摘要:

We present a method for determining the δ18O of seawater in the deep ocean during the last glacial maximum from the measured δ18O values of deep sea pore fluids. Using data from Deep Sea Drilling Project (DSDP) site 576 in the Western Pacific, this method yields a glacial to interglacial change in δ18Oswof 1.0±0.25‰. This value for Δδ18Oswis the first direct measurement of deep ocean δ18O for the last glacial maximum and avoids the problems of spatial and temporal variability of the δ18O of surface water implicit in previous determinations. More precise, higher‐resolution pore fluid measurements are required to improve this de

ISSN:0883-8305    

DOI:10.1029/92PA02796

年代:1993

数据来源: WILEY

摘要:

Measurements of opal preservation in deep sea sediment cores have been presented in three ways: the opal concentration as a fraction of total dry weight (%opaltot), the opal concentration normalized to calcite‐free dry weight (%opalcalcite‐free), and me opal accumulation rate (opal MAR). It is tempting to interpret changes in these indices as indicators of rates of biological production in past oceans. Based on theoretical constraints, we argue that in typical tropical and subtropical sediments, both %Opalcalcite‐freeand opal MAR reflect a significant artifact of dilution by other phases. Thus the band of high %Opalcalcite‐freein the equatorial Pacific appears to be caused in large part by the high %Calcite in that region, rather than by high opal productivity. The best candidate for a reliable paleoproductivity proxy appears to be %Opaltot. Unfortunately, present‐day %Opaltotdata from tropical and subtropical regions show little or no systematic trend with the rain rate of opal. Pore water silica concentration data reveal that the apparent pore water opal solubility is not constant but correlates regionally with the rain rate of opal to the seafloor. A model that treats opal as a single homogeneous phase with a single well‐defined solubility product predicts a strong dependence of opal concentration on rain rate (in stark contrast to the data), and a constant asymptotic pore water Si. Two models representing opal as multiple heterogeneous phases with different solubilities are able to reproduce the observed asymptotic pore water Si/rain rate relationship, but not the lack of rain rate trend in the opal concentration data. Only by assuming a systematic trend in the quality of opal (i.e., the solubility) as a function of opal production, can we reproduce the observed pattern of opal preservation. The implication of this study is that changes in opal preservation in the geologic record cannot simply be interpreted in terms of changes in surface ocean productivity until our understanding of opal diagenesis can

ISSN:0883-8305    

DOI:10.1029/92PA02803

年代:1993

数据来源: WILEY

摘要:

An ocean general circulation model (OGCM) of the Indian Ocean is forced by paleoclimate simulations for five time slices over the last 126,000 years. Using an annual period sinusoidal fit to the perpetual January and July paleoclimatic model forcing, the OGCM produces seasonal cycles in velocity fields, both vertical and horizontal, and temperature/salinity fields at these five times. The simulations indicate that the southern hemisphere upper ocean circulation of the Indian Ocean is essentially/invariant to climatic fluctuation on this time scale. They also suggest that the low‐insolation (18,000 years B.P.) northern hemisphere upper ocean summer circulation was very much weaker compared to high‐insolation periods (such as 9000 years B.P. and 126,000 years B.P.), by perhaps an order of magnitude. By contrast, the simulated low‐insolation winter northern hemisphere upper ocean circulation was substantially stronger than during high insolation periods. Equatorial upwelling, as simulated, is shown to be a mainly boreal summer event, reduced by an order of magnitude during low‐insolation conditions. A similar reduction in upwelling is seen along the Arabian coast, but a less severe decrease along the Somalian coast. The model simulation of February and August sea surface temperature at 18,000 years B.P., the height of the last glaciation, is generally within 1°–2°C of paleoclimate reconstructions. The effect of lower glacial sea level on the circulation is shown to be

ISSN:0883-8305    

DOI:10.1029/92PA02527

年代:1993

数据来源: WILEY

摘要:

Isotopic and sedimentologic data from Ocean Drilling Program hole 704A suggest that isotopic stages 7, 9, and 11 were marked by unusually strong interglacial conditions in surface waters of the southern ocean. During interglacial stages 9 and 11, warm surface waters penetrated far poleward and may have led to destabilization of the West Antarctic Ice Sheet. In contrast, the strongest glacial conditions in surface waters of the subantarctic South Atlantic occurred during oxygen isotopic stage 12. Comparisons of benthic carbon isotopic gradients between sites located in the North Atlantic, southern ocean, and Pacific indicate that the production of upper North Atlantic Deep Water (uNADW) was strongest during stages 7,9, and 11 and weakest during stage 12, These results suggest a possible link between the flux of uNADW and paleoceanographic change in the southern ocean and support the traditional NADW‐Antarctic connection whereby increased NADW leads to warming of the southern ocea

ISSN:0883-8305    

DOI:10.1029/92PA02774

年代:1993

数据来源: WILEY

摘要:

Seven marine sediment cores were collected from the Kangerdlugssuaq Trough off the southeast coast of Greenland. The trough cuts across the continental shelf from the fjord mouth to the upper slope in Denmark Strait and provides a sedimentary environment sheltered from the East Greenland Current flowing along the shelf. The cores have been14C dated (accelerator mass spectroscopy dates) and analyzed for diatom accumulation rates and changes in floral composition. It was found that the shelf probably was not glaciated by grounded ice but occupied by an ice shelf and/or permanent sea ice between 14,000 years B.P. and the present. Average retreat rates of glacial/permanent sea ice were circa 60 m/yr between circa 13,500 and 11,000 years B.P. The retreat rates increased to about 150 m/yr between 11,000 to 10,000 years B.P. The increase is unexpected because this correlates with Fairbanks' (1989) projected low glacial meltwater discharge for the same time which is coeval with the Younger Dryas. Several events of sea surface warming/cooling took place on the outer shelf from 12,000 to 9000 years B.P. followed by cooling to the present, with a possible warming between 3000–4000 years BP. On the inner shelf pulses of warm/cool surface water occurred from 9000 to 6000 years B.P., which was succeeded by surface water cooling until presen

ISSN:0883-8305    

DOI:10.1029/92PA02591

年代:1993

数据来源: WILEY

摘要:

Twelve upper Pliocene‐lower Pleistocene calcareous nannofossil events are calibrated with oxygen isotope stratigraphy at eight DSDP/ODP sites ranging from 53°N to 41°S latitudes in different oceans in order to determine their ages and to reveal their synchrony or diachrony within a significantly higher‐resolution time scale than the geomagnetic polarity time scale allows. Six nannofossil events appear to be synchronous from low to middle latitudes (the last occurrences ofReticulofenestra asanoi(oxygen isotope stage 22),Gephyrocapsaspp. A‐B (stage 37),Discoaster brouweri(stage 71),Discoaster surculus(stage 99),Discoaster tamalis(stage 111), and the first occurrence ofGephyrocapsaspp. A‐B (stage 59)), five appear to be diachronous (the first occurrences ofGephyrocapsaspp. C‐D,Reticulofenestra asanoi, andGephyrocapsaspp. A‐B larger than 5.5 µm, and the last occurrences ofHelicosphaera selliiandDiscoaster pentaradiatus), and one is synchronous at low latitudes (the last occurrence ofCalcidisc

ISSN:0883-8305    

DOI:10.1029/92PA02504

年代:1993

数据来源: WILEY

摘要:

The Cenozoic strontium isotope (87Sr/86Sr) variation in seawater is largely defined using Deep Sea Drilling Project (DSDP) samples. Examination of non‐DSDP results, particularly for the Paleogene, suggest that the original seawater curve of Burke et al. (1982) may lie above the true seawater ratio during this time. To investigate this possibility, we analyzed the87Sr/86Sr for 230 Paleogene rock and shell samples from the 83 sites in Alabama, Mississippi, and Louisiana. Many of the samples are from classic sections on which the Gulf Coast Paleogene stratigraphy was originally defined. Multiple analyses of samples from these units show very little scatter, and in most cases, precise agreement is obtained among geographically separated collection sites. We believe that analysis of multiple samples from a unit of established age, collected at more than one site, yields results that most accurately define the actual seawater Sr isotope ratio at any given time. Results from these units define a seawater variation line that tracks well within the range of DSDP data, but is generally below the original seawater line of Burke et al. (1982). It is often assumed that samples from continental settings will exchange with high‐ratio Sr thereby raising87Sr/86Sr values. Our Gulf Coast samples, which are derived from a continental setting, yield individual87Sr/86Sr values that are both higher and lower than the estimated contemporaneous seawater ratio. An analysis of our Gulf Coast data and DSDP results show that the direction of isotopic exchange cannot be confidently predicted for individual samples based solely on their derivation from either continental or oceanic setti

ISSN:0883-8305    

DOI:10.1029/92PA02338

年代:1993

数据来源: WILEY

摘要:

The deposition of marine sediments in the deep Mediterranean basins following the Messinian salinity crisis (“Zanclean Deluge”) began at 4.9 Ma and continued until normal marine conditions prevailed at 4.6 Ma. The Betic Strait of southeast Spain, the Rif Strait of northern Morocco, and the Strait of Gibraltar have been postulated as possible entry ways for the marine waters into the Mediterranean. Two possible explanations have been offered for the timing of the deposition of marine hemipelagic marls in the Vera Basin (Cuevas del Almanzora section) within the Betic Strait: (1) early Pliocene, representing the “Zanclean Deluge” (Cita et al., 1980), (2) Messinian (∼5.7 Ma), representing the last marine stage before the Messinian salinity crisis (Benson and Rakic‐El Bied, 1991; Benson et al., 1991). The strontium isotopic ratio on mixed assemblages of foraminifers from these marine marls was determined in order to assess their ages. The average87Sr/86Sr (0.709033±24) of the seven measured samples is, compared to the base of the Miocene/Pliocene boundary stratotype at Capo Rossello, Sicily, higher by 38 × 10−6and yields an age of 4.6±0.5 Ma when compared to the established open ocean Sr seawater curve. The determined age indicates that (1) the first 3.6 m of marine deposits of the Cuevas del Almanzora section sensu Cita et al. (1980) including the passage zone are Pliocene, (2) the earliest marine Pliocene might not be represented at Cuevas del Almanzora indicating that this marginal basin at the edge of the Mediterranean Sea, yet within the former Betic Strait, was filled progressively by a marine transgression originating from the Mediterranean Sea around 4.6 Ma. The absence of lowermost Pliocene sediments could suggest that the Betic Strait did not serve as a conduit for the water masses entering the Mediterranean basins during the early Pliocene; and (3) the presence of an early evolutionary stage ofGloborotalia margaritaein the Moroccan Bou Regreg section (approximately 5.8–5.6 Ma) and in the Spanish Cuevas del Almanzora section (later than 4.6±0.5 Ma) are diachronous suggesting that straightforward correlations of micropaleontological events within the Mediterranean area with micropaleontologic events of open marine sections should be tested thoroughly by independent metho

ISSN:0883-8305    

DOI:10.1029/92PA02795

年代:1993

数据来源: WILEY