摘要:

Detailed planktonic foraminiferal biostratigraphy from eight measured sections of Cretaceous limestone near Laytonville, California, indicates a composite sequence that extends in age from late Albian to early Turonian. The sequence contains seven biozones and two subzones based on the first and last appearance datums of planktonic foraminifers examined in thin section. Unequivocal biostratigraphic facing directions show four sections are right side up and four are reversed, and confirm the stratigraphic polarity employed in the paleomagnetic studies of Alvarez et al. (1980) and Tarduno et al. (1986). Temporal changes in the microfauna deviate from the global trend. Early Albian through early Cenomanian planktonic foraminiferal assemblages dominated by opportunistic, eurytopic species ofHedbergellaandGlobigerinelloidesgive way in the late Cenomanian to assemblages containing more abundant larger, heavily ornamented stenotopic species ofRotaliporaandPraeglobotruncanatypical of stratified Tethyan oceans. Radiolarian replacement chert increases slightly in the late Albian to early Cenomanian part of the Laytonville sequence and then decreases in the late Cenomanian and early Turonian. Benthic foraminifers show a similar decrease in abundance and diversity. We propose that this complex of biogenic and lithogenic patterns records transit via oceanic plate motion from a depositional site in the southern part of the paleoequatorial zone of high productivity, characterized by pronounced upwelling and habitat destabilization, perhaps augmented by topographic upwelling, to the central part of the equatorial zone dominated by biogenic calcite deposition. Our model of northward transit from below the equator supports the paleomagnetic determinations of Alvarez et al. (1980) and Tarduno et al. (1986). Arrival at the paleoequator coincided with the onset of stratification in the world ocean during the middle Cenomanian, indicated by the diversification of the more complex rotaliporids, and the subsequent intensification of oxygen‐depleted intermediate waters in the latest Cenomanian to early Turonian. Two pulses in heterohelicid abundance signify the onset or intensification of an oxygen‐minimum layer if Cretaceous heterohelicids occupied habitats analagous to Tertiary biserial heterohelicids. The first pulse beginning in the middle CenomanianRotalipora reicheliZone may indicate a minor expansion of the oxygen‐minimum zone or regional upwelling due to transit of the site beneath the equatorial divergence. The second pulse in the late CenomanianDicarinella algerianaSubzone signals the onset of upwelling of deeper oceanic water masses that characterized the succeedingWhiteinella archaeocretaceaZone. Organic‐rich black shales typical of theWhiteinella archaeocretaceaZone are missing at Laytonville, although samples do contain the low‐diversity, partially dissolved, planktonic assemblages that characterize this zone. The lack of black shales suggests that deposition occurred at depths greater than the oxygen‐depleted intermediate water depths or alternately indicates paleoceanographic conditions unique to

ISSN:0883-8305    

DOI:10.1029/PA005i005p00639

年代:1990

数据来源: WILEY

摘要:

In the western equatorial Pacific, the El Niño/Southern Oscillation (ENSO) phenomenon is characterized by precipitation variability associated with the migration of the Indonesian low pressure cell to the region of the date line and the equator. During ENSO events, Tarawa Atoll (1°N, 172°E) experiences heavy rainfall which has an estimated δ18O of about −8 to −10‰ δ18OSMOW. At Tarawa, sufficient precipitation of this composition falls during ENSO events to alter the δ18O and the salinity of the surface waters. Oxygen isotope records from two corals collected off the reef crest of Tarawa reflect rainfall variations associated with both weak and strong ENSO conditions, with approximately monthly resolution. Coral skeletal δ18O variations due to small sea surface temperature (SST) changes are secondary. These records demonstrate the remarkable ability of this technique to reconstruct variations in the position of the Indonesian Low from coral δ18O records in the western equatorial Pacific, a region which has few paleoclimatic records. The coral isotopic data correctly resolve the relative magnitudes of recent variations in the Southern Oscillation Index. Combining the Tarawa record with an oxygen isotopic history from a Galápagos Islands coral demonstrates the ability to distinguish the meteorologic (precipitation) and oceanographic (SST) anomalies that characterize ENSO events across the Pacific Basin over the period of common record (1960–1979). Comparison of the intensity of climatic anomalies at these two sites yields insight into the spatial variability of ENSO events. Isotope records from older corals can provide high‐resolution, Pacific‐wide reconstructions of ENSO behavior during periods of different climate

ISSN:0883-8305    

DOI:10.1029/PA005i005p00669

年代:1990

数据来源: WILEY

摘要:

The history of the northward growth and of the fluctuations of the Benguela Current from the middle Miocene (ca. 14 Ma) to the uppermost late Miocene (ca. 5 Ma) has been reconstructed from analyses of the coarse and clay fractions and geochemical analyses of the organic matter in sediments from Deep Sea Drilling Project Sites 362 and 532. In the middle Miocene the Benguela Current had not yet reached the Walvis Ridge, and consequently, no local upwelling occurred. Carbonate preservation was good during this period of low productivity. Rhythmic changes related to Antarctic ice volumes were weak, leading to slightly higher organic matter concentrations and higher montmorillonite/illite (M/I) ratios in sediments deposited during times of greater continental ice. Montmorillonite was carried north from the Orange River by the Benguela Current, whereas illite originated from the nearby Namib Desert. In the early late Miocene the Benguela Current reached the Walvis Ridge in colder periods and led to well‐developed rhythmic increases in opal, which was transported from near‐coastal upwelling areas. Organic matter concentration, carbonate dissolution, and M/I ratios were also enhanced during periods of greater ice volume. During the late Miocene, farther northward migration of the Benguela Current led to a change in the uppermost Miocene sediments: opal contents and M/I ratios were high during times of low ice volume rather than in colder times. During the cold periods, local winds from the Namib desert increased the illite supply to the Walvis Ridge and depressed the M/I ratio. In these periods, the Benguela Current flowed farther to the north, reaching the Angola Basin as it did in Quaternary glacial periods. Throughout these changes, sediment organic carbon content was always enhanced in high‐ice‐volume periods from near‐bottom downslope transport of organic matter from shelf regions during regressions, and carbonate dissolution in the sediments was increased as a result of oxidation of this organ

ISSN:0883-8305    

DOI:10.1029/PA005i005p00685

年代:1990

数据来源: WILEY

ISSN:0883-8305    

DOI:10.1029/PA005i005p00709

年代:1990

数据来源: WILEY

摘要:

New developments in ocean paleoproductivity and paleochemistry have involved the application of tracers that record seawater composition, that document sedimentary fluxes, and that may be used to infer oceanic processes. These include the use of Cd/Ca and Ba/Ca ratios in benthic foraminifera, rare earth element patterns of chemical sediments, uranium‐series nuclides in carbonate sediments and studies of the origins of chemical fluxes and anomalies in sedimentary profile

ISSN:0883-8305    

DOI:10.1029/PA005i005p00711

年代:1990

数据来源: WILEY

摘要:

Foraminiferal Ba can be used to reconstruct the distribution of deeply regenerated chemical components in past oceans. The Ba contents of benthic foraminifera recovered from cores in the Atlantic Ocean indicate that waters deeper than 2900 m had ∼30–60% higher Ba during the last glacial maximum (LGM). These changes are consistent with previously observed glacial nutrient enrichments based on foraminiferal Cd and δ13C. Increases in deepwater nutrient contents in the Atlantic can be explained by reductions in North Atlantic Deep Water formation during the LGM. Ba/Ca of benthic foraminifera from the glacial sections of intermediate depth Atlantic cores are equal to or lower than Holocene values. This Ba evidence argues against the Mediterranean as a greatly increased source to Atlantic intermediate waters during the LGM, since the Mediterranean is enriched in Ba today and apparently remained enriched during the LGM. Benthic foraminiferal Ba from the glacial sections of cores from the eastern equatorial Pacific suggest that deep waters of the Glacial Pacific were about 25% lower in Ba (at ∼3000 m). Taken together, the foraminiferal evidence indicates that the Ba content of deep waters of the Atlantic, Antarctic, and Pacific were similar at the LGM. Since foraminiferal Cd distributions indicate that Cd remained significantly lower in the deep Atlantic relative to the Pacific at the last glacial maximum, a seven‐box ocean model is used to explore several scenarios for reconciling LGM Ba and Cd distributions. While the changed distribution of both tracers suggests diminishment in the flux of nutrient depleted waters to the deep Atlantic during the LGM, increased Atlantic upwelling rates and consequently enhanced Ba particle fluxes can account for the the lack of Ba fractionation between the deep Atlantic and Pacific. The model suggests that Ba can be transferred efficiently to the deep Atlantic by enhanced upwelling because the vast majority of the Ba is regenerated in the deep Atla

ISSN:0883-8305    

DOI:10.1029/PA005i005p00719

年代:1990

数据来源: WILEY

摘要:

Cadmium‐to‐calcium records in benthic foraminifera, interpreted in conjunction with carbon isotope records, reflect oceanic circulation patterns and oceanic inventories of Cd, carbon 13/carbon 12, and indirectly, phosphorus. Cd/Ca ratios have been determined in benthic foraminiferaCibicidoidesspp. for the late, middle, and early Miocene from a site in the South Atlantic (Deep Sea Drilling Project (DSDP) site 525, leg 74, ∼2500 m, Walvis Ridge) and from a site in the western equatorial Pacific (DSDP site 289, leg 30, ∼2200 m, Ontong‐Java Plateau). Both sites have well‐established carbon isotope records for benthic foraminifera typical of the global record dominated by inventory changes. Cd/Ca ratios and carbon isotope values were averaged for the Miocene in time intervals substantially longer than estimated oceanic residence times of Cd, C, or P. Oceanic mean Cd/Ca and carbon isotopes were estimated as weighted means of the values from both sites using weighting factors based on replicating modern oceanic means; this may tend to underestimate oceanic inventories earlier than the late Miocene because of changing water mass dominance at the South Atlantic site. From these means, the oceanic Cd inventory appears relatively stable during the Miocene, while there are large changes in carbon isotope inventories. Miocene Cd inventory estimates are at least 20% lower than published estimates for Quaternary stages 1 and 2. Increasing oceanic Cd inventories over the past 10 m.y. are consistent with hypotheses of increased river fluxes in the recent geologic past. Cd/Ca differences between sites may be indicative of the extent of basin‐basin fractionation from bottom water circulation, although the validity of this comparison depends on how representative each of these two sites is through time for its respective ocean basin. The Pacific‐Atlantic Cd/Ca difference based on these two sites was larger in the late Miocene compared to the middle and early Miocene. The relationship between Cd/Ca and carbon isotope inventories and intersite gradients for these sites suggests that either oceanic phosphorus inventories or parameters related to marine organic matter composition must have been different in the Miocene than at present. Large changes in the carbon isotope balance, reflected as both positive and negative excursions in the carbon isotope inventories, occurred with little effect on Cd inventories. Changes in phosphorus fluxes associated with these carbon isotope flux changes may have been compensated for by changing deposition fluxes to other phosphorus sinks. Phosphorus inventories may have increased from the middle to late Miocene with increasingly negative oceanic carbon isotope inventories. However, confirmation of these suggestions depend on collecting more Cd/Ca (and δ13C) data throughout the Miocene oceans and on developing the application of other constraints for these

ISSN:0883-8305    

DOI:10.1029/PA005i005p00743

年代:1990

数据来源: WILEY

摘要:

Variations in carbonate flux and dissolution, which occurred in the equatorial Atlantic during the last 24,000 years, have been estimated by a new approach that allows the point‐by‐point determination of paleofluxes to the seafloor. An unprecedented time resolution can thus be obtained which allows sequencing of the relatively rapid events occurring during deglaciation. The method is based on observations that the flux of unsupported230Th into deep‐sea sediments is nearly independent of the total mass flux and is close to the production rate. Thus excess230Th activity in sediments can be used as a reference against which fluxes of other sedimentary components can be estimated. The study was conducted at two sites (Ceará Rise; western equatorial Atlantic, and Sierra Leone Rise; eastern equatorial Atlantic) in cores raised from three different depths at each site. From measurements of230Th and CaCO3, changes in carbonate flux with time and depth were obtained. A rapid increase in carbonate production, starting at the onset of deglaciation, was found in both areas. This event may have important implications for the postglacial increase in atmospheric CO2by increasing the global carbonate carbon to organic carbon rain ratio and decreasing the alkalinity of surface waters (and possibly the North Atlantic Deep Water). Increased carbonate dissolution occurred in the two regions during deglaciation, followed by a minimum during mid‐Holocene and renewed intensification of dissolution in late Holocene. During the last 16,000 years, carbonate dissolution was consistently more pronounced in the western than in the eastern basin, reflecting the influence of Antarctic Bottom Water in the west. This trend was reversed during stage 2, possibly due to the accumulation of metabolic CO2below the level of the Romanche Fracture Zone in the easte

ISSN:0883-8305    

DOI:10.1029/PA005i005p00761

年代:1990

数据来源: WILEY

摘要:

Uranium series nuclide concentrations have been measured on sediments from five box cores from an equatorial Pacific transect.230Thexcessactivities show discontinuities at the Holocene‐glacial boundary as dated by14C. The glacial sedimentation rates determined by230Th and14C are 2.5‐3.0 cm/kyr. The Holocene rates from230Th are much lower than those dated by14C (1.9–2.3 cm/kyr) because of carbonate dissolution.230Th sedimentation fluxes exceed water column supply by factors of 1.2–1.8 in the Holocene and 1.8–3.0 in the glacial sections. A number of models have been applied to calculate carbonate dissolution rates. The results show that carbonate dissolution rates in the Holocene (in g/cm²kyr) equal 1.5 × 10−3exp (1.4D) where D is water depth in kilometers. A point‐by‐point estimation of sediment fluxes through time show that clay accumulation rates in the area have been near constant at 0.1–0.2 g/cm²kyr over the past 20 kyr whereas carbonate accumulation rates have decreased dramatically from 0.6–1.0 g/cm²kyr in the glacial sections of the cores to 0.2–0.6 g/cm²kyr in the Holocene. The errors caused by the uncertainties in the age of the termination of the last glacial period have been investigated and results show that a range of 11–14 kyr leads to an error upper limit of about 30% in the estimation of CaCO3dissolution rates. The response time of CaCO3and230Thexconcentrations in the mixed layer of sediments due to an impulse of change in CaCO3dissolution rate has also been discussed, showing that the observed changes in carbonate dissolution may be explained in terms of a single or a continuous change, depending upon the

ISSN:0883-8305    

DOI:10.1029/PA005i005p00789

年代:1990

数据来源: WILEY

摘要:

Profiles of the230Th concentration in Mn crusts from the central Pacific Ocean measured at extremely high depth resolution reveal that the growth rates of Mn crusts are influenced by climate. Based on a “constant flux model” the sections of maximum230Th concentration correspond to periods of slow growth during glacial stages. Fast growth occurred during interglacial stages 1, 5, and 7, probably due to a larger supply of Mn oxides from the water column. High‐resolution profiles of230Th and10Be in sediment cores from high biological productivity areas display radioisotope maxima in the interglacial stages and minima during glacial periods, the ratio of the fluxes of10Be/230Th being ≥4. The only exceptions are observed at 135 and 270 kyr B.P., where the ratio of the fluxes is as low as 0.1 to 0.3. We presume that this “230Th anomaly” reflects short periods of time at the end of glacials when precipitation of MnO2occurred. This hypothesis is confirmed by peaks of Mn observed in sediment cores mainly at the transitions from glacial to interglacial stages. The standing crops of Mn in these layers suggest release of Mn2+from the sediments during glacial stages and buildup of Mn in the water column to concentrations of up t

ISSN:0883-8305    

DOI:10.1029/PA005i005p00811

年代:1990

数据来源: WILEY