『Abstract
Describing, characterizing and interpreting the nearly infinite
variety of carbonate rocks are conundrums - intricate and difficult
problems having only conjectural answers - that have occupied
geologists for more than two centuries. Depositional features
including components, rock textures, lithofacies, platform types
and architecture, all vary in space and time, as do the results
of diagenetic processes on those primary features. Approaches
to the study of carbonate rocks have become progressively more
analytical. One focus has evolved from efforts to build reference
models for specific Phanerozoic windows to scrutinize the effect
of climate and long-term oscillations of the ocean-atmosphere
system in influencing the mineralogy of carbonate components.
This paper adds to the ongoing lively debates by attempting to
under stand changes in the predominant types of carbonate-producing
organisms during the Mesozoic-Cenozoic, while striving to minimize
the uniformitarian bias. Our approach integrates estimates of
changes in Ca2+ concentration in seawater and atmospheric
CO2, with biological evolution and ecological
requirements of characteristic carbonate-producing marine communities.
The underlying rationale for our approach is the fact that CO2 is basic to both carbonates and organic matter,
and that photosynthesis is a fundamental biological process responsible
for both primary production of organic matter and providing chemical
environments that promote calcification. Gross photosynthesis
and hypercalcification are dependent largely upon sunlight, while
net primary production and, e.g., subsequent burial of organic
matter typically requires sources of new nutrients (N, P and trace
elements). Our approach plausibly explains the changing character
of carbonate production as an evolving response to changing environmental
conditions driven by the geotectonic cycle, while identifying
uncertainties that deserve further research.
With metazoan consumer diversity reduced by the end-Permian extinctions,
excess photosynthesis by phytoplankton and microbial assemblages
in surface waters, induced by moderately high CO2
and temperature during the Early Mesozoic, supported proliferation
of non-tissular metazoans (e.g., sponges) and heterotrophic bacteria
at the sea floor. Metabolic activity by those microbes, especially
sulfate reduction, resulted in abundant biologically-induced geochemical
carbonate precipitation on and within the sea floor. For example,
with the opening of Tethyan seaways during the Triassic, massive
sponge/microbe boundstones (the benthic automicrite factory) formed
steep, massive and thick progradational slopes and, locally, mud-mounds.
As tectonic processes created shallow epicontinental seas, photosynthesis
drove lime-mud precipitation in the illuminated zone of the water
column. The resulting neritic lime-mud component of the shallow-water
carbonate factory became predominant during the Jurassic, paralleling
the increase in atmospheric pCO2, while the
decreasing importance of the benthic automicrite factory parallels
the diversification of calcifying metazoans, phytoplankton and
zooplankton.
With atmospheric pCO2 declining through
the Cretaceous, the potential habitats for neritic lime-mud precipitation
declined. At the same time, peak oceanic Ca2+ concentrations
promoted biotically-controlled calcification by the skeletal factory.
With changes produced by extinctions and turnovers at the Cretaceous-Tertiary
boundary, adaptations to decreasing Ca2+ and pCO2, coupled with increasing global temperature
gradients (i.e., high-latitude and deep-water cooling), and strategies
that efficiently linked photosynthesis and calcification, promoted
successive changes of the dominant skeletal factory through the
Cenozoic: larger benthic foraminifers (protist-protist symbiosis)
during the Paleogene, red algae during the Miocene and modern
coral reefs (metazoan-protist symbiosis) since Late Miocene.
Keywords: carbonate factories; atmospheric CO2;
dissolved Ca; photosynthesis
1. Introduction
1.1. Goals of this paper
2. Biogenic carbonate factories
2.1. Microbially induced calcification mechanisms
2.2. Photosynthesis and calcification
2.3. The neritic lime-mud factory
2.4. The microbial loop
2.5. Algal symbiosis, mixotrophy and hypercalcification
2.6. The benthic automicrite factory
3. Carbonate factories through the Mesozoic and Cenozoic
3.1. Triassic carbonate platforms
3.1.1. Predominance of the benthic automicrite factory
3.2. Jurassic platforms
3.2.1. The re-emerging shallow-water carbonate factory: oolites
and neritic lime muds
3.3. Early to Middle Cretaceous
3.3.1. The shift to predominance of the skeletal factory
3.4. Late Cretaceous
3.4.1. The skeletal factory predominates
3.5. Paleogene
3.5.1. Algal symbiosis-driven skeletal factories
3.6. Neogene
3.6.1. Shallowing of the skeletal carbonate factory
4. Discussion
5. Conclusions
Acknowledgements
References