The Late Ordovician was characterized by glaciation, global cooling, and mass extinction. It was assumed that these events were triggered by increased delivery of the nutrient phosphorus (P). However, why this took place in two pulses is not yet clear. Looking at Longman, J. et al.'s journal paper, this article analyzes the connection between Late Ordovician marine productivity and cooling episodes to subaerial volcanic activity.
Image Credit: Nick Greaves/Shutterstock.com
The Late Ordovician mass extinction (LOME) took place in two phases and was the second-largest extinction event in Earth’s history in terms of species loss. The Late Ordovician is characterized by numerous carbon isotope excursions (CIEs)—two globally represented ones are the Guttenburg isotopic carbon excursion (GICE) at ~454 million years ago (Ma) and the Hirnantian isotopic carbon excursion (HICE) at ~445 Ma.
However, the major factor behind the CIEs and linked cooling is uncertain. It is assumed that the advent of early non-vascular land plants increased terrestrial weathering and also the delivery of the vital limiting nutrient phosphorus (P). The increased amount of P increases organic carbon burial and marine productivity, inducing a decrease in atmospheric CO2.
There are also other proposals, however, many observations support this concept of the Late Ordovician cooling triggered by organic carbon burial. But it is yet unclear why this took place in two distinct pulses during the GICE and HICE. This article investigates if Late Ordovician marine productivity and cooling episodes were directly related to subaerial volcanic activity. There are numerous records of volcanic eruptions in the Late Ordovician.
Recent studies use total organic carbon to mercury ratio to associate volcanic mercury emissions to Late Ordovician climatic change. It is also not known about the amount of P supplied from ash during the Late Ordovician or its influence on the marine environment. This study collates global data on P depletion in tephra layers as a means of quantifying P release to the ocean at the time of ash deposition and diagenesis.
To analyze the timing of volcanic activity, 43 Ar-Ar, and U–Pb dates from North American and Scandinavian bentonites (see Figure 1a) and 24 dates from Chinese bentonites of the Late Ordovician age (see Figure 1b) were compiled. The reconstruction indicated the occurrence of bentonite deposition in two discrete pulses (see Figure 1c).
Figure 1. Compilation of Late Ordovician bentonite ages from North America and China. (a, b), Bentonite ages in North America/Scandinavia (a) and China (b). Each age is represented by a probability density curve derived from published mean and standard deviation, from which 10,000 Monte Carlo simulations were completed and binned at 0.25 Myr intervals to attain probability densities of the eruption occurring in each bin. Colors correspond to the studies from which each age is obtained. Average probability densities for each 0.25 Myr bin for the North American (blue) and Chinese (red) bentonites. Vertical lines indicate the bin in which bentonite deposition is most likely. Image Credit: Longman, et al., 2021
Figure 2 depicts the eruption of two geographically diverse volcanic provinces.
Figure 2. Palaeogeographic reconstruction for the Late Ordovician at ~450 Ma (Katian). Ellipses mark the two volcanic provinces investigated in this study, with blue ellipses representing the North American and Scandinavian provinces and a green ellipse representing the Chinese province. The base map was constructed using the plate tectonic reconstructions from Merdith, et al., 2021 and is based partly on Cocks, and Torsvik, 2020.
The first pulse indicates North American/Scandinavian volcanism and the Chinese bentonite ages showed more spread with less accurate dates. The compilation indicates that the most intense volcanism in the China region took place between 445.25Ma and 442.5Ma and the two volcanic pulses correlate to the two primary CIEs of the Late Ordovician, the GICE and HICE, and would thus assist a connection between volcanism and climate change.
To examine P release at the time of ash deposition, diagenesis, and weathering the amount of P supplied by the two key pulses of volcanism was estimated. Data from marine sediment-hosted tephras were examined and compared to data from eight additional modern volcanic provinces (see Figure 3).
Figure 3. P depletion, an indicator of the amount of P lost to the ocean, from ten present-day representative volcanic provinces. (a) Boxes are defined between the first and third quartiles (the interquartile range), with minimum and maximum whiskers representative of 1.5 times the interquartile range. (b), A map of each volcanic province used for this reconstruction with the provinces identified by the numbers and colors used in a. Image Credit: Basemap from P. Wessel, University of Hawai’i, and W. H. F. Smith, NOAA Laboratory for Satellite Altimetry
The scale of P flux was estimated from a Monte Carlo simulation of inputs and was found that the annual P flux from diagenesis and ash deposition was 3 × 1010 mol P yr−1.
The depletion factors and analysis of ash supply in the Late Ordovician could be used for the quantification of P supply during the two investigated events. In the case of GICE, the simulations indicated a mean of 2.29 × 1015 mol P (see Figure 4), which increased further, and for the HICE, a mean supply of 2.89 × 1015 mol P was noted.
Figure 4. Monte Carlo simulations of P supply from volcanic weathering during the Late Ordovician with variable distributions defined by our ash-depletion and weathering model. (a, b), P supply from ash deposition and diagenesis for the two pulses of volcanism. Amount of P supplied from the volcanism at 453.5 Ma (a) and 444 Ma (b). The total ash weight is presented along the x-axis and the total P supply on the y axis. Each Monte Carlo simulation is indicated by a circle with the color indicating the depletion factor. (c), Estimate of P flux resulting from weathering of terrestrial volcanic matter (y-axis) plotted against the area covered by this ash and lava. Again, each simulation is indicated by a filled circle with the color in this case denoting the rate of P supply. Image Credit: Longman, et al., 2021
Weathering flux of P was estimated and the GICE and HICE P inputs were represented by Gaussian functions with their maxima at the times of greatest depositional intensity. The total P input is evaluated for both the means and 95th percentiles. As the COPSE model does not indicate the feedbacks well, it was concluded that a fivefold-larger P input is needed in COPSE to generate the same spike in marine P concentration seen in the multi-box model.
Figure 5 indicates the model outputs for average surface temperature, atmospheric CO2, marine anoxia, and δ13C of novel sedimentary carbonates.
Figure 5. Biogeochemical model outputs for impacts of volcanism during the GICE and HICE. COPSE model baseline runs38 plus P supply from ash. (a) The P input Gaussian functions. The P input magnitude follows the mean or 95th percentile of the values derived for ash supply and weathering combined, with or without recycling of P from sediments. (b) Modeled δ13C of carbonates (lines and colors defined in e) compared with data49 (yellow circles). (c) Modeled atmospheric CO2. (d) Modeled global average surface temperature. (e) Degree of marine anoxia (represented as the modeled proportion of anoxic seafloor). Solid lines show the same simulations as the dashed lines but with additional P input to represent sedimentary recycling of P. Image Credit: Longman, et al., 2021
The outputs indicate that the P release from volcanic ash deposition and weathering, along with the recycling of P from sediments, is adequate to trigger big changes in climate and biogeochemistry as seen in the geological record. The maximum global cooling at the HICE is approximately 3 °C.
The temperature predictions are in accordance with clumped isotope thermometry indicating that the Hirnantian icehouse was relatively short. A key feature of the HICE in the geological record is the extensive formation of organic-rich shales—particularly in China—potentially associated with widespread ocean anoxia.
The observations indicate that volcanic ash diagenesis and weathering performed a major role in the Late Ordovician Earth system. The results might detail various features of the LOME that do not follow trends linked with other mass extinctions particularly the association to cooling instead of warming.
In the Late Ordovician, it seems that the lasting nature of nutrient supply from weathering of eruptive products such as volcanic ash performs a more dominant role compared to the medium-term warming linked with CO2 injection. The super-eruptions indicated by bentonites would have led to initial cooling, followed by warming. These warming/cooling cycles are dangerous for organisms resulting in biodiversity loss explaining the LOME initiation.
Along with nutrients, there is also a possibility of toxic metal release. This is indicated by the evidence for metal-induced malformations in the Hirnantian glaciation. Volcanic ash would have led to the formation of anoxic conditions which further enhanced redox-based recycling of toxic metals.
Tephra layers were investigated for their major elemental content—including P content. Tephras were pinpointed visually and microscopically and P was estimated in the layers. The GEOROC database was employed to evaluate the protolith composition of volcanic material from the source regions. The data was filtered and the original composition of altered tephra was calculated along with depletion factors.
Monte Carlo simulations of variables linked with bentonite deposition were employed to evaluate the size of the volcanic eruptions and ash deposition. The study used a state-of-the-art COPSE biogeochemical model.
With the results obtained it was evident that the pulsed nature of global cooling was the result of the eruption of two diverse volcanic provinces—North America and the Baltic and Southern China. The proposed models indicate the deposition of ash blankets and weathering of lavas during Late Ordovician volcanism provided sufficient P to trigger glaciation, global cooling, and the LOME.
Longman, J., Mills, B. J. W., Manners, H. R., Gernon, T. M., Palmer, M. R. (2021) Late Ordovician climate change and extinctions driven by elevated volcanic nutrient supply. Nature Geoscience, 14, pp. 924–929. Available online: https://www.nature.com/articles/s41561-021-00855-5.
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