Yi Ge Zhang, Mark Pagani, Zhonghui Liu, Steven M. Bohaty, Robert DeConto
The alkenone–pCO2 methodology has been used to reconstruct the partial pressure of ancient atmospheric carbon dioxide (pCO2) for the past 45 million years of Earth’s history (Middle Eocene to Pleistocene epochs). The present long-term CO2 record is a composite of data from multiple ocean localities that express a wide range of oceanographic and algal growth conditions that potentially bias CO2 results. In this study, we present a pCO2 record spanning the past 40 million years from a single marine locality, Ocean Drilling Program Site 925 located in the western equatorial Atlantic Ocean. The trends and absolute values of our new CO2 record site are broadly consistent with previously published multi-site alkenone–CO2 results. However, new pCO2 estimates for the Middle Miocene are notably higher than published records, with average pCO2 concentrations in the range of 400–500 ppm. Our results are generally consistent with recent pCO2 estimates based on boron isotope pH data and stomatal index records, and suggest that CO2 levels were highest during a period of global warmth associated with the Middle Miocene Climatic Optimum (17–14 million years ago, Ma), followed by a decline in CO2 during the Middle Miocene Climate Transition (approx. 14 Ma). Several relationships remain contrary to expectations. For example, benthic foraminiferal δ18O records suggest a period of deglaciation and/or high-latitude warming during the latest Oligocene (27–23 Ma) that, based on our results, occurred concurrently with a long-term decrease in CO2 levels. Additionally, a large positive δ18O excursion near the Oligocene–Miocene boundary (the Mi-1 event, approx. 23Ma), assumed to represent a period of glacial advance and retreat on Antarctica, is difficult to explain by our CO2 record alone given what is known of Antarctic ice sheet history and the strong hysteresis of the East Antarctic Ice Sheet once it has grown to continental dimensions. We also demonstrate that in the Neogene with low CO2 levels, algal carbon concentrating mechanisms and spontaneous bicarbonate–CO2 conversions are likely to play a more important role in algal carbon fixation, which provides a potential bias to the alkenone–pCO2 method.
Mark Pagani, Matthew Huber, Zhonghui Liu, Steven M. Bohaty, Jorijntje Henderiks, Willem Sijp, Srinath Krishnan, Robert M. DeConto
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.
Mark Pagani, Michael A. Arthur, Katherine H. Freeman
Changes in carbon dioxide concentration or ocean circulation are generally invoked to explain equable early Miocene climates and a rapid East Antarctic ice sheet (EAIS) expansion in the middle Miocene. This study reconstructs late Oligocene to late Miocene pCO2 estimates from εp values based on carbon isotopic analyses of di-unsaturated alkenones and planktonic foraminifera from Deep Sea Drilling Project Sites 588 and 608 and Ocean Drilling Program Site 730. Our results indicate that highest pCO2 occurred during the latest Oligocene (ca. 350 ppmv) but decreased rapidly at ca. 25 Ma. The early and middle Miocene was characterized by low pCO2 (260-190 ppmv). Lower intervals of pCO2 correspond to inferred organic-carbon burial events and glacial episodes, with the lowest concentrations occurring during the middle Miocene. There is no evidence for either high pCO2 during the late early Miocene climatic optimum or a sharp pCO2 decrease associated with EAIS growth. Paradoxically, pCO2 rises following EAIS growth and obtained pre-industrial levels by ca. 10 Ma. Although we emphasize an oceanographic control on Miocene climate, low pCO2 could have primed the climate system to respond sensitively to changes in heat and vapor transport.
Mark Pagani, Katherine H. Freeman, Michael A. Arthur
Trends in carbon isotope compositions of soil carbonates and equid teeth enamel support a global expansion of C4 grasses in the late Miocene. This event was originally attributed to a large-scale decrease in pCO2 which would have favored C4 over C3 plant metabolism. Here we present alkenone-based pCO2 estimates for the late Miocene from Deep Sea Drilling Project (DSDP) Site 588. Our results indicate that there was no major change in pCO2 during this time (10-5 Ma). Instead, pCO2 steadily increased from ca. 14 to 9 Ma and stabilized at approximately pre-industrial values. We propose that C4 expansion was triggered by an episode of regional aridity on a global scale caused by both existing low pCO2 conditions and a late Miocene phase of Asian orogeny.
Mark Pagani, James C. Zachos, Katherine H. Freeman, Brett Tipple, Stephen Bohaty
The relationship between the partial pressure of atmospheric carbon dioxide (pCO2) and Paleogene climate is poorly resolved. Here we use stable carbon isotopic values of di-unsaturated alkenones extracted from deep sea cores to reconstruct pCO2 from the middle Eocene to the late Oligocene (~45-25 Ma). Our results demonstrate that pCO2 ranged between 1000 to 1500 ppmv in the middle to late Eocene, and then decreased in several steps during the Oligocene, and reached modern levels by the latest Oligocene. The fall in pCO2 likely allowed for a critical expansion of ice sheets on Antarctica, and promoted conditions that forced the onset of terrestrial C4 photosynthesis.
Mark Pagani, Zhonghui Liu, Jonathan LaRiviere, Ana Christina Ravelo
Climate sensitivity refers to the mean-annual global temperature response to CO2 doubling due to the radiative effects of CO2 and associated feedbacks. The proposed range of climate sensitivity, ~1.5 to 4.5oC, represents fast-feedback sensitivity that incorporates changes in atmospheric water vapor, sea ice, and cloud and aerosol distributions. However, other feedbacks involving changes in continental ice extent, terrestrial ecosystems, non-CO2 greenhouse gas production, and other climate system parameters, operate on longer timescales and impact the temperature of the Earth. Warming related to a doubling of CO2 including all short- and long-term feedbacks is the Earth-system climate sensitivity. For this study, we evaluate the Earth-system climate sensitivity by reconstructing middle and early Pliocene CO2 concentrations when global temperatures were ~3 to 4oC warmer than pre-industrial conditions. We demonstrate that only a minor change in CO2 was associated with substantial global warming ~4.5 million years ago, with CO2 levels in the range of ~365 to 415 ppm during peak temperatures. Given estimates of global temperatures during the Pliocene, our results support a high Earth-system climate sensitivity for at least the past ~5 million.
Brett J. Tipple, Stephen R. Meyers, Mark Pagani
The carbon isotope ratio (δ13C) of plant material is commonly used to reconstruct the relative distribution of C3 and C4 plants in ancient ecosystems. However, such estimates fundamentally depend on the δ13C of atmospheric CO2 at the time, which likely varied throughout Earth history. For this study, we translate long-term benthic and planktonic δ13C and δ18O records into a reconstruction of Cenozoic δ13CCO2 by modeling equilibrium and non-equilibrium processes. Confidence intervals for the reconstructed δ13CCO2 estimates are assigned after careful consideration of these processes, as well as the sampling resolution of the data. We find that the influence of photosymbiotes, depth of production, seasonal variability, and preservation limit the utility of planktonic foraminiferal records, whereas benthic records provide a more well constrained δ13CCO2 estimate. Furthermore, sensitivity analyses designed to quantify the effects of temperature uncertainty and diagenesis on benthic foraminifera δ13C and δ18O values indicate that these factors act to offset one another, yielding a resilient estimate of δ13CCO2. Our reconstruction suggests that Cenozoic δ13CCO2 averaged -6.1 permil, while only 11.2 million of the last 65.5 million years correspond to the pre-Industrial value of -6.5 permil (with 90% confidence). δ13CCO2 also displays significant variations throughout the record, at times departing from the pre-Industrial value by more than 2 permil. Thus, the observed variability in δ13CCO2 should be considered in isotopic reconstructions of ancient terrestrial-plant ecosystems, especially during the Late and Middle Miocene, times of presumed C4 grassland expansion.
Katherine H. Freeman and Mark Pagani
The reconstruction of CO2 values based on biomarker and isotopic analyses provides many challenges and as many opportunities. The method is best applied when CO2 levels were relatively low, when phosphate concentrations can be constrained, and with multiple sample localities representing stable oceanographic regimes. With our currently available understanding of alkenone isotopic biogeochemistry and under the best of circumstances, this will lead to uncertainties of approximately 20%. The relative utility of this information depends entirely on the question being addressed. It is inappropriate to expect precise reconstructions on very long timescales because the quality of our understanding of ancient oceanic and biological processes diminishes back in time, while the uncertainty increases at times of high CO2. However, isotopic variations do record information about very ancient CO2. Observed variations in ep can be used to suggest relative changes in CO2 and whether levels were above or below a threshold level of sensitivity for isotopic fractionation during carbon fixation. Elevated ep values approaching the maximum values are associated with significant uncertainty, although when constrained by phosphate estimates, they nonetheless indicate elevated CO2 levels sufficient to result in the expression of maximum fractionation by enzymatic fixation. In such situations, it is especially necessary to establish multiple records from both high and low phosphate environments to reduce the uncertainty and constrain the range of surface water CO2 concentrations.
Cenozoic climates have varied across a variety of timescales, including slow, unidirectional change over tens of millions of years, as well as severe, geologically abrupt shifts in Earth’s climatic state. Establishing the history of atmospheric carbon dioxide is critical in prioritizing the factors responsible for past climatic events, and integral in positioning future climate change within a geological context. One approach in this pursuit utilizes the stable carbon isotopic composition of marine organic molecules known as alkenones. The following report represents a summary of the factors affecting alkenone carbon isotopic compositions, the underlying assumptions and accuracy of short- and long-term CO2 records established from these sedimentary molecules, and their implications for the controls on the evolution of Cenozoic climates.
Mark Pagani, Damien Lemarchand, Arthur Spivack, Jerome Gaillardet
The boron isotope-pH technique is founded on a theoretical model of carbonate δ11B variation with pH that assumes that the boron isotopic composition of carbonates mirrors the boron isotopic composition of borate in solution. Knowledge of the fractionation factor for isotope exchange between boric acid and borate in solution, the equilibrium constant for the dissociation of boric acid, as well as the isotopic composition of boron in seawater are required parameters of the model. The available data suggests that both the value of the fractionation factor for isotope exchange between boric acid and borate and the history of the isotopic composition of boron in seawater are poorly constrained. However, an empirical value for the equilibrium constant for the dissociation of boric acid can be estimated from the results of inorganic carbonate precipitation experiments. This exercise yields a value of ~0.974 in accordance with recent theoretical estimates, but substantially deviates from the theoretical value of 0.981 often used to estimate paleocean pH. Uncertainty in pH reconstructions increases as foraminiferal ‘vital effects’ are considered and different models for secular changes in seawater d11B are applied. Given the current understanding of boron systematics, pH values estimated using this technique have considerable uncertainty, particularly when reconstructions exceed the residence time of boron in the ocean.
Jorijntje Henderiks and Mark Pagani
Long-term alkenone-based pCO2 records are widely applied in paleoclimate evaluations. These pCO2 estimates are based on records of the carbon isotope fractionation that occurs during marine haptophyte photosynthesis (εp37:2). In addition to the concentration of aqueous CO2 (CO2(aq)) the magnitude of εp37:2 is also influenced by algal growth rates and cell geometry. To date, the influence of haptophyte cell geometry on the expression of ancient εp37:2 values has received little attention. This study evaluates changes in cell geometry of ancient alkenone-producing algae at Deep See Drilling Project Site 516 in the southwest Atlantic Ocean by analyzing individual coccolith dimensions, which are proportional to algal cell volume and surface area. We show that during part of the early Miocene, mean cell sizes of alkenone-producing algae were smaller relative to modern Emiliania huxleyi. Cell size variations coincide with significant changes in εp37:2, with a distinct 6% decrease in εp37:2 at 20.3 Ma associated with a 27% increase in haptophyte cell sizes. These changes in cell size impact εp37:2-based interpretations of growth rate variation and CO2(aq) estimates for this southwest Atlantic site. After correcting for cell geometry, CO2(aq) estimates at Site 516 are consistent with those reported from other oligotrophic sites during this time, resulting in overall low atmospheric pCO2 estimates (<350 ppmv) for the early Miocene.
Jorijntje Henderiks and Mark Pagani
Alkenone-based Cenozoic records of the partial pressure of atmospheric carbon dioxide (pCO2) are founded on the carbon isotope fractionation that occurred during marine photosynthesis (εp37:2). However, the magnitude of εp37:2 is also influenced by phytoplankton cell size - a consideration lacking in previous alkenone-based CO2 estimates. In this study, we reconstruct cell size trends in ancient alkenone-producing coccolithophores (the reticulofenestrids) to test the influence that cell size variability played in determining εp37:2 trends and pCO2 estimates during the middle Eocene to early Miocene. At the investigated deep-sea sites, the reticulofenestrids experienced high diversity and largest mean cell sizes during the late Eocene, followed by a long-term decrease in maximum cell size since the earliest Oligocene. Decreasing haptophyte cell sizes do not account for the long-term increase in the stable carbon isotopic composition of alkenones and associated decrease in εp37:2 values during the Paleogene, supporting the conclusion that the secular pattern of εp37:2 values is primarily controlled by decreasing CO2 concentration since the earliest Oligocene. Further, given the physiology of modern alkenone producers, and considering the timings of coccolithophorid cell size change, extinctions, and changes in reconstructed pCO2 and temperature, we speculate that the selection of smaller reticulofenestrid cells during the Oligocene primarily reflects an adaptive response to increased [CO2(aq)] limitation.