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Eutrophic
Mesotrophic
Oligotrophic
Shallow upward
Shoal upward
Nutrient
productivity
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Rates
of carbonate sediment production respond to photosynthesis and this production
rate increases as the water becomes shallower . For this reason carbonate depositional
systems commonly fill towards sea level and so "shoal" or "shallow"
upward. The result is the depositional cycles of Goldhammer et, 1990), and
the "simple" carbonate sequence cycles of the platform carbonate
cycles of the Upper Miocene of Mallorca (Pomar, personal communication).
However working in the Murray Basin Lukasik & James, 2003 have established
that varying nutrient levels affected the carbonate fill of this basin.
As can be seen in the animated gifs and summary diagrams below the carbonate
depositional surface of the Murray Basin built towards sea level but did
not reach it. The carbonates responded not only to base level change but
also the varying nutrient levels in the basin. Thus during sea level "lows"
the shallower and more isolated the basin caused nutrient productivity to
rise (the setting became eutrophic) while carbonate accumulation slowed.
In contrast as the basin deepened during the onset of the following sea
level rise, the rate of nutrient productivity fell (the setting became oligotrophic)
while the rate of carbonate production increased. Eventually during the
greatest rate of base level rise the rate of carbonate production was reduced
by the increasing depth of water. Simultaneously as the Murray Basin increased
in size and deepened upward in response to this sea level rise with the
result that the increased fetch enabled storms to scour the sediment surface
while it was starved of higher rates of carbonate production. The effect
that Lukasik & James, 2003 describe is probably not uncommon in the
many deepening upward carbonate cycles of the geological record.
References
Cited
Goldhammer,
R. K., Dunn, P. A., and Hardie, L. A., 1990, Depositional
cycles, composite sea-level changes, cycle stacking patterns, and the
hierarchy of stratigraphic forcing: Examples from Alpine Triassic platform
carbonates: Geological Society of America Bulletin, v. 102, p.
535–562.
[Landmark paper on the use of the character of cycles to determine
their origins?].
Lukasik,
Jeff J., and Noel P. James, (2003), Deepening-Upward Subtidal
Cycles, Murray Basin, South Australia, Journal of Sedimentary
Research, Vol. 73, No. 5, P. 653–671
[Mix of ichnology & faunal diversity are used to determine deepening
upward cycles, depositional setting & correlate lithofacies and chronostratigraphic
surfaces.].
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