Biogeochemistry of sediments in the Aegean Sea

Biochemical composition of sedimentary organic matter (OM), vertical fluxes and bacterial distribution were studied at 15 stations (95-2270 m depth) in the Aegean Sea during spring and summer. Downward fluxes of labile OM were significantly higher in the northern than in the southern part and were higher in summer than in spring. Primary inputs of OM were not related to sedimentary OM concentrations, which had highest values in summer. Sedimentary chlorophyll-a concentrations were similar in the northern and southern parts. Carbohydrates, the main component of sedimentary OM, were about 1.2 times higher in the southern part than in the northern, without significant temporal changes. Total proteins were higher in summer and about double in the northern part. Sedimentary proteins appeared more dependent upon the downward flux of phytopigment than of proteins. Sedimentary OM was characterised by a relatively large fraction of soluble compounds and showed better quality in the northern part. The lack of a depth-related pattern in sedimentary OM and the similar concentrations in the two areas suggest that differences in sedimentary OM quality in the Aegean basin are dependent on system productivity; the bulk of sedimentary OM is largely conservative. Sedimentary bacterial density was about double in the northern part and higher in spring than in summer, but bacterial size was about three times higher in summer, resulting in a larger bacterial biomass in summer. Bacterial density was coupled with total and protein fluxes, indicating a rapid bacterial response to pelagic production. Bacterial biomass was significantly correlated with sedimentary protein and phytopigment concentrations, indicating a clear response to accumulation of labile OM in the sediments. In all cases bacteria accounted for <5% of the organic C and N pools. The efficiency of benthic bacteria in exploiting protein pools, estimated as amounts of protein available per unit bacterial biomass, indicates a constant ratio of about 70 µg proteins/µg C. This suggests a similar bacterial efficiency all over the area studied, unaffected by different trophic conditions.

Petrography and geochemistry of lithic clasts in sediments obtained from ODP Leg 180 sites

This report includes the petrographic description and reviews the distribution of lithic clasts in sediments drilled during Leg 180 in the Woodlark Basin (southwest Pacific). The lithic clasts include (1) metamorphic rocks; (2) granites; (3) serpentinites, gabbros, dolerites, and basalts likely derived from the Papuan ophiolite belt; (4) rare alkaline volcanites reworked in middle Miocene sediments; (5) medium- to high-K calc-alkaline island arc volcanites, in part as reworked clasts, and explosive products deposited by fallout or reworked by turbiditic currents; and (6) rare sedimentary fragments. At the footwall sites the clast assemblage evidences the association of dolerites and evolved gabbroic rocks; the serpentinite likely pertaining to the same ophiolitic complex are likely derived from onland outcrops and transported by means of turbidity currents. On the whole, extensional tectonics active at least since the middle Pliocene can be inferred. The calc-alkaline volcanism is in continuity with the arc-related products from the Papua Peninsula and D'Entrecasteaux Islands and with the latest volcanics of the Miocene Trobrian arc. However, the medium- to high-K and shoshonitic products do not display a significant temporal evolution within the stratigraphic setting. Lava clasts, volcanogenic grains, and glass shards are associated with turbidity currents, whereas in the Pliocene of northern margin the increasing frequency of tephra (glass shards and vesicular silicic fragments) suggests more explosive activity and increasing contribution to the sediments from aerial fallout materials. Evidence of localized alkalic volcanism of presumable early to middle Miocene age is a new finding. It could represent a rift phase earlier than or coeval to the first opening of the Woodlark Basin or, less probably, could derive from depositional trajectories diverted from an adjacent basin.

Geochemistry of continental slope and deep sea basin sediments off Uruguay

Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mössbauer spectroscopy. Both methods showed good agreement (<6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1 N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24 h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe2+. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.

Lithic grain and mineral composition of sand and sandstones from the Japan and Mariana forearc and backarc (Table 3)

One hundred and twenty point counts of Oligocene to Recent sands and sandstones from DSDP sites in the Japan and Mariana intraoceanic forearc and backarc basins demonstrate that there is a clear compositional difference between the continentally influenced Japan forearc and backarc sediments, and the totally oceanic Mariana forearc and backarc sediments. Japan forearc sediments average 10 QFL%Q, 0.82 P/F, 2 Framework%Mica, 74 LmLvLst%Lv, and 19 LmLvLst%Lst. In contrast, the Mariana forearc and backarc sediments average 0 QFL%Q, 1.00 P/F, 0 Framework%Mica, 98 LmLvLst%Lv, and 1 LmLvLst%Lst. Sediment compositions in the Japan region are variable. The Honshu forearc sediments average 5 QFL%Q, 0.94 P/F, 1 Framework%Mica, 82 LmLvLst%Lv, and 15 LmLvLst%Lst. The Yamato Basin sediments (DSDP Site 299) average 13 QFL%Q, 0.70 P/F, 3 Framework%Mica, 78 LmLvLst%Lv, and 14 LmLvLst%Lst. The Japan Basin sediments (DSDP Site 301) average 24 QFL%Q, 0.54 P/F, 9 Framework%Mica, 58 LmLvLst%Lv, and 21 LmLvLst%Lst. P/F and Framework%Mica are higher in the Yamato Basin sediments than in the forearc sediments due to an increase in modal potassium content of volcanic rocks from east to west, on the island of Honshu. Site 301 possesses a higher QFL%Q and LmLvLst%Lst, and lower LmLvLst%Lv than Site 299 because it receives sediment from the Asian mainland as well as the island of Honshu. DSDP Site 293 sediments, in the Mariana region, average 0.97 P/F, 1 Framework%Mica, 13 LmLvLst%Lm and 83 LmLvLst%Lv, due to their proximity to the island of Luzon. The remaining Mariana forearc and backarc sediments show a uniform composition.

Strontium isotopes in pore waters of east equatorial Pacific sediments

Pore-water samples from the equatorial sedimentary bulge area show reversals in depth profiles of 87Sr/86Sr ratios at the sediment/basement interface. Results of this work support inferences made from previous pore-water data (from DSDP drilling in the area) that large-scale horizontal advection of seawater has occurred through the basement underlying the thick sedimentary sequence in this region. The area of apparent advection includes the eastern part of the equatorial high-productivity zone and part of the Guatemala Basin. We attempted to find links between the observed near-basement reversals in pore-water chemistry and sedimentary thickness, age, and topography of the area. Most of the sites that show horizontal advection have disturbed basement topography or outcrops within 10 to 20 km, suggesting that the cooling effects of outcrops may extend for at least 20 km horizontally. Heat-flow data from the area were compared to determine whether sites showing near-bottom chemistry reversals were consistent with areas of low conductive heat flow. This was generally true for the area of the sedimentary bulge and Guatemala Basin. Not enough pore-water data from the Nazca Plate were available to establish any reliable systematics. Because the high-productivity area is well-sealed from hydrothermal circulation, the missing heat must be lost by horizontal advective heat transport. From profiles of strontium isotopes and other elements that show departure from seawater values with increasing depth in the sediments, but return to seawater values near the basement, it appears that water flows relatively freely through much of the oceanic crust, even when sealed by considerable sedimentary cover.

Heat flow values in the Sea of Okhotsk region

Tectonic structure and anomalous distributions of geophysical fields of the Sea of Okhotsk region are considered; the lack of reliable data on age of the lithosphere beneath basins of various origin in the Sea of Okhotsk is noted. Model calculations based on geological and geophysical data yielded 65 Ma (Cretaceous-Paleocene boundary) age for the Central Okhotsk rise underlain by the continental lithosphere. This estimate agrees with the age (the end of Cretaceous) derived from seismostratigraphic data. A comparative analysis of theoretical and measured heat flows in the Akademii Nauk Rise, underlain by the thinned continental crust, is performed. The analysis points to a higher (by 20%) value of the measured thermal background of the rise, which is consistent with high negative gradient of gravity anomalies in this area. Calculations yielded 36 Ma (Early Oligocene) age and lithosphere thickness of 50 km for the South Okhotsk depression, whose seafloor was formed by processes of back-arc spreading. The estimated age of the depression is supported by kinematic data on the region; the calculated thickness of the lithosphere coincides with the value estimated from data of magnetotelluric sounding here. This indicates that formation time (36 Ma) of the South Okhotsk depression was estimated correctly. Numerical modeling performed for determination of the basement age of rifting basins in the Sea of Okhotsk gave the following estimates: 18 Ma (Early Miocene) for the Deryugin Basin, 12 Ma (Middle Miocene) for the TINRO Basin, and 23 Ma (Late Oligocene) for the West Kamchatka Trough. These estimates agree with formation time (Oligocene-Quaternary) of the sedimentary cover in rifting basins of the Sea of Okhotsk derived from geological and geophysical data. Model temperature estimates are obtained for lithologic and stratigraphic boundaries of the sedimentary cover in the Deryugin and TINRO Basins and the West Kamchatka Trough; the temperature analysis indicates that the latter two structures are promising for oil and hydrocarbon gas generation; the West Kamchatka Trough possesses better reservoir properties compared to the TINRO and Deryugin Basins. The latter is promising for generation of hydrocarbon gas. Paleogeodynamic reconstructions of the Sea of Okhotsk region evolution are obtained for times of 90, 66, and 36 Ma on the base of kinematic, geomagnetic, structural, tectonic, geothermal, and other geological and geophysical data.

Sedimentary lipids and palynomorphs of sediment core ABC26 from the Eastern Mediterranean

Selective degradation of organic matter in sediments is important for reconstructing past environments and understanding the carbon cycle. Here, we report on compositional changes between and within lipid classes and kerogen types (represented by palynomorph groups) in relation to the organic matter flux to the sea floor and oxidation state of the sediments since the early Holocene for central Eastern Mediterranean site ABC26. This includes the initially oxic but nowadays anoxic presapropelic interval, the still unoxidised lower part of the organic rich S1 sapropel, its postdepositionally oxidised and nowadays organic-poor upper part as well as the overlying postsapropelic sediments which have always been oxic. A general ~ 2.3 times increase in terrestrial and marine input during sapropel formation is estimated on the basis of the total organic carbon (TOC), pollen, spore, dinoflagellate cyst, n-alkane, n-alkanol and n-alkanoic acid concentration changes in the unoxidised part of the sapropel. The long-chain alkenones, 1,15 diols and keto-ols, loliolides and sterols indicate that some plankton groups, notably dinoflagellates, may have increased much more. Apart from the terrestrial and surface water contributions to the sedimentary organic matter, anomalous distributions and preservation of some C23-C27 alkanes, alkanols and alkanoic acids have been observed, which are interpreted as a contribution by organisms living in situ. Comparison of the unoxidised S1 sapropel with the overlying oxidised sapropel and the organic matter concentration profiles in the oxidised postsapropelic sediments demonstrates strong and highly selective aerobic degradation of lipids and palynomorphs. There seems to be a fundamental difference in degradation kinetics between lipids and pollen which may be possibly related with the absence of sorptive preservation as a protective mechanism for palynomorph degradation. The n-alkanes, Impagidinium, and Nematosphaeropsis are clearly more resistant than TOC. The n-alkanols and n-carboxylic acids are about equally resistant whereas the pollen, all other dinoflagellate cysts and other lipids appear to degrade considerably faster, which questions the practice of normalising to TOC without taking diagenesis into account. Selective degradation also modifies the relative distributions within lipid classes, whereby the longer-chain alkanes, alcohols and fatty acids disappear faster than their shorter-chain equivalents. Accordingly, interpretation of lipid and palynomorph assemblages in terms of pre- or syndepositional environmental change should be done carefully when proper knowledge of the postdepositional preservation history is absent. Two lipid-based preservation proxies are tested the diol-keto-ol oxidation index based on the 1,15C30 diol and keto-ols (DOXI) and the alcohol preservation index (API) whereby the former seems to be the most promising.

Planktonic assemblages during sapropel S5 deposits in the Urania Basin

A micropaleontological study of planktonic assemblages on the partially laminated sapropel S5 (late Pleistocene, marine isotope stage (MIS) 5e) was performed in two piston cores from Urania Basin area (eastern Mediterranean, west of Crete): UM94PC16 and UM94PC31 recovered during a PALEOFLUX Project Cruise. The abundance of Florisphaera profunda indicates the development of a deep chlorophyll maximum (DCM) before the anoxic condition at bottom were established, whereas patterns of upper photic zone coccoliths suggest extreme oligotrophy in surface water. The short appearance of Globorotalia scitula and the presence of Globigerinoides ruber in the lower part of sapropel testify to a thermal stratification, also recorded by changes in primary producers. During G. scitula occurrence, diatoms, mainly represented by Pseudosolenia calcar-avis, appear and bloom because of their capability in using nutrients from DCM. Scanning electron microscope analyses performed on selected intervals from UM94PC16 show that the sapropel is organized in microlaminae mostly composed by siliceous microfossils. In particular, sapropel S5 could be related to an enhanced nutrient availability in the lower-middle part of the photic zone, stratified conditions, and a higher continental input.