摘要:

A proposal has been made by Broecker et al. (1990) that rapid changes on a time scale of a thousand years or so, seen over much of the last major glacial in the Greenland ice core record, represent significant climate changes and are caused by a salt oscillator in the glacial Atlantic. This proposal is examined in terms of a rudimentary quantitative model. Scale analysis asserts that heat transported to the high‐latitude atmosphere when the thermohaline circulation is turned on, is large enough to produce the melting rates found by Fairbanks (1989) for the time interval around that of the Younger Dryas event and that these melting rates are of the same order of magnitude as the mass flux associated with water vapor flux to the Pacific Ocean estimated by Broecker (1989). Scale analysis also suggests that the salinity fluxes associated with 1) the water vapor flux mechanism, 2) the rapid melting episodes of Fairbanks, 3) possibly ice sheet growth events, 4) net transport by the thermohaline circulation and 5) net transport by turbulent eddy mixing are roughly of the same order of magnitude and therefore may be important mechanisms for producing salinity oscillations on a time scale of a few thousands of years, (see Broecker, 1990). By integration of simple salt conservation equations, it is found that model oscillations with a period of a few thousand years occur over a significant range of salinity fluxes; estimated fluxes fall well within the range for which oscillations exist. The model also suggests that there may exist close coupling between the European‐Scandinavian ice sheets and the bimodal response of the oscillator; that is, significant increases or decreases in continental ice volume may accompany each complete cycle of the oscillator. In addition, it appears that continental ice may be required for the salt oscillator to function. A crucial element, which cannot adequately be investigated with the present model, concerns the local effect of salinity source/sinks associated with melt water production. The proximity of these source regions on the neighboring ice sheets to the local regions where production of deep water occurs may play a critical role in the functioning of the proposed salt oscillator. In addition, further treatment of thermodynamics is needed to investigate the feasibility of a salinity driven oscilla

ISSN:0883-8305    

DOI:10.1029/PA005i006p00835

年代:1990

数据来源: WILEY

摘要:

Fluctuations in abundance between the two calcareous nannofossil speciesCoccolithus pelagicusandReticulofenestra pseudoumbilicaare observed in the upper Miocene sediments recovered from Deep Sea Drilling Project hole 552A. A record of these fluctuations (Cp/Rp) is established for a time interval of ∼1.35 m.y. (from 4.9 to 6.25 Ma) and is shown to yield periodicities which correspond to Milankovitch periodicities. A change in regime occurred at 5.72 Ma, as indicated by a shift from a series dominated by the 20‐kyr cycle to a series dominated by the 100‐kyr cycle. The Cp/Rp is compared with the isotopic δ18O and δ13O records from benthic and planktonic foraminifera. It is shown that a good correlation between the Cp/Rp and the isotopic records occurs only at times of major global changes. This suggests that the Cp/Rp, which reflects water masses movements, yields a complex paleoclimatic/paleoceanographic message which can help improving our understanding of the late Miocene

ISSN:0883-8305    

DOI:10.1029/PA005i006p00845

年代:1990

数据来源: WILEY

摘要:

High‐resolution δ13C and δ18O records have been generated from analyses of the planktonic foraminiferal speciesHeterohelix globulosaand the benthonic foraminiferal taxonLenticulinaspp from 3 m of a cored section spanning the Cretaceous/Tertiary (K/T) boundary at Brazos River, Texas. These are the first stable isotope records across the K/T boundary based on monospecific and monogeneric foraminiferal samples. They show a gradual decrease in δ13C values of about 2.5 permil beginning at the K/T boundary, as defined by the first appearance of Tertiary planktonic foraminifera, and continuing 17–20 cm above the boundary, approximately 40,000 years later. Gradual13C depletion contrasts with the sudden δ13C drop at the K/T boundary observed in many deep‐sea sections. The surface‐to‐bottom δ13C gradient decreased to less than zero approximately 25,000–30,000 years after the K/T boundary and remained negative for at least the next 140,000 years. Concomitant with change in δ13C values is a gradual decrease of about 2.5 permil in δ18C values which has not been observed at other localities. This18O depletion suggests changes in temperature and/or salinity in the earliest Paleocene Gulf of Mexico. No extinction of foraminiferal species is associated with the K/T boundary or the onset of18O and13C depletions. Instead, two phases of Cretaceous species extinctions occur. One extinction phase is below the K/T boundary and below the tsunami bed of Bourgeois et al. [1988] and may be linked to sea level regression and environmental perturbations. The second extinction phase coincides with the minimum in δ13C and δ18O values in the Early Danian (Zone P0/Pla) and appears directly related to environmental changes reflected in the isotopic record.H. globulosa, which is commonly present in Maastrichtian and Danian sediments, exhibits significantly lower18O/16O and13C/12C ratios in Tertiary sediments relative to specimens from Maastrichtian sediments, demonstrating the survival of this important Cretaceous taxon after th

ISSN:0883-8305    

DOI:10.1029/PA005i006p00867

年代:1990

数据来源: WILEY

ISSN:0883-8305    

DOI:10.1029/PA005i006p00891

年代:1990

数据来源: WILEY

摘要:

Runoff from North America may have played a significant role in ocean circulation and climate change during the last deglaciation. Because the driving force behind such changes may have been related to salinity of the north flowing Atlantic Ocean conveyor circulation, it is critical to know the volume, timing, and location of fresh water entering the North Atlantic from the melting Laurentide Ice Sheet. During the Younger Dryas cold episode, 11,000–10,000 years B.P., there was a two‐fold increase in the volume of meltwater plus precipitation runoff, to more than 1700 km³ yr−1, flowing through the St. Lawrence valley to the North Atlantic, mainly because retreating ice allowed the glacial Lake Agassiz basin to drain eastward into the Great Lakes at this time. There was a corresponding decline in discharge from Lake Agassiz through the Mississippi River to the Gulf of Mexico. Runoff to the Arctic Ocean also increased at about the beginning of the Younger Dryas, from 740 to 900 km³ yr−1, because of the capture of what is now the headwater region of the Mackenzie River watershed. This, in combination with rising sea level and warming climate, may have increased the amount of pack ice reaching the North Atlantic through the Norwegian Sea from the Arctic Ocean. At 10,000 years B.P., eastward overflow from the western interior of North America was blocked by advancing ice, again forcing overflow to the Gulf of Mexico and, possibly, to the northwest into the Arctic Ocean. Although total runoff to the oceans from all regions draining from the Laurentide Ice Sheet did not vary substantially between 12,000 and 9000 years B.P., if discharge to the Gulf of Mexico is excluded, fresh water reaching the North Atlantic averaged 4000 km³ yr−1during the Younger Dryas, in contrast to 2870 km³ yr−1just before this cold episode and 3440 km³ y

ISSN:0883-8305    

DOI:10.1029/PA005i006p00897

年代:1990

数据来源: WILEY

摘要:

Ice‐directional features and erratic lithologies of the last glaciation demonstrate that a major outlet glacier of the Laurentide Ice Sheet flowed NNE across outer Hudson Strait rather than SE down the Strait as previously hypothesized. Ice advanced more than 600 km from a Labradorean source, crossing Hudson Strait and outer Frobisher Bay, and advancing onto Hall Peninsula; the highest summits on Loks Land (circa 400 m asl), a large island at the SE tip of Hall Peninsula, were inundated by Labradorean ice. Three distinct Late Wisconsin advances have been recognized. The earliest, most extensive advance crossed Frobisher about 11.5 kyr B.P. and was maintained for circa 1 kyr. Differences in the extent of amino acid racemization in shells from interstadial and deglacial sites on eastern Loks Land limit the duration of the most extensive Late Wisconsin advance(s) to a brief interval of no more than 2 kyr. Radiocarbon dates on in situ shells and on shells incorporated in till indicate that Frobisher Bay was ice‐free from at least 10.5 to 10.2 kyr B.P. after which a second Labradorean advance, the Gold Cove readvance, attained nearly the same extent as the earlier advance. The readvance was short lived, and the Bay was deglaciated before 9.5 kyr B.P. A final readvance occurred during the Cockburn Substage (9 to 8 kyr B.P.), but ice failed to cross Frobisher Bay.The maximum extent of the Labradorean advance coincides with the onset of Younger Dryas cooling. Based on a minimum cross‐sectional area and reasonable estimates of ice velocity, the iceberg flux to the North Atlantic Ocean at this time was between 300 to 2400 km³ year−1, about the same as the freshwater influx from the diversion of Lake Agassiz drainage to the St. Lawrence. The input of a significant volume of ice into the North Atlantic via the Labrador Sea would have cooled sea surface and air temperatures, increased sea‐surface albedo, and diminished the effectiveness of wind mixing of surface waters, thereby reducing the volume of water required to effectively cap the North Atlantic. We suggest that a combination of a massive iceberg flux and increased St. Lawrence discharge may have been required to initiate Younger Dryas cooling. When the outflow of Lake Agassiz was again routed down the St. Lawrence at 9.5 kyr B.P., a second “Younger Dryas” event failed to occur, because by that time Labradorean ice had retreated within Hudson Strait and was unable to generate a substantial (climate‐altering) volume of icebergs. The possibility of an iceberg flood in the North Atlantic could be tested by measuring the concentration of ice‐rafted detritus (IRD; traditionally defined as quartz/feldspar grains>125 µm) in deep‐sea cores. However, sediment entrained by the Labradorean ice is dominated by detrital carbonate, and only a small fraction of the noncarbonate component is>125 µm, hence a revised definition of IRD is required to evaluate the ic

ISSN:0883-8305    

DOI:10.1029/PA005i006p00907

年代:1990

数据来源: WILEY

摘要:

Cores HU82‐034‐057 and HU84‐035‐008, Resolution Basin, SE Baffin Shelf, contain 200 and 450 cm, respectively, of sediment that spans the Younger Dryas chron. In both cores the interval is bracketed by14C dates on foraminifera or molluscs. These sites were close to the margin of the late Wisconsin (Foxe) ice sheet as it flowed toward the Labrador Sea. Prior to 11 ka, both cores record moderate to high accumulations of foraminifera, relatively high del18O values in planktonic foraminifera, and low values of detrital carbonate. The diatom and percent opal records imply occasional seasonally open water conditions. During part of the Younger Dryas chron both the diatom and opal analyses imply a shutoff of biogenic silica production, suggesting surface water conditions affected by increased sea ice and/or reduced nutrients. In addition, the Younger Dryas interval is marked by an increase in coarse sand and detrital carbonate, a decrease in total organic carbon and foraminifera, and high rates of sediment accumulation. The inferred environment during the Younger Dryas is ice‐proximal. In HU82‐034‐057, the foraminifera and other data suggest a change in conditions during the middle part of the Younge

ISSN:0883-8305    

DOI:10.1029/PA005i006p00921

年代:1990

数据来源: WILEY

摘要:

230Th/234U and14C dating of Barbados corals has extended the calibration of14C years B.P. to calendar years B.P. beyond the 9200 year tree ring series (Bard et al., 1990). This now permits the conversion of14C chronozones, which delimit major climate shifts in western Europe, to calendar years. The Younger Dryas chronozone, defined as 11,000 to 10,00014C years B.P., corresponds to 13,000 to 11,700 calendar years B.P. This calibration affects the interpretation of an intensely studied example of the “Younger Dryas climate event,” the δ18O anomaly between 1785 and 1793 m in Dye 3 ice core. The end of the δ18O anomaly in Dye 3 ice core has been dated by measurements of14C in air bubbles (Andree et al., 1984, 1986) and by annual layer counting (Hammer et al., 1986). The older14C dates fall out of the range of the tree ring calibration series but can now be calibrated to calendar years using the Barbados230Th/234U calibration. The14Ccorrectedage for the end of the δ18O event is 10,300 ± 400 calendar years B.P. compared to the annual layer counting age of 10,720 ± 150 years B.P. Thus, the “Younger Dryas” event in the Dye 3 ice core ends in the Preboreal chronozone (11,700 to 10,000 calendar years B.P.) and is not correlative with the end of the Younger Dryas event identified in pollen records marking European vegetation changes. The end of the Dye 3 δ18O event is, however, correlative with the end of meltwater pulse IB (Fairbanks, 1989), marking a period of intense deglaciation with meltwater discharge rates exceeding

ISSN:0883-8305    

DOI:10.1029/PA005i006p00937

年代:1990

数据来源: WILEY

摘要:

Data are presented from Orca Basin piston core EN32‐PC4 in the Gulf of Mexico that confirm the existence of surface water cooling during the Younger Dry as chronozone (11–10 ka). Late glacial planktonic foraminiferal species made a reappearance between 11.4 and 9.8 ka, an episode also marked by distinctly higher oxygen isotopic values derived from the planktonic foraminiferGlobigerinoides ruber. The presence of the Younger Dryas event in the gulf at 27°N demonstrates that surface water cooling extended to mid‐latitude regions in the North Atlantic. The cool surface water interval is bracketed by rapid shifts in δ18O related to changes in the influx of meltwater to the Gulf of Mexico. A chronology based upon seven accelerator radiocarbon dates indicates that cooling commenced over a ∼500 year period and ended in less than 200 years. These results are among the first deep‐sea sediment data documenting the climatic transitions bracketing the Younger Dryas with a rapidity observed in ice core records. A rapid decrease in δ18O values measured in the white form ofGs. ruberat 10.2 ka is explained by significant meltwater influx into the gulf and rapid increase in sea surface temperatures. Surprisingly, a similar decrease is not observed in the pink form ofGs. ruber, a summer surface water dweller in the gulf. This discrepancy may be explained by continued meltwater influx throughout the Younger Dryas during the summers only, such that there was no change in the δ18O of the pink form at the end of the episode. An additional possibility is that warming at the end of the Younger Dryas raised year‐average temperatures and summer temperatures remained constant. The coincidence of rapid shifts in δ18O with the Younger Dryas strongly suggests a dynamic causal relationship and therefore supports a model for the cause of the Younger Dryas cooling based on changes in the routing of Laurentide g

ISSN:0883-8305    

DOI:10.1029/PA005i006p00949

年代:1990

数据来源: WILEY

摘要:

Oxygen isotopic analyses of individualOrbulina universafrom Orca Basin core EN32‐PC6 document the presence of low salinity surface waters in the northern Gulf of Mexico over the past 16 kyr. Isotopic data from an interval immediately following the Younger Dryas Event indicate the rapid decrease in δ18O values at the conclusion of the Younger Dryas was due to a year‐round return of meltwater to the Gulf of Mexico. Data indicate periodic or seasonal low‐salinity waters existed over the region of the Orca Basin prior to the initiation of the meltwater spike. Estimates suggestO. universagrew its shell in salinities at least 4.5 ‰ below ambient. SinceO. universamay have calcified deep in the mixed layer during periods of low salinity, surface salinities could have been even lower. Comparison of the average of individualO. universaoxygen isotopic values with data from multiple shell samples of whiteGs. ruberfrom the same core samples demonstrates that the two species record similar values during the late Holocene. In contrast,O. universarecords lower oxygen isotopic values during the late glacial/deglacial intervals, possibly due to differences in seasonal distribution or shell ontogeny between the two

ISSN:0883-8305    

DOI:10.1029/PA005i006p00963

年代:1990

数据来源: WILEY