TEX₈₆

TEX₈₆ is an organic paleothermometer based upon the membrane lipids of mesophilic marine Thaumarchaeota (formerly Marine Group 1 Crenarchaeota).[1][2]

Molecular structures and HPLC detection of GDGTs

Basics

The membrane lipids of Thaumarchaeota are composed of glycerol dialkyl glycerol tetraethers (GDGTs) which contain 0-3 cyclopentane moieties. Thaumarchaeota also synthesise crenarchaeol which contains four cyclopentane moieties and a single cyclohexane moiety and a regio-isomer. The cyclohexane and cyclopentane rings, formed by internal cyclisation of one of the biphytane chains,[3] have a pronounced effect on the thermal transition points of the Thaumarchaeotal cell membrane. Mesocosm studies demonstrate that the degree of cyclisation is generally governed by growth temperature [4]

Calibrations

Based upon the relative distribution of isoprenoidal GDGTs, Schouten et al. (2002) proposed the tetraether index of 86 carbon atoms (TEX₈₆) as a proxy for sea surface temperature (SST). GDGT-0 is excluded from the calibration as it can have multiple sources [5] while GDGT-4 is omitted as it exhibits no correlation with SST and is often an order of magnitude more abundant than its isomer and the other GDGTs. The most recent TEX₈₆ calibration invokes two separate indices and calibrations:[6] TEX₈₆^H uses the same combination of GDGTs as in the original TEX₈₆ relationship:

${\displaystyle {\text{GDGT ratio-2}}={\tfrac {[{\text{GDGT-2}}]+[{\text{GDGT-3}}]+[{\text{GDGT-4}}']}{[{\text{GDGT-1}}]+[{\text{GDGT-2}}]+[{\text{GDGT-3}}]+[{\text{GDGT-4}}']}}}$

GDGT ratio-2 is correlated to SST using the calibration equation:

TEX₈₆^H = 68.4×log(GDGT ratio-2) + 38.6.

TEX₈₆^H has a calibration error of ±2.5 °C and is based upon 255 core-top sediments.

TEX₈₆^L employs a combination of GDGTs that is different from TEX₈₆^H, removing GDGT-3 from the numerator and excluding GDGT-4’ entirely:

${\displaystyle {\text{GDGT ratio-1}}={\tfrac {[{\text{GDGT-2}}]}{[{\text{GDGT-1}}]+[{\text{GDGT-2}}]+[{\text{GDGT-3}}]}}}$

GDGT ratio-1 is correlated to SST using the calibration equation:

TEX₈₆^L = 67.5×log(GDGT ratio-1) + 46.9.

TEX₈₆^Lhas a calibration error of ±4 °C and is based upon 396 core-top sediment samples.

Other calibrations exist (including 1/TEX₈₆,[7] TEX₈₆'[8] and pTEX₈₆ [9]) and should be considered when reconstructing temperature.

Caveats

There are several caveats to this proxy and this list is by no means exhaustive. For more information, consult [10]

Terrestrial input

The branched vs isoprenoidal tetratether (BIT) index can used to measure the relative fluvial input of terrestrial organic matter (TOM) into the marine realm (Hopmans et al., 2004). The BIT index is based upon the premise that GDGT-4 (also known as crenarchaeol) is derived from marine-dwelling Thaumarchaeota and branched GDGTs are derived from terrestrial soil bacteria. When BIT values exceed 0.4, a deviation of >2 °C is incorporated into TEX₈₆ SST estimates. However, isoprenoidal GDGTs can be synthesised in the terrestrial environment and can render BIT values unreliable (Weijers et al., 2006; Sluijs et al., 2007; Xie et al., 2012). A strong co-variation between GDGT-4 and branched GDGTs in modern marine and freshwater environments also suggests a common or mixed source for isoprenoidal and branched GDGTs (Fietz et al., 2012).

Anaerobic Oxidation of Methane (AOM)

The Methane Index (MI) was proposed to help distinguish the relative input of methanotrophic Euryarchaeota in settings characterised by diffuse methane flux and anaerobic oxidation of methane (AOM) (Zhang et al., 2011).[11] These sites are characterised by a distinct GDGT distribution, namely the predominance of GDGT-1. -2 and -3. High MI values (>0.5) reflect high rates of gas-hydrate-related AOM.

Degradation

Thermal maturity is only thought to affect GDGTs when temperature exceed 240 °C. This can be tested using a ratio of specific hopane isomers. Oxic degradation, which is a selective process and degrades compounds at different rates, has been shown to affect TEX₈₆ values and can bias SST values by up to 6 °C.

Application

The oldest TEX₈₆ record is from the middle Jurassic (~160Ma) and indicates relatively warm sea surface temperatures.[12] TEX₈₆ has been used to reconstruct temperature throughout the Cenozoic era (65-0Ma)[13][14] and is useful when other SST proxies are diagenetically altered (e.g. planktonic foraminifera[15]) or absent (e.g. alkenones[16])

Eocene

TEX₈₆ has been extensively used to reconstruct Eocene (55-34Ma) SST. During the early Eocene, TEX₈₆ values indicate warm high southern hemisphere latitude SSTs (20-25 °C) in agreement with other, independently derived proxies (e.g. alkenones, CLAMP, Mg/Ca). During the middle and late Eocene, high southern latitude sites cooled while the tropics remained stable and warm. Possible reasons for this cooling include long-term changes in carbon dioxide and/or changes in gateway reorganisation (e.g. Tasman Gateway, Drake Passage).

References

1. Schouten, S., Hopmans, E.C., Schefus, E., and Sinninghe Damste. (2002) Distributional variation in marine crenarchaeotal membrane lipids: a new tool for reconstructing ancient sea water temperatures?. Earth and Planetary Science Letters, 204, 265.
2. Kim, J.-H., S. Schouten, E. C. Hopmans, B. Donner, J. S. Sinninghe Damsté. (2008) Global sediment core-top calibration of the TEX86 paleothermometer in the ocean. Geochimica et Cosmochimica Acta, 72, 1154.
3. Schouten, S., Hopmans, E. C., and Sinninghe Damsté, J. S., 2013, The organic geochemistry of glycerol dialkyl glycerol tetraether lipids: A review: Organic Geochemistry, v. 54, no. 0, p. 19-61.
4. Wuchter, C., Schouten, S., Coolen, M. J. L., and Sinninghe Damsté, J. S., 2004, Temperature-dependent variation in the distribution of tetraether membrane lipids of marine Crenarchaeota: Implications for TEX86 paleothermometry: Paleoceanography, v. 19, no. 4, p. PA4028
5. Koga, Y., Nishihara, M., Morii, H., and Akagawa-Matsushita, M., 1993, Ether polar lipids of methanogenic bacteria: structures, comparative aspects, and biosyntheses: Microbiological Reviews, v. 57, no. 1, p. 164-182
6. Kim, J.-H., van der Meer, J., Schouten, S., Helmke, P., Willmott, V., Sangiorgi, F., Koç, N., Hopmans, E. C., and Damsté, J. S. S., 2010, New indices and calibrations derived from the distribution of crenarchaeal isoprenoid tetraether lipids: Implications for past sea surface temperature reconstructions: Geochimica et Cosmochimica Acta, v. 74, no. 16, p. 4639-4654.
7. Liu, Z., Pagani, M., Zinniker, D., DeConto, R., Huber, M., Brinkhuis, H., Shah, S. R., Leckie, R. M., and Pearson, A., 2009, Global Cooling During the Eocene-Oligocene Climate Transition: Science, v. 323, no. 5918, p. 1187-1190
8. Sluijs, A., Schouten, S., Pagani, M., Woltering, M., Brinkhuis, H., Damsté, J. S. S., Dickens, G. R., Huber, M., Reichart, G.-J., Stein, R., Matthiessen, J., Lourens, L. J., Pedentchouk, N., Backman, J., Moran, K., and the Expedition, S., 2006, Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum: Nature, v. 441, no. 7093, p. 610-613.
9. Hollis, C. J., Taylor, K. W. R., Handley, L., Pancost, R. D., Huber, M., Creech, J. B., Hines, B. R., Crouch, E. M., Morgans, H. E. G., Crampton, J. S., Gibbs, S., Pearson, P. N., and Zachos, J. C., 2012, Early Paleogene temperature history of the Southwest Pacific Ocean: Reconciling proxies and models: Earth and Planetary Science Letters, v. 349–350, no. 0, p. 53-66.
10. Schouten, S., Hopmans, E. C., and Sinninghe Damsté, J. S., 2013, The organic geochemistry of glycerol dialkyl glycerol tetraether lipids: A review: Organic Geochemistry, v. 54, no. 0, p. 19-61.
11. Zhang, Yi Ge; Zhang, Chuanlun L.; Liu, Xiao-Lei; Li, Li; Hinrichs, Kai-Uwe; Noakes, John E. (2011). "Methane Index: A tetraether archaeal lipid biomarker indicator for detecting the instability of marine gas hydrates". Earth and Planetary Science Letters. 307 (3–4): 525–534. Bibcode:2011E&PSL.307..525Z. doi:10.1016/j.epsl.2011.05.031.
12. Jenkyns, H., Schouten-Huibers, L., Schouten S. and Sinninghe-Damste, J.S., 2012, Warm Middle Jurassic-early Cretaceous high-latitude sea surface temperature from the Southern Ocean. Climate of the Past, v. 8, p.215-226
13. Sluijs, A., Schouten, S., Donders, T. H., Schoon, P. L., Rohl, U., Reichart, G.-J., Sangiorgi, F., Kim, J.-H., Sinninghe Damste, J. S., and Brinkhuis, H., 2009, Warm and wet conditions in the Arctic region during Eocene Thermal Maximum 2: Nature Geosci, v. 2, no. 11, p. 777-780.
14. Zachos, J. C., Schouten, S., Bohaty, S., Quattlebaum, T., Sluijs, A., Brinkhuis, H., Gibbs, S. J., and Bralower, T. J., 2006, Extreme warming of mid-latitude coastal ocean during the Paleocene-Eocene Thermal Maximum: Inferences from TEX86 and isotope data: Geology, v. 34, no. 9, p. 737-740.
15. Pearson, P. N., van Dongen, B. E., Nicholas, C. J., Pancost, R. D., Schouten, S., Singano, J. M., and Wade, B. S., 2007, Stable warm tropical climate through the Eocene Epoch: Geology, v. 35, no. 3, p. 211-214.
16. Bijl, P. K., Schouten, S., Sluijs, A., Reichart, G.-J., Zachos, J. C., and Brinkhuis, H., 2009, Early Palaeogene temperature evolution of the southwest Pacific Ocean: Nature, v. 461, no. 7265, p. 776-779.

Further reading