ISSN:0883-8305    

DOI:10.1029/93PA00252

年代:1993

数据来源: WILEY

摘要:

The seeming discordance between the glacial to interglacial changes in the carbon isotope ratio and cadmium content of benthic foraminifera from the deep Antarctic has cast a shadow on the reliability of these proxies as valid of paleocirculation indicators. The problem is that while cadmium content shows little glacial to interglacial change [Lea and Boyle, 1990; Boyle, 1992], the carbon isotope change is larger than at any other place in the ocean [Curry et al., 1988; Ïññü et al., 1990;Charles and Fairbanks, 199O]. Even after correction for the 0.35‰ glacial to interglacial change in the isotope ratio for mean ocean‐atmosphere carbon, a difference of 0.5 to 0.6‰ remains. On the basis of Redfield ratios for today's ocean, this is equivalent to a 25% increase in cadmium content which is not seen. This puzzle has been with us for seve

ISSN:0883-8305    

DOI:10.1029/93PA00262

年代:1993

数据来源: WILEY

摘要:

A high‐quality δ18O record has been obtained from a deep sea core in the Somali Basin in a water depth of 4020 m, well below the foraminiferal lysocline, by analyzing bulk sediment instead of picked foraminifera. In some areas this may be a valuable tool for extending the applicability of the δ18O stratigraphy, for obtaining data by a less labor‐intensive procedure, and for conserving valuable core material. Additionally, we draw attention to the need for more data on δ13C in bulk carbonate if we are to fully utilize the ocean δ13C record either in relation to long‐term global carbon budget, or in relation to changes in carbon cycling in the ocean and its effect on atmosph

ISSN:0883-8305    

DOI:10.1029/93PA00123

年代:1993

数据来源: WILEY

摘要:

Core NWR 5 from Northwind Ridge in the western Arctic Ocean contains a climatic record that extends back for over 1 million years. Dark brown beds with abundant planktonic foraminifers represent interglacial conditions, whereas glacial and transitional conditions are represented by lithologies barren or nearly barren of planktonic foraminifers. We conclude that seven, and possibly all nine, interglacials of the Brunhes magnetic chronozone (the last 780,000 years) are represented in NWR 5. For about the last 800,000 years, the western Arctic Ocean apparently had a permanent, thick ice cover and was unable to support significant populations of planktonic foraminifers except during major interglacials when seasonally open and high‐productivity surface waters occurred at least along the basin margins. Our results support previous interpretations that climate variations in the Arctic are in phase with glacial/interglacial cycles observed in other proxy records of the Brunhe

ISSN:0883-8305    

DOI:10.1029/93PA00146

年代:1993

数据来源: WILEY

摘要:

The migration of thermophilic marine Ostracoda into the Arctic Ocean during the Pliocene indicates that winter and summer ocean temperatures around Arctic margins were ≥ 0 °C and>3 °C, respectively, and that ice‐free conditions existed for most or all of the Arctic. By at least 3.5–3.0 Ma, probably earlier, the opening of the Bering Strait allowed marine organisms to migrate through the Arctic Ocean, mostly from the Pacific Ocean. Migrant taxa such asCythere, Hemicythere,andNeomonoceratinaare known from Pliocene deposits of Alaska and Canada as well as Neogene deposits of the North Pacific and Atlantic oceans. On the basis of ecological and Zoogeographic information on ostracode species from more than 800 modern “core top” samples for the North Atlantic, North Pacific, and Arctic Oceans, we determined winter and summer temperature tolerances for migrant taxa to be at or above about 0 °C and 3 °C. This suggests ice‐free summers, and probably, a perennially ice‐free Arctic Ocean in some regions. Elevated water temperatures in the Arctic Ocean between 3.5 and 2.0 Ma is supported by evidence for late Pliocene increased meridional heat transport in the Nor

ISSN:0883-8305    

DOI:10.1029/93PA00060

年代:1993

数据来源: WILEY

摘要:

The observation by Heinrich (1988) that, during the last glacial period, much of the input of ice‐rafted detritus to the North Atlantic sediments may have occurred as a succession of catastrophic events, rekindled interest on the history of the northern ice sheets over the last glacial period. In this paper, we present a rapid method to study the distribution of these events (both in space and time) using whole core low‐field magnetic susceptibility. We report on approximately 20 cores covering the last 150 to 250 kyr. Well‐defined patterns of ice‐rafted detritus appear during periods of large continental ice‐sheet extent, although these are not always associated within their maxima. Most of the events may be traced across the North Atlantic Ocean. For the six most recent Heinrich layers (HL), two distinct patterns exist: HL1, HL2, HL4, HL5 are distributed along the northern boundary of the Glacial Polar Front, over most of the North Atlantic between ≈40° and 50°N; HL3 is more restricted to the central and eastern part of the northern Atlantic. The Nd‐Sr isotopic composition of the material constituting different Heinrich events indicates the different provenance of the two patterns: HL3 has a typical Scandinavia‐Arctic‐Icelandic “young crust” signature, and the others have a large component of northern Quebec and northern West Greenland “old crust” material. These isotopic results, obtained on core SU‐9008 from the North American basin, are in agreement with the study by Jantschik and Huon (1992), who used K‐Ar dating of silt‐ and clay‐size fractions of an eastern basin core (ME‐68‐89). These data confirm the large spatial scale of these events, and the enormous amoun

ISSN:0883-8305    

DOI:10.1029/92PA02923

年代:1993

数据来源: WILEY

摘要:

In the northwest Arabian Sea upwelling occurs each summer, driven by the strong SW monsoon winds. Upwelling results in high biological productivity and a distinctive assemblage of plankton species in the surface waters off Oman that are preserved in the sediments along the Oman continental margin, creating a geologic record of monsoon‐driven upwelling. Sediments recovered from the Oman continental margin during Ocean Drilling Program leg 117 provide an opportunity to examine how upwelling has varied during the late Quaternary, spanning a longer interval than piston cores recovered prior to the ODP cruise. Variations in foraminifer shell accumulation and in the relative abundance ofGlobigerina bulloidesindicate dominant cycles of variation at 1/100 kyr, the dominant frequency of glacial‐interglacial variations, and at 1/23 kyr, the frequency of precessionally driven cycles in seasonal insolation. The strongest monsoon winds (indicated by increased upwelling) occurred during interglacial times when perihelion was aligned with the summer solstice, an orbital change that increased the insolation received during summer in the northern hemisphere. During glacial times upwelling was reduced, and although the precessional cycles were still present their amplitude was smaller. At both frequencies the upwelling cycles are in phase with minimum ice volume, evidence that glacial‐interglacial climate changes also include changes to the climate system that influence the low‐latitude monsoon. We attribute the decrease in the monsoon winds observed during glacial times to changes in bare land albedo over Asia and/or to changes in the areal extent and seasonal cycle in Asian snow cover that decrease the summer land‐sea temperature contrast. Other mechanisms may also be involved. These new upwelling time series differ substantially from previous results, however the previous work relied on cores located farther offshore where upwelling is less intense and other physical mechanisms become important. Our results support the observations derived from atmospheric general circulation models of the atmosphere that indicate that both glacial boundary conditions, and the strength of summer insolation are important variables contributing to cycles in the monsoon winds during the late Q

ISSN:0883-8305    

DOI:10.1029/93PA00256

年代:1993

数据来源: WILEY

摘要:

High‐ and low‐latitude forcing of terrestrial African paleoclimate variability is demonstrated using 900 ka eolian and biogenic component records from Ocean Drilling Program site 663 in the eastern equatorial Atlantic. Terrigenous (eolian dust) and phytolith (savannah grass cuticle) accumulation rate records vary predominantly at 100 and 41 kyr periodicities and spectral phase estimates implicate high‐latitude forcing. The abundance of freshwater diatoms (Melosira) transported from dry African lake beds varies coherently at 23–19 kyr orbital periodicities and at a phasing which implicates low‐latitude precessional monsoon forcing. Modeling studies demonstrate that African climate is sensitive to both high‐ and low‐latitude boundary conditions. African monsoon intensity is modulated by direct insolation variations due to orbital precession, whereas remote high‐latitude forcing can be related to cool North Atlantic sea surface temperatures (SSTs) which promote African aridity and enhance dust‐transporting wind speeds. The site 663 terrigenous and phytolith records covary with North Atlantic SST variability at 41 °N (site 607). We suggest that Pleistocene African climate has responded to both high‐latitude North Atlantic SST variability as well as low‐latitude precessional monsoon forcing; the high‐latitude influence dominates the sedimentary record. Prior to circa 2.4 Ma, terrigenous variations occurred primarily at precessional periodicities (23–19 kyr), indicating that African climate was largely controlled by low‐latitude insolation variations prior to the onset of high‐amplitude glaci

ISSN:0883-8305    

DOI:10.1029/93PA02688

年代:1993

数据来源: WILEY

摘要:

The late Quaternary sedimentary record in periplatform oozes and muds deposited on the northern Nicaragua Rise results from the interplay of four main controls: (1) input of pelagic carbonates and bank‐derived fine aragonite and magnesian calcite; (2) input of siliciclastic sediments; (3) dispersal and removal of sediments by the Caribbean Current; and (4) partial seafloor dissolution of metastable carbonates. High accumulation rates of the calcite coarse sediment fraction throughout the study area demonstrate that planktonic foraminiferal productivity peaked during interglacial stages. Neritic carbonate productivity also peaked during interglacial stages, when bank and shelf tops along the Nicaragua Rise were submerged within the photic zone. Because the bank and shelf tops remained mostly exposed during glacial stages, the surface area available for the neritic carbonate productivity was drastically reduced to a narrow band along the margins of Pedro Bank and the southern shelf of Jamaica. A large volume of siliciclastic sediments, transported from the coastal area of South America and the eastern and western regions of Jamaica, were deposited during glacial stages within the eastern and western deep extensions of Walton Basin. This finding contrasts with the input pattern of siliciclastic sediments in Walton Basin itself, where the largest input of siliciclastic sediments occurred at each of the interglacial climatic optima, corresponding to intervals of maximum sea level transgression. Finally, cores in water depths exceeding 1100 m display during interglacial stages aragonite accumulation rates systematically lower than rates in cores from water depths shallower than 1100 m. This indicates that some bank‐derived aragonite and magnesian calcite has been partially removed by seafloor dissolution in areas from water depths exceeding 1100 m. Results from this study on the northeastern Nicaragua Rise indicate that not only do offbank transport and water column saturation state influence the late Quaternary record but also that the variable strength of the Caribbean Current and the proximity of sources for siliciclastic sediments have played a major role in the development of the late Quaternary periplatform high resolution stratigra

ISSN:0883-8305    

DOI:10.1029/92PA02876

年代:1993

数据来源: WILEY

摘要:

Milankovitch‐type periodicity was identified within Lower Cretaceous cyclic sequences from the western Central Atlantic Ocean and the Vocontian Basin (southeast France) through application of the Walsh spectral method to detailed stratigraphic sections. The repetitious bedding in these sequences appears to be the product of cyclic climatic changes controlled by the orbital forcing. Obliquity seems to be the most important and persistent orbital element responsible for cyclic sedimentation, whereas precession is less important. Upper Berriasian and Valanginian strata from the Atlantic Ocean contain indications of eccentricity‐induced periodicity and of a nonlinear response of the climatic system. Variations in orbitally tuned pelagic sedimentation rates in the Atlantic Ocean correlate with the changes in the character of cycles and with the occurrence of calcareous turbidites; therefore this suite of sedimentary features are responding to evolving paleoceanographic conditions and possible relative sea level fluctuations. The orbitally tuned durations of the Valanginian and Hauterivian stages in the Vocontian Basin are a minimum of 5.9 and 5.3 m.y., respectiv

ISSN:0883-8305    

DOI:10.1029/93PA00253

年代:1993

数据来源: WILEY