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Showing papers in "Paleoceanography in 1988"


Journal ArticleDOI
TL;DR: In this paper, a detailed reconstruction of the geographic distribution of ∂13C in benthic foraminifera in the Atlantic Ocean during the last glacial maximum was presented.
Abstract: The degree of similarity of the ∂13C records of the planktonic foraminiferal species N. pachyderma and of the benthic foraminiferal genus Cibicides in the high-latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle. Whereas continuous formation of deep water is recognized in the southern ocean, the Norwegian Sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation. However, deep water formed in the north Atlantic south of the Norwegian Sea during the last climatic cycle as early as isotope substage 5d, and this area was also the only active northern source during stages 4–2. A detailed reconstruction of the geographic distribution of ∂13C in benthic foraminifera in the Atlantic Ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean, whereas the Glacial North Atlantic Deep Water cannot be traced south of 40°N. At shallower depth an oxygenated 13C rich Intermediate Water mass extended from 45°N to 15°S. In the Pacific Ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700–2600 m and is best explained by stronger formation of Intermediate Water in high northern latitudes.

1,065 citations



Journal ArticleDOI
TL;DR: Carbon isotopic measurements on the benthic foraminiferal genus Cibicidoides indicate that mean deep ocean δ13C values were 0.46 ‰ lower during the last glacial maximum than during the Late Holocene as mentioned in this paper.
Abstract: Carbon isotopic measurements on the benthic foraminiferal genus Cibicidoides document that mean deep ocean δ13C values were 0.46 ‰ lower during the last glacial maximum than during the Late Holocene. The geographic distribution of δ13C was altered by changes in the production rate of nutrient-depleted deep water in the North Atlantic. During the Late Holocene, North Atlantic Deep Water, with high δ13C values and low nutrient values, can be found throughout the Atlantic Ocean, and its effects can be traced into the southern ocean where it mixes with recirculated Pacific deep water. During the glaciation, decreased production of North Atlantic Deep Water allowed southern ocean deep water to penetrate farther into the North Atlantic and across low-latitude fracture zones into the eastern Atlantic. Mean southern ocean δ13C values during the glaciation are lower than both North Atlantic and Pacific δ13C values, suggesting that production of nutrient-depleted water occurred in both oceans during the glaciation. Enriched 13C values in shallow cores within the Atlantic Ocean indicate the existence of a nutrient-depleted water mass above 2000 m in this ocean.

565 citations


Journal ArticleDOI
TL;DR: Based on detailed reconstructions of global distribution patterns, both paleoproductivity and the benthic δ13C record of CO2, which is dissolved in the deep ocean, strongly differed between the Last Glacial Maximum and the Holocene.
Abstract: Based on detailed reconstructions of global distribution patterns, both paleoproductivity and the benthic δ13C record of CO2, which is dissolved in the deep ocean, strongly differed between the Last Glacial Maximum and the Holocene. With the onset of Termination I about 15,000 years ago, the new (export) production of low- and mid-latitude upwelling cells started to decline by more than 2-4 Gt carbon/year. This reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric CO2 as recorded in ice cores and, with a slight delay of more than 1000 years, to a large-scale gradual CO2 depletion of the deep ocean by about 650 Gt C. This estimate is based on an average increase in benthic δ13C by 0.4–0.5‰. The decrease in new production also matches a clear 13C depletion of organic matter, possibly recording an end of extreme nutrient utilization in upwelling cells. As shown by Sarnthein et al., [1987], the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades, which in turn was linked via a shrinking extent of sea ice to a massive increase in high-latitude insolation, i.e., to orbital forcing as primary cause.

487 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate that cadmium in benthic foraminiferal shells is related to the bottom water composition through a proportionality constant D ≃ 2.9.
Abstract: The oceanic distribution of cadmium resembles that of phosphorus. Because the cadmium content of foraminiferal shells is governed by the cadmium content of seawater, planktonic and benthic fossil shells can be used to infer nutrient distributions within ancient oceans. Empirical studies demonstrate that cadmium in benthic foraminiferal shells is related to the bottom water composition through a proportionality constant D ≃ 2.9. This constant is the same for each of the species studied: Cibicidoides wuellerstorfi, Cibicidoides kullenbergi, Nuttallides umbonifera, and Uvigerina.spp. Downcore cadmium data from high-quality Pacific and Atlantic sediment cores suggest that the cadmium inventory of the ocean did not change significantly between the most recent glacial maximum and the present. Hence changes in the cadmium content of fossils at a site directly reflect changes in nutrient distributions due to altered oceanic circulation patterns. Studies of cadmium in Pleistocene sediments show that deep ocean circulation patterns were significantly different during the most recent glacial maximum. In the western North Atlantic Ocean, the nutrient content of waters from 2500 to 3500 m was twice as high during glacials as during interglacial periods, signifying an increase in the proportion of waters of Antarctic origin in this depth interval and a decrease (but not a cessation) in the flux of North Atlantic Deep Waters through this depth range. Below 3500 m, the nutrient increase was greater, indicating a higher proportion of Antarctic Bottom Water. Above 2500 m, the nutrient content was lower during glacial periods than it has been during interglacial times. The Cd content of high-latitude North Atlantic and southern ocean surface waters does not show a significant glacial/interglacial change, which argues against theories for changes atmospheric carbon dioxide that require changes in high-latitude nutrient concentrations.

473 citations


Journal ArticleDOI
TL;DR: In this article, the Δ(14C atmosphere, 14C sea surface) tracer was used as a new paleoceanographic tracer which may provide additional information about high latitude surface waters complementary to those obtained with 13C/12C and Cd/Ca ratios measured in planktonic foraminifera.
Abstract: Carbon 14 dates obtained by accelerator mass spectrometry (AMS) on foraminiferal samples from deep-sea sediment cores must be corrected for the difference in 14C composition between the atmosphere and the sea surface. In the modern ocean, the “apparent age” of carbonate shells formed in surface waters varies between 300 and 1200 years and depends mainly on latitude. The time variation of this parameter during climate oscillations of the last 40,000 years may have been significant: there should have been small changes for most of the ocean between 40°S and 40°N, but an increase of the apparent age by several hundred years should be expected at high latitudes in response to subpolar/subtropical front movements. The North Atlantic is likely to have experienced the most significant changes, due to large variations in the mode and rate of North Atlantic Deep Water production. These hypothetical changes may be measured by coupled AMS 14C dating of contemporaneous planktonic foraminifera and terrestrial organic matter (pollen, charcoal, wood, etc.) which occur in the same core or are stratigraphically linked by the same volcanic ash layer. The Δ(14C atmosphere, 14C sea surface) can be viewed as a new paleoceanographic tracer which may provide additional information about high latitude surface waters complementary to those obtained with 13C/12C and Cd/Ca ratios measured in planktonic foraminifera.

471 citations


Journal ArticleDOI
TL;DR: In this paper, Accelerator Mass Spectrometry (AMS) radiocarbon results on two strategically located deep-sea cores were used to provide a chronology for surface water temperatures in the northern Atlantic and the other for the meltwater discharge from the Mississippi River.
Abstract: 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 sheet.

389 citations


Journal ArticleDOI
TL;DR: In this paper, the authors generated ∼300 Kyr records of biogenic opal, calcite, and organic carbon (Corg) for three cores in the eastern and central equatorial Pacific Ocean and compared the records to determine whether common periods of sedimentation have occurred throughout the region.
Abstract: 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 Corg has 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 Corg peak occurs at 18 ka and can have greater than 2 times the Holocene Corg content. Other major Corg peaks occur 150 ka and perhaps at 280 ka. We have compared the Corg record 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 Corg record 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 Corg record is primarily a measure of productivity. By comparing the opal, calcite, and Corg records in V19–28, a core which is at or above the lysocline, we found that both increased calcite and opal deposition matches high Corg accumulation. 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 must accompany order.

271 citations


Journal ArticleDOI
TL;DR: In this paper, a box model of the atmosphere and ocean was developed to investigate how geochemical distributions extant during the late Pleistocene may have come about, and the model predicts several changes that seem to be recorded in the sedimentary record, as follows: (1) a global redistribution of nutrients and 12C from the intermediate to deep water takes place with the Atlantic intermediate water phosphate decreasing 0.6 µmole kg−1 and the δ13C increasing 0.3 to 0.5
Abstract: A box model of the atmosphere and ocean was developed to investigate how geochemical distributions extant during the late Pleistocene may have come about. The model simulates the regional distribution of calcium carbonate dissolution as well as the chemical oceanography and atmospheric CO2, δ13C, and radiocarbon. If the downward biological flux of particulate carbon increases by a factor of 2 to 3 in the Antarctic and if this increase is combined with a relative increase of the Atlantic sector Antarctic Bottom Water (AABW) versus North Atlantic Deep Water (NADW) source ratio from 1∶3 to about 2∶1, then the model predicts several changes that seem to be recorded in the sedimentary record, as follows: (1) A global redistribution of nutrients and 12C from the intermediate to deep water takes place with the Atlantic intermediate water phosphate decreasing 0.6 µmole kg−1 and the δ13C increasing 0.3 to 0.5‰. (2) The dissolved oxygen level of the deep sea is reduced from an average of about 180 to 70 µmole kg−1, but the intermediate water oxygen declines only a small amount. (3) The decrease in intermediate water nutrient concentration results in lower average organic carbon and calcium carbonate production in the warm surface ocean. (4) The atmospheric CO2 decreases by 90 to 110 ppm. (5) Initially, a global increase in calcium carbonate dissolution occurs, which is followed by a relaxation toward better preservation than exists for the present ocean. In the model in this paper the reduction of NADW by itself does not produce these effects. Rather, the nutrient decrease that does occur is found mostly in North Pacific intermediate water, and the model atmospheric CO2 decrease is only 10 to 30 ppm. It is observed that 92% of the atmospheric CO2 change takes place according to a 200-year time constant in the model. This corresponds to the response time of the upper ocean and atmosphere to a change in the stationary state atmospheric PCO2. Thus, according to this model, the time lag between the nutrient-based cause and the atmospheric CO2 response is not expected to be particularly large.

204 citations


Journal ArticleDOI
TL;DR: In this article, the SPECMAP oxygen isotope master curve and carbonate content of late Quaternary sediment cores from the Senegal continental slope and the Sierra Leone Rise in the eastern equatorial Atlantic correlate strongly with the specular magnetic properties.
Abstract: 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 techniques.

178 citations


Journal ArticleDOI
TL;DR: Sedimentological, isotopic and magnetostratigraphic investigations of Ocean Drilling Program and Deep Sea Drilling Project sites 642, 643, 644 and 610 document the oceanographic and climatic evolution of the Norwegian Sea and the northeastern Atlantic over the last 2.8 m as discussed by the authors.
Abstract: Sedimentological, isotopic and magnetostratigraphic investigations of Ocean Drilling Program and Deep Sea Drilling Project sites 642, 643, 644 and 610 document the oceanographic and climatic evolution of the Norwegian Sea and the northeastern Atlantic over the last 2.8 m.y.. The results show that a major expansion of the Scandinavian Ice Sheet to the coastal areas took place at about 2.56 Ma. Relatively severe glacials appeared until about 2 Ma. The period 2.6 - 1.2 Ma experienced in general cold surface water conditions with only a weak influx of temperate Atlantic water as compared with late Quaternary interglacials. The Norwegian Sea was a sink of deep water through this period but deepwater ventilation was reduced and calcite dissolution was high compared with the Holocene. Deep water formed by other mechanisms than it does today. Between 2 and 1.2 Ma the glaciations in Scandinavia were small. A transition toward larger glacials took place during the period 1.2 to 0.6 Ma, corresponding to warmer interglacials and reduced calcite dissolution. Only during the last 0.6 m.y. has the oceanographic and climatic system of the Norwegian Sea varied in the manner described in previous studies of the late Quaternary. A strong thermal gradient was present between the Norwegian Sea and the northeastern Atlantic during the Matuyama (2.5–0.7 Ma). This is interpreted as a sign of a more zonal and less meridional climatic system over the region compared with the present situation.

Journal ArticleDOI
TL;DR: The authors examined the paleoceanography and paleogeography of the Barents Sea during the late Cenozoic glacial regime by seismostratigraphy and core analysis.
Abstract: We examine the paleoceanography and paleogeography of the Barents Sea during the late Cenozoic glacial regime by seismostratigraphy and core analysis. During the smaller glaciations which probably dominated the first period of the glaciogenic regime (approximately 2.5–0.8 Ma?), glaciomarine processes with sediment influx from the adjacent mainland prevailed. The Barents Sea has been covered by grounded ice sheets several (five to 10) times during the late Cenozoic. We speculate that these glaciations occurred during the last 0.8 Ma. During these larger glaciations the margin of the ice sheet expanded to the shelf edge, and the continental shelf break and slope acted as a depocenter and prograded rapidly. Frequent sediment gravity flows accumulated on the lower continental slope. During interglacials, the Barents Sea was converted to a starved continental margin. Most of the Barents Sea was glaciated during the maximum of the last glaciation (19–16 ka). A two-stepped deglaciation, 16 to 13 ka and 13 to 10 ka, occurred. Incipient Atlantic water reached the western areas about 13 ka and totally replaced the Arctic water by 10 ka. These paleoceanographic events were delayed in the eastern Barents Sea.

Journal ArticleDOI
TL;DR: A detailed nannofossil zonation has been derived for the Cenomanian-Turonian boundary interval consisting of three zones, two subzones, and nine additional biohorizons as discussed by the authors.
Abstract: A detailed nannofossil zonation has been derived for the Cenomanian-Turonian boundary interval consisting of three zones, two subzones, and nine additional biohorizons. This biostratigraphy, based on the investigation of numerous sections from Europe, North America, and Africa, allows accurate correlation of sections from shelf and deep-sea environments. The increased resolution helps solve several pertinent paleoceanographic and sedimentologic problems in this time period. Facies patterns are variable in this interval for numerous reasons. A widespread hiatus occurred in the latest Cenomanian and earliest Turonian in much of northwest Europe and may have been an indirect result of peak marine transgression. In parts of the deep Tethys, dissolution of carbonate took place in undersaturated bottom waters or during early diagenesis. Only a few boundary sections, mostly within the Western Interior, United States, are characterized by continuous carbonate deposition. In many sections a ∥13C excursion (which has been associated with the burial of large amounts of marine organic matter) coincides with a facies change, lower ratios of coccolith carbonate to micrite, and poorer nannofossil preservation. This excursion therefore may be strongly overprinted by diagenesis. Numerous nannofossil events indicate that the carbon excursion is slightly diachronous between sequences of the Western Interior, and that the shift occurred distinctly later in this region than in northwest Germany. The data obtained do not invalidate the concept of a Cenomanian-Turonian ocean-wide anoxic event but show how local tectonic, climatic, and oceanographic perturbations have masked the global scenario. In particular, they indicate that carbon shifts may be basinal and not always oceanic phenomena, a conclusion which can be rationalized with overall sluggish middle Cretaceous circulation. Nannofossil assemblages in almost all sequences observed contain exceptionally high abundances of two dissolution-resistant taxa in particular samples: Eprolithus floralis and Broinsonia sp.. However, the overall composition of assemblages does not change dramatically in the boundary interval, in contrast to the macrofossils and foraminifera, which suffered widespread extinctions.

Journal ArticleDOI
TL;DR: In this article, the spectral properties of the eolian grain size and the sea surface temperature were analyzed for the period from 402,000-774,000 years to the mid-Brunhes event.
Abstract: 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 records, but its cause remains unknown.

Journal ArticleDOI
TL;DR: In this article, two gravity cores collected from the central Panama Basin have been used to investigate whether oxygen was substantially depleted in the upper centimeter where Uvigerina spp. are thought to live during the last glacial period.
Abstract: The Stage 2 glacial maximum in eastern equatorial Pacific Ocean sediments is characterized by a pronounced increase in the organic carbon content which has been attributed either to increased upwelling and higher production along the equatorial divergence during the last glacial period or to enhanced preservation as a consequence of a lower concentration of dissolved O2 in glacial bottom water. Several lines of evidence are presented, based on data from two gravity cores collected from the central Panama Basin, which support the former interpretation. First, both the abundance and the size of benthic foraminifera of the genus Uvigerina increase in Stage 2, coincident with the increase in organic carbon concentration, most probably because of the greater availability of organic detritus during the glacial period. The increase in foraminiferal size and associated greater requirement of oxygen for respiration argues against the probability that oxygen was substantially depleted in the upper centimeter where Uvigerina spp. are thought to live. Second, the number of meiofaunal faecal pellets increases markedly during the Stage 2 maximum, indicating a more populous and active infauna, which again argues against oxygen depletion. Third, there is no significant decrease in the iodine:organic carbon (I:Corg) ratio in the Stage 2 sediments. Because iodine is depleted relative to carbon in anoxic basins but enriched in association with organic matter in oxic environments, the absence of a depletion in the I:Corg ratio in Stage 2 is evidence that bottom water remained oxygen- replete during the last glacial. Fourth, molybdenum is not enriched in sediments deposited in Stage 2; the presence of anoxic or dysaerobic bottom water or shallow pore waters during that time would have been recorded by the addition of Mo to the sediments via coprecipitation with authigenic iron sulphides. The combined faunal and geochemical data are consistent with the notion that Panama Basin deep waters remained oxygen-replete during the last glacial maximum. It is concluded that the late Stage 2 carbon enrichment in the eastern equatorial Pacific more probably reflects higher productivity rather than enhanced preservation.

Journal ArticleDOI
TL;DR: Gravity cores recovered from Manganese Nodule Project site H (6°33′N, 92°49′W) show marked downcore variations in the abundance of calcium carbonate, organic carbon, opal, manganese, and other components deposited over the past 400,000 years as mentioned in this paper.
Abstract: Gravity cores recovered from Manganese Nodule Project site H (6°33′N, 92°49′W) show marked downcore variations in the abundance of calcium carbonate, organic carbon, opal, manganese, and other components deposited over the past 400,000 years. Variations in the downcore abundance of organic carbon, which ranges from 0.2 to 1.0%, can be used to hindcast redox conditions in the surface sediments over this time. The results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles. Downcore variations in solid phase Mn, Ni, and Cu can be produced by such changes in redox conditions. A model which predicts that solid phase Mn can be trapped and buried when the Mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and Mn records and supports the reconstructed redox variations. The history of redox variations at site H can be explained by changes with time in surface water productivity. Major productivity variations at the site occur over 100-kyr cycles, with relatively higher productivity occurring during glacial stages. Thus Quaternary climate changes influence surface water productivity, redox conditions in sediments, and the cycling of transition metals.

Journal ArticleDOI
TL;DR: In this article, an approach which integrates four independent data sets, magnetostratigraphy, seafloor spreading magnetic lineation patterns, biostratigations, and isotopic ages, is presented.
Abstract: Geochronology is the conceptual division of continuous time as measured (geochronometry) by the progression in an ordinal series of events. This is best achieved by an approach which integrates four independent data sets: magnetostratigraphy, seafloor spreading magnetic lineation patterns, biostratigraphy, and isotopic ages. This integrated approach results in an ordinal framework which can measure time with greater resolution, though perhaps less accuracy, than a radiometric approach alone. A comparative analysis of two recently proposed Paleogene geochronologic time scales is presented.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the role of sedimentary environment surface area in predicating the differential partitioning of finite carbonate fluxes to Phanerozoic global oceans.
Abstract: Sites of calcium-magnesium carbonate accumulation have been shifting from shallow shelf to deep marine settings since the late Cretaceous. During this time, total area of flooded epicratonic shelf has decreased about 50%, and areas of shelf carbonate deposition have declined from 27 × 106 km² to present areas of 1.4 × 106 km² while the maximum extent of platform carbonate accumulation has decreased from about 50° to 28° north latitude. In terms of cratonic carbonate, this decrease in tropical shelf area has resulted in a sixfold reduction in shoal-water limestone accumulation from a Cretaceous rate of 0.6 × km³/m.y. to a present value of about 0.1 × 106 km³/m.y.. Over the same time period, the global carbonate compensation depth has deepened by about 1.5 km, attendant with a threefold areal increase of calcite ooze depositionn (40 to 130 × 106 km²). This expansion has resulted in an eightfold increase in pelagic limestone accumulation from a Cretaceous rate of 0.05 × 106 km³/m.y. to a present rate of about 0.4 × 106 km³/m.y. Transfer of accumulation from shallow cratonic to deep marine depositional settings records the central role of sedimentary environment surface area in predicating the differential partitioning of finite carbonate fluxes to Phanerozoic global oceans.

Journal ArticleDOI
TL;DR: In this article, radiocarbon ages for benthic and planktonic foraminifera from the late glacial sections of two Atlantic and two Pacific cores are reported.
Abstract: Radiocarbon ages for benthic and planktonic foraminifera from the late glacial sections of two Atlantic and two Pacific cores are reported. The differences for benthic-planktonic pairs suggest that the radiocarbon age for deep Atlantic water was somewhat larger than today's (i.e., 600±250, as opposed to 400 years) and that the radiocarbon age for deep Pacific water was also slightly larger than today's (2100±400, as opposed to 1600, years). Our results suggest that during glacial time, the deep Pacific was, as it is today, significantly depleted in radiocarbon relative to the deep Atlantic. As many questions remain unanswered regarding the reliability of this approach, these conclusions must be considered to be preliminary.

Journal ArticleDOI
TL;DR: In this article, the authors improved upper Eocene to Oligocene deep-sea chronostratigraphic control by integrating isotope (87Sr/86Sr, δ18O, Δ13C) stratigraphy and magnetostratigraphy.
Abstract: We improved upper Eocene to Oligocene deep-sea chronostratigraphic control by integrating isotope (87Sr/86Sr, δ18O, δ13C) stratigraphy and magnetostratigraphy. Most previous attempts to establish the timing of isotope fluctuations have relied upon biostratigraphic age estimates which have uncertainties of 0.5 to over 4.0 m.y. Deep Sea Drilling Project (DSDP) Site 522 contains the best available upper Eocene to Oligocene magnetostratigraphic record which allows first-order correlations of isotope records (87Sr/86Sr, δ18O, δ13C) to the Geomagnetic Polarity Time Scale (GPTS). Empirical calibrations between the 87Sr/86Sr of foraminifera and magnetochronology at Site 522 allow more precise correlation of “unknown” samples with the GPTS. For example, shallow water and high-latitude sections may be tied into the deep-sea record. Sr-isotope stratigraphic resolution for the latest Eocene to Oligocene is approximately 2 m.y.

Journal ArticleDOI
TL;DR: In this article, the postdepositional particle flux (accumulation rate) over the last 5100 years was studied in the anoxic sediment units I and II from the central western Black Sea.
Abstract: The postdepositional particle flux (accumulation rate) over the last 5100 years was studied in the anoxic sediment units I and II from the central western Black Sea. Deposition of unit II (sapropel) in the deep Black Sea basin was divided further into two subunits. The beginning of the deposition of the sapropel in the Black Sea (5100–3100 years B.P., subunit IIb) was characterized by higher organic carbon accumulation rates due largely to higher primary production. Accumulation rates of lithogenic matter were at a minimum compared to younger units. During the subsequent 2100 years (3100–1000 years B.P., subunit IIa) organic carbon accumulation rates decreased considerably while accumulation rates of lithogenic matter increased to the present level. At 1000 years B.P. (unit I), total mass accumulation rates increased by a factor of 3. This increase was largely a result of an increase in carbonate accumulation rates by a factor of 11, which was caused by the rapid appearance of Emiliania huxleyi in the Black Sea. Higher carbonate accumulation rates in unit I are mainly the cause for the considerably lower organic carbon content compared to unit II, since the organic carbon accumulation rate in unit I did not decrease. General correspondence of accumulation rates of lithogenic matter in different cores through time suggests overregional long-term (100-year scale) climatic effects on the terrigenous particle supply to the Black Sea.

Journal ArticleDOI
TL;DR: An analysis of diatom assemblages within surface sediments from McMurdo Sound, Antarctica, document the regional circulation patterns in this large embayment in the southwestern Ross Sea as mentioned in this paper.
Abstract: Analyses of diatom assemblages within surface sediments from McMurdo Sound, Antarctica, document the regional circulation patterns in this large embayment in the southwestern Ross Sea. Thalassiosira spp., indicative of water column primary productivity, is most common in eastern and portions of northwestern McMurdo Sound as a result of its advection into these regions from areas of open water primary productivity. Surface sediments from southwestern McMurdo Sound are composed mainly of Nitzschia curta, a small pennate form commonly found both as a member of the sea ice microbial community as well as a dominant component in ice edge blooms in the Ross Sea. The northward advection of oligotrophic water into the western Sound from beneath the McMurdo Ice Shelf results in the dominance of an autochthonous flora in the southwestern Sound. Sediments deposited during the past 500 years also have been dominated by Thalassiosira spp. and Nitzschia curta. Thalassiosira spp. concentrations reached a maximum between 1600 and 1875 A.D., corresponding to the time of the “Little Ice Age.” More persistent winds may have been responsible for more prevalent polynyas at that time, resulting in greater amounts of open water primary productivity and subsequent higher percentages of Thalassiosira spp. Increased areal extent and/or duration of coastal polynyas during the “Little Ice Age” suggests that within the southwestern Ross Sea, the production of High Salinity Shelf Water, and hence Antarctic Bottom Water, may have been greater at that time.

Journal ArticleDOI
TL;DR: In this article, five lines of concrete, quantitative evidence about oceanic chemical composition during the past 65 m.y.s. can provide insights into the controlling processes and their temporal variations, but the system constraints are not yet fully understood.
Abstract: Five lines of concrete, quantitative evidence about oceanic chemical composition during the past 65 m.y. can provide insights into the controlling processes and their temporal variations, but the system constraints are not yet fully understood. The calcium carbonate compensation depth, mean carbon isotopic ratio, and strontium isotopic ratio have all changed significantly during this time, while changes in lithium-to-calcium and strontium-to-calcium ratios have been much smaller. When these tracers are considered separately, simple scenarios of changes in geochemical and geological factors can be used to explain the historical records. These factors include: (1) increases in continental chemical weathering rates and net continental fluxes to the oceans, (2) changes in the isotopic composition of material weathered from continents to the oceans, (3) decreases in the proportion of calcium carbonate deposited on continental shelves, (4) changes in the deposition or weathering rates of certain types of sediments, and (5) changes in hydrothermal circulation. Because these geochemical cycles are linked to each other (and to others, such as nutrients and global atmospheric carbon dioxide), an explanation invoked for one tracer must satisfy the direction, magnitude, and timing of the patterns for all the tracers. Several simple scenarios for geochemical variations have been evaluated for their consequences (both intended and unintended) and compared to the historical records. Despite the attractiveness and possible justifications of some simple hypotheses, for example, decreases in organic carbon burial rates, increases in carbonate weathering rates on the continents, or increases in all continental weathering rates, no one simple scenario appears to be consistent with both the timing and magnitude of the changes observed during the past 65 m.y. for all tracers considered. The contradictions uncovered can be useful in defining what knowledge is crucial to further understanding of oceanic chemical history. Better knowledge is needed of absolute deep-ocean sediment accumulation rates and their temporal variability, of the extent and time scale of variability in the net carbon isotope input to the ocean from continental weathering, of steady state fluxes for oceanic mass balances and the extent to which these mass balances approach steady state behavior, and of the links and interactions between the geochemical cycles of various elements.

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TL;DR: In this article, a rock-magnetic study of core LL44-GPC3 was carried out to determine whether rockmagnetic 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.
Abstract: 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 indicator for eolian activity.

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TL;DR: In this article, the authors address the aliasing problem relevant to the search for orbital variations recorded in sedimentary sections, and illustrate three cases of aliasing, i.e., aliasing of the 23,000-year period orbital precession rhythm which is common in late Quaternary paleoclimatic records.
Abstract: Aliasing of a time series shifts high-frequency variance into lower frequencies. It is caused by sampling at an interval too broad to resolve true high-frequency signals. This effect is well understood in classical time series analysis, where sample intervals are known and constant, but in a geologic context, where sample intervals may vary, aliasing remains poorly known. We address here the aliasing problem relevant to the search for orbital variations recorded in sedimentary sections. Our example is aliasing of the 23,000-year period orbital precession rhythm which is common in late Quaternary paleoclimatic records. We illustrate three cases of aliasing. First, we sample precession at a constant interval of 25,000 years. This is a typical target for many “high-resolution” studies of Neogene sections. This sampling interval translates precessional variance into predictable spectral peaks near 400 ka and 100 ka, which could be mistaken for the longer-period eccentricity rhythms. Second, random variations in the sampling interval around the 25,000-year target spread the aliased variance over a range of frequencies. This induces either unpredictable long-period spectral peaks or, in the extreme, a white noise spectrum. In the third case, variations of the sampling interval are autocorrelated. This simulates a section with long-period variations in sedimentation rate sampled at constant depth intervals. Here the single aliased peaks of case 1 are split into two or more peaks of slightly higher and lower frequencies. In all three cases, for long enough time series, the total variance recorded is the same. We compare these numerical experiments to a Miocene oxygen isotope record from Deep Sea Drilling Project site 577, sampled at ∼25,000-year intervals. With these data it is impossible to tell whether the long-period variations are due to the direct effects of eccentricity or the aliased effects of precession. In theory it should be possible to test for eccentricity signals at low resolution by randomizing the sampling intervals. In practice, however, it is only through high-density sampling that we can define intervals well enough to assess the effects of aliasing.

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TL;DR: In this article, the assumption of synchrony of first and last occurrences of fossil taxa can be tested using graphic correlation procedures which, by allowing measured stratigraphic sections to be compared on a common depth scale, make it possible to develop a correlation model which integrates information from a number of cores.
Abstract: The assumption of synchrony of first and last occurrences of fossil taxa can be tested using graphic correlation procedures which, by allowing measured stratigraphic sections to be compared on a common depth scale, make it possible to develop a correlation model which integrates information from a number of cores The strategy of the test presented here is to use a graphic correlation model that is based on data from the Atlantic (Deep Sea Drilling Project (DSDP) sites 502, 516A) and north Pacific (DSDP site 577A) as a basis for determining to what extent fossil datums in the southwest Pacific are synchronous First and last occurrences of Pliocene calcareous nannofossils and planktonic foraminifers have been compared in five DSDP cores from the southwest Pacific ocean (sites 586, 587, 588, 590A, and 592) All cores were recovered using hydraulic piston coring technology, which assures the best recovery and minimal disturbance Most of these cores contain abundant, well-preserved foraminifers and nannofossils, as well as a partial record of many of the expected magnetic polarity reversals in this part of the section To assure taxonomic consistency, all taxonomic identifications were made by the author Graphic correlation of this data set suggests that several important biostratigraphic markers are highly diachronous For example, this study confirms that Globorotalia truncatulinoides first occurs at approximately 24 Ma between 20° and 35° south latitude in the southwest Pacific, approximately 05 my earlier than it is found elsewhere in the Atlantic and Pacific Other datums, such as the last occurrence of Discoaster brouweri, are essentially synchronous These findings suggest that biostratigraphic models based on the assumption of synchrony of first and last occurrences of fossil taxa may be incorrect Biostratigraphic models created with the Graphic Correlation method offer an opportunity to examine the biogeographic dimensions of origination, migration, and extinction of planktonic taxa

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TL;DR: In this article, the authors discuss the possible causes for these differences and attempt to evaluate their impact on the interpretation of benthic-planktonic age differences and evaluate the impact of these differences on the ventilation rate of planktonic foraminifera.
Abstract: As a test of the reliability of paleocean ventilation rates reconstructed from radiocarbon age differences between planktonic and benthic foraminifera, measurements have been made on coexisting species of planktonic foraminifera While ideally no differences should exist, we do find them In this paper we discuss the possible causes for these differences and attempt to evaluate their impact on the interpretation of benthic-planktonic age differences

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TL;DR: In this paper, the use of glauconite as a chronometer for geologic time scale studies has been discussed, and it is argued that if the geohistory of the sample in question is known in detail, one can ascertain before hand that the sample will yield a radiometric result that is accurate in the geological sense.
Abstract: There are advocates of the use of glauconite as a chronometer for geologic time scale studies who insist that if the geohistory of the sample in question is known in detail, one can ascertain before hand that the sample will yield a radiometric result that is accurate in the geological sense. Examples are presented indicating that while some glauconite samples may yield the correct age within their assigned analytical uncertainties when compared to high-temperature minerals, others clearly do not and are too young by varying degrees. The outright acceptance of published results on glauconite can result in erroneous age estimates of various biostratigraphic levels and stage or epoch boundaries.

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TL;DR: The mean grain size of the detrital silt fraction was analyzed in two piston cores from the crest of the Blake Outer Ridge using electronic particle analysis as discussed by the authors, and the resultant profiles in the two cores are highly correlatable and are interpreted to reflect the paleospeed of the Western Boundary Undercurrent (WBUC) for the past 90 kyr.
Abstract: The mean grain size of the detrital silt fraction was analyzed in two piston cores from the crest of the Blake Outer Ridge using electronic particle analysis. The resultant profiles in the two cores are highly correlatable and are interpreted to reflect the paleospeed of the Western Boundary Undercurrent (WBUC) for the past 90 kyr. The record can be divided into three periods. The first encompasses the past 12,000 years and begins with an abrupt acceleration marking the transition from slow, glacial to more vigorous, Holocene conditions. The later Holocene is a period of strong but variable flow. The second period extends from 30 to 12 × 10³ years and is characterized by a steady reduction of WBUC strength through the last ice age. The current continued to decelerate until 12 × 10³ years when the polar front finally retreated northward past the passages connecting the Norwegian Sea to the north Atlantic. The third period, from 30 to about 90 × 10³ years, was a time of strong and variable flow that was independent of the glacial/interglacial signal that is seen in the oxygen isotope record. During this time, the core of maximum flow appears to have shifted up and down the flank of the Blake Outer Ridge and may have been influenced as much by salinity as by temperature variations in the source areas of the WBUC.

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TL;DR: The seawater curve for the Cenozoic is subdivided into three segments: Quaternary to mid-Miocene, mid-miocene to late Eocene, and late eocene to early Paleocene as mentioned in this paper.
Abstract: The strontium isotopic ratio (87Sr/86Sr) in seawater changes slowly over geologic time. This variation is caused by changes in the relative contribution of Sr from various isotopically distinct sources within the crust. The most important of these are high-ratio sialic rocks from continents and low-ratio mafic volcanic and mafic intrusive rocks from continental margins and ocean basins. A plot of Sr isotope ratio versus age for Phanerozoic marine samples produces a curve exhibiting many episodes of increasing and decreasing values. This variation can be used as a basis for temporal correlation of marine carbonate, sulfate, and phosphate sediments. Temporal correlations can be made between high-latitude and low-latitude sequences, deepwater and shallow-water sequences, and normal-marine and restricted-marine (hypersaline/hyposaline) sequences. Satisfactory biostratigraphic correlations between such sequences are often hampered by either the absence of age-diagnostic fossils or by the provinciality of faunal and floral assemblages. Rapid change that took place in the 87Sr/86Sr of seawater during most of the Cenozoic makes this era particularly well suited for precise temporal correlation. The seawater curve for the Cenozoic is subdivided into three segments: Quaternary to mid-Miocene, mid-Miocene to late Eocene, and late Eocene to early Paleocene. The mid-Miocene to late Eocene curve segment exhibits a particularly steep slope, making this a promising interval for high-resolution stratigraphic correlation. Although current data generally support the present configuration of the seawater curve, some revision of the curve is probably required in the vicinity of the Oligocene-Eocene boundary. Establishment of the general configuration of the seawater curve for the Cenozoic has promoted efforts to refine the curve on the basis of construction of detailed Sr isotope profiles within individual stratigraphic sequences. A Sr isotope profile at Deep Sea Drilling Project (DSDP) site 590B suggests a complex Neogene seawater curve characterized by minor slope changes in the Pliocene and Miocene. These slope changes are not specifically identified in the seawater curve constructed from multilocation data. On the basis of this more complex curve, and in the absence of diagenetic complications, the ultimate Neogene stratigraphic resolution is estimated to range from 0.1 to 2 million years. Both the verification and the general stratigraphic applicability of this more complex Neogene curve are largely dependent on the degree of preservation of the original seawater ratio in marine samples.