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1.
Tonle Sap Lake, Cambodia, possesses one of the most productive inland fisheries in the world and is a vital natural resource for the country. The lake is connected to the Mekong River via the Tonle Sap River. Flow in the Tonle Sap River reverses seasonally, with water exiting the lake in the dry season and entering the lake during the summer monsoon. This flood pulse drives the lake’s biological productivity. We used Sr, Nd, and Pb isotopes and elemental concentrations in lake sediment cores to track changes in the provenance of deposits in Tonle Sap Lake. We sought to determine when the lake first began to receive water and sediment input via the Mekong River, which initiated flood pulse processes. The transition from a non-pulsing lake to the Mekong-connected system is marked by shifts to values of 87Sr/86Sr, εNd, and 207Pb/204Pb that are characteristic of Mekong River sediments. In addition, magnetic susceptibility increased and sediment elemental composition changed. Elemental (P) measures point to enhanced phosphorus loading and C/N and isotope ratios of bulk organic matter indicate a shift to greater relative contribution of organic material from aquatic versus terrestrial environments, coinciding with the initiation of flood pulse processes. On the basis of radiocarbon dating in two cores, we estimate the initiation of the annual flood pulse occurred between ~4,450 and 3,910 cal year BP.  相似文献   

2.
Origin of the sedimentary deposits of the Naracoorte Caves, South Australia   总被引：1，自引：0，他引：1  
M.S. Forbes  E.A. Bestland   《Geomorphology》2007,86(3-4):369-392
The origin of the sediments located in the Naracoorte Caves (South Australia) was investigated via the analysis of strontium isotope ratios (87Sr/86Sr), elemental geochemistry, and mineralogy. Sedimentary deposits located in Robertson, Wet, Blanche and several other chambers in Victoria Cave are all variable mixes of fine sand and coarse silts, which display similar and consistent strontium isotope ratios (0.717–0.725). This suggests that over the 400 ka time frame that these deposits span there has been minimal variation in the source of the clastic sediments. Increased strontium concentrations for these cave sediments correspond with increasing silt content, yet there is no correlation between 87Sr/86Sr ratios and silt content. This implies that the silt-sized component of the sediments is the main contributor of strontium to the cave sediments. Comparisons of 87Sr/86Sr with regional surficial deposits show a significant correlation between the cave sediments (avg: 0.7228; n = 27), the fine silt lunettes of the Bool Lagoon area (avg: 0.7224; n = 4), the sandy A horizons of the Coonawarra Red Brown Earths (RBEs; avg: 0.726; n = 5), and Holocene age podsolic sand deposits (0.723). These data suggest that there has been substantial flux from this group of deposits to the caves, as would be expected considering prevailing winds. This relationship is further supported by a strong correlation between many trace elements, including Ti, Zr, Ce, and Y; however, variations in clay mineralogy suggest that the fine silt-dominated lunettes and Padthaway RBEs were not significant contributors to the cave deposits. Hence, the detritus entering the caves was more than likely from areas proximal to the cave entrance and was dominated by medium grain-sized materials. Major regional deposits, including the coarser-grained, calcite-rich Bridgewater Formation sands, basalts from the lower SE, Padthaway Horst granites, Gambier limestone, and metamorphics from the Adelaide geosyncline show minimal correlation in 87Sr/86Sr ratios, elemental geochemistry, and mineralogy with the cave sediments, and are discounted as significant sources. In comparison, 87Sr/86Sr ratios for the Coorong silty sands (0.717–0.724), Lower Murray sands (0.727–0.730), and the medium size silt component of the Murray–Darling River system (0.71–0.72), compare favourably with the cave sediments. This relationship is further supported by similarities in elemental chemistry and mineralogy. Thus, much of the strontium-rich silt that is now located in the Naracoorte Cave sediments likely originated from the Murray–Darling basin. Over time, this material has been transported to the SE of South Australia, where it mixed with the medium sand component of the regressive dune ridge sequence, locally derived organic matter, limestone fragments, and fossil material to produce the unique deposits that we see evident in many of the chambers of the Naracoorte Cave system today.  相似文献   

3.
J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 14C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the Lake Stanley phase. A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.314C ka, determined from the inferred 14C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time. A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand, late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes. This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special issue.  相似文献   

4.
We explored the potential for using Pediastrum (Meyen), a genus of green alga commonly found in palaeoecological studies, as a proxy for lake-level change in tropical South America. The study site, Laguna La Gaiba (LLG) (17°45′S, 57°40′W), is a broad, shallow lake located along the course of the Paraguay River in the Pantanal, a 135,000-km2 tropical wetland located mostly in western Brazil, but extending into eastern Bolivia. Fourteen surface sediment samples were taken from LLG across a range of lake depths (2–5.2 m) and analyzed for Pediastrum. We found seven species, of which P. musteri (Tell et Mataloni), P. argentiniense (Bourr. et Tell), and P. cf. angulosum (Ehrenb.) ex Menegh. were identified as potential indicators of lake level. Results of the modern dataset were applied to 31 fossil Pediastrum assemblages spanning the early Holocene (12.0 kyr BP) to present to infer past lake level changes qualitatively. Early Holocene (12.0–9.8 kyr BP) assemblages do not show a clear signal, though abundance of P. simplex (Meyen) suggests relatively high lake levels. Absence of P. musteri, characteristic of deep, open water, and abundance of macrophyte-associated taxa indicate lake levels were lowest from 9.8 to 3.0 kyr BP. A shift to wetter conditions began at 4.4 kyr BP, indicated by the appearance of P. musteri, though inferred lake levels did not reach modern values until 1.4 kyr BP. The Pediastrum-inferred mid-Holocene lowstand is consistent with lower precipitation, previously inferred using pollen from this site, and is also in agreement with evidence for widespread drought in the South American tropics during the middle Holocene. An inference for steadily increasing lake level from 4.4 kyr BP to present is consistent with diatom-inferred water level rise at Lake Titicaca, and demonstrates coherence with the broad pattern of increasing monsoon strength from the late Holocene until present in tropical South America.  相似文献   

5.
We present a Holocene record of climate and environmental change in central New York (USA) inferred using lithologic and stable isotope data from two sediment cores recovered in Cayuga Lake. The record was divided into three intervals: (1) early Holocene (~11.6–8.8 ka), (2) Hypsithermal (~8.8–4.4 ka), and (3) Neoglacial (~4.4 ka to present). The early Holocene began abruptly, with rising lake level and relatively deep water. Between ~10.8 and 9.2 ka, cool and dry conditions prevailed at a time of maximum solar insolation. This anomaly has been referred to as the “post-Younger Dryas climate interval” and lasted ~1,600 years, the approximate length of one “Bond cycle.” The Hypsithermal was the warmest, wettest and most biologically productive interval of the Holocene in central New York. The Hypsithermal was characterized by centennial to multi-centennial-scale variability. The 8.2 ka event is one such variation. The Neoglacial was an interval of generally cooler and dryer conditions, falling lake levels, and several prominent climate anomalies. At approximately 2.4 ka, δ13C of bulk organic matter increased abruptly by 5‰ as lake level declined, and the lake flora was dominated by Chara sp. during the coldest interval of the Neoglacial. Numerous sediment variables display increased variability ~2.0 ka, which continues today. Archaeological data from the literature suggest that Native American populations may have been large enough to impact land cover by about 2.4 ka and we hypothesize that the “Anthropocene” began at about that time in central New York. We also found paleolimnological evidence for the Medieval Warm Period (~1.4–0.5 ka), which was warmer and wetter than today, and for the Little Ice Age (~500–150 years ago), a period with temperatures colder than today.  相似文献   

6.
The utility of ostracod-based palaeoenvironmental reconstruction was evaluated using instrumental data for Lake Qarun, Egypt. The euryhaline ostracod Cyprideis torosa was the only species found in the lake’s recent sediment record. This species is known to tolerate salinity levels and water solute compositions that may prevent colonisation by other species. Oxygen and carbon isotope ratios of ostracod carbonate from lake sediments covary with changes in instrumental values for lake level and salinity for the period 1890–1974. δ13C-values correlate negatively with lake water salinity (r 2 = 0.87) and δ18O-values correlate negatively with measured lake level changes (r 2 = 0.41). Other ostracod proxy data provide qualitative information on lake level trends. Fossil assemblage data (juvenile/adult and valve/carapace ratios and valve preservation) provide information on wave energy. Ecophenotypic variation of C. torosa valves provided some useful palaeolimnological information. Sr/Ca and Mg/Ca ratios in ostracods were not found to reflect water composition, due to the uncoupling of these ratios with salinity in Lake Qarun. Overall, our results highlight the need to calibrate ostracod proxy data in modern systems prior to their use for palaeoenvironmental reconstruction.  相似文献   

7.
The water level of the Lake Michigan basin is currently 177 m above sea level. Around 9,800 14C years B.P., the lake level in the Lake Michigan basin had dropped to its lowest level in prehistory, about 70 m above sea level. This low level (Lake Chippewa) had profound effects on the rivers flowing directly into the basin. Recent studies of the St. Joseph River indicate that the extreme low lake level rejuvenated the river, causing massive incision of up to 43 m in a valley no more than 1.6 km wide. The incision is seen 25 km upstream of the present shoreline. As lake level rose from the Chippewa low, the St. Joseph River lost competence and its estuary migrated back upstream. Floodplain and channel sediments partially refilled the recently excavated valley leaving a distinctly non-classical morphology of steep sides with a broad, flat bottom. The valley walls of the lower St. Joseph River are 12–18 m tall and borings reveal up to 30 m of infill sediment below the modern floodplain. About 3 × 108 m3 of sediment was removed from the St. Joseph River valley during the Chippewa phase lowstand, a massive volume, some of which likely resides in a lowstand delta approximately 30 km off-shore in Lake Michigan. The active floodplain below Niles, Michigan, is inset into an upper terrace and delta graded to the Calumet level (189 m) of Lake Chicago. In the lower portion of the terrace stratigraphy a 1.5–2.0 m thick section of clast-supported gravel marks the entry of the main St. Joseph River drainage above South Bend, Indiana, into the Lake Michigan basin. This gravel layer represents the consolidation of drainage that probably occurred during final melting out of ice-marginal kettle chains allowing stream piracy to proceed between Niles and South Bend. It is unlikely that the St. Joseph River is palimpsest upon a bedrock valley. The landform it cuts across is a glaciofluvial-deltaic feature rather than a classic unsorted moraine that would drape over pre-glacial topography. This is the fifth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.  相似文献   

8.
A 12.87-m-long sediment core was retrieved from closed-basin Lake Daihai in the monsoon–arid transition zone of north-central China. Oxides of major elements and their ratios normalized to Al in the AMS-14C-dated core were employed to evaluate chemical weathering intensity (CWI) in the lake drainage basin, which reflects hydrothermal conditions in the study area. Lower CWI periods occurred prior to 14.5 ka BP, and during the intervals ca. 11.7–10.3, 3.5–3.2, 2.6–1.7 ka BP, and 1.2–0 ka BP, indicating relatively low temperatures and moisture availability. Greater CWI during the intervening periods ca. 14.5–11.7, 10.3–9.0, 3.2–2.6, and 1.7–1.2 ka BP, with the maximum CWI at ca. 6.7–3.5 ka BP, imply ameliorated hydrothermal conditions in the lake basin, i.e. higher temperatures and precipitation. Exceptionally low CWI, associated with high CaO/MgO ratio during ca. 9.0–6.7 ka BP, suggests higher evaporation rates in the area under warmer temperature. Overall, CWI displays in-phase variations with changes in organic matter (TOC, TN), carbonate (CaCO3) and pollen assemblages, all of which are related to variations in monsoon effective precipitation. High CWI indicates strong monsoon-induced precipitation, whereas low CWI reflects a weak precipitation regime. The optimum hydrothermal status, recorded by the strongest CWI and maximum monsoon effective precipitation during ca. 6.7–3.5 ka BP defines the Holocene climate optimum (HCO) in the Lake Daihai region. These results indicate that the HCO prevails after the early Holocene in the monsoon–arid transition zone of north-central China. Temperature and precipitation variations during most of the Holocene, inferred from the lake sediments, are due largely to insolation forcing. Dry but warm conditions ca. 9.0–6.7 ka BP, however, probably reflect the complex interactions between insolation and geography (e.g. altitude and local topography).  相似文献   

9.
West Hawk Lake (WHL) is located within the glacial Lake Agassiz basin, 140 km east of Winnipeg, Manitoba. The small lake lies in a deep, steep-sided, meteorite impact crater, which has been partly filled by 60 m of sediment that today forms a flat floor in the central part of the basin below 111 m of water. Four cores, 5–11 m in length, were collected using a Kullenberg piston gravity corer. All sediment is clay, contains no unconformities, and has low organic content in all but the upper meter. Sample analyses include bulk and clay mineralogy, major and minor elements, TOC, stable isotopes of C, N, and O, pollen, charcoal, diatoms, and floral and faunal macrofossils. The sequence is divided into four units based mainly on thickness and style of lamination, diatoms, and pollen. AMS radiocarbon dates do not provide a clear indication of age in the postglacial sequence; possible explanations include contamination by older organic inwash and downward movement of younger organic acids. A chronological framework was established using only selected AMS dates on plant macrofossils, combined with correlations to dated events outside the basin and paleotopographic reconstructions of Lake Agassiz. The 822 1-cm-thick varves in the lower 8 m of the cored WHL sequence were deposited just prior to 10,000 cal years BP (∼8,900 14C years BP), during the glacial Lake Agassiz phase of the lake. The disappearance of dolomite near the top of the varved sequence reflects the reduced influence of Lake Agassiz and the carbonate bedrock and glacial sediment in its catchment. The lowermost varves are barren of organisms, indicating cold and turbid glacial lake waters, but the presence of benthic and planktonic algae in the upper 520 varves indicates warming; this lake phase coincides with a change in clay mineralogy, δ18O and δ13C in cellulose, and in some other parameters. This change may have resulted from a major drawdown in Lake Agassiz when its overflow switched from northwest to east after formation of the Upper Campbell beach of that lake 9,300–9,400 14C years ago. The end of thick varve deposition at ∼10,000 cal years BP is related to the opening of a lower eastern outlet of Lake Agassiz and an accompanying drop in West Hawk Lake level. WHL became independent from Lake Agassiz at this time, sedimentation rates dropped, and only ∼2.5 m of sediment was deposited in the next 10,000 years. During the first two centuries of post-Lake Agassiz history, there were anomalies in the diatom assemblage, stable O and C isotopes, magnetic susceptibility, and other parameters, reflecting an unstable watershed. Modern oligotrophic conditions were soon established; charcoal abundance increased in response to the reduced distance to the shoreline and to warmer conditions. Regional warming after ∼9,500 cal years BP is indicated by pollen and diatoms as well as C and O isotope values. Relatively dry conditions are suggested by a rise in pine and decrease in spruce and other vegetation types between 9,500 and 5,000 cal years BP (∼8,500–4,400 14C years BP), plus a decrease in δ13Ccell values. After this, there was a shift to slightly cooler and wetter conditions. A large increase in organic content and change in elemental concentration in the past several thousand years probably reflects a decline in supply of mineral detritus to the basin and possibly an increase in productivity.  相似文献   

10.
Dolomitization in the Western Canadian Sedimentary Basin has been extensively researched, producing vast geochemical datasets. This provides a unique opportunity to assess the regional sources and flux of dolomitizing fluids on a larger scale than previous studies. A meta‐analysis was conducted on stable isotope, strontium isotope (87Sr/86Sr), fluid inclusion and lithium‐rich formation water data published over 30 years, with new petrographic, X‐ray diffraction, stable isotope and rare‐earth element (REE+Y) data. The Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group contain replacement dolomite (RD) cross‐cut by stylolites, suggesting replacement dolomitization occurred during shallow burial. Stable isotope, REE+Y and 87Sr/86Sr data indicate RD formed from Devonian seawater, then recrystallized during burial. Apart from the Wabamun Group of the Peace River Arch (PRA), saddle dolomite cement (SDC) is more δ18O(PDB) depleted than RD, and cross‐cuts stylolites, suggesting precipitation during deep burial. SDC 87Sr/86Sr data indicate contributions from 87Sr‐rich basinal brines in the West Shale Basin (WSB) and PRA, and authigenic quartz/albite suggests basinal brines interacted with underlying clastic aquifers before ascending faults into carbonate strata. The absence of quartz/albite within dolomites of the East Shale Basin (ESB) suggests dolomitizing fluids only interacted with carbonate strata. We conclude that replacement dolomitization resulted from connate Devonian seawater circulating through aquifers and faults during shallow burial. SDC precipitated during deep burial from basinal brines sourced from basal carbonates (ESB) and clastic aquifers (WSB, PRA). Lithium‐rich formation waters suggest basinal brines originated as residual evapo‐concentrated Middle Devonian seawater that interacted with basal aquifers and ascended faults during the Antler and Laramide Orogenies. These results corroborate those of previous studies but are verified by new integrated analysis of multiple datasets. New insights emphasize the importance of basal aquifers and residual evapo‐concentrated seawater in dolomitization, which is potentially applicable to other regionally dolomitized basins.  相似文献   

11.
A combination of water and sediment chemistry was used to investigate carbonate production and preservation in Lake Pumayum Co (altitude 5,030 m a.s.l.), south Tibet, China. We compared the chemical composition of lake water in various parts of the lake with that of input rivers and found that the loss of Ca2+ results from calcite sedimentation induced by evaporation and biogenic precipitation. This is supported by evaporation data from the catchment and δ18O measurements on water. Results suggest that CaCO3 is the predominant carbonate in this lake. There is a positive correlation in the sediments among concentrations of total inorganic carbon (TIC), Ca, total organic carbon (TOC), and total nitrogen, confirming that most carbonates in sediment are endogenic. The Jiaqu River is the largest inflow to Lake Pumayum Co and has a strong influence on both lake water chemistry and sediment composition. The river and lake bathymetry influence carbonate sedimentation by affecting water flow velocity and growing conditions for macrophytes. Different carbon contents and relationships between TIC and TOC in the two long cores from different depths in the lake reveal that hypolimnetic conditions also influence carbonate precipitation and preservation.  相似文献   

12.
Sub-bottom profiling was conducted at eight sub-basins within the lower French River area, Ontario, to investigate deposits preserved within the ancient North Bay outlet. Ten cores were collected that targeted the four depositional acoustic facies identified in the sub-bottom profiling records. The rhythmically laminated/bedded glaciolacustrine deposits of facies I are interpreted to have aggraded within glacial Lake Algonquin and its associated recessional lakes that persisted between 13,000 and 11,300 cal BP (~11,100 and 9,900 BP). The majority of the facies II, III and IV lacustrine deposits accumulated between about 9,500 cal BP (~8,500 BP) and the mid-Holocene, based on radiocarbon-dated organic materials. These deposits represent sedimentation within a ‘large’ lake during the late portion of the Mattawa-Stanley phase, and the Nipissing transgression, Nipissing Great Lakes and post-Nipissing recession phases of lake levels. Two sets of organic-rich sand beds are preserved within facies II deposits and reveal that the large lake lacustrine depositional environment was interrupted during the late Mattawa-Stanley phase between 9,500–9,300 and 9,000–8,400 cal BP (~8,500–8,300 and ~8,000–7,600 BP), when the water surface of Lake Hough fell below the outlet threshold and the lake basin became hydrologically closed. Pre-9,500 cal BP (~8,500 BP), the early and middle portions of the Mattawa-Stanley phase were dominated by erosion, as reflected by an unconformity at the base of facies II that occurs widely in the sub-basins and the general lack of preserved deposits for these intervals in the cores. This erosion is attributed to wave action and fluvial scouring within the outlet mouth during the early and mid-Stanley-Hough low stages and relates specifically to the period when the flowing portion of the North Bay outlet was situated over the lower French River area. This study reveals that the majority of the post-glacial deposits accumulated after the outlet threshold had shifted permanently eastwards and the lower French River area was inundated under the multiple phases of the large lake occupying the Nipissing Lowlands and Georgian-Huron basins, extending well into the mid-Holocene. The occurrence of deposits marking two closed-basin intervals during the late Stanley-Hough stage are well preserved locally within the lacustrine depositional sequence, but identifying earlier closed-basin intervals from the French River stratigraphy is hindered by the lack of preserved pre-9,500 cal BP (~8,500 BP) post-glacial deposits.  相似文献   

13.
A combination of carbon-to-nitrogen ratios (TOC/TN), Rock Eval-analyses, and stable isotope values of bulk nitrogen (δ15N) and organic carbon (δ13Corg) was used to characterize bulk organic matter (OM) of a piston core from the Patagonian maar lake Laguna Potrok Aike (Argentina) for the purpose of palaeoenvironmental reconstruction. Sedimentary data were compared with geochemical signatures of potential OM sources from Laguna Potrok Aike and its catchment area to identify the sources of sedimentary OM. Correlation patterns between isotopic data and TOC/TN ratios allowed differentiation of five distinct phases with different OM composition. Before 8470 calibrated 14C years before present (cal. yrs BP) and after 7400 cal. yrs BP, isotopic and organo-geochemical fingerprints indicate that the sediments of Laguna Potrok Aike consist predominantly of soil and diatom OM with varying admixtures of cyanobacterial and aquatic macrophyte OM. For a short phase of the early Holocene (ca. 8470–7400 cal. yrs BP), however, extremely high input of soil OM is implied by isotopic fingerprints. Previous seismic and geochronological results indicate a severe lake-level drop of 33 m below present-day shortly before 6590 cal. yrs BP. It is suggested that this lake level drop was accompanied by increased erosion of shore banks and channel incision enhancing soil OM deposition in the lake basin. Thus, isotopic data can be linked to hydrological variations at Laguna Potrok Aike and allow a more precise dating of this extremely low lake level. An isotopic mixing model was used including four different sources (soil, cyanobacteria, diatom and aquatic macrophyte OM) to model OM variations and the model results were compared with quantitative microfossil data.  相似文献   

14.
海鸟活动在东南极中山站区莫愁湖沉积物中的记录   总被引：2，自引：0，他引：2       下载免费PDF全文
本文对采自东南极拉斯曼丘陵中山站区的莫愁湖沉积物的元素、氨基酸和Sr同位素组成进行了详细的地球化学分析。结果表明莫愁湖沉积物中Zn、Cu、F、S、P、Se等六种元素具有共同的物质来源,其标型组合特征很可能揭示了莫愁湖沉积物在历史时期曾受到过企鹅等海鸟粪的影响,沉积物中酸溶相Sr同位素组成则确认了海洋源Sr的生物地球化学转移过程。酸溶相87Sr/86Sr值和研究区的新鲜海鸟粪酸溶相Sr同位素组成基本一致,都非常接近于海洋来源锶同位素组成(87Sr/86Sr=0.7092),并且沉积物中的生物元素Zn、Cu、F、S、P、Se含量和酸溶相Sr浓度具有非常高的统计相关性。酸溶相的87Sr/86Sr比值和反映湖泊中藻类等生物自然生成量的氨基酸含量表现出明显的负相关关系,进一步揭示了莫愁湖沉积物在历史时期曾受到企鹅等海鸟活动的影响。上述研究结果为更深入地研究该地区的生态、气候和环境的演变历史及其相互作用提供了有价值的参考资料。  相似文献   

15.
The spatial and temporal variations of stable oxygen isotope in precipitation on different time scales are analyzed according to the data from the IAEA/WMO stations with long survey series in the Northern Hemisphere. Temperature effect is mainly distributed in mid-high latitudes on seasonal scale except for Bamako and Addisababa stations. The δ18O/temperature slope displays the positive correlation against altitude for most of the statistical stations. Amount effect appears primarily in the region south of 30°N and coastal areas. The δ18O/precipitation slope is indirectly proportional to precipitation amount. For some of the sampling stations at mid-high latitudes where their seasonal distribution of precipitation is contrary to that of temperature, coupled with temperature effect, the amount effect appears synchronistically. Either the temperature effect or the amount effect on seasonal scale, there are positive correlations to a certain extent between the annual weighted mean δ18O and the annual mean temperature for almost all the stations. The correlation between composite δ18O and temperature on spatial scale is much more marked, compared with that of individual station. There is a good agreement between 10-year moving average temperature curves I and II, with the values of the former all markedly smaller than corresponding ones of the latter, calculated by the monthly mean series group I and the annual mean series group II, respectively. However, two calculated dδ18O/dT curves display the distinct difference: the variation amplitude of slope series II is larger than that of slope series I. Both curves had similar ascending trend from the 1960s to the 1970s, and then, their variations display the anti-phase. Moreover, the analyses show that there is negative correlation between slope series FI and temperature series II. However, the status is different for slope series I and temperature series I. Both series have contrary trend from the 1960s to the 1970s, whereas the same trend since the 1980s.  相似文献   

16.
Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns about sediment and water pollution. Lead-210 dated sediment cores from the lake represent ~95 years of accumulation and yield a mean sedimentation rate of ~4.7 mm year−1. Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (δ13C, δ15N, and δ34S), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC flux varies from 1.0 to 3.5 g m−2 year−1. Sediments are anoxic (Eh −328 to −187 mV) and have low (0.10–0.30 g m−2 year−1) N, but high (0.37–1.0 g m−2 year−1) S flux. Shifts in δ13C, δ15N, and δ34S suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the lake has remained largely unchanged, and the perceived human-induced impacts are limited.  相似文献   

17.
Freshwater Lake Ulubat (z mean = 1.5–2.0 m and Area = ~138 km2), NW Anatolia, Turkey was filled in by fine-to-medium-grain silts during the late Holocene. Deposition in Lake Ulubat has been 1.6 cm year−1 for the last 50 years, but the sedimentation rate over the last ~1,600 years was lower (0.37 mm year−1). The organic matter and carbonate contents of the infill show cyclic changes that reflect environmental fluctuations. The silt-dominated lithology and the vertically uniform heavy metal distributions are probably due to wind-controlled sedimentation in the lake. Heterogeneous mud, derived from a large, mountainous drainage basin, is deposited in the lake mostly during summer, June to October, when conditions are hot and calm. Winter months are stormier and sediments are re-suspended due to the shallow water depth and the effect of waves on the lake bottom. It is likely that re-suspended sediments, particularly fine-grained particles, together with the heavy metals, are transported out of the lake via the outlet, especially during periods of high lake level. This resuspension and removal process probably caused the lake sediments to become silt-dominated and depleted in heavy metals. The role of broad shallow lakes in sequestering sediments and heavy metals can be described more accurately when wind data are considered. Such information may also be helpful for land-use planning in downstream areas.  相似文献   

18.
Sediment accumulation rate (SAR) is an important physical parameter in all lakes and increases have been observed in many over the last c.100 years. This has been ascribed to changes in land-use and land-management causing accelerated catchment soil erosion and an increase in autochthonous organic matter production. The EU Water Framework Directive requires that assessment of biological, hydromorphological and chemical elements of water quality should be based on the degree to which present day conditions deviate from those expected in the absence of significant anthropogenic influence, termed reference conditions. Currently however, the reference condition for sediment accumulation rate for lakes of different types is undefined. To improve our understanding of the controls on SARs we compiled SAR and lake typology data for 207 European lakes derived from 210Pb dated cores to assess how rates have changed through time (in 25 year classes) both overall and for lakes of different types. Seventy-one percent of these sediment cores showed surface SARs higher than “basal” (mainly nineteenth century) rates, 11% showed no change while 18% showed a decline. Lakes were then classified into lake-types using four variables: alkalinity (3 classes), altitude (3 classes), maximum depth (2 classes) and lake area (2 classes). This generated a possible 36 lake classes of which 25 were represented in the dataset. Nine lake-types contained >10 lakes. Little change in SAR occurred prior to 1900 and most increases occurred in more recent periods, in particular 1950–1975 and post-1975. This indicates a general acceleration in SAR in European lakes during the second half of the twentieth century. Reference SARs were estimated for six lake-types with the highest number of sites. European mountain lakes had the lowest reference SAR (0.005 ± 0.003 g cm−2 yr−1) while lowland, high alkalinity sites had the highest (0.03–0.04 g cm−2 yr−1). SARs for other lake-types ranged between 0.012 and 0.024 g cm−2 yr−1. Using the mountain lake-type as an example, the 1850 reference SAR appears to show good agreement with available data for lakes beyond Europe indicating these values may be more broadly applicable. Contemporary SARs in lakes of all classes showed exceedence over their defined reference SAR. This may be partly due to diagenetic processes. Greatest exceedences were found in shallow, low altitude lakes and these are considered to be the ones under the greatest threat from continued elevation of SAR. It is considered that climate change may play a progressively more important role in driving SAR in the future.  相似文献   

19.
Groundwater is the major source of drinking water in Nahavand city. However, the groundwater quality at the agricultural areas has been deteriorating in recent years. Ground water quality monitoring is a tool which provides important information for water management and sustainable development of the water resources in Nahavand. Hydrochemical investigations were carried out in an agricultural area in Nahavand, western Iran, to assess chemical composition of groundwater. In this study, 64 representative groundwater samples were collected from different irrigation wells and analyzed for pH, electrical conductivity, major ions, and nitrate. The results of the chemical analysis of the groundwater showed that concentrations of ions vary widely and the most prevalent water type is Ca–Mg–HCO3, followed by other water types: Ca–HCO3, Ca–Na–HCO3, and Na–Cl, which is in relation with their interactions with the geological formations of the basin, dissolution of feldspars and chloride and bicarbonate minerals, and anthropogenic activities. Thirty-seven percent of the water samples showed nitrate (NO3 −) concentrations above the human affected value (13 mg L−1). The phosphorous (P) concentration in groundwater was between 0.11 and 0.90 mg L−1, with an average value of 0.30 mg L−1, with all of the samples over 0.05 mg L−1. The most dominant class C2-S1 (76.5%) was found in the studied area, indicating that sodicity is very low and salinity is medium, and that these waters are suitable for irrigation in almost all soils. Agronomic practices, such as cultivation, cropping, and irrigation water management may decrease the average NO3 − concentration in water draining from the soil zone.  相似文献   

20.
Exposures along the lower Kaministiquia River (near Thunder Bay, Ontario, Canada) provide insight into early Holocene lake level fluctuations and paleoenvironmental conditions in the northwestern Lake Superior basin. These exposures show at least two large paleochannels which were downcut into offshore sediments, and were later filled with >2 m of sand, ~3 m of rhythmically laminated silt and clay, and ~6 m of interbedded silt and sand. Buried by the rhythmically laminated silty clay unit is a well-preserved organic deposit with abundant plant macrofossils from terrestrial and emergent taxa, including several upright tree trunks. Three AMS radiocarbon ages were obtained on wood and conifer cones from this deposit: 8,135 ± 25 (9,130–9,010 cal), 8,010 ± 25 (9,010–8,780 cal), and 7,990 ± 20 (8,990–8,770 cal) BP. This sequence records an early postglacial high-water phase, followed by the Houghton lowstand, and reflooding of the lower Kaministiquia River Valley. The drop in lake level associated with the Houghton phase forced the ancestral Kaministiquia River to downcut. By ~9,100 cal (~8,100) BP, older channels eroded into subaqueous underflow fan deposits in the Thunder Bay area near Fort William Historical Park (FWHP) were abandoned and colonized by a Picea-Abies-Larix forest. Based on stratigraphic data corrected for differential isostatic rebound, the lake was below the Sault Ste. Marie bedrock sill between at least 9,100 cal (8,100) and 8,900 cal (8,000) BP. Shortly after 8,900 cal BP, the lake quickly rose and buried in situ lowland vegetation at FWHP with varved sediments. We argue that this transgression was due to overflow from glacial Lakes Agassiz or Ojibway associated with the retreat of the Laurentide Ice Sheet from the Nakina moraine and/or the Cochrane surge margins in the Hudson Bay Lowlands. A continued rise in lake level after 6,420 ± 20 (7,400 cal) BP at FWHP may record uplift of the North Bay outlet above the Sault Ste. Marie bedrock sill and the onset of the Nipissing transgression in the Lake Superior basin.  相似文献   

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