Eocene Temperature Gradients

correspondence Eocene temperature gradients To the Editor — Sze Ling Ho and a b 1 Thomas Laepple argue that the TEX86 C) 0 ° palaeothermometer should be calibrated 45 to deep subsurface ocean temperature −2 40 and that doing so resolves a discrepancy C) −4 ° between data and climate model 35 simulations for the early Eocene. Here −6 we argue that their proposed calibration 30 −8 of TEX86 is incompatible with ecological 25 evidence and inappropriate for the largely −10 shallow-water Eocene data. In addition, 20 −12 early Eocene TEX86 data agree reasonably 15 well with other proxy data, such that warm ( temperature Sea-surface −14 poles and a flat meridional temperature 10 gradient are not unique to TEX86. −16 5 Dierence between temperatures predicted by the by predicted temperatures between Dierence The primary assumption behind Ho and ( temperatures 1 and observed in ref. calibration 14 16 18 20 22 24 26 85° S 55° S 25° S 0° 25° N 55° N 85° N Observed temperature (°C) Latitude Laepple’s reinterpretation of Eocene TEX86 data is that the Thaumarchaeotal lipids BAYSPAR median BAYSPAR 1σ (known as GDGTs) that comprise the BAYSPAR 90% Cl Mg/Ca Glassy δ18O ∆47 proxy are derived from 0–1,000 m water 18 depth. However, microbial, ecological Non-glassy δ O Model Eocene SST and oceanographic observations indicate Bias in the calibration by Ho and Laepple1 and Eocene temperature reconstructions. , In that the sedimentary TEX86 signal Figure 1 | a predominantly represents the shallow warm (>15 °C) and shallow (<200 m) sites in the modern ocean, the Ho and Laepple calibration subsurface (about 50–300 m). This is systematically underestimates depth-averaged temperatures (median values presented as points, where maxima in intact polar GDGT and linear regression as the red line, and maximum and minimum regressions based on the calibration Thaumarchaeotal gene (16S rRNA and ensemble as dashed red lines). b, Eocene SST predictions based on the BAYSPAR5 calibration (blue amoA) abundances occur2,3, consistent dots, the median Gaussian fit is shown as a blue line with shading representing σ1 and the 90% with their ecological niche as ammonium CI) are comparable to other proxies (90% CI shown) and indicate higher temperatures at mid- to oxidizers. Although Thaumarchaeota high-latitudes than an ensemble of simulations of early Eocene climate11 (1σ range of SSTs shown as exist in deeper waters, their cell numbers grey shading). decline sharply below ~300 m (ref. 3) and export to the sediment is less efficient4. Hence, sedimentary TEX86 values to the surface and shallow subsurface appropriate comparison between proxies closely match those of the upper water calibrations (slope ~ 0.015 TEX86 units requires formal modelling of uncertainties column, indicating preservation of an per °C)7, supporting a shallow-water depth associated with both systems, including 4 Kʹ upper-ocean signal . origin for sedimentary GDGTs. structural errors relevant to U37 such This evidence justifies several Despite this observational evidence, as non-linear temperature sensitivity, 8 approaches to TEX86 calibration. Despite Ho and Laepple compare the power diagenetic alteration and lateral advection . Kʹ Kʹ the Thaumarchaeotal niche in the shallow spectra of paired alkenone U37 and TEX86 Furthermore, even if U37 and TEX86 do subsurface, TEX86 can be used as a sea- records, and use this result as the basis of show different variability in some places, surface temperature (SST) proxy due to their argument that TEX86 must integrate it does not follow that TEX86 is incorrectly the high correlation between subsurface temperatures to at least 550 m and possibly calibrated; slight differences in seasonality temperatures and SSTs5. Although the as deep as 950 m. The result is a calibration and depth of production can and do impart assumption that surface and subsurface with a TEX86-temperature sensitivity that variance differences between proxies. temperatures maintain similar variability is twice as large as that found in previous Using their deep-water calibration, through time can be violated, it is work (slope ~0.030), which is difficult Ho and Laepple revisit published Eocene reasonable for inference of long-term to reconcile with the evidence discussed TEX86 data and argue that they can resolve mean SSTs. A similar assumption is made above. Their approach is flawed in several discrepancies between proxies and climate in δ18O palaeothermometry, which relies ways. First, their Fig. 2a demonstrates models. We identify two major fallacies on foraminifera that reside throughout that all of the variance and explanatory in their approach: first, the majority of the upper mixed layer. Shallow subsurface power derives from shallow waters sites with early Eocene TEX86 data are (0–200 m) instead of surface calibrations (0–300 m); extending the calibration down shallow (water depths between 0 and for TEX86 are also appropriate and have to 1,000 m merely dilutes the regression 200 m; Supplementary Table 1). Applying a been applied5,6. Laboratory mesocosm with relatively invariant data. Second, calibration that predicts water temperature Kʹ experiments independently confirm a they assume that U37 represents true averaged over a range of 0–550 m or TEX86-temperature sensitivity similar SST variability, which is unjustified. An 0–950 m is inappropriate. By way of 538 NATURE GEOSCIENCE | VOL 10 | AUGUST 2017 | www.nature.com/naturegeoscience ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. correspondence example, we apply the Ho and Laepple were near 20 °C, as indicated by the 5. Tierney, J. E. & Tingley, M. P. Sci. Data 2, 150029 (2015). 6. Kim, J.-H. et al. Geophys. Res. Lett. 39, L06705 (2012). calibration to warm (T > 15 °C), shallow Arctic site (Fig. 1b), these are difficult to 7. Wuchter, C., Schouten, S., Coolen, M. & Sinninghe Damsté, J. S. 9 (0–200 m) sites in the modern surface reproduce with models (Fig. 1b) . Such Paleoceanography 19, PA4028 (2004). sediment dataset (see Supplementary temperatures are not at all unlikely and are 8. Conte, M. H. et al. Geochem. Geophys. Geosys. 7, Q02005 (2006). Information), which have TEX indices corroborated by independent evidence, 9. Lunt, D. J. et al. Clim. Past 8, 1717–1736 (2012). 86 10. Markwick, P. J. Palaeogeogr. Palaeoclimatol. Palaeoecol. analogous to the Eocene values. Their such as the presence of palms, baobab 137, 205–271 (1998). calibration underestimates depth-averaged and crocodiles at polar latitudes10,11. The 11. Pross, J. et al. Nature 488, 73–77 (2012). temperatures by 7.5 °C in the modern equable climate problem — the lingering 12. Ingalls, A. Nat. Geosci. 9, 572–573 (2016). ocean (Fig. 1a), and the underestimation mismatch between proxy data and model trends with temperature, such that the simulations — cannot be put to rest. Additional information warmest sites are under-predicted by a TEX data have played a critical role in Supplementary information is available in the online 86 version of the paper. greater amount (~12 °C, Fig. 1a). Their constraining early Cenozoic temperatures. calibration will therefore produce an Indeed, although we disagree with their artificially flat gradient between tropical approach and conclusions, Ho and Laepple Jessica E. Tierney1*, Jaap S. Sinninghe Damsté2,3, and subtropical temperatures, both in the never question the fundamental utility Richard D. Pancost4, Appy Sluijs3 and modern, and in the Eocene, oceans. of the proxy. This contrasts with the James C. Zachos5 12 1 Second, TEX86-inferred SSTs are not News & Views accompanying the article , University of Arizona, Department of significantly different from those provided which suggests that temperature is not Geosciences, 1040 E 4th Street, Tucson, 2 by other proxies (Fig. 1b). There is no the dominant control on TEX86. This view Arizona 85721, USA. NIOZ Royal Netherlands evidence that TEX86-based temperatures discounts the large body of literature that Institute for Sea Research, Department of are too high — or produce a gradient demonstrates the validity and robustness Marine Microbiology and Biogeochemistry, that is too flat — relative to other proxies of the palaeothermometer, as well as the and University of Utrecht, PO Box 59, 1797 AB within uncertainties. Only foraminiferal simple fact that ocean temperature explains Den Burg, Texel, The Netherlands. 3Department 18 δ O data from Ocean Drilling Program over 70% of the variance in modern TEX86 of Earth Sciences, Faculty of Geosciences, 5 sites 690 and 738 indicate substantially data . TEX86 has revolutionized our view of University of Utrecht, Heidelberglaan 2, lower temperatures (Fig. 1b); however, past warm climates, and we expect that it 3584C Utrecht, The Netherlands. 4Organic these data are probably biased by will continue to do so. ❐ Geochemistry Unit, School of Chemistry, Cabot diagenetic alteration. In the Southwest Institute, University of Bristol, Bristol BS8 1TS, 5 Pacific, Mg/Ca and TEX86 both indicate References UK. University of California, Santa Cruz, remarkably high SSTs (~30 °C). These 1. Ho, S. L. & Laepple, T. Nat. Geosci. 9, 606–610 (2016). Department of Earth and Planetary Sciences, locations may not be representative of 2. Church, M. J. et al. Environ. Microbiol. 12, 679–688 (2010). 1156 High Street, Santa Cruz, California 3. Schouten, S. et al. Geochim. Cosmochim. Acta 98, 228–243 (2012). their latitudinal band globally; however, 4. Wuchter, C., Schouten, S., Wakeham, S. G. & 95064, USA. even if early Eocene polar temperatures Sinninghe Damsté, J. S. Paleoceanography 21, PA3013 (2005). *e-mail: [email protected] Reply to ‘Eocene temperature gradients’ 4 H Sze Ling Ho and Thomas Laepple reply — calibration (68.4 °C per unit TEX86) or evidence (Supplementary Fig. 6) that 5 Kʹ Jessica Tierney and colleagues question the the effective slope of BAYSPAR (80.8 °C supports the use of U37 to constrain TEX86.

Load more