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1.
During ancient times the natural deposition fluxes of lead which can be leached with dilute acid from sediments in Santa Barbara, Santa Monica and San Pedro basins offshore from the Los Angeles Urban complex, were about 0.7, 0.1 and 0.2 μg Pb/cm2 yr respectively. Since there was little difference in biological productivity in surface waters of these basins, it is proposed that clay is a major transport vehicle for sequestered soluble lead, which then explains why the lead deposition flux within the Santa Barbara basin was so much larger compared to the other basins. The fluxes of silicate mud in the basins in ancient times were about 92, 19 and 30 mg/cm2 yr in Santa Barbara, Santa Monica and San Pedro basins respectively. Today deposition fluxes of acid soluble lead within these three basins are 3- to 9-fold greater, being about 2.1, 1.1 and 1.8 μg Pb/cm2 yr respectively, partly in the form of directly deposited large sewage particles, which account for none, 23 and 34 of the total industrial lead deposition fluxes in the respective basins. Concentrations of leachable lead in varve dated sediment layers increase with time and isotopic compositions of these leads change in accordance with corresponding known changes of isotopic compositions of industrial lead in the Los Angeles atmosphere. Lead remaining in acid leached sediment residues originates from igneous and clay minerals, exhibiting no change in concentration or isotopic composition since pre-industrial times.Deposition fluxes of total barium in sediments among the three basins were proportional to mass deposition fluxes before 1950 in the same manner as for lead. Afterwards, there are barium concentration maxima with time in both Santa Monica and San Pedro Basin sediments which are attributable to industrial sewage rather than to episodic erosion from barium-rich sedimentary evaporite strata exposed locally along the shore. An increase of barium concentrations in present day Santa Barbara basin sediments may reflect dispersal of barium-rich drilling mud from local drilling operations.  相似文献   

2.
The distributions of hydrocarbons in sediment cores dated by 210Pb (1845–1977) from San Pedro and San Nicolas Basins in the Southern California Bight have been determined by gas Chromatographic and combined gas chromatographic-mass spectrometric analysis. The chromatograms of the hydrocarbons contain peaks of resolved alkanes and cycloalkanes, as well as an unresolved complex mixture which decrease in content with increasing depth in both of the cores. The concentrations of o,p' andp,p'-DDE are highest in the top 50 mm of the San Pedro core section. The 45–50 mm segment of this core was deposited in the 1945–1950 period when DDT (the precursor of DDE compounds) came into common use. The relatively high content of hydrocarbons and DDE in this core is attributable to the proximity of the site to the San Pedro Harbor which receives petroleum residues from shipping, sewage outfalls and industrial effluents. The presence of only traces of DDE throughout the San Nicolas core, and the low hydrocarbon content are explainable by the greater distance of this basin from anthropogenic inputs and probably a greater rate of degradation of deposited organic matter during bioturbation. δ13C, δ15N and electron-spin resonance analyses of kerogens and humic substances in these cores, indicate that most of this organic matter in these sediments is of a marine origin.  相似文献   

3.
Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity‐current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high‐resolution seismic‐reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr?1, with increases at times of extreme relative sea‐level lowstand. Coarser‐grained mid‐fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea‐level fall. These pulses of coarse‐grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr?1 on the mid‐ and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr?1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer‐term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level.  相似文献   

4.
The triterpenol geochemistry of the Santa Monica Basin from the Southern California Borderland, off the U.S.A., is described from the study of two sets of trap deployments, five box cores (≈30 cm) and a hydroplastic core (≈1 m). The biogenic sources and diagenetic stability of the triterpenols are discussed.The 17β(H), 21β(H)-hopanols (22R isomer) occur in the carbon number range from 30 to 32 and their abundance is nearly uniform in the shallow sediment sections. However, the three hopanols follow the order of abundance, C32 > C31 C30, in deeper sections. Their concentrations spans from trace levels to 156 μg/g organic carbon (<15 ng to 7 μg/g dry sediment). Tetrahymanol (gammaceran-3β-ol) has been identified in all the samples except in one set of trap particles collected at 100 m water depth, from trace level (<1 μg) to 215 μg/g organic carbon (<20 ng to 9 μg/g dry sediment). Diplopterol is also detected in trace amounts in some samples. The triterpenols in the trap material generally increase with the water column depth and decrease with the subbottom depth in the sediment cores.The extended hopanols are either degradation products of polyhydroxybacteriohopanes or are biosynthesized by bacteria. Tetrahymanol is probably the only suggested biological precursor of gammacerane (the reduced counterpart of tetrahymanol), which has been recognized in numerous crude oils and lithified sediments. Although it has been reported earlier from Green River Shale and from a residual Pleistocene lake sediment, tetrahymanol has so far been positively identified from recent marine sediments only in two recent studies. The decreasing content of tetrahymanol in sedimentary depth profiles in the Santa Monica Basin would favor an origin for this compound in the water column or at the sediment surface. The ubiquitous occurrence of this compound throughout the study area suggests that this triterpenol most probably originates from primitive organisms (protozoa, bacteria?), hitherto not identified or, more likely, not yet analyzed for their lipid composition.  相似文献   

5.
We conducted an extensive survey of hydrogen-isotopic compositions (D/H ratios) of diverse sedimentary lipids from the Santa Barbara Basin (SBB), offshore southern California. The main goal of this survey was to assess the diversity of D/H ratios in lipids from marine sediments, in order to provide a more detailed understanding of relevant biological and geochemical factors impacting lipid isotopic variability. A total of 1182 individual δD values are reported from two stations in SBB, one located in the suboxic basin depocenter and the other on the fully oxic flank of the basin. Sediments collected from the basin depocenter span a depth of ∼2.5 m and reach the methanogenic zone. Lipids that were analyzed include n-alkanes, n-alkanols and alkenols, short- and long-chain fatty acids, linear isoprenoids, steroids, and hopanoids, and exhibit several systematic patterns. First, there are no significant differences in δD values between the two sampling locations, nor with increasing depth for most lipids, indicating that degradation does not influence sedimentary lipid δD values. Second, relatively large differences in δD values among differing molecular structures are observed in all samples. n-Alkyl lipids of probable marine origin have typical δD values between −150 and −200‰, those from terrestrial leaf waxes and aquatic plants range from −80 to −170‰, while petroleum n-alkanes are typically −90 to −150‰. Third, lipids inferred to derive from bacteria (branched fatty acids and hopanols) living at the sediment surface or in the water column tend to be D-enriched relative to similar algal products by 30‰ or more. At the same time, several other lipids have δD values that decrease strongly with depth, presumably as a result of in situ production by anaerobic bacteria. This dichotomy in isotopic compositions of bacterial lipids is inconsistent with a nearly constant D/H fractionation during lipid biosynthesis, and likely reflects significant variations associated with metabolism.  相似文献   

6.
Microscopic charcoal from varved Santa Barbara Basin sediments was used to reconstruct a 560-yr record (A.D. 1425 to 1985) of Santa Ana fires. Comparison of large (>3750 μm2) charcoal with documented fire records in the Santa Barbara Ranger District shows that high accumulations correspond to large fires (>20,000 ha) that occurred during Santa Ana conditions. The charcoal record reconstructed a minimum of 20 large fires in the Santa Barbara region during the study period. The average time between fires shows no distinct change across three different land use periods: the Chumash period, apparently characterized by frequent burning, the Spanish/Early American period with nominal fire control, and the 20th century with active fire suppression. Pollen data support the conclusion that the fire regime has not dramatically changed during the last 500 yr. Comparison of large charcoal particle accumulation rates and precipitation reconstructed from tree rings show a strong relationship between climate and fire history, with large fires consistently occurring at the end of wet periods and the beginning of droughts.  相似文献   

7.
Geochemical and isotopic data for the uppermost 1.2 m of the sediments of the central Santa Monica Basin plain were examined to better understand organic matter deposition and recycling at this site. Isotopic signatures (Δ14C and δ13C) of methane (CH4) and dissolved inorganic carbon (DIC) indicate the occurrence of anaerobic oxidation of CH4 that is fueled by CH4 supplied from a relict reservoir that is decoupled from local organic carbon (Corg) degradation and methanogenesis. This finding was corroborated by a flux budget of pore-water solutes across the basal horizon of the profile. Together these results provide a plausible explanation for the anomalously low ratio between alkalinity production and sulfate consumption reported for these sediments over two decades ago. Shifts in Δ14C and δ13C signatures of Corg have previously been reported across the 20-cm depth horizon for this site and attributed to a transition from oxic to anoxic bottom water that occurred ~350 years BP. However, we show that this horizon also coincides with a boundary between the base of a hemipelagic mud section and the top of a turbidite interval, complicating the interpretation of organic geochemical data across this boundary. Radiocarbon signatures of DIC diffusing upward into surface sediments indicate that remineralization at depth is supported by relatively 14C-enriched Corg within the sedimentary matrix. While the exact nature of this Corg is unclear, possible sources are hemipelagic mud sections that were buried rapidly under thick turbidites, and 14C-rich moieties dispersed within Corg-poor turbidite sections.  相似文献   

8.
The reported source rocks for the abundant petroleum in the Tarim Basin, China range from Cambrian to Lower Ordovician and/or Upper Ordovician in age. However, the difference between the two groups of source rocks is not well characterized. In this study, pyrite was removed from eleven mature to over mature kerogen samples from source rocks using the method of CrCl2 reduction and grinding. The kerogen and coexisting pyrite samples were then analyzed for δ34S values. Results show that the kerogen samples from the Cambrian have δ34S values between +10.4‰ and +19.4‰. The values are significantly higher than those from the Lower Ordovician kerogen (δ34S of between +6.7‰ and +8.7‰), which in turn are generally higher than from the Upper Ordovician kerogen samples (δ34S of between ?15.3 and +6.8‰). The associated pyrite shows a similar trend but with much lower δ34S values. This stratigraphically controlled sulfur isotope variation parallels the evolving contemporary marine sulfate and dated oil δ34S values from other basins, suggesting that seawater sulfate and source rock age have an important influence on kerogen and pyrite δ34S values. The relatively high δ34S values in the Cambrian to Lower Ordovician source rocks are associated with abundant aryl isoprenoids, gammacerane and C35 homohopanes in the extractable organic matter, indicating that these source rocks were deposited in a bottom water euxinic environment with water stratification. Compared with the Upper Ordovician, the Cambrian to Lower Ordovician source rocks show abundance in C28 20R sterane, C23 tricyclic terpanes, 4,23,24-trimethyl triaromatic dinosteroids and depletion in C24 tetracyclic terpane, C29 hopane. Thus, δ34S values and biomarkers of source rock organic matter can be used for distinguishing the Cambrian and Upper Ordovician source rocks in the Tarim Basin.  相似文献   

9.
The Upper Triassic Xujiahe Formation in the Sichuan Basin, SW China consists of a series of coal measures. The first, third and fifth members of this formation are dominated by gas prone dark mudstones and coals. The mudstones contain Type II and III kerogens with average organic carbon contents around 1.96%. These source rocks are mature in the central Sichuan and highly mature in the western Sichuan Basin, characterized by gas generation with subordinate amounts of light oil or condensate oils. The source rocks are intercalated with the sandstone dominated second, fourth and sixth members of the Xujiahe Formation, thus leading to three separate self contained petroleum systems in the region. The proven gas reserves in the Xujiahe Formation are only less than that of the Triassic Feixianguan Formation and the Xujiahe Formation has the second largest gas field (Guang’an gas field) in the basin. Gases derived from the Xujiahe Formation coals generally show a normal stable carbon isotopic trend for C1–C4 n-alkanes, with the highest δ13C2 values among the nine gas pay zones in the basin (?20.7‰ to ?28.3‰), and δ13C1 values as low as ?43.0‰ in the central Sichuan. Gas accumulations with an oil leg have also been found in the eastern and southern Sichuan where the thickness of the Xujiahe Formation is significantly reduced. Gases in these accumulations tend to show low δ13C2 values (?30.0‰ to ?36.3‰), characteristic of oil prone source rocks.  相似文献   

10.
The Transverse Ranges of southern California represent an uplifted and variably dissected Mesozoic magmatic arc, and Mesozoic to Holocene sedimentary and volcanic strata deposited in convergent and transform tectonic settings. Modern sand within part of the Western Transverse Ranges represents: first-order sampling scale of the Santa Monica and the San Gabriel Mountains; second-order sampling scale of the Santa Clara River draining both mountain ranges; and third-order sampling scale of the beach system between the mouth of the Santa Clara River and the eastern Santa Monica Mountains, and turbidite sand of the Hueneme-Mugu submarine fan. Source lithology includes plutonic and metamorphic rocks of the San Gabriel Mountains, and sedimentary and volcanic rocks of the Santa Monica Mountains. First-order sands have large compositional variability. Sand from local coastal drainage of the Santa Monica Mountains ranges from basaltic feldspatholithic to quartzofeldspathic. Sand of the San Gabriel Mountains local drainages has three distinct petrofacies, ranging from metamorphiclastic feldspatholithic to mixed metamorphi/plutoniclastic and plutoniclastic quartzofeldspathic. Second-order sand is represented by the main channel of the Santa Clara River; the sand has an abrupt downstream compositional change, from feldspathic to quartzofeldspathic. Third-order sand (beaches and deep-sea turbidite samples) of the Santa Monica Basin is quartzofeldspathic. Beach sand is more quartz-rich than is Santa Clara river sand, whereas turbidite sand is more feldspar-rich than is beach sand. Deep-sea sand has intermediate composition with respect to second-order samples of the Santa Clara River and third-order samples of the beach system, suggesting that (1) the Santa Clara River is the main source of sediments to the marine environment; and (2) local entry points from canyons located near local drainages may generate turbidity currents during exceptional flood conditions. Petrologic data of modern sand of the study area are highly variable at first- and second-order scale, whereas third-order sand is homogenized. The homogenized composition of deep-marine sand is similar to the composition of most ancient sandstone derived primarily from the Mesozoic dissected magmatic arc of southern California. This study of the Western Transverse Ranges illustrates the effects of source lithology, transport, depositional environment, and sampling scale on sand composition of a complex system, which provides insights regarding actualistic petrofacies models.  相似文献   

11.
Particulate Fe and Mn may be important trace metal scavengers in the water column as well as being probable indicators of biologically mediated redox processes. A study has been made of suspended particulate composition in the Santa Barbara Basin, a shallow near-shore basin off southern California with sub-oxic conditions below sill depth. Observations have revealed several interesting phenomena relating to the geochemistry of Fe and Mn. Most striking is a profound enrichment of particulate Fe in samples from the bottom two hundred meters. These particulates have a constant Fe/P mole ratio of about three and may originate at the sediment-water interface or may be transported to the basin from local marshes. For particulate Mn, enrichments are observed both in the sub-sill waters and near the base of the euphotic zone. A consideration of particle removal rates suggests that the sub-photic zone enrichment has a biogenic origin. In the sub-sill waters, enrichment in Mn is apparently due to the precipitation of dissolved Mn diffusing from the anoxic basin sediments. A simple mass balance suggests that most of the Mn lost from the sediments is transported from the Santa Barbara Basin in dissolved form.  相似文献   

12.
δ13C values of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate organic C (POC) together with δ18O and δ2H values of water, δ34S values of dissolved SO4, and major ion concentrations were measured in the Murray River and its tributaries between November 2005 and April 2007 to constrain the origins and behaviour of riverine C. δ13CDIC values in the Murray River vary between −9.5 and −4.7‰ with a range of <3‰ within any sampling round. δ13CDIC values of the tributaries are −11.0‰ to −5.1‰. DIC concentrations of the Murray River increase from ∼25 mg/L in the middle and upper reaches of the river to 45–55 mg/L in the lower reaches. However, the mass ratio of DIC as a proportion of the total dissolved solids (TDS) decreases from ∼0.6–0.7 in the headwaters to ∼0.2–0.3 in the lower reaches of the river, with similar downstream changes in DIC/Cl ratios. This precludes simple evaporative concentration of DIC and is interpreted as the river evading CO2; this interpretation is consistent with pCO2 values that are in the range 550–11,200 ppm volume (ppmv), which are far higher than those in equilibrium with the atmosphere (∼360 ppmv). The δ13CDIC values are similar to those that would be produced by the weathering of marine limestone (δ13C ∼ 0‰). However, the lack of marine limestones cropping out in the Murray–Darling Basin and the relatively uniform δ13CDIC values of the Murray River (even in upland reaches where the dominant rock types are metamorphosed silicates and granites) make this unlikely. Rather the high pCO2 values and δ13CDIC values are best explained by a combination of mineralisation of low δ13C organic C and evasion to the atmosphere. The rate of these two processes may attain near steady state and control both DIC concentrations and δ13C values.  相似文献   

13.
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14.
Understanding the supply and preservation of glycerol dibiphytanyl glycerol tetraethers (GDGTs) in marine sediments helps inform their use in paleoceanography. Compound-specific radiocarbon measurements of sedimentary alkenones from multiple environments have been used to gain insight into processes that affect paleotemperature reconstructions. Similar analyses are warranted to investigate how analogous processes affecting GDGTs impact TEX86 paleotemperatures. Here we present radiocarbon measurements on individual GDGTs from Bermuda Rise and Santa Monica Basin sediments and discuss the results in the context of previous studies of co-depositional alkenones and foraminifera. The 14C contents of GDGTs and planktonic foraminifera in Bermuda Rise are very similar, suggesting a local source; and TEX86-derived temperatures agree more closely with foraminiferal temperatures than do temperatures. In contrast, GDGTs in Santa Monica Basin are depleted in 14C relative to both alkenones and foraminifera, and TEX86 temperatures agree poorly with known surface water values. We propose three possible factors that could explain these results: (i) GDGTs may be labile relative to alkenones during advective transport through oxic waters; (ii) archaeal production deep in the water column may contribute 14C-depleted GDGTs to sediments; and (iii) some GDGTs also may derive from sedimentary archaeal communities. Each of these three processes is likely to occur with varying relative importance depending on geographic location. The latter two may help to explain why TEX86 temperature reconstructions from Santa Monica Basin do not appear to reflect actual sea surface temperatures. Terrigenous GDGTs are unlikely to be major contributors to Bermuda Rise or Santa Monica Basin sediments, based on values of the BIT index. The results also indicate that the crenarchaeol regioisomer is governed by processes different from other GDGTs. Individual measurements of the crenarchaeol regioisomer are significantly depleted in 14C relative to co-occurring GDGTs, indicating an alternative origin for this compound that presently remains unknown. Re-examination of the contribution of crenarchaeol regioisomer to the TEX86 index shows that it is a significant influence on the sensitivity of temperature reconstructions.  相似文献   

15.
Plant macrofossils from 38 packrat middens spanning the last ~ 33,000 cal yr BP record vegetation between ~ 650 and 900 m elevation along the eastern escarpment of the Sierra San Pedro Mártir, northern Baja California. The middens span most of the Holocene, with a gap between ~ 4600 and 1800 cal yr BP, but coverage in the Pleistocene is uneven with a larger hiatus between 23,100 and 14,400 cal yr BP. The midden flora is relatively stable from the Pleistocene to Holocene. Exceptions include Pinus californiarum, Juniperus californica and other chaparral elements that were most abundant > 23,100 cal yr BP and declined after 14,400 cal yr BP. Despite being near the chaparral/woodland-desertscrub ecotone during glacial times, the midden assemblages reflect none of the climatic reversals evident in the glacial or marine record, and this is corroborated by a nearby semi-continuous pollen stratigraphy from lake sediments. Regular appearance of C4 grasses and summer-flowering annuals since 13,600 cal yr BP indicates occurrence of summer rainfall equivalent to modern (JAS average of ~ 80–90 mm). This casts doubt on the claim, based on temperature proxies from marine sediments in the Guaymas Basin, that monsoonal development in the northern Gulf and Arizona was delayed until after 6200 cal yr BP.  相似文献   

16.
Biodegradation is one of the main natural attenuation processes in groundwater contaminated with petroleum hydrocarbons. In this work, preliminary studies have been carried out by analyzing the concentrations of total petroleum hydrocarbons (TPH), dissolved inorganic carbon (DIC), dominant terminal electron accepters or donors, as well as δ 13CDIC and δ 34SSO4, to reveal the biodegradation mechanism of petroleum hydrocarbons in a contaminated site. The results show that along groundwater flow in the central line of the plume, the concentrations of electron acceptors, pH, and E h increased but TPH and DIC decreased. The δ 13CDIC values of the contaminated groundwater were in the range of ?14.02 to ?22.28 ‰PDB and ?7.71 to 8.36 ‰PDB, which reflected a significant depletion and enrichment of 13C, respectively. The increase of DIC is believed to result from the non-methanogenic and methanogenic biodegradation of petroleum hydrocarbon in groundwater. Meanwhile, from the contaminated source to the downgradient of the plume, the 34S in the contaminated groundwater became more depleted. The Rayleigh model calculation confirmed the occurrence of bacterial sulfate reduction as a biodegradation pathway of the petroleum hydrocarbon in the contaminated aquifers. It was concluded that stable isotope measurements, combined with other biogeochemical measurements, can be a useful tool to prove the occurrence of the biodegradation process and to identify the dominant terminal electron-accepting process in contaminated aquifers.  相似文献   

17.
The varved sediments of the Santa Barbara Basin off southern California, offer a unique opportunity to study the changes in oceanographic conditions of this nearshore area during the last 8000 yr. Quantitative analysis of Radiolaria found in recent surface sediment samples from the eastern North Pacific allows the identification of four “assemblages” which can be related to the physical oceanography of the California Current. Two assemblages are associated with the southerly flowing California Current, one with the main stream of the current (California Current Assemblage) and the other with the offshore flow along northern California (Central Assemblage). The two other assemblages are associated with the subtropical region of the eastern North Pacific (Subtropical Assemblage) and one found mostly off the coast of Baja California (Baja Assemblage). Analysis of the Radiolaria found in the varved sediments of a core from the Santa Barbara Basin give an 8000-yr continuous record of these four assemblages. The California and Baja Assemblages show only minor fluctuations in their importance in the sediments of the Santa Barbara Basin. The California Assemblage, however, shows a steady increase during this time period. Prior to 5400 yr B.P. the Radiolaria were predominately subtropical in character, whereas after 5400 yr B.P. the Central Assemblage becomes more important. Since 5400 yr B.P. most of the changes in the radiolarian fauna consist of fluctuations in the importance of these two assemblages. Past sea-surface temperatures for the month of February were calculated using the transfer function technique of J. Imbrie and N. G. Kipp (1971, In “The Late Cenozore Glacial Ages” L. K. Turekian, Ed.), (Chap. 5, Yale Univ. Press, New Haven, Conn.). The time series of paleotemperature estimates show major changes in the average February temperature of Santa Barbara Basin waters. The range of estimated temperatures (12°C) exceeds that of the historical observations of February temperatures in the Santa Barbara Basin but does not exceed the observed range for the California Current region. The intervals from 800 to 1800 yr B.P. 3600 to 3800 yr B.P. and 5400 to the end of the record appear to have been generally warmer than today. Comparison of the Holocene record of alpine glacial advances with the radiolarian assemblage and paleotemperature time series shows that the initiations of advances was coincident with a decrease in sea-surface temperatures and an increase in the importance of the Central Assemblage in the Santa Barbara Basin. The terminations of these advances were not marked by any consistent characteristic in the Santa Barbara Basin time series.  相似文献   

18.
This study investigated the geochemical features of the lower Paleozoic strata of Yaerdang Mountain outcrop along with the core samples from well TD2∈ in the eastern Tarim Basin,NW China.The total organic carbon abundance,hydrocarbon-generating precursor biospecies,and stable isotope ratios of organics and carbonate(δ~(13)C_(ker),δ~(13)C_(carb) and δ~(18)O_(carb)) were comprehensively studied for their possible correlative constraints during sedimentary evolution.The results revealed that the δ~(13)C_(ker)(VPDB) of Cambrian kerogens along the outcrop section varied from-34.6‰ to-28.4‰,indicating an increasing tendency from the lower Cambrian to the upper Cambrian.This was on the whole accompanied by the variation in the δ~(13)C_(carb) and δ~(18)O_(carb) along the profile,which might be associated with the changes in the sea level and also in the compositional variation of benthic and planktonic biomass.The large variation in the stable carbon isotope ratios up to 6‰ along the outcrop section reflected the heterogeneity of the Cambrian source rocks from the eastern Tarim Basin.Hence,the ~(13)C-enriched crude oils from well TD2∈might have been derived from a localized stratum of Cambrian source rocks.The results from this study showed the possibility of multiple source kitchens in the Cambrian-lower Ordovician portion of Tarim Basin.  相似文献   

19.
The Paleocene-Eocene Thermal Maximum(PETM) event was a dramatic global warming w55.93 Ma ago that resulted in biological extinction events, lithological changes, and major deviations in σ13 C and σ18 O.The southwestern Tarim Basin of China exposes successive Paleogene strata as a result of Tethys evolution and is considered an ideal region for PETM research.Based on calcareous nannoplankton biostratigraphy, we also used stable isotopes and XRD to analyse the Paleocene-Eocene transition in the Tarim Basin. At the Bashibulake Section, the PETM interval is characterized by(1) an abrupt negative shifts in σ13 C_(org), σ13 C_(carb) and σ18 O(-3%, -4.5% and -3%respectively);(2) an obvious negative correlation between the K-mode(Discoaster, Fasciculithus, Ericsonia, Sphenolithus and Rhomboaster) and r-mode(Biscutum, Chiasmolithus, Toweius) nannofossil taxa coincident with a robust Rhomboaster-Discoaster assemblage; and(3) a significant increase in the percentage of detrital input along with an increase in gypsum content. In the upper part of the Qimugen Formation Micrantholithus and Braarudosphaera are commonly found right up to the top where most of the nannofloras suffer a sharp decrease. In the overlying Gaijitage Formation, calcareous nannofossils disappear completely. These events indicate that the southwestern Tarim Basin was a warm shallow continental shelf during the deposition of the Qimugen Formation. From the early Eocene, the environment changed conspicuously. Evaporation increased and sea level fell, which led to an acid climate.This climate mode continued within the youngest unit studied, the Gaijitage Formation, characterized by the deposition of thick evaporates. Consequently, most of the marine plankton, i.e. calcareous nannoplankton, became disappear, because of the significant climate shift.  相似文献   

20.
《Sedimentary Geology》2006,183(1-2):51-69
The Chicxulub Sedimentary Basin of the northwestern Yucatan Peninsula, Mexico, which was formed because of the largest identified Phanerozoic bolide impact on Earth, became a site of deposition of dominantly marine carbonate sediments during most of the Cenozoic Era. This is a study of the filling and diagenetic history of this basin and surrounding areas. The study makes use of lithologic, biostratigraphic, petrographic, and geochemical data obtained on core samples from boreholes drilled throughout the northwestern Yucatan Peninsula.The core sample data indicate that: 1) The Chicxulub Sedimentary Basin concentrated the deposition of pelagic and outer-platform sediments during the Paleocene and Eocene, and, in places, during the Early Oligocene, as well, and filled during the Middle Miocene, 2) deeper-water limestone also is present within the Paleocene and Lower Eocene of the proposed Santa Elena Depression, which is located immediately south of the Basin, 3) shallow-water deposits are relatively more abundant outside the Basin and Depression than inside, 4) the autigenic and allogenic silicates from the Paleogene formations are the most abundant inside the Depression, 5) sediment deposition and diagenesis within the Basin also were controlled by impact crater topography, 6) the abundance of the possible features of subaerial exposure increases upward and outward from the center of the Basin, and 7) the formation of replacive low-magnesium calcite and dolomite, dedolomitization, dissolution, and precipitation of vug-filling calcite and dolomite cement have been more common outside the Basin than inside.δ18O in whole-rock (excluding vug-filling) calcite from core samples ranges from − 7.14‰ to + 0.85‰ PDB. δ13C varies from − 6.92‰ to + 3.30‰ PDB. Both stable isotopes correlate inversely with the abundance of subaerial exposure features indicating that freshwater diagenesis has been extensive especially outside and at the edge of the Chicxulub Sedimentary Basin.δ18O and δ13C in whole-rock (excluding vug-filling) dolomite ranges from − 5.54‰ to + 0.87‰ PDB and − 4.63‰ to + 3.38‰ PDB, respectively. Most dolomite samples have negative δ18O and positive δ13C suggesting that replacive dolomitization involved the presence of a fluid dominated by freshwater and/or an anomalously high geothermal gradient.Most dolomite XRD-determined mole percent CaCO3 varies between 51 and 56. Replacive dolomite is larger, more euhedral, and less stoichiometric inside the Chicxulub Sedimentary Basin than outside.  相似文献   

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