Lake sediment evidence for late Holocene climate change and landscape erosion in western Iceland     

Freddy J. Gathorne-Hardy  Egill Erlendsson  Peter G. Langdon  Kevin J. Edwards 《Journal of Paleolimnology》2009,42(3):413-426

Ecosystem variability must be assessed over a range of timescales in order to fully understand natural ecosystem processes. Long-term climate change, at millennial and centennial scales, is a major driver of natural ecosystem variability, but identifying evidence of past climate change is frequently confounded by human-induced impacts on the ecosystem. Iceland is a location where it is possible to separate natural from anthropogenic change in environmental archives, as the date of settlement is accepted to be around AD 874, prior to which the island was free from proven human impacts. We used a lake sediment core from Breieavatn, near Reykholt, a major farm of the Norse period in western Iceland, to examine landscape development. A change in pollen concentration in the sediments, especially the decline in Betula, indicated initial landscape degradation immediately post-settlement, whereas the chironomid fauna and reconstructed temperatures were relatively complacent during this period. The pollen evidence is corroborated by ¹⁴C analyses, which indicate an increase in older carbon entering the lake, inferred to have been caused by increased erosion following settlement. Further decreases in Betula pollen occurred around AD 1300, pre-dating a drop in chironomid-inferred temperatures (CI-T) of ~1°C over 100–200 years. The CI-T reconstruction also shows a significant cooling after ~AD 1800, likely indicative of the coldest phase of the Little Ice Age. The evidence suggests that the chironomid record was relatively unaffected by the increased landscape degradation and hence reveals a temperature reconstruction independent of human impact.  相似文献   

Monitoring the freely dissolved concentrations of polycyclic aromatic hydrocarbons (PAH) and alkylphenols (AP) around a Norwegian oil platform by holistic passive sampling     

Christopher Harman  Kevin V. Thomas  Knut Erik Tollefsen  Sonnich Meier  Olav Byum  Merete Grung 《Marine pollution bulletin》2009,58(11):1671-1679

In order to assess the environmental impact of aquatic discharges from the offshore oil industry, polar organic chemical integrative samplers (POCIS) and semipermeable membrane devices (SPMDs) were deployed around an oil platform and at reference locations in the North Sea. Exposure to polycyclic aromatic hydrocarbons (PAH) and alkylated phenols (AP) was determined from passive sampler accumulations using an empirical uptake model, the dissipation of performance reference compounds and adjusted laboratory derived sampling rates. Exposure was relatively similar within 1–2 km of the discharge point, with levels dominated by short chained C1–C3 AP isomers (19–51 ng L⁻¹) and alkylated naphthalenes, phenanthrenes and dibenzothiophenes (NPD, 29–45 ng L⁻¹). Exposure stations showed significant differences to reference sites for NPD, but not always for more hydrophobic PAH. These concentrations are several orders of magnitude lower than those reported to give both acute and sub-lethal effects, although their long term consequences are unknown.  相似文献   

A Bayesian hierarchical modeling approach for analyzing observational data from marine ecological studies   总被引：1，自引：0，他引：1  

Song S. Qian  J. Kevin Craig  Melissa M. Baustian  Nancy N. Rabalais 《Marine pollution bulletin》2009,(12):1916-1921

We introduce the Bayesian hierarchical modeling approach for analyzing observational data from marine ecological studies using a data set intended for inference on the effects of bottom-water hypoxia on macrobenthic communities in the northern Gulf of Mexico off the coast of Louisiana, USA. We illustrate (1) the process of developing a model, (2) the use of the hierarchical model results for statistical inference through innovative graphical presentation, and (3) a comparison to the conventional linear modeling approach (ANOVA). Our results indicate that the Bayesian hierarchical approach is better able to detect a “treatment” effect than classical ANOVA while avoiding several arbitrary assumptions necessary for linear models, and is also more easily interpreted when presented graphically. These results suggest that the hierarchical modeling approach is a better alternative than conventional linear models and should be considered for the analysis of observational field data from marine systems.  相似文献   

Equilibrium Fe isotope fractionation between inorganic aqueous Fe(III) and the siderophore complex, Fe(III)-desferrioxamine B     

K. Dideriksen  J.A. Baker  S.L.S. Stipp   《Earth and Planetary Science Letters》2008,269(1-2):280-290

In oxic oceans, most of the dissolved iron (Fe) exists as complexes with siderophore-like, strongly coordinating organic ligands. Thus, the isotope composition of the little amount of free inorganic Fe that is available for precipitation and preservation in the geological record may largely be controlled by isotope fractionation between the free and complexed iron. We have determined the equilibrium Fe isotope fractionation induced by organic ligand activity in experiments with solutions having co-existing inorganic Fe(III) species and siderophore complexes, Fe-desferrioxamine B (at pH 2). The two differently complexed Fe(III) pools were separated by addition of Na₂CO₃, which led to immediate precipitation of the inorganic Fe without causing significant dissociation of Fe-desferrioxamine complexes. Experiments using enriched ⁵⁷Fe tracer showed that isotopic equilibration between the ⁵⁷Fe-labelled inorganic species and the isotopically “normal” siderophore-bound Fe was rapid during the first few seconds and then became slower. Consequently, the data fitted poorly to first and second order reaction equations. However, with a two-stage reaction, the data fit perfectly with a first order equation for the slower stage, indicating that approximately 40% re-equilibration may take place during the separation of the two pools. To further test if the induced precipitation leads to experimental artefacts, the fractionation during precipitation of inorganic Fe was determined. Assuming a Rayleigh-type fractionation during precipitation, this experiment yielded an isotope fractionation factor of ⁵⁶Fe_{solution-solid} = 1.00027. Calculations based on these results indicate that isotopic re-equilibration is unlikely to significantly affect our determined equilibrium Fe isotope fractionation between inorganically and organically complexed Fe. To determine the equilibrium Fe isotope fractionation between inorganically and organically bound Fe(III), experiments with variable proportions of inorganic Fe were carried out at 25 °C. Irrespective of the proportion of inorganic Fe, equilibrium fractionation factors were within experimental uncertainty, yielding an average fractionation factor, Δ⁵⁶Fe_DFOB-inorg of 0.60 ± 0.15‰. The results indicate that equilibrium Fe isotope fractionation induced by strongly coordinating organic ligands may fractionate Fe isotopes in nature. The fractionation is likely to be important in oxic, Fe(III)-bearing environments, such as soils and rivers, and may, for example, largely control the Fe isotope composition of marine Fe–Mn crusts.  相似文献   

Chicxulub impact ejecta from Albion Island, Belize     

Kevin O Pope  Adriana C Ocampo  Alfred G Fischer  Walter Alvarez  Bruce W Fouke  Clyde L Webster  Francisco J Vega  Jan Smit  A.Eugene Fritsche  Philippe Claeys 《Earth and Planetary Science Letters》1999,170(4):275-364

Impact ejecta from the Albion Formation are exposed in northern Belize. The ejecta come from the outer portion of the continuous ejecta blanket of the Chicxulub crater, which is located 360 km to the northwest. The basal unit of the Albion Formation is a 1-m-thick clay and dolomite spheroid bed composed of up to four discrete flows. The clay spheroids are altered impact glass, and the dolomite spheroids are accretionary lapilli. The upper unit is a 15-m-thick coarse diamictite bed containing altered glass, large accretionary blocks, striated, polished, and impacted cobbles, and rare shocked quartz. The abundance of accretionary clasts, evidence for atmospheric drag sorting, and the presence of multiple flows in the Albion Formation indicate that atmospheres play an important role in the formation of the outer portions of continuous ejecta blankets of large craters.  相似文献   

Researching local elites: reflexivity, ‘situatedness’ and political-temporal contingency     

Kevin G. Ward  Martin Jones   《Geoforum》1999,30(4):70

Since the late 1970s, the UK has experienced a series of changes in the mechanisms of governing local economic development. This paper considers the methodological consequences of these recent changes. Drawing on two local elite-based case studies, involving semi-structured interviews, we reflect on our acquired ‘situated knowledge’ and argue that this is related to the contingent nature of our respective research access. We advocate that an approach sensitive to ‘research situatedness’ (with respect to the politics of time as a ‘research moment’) is essential to avoid over generalising from our experiences. To address this concern, the paper offers two concepts for debate – political-temporal contingency and the mode of entry.  相似文献   

Petaloid Organs Preserved in an Arctic Plant Macrofossil Assemblage from Full-Glacial Sediments in Southeastern Minnesota     

R. G. Baker  J. A. Mason  L. J. Maher  Jr. 《Quaternary Research》1999,52(3):388

A small suite of plant macrofossils indicates that southeastern Minnesota supported subarctic to arctic vegetation 18,700 yr B.P. Fossil tepals of Polygonum viviparum are exceptionally well preserved; they occur with more fragmentary remains of Dryas integrifolia, Vaccinium uliginosum var. alpinum, and probable species of arctic Salix, S. cf. herbacea, and S. cf. arctica. The pollen spectrum from this site is dominated by Picea, Pinus, and Cyperaceae, which are typical of midwestern full-glacial sequences. Tundra-like conditions with permafrost were present in southeastern Minnesota during full-glacial time. Local environments 18,700 yr B.P. reconstructed from both physical and paleobotanical evidence include wind-swept ridge tops with thin loess, outcrops of dolostone and sandstone on valley walls, colluvial slopes, sandy to gravelly floodplains, shallow floodplain pools, wet meadows, and peaty turfs, all in a treeless or nearly treeless environment.  相似文献   

Petrology of unique achondrite Queen Alexandra Range 93148: A piece of the pallasite (howardite‐eucrite‐diogenite?) parent body?     

Cyrena Anne GOODRICH  Kevin RIGHTER 《Meteoritics & planetary science》2000,35(3):521-535

Abstract— Queen Alexandra Range (QUE) 93148 is a small (1.1 g) olivine‐rich achondrite (mg 86) that contains variable amounts of orthopyroxene (mg 87) and kamacite (6.7 wt% Ni), with minor augite. Olivine in QUE 93148 contains an unusual suite of inclusions: (1) 5 × 100 μm sized lamellae with a CaO‐ and Cr₂O₃‐rich (～10 and 22 wt%, respectively) composition that may represent a submicrometer‐scale intergrowth of chromite and pyroxene(s); (2) 75 × 500 μm sized lamellar symplectites composed of chromite and two pyroxenes, with minor metal; (3) 15–20 μm sized, irregularly‐shaped symplectites composed of chromite and pyroxene(s); (4) 100–150 μm sized, elliptical inclusions composed of chromite, two pyroxenes, metal, troilite, and rare whitlockite. Type 1, 2, and 3 inclusions probably formed by exsolution from the host olivine during slow cooling, whereas type 4 more likely resulted from early entrapment of silicate and metallic melts followed by closed‐system oxidation. Queen Alexandra Range 93148 can be distinguished from most other olivine‐rich achondrites (ureilites, winonaites, lodranites, acapulcoites, brachinites, Eagle‐Station‐type pallasites, and pyroxene pallasites), as well as from mesosiderites, by some or all of the following properties: O‐isotopic composition, Fe‐Mn‐Mg relations of olivine, CaO and Cr₂O₃ contents of olivine, orthopyroxene compositions, molar Cr/(Cr + Al) ratios of chromite, metal composition, texture, and the presence of the inclusions. In terms of many of these properties, it shows an affinity to main‐group pallasites. Nevertheless, it cannot be identified as belonging to this group. Meteorite QUE 93148 appears to be a unique achondrite. Possibly it should be considered to be a pyroxene pallasite that is genetically related to main‐group pallasites. Alternatively, it may be derived from the mantle of the pallasite (howardite‐eucrite‐diogenite?) parent body.  相似文献   

Early diagenetic alteration of chlorophyll-a and bacteriochlorophyll-a in a contemporaneous marl ecosystem; Florida Bay     

J. William Louda  Joseph W. Loitz  David T. Rudnick  Earl W. Baker 《Organic Geochemistry》2000,31(12)

This report covers the analyses of tetrapyrrole pigments in sediments and numerous source biota from northern central Florida Bay. Sediment cores were all carbonate marls and ‘bedrock’ (Pleistocene limestones) was typically at 0.6–1.2 m bsf. (below sea floor). Extraction of the sediments was performed using tetrahydrofuran, shown to be 3–7 times as effective as more common solvents. Surficial sediments were found to contain a pigment distribution indicating a diatomaceous-cyanobacterial biofilm/mat underlain with purple sulfur bacteria as the microphytobenthos. Downhole trends in pigment diagenesis revealed differences due to paleoenvironment. That is, when bacteriochlorophyll-a (BCHLa) plus derivatives were elevated, indicating strong syn-depositional anoxia, the pheophytins-a (PTINsa) were the major chlorophyll-a (CHLa) derivatives. Conversely, when BCHL-a plus derivatives were low, indicating oxic to dys-oxic syn-depositional conditions, then pyropheophorbide-a (pPBIDa) and 13², 17²-cyclopheophorbide-a (CYCLO) were strongly dominant. The generation of cyclopheophorbide in carbonate marls with weakly anoxic to dysoxic syn-depositional conditions may yield an oil-source paleoenvironmental marker for bitumen containing compounds such as bicycloalkanoporphyrins (BiCAPs). Downhole conversions were observed for the following diagenetic steps; CHLa to pheophytin-a (PTINa), PTINa to pyropheophytin-a (pPTINa), pPBIDa to CYCLO, BCHLa to bacteriopheophytin-a (BPTINa), and BPTINa to bacteriopyropheophytin-a (BpPTINa).  相似文献   

Chironomid grazing on benthic microalgae in a Louisiana salt marsh     

April C. Goldfinch  Kevin R. Carman 《Estuaries and Coasts》2000,23(4):536-547

Field experiments were conducted to examine spatial and temporal variation in chironomid (predominantlyTanypus clavatus) abundance, and their trophic relationship with benthic microalgae. High performance liquid chromatography (HPLC) analysis of chironomid gut pigments indicated that diatoms comprised the bulk of the microalgae ingested by chironomids.¹⁴C-feeding studies were used to obtain quantitative estimates of chironomid, copepod, ostracod, and nematode grazing on benthic microalgae. Daily consumption of standing microalgal biomass by chironomids ranged from 0.12% (January) to 125% (May), but was highly variable. There were no significant diel, temporal (over the scale of months), or spatial patterns in individual chironomid grazing rates. There was significant temporal variation in the proportion of microalgal biomass consumed by the total meiofaunal assemblage, and highest grazing impacts occurred in May, coincident with high abundances of chironomids, harpacticoid copepods, and ostracods. The grazing impact of chironomids was comparable to or greater than that of other known grazers of microalgae (copepods, ostracods). Functional-response experiments performed in the laboratory revealed that chironomid ingestion rates increased with increasing food availability over short (1 to 2 h) time scales. Field data did not indicate a functional response to food availability over longer (mo) time scales, possibly because of other environmental incluences. Gut residence time (determined using fluorescently labeled beads) changed with variable feeding rates, which were in turn a function of variable food availability. Chironomid larvae have the ability to consume significant fraction of the microphytobenthos in absolute terms, and relative to other meiofauna, indicating that they are an important component of the salt marsh food web.  相似文献