摘要:

Geologic studies indicate that prior to ∼40 Ma the Drake Passage was closed and the Central American Isthmus was open. The effect of these changes has been examined in an ocean general circulation model. Several sensitivity experiments were conducted, all with atmospheric forcing and other boundary conditions from the present climate, but with different combinations of closed and open gateways. In the first experiment, the only change involved closure of the Drake Passage. In agreement with earlier studies the barrier modified the geostrophic balance that now maintains the circumpolar flow in the southern ocean, with the net effect being decreased transport of the Antarctic Current and an approximate fourfold increase in outflow of Antarctic deep‐bottom waters. The very large increase in Antarctic outflow suppresses North Atlantic Deep Water (NADW) formation. In addition to corroboration of results from earlier studies, our simulations provide several new insights into the role of a closed Drake Passage. A more geologically realistic closed Drake/open central American isthmus experiment produces essentially the same pattern of deepwater circulation from the first experiment, except that Antarctic outflow is about 20% less than the first experiment. The resultant unipolar deepwater circulation pattern for the second experiment is consistent with paleoceanographic observations from the early Cenozoic. A third experiment involved an open Drake and open central American isthmus. In this experiment, Antarctic outflow is diminished to slightly above present levels but NADW production is still low due to free exchange of low‐salinity surface water between the North Pacific and North Atlantic. The low level of thermohaline overturn should have reduced oceanic productivity in the Oligocene (∼30 Ma), a result in agreement with geologic observations. Finally, simulations with an energy balance model demonstrate that the changes in surface heat flux south of 60°S due to breaching of the Drake barrier do not result in temperature changes large enough to have triggered Antarctic glaciation. This last result suggests that some other factor (CO2?) may be required for Antarctic ice sheet expansion in the Oligocene (∼30–34 Ma). Our results lend further support to the concept that even in the absence of changing boundary conditions due to ice sheet growth, variations in the geometry of the ocean basins can significantly influence ocean circulation patterns and the sediment record. The results also suggest that the primary polarities of the Cenozoic deepwater circulation may have been controlled by opening and closing of these

ISSN:0883-8305    

DOI:10.1029/93PA00893

年代:1993

数据来源: WILEY

摘要:

Present‐day surface wind stress climatology is manipulated to simulate wind conditions during the last glacial maximum. These estimated wind fields force a one‐layer, wind‐driven numerical model of the southern ocean to determine if a change in the strength of the surface wind stress can shift the location of the Antarctic Polar Front, which is part of the Antarctic Circumpolar Current. A change in the forcing by a factor of 0.5–2.0 results in a change in the speed of the flow by an identical factor with no change in position. However, if the present‐day wind climatology is shifted meridionally, there is a change in both strength of the circulation and spatial pattern. A shift of the wind stress of more than 5° of latitude is required to produce a shift in the location of the p

ISSN:0883-8305    

DOI:10.1029/93PA01046

年代:1993

数据来源: WILEY

摘要:

High‐resolution textural, carbonate, microfossil, and oxygen (δ18O) and carbon (δ13C) stable isotopic analyses are presented for the late Quaternary (isotopic stages 1 to 6) interval of a core at Deep Sea Drilling Project site 594, situated just south of the present Subtropical Convergence in northernmost Subantarctic surface waters on the southern flank of Chatham Rise in the southwest Pacific. Downcore alternations of pelagic and hemipelagic oozes correspond to interglacial and glacial episodes, respectively. Interglacial oozes contain a northern Subantarctic assemblage of planktonic foraminifera, with rare cool subtropical species, while glacial oozes are characterized by species typical of southern(most) Subantarctic waters and include radiolaria with affinity for Antarctic waters. The planktonic δ18O record for the site supports a 3°–6°C temperature change in near‐surface waters between interglacial and glacial stages and indicates that during stage 5e, the near‐surface waters were about 1°C warmer than at present. A pronounced cooling during stage 5d matches that of the Vostok ice core δD record, and negative excursions at the end of stages 2 and 6 support a southern latitude warming preceding northern hemisphere deglaciation. Benthic foraminifera typical of cold intermediate to deep waters can increase dramatically in abundance in the hemipelagic ooze intervals when δ18O results suggest a temperature drop in glacial stage bottom waters of 2°–4°C, possibly a result of upward displacement of Antarctic Intermediate Water by Circumpolar Deep Water at the site. The foraminiferal δ13C records support a reduced influence of North Atlantic Deep Water in the southwest Pacific during glacial stages, when nutrient enhancement occurred in both bottom and surface waters. Despite the pronounced changes between interglacial and glacial conditions inferred at site 594, contributed to by a substantial northward shift by at least 5° of latitude in the position of the Antarctic Convergence south of New Zealand during glacial episodes, we conclude that the Subtropical Convergence remained locked to Chatham Rise (approximately 44°S) throughout stages 1 to 6. Major compression of the intervening belt of Subantarctic water during glacial episodes, and the associated very steep thermal gradients and intensified atmospheric and oceanic circulation patterns that developed in this part of the southwest Pacific, account for the harsh, frigid environment reported for on‐land southern Ne

ISSN:0883-8305    

DOI:10.1029/93PA01162

年代:1993

数据来源: WILEY

摘要:

A quantitative analysis of calcareous nannofossil assemblages was carried out, with an optical microscope, on four deep‐sea cores raised from the eastern Mediterranean and selected to form a NW‐SE transect. A total of 748 samples were studied, including a very detailed analysis of sapropels S5 and S6 in two, out of the four cores. This paper is restricted to the discussion of the results obtained onFlorisphaera profundawhich provided important insights on the mechanisms of sapropel formation, an intricated and unravelled puzzle for the scientists of the last decades. On the basis of abundance variations ofF. profunda, the only species of calcareous nannoplankton which inhabits the lower part of the photic zone, I suggest that sapropel formation was triggered by increased primary production, either confined to a Deep Chlorophyll Maximum (DCM) (a concept previously introduced by Rohling and Gieskes, [1989]) or extended to a thicker layer of the photic zone. When increased primary production was confined to the DCM (sapropel S3, S4, S5, S7, and S1, this latter only in the eastern part of the basin), the abundance ofF. profundawas enhanced greatly with respect to all other species living in the higher part of the photic zone. On the contrary, when the increased primary production was extended to a thicker layer of the photic zone (“glacial” sapropels S6 and S8) the production of most nannoplankton species was favored, decreasing the relative proportion ofF. p

ISSN:0883-8305    

DOI:10.1029/93PA00756

年代:1993

数据来源: WILEY

摘要:

Using stable isotopic analyses of individual tests of planktonic foraminifera with different seasonal and depth habitats, we have investigated temporal variations in surface water salinities in the eastern Mediterranean during the past 13,000 years. Low oxygen isotopic values document the presence of anomalously low salinity conditions in the eastern Mediterranean at about 13,000 years B.P., 4000 years before the deposition of the youngest sapropel. By 11,000 years B.P., during the deposition of presapropel sediments, low‐salinity surface waters were more pervasive but were restricted to summer months and only affected the surface of the water column. At the onset of sapropel deposition about 9000 years B.P., surface salinities in the eastern Mediterranean had decreased by almost 4‰ and had extended to depths of at least 75 m. The low‐salinity surface lens was not, however, a permanent feature of the eastern Mediterranean during sapropel deposition. Low‐salinity surface conditions occurred in the fall and winter seasons, but normal sea surface salinities returned during spring months on a seasonal and/or interannual basis. Thus, a low‐salinity surface lens could have existed continuously for months or decades at a time but would have been interrupted intermittently by a return to normal conditions. Any triggering mechanism proposed for Mediterranean sapropels must account for the seasonal fluctuations and ephemeral nature of low‐salinity conditions in the eastern Mediterranean. Despite normal salinities returning occasionally during spring months, a low‐salinity surface lens may have greatly inhibited the production of Levantine intermediate water, prevented the formation of Adriatic deep water, and consequently, reduced ventilation of the eastern

ISSN:0883-8305    

DOI:10.1029/93PA01319

年代:1993

数据来源: WILEY

摘要:

In the southwestern New Jersey Coastal Plain, four drill holes contain continuous neritic sedimentation across the Paleocene/Eocene boundary (calcareous nannofossil Zone NP 9/NP 10 boundary). Significant lithologic and biotic changes occur in these strata near the top of the Paleocene. Global warming, increased precipitation, and other oceanographic and climatic events that have been recognized in high‐latitude, deep‐oceanic deposits of the latest Paleocene also influenced mid‐latitude, shallow‐marine, and terrestrial environments of the western North Atlantic. The diverse, well‐preserved calcareous nannofossil flora that is present throughout the entire New Jersey boundary section accurately places these events within the uppermost part of the upper Paleocene Zone NP 9. Several rapid but gradational changes occur within a 1.1‐m interval near the top of Zone NP 9. The changes include (1) a change in lithology from glauconitic quartz sand to clay, (2) a change in clay mineral suites from illite/smectite‐dominated to kaolinite‐dominated, (3) a change in benthic foraminiferal assemblages to a lower diversity fauna suggestive of low‐oxygen environments, (4) a significant increase in planktonic foraminiferal abundance, and (5) an increased species turnover rate in marine calcareous nannofossils. Pollen was sparse in the New Jersey drill holes, but terrestrial sporomorph species in Virginia exhibit increased turnover rates at a correlative level. Foraminiferal assemblages and lithology indicate that relative sea level rose in New Jersey at the same time as these late Paleocene events occurred in late Biochron NP 9. The higher sea levels influenced sediment type and absolute abundance of planktonic foraminifers in the deposits. Above the initial increase of kaolinite in the upper part of Zone NP 9, the kaolinite percentage continues to increase, and the maximum kaolinite value occurs in the uppermost part of Zone NP 9. There are few changes in either the sediments or the biota precisely at the Zone NP 9/NP 10 boundary in New Jersey. The clay‐rich deposits with a high kaolinite clay mineral suite, the lowered diversity benthic foraminiferal assemblages, the abundant planktonic foraminiferal specimens, and the calcareous nannofossil assemblages continued essentially unchanged into the earliest Eocene Zone NP 10. Within the lower part of Zone NP 10, the kaolinite percentage decreased

ISSN:0883-8305    

DOI:10.1029/93PA01367

年代:1993

数据来源: WILEY

摘要:

In the aftermath of the Cretaceous/Tertiary (K/T) boundary event (∼65 m.y. ago), pelagic carbonate productivity was greatly reduced for several hundred thousand years. A decrease in carbonate productivity by a factor greater than 3, in the absence of some mechanism to remove excess carbonate from the ocean, should have resulted in the accumulation of carbon and alkalinity in the oceans. This would cause the atmospheric partial pressure of CO2to fall dramatically and the deep ocean to become fully saturated with respect to calcite. Evidence of such a period of highly calcite‐saturated oceans with low atmosphericpCO2in the earliest Tertiary is lacking, suggesting that ocean chemistry may have been buffered by some process or processes. Shallow‐water carbonate accumulation rates may depend, in part, on carbonate ion concentrations, and thus shallow‐water carbonate deposition might act to stabilize ocean chemistry in the face of a dramatic reduction in pelagic productivity. In our four‐box ocean model, as the oceanic carbonate ion concentration rises in the face of diminished pelagic carbonate accumulation, the shallow‐water carbonate accumulation rate increases, compensating for the reduction in pelagic carbonate accumulation. These model results indicate that the carbonate‐ion feedback on shallow‐water carbonate sedimentation may have acted to balance oceanic carbon and alkalinity budgets at the K/T boundary, and, furthermore, may have been a primary mechanism maintaining high shallow‐water carbonate accumulation rates prior to the Jurassic onset of widespread pelagic carbo

ISSN:0883-8305    

DOI:10.1029/93PA01163

年代:1993

数据来源: WILEY

摘要:

Extant planktonic foraminifera display positive covariance between δ13C signals and test size. As documented by other studies, primary causes of increased δ13C values with increased test size may include increased reliance on ambient CO2for calcification at larger test sizes, decreased kinetic fractionation during calcification at larger test sizes, and increased photosymbiotic activity in larger symbiont‐bearing planktonic foraminifera. Planktonic foraminiferal δ18O values also often covary with test size, although the direction of this covariance is taxon dependent. Possible explanations for relationships between δ18O signals and test size include changing habitat depth over ontogeny, correlations between adult test size and environmental conditions, and changing isotopic disequilibrium with size, ontogenetic stage, or photosymbiont density. In order to assess the magnitude and implications of similar size dependence in earliest Paleocene planktonic foraminifera, we measured the stable isotopic signals of multiple size fractions of 10 earliest Paleocene species. All of these taxa exhibit a strong positive correlation between δ13C and test size. The slope and magnitude of this trend varies between species, withWoodringina claytonensisdisplaying the largest shift (1.1‰ over a 130 µm range in mean sieve size) andGuembelitria cretaceadisplaying the smallest (0.2‰ over a 38 µm range). By analogy with modern planktonic foraminifera, this general relationship between δ13C and size probably resulted from increased reliance on ambient CO2for calcification at larger test sizes. The high magnitude of this shift in some taxa may reflect either photosymbiotic enhancement of the general trend or relatively greater changes in the proportions of metabolic and ambient CO2used for calcification at different test sizes. Failure to account for relationships between test size and δ13C signals can lead to underestimation of early Paleocene surface ocean δ13C values by 1‰ or more. These size‐related δ13C effects provide an alternative explanation for decreases in whole‐rock δ13C values and some decreases in planktonic‐tobenthic foraminiferal δ13C gradients documented at marine K/T boundary sequences. At all size fractions, the 10 Paleocene taxa display a very limited interspecies range of δ18O derived paleotemperatures. Despite this limited range, paleobiogeographic patterns and δ18O signals appear to provide realistic estimates of relative paleodepth and seasonal affinities of earliest Paleocene planktonic foraminiferal species. Earliest Paleocene δ18O and biogeographic data are consistent with a general trend of surface‐to‐deep diversification of microperforate planktonic foraminifera following the K/T boundary. Such a trend may simply result from exploitation of a near‐surface open‐ocean habitat by the e

ISSN:0883-8305    

DOI:10.1029/93PA00952

年代:1993

数据来源: WILEY