Srinath Krishnan

The role of carbon dioxide during the onset of Antarctic glaciation

Antarctica glaciation began soon after a large decrease in the concentration of atmospheric carbon dioxide around 35 million years ago. Earth’s modern climate, characterized by polar ice sheets and large equator-to-pole temperature gradients, is rooted in environmental changes that promoted Antarctic glaciation ~33.7 million years ago. Onset of Antarctic glaciation reflects a critical tipping point for Earth’s climate and provides a framework for investigating the role of atmospheric carbon dioxide (CO2) during major climatic change. Previously published records of alkenone-based CO2 from high- and low-latitude ocean localities suggested that CO2 increased during glaciation, in contradiction to theory. Here, we further investigate alkenone records and demonstrate that Antarctic and subantarctic data overestimate atmospheric CO2 levels, biasing long-term trends. Our results show that CO2 declined before and during Antarctic glaciation and support a substantial CO2 decrease as the primary agent forcing Antarctic glaciation, consistent with model-derived CO2 thresholds.

Effects of Rapid Global Warming at the Paleocene-Eocene Boundary on Neotropical Vegetation

Hot Tropical Explosion The Paleocene-Eocene Thermal Maximum (PETM), 55 million years ago, was a unique episode of rapid global warming (∼5°C), often used as an ancient analog for future global climate change. Climate alteration during the PETM has been extensively studied in the marine realm, and from a few temperate to polar terrestrial localities, but little is known about how the tropics responded to the high temperatures and high levels of CO2. Using evidence from pollen analysis, Jaramillo et al. (p. 957) show that rapid tropical forest diversification occurred during the PETM, without plant extinction or regional aridity. Unexpectedly, diversity seemed to increase at higher temperatures, contradicting previous assumptions that tropical flora will succumb if temperatures become excessive. Palynology shows that tropical forests persisted under conditions of rapid climate warming 55 million years ago. Temperatures in tropical regions are estimated to have increased by 3° to 5°C, compared with Late Paleocene values, during the Paleocene-Eocene Thermal Maximum (PETM, 56.3 million years ago) event. We investigated the tropical forest response to this rapid warming by evaluating the palynological record of three stratigraphic sections in eastern Colombia and western Venezuela. We observed a rapid and distinct increase in plant diversity and origination rates, with a set of new taxa, mostly angiosperms, added to the existing stock of low-diversity Paleocene flora. There is no evidence for enhanced aridity in the northern Neotropics. The tropical rainforest was able to persist under elevated temperatures and high levels of atmospheric carbon dioxide, in contrast to speculations that tropical ecosystems were severely compromised by heat stress.

An interlaboratory study of TEX86 and BIT analysis of sediments, extracts, and standard mixtures

Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX86 values (reproducibility +/- 3-4 degrees C when translated to temperature) but a large spread in BIT measurements (reproducibility +/- 0.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX86 values ranged from 1.3 to 3.0 degrees C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX86 and BIT indices are not significantly affected by interlaboratory differences in sediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poor reproducibility and a large overestimation of the true (i.e., molar-based) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values.