ISSN:0883-8305    

DOI:10.1029/94PA02581

年代:1995

数据来源: WILEY

摘要:

The rhythm of sedimentary cycles reflecting the climatic precession signal (19 and 23 kyr components) from the upper Miocene of the Bou Regreg section at Ain el Beida near Rabat, Morocco, had been analyzed to determine the age of the geomagnetic polarity reversals of Subchron 5N1 (C3An.1n). A new method of analysis of cycle “signatures” uses adjusted data series as represented by a signal obtained from grey‐level traces through image‐enhanced photographs of the sediment cycles. The measured, “timeless” signal is reduced and geometrically transformed to a signature template in order to compare its pattern with similar time constrained segments of the target precession signal (retrodicted for 35° N). Conventional statistical and spectral analysis tests are used to regress to the best possible fit. A date estimate of 5.94 Ma for the Chron 5/Gilbert boundary (C3An.1n/C3r), which occurs near the times of the closure of the Rifian Corridor and the beginning of the “drawdown” phase of the Messinian Salinity Crisis, is found to be in close agreement with independent approximations derived from extrapolation of sea floor spreading, radiometric dating, and younger Earth‐o

ISSN:0883-8305    

DOI:10.1029/94PA02539

年代:1995

数据来源: WILEY

摘要:

Numerous studies have shown that δ18O records from benthic and planktonic foraminifera, primarily a proxy of global ice volume variations, reflect Milankovitch periodicities. To study climatic response to orbital forcing at Ocean Drilling Program site 758, we have generated continuous δ18O and δ13C records from a single benthic foraminiferal speciesCibicides wuellerstorfifor the last 3.6 m.y. and extended the planktonic foraminiferal isotope records of Farrell and Janecek (1991) (0‐2.5 Ma, based onGlobigerinoides sacculifer) to 3.6 Ma (Chen, 1994). We then constructed an age model by matching, correlating and tuning the benthic δ18O record to a model simulation of ice volume (Imbrie and Imbrie, 1980). The filtered 41‐ and 23‐kyr signals based on the resultant astronomically tuned age model are highly correlated to obliquity (r=0.83) and precession (r=0.75), respectively. Although derived with methodology different from Shackleton et al. (1990) and Hilgen (1991a, b), our results generally agree with their published astronomical timescales for the time interval from 0 to 3.0 Ma, providing additional support for the newly emerging chronology based on orbital tuning. Slight discrepancies exist in the time interval from 3.0 to 3.6 Ma, suggesting several possibilities, including differences in the approaches of orbital tuning and the relatively low amplitude of δ18O variations in our record. However, even if the discrepancies are due to the relatively low amplitude of the isotope signals in our record at 3.0–3.6 Ma, our resultant timescale as a whole does not adversely affect our evaluation of the paleoclimatology and paleoceanography of the Indian Ocean, such as the evolution of the 100‐, 41‐ and 23‐kyr cycles, and variation of global ice volume and deepwater temperature durin

ISSN:0883-8305    

DOI:10.1029/94PA02290

年代:1995

数据来源: WILEY

摘要:

Equilibrium integrations with the three‐dimensional Hamburg ocean carbon cycle circulation model show the effect of a Southern Ocean surface outcrop that is closed off for gas exchange with the atmosphere and export production of particulate organic carbon and CaCO3south of 32.5 °S. For such a close‐off, the northern Atlantic outcrop assumes a greater degree of control of atmospheric CO2partial pressure. Contrary to expectations, the atmospheric pCO2increases compared to a control run without close‐off. Maximizing the nutrient utilization in the North Atlantic can compensate for this increase. However, it does not draw down the pCO2to glacial

ISSN:0883-8305    

DOI:10.1029/94PA02353

年代:1995

数据来源: WILEY

摘要:

The episodes of ice‐rafted detritus (IRD) accumulation identified in North Atlantic sediment cores indicate periods of abnormal iceberg production mainly of the Laurentide Ice Sheet. In this paper we develop further a scaling theory of these so‐called Heinrich events, leading to a general criterion for basal melting of an ice sheet which we take as a necessary condition for surging and streaming. This criterion depends not only on the physical properties of ice sheets and the magnitude of the geothermal and viscous heating but also on the history of changes in ice elevation, atmospheric carbon dioxide, and sea surface temperature, the thermal effects of which at the top of an ice sheet are transmitted with a time delay to the basal boundary by thermal advection. We apply this criterion to various past and present ice sheets showing their general vulnerability to surging. In greater detail, we use estimates of the time variations of ice volume (SPECMAP), carbon dioxide (Vostok), and North Atlantic sea surface temperature (site K708‐1) to estimate when basal melting conditions prevailed for the Laurentide Ice Sheet over the past 140,000 years, thereby making it vulnerable to Heinrich‐type surges. A reasonable level of agreement is obtained with the observed record of North Atlantic IRD activity, including the six numbered Heinrich events as well as earlier activity of this kind that occurred when the ice volume wa

ISSN:0883-8305    

DOI:10.1029/94PA02815

年代:1995

数据来源: WILEY

摘要:

Flux and taxonomic composition of the diatom assemblage was determined for biweekly sediment traps in the central Gulf of California over a 2‐year period. Fluxes and relative abundance of taxa are not significantly different in traps from the eastern side of the Guaymas Basin and the western side of the Carmen Basin, indicating that patterns of diatom production are uniform across the central gulf at all seasons of the year. The annual maximum in diatom flux occurred during the winter season of both years, either as a sustained event over many weeks (1990) or as a series of short blooms (1991–1992). Of the most abundant taxa, only hyalochaeteChaetocerosspecies showed a clear seasonal cycle, with a flux maximum in the spring of both years. Factor analysis defines five assemblages representing (1) summer and fall conditions of surface stratification and nutrient limitation, (2) winter bloom triggered by onset of strong northwesterly winds, (3) late spring to summer production probably related to sporadic events during periods of weak southeasterly winds, (4) spring 1991 inferred to reflect the distal edge of a coastal upwelling phase, (5) lateral advection of resuspended sediments from the continental shelf, possibly resulting from trapped coastal waves during summer hurricanes and from storms during the 1991–1992 El Niño‐Southern Oscillation (ENSO). The diatom data support the inference that subtropical Pacific water penetrates deeper into the gulf during ENSO events. In order to compare the sediment trap record to a sediment core, a second factor analysis was done using only the taxa which are preserved in the sediment and omitting the most abundant taxa, which showed little downcore variation. This produces five factors representing (1) summer‐fall low production, (2) early winter bloom, (3) late winter‐spring upwelling, (4) ENSO conditions and (5) a late spring‐summer assemblage of uncertain significance. Variation of species characterisic of each factor was examined in piston core Atlantis II 125‐8 56JPC taken from the oxygen minimum zone in Guaymas Basin. Correlation to nearby Deep Sea Drilling Project Hole 480 indicates that the core penetrates to about 15 ka, with laminated sediments in the Holocene (0–10 ka) and late deglacial (11–13 ka) and massive (nonlaminated) sediments around Younger Dryas time (10–11 ka) and before 13 ka. Downcore variations in the diatom assemblage do not correspond directly to presence of laminations, implying that the processes controlling bottom water oxygen content are not determined by conditions of surface water production. Presence of calcareous nannofossils downcore correlates inversely with two diatom assemblages (ENSO and winter bloom indicators) and not with occurrence of laminations, suggesting that occurrence of calcareous nannofossils in the sediment is at least partly a function of surface water production. Prior to 12 ka, production was lower than at present during all seasons, implying weaker winds throughout the year; subtropical waters extended to the midgulf as they do today during ENSO events. Between 11 and 12 ka, conditions were similar to those of the present, with relatively high winter and spring production resulting from northwesterly winds. ENSO events were still common, although less so than during the preceding period. Younger Dryas time was one of transition with intermediate conditions: a slackening of winter‐spring northwesterly winds and reduction of total annual production, and increased occurrence of ENSO conditions. In the early to middle Holocene winter, northwesterlies increased to a maximum, resulting in very high production, but apparently did not persist into spring; subtropical waters mostly disappeared from the gulf. In the late Holocene, northwesterly winds have supported high production during both w

ISSN:0883-8305    

DOI:10.1029/94PA02796

年代:1995

数据来源: WILEY

摘要:

We have studied the sediment geochemistry and benthic foraminiferal fauna in a piston core from the northwest Arabian Sea (covering the last 150,000 years), and compared the results with the stable isotopic record from both benthic (Uvigerina peregrina) and planktonic (Globigerinoides sacculifer) foraminifera. The δ18O record of the planktonic foraminiferal species shows excursions towards heavier δ18O values when compared to the benthic record and the SPECMAP stack, particularly in stage 3, between approximately 30 and 45–50 kyr. We call this the “stage 3 event.” Species diversity and relative abundance data on benthic foraminifera reveal that some species, e.g.Cibicidoides wuellerstorfi, Oridorsalis umbonatus, Pullenia bulloides, Pyrgospp., and the uvigerinids increased in relative abundance during glacial stages 2 and 4 and during the stage 3 event, whereas other (e.g.,Astrononion novozealandicum, Cibicidoides bradyi, Eggerella bradyi, Gyroidina altiformis, Hoeglundina elegans, Pullenia subcarinata, andSigmoilina edwardsii) decreased. Using correspondence analysis we determined that approximately 50% of the total sample variation may be accounted for by three first factors which all are interpreted as primarily related to surface water productivity and the quality of the organic matter that reaches the seafloor. The relative abundance of some species appears to vary on a precessional cycle, perhaps responding to monsoon‐driven upwelling and associated increased food supply, whilst the abundances of other species are related to the 100 kyr ice volume cycle, and may be responding to changes in intermediate and deep water chemistry. Geochemical paleoproductivity tracers, such as the Ba content of the sediment, have a very similar distribution to specific foraminiferal factors in both time and frequency domains. Certain planktonic foraminifera, (e.g.,Globigerina bulloides), also display obvious maxima in the stage 3 event. The stage 3 event occurs at a time of reduced rate of amplitude change in both solar radiation and monsoon pressure index. We conclude, that there was a period 15 to 20 kyr duration during stage 3 with cooler‐than‐normal surface waters, enhanced productivity and rapid settling of bi

ISSN:0883-8305    

DOI:10.1029/94PA02514

年代:1995

数据来源: WILEY

摘要:

Variation in upwelling may be monitored using various micropaleontological indices, such as the relative abundance or accumulation rate of the planktonic foraminiferal speciesGlobigerina bulloidesor mass accumulation rate of planktonic foraminifera. We have analyzed the relationships between the upwelling history and mean test size variations in the planktonic foraminiferal speciesGlobigerinoides ruber, Globigerinita glutinata, G. bulloides,andNeogloboquadrina dutertreithrough the last 19,000 years in Ocean Drilling Program hole 723 A from the Oman Margin, Arabian Sea. The study reveals that these species were larger on average between 11 and 5 ka, which coincides with a period of strong upwelling. On the other hand, these species showed smaller mean test sizes during the last glacial period and late Holocene when upwelling was weaker. Mass accumulation rates of these species are linearly proportional to their mean test sizes, indicating that both mean size and productivity of individual species are related to the upwelling intensity. Therefore we propose that the size variations inG. ruber, G. glutinata, G. bulloides, andN. dutertreican be used as an additional micropaleontological index of upwelling intensity in the Arabian Sea.

ISSN:0883-8305    

DOI:10.1029/94PA02682

年代:1995

数据来源: WILEY

摘要:

We measured stable isotopic signals of the largest specimens of all abundant species of planktonic foraminifera in late Maastrichtian samples from three Atlantic mid‐latitude and high‐latitude sites. Each sample is marked by a relatively small interspecies range of δ18O values (1.2 to 0.8‰). Each also exhibits an interspecies range of δ13C values that resembles those of modern assemblages at similar latitudes (2.8 to 0.9‰, with greater ranges at lower latitudes). These results indicate that Late Cretaceous planktonic foraminifera inhabited water masses characterized by a relatively narrow range of temperatures but may have occupied an array of niches as ecologically diverse as that of modern taxa. Maastrichtian taxa exhibit more complex relationships between skeletal morphology and vertical and seasonal paleohabitat affinities than previous paleoecologic models have assumed. For example, keeled taxa were not limited to cold or deep habitats and serial taxa appear to have occupied both warm near‐surface niches and the coolest or most saline habitats. The isotopic differences between species indicate that apparent variation in Maastrichtian stable isotopic signals strongly depends on the taxa analyzed. Narrowly constrained size fractions ofRugoglobigerinaspecies may provide a good estimate of relative variation in isotopic equilibria of near‐surface summer paleoconditions at low and middle latitudes.Gublerinaspecies,Laeviheterohelixspecies, andPlanoglobulina multicamerataappear the likeliest candidates for consistently tracking relative variation in isotopic equilibria of the coldest or most saline paleohabitats of Maastrichtian planktonic foraminifera. These isotopic results suggest that upper water column thermal gradients may have been only 3 to 5°C in mid‐latitude and high‐latitude regions. They also suggest that the western and central Atlantic mid‐latitude summer seasurface may have been much cooler or saltier in the Maastrichtian than it is today. Finally, they corroborate previous estimates of ∼ 10°C surface water at

ISSN:0883-8305    

DOI:10.1029/94PA02671

年代:1995

数据来源: WILEY

摘要:

Sea level low stands are associated with the growth of glacial ice sheets. During maximum stages of glaciation, ice sheets may extend to the outer continental shelf. These glacial maxima normally correspond to sea level low stands. Consequently, ice sheets may be supported directly by the seafloor inducing increased pore pressures in the sediment and increasing the overall bearing stresses on the margin. These stresses trigger undrained failure in upper slope sediment and bearing capacity failure in the middle to lower slope. After complete ice melting and dissipation of excess pore pressure, the inter‐glacial sediments become overconsolidated. Shear resistance is higher, and no gravity or earthquake‐induced failure occurs. The gravity events can only occur in underconsolidated and normally consolidated glacial and postglacial deposits. This explains the variation in volume and activity of mass wasting events during low and high sea level periods in areas where a glacial ice exists. Analyses of maximum ice loading events are presented for the Scotian slope. In contrast with traditional thought, these analyses suggest that ice loading, rather than high sedimentation rates, is the dominant mechanism for slope instability on glaciated marg

ISSN:0883-8305    

DOI:10.1029/94PA02352

年代:1995

数据来源: WILEY