www.acsprf.org

ACS-PRF # 49235-DNI2

Dr. James M. Russell, Brown University

The Methylation index of Branched Tetraethers (MBT) as a temperature proxy in lakes 

The goals of this award are to: 1) Investigate the sources and environmental controls on the relative abundances of nine branched glycerol dialkyl glycerol tetraethers (GDGTs) in lake sediments; 2) Develop new models to quantitatively relate GDGT abudances in lake sediments to environmental parameters, particularly temperature, for paleoenvironmental reconstruction, and 3) Test the applicability of these models to reconstructing past temperature by applying the new GDGT proxy to lake sediment cores.

Sources and Environmental Controls

In 2011, we measured the relative abundances of branched GDGTs in a set of 110 surface sediment samples from East African lakes.  This significantly expands upon our work in 2010 (Tierney et al., 2010), which analyzed 46 lake samples.  In the smaller calibration set of Tierney et al. (2010), lake temperature and lake pH are correlated, making it difficult to separate the influence of these two variables on the relative abundances of GDGTs.  Lake temperature and pH are poorly correlated in our large sample set, and we have found that lake temperature has a much stronger influence on GDGTs than does lake pH.  pH, in fact, has little effect on branched GDGT distributions, in contrast to branched GDGT distributions in soils (Weijers et al., 2007).

Tierney et al. (2010) showed that calibrations relating branched GDGTs temperature and pH based upon GDGTs in soils are not applicable to lake sediments, indicating different microbial sources of branched GDGTs in lakes and soils.  To test the hypothesis for in-situ formation of branched GDGTs in lakes, we analyzed a set of ~60 soil samples from East Africa collected within the lake catchments of our lake calibration dataset. There are statistically significant differences in GDGT distributions between soils and adjacent lake sediments at all elevations, strong support for in-situ lacustrine prodution.  In highly water-saturated, high-elevation soils, branched GDGTs begin to resemble the GDGTs in nearby lakes (although they still retain significant differences), indicating that the presence of water is a key control on the relationship between microbial membrane lipids and environmental parameters.  This likely occurs through the effects of water on ionic transport across the microbial membrane.  This work has been published by my graduate student (Loomis et al., 2011).

In May, 2009, we installed a time-series sediment trap equipped in South King Pond, a small, deep, alkaline lake in northern Vermont.  For the past two years we collected monthly samples of the material sedimenting to the bottom of the pond, measured water column chemistry and temperature, and have filtered particulate organic matter from the surface waters and hypolimnion to test for the presence and relative abundances of branched GDGTs.  We have found that, as in the East African lakes, branched GDGTs within South King Pond have very different distributions than GDGTs in the surrounding soils, indicating production of these compounds within the lake.  Analysis of the sediment trap samples indicates that the flux of branched GDGTs to the sediments increases by nearly an order of magnitude during periods of lake mixing, when the overall lake productivity is high.  This suggests that the bacteria producing branched GDGTs respond strongly to nutrient concentrations and other aquatic ecosystem properties associated with lake mixing.  Interestingly, the branched GDGTs exhibit only weak distributional variations across the seasonal cycle- in nearly all seasons both filtered and sedimenting GDGTs resemble the material produced during lake mixing.  This suggests that temperatures during mixing largely ‘set’ the GDGT signals in temperate zone lakes.  This work is in preparation for publication.

Temperature Calibration and Validation

We have developed new calibrations of branched GDGTs to air temperature using our expanded set of 110 East African lake sediments.  We have experimented with a number of different numerical methods for this calibration, from various linear and multiple regressions of different subsets of the branched GDGTs against temperature, to inference modeling approaches commonly used in paleoecology.  The latter, and in particular the weighted averaging method (Birks, 1995), provide the most precise temperature calibrations, with low error of prediction and bias, and bootstrap correlation coefficients of 0.91 between observed and reconstructed temperatures. 

We applied branched GDGTs to reconstruct temperature variations in the sediments of Sacred Lake, East Africa, during the past 30,000 years BP to develop the first quantitative temperature reconstruction from a small lake using branched GDGTs.  We have found, using our new calibration, that the magnitude and patterns of temperature variations during the last glacial termination (~20-10 kyr BP) are similar to those established from nearby lakes and fossil groundwater deposits, providing strong support that our new method can be used for quantitative temperature reconstruction.  This work is in preparation for publication.

Student Training
This award formed the basis for one thesis chapter Dr. Jessica Tierney, who earned a Ph.D. in May 2010 and is now a post-doc at Lamont-Doherty Earth Observatory.  This award will form the basis for the majority of the Ph.D. of Shannon Loomis, who is starting her fourth year at Brown in 2011/2012 and has 1 paper published and three in preparation.  The award also provided senior thesis opportunities to two Brown undergraduate students who completed their degrees in May, 2011.

References:

Birks, H.J.B., 1995, Quantiative paleoenvironmental reconstruction, in Maddy, D., and Brew, J.S., eds., Statistical modeling of Quaternary Science Data: Cambridge, Cambridge Quaternary Research Association, p. 161-254.

Loomis, S. E., Russell, J. M., Sinninghe Damsté, J. S., 2011.  Distributions of branched GDGTs in soils and lake sediments from western Uganda: Implications for a lacustrine paleothermometer. Organic Geochemistry, doi:10.1016/j.orggeochem.2011.06.004

Tierney, J. E., Russell, J. M. Eggermont, H. E., Hopmans, E. C., Verschuren, D., Sinninghe Damsté, J S., 2010.  Environmental controls on branched tetraether lipid distributions in tropical East African lake sediments.  Geochimica et Cosmochimica Acta 74: 4902-4918.

Weijers, J.H., Schouten, S., van den Donker, J.C., Hopmans, E.C., and Sinninghe

Damste, J.S., 2007, Environmental controls on bacterial tetraether membrane lipid

distribution in soils: Geochimica et Cosmochimica Acta, v. 71, p. 703-713.