Reconstructing atmospheric CO₂ during the Plio–Pleistocene transition by fossil Typha

Abstract

The Earth has undergone a significant climate switch from greenhouse to icehouse during the Plio-Pleistocene transition (PPT) around 2.7-2.4 million years ago (Ma), marked by the intensification of the Northern Hemisphere glaciation (NHG) ~2.7 Ma. Evidence based on oceanic CO2 [(CO2)aq], supposed to be in close equilibrium with the atmospheric CO2 [(CO2)atm], suggests that the CO2 decline might drive such climate cooling. However, the rarity of direct evidence from [CO2]atm during the interval prevents determination of the atmospheric CO2 level and further assessment on the impact of its fluctuation. Here, we reconstruct the [CO2]atm level during 2.77-2.52 Ma based on a new developed proxy of stomatal index on Typha orientalis leaves from Shanxi, North China, and depict the first [CO2]atm curve over the past 5 Ma by using stomata-based [CO2]atm data. Comparisons of the terrestrial-based [CO2]atm and the existed marine-based [CO2]aq curves show a similar general trend but with different intensity of fluctuations. Our data reveal that the high peak of [CO2]atm occurred at 2.77-2.52 Ma with a lower [CO2]aq background. The subsequent sharp fall in [CO2]atm level might be responsible for the intensification of the NHG based on their general temporal synchronism. These findings shed a significant light for our understanding toward the [CO2]atm changes and its ecological impact since 5 Ma.

Contrasting responses of leaf stomatal characteristics to climate change: a considerable challenge to predict carbon and water cycles

“…The threshold CO 2 concentration for effects on SD and SI is based on all plant life forms considered in this study; accurate assessment of the individual threshold of each plant life form will require additional data. In contrast, the RR of the SI showed a negative response to increasing CO 2 up to approximately 700 ppm; thus, SI may be a better and more reliable CO 2 proxy than SD because it responds in a sensitive way to CO 2 changes, as reported in previous studies (Haworth et al ., ; Rivera et al ., ; Bai et al ., ; Hu et al ., ). In addition, SD is sensitive to factors that affect the initiation of stomata and cell expansion, and cell expansion is affected by many variables (e.g., light, temperature, and water status); thus, changes in these variables can mask the effects of signals that cause stomatal initiation.…”

“…Elevated CO 2 can stimulate cell division and expansion, resulting in thicker leaves with more numerous cells or cell layers and/or larger cells (Ferris et al, 2001;Luomala et al, 2005). In addition, different responses of stomatal density (SD, the number of stomatal pores per unit leaf area) to changes in the CO 2 concentration have also been reported (Haworth et al, 2011a;Bai et al, 2015). Experimental results have shown decreases, no effect or even increases in SD as the CO 2 concentration increases (Ferris & Taylor, 1994;Amthor, 1995;Dixon et al, 1995;Reddy et al, 1998;Ferris et al, 2002;Marchi et al, 2004;Luomala et al, 2005;Xu & Zhou, 2005).…”

“…Thus, considerable caution is required when using SD as an indicator of a plant's response to elevated atmospheric CO 2 (Haworth et al, 2015;Xu et al, 2016). However, with the use of SI, the effects of other environmental factors, such as temperature, water stress, and humidity (Bai et al, 2015), can be eliminated because SI is a direct measure of the proportion of epidermal cells that have differentiated into stomata, whereas SD is also influenced by epidermal cell size, which can be modified by other factors (Ferris et al, 2001;Luomala et al, 2005). Thus, SI has been shown to be a reliable CO 2 proxy as it responds positively to CO 2 changes, and has been used to estimate the effects of increased atmospheric CO 2 (Haworth et al, 2011a;Rivera et al, 2014;Bai et al, 2015;Hu et al, 2015).…”

Estimates of late middle Eocene <i>p</i>CO<sub>2</sub> based on stomatal density of modern and fossil <i>Nageia</i> leaves

“…Liu et al: Late middle Eocene pCO 2 based on the Nageia leaves cus (Kürschner et al, 1996(Kürschner et al, , 2001, Laurus and Ocotea (Kürschner et al, 2008). Recently, positive correlations between stomatal index or stomatal frequency and pCO 2 have been reported based on fossil Typha and Quercus (Bai et al, 2015;Hu et al, 2015). However, the tropical and subtropical moist broadleaf forest conifer tree Nageia has not been used previously in paleobotanical estimates of pCO 2 concentration.…”

Paleoecology, Ploidy, Paleoatmospheric Composition, and Developmental Biology: A Review of the Multiple Uses of Fossil Stomata

“…Over the past three decades, since publication of Woodward's (Woodward, 1987) seminal article on the inverse relationship between stomatal density and atmospheric CO 2 concentration, the "uncontrolled variability" in stomatal traits such as stomatal density (SD) and stomatal index (SI) has been seized on by paleobiologists as an opportunity to extract meaningful paleoclimatic and paleoecological information from fossil plant stomata (McElwain et al, 2005;Roth-Nebelsick, 2005;Wagner et al, 2005;Kürschner et al, 2008;Lammertsma et al, 2011;Steinthorsdottir et al, 2011b;Franks et al, 2014;Maxbauer et al, 2014;Bai et al, 2015;Montañez et al, 2016;Steinthorsdottir et al, 2016aSteinthorsdottir et al, , 2016b. This has led to a subtle tension in the field of paleobotany, where at one extreme some studies have focused almost exclusively on the taxonomic and systematic utility of stomata with insufficient consideration of environmentally driven variability, while at the other extreme some reconstructions of paleoatmospheric composition have been undertaken using fossil stomatal traits without due consideration for taxonomic determination of the fossils used.…”