Recent Talks

Microbial carbonates (2010)
Marrying stromatolite perspectives: 3500 million years of history and a century of research (2008)
CO2-concentrating mechanisms, harmful blooms, and Late Devonian reef extinction 375 million years ago (2008)
Microbial carbonates: processes and products in time and space (2006)
Atmospheric trigger for Early Carboniferous carbonate mud mounds? (2006)
Secular variations in abundance of calcified algae and bacteria: how biomineralization can reflect global changes in temperature and water chemistry (2005)

Riding, R. 2010. Microbial carbonates. 18th International Sedimentological Congress – Mendoza, Argentina, 26 September–1 October, 2010. Abstracts, p. 87.

Microbial carbonates

Robert Riding
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, USA.
(riding@cardiff.ac.uk)

Synsedimentary microbial carbonates include products of bacterially bio-induced precipitation that form within sediments (e.g., at cold methane seeps), at the sediment–water interface (e.g., in biofilm–mats), and as allochthonous water–column precipitates (e.g., biogenic whitings). They have a long record and wide distribution in aquatic environments. Microbial carbonate development particularly reflects: (i) environmental controls (e.g., carbon dioxide, light and nutrient availability; carbonate saturation); (ii) evolution of bacterial metabolisms that promote precipitation (e.g., sulphate reduction, oxygenic photosynthesis and carbon dioxide concentrating mechanisms, CCM); and (iii) interactions with eukaryotes (e.g., grazers, mat– and reef–builders). Key questions include microfabric and macrofabric development, controls on spatial and secular abundance, and the record of metabolic development and seawater carbonate saturation state reflected in microbial carbonates.

Precambrian stromatolites reflect the rise of bacterial mats and metabolisms that influenced microfabric development, and progressive decline in abiogenic seafloor crust precipitation. Suggestions for investigation include: (i) millimetric dark–light couplets are seasonal alternations of abiogenic crust and lithified microbial mat (Hybrid Stromatolites); (ii) clotted–peloidal microfabrics increased with sulphate availability that promoted bacterial sulphate reduction; (iii) cyanobacterial in vivo sheath calcification was triggered by CCM induction in response to decline in carbon dioxide. These latter filamentous fabrics contributed to thrombolite development that transformed macrofabrics in the mid–Proterozoic. Phytoplanktic CCM induction at this time could also have increased whiting precipitation, contributing to carbonate mud substrates in which 'molar tooth' structures developed. In addition, increase in background sedimentation would have reduced stromatolite relative accretion rate, promoting late–Proterozoic diversification of digitate forms.

In the Early Palaeozoic, cyanobacteria and other calcimicrobes contributed significantly to thrombolite–dendrolite formation. Domes and columns declined in the Ordovician as algal–metazoan reefs increased. Subsequently, these morphotypes only developed extensively in marine environments when or where skeletal encrusters were reduced, as in the immediate aftermaths of Mass Extinction events and in ecologic refuges. Reefs limited overall microbial carbonate habitats, but provided cryptic substrates where heterotrophic communities developed reefal microbial crusts, often with distinctive clotted fabrics. Late Devonian decline in carbon dioxide stimulated cyanobacterial sheath calcification, and also whitings that contributed to Late Devonian–Early Carboniferous carbonate mud mound formation. In addition to competition, episodic Phanerozoic decline in microbial carbonate abundance reflects fluctuating reduction in seawater carbonate saturation that slowed lithification and therefore accretion.

Present–day thrombolitic stromatolites with weak initial lithification that largely accrete by grain trapping are grazing prone; but grazing probably has reduced influence on early–lithified mats. Present–day examples provide valuable insights. Cyanobacterial mat calcification is well developed in calcareous streams and lakes but weak in marine environments, reflecting dependence on elevated carbonate saturation. Marine calcification is sustained better by sulphate reducers in more enclosed substrates, as in methane seeps and reefal crusts. Large marine domes are restricted to shallow wave–swept bays and channels, protected from reef encrustation by hypersalinity (Shark Bay) and/or mobile sediment (Lee Stocking Island). Their accretion relies on production of extracellular polymeric substances, stimulated by high illumination, that promote grain trapping and on heterotroph–dominated sub–mat lithification.

Riding, R. 2008. Marrying stromatolite perspectives: 3500 million years of history and a century of research. Geobiology of Stromatolites, International Kalkowsky Symposium, Göttingen, Germany, October 4–11, 2008. Abstract Volume and Field Guide to Excursions: 29–30. ISBN 978–3–940344–52–6

Marrying stromatolite perspectives: 3500 million years of history and a century of research

Robert Riding
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA.
(riding@cardiff.ac.uk)

Stromatolites combine an extraordinarily long geological record with an unusual mixture of challenges. How they have formed and changed through time are key areas for research. Approaches to these questions hinge on stromatolite definition, and continuing lack of resolution of this central issue compounds the difficulties. Reflecting on the century since Ernst Kalkowsky introduced 'Stromatolith', a pessimist might emphasize dogma and confusion. An optimist, on the other hand, could highlight significant advances that have laid the groundwork for many fruitful insights. Given their long history, the time period from which stromatolites are viewed is critically important. It is not difficult to regard many Phanerozoic stromatolites as essentially lithified microbial mats. In contrast, many Precambrian examples contain precipitated abiogenic crusts, both with and without microbial mats. The immediate challenge is to marry these dual abiogenic and biogenic perspectives of stromatolites resulting from a century of research.

Regular, even layering, common in Archaean stromatolites, probably reflects significant abiogenic precipitation, and early-mid Proterozoic stromatolites, including Conophyton, commonly show dark-light layers that appear to represent alternations of lithified mat and abiogenic crust. In contrast, Phanerozoic marine stromatolites are characterized by uneven fine-grained layers. These successive developments suggest decline in abiogenic precipitation and increase in lithified mat components through time. Presumably these in turn reflect changes in seawater chemistry and mat growth. Large - metric, even decametric - evenly layered Precambrian cones and domes lack present-day analogues. What factors determined their size and morphology, and promoted alternation of mat and abiogenic precipitate?

Abiogenic precipitation and mat lithification are dependent on carbonate saturation state with respect to CaCO3 minerals. Two important processes promoting carbonate precipitation in mats are sulphate reduction and photosynthesis. These could respectively be largely responsible for clotted/peloidal (spongiostrome) and tubiform (porostromate) microfabrics, and their inceptions could reflect changes in seawater composition. Palaeoproterozoic development of clotted/peloidal fabrics might reflect increased SO42- level. Mesoproterozoic appearance of cyanobacterial sheath calcification could reflect development of CO2-concentrating mechanisms stimulated by decline in atmospheric CO2.

Stromatolite shape reflects original synoptic relief, determined by accretion rate relative to adjacent sediment. Low relative accretion rate results in low relief and complex shape; high relative accretion rate results in high relief and simple shape. In this view, mid-Proterozoic increase in morphotypic diversity, e.g., in branched stromatolites, is not a proxy for abundance. Instead it reflects lower synoptic relief due to reduced relative accretion rate. It could reflect reduced microbial growth and/or reduction in synsedimentary lithification.

Phanerozoic development of algal and metazoan reef builders inhibited microbial dome-column formation in favour of less regular reefal crusts, except during Mass Extinction aftermaths and in ecological refuges such as Shark Bay and Lee Stocking Island. Long-term overall decline in microbial carbonate abundance probably mainly reflects decline in seawater saturation state that slowed lithification and therefore accretion. Grazing may have been a subordinate factor so long as microbial mats were well lithified. Fluctuations in seawater saturation state are reflected by changes in microbial carbonate abundance and episodic development of dendrolite and thrombolite fabrics in the Cambrian-Early Ordovician, Late Devonian, and Permian-Triassic. Present-day high metazoan diversity and generally low saturation state result in scarcity of marine microbial carbonates. Diatoms transformed Cenozoic mat communities, significantly enhancing trapping ability, reflected in the formation of coarse-grained columns where stress inhibits metazoan overgrowth, as at Shark Bay and Lee Stocking Island. The coarse and crudely layered fabrics prominent in some of these columns are not closely comparable with most ancient stromatolites.

From this perspective, stromatolites emerge as abiogenic, biogenic, and combinations of the two. Their alterations in shape, size, fabric and abundance through time archive significant changes in atmospheric composition, seawater chemistry, mat evolution and biotic interaction. The key to ancient stromatolites lies in present-day examples. The key to present-day ones resides in the past.

Riding, R. 2008. CO2-concentrating mechanisms, harmful blooms, and Late Devonian reef extinction 375 million years ago. 11th International Coral Reef Symposium, Fort Lauderdale, Florida, USA, July 7-11, 2008. Abstracts: 8.

CO2-concentrating mechanisms, harmful blooms, and Late Devonian reef extinction 375 million years ago

Robert Riding
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA.
(riding@cardiff.ac.uk)

Late Devonian Mass Extinction caused the largest change experienced by reef biotas in the entire Phanerozoic. Stromatoporoid-coral communities that had dominated metazoan reefs since the mid-Ordovician disappeared. In the extinction's immediate aftermath they were replaced by microbial reefs. By the Early Mississippian rimmed shelves had given way to ramps dominated by carbonate mud mounds. The Mass Extinction selectively eliminated shallow marine organisms, including acritarch phytoplankton as well as reef biotas.

Here I suggest induction of CO2-concentrating mechanisms (CCMs) in marine phytoplankton as a factor in Late Devonian Mass Extinction. CCMs help maintain photosynthesis when levels of dissolved inorganic carbon are low. Late Devonian decline in atmospheric CO2 level was sufficient to induce CCMs in aquatic algae and cyanobacteria. This is likely to have had several geologically recognizable effects. Firstly, CCMs promote phytoplankton productivity and bloom conditions by helping to overcome carbon limitation. Productivity enhances organic matter burial, and harmful blooms kill shallow-water reefs and pelagic organisms. Secondly, conditions favouring photosynthetic groups with effective CCMs can promote community restructuring, resulting in extinction of some phytoplankton groups. Thirdly, cyanobacterial CCMs can stimulate calcification, both in the water column as 'whitings', and in benthic mats as in situ microbial carbonates.

The Late Devonian marine realm underwent marked changes in addition to Mass Extinction. Organic carbon-rich sediments, microbial carbonates, and carbonate mud mounds significantly increased in abundance. These, at first sight disparate, developments all have potential links to CCM induction in phytoplankton. Harmful blooms could have contributed to extinction of shallow marine metazoans, and increased phytoplankton productivity would have increased organic carbon-rich sediments. Phytoplankton community restructuring could have led to acritarch extinction. Combination of increased microbial productivity and enhanced cyanobacterial calcification would have promoted both microbial carbonate and carbonate mud mound development. These possibilities do not exclude the likely involvement of additional causative factors.

Riding, R. 2006. Microbial carbonates: processes and products in time and space. 17th International Sedimentological Congress, Fukuoka, Japan, August 27-1 September, 2006. Abstracts Volume A: 12.

Microbial carbonates: processes and products in time and space

Robert Riding
School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF14 0XH, Wales, UK
(riding@cardiff.ac.uk)

Microbial carbonates reflect the influences of microbial metabolism, cell surface properties, and extracellular polymeric substances (EPS) on mineral precipitation, in combination with grain stabilization at the sediment-water interface by mat-forming communities. The early lithification essential for the accretion and preservation of benthic microbial carbonates is biologically mediated but also environmentally dependent.

Significance of column-dome morphology Microbial carbonate morphotypes reflect synoptic relief during formation determined by accretion rate relative to adjacent sediment. Low relative accretion rate results in low relief and complex shape; high relative accretion rate results in high relief and simple shape. It follows that increase in morphotypic diversity, e.g., in Proterozoic branching stromatolites, reflects low synoptic relief due to low relative accretion rate and is not a measure of abundance.

Microfabrics 'Porostromate' (calcified filament) and 'spongiostromate' (clotted) microfabrics mainly appear to respectively reflect aerobic surface (e.g., photosynthetic) and anaerobic subsurface (e.g., sulphate reduction) calcification processes in benthic microbial mat communities. Photosynthetically induced calcification of cyanobacterial sheaths is enhanced by carbon-concentrating mechanisms (CCMs) and elevated saturation state of overlying waters. At or immediately below this level, bacterial degradation of residual cell material and EPS creates clotted and peloidal 'spongiostrome' microfabrics. Dolomite precipitation can also result from sulphate reduction. Coarse agglutinated fabrics may reflect enhanced trapping due to incorporation of algae into microbial mats.

Macrofabrics Macrofabrics reflect the degree of uniformity of components and their patterns of accretion or disturbance. The processes of calcification and agglutination that determine microfabrics also result in distinctive macrofabrics. Typically, stromatolites (laminated) have spongiostrome microfabrics, whereas thrombolites (clotted) and dendrolites (dendritic) have porostromate and related microfabrics. In coarse agglutinated stromatolites (e.g., Lee Stocking, Shark Bay) crude lamination results from coarse sediment supply, but also suggests irregular and uneven accretion.

Development in time and space Processes of mat accretion and calcification change through time in response to changes in (i) atmosphere-hydrosphere composition and their effects on microbial metabolism (e.g., sulphate reduction, photosynthetic calcification), and (ii) in microbial evolution. These changes are reflected by micro- and macrofabrics. Scarcity of micrite in Archaean stromatolites may reflect limitation of mat sulphate reduction by low seawater SO₄^2- levels, and inhibition of planktic whiting precipitation by elevated CO₂. Clotted and other micritic fabrics could reflect changes in these conditions during the Proterozoic, as SO₄^2- levels rose and CO₂ declined. Falling CO₂ and rising O₂, stimulating whiting production and inducing carbon-concentrating mechanisms (CCM) in cyanobacteria, would account for Mesoproterozoic increase in micrite and Neoproterozoic cyanobacterial sheath-calcification. Global cooling ~700-570 Myr ago would have favoured diffusive entry of CO₂ into cells, slowing CCM development, and lower levels of temperature and p_CO2 would have reduced seawater saturation state, hindering cyanobacterial calcification. As Earth emerged from 'Snowball' glaciations in the late Neoproterozoic, global warming and O₂ rise could have reactivated CCM development. At the same time, rising levels of temperature, calcium and p_CO2 are likely to have increased seawater saturation state. These changes would have stimulated widespread cyanobacterial sheath calcification in the Early Cambrian. This biocalcification event promoted rapid widespread development of calcified cyanobacterial reefs and transformed benthic microbial carbonate fabrics. Rise of algal and metazoan reef builders during the Phanerozoic inhibited microbial dome-column formation in favour of reefal crusts and irregular masses, except during Mass Extinction aftermaths and in ecological refuges such as Shark Bay. Long-term decline in overall abundance mainly reflects decline in seawater saturation state that slows lithification and therefore growth. Grazing may not have been a significant factor so long as microbial mats were well-lithified, but is likely to have increased as saturation state declined. Fluctuations in seawater saturation state are reflected in Phanerozoic changes in microbial carbonate abundance and episodic development of dendrolites and thrombolites in the Cambrian-Early Ordovician, Late Devonian, and Permo-Trias. Present-day coincidence of high metazoan diversity and low saturation state results in reefal microbial carbonates being scarcer than at almost any other time during the Phanerozoic. Diversification of diatoms has transformed some mat communities during the Cenozoic. Present-day examples of marine columnar microbial carbonates at Shark Bay and Lee Stocking Island have been depicted as ancient survivors in ecological refuges protected from metazoan competition. However, the coarsely agglutinated and crudely layered fabrics prominent in these columns are not closely comparable with the fabrics in most pre-Neogene microbial carbonates. These examples appear to be a relatively recent development of algal-cyanobacterial mats adapted to trapping coarse grains in current- and wave-swept environments.

Microbial carbonates are therefore sensitive archives of geobiological change over much of Earth history.

Riding, R. 2006. Atmospheric trigger for Early Carboniferous carbonate mud mounds? 17th International Sedimentological Congress, Fukuoka, Japan, August 27-1 September, 2006. Abstracts Volume A: 131.

Atmospheric trigger for Early Carboniferous carbonate mud mounds?

Robert Riding
School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF14 0XH, Wales, UK
(riding@cardiff.ac.uk)

Extensive development of carbonate mud mounds on shallow marine shelves in the Early Carboniferous, ~340 Myr ago, coincided with increases in calcified cyanobacteria and dasycladalean algae. These three, at first sight disparate, developments can tentatively be linked to changes in atmospheric CO₂ and O₂ levels that influenced photosynthetically induced calcification.

In extant algae and cyanobacteria, carbon-concentrating mechanisms pump CO₂ into cells to maintain photosynthesis. Carbon-concentrating mechanisms are triggered by low levels of atmospheric CO₂ and high levels of O₂. The need for carbon-concentrating mechanisms very likely arises from inefficiency of RUBISCO, the primary carbon fixing enzyme [1, 2]. RUBISCO's ability to bind CO₂ is limited because it can also bind O₂. When this occurs, oxygenase activity competitively inhibits carbon fixation, resulting in loss of CO₂ from the cell by photorespiration.

Carbon-concentrating mechanisms ameliorate this obstacle to photosynthesis by improving carbon uptake and concentrating CO₂ in the cell [2, 3]. Carbon-concentrating mechanisms import CO₂ and HCO3- into the cell. A side effect is extracellular increase in pH that favours sheath calcification in cyanobacteria [7]. A similar side effect can be inferred for dasycladalean calcification and for the precipitation of numerous small CaCO₃ crystals in the water column (whitings) adjacent to cyanobacterial picoplankton cells.

It has been speculated that, at some point following the appearance of RUBISCO in the Archaean, declining atmospheric CO₂ and increasing O₂ levels led photoautotrophs to develop carbon-concentrating mechanisms [3]. Adopting this approach, it has been proposed [4] that cyanobacterial carbon-concentrating mechanisms developed ~400-300 Myr ago in response to marked fall in CO₂ [5] and rise in O₂ [6].

The marine Palaeozoic records of calcified benthic cyanobacteria [8] and dasycladalean algae [9] show that both these groups increased in abundance and/or diversity in the Early Carboniferous. It is proposed here that these increases reflect enhanced calcification due to induction of carbon-concentratingmechanisms in these groups in response to atmospheric fall in CO₂ and increase in O₂. It is also proposed that, at the same time, cyanobacterial picoplankton also acquired carbon-concentrating mechanisms and that this led to whiting precipitation in nutrient-rich shelf seas where these plankton bloomed.

Extensive whiting precipitation would result in accumulation of large quantities of mud-grade carbonate on shelf sea-floors, facilitating mud mound formation. It is possible that the enhanced calcification of benthic cyanobacteria and dasycladaleans also contributed to mud mound deposition. Weaker calcification in dasycladaleans prior to the Carboniferous could account for their relatively poor record as calcified fossils in the Early and Mid-Palaeozoic.

Thus, the following sequence of geobiological events is suggested here to account for the widespread development of Early Carboniferous carbonate mud mounds: (i) Substantial decreases in atmospheric CO₂ levels and increases in O₂ levels occurred in the Early Carboniferous. (ii) These changes stimulated acquisition of carbon-concentrating mechanisms in cyanobacterial picoplankton that (iii) raised pH adjacent to the cells, promoting extensive precipitation of small CaCO₃ crystals in the water column in the vicinity of the cells. (iv) This whiting precipitation resulted in widespread and thick accumulations of carbonate mud on marine shelves.

At the same time, induction of carbon-concentrating mechanisms in benthic cyanobacteria and dasycladalean algae also stimulated calcification in these groups.

The processes responsible for the localization of the fine-grained whiting sediment into discrete mud mounds remain to be elucidated.

References

[1] Badger, M.R. (1987) The CO₂-concentrating mechanism in aquatic phototrophs. In: The Biochemistry of Plants: a Comprehensive Treatise, Vol. 10. Photosynthesis (Ed. by M.D. Hatch and N.K. Boardman), Academic Press, San Diego, 219-274.
[2] Kaplan, A., Badger, M. R. and Berry, J. A. (1980) Photosynthesis and the intracellular inorganic carbon-pool in the blue green algae Anabaena variabilis: response to external CO₂ concentration. Planta, 149, 219-226.
[3] Raven, J.A. (1997) Putting the C in phycology. European Journal of Phycology, 32, 319-333.
[4] Badger, M.R., Hanson, D. and Price, G.D. (2002) Evolution and diversity of CO₂ concentrating mechanisms in cyanobacteria. Functional Plant Biology, 29, 161 - 173.
[5] Berner, R.A. and Kothavala, Z. (2001) GEOCARB III: a revised model of atmospheric CO₂ over Phanerozoic time. American Journal of Science, 301, 182-204.
[6] Berner, R.A. (2001) Modeling atmospheric O₂ over Phanerozoic time. Geochimica et Cosmochimica Acta, 65, 685-694.
[7] Merz, M.U.E. (1992) The biology of carbonate precipitation by cyanobacteria. Facies, 26, 81-102.
[8] Arp, G., Reimer, A., and Reitner, J. (2001) Photosynthesis-induced biofilm calcification and calcium concentrations in Phanerozoic oceans. Science, 292, 1701-1704.
[9] Bucur, I. (1999) Stratigraphic significance of some skeletal algae (Dasycladales, Caulerpales) of the Phanerozoic. In: Depositional Episodes and Bioevents (Ed. by A. Farinacci and A.R. Lord), Palaeopelagos Special Publication, No. 2, 53-104.

Riding, R. 2005. Secular variations in abundance of calcified algae and cyanobacteria: how biomineralization can reflect global changes in temperature and water chemistry. System Earth-Biosphere Coupling, Erlangen, September 24-29, 2005. Schriftenreihe der Deutschen Gesellschaft für Geowissenschaften Heft 39: 312.

Secular variations in abundance of calcified algae and bacteria: how biomineralization can reflect global changes in temperature and water chemistry

Robert Riding
School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF14 0XH, Wales, UK
(riding@cardiff.ac.uk)

In calcareous algae and cyanobacteria, cellular site and mineralogy of calcification, together with biogeographic range, relate to the degree of control exerted by the organism over calcification. With decreasing control, site of calcification moves from intra- to extra-cellular, mineralogy shifts towards that of ambient abiotic carbonate precipitates, and environmental distribution becomes restricted to environments in which inorganic precipitation is favoured (warmer water in marine environments). Strong control (e.g., corallines) allows wide environmental distribution of biocalcification, is linked to intracellular site of CaCO₃ nucleation, and results in a polymorph not necessarily in equilibrium with the ambient environment. In contrast, weak control (e.g., cyanobacteria and green algae such as dasycladaleans and halimedaceans) limits the environmental distribution of biocalcification, is linked to extracellular site of CaCO₃ nucleation, and results in a polymorph in equilibrium with the ambient environment.

Coccolithophores, corallines, chlorophytes, and cyanobacteria exhibit decreasing degrees of control over calcification. The mineralogy and composition of calcification products towards the end of this series more and more approximate those of ambient abiotic carbonates. Simultaneously, the site of calcification becomes more external to the cell, and the environment of distribution becomes more restricted.

Through time, cyanobacteria and green algae that weakly control calcification respond sensitively to changes in sea-water chemistry affecting CaCO₃ precipitation. Periods when these groups were abundant and diverse, and show wide latitudinal ranges, correspond with times of elevated seawater saturation state, and also with high global temperature, which further increases saturation state for CaCO₃ minerals. They therefore behave as indicators of environmentally controlled precipitation. Groups such as corallines and coccolithophores that closely control calcification are less subject to these factors.

In common with all marine organisms, the geological history of calcified algae and bacteria will reflect evolutionary responses to changing ecological and environmental conditions. Here I suggest that it also reflect differences in biomineralization. Cyanobacteria and green algae such as dasycladaleans and halimedaceans control their calcification less than coralline red algae and coccolithophores. Consequently, calcified cyanobacteria and green algae have been more widespread and diverse at times when global temperatures and seawater saturation state have been higher. Calcified algae and bacteria therefore have great potential to reflect past fluctuations in environmental controls over carbonate precipitation.