshine hours. Annually, peak δ18O values are often observed to precede peak δ13C values by one sample (i.e., about 2 months). We infer from the isotope pattern that periods of reduced SST were followed about 2 months later by periods of increased number of sunshine hours. This seasonal pattern is similar to that of southwest Florida today. Cross‐spectral analysis of fossil coral δ18O and δ13C documents coherency peaks, above the 95% confidence interval, at annual and subdecadal frequencies (1.0, 1.6, 2.9 and 5 year). The spectral phase angle between δ18O and δ13C at the annual frequency is −60 ± 10°, at the 95% confidence interval. The temporal equivalent of this angle is approximately 2 months, which confirms visual o

ISSN:0883-8305    

DOI:10.1029/95PA00374

年代:1995

数据来源: WILEY

摘要:

A quantitative model for describing the within‐population variation in planktonic foraminifer shell chemistry that results from secondary calcification and selective dissolution is presented. The objective is to construct a basis for inferring the chemistries of different shell components and estimating the extent of shell dissolution. Variation is modeled as mixtures of two kinds of shell calcite, a primary calcite that forms the chambers and a secondary calcite that forms crust. Bulk shell chemistries are intermediate between the chemistries of these calcites and lie on mixing lines between them. For two component systems, mass balance relationships can be reformulated as a linear regression and solved for the chemistries of the primary and secondary calcites and for the uncertainties associated with these estimates. To apply this model, one needs measurements of bulk shell chemistries and estimates of the relative proportions of secondary and primary calcites. For most planktonic foraminifer species the proportion of secondary calcite can be estimated from differences in the relationship between shell size and mass before and after crusting. Preliminary results are consistent with previous work showing that secondary calcites are added at depth. However, even deep‐dwelling species appear to grow most of their primary shell in surface waters and some surface‐dwelling species add secondary calcite in the deep ocean. In contrast to the model's simple description of secondary calcification, the variation in chemistry from selective dissolution is more complicated because undissolved shells are themselves mixtures of primary and secondary calcites and therefore present a wide range of initial shell compositions. Nevertheless, the model allows both the compositions of different components to be inferred and the amount of dissolution to be estimated. Preliminary results indicate that dissolution of planktonic foraminifera is apparent nearly 2 km above the foraminifer lysocline and even apparently well‐preserved shells may be over 50% di

ISSN:0883-8305    

DOI:10.1029/95PA00059

年代:1995

数据来源: WILEY

摘要:

The magnetic concentrations and magnetic fluxes of marine sediments from the Somali Basin show positive correlations with insolation, using oxygen isotopes for calibration. Investigations of the rock magnetic properties indicate an increase in the magnetite/hematite ratio during warmer episodes and the presence of additional fine grains of magnetite. The magnetic susceptibility profiles of several other cores suggest that the same characteristics prevail over a large area within the basin. These features are opposite to the variations observed to the north of this area (DeMenocalet al.,1991) where eolian deposition is dominant. Magnetic measurements performed on the coarse and fine fractions of the sediment show that the magnetic signal is carried by the finer fraction. Analyses of the principal components of clay mineralogy show that river transported sediments are geographically very limited and confirm that there is no indication of significant eolian deposition. It is most likely that the Antarctic bottom currents were responsible for the transport of the magnetic particles and thus for the correlation between the magnetic and climatic records.

ISSN:0883-8305    

DOI:10.1029/94PA03151

年代:1995

数据来源: WILEY

摘要:

This study documents the biological signatures impressed upon the sedimentary record underlying both the 5°N upwelling system of the Somali Current and the equatorial area of the Somali Basin out of the upwelling influence. The evolution of these two distinct hydrographic systems is compared for the last 160 kyr. Correspondence and cluster analyses are performed on combined radiolarian and planktonic foraminiferal quantitative data in order to study the changes of the planktonic assemblages through time and space. The Upwelling Radiolarian Index (URI) is used as a productivity proxy. The water temperature and hydrographic structure of the upper water masses appear to be the major factors controlling the distribution patterns of the fauna. The relative abundances of three groups of foraminifera, cold water form (dextralN. pachyderma), mixed layer dwellers (G. trilobus, G. ruber, G. sacculifer, G. conglobatus, andG. glutinata), and thermocline dwellers (G. menardii, G. tumida, N. dutertrei, G. crassaformis, andP. obliquiloculata), follow distinct evolutionary patterns at the two sites during the last 160 kyr. At the equatorial site (core MD 85668), downcore fluctuations in the relative abundances of the three groups are closely related to the glacial/interglacial cyclicity and provide some insights into the interpretation of hydrographic changes. The dominance of the mixed layer foraminifera at the transition intervals between isotope stages 6/5 and 2/1, combined with weak URI values, is thought to reflect the reorganization of the oceanographic circulation. These short‐term events (with a duration of<5000 year) could be related to the rapid inflow of oxygen‐depleted water through the Indonesian straits as a result of sea level rise during deglaciation. Underneath the 5°N gyre (core MD 85674), the response to global climatic changes is overprinted by the regional effect of the Somalian upwelling, which has been persistent over the last 16

ISSN:0883-8305    

DOI:10.1029/95PA00420

年代:1995

数据来源: WILEY

摘要:

Eolian dust derived from the desert regions of North Africa is blown far into the tropical Atlantic Ocean by persistent easterly and northeasterly winds. In this paper, we demonstrate that the iron oxides, hematite and goethite, are a worthwhile addition to proxy monitors of eolian sedimentation in the tropical Atlantic. Iron oxides are identified by diffuse reflectance spectrophotometry, a technique capable of identifying these minerals in concentrations as low as 0.01% by weight. We analyze samples from both the modern and last glacial maximum (LGM) synoptic levels from 178 sample locations yielding a total of 356 samples distributed throughout the Atlantic Ocean. To determine the relative contribution of the iron oxides, we factor analyzed the modern and LGM levels as a single data set. The iron oxide factor explains about 25% of the variance in the combined core top and LGM data set. Mapped factor scores for the LGM and modern ocean indicate high iron oxide values are present in just two regions, one off eastern North America and the other off northwest Africa. In the region off eastern North America, iron oxide occurs primarily during the LGM as the previously noted “brick red lutite,” a unique sediment type derived from the erosion of Permo‐Carboniferous red beds in Atlantic Canada. A larger, lobe‐shaped area of iron oxide rich sediment is present off northwest Africa in both the modern and LGM levels. The modern iron oxide lobe is coincident with the distribution of eolian dust as determined by observations from ships, satellites, and analysis of air samples. During the LGM, iron oxides exhibit a similar distribution except the southern margin of the region shifts equatorward and iron oxide concentration in

ISSN:0883-8305    

DOI:10.1029/95PA00421

年代:1995

数据来源: WILEY