Table 1. Model-predicted climate and maximum survivable paleoelevations for an array of topographic scenarios (V1 to V8 and P0 to P4) for Tibet in the Late Paleogene.
Thermal regimes regarded as fatal to palms (21) or failing to reproduce proxy values or occurring below prescribed land surfaces are shown in red. Blue shading indicates unconstrained (by proxy data) model-predicted CMMTs fatal to palms, and gray shading indicates model failure to reproduce proxy thermal conditions. Green shading indicates palm-survivable scenarios. The total number of frost days, number of 5-day cold events <0°C, and number of 5-day cold events <−5°C during a 30-year period are shown. These indicate that above 2 km, the frequency of potentially fatal frost periods increases markedly. The highest survivable paleoelevations of the palm fossil locality are based on the model-derived cold month mean temperature terrestrial lapse rate γ, a conservative proxy-based CMMT at mean sea level Tmsl obtained from the Tirap site (20.4°C) using the PhysgAsia2 and HiResGridMetAsia2 calibration (47) and a minimum palm-survivable CMMT of 5.2°C. Note that because Tirap is to the south of Lunpola, and Lunpola is not coastal, the theoretical CMMT at the Lunpola site may have been slightly cooler than this value. Asterisk (*) values indicate elevations lower than the prescribed land surface, indicating that no palm could live on that surface. Because of the uncertainty in paleopositioning of the Lunpola Basin, we ran a latitude sensitivity test (scenarios V5 to V8) where the location of the valley topography was moved one grid cell to the south. None of the plateau scenarios (P0 to P4) produce survivable conditions. The most plausible scenarios (green fill) are those consisting of a deep valley system with the valley floor set at 2 km above mean sea level (AMSL) at paleolatitude ~30°N bounded by high (at least 4 km) mountain systems.
|
Topography scenario |
MAT (°C) |
CMMT (°C) |
Total frost days (<0°C) |
No. 5-day events <0°C |
No. 5-day events <−5°C |
Tirap MAT (°C) msl (proxy, 26.2° ± 2.3°C) |
Tirap CMMT (°C) msl (proxy, 20.4° ± 3.6°C) |
Lunpola CMMT lapse rate (γ) (°C/km) |
Predicted highest survivable elevation (m) |
|
| V1 | Valley (2 km), 4 km uplands, open |
22.4 | 4.0 | 2504 | 161 | 68 | 28 | 19.3 | 5.3 | 2867 |
| V2 | Valley (3.2 km), 4 km uplands, open |
15.9 | 0.4 | 3662 | 165 | 140 | 26.5 | 16.6 | 7.1 | 2132* |
| V3 | Valley (2 km), 4 km uplands, closed |
21.2 | 1.2 | 2854 | 160 | 91 | 27.8 | 19.1 | 4.1 | 3719 |
| V4 | Valley (3.2 km), 4 km uplands, closed |
16.3 | 0.1 | 3695 | 185 | 146 | 27.8 | 19.1 | 6.8 | 2233* |
| V5 | V1 latitude sensitivity | 29.4 | 10.1 | 962 | 80 | 2 | 27.8 | 19.5 | 6.9 | 2194 |
| V6 | V2 latitude sensitivity | 18.1 | 4.3 | 2590 | 161 | 81 | 26.4 | 17.0 | 7.2 | 2100* |
| V7 | V3 latitude sensitivity | 24.7 | 9.6 | 1034 | 81 | 6 | 27.8 | 19.1 | 6.7 | 2246 |
| V8 | V4 latitude sensitivity | 17.8 | 3.5 | 2716 | 183 | 76 | 26.4 | 16.8 | 7.2 | 2094* |
| P0 | 0-km plateau | 29.4 | 10.2 | 674 | 51 | 1 | 31.5 | 17.7 | 4.5 | 3380 |
| P1 | 1-km plateau | 26.1 | 8.9 | 1084 | 86 | 10 | 29.0 | 18.1 | 6.3 | 2396 |
| P2 | 2-km plateau | 23.2 | 8.0 | 1280 | 107 | 20 | 28.6 | 20.3 | 6.7 | 2260 |
| P3 | 3-km plateau | 18.6 | 4.9 | 2346 | 172 | 47 | 26.9 | 18.2 | 6.6 | 2304* |
| P4 | 4-km plateau | 8.9 | −1.7 | 4718 | 196 | 177 | 27.0 | 19.7 | 7.6 | 1982* |