摘要:

The standard model of Pleistocene climate fluctuations being driven by orbital insolation changes has been challenged by results from a high‐precision, radiometrically dated deposit from Devils Hole, Nevada. Although the Devils Hole record agrees with a previously derived ice volume record for most of the last 500,000 years, clear indication of warming prior to the orbitally dated onset of the last interglacial forms the prime basis for this conclusion. I suggest that “early warming” at Devils Hole needs to be understood in light of the prior very extensive glaciation at 150,000 B.P. Marine records in both the North Pacific and North Atlantic, from the same latitude as Devils Hole, indicate that the subpolar fronts during the penultimate glaciation (150,000 years ago) were 5° father south than during the last glaciation. Continental glaciation may also have been more extensive at this time. The first phase of ice retreat ∼ 145,000 years ago is associated with a northward shift of these fronts, which results in full‐interglacial sea surface temperatures in the North Atlantic. The early warming at Devils Hole may reflect the northward movement of the ice margins and associated fronts. If so, both the Devils Hole and deep‐sea chronologies may be correct, with differences between 130,000 and 150,000 years ago reflecting a regionally varying response to climate shifts associated with a very extensiv

ISSN:0883-8305    

DOI:10.1029/93PA02558

年代:1994

数据来源: WILEY

摘要:

Sediment cores from the southern continental margin of Australia are near the formation region of Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water and record the changes in these water masses from the last glacial maximum through the present. Carbon and oxygen isotopes were measured on the benthic foraminiferal speciesPlanulina wuellerstrorfifor both the Recent and last glacial maximum sections of the cores and were then used to reconstruct temperature and carbon isotopic water column profiles. The glacial oxygen isotope profile indicates a vertical temperature structure for this region similar to that in today's Subantarctic Zone. Although intermediate water δ13C cannot be used as a nutrient tracer in this region because of the large influence of air‐sea carbon isotopic exchange on this water mass, δ13C can be used as a water mass tracer. Today, AAIW properties reflect contributions from cool, fresh Antarctic Surface Waters (2/3) and warm, salty waters from the Indian Ocean (1/3). When examined in conjuction with the glacial δ13C and δ18C data from the north Indian and Southern Oceans, our data suggest a much reduced contribution of North Indian Ocean intermediate water to glacial Antarctic Intermediate Water relative to the contribution of Antarctic Surface Water. This fresher, cooler glacial Antarctic Intermediate Water would be distributed to the intermediate‐depth ocean, thus decreasing the transport of salt produced in the North Indian Ocean to the rest of the world's oceans. Combined with evidence for a reduced influence of North Atlantic Deep Water, these results suggest major changes in the pathways for the redistribution of heat and salt in the glacia

ISSN:0883-8305    

DOI:10.1029/93PA02446

年代:1994

数据来源: WILEY

摘要:

A recently developed technique for determining past sea surface temperatures (SST), based on an analysis of the unsaturation ratio of long chain C37methyl alkenones produced byPrymnesiophyceaephytoplankton (U37k′), has been applied to an upper Quaternary sediment core from the equatorial Atlantic. U37k′temperature estimates were compared to those obtained from δ18O of the planktonic foraminiferGlobigerinoides sacculiferand of planktonic foraminiferal assemblages for the last glacial cycle. The alkenone method showed 1.8°C cooling at the last glacial maximum, about 1/2 to 1/3 of the decrease shown by the isotopic method (6.3°C) and foraminiferal modern analogue technique estimates for the warm season (3.8°C). Warm season foraminiferal assemblage estimates based on transfer functions are out of phase with the other estimates, showing a 1.4°C drop at the last glacial maximum with an additional 0.9°C drop in the deglaciation. Increased alkenone abundances, total organic carbon percentage and foraminiferal accumulation rates in the last glaciation indicate an increase in productivity of as much as 4 times over present day. These changes are thought to be due to increased upwelling caused by enhanced winds during the glaciation. If U37k′estimates are correct, as much as 50–70% (up to 4.5°C) of estimated δ18O and modern analogue temperature changes in the last glaciation may have been due to changes in thermocline depth, whereas transfer functions seem more strongly influenced by seasonality changes. This indicates these estimates may be influenced as strongly by other factors as they are by SST, which in the equatorial Atlantic was only reduced slightly in the

ISSN:0883-8305    

DOI:10.1029/93PA02198

年代:1994

数据来源: WILEY

摘要:

The present upper water mass of the northeastern Pacific Ocean off California has a well‐developed oxygen minimum zone between 600 and 1200 m wherein concentrations of dissolved oxygen are less than 0.5 mL/L. Even at such low concentrations of dissolved oxygen, benthic burrowing organisms are abundant enough to thoroughly bioturbate the surface and near‐surface sediments. These macro organisms, together with micro organisms, also consume large quantities of organic carbon produced by large seasonal stocks of plankton in the overlying surface waters, which are supported by high concentrations of nutrients within the California Current upwelling system. In contrast to modern conditions of bioturbation, laminated sediments are preserved in upper Pleistocene sections of cores collected on the continental slope at water depths within the present oxygen minimum zone from at least as far north as the California‐Oregon border and as far south as Point Conception. Comparison of sediment components in the laminae with those delivered to sediment traps as pelagic marine “snow” demonstrates that the dark‐light lamination couplets are indeed annual (varves). These upper Pleistocene varved sediments contain more abundant lipid‐rich “sapropelic” (type II) organic matter than the overlying bioturbated, oxidized Holocene sediments. The baseline of stable carbon isotopic composition of the organic matter in these slope cores does not change with time, indicating that the higher concentrations of type II organic matter in the varved sediments represent better preservation of organic matter rather than any change in the source

ISSN:0883-8305    

DOI:10.1029/93PA02829

年代:1994

数据来源: WILEY

摘要:

Late Pleistocene signals of calcium carbonate, organic carbon, and opaline silica concentration and accumulation are documented in a series of cores from a zonal/meridional/depth transect in the equatorial Atlantic Ocean to reconstruct the regional sedimentary history. Spectral analysis reveals that maxima and minima in biogenous sedimentation occur with glacial‐interglacial cyclicity as a function of both (1) primary production at the sea surface modulated by orbitally forced variation in trade wind zonality and (2) destruction at the seafloor by variation in the chemical character of advected intermediate and deep water from high latitudes modulated by high‐latitude ice volume. From these results a pattern emerges in which the relative proportion of signal variance from the productivity signal centered on the precessional (23 kyr) band decreases while that of the destruction signal centered on the obliquity (41 kyr) and eccentricity (100 kyr) periods increases below ∼3600‐m ocea

ISSN:0883-8305    

DOI:10.1029/93PA02901

年代:1994

数据来源: WILEY

摘要:

The distribution of living (rose bengal stained) deep‐sea benthic foraminifera was determined in the upper 20 cm of sediments of eight Soutar box cores taken from two depth transects (510‐4515 m) in the thermospheric (>10°C) Sulu Sea. Despite the uniformity of bottom water temperatures, salinities, and dissolved oxygen levels below 1000 m, significant faunal differences exist at different depths in the low‐oxygen (∼1.25 mL/L below 1000 m) basin. The shallowest site (510 m) is dominated (>10% of the calcareous fauna) byCibicidoides, Uvigerina, (>150 µm) andBolivina(>63 µm), whileSiphoninais codominant withCibicidoidesandUvigerinain the 1005‐m core. The 2000‐m cores are dominated byCibicidoides, Gyroidinoides, andOridorsalis, whileCibicidoides bradyiandOridorsalis umbonatusdominate the 3000‐ and 4000‐m cores. Infaunal assemblages ofValvulineria mexicanaare found in the sediments of the 4515‐m core. Relatively low bottom water oxygen values do not necessarily yield "typical low‐oxygen taxa" such asBolivina, Uvigerina, Chilostomella, Bulimina, andGlobobulimina. Changes in the abundances of these taxa in fossil assemblages have been used as indicators of changes in ancient bottom water oxygen levels but may instead reflect organic carbon contents of the sediments. An examination of the vertical distributions of foraminiferal assemblages from the>63‐µm and>150‐µm fractions reveals that taxa have microhabitat preferences similar to those observed in other regions. Taxa found in the upper 0‐ to 1‐cm interval (epifaunal) includeCibicidoides wuellerstorfiandHoeglundina elegans, while taxa such asChilostomellaandGlobobuliminareach maximum abundances in subsurface sediments and have infaunal microhabitat preferences.Cibicidoides bradyiandO. umbonatuslive in sediment depths from 0‐ to 4‐cm and have transitional preferences with both epifaunal and infaunal occurrences. Intrageneric differences in test morphologies, including pore distribution, rounded peripheries, and variable spire height, are observed inCibicidoidesandGyroidinoidesand are suggested to be related to microhabitat preferences. Vertical distributions of a number of taxa found in both the 63‐ to 150‐µm and>150‐µm fractions are similar, suggesting that juveniles and adults live under similar microhabitat conditions. Ontogenetic changes in microhabitat preferences of most species are not observed in this study and therefore would not be expected to account for isotopic vital effect

ISSN:0883-8305    

DOI:10.1029/93PA02327

年代:1994

数据来源: WILEY

摘要:

Geochemical analyses as well as X ray diffraction measurements were carried out on five sediment cores from the eastern Angola Basin and the equatorial divergence of the South Atlantic. Barite concentrations were calculated from total barium concentrations by subtracting the estimated barium background supplied by “nonbarite” barium carriers. Barite concentrations assessed by this geochemical method show a good correspondence to barite concentrations determined by quantitative X ray diffraction measurements. Barite proved to be an important carrier of barium in the pelagic cores, contributing up to 90% of the total barium concentrations in the sediment, while clastic material provides an important source of barium at sites closer to the African continent. Barite accumulation rates were calculated in order to eliminate the diluting effects of varying inputs of terrigenous and biogenic material. Barite accumulation rates show cyclic variations with maxima corresponding to glacial and minima to interglacial stages. Absolute paleoproduction rates were computed from barite accumulation rates. At the Congo fan and the equatorial divergence they are consistent with calculations based on total organic carbon (TOC) accumulation. At the Walvis Ridge, glacial‐interglacial cycles contrast to a constant paleoproductivity computed from TOC accumul

ISSN:0883-8305    

DOI:10.1029/93PA02559

年代:1994

数据来源: WILEY

摘要:

Decomposition of organic matter combined with density stratification generate a pronounced intermediate water oxygen minimum zone (OMZ) in the northwest Indian Ocean. This zone currently lies between water depths of 200 and 2000 m and extends approximately 5000 km southeast from the Arabian coast. Based upon benthic foraminiferal assemblage changes, it has been suggested that this OMZ was even more extensive during the late Miocene‐early Pliocene (6.5–3.0 Ma), with a maximum volume and/or intensity at approximately 5.0 Ma. While this inference may contribute to an understanding of the history of northwest Indian Ocean upwelling, corroborating geochemical evidence for this interpretation has heretofore been lacking. Ocean Drilling Program (ODP) sites 752, 754, and 757 on Broken and Ninetyeast ridges are located within central Indian Ocean intermediate water depths (1086–1650 m) but outside the present lateral dimensions of the Indian Ocean OMZ. High‐resolution chemical analyses of sediment from these sites indicate significant reductions in the flux of Mn and normalized Mn concentrations between 6.5 and 3.0 Ma that are most pronounced at approximately 5.0 Ma. Because late Miocene‐Pliocene paleodepths for these sites were essentially the same as at present and because extremely low sedimentation rates (0.3–1.3 cm/ky) most likely precluded sedimentary metal oxide diagenesis, we suggest that the observed Mn depletions reflect diminished deposition of reducible Mn oxyhydroxide phases within O2deficient intermediate waters and that this effect was most intense at approximately 5.0 Ma. This interpretation implies that waters with less than 2.0 mL/L O2extended at least 1500 km beyond their present limits and is consistent with changes in benthic foraminifera assemblages. We further suggest this expanded Indian Ocean OMZ is related to regionally and/or globally increased biological p

ISSN:0883-8305    

DOI:10.1029/93PA02699

年代:1994

数据来源: WILEY