Organic carbon and biomarker record from the Laptev Sea continental margin

In order to understand the processes controlling organic carbon deposition (i.e., primary productivity vs. terrigenous supply) and their paleoceanographic significance, three sediment cores (PS2471, PS2474. and PS2476) from the Laptev Sea continental margin were investigated for their content and composition of organic carbon. The characterization of organic matter indudes the determination of buk parameters (hydrogen index values and C/N ratios) and the analysis of specific biomarkers (n-alaknes, fatty acids, alkenones, and pigments). Total organic carbon (TOC) values vary between 0.3 and 2%. In general, the organic matter from the Laptev Sea continental margin is dominated by terrigenous matter throughout. However. significant amounts of marine organic carbon occur. The turbidites, according to a still preliminary stratigraphy probably deposited during glacial Oxygen Isotope Stages 2 and 4, are characterized by maximum amounts of organic carbon of terrigenous origin. Marine organic carbon appears to show enhanced relative abundances in the Termination I (?) and early Holocene time intervals, as indicated by maximum amounts of short chain n-alkanes, short-chain fatty acids, and alkenones. The increased amounts of faity acids, however, may also have a freshwater origin due to increased river discharge at that time. The occurrence of alkenones is suggested to indicate an intensification of Atlantic water inflow along the Eurasian continental margin starting at that time. Oxygen Isotope Stage l accumutation rates of total organic carhon are 0.3, 0.17, and 0.02 C/cm**2/ky in cores PS2476, PS2474, and PS2471, respectively.

Particle flux in the NE Atlantic as recorded by sediment traps

Downward particle flux was measured using sediment traps at various depths over the Porcupine Abyssal Plain (water depth ab. 4850 m) for prolonged periods from 1989 to 1999. A strong seasonal pattern of flux was evident reaching a maximum in mid-summer. The composition of the material changed with depth, reflecting the processes of remineralisation and dissolution as the material sank through the water column. However, there was surprisingly little seasonal variation in its composition to reflect changes in the biology of the euphotic zone. Currents at the site have a strong tidal component with speeds almost always less than 15 cm/sec. In the deeper part of the water column they tend to be northerly in direction, when averaged over periods of several months. A model of upper ocean biogeochemistry forced by meteorology was run for the decade in order to provide an estimate of flux at 3000 m depth. Agreement with measured organic carbon flux is good, both in terms of the timings of the annual peaks and in the integrated annual flux. Interannual variations in the integrated flux are of similar magnitude for both the model output and sediment trap measurements, but there is no significant relationship between these two sets of estimates. No long-term trend in flux is evident, either from the model, or from the measurements. During two spring/summer periods, the marine snow concentration in the water column was assessed by time-lapse photography and showed a strong peak at the start of the downward pulse of material at 3000 m. This emphasises the importance of large particles during periods of maximum flux and at the start of flux peaks. Time lapse photographs of the seabed show a seasonal cycle of coverage of phytodetrital material, in agreement with the model output both in terms of timing and magnitude of coverage prior to 1996. However, after a change in the structure of the benthic community in 1996 no phytodetritus was evident on the seabed. The model output shows only a single peak in flux each year, whereas the measured data usually indicated a double peak. It is concluded that the observed double peak may be a reflection of lowered sediment trap efficiency when flux is very high and is dominated by large marine snow particles. Resuspension into the trap 100 m above the seabed, when compared to the primary flux at 3000 m depth (1800 mab) was lower during periods of high primary flux probably because of a reduction in the height of resuspension when the material is fresh. At 2 mab, the picture is more complex with resuspension being enhanced during the periods of higher flux in 1997, which is consistent with this hypothesis. However there was rather little relationship to flux at 3000 m in 1998. At 3000 m depth, the Flux Stability Index (FSI), which provides a measure of the constancy of the seasonal cycle of flux, exhibited an inverse relationship with flux, such that the highest flux of organic carbon was recorded during the year with the greatest seasonal variation.

(Table 3) Oxygen and carbon isotopic record of living (Rose Bengal Stained) benthic foraminifera of North Atlantic surface sediments

Variation of the d13C of living (Rose Bengal stained) deep-sea benthic foraminifera is documented from two deep-water sites (~2430 and ~3010 m) from a northwest Atlantic Ocean study area 275 km south of Nantucket Island. The carbon isotopic data of Hoeglundina elegans and Uvigerina peregrina from five sets of Multicorer and Soutar Box Core samples taken over a 10-month interval (March, May, July, and October 1996 and January 1997) are compared with an 11.5 month time series of organic carbon flux to assess the effect of organic carbon flux on the carbon isotopic composition of dominant taxa. Carbon isotopic data of Hoeglundina elegans at 3010 m show 0.3 per mil lower mean values following an organic carbon flux maximum resulting from a spring phytoplankton bloom. This d13C change following the spring bloom is suggested to be due to the presence of a phytodetritus layer on the seafloor and the subsequent depletion of d13C in the pore waters within the phytodetritus and overlying the sediment surface. Carbon isotopic data of H. elegans from the 2430 m site show an opposite pattern to that found at 3010 m with a d13C enrichment following the spring bloom. This different pattern may be due to spatial variation in phytodetritus deposition and resuspension or to a limited number of specimens recovered from the March 1996 cruise. The d13C of Uvigerina peregrina at 2430 m shows variation over the 10 month interval, but an analysis of variance shows that the variability is more consistent with core and subcore variability than with seasonal changes. The isotopic analyses are grouped into 100 µm size classes on the basis of length measurements of individual specimens to evaluate d13C ontogenetic changes of each species. The data show no consistent patterns between size classes in the d13C of either H. elegans or U. peregrina. These results suggest that variation in organic carbon flux does not preferentially affect particular size classes, nor do d13C ontogenetic changes exist within the >250 to >750 µm size range for these species at this locality. On the basis of the lack of ontogenetic changes a range of sizes of specimens from a sample can be used to reconstruct d13C in paleoceanographic studies. The prediction standard deviation, which is composed of cruise, core, subcore, and residual (replicate) variability, provides an estimate of the magnitude of variability in fossil d13C data; it is 0.27 per mil for H. elegans at 3010 m and 0.4 per mil for U. peregrina at the 2430 m site. Since these standard deviations are based on living specimens, they should be regarded as minimum estimates of variability for fossil data based on single specimen analyses. Most paleoceanographic reconstructions are based on the analysis of multiple specimens, and as a result, the standard error would be expected to be reduced for any particular sample. The reduced standard error resulting from the analysis of multiple specimens would result in the seasonal and spatial variability observed in this study having little impact on carbon isotopic records.

Stable carbon and oxygen isotope ratios of benthic foraminifera from the East Atlantic Ocean

The glacial to interglacial delta13C records of the benthic foraminifera Cibicidoides wuellerstorfi and the Uvigerina peregrina group from deep-sea cores cannot be adjusted by a generally valid constant. The delta13C values of the U. peregrina group largely correlate with the accumulation rates of organic carbon, suggesting a local "habitat effect"; those of C. wuellerstorfi vary independently with respect to the carbon flux and record fluctuations in the delta13C of the ambient bottom water isotopic composition.

Flux data in the Southern Ocean

Since 1983 time-series traps have been deployed in the Atlantic sector of the Southern Ocean to measure the flux of organic carbon, biogenic silica and carbonate. The organic carbon flux data are used to calculate primary production rates and organic carbon fluxes at 100 m water depth. From these calculations, annual primary production rates range from about 170 g C m**-2 in the coastal area (Bransfield Strait) to almost zero in the Permanent Sea-Ice Zone. High rates (of about 80 g C m**-2 year**-1 ) were calculated for the Polar Front Zone and rather low values (about 20 g C m**-2 year**-1 ) characterize the Maud Rise area. The estimated primary production for the entire Southern Ocean (south of 50°S), using various subsystems with characteristic carbon fluxes, is in the order of 1 * 10**9tons year**-1; the organic carbon flux out of the photic layer is 0.17 * 10**9tons year**-1. Our calculation of the Southern Ocean total annual primary production is substantially lower than previously reported values.

(Table 2) Organic residue at DSDP Hole 76-534A

The kind, sedimentation rate, and diagenesis of organic particles delivered to the North Atlantic seafloor during the Middle Jurassic-Early Cretaceous were responsible for the presence of carbonaceous sediments in Hole 534A. Organic-rich black clays formed from the rapid supply of organic matter; this organic matter was composed of either abundant, well-preserved, and poorly sorted particles of land plants deposited in clays and silty clays within terrigenous turbiditic sequences (tracheal facies) or abundant amorphous debris (xenomorphic facies) generated through the digestive tracts of marine zooplankton and sedimented as fecal pellets. Evidence for the fecal-pellet origin of xenomorphic debris is illustrated. Black clays were also produced in sediments containing less organic matter as a result of the black color of carbonized particles composing all or most of the residues (micrinitic facies). Slowly sedimented hematitic Aptian clays contain very little carbonized, organic debris that survived diagenetic oxidation. In the red calcareous clay sequence of the Late Jurassic, larger amounts of this oxidized debris turned several clay layers black or blackish red. Carbonized debris also dominates the residues recovered in interbedded black and green Albian clays. Carbonization of organic matter in these sediments either turned them black or provided the diagenetic environment for reduced iron. Carbonized debris is also appreciable in burrow-mottled black-green Kimmeridgian clay. The study of Hole 534A organic matter indicates that during the middle Callovian there was a rapid supply of terrigenous organic matter, followed by a late Callovian episode of rapidly supplied xenomorphic debris deposited as fecal pellets. The Late Jurassic-Berriasian was a time of slower sedimentation of organic matter, primarily of a marine dinoflagellate flora in a poorly preserved xenomorphic facies variously affected by diagenetic oxidation. Several intervals of carbonized tracheal tissue in the Oxfordian and Kimmeridgian suggest episodes of oxidized terrigenous matter. The same sequence of Callovian organic events is evident in much of the Early Cretaceous