Published alkenone  p records spanning known glacial pCO 2 cycles show considerably less variability than predicted by the diffusive model for cellular carbon acquisition and isotope fractionation. We suggest this pattern is consistent with a systematic cellular enhancement of the carbon supply to photosynthesis via carbon concentrating mechanisms under the case of carbon limitation during low pCO 2 glacial time periods, an effect also manifest under carbon limitation in experimental cultures of coccolithophores as well as diatoms. While the low-amplitude  p signal over glacial pCO 2 cycles has led some to question the reliability of  p for reconstructing long-term pCO 2 , the [CO 2 ] aq in the tropical oceans during glacial pCO 2 minima represents the most extreme low CO 2 conditions likely experienced by phytoplankton in the Cenozoic, and the strongest upregulation of carbon concentrating mechanisms. Using a statistical multilinear regression model, we quantitatively parse out the factors (namely light, growth rate, and [CO 2 ] aq ), that contribute to variation in  p in alkenone-producing algae, which confirms a much smaller dependence of  p on [CO 2 ] aq in the low [CO 2 ] aq range, than inferred from the hyperbolic form of the diffusive model. Application of the new statistical model to two published tropical  p records spanning the late Neogene produces much more dynamic pCO 2 estimates than the conventional diffusive model and reveals a significant pCO 2 decline over the last 15 Ma, which is broadly consistent with recent results from boron isotopes of foraminifera. The stable isotopic fractionation between coccolith calcite and seawater dissolved inorganic carbon (here ∆ coccolith-DIC ) also shows systematic variations over glacial-interglacial cycles which may, following future experimental constraints, help estimate the degree of upregulation of parts of the algal carbon concentrating mechanism over glacial cycles.This simplified formulation clarifies the dependence of b on variation in the cellular C content and surface area, which scale with cell size; as well as variation in the growth rate and the effective permeability to CO 2 . When the effects of these factors are considered in aggregate, e.g. by empirical derivations of b from photic zone or culture samples, it must be remembered that the covariation and relative weight of each of these factors spatially in the modern ocean, or in culture experiments, may differ from past temporal significance and covariation of these factors. In practice, however, most previous work has interpreted variation in b to reflect either changes only in the growth rate parameter (Bidigare et al., 1997;Seki et al., 2010), or over long timescales also changes in the cell size and consequently in /S (Henderiks and Pagani, 2008;Seki et al., 2010). Potential variations in P have not been evaluated for glacial samples or the full range of published experiments with  p determinations in experimental culture, although some previous studies have acknowledged that the b...

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Since the late Miocene, plants using the C4 photosynthetic pathway have increased to become major components of many tropical and subtropical ecosystems. However, the drivers for this expansion remain under debate, in part because of the varied histories of C4 vegetation on different continents. Australia hosts the highest dominance of C4 vegetation of all continents, but little is known about the history of C4 vegetation there. Carbon isotope ratios of plant waxes from scientific ocean drilling sediments off north‐western Australia reveal the onset of Australian C4 expansion at ~3.5 Ma, later than in many other regions. Pollen analysis from the same sediments reveals increasingly open C3‐dominated biomes preceding the shift to open C4‐dominated biomes by several million years. We hypothesize that the development of a summer monsoon climate beginning in the late Pliocene promoted a highly seasonal precipitation regime favorable to the expansion of C4 vegetation.

During the Cenozoic (66 Ma to present), grasslands and open habitats replaced forests across large swaths of the tropics and subtropics (Jacobs et al., 1999;Strömberg, 2011), reshaping the vegetation structure and faunal communities in terrestrial ecosystems around the world (Anderson, 2006). In many regions, the late Miocene (∼11-5 Ma) conclusion of this ecological trajectory was the establishment of grasslands dominated by grass species using the C 4 photosynthetic pathway (Edwards et al., 2010). A substantial number of studies have mapped out when C 4 ecosystems became established around the globe. The earliest onset of C 4 ecosystem expansion occurred around 10