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Paired 230Th/U and 14C dating were performed on deep-sea corals (Lophelia pertusa and Madrepora oculata) from the northeastern North Atlantic at 730 m bsl to investigate past changes of the thermohaline circulation. These were estimated using the ?14C value of the upper intermediate waters, based on the 14C ages of the top and base of each coral, where possible, and the 230Th/U dating. The reliability of these estimates was checked by dating two very young corals of the species L. pertusa. One of these corals, collected alive in 1999 AD, gave a 230Th/U age of 1995±4 AD after correction for non-radiogenic 230Th. Another coral, the top of which dated to 1969±6 AD, recorded the atmospheric 14C/12C increase due to the nuclear tests in the early 1960s. The calculated ?14C values from these two corals agree with those measured at GEOSECS Station 23 in 1972–1973 [Östlund et al., Earth Planet. Sci. Lett. 23 (1974) 69–86] and 1991–1992 [Nydal and Gisfelos, Radiocarbon 38 (1996) 389–406]. This, together with the 100% aragonite content and the d234U and 230Th/232Th values of all the dated corals, indicates that none of the corals behaved as open systems with respect to their U-series nuclides and that they closely represent the water mass properties in which they lived. The pre-anthropogenic ?14C value of the North Atlantic intermediate waters was estimated at -69±4‰. The reservoir age varies from 400 years to 600 years, and this variation is due to atmospheric 14C/12C changes. A reservoir age of 610±80 years, close to the pre-anthropogenic value, was determined from one coral dated at 10 430±120 cal yr BP, when the global sea level was approximately at -35 m [Bard et al., Nature 382 (1996) 241–244]. This suggests a modern-like pattern of the oceanic circulation prevailed in the Northeast Atlantic Ocean at this time although the deglaciation was not completely achieved.

We present combined 230Th/U and 14C dating and stable isotope analyses on benthic corals from the northeastern North Atlantic in order to investigate past changes of the thermohaline circulation. The reef forming cold-water corals Lophelia pertusa and Madrepora oculata were raised from intermediate depth (~750m bsl) from carbonate mounds along Rockall and Porcupine Bank and Porcupine Seabight. The 230Th/U ages range from today to 247400yr. The d234U, 230Th/232Th and X-ray images indicate negligible alteration of the investigated corals, i.e. open system behavior. Very young deep-sea corals were accurately dated by means of 230Th/U dating. One in situ living Lophelia coral yielded a mean age of 1995AD, matching the date of collection in 1999AD. From this coral. the measured and calculated seawater d14C values are indistinguishable and the reservoir age Rinterm of the upper intermediate waters is 710±80 years. Several modern corals, being dated between 1950AD and 1986AD, recorded the atmospheric 14C/12C increase due to the nuclear tests in the early 60s. The modern pre-bomb d14C value of the North Atlantic intermediate waters was determined at an average of -65±7‰, and the mean reservoir age at 448±62 years. Finally, several investigated benthic coral grew during the second step of the deglaciation and during the Holocene climate optimum (from 10900 to about 8000 CAL yr BP). The reservoir age of average 500±150 years is equivalent to that of today indicating that, during the studied coral growth episodes, a modern type oceanic circulation, as well as the air-sea and surface to deeper adjacent water 14CO2 exchanges prevaled in the Northeast Atlantic Ocean.

We have determined the d18O and d13C values of azooxanthellate corals (Lophelia pertusa ) at a micrometre scale using an ion microprobe (SIMS-Secondary Ion Mass Spectrometry). Coral skeletons are composed of two different microstructures:Centres of calcification, and surrounding fibres.In Lophelia pertusa centres of calcification are large (50µm) and arranged in lines of centres of calcification. Our results show that centres of calcification in Lophelia pertusa have a restricted range of variation in d18O (-2.8 ± 0.3‰ (V-PDB)), and a larger range in d13C (14.3 to 10.9‰ (V-PDB)). Surrounding skeletal fibres exhibit large isotopic variation both for C and O (up to 12‰), and d13C and d18O are positively correlated. The C and O isotopic compositions of the centre of calcification deviate from this linear trend at the lightest d18O values of the surrounding fibres. The variation of d18O at a micrometre scale is probably the result of two processes: An isotopic equilibrium calcification with at least 1 pH unit variation in the calcification fluid as indicated by direct measurements of coelenteron pH in the coral Galaxea fascicularis (Al-Horani et al., 2003), and a kinetic fractionation.The d13 apparent disequilibrium in Lophelia pertusa may be the result of mixing between depleted d13C metabolic CO2 (respiration) and DIC coming directly from seawater.

Planktic foraminiferal faunas from different environments in the Arabian Sea were size fractionated using 14 sieves with meshes between 100 and 710 μm, to assess the effect of the sieve mesh size cut off level on the faunal composition and to determine the size frequency distribution of individual species. Nine samples from a plankton pump and a towed net, a sediment trap, a box-core and a piston core were selected, to cover living and settling flux faunas as well as fossil faunas from the sediment. In living faunas, most species show an exponential size frequency distribution, with highest numbers in the finest interval of the size spectrum. In sediment trap and core samples, individual species size frequency distributions may consist of: (1) an exponential distribution of relatively small pre-adult specimens; (2) a Gaussian-shaped distribution of larger specimens, which may be classified as adult or terminal; or (3) a combination of both. The distributions are separated using a best fit technique. The composition of the total planktic foraminiferal fauna strongly changes along the size spectrum. Dominant taxa in >355 μm fractions are Orbulina universa, Globorotalia menardii, Globorotalia tumida, Globigerinella siphonifera and Globigerinoides sacculifer, in 125-355 μm fractions Globigerina bulloides, Globigerinoides ruber, Neogloboquadrina dutertrei and Globigerinita glutinata, and in 100 μm) is present in the 100-125 μm fraction and 1-6% is larger than 250 μm. In samples representing a settling flux (sediment trap and sediment samples) 29-57% of the fauna is present in the 100-125 μm fraction, while 6-23% is larger than 250 μm. Size frequency distributions of the dextral Neogloboquadrina complex (= Neogloboquadrina dutertrei and Neogloboquadrina pachyderma + P-D intergrades) show a bimodal pattern; a smaller peak reflecting dextral Neogloboquadrina pachyderma, and a larger peak of adult Neogloboquadrina dutertrei. By applying a best fit technique to the data, the two species may be separated from each other. In size fractions larger than 150 μm most species have reached the adult stage of ontogeny and we recommend this mesh size for standard faunal analysis. In addition, sieve mesh sizes of 125 and 250 μm have to be used to obtain a reliable estimate of the abundance of small and large species, respectively.