59th Annual Report on Research 2014

The patterns of methylation and cyclization (MBT and CBT indices) in the ubiquitous, bacteria-derived branched glycerol dialkyl glycerol tetraethers (b-GDGT) have been shown to vary systematically with temperature and soil pH. There have been major efforts to calibrate b-GDGT distributions against environmental parameters in globally distributed soils (including ~ 135 North American soils). However, there are two problems regarding soil b-GDGTs in the North American: 1) published data so far are geographically limited and insufficient for establishing robust regional calibrations cross key environmental boundaries; and 2) there have been no attempts to apply multiple linear regression (MLR), which has been well demonstrated in African and Chinese soils, to North American soils. To address these problems, we analyze b-GDGTs in 103 strategically selected soil samples across USA (Fig.1), and integrate all available data to re-assess environmental impact on GDGT distribution. In almost all the cases, MLR yielded higher R² values and smaller errors than the empirical MBT/CBT indices, supporting the use of MLR over MBT/CBT indices for paleoclimate reconstructions. Using MLR for all samples, we obtain a series of new regressions for the relationship between fractional abundances of branched GDGTs (FAB) and environmental variables (Annual average temperature or AAT, annual average precipitation or AAP, or soil pH values:

AAT = 7.853 + 6.925 × f(I)

 (R² = 0.132; RMSE = 4.45℃; p < 0.001; n = 238) (1)

pH = 8.466 - 4.182 × f(I) – 1.797 × f(II)

(R² = 0.562; RMSE = 0.82 pH unit; p < 0.001; n = 230) (2)

AAP = 1.05 + 2.862 × f(I) + 3.6 × f(Ib) – 13.799 × f(IIIb) + 14.08 × f(Ic) – 0.875 × f(II)

 (R² = 0.74; RMSE = 0.53 mm/day; p < 0.001; n = 238) (3)

The strong relationship between FAB and precipitation is surprising, since published data so far have mostly emphasized temperature and pH relationship (Weijers et al., 2007; Peterse et al., 2012).

We note, however, if we divide samples according to precipitation, distinctive differences between different regions are revealed. Importantly, annual average precipitation (AAP) played a major role, in most cases stronger than annual average temperature (AAT), in controlling the b-GDGT distributions. The strongest correlation between AAP and b-GDGT is found in mid-latitude grassland (R² = 0.78), or when soil pH values are greater than 8 (R² = 0.88). Temperature effect is only apparent in regions of high AAP (e.g., >1200 mm) and mixed forest. The effect of pH is strong in almost all regions. Our results suggest that temperature reconstructions involve a more complex set of controls than previously realized, including possibly vegetation effects and almost certainly AAP effects on the development of branched GDGT-producing bacteria.

Some examples of strong relationship between environmental variables and FAB is listed as follows:

AAP > 1200 mm/yr

AAT = 34.69 – 55.327 × f(II) – 14.115 × f(I)

 (R² = 0.75; RMSE = 2.24℃; p < 0.001; n = 35) (4)

Midlatitude grassland

AAP = 4.195 – 4.724 × f(II) – 2.604 × f(III) – 23.425 × f(IIc)

(R² = 0.78; RMSE = 0.31 mm/day; p < 0.001; n = 43) (5)

 

pH > 8

AAP = 0.93 + 9.452 × f(Ib) – 2.149 × f(I)

(R² = 0.88; RMSE = 0.16 mm/day; p < 0.001; n = 19) (6)

 

AAT > 15

pH = 8.225 – 3.743 × f(I)

(R² = 0.789; RMSE = 0.583 pH unit; p < 0.001; n = 35) (7)

AAP = 3.694 – 3.442 × f(II) – 4.192 × f(III)

(R² = 0.766; RMSE = 0.557 mm/day; p < 0.001; n = 35) (8)

Our results indicate distributions of branched GDGTs in different environments differ significantly: the predominant controlling factors differ and the strength of the relationship differs as well. We have already completed all the data analysis and are currently working up the manuscript for submission.

References:

Dirghangi S. S., Pagani M., Hren M. T. and Tipple B. J. (2013) Distribution of glycerol dialkyl glycerol tetraethers in soils from two environmental transects in the USA. Organic Geochemistry 59, 49-60.

Peterse F., van der Meer J., Schouten S., Weijers J. W. H., Fierer N., Jackson R. B., Kim J.-H. and Sinninghe Damsté J. S. (2012) Revised calibration of the MBT-CBT paleotemperature proxy based on branched tetraether membrane lipids in surface soils. Geochimica et Cosmochimica Acta 96, 215-229.

Weijers J. W. H., Schouten S., van den Donker J. C., Hopmans E. C. and Sinninghe Damsté J. S. (2007b) Environmental controls on bacterial tetraether membrane lipid distribution in soils. Geochimica et Cosmochimica Acta 71, 703–713.