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.

Age models and stable isotopes of sediment cores from the northern seas

Oxygen and carbon isotope measurements were carried out on tests of planktic foraminifers N. pachyderma (sin.) from eight sediment cores taken from the eastern Arctic Ocean, the Fram Strait, and the lceland Sea, in order to reconstruct Arctic Ocean and Norwegian-Greenland Sea circulation patterns and ice covers during the last 130,000 years. In addition, the influence of ice, temperature and salinity effects on the isotopic signal was quantified. Isotope measurements on foraminifers from sediment surface samples were used to elucidate the ecology of N. pachyderma (sin.). Changes in the oxygen and carbon isotope composition of N. pachyderma (sin.) from sediment surface samples document the horizontal and vertical changes of water mass boundaries controlled by water temperature and salinity, because N. pachyderma (sin.) shows drastic changes in depth habitats, depending on the water mass properties. It was able to be shown that in the investigated areas a regional and spatial apparent increase of the ice effect occurred. This happened especially during the termination I by direct advection of meltwaters from nearby continents or during the termination and in interglacials by supply of isotopically light water from rivers. A northwardly proceeding overprint of the 'global' ice effect, increasing from the Norwegian-Greenland Sea to the Arctic Ocean, was not able to be demonstrated. By means of a model the influence of temperature and salinity on the global ice volume signal during the last 130,000 years was recorded. In combination with the results of this study, the model was the basis for a reconstruction of the paleoceanographic development of the Arctic Ocean and the Norwegian-Greenland Sea during this time interval. The conception of a relatively thick and permanent sea ice cover in the Nordic Seas during glacial times should be replaced by the model of a seasonally and regionally highly variable ice cover. Only during isotope stage 5e may there have been a local deep water formation in the Fram Strait.

Sedimentology, age models and stable isotope ratios of sediment cores from the equatorial Pacific

Based on detailed reconstructions of global distribution patterns, both paleoproductivity and the benthic d13C 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 d13C by 0.4-0.5 per mil. 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.