GEOL2301: Palaeoecosystems

Identifying ancient reefs

What is a reef? It might be easy to know when you're swimming above one, but identifying a reef in the fossil record is a little more tricky: not least because the inside doesn't look quite like the outside.

Challenging the conception that a reef is purely a stack of corals growing ever upwards towards the light, coring studies have shown that the 'inside' of a reef is a jumble of broken fragments and cavities filled with rubble and mud. A 'framework' of skeletons is not always easy to recognize.

Reefs are primarily constituted of the skeletons of constructors, bound by binder organisms. Bafflers reduce water speed close to the sediment surface, thus trapping fine particles, including those removed or excreted by destroyers. Dwellers take advantage of the many open cavities within a reef framework – which may, on incorporation into the rock record, later house oil and gas deposits: reef rock has an excellent reservoir potential.

• Suggest Palaeozoic organisms that may have occupied each ecological guild.
• For a hint, click this text to reveal one possible example for each class. You still have to match each example to the correct guild though!

Constructor	Nautiloid
Binder	Trilobite
Baffler	Fenestrate bryozoan
Destroyer	Encrusting bryozoan
Dweller	Tabulate coral

• • Important Palaeozoic constructors are tabluate and rugose corals, and solenoporoid sponges.
  • Encrusting bryozoans and calcifying red algae are dominant Palaeozoic binders
  • Anything erect, protruding into the water column, might be a baffler: fenetrate bryozoans and crinoids would be good examples.
  • Destroyers were less abundant before the Mesozoic Marine Revolution: boring organisms did exist, but shell crushing was rare. Nautiloids are the most likely candidate I can think of.
  • Lots of things might have dwelt in reefs: fish, trilobites, gastropods, potentially some vagrant bivalves…
• Why are there no reefs in the present-day Mediterranean?
• It may help to look up geographical properties of Mediterranean cities such as Cairo or Athens.
• Despite warm temperatures through the summer, the air temperature falls slightly below 15° C in winter across most of the Med. If the sea water retained some of its summer warmth through the winter, reefs might perhaps scrape by: but the high latitude (just over 30° N) means that there's not enough winter sunlight to power zooxanthellae.
  As a secondary consideration, much of the north and east basin is also clouded by riverine input – less so the south of the basin, into which the dry Sahara drains. And a slightly elevated salinity, particularly in the evaporative summer months, represents a further headwind.

Reefs are a prominent example of biogeology: the capacity of life to account for first-order patterns in the lithosphere.

• How might the rock record differ in periods of geological time when large-scale reefs were present or absent?

• The primary impact here is the incorporation of prodigous quantities of calcium carbonate into the rock record, filling sedimentary basins at their margins (and potentially accellerating their deepening and development).

  A concomitant impact is the reduction of atmospheric CO₂ levels, which will be picked up in geochemical tracers and through its impact on biological and geological processes.

Here's a video that testifies to the capacity of metazoans to process and repackage large volumes of sediment.

Has there ever been a more powerful #echinoderm fart witnessed by mankind? Is this the greatest invertefart of all time? PS: I was behind the camera, but due to supreme scuba skills and impeccable nerves I escaped the intestinal tsunami in one piece. @echinoblog check it out. pic.twitter.com/KpOexOksBo

— Klaus Stiefel (@Pacificklaus) February 11, 2021

Mutualistic living

The presence of photosymbiotic zooxanthellae is what allows extant reef-building animals to maintain their phenomenal growth rates and produce reef rock in step with sea level change.

Zooxanthellae are dinoflagellates: single-celled red algae better known among palaeontologists for their rich palynological (i.e. microfossil) record. Although dinoflagellates and their hosts can survive independently, both prosper when enjoying a mutualistic relationship: the algae process their hosts’ waste nutrients and carbonate ions to produce sugars (i.e. energy) and oxygen.

The productivity boost is so pronounced that a previous scheme classified corals as hermatypic (zooxanthellate and constructional, i.e. reef-building) or ahermatypic (non-zooxanthellate and non-constructional). This represents a slight oversimplification: there exist zooxanthellate, non-constructional corals, and non-zooxanthellate constructional corals; and in the fossil record, the presence of zooxanthellae is necessarily conjectural. The strength of the association is nevertheless instructive; it is diffcult to manufacture enough carbonate to form a reef without assistance from photosynthesis.

• How does the zooxanthella symbiosis explain carbonate reefs' requirements for clear waters?
• Turbitidy, perhaps arising from phytoplankton where nutrients are sufficient to support blooms, blocks sunlight from penetrating to the depth of the reef surface, limiting photosynthesis.

Increased nutrient levels (right) allow free-living algae to prosper at corals' expense. Source

Rises and demises of reefs

Reefs as we recognize them today could not exist until constructional and binding organisms evolved, and so did not exist until the Cambrian 'explosion' of marine biodiversity.

Even then, coral-dominated reefs bound by coralline red algae and bryozoans did not arise until the Ordovician, and have experienced a number of hiatuses, interrupted by reefs with very different framework organisms (rudist bivalves in the Cretaceous; none, or bacterial, in the Carboniferous/Permian) and periods in the Triassic and Jurassic (romantically termed 'eclipses') where no substantial reefs seem to have existed at all.

• What are the framework and binding elements of a stromatolite reef?
• Cyanobacteria bind together mud particles, which might be considered to provide the (abiotic) framework.
• Justify the classification of a stromatolite as a reef.
• A stromatolite is a thickened mass of typically limestone mud that is distinct from the surrounding strata, thus satisfying Dunham's weak stratigraphic definition of a reef. But with the framework not comprising calcified organisms, and the scarcity of cavities, it is a bit of a squeeze to fit a stromatolite into the ecological definition. But might the same be true of Carboniferous/Permian "reefs"?

First-order controls on reefs through time. Source: Hubbard et al. in Stanley 2001

• To what extent should (i) Ordovician→Devonian; (ii) Late Triassic; and (iii) Tertiary reefs be grouped into a single category?

• Common ground: predominantly coral (i.e. anthozoan) framework; bound by calcifying red algae and bryozoans. Functionally and mechanically there seems to be a strong correspondence: it would take a time-traveller some careful inspection to work out whether they were snorkelling over a Palaeozoic, Mesozoic or Cenozoic reef.

  Differences: Scleractinians are aragonitic, rather than calcitic (as rugose & tabulate corals); they re-invented mineralization (convergent evolution!) so cannot be seen as a continuation of the Palaeozoic tradition. Stromatoporoids (filter feeders) and calcareous green algae (photosynthesizers) are obviously not direct equivalents or 'replacements'.
  There is a stronger argument that Mesozoic and Cenozoic reefs are continuous, owing to their common taxonomic constitution: but where did these taxa reside during the Jurassic eclipse and the domanince of rudists during the Cretaceous? Were there holdouts that didn't get preserved for some reason, perhaps around remote island arcs? Or did the key taxa find non-reef roles and only "get back together" after the K-Pg mass extinction cleared the rudists from the scene?

Extension: Anatomy of a reef system

Explore the anatomy of a reef with these external resources and the accompanying video (embeded here).

Describe the characteristics you would expect to see of sedimentary rocks formed in the:

• Back reef zone:
• The back reef zone comprises a calm, shallow lagoon. Erosive forces will be tembered by the shelter afforded by the reef. There may be a high component of relatively intact skeletal material, perhaps accompanied by carbonate grains (peloids/ooids/rock fragments), or siliciclastics from a neighbouring continent. Burrowing is likely to be intense.
• Reef crest / flat:
• The majority of the sediment will comprise disarticulated elements of reef organisms, though these may well be highly broken down by attrition (storms?) and ground down finer by bioeroders (sea cucumbers?). Expect large voids, partially filled by rubble and muds.
• Reef front → Forereef:
• This is a steep slope, so sediments may be transient, and will be dominated by talus material from the reef – perhaps including large blocks as well as smaller storm-generated debris.

It's not all hot

Reefs may secrete carbonate apace, but carbonates also form in cooler waters, and in settings that are not nutrient-starved. Cool-water carbonates are now recognized as an important ecological and sedimentary setting in modern oceans, though a longstanding resistance to the concept has not been entirely purged from the literature; many settings traditionally interpreted as warm-water reefs await reevaluation through a cool-water carbonate lens.

Cool-water carbonates are dominated by the heterozoan assemblage.

• How does the taxonomic composition of this assemblage differ from the photozoan assemblage in modern oceans?
• Abundance of benthic forams, molluscs, bryozoans and echinoderms. Absence or scarcity of calcifying green algae. (Coralline reds are more environmentally tolerant and may be locally abundant.) Barnacles are not always present, but are characteristic of cool-water carbonates.
• To what extent can these taxonomic trends be extrapolated to deep time?

• Insofar as this is an ecological distinction, we might expect to substitute extant heterotrophs for extinct analogues: e.g. sea fans for crinoids?

  We know that there are some differences. Can we assume that the substitution of scleractinians by Palaeozoic corals represents a direct swap? Possibly. Do modern benthic forams and bryozoans have the same ecology as their Mesozoic/Palaeozoic forebears? We might hope so, but we'd be speculating. It may be riskier still to interpret stromatoporoids as an exclusively warm-water taxon, simply because they contribute to the framework of Palaeozoic reefs.

Anatomy of a cool-water carbonate system

Cool-water carbonates exhibit a more continuous relationship between distance-from-shore and water depth than is observed in a reef system. (Why?)

• Because the growth of a reef creates topography that elevates the natural fore-reef gradient whist protecting the back-reef setting from the erosive power of storms and tides.
• How can water depth be predicted from the composition of carbonate sediment deposited in a cool-water environment?

• Shallow water: water energy grinds down carbonate material to dust, except in sheltered microtidal environments (e.g. the Mediterranean).

  Below fair weather wave base, shelly bottom dwellers thrive; if storms are infrequent enough, they may be preserved reasonably intact.

  Below storm wave base, there may be fewer land-derived nutrients to sustain suspension feeding organisms; calcifying photosynthesizers begin to proliferate. These will have increasingly delicate forms as water depth (and thus interval between storms) increases.

  Below the photic zone we see a lower abundance of life dominated by suspension feeders (sediment is being transported down slope and raining from the water column) and characteristically bryozoans, also known as "moss animals". These will be very delicate forms, to maximize the amount of water they can process – no need to worry about storm damage this deep. Like moss on bare rock, bryozoans can trap fine particles, which will be integrated into the sedimentary record.