Biogeochemical significance of plant-microbial interactions in the Critical Zone

In Critical Zone research - https://czo-archive.criticalzone.org/national/research/the-critical-zone-1national/ - the microbial interaction of plants in the rhizosphere is most important. Apart from legumes that host nitrogen-fixing bacterial species, the rhizospheres of other plants have a symbiotic relationship with mycorrhizal fungi. I believe that in biological weathering and resulting biogeochemical interactions, the microbial interaction with the plants is of great importance — the structure of the microbiome in the rhizosphere governs the mobility and speciation of chemical elements.

A recent revelation of positive interactions among bacterial species Kehe et al. (2021) and the influence of endophytic bacteria over epiphytic population in the rhizosphere Zgadzaj et al. (2016) form a critical component of future research in the Critical Zone. Further, Critical Zone research should focus more on the community structure of the plants and their endophytic and epiphytic microbiome in the soil of the rhizosphere of a given area. The ambient temperature and moisture conditions shall profoundly impact such a population and their biogeochemical interactions. Those results are very much essential in the understanding of ecosystem functioning under the changing climate.
 

Further reading:

Heintz-Buschart, A., Guerra, C., Djukic, I., Cesarz, S., Chatzinotas, A., Patoine, G., Sikorski, J., Buscot, F., Küsel, K., Wegner, C.-E., Eisenhauer, N., 2020. Microbial diversity-ecosystem function relationships across environmental gradients. RIO 6, e52217. https://doi.org/10.3897/rio.6.e52217

Kehe, J., Ortiz, A., Kulesa, A., Gore, J., Blainey, P.C., Friedman, J., 2021. Positive interactions are common among culturable bacteria. Sci. Adv. 7, eabi7159. https://doi.org/10.1126/sciadv.abi7159

Zgadzaj, R., Garrido-Oter, R., Jensen, D.B., Koprivova, A., Schulze-Lefert, P., Radutoiu, S., 2016. Root nodule symbiosis in Lotus japonicus drives the establishment of distinctive rhizosphere, root, and nodule bacterial communities. Proc Natl Acad Sci USA 113, E7996–E8005. https://doi.org/10.1073/pnas.1616564113

 

Plant-microbial symbiosis and Earth's climate

The role of symbiotic relationships of plants with the rhizosphere bacteria and fungi on Earth’s climate has not been given due attention. A recent study by Mohr et al. (2021) suggests that seagrasses met their nitrogen deficiency through symbiosis with the bacterial population in their root (endophytic bacteria), the first observation in seagrass. Thus, they can grow well and oxygenate the water column in nitrogen-limited coastal areas like the Meditterennian Sea.

In an earlier work by Pohl et al. (2021) about the vertical decoupling of the ocean in Late Ordovician anoxia (around 440-460 million years ago), there was an expansion of bottom water deoxygenation, though upper oceanic water remained well-oxygenated. Please note seagrasses evolved only around 100 million years ago, and we are not aware of anything that contributed to a well-oxygenated water column during Late Ordovician anoxia.

Seagrasses are one of the significant contributors to the global carbon cycle akin to the mangroves. As nitrogen limitation is more prevalent than phosphorus limitation in the world’s oceans, symbiotic relations promoting nitrogen fixation and its availability to plants have wide-ranging implications in understanding and predicting future climate. Especially under rising sea levels, if the submerged coastal areas tend to favour the growth of seagrass, they can have the potential to subdue or reverse global warming.

Now the question remains when did the symbiosis start in Earth’s history? What are its implications in nutrient cycling and global climate - both on the continents and oceans?
 

Further reading:

Mohr, W., Lehnen, N., Ahmerkamp, S., Marchant, H.K., Graf, J.S., Tschitschko, B., Yilmaz, P., Littmann, S., Gruber-Vodicka, H., Leisch, N., Weber, M., Lott, C., Schubert, C.J., Milucka, J., Kuypers, M.M.M., 2021. Terrestrial-type nitrogen-fixing symbiosis between seagrass and a marine bacterium. Nature. https://doi.org/10.1038/s41586-021-04063-4

Pohl, A., Lu, Z., Lu, W., Stockey, R.G., Elrick, M., Li, M., Desrochers, A., Shen, Y., He, R., Finnegan, S., Ridgwell, A., 2021. Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation. Nat. Geosci. 14, 868–873. https://doi.org/10.1038/s41561-021-00843-9

 

Causal mechanism of ocean deoxygenation, mass extinction, and climate change

A recent work of Pohl et al. (2021) 

"........seafloor anoxia occurred during the latest Ordovician glacial maximum, coincident with the second pulse of the Late Ordovician mass extinction. However, expanded anoxia in a glacial climate strikingly contrasts with the warming-associated Mesozoic anoxic events and raises questions as to both the causal mechanism of ocean deoxygenation and its relationship with extinction."

I believe that the rate of change of the sea surface temperature is more critical in determining the expansion of bottom ocean deoxygenation, and it should also affect the ocean circulation patterns. Under two different rates of sea surface temperature changes, the emerging ocean circulation patterns and expansion of bottom ocean deoxygenation should be different. Thus, there won't be a simple relationship between Earth's climate and mass extinctions. In a nutshell, how fast the sea surface temperature changes occur can generate entirely different ocean circulation patterns, coevolving deoxygenation and mass extinctions.
 

Further reading:
 

Harper, D.A.T., 2021. Late Ordovician Extinctions, in: Encyclopedia of Geology. Elsevier, pp. 617–627. https://doi.org/10.1016/B978-0-12-409548-9.12530-8

Luo, G., Algeo, T.J., Zhan, R., Yan, D., Huang, J., Liu, J., Xie, S., 2016. Perturbation of the marine nitrogen cycle during the Late Ordovician glaciation and mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology 448, 339–348. https://doi.org/10.1016/j.palaeo.2015.07.018

Pohl, A., Donnadieu, Y., Le Hir, G., Ferreira, D., 2017. The climatic significance of Late Ordovician-early Silurian black shales: ORDOVICIAN CLIMATE AND BLACK SHALES. Paleoceanography 32, 397–423. https://doi.org/10.1002/2016PA003064

Pohl, A., Lu, Z., Lu, W., Stockey, R.G., Elrick, M., Li, M., Desrochers, A., Shen, Y., He, R., Finnegan, S., Ridgwell, A., 2021. Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation. Nat. Geosci. 14, 868–873. https://doi.org/10.1038/s41561-021-00843-9

Yang, X., Yan, D., Li, T., Zhang, L., Zhang, B., He, J., Fan, H., Shangguan, Y., 2020. Oceanic environment changes caused the Late Ordovician extinction: evidence from geochemical and Nd isotopic composition in the Yangtze area, South China. Geol. Mag. 157, 651–665. https://doi.org/10.1017/S0016756819001237

Non-steady state diagenesis and Earth's climate

There is a definite role for non-steady-state diagenesis in exercising control over marine organic carbon accumulation and remobilisation and transport of biologically essential elements. It sets up a transient process in the biogeochemical cycling of the elements in the ocean, and such a transient process can implicate an unpredictable impact on Earth's climate. Most of the contradictions observed in the palaeoclimate of the Earth and its consequences, for example, the mass extinction during Late Ordovician glacial maximum and the warming-associated Mesozoic anoxic events (Pohl et al., 2021), requires a reinvestigation into the role of non-steady-state diagenesis that might have occurred during those times.
 

Many studies have assumed steady-state diagenesis; however, non-steady-state diagenesis is most common in several parts of the world's oceans, for example, the Peruvian Shelf (Sellappa Gounder et al., 2020; and references cited in the supplement; Sulpis et al., 2021). The non-steady-state diagenesis affects the extent and intensity of the Oxygen Minimum Zone (OMZ). It also impacts the quantity of trace elements diffusing across the sediment-water interface in the continental margin and deep oceanic regions and affects the cycling of many elements, including sulphur, silicon, and aluminium.

Further reading:
 

Pohl, A., Lu, Z., Lu, W., Stockey, R.G., Elrick, M., Li, M., Desrochers, A., Shen, Y., He, R., Finnegan, S., Ridgwell, A., 2021. Vertical decoupling in Late Ordovician anoxia due to reorganisation of ocean circulation. Nat. Geosci. 14, 868–873. https://doi.org/10.1038/s41561-021-00843-9
 

Sellappa Gounder, E., Sundaramurthy, S., Ramasamy, N., Palanivel, P., 2020. Palaeoenvironmental applications of chromium and aluminium: Concerns on partitioning and early diagenetic remobilisation. Geological Journal 56, 2379–2397. https://doi.org/10.1002/gj.3913
 

Sulpis, O., Humphreys, M., Wilhelmus, M., Carroll, D., Berelson, W., Menemenlis, D., Middelburg, J., Adkins, J., 2021. RADIv1: a non-steady-state early diagenetic model for ocean sediments in Julia and MATLAB/GNU Octave (preprint). Oceanography. https://doi.org/10.5194/gmd-2021-211

Climate transitions: application of "Universality Principle"

Regarding climate transitions, different scenario emerges from another recent work (Shackleton et al., 2020) that describes the change from glacial to interglacial period as follows:

"Mean ocean temperature reached its maximum value of 1.1 ± 0.3 °C warmer-than-modern values at the end of the penultimate deglaciation at 129 ka, which resulted in 0.7 ± 0.3 m of thermosteric sea-level rise relative to the present level. However, this maximum in ocean heat content was a transient feature; mean ocean temperature decreased in the first several thousand years of the interglacial and achieved a stable, comparable-to-modern value by ~127 ka."

Thus, the Earth system's net "heat content" is critical in climate transitions. In comparison, the transition from the interglacial to glacial period is also a transient process (Ramadhin and Yi, 2020). They state that "The termination of a glacial period occurs rapidly while the changeover to a glacial period takes tens of thousands of years to be completed, resulting in an interesting asymmetrical shape for which there is yet no consensus on the mechanism(s) (Tziperman and Gildor, 2003)".
 

Transient processes are non-linear and chaotic. However, an order exists within that chaos in the realm of the "Universality Principle." I suppose that at the state of criticality during climate transitions, the energy condenses within a specific domain when the "tipping point" is reached. Then, "thermalization" - the tendency for energy to spread throughout a system - takes the lead, and that slows down the rapid change in the climate. The intensity of the negative-feedback mechanisms, namely the "sea-ice precipitation feedback" and "sea ice-insulation feedback", limits the rate of thermalization. And that results in asymmetrical transitions between icehouse and greenhouse states.
 

The large-scale involvement of ice probably reduces the transient time required for glacial to interglacial transition. The extension of ice diminishes during interglacials. Thus, the change from interglacial to glacial periods might occur slowly due to active negative feedback.

For further background on the "Universality Principle" and "Thermalization", please read:

1) The Universal Law That Aims Time's Arrow

2) In Mysterious Pattern, Math and Nature Converge
 

References:

Ramadhin, C., Yi, C., 2020. ESD Ideas: Why are glaciations slower than deglaciations? Earth Syst. Dynam. 11, 13–16. https://doi.org/10.5194/esd-11-13-2020

Shackleton, S., Baggenstos, D., Menking, J.A., Dyonisius, M.N., Bereiter, B., Bauska, T.K., Rhodes, R.H., Brook, E.J., Petrenko, V.V., McConnell, J.R., Kellerhals, T., Häberli, M., Schmitt, J., Fischer, H., Severinghaus, J.P., 2020. Global ocean heat content in the Last Interglacial. Nat. Geosci. 13, 77–81. https://doi.org/10.1038/s41561-019-0498-0

The bimodal distribution of river sediments

The bimodal distribution of sediments in the river beds has been widely reported. The most exciting work belongs to

Shaw and Kellerhalls (1982) The Composition of Recent Alluvial Gravels in Alberta River Beds, Bulletin 41, Alberta Research Council, Canada.

They have studied the bimodal distribution of gravel and sand in the 12 mountainous rivers of Alberta, spanning 7,500 km in length. That has brought out the presence of three distinct zones:

1) A mountain zone where the grain size increases with distance.
2) The central reach where the grain size decreases exponentially with distance (Stenberg's relationship).
3) The lower reach is the sand bedded river.

Other works report the bimodal distribution of the sediments in the sand and silt fraction, and references are available in the above publication.

They have proposed several inherent mechanisms that may account for the origin of bimodal distribution of the grain size in the river sediments. Though I am not contradicting their interpretations, I suppose that the very basis of such a mechanism lies in Boltzmann distribution that describes the energy function of the sediments across their size spectrum. Day by day, I am finding more pieces of evidence of the involvement of Boltzman distribution in natural processes, including adsorption - the most common phenomenon observed in the interacting sediment particles with trace metals dissolved in solution (natural water). For example, please refer to our publication:

Anandaraj, B., Eswaramoorthi, S., Rajesh, T.P., Aravind, J., Suresh Babu, P., 2018. Chromium(VI) adsorption by Codium tomentosum: evidence for adsorption by porous media from sigmoidal dose-response curve. Int. J. Environ. Sci. Technol. 15, 2595–2606.

The geochemical implications of the size sorting of the sediments that form three different mountain zones are monumental. They have implications in understanding sediment provenance, weathering intensity, source apportionment (quantitative provenance analysis), reservoir capacity, and aquifer characteristics worldwide.
 

I firmly believe that Shaw and Kellerhalls (1982) findings are extensible to global rivers.

Further reading:

Bolton, A.J., Maltman, A.J., Fisher, Q., 2000. Anisotropic permeability and bimodal pore-size distributions of fine-grained marine sediments. Marine and Petroleum Geology 17, 657–672. https://doi.org/10.1016/S0264-8172(00)00019-2

Colombini, M., Carbonari, C., 2020. Sorting and bed waves in unidirectional shallow-water flows. J. Fluid Mech. 885, A46. https://doi.org/10.1017/jfm.2019.1039

Houssais, M., Lajeunesse, E., 2012. Bedload transport of a bimodal sediment bed. J. Geophys. Res. 117, https://doi.org/10.1029/2012JF002490

Lee, B.J., Toorman, E., Molz, F.J., Wang, J., 2011. A two-class population balance equation yielding bimodal flocculation of marine or estuarine sediments. Water Research 45, 2131–2145. https://doi.org/10.1016/j.watres.2010.12.028

Sambrook Smith, G.H., 1996. Bimodal fluvial bed sediments: origin, spatial extent and processes. Progress in Physical Geography: Earth and Environment 20, 402–417. https://doi.org/10.1177/030913339602000402

Smith, G.H.S., Nicholas, A.P., Ferguson, R.I., 1997. Measuring and defining bimodal sediments: Problems and implications. Water Resour. Res. 33, 1179–1185. https://doi.org/10.1029/97WR00365

Sonu, C.J., 1972. Bimodal composition and cyclic characteristics of beach sediment in continuously changing profiles. Journal of Sedimentary Research 42, 852–857. https://doi.org/10.1306/74D72653-2B21-11D7-8648000102C1865D

Wathen, S.J., Ferguson, R.I., Hoey, T.B., Werritty, A., 1995. Unequal mobility of gravel and sand in weakly bimodal river sediments. Water Resour. Res. 31, 2087–2096. https://doi.org/10.1029/95WR01229

Wolcott, J., 1988. Nonfluvial control of bimodal grain-size distributions in river-bed gravels. Journal of Sedimentary Research 58, 979–984. https://doi.org/10.1306/212F8ED6-2B24-11D7-8648000102C1865D

Wu, W., Li, W., 2017. Porosity of bimodal sediment mixture with particle filling. International Journal of Sediment Research 32, 253–259. https://doi.org/10.1016/j.ijsrc.2017.03.005

What drives the short-term oscillations of phosphorus burial in the continental margin sediments of the Arabian Sea?

Two things emerge from the works of Jung et al. (2002) and Beasley et al. (2021):
 

1) Due to solar insolation, there have been decadal to centennial-scale changes in the upper oceanic water temperatures (2 to 3 deg.C) during the early Holocene in the Arabian Sea that modulated upwelling and monsoon precipitation (Jung et al., 2002);

2) Such monsoonal precipitation and upwelling changes might have occurred since the Oligocene-Miocene transition when the monsoon system got established (Beasley et al., 2021).

Our earlier work indicated short-term oscillations of phosphorus burial in the continental margins of the Arabian Sea (Phosphorus Deposition in Arabian Sea Sediments through Time - https://www.prl.res.in/~library/planetary_and_geosciences.htm that I interpreted as:

“Contrary to the widely held view that phosphorus could affect primary productivity in the long run, here we show evidence to believe that phosphorus may become a limiting nutrient on centennial to millennial scales, provided that its supply to the water column is restricted during high productivity episodes. Such evidence comes from the spectral analysis of phosphorus data obtained from the analysis of core sediments collected from the continental margin sediments of the eastern Arabian Sea. The results show century to millennial-scale oscillations in the burial flux of phosphorus to the sediments, which can be attributed to ocean circulation changes and intensification of SW monsoonal wind strength, which together modulates upwelling of remobilised nutrients and water column productivity. .........These results suggest that short-term solar oscillations can influence water column primary productivity and thereby phosphorus burial in the continental margin sediments of the Arabian Sea. When the phosphorus burial rate is high and the phosphorus supply to the water column is restricted (low river discharge and reduced upwelling), it may become a limiting nutrient. The century and millennial-scale oscillations in phosphorus burial rate imply that such a possibility can arise in the short term, contrary to the widely held belief that phosphorus limits productivity only on geological time scales.”

Now a few things have become clear. The oscillations in phosphorus burial may also have arisen from centennial-scale solar insolation changes that modulated the strength of the monsoon and the delivery of riverine supply of phosphorus. Moreover, changes in the upwelling may have regulated productivity and the observed phosphorus burial signal.

A further complication to this interpretation arises from a recent study suggesting that adsorption of phosphorus by iron oxides and its release during hypoxic events - the iron-phosphorus feedback - can drive multidecadal oscillations in hypoxia. The authors wrote:

“Our study shows that changes in the distribution of iron oxides between deep and shallow areas of the Baltic Sea led to self-sustaining variability (oscillations) in oxygen stress on decadal timescales during past intervals in the Sea’s 8000-year history. We use a model to demonstrate that under certain conditions of climate and nutrient pressure, such variability may occur naturally........” (Jilbert et al., 2021).

Therefore, more insight is needed to assess the short-term limitation of oceanic productivity by phosphorus in circulation-limited or enclosed oceanic regions.

Further reading:
 

Beasley, C., Kender, S., Giosan, L., Bolton, C.T., Anand, P., Leng, M.J., Nilsson‐Kerr, K., Ullmann, C.V., Hesselbo, S.P., Littler, K., 2021. Evidence of a South Asian proto‐monsoon during the Oligocene–Miocene transition. Paleoceanogr Paleoclimatol. https://doi.org/10.1029/2021PA004278
 

Jilbert, T., Gustafsson, B.G., Veldhuijzen, S., Reed, D.C., Helmond, N.A.G.M., Hermans, M., Slomp, C.P., 2021. Iron‐phosphorus feedbacks drive multidecadal oscillations in Baltic Sea hypoxia. Geophys Res Lett. https://doi.org/10.1029/2021GL095908
 

Jung, S.J.A., Davies, G.R., Ganssen, G., Kroon, D., 2002. Decadal-centennial scale monsoon variations in the Arabian Sea during the Early Holocene. Geochem.-Geophys.-Geosyst. 3, 1–10. https://doi.org/10.1029/2002GC000348