Age determination, planktic foraminifera and stable istopes of sediment core MSM05/5_712-2

A sediment core from the West Spitsbergen continental margin was studied to reconstruct climate and paleoceanographic variability during the last ~9 ka in the eastern Fram Strait. Our multiproxy evidence suggests that the establishment of the modern oceanographic configuration in the eastern Fram Strait occurred stepwise, in response to the postglacial sea-level rise and the related onset of modern sea-ice production on the shallow Siberian shelves. The late Early and Mid Holocene interval (9 to 5 ka) was generally characterized by relatively unstable conditions. High abundance of the subpolar planktic foraminifer species Turborotalita quinqueloba implies strong intensity of Atlantic Water (AW) inflow with high productivity and/or high AW temperatures, resulting in a strong heat flux to the Arctic. A series of short-lived cooling events (8.2, 6.9. and 6.1 ka) occurred superimposed on the warm late Early and Mid Holocene conditions. Our proxy data imply that simultaneous to the complete postglacial flooding of Arctic shallow shelves and the initiation of modern sea-ice production, strong advance of polar waters initiated modern oceanographic conditions in the eastern Fram Strait at ~5.2 ka. The Late Holocene was marked by the dominance of the polar planktic foraminifer species Neogloboquadrina pachyderma, a significant expansion of sea ice/icebergs, and strong stratification of the water column. Although planktic foraminiferal assemblages as well as sea surface and subsurface temperatures suggest a return of slightly strengthened advection of subsurface Atlantic Water after 3 ka, a relatively stable cold-water layer prevailed at the sea surface and the study site was probably located within the seasonally fluctuating marginal ice zone during the Neoglacial period.

Age model for the time between 3.45 and 2.45 Ma of ODP Site 189-1172

Calcareous nannoplankton assemblages and benthic d18O isotopes of Pliocene deep-sea sediments of ODP site 1172 (East of Tasmania) have been studied to improve our knowledge of the Southern Ocean paleoceanography. Our study site is located just north of the Subtropical Front (STF), an ideal setting to monitor migrations of the STF during our study period, between 3.45 and 2.45 Ma. The assemblage identified at ODP site 1172 has been interpreted as characteristic for the transitional zone water mass, located south of the STF, based on: (i) the low abundances (< 1%) of subtropical taxa, (ii) relatively high percentages of Coccolithus pelagicus, a subpolar type species, (iii) abundances from 2-10% of Calcidiscus leptoporus, a species that frequently inhabits the zone south of the STF and (iv) the high abundances of small Noelaerhabdaceae which at present dominates the zone south of the STF. Across our interval the calcareous nannoplankton manifests glacial-interglacial variability. We have identified cold events, characterized by high abundances of C. pelagicus which coincide with glacial periods, except during G7. After 3.1 Ma cold events are more frequent, in concordance with global cooling trends. Around 2.75 Ma, the interglacial stage G7 is characterized by anomalous low temperatures which most likely are linked to definite closure of the Central American Seaway (CAS), an event that is believed to have had global consequences. A gradual increase of very small Reticulofenestra across our section marks a significant trend in the small Noelaerhabdaceae species group and has been linked to a general enhanced mixing of the water column in agreement with previous studies. It is suggested that a rapid decline of small Gephyrocapsa after isotopic stage G7 might be related to the cooling observed in our study site after the closure of the CAS.

Basalt density, basement age, and intrusive/extrusive relations from DSDP Legs 2 through 7, 9, and 14

Densities of layer 2 basalt recovered during the Deep Sea Drilling Project have been found to decrease steadily with age, a finding ascribed to progressive submarine weathering in the context of sea-floor spreading. The least-squares solution for 52 density measurements gives a rate of decrease in density of (Delta p)/(Delta t) = -0.0046 g per ccm m.y. = -16 percent per 100 m.y., which is in excellent agreement with earlier estimates based on observed chemical depletion rates of dredged oceanic basalt. Weathering of sea-floor basalt, should it penetrate to any considerable depth in layer 2, will decrease layer 2 seismic refraction velocities, act as a source of geothermal heat, and substantially influence the chemistry of sea water and the overlying column of sediment.

Abundance of planktic foraminiferal species in sediments at DSDP Sites 10-95, 77-536, and 77-540

Over most of the Gulf of Mexico and Caribbean a hiatus is present between the lower upper Maastrichtian and lowermost Tertiary deposits; sedimentation resumed ~200 ka (upper zone Pla) after the K-T boundary. Current-bedded volcaniclastic sedimentary rocks at Deep Sea Drilling Project (DSDP) Sites 536 and 540, which were previously interpreted as impact-generated megawave deposits of K-T boundary age, are biostratigraphically of pre-K-T boundary age and probably represent turbidite or gravity-How deposits. The top 10 to 20 cm of this deposit at Site 536 contains very rare Micula prinsii, the uppermost Maastrichtian index taxon, as well as low values of Ir (0.6 pbb) and rare Ni-rich spinels. These indicate possible reworking of sediments of K-T boundary age at the hiatus. Absence of continuous sediment accumulation across the K-T boundary in the 16 Gulf of Mexico and Caribbean sections examined prevents their providing evidence of impact-generated megawave deposits in this region. Our study indicates that the most complete trans-K-T stratigraphic records may be found in onshore marine sections of Mexico, Cuba, and Haiti. The stratigraphic records of these areas should be investigated further for evidence of impact deposits.

K-Ar dating of altered deep-sea igneous rocks from DSDP Legs 2, 34, and 35

In an attempt to establish criteria for obtaining reliable K-Ar dates, conventional K-Ar studies of several Deep Sea Drilling Project sites were undertaken. K-Ar dates of these rocks may be subject to inaccuracies as the result of sea-water alteration. Inaccuracies may also result from the presence of excess radiogenic 40Ar trapped in rapidly cooled rocks at the time of their formation. The results obtained for DSDP Leg 34 basalts indicate that lowering of K-Ar dates, which is related to potassium addition by weathering, is a major cause of uncertainty in obtaining reliable K-Ar dates for deep-sea rocks. It could not be determined if the potassium addition to the basalts occurred at the time of formation, t_o, or continuously from t_o to the present. Calculations show that sediment cover is not a significant barrier to the diffusion of potassium into the basalt. 40Ar loss contributes, at least in part, to the lowering of the K-Ar date in rocks that have added potassium. The meaning of the K-Ar results obtained for DSDP Legs 35 and 2 basalts could not be unambiguously established. Because of the problems involved, caution must be used in interpreting the meaning of conventional K-Ar dates for deep-sea rocks.