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Michael Zech - One of the best experts on this subject based on the ideXlab platform.
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oxygen isotope ratios 18o 16o of hemicellulose derived sugar biomarkers in plants soils and sediments as Paleoclimate Proxy i insight from a climate chamber experiment
Geochimica et Cosmochimica Acta, 2014Co-Authors: Michael Zech, Christoph Mayr, Mario Tuthorn, Katharina Leibersauheitl, Bruno GlaserAbstract:Abstract The oxygen isotopic composition of cellulose is a valuable Proxy in Paleoclimate research. However, its application to sedimentary archives is challenging due to extraction and purification of cellulose. Here we present compound-specific δ 18 O results of hemicellulose-derived sugar biomarkers determined using gas chromatography–pyrolysis–isotope ratio mass spectrometry, which is a method that overcomes the above-mentioned analytical challenges. The biomarkers were extracted from stem material of different plants ( Eucalyptus globulus , Vicia faba and Brassica oleracea ) grown in climate chamber experiments under different climatic conditions. The δ 18 O values of arabinose and xylose range from 31.4‰ to 45.9‰ and from 28.7‰ to 40.8‰, respectively, and correlate highly significantly with each other ( R = 0.91, p 18 O hemicellulose (mean of arabinose and xylose) correlate highly significantly with δ 18 O leaf water ( R = 0.66, p 18 O cellulose ( R = 0.42, p R = −0.79, p R = −0.66, p 18 O enrichment of leaf water. While relative air humidity controls most rigorously the evapotranspirative 18 O enrichment, the direct temperature effect is less important. However, temperature can indirectly exert influence via plant physiological reactions, namely by influencing the transpiration rate which affects δ 18 O leaf water due to the Peclet effect. In a companion paper (Tuthorn et al., this issue) we demonstrate the applicability of the hemicellulose-derived sugar biomarker δ 18 O method to soils and provide evidence from a climate transect study confirming that relative air humidity exerts the dominant control on evapotranspirative 18 O enrichment of leaf water. Finally, we present a conceptual model for the interpretation of δ 18 O hemicellulose records and propose that a combined δ 18 O hemicellulose and δ 2 H n -alkane biomarker approach is promising for disentangling δ 18 O precipitation variability from evapotranspirative 18 O enrichment variability in future Paleoclimate studies.
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oxygen isotope ratios 18o 16o of hemicellulose derived sugar biomarkers in plants soils and sediments as Paleoclimate Proxy ii insight from a climate transect study
Geochimica et Cosmochimica Acta, 2014Co-Authors: Mario Tuthorn, Michael Zech, Marc Ruppenthal, Yvonne Oelmann, Ansgar Kahmen, Hector Del Valle, Wolfgang Wilcke, Bruno GlaserAbstract:The oxygen isotopic composition of precipitation (δ18Oprec) is well known to be a valuable (paleo-)climate Proxy. Paleosols and sediments and hemicelluloses therein have the potential to serve as archives recording the isotopic composition of paleoprecipitation. In a companion paper (Zech et al., 2014) we investigated δ18Ohemicellulose values of plants grown under different climatic conditions in a climate chamber experiment. Here we present results of compound-specific δ18O analyses of arabinose, fucose and xylose extracted from modern topsoils (n = 56) along a large humid-arid climate transect in Argentina in order to answer the question whether hemicellulose biomarkers in soils reflect δ18Oprec. The results from the field replications indicate that the homogeneity of topsoils with regard to δ18Ohemicellulose is very high for most of the 20 sampling sites. Standard deviations for the field replications are 1.5‰, 2.2‰ and 1.7‰, for arabinose, fucose and xylose, respectively. Furthermore, all three hemicellulose biomarkers reveal systematic and similar trends along the climate gradient. However, the δ18Ohemicellulose values (mean of the three sugars) do not correlate positively with δ18Oprec (r = −0.54, p < 0.014, n = 20). By using a Peclet-modified Craig-Gordon (PMCG) model it can be shown that the δ18Ohemicellulose values correlate highly significantly with modeled δ18Oleaf water values (r = 0.81, p < 0.001, n = 20). This finding suggests that hemicellulose biomarkers in (paleo-)soils do not simply reflect δ18Oprec but rather δ18Oprec altered by evaporative 18O enrichment of leaf water due to evapotranspiration. According to the modeling results, evaporative 18O enrichment of leaf water is relatively low (∼10‰) in the humid northern part of the Argentinian transect and much higher (up to 19‰) in the arid middle and southern part of the transect. Model sensitivity tests corroborate that changes in relative air humidity exert a dominant control on evaporative 18O enrichment of leaf water and thus δ18Ohemicellulose, whereas the effect of temperature changes is of minor importance. While oxygen exchange and degradation effects seem to be negligible, further factors needing consideration when interpreting δ18Ohemicellulose values obtained from (paleo-)soils are evaporative 18O enrichment of soil water, seasonality effects, wind effects and in case of abundant stem/root-derived organic matter input a partial loss of the evaporative 18O enrichment of leaf water. Overall, our results prove that compound-specific δ18O analyses of hemicellulose biomarkers in soils and sediments are a promising tool for Paleoclimate research. However, disentangling the two major factors influencing δ18Ohemicellulose, namely δ18Oprec and relative air humidity controlled evaporative 18O enrichment of leaf water, is challenging based on δ18O analyses alone.
Bruno Glaser - One of the best experts on this subject based on the ideXlab platform.
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oxygen isotope ratios 18o 16o of hemicellulose derived sugar biomarkers in plants soils and sediments as Paleoclimate Proxy i insight from a climate chamber experiment
Geochimica et Cosmochimica Acta, 2014Co-Authors: Michael Zech, Christoph Mayr, Mario Tuthorn, Katharina Leibersauheitl, Bruno GlaserAbstract:Abstract The oxygen isotopic composition of cellulose is a valuable Proxy in Paleoclimate research. However, its application to sedimentary archives is challenging due to extraction and purification of cellulose. Here we present compound-specific δ 18 O results of hemicellulose-derived sugar biomarkers determined using gas chromatography–pyrolysis–isotope ratio mass spectrometry, which is a method that overcomes the above-mentioned analytical challenges. The biomarkers were extracted from stem material of different plants ( Eucalyptus globulus , Vicia faba and Brassica oleracea ) grown in climate chamber experiments under different climatic conditions. The δ 18 O values of arabinose and xylose range from 31.4‰ to 45.9‰ and from 28.7‰ to 40.8‰, respectively, and correlate highly significantly with each other ( R = 0.91, p 18 O hemicellulose (mean of arabinose and xylose) correlate highly significantly with δ 18 O leaf water ( R = 0.66, p 18 O cellulose ( R = 0.42, p R = −0.79, p R = −0.66, p 18 O enrichment of leaf water. While relative air humidity controls most rigorously the evapotranspirative 18 O enrichment, the direct temperature effect is less important. However, temperature can indirectly exert influence via plant physiological reactions, namely by influencing the transpiration rate which affects δ 18 O leaf water due to the Peclet effect. In a companion paper (Tuthorn et al., this issue) we demonstrate the applicability of the hemicellulose-derived sugar biomarker δ 18 O method to soils and provide evidence from a climate transect study confirming that relative air humidity exerts the dominant control on evapotranspirative 18 O enrichment of leaf water. Finally, we present a conceptual model for the interpretation of δ 18 O hemicellulose records and propose that a combined δ 18 O hemicellulose and δ 2 H n -alkane biomarker approach is promising for disentangling δ 18 O precipitation variability from evapotranspirative 18 O enrichment variability in future Paleoclimate studies.
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oxygen isotope ratios 18o 16o of hemicellulose derived sugar biomarkers in plants soils and sediments as Paleoclimate Proxy ii insight from a climate transect study
Geochimica et Cosmochimica Acta, 2014Co-Authors: Mario Tuthorn, Michael Zech, Marc Ruppenthal, Yvonne Oelmann, Ansgar Kahmen, Hector Del Valle, Wolfgang Wilcke, Bruno GlaserAbstract:The oxygen isotopic composition of precipitation (δ18Oprec) is well known to be a valuable (paleo-)climate Proxy. Paleosols and sediments and hemicelluloses therein have the potential to serve as archives recording the isotopic composition of paleoprecipitation. In a companion paper (Zech et al., 2014) we investigated δ18Ohemicellulose values of plants grown under different climatic conditions in a climate chamber experiment. Here we present results of compound-specific δ18O analyses of arabinose, fucose and xylose extracted from modern topsoils (n = 56) along a large humid-arid climate transect in Argentina in order to answer the question whether hemicellulose biomarkers in soils reflect δ18Oprec. The results from the field replications indicate that the homogeneity of topsoils with regard to δ18Ohemicellulose is very high for most of the 20 sampling sites. Standard deviations for the field replications are 1.5‰, 2.2‰ and 1.7‰, for arabinose, fucose and xylose, respectively. Furthermore, all three hemicellulose biomarkers reveal systematic and similar trends along the climate gradient. However, the δ18Ohemicellulose values (mean of the three sugars) do not correlate positively with δ18Oprec (r = −0.54, p < 0.014, n = 20). By using a Peclet-modified Craig-Gordon (PMCG) model it can be shown that the δ18Ohemicellulose values correlate highly significantly with modeled δ18Oleaf water values (r = 0.81, p < 0.001, n = 20). This finding suggests that hemicellulose biomarkers in (paleo-)soils do not simply reflect δ18Oprec but rather δ18Oprec altered by evaporative 18O enrichment of leaf water due to evapotranspiration. According to the modeling results, evaporative 18O enrichment of leaf water is relatively low (∼10‰) in the humid northern part of the Argentinian transect and much higher (up to 19‰) in the arid middle and southern part of the transect. Model sensitivity tests corroborate that changes in relative air humidity exert a dominant control on evaporative 18O enrichment of leaf water and thus δ18Ohemicellulose, whereas the effect of temperature changes is of minor importance. While oxygen exchange and degradation effects seem to be negligible, further factors needing consideration when interpreting δ18Ohemicellulose values obtained from (paleo-)soils are evaporative 18O enrichment of soil water, seasonality effects, wind effects and in case of abundant stem/root-derived organic matter input a partial loss of the evaporative 18O enrichment of leaf water. Overall, our results prove that compound-specific δ18O analyses of hemicellulose biomarkers in soils and sediments are a promising tool for Paleoclimate research. However, disentangling the two major factors influencing δ18Ohemicellulose, namely δ18Oprec and relative air humidity controlled evaporative 18O enrichment of leaf water, is challenging based on δ18O analyses alone.
Andy Breckenridge - One of the best experts on this subject based on the ideXlab platform.
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pre aged plant waxes in tropical lake sediments and their influence on the chronology of molecular Paleoclimate Proxy records
Geochimica et Cosmochimica Acta, 2014Co-Authors: Peter M J Douglas, Timothy I. Eglinton, Mark Pagani, Mark Brenner, David A Hodell, Jason H Curtis, Keith F, Andy BreckenridgeAbstract:Sedimentary records of plant-wax hydrogen (dDwax) and carbon (d 13 Cwax) stable isotopes are increasingly applied to infer past climate change. Compound-specific radiocarbon analyses, however, indicate that long time lags can occur between the synthesis of plant waxes and their subsequent deposition in marginal marine sediments. The influence of these time lags on interpretations of plant-wax stable isotope records is presently unconstrained, and it is unclear whether such time lags also affect lacustrine sediments. We present compound-specific radiocarbon ( 14 Cwax) data for n-alkanoic acid plant waxes (n-C26 to n-C32) from: (1) a sediment core from Lake Chichancanab, Yucatan Peninsula, Mexico, (2) soils in the Lake Chichancanab catchment, and (3) surface sediments from three other lakes in southeastern Mexico and northern Guatemala. 14 Cwax ages in the surface sediments are consistently older than modern, and may be negatively correlated with mean annual precipitation and positively correlated with lake catchment area. 14 Cwax ages in soils surrounding Lake Chichancanab increase with soil depth, consistent with deep, subsoil horizons being the primary source of lacustrine aged plant waxes, which are likely delivered to lake sediments through subsurface transport. Plant waxes in the Lake Chichancanab core are 350–1200 years older than corresponding ages of bulk sediment deposition, determined by 14 C dates on terrestrial plant macrofossils in the core. A dDwax time series is in closer agreement with other regional Proxy hydroclimate records when a plant-wax 14 C age model is applied, as opposed to the macrofossil-based core chronology. Inverse modeling of plant-wax age distribution parameters suggests that plant waxes in the Lake Chichancanab sediment core derive predominantly from millennial-age soil carbon pools that exhibit relatively little age variance (<200 years). Our findings demonstrate that high-temporal-resolution climate records inferred from stable isotope measures on plant waxes in lacustrine sediments may suffer from possible chronologic distortions as a consequence of long residence times of plant waxes in soils. They also underscore the importance of direct radiocarbon dating of these organic molecules.
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Pre-aged plant waxes in tropical lake sediments and their influence on the chronology of molecular Paleoclimate Proxy records
Geochimica et Cosmochimica Acta, 2014Co-Authors: Peter M J Douglas, Timothy I. Eglinton, Mark Pagani, Mark Brenner, David A Hodell, Jason H Curtis, Keith F, Andy BreckenridgeAbstract:Sedimentary records of plant-wax hydrogen (dDwax) and carbon (d 13 Cwax) stable isotopes are increasingly applied to infer past climate change. Compound-specific radiocarbon analyses, however, indicate that long time lags can occur between the synthesis of plant waxes and their subsequent deposition in marginal marine sediments. The influence of these time lags on interpretations of plant-wax stable isotope records is presently unconstrained, and it is unclear whether such time lags also affect lacustrine sediments. We present compound-specific radiocarbon ( 14 Cwax) data for n-alkanoic acid plant waxes (n-C26 to n-C32) from: (1) a sediment core from Lake Chichancanab, Yucatan Peninsula, Mexico, (2) soils in the Lake Chichancanab catchment, and (3) surface sediments from three other lakes in southeastern Mexico and northern Guatemala. 14 Cwax ages in the surface sediments are consistently older than modern, and may be negatively correlated with mean annual precipitation and positively correlated with lake catchment area. 14 Cwax ages in soils surrounding Lake Chichancanab increase with soil depth, consistent with deep, subsoil horizons being the primary source of lacustrine aged plant waxes, which are likely delivered to lake sediments through subsurface transport. Plant waxes in the Lake Chichancanab core are 350–1200 years older than corresponding ages of bulk sediment deposition, determined by 14 C dates on terrestrial plant macrofossils in the core. A dDwax time series is in closer agreement with other regional Proxy hydroclimate records when a plant-wax 14 C age model is applied, as opposed to the macrofossil-based core chronology. Inverse modeling of plant-wax age distribution parameters suggests that plant waxes in the Lake Chichancanab sediment core derive predominantly from millennial-age soil carbon pools that exhibit relatively little age variance (
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Freshwater outburst from lake superior as a trigger for the cold event 9300 years ago.
Science (New York N.Y.), 2010Co-Authors: Steven M. Colman, Thomas V. Lowell, Glenn A. Milne, Timothy G. Fisher, Andy Breckenridge, Matthew Boyd, James T. TellerAbstract:Paleoclimate Proxy records reveal a pervasive cooling event with a Northern Hemispheric extent approximately 9300 years ago. Coeval changes in the oceanic circulation of the North Atlantic imply freshwater forcing. However, the source, magnitude, and routing of meltwater have remained unknown. Located in central North America, Lake Superior is a key site for regulating the outflow of glacial meltwater to the oceans. Here, we show evidence for an approximately 45-meter rapid lake-level fall in this basin, centered on 9300 calibrated years before the present, due to the failure of a glacial drift dam on the southeast corner of the lake. We ascribe the widespread climate anomaly approximately 9300 years ago to this freshwater outburst delivered to the North Atlantic Ocean through the Lake Huron-North Bay-Ottawa River-St. Lawrence River valleys.
Michael E Mann - One of the best experts on this subject based on the ideXlab platform.
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tracking variable sedimentation rates and astronomical forcing in phanerozoic Paleoclimate Proxy series with evolutionary correlation coefficients and hypothesis testing
Earth and Planetary Science Letters, 2018Co-Authors: Mingsong Li, Lee R Kump, Linda A Hinnov, Michael E MannAbstract:Abstract This paper addresses two fundamental issues in cyclostratigraphy and paleoclimatology: identification of astronomical forcing in sequences of stratigraphic cycles, and accurate evaluation of variable sedimentation rates. The technique presented here considers these issues part of an inverse problem and estimates the product-moment correlation coefficient between the power spectra of astronomical solutions and Paleoclimate Proxy series across a range of test sedimentation rates. The number of contributing astronomical parameters in the estimate is also considered. Our estimation procedure tests the hypothesis that astronomical forcing had a significant impact on Proxy records. The null hypothesis of no astronomical forcing is evaluated using a Monte Carlo simulation approach. The test is applied using a sliding stratigraphic window to track variable sedimentation rates along the Paleoclimate Proxy series, in a procedure termed “eCOCO” (evolutionary correlation coefficient) analysis. Representative models with constant and variable sedimentation rates, and pure noise and mixed signal and noise series are evaluated to demonstrate the robustness of the approach. The method is then applied to Cenozoic, Mesozoic and Paleozoic Paleoclimate series. The Cenozoic case study focuses on a high-resolution Paleocene–Eocene iron concentration series from ODP Site 1262 (Leg 208) covering the Paleocene–Eocene Thermal Maximum and Eocene Thermal Maximum 2 events. The eCOCO time-calibrated iron series confirms previous findings of a role for long-term astronomical forcing of these Eocene events. The Mesozoic case study applies eCOCO to the classic Late Triassic Newark depth rank series of eastern North America. The estimated high-resolution sedimentation rate map in this case demonstrates a causal link between variations in depositional environment and sedimentation rate. Finally, the Paleozoic case study supports the cyclostratigraphic interpretation of a Devonian magnetic susceptibility series at La Thure, Belgium and provides new insights into changes of the depositional setting at this location. Taken together, eCOCO is a powerful tool for simultaneously evaluating sedimentation rates and astronomical forcing for Paleoclimate series throughout the Phanerozoic.
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The role of forcing and internal dynamics in explaining the Medieval Climate Anomaly
Climate Dynamics, 2012Co-Authors: Hugues Goosse, Michael E Mann, Elisabeth Crespin, Svetlana Dubinkina, Marie-france Loutre, Hans Renssen, Y. Sallaz-damaz, Drew T. ShindellAbstract:Proxy reconstructions suggest that peak global temperature during the past warm interval known as the Medieval Climate Anomaly (MCA, roughly 950–1250 AD) has been exceeded only during the most recent decades. To better understand the origin of this warm period, we use model simulations constrained by data assimilation establishing the spatial pattern of temperature changes that is most consistent with forcing estimates, model physics and the empirical information contained in Paleoclimate Proxy records. These numerical experiments demonstrate that the reconstructed spatial temperature pattern of the MCA can be explained by a simple thermodynamical response of the climate system to relatively weak changes in radiative forcing combined with a modification of the atmospheric circulation, displaying some similarities with the positive phase of the so-called Arctic Oscillation, and with northward shifts in the position of the Gulf Stream and Kuroshio currents. The mechanisms underlying the MCA are thus quite different from anthropogenic mechanisms responsible for modern global warming.
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Using Paleoclimate Proxy-data to select optimal realisations in an ensemble of simulations of the climate of the past millennium
Climate Dynamics, 2006Co-Authors: Hugues Goosse, Hans Renssen, Axel Timmermann, Raymond S. Bradley, Michael E MannAbstract:We present and describe in detail the advantages and limitations of a technique that combines in an optimal way model results and Proxy-data time series in order to obtain states of the climate system consistent with model physics, reconstruction of past radiative forcing and Proxy records. To achieve this goal, we select among an ensemble of simulations covering the last millennium performed with a low-resolution 3-D climate model the ones that minimise a cost function. This cost function measures the misfit between model results and Proxy records. In the framework of the tests performed here, an ensemble of 30 to 40 simulations appears sufficient to reach reasonable correlations between model results and reconstructions, in configurations for which a small amount of data is available as well as in data-rich areas. Preliminary applications of the technique show that it can be used to provide reconstructions of past large-scale temperature changes, complementary to the ones obtained by statistical methods. Furthermore, as model results include a representation of atmospheric and oceanic circulations, it can be used to provide insights into some amplification mechanisms responsible for past temperature changes. On the other hand, if the number of Proxy records is too low, it could not be used to provide reconstructions of past changes at a regional scale.
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Using Paleoclimate Proxy-data to select optimal realisations in an ensemble of simulations of the climate of the past millennium
Climate Dynamics, 2006Co-Authors: Hugues Goosse, Hans Renssen, Axel Timmermann, Raymond S. Bradley, Michael E MannAbstract:We present and describe in detail the advantages and limitations of a technique that combines in an optimal way model results and Proxy-data time series in order to obtain states of the climate system consistent with model physics, reconstruction of past radiative forcing and Proxy records. To achieve this goal, we select among an ensemble of simulations covering the last millennium performed with a low-resolution 3-D climate model the ones that minimise a cost function. This cost function measures the misfit between model results and Proxy records. In the framework of the tests performed here, an ensemble of 30 to 40 simulations appears sufficient to reach reasonable correlations between model results and reconstructions, in configurations for which a small amount of data is available as well as in data-rich areas. Preliminary applications of the technique show that it can be used to provide reconstructions of past large-scale temperature changes, complementary to the ones obtained by statistical methods. Furthermore, as model results include a representation of atmospheric and oceanic circulations, it can be used to provide insights into some amplification mechanisms responsible for past temperature changes. On the other hand, if the number of Proxy records is too low, it could not be used to provide reconstructions of past changes at a regional scale. © Springer-Verlag 2006
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Past millennia climate variability
Eos Transactions American Geophysical Union, 2006Co-Authors: Michael E Mann, Christoph Kull, Keith R. Briffa, Philip Jones, Thorsten Kiefer, Heinz WannerAbstract:Human influences on climate operate against a background of long-term natural climate variability. Our ability to characterize this long-term variability and to distinguish it from climate change due to human activities is limited by the relative shortness of the instrumental record. Thus, investigators turn to a combination of indirect Paleoclimate Proxy evidence and theoretical climate models to ascertain the nature and causes of climate changes on centennial and longer timescales. Particularly relevant in this context is the time frame of the last few millennia, which is termed the ‘Late Holocene.’ During this period, the fundamental external boundary conditions on the climate, such as the configuration of the continents, the size and locations of the major ice sheets, and the mean radiative forcing due to changes in Earth-orbital geometry, are similar to those today. Study of this interval thus allows insights into the natural variability that might be expected today in the absence of human influences.
Timothy I. Eglinton - One of the best experts on this subject based on the ideXlab platform.
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pre aged plant waxes in tropical lake sediments and their influence on the chronology of molecular Paleoclimate Proxy records
Geochimica et Cosmochimica Acta, 2014Co-Authors: Peter M J Douglas, Timothy I. Eglinton, Mark Pagani, Mark Brenner, David A Hodell, Jason H Curtis, Keith F, Andy BreckenridgeAbstract:Sedimentary records of plant-wax hydrogen (dDwax) and carbon (d 13 Cwax) stable isotopes are increasingly applied to infer past climate change. Compound-specific radiocarbon analyses, however, indicate that long time lags can occur between the synthesis of plant waxes and their subsequent deposition in marginal marine sediments. The influence of these time lags on interpretations of plant-wax stable isotope records is presently unconstrained, and it is unclear whether such time lags also affect lacustrine sediments. We present compound-specific radiocarbon ( 14 Cwax) data for n-alkanoic acid plant waxes (n-C26 to n-C32) from: (1) a sediment core from Lake Chichancanab, Yucatan Peninsula, Mexico, (2) soils in the Lake Chichancanab catchment, and (3) surface sediments from three other lakes in southeastern Mexico and northern Guatemala. 14 Cwax ages in the surface sediments are consistently older than modern, and may be negatively correlated with mean annual precipitation and positively correlated with lake catchment area. 14 Cwax ages in soils surrounding Lake Chichancanab increase with soil depth, consistent with deep, subsoil horizons being the primary source of lacustrine aged plant waxes, which are likely delivered to lake sediments through subsurface transport. Plant waxes in the Lake Chichancanab core are 350–1200 years older than corresponding ages of bulk sediment deposition, determined by 14 C dates on terrestrial plant macrofossils in the core. A dDwax time series is in closer agreement with other regional Proxy hydroclimate records when a plant-wax 14 C age model is applied, as opposed to the macrofossil-based core chronology. Inverse modeling of plant-wax age distribution parameters suggests that plant waxes in the Lake Chichancanab sediment core derive predominantly from millennial-age soil carbon pools that exhibit relatively little age variance (<200 years). Our findings demonstrate that high-temporal-resolution climate records inferred from stable isotope measures on plant waxes in lacustrine sediments may suffer from possible chronologic distortions as a consequence of long residence times of plant waxes in soils. They also underscore the importance of direct radiocarbon dating of these organic molecules.
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Pre-aged plant waxes in tropical lake sediments and their influence on the chronology of molecular Paleoclimate Proxy records
Geochimica et Cosmochimica Acta, 2014Co-Authors: Peter M J Douglas, Timothy I. Eglinton, Mark Pagani, Mark Brenner, David A Hodell, Jason H Curtis, Keith F, Andy BreckenridgeAbstract:Sedimentary records of plant-wax hydrogen (dDwax) and carbon (d 13 Cwax) stable isotopes are increasingly applied to infer past climate change. Compound-specific radiocarbon analyses, however, indicate that long time lags can occur between the synthesis of plant waxes and their subsequent deposition in marginal marine sediments. The influence of these time lags on interpretations of plant-wax stable isotope records is presently unconstrained, and it is unclear whether such time lags also affect lacustrine sediments. We present compound-specific radiocarbon ( 14 Cwax) data for n-alkanoic acid plant waxes (n-C26 to n-C32) from: (1) a sediment core from Lake Chichancanab, Yucatan Peninsula, Mexico, (2) soils in the Lake Chichancanab catchment, and (3) surface sediments from three other lakes in southeastern Mexico and northern Guatemala. 14 Cwax ages in the surface sediments are consistently older than modern, and may be negatively correlated with mean annual precipitation and positively correlated with lake catchment area. 14 Cwax ages in soils surrounding Lake Chichancanab increase with soil depth, consistent with deep, subsoil horizons being the primary source of lacustrine aged plant waxes, which are likely delivered to lake sediments through subsurface transport. Plant waxes in the Lake Chichancanab core are 350–1200 years older than corresponding ages of bulk sediment deposition, determined by 14 C dates on terrestrial plant macrofossils in the core. A dDwax time series is in closer agreement with other regional Proxy hydroclimate records when a plant-wax 14 C age model is applied, as opposed to the macrofossil-based core chronology. Inverse modeling of plant-wax age distribution parameters suggests that plant waxes in the Lake Chichancanab sediment core derive predominantly from millennial-age soil carbon pools that exhibit relatively little age variance (
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Alkenones as paleoceanographic proxies
Geochemistry Geophysics Geosystems, 2000Co-Authors: Julian P. Sachs, Ralph R Schneider, Timothy I. Eglinton, Katherine H. Freeman, Gerald Ganssen, Jerry F. Mcmanus, Delia W OppoAbstract:[1] Alkenone-derived estimates of temperature and CO2 are contributing to our understanding of Earth's climate history. In order to increase confidence in alkenone-based climate proxies we recommend the following actions. First, the sedimentary component or fraction containing alkenones should be identified in order to assess the impact of horizontal advection and vertical mixing on alkenone-derived temperature and CO2 estimates. Second, differential mixing rates of alkenone-containing particles and sand-sized foraminifera should be quantified by independent dating of the two phases. Until that is accomplished, apparent temporal offsets of climate proxies in the two phases should be interpreted cautiously. Third, the stability of the unsaturation ratio and carbon isotopic compositions of alkenones during all phases of diagenesis should be confirmed. Both field and laboratory observations are required. Fourth, future alkenone investigations should be coupled with other Paleoclimate Proxy measurements at high-deposition-rate-sites in a variety of oceanographic settings. In upwelling regions and in the vicinity of river plumes, salinity and nutrient proxies should be measured since changes in these parameters may affect alkenone biosynthesis.