Department of geology northwest university

Trace of paleoatmospheric CO2 level in Ordovician: evidence from calcified cyanobacteria fossils

The atmospheric CO2 level has an important influence on the climate change at present and in the geological history. In the past 800000 years, CO2 levels could be measured directly from the ice core. Before that, in the longer time scale, the atmospheric CO2 level was estimated by geochemical model, or by biological fossil indicators (such as leaf stomatal density, etc.) or geochemical indicators. Although each indicator has its advantages and limitations, these indicators are widely used to assess atmospheric CO2 levels in the past 420myr. At present, it is believed that the level of atmospheric CO2 in Ordovician is closely related to climate change and ocean oxidation, and is the main controlling factor for the great radiation of Ordovician organisms and the great extinction of Late Ordovician. However, the existing Ordovician atmospheric CO2 geochemical models show great differences, the level of atmospheric CO2 is still not determined, and lack of fossil and geochemical indicators. Liu Lijing, a young teacher of paleontology teaching and research section, Department of geology, Northwest University, and her domestic collaborators, together with Professor Liyuan Liang, an environmental geochemical expert of Kewell geological consulting company, Kentucky, USA, and Robert, an international famous microbial carbonate expert of the University of Tennessee, USA Professor riding, using the cyanobacteria calcification index to indicate the level and change of atmospheric CO2 in this key geological history period, for the first time, restricted different geochemical models with fossil index. This research result was recently published online in earth and Planetary Science Letters, a top geoscience journal.

The photosynthesis of cyanobacteria requires carbon dioxide accumulating mechanisms (CCMS), which absorb HCO3 - in water and convert it into CO2 for photosynthesis. This process will release Oh - to the water, and induce the deposition of CaCO3 in the inner or surface of the sheath in the water with high calcium carbonate saturation. The calcified scabbard formed by this process can be preserved as the fossilized scabbard of cyanobacteria. CCMS are believed to be evolved by cyanobacteria in the face of environmental changes with low CO2 and high O2 levels. The experimental results show that CCM expression occurs when the partial pressure of CO2 in the external atmosphere is lower than 10 pal. if the level of CO2 in the air is higher than 10 pal, there is no HCO3 - absorption and oh - release. Therefore, the calcification of cyanobacteria can reflect the atmospheric PCO2 & lt; 10pal. At the same time, although the mechanism is still unclear, the increase of atmospheric O2 is also believed to promote the expression of CCMS. Therefore, the author uses the Ordovician Marine calcified cyanobacteria gel sheath fossil to indicate the change of atmospheric CO2, which is called "cyanobacteria calcification index", as the index reflecting the CO2 level lower than 10pal.

In the past, the author has made a detailed study on the systematic paleontology, sedimentary facies distribution and stratigraphic distribution of calcified cyanobacteria in Ordovician carbonate platform of Tarim Basin and Ordos Basin. On the basis of previous studies, this study conducted detailed statistics on the diversity, classification location, sedimentary facies distribution and stratigraphic distribution of calcified cyanobacteria fossils around the world through literature research, and ensured the correctness of all data through the verification, analysis and correction of literature data. It is found that there are 19 genera of Ordovician calcified cyanobacteria, belonging to tremella and Candida (Fig. 1 and Fig. 2). The global stratigraphic distribution shows that the diversity of Early Ordovician calcified cyanobacteria fossils is very low and Limited (in microbial reefs and metazoan reefs). From the late Middle Ordovician to the late Ordovician (from the late dariwell to the late Kaidi), many calcified cyanobacteria fossils have reappeared or appeared for the first time since the second Cambrian. The global calcified cyanobacteria fossils belong to the first level of diversity It has experienced a tenfold increase and is widely distributed in various sedimentary facies (metazoan algal reef, microbial reef, open platform, lagoon and tidal flat) on carbonate platform (Fig. 3). This indicates that the calcification of cyanobacteria has entered the intensive CCE from the early Ordovician to early Middle Ordovician (including the middle late Cambrian). The strong increase of the calcification indicates that the expression of CCM of cyanobacteria is widely open. It shows that the atmospheric CO2 level experienced a continuous decline in the late dariwell, samby and Kaidi periods (460myr-445myr) and was lower than 10pal. Comparing this result with the current estimation results of all the Ordovician atmospheric CO2 and O2 geochemical models, we find that the estimation values of copse model and the latest model derived from geocarb are more consistent with our fossil indicators than those of geoclip and Magic (Fig. 4b, d). This may be related to the copse model and the latest geocarb model, which more reasonably and comprehensively consider a variety of key organic and inorganic processes affecting atmospheric CO2 and O2 in the earth system. This study not only provides new evidence for the climate driven biological radiation and extinction during the Ordovician period, but also shows that the variation of calcified cyanobacteria diversity can more clearly reflect the change of calcification cycle. The calcification index of cyanobacteria should be better studied and applied in the future research on the level of CO2 in the deep atmosphere of the earth.

Article information: Odovician cyanobacterial call: a marine fossil proxy for atmospheric CO2

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F I g. 1. A, girvanella; B, subtifloria; C,? Batinevia; d), bevocastria; E, f), xiangella; G, obrucchevella; h, J), razumovskia; I, acuasiphonoria; K), osillatoriaceae Gen. indent. 1; L), m), osillatoriaceae Gen. indent. 2

Figure 2. Ordovician calcified Candida cyanobacteria. A) proaulopora; b) phapelophyton; c) gomphosiphon; d) hedstroeia; E), Bija; F), cayeuxia; g), zonotrichites; H), ortonella; I, apophoretella

Fig. 3 A) paleogeographic location of Ordovician calcified cyanobacteria; b) distribution of global calcified cyanobacteria in Ordovician strata and sedimentary facies

Fig. 4 Comparison of Ordovician calcified cyanobacteria with the estimated values of atmospheric CO2 geochemical model, dissolved oxygen and sea water surface temperature. A) the number of calcified cyanobacteria and the calcification period of cyanobacteria (CCE). B) the estimated values of each atmospheric CO2 geochemical model, red: geocarb-sulfvolc; yellow: geocarbsulf + GCM; green: upgraded geocarb; blue: copse; Purple: magic; Brown: geolim. C) sea water surface temperature estimate. D) dissolved oxygen estimate, red: geocarb-sulfvolc; yellow: geocarbsulf + GCM; green, upgraded geocarb; blue: copse; Purple: magic; Brown: geolim; green dotted line: fractionation effect model for optical cooperation.

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