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1.
The stability of a mangrove ecosystem in Cananeia, Brazil, is assessed based on investigations of the site-specific temporal rise in relative sea level during the past 50 years, 100-year sediment accumulation rates (SAR) and sources of organic matter (OM). Addressing this, three sediment cores were collected in a transect, intertidal mud flat, mangrove margin and well into the forest. The net SAR, as estimated by the age–depth relationships of 210Pb and 137Cs, is between 2.5 and 3.9 mm year−1. These rates are comparable to the estimates based on the Pb and Zn contaminant markers corresponding to mining initiation in the region in 1918. Further, the SARs are lower than the rate of regional relative sea level rise (4 mm year−1) as indicated by the past 50-year tide gauge record, but the rate is higher than the eustatic sea level rise (1.7 ± 0.3 mm year−1). The stratigraphies of TOC/TN, δ13C(OC), OP and δ15N indicate site-specific mangal vegetal litter, which is the predominant source of OM at all core sites, during the past century and reflects a stable mangal system over that time span.  相似文献   

2.
The distribution and nature of sedimentary organic matter (OM) have been examined in sediment cores collected from the lower reaches, including estuary, of the Godavari River in order to understand sedimentation patterns, sources, and diagenesis of OM. The samples were analyzed for organic carbon (Corg), total nitrogen, amino acids and hexosamines. The observed irregular trends in Corg distribution with depth indicate the unstable nature of bed sediment in the lower reaches of the river. Yet, in the lower estuarine region, regular trends in Corg distribution with depth reflect the deposition of sediment. The atomic ratio of Corg and total nitrogen (C/N; 10.5–16.1) also supported this observation. The distribution of amino acids and diagenetic indicators (β-ala+γ-aba mol.%, AA/HA and Glc-NH2/Gal-NH2) in individual cores revealed the post-depositional changes in the OM. In the core sediment from the lower reaches, there was no clear cut trend in amino acid content with depth. In the estuarine region, however, amino acid content was very low (50.5 and 186.5 μg g−1) in the upper layers compared to that in the lower layers (558.5 and 1099.3 μg g−1). Reactivity index (range 0.3–3.7) revealed that OM in the upper few centimeters, especially in the lower estuarine region, was more reactive relative to that in the deeper layers.  相似文献   

3.
The stable isotope compositions of organic carbon and nitrogen, the contents of organic carbon and nitrogen and C/N ratios for two cores recovered from the Empakai Crater at water depths of 11 and 20 m are used to document climatic changes in northern Tanzania. Eight 14C AMS dates determined on total organic matter (OM) indicate that the sedimentation rate in this lake is about 30 cm/ka for the late Pleistocene to early Holocene period. There are differences in the δ13C values of organic carbon between the two cores, which may be a result of differences in location from the present shoreline and of different water depths. In the deeper-water core the δ13C values show a general downcore decrease to the base of the core with a sharp change to lower values of about 4‰ at a depth of 100 cm (8.7 ka). The general trend of downcore decrease in 13C values can be attributed either to a systematic decrease in the relative proportion of C4 type of OM, owing to an increase in precipitation and change in vegetation cover from grassland to forest, or to utilization of isotopically enriched carbon during photosynthesis. The δ15N values show a general downcore increase with again a sharp change of about 5‰ to lower values at about 8.7 ka. A sharp change of about 5‰ and 4‰ to more depleted values at a depth of 100 cm of both 15N and 13C, respectively, suggests either hiatus or abrupt change in climatic condition from wetter conditions to drier conditions. There is enhanced preservation of OM in the lake as depicted by high mean values of organic carbon and nitrogen at both sites.  相似文献   

4.
Gaseous methane loss from a brackish, intertidal salt marsh sediment was measured in April, June, August, and October 1977. Twenty-four sediment cores were taken on each date. Annual loss of methane carbon from the mud flats was 10.7 g CH4?C per m2 per year, a value closer to freshwater values than marine systems. Loss of methane fromSpartina peat was low.  相似文献   

5.
Widespread mud volcanism across the thick (≤ 14 km) seismically active sedimentary prism of the Gulf of Cadiz is driven by tectonic activity along extensive strike–slip faults and thrusts associated with the accommodation of the Africa–Eurasia convergence and building of the Arc of Gibraltar, respectively. An investigation of eleven active sites located on the Moroccan Margin and in deeper waters across the wedge showed that light volatile hydrocarbon gases vented at the mud volcanoes (MVs) have distinct, mainly thermogenic, origins. Gases of higher and lower thermal maturities are mixed at Ginsburg and Mercator MVs on the Moroccan Margin, probably because high maturity gases that are trapped beneath evaporite deposits are transported upwards at the MVs and mixed with shallower, less mature, thermogenic gases during migration. At all other sites except for the westernmost Porto MV, δ13C–CH4 and δ2H–CH4 values of ~ − 50‰ and − 200‰, respectively, suggest a common origin for methane; however, the ratio of CH4/(C2H6 + C3H8) varies from ~ 10 to > 7000 between sites. Mixing of shallow biogenic and deep thermogenic gases cannot account for the observed compositions which instead result mainly from extensive migration of thermogenic gases in the deeply-buried sediments, possibly associated with biodegradation of C2+ homologues and secondary methane production at Captain Arutyunov and Carlos Ribeiro MVs. At the deep-water Bonjardim, Olenin and Carlos Ribeiro MVs, generation of C2+-enriched gases is probably promoted by high heat flux anomalies which have been measured in the western area of the wedge. At Porto MV, gases are highly enriched in CH4 having δ13C–CH4 ~ − 50‰, as at most sites, but markedly lower δ2H–CH4 values < − 250‰, suggesting that it is not generated by thermal cracking of n-alkanes but rather that it has a deep Archaeal origin. The presence of petroleum-type hydrocarbons is consistent with a thermogenic origin, and at sites where CH4 is predominant support the suggestion that gases have experienced extensive transport during which they mobilized oil from sediments ~ 2–4 km deep. These fluids then migrate into shallower, thermally immature muds, driving their mobilization and extrusion at the seafloor. At Porto MV, the limited presence of petroleum in mud breccia sediments further supports the hypothesis of a predominantly deep microbial origin of CH4.  相似文献   

6.
In this work, fossil molluscan assemblages are analyzed in order to reconstruct the evolution of the Northern Gulf of Thailand during the Holocene. The marine sediments (Bangkok Clay Formation) of the Lower Central Plain of Bangkok and the coastal plain of Phetchaburi were sampled at 16 localities, obtaining fossil shells and mangrove peat whose 14C ages range from 9000 to 2000 CYBP. A statistical treatment of abundance data returned four major groups, namely the Dendostrea rosacea association (intertidal mud), the Corbula fortisulcata–Mactra luzonica association (shallow infralittoral sandy mud), the Nuculana mauritiana–Timoclea scabra association (infralittoral sand spit) and the Timoclea scabra–Arcopagia pudica association (infralittoral mud). The data allowed both a stratigraphic correlation along two transects covering all of the Holocene basin and the creation of digitalized maps showing the presumable extension of the Thai paleogulf around the apex of Flandrian transgression event (about 5500 CYBP).  相似文献   

7.
Flax Pond is a small (0.5 km2) salt marsh on the north shore of Long Island, New York. Two 1 m2 plots within each of the following environments were covered with a marker layer of either brick dust or aluminum glitter: 1) bare mud flats; 2) areas newly colonized by Spartina alterniflora; and 3) high intertidal. S. alterniflora peat surfaces. Monthly cores revealed the amount of sediment that accumulated since placement of the marker. Accretion rates from October, 1974 to February, 1976 were as follows: bare mud flats ?20.5 to 45.5 mm/yr; recently vegetated mud flats ?9.5 to 37.0 mm/yr; and high intertidal peat surfaces ?2.0 to 4.25 mm/yr. Sedimentation rates decrease with increasing elevation because of the reduced tidal submergence time and decreased height of the overlying water column. In areas of low elevation, ice and storms cause either erosion or a reduced rate of accretion during the winter months. The average mud accretion rate over the past 173 years is 3.4 mm/yr. Differences between the short-term rate and the long-term rate indicate substantial annual variation in the accumulation of mud in salt marshes. Short-term rates of peat accretion are similar to long-term estimates, indicating that rates of peat accretion are relatively constant over long intervals.  相似文献   

8.
Structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite, a bituminous shale and an oil shale of the Kimmeridge Clay Formation (KCF) in Dorset were studied in order to gain insight into (i) the type and extent of water column anoxia and (ii) changes in the concentration and isotopic composition of dissolved inorganic carbon (DIC) in the palaeowater column. The samples studied fit into the curve of increasing δ13C of the kerogen (δ13CTOC) with increasing TOC, reported by Huc et al. (1992). Their hypothesis, that the positive correlation between TOC and δ13CTOC is the result of differing degrees of organic matter (OM) mineralisation in the water column, was tested by measuring the δ13C values of primary production markers. These δ13C values were found to differ on average by only 1‰ among the samples, implying that differences in the extent of OM mineralisation cannot fully account for the 3‰ difference in δ13CTOC. The extractable OM in the oil shale differs from that in the other sediments due to both differences in maturity, and differences in the planktonic community. These differences, however, are not likely to have significantly influenced δ13CTOC either. All three sediments contain abundant derivatives of isorenieratene, indicating that periodically euxinia was extending into the photic zone. The sediments are rich in organic sulfur, as revealed by the abundant sulfur compounds in the pyrolysates. The prominence of C1-C3 alkylated thiophenes over n-alkanes and n-alkenes is most pronounced in the pyrolysate of the sediment richest in TOC. This suggests that sulfurisation of OM may have played an important role in determining the TOC-δ13CTOC relationship reported by Huc et al. (1992).  相似文献   

9.
To assess whether the biochemical characteristics of peat can provide clues for past ecosystem changes or not, a study was carried out combining elemental analysis, micro-morphological counts and neutral monosaccharide determination of peat organic matter (OM) and the dominant living plants from a formerly cut-over peat bog in the Jura Mountains. Peat profiles (up to 50 cm depth) from two distinctive zones (bare peat, FRA and a regenerating stage, FRC) were compared with the reference profile (FRD) taken from an unexploited area of the bog. The results show contrasting OM composition along the profiles. In the upper sections of the FRD and FRC profiles, high C/N ratios and sugar contents (in the same range as in the source plants) and the large predominance of well preserved plant tissues indicate good preservation of primary biological inputs. In contrast, in peat from the FRA profile and deeper levels of the FRC profile, lower C/N ratios, lower amounts of sugars and a predominance of amorphous OM and mucilage suggest more extensive OM degradation. These features delineate a clear threshold between an uppermost “new” regenerating peat section and an “old” catotelm peat below. Nevertheless, even in the latter, the sugar contents remain relatively high (>80 mg/g) compared with other organic and mineral soils. Analysis of typical peat-forming plants and of bulk peat and fine grained fractions allowed identification of the following source indicators: xylose and arabinose for Cyperaceae; rhamnose, galactose and mannose for mosses; and ribose (and to a lesser extent, hemicellulosic glucose) possibly for microbial synthesis.  相似文献   

10.
Coastal environments of northwest Alaska preserve a detailed record of sea level change during the past 5000 14C yr. Rapid burial of intertidal marshes by storm overwash processes, a short open water period, and the arctic maritime climate contribute to the preservation of marine and eolian facies. Eustatic and storm-controlled changes in sea level can be identified from interbedded sequences of marsh peat and coastal flood deposits on barrier islands and estuaries along northwest Seward Peninsula. Mean eustatic sea level has risen about 1.5 m during the last 5000 years, at an average of ca. 0.028 cm yr−1, based on the depth and age relationship of a suite of 23 peat horizons sampled from a 140 km-long reach of coast. Nearshore erosion and sedimentation are controlled by secular variations in wave climate. Peaks in tidal amplitude and storminess correlate with transgressive sedimentary regimes and occurred during the periods 3300–1700, 1200–900, and since about 400 14C yr BP. Short-term fluctuations in relative sea level are superimposed on the long-term regional eustatic trend, and record the coastal response to variable rates of sea-level change during the late Holocene.  相似文献   

11.
A number of studies have revealed that the climate in the eastern margin of the Tibetan Plateau and Northeast China is sensitive to postglacial changes. Unfortunately, the link of the past climate evolution between the two regions is not well understood. In this study, two cores are analyzed to determine this link directly. The high-resolution n-alkanol distribution patterns from two typical peat sequences covering the past 16,000 cal years in the northeastern margin of the Tibetan Plateau and Northeast China, respectively, are closely examined by gas chromatograph–mass spectrometry analysis. In combination with other palaeoclimatic proxies, it is proposed that the n-alkanol average chain length and (C22 + C24)/(C26 + C28) ratio could reflect past climate changes in the two peat sequences. The n-alkanol proxies reveal several climatic intervals in the period from the last deglaciation through the Holocene. A comparison of n-alkanol records between the northeastern margin of the Tibetan Plateau and Northeast China indicates that the start and end of the warm Holocene Optimum differed at the two locations. The spatially asynchronous pattern of climatic change is possibly a result of different responses to change in solar radiation. The evolution of the Holocene paleoclimate is more consistent with changes in Northern Hemisphere solar radiation in Northeast China than on the Tibetan Plateau. The Holocene Optimum began and terminated earlier in Northeast China than in the northeastern margin of the Tibetan Plateau. Thus, the two n-alkanol proxies provide valuable insights into the regional Holocene climate and local environmental conditions.  相似文献   

12.
As a contribution to a project on the role of aerobic heterotrophic bacteria in the organic matter turnover within cyanobacterial mats, we investigated selected lipid classes of five morphologically different microbial mats from the intertidal area of Abu Dhabi, United Arab Emirates. The mats thrive under extreme environmental conditions as they are exposed to salinity up to 200 and shadowless sunshine for nearly 12 h per day. The total extractable organic matter is characterised by heavy δ13C values of −17‰ to −10‰, suggesting CO2 limitation. The lipids reflect the biomass of the principal mat-building phototrophic and heterotrophic microorganisms. Moderate amounts of hydrocarbons, mainly cyanobacterial n-heptadecane, short chain monomethyl alkanes and diploptene were detected in all the mats. The ubiquitous n-hexadecanoic acid is dominant amongst the free and ester-bound phospholipid fatty acids (PLFAs), whereas the overall acid composition varies due to differences in the extreme environmental conditions. Additionally, the top layers of two of the five mats were analysed separately to compare their lipid compositions with those of the whole mats. As would be expected, the concentrations of cyanobacterial biomarkers exceed those found in the total mats. Noteworthy is the presence of n-heneicosadiene as predominant hydrocarbon in the top layer of the so called gelatinous mat, which thrives under the locally most extreme conditions. As the n-alkadiene was present to a much lesser extent in the intact mat, it may provide a new biomarker for a so far unknown important aerobic organism.  相似文献   

13.
Carbon isotope and molecular compositions of Mississippian to Upper Cretaceous mud gases have been examined from four depth profiles across the Western Canada Sedimentary Basin (WCSB). The profiles range from the shallow oil sands in the east (R0 = 0.25) to the very mature sediments in the overthrust zone to the west (R0 = 2.5). In the undisturbed WCSB, δ13C1–δ13C2 and δ13C2–δ13C3 cross-plots show three maturity and alteration trends: (1) pre-Cretaceous gas sourced from type II kerogen; (2) Cretaceous Colorado Group gas; and (3) Lower Cretaceous Mannville Group biodegraded gas. A fourth set of distinctly different maturity trends is recognized for Lower Cretaceous gas sourced from type III kerogen in the disturbed belt of the WCSB. Displacement of these latter maturity trends to high δ13C2 values suggests that the sampled gas was trapped after earlier formed gas escaped, probably as a result of overthrusting. Unusually 13C-enriched gas (δ13C1 = −34‰, δ13C2 = −13‰, and δ13C3 = 0‰), from the Gething Formation in the disturbed belt, is the result of late stage gas cracking in a closed system. In general, gas maturity is consistent with the maturity of the host sediments in the WCSB, suggesting that migration and mixing of gases was not pervasive on a broad regional and stratigraphic scale. The ‘Deep Basin’ portion of the WCSB is an exception. Here extensive cross-formational homogenization of gases has occurred, in addition to updip migration along the most permeable stratigraphic units.  相似文献   

14.
《Organic Geochemistry》2012,42(12):1277-1284
Compound-specific isotope measurements of organic compounds are increasingly important in palaeoclimate reconstruction. Searching for more accurate peat-based palaeoenvironmental proxies, compound-specific fractionation of stable C, H and O isotopes of organic compounds synthesized by Sphagnum were determined in a greenhouse study. Three Sphagnum species were grown under controlled climate conditions. Stable isotope ratios of cellulose, bulk organic matter (OM) and C21–C25 n-alkanes were measured to explore whether fractionation in Sphagnum is species-specific, as a result of either environmental conditions or genetic variation. The oxygen isotopic composition (δ18O) of cellulose was equal for all species and all treatments. The hydrogen isotopic composition (δD) of the n-alkanes displayed an unexpected variation among the species, with values between −154‰ for Sphagnum rubellum and −184‰ for Sphagnum fallax for the C23 n-alkane, irrespective of groundwater level. The stable carbon isotopic composition (δ13C) of the latter also showed a species-specific pattern. The pattern was similar for the carbon isotope fractionation of bulk OM, although the C23 n-alkane was >10‰ more depleted than the bulk OM. The variation in H fractionation may originate in the lipid biosynthesis, whereas C fractionation is also related to humidity conditions. Our findings clearly emphasize the importance of species identification in palaeoclimate studies based on stable isotopes from peat cores.  相似文献   

15.
Lipid biomarkers from a peat plateau profile from the Northeast European Russian Arctic were analyzed. The peat originated as a wet fen ca. 9 ka BP and developed into a peat bog after the onset of permafrost ca. 2.5 ka BP. The distributions and abundances of n-alkanols, n-alkanoic acids, n-alkanes, n-alkan-2-ones and sterols were determined to study the effect of degradation on their paleoclimate proxy information. Plant macrofossil analysis was also used in combination with the lipid distributions. The n-alkanol and n-alkanoic acid distributions in the upper part of the sequence generally correspond to compositions expected from plant macrofossil assemblages. Their carbon preference index (CPI) values increase with depth and age, whereas those of the n-alkanes decrease. The different CPI patterns suggest that n-alkanoic acids and n-alkanols deeper in the sequence may be produced during humification through alteration of other lipids. Excursions in the n-alkanoic acid content also suggest an important contribution of invasive roots to the lipid biomarker composition. The CPIs associated with these compounds show that under permafrost conditions organic material from Sphagnum is better preserved than material from vascular plants. Increasing stanol/stenol ratio values and decreasing n-alkane CPI values indicate progressive degradation of organic matter (OM) with depth. The n-alkan-2-one/n-alkane and n-alkan-2-one/n-alkanoic acid ratios were shown to be useful proxies that can reflect the degree of OM preservation and suggest that both microbial oxidation of n-alkanes and decarboxylation of n-alkanoic acids produce n-alkan-2-ones in this peat sequence.  相似文献   

16.
Surface sediments from the subtropical Pearl River estuary and adjacent South China Sea were investigated by molecular organic geochemical methods to determine the composition, distribution and origin of extractable lipids (n-alkanes, n-alkanols and sterols). The absolute and organic C normalized concentrations of total alkane, n-alkanol, and sterol ranged from 0.16 to 2.67 μg g−1 and 0.9 to 12.3 μg g−1 OC, 24.4 to 427.3 ng g−1 and 63.2 to 1966.7 ng g−1 OC, and 9.0 to 493.5 ng g−1 and 58.4 to 1042.4 ng g−1 OC, respectively. The spatial distributions of these biomarkers indicated that terrestrial-derived molecular biomarkers such as long-chain n-alkanes, n-alkanols and plant-derived sterols were higher at the river mouth and along the coastline, suggesting that a higher proportion of terrestrial particulate organic matter was deposited there. Relatively lower amounts of marine-derived biomarkers such as short-chain n-alkanes, algal sterols at the river mouth reflected the lower primary productivity due to high turbidity. The spatial patterns of these biomarkers were partially related to the estuarine processes and conditions, evidencing an increased terrestrial signal from the Pearl River mouth to the inner estuary, and enhanced marine conditions further offshore.  相似文献   

17.
Models of peat accumulation assume that peat decomposition occurs mostly above the water table, with little or no decomposition once it enters the deeper, saturated, anoxic zone. Few studies have used molecular biomarkers for tracing post depositional, decomposition-related trends in peat deposits. We studied the major diagenetic changes in the phenolic constituents within a Sphagnum-dominated ombrotrophic bog deposit. The yield of lignin-derived phenols and degree of decomposition, measured using alkaline cupric oxide oxidation of bulk peat samples and their corresponding humic acids, revealed that most of the degradation takes place in the surface layers corresponding to the acrotelm. In fact, total phenolic constituents of peat samples decrease from 36.1 to 21.6 mg g−1 OC (organic carbon) over the first 36 cm, whereas in the deeper anoxic layers, phenolic constituents tend to accumulate, reaching a highest concentration of 71.0 mg g−1 OC. The diagenetic alteration of these phenolic constituents during peat accumulation involves significant demethoxylation and an increasing yield of vanillyl oxidation products despite the low redox potential. Syringyl phenols tend to be particularly resistant to diagenetic alteration and are significantly enriched within the humic acid fraction (twofold) with respect to the bulk peat. This, together with the higher degree of oxidation, suggests that this organic matter fraction is enriched in more resistant, but nonetheless diagenetically altered, phenolic constituents. This suggests that humic acids constitute a refractory pool of organic C with a relatively low turnover rate. Our results confirm that the major processes involved in the variation in phenolic constituents with depth are strongly related to the post depositional environment and that evaluation of diagenetic trends in phenolic constituents may provide molecular-level information on the changes that fresh biomass undergoes during early diagenesis in peatlands.  相似文献   

18.
Lead isotope amount ratios are commonly used in diverse fields such as archaeometry, geochemistry and forensic science. Currently, five reference materials with certified lead isotope amount ratios are available, namely NIST SRM 981, 982 and 983, GBW‐04442 and NMIJ 3681‐a. Only NIST SRM 981 and NMIJ 3681‐a have approximately natural isotopic compositions, and NIST SRM 981 is predominantly used for correcting mass discrimination/mass fractionation in the applied mass spectrometric procedures. Consequently, there is no other certified reference material available to be used for validation and/or quality control of the analytical procedures applied to lead isotope amount ratio measurements. To fill this gap, two new reference materials have been produced and certified for their lead isotope amount ratios. For both certified reference materials, complete uncertainty budgets have been calculated and SI traceability has been established. This provides the users with independent means for validating and verifying their analytical procedures and for conducting quality control measures. ERM‐EB400 is a bronze material with a nominal lead mass fraction of 45 mg kg?1 and certified lead isotope amount ratios of n(206Pb)/n(204Pb) = 18.072(17) mol mol?1, n(207Pb)/n(204Pb) = 15.578(18) mol mol?1 and n(208Pb)/n(204Pb) = 38.075(46) mol mol?1 with the associated expanded uncertainties (k = 2) given in brackets. ERM‐AE142 is a high‐purity solution of lead in 2% nitric acid with a nominal mass fraction of 100 mg kg?1 and certified Pb isotope amount ratios of n(206Pb)/n(204Pb) = 21.114(17) mol mol?1, n(207Pb)/n(204Pb) = 15.944(17) mol mol?1 and n(208Pb)/n(204Pb) = 39.850(44) mol mol?1 with the associated expanded uncertainties (k = 2) given in brackets. Both materials are specifically designed to fall within the natural lead isotopic variation and to assist users with the validation and verification of their analytical procedures. Note that while one of these reference materials requires the chemical separation of Pb from its matrix (ERM‐EB400), the other does not (ERM‐AE142). As additional information, δ208/206PbNIST SRM981 values are provided for both materials. For ERM‐AE142, a delta value of δ208/206PbNIST SRM981 = ?28.21(30)‰ was obtained, and for ERM‐EB400, a delta value of δ208/206PbNIST SRM981= ?129.47(38)‰ was obtained, with the associated expanded uncertainties (k = 2) given in brackets.  相似文献   

19.
We determined biomarker concentrations and distributions for surface sediments from 54 sites in the Pearl River Estuary, China. We focus on a suite of four biomarker-based indicators for relative terrestrial to marine organic matter (OM) source: the branched-isoprenoid tetraether (BIT) index, the ratio of high/low molecular weight n-alcohols [(ΣC26–34/(ΣC16+18 + ΣC26–34)], an analogous ratio for n-fatty acids and the ΣC29-steroids/(ΣC29-steroids + brassicasterol) ratio. All four exhibit the same terrestrial to marine transition seen in previous bulk δ13C studies, but with an abrupt decrease in the relative terrestrial contribution across the delta front to pro-delta transition. Concentrations of terrestrially-derived biomarkers show no systematic decrease across the transition. Instead, the decrease in the proportion of terrestrial OM is due to a decrease in the sedimentation rate and associated terrestrial OM burial across the delta toe. This suggests that diagenetic controls on the fate of terrestrial OM, such as increased biodegradation where sedimentation rate is low, are subordinate to sedimentological processes. Biomarker-derived temperature values are cooler than expected for the lower Pearl River catchment, suggesting that the dominant component of the terrestrial OM is derived from the cooler upland regions of the catchment. The dominance of input from more distal terrain with greater topographic relief is evidence for the importance of geomorphological control on terrigenous OM transport. Collectively, the results demonstrate the importance of sedimentological processes in the supply, deposition and transport of terrestrial OM.  相似文献   

20.
The influence of the northern Atlantic and tropical monsoonal systems, as recorded by the River Nile, on the climate variability of the southeastern Mediterranean was studied in two cores taken by the R/V Marion Dufresne: one core taken SE of Cyprus representing the northern Levantine Basin (core 9501, 980 m water depth) and the other located ˜380 km further south, represents the southeastern Levantine Basin in an area influenced by the River Nile plume (core 9509, 884 m water depth). The study was performed at relatively high resolution using several proxies: δ18O of Globigerinoides ruber, sediment characteristics and index colour parameters in core sections representing the last 86 ka. A low-resolution alkenone sea surface temperature record was also measured. The time frame in both cores was mostly constrained by ‘wiggle’ matching with the nearby well-dated δ18O and δ13C record of the Soreq Cave, which is mainly influenced by the eastern Mediterranean water vapor. The sedimentary record of the southern core is strongly influenced by the River Nile contribution throughout the last 86 ka, as evidenced by the higher sedimentation rates compared with the northern core (20 cm/ka vs. 5 cm/ka), continuously darker sediment colour, and higher TOC values (0.6–0.9 vs. 0.25 wt% not including sapropels). During sapropels S1 and S3, present in both cores, the influence of the River Nile became more widespread, reaching as far as Cyprus. Yet, the influence of the River Nile remained stronger in the south, as evident by the higher TOC values in the southern core throughout the entire 90 ka period and the longer duration of S1 in the southern core. An anomalous low δ18O interval that is not recorded in western Mediterranean occurred between 58 and 49 ka in the Levantine Basin and is more developed in the northern core. This period correlates with D-O interstadial 14 and maximum northern hemisphere insolation during the lastglacial cycle, suggesting that the warming mainly impacted the northern Levant.The Eastern Mediterranean Sea and land area was considerably warmer than the western Mediterranean throughout the LGM – Holocene transition, and the δ18OG. ruber drop of 4.5‰ is significantly greater than the 3‰ shift found for the western Mediterranean δ18OG. bulloides, both differences reflecting an increased continental effect from the western to eastern Mediterranean. Comparison between the marine and the land δ18O records suggests that the origin of rain over the land is composed of mixed signal from the southern and northern Levantine Basin. The study of Δδ18Osea–land variations demonstrates that various factors have influenced the sea–land relationship during the last 90 ka. The ‘amount effect’ has an important influence on rainfall δ18O during interglacial periods (particularly sapropel periods), whereas during glacial periods, increased land distances and elevation differences arising from decrease in sea level may have brought about decrease in δ18O of rainfall due to Rayleigh distillation processes. These influences were superimposed on those of sea surface water δ18O changes brought about by continental ice melting, and the strong effects felt in the southern Levantine Basin of the high River Nile input during periods of enhanced monsoonal activity.  相似文献   

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