ISSN:0883-8305    

DOI:10.1029/94PA00442

年代:1994

数据来源: WILEY

ISSN:0883-8305    

DOI:10.1029/94PA00444

年代:1994

数据来源: WILEY

ISSN:0883-8305    

DOI:10.1029/94PA00291

年代:1994

数据来源: WILEY

摘要:

Cooling ages of rock in the Himalayas imply that rapid exhumation between the Main Central thrust system and the South Tibetan detachment system occurred between 21 and 17 Ma. The generation of relief and enhanced weathering which followed this event may have resulted in a pronounced increase in the delivery of dissolved strontium, carbon, phosphorus, and other chemical weathering products to the ocean (Richter et al., 1992). The increased supply of nutrients stimulated productivity in oceanic upwelling zones and expansion of the oxygen minimum zone leading to enhanced burial and preservation of organic matter in the Monterey formation and other deposits from this interval. A downdraw of atmospheric CO2associated with enhanced chemical weathering rates and organic matter burial may have led to global cooling and the expansion of the Antarctic ice sheet by 15 Ma. The above scenario differs from the “Monterey hypothesis” of Vincent and Berger in that CO2downdraw is primarily via silicate weathering rather than organic carbon burial and that organic carbon burial is driven by increased delivery of nutrients to the ocean rather than by stronger upwelling. A carbon mass balance calculation which assumes that river fluxes have been increasing over the last 40 Ma predicts that absolute organic carbon burial increased over this interval while, at the same time, the fraction of carbon buried as organic matter versus carbonate decreased. This implies that the organic carbon cycle has acted as a net source of CO2to the atmosphere over the late Cenoz

ISSN:0883-8305    

DOI:10.1029/94PA00289

年代:1994

数据来源: WILEY

摘要:

A composite strontium isotopic seawater curve was constructed for the Miocene between 24 and 6 Ma by combining87Sr/86Sr measurements of planktonic foraminifera from Deep Sea Drilling Project sites 289 and 588. Site 289, with its virtually continuous sedimentary record and high sedimentation rates (26 m/m.y.), was used for constructing the Oligocene to mid‐Miocene part of the record, which included the calibration of 63 biostratigraphic datums to the Sr seawater curve using the timescale of Cande and Kent (1992). Across the Oligocene/Miocene boundary, a brief plateau occurred in the Sr seawater curve (87Sr/86Sr values averaged 0.70824) which is coincident with a carbon isotopic maximum (CM‐O/M) from 24.3 to 22.6 Ma. During the early Miocene, the strontium isotopic curve was marked by a steep rise in87Sr/86Sr that included a break in slope near 19 Ma. The rate of growth was about 60 ppm/m.y. between 22.5 and 19.0 Ma and increased to over 80 ppm/m.y. between 19.0 and 16 Ma. Beginning at ∼16 Ma (between carbon isotopic maxima CM3 and CM4 of Woodruff and Savin (1991)), the rate of87Sr/86Sr growth slowed and87Sr/86Sr values were near constant from 15 to 13 Ma. After 13 Ma, growth in87Sr/86Sr resumed and continued until ∼9 Ma, when the rate of87Sr/86Sr growth decreased to zero once again. The entire Miocene seawater curve can be described by a high‐order function, and the first derivative (d87Sr/86Sr/dt) of this function reveals two periods of increased slope. The greatest rate of87Sr/86Sr change occurred during the early Miocene between ∼20 and 16 Ma, and a smaller, but distinct, period of increased slope also occurred during the late Miocene between ∼12 and 9 Ma. These periods of steepened slope coincide with major phases of uplift and denudation of the Himalayan‐Tibetan Plateau region, supporting previous interpretations that the primary control on seawater87Sr/86Sr during the Miocene was related to the collision of India and Asia. The rapid increase in87Sr/86Sr values during the early Miocene from 20 to 16 Ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans. In the absence of another source of CO2, these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric CO2and climatic cooling through a reversed greenhouse effect. The paleoclimatic record, however, indicates a warming trend during the early Miocene, culminating in a climatic optimum between 17 and 14.5 Ma. We suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic CO2from the eruption of the Columbia River Flood Basalts (CRFB) between 17 and 15 Ma. The decrease in the rate of CRFB eruptions at 15 Ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in Monterey deposits eventually led to cooling and increased glaciation between ∼14.5 and 13 Ma. The CRFB hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the Monterey at 17.5 Ma (as marked by increased δ13C values) and the climatic cooling and glaciation during the middle Miocene (as marked by the increase in δ18O values), which did not b

ISSN:0883-8305    

DOI:10.1029/94PA00292

年代:1994

数据来源: WILEY

摘要:

This study tests and improves on previously published early and middle Miocene87Sr/86Sr marine correlations, presents Sr isotopic age correlations for this interval using the new timescale ofCande and Kent[1992], and evaluates Sr isotopic changes against an inferred glacioeustatic proxy. We generated a latest Oligocene to early late Miocene87Sr/86Sr isotope record from Ocean Drilling Program (ODP) Hole 747A; this site provides an excellent magnetostratigraphic record during most of this interval for independent age estimates, very good foraminiferal preservation, and excellent core recovery. Comparisons of new87Sr/86Sr data from Hole 747A with previously published data from Deep Sea Drilling Project (DSDP) Sites 608 [Miller et al., 1991a] and 588 [Hodell et al., 1991] yield the following results: (1) confirmation and refinement of the early Miocene Sr isotope changes, (2) improved definition of the timing of the changes in slope of87Sr/86Sr near 15.4 Ma and 22.8 Ma, (3) improved Sr isotopic age resolution for the middle Miocene with resolution as good as ±0.7 m.y., and (4) identification of an inflection in the Sr isotope record at 28.0 Ma based on the combined records from DSDP Site 522 [Miller et al., 1988] and ODP Hole 747A. We have been unable to determine the cause of middle Miocene offset between Site 588 and Hole 747A data, although we believe it may be attributed to problems in the age assignments for Hole 588A for the interval ∼14‐11 Ma and Site 747 for the interval 11–8 Ma. Because Hole 747A results provide a better chronology than Site 588 for most of the Miocene and a better middle Miocene Sr isotope record than Site 608, we propose that Hole 747A serves as the best reference section for Miocene87Sr/86Sr variations from ca. 23 to 11 Ma. Using87Sr/86Sr data from Sites 522, 608, and 747A, we relate late Eocene to early Miocene inflections in the87Sr/86Sr isotope record to oxygen isotope increases and decreases inferred to represent glacioeustatic events. The decreases (deglaciations) observed in the δ18O record apparently lead the87Sr/86Sr inflections by 1 to

ISSN:0883-8305    

DOI:10.1029/94PA00249

年代:1994

数据来源: WILEY

摘要:

The trace metal composition of foraminiferal shells preserved in deep‐sea sediments is a critical source of data used in reconstructing the physical and chemical characteristics of ancient oceans. However, paleoceanographic information derived from these tracers is limited by the accuracy with which calcite shells record ambient seawater chemistry and the question of whether changing environmental conditions or interspecific differences affect shell chemistry. Experiments with living foraminifera are one of the most direct ways to quantitatively evaluate the relationship between shell and seawater chemistry and the influence, if any, of environmental variables. Here we present results from laboratory experiments on the living planktonic foraminifersGlobigerina sacculiferandOrbulina universathat demonstrate that precipitated shells provide an accurate record of seawater barium, a tracer used in reconstructing nutrient and alkalinity distributions in past oceans. On the basis of culturing data at 29°C, 36.7‰ salinity, the best estimate of the partition coefficient D relating foraminifera shell Ba/Ca to seawater Ba/Ca is 0.147 ± 0.004. Data show that when environmental conditions are held constant, there is no significant variability in Ba uptake between individual shells of one or both species. Results also demonstrate that temperature varying over a 7°C range and salinity varying over a 3‰ range do not appreciably influence B

ISSN:0883-8305    

DOI:10.1029/94PA00151

年代:1994

数据来源: WILEY

摘要:

Recent geochemical models invoke ocean alkalinity changes, particularly in the surface Southern Ocean, to explain glacial agepCO2reduction. In such models, alkalinity increases in glacial periods are driven by reductions in North Atlantic Deep Water (NADW) supply, which lead to increases in deep‐water nutrients and dissolution of carbonate sediments, and to increased alkalinity of Circumpolar Deep Water upwelling in the surface Southern Ocean. We use cores from the Southeast Indian Ridge and from the deep Cape Basin in the South Atlantic to show that carbonate dissolution was enhanced during glacial stages in areas now bathed by Circumpolar Deep Water. This suggests that deep Southern Ocean carbonate ion concentrations were lower in glacial stages than in interglacials, rather than higher as suggested by the polar alkalinity model [Broecker and Peng, 1989]. Our observations show that changes in Southern Ocean CaCO3preservation are coherent with changes in the relative flux of NADW, suggesting that Southern Ocean carbonate chemistry is closely linked to changes in deepwater circulation. The pattern of enhanced dissolution in glacials is consistent with a reduction in the supply of nutrient‐depleted water (NADW) to the Southern Ocean and with an increase of nutrients in deep water masses. Carbonate mass accumulation rates on the Southeast Indian Ridge (3200–3800 m), and in relatively shallow cores (<3000 m) from the Kerguelen Plateau and the South Pacific were significantly reduced during glacial stages, by about 50%. The reduced carbonate mass accumulation rates and enhanced dissolution during glacials may be partly due to decreases in CaCO3:Corgflux ratios, acting as another mechanism which would raise the alkalinity of Southern Ocean surface waters. The polar alkalinity model assumes that the ratio of organic carbon to carbonate production on surface alkalinity is constant. Even if overall productivity in the Southern Ocean were held constant, a decrease in the CaCO3:Corgratio would result in increased alkalinity and reducedpCO2in Southern Ocean surface waters during glacials. This ecologically driven surface alkalinity change may enhance deepwater‐mediated changes in alkalinity, and amplify rapid changes

ISSN:0883-8305    

DOI:10.1029/93PA03524

年代:1994

数据来源: WILEY

摘要:

A group of calcareous nannoplankton named nannoconids experienced a crisis in the early Aptian and recovered only later in the late Aptian after a period of virtual absence. Although no extinctions occurred, the widespread nature of the “nannoconid crisis” suggests a global causal factor. This crisis is recorded within theChiastozygus litterariusnannofossil andGlobigerinelloides blowiplanktonic foraminiferal zones, postdates magnetic chronozone M0 by approximately 300 kyr, and precedes the oceanic anoxic subevent 1a and associated δ13C anomaly by some 40–100 kyr. Selective dissolution and anoxia cannot explain the crisis, because nannoconids are dissolution‐resistant forms and their crisis clearly precedes the deposition of anoxic sediments. At least 1 m.y. prior to the “nannoconid crisis,” the onset of a nannoplankton speciation event may be the response of nannofloras to a major rise in relative sea level. The “nannoconid crisis” seems to be synchronous with the early Aptian volcanic eruptions in the Pacific Ocean. Hence calcareous nannoplankton were severely affected by the “superplume” volcanic episode. The coccolithophorid bloom/nannoconid crisis was possibly induced by the excessive CO2levels in the atmosphere and/or caused by changes in nutrient content of oceanic surface waters. Fertility was enhanced by rapid turnover of nutrients due to the abnormal volcanic activity and accelerated transfer of nutrients from the continents into the oceans under warm and humid conditions of the mid‐Cretaceous greenhouse climate. The “nannoconid crisis” may represent a competition between phytoplankton groups for nutrients or, more likely, competition between different calcareous nannoplankton. The biologic affinity and mode of life ofNannoconusare unknown, because there is no modern analog of this genus. However, comparison of Lower Cretaceous nannofossil assemblages with modern nannoplankton cummunities suggests that nannoconids, like extantFlorisphaera profunda, possibly inhabited the lower photic zone. Concentrations of nutrients in the upper euphotic zone may have triggered blooms of coccolithophorids and nannoconid depletion. This model implies that the “nannoconid crisis” is the result of an abrupt, major change in the structure of surface waters caused directly or indirectly by the “superplume.” The adjustments of the biosphere to the new paleoceanographic and climatic conditions required some 40–100 kyr before changing into abnormally high primary productivity and deposition of organic carbon‐rich sediments with dino

ISSN:0883-8305    

DOI:10.1029/94PA00258

年代:1994

数据来源: WILEY