Age models of sediment cores from the North-West African continental margin

A statistical analysis ol 15 deep sea cores in the eastern North Atlantic off NW Africa revealed the typical fluctuation pattern of distinct species proups as has been described from various parts of the world ocean. Only the "WBF-group" appears to be correlated with global climatic changes, i.e. warmer periods as the Eemian and the Atlanticum. A partly antagonistic "High Productivity group" (HPR-group) is in general not linked with global changes but times of increased fertility in the surface water and the resulting flux of organic matter reaching the bottom. The groups were extracted from cluster analysis of more than 150 surface samples (HPR-group) and a factor analysis of selected cores (WBF-group). In contrast to previous studies the observed fluctuations can not be explained by drastic changes in bottom water masses, but by the pulsation of a distinct "High Productivity Patch" in space and time. At present, this patch is located below the well known upwelling area between 22° and 12° northern latitude. It shifted to the north (up to 27 °N) during the latest glacial period ( 18 ky), indicating an equivalent shift of upwelling productivity caused by advection of nutrient rich upwelling SACW-waters, probably during most of isotopic stages 2 and 3.

Benthic foraminifera of late Quaternary sediments in the northern North Atlantic

Four long sediment cores from locations in the Framstrait, the Norwegian-Greenland Seas and the northern North Atlantic were analysed in a high resolution sampling mode (1 - 2 cm density) for their benthic foraminiferal content. In particular the impact of the intense climatic changes at glacial/interglacial transitions (terminations I and II) on the benthic community have been of special interest. The faunal data were investigated by means of multivariate analysis and represented in their chronological occurence. The most prominent species of benthic foraminifera in the Norwegian-Greenland Seas are Oridorsalis umbonatus, Cibicidoides wuellerstorfi, the group of Cassidulina, Pyrgo rotalaria, Globocassidulina subglobosa and fragmented tubes of arenaceous species. The climatic signal of termination I as well as termination II is recorded in the fossil foraminiferal tests as divided transition from glacial to interglacial. The elder INDAR maximum (individuals accumulation rate = individuals/sq cm * 1.000 y; Norwegian-Greenland Seas: average 3.000 - 6.000 individuals/sq cm * 1.000 y; northern North Atlantic: average 150 individuals/sq cm * 1.000 y) is followed by a period of decreased values. The second, younger maximum reaches comparable values as the elder maximum. The interglacial INDAR are in average 700 individuals/sq cm * 1.000 y in the Norwegian-Greenland Seas and 200 individuals/sq cm * 1.000 y in average in the northern North Atlantic. The occurence of the elder INDAR maximum shows a distinct chronological transgressivity between the northern North Atlantic (12.400 ybp.) and the Framstrait (8.900 ybp.). The time shift from south to north amounts 3.500 yrs., the average expanding velocity 0,78 km per year. Within the Norwegian-Greenland Seas the average expanding velocity amounts 0,48 km per year. This chronological transgressivity is interpreted as impact of the progressive expanding of the North Atlantic and the Norwegian Current during the deglaciation. The dynamic of the faunal development is defined as increasing INDAR per time. The elder INDAR maximum shows in both glacial/interglacial transitions an exponential increase from south to north. Termination II is characterized by a general higher dynamic as termination I. By means of the high resolution sampling density the impact of regional isotopic recognized melt-water events is recognized by an increase of endobenthic and t-ubiquitous species in the Norwegian-Greenland Seas sediments. During termination I the relative minimum between both INDAR maxima occur chronological with an decrease of calculated sea surface temperatures. This is interpreted as indication of the close pelagic - benthic coupling. The climatic signal in the northern North Atlantic recorded in the fossil benthic foraminiferal community shows a lower amplitude as in the Norwegian-Greenland Seas. The occurence of the epibenthic Cibicidoides wuellersforfi allows to evaluate the variability of the bottom water mass. In general at all core locations increasing lateral bottom currents are recognized with the occurence of the second younger INDAR maximum. In comparison with various paleo-climatological data sets fossil benthic foraminifers show a distinct koherence with changes of the atmospheric temperatures, the SSTs and the postglacial sea level increase. The benthic foraminiferal fauna is bound indirectly on and indicative for regional climatic changes, but principal dependent upon global climatic changes.

Sea surface temperture reconstruction and planktonic foraminifera of ODP Hole 117-723A

In the western Arabian Sea (WAS), the highest seasonal sea surface temperature (SST) difference presently occurs between May and August. In order to gain an understanding on how monsoonal upwelling modulates the SST difference between these two months, we have computed SST for the months of May and August based on census counts of planktonic foraminifers by using the artificial neural network (ANN) technique. The SST difference between May and August exhibits three distinct phases: i) a moderate SST difference in the late Holocene (0-3.5 ka) is attributable to intense upwelling during August, ii) a minimum SST difference from 4 to 12 ka is due to weak upwelling during the month of August, and iii) the highest SST difference during the last glacial interval (19 to 22 ka) with high Globigerina bulloides % could have been caused by the occurrence of a prolonged upwelling season (from May through July) and maximum difference in the incoming solar radiation between May and August. Overall, variations in the SST difference between May and August show that the timing of intense upwelling in the Western Arabian Sea over the last 22 kyr has been variable over the months of June, July and August.

(Table 1) Abundances of Discoaster species in ODP Hole 130-806C

Members of the calcareous nannofossil genus Discoaster have been used extensively to subdivide Tertiary deep-sea sediments into biostratigraphic zones or subzones (e.g., Martini, 1971; Bukry, 1973). Haq and Lohmann (1976) mapped biogeographic migrations of this group through time and over latitude. They suggested that expansions and contractions of Discoaster-dominated assemblages across latitudes reflect sea-surface temperature changes. Subsequently, late Pliocene Discoaster species were counted at closely spaced sample intervals from various Atlantic sites (Backman et al., 1986; Backman and Pestiaux, 1987; Chepstow-Lusty et al., 1989, 1991), and Indian Ocean as well as Pacific Ocean sites (Chepstow-Lusty, 1990). In addition to the biostratigraphic information revealing positions and the precision by which the different late Pliocene Discoaster species can be determined, these studies also demonstrated that discoasters strongly fluctuate in abundance as a function of time. These abundance variations occur in equatorial as well as temperate temperature regimes, and show periodicities that reflect orbital frequencies. Chepstow-Lusty et al. (1989, 1991) also suggested that the oscillating abundances partly represent productivity pressure, because discoasters tend to show low abundances under high productivity conditions and vice versa. In the Pacific Ocean, counts showing late Pliocene Discoaster abundances exist from three sites, namely Ocean Drilling Program (ODP) Site 677 in the eastern equatorial upwelling region, Core V28-179 from the central equatorial region, and Core V32-127 from the mid-latitude Hess Rise. The two Vema cores are condensed and show sedimentation rates below 0.5 cm/1000 yr, thus offering a poorly resolved stratigraphy. Hole 806C from the Ontong Java Plateau provided an opportunity to establish a highly resolved Discoaster record from the western extreme of the equatorial Pacific under an environmental setting that differed from ODP Site 677 by being less influenced by intense upwelling. The Discoaster counting technique is described by Backman and Shackleton (1983).

Sedimentology and paleontology on core Lz1104 from Lake Hjort, Greenland

Two sediment cores of 70 and 252 cm length were recovered from Hjort Sø, a small lake on Store Koldewey, Northeast Greenland, and studied with a multidisciplinary approach in order to reconstruct the local environmental history and to test the relevance of proxies for paleoenvironmental information. The basal sediments from the longer core are dominated by clastic matter, which was likely deposited during deglaciation of the lake basin. These clastic sediments are overlain by gyttja, which is also present throughout the shorter core. AMS radiocarbon dating was conducted on plant macrofossils of 11 samples from the gyttja in both cores. A reliable chronology was established for both cores, which dated the onset of organic accumulation at 9,500 cal. year BP. The Holocene temperature development, with an early to mid Holocene thermal maximum, is best reflected in the grain-size composition. Nutrient availability was apparently low during the early Holocene and led to low productivity in the lake and its vicinity. From ca. 7,000 cal. year BP, productivity in the lake increased significantly, probably induced by external nutrient input from goose excrements. From this time, micro- and macrofossil remains reflect relatively well the climate history of East Greenland, with a cooling during the middle Holocene, the medieval warming, and the Little Ice Age. The amount of organic matter in the sequence seems to be more affected by lake ice cover or by nutrient supply from the catchment than by temperature changes. The record from Hjort Sø thus reveals the difficulties in interpreting sedimentary records from high arctic regions.

Age determination and stable isotope ratios of planktonic foraminifera from DSDP Hole 94-609

As shown by the work of Dansgaard and his colleagues, climate oscillations of one or so millennia duration punctuate much of glacial section of the Greenland ice cores. These oscillations are characterized by 5°C air temperature changes, severalfold dust content changes and 50 ppm CO2 changes. Both the temperature and CO2 change are best explained by changes in the mode of operation of the ocean. In this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50°N. Based on this finding, we suggest that the Greenland climate changes are driven by oscillations in the salinity of the Atlantic Ocean which modulate the strength of the Atlantic's conveyor circulation.

(Fig. 1) Stable carbon and oxygen isotope ratios of foraminifera of ODP Hole 113-690B

A remarkable oxygen and carbon isotope excursion occurred in Antarctic waters near the end of the Palaeocene (~57.33 Myr ago), indicating rapid global warming and oceanographic changes that caused one of the largest deep-sea benthic extinctions of the past 90 million years. In contrast, the oceanic plankton were largely unaffected, implying a decoupling of the deep and shallow ecosystems. The data suggest that for a few thousand years, ocean circulation underwent fundamental changes producing a transient state that, although brief, had long-term effects on environmental and biotic evolution.

Pleistocene planktonic foraminifera at ODP Site 128-798

ODP Site 798 on the Oki Ridge in the Southern Japan Sea yielded the first continuous and well-preserved record of Pleistocene planktonic foraminifers in the Northwestern Pacific Ocean region. Quantitative analysis of planktonic foraminifers completed for 122 samples from the 200-m-thick Pleistocene section cored at ODP Site 798 provides a proxy record of variations in sea-surface temperature, productivity, and circulation during the past 1.6 m.y. in an area beneath the track of the Tsushima Current. Faunal census data allow recognition of five distinct assemblages: (1) type A assemblages dominated by sinistrally coiling forms of Neogloboquadrina pachyderma representing polar-subpolar surface temperatures, (2) type B assemblages dominated by Globigerina bulloides and thought to represent periods of increased surface productivity and upwelling, (3) type C assemblages marked by significant abundances of dextrally coiling forms of N. pachyderma thought to represent the warm transitional waters of the Tsushima Current, (4) type D assemblages distinguished by relatively high percentages of dextral N. pachyderma and Globorotalia inflata that also represent warmer surface temperatures and increased flow of the Tsushima Current, and (5) type E assemblages marked by relatively large numbers of the delicate species Globigerina quinqueloba and Globigerinita spp., indicative of exceptional preservation conditions and/or episodic high production of these taxa. Early and middle Pleistocene coiling patterns of Neogloboquadrina pachyderma at Site 798 can be correlated with Pleistocene coiling trends and planktonic foraminiferal datums established in the onshore Oga Peninsula sequence of Northern Honshu and open-ocean N. pachyderma coiling dominance shifts in the North Pacific region. A sustained early Pleistocene warm period recognized in both the Oga Peninsula sequence and the Northern Pacific can clearly be recognized at Site 798. In addition, the late Pleistocene planktonic foraminiferal record at Site 798 shows good correlation with glaciation/deglaciation events for the Northern Hemisphere as delineated by oxygen isotopes and represents the first detailed analysis of Pleistocene sea-surface temperature changes in the Northwestern Pacific Ocean region.

(Table 1) Age determination of DSDP Hole 94-607 and sediment cores V30-101 and V23-81

Oxygen isotope measurements in Greenland ice demonstrate that a series of rapid warm-cold oscillations -called Dansgaard-Oeschger events- punctuated the last glaciation (Dansgard et al., 1993, doi:10.1038/364218a0). Here we present records of sea surface temperature from North Atlantic sediments spanning the past 90 kyr which contain a series of rapid temperature oscillations closely matching those in the ice-core record, confirming predictions that the ocean must bear the imprint of the Dansgaard-Oeschger events (Broecker et al., 1988, doi:10.1016/0033-5894(88)90082-8; 1990, doi:10.1029/PA005i004p00469). Moreover, we show that between 20 and 80 kyr ago, the shifts in ocean-atmosphere temperature are bundled into cooling cycles, lasting on average 10 to 15 kyr, with asymmetrical saw-tooth shapes. Each cycle culminated in an enormous discharge of icebergs into the North Atlantic (a 'Hein-rich event' (Bond et al., 1992, doi:10.1038/360245a0; Broecker et al., 1992, doi:10.1007/BF00193540), followed by an abrupt shift to a warmer climate. These cycles document a previously unrecognized link between ice sheet behaviour and ocean-atmosphere temperature changes. An important question that remains to be resolved is whether the cycles are driven by external factors, such as orbital forcing, or by inter-nal ice-sheet dynamics.

(Table 1) Age determination of sediment core SO136-003GC

We compile and compare data for the last 150,000 years from four deep-sea cores in the midlatitude zone of the Southern Hemisphere. We recalculate sea surface temperature estimates derived from foraminifera and compare these with estimates derived from alkenones and magnesium/calcium ratios in foraminiferal carbonate and with accompanying sedimentological and pollen records on a common absolute timescale. Using a stack of the highest-resolution records, we find that first-order climate change occurs in concert with changes in insolation in the Northern Hemisphere. Glacier extent and inferred vegetation changes in Australia and New Zealand vary in tandem with sea surface temperatures, signifying close links between oceanic and terrestrial temperature. In the Southern Ocean, rapid temperature change of the order of 6°C occurs within a few centuries and appears to have played an important role in midlatitude climate change. Sea surface temperature changes over longer periods closely match proxy temperature records from Antarctic ice cores. Warm events correlate with Antarctic events A1-A4 and appear to occur just before Dansgaard-Oeschger events 8, 12, 14, and 17 in Greenland.

Foraminiferal faunal estimates of paleotemperature: Circumventing the no-analog problem yields cool ice age tropics

The sensitivity of the tropics to climate change, particularly the amplitude of glacial-to-interglacial changes in sea surface temperature (SST), is one of the great controversies in paleoclimatology. Here we reassess faunal estimates of ice age SSTs, focusing on the problem of no-analog planktonic foraminiferal assemblages in the equatorial oceans that confounds both classical transfer function and modern analog methods. A new calibration strategy developed here, which uses past variability of species to define robust faunal assemblages, solves the no-analog problem and reveals ice age cooling of 5° to 6°C in the equatorial current systems of the Atlantic and eastern Pacific Oceans. Classical transfer functions underestimated temperature changes in some areas of the tropical oceans because core-top assemblages misrepresented the ice age faunal assemblages. Our finding is consistent with some geochemical estimates and model predictions of greater ice age cooling in the tropics than was inferred by Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) [1981] and thus may help to resolve a long-standing controversy. Our new foraminiferal transfer function suggests that such cooling was limited to the equatorial current systems, however, and supports CLIMAP's inference of stability of the subtropical gyre centers.

Oceanographic investiations from the Sea of Ochotsk

The Sea of Okhotsk is a marginal sea of the Pacific Ocean, which is characterized by strong variations in the productivity and sediment supply due to sea ice transport and river input. Furthermore the variations in the hydrological cycle determine the formation of the SOIW (Sea of Okhotsk Intermediate Water) which plays an important role in the ventilation processes in the intermediate water of the N-Pacific. Isotope data measured on planktonic and benthic foraminifera, sedimentological and geochemical studies of sediment cores and surface samples from the Sea of Okhotsk are used to reconstruct the paleoceanography during the past 350.000 years. The dating and correlation of the sediments are based on oxygen isotope stratigraphy, absolute ages, magnetic susceptibility as well as a detailled tephrachronology of the entire basin. The sedimentation rates are characterized by temporal and spatial variations. The maximum sedimentation rate takes place at the continental slope off Sakhalin due to the input of the Amur River, the sea ice drift and the high productivity. The sedimentation rate in the eastern part of the Sea of Okhotsk is generelly high because of the influence of the nutrient-rich Kamchatka Current. In the central and northern parts of the Sea of Okhotsk, areas with low productivity and reduced terrestrial supply, the sedimentation rate is the lowest. The analyses of the surface sediment samples make it possible to characterize the (sub)- recent sediment supply and transportation processes. The bulk sediment measurements, isotope data and the accumulation rate of ice-rafted debris (IRD) show a dominant sea ice cover and a region with a high productivity as well as a high Amur River input in the western part of the sea. The eastern part of the Sea of Okhotsk, however, is marked by the predominance of warm and nutrient-rich water masses coming from the Kamchatka Current which restricts the sea ice cover. This is reflected in low content of ice-rafted debris and high productivity proxies as well as in isotope data. The deposits of the Sea of Okhotsk are characterized by terrestrial, biogenic and volcanogenic sediment input which varies temporally and spatially. Here, the sedimentation pattern is dominated by the terrestrial input. Bulk sediment measurements and sample analyses of the > 63 micron particle input make it possible to distinguish glacial and interglacial fluctuations. The sedimentation processes during glacial times are determined by a high content of ice-rafted debris, whereas the primary production is higher during interglacial periods. During the last glacial/interglacial cycle the IRD-distribution pattern indicates a strong sea ice transport in the western part and in large areas of the open sea in the eastern part of the Sea of Okhotsk with a relatively constant ice-drift system. The IRD flux in sediments of the oxygen isotope Stage 6 reflects a new sedimentation pattern in the eastern part of the sea. This high IRD accumulation rate indicates ice advances beyond the shelf margin and an iceberg transport from NE-E direction into the Sea of Okhotsk. The several large, brief, negative anomalies in d13C values of Neogloboquadrina pachyderma (s) show releases of methane from basin sediments which correspond to periods of relative sea level falls. The high sedimentation rates on the Sakhalin slope allow insights into the climatic history in Holocene and indicate shorter-scale variations oscillation in Stage 3, which correlate with the global climatic changes. These variations are described as Dansgaard-Oeschger cycles in Greenland ice cores and as Heinrich-Events in several marine sediment cores from the N-Atlantic.

(Table 1) Age determination on monospecies planktonic foraminifera of sediment core AN25-934

Reconstruction of regional climate and the Okhotsk Sea (OS) environment for the Last Glacial Maximum (LGM), deglaciation and Holocene were performed on the basis of high-resolution records of ice rafted debris (IRD), CaCO3, opal, total organic carbon (TOC), biogenic Ba (Ba_bio) and redox sensitive element (Mn, Mo) content, and diatom and pollen results of four cores that form a north-southern transect. Age models of the studied cores were earlier established by AMS 14C data, oxygen - isotope chronostratigraphy and tephrochronology. According to received results, since 25 ka the regional climate and OS environmental conditions have changed synchronously with LGM condition, cold Heinrich event 1, Bølling -Allerød (BA) warming, Younger Dryas (YD) cooling and Pre-Boreal (PB) warming recorded in the Greenland ice core, North Atlantic sediment, and China cave stalagmites. Calculation of IRD MAR in sediment of north-south transect cores indicate an increase of sea ice formation several times in the glacial OS as compared to the Late Holocene. Accompanying ice formation, increased brine rejection and the larger potential density of surface water at the north shelf due to a drop of glacial East Asia summer monsoon precipitation and Amur River run off, led to strong enhancement of the role of the OS in glacial North Pacific Intermediate Water (NPIW) formation. The remarkable increase in OS productivity during BA and PB warming was probably related with significant reorganisation of the North Pacific deep water ventilation and nutrient input into the NPIW and OS Intermediate Water (OSIW). Seven Holocene OS millennial cold events based on the elevated values of the detrended IRD stack record over the IRD broad trend in the sediments of the studied cores have occurred synchronously with cold events recorded in the North Atlantic, Greenland ice cores and China cave stalagmites after 9 ka. Diatom production in the OS were mostly controlled by sea ice cover changes and surface water stratification induced by sea-ice melting; therefore significant opal accumulation in sediments of this basin begin from 4-6 ka ago simultaneously with a remarkable decrease of sea ice cover.

Accumulation rates of plant-wax-derived long-chain C25 to C35 odd-numbered n-alkanes and d13C values of the n-C31 alkane of ODP Hole 175-1077B

The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes in the distribution of C4 plants-adapted to warm, dry conditions and low atmospheric CO2 concentrations (Collatz et al., 1998, doi:10.1007/s004420050468) -have been attributed to marked changes in environmental conditions, but the relative impacts of changes in aridity, temperature (Pagani et al., 1999, doi:10.1126/science.285.5429.876; Huang et al., 2001, doi:10.1126/science.1060143) and CO2 concentration (Cerling et al., 1993, doi:10.1038/361344a0; Kuypers et al., 1999, doi:10.1038/20659) are not well understood. Here, we present a record of African C4 plant abundance between 1.2 and 0.45 million years ago, derived from compound-specific carbon isotope analyses of wind-transported terrigenous plant waxes. We find that large-scale changes in African vegetation are linked closely to sea surface temperatures in the tropical Atlantic Ocean. We conclude that, in the mid-Pleistocene, changes in atmospheric moisture content - driven by tropical sea surface temperature changes and the strength of the African monsoon - controlled aridity on the African continent, and hence large-scale vegetation changes.