首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Glycerol dialkyl glycerol tetraethers (GDGTs) are increasingly used as proxies for paleoclimate studies of marine and lacustrine environments. While GDGT-based proxies have been applied to a number of lake environments globally, little is known about the distribution of GDGTs on the Qinghai-Tibet Plateau. We have investigated the isoprenoid GDGTs (iGDGTs) and branched GDGTs (bGDGTs) in Lake Qinghai sediments and the surrounding surface soil in order to examine their potential use as paleoclimate proxies on the Qinghai-Tibet Plateau. The results show that (i) the values of the iGDGT/bGDGT ratio for surrounding soil were at the higher end among globally distributed soils and consequently BIT values (avg. 0.71) at the lower end, (ii) the TEX86 values decreased while the MBT and CBT values increased along an onshore soil–nearshore sediment–offshore sediment transect, (iii) the TEX86 values for the offshore sediments were almost identical and their inferred temperatures were close to mean summer surface water temperature and (iv) the bGDGT-inferred mean annual air temperature (MAAT) and pH for soil were consistent with measured MAAT and pH. However, the CBT-inferred pH for offshore sediments seemed inconsistent with the pH of lake water or sediment. Our results suggest that (i) the higher pH may be an important factor leading to the higher iGDGT/bGDGT values (and lower BIT values) in surrounding surface soil, (ii) both iGDGTs and bGDGTs may originate from terrestrial input and in situ production for this saline lake, especially for nearshore sediments. However, for offshore sediments, terrestrial iGDGT input seems minor, and TEX86 may be useful for paleoclimate studies of Lake Qinghai.  相似文献   

2.
Glycerol dialkyl glycerol tetraether (GDGT) lipids are membrane lipids which were long thought to be synthesized mainly by archaea, organisms thought to be limited to extreme environments. Analysis of environmental samples over the last decade has shown, however, that their structural diversity and sources are much wider than anticipated and that they occur ubiquitously in a wide range of environments, such as oceans and lakes, and their (sub)surface sediments, as well as soils. Several GDGTs have been unambiguously identified and can be used as biomarker lipids, since they are preserved in immature sediments <140 Ma. Close examination of the distributions has led to the discovery that GDGTs might be used as proxies for certain environmental parameters, such as the input of soil organic matter to marine environments, soil pH, air temperature and sea and lake water temperature. Here, we review the progress made over the last decade in the analysis, occurrence and recognition of sources of GDGTs, their applications as biomarker lipids, and the development and application of proxies based on their distributions.  相似文献   

3.
We measured archaeal lipid distributions from globally distributed samples of freshwater, marine, and hypersaline suspended particulate matter. Cluster analysis of relative lipid distributions identified four distinct groups, including: (1) marine epipelagic (<100 m) waters, (2) marine mesopelagic (200-1500 m) and upwelling waters, (3) freshwater/estuarine waters, and (4) hypersaline waters. A pronounced difference in lipid composition patterns is the near absence of ring-containing glycerol dialkyl glycerol tetraethers (GDGTs) at high salinity. Different archaeal communities populate marine (mesophilic Crenarchaeota and Euryarchaeota), and hypersaline environments (halophilic Euryarchaeota) and community shifts can regulate differences in lipid patterns between marine and hypersaline waters. We propose that community changes within meosphilic marine Archaea also regulate the lipid patterns distinguishing epipelagic and mesopelagic/upwelling zones. Changes in the relative amounts of crenarchaeol and caldarchaeol and low relative abundances of ringed structures in surface waters differentiate lipids from the epipelagic and mesopelagic/upwelling waters. Patterns of lipids in mesopelagic (and upwelling) waters are similar to those expected of the ammonia-oxidizing Group I Crenarchaeota, with predominance of crenarchaeol and abundant cyclic GDGTs; non-metric multidimensional analysis (NMDS) shows this pattern is associated with high nitrate concentrations. In contrast, limited culture evidence indicates marine Group II Euryarchaeota may be capable of producing mainly caldarchaeol and some, but not all, of the ringed GDGTs and we suggest that these organisms, along with the Crenarchaeota, contribute to lipids in epipelagic marine waters. Calculated TEX86 temperatures in mesopelagic samples (reported here and in published data sets) are always much warmer than measured in situ temperatures. We propose lipids used in the temperature proxy derive from both Euryarchaeaota and Crenarchaeota, and observed values of TEX86 are subject to changes in their ecology as influenced by nutrient fluctuations or other perturbations. Applications of published core-top TEX86-SST correlations require that (1) the surface waters are always composed of similar communities with the same temperature response and (2) that deeper water GDGT production is not transported to the sediments. Our lipid distribution patterns demonstrate both surface-water archaeal community differences (which accompany greater nutrient influxes, shoaling of mesopelagic Crenarchaeota during upwelling periods, and possibly due to an influx of terrestrial Archaea), and changes in organic matter transport through the water column can affect the distribution of lipids recorded in sediments. We therefore suggest that reported temperature shifts in ancient applications indicate TEX86 lipids recorded not only temperature changes, but also changes in archaeal ecology, nutrient concentrations, and possibly oceanographic conditions.  相似文献   

4.
Branched glycerol dialkyl glycerol tetraethers (GDGTs) are bacterial membrane lipids, ubiquitously present in soils and peat bogs, as well as in rivers, lakes and lake sediments. Their distribution in soil is controlled mainly by pH and mean annual air temperature, but the controls on their distribution in lake sediments are less well understood. Several studies have found a relationship between the distribution of branched GDGTs in lake sediments and average lake water pH, suggesting an aquatic source for them, besides that for soil transported to the lake via erosion. We sampled the surface water suspended particulate matter (SPM) from 23 lakes in Minnesota and Iowa (USA), that vary widely in pH, alkalinity and trophic state. The SPM was analyzed for the concentration and distributions of core lipid (presumed fossil origin) and intact polar lipid (IPL, presumed to derive from living cells) branched GDGTs. The presence of substantial amounts (18–48%) of IPL-derived branched GDGTs suggests that branched GDGTs are likely of autochthonous origin. Temperature estimates based on their distribution using lake-specific calibrations agree reasonably with water temperature at time of sampling and average air temperature of the season of sampling. Importantly, a strong correlation between the distribution of branched GDGTs and lake water pH was found (r2 0.72), in agreement with a predominant in situ production. An stronger correlation was found with lake water alkalinity (r2 0.83), although the underlying mechanism that controls the relationship is not understood. Our results raise the potential for reconstructing pH/alkalinity of past lake environments, which could provide important knowledge on past developments in lake water chemistry.  相似文献   

5.
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   

6.
The concentrations of Br and I in marine sediments have been used to categorize the sedimentary environments of different coastal regions of the world with respect to organic matter contents. The present study uses the concentrations of Br, I and P2O5 of different marine settings as a new proxy to interpret the depositional environments. A total of 150 coastal lagoon sediment samples (suspended sediments, surface sediments and sediment cores) were analyzed for Br, I and P2O5 concentrations by X-ray fluorescence spectrometry. They were compared with the Br, I and P2O5 concentrations of the 2004 Indian Ocean tsunami sediments. Sediments from various sources are separately clustered in I–Br plot and a trivial negative correlation is found for the whole plot. A similar correlation pattern exists in the I–P2O5 diagram. This correlation is explained by the distribution of marine plants (higher and lower) in different sedimentary environments of the coastal profile. Therefore, the concentration of I and its relation to P2O5 can be used as a tool to identify sediment depositional environments in marine settings.  相似文献   

7.
Anammox, the microbial anaerobic oxidation of ammonium by nitrite to produce dinitrogen gas, has been recognized as a key process in both the marine and freshwater nitrogen cycles, and found to be a major sink for fixed inorganic nitrogen in the oceans. Ladderane lipids are unique anammox bacterial membrane lipids that have been used as biomarkers for anammox bacteria in recent and past environmental settings. However, the fate of ladderane lipids during diagenesis is as of yet unknown. In this study, we performed oxic degradation experiments (at 20-100 °C) with anammox bacterial biomass to simulate early diagenetic processes occurring in the water column and at the sediment-water interface. Abundances of C18 and C20 ladderane lipids decreased with increasing temperatures, testifying to their labile nature. The most abundant products formed were ladderane lipids with a shorter alkyl side chain (C14 and C16 ladderane fatty acids), which was unambiguously established using two-dimensional NMR techniques on an isolated C14-[3]-ladderane fatty acid. The most pronounced production of these short-chain lipids was at 40 °C, suggesting that degradation of ladderane lipids was microbially mediated, likely through a β-oxidation pathway. An HPLC-MS/MS method was developed for the detection of these ladderane alteration products in environmental samples and positively tested on various sediments. This showed that the ladderanes formed during degradation experiments also naturally occur in the marine environment. Thus, short-chain ladderane lipids may complement the original longer-chain ladderane lipids as suitable biomarkers for the detection of anammox processes in past depositional environments.  相似文献   

8.
We studied the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) in suspended particulate matter from the water column of Lake Tanganyika (East Africa), where sediment studies had shown the applicability of the TEX86 proxy for reconstructing surface lake water temperature. GDGTs, in particular crenarchaeol, showed maximum abundance within the suboxic zone (100–180 m), suggesting that this is the preferred niche of ammonia-oxidizing Thaumarchaeota. Despite evidence for anaerobic methane oxidation in deep anoxic water (300–1200 m) no unambiguous evidence for an imprint of methanotrophic archaea on GDGT distribution was found. Comparison of TEX86 and BIT indices with those of surface sediments suggests that the sedimentary GDGTs are derived predominantly from the oxic zone and suboxic zone of the lake.  相似文献   

9.
Long-chain alkenones (LCK) of lacustrine surface sediments were analyzed in 37 lakes from China. The results obtained were complemented by published data from 13 other Chinese lakes. These lakes are located across large temperature and precipitation gradients, therefore allowing for an assessment of the distribution pattern of LCK and their temperature dependency. Different distribution patterns of LCK (C37 predominant pattern and C38 predominant pattern) were detected in the surface sediment samples. The ratio of C37:4 methyl ketone to the sum of C37 alkenones observed in the different lakes is highly variable (5%-96%, with mean value of 55%), and more than that seen in marine systems. The finding that some of the ocean LCK precursor algae (Gephyrocapsa oceanica, Coccolithus pelagicus) were also present in the limnic systems suggested that both systems might have similar biosynthetic sources. Empirical relationships between the alkenone unsaturation index U37k′ and different temperature sets (mean annual air temperature, mean annual air temperature in different seasons, and lake surface water temperature of July) were tested. The best correlation between U37k′ and temperature was obtained using mean annual air temperature. A general linear regression of U37k′ and MAAT can be expressed as U37k′ = 0.0328 × T + 0.126 (n = 38, r2 = 0.83). Although questions such as species-uncertainty and other unknown factors for U37k′ temperature dependence still remain, the equation might be representative of the average contribution of LCK to sediments for these data over a wide range of surface temperatures, water chemistry and different alkenones-producer algal populations. The general relationship of U37k′ and mean annual air temperature is consistent with that in marine systems. It supports the suggestion that the biosynthetic pathway of alkenones and the mechanism of their temperature signal may be similar in both marine and limnic systems. LCK might be used as an important paleotemperature proxy in limnic environment.  相似文献   

10.
A series of even carbon numbered α, ω-dicarboxylic acids ranging from C16 to C24 has been identified in recent sediments from various environments and sampling localities. A lacustrine sediment did not show detectable quantities of diacids. Consideration of quantitative relationships involving the diacids leads us to propose a dual origin for these diacids: deposition by mangroves is their main source in mangal areas while in situ production by sedimentary organisms is the only important source of diacids in a non-mangal marine environment. A fresh water lagoon shows an intermediate situation between these extremes.  相似文献   

11.
海洋沉积物中色素生物标志物研究进展   总被引：1，自引：0，他引：1  
海洋沉积物中的光合色素包含着水体、沉积物中浮游和底栖植物以及微生物群落的丰富信息,能表征特定生物来源,在埋藏到沉积物甚至发生某些改变之后仍然保留其源信息,是一类重要的化学生物标志物.结合总有机碳、总氮等其他海洋地球化学参数,沉积色素可用来研究海洋浮游植物和光合细菌的群落组成和丰度,反演海洋初级生产、水体富营养化水平及其历史趋势,指示水体和沉积物氧化还原条件,揭示海域气候条件等现状及其历史变化.沉积色素的研究,对于掌握海洋中碳的生物地球化学循环过程,回溯古环境、古海洋、古生态以及古气候记录,制定合理的海洋管理政策具有十分重要的意义.阐述了沉积物中色素的分类、来源、性质和分析方法,分析了色素在沉积物中的保存和变化规律,探讨总结了沉积色素作为化学生物标志物在海洋学研究中的应用.  相似文献   

12.
The abundance and structural diversity of bacteriohopanepolyols (BHPs) was examined in three marine pelagic environments that are characterized by strong vertical redox gradients and water column suboxia or anoxia. The abundance and, in most instances, structural diversity of BHPs was highest at depths where conditions were suboxic or anoxic. However, the majority of the BHP structures that were identified are environmentally cosmopolitan and their biological sources are presently not well constrained. An isomer of bacteriohopanetetrol (denoted BHT II) was observed at all three study sites in association with anoxic and suboxic conditions within the water column. Based on the absence of BHT II from terrigenous and oxic marine environments studied to date, and its strong association with suboxic and anoxic marine pelagic environments, we propose that BHT II is a promising candidate biomarker for water column suboxia and anoxia in the marine geologic record. The molecular fingerprint of BHPs in suspended and sinking particles and core-top sediments indicates that hopanoids produced within the water column are exported to marine sediments and that their biological source is most likely associated with settling particles and not the free-water phase. Based on our observations, BHPs likely represent an important input to the sedimentary hopanoid inventory, particularly in upwelling environments characterized by pelagic oxygen minimum zones (OMZ) and anoxic marine basins.  相似文献   

13.
A complete interglacial cycle, named the Fjøsangerian and correlated with the Eemian by means of its pollen stratigraphy, is found in marine sediments just above the present day sea level outside Bergen, western Norway. At the base of the section there are two basal tills of assumed Saalian ( sensu lato ) age in which the mineralogy and geochemistry indicate local provenance. Above occur beds of marine silt, sand and gravel, deposited at water depths of between 10 and 50 m. The terrestrial pollen and the marine foraminifera and molluscs indicate a cold-warm-cold sequence with parallel development of the atmospheric and sea surface temperatures. In both environments the flora/fauna indicate an interglacial climatic optimum at least as warm as that during the Holocene. The high relative sea level during the Eemian (at least 30 m above sea level) requires younger neotectonic uplift. The uppermost marine beds are partly glaciomarine silts, as indicated by their mineralogy, drop stones and fauna, and partly interstadial gravels. The pollen indicates an open vegetation throughout these upper beds, and the correlation of the described interstadial with Early Weichselian interstadials elsewhere is essentially unknown. The section is capped by an Early Weichselian basal till containing redeposited fossils, sediments, and weathering products. Several clastic dikes injected from the glacier sole penetrate the till and the interglacial sediments. Radiocarbon dates on wood and shells gave infinite ages. Amino acid epimerization ratios in molluscs support the inferred Eemian age of the deposit. The Fjøsangerian is correlated with the Eemian and deep sea oxygen isotope stage 5e; other possible correlations are also discussed.  相似文献   

14.
Steryl alkyl ethers (SAEs) have been reported from marine sediments ranging from Holocene to Lower Cretaceous and their production has been linked to upwelling of cold, nutrient-rich water. However, the lack of continuous stratigraphic records for them has complicated validation of the precise connection of their production, sedimentary distribution and survival to climatic conditions. Here, we report an SAE record that spans the last ca. 135 ka from a core taken offshore southeastern Australia in the Murray Canyons Group region. The SAEs are composed predominantly of C27–C30 steroid moieties with one or two double bonds, ether-bound to C10–C11 alkyl moieties. Some of the SAEs are identical to those reported in marine sediments, whereas others have not been identified previously. They do not exhibit any systematic change in distribution over the past 110 ka but sediments older than 110 ka possess a different assemblage, with a higher amount of constituents containing a C11 alkyl moiety. The SAE accumulation rate increases rapidly during glacial episodes and for intervals characterized by a sharp decline in sea-surface temperature. The results suggest that SAEs may be related to marine eukaryotes that thrive when there is an influx of cold, deep waters in the Murray Canyons Group region, likely triggered by stronger westerlies over southeastern Australia forcing deeper water to reach the surface during glacials.  相似文献   

15.
To determine where and when glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in lakes are produced, we collected descending particles in Lake Lucerne (Switzerland) using two sediment traps (at 42 and 72 m water depth) with a monthly resolution from January 2008 to late March 2009. Suspended particulate matter (SPM) was monthly filtered from the water column at three different depths. The potential application of GDGTs in palaeoenvironmental and palaeoclimatic reconstructions was investigated by comparing core lipids and their relative GDGT distribution, with lake water temperatures throughout the year. Fluxes of GDGTs and their concentrations in the water column vary according to a seasonal pattern, showing a similar trend in the SPM and sediment traps. Fluxes and concentrations of isoprenoid GDGTs increase with depth, maximum values being observed in the deeper part of the water column, indicating production of isoprenoid GDGTs by Thaumarchaeota in the deep (∼50 m), aphotic zone of Lake Lucerne. The flux-weighted averages of the proxies TEX86 (0.27) and BIT (0.03) based on the total extracted GDGTs are similar at both trap depths. A sediment core from the same location showed that in the first few centimetres of the core TEX86 and BIT values of 0.29 and 0.07, respectively, are similar to those recorded for descending particles and SPM, indicating that the sedimentary TEX86 records the annual mean temperature of deeper waters in Lake Lucerne. TEX86 values are slightly higher below 20 cm in the core. This offset is interpreted to be caused by the present-day trophic state of the lake, which probably resulted in a deeper niche of the Thaumarchaeota. Branched GDGTs represent only a minor fraction of the total GDGTs in the lake and their origin remains unclear. Our data reveal that GDGTs in lakes have a large potential for palaeoclimatic studies but indicate that knowledge of the system is important for accurate interpretation.  相似文献   

16.
A diverse collection of globally distributed soil samples was analyzed for its glycerol dialkyl glycerol tetraether (GDGT) membrane lipid content. Branched GDGTs, derived from anaerobic soil bacteria, were the most dominant and were found in all soils. Isoprenoid GDGTs, membrane lipids of Archaea, were also present, although in considerably lower concentration. Crenarchaeol, a specific isoprenoid membrane lipid of the non-thermophilic Crenarchaeota, was also regularly detected and its abundance might be related to soil pH. The detection of crenarchaeol in nearly all of the samples is the first report of this type of GDGT membrane lipid in soils and is in agreement with molecular ecological studies, confirming the widespread occurrence of non-thermophilic Crenarchaeota in the terrestrial realm. The fluvial transport of crenarchaeol and other isoprenoid GDGTs to marine and lacustrine environments could possibly bias the BIT index, a ratio between branched GDGTs and crenarchaeol used to determine relative terrestrial organic matter (TOM) input. However, as crenarchaeol in soils is only present in low concentration compared to branched GDGTs, no large effect is expected for the BIT index. The fluvial input of terrestrially derived isoprenoid GDGTs could also bias the TEX86, a proxy used to determine palaeo surface temperatures in marine and lacustrine settings and based on the ratio of cyclopentane-containing isoprenoid GDGTs in marine and lacustrine Crenarchaeota. Indeed, it is shown that a substantial bias in TEX86-reconstructed sea and lake surface temperatures can occur if TOM input is high, e.g. near large river outflows.  相似文献   

17.
Long-term carbon cycling and climate change are strongly dependent on organic carbon (OC) burial in marine sediments. Radiocarbon (14C) has been widely used to constrain the sources, sinks, and processing of sedimentary OC. To elucidate the dominant controls on the radiocarbon content of total organic carbon (14CTOC) accumulating in surface sediments we construct a box model that predicts 14CTOC in the sediment mixed layer (measured as fraction modern, Fm). Our model defines three distinct OC pools (“degradable,” “semi-labile,” and “refractory”) and assumes that 14CTOC flux to sediments is exclusively derived from surface ocean primary productivity, and hence follows a “generic” surface ocean dissolved inorganic carbon (DIC) bomb curve. Model predictions are compared to a set of 75 surface sediment samples, which span a wide geographic range and reflect diverse water column and depositional conditions, and for which sedimentation rate and mixed-layer depth are well characterized. Our model overestimates the Fm value for a majority (65%) of these sites, especially at shallow water depths and for sites characterized by depleted δ13CTOC values. The model is most sensitive to sedimentation rate and mixed-layer depth. Therefore, slight changes to these parameters can lead to a match between modeled and measured Fm values at many sites. Yet, in some cases, we find that measured Fm values cannot be simulated without large and unrealistic changes to sedimentation rate and mixed-layer depth. These results point to sources of pre-aged OC to surface sediments and implicate soil-derived terrestrial OC, reworked marine OC, and/or anthropogenic carbon as important components of the organic matter present in surface sediments. This approach provides a valuable framework within which to explore controls on sedimentary organic matter composition and carbon burial over a range of spatial and temporal scales.  相似文献   

18.
Sedimentological, micropalaeontological (benthic foraminifers and dinoflagellate cysts), stable isotope data and AMS 14C datings on cores and surface samples, in addition to acoustic data, have been obtained from Voldafjorden, western Norway. Based on these data the late glacial and Holocene sedimentological processes and variability in circulation and fjord environments are outlined. Glacial marine sedimentation prevailed in the Voldafjorden between 11.0 kyr and 9.2 kyr BP (radiocarbon years). In the later part of the Allerød period, and for the rest of the Holocene, there was deposition of fine‐grained normal marine sediments in the fjord basin. Turbidite layers, recorded in core material and on acoustic profiles, dated to ca. 2.1, 6.9–7.6, ca. 9.6 and ca. 11.0 kyr BP, interrupted the marine sedimentation. The event dated to between 6.9 and 7.6 kyr BP probably corresponds to a tsunami resulting from large‐scale sliding on the continental margin off Norway (the Storegga Tsunami). During the later part of the Allerød period, Voldafjorden had a strongly stratified water column with cold bottom water and warm surface water, reaching interglacial temperatures during the summer seasons. During the Younger Dryas cold event there was a return to arctic sea‐surface summer temperatures, possibly with year‐round sea‐ice cover, the entire benthic fauna being composed of arctic species. The first strong Holocene warming, observed simultaneously in bottom and sea‐surface temperature proxies, occurred at ca. 10.1 kyr BP. Bottom water proxies indicate two cold periods, possibly with 2°C lowering of temperatures, at ca. 10.0 (PBO 1) and at 9.8 kyr BP (PBO 2). These events may both result from catastrophic outbursts of Baltic glacial lake water. The remainder of the Holocene experienced variability in basin water temperature, indicated by oxygen isotope measurements with an amplitude of ca. 2°C, with cooler periods at ca. 8.4–9.0, 5.6, 5.2, 4.6, 4.2, 3.5, 2.2, 1.2 and 0.4–0.8 kyr BP. Changes in the fjord hydrology through the past 11.3 kyr show a close correspondence, both in amplitude and timing of events, recorded in cores from the Norwegian Sea region and the North Atlantic. These data suggest a close relationship between fjord environments and variability in large‐scale oceanic circulation. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

19.
With this study, I suggest that changes in biogeochemical processes in northern Norwegian fjords during the last glacial/interglacial transition (14.3 to 6.3 ka B.P.) are predominantly climate induced. Variable strength in Atlantic water inflow, intrusion of Arctic waters and sea-ice coverage, various nutrient supply, as well as several re-advances of continental ice-sheets are recorded by both bulk organic and selected biomarkers in sediments of the Andfjord. Stable isotope (δ13Corg, δ15N) and Rock-Eval pyrolysis data indicate a strong supply of terrestrial organic matter during stadials (e.g., Younger Dryas, Older Dryas) whereas enhanced input of marine organic matter prevailed during the interstadials (Bølling/Allerød). Profiles of selected biomarkers of marine origin show a characteristic climate-induced variation during the whole time interval, where relatively low concentrations of dinosterols and C37 alkenones may indicate—similar to today—a minor contribution of their respective plankton groups (coccolithophorids, dinoflagellates). The prevalence of Arctic waters and seasonal sea-ice coverage during the Younger Dryas is indicated by a single maximum of 24-methylenecholesterol probably indicating the dominance of sea-ice diatoms and/or the prevalence of diatoms of Thalassiosiraceae in surface waters. The sediments of the Allerød, in contrast, show a pronounced maximum in dinosterols, indicating a better adaptation of dinoflagellates to an oceanic regime that is characterized by incipient intrusion of Atlantic water, highly variable sea surface temperatures, and well stratified water masses. The concentration of higher-molecular-weight n-alkanes, however, follows the deglacial-early Holocene trend of climate amelioration from the last deglaciation to the middle Holocene, with maxima during ice advances (stadials) and minima during the middle Holocene warm period.These new results significantly improve the understanding of climate-induced response on organic matter supply in fjords and provide a better knowledge of past variations of biogeochemical processes in high-latitude coastal environments.  相似文献   

20.
Bacterially produced branched glycerol dialkyl glycerol tetraethers (GDGTs) are ubiquitous in soils and lake sediments and can potentially be used to reconstruct past temperatures. In lakes, however, it is still unclear if these compounds are derived from eroded soils or if they are produced in situ. To better understand environmental controls on the distributions of these compounds and the sources of branched GDGTs to lake sediments, we compare branched GDGT distributions and concentrations in lake sediments and catchment soils within a 3600 m altitudinal transect in western Uganda. Reconstructed mean annual air temperature (MAAT), determined from the degree of methylation (MBT) and cyclisation (CBT) of branched GDGTs in soils, decreases with increasing altitude, as is expected from the air temperature gradient in our transect. However, we observe significant offsets between observed and reconstructed temperatures in soils from wet, high elevation soils but not in most dry, low elevation soils. Branched GDGT distributions differ significantly between lake sediments and soils at all elevations, with greater differences at low elevations than at high elevations. These data support previous hypotheses that branched GDGTs are produced in situ in lakes and suggest that the abundance of water in soil environments may play a role in controlling the distribution of branched GDGTs. While branched GDGTs in lacustrine sediments can be used to reconstruct temporal temperature variations in lakes, we urge caution in utilizing this proxy in lacustrine systems with high soil loadings, especially if there is evidence of changing clastic fluxes through time.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号