摘要:

An astronomically calibrated time scale was recently established (Hilgen, 1991a, b) for the Pliocene and earliest Pleistocene, based on the correlation of dominantly precession‐controlled sedimentary cycles (sapropels and CaCO3cycles) in Mediterranean deep‐sea sections to astronomical solution Ber90 (Berger and Loutre, 1991). This time scale, which differed substantially from the conventional time scales then widely used, is essentially confirmed by a new generation of radiometric dates using the40Ar/39Ar laser fusion technique. To evaluate this time scale, we here extract both precession‐ and obliquity‐related components from late Pliocene‐earliest Pleistocene climatic proxy records in the Mediterranean and determine coherencies and phase (time) lags between these components and the respective orbital variations. This work is found to support the new time scale, because significant coherencies are found between the astronomically related components in our proxy records and the orbital variations, not only at the main period of precession but also at the main period of obliquity and, because of the eccentricity modulation of precession, in the low‐frequency eccentricity bands of the spectrum. But the calculated time lag between obliquity and 41‐kyr components in the climatic proxy records shows a significant increase of 5.6 (±1) kyr with respect to the late Pleistocene if the phase relation with precession is kept constant. This increase can be explained by a change in the climate response time to obliquity or to precession, by a small uncertainty in the astronomical solution, or by a combination of

ISSN:0883-8305    

DOI:10.1029/93PA01248

年代:1993

数据来源: WILEY

摘要:

The development of an orbitally tuned time scale for the ODP leg 138 sites provides biostratigraphers a very high resolution chronostratigraphic framework. With this framework we are better able to define which of the first and last appearances of species appear to be synchronous. In addition, the geographic distribution of sites provides the means with which the detailed spatial patterns of invasion of new species and the extinction of older species can be mapped. These maps not only provide information on the process of evolution, migration, and extinction, they can also be related to water mass distributions and near‐surface circulation of the ocean. Of 39 radiolarian events studied at 11 sites in the eastern equatorial Pacific, 28 were found to have a minimum range in their estimated age that exceeded 0.15 m.y. The temporal pattern of first and last appearances of these diachronous events have coherent spatial patterns that indicate shifts in the areas of high oceanographic gradients over the past 10 Ma. These changes in the locations of high gradient regions suggest that the South Equatorial Current (SEC) was north of its present position prior to approximately 7 Ma. There was a southward shift in the northern boundary of this current between approximately 6 and 7 Ma, and the development of a relatively strong gradient between the northeastern and northwestern sites. Between approximately 3.7 and 3.4 Ma, there was a very slight northward shift in the northern boundary of the SEC and the steep gradients between the northeastern and northwestern sites may have disappeared. This change is thought to be associated with the closing of the Isthmus of Panama. The temporal‐spatial patterns of diachronous events younger than 3.4 Ma are consistent with patterns of circulation in the modern oc

ISSN:0883-8305    

DOI:10.1029/93PA01328

年代:1993

数据来源: WILEY

摘要:

On a transect between 20° and 70°S in the eastern Atlantic Ocean and Weddell Sea, water samples from 19 hydrographic stations and bottom water from 55 surface sediment samples taken with a multiple corer were investigated for the stable carbon isotopic composition of the total dissolved inorganic carbon (δ13CΣCO2). These measurements were compared to δ13C values determined on live specimens of the benthic foraminiferFontbotia wuellerstorfiand closely related genera from the same stations. In addition, at 16 stations the stable carbon isotope composition of sedimentary organic carbon was measured. General deepwater and bottom‐water mass circulation patterns as inferred from the δ13CΣCO2are in close agreement with those known from other nonconservative tracers. Very low δ13C values of upper Circumpolar Deep Water (<0.3‰ Pee Dee belemnite (PDB)) in the Polar Front region and the eastern limb of the Weddell gyre coincide with nutrient maxima. However, a significant decoupling of the dissolved phosphate signal from the δ13CΣCO2signal is indicated in the abyssal Weddell Sea. We attribute this to temperature‐dependent fractionation processes during gas exchange of surface waters with the atmosphere at sites of bottom‐water formation. Multiple corer water from the sediment/water interface is slightly δ13C depleted relative to deepwater and bottom‐water δ13ΣCO2. The surface sediment organic carbon δ13C is 3 to 4‰ lower south of the Polar Front than north of it, and the δ13Corgin freshly accumulated phytodetritus is 3 to 4‰ lower than surface sediment organic carbon δ13C. Comparison of liveF. wuellerstorfiδ13C and related genera with bottom‐water δ13CΣCO2exhibits at most stations between the Subtropical Front (≈41°S) and the southern boundary of the Antarctic Circumpolar Current (≈55°S) a significant lowering of foraminiferal δ13C values. Compilation of a mean last glacial/interglacial δ13C amplitude (Δδ13C) from six published southern ocean cores results in a shift of −0.99± 0.13‰ PDB; this shift is greater than that in all other regions. However, all of these cores are from positions close to Recent oceanic fronts. Thus, for these peripheral areas of the southern ocean, we suggest about half of the glacial/interglacial shift can be explained by varying frontal zone positions and widths accompanied by

ISSN:0883-8305    

DOI:10.1029/93PA01291

年代:1993

数据来源: WILEY

摘要:

High‐resolution records of opal, carbonate, and terrigenous fluxes have been obtained from a high‐sedimentation rate core (MD84‐527: 43°50′S; 51°19;′E; 3269 m) by normalization to230Th. This method estimates paleofluxes to the seafloor on a point‐by‐point basis and distinguishes changes in sediment accumulation due to variations in vertical rain rates from those due to changes in syndepositional sediment redistribution by bottom currents. We also measured sediment δ15N to evaluate the changes in nitrate utilization in the overlying surface waters associated with paleoflux variations. Our results show that opal accumulation rates on the seafloor during the Holocene and stage 3, based on14C dating, were respectively tenfold and fivefold higher than the vertical rain rates, At this particular location, changes in opal accumulation on the seafloor appear to be mainly controlled by sediment redistribution by bottom currents rather than variations in opal fluxes from the overlying water column. Correction for syndepositional sediment redistribution and the improved time resolution that can be achieved by normalization to230Th disclose important variations in opal rain rates. We found relatively high but variable opal paleoflux during stage 3, with two maxima centered at 36 and 30 kyr B.P., low opal paleoflux during stage 2 and deglaciation and a pronounced maximum during the early Holocene, We interpret this record as reflecting variations in opal production rates associated with climate‐induced latitudinal migration of the southern ocean frontal system. Sediments deposited during periods of high opal paleoflux also have high authigenic U concentrations, suggesting more reducing conditions in the sediment, and high Pa‐231/Th‐230 ratios, suggesting increased scavenging from the water column. Sediment δ15N is circa 1.5 per mil higher during isotopic stage 2 and deglaciation. The low opal rain rates recorded during that period appear to have been associated with increased nitrate depletion. This suggests that opal paleofluxes do not simply reflect latitudinal migration of the frontal system but also changes in the structure of the upper water column. Increased stratification during isotopic stage 2 and deglaciation could have been produced by a meltwater lid, leading to lower nitrate supply

ISSN:0883-8305    

DOI:10.1029/93PA00784

年代:1993

数据来源: WILEY

摘要:

Radiocarbon and230Thexcessdata from six NE Atlantic box cores are considered. The cores form a transect from the Porcupine Abyssal Plain over the East Thulean Rise to the southern end of Feni Drift. The chronology for the cores is established from bulk sediment carbonate radiocarbon data and reveals that sections exhibiting constant accumulation rates can be identified in all the cores, with rates of 3.0‐3.5 cm kyr−1on the plain through the Holocene and late Holocene rates of 4.3–6.6 cm kyr−1elsewhere. Five out of the six cores show accumulations of more230Thexcessthan is produced in the overlying water column, with the greatest inventories (up to 225% of production) in the cores from the rise and drift. A size fraction comparison between two cores from the plain and rise reveals that the higher overall accumulation rates and230Thexcessinventories in the off‐plain cores are due to an increased fine (<5 µm) component fraction, whereas the flux of coarser material is similar to that received on the plain. This suggests that the higher fluxes of materials observed are physically (rather than biogeochemically) driven and also that drift formation has been continuously active in the late Holocene. Sections of all the cores where regular accumulation is defined by the radiocarbon data are modeled first by a linear radiocarbon age/depth model and second by a constant rain (230Thexcess)0model prorated for the observed core inventories. These modeling approaches yield historical mass accumulation rate estimates which are generally in reasonable agreement (±30%), but the differences observed appear to be well organized in time rather

ISSN:0883-8305    

DOI:10.1029/93PA01366

年代:1993

数据来源: WILEY

摘要:

We examined the flux of Al to sediment accumulating beneath the zone of elevated productivity in the central equatorial Pacific Ocean, along a surface sediment transect at 135°W as well as downcore for a 650 kyr record at 1.3°N, 133.6°W. Across the surface transect, a pronounced, broadly equatorially symmetric increase in Al accumulation is observed, relative to Ti, with Al/Ti ratios reaching values 3–4 times that of potential detrital sources. The profile parallels biogenic accumulation and the modeled flux of particulate234Th, suggesting rapid and preferential adsorptive removal of Al from seawater by settling biogenic particles. Normative calculations confirm that most Al is unsupported by the terrigenous fraction. The observed distributions are consistent with previous observations of the relative and absolute behavior of Al and Ti in seawater, and we can construct a reasonable mass balance between the amount of seawater‐sourced Al retained in the sediment and the amount of seawater Al available in the overlying column. The close tie between Al/Ti and biogenic accumulation (as opposed to concentration) emphasizes that biogenic sedimentary Al/Ti responds to removal‐transport phenomena and not bulk sediment composition. Thus, in these sediments dominated by the biogenic component, the bulk Al/Ti ratio reflects biogenic particle flux, and by extension, productivity of the overlying seawater. The downcore profile of Al/Ti at 1.3°N displays marked increases during glacial episodes, similar to that observed across the surface transect, from a background value near Al/Ti of average upper crust. The excursions in Al/Ti are stratigraphically coincident with maxima in both bulk and CaCO3accumulation and the excess Al appears to not be preferentially affiliated with opaline or organic phases. Consistent with the similar behavioral removal of Al and234Th, the latter of which responds to the total particle flux, the Al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits. These results collectively indicate that (1) Al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater. Therefore, the common tenet that Al is dominantly associated with terrestrial particulate matter, and the subsequent use of Al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment, needs to be reevaluated. (2) The Al/Ti ratio in biogenic sediment can be used to trace the productivity of the overlying water, providing a powerful new paleochemical tool to investigate oceanic response to climatic variation. (3) The close correlation between the Al/Ti productivity signal and carbonate maxima downcore at 1.3°N suggests that the sedimentary carbonate maxima in the central equatorial Pacific Ocean record increased productivity during glacia

ISSN:0883-8305    

DOI:10.1029/93PA02195

年代:1993

数据来源: WILEY

摘要:

Organic‐rich Upper Cretaceous sequences in Israel were deposited in an extensive, highly productive upwelling‐linked system which prevailed along the southern Tethys margin, and lasted for ∼19 m.y. (Santonian to late Maastrichtian). An understanding of the spatial and temporal characteristics of this system was gained through detailed paleontological and geochemical analyses of subsurface sequences in two basins in Israel, representing an outer (Shefela) and an inner (Zin) facies belt. The nature of the upwelling system, and its effect on the sedimentary record, is related to two basic environmental parameters, namely paleoproductivity intensity and oxygen levels at the bottom. The assessment of these parameters and their interrelationship has been performed through the development of paleontological (foraminiferal) criteria, which are independent of the organic matter content. Following the establishment of these criteria, it is concluded that the productivity reached its maximum intensity during the late Campanian, which was also the time of most notable differentiation between the center of the upwelling system in the inner belt and the less intense conditions in the outer basin. This distribution is expressed in varied lithology (organic‐rich carbonates, phosphorites, and siliceous rocks) at the core of upwelling and a uniform lithology (organic‐rich carbonates) at the margin of this system. The uniform lithology of the Maastrichtian in both basins, composed of organic‐rich carbonates, is ascribed to a gradual weakening of productivity. The bottom conditions in the inner belt during the late Campanian (the time of maximum surface productivity) were near anoxic, changing to more aerated (dysaerobic) conditions during the early Maastrichtian. In the outer belt a more aerated bottom (dysaerobic) prevailed throughout the late Campanian to late Maastrichtian. The elevated organic matter content in both basins reflects the overall environment of high productivity; its actual variations, however, seem to be unrelated to changes in surface productivity and bottom ox

ISSN:0883-8305    

DOI:10.1029/93PA02197

年代:1993

数据来源: WILEY