Salinity and concentrations of organic carbon in bays of the White Sea

Organic carbon in bays of the White Sea was studied for the first time in 1987. Bays of various types in the Kandalaksha Gulf and the Onega Gulf were investigated. Concentration of C_org ranged from 3.5 to 9 mg/l. The highest weighted-mean concentration of C_org occurred in shallow bays of the Onega Gulf (Suma Bay - 6.17 mg/l, Kolezhma Bay - 5.25 mg/1); slightly lower levels occurred in the Soroka Bay (4.85 mg/l) and Kem' Bay (4.78 mg/l). The lowest concentrations were in deep bays of the Kandalaksha Gulf (Chupa Bay - 4.35 mg/l, Velikaya Salma Bay - 4.10 mg/l). As a rule C_org concentration decreases with depth in deep-water bays (but increases slightly in the thermocline layer). The key factor governing organic matter concentration in the bays of the Onega Gulf with river runoff is allochthonous terrigenous organic matter, as indicated by negative correlation of C_org with salinity (R=-0.83+/-0.07, p=0.96) and nonsignificant correlation with primary production.

Organic compounds at DSDP Hole 90-593

Corg and Norg contents in the acid insoluble mineral fraction were studied in sediments of Site 593. Both decrease systematically from Recent to early Miocene over 425 m of carbonate facies. C/N ratios (7-11) are typically marine and indicate that residual organic matter, bound to clay minerals, was originally scavenged from the marine habitat rather than being of terrigenous origin. Variations of Corg and Norg are almost entirely controlled by rates of sedimentation, which gradually increase from Recent to early Miocene. Preliminary results of carbohydrate distribution indicate that epigenetic and diagenetic processes alter both the concentrations and the ratios of individual monomers with depth. Total carbohydrate concentrations in the samples diminish from 91 µg/g sediment at 18 m sub-bottom depth to 49 µg/g at 335 m. In contrast, sugars in the acid insoluble residue increase with depth, suggesting release of structural polysaccharides and their subsequent association with clay minerals. Ratios of arabinose to fucose, which are about 6:1 in Recent carbonaceous sediments intercepted by sediment traps, vary from 1:1 in the youngest sample to 1:2.5 in the oldest.

Sediment investigations in the Kara Sea

AMS-14C dated sediment cores from the Ob and Yenisei estuaries and the adjacent inner Kara Sea were investigated to determine the siliclastic and organic carbon fluxes and their relationship to paleoenvironmental changes. The variability of sediment fluxes during Holocene times is related to the post-glacial sea-level rise and changes in river discharge and coastal erosion input. Whereas during the late/middle Holocene most of the terrigenous sediments were deposited in the estuaries and the areas directly off the estuaries, huge amounts of sediments accumulated on the Kara Sea shelf farther north during the early Holocene before about 9 Cal. kyrs. BP. The maximum accumulation at that time is related to the lowered sea level, increased coastal erosion, and increased river discharge due to the final stage of mountain deglaciation of the Putoran Massif. Increased supply of Yenisei-derived material indicated by peak magnetic susceptibility values probably occurred in climate-related pulses culminating near 11, 10, and 9 Cal. kyrs. BP. As sea level rose, the main Holocene depocenter migrated southward. Based on hydrogen index values and n-alkanes, the organic matter is predominantly of terrigenous origin. Maximum accumulation rates of 1.5 to more than 6 g/cm**2/y occurred in the early Holocene sediments, suggesting more humid climatic conditions with an increased vegetation cover in the source area at that time. In general, high organic carbon accumulation rates characterize the estuaries and the inner Kara Sea as important sink for terrigenous organic carbon. A high-resolution record of Holocene variability of magnetic susceptibility (MS) in an AMS14C-dated sediment core from the northern Yenisei estuary may indicate natural variability of Arctic climate change and river discharge on a centennial to millenial time scale. Short-term maxima in MS probably related to warmer climate, enhanced precipitation, intensified weathering/erosion and increased river discharge, display a frequency of about 300 to 700 years.

Organic geochemical studies of DSDP Leg 64 Holes

Twenty-six core samples from Leg 64, Holes 474, 474A, 477, 478, 479, and 481A in the Gulf of California, were provided by the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES) Advisory Panel on Organic Geochemistry for analysis. The high heat flow characteristic of the basin provides an opportunity to study the effect of temperature on the diagenesis of organic matter. The contents and carbon isotope compositions of the organic matter and bitumen fractions of different polarity, isoprenoid and normal alkane distributions, and the nature of tetrapyrrole pigments were studied. Relative contents of hydrocarbons and bitumens depend on the thermal history of the deposits. Among other criteria, the nature and content of tetrapyrrole pigments appear to be most sensitive to thermal stress. Whereas only chlorins are present in the immature samples, porphyrins, including VO-porphyrins, appear in the thermally altered deposits, despite the shallow burial depth. Alkane distributions in thermally changed samples are characterized by low values of phytane to 2-C18 ratios and an odd/even carbon preference index close to unity. The thermally altered samples show unusual carbon isotope distributions of the bitumen fractions. The data also provide some evidence concerning the source of the organic matter and the degree of diagenesis.

CPI, d13C, Corg and diploptene from ODP Hole 169-1034B varved sediments

Saanich Inlet has been a highly productive fjord since the last glaciation. During ODP Leg 169S, nearly 70 m of Holocene sediments were recovered from Hole 1034 at the center of the inlet. The younger sediments are laminated, anaerobic, and rich in organic material (1-2.5 wt.% Corg), whereas the older sediments below 70 mbsf are non-laminated, aerobic, with glacio-marine characteristics and have a significantly lower organic matter content. This difference is also reflected in the changes of interstitial fluids, and in biomarker compositions and their carbon isotope signals. The bacterially-derived hopanoid 17alpha(H),21beta(H)-hop-22(29)-ene (diploptene) occurs in Saanich Inlet sediments throughout the Holocene but is not present in Pleistocene glacio-marine sediments. Its concentration increases after ~6000 years BP up to present time to about 70 µg/g Corg, whereas terrigenous biomarkers such as the n-alkane C31 are low throughout the Holocene (<51 µg/g Corg) and even slightly decrease to 36 µg/g Corg at the most recent time. The increasing concentrations of diploptene in sediments younger than ~6000 years BP separate a recent period of higher primary productivity, stronger anoxic bottom waters, and higher bacterial activity from an older period with lesser activity, heretofore undifferentiated. Carbon isotopic compositions of diploptene in the Holocene are between ~31.5 and ~39.6 per mil PDB after ~6000 years BP. These differences in the carbon isotopic record of diploptene probably reflect changes in microbial community structure of bacteria living at the oxic-anoxic interface of the overlying water column. The heavier isotope values are consistent with the activity of nitrifying bacteria and the lighter isotope values with that of aerobic methanotrophic bacteria. Therefore, intermediate delta13C values probably represent mixtures between the populations. In contrast, carbon isotopic compositions of n-C31 are roughly constant at ~31.4 ± 1.1 per mil PDB throughout the Holocene, indicating a uniform input from cuticular waxes of higher plants. Prior to ~6000 years BP, diploptene enriched in 13C of up to -26.3 per mil PDB is indicative of cyanobacteria living in the photic zone and suggests a period of lower primary productivity, more oxygenated bottom waters, and hence lower bacterial activity during the earliest Holocene.