Temperature and d18O measurements from different ODP Sites at the shelf and upper slope of the Peru Margin

The first experimentally determined temperature dependent oxygen-18 fractionation factor between dolomite and water at low temperatures [Vasconcelos et al. 1995 doi:10.1130/G20992.1] allows now the precise calculation of temperatures during early diagenetic dolomite precipitation. We use d18O values of early diagenetic dolomite beds sampled during ODP Legs 112 and 201 on the Peru continental margin (Sites 1227, 1228 and 1229) [Meister et al. 2007, doi:10.1111/j.1365-3091.2007.00870.x] to calculate paleo-porewater temperatures at the time of dolomite precipitation. We assumed unaltered seawater d18O values in the porewater, which is supported by d18O values of the modern porewater presented in this study. The dolomite layers in the Pleistocene part of the sedimentary columns showed oxygen isotope temperatures up to 5 °C lower than today. Since Sites 1228 and 1229 are located at 150 and 250 m below sealevel, respectively, their paleo-porewater temperatures would be influenced by considerably colder surface water during glacial sealevel lowstands. Thus, Pleistocene dolomite layers in the Peru Continental margin probably formed during glacial times. This finding is consistent with a model for dolomite precipitation in the Peru Margin recently discussed by Meister et al. [Meister et al. 2007, doi:10.1111/j.1365-3091.2007.00870.x], where dolomite forms episodically at the sulphate methane interface. It was shown that the sulphate methane interface migrates upwards and downwards within the sedimentary column, but dolomite layers may only form when the sulphate-methane interface stays at a fixed depth for a sufficient amount of time. We hypothesize that the sulphate-methane interface persists within TOC-rich interglacial sediments, while this zone is buried by TOC-poor sedimentation during glacial times. Thus, the presented oxygen isotope data provide additional information on the timing of early diagenetic dolomite formation and a possible link between episodicity in dolomite formation and sealevel variations. A similar link between early diagenesis and oceanography may also explain spacing of dolomite layers in a Milankovitch type pattern observed in the geological record, such as in the Miocene Monterey Formation.

Calcareous nannofossils in ODP Hole 131-808C

ODP Leg 131 recovered nannofossil-bearing sediments from Site 808 in the Nankai Trough, western Pacific Ocean. Three holes were examined for nannofossils, 808A, 808B, and 808C. A total of 22 nannofossil events were recognized, of which 10 are used as zonal markers. The sediments recovered from Hole 808A (0-111.4 mbsf) contain Pleistocene nannofossil assemblages that are mostly well preserved. All samples from this hole were assigned to nannofossil Zone NN21. The nannofossil assemblages observed in Hole 808B (111.0-358.8 mbsf) are poorly to well preserved and were all assigned to the Pleistocene. The NN21/NN20 Boundary is placed at 230.7 ± 4.4 mbsf. Hole 808C was cored from 298.5 to 1327 mbsf and basalt was reached at 1289.9 mbsf. The sediments recovered range in age from the upper part of Zone NN20 of the Pleistocene to Zone NN5 of the middle Miocene and contain poorly to well-preserved nannofossil assemblages. The Pliocene/Pleistocene Boundary, marked by the FO Gephyrocapsa caribbeanica, was placed at 776.3 ±1.6 mbsf, and the Miocene/Pliocene Boundary is tentatively placed at 955.9 ±1.5 mbsf. The lowermost sediments above basement as well as a sediment sample intercalated between basalt flows are assigned to Zone NN5, with an age of approximately 15 Ma. Age estimates provided by nannofossils show that the sedimentation rate in the trench-fill deposits of the Nankai Trough was very high, 800-1350 m/m.y (0-0.46 Ma), whereas in the Shikoku Basin deposits (> 0.46 Ma), the sedimentation rate was much lower (24-200 m/m.y). These age estimates also provide an extrapolated age of approximately 15 Ma for the basaltic basement at Site 808.

Grain size composition of 275 sediment profiles from the Arctic seas

This data set contains grain size analyses of bottom sediments collected by scientists from the V.P. Zenkovich Laboratory of Shelf and Sea Coasts (P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences) during the Project ''Arctic Shelf of the Eurasia in the Late Quaternary'' in a number of expeditions to the Barents, Kara, East Siberian and Chukchi Seas on board the research vessels R/V Professor Shtokman, H/V Dmitry Laptev, H/V Malygin, and icebreaker Georgy Sedov since 1978. The analyses have been carried out according to the methods published by Petelin (1967) in the Analytical Laboratory of the P.P. Shirshov Institute of Oceanology. Archiving and electronic publication was performed through a data rescue by Evgeny Gurvich in 2003.