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1.
Accumulation rates of (marine) organic carbon and estimates of paleoproductivity from organic carbon data indicate distinct late Quaternary glacial/interglacial cycles at Ocean Drilling Program (ODP) Site 646. During the last 240,000 years, surface-water productivity was significantly higher in the northern Labrador Sea during interglacial than during glacial times (70 to 170 gC/m2/yr vs. 30 to 70 gC/m2/yr). The reduced glacial productivity is probably caused by a closed sea-ice cover dominating the northern Labrador Sea during glacial intervals.  相似文献   

2.
We investigated marine and terrestrial environmental changes at the northern Japan margin in the northwestern Pacific during the last 23,000 years by analyzing biomarkers (alkenones, long-chain n-alkanes, long-chain n-fatty acids, and lignin-derived materials) in Core GH02-1030. The U 37K′-derived temperature in the last glacial maximum (LGM) centered at 21 ka was ∼10°C, which was 2°C lower than the core-top temperature (∼12°C). This small temperature drop does not agree with pollen evidence of a large air temperature drop (more than 4°C) in the Tokachi area. This disagreement might be attributed to a bias of U 37K′-derived temperature within 2.5°C by a seasonal shift in alkenone production. The U 37K′-derived temperature was significantly low during the last deglaciation. Because this cooling was significant in the Kuroshio-Oyashio transition zone, the temperature drops are attributable to the southward displacement of the Kuroshio-Oyashio boundary. Abundant lignin-derived materials, long-chain n-alkanes and long-chain n-fatty acids indicate a higher contribution of terrigenous organic matter from 17 to 12 ka. This phenomenon might have resulted from an enhanced coastal erosion of terrestrial soils due to marine transgression and/or an efficient inflow of higher plant debris to river waters from 17 to 12 ka.  相似文献   

3.
CaCO3 and total organic carbon concentrations, organic matter C/N and carbon isotope ratios, and sediment accumulation rates in late Quaternary sediments from DSDP Site 594 provide information about glacial–interglacial variations in the delivery of organic matter to the Chatham Rise offshore of southeastern New Zealand. Low C/N ratios and nearly constant organic δ13C values of −23‰ indicate that marine production dominates organic matter supply in both glacial and interglacial times during oxygen isotope stages 1 through 6 (0–140 ka) and 17 through 19 (660–790 ka). Increased organic carbon mass accumulation rates in isotope stages 2, 4, 6, and 18 record enhanced marine productivity during glacial maxima. Excursions of organic δ13C values to ca. −29‰ in portions of isotope stage 2 suggest that the local concentration of dissolved CO2 was occasionally elevated during the last glacial maximum, probably as a result of short periods of lowered sea-surface temperature. Dilution of carbonates by clastic continental sediment generally increases at this location during glacial maxima, but enhanced delivery of land-derived organic matter does not accompany the increased accumulation of clastic sediments.  相似文献   

4.
Cycladophora davisiana, a radiolarian species dwelling at mesopelagic depths, is known as a representative glacial fauna due to its unique distribution during glacial periods. In the present ocean, abundant production of C. davisiana is only observed in the Okhotsk Sea, indicating an adaptation of C. davisiana for seasonal sea-ice covered conditions. We found pronounced abundant production of C. davisiana during the early to middle Holocene in the Okhotsk Sea, suggesting more favorable conditions for C. davisiana than the present Okhotsk Sea. In order to clarify the reason, oceanographic conditions during the Holocene were reconstructed based on biomarkers, lithogenic grains including ice-rafted debris (IRD), biogenic opal, and total organic carbon (TOC) in two sediment cores from the Okhotsk Sea. These indicators suggest that the pronounced C. davisiana production may be attributed to: 1) a supply to mesopelagic depths under intensified stratification of fine organic particles derived from coccolithophorids, bacteria, and detrital materials; and 2) cold, well-ventilated intermediate water formation.  相似文献   

5.
Sedimentation rates were determined for the northern Gulf of Mexico margin sediments at water depths ranging from 770 to 3560 m, using radiocarbon determinations of organic matter. Resulting sedimentation rates ranged from 3 to 15 cm/kyr, decreasing with increasing water depth. These rates agree with long-term sedimentation rates estimated previously using stratigraphic methods, and with estimates of sediment delivery rates by the Mississippi River to the northern Gulf of Mexico, but are generally higher by 1–2 orders of magnitude than those estimated by 210Pbxs methods. Near-surface slope sediments from 2737 m water depth in the Mississippi River fan were much older than the rest. They had minimum 14C ages of 16–27 kyr and δ13C values ranging from −24‰ to −26.5‰, indicating a terrestrial origin of organic matter. The sediments from this site were thus likely deposited by episodic mass wasting of slope sediment through the canyon, delineating the previously suggested main pathway of sediment and clay movement to abyssal Gulf sediments.  相似文献   

6.
We have measured inorganic nitrogen (IN) content and the isotope ratio of IN (δ15NIN) in a sediment core covering the last 145 kyr in the western subarctic Pacific (WSAP). IN content was generally high during glacial periods and shows positive correlations with both eolian dust content and the ratio of organic carbon (C) to organic nitrogen (ON) (C/ON) found in our previous studies. This means that IN was transported from continental areas to the WSAP together with eolian dust and that the IN was not contaminated by volcanic materials, because the eolian dust content was reconstructed using metal components to remove contaminating volcanic materials. Therefore, IN content in the WSAP sediments, the clay fraction of which is not greatly affected by drift deposits seen at the other sites in this region, may potentially be an effective proxy for eolian dust, without the need to consider contamination by volcanic materials. δ15NIN was generally low during glacial periods and shows negative correlations with IN, eolian dust, and C/ON. The possible causes of the observed variations in δ15NIN are as follows: (1) authigenic fixation of NH4 + in water-column and pore water of sea-floor sediments to clay minerals; (2) contamination of measured IN by highly resistant organic matter; or (3) variations in the continental source region of the eolian dust supplied to the WSAP and climatically induced changes in δ15N of soil organic matter there. The last mechanism shows the potential for δ15NIN to be used as a proxy for climate change on land, and is consistent with other published explanations of the spatial distribution of δ15NIN in modern sea-floor sediments.  相似文献   

7.
The Storegga Slide complex is a multi-stage slope failure on the Norwegian continental margin where the most recent major event occurred 8.1 ka b.p. (calendar years before present). Its northern flank contains pockmark features that are commonly inferred to be related to the historical and modern venting of methane-bearing fluids. Three jumbo piston cores (JPC), one from a pockmark and two background cores at variable distances from this site (proximal, 5 km, and distal, 15 km) on the northern flank of the slide (806–1,524 m water depths), were sampled at 10 cm resolution to assess the geologic record of methane venting in the Nyegga pockmark field. Six down-core radiocarbon measurements on mixed planktonic foraminifer species reveal ages of 9.4–16.4 ka b.p. Bathymodiolus mussel shell horizons, indicators of methane-rich environments, have been dated at 15.8–17.6 and ~22 ka b.p. in the pockmark core. Stable isotope analyses on planktonic (Neogloboquadrina pachyderma sinistral) and benthic (Islandiella norcrossi, Melonis barleeanum) Foraminifera reveal δ18O values indicative of a clear glacial/deglacial transition (−1.5‰ shift in planktonic species). Both planktonic and benthic δ13C signatures record multiple excursions, interpreted to reflect the influence of methane in the environment; these δ13C excursions occur in the pockmark core and also in the distal background core. While authigenic calcite formation on the seafloor may play an important role in producing such excursions, these data together suggest the influence of methane seepage within the pockmark field over the past 25 ka, whereby seepage was particularly active between 13 and 15 ka. This is consistent with previously inferred regional increases in porewater pressure associated with glacial loading and higher sedimentation rates, which can cause gas hydrate and slope instability.  相似文献   

8.
Sediments were sampled and oxygen profiles of the water column were determined in the Indian Ocean off west and south Indonesia in order to obtain information on the production, transformation, and accumulation of organic matter (OM). The stable carbon isotope composition (δ13Corg) in combination with C/N ratios depicts the almost exclusively marine origin of sedimentary organic matter in the entire study area. Maximum concentrations of organic carbon (Corg) and nitrogen (N) of 3.0% and 0.31%, respectively, were observed in the northern Mentawai Basin and in the Savu and Lombok basins. Minimum δ15N values of 3.7‰ were measured in the northern Mentawai Basin, whereas they varied around 5.4‰ at stations outside this region. Minimum bottom water oxygen concentrations of 1.1 mL L?1, corresponding to an oxygen saturation of 16.1%, indicate reduced ventilation of bottom water in the northern Mentawai Basin. This low bottom water oxygen reduces organic matter decomposition, which is demonstrated by the almost unaltered isotopic composition of nitrogen during early diagenesis. Maximum Corg accumulation rates (CARs) were measured in the Lombok (10.4 g C m?2 yr?1) and northern Mentawai basins (5.2 g C m?2 yr?1). Upwelling-induced high productivity is responsible for the high CAR off East Java, Lombok, and Savu Basins, while a better OM preservation caused by reduced ventilation contributes to the high CAR observed in the northern Mentawai Basin. The interplay between primary production, remineralisation, and organic carbon burial determines the regional heterogeneity. CAR in the Indian Ocean upwelling region off Indonesia is lower than in the Peru and Chile upwellings, but in the same order of magnitude as in the Arabian Sea, the Benguela, and Gulf of California upwellings, and corresponds to 0.1–7.1% of the global ocean carbon burial. This demonstrates the relevance of the Indian Ocean margin off Indonesia for the global OM burial.  相似文献   

9.
The early Miocene Pedregoso Formation is one of the numerous formations rich in organic matter within the stratigraphic record of the Urumaco Trough, in the central area of the Falcón Basin. Due to its lithological characteristics and stratigraphic position, this formation is of great interest regarding the basin's petroliferous systems. The evaluation of various inorganic and organic geochemical parameters indicates that the organic matter is primarily of marine origin, deposited in a marine carbonate environment typical of reefal systems, under oxic-to-dysoxic conditions. The low variability in the TOC concentrations and in the distributions of the biomarkers extracted from the samples suggests that the paleoenvironmental conditions and the organic-matter supply remained approximately constant throughout the sedimentation of this unit. The Pedregoso type-II organic matter (marine origin) and initial organic richness value (∼1.8%) suggest that this unit has probably generated hydrocarbons within the Urumaco Trough. However, present-day thermal maturity parameters reveal that the Pedregoso organic matter is overmature (dry gas window), indicating that this unit is only capable to generate gas. In addition, the geothermal gradient, maturity parameters, and the maximum paleotemperature estimated in this study suggest that the Pedregoso Formation reached a maximum burial depth the ∼6.5 km, consistent with the value obtained from data of stratigraphic thickness in the Urumaco Trough. This implies that the thermal anomaly that affected the basin during the Late Eocene–Early Miocene did not reach the central part of the basin, and therefore, the organic matter maturation in this unit is due to the sedimentary burial.  相似文献   

10.
Ba and Ti in a sediment core (10 m long) from the Okhotsk Sea, covering the last 120 kyrs, were measured. The authigenic Ba (Baex) contents were calculated and compared with the opal. The correlation coefficient between Baex and opal was quite small (r = 0.34), but it greatly grew larger (r = 0.90), if the Baex contents were multiplied by a simple function increasing with depth, except for two intervals. This may be due to the gradual change in the sedimentation environment during the glacial ages. One of the exceptional interval is found at 60–170 cm in depth, corresponding to 10–17 calendar kyr ago, the last termination period of glacial ages, where the Baex began to increase prior to opal. Since the calcium carbonate contents similarly increased prior to the opal increase, the Baex may be also related to calcareous organisms besides siliceous ones. The other is the last interglacial period around 120 kyr ago when the opal contents were high, but those of the Baex were not increased. This can be explained, if the Baex was reduced to sulfide and dissolved away in a strongly anoxic environment during the biologically productive period. During the glacial ages, the mass accumulation rate (MAR) of lithogenic Ti was about twice the amount of that during the interglacial ages. For opal, however, the contrast between the glacial and interglacial ages was more remarkable in its MAR than in its concentration in sediments, due to the larger variation in the bulk sedimentation rates. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
Eucampia antarctica (Castr.) Mangin abundance curves for two piston cores from the western Agulhas Basin (southeast Atlantic sector of the Southern Ocean) were used to place volcanic glass shard and ice-rafted debris abundance curves into a stratigraphic framework for the last 100,000 years. A direct correlation is shown between increased abundance of E. antarctica, tephra and ice-rafted debris; low abundances are often characterized by calcareous sediment in the northern core. Peaks in abundance of E. antarctica have been interpreted as an indication of glacial periods and the increased ice-rafted debris and tephra during glacial periods is evidence for an ice-rafted origin for the tephra.Microprobe geochemical analysis of volcanic glass shards provides no single distinct source for the ash. The geochemical data is consistent, but not definitive, with a primary source in the Scotia Arc and a minor source from Bouvet Island. Between core correlations of geochemically “fingerprinted” dispersed tephra zones were not successful due to variability of glass shard geochemistry and limited sample size.  相似文献   

12.
Soil core samples fromRhizophora mangleL. andAvicennia schauerianaStapf & Leech forests from south-eastern Brazil were analysed for their total organic matter content and their sugars, amino acid and amino sugars composition. Organic carbon and nitrogen contents were higher inAvicenniathan inRhizophorasoils. The contribution of sugars and amino acids to the total organic carbon pool was constant with depth inRhizophorasoils whereas inAvicenniasoils it increased. Spectral distribution of sugars and amino acids showed a dominance of Ca-affine monomers, particularly acidic amino acids, and the sugar arabinose. Biogeochemical indicators derived from ratios of individual sugar and amino acid monomers confirm previous studies which showed that organic matter in both soils is mainly of mangrove origin. The results further suggest accumulation of organic matter inRhizophorasoils and a continuing degradation of organic matter inAvicenniasoils. The latter may thus release more nutrients to adjacent ecosystems thanRhizophorasoils.  相似文献   

13.
Organic carbon (C) and total nitrogen (N) contents and corresponding isotope ratios were determined in surficial sediment (0–3 cm) at 94 stations ranging from 21 to 1995 m water depth off Tokachi, Hokkaido, Japan, to elucidate the distribution and source of sedimentary organic matter. Suspended particulate organic matter (POM) in the seawater and suspended POM and sediment in the Tokachi River were also examined. δ13C, δ15N and C / N ratios of the samples in the Tokachi River suggest that the spring snowmelt is an important process for the transport of terrestrial organic matter to the coastal waters. δ13C values of suspended POM in the surface seawater were higher in May and November than in August, while δ15N values of the POM were higher in May and August than in November. These changes are attributed to seasonal changes in phytoplankton growth rate and nitrate availability. δ13C and δ15N values in the sediments off Tokachi were lowest near the Tokachi River mouth, and increased offshore to constant values that persisted from 134 to 1995 m water depth. The spatial variation in C / N ratios in the sediment mirrored those of δ13C and δ15N. Comparison of δ13C, δ15N and C / N ratios in the sediments off Tokachi with those in the Tokachi River and seawater indicates that about half of the organic matter in the sediment was of terrestrial origin near the Tokachi River mouth, and the sedimentary organic matter from 134 to 1995 m water depth was of marine origin. The organic C content in the sediment was high near the Tokachi River mouth, and also around 1000 m water depth. The C content was significantly correlated with silt plus clay content, with different regression lines for those stations shallower and deeper than 134 m, owing to several stations of higher C content with the elevated C / N ratio on the inner shelf. These results suggest that transport and deposition of organic-rich fine sediment particles by hydrodynamic processes were major factors controlling C content off Tokachi. In addition, the supply of a fraction of terrestrial organic matter with high C / N probably also affected C content on the inner shelf.  相似文献   

14.
高原湖泊溶解有机质的三维荧光光谱特性初步研究   总被引:9,自引:0,他引:9  
提要近年来,荧光光谱技术被广泛应用于研究天然水体中溶解有机质(Dissolved Organic Matter,DOM)的物理化学特性。为了理解高原湖泊中DOM的组成、荧光光谱特性及其在湖泊水体中的垂直分布情况,作者利用荧光发射光谱、三维荧光光谱研究云贵高原湖泊红枫湖和百花湖中的DOM。结果显示,高原湖泊DOM主要表现为类富里酸荧光,包括可见区和紫外区两种类型的荧光峰,各种天然水体中都有报道的类蛋白荧光在红枫湖DOM中并不明显,而在百花湖DOM中则有较强的类蛋白荧光。溶解有机质所含三种类型荧光峰(PeakA:紫外区类富里酸荧光峰;PeakB:可见区类富里酸荧光峰;PeakC:类蛋白荧光峰)的荧光强度与溶解有机碳之间没有明显的线性相关关系,可能与高原湖泊水体pH值、DOM在湖泊不同深度由于受到光降解、微生物降解等作用存在差异等因素有关。与有机质结构和成熟度有关的荧光峰比值r(A,C)在1.40—2.09范围内,红枫湖南湖、百花湖DOM样品的r(A,C)值随着水体深度有下降的趋势,而红枫湖北湖DOM的r(A,C)值则随着水体深度有较大起伏,揭示了高原湖泊水体中DOM的结构以及分布情况存在差异。另外,r(A,C)值与pH之间具有良好的正相关关系,其相关系数红枫湖南湖DOM为0.95,百花湖两个采样点DOM分别为0.67、0.75,而红枫湖北湖DOM则显示出一定的负相关性(R2=0.45)。  相似文献   

15.
To examine the source and preservation of organic matter in the shelf sediments of the East China Sea (ECS), we measured bulk C/N and isotopes, organic biomarkers (n-alkanes and fatty acids) and compound-specific (fatty acids) stable carbon isotope ratios in three sediment cores collected from two sites near the Changjiang Estuary and one in the ECS shelf. Contrasting chemical and isotopic compositions of organic matter were observed between the estuarine and shelf sediments. The concentrations of total n-alkanes and fatty acids in the shelf surface sediments (0–2 cm) were 5–10 times higher than those in estuarine surface sediments but they all decreased rapidly to comparable levels below the surface layer. The compositions of n-alkanes in the estuarine sediments were dominated by C26-C33 long-chain n-alkanes with a strong odd-to-even carbon number predominance. In contrast, the composition of n-alkanes in the shelf sediment was dominated by nC15 to nC22 compounds. Long-chain (>C20) fatty acids (terrestrial biomarkers) accounted for a significantly higher fraction in the estuarine sediments compared to that in the shelf sediment, while short-chain (<C20) saturated and unsaturated fatty acids were more abundant in the shelf surface sediments than in the estuarine sediments. Stable carbon isotopic ratios of individual fatty acids showed a general positive shift from estuarine to shelf sediments, consistent with the variations in bulk δ 13CTOCTOC. These contrasts between the estuarine and shelf sediments indicate that terrestrial organic matter was mainly deposited within the Changjiang Estuary and inner shelf of ECS. Post-depositional diagenetic processes in the surface sediments rapidly altered the chemical compositions and control the preservation of organic matter in the region.  相似文献   

16.
Seafloor geomorphology and surficial stratigraphy of the New Jersey middle continental shelf provide a detailed record of sea-level change during the last advance and retreat of the Laurentide ice sheet (120 kyr B.P. to Present). A NW–SE-oriented corridor on the middle shelf between water depths of 40 m (the mid-shelf “paleo-shore”) and 100 m (the Franklin “paleo-shore”) encompasses 500 line-km of 2D Huntec boomer profiles (500–3500 Hz), an embedded 4.6 km2 3D volume, and a 490 km2 swath bathymetry map. We use these data to develop a relative stratigraphy. Core samples from published studies also provide some chronological and sedimentological constraints on the upper <5 m of the stratigraphic succession.The following stratigraphic units and surfaces occur (from bottom to top): (1) “R”, a high-amplitude reflection that separates sediment >46.5 kyr old (by AMS 14C dating) from overlying sediment wedges; (2) the outer shelf wedge, a marine unit up to 50 m thick that onlaps “R”; (3) “Channels”, a reflection sub-parallel to the seafloor that incises “R”, and appears as a dendritic system of channels in map view; (4) “Channels” fill, the upper portion of which is sampled and known to represent deepening-upward marine sediments 12.3 kyr in age; (5) the “T” horizon, a seismically discontinuous surface that caps “Channels” fill; (6) oblique ridge deposits, coarse-grained shelly units comprised of km-scale, shallow shelf bedforms; and (7) ribbon-floored swales, bathymetric depressions parallel to modern shelf currents that truncate the oblique ridges and cut into surficial deposits.We interpret this succession of features in light of a global eustatic sea-level curve and the consequent migration of the coastline across the middle shelf during the last 120 kyr. The morphology of the New Jersey middle shelf shows a discrete sequence of stratigraphic elements, and reflects the pulsed episodicity of the last sea-level cycle. “R” is a complicated marine/non-marine erosional surface formed during the last regression, while the outer shelf wedge represents a shelf wedge emplaced during a minor glacial retreat before maximum Wisconsin lowstand (i.e., marine oxygen isotope stage 3.1). “Channels” is a widespread fluvial subarial erosion surface formed at the late Wisconsin glacial maximum 22 kyr B.P. The shoreline migrated back across the mid-shelf corridor non-uniformly during the period represented by “Channels” fill. Oblique ridges are relict features on the New Jersey middle shelf, while the ribbon-floored swales represent modern shelf erosion. There is no systematic relationship between modern seafloor morphology and the very shallowly buried stratigraphic succession.  相似文献   

17.
Elemental (TOC, TN, C/N) and stable carbon isotopic (δ13C) compositions and n-alkane (nC16–38) concentrations were measured for Spartina alterniflora, a C4 marsh grass, Typha latifolia, a C3 marsh grass, and three sediment cores collected from middle and upper estuarine sites from the Plum Island salt marshes. Our results indicated that the organic matter preserved in the sediments was highly affected by the marsh plants that dominated the sampling sites. δ13C values of organic matter preserved in the upper fresh water site sediment were more negative (−23.0±0.3‰) as affected by the C3 plants than the values of organic matter preserved in the sediments of middle (−18.9±0.8‰) and mud flat sites (−19.4±0.1‰) as influenced mainly by the C4 marsh plants. The distribution of n-alkanes measured in all sediments showed similar patterns as those determined in the marsh grasses S. alterniflora and T. latifolia, and nC21 to nC33 long-chain n-alkanes were the major compounds determined in all sediment samples. The strong odd-to-even carbon numbered n-alkane predominance was found in all three sediments and nC29 was the most abundant homologue in all samples measured. Both δ13C compositions of organic matter and n-alkane distributions in these sediments indicate that the marsh plants could contribute significant amount of organic matter preserved in Plum Island salt marsh sediments. This suggests that salt marshes play an important role in the cycling of nutrients and organic carbon in the estuary and adjacent coastal waters.  相似文献   

18.
Abstract

An analysis of the relationship between organic matter and liquid and plastic limits, and grain‐specific gravity of a marine sediment was accomplished by making a stepwise adjustment in the organic content of that sediment. The sample used was from Santa Barbara Basin (off southern California) and is typical of fine‐grained marine sediments: it is a clayey silt with a common suite of minerals and other constituents. During the experiment, texture and composition (except organic content) were constant; only the quantity of natural, indigenous organic matter was changed.

A strong linear relationship exists between the independent variable, the amount of organic carbon present in the sample, and the dependent variables. Liquid limit, plastic limit, and plasticity index all increased with increasing organic content over the range studied (0.57–3.20% organic carbon). Grain‐specific gravity decreased. All had linear correlation coefficients (r) greater than |0.90| and r 2 values greater than 90%, except the plasticity index (83%).

Comparing the results of regression analyses from this and several similar studies shows that although there is good qualitative agreement, there are quantitative inconsistencies. In particular there is considerable overall variability in the regression coefficients. Among studies on marine sediments the inconsistencies are less pronounced, yet still evident. The increase in liquid limit as organic carbon increased by 1 % sediment dry weight ranged from 9 to 28% water content; in the plastic limit the range was from 4 to 18%. However, in these marine studies regression coefficients are relatively close in value in some cases, levels of significance of the regressions are high in most cases, and in all cases the relationships appear to be linear over the range of organic carbon percentage studied. Finally, we believe that a relatively clear relationship between plasticity and organic carbon begins to emerge when the latter exceeds a value of 2%.  相似文献   

19.
Multi-biomarker indexes were analyzed for two piston cores from potential cold seep areas of the South China Sea off southwestern Taiwan. Total organic carbon(TOC) normalized terrestrial(n-alkanes) and marine(brassicasterol, dinosterol, alkenones and iso-GDGTs) biomarker contents and ratios(TMBR, 1/Pmar-aq, BIT) were used to evaluate the contributions of terrestrial and marine organic matter(TOM and MOM respectively) to the sedimentary organic matter, indicating that MOM dominated the organic sources in Core MD052911 and the sedimentary organic matter in Core ORI-_(86)0-22 was mainly derived from terrestrial inputs, and different morphologies were the likely reason for TOM percentage differences. BIT results suggested that river-transported terrestrial soil organic matter was not a major source of TOM of sedimentary organic matter around these settings.Diagnostic biomarkers for methane-oxidizing archaea(MOA) were only detected in one sample at 172 cm depth of Core ORI-_(86)0-22, with abnormally high iso-GDGTs content and Methane Index(MI) value(0.94). These results indicated high anaerobic oxidation of methane(AOM) activities at or around 172 cm in Core ORI-_(86)0-22.However in Core MD052911, MOA biomarkers were not detected and MI values were lower(0.19–0.38), indicated insignificant contributions of iso-GDGTs from methanotrophic archaea and the absence of significant AOM activities. Biomarker results thus indicated that the discontinuous upward methane seepage and insufficient methane flux could not induce high AOM activities in our sampling sites. In addition, the different patterns of TEX_(86) and U_(37)~(K′) temperature in two cores suggested that AOM activities affected TEX_(86)37 temperature estimates with lower values in Core ORI-_(86)0-22, but not significantly on TEX_(86) temperature estimates in Core MD052911.  相似文献   

20.
Benthic foraminifera are investigated in sediment core LV28-34-2 (53°51.971′N, 146°47.499′E, sea depth 1431 m, core length 965 cm). The distribution of foraminifera is studied in coarse-grained (>0.125 mm) sediment fractions of 191 samples taken with a step of 5 cm. The core covers the interval from oxygen isotope stage (OIS) 6 up to the Holocene. The foraminiferal assemblages of the penultimate (OIS 6) and last (OIS 5d-2) glaciations are characterized by low abundances and prevalence of Uvigerina auberiana. The specific structure of the OIS 6 assemblages differs from the last glaciation ones by the mass presence of Cassidulina teretis, which characterizes low temperatures and a high influx of organic matter to the sea floor. The major factor responsible for the weak development of benthic foraminifers during the glacial time is the deficiency of food resources. The foraminiferal assemblage of the interglacial optimum (OIS 5e) is composed of both calcareous and agglutinated species (Martinottiella communis).The inflow of Pacific waters was probably more intensive, the bioproductivity was higher, and the critical carbonate compensation depth was shallower at that time than during the glaciations. During the deglaciation, the quantity and structure of the foraminiferal assemblages reflected two pulses of warming (terminations 1B and 1A) with an intervenient cooling event (Younger Dryassic). The assemblages of warm periods were characterized by exclusively high foraminiferal abundances, which sharply decreased during the Younger Dryassic cooling. The specific structure of the foraminiferal assemblage during deglaciation was relatively uniform, being composed of only calcareous taxa typical of highly productive areas of the ocean. The paleoenvironmental conditions were similar to the conditions of the interglacial optimum (OIS 5e), although the preservation of calcareous tests is better. In the Holocene sediments, the concentration of benthic foraminifera is substantially lower than during the deglaciation, which is explainable by their dilution in the sediments by diatoms; the composition of the foraminiferal assemblages is similar to that of their glacial counterparts.  相似文献   

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