Testing algal-based pCO2 proxies at a modern CO2 seep (Vulcano, Italy)

Abstract

Understanding long-term trends in atmospheric concentrations of carbon dioxide (pco 2) has become increasingly relevant as modern concentrations surpass recent historic trends. one method for estimating past pco 2 , the stable carbon isotopic fractionation associated with photosynthesis (Ɛ p) has shown promise over the past several decades, in particular using species-specific biomarker lipids such as alkenones. Recently, the Ɛ p of more general biomarker lipids, organic compounds derived from a multitude of species, have been applied to generate longer-spanning, more ubiquitous records than those of alkenones but the sensitivity of this proxy to changes in pco 2 has not been constrained in modern settings. Here, we test Ɛ p using a variety of general biomarkers along a transect taken from a naturally occurring marine co 2 seep in Levante Bay of the Aeolian island of Vulcano in italy. the studied general biomarkers, loliolide, cholesterol, and phytol, all show increasing depletion in 13 c over the transect from the control site towards the seep, suggesting that co 2 exerts a strong control on isotopic fractionation in natural phytoplankton communities. the strongest shift in fractionation was seen in phytol, and pco 2 estimates derived from phytol confirm the utility of this biomarker as a proxy for pco 2 reconstruction. The concentration of atmospheric carbon dioxide (pCO 2 , expressed in partial pressure µatm), as directly measured from air trapped in ice cores, has had a major influence on climate over the past 800 thousand years (ka) 1. During this period, pCO 2 and temperature oscillated together between stable bounds every 100 ka 2. In the past two centuries, the rise of pCO 2 has broken those bounds from the pre-industrial values, previously only ranging between ca. 180 to 280 µatm, to the 410 µatm of today 3. This rapid rise in pCO 2 causes concern that climate, particularly temperature, will accordingly change. In order to better understand how changes may occur, reconstructing longer trends in pCO 2 over the geologic record could offer context for evaluating the direction and magnitude of climate change. Many proxies have been developed for reconstructing past pCO 2 and applied with mixed success over the past several decades 4. One method for studying past pCO 2 makes use of the stable carbon isotopic fractionation due to CO 2-fixation (Ɛ p), where biomass of photoautotrophs becomes increasingly depleted in 13 C as pCO 2 increases due to kinetic discrimination by the CO 2-fixing enzyme Rubisco 5-7. Ɛ p can be derived from the δ 13 C of photoautotrophic biomass, recorded in sedimentary organic matter, and the δ 13 C of inorganic CO 2 derived from the carbonate in the shells of planktonic foraminifera 8. Although pCO 2 has been shown to be one of the dominant physiological control on the δ 13 C of photoautotrophic biomass 9 , studies on Ɛ p in algae have shown that other factors may influence this value, primarily growth rate 10 and cell size and shape 11 , as well as minor influences such as light, and ...

Low atmospheric CO2 levels before the rise of forested ecosystems

“…[19][20][21]. The phytane proxy shows the expected response at elevated CO 2 concentrations and predicts declining CO 2 levels as a function of distance to a modern CO 2 seep 25 (albeit with larger errors than reported from Palaeozoic phytane). When applied to modern phytoplankton, the phytane proxy predicts a wide range of atmospheric CO 2 levels even today (~300-1200 ppm) 26 .…”

Estimation of Physiological Factors Controlling Carbon Isotope Fractionation in Coccolithophores in Photic Zone and Core‐Top Samples

“…This focus is motivated by the goal of estimating the climate sensitivity to changes in the carbon cycle and related feedbacks for higher than modern p CO 2 (Flato et al., 2013), as the ice core archive of atmospheric p CO 2 of the last 800 ka (Lüthi, 2008) samples only times of preindustrial or lower p CO 2 . A few studies have also sought to exploit the sensitivity of Ɛ p to growth rates of marine phytoplankton in the modern ocean, when the dissolved CO 2 concentration is independently measured (Burkhardt et al., 1999; Laws et al., 1995; Witkowski et al., 2020). Although carbon isotopic fractionation in all marine phytoplankton is expected to be governed by similar processes, isolating the Ɛ p signal from a specific group of taxa can be advantageous because it limits variation of some parameters affecting Ɛ p, such as cell permeability or geometry.…”

Continuous sterane and phytane δ13C record reveals a substantial pCO2 decline since the mid-Miocene

“…Several recent studies explored the potential of GPBs across a modern environmental transect from high pCO 2 near a naturally-occurring marine CO 2 seep towards control values in two drastically different geographic locations (i.e., off the coasts of Vulcano Island, Italy 25 and Shikine Island, Japan 26 ). The applied GPBs known as phytol (i.e., the side-chain of the vital photoautotrophic pigment chlorophyll-a 27 ) and cholesterol (i.e., a sterol that all eukaryotes synthesize or produce from ingested sterols with minimal isotopic fractionation 28,29 ) demonstrate that mixed phytoplankton communities with varying cell sizes and growth rates still exhibit a strong isotopic response to CO 2[aq] . Phytol has further been tested across glacial-interglacial cycles, which suggest phytol reconstructions were within error of the ice core-based CO 2 records and showed nearly identical values as the alkenone-based reconstructions 30 .…”