Late Oligocene to Recent benthic foraminifers from the northeastern North Atlantic

Deep-sea benthic foraminiferal faunas were studied from Sites 608 (depth 3534 m, 42°50'N, 23°05'W) and 610 (depth 2427 m, 53°13'N, 18°53'W). The sampling interval corresponded to 0.1 to 0.5 m.y. at Site 608 and in the sections of Site 610 from which core recovery was continuous. First and last appearances of benthic foraminiferal taxa are generally not coeval at the two sites, although the faunal patterns are similar and many species occur at both sites. Major periods of changes in the benthic faunas, as indicated by the numbers of first and last appearances and changes in relative abundances, occurred in the early Miocene (19.2-17 Ma), the middle Miocene (15.5-13.5 Ma), the late Miocene (7-5.5 Ma), and the Pliocene-Pleistocene (3.5-0.7 Ma). A period of minor changes in the middle to late Miocene (10-9 Ma) was recognized at Site 608 only. These periods of faunal changes can be correlated with periods of paleoceanographic changes: there was a period of sluggish circulation in the northeastern North Atlantic from 19.2 to 17 Ma, and the deep waters of the oceans probably cooled between 15.5 and 13.5 Ma, as indicated by an increase in delta18O values in benthic foraminiferal tests. The period between 10 and 9 Ma was probably characterized by relatively vigorous bottom-water circulation in the northeastern Atlantic, as indicated by the presence of a widespread reflector. The faunal change at 7 to 5.5 Ma corresponds in time with a worldwide change in delta13C values, and with the Messinian closing of the Mediterranean. The last and largest faunal changes correspond in time with the onset and intensification of Northern Hemisphere glaciation.

Stable carbon and oxygen isotope record of foraminifera from ODP holes 104-643A and 104-644A

Continuous sediment sections spanning the last 2.8 Ma have been studied using stable isotope stratigraphy and sedimentological methods. By using paleomagnetic reversals as a chronostratigraphic tool, climatic and paleoceanographic changes have been placed in a time framework. The results show that the major expansion of the Scandinavian Ice Sheet to the coastal areas occurred in the late Neogene period at about 2.8 Ma. Relatively high-amplitude glacials appeared until about 2 Ma. The period between 2.8 and 1.2 Ma was marked by cold surface water conditions with only weak influx of temperate Atlantic water as compared with late Quaternary interglacials. During this period, climatic variations were smaller in amplitude than in the late Quaternary. The Norwegian Sea was a sink of deep water throughout the studied period but deep water ventilation was reduced and calcite dissolution was high compared with the Holocene. Deep water formed by other processes than today. Between 2 and 1.2 Ma, glaciations in Scandinavia were relatively small. A transition toward larger glacials took place during the period 1.2 to 0.6 Ma, corresponding with warmer interglacials and increasing influx of temperate surface water during interglacials. 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 as compared with the present situation. The transition towards more meridionality took place over several hundred thousand yr. Only during the last 0.6 Ma has the oceanographic and climatic system of the Norwegian Sea varied in the manner described from previous studies of the late Quaternary.

Benthic foraminifera in the Arctic Ocean

We examine the quantitative composition of benthic foraminiferal assemblages of Rose Bengal-stained surface samples from 37 stations in the Laptev Sea, and combine this data set with an existing data set along a transect from Spitsbergen to the central Arctic Ocean. Foraminiferal test accumulation rates, diversity, faunal composition and statistically defined foraminiferal associations are analysed for living (Rose Bengal-stained) and dead foraminifers. We compare the results of several benthic foraminiferal diversity indices and statistically defined foraminiferal associations, including Fisher's alpha and Shannon-Wiener diversity indices, Q-mode principal component analysis and correspondence analysis. Diversity and faunal density (standing stock) of living benthic foraminifers are positively correlated to trophic resources. In contrast, the accumulation rate of dead foraminifers (BFAR) shows fluctuating values depending on test disintegration processes. Foraminiferal associations defined by Q-mode principal component analysis and correspondence analysis are comparable. The factor values of the correspondence analysis allow a quantitative correlation between the foraminiferal fauna and the local carbon flux, which may be used as a tool to estimate changes in primary productivity.

Vertical distribution of benthic foraminifers in the Arctic Ocean

The vertical distribution of living (Rose Bengal stained) benthic foraminifers was determined in the upper 15 cm of sediment cores taken along transects extending from the continental shelf of Spitsbergen through the Eurasian Basin of the Arctic Ocean. Cores taken by a multiple corer were raised from 50 stations with water depths between 94 and 4427 m, from areas with moderate primary production values to areas that are among the least productive ones in the world. We believe, that in the Arctic Ocean the vertical distribution of living foraminifers is determined by the restricted availability of food. Live foraminiferal faunas are dominated by potentially infaunal species or epifaunal species. Species confined to the infaunal microhabitat are absent in Arctic sediments that we examined, and predominantly infaunal living species are nowhere dominant. In general, an infaunal mode of life is restricted to the seasonally ice-free areas and thus to areas with at least moderate primary production during the summer period. Under the permanent ice cover living species are usually restricted to the top centimeter of the sediment surface, even though some are able to dwell deeper in the sediment under ice-free conditions.

Isotope measurements from ODP Hole 104-642B

The Pliocene (5.3-2.6 Ma) is often described as a relatively stable climatic period, with warm temperatures characterizing high latitudes. New suborbital resolved stable isotope records from ODP Hole 642B in the Eastern Nordic Seas document that the Pliocene was not a stable period characterized by one climate. Rather, seven distinct climate phases, each lasting between 150,000 and 400,000 years, are identified and characterized in the time interval 5.1-3.1 Ma. Four of the transitions between the defined climate phases occurred close to an eccentricity minimum and a minimum in amplitude of change for Northern Hemisphere summer insolation, while two occurred around an eccentricity maximum and a maximum in amplitude in insolation change. Hence, a low frequency response of the Nordic Seas to insolation forcing is indicated. In addition, paleogeographic and related paleoceanographic changes, expansion of the Arctic sea ice cover and onset of NHG were important factors behind the evolving Pliocene low frequency variability in the eastern Nordic Seas. It is likely that the identified climate phases and transitions are important beyond the Nordic Seas, due to their association with changes to both insolation and paleogeography. Also, a strong and variable degree of diagenetic calcite overgrowth is documented for the planktic foraminifera, especially influencing the planktic d18O results; the absolute values and amplitude of change cannot be taken at face value.

Relative contents of foraminifera species in bottom sediments of northern and southern seas

Correlation of paleoceanographic events in several key regions of the World Ocean: North Atlantic, Antarctic, West Arctic Seas, North Pacific and tropical Indo-Pacific has been carried out for the last 135 ka based on micropaleontological, stable isotope, geochronological (AMS-14C) and other data. It has been shown that the global thermohaline circulation controls remote climatic teleconnections on millennial-scale and partly on centennial-scale, while short-term climate changes are mainly transferred by the atmosphere. The basic information is given about the recent thermohaline circulation and stages of its development during Neogene.

Distribution of planktonic and benthic foraminifers in bottom sediments from Core LV28-42-5, Sea of Okhotsk

Distribution of diatoms and planktonic and benthic foraminifers, as well as correlation of components of sandy grain size fraction were studied in the Quaternary sediment core LV28-42-5 (720 cm long) col¬lected on the southeastern slope (1045 m depth) of the Institute of Oceanology Rise, Sea of Okhotsk. This study allowed to reconstruct principle features of paleoceanographic evolution. In the course of penultimate and last continental glaciations (isotope stages 6 and 4-2) and during the later period of the last interglacial (substages 5.d-5.a) the following conditions were characteristic of this area: low temperatures of surface water, terrigenous sediment accumulation including coarse grained ice-rafted material, minimum bioproductivity and microfossil content in sediments, low sea level, reduced water exchange with the ocean, low position of old deep Pacific water. During the interglacial optimum (substage 5.e), as well as in the last deglaciation and Holocene (stage 1) water temperature and bioproductivity increased, sea level rose, and active surface water exchange between the Sea of Okhotsk and the Pacific Ocean and the Sea of Japan took place. This resulted in intensive inflow of the old deep Pacific water into the Sea of Okhotsk and elevation of its upper boundary by few hundred meters. During the later intervals of these warm periods a dichothermal structure of the upper water layer formed and diatom oozes accumulated.

Stable isotope record, foraminifera and susceptibility determination of sediment core ENAM93-21

A high-resolution piston core, ENAM93-21, from a water depth of 1020 m near the Faeroe-Shetland Channel is investigated for variations in magnetic susceptibility, surface oxygen isotopes, grain size distribution, content of ice-rafted detritus (IRD), and distribution of planktonic and benthic foraminifera. The core, covering the last 58,000 years, is correlated with the Greenland ice cores and compared with paleorecords from the Norwegian Sea and the North Atlantic Ocean. All fifteen Dansgaard-Oeschger climatic cycles recognized from the investigated time period in the Greenland ice cores have been identified in the ENAM93-21 core. Each cycle is subdivided into three intervals on the basis of characteristic benthic and planktonic faunas. Interstadial intervals contain a relatively warm planktonic fauna and a benthic fauna similar to the modern fauna in the Norwegian Sea. This indicates thermohaline convection as at present, with a significant contribution of deep water to the North Atlantic Deep Water (NADW). Transitional cooling intervals are characterized by more cold water planktonic foraminfera and ice-related benthic species. The benthic fauna signifies restricted bottom water conditions and a reduced contribution to the NADW. The peak abundance of N. pachyderma (s.) and the coldest surface water conditions are found in the stadial intervals. The benthic fauna is dominated by species with an association to Atlantic Intermediate Water, suggesting an increased Atlantic influence in the Norwegian Sea, and there was probably no contribution to the NADW through the Faeroe-Shetland Channel. The three different modes of circulation can be correlated to paleoceanographic events in the Norwegian Sea and the North Atlantic Ocean.

Investigations on two sediment cores from the Arctic Ocean

During the "RV Polarstem"-Expedition ARK VIII/2 sediment samples were obtained at the continental slope of NW-Spitsbergen. Detailed sedimentological and geochemical analysis were carried out at two undisturbed box cores (PS2122-1GKG, PS2123-2GKG) as well as two gravity cores (PS2122-1SL, PS2123-2SL). The following parameters were deterrnined: Organic carbon, nitrogen and carbonate contents, hydrogen index, stable isotopes, ice rafted debris, grain-size distribution and biogenic opal. The main objective of this study was the reconstruction of paleoenvironmental changes off the northwest coast of Spitsbergen during the last glacial/interglacial-cycle, i.e., during the last about 128.000 years. The results of the investigations can be summarized as follows: - During isotope stage 1 (Holocene) and 5.5 (Eemian Interglacial), light stable isotopes (d180: 3.4-2 %o; d13C: 0.26-0.5 %o), increased bioturbation, high content of planktonic foraminifera and biogenic opal and low quantity of ice-rafted material, indicate seasonally ice-free conditions along the northwest coast due to the intfluence of the Westspitsbergen Current. - Additionally, the sediment characteristics of the middle of isotope stage 2 (Last Glacial Maximum) and at the end of stage 3 confirms an inflow of warmer Atlantic water. The highest production of planktonic and benthic foraminifera (N. pachyderma sin., Cassidulina teretis) (CaC03: 10 %) may reflect the expansion of the 'Whalers Bay'-Polynya as a result of the influence of the Westspitsbergen Current. Presumably, occasionally open-ice conditions provide sufficient precipitation to buildup the Svalbard/Barents Ice Sheet. - The time intervals for the glacier advances on Svalbard given by Mangerud et al. (1992), can be correlated with increased accumulation of ice-rafted material in the sediments at the northwest coast of Spitsbergen. Especially during isotope stage 4 and at the beginning of the Last Glacial Maximum (isotope stage 2), a drastically increased supply of coarse terrigenous material occurs. The high accumulation rate (0.18-0.21 g/cm**2/ka) of terrigenous organic carbon is indicated by high C/N ratios (until 16) and low hydrogen index (50 mg HC/gC). In constrast to deep sea sediments in the Fram-Strait (Hebbeln 1992), the glacier advance between 118.000 and 108.000 years B.P. ist documented in the continental slope sediments. - At the end of the Weichselian ice age, the deglaciation at the northwest coast starts with a typical melt-water signal in the stables isotope record (d18O: 3.5 %o; d13C: -0.16 %o) and high contents of gravel (6-13 %). The signal can be assigned to an event at the westcoast of Spitsbergen (core NP90-39), dated to 14.500 years B.P. (Andersen et al. 1993).

Recent benthic foraminifera of the Scotia Sea and Argentine Basin

We investigated 88 surface sediment samples taken with a multiple corer from the southwestern South Atlantic Ocean for their live (Rose Bengal stained) and dead benthic foraminiferal content. Using Q-Mode Principal Component Analysis six live and six dead associations are differentiated. Live and dead association distributions correspond fairly well; differences are mainly caused by downslope transport and selective test destruction. In addition, four potential fossil associations are calculated from the dead data set after removal of non-fossilizable species. These potential fossil associations are expected to be useful for paleoceanographic reconstructions. Environments are described in detail for the live and potential fossil associations and for selected species. Along the upper Argentine continental slope strong bottom currents control the occurrence of live, dead and potential fossil Angulogerina angulosa associations. Here, particles of a high organic carbon flux rate remain suspended. Below this high energy environment live, dead and potential fossil Uvigerina peregrina dominated associations correlate with enhanced sediment organic carbon content and still high organic carbon flux rates. The live A. angulosa and U. peregrina associations correlate with high standing crops. Furthermore, live and dead Epistominella exigua-Nuttallides umbonifer associations were separated. Dominance of a Nuttallides umbonifer potential fossil association relates to coverage by Antarctic Bottom Water (AABW) and Lower Circumpolar Deep Water (LCDW), above the Calcite Compensation Depth (CCD). Three associations of mainly agglutinated foraminifera occur in sediments bathed mainly by AABW or CDW. A Reophax difflugiformis association was found in mud-rich and diatomaceous sediments. Below the CCD, a Psammosphaera fusca association occurs in coarse sediments poor in organic carbon while a Cribrostomoides subglobosus-Ammobaculites agglutinans association covers a more variable environmental range with mud contents exceeding 30%. One single Eggerella bradyi-Martinottiella communis association poor in both species and individuals remains from the agglutinated associations below the CCD if only preservable species are considered for calculation.

Living benthic foraminifers from the central Arctic Ocean

Fifty short sediment cores collected with a multiple corer and five box cores from the central Arctic Ocean were analysed to study the ecology and distribution of benthic foraminifers. To work out living faunal associations, standing stock and diversity, separate analyses of living (Rose Bengal stained) and dead foraminifers were carried out for the sediment surface. The size fractions between 63 and 125 µm and >125 µm were counted separately to allow comparison with former Arctic studies and with studies from the adjacent Norwegian-Greenland Sea, Barents Sea and the North Atlantic Ocean. Benthic foraminiferal associations are mainly controlled by the availability of food, and competition for food, while water mass characteristics, bottom current activity, substrate composition, and water depth are of minor importance. Off Spitsbergen in seasonally ice-free areas, high primary production rates are reflected by high standing stocks, high diversities, and foraminiferal associations (>125 µm) that are similar to those of the Norwegian-Greenland Sea. Generally, in seasonally ice-free areas standing stock and diversity increase with increasing food supply. In the central Arctic Ocean, the oligotrophic permanently ice-covered areas are dominated by epibenthic species. The limited food availability is reflected by very low standing stocks and low diversities. Most of these foraminiferal associations do not correspond to those of the Norwegian-Greenland Sea. The dominant associations include simple agglutinated species such as Sorosphaerae, Placopsilinellae, Komokiacea and Aschemonellae, as well as small calcareous species such as Stetsonia horvathi and Epistominella arctica. Those of the foraminiferal species that usually thrive under seasonally ice-free conditions in middle bathyal to lower bathyal water depth are found under permanently ice-covered conditions in water depths about 1000 m shallower, if present at all.