Distribution and stable isotopic compositions of hydrogen and carbon of plant-wax derived n-C29 and -C31 alkanes in terrestrial, coastal and offshore surface sediments

Reconstructing terrestrial water budgets is of prime importance for understanding past climate and environment. To shed more light on how plant-wax derived n-alkanes may be used for this purpose we investigated the distribution and stable isotopic compositions of hydrogen (dD) and carbon (d13C) of plant-wax derived n-C29 and -C31 alkanes in terrestrial, coastal and offshore surface sediments in relation to hydrology along a NW-SE transect east of the Italian Apennines from the Po River to the Eastern Gulf of Taranto. The plant wax average chain length increases southward and may relate to increasing temperature and/or aridity. The plant wax dD of the terrestrial and coastal samples also increases southward and mainly reflects changes in the dD of precipitation. The d13C of plant waxes is primarily interpreted in terms of C3 vegetation changes rather than varying contributions by C4 plants. The plant wax d13C-dD composition of the Po River and Apennine rivers differs considerably from that in southern Italy, and suggests a mainly southern source for plant waxes in marine sediments of the Gulf of Taranto. This calibration provides a basis for the reconstruction of past changes in the Italian water balance and n-alkane source areas.

Hydrogen and oxygen isotopes in ground lce of Bykovsky Peninsula, North Siberia

In wide areas of Northern Siberia, glaciers have been absent since the Late Pleistocene. Therefore, ground ice and especially ice wedges are used as archives for paleoclimatic studies. In the present study, carried out on the Bykovsky Peninsula, eastern Lena Delta, we were able to distinguish ice wedges of different genetic units by means of oxygen and hydrogen isotopes. The results obtained by this study on the Ice Complex, a peculiar periglacial phenomenon, allowed the reconstruction of the climate history with a subdivision of a period of very cold winters (60-55 ka), followed by a long stable period of cold winter temperatures (50-24 ka), Between 20 ka and I I ka, climate warming is indicated in stable isotope compositions, most probably after the Late Glacial Maximum. At that time, a change of the marine source of the precipitation from a more humid source to the present North Atlantic source region was assumed. For the Ice Complex, a continuous age-height relationship was established, indicating syngenetic vertical ice wedge growth and sediment accumulation rates of 0.7 m/ky. During the Holocene optimum, ice wedge growth was probably limited due to the extensive formation of lacustrine environments. Holocene ice wedges in thermokarst depressions (alases) and thermoerosional valleys (logs) were formed after climate deterioration from about 4.5 ka until the present. Winter temperatures were warmer at this time as compared to the cooler Pleistocene. Migration of bound water between ice wedges and segregated ice may have altered the isotopic composition of old ice wedges. The presence of ice wedges as diagnostic features for permafrost conditions since 60 ka, implies that a large glacier extending over the Laptev Sea shelf did not exist. For the remote non-glaciated areas of Northern Siberia, ice wedges were established as a powerful climate archive.

Isotope record and major element chemistry of intestitial waters of ODP Leg 104 sites

The variations in major elements and isotope composition (87Sr/86Sr, delta18O, deltaD) of interstitial waters in Leg 104 sediments is most probably caused by the alteration of volcanic matter. A reaction scheme where volcanic glass reacts with pore-water magnesium and potassium to form trioctahedral smectite, phillipsite, and chert is proposed. Model calculations demonstrate that the pore waters may evolve their negative 6180 signatures without recourse to unreasonably large amounts of volcanic detritus or external sources.

Borehole temperature, submarine permafrost methane concentrations and pore water chemistry measured on borehole BK2, central Laptev Sea shelf

Submarine permafrost degradation has been invoked as a cause for recent observations of methane emissions from the seabed to the water column and atmosphere of the East Siberian shelf. Sediment drilled 52 m down from the sea ice in Buor Khaya Bay, central Laptev Sea revealed unfrozen sediment overlying ice-bonded permafrost. Methane concentrations in the overlying unfrozen sediment were low (mean 20 µM) but higher in the underlying ice-bonded submarine permafrost (mean 380 µM). In contrast, sulfate concentrations were substantially higher in the unfrozen sediment (mean 2.5 mM) than in the underlying submarine permafrost (mean 0.1 mM). Using deduced permafrost degradation rates, we calculate potential mean methane efflux from degrading permafrost of 120 mg/m**2 per year at this site. However, a drop of methane concentrations from 190 µM to 19 µM and a concomitant increase of methane d13C from -63 per mil to -35 per mil directly above the ice-bonded permafrost suggest that methane is effectively oxidized within the overlying unfrozen sediment before it reaches the water column. High rates of methane ebullition into the water column observed elsewhere are thus unlikely to have ice-bonded permafrost as their source.

Stable and radiogenic isotope ratios in pore waters from ODP Leg 127 sites

Interstitial waters from four sites of the Japan Sea (794 to 797) have been analyzed for stable isotopes (delta D, delta11B, delta18O, and delta34S) and 87Sr/86Sr, besides major and minor ions. The isotopic composition is dominated by organic matter degradation, alteration of ash layers and volcaniclastic sands, silica transformation (opal A/CT), and basement alteration. Organic matter degradation and corresponding sulfate reduction leads to 32S depletion and is dependent upon sedimentation rate. The remaining sulfate reservoir is characterized by very "heavy" delta34S ratios, up to +93 ? (rel. CDT = Canyon Diabolo Troilite). "Barite fronts," which may develop in such sediments, should also be characterized by very "heavy" sulfur isotopes. The alteration of volcaniclastic material in the Quaternary sections influences the delta18O (-1.5 ? shift) and delta11B (desorption and later adsorption of "labile"11B). A pronounced positive delta11B anomaly at Site 795 represents the depth range of preferential 10B uptake by alteration products of the ash layers. At Site 796 delta D, delta11B, and 87Sr/86Sr are severely affected by alteration processes of volcaniclastic sands. The opal A/CT transformation may influence the oxygen isotopes and serves as a potential source for B, which is liberated at this interval at Site 795. This positive B anomaly is not reflected in the delta11B profile. Basement alteration processes dominate the sedimentary sequence below the opal A/CT transition, which serves as a chemical and physical boundary. The decreases in delta D and delta18O are probably related to a "paleo ocean water reservoir" situated in the permeable Layer II of the oceanic crust, as is indicated by the positive correlation between these two parameters. Besides Mg, alkalies and delta18O basement rocks also serve as a sink for 11 B (Site 795) and are the source for the Ca and Sr increases, as is documented by the less radiogenic 87Sr/86Sr ratio. 87Sr/86Sr ratios for the lowermost pore waters from Site 795 (0.70529) are comparable to those from volcaniclastic rocks from the "Green Tuff' region (0.704 to 0.706) and oil field brines from the Niigata Oil Field.