Stable isotope record of foraminifera from South Atlantic sediments with reconstruction of paleotemperatures and paleosalinities

Changes in surface water hydrography in the Southern Ocean (eastern Atlantic sector) could be reconstructed on the basis of isotope-geochemical and micropaleontological studies. A total of 75 high quality multicorer sediment surface samples from the southern South Atlantic Ocean and three Quaternary sediment cores, taken on a meridional transect across the Antarctic Circumpolar Current, have been investigated. The results of examining stable oxygen isotope compositions of 24 foraminiferal species and morphotypes were compared to the near-surface hydrography. The different foraminifera have been divided into four groups living at different depths in the upper water column. The 8180 differences between shallow-living (e.g. G. bulloides, N. pachyderma) and deeper-dwelling (e. g. G. inflata) species reflect the measured temperature gradient of the upper 250 m in the water column. Thus, the 6180 difference between shallow-living and deeper-living foraminifera can be used as an indicator for the vertical temperature gradient in the surface water of the Antarctic Circumpolar Current, which is independent of ice volume. All planktonic foraminifera in the surface sediment samples have been counted. 27 species and morphotypes have been selected, to form a reference data Set for statistical purposes. By using R- and Q-mode principal component analysis these planktonic foraminifera have been divided into four and five assemblages, respectively. The geographic distribution of these assemblages is mainly linked to the temperature of sea-surface waters. The five assemblages (factors) of the Q-mode principal component analysis account for 97.l % of the variance of original data. Following the transferfunction- technique a multiple regression between the Q-mode factors and the actual mean sea-surface environmental parameters resulted in a set of equations. The new transfer function can be used to estimate past sea-surface seasonal temperatures for paleoassemblages of planktonic foraminifera with a precision of approximately ±1.2°C. This transfer function F75-27-5 encompasses in particular the environmental conditions in the Atlantic sector of the Antarctic Circumpolar Current. During the last 140,000 years reconstructed sea-surface temperatures fluctuated in the present northern Subantarctic Zone (PS2076-1/3) at an amplitude of up to 7.5°C in summer and of up to 8.5°C in winter. In the present Polarfrontal Zone (PS1754-1) these fluctuations between glacials and interglacials show lower temperatures from 2.5 to 8.5°C in summer and from 1.0 to 5.0°C in winter, respectively. Compared to today, calculated oxygen isotope temperature gradients in the present Subantarctic Zone were lower during the last 140,000 years. This is an indicator for a good mixing of the upper water column. In the Polarfrontal Zone also lower oxygen isotope temperature gradients were found for the glacials 6, 4 and 2. But almost similar temperature gradients as today were found during the interglacial stages 5, 3 and the Holocene, which implicates a mixing of the upper water column compared to present. Paleosalinities were reconstructed by combining d18O-data and the evaluated transfer function paleotemperatures. Especially in the present Polarfrontal Zone (PS1754-1) and in the Antarctic Zone (PS1768-8), a short-term reduction of salinity up to 4 %o, could be detected. This significant reduction in sea-surface water salinity indicates the increased influx of melt-water at the beginning of deglaciation in the southern hemisphere at the end of the last glacial, approximately 16,500-13,000 years ago. The reconstruction of environmental Parameters indicates only small changes in the position of the frontal Systems in the eastern sector of the Antarctic Circumpolar Current during the last 140,000 years. The average position of the Subtropical Front and Subantarctic Front shifted approximately three latitudes between interglacials and glacials. The Antarctic Polar Front shifted approximately four latitudes. But substantial modifications of this scenario have been interpreted for the reconstruction of cold sea-surface temperatures at 41Â S during the oxygen isotope stages 16 and 14 to 12. During these times the Subtropical Front was probably shified up to seven latitudes northwards.

Elemental and stable isotopic composition of fine fraction samples of ODP Site 115-709 (Table 1)

Stable isotopic and minor element compositions were measured on the fine fraction of pelagic carbonate sediments from Ocean Drilling Program Site 709 in the central Indian Ocean. This section ranges in age from 47 Ma to the present. The observed compositional variations are the result of either paleoceanographic changes (past oceanic chemical or temperature variations) or diagenetic changes. The CaCO3 record is little affected by diagenesis. From previous work, carbonate content is known to be determined by the interplay of biological productivity, water column dissolution, and dilution. The carbon isotopic record is generally similar to previously published curves. A good correlation was observed between sea-level high stands and high 13C/12C ratios. This supports Shackleton's hypothesis that as the proportion of organic carbon buried in marine sediments becomes larger, oceanic-dissolved inorganic carbon becomes isotopically heavier. This proportion appears to be higher when sea level is higher and organic carbon is buried in more extensive shallow-shelf sediments. The strontium content and oxygen isotopic composition of carbonate sediments are much more affected by burial diagenesis. Low strontium concentrations are invariably associated with high values of d18O, probably indicating zones of greater carbonate recrystallization. Nevertheless, there is an inverse correlation between strontium concentration and sea level that is thought to be a result of high-strontium aragonitic sedimentation on shallow banks and shelves during high stands. Iron and manganese concentrations and, to a lesser extent, magnesium and strontium concentrations and carbon isotopic ratios are affected by early diagenetic reactions. These reactions are best observed in a slumped interval of sediments that occurs between 13.0 and 17.5 Ma. As a result of microbial reduction of manganese and iron oxides and dissolved sulfate, it is hypothesized that small amounts of mixed-metal carbonate cements are precipitated. These have low carbon isotopic ratios and high concentrations of metals.

(Table 1) Stable oxygen isotope ratios of Globigerinoides sacculifer of ODP Hole 130-806B

A detailed Pliocene oxygen isotope record from the Ontong Java Plateau, based on measurements of the surface-dwelling planktonic foraminifer Globigerinoides sacculifer, was produced for the period from 5 to 2 Ma. The record documents major long- and short-term climate changes. The results show periods of enhanced ice volume at 4.6 to 4.3 Ma and after 2.85 Ma, a long-term warming trend from 4.1 to 3.7 Ma, and a distinct cooling trend that was initiated at 3.5 Ma and progressed through the initiation of large-scale Northern Hemisphere glaciation after 2.85 Ma (according to the time scale of Shackleton and others proposed in 1990). Periods of high average ice volumes also show the highest d18O amplitudes. The pattern of climate cyclicity changed markedly at about 2.85 Ma. Earlier times were marked by high-frequency variability at the precessional frequencies or even higher frequencies, pointing to low-latitude processes as a main controlling factor driving planktonic d18O variability in this period. The high-frequency variability is not coherent with insolation and points to strong nonlinearity in the way the climate system responded to orbital forcing before the onset of large scale Northern Hemisphere glaciation. After 3 Ma, stronger 41-k.y. cyclicity appears in the record. The shift in pattern is clearest around 2.85 Ma (according to the time scale proposed by Shackleton and others in 1990), 100-200 k.y. before the most dramatic spread of Northern Hemisphere ice sheets. This indicates that high-latitude processes from this point on began to take over and influence most strongly the d18O record, which now reflects ice-volume fluctuations related to the climatic effects of obliquity forcing on the seasonality of high-latitude areas, most probably in the Northern Hemisphere. The general Pliocene trend is that high-latitude climate sensitivity and instability was increasing, and the causal factors producing the intensified glacial cyclicity during the Pliocene must be factors that enhance cooling and climate sensitivity in the subarctic areas.

Foraminifera and stable isotope record of ODP Leg 105 sites

Faunal and stable isotopic data in Sites 646 and 647 provide a ~0.9-Ma paleoclimatic and paleoceanographic record for the Labrador Sea, that is supported by a floral record for the past ~0.3 Ma. At both sites, most glacial stages generally are dominated by polar fauna and flora with low species diversity. Although minor occurrences of subpolar species also were observed in lowermost parts of several glacial stages in Site 646, the faunal classification of Ruddiman and Mclntyre (1976) suggested the presence of polar ecological water masses in the area during most of the glacial periods. In several glacial stages at Site 647, both the faunal and floral data indicate that early periods were marked by subpolar and transitional ecological water masses. The interglacials are characterized by a polar fauna at Site 646 and by polar and transitional faunas and floras at Site 647. However, several interglacial stages in Site 646 include a subpolar flora, in contrast to a planktonic foraminifer fauna similar to that found in the glacial stages. The occurrence of subpolar water masses in several glacial isotopic stages indicates significant northward advection of warmer waters into the Labrador Sea during the early glacial periods, which provided a corridor of oceanic warmth extending from mid- to high latitudes and contributed an additional source of moisture for continental ice-sheet growth. Similar conditions also were documented in the northwest Labrador Sea, Grand Banks, and the North Atlantic.

Age models and stable isotope record of sediment cores from the Atlantic Ocean

The high-resolution delta18O and delta13C records of benthic foraminifera from a 150,000-year long core from the Caribbean Sea indicate that there was generally high delta13C during glaciations and low delta13C during interglaciations. Due to its 1800-m sill depth, the properties of deep water in the Caribbean Sea are similar to those of middepth tropical Atlantic water. During interglaciations, the water filling the deep Caribbean Sea is an admixture of low delta13C Upper Circumpolar Water (UCPW) and high delta13C Upper North Atlantic Deep Water (UNADW). By contrast, only high delta13C UNADW enters during glaciations. Deep ocean circulation changes can influence atmospheric CO2 levels (Broecker and Takahashi, 1985; Boyle, 1988 doi:10.1029/JC093iC12p15701; Keir, 1988 doi:10.1029/PA003i004p00413; Broecker and Peng, 1989 doi:10.1029/GB003i003p00215). By comparing delta13C records of benthic foraminifera from cores lying in Southern Ocean Water, the Caribbean Sea, and at several other Atlantic Ocean sites, the thermohaline state of the Atlantic Ocean (how close it was to a full glacial or full interglacial configuration) is characterized. A continuum of circulation patterns between the glacial and interglacial extremes appears to have existed in the past. Subtracting the deep Pacific (~mean ocean water) delta13C record from the Caribbean delta13C record yields a record which describes large changes in the Atlantic Ocean thermohaline circulation. The delta13C difference varies as the vertical nutrient distribution changes. This new proxy record bears a striking resemblance to the 150,000-year-long atmospheric CO2 record (Barnola et al., 1987 doi:10.1038/329408a0). This favorable comparison between the new proxy record and the atmospheric CO2 record is consistent with Boyle's (1988a) model that vertical nutrient redistribution has driven large atmospheric CO2 changes in the past. Changes in the relative contribution of NADW and Pacific outflow water to the Southern Ocean are also consistent with Broecker and Peng's (1989) recent model for atmospheric CO2 changes.

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.