Molecular and stable isotope composition of headspace and total hydrogencarbon gases of ODP Leg 104 sites

Molecular and isotope compositions of headspace and total (free + sorbed) hydrocarbon gases from drilled cores of the three ODP Leg 104 Sites 642, 643, and 644 of the Voring Plateau are used to characterize the origin and distribution of these gases in Holocene to Eocene sediments. Only minor amounts of methane were found in the headspace (0.1 to < 0.001 vol%). Although methane through propane are present in all of the total gas samples, different origins account for the concentration and composition variations found. Site 643 at the foot of the outer Voring Plateau represents a geological setting with poor hydrocarbon generating potential, (sediments with low TOC and maturity overlying oceanic basement). Correspondingly, the total gas concentrations are low, typical for background gases (yield C1 - 4 = 31 to 232 ppb, C1/C2+ = 0.6 to 4; delta13C(CH4) -22 per mil to -42 per mil) probably of a diagenetic origin. Holocene to Eocene sediments, which overlie volcanic units, were drilled on the outer Vdring Plateau, at Holes 642B and D. Similar to Site 643, these sediments possess a poor hydrocarbon generating potential. The total gas character (yield C1 - 4 = 20 to 410 ppb; C1/C2+ = 1.7 to 13.3; delta13C(CH4) ca. -23 per mil to -40 per mil) again indicates a diagenetic origin, perhaps with the addition of some biogenic gas. The higher geothermal gradient and the underlying volcanics do not appear to have any influence on the gas geochemistry. The free gas (Vacutainer TM) in the sediments at Site 644 are dominated by biogenic gas (C1/C2+ > 104; delta13C(CH4) -77 per mil). Indications, in the total gas, of hydrocarbons with a thermogenic signature (yield C1 - 4 = 121 to 769 ppb, C1/ C2+ = 3 to 8; delta13C(CH4) = -39 per mil to -71 per mil), could not be unequivocally confirmed as such. Alternatively, these gases may represent mixtures of diagenetic and biogenic gases.

Morphology and size of Neogloboquadrina pachyderma in the North Atlantic

The variability in size and shape of shells of the polar planktonic foraminifer Neogloboquadrina pachyderma have been quantified in 33 recent surface sediment samples throughout the northern Atlantic Ocean and correlated with the properties of the ambient surface waters. The aim of the study was to determine whether any of the morphological features could be used to reconstruct sea surface properties in the polar realm of the North Atlantic, where most paleotemperature proxies appear to fail. The analyses revealed that shell morphology is only weakly controlled by habitat properties, whereas shell size showed a strong correlation with sea surface temperature. The regression of mean shell size on sea surface temperature revealed the presence of two trends among the sinistrally coiled shells: a continuous increase in shell size with decreasing SST in sediments deposited under polar water masses and a continuous increase in shell size with increasing SST in samples from transitional waters. The second trend mirrors the trend observed for dextrally coiled shells, which are frequent in the same samples and signal the presence of N. incompta. The identical mean shell size trends among the sinistral and dextral specimens in the temperate samples confirms the results of earlier genetic studies which indicated the existence of a small but distinct proportion of opposite coiling in N. incompta, to which the sinistral shells in the temperate samples could be attributed. The linear correlation between mean shell size and sea surface temperature in the polar domain (summer SST < 9 °C) has been used to develop an empirical formula for the reconstruction of past sea surface temperatures from shell sizes in fossil samples. The standard error of the residuals of the linear regression is 2.36 °C (1 sigma), which implies a much larger error than for most paleothermometers, but enough precision to allow resolution between results by individual paleothermometers in the polar domain. The resulting regression model has been applied on two sediment cores spanning the interval from the Last Glacial Maximum (LGM) to the present day. The results from core PS1906-1 are consistent with ice-free conditions during the LGM in the Norwegian Sea. The SST estimates for the LGM inferred from N. pachyderma shell size are similar or slightly higher than those for the latest Holocene. The results do not indicate anomalously high SST during the glacial and the LGM reconstructions thus appear more consistent with the results from foraminiferal transfer functions and geochemical proxies. Both sediment cores show the highest reconstructed SST during the early Holocene insolation optimum.