Algal constraints on the cenozoic history of atmospheric CO2 (original) (raw)
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Decrease in coccolithophore calcification and CO2 since the middle Miocene
Nature Communications, 2016
Marine algae are instrumental in carbon cycling and atmospheric carbon dioxide (CO 2) regulation. One group, coccolithophores, uses carbon to photosynthesize and to calcify, covering their cells with chalk platelets (coccoliths). How ocean acidification influences coccolithophore calcification is strongly debated, and the effects of carbonate chemistry changes in the geological past are poorly understood. This paper relates degree of coccolith calcification to cellular calcification, and presents the first records of size-normalized coccolith thickness spanning the last 14 Myr from tropical oceans. Degree of calcification was highest in the low-pH, high-CO 2 Miocene ocean, but decreased significantly between 6 and 4 Myr ago. Based on this and concurrent trends in a new alkenone e p record, we propose that decreasing CO 2 partly drove the observed trend via reduced cellular bicarbonate allocation to calcification. This trend reversed in the late Pleistocene despite low CO 2 , suggesting an additional regulator of calcification such as alkalinity.
A coccolithophore concept for constraining the Cenozoic carbon cycle
Biogeosciences, 2007
An urgent question for future climate, in light of increased burning of fossil fuels, is the temperature sensitivity of the climate system to atmospheric carbon dioxide (pCO 2 ). To date, no direct proxy for past levels of pCO 2 exists beyond the reach of the polar ice core records. We propose a new methodology for placing a constraint on pCO 2 over the Cenozoic based on the physiological plasticity of extant coccolithophores. Specifically, our premise is that the contrasting calcification tolerance 1 of various extant species of coccolithophore to raised pCO 2 reflects an "evolutionary memory" of past atmospheric composition. The different times of evolution of certain morphospecies allows an upper constraint of past pCO 2 to be placed on Cenozoic timeslices. Further, our hypothesis has implications for the response of marine calcifiers to ocean acidification. Geologically "ancient" species, which have survived large changes in ocean chemistry, are likely more resilient to predicted acidification.
Sensitivity of coccolithophores to carbonate chemistry and ocean acidification
Nature, 2011
About one-third of the carbon dioxide (CO 2 ) released into the atmosphere as a result of human activity has been absorbed by the oceans 1 , where it partitions into the constituent ions of carbonic acid. This leads to ocean acidification, one of the major threats to marine ecosystems 2 and particularly to calcifying organisms such as corals 3,4 , foraminifera 5-7 and coccolithophores 8 . Coccolithophores are abundant phytoplankton that are responsible for a large part of modern oceanic carbonate production. Culture experiments investigating the physiological response of coccolithophore calcification to increased CO 2 have yielded contradictory results between and even within species . Here we quantified the calcite mass of dominant coccolithophores in the present ocean and over the past forty thousand years, and found a marked pattern of decreasing calcification with increasing partial pressure of CO 2 and concomitant decreasing concentrations of CO 3 22 . Our analyses revealed that differentially calcified species and morphotypes are distributed in the ocean according to carbonate chemistry. A substantial impact on the marine carbon cycle might be expected upon extrapolation of this correlation to predicted ocean acidification in the future. However, our discovery of a heavily calcified Emiliania huxleyi morphotype in modern waters with low pH highlights the complexity of assemblage-level responses to environmental forcing factors.
Global Change Biology, 2012
Calcifying phytoplankton play an important role in marine ecosystems and global biogeochemical cycles, affecting the transfer of both organic and inorganic carbon from the surface to the deep ocean. Coccolithophores are the most prominent members of this group, being well adapted to low-nutrients environments (e.g., subtropical gyres). Despite urgent concerns, their response to rising atmospheric carbon dioxide levels (pCO 2 ) and ocean acidification is still poorly understood, and short-term experiments may not extrapolate into longer-term climatic adaptation. Current atmospheric pCO 2 (~390 ppmv) is unprecedented since at least 3 million years ago (Ma), and levels projected for the next century were last seen more than 34 Ma. Hence, a deep-time perspective is needed to understand the longterm effects of high pCO 2 on the biosphere. Here we combine a comprehensive fossil data set on coccolithophore cell size with a novel measure of ecological prominence: Summed Common Species Occurrence Rate (SCOR). The SCOR is decoupled from species richness, and captures changes in the extent to which coccolithophores were common and widespread, based on global occurrences in deep-sea sediments. The size and SCOR records are compared to stateof-the-art data on climatic and environmental changes from 50 to 5 Ma. We advance beyond simple correlations and trends to quantify the relative strength and directionality of information transfer among these records. Coccolithophores were globally more common and widespread, larger, and more heavily calcified in the pre-34 Ma greenhouse world, and declined along with pCO 2 during the Oligocene (34-23 Ma). Our results suggest that atmospheric pCO 2 has exerted an important long-term control on coccolithophores, directly through its availability for photosynthesis or indirectly via weathering supply of resources for growth and calcification.
Species-specific growth response of coccolithophores to Palaeocene–Eocene environmental change
Nature Geoscience, 2013
Coccolithophores-single-celled calcifying phytoplankton -represent an essential footing to marine ecosystems, yet their sensitivity to environmental change, and in particular increases in atmospheric CO 2 , is poorly understood 1 . During the Palaeocene-Eocene Thermal Maximum (PETM), about 56 million years ago, atmospheric CO 2 concentrations rose rapidly and the oceans acidified 2,3 , making this an ideal time interval to examine coccolithophore responses to environmental change.
Species-specific responses of calcifying algae to changing seawater carbonate chemistry
Geochemistry, Geophysics, Geosystems, 2006
1] Uptake of half of the fossil fuel CO 2 into the ocean causes gradual seawater acidification. This has been shown to slow down calcification of major calcifying groups, such as corals, foraminifera, and coccolithophores. Here we show that two of the most productive marine calcifying species, the coccolithophores Coccolithus pelagicus and Calcidiscus leptoporus, do not follow the CO 2 -related calcification response previously found. In batch culture experiments, particulate inorganic carbon (PIC) of C. leptoporus changes with increasing CO 2 concentration in a nonlinear relationship. A PIC optimum curve is obtained, with a maximum value at present-day surface ocean pCO 2 levels ($360 ppm CO 2 ). With particulate organic carbon (POC) remaining constant over the range of CO 2 concentrations, the PIC/POC ratio also shows an optimum curve. In the C. pelagicus cultures, neither PIC nor POC changes significantly over the CO 2 range tested, yielding a stable PIC/POC ratio. Since growth rate in both species did not change with pCO 2 , POC and PIC production show the same pattern as POC and PIC. The two investigated species respond differently to changes in the seawater carbonate chemistry, highlighting the need to consider species-specific effects when evaluating whole ecosystem responses. Changes of calcification rate (PIC production) were highly correlated to changes in coccolith morphology. Since our experimental results suggest altered coccolith morphology (at least in the case of C. leptoporus) in the geological past, coccoliths originating from sedimentary records of periods with different CO 2 levels were analyzed. Analysis of sediment samples was performed on six cores obtained from locations well above the lysocline and covering a range of latitudes throughout the Atlantic Ocean. Scanning electron micrograph analysis of coccolith morphologies did not reveal any evidence for significant numbers of incomplete or malformed coccoliths of C. pelagicus and C. leptoporus in last glacial maximum and Holocene sediments. The discrepancy between experimental and geological results might be explained by adaptation to changing carbonate chemistry.
Research Square (Research Square), 2023
Evolutionary or adaptative changes in Noelaerhabdaceae coccolithophores occurred in parallel with major changes in carbonate export and burial during scenarios of low orbital eccentricity, with a ~ 400 kyr recurrence, during the Pleistocene. Coeval with these conditions of enhanced proliferation, here we report a globally enhanced calci cation intensity of specimens across multiple species or morphotypes within the Gephyrocapsa complex during the Mid-Brunhes (MB) interval, 400 ka. Seawater alkalinity is proposed as the environmental trigger for the increased production of both the inorganic and organic carbon, possibly implemented by a coupled increase in nutrient delivery. The strong biological pump triggered by the enhanced proliferation of highly calci ed Gephyrocapsa, together with respiration dissolution, would have contributed to the associated deep sea dissolution event at the ~ 400 kyr scale, limiting the removal of alkalinity by burial, and maintaining constant levels at this scale. This new perspective highlights, rst, the role of orbital forcing in phytoplankton evolution or adaptation through changes in the seawater carbon chemistry. Second, the capacity of the Noelaerhabdaceae acmes to modify the typical behavior of carbonate compensation in the ocean. Our ndings suggests that changes in coccolith calci cation intensity may indicate changes in past ocean carbonate chemistry and the operation of the global carbon cycle under contrasting background conditions during the Cenozoic.
Coccolithophore cell size and the Paleogene decline in atmospheric CO2
Earth and Planetary Science Letters, 2008
Alkenone-based Cenozoic records of the partial pressure of atmospheric carbon dioxide (pCO 2 ) are founded on the carbon isotope fractionation that occurred during marine photosynthesis (ε p37:2 ). However, the magnitude of ε p37:2 is also influenced by phytoplankton cell sizea consideration lacking in previous alkenone-based CO 2 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 ε p37:2 trends and pCO 2 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 ε p37:2 values during the Paleogene, supporting the conclusion that the secular pattern of ε p37:2 values is primarily controlled by decreasing CO 2 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 pCO 2 and temperature, we speculate that the selection of smaller reticulofenestrid cells during the Oligocene primarily reflects an adaptive response to increased [CO 2(aq) ] limitation.