摘要:

It has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions. This extraordinary event, referred to by European palynologists as the Younger Dryas, was centered in the northern Atlantic basin. Evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern Atlantic Ocean. Further, it appears that these mode changes may have been triggered by diversions of glacial meltwater between the Mississippi River and the St. Lawrence River drainage systems. We report here Accelerator Mass Spectrometry (AMS) radiocarbon results on two strategically located deep‐sea cores. One provides a chronology for surface water temperatures in the northern Atlantic and the other for the meltwater discharge from the Mississippi River. Our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern Atlantic basin with the meltwater history for the Laurentian ice shee

ISSN:0883-8305    

DOI:10.1029/PA003i001p00001

年代:1988

数据来源: WILEY

摘要:

Proxy indicators of sea surface temperature and equatorial divergence based on radiolarian assemblage data, and of trade wind intensity based on eolian grain size data show similar aspects of variability during the late Pleistocene: All indicators fluctuate at higher frequencies than the 100,000‐year glacial‐interglacial cycle, display reduced amplitude variations since 300,000 years ago, exhibit a change in the record character at about 300,000 years ago (the mid‐Brunhes climatic event), and have higher amplitude variations in sediments 300,000–850,000 years old. Time series analyses were conducted to determine the spectral character of each record (δ18O of planktonic foraminifer, sea surface temperature values, equatorial divergence indicators, and wind intensity indicators) and to quantify interrecord coherence and phase relationships. The record was divided at the 300,000‐year clear change in climatic variability (nonstationarity). The δ18O‐based time scale is better lower in the core so our spectral analyses concentrated on the interval from 402,000–774,000 years. The δ18O spectra show 100,000‐ and 41,000‐year power in the younger portion, 0–300,000 years, and 100,000‐, 41,000‐ and 23,000‐year power in the older interval, all highly coherent and in phase with the SPECMAP average stacked isotope record. Unlike the isotope record the dominant period in both the eolian grain size and equatorial divergence indicators is 31,000 years. This period is also important in the sea surface temperature signal where the dominant spectral peak is 100,000 years. The 31,000‐year spectral component is coherent and in phase between the eolian and divergence records, confirming the link between atmospheric and ocean surface circulation for the first time in the paleoclimate record. Since the 31,000‐year power appears in independent data sets within this core and also appears in other equatorial records [J. Imbrie personal communication, 1987], we assume it to be real and representative of both a nonlinear response to orbital forcing, possibly a combination of orbital tilt and eccentricity, and some resonance phenomenon required to amplify the response at this period so that it appears as a dominant frequency component. The mid‐Brunhes climatic event is an important aspect of these reco

ISSN:0883-8305    

DOI:10.1029/PA003i001p00021

年代:1988

数据来源: WILEY

摘要:

We have generated ∼300 Kyr records of biogenic opal, calcite, and organic carbon (Corg) for three cores in the eastern and central equatorial Pacific Ocean and have compared the records to determine whether common periods of biogenic sedimentation have occurred throughout the region. We find that Corghas been deposited in common pulses throughout the area, while opal has a much more local pattern of variation. Calcite varies regionally, but the record is shaped by superimposed dissolution and productivity processes. The most intense Corgpeak occurs at 18 ka and can have greater than 2 times the Holocene Corgcontent. Other major Corgpeaks occur 150 ka and perhaps at 280 ka. We have compared the Corgrecord in one of the cores, V19–28, to a model deepwater oxygen record developed from ∂13C data in the nearby V19–30 to test whether the Corgrecord has been mostly shaped by degradation or by the rain of organic matter from the euphotic zone. We found no coherence between the two records, implying that the Corgrecord is primarily a measure of productivity. By comparing the opal, calcite, and Corgrecords in V19–28, a core which is at or above the lysocline, we found that both increased calcite and opal deposition matches high Corgaccumulation. We also found, however, that the calcite and opal records were uncorrelated, so that episodes of high opal deposition do not necessarily accumulate calcite rapidly. We hypothesize that at least two different plankton communities have been dominant in the waters above this site, one rich in opal‐secreting plankton and one more dominated by calcite producers. The opal‐rich plankton community was dominant during the intervals 10–15 ka and 35–60 ka.Supplement data (Table 2) is available with entire articleon microfiche. Order from American Geophysical Union,2000 Florida Avenue, N.W., Washington, DC 20009.Document P88001; $2.50. Payment mus

ISSN:0883-8305    

DOI:10.1029/PA003i001p00039

年代:1988

数据来源: WILEY

摘要:

Rapid and nondestructive rock‐magnetic measurements of late Quaternary sediment cores from the Senegal continental slope and the Sierra Leone Rise in the eastern equatorial Atlantic correlate strongly with the SPECMAP oxygen isotope master curve and carbonate content. The rock‐magnetic‐paleoclimatic relationship results from differences in the concentration, accumulation rate, grain size, and composition of magnetic minerals between glacials and interglacials. At both sites, magnetic mineral assemblages of glacial episodes are characterized by higher concentrations (and accumulation rates) of magnetic minerals, by coarser‐grained ferrimagnetic minerals, e.g., magnetite, and by a lower ratio of ferrimagnetic to antiferromagnetic, e.g., goethite, hematite, minerals. By serving as a sensitive tracer for various deep‐sea sedimentary components, rock‐magnetic properties can provide insights into changing sediment sources and fluxes and therefore are a useful addition to more traditional chemically and mineralogically based paleoceanographic

ISSN:0883-8305    

DOI:10.1029/PA003i001p00061

年代:1988

数据来源: WILEY

摘要:

Previous magnetostratigraphic studies of nonfossiliferous pre‐Pliocene age pelagic clay cores from the central north Pacific have proven unsuccessful because of the unstable behavior of the natural remanent magnetization. The inability to obtain a reliable magnetostratigraphy has severely limited the temporal resolution that can be achieved in paleoceanographic studies of these nonfossiliferous pelagic clays. We carried out a rock‐magnetic study of core LL44‐GPC3, which spans the interval 0‐70 m.y. ago, to determine whether rock‐magnetic parameters can be used to construct a high‐resolution stratigraphic framework for paleoceanographic studies of central north Pacific pelagic clays despite the instability of the natural remanence. In addition, we tested the ability of rock‐magnetic methods to detect and characterize the paleoceanographic changes that are recorded in the sediments of LL44‐GPC3. Stratigraphic variations in rapid and nondestructive rock‐magnetic parameters and related ratios reflect changes in the concentration, mineralogy, and grain size of the magnetic minerals within the sediments. Rock‐magnetic parameters exhibit coherent fluctuations within both the stable and unstable sections of LL44‐GPC3. This result suggests the potential use of these parameters for regional correlation and relative dating of the nonfossil‐bearing pelagic clays of the central north Pacific gyre. Major fluctuations in rock‐magnetic parameters plotted as accumulation rates correspond to intervals of paleoenvironmental change that have been previously detected by other paleoceanographic methods. In addition, variations in a parameter proportional to the concentration of goethite/hematite correlate remarkably well with the mass accumulation rate of the total eolian component, suggesting that in some situations it can be used as a proxy indi

ISSN:0883-8305    

DOI:10.1029/PA003i001p00089

年代:1988

数据来源: WILEY

摘要:

As a limiting nutrient to marine life, phosphorus (P) is an effective tracer of today's marine productivity. The distribution of P in marine sediments likewise tracks the history of marine productivity because of its relative insolubility in seawater. CaCO3, biogenic opal, terrigenous sediment, and total P have been measured in cores from nine Pacific sites (Deep Sea Drilling Project (DSDP) 65, 66, 310, 77, 62, 572, 463, 586, and GPC‐3) and one subantarctic (DSDP 266) site. These sites were specifically chosen to provide information on biota burial flux changes with time for sedimentary sinks that represent key oceanographic variables, i.e., rate of upwelling, water depth, and carbonate dissolution gradient. The accumulation rates of these components for the last 10 Ma were then calculated from determined core age versus depth plots, core bulk density, and porosity data. The accumulation of P weakly correlates with that of CaCO3, moderately with that of total sediment, and very strongly with carbonate‐free accumulation. Two prominent peaks for all components occur at 2–3 Ma and 5–6 Ma, and record the chemical loading of dissolved CaCO3, SiO2, and P from glacially emergent continental shelves. These results indicate that continental shelf phosphorites form during interglacially high sea levels and correspond to low deep‐sea P accumulation rates, whereas glacially lowered sea levels allow for shelf bypassing and greater deep‐sea P accumul

ISSN:0883-8305    

DOI:10.1029/PA003i001p00113

年代:1988

数据来源: WILEY