Magnetic properties of basalts at DSDP Hole 83-504B

We have studied the magnetic properties of 22 samples from DSDP Leg 83 to determine the origin of remanence and its relationship to such problems as the tectonic and chemical evolution of the section, the depth of the magnetized layer, and the applicability of magnetic properties of ophiolites to the marine crust. The magnitude of natural remanence has fairly typical values in the uppermost part of the section, falls two to three orders of magnitude in the transition zone, and returns to values slightly less than the upper part in the dike complex. This behavior reflects, for the most part, variations in the amount of magnetic minerals present. Directional behavior is highly variable throughout the section and often shows complexity even on the level of a single sample. Curie temperature measurements and preliminary opaque petrography indicate that the remanence is chemical in origin and probably involves a resetting of the original thermal remanent magnetization (TRM) direction. Selective destructive demagnetization of four breccia samples shows that the remanence of the clasts was acquired prior to consolidation and did not change significantly thereafter. There are also indications that some of the remanence may be carried by secondary magnetic phases. A comparison of these samples with comparable ophiolite rocks is equivocal, with similarities in remanence characteristics but differences in magnetic mineralogy. As for magnetic anomalies, the transition zone is too weakly magnetized to contribute significantly. The available data on the dike complex are inconclusive and their contribution is still open to debate.

Organic carbon flux and remineralization in surface sediments

Organic carbon fluxes through the sediment/water interface in the high-latitude North Atlantic were calculated from oxygen microprofiles. A wire-operated in situ oxygen bottom profiler was deployed, and oxygen profiles were also measured onboard (ex situ). Diffusive oxygen fluxes, obtained by fitting exponential functions to the oxygen profiles, were translated into organic carbon fluxes and organic carbon degradation rates. The mean Corg input to the abyssal plain sediments of the Norwegian and Greenland Seas was found to be 1.9 mg C/m**2/d. Typical values at the seasonally ice-covered East Greenland continental margin are between 1.3 and 10.9 mg C/m**2/d (mean 3.7 mg C/m**2/d), whereas fluxes on the East Greenland shelf are considerably higher, 9.1-22.5 mg C/m**2/d. On the Norwegian continental slope Corg fluxes of 3.3-13.9 mg C/m**2/d (mean 6.5 mg C/m**2/d) were found. Fluxes are considerably higher here compared to stations on the East Greenland slope at similar water depths. By repeated occupation of three sites off southern Norway in 1997 the temporal variability of diffusive O2 fluxes was found to be quite low. The seasonal signal of primary and export production from the upper water column appears to be strongly damped at the seafloor. Degradation rates of 0.004-1.1 mg C/cm**3/a at the sediment surface were calculated from the oxygen profiles. First-order degradation constants, obtained from Corg degradation rates and sediment organic carbon content, are in the range 0.03-0.6/a. Thus, the corresponding mean lifetime of organic carbon lies between 1.7 and 33.2 years, which also suggests that seasonal variations in Corg flux are small. The data presented here characterize the Norwegian and Greenland Seas as oligotrophic and relatively low organic carbon deep-sea environments.

Oxygen Utilization and Downward Carbon Flux in an Oxygen-Depleted Eddy in the Eastern Tropical North Atlantic

The occurrence of mesoscale eddies that develop suboxic environments at shallow depth (about 40-100 m) has recently been reported for the eastern tropical North Atlantic (ETNA). Their hydrographic structure suggests that the water mass inside the eddy is well isolated from ambient waters supporting the development of severe near-surface oxygen deficits. So far, hydrographic and biogeochemical characterization of these eddies was limited to a few autonomous surveys, with the use of moorings, under water gliders and profiling floats. In this study we present results from the first dedicated biogeochemical survey of one of these eddies conducted in March 2014 near the Cape Verde Ocean Observatory (CVOO). During the survey the eddy core showed oxygen concentrations as low as 5 µmol kg-1 with a pH of around 7.6 at approximately 100 m depth. Correspondingly, the aragonite saturation level dropped to 1 at the same depth, thereby creating unfavorable conditions for calcifying organisms. To our knowledge, such enhanced acidity within near-surface waters has never been reported before for the open Atlantic Ocean. Vertical distributions of particulate organic matter and dissolved organic matter (POM and DOM), generally showed elevated concentrations in the surface mixed layer (0-70 m), with DOM also accumulating beneath the oxygen minimum. With the use of reference data from the upwelling region where these eddies are formed, the oxygen utilization rate was calculated by determining oxygen consumption through the remineralization of organic matter. Inside the core, we found these rates were almost 1 order of magnitude higher (apparent oxygen utilization rate (aOUR); 0.26 µmol kg-1 day-1) than typical values for the open North Atlantic. Computed downward fluxes for particulate organic carbon (POC), were around 0.19 to 0.23 g C m-2 day-1 at 100 m depth, clearly exceeding fluxes typical for an oligotrophic open-ocean setting. The observations support the view that the oxygen-depleted eddies can be viewed as isolated, westwards propagating upwelling systems of their own, thereby represent re-occurring alien biogeochemical environments in the ETNA.

87Sr/86Sr ratios, Sr and Rb concentrations and stable oxygen isotope values for some basalts from ODP Hole 504B (Table 1)

Seventeen whole-rock samples, generally taken at 25-50 m intervals from 5 to 560 m sub-basement in Hole 504B, drilled in 6.2 m.y. old crust, were analysed for 87Sr/86Sr ratios, Sr and Rb concentrations, and 18O/16O ratios. Sr isotope ratios for 8 samples from the upper 260 m of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the 330-560 m interval, 5 samples have a restricted range of 0.70255-0.70279, with a mean of 0.70266, the average value for fresh mid-ocean ridge basalts (MORB). In the 260-330 m interval, approximately intermediate Sr isotopic ratios are found. Delta18O values (?) range from 6.4 to 7.8 in the upper 260 m, 6.2-6.4 in the 270-320 m interval, and 5.8-6.2 in the 320-560 m interval. The values in the upper 260 m are typical for basalts which have undergone low-temperature seawater alteration, whereas the values for the 320-560 m interval correspond to MORB which have experienced essentially no oxygen isotopic alteration. The higher 87Sr/86Sr and 18O/16O ratios in the upper part of the hole can be interpreted as the result of a greater overall water/rock ratio in the upper part of the Hole 504B crust than in the lower part. Interaction of basalt with seawater (87Sr/86Sr = 0.7091) increased basalt 87Sr/86Sr ratios and produced smectitic alteration products which raised whole-rock delta18O values. Seawater circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below about 230 m sub-basement. These flows may have helped to seal off lower basalts from through-flowing seawater.

Abundances and d13C values of n-alkanes extracted from sediment samples of ODP Hole 116-717C

Siwalik paleosol and Bengal Fan sediment samples were analyzed for the abundance and isotopic composition of n-alkanes in order to test for molecular evidence of the expansion of C4 grasslands on the Indian subcontinent. The carbon isotopic compositions of high-molecular-weight alkanes in both the ancient soils and sediments record a shift from low d13C values (ca. -30 per mil) to higher values (ca. -22 per mil) prior to 6 Ma. This shift is similar in magnitude to that recorded by paleosol carbonate and fossil teeth, and is consistent with a relatively rapid transition from dominantly C3 vegetation to an ecosystem dominated by C4 plants typical of semi-arid grasslands. The n-alkane values from our paleosol samples indicate that the isotopic change began as early as 9 Ma, reflecting either a growing contribution of C4 plants to a dominantly C3 biomass or a decrease in water availability to C3 plants. Molecular and isotopic analyses of other compounds, including n-alcohols and low-molecular weight n-alkanes indicate paleosol organic matter contains contributions from a mixture of sources, including vascular plants, algae and/or cyanobacteria and microorganisms. A range of inputs is likewise reflected in the isotopic composition of the total organic carbon from these samples. In addition, the n-alkanes from two samples show little evidence for pedegenic inputs and we suggest the compounds were derived instead from the paleosol's parent materials. We suggest the record of vegetation in ancient terrestrial ecosystems is better reconstructed using isotopic signatures of molecular markers, rather than bulk organic carbon. This approach provides a means of expanding the spatial and temporal records of C4 plant biomass which will help to resolve possible tectonic, climatic or biological controls on the rise of this important component of the terrestrial biosphere.

(Table 1) Typical trace-fossil occurrences in DSDP Holes 56-436 and 56-435A

Trace fossils are in places abundant in cores from DSDP Leg 56 sites. They are particularly rich in the pelagic-clay sequence at Site 436. Some significant trace fossils, including Zoophycos, Teichichnus, Chondrites, rind and solid burrows, and pellet-armored rods, are described. The ichnofauna, except for pellet-armored rods in diatomaceous mudstone of the landward trench slope, is characterized by cosmopolitan bathyal to abyssal forms.