Stable carbon and oxygen isotopic evidence for late Pleistocene to middle Holocene climatic fluctuations in the interior of southern Africa     

Jeannette M. Smith  Julia A. Lee‐Thorp  Judith C. Sealy 《第四纪科学杂志》2002,17(7):683-695

Stable carbon and oxygen isotope analyses of ungulate grazers from four archaeological sites located in different environs within the Caledon River Valley have provided a relatively well‐dated proxy palaeoenvironmental and palaeoclimatic sequence for the period between 16 000 and 6000 calendar (cal.) yr BP. Within the overall trend towards hot mid‐Holocene temperatures and a summer rainfall pattern, stable carbon isotope results show that there were three periods when growth season temperatures were cool enough for C₃ grasses to be present: 16 000–14 000; 10 200–9600, and 8400–8000 cal. yr BP. Similar trends were recorded in stable oxygen isotope values, reflecting shifts in either temperature or available moisture. Although having a similar pattern to that of the lower altitude site, sites situated in foothills and montane portions of the valley consistently maintained lower temperatures until the mid‐Holocene altithermal. At this time growth season temperatures warmed sufficiently for a 100% C₄ grassland to expand in altitude from the warmer low lying localities. In relation to present understanding of synoptic and global climatic patterning, these findings suggest that the early to middle Holocene transition was not a gradual warming trend, but rather it was marked by a series of climatic fluctuations. Of particular note is the possible global, rather than regional, occurrence of the 8200 cal. yr BP ‘event’. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

GCM Hindcasts of SST Forced Climate Variability over Agriculturally Intensive Regions     

Leonard M. Druyan  Kathryn P. Shah  Mark A. Chandler  David Rind 《Climatic change》2000,45(2):279-322

The NASA/Goddard Institute for Space Studies (GISS) climatemodel is forced with globally observed sea-surfacetemperatures (SST) in five simulations, 1969–1991,with individual runs beginning from altered initialatmospheric conditions. The interannual variability ofmodeled anomalies of the Southern Oscillation Index,mid-tropospheric temperatures, 850 mb zonal winds andOutgoing Longwave Radiation over the tropical PacificOcean, which has the largest SST anomaly forcing, arestrongly correlated with observed trends which reflectENSO cycles. The model's rainfall variability overthree agriculturally intensive regions, two tropicaland one mid-latitude, is investigated in order toevaluate the potential usefulness of GCM predictionsfor agricultural planning. The correct sign ofZimbabwe seasonal precipitation anomalies was hindcastwithin a useful range of consensus only for selectseasons corresponding to extreme ENSO events for whichanomalous circulation patterns were ratherrealistically simulated. The correlation betweenhindcasts of Nordeste monthly precipitation andobservations increases with time smoothing, reaching0.64 for 5-month running means. Consensus betweenindividual runs is directly proportional to theabsolute value of Niño3 SST so that during ElNiño and La Niña years most simulations agreeon the sign of predicted Nordeste rainfall anomalies.We show that during selected seasons the uppertropospheric divergent circulation and near surfacemeridional displacements of the ITCZ are realisticallyrepresented by the ensemble mean of the simulations.This realistic simulation of both the synopticmechanisms and the resulting precipitation changesincreases confidence in the GCM's potential forseasonal climate prediction.  相似文献   

Stable isotope evidence of late MIS 3 to middle Holocene palaeoenvironments from Sehonghong Rockshelter,eastern Lesotho          下载免费PDF全文

Emma Loftus  Brian A. Stewart  Genevieve Dewar  Julia Lee‐Thorp 《第四纪科学杂志》2015,30(8):805-816

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Hydrological principles for sustainable management of forest ecosystems     

Irena F. Creed  Gabor Z. Sass  Jim M. Buttle  Julia A. Jones 《水文研究》2011,25(13):2152-2160

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Fifty years of runoff response to conversion of old-growth forest to planted forest in the H. J. Andrews Forest,Oregon, USA     

Emily A. Crampe  Catalina Segura  Julia A. Jones 《水文研究》2021,35(5):e14168

Long-term watershed experiments provide the opportunity to understand forest hydrology responses to past logging, road construction, forest regrowth, and their interactions with climate and geomorphic processes such as road-related landslides. We examined a 50-year record from paired-watershed experiments in the H. J. Andrews Experimental Forest, Oregon, USA in which 125 to 450-year-old conifer forests were harvested in the 1960s and 1970s and converted to planted conifer forests. We evaluated how quickflow and delayed flow for 1222 events in treated and reference watersheds changed by season after clearcutting and road construction, including 50 years of growth of planted forest, major floods, and multi-decade reductions in snowpack. Quickflow runoff early in the water year (fall) increased by up to +99% in the first decade, declining to below pre-harvest levels (−1% to −15%) by the third to fifth decade after clearcutting. Fall delayed flow responded more dramatically than quickflow and fell below pre-treatment levels in all watersheds by the fifth decade, consistent with increased transpiration in the planted forests. Quickflow increased less (+12% to 70%) during the winter and spring but remained higher than pre-treatment levels throughout the fourth or fifth decade, potentially impacted by post-harvest burning, roads, and landslides. Quickflow remained high throughout the 50-year period of study, and much higher than delayed flow in the last two decades in a watershed in which road-related changes in flow routing and debris flows after the flood of record increased network connectivity. A long-term decline in regional snowpack was not clearly associated with responses of treated vs. reference watersheds. Hydrologic processes altered by harvest of old-growth conifer forest more than 50 years ago (transpiration, interception, snowmelt, and flow routing) continued to modify streamflow, with no clear evidence of hydrologic recovery. These findings underscore the importance of continued long-term watershed experiments.  相似文献   

Askival: An altered feldspathic cumulate sample in Gale crater     

Donald Lewis Bowden  John C. Bridges  Agnes Cousin  William Rapin  Julia Semprich  Olivier Gasnault  Olivier Forni  Patrick Gasda  Debarati Das  Valerie Payré  Violaine Sautter  Candice C. Bedford  Roger C. Wiens  Patrick Pinet  Jens Frydenvang 《Meteoritics & planetary science》2023,58(1):41-62

Askival is a light-toned, coarsely crystalline float rock, which was identified near the base of Vera Rubin Ridge in Gale crater. We have studied Askival, principally with the ChemCam instrument but also using APXS compositional data and MAHLI images. Askival and an earlier identified sample, Bindi, represent two rare examples of feldspathic cumulate float rocks in Gale crater with >65% relict plagioclase. Bindi appears unaltered whereas Askival shows textural and compositional signatures of silicification, along with alkali remobilization and hydration. Askival likely experienced multiple stages of alteration, occurring first through acidic hydrolysis of metal cations, followed by deposition of silica and possible phyllosilicates at low T and neutral-alkaline pH. Through laser-induced breakdown spectroscopy compositional analyses and normative calculations, we suggest that an assemblage of Fe-Mg silicates including amphibole and pyroxene, Fe phases, and possibly Mg-rich phyllosilicate are present. Thermodynamic modeling of the more pristine Bindi composition predicts that amphibole and feldspar are stable within an upper crustal setting. This is consistent with the presence of amphibole in the parent igneous rocks of Askival and suggests that the paucity of amphiboles in other known Martian samples reflects the lack of representative samples of the Martian crust rather than their absence on Mars.  相似文献   

Recent anthropogenic impact in ancient Lake Ohrid (Macedonia/Albania): a palaeolimnological approach     

Julia Lorenschat  Xiaosen Zhang  Flavio S. Anselmetti  Jane M. Reed  Martin Wessels  Antje Schwalb 《Journal of Paleolimnology》2014,52(3):139-154

Ancient lakes, which are important centres of biodiversity and endemism, are threatened by a wide variety of human impacts. To assess environmental impact on ancient Lake Ohrid we have taken short sediment cores from two contrasting site locations, comprising a site of urban pollution and an apparently pristine area. Recent impacts on water quality and ecology were assessed using sediment, geochemical, ostracode, and diatom data derived from analysis of two ²¹⁰Pb-dated sediment cores spanning the period from 1918 to 2009. According to the index of geoaccumulation, sediments were often moderately contaminated with As. Fe and Ni concentrations often exceeded reported maximum limits above which harmful effects on sediment-dwelling organisms are expected. Productivity in the (pristine) south-eastern part of Lake Ohrid (Sveti Naum) is generally lower than in the north, probably due to the strong influence of spring discharge. Low ostracode and diatom concentrations, low abundance of the epilimnetic diatom Cyclotella ocellata, and low values of TOC and TIC indicate a lower productivity from the early 1920s to the late 1980s. Since the mid 1970s, increased relative abundance of C. ocellata and increasing diatom concentration indicate increasing productivity in the south-eastern part. Rising numbers of ostracode valves and higher TIC and TOC contents in both sediment cores indicate an increase in productivity during the late 1980s. A slight increase in productivity near Sveti Naum continued from the early 1990s until 2009, witnessed by rising TC, TIC, and TOC content and a generally high number of ostracode valves and ostracode diversity. The area near the City of Struga (site of urban pollution) is also characterized by rising TOC and TIC contents and, furthermore, by increasing Cu, Fe, Pb, and Zn concentrations since the early 1990s. The recent reduction in the number of ostracode valves and ostracode diversity is probably caused by a higher heavy metal load into the lake. This suggests that living conditions for the endemic species in Lake Ohrid have become less favourable in the northern part of the lake, which might threaten the unique flora and fauna of Lake Ohrid.  相似文献   

Climate influences on water and sediment properties of Genovesa Crater Lake,Galápagos     

Jessica L. Conroy  Diane M. Thompson  Aaron Collins  Jonathan T. Overpeck  Mark B. Bush  Julia E. Cole 《Journal of Paleolimnology》2014,52(4):331-347

Genovesa Crater Lake is a remote, hypersaline lake in the northern Galápagos archipelago that contains a finely laminated sediment record. This sediment record has the potential to provide a high-resolution history of past climate variability in the eastern tropical Pacific. Here we present modern climate, lake, and sediment observations from 2009 to 2012 to explore how local climate variability influences Genovesa Crater Lake and its sediments. Surface lake temperature is strongly linked to air temperature and is highly seasonal. Temperature stratification is strongest during the warm season, whereas temperature becomes more uniform through the water column in the cool season. Deeper and earlier mixing occurred during the 2010 La Niña, which subsequently delayed 2011 cool season mixing and maximum warm season surface temperatures in 2011 and 2012. Lake salinity changes are influenced by precipitation, evaporation and persistent seawater influx. The largest declines in subsurface salinity follow months after the rainy season, when temperatures cool and fresher surface water from the previous warm/wet season mixes into the subsurface. Between 2009 and 2012, more calcium carbonate precipitated during a period of higher salinity. The period of highest calcium carbonate abundance measured in sediment records that span the late nineteenth to twentieth century coincides with the failure of two consecutive rainy seasons in 1988 and 1989 as well as the coldest monthly sea surface temperature measured at Puerto Ayora in 1989. More calcium carbonate-rich laminae from AD 1550 ± 70 to 1675 ± 90 may indicate a greater frequency of prolonged droughts or cooler temperatures, although enhanced productivity may also modulate carbonate precipitation. More Ca-rich laminae in Genovesa coincide with dry conditions inferred from other Galápagos sediment proxies, as well as prolonged dry and cool conditions inferred from reconstructions of the Southern Oscillation Index and NINO3 sea surface temperatures.  相似文献   

Validation of Arenicola marina in field toxicity bioassays using benthic cages: biomarkers as tools for assessing sediment quality     

Ramos-Gómez J  Martins M  Raimundo J  Vale C  Martín-Díaz ML  DelValls TA 《Marine pollution bulletin》2011,62(7):1538-1549

Sediment toxicity assessments using caged organisms present advantages over using laboratory and native community studies. The use of caged Arenicola marina in sediment toxicity assessments was evaluated. Lugworms were exposed in situ to sediments from coastal and port areas in Spain for seven days, and the activities of the biotransformation enzymes ethoxyresorufin O-deethylase, dibenzylfluorescein dealkylase and glutathione S-transferase, the activities of the antioxidant enzymes glutathione reductase and glutathione peroxidase and lipid peroxidation were then analyzed as biomarkers. Biomarker results and sediment physicochemical data were integrated. Cádiz Bay (SW Spain) sediments presented metal contamination that was not linked to a biochemical response. In LPGC Port (SW Spain), Pb contamination exhibited a moderate toxic potential, while PAHs, and presumably pharmaceuticals, provoked biochemical responses that efficiently prevented lipid peroxidation. In Santander Bay (N Spain), exposure to PAHs and, presumably, pharmaceuticals induced biomarker responses, but lipid peroxidation occurred nevertheless. These results indicated that caged A. marina were effective for the assessment of sediment quality and that the selected biomarkers were sufficiently sensitive to identify chemical exposure and toxicity.  相似文献   

Solubility and solution mechanisms of C-O-H volatiles in silicate melt with variable redox conditions and melt composition at upper mantle temperatures and pressures     

Bjorn O. Mysen  Kathryn Kumamoto  Marilyn L. Fogel 《Geochimica et cosmochimica acta》2011,75(20):6183-6199

Solubility and solution mechanisms in silicate melts of oxidized and reduced C-bearing species in the C-O-H system have been determined experimentally at 1.5 GPa and 1400 °C with mass spectrometric, NMR, and Raman spectroscopic methods. The hydrogen fugacity, f_H2, was controlled in the range between that of the iron-wüstite-H₂O (IW) and the magnetite-hematite-H₂O (MH) buffers. The melt polymerization varied between those typical of tholeiitic and andesitic melts.The solubility of oxidized (on the order of 1-2 wt% as C) and reduced carbon (on the order of 0.15-0.35 wt% as C) is positively correlated with the NBO/Si (nonbridging oxygen per silicon) of the melt. At given NBO/Si-value, the solubility of oxidized carbon is 2-4 times greater than under reducing conditions. Oxidized carbon dioxide is dissolved as complexes, whereas the dominant reduced species in melts are CH₃-groups forming bonds with Si⁴⁺ together with molecular CH₄. Formation of complexes results in silicate melt polymerization (decreasing NBO/Si), whereas solution of reduced carbon results in depolymerization of melts (increasing NBO/Si).Redox melting in the Earth’s interior has been explained with the aid of the different solution mechanisms of oxidized and reduced carbon in silicate melts. Further, effects of oxidized and reduced carbon on melt viscosity and on element partitioning between melts and minerals have been evaluated from relationships between melt polymerization and dissolved carbon combined with existing experimental data that link melt properties and melt polymerization. With total carbon contents in the melts on the order of several mol%, mineral/melt element partition coefficients and melt viscosity can change by several tens to several hundred percent with variable redox conditions in the range of the Earth’s deep crust and upper mantle.  相似文献