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Reefs leave distinctive sedimentary deposits in the geological record that commonly contain high primary porosity and, therefore, can serve as important reservoirs for petroleum.  Consequently, controls on reef development are of interest not only to biologists and paleontologists but also to carbonate sedimentologists and industry geologists.  The re-establishment of reefs following the end-Permian mass extinction (252 Mya) provides an ideal opportunity to study the controls on reef formation from a sedimentary and biological perspective.

Metazoan and algal reefs are absent from the margins of Lower Triassic carbonate platforms.  When reefs reappeared in the Middle Triassic, they contained potentially reef-building organisms such as scleractinian corals, calcareous sponges, and calcareous algae. However, the precise timing of reappearance of these groups is poorly constrained.  Moreover, quantitative analyses suggest these organisms were not volumetrically important within the reef framework.  Instead, organisms of unknown biological affinities, particularly Tubiphytes, and large volumes of marine cements contribute most to the structure and volume of Middle Triassic reefs.  Thus, the recovery reef-building metazoans and algae may be more a reflection of the return of reefs than a cause.  The key controls may instead include, for example, platform geometry or carbonate chemistry of seawater (including inhibitors of carbonate precipitation).  Assessing the controls on reef recovery has been a challenge, however, because the best-studied Middle Triassic reefs do not date to the earliest part of the Middle Triassic and pervasive dolomitization has further limited detailed analyses of these reefs in situ. 

In this project, we have assessed three aspects of reef recovery: pattern, timing, and environmental context.

During the fall of 2008 and spring of 2009, we analyzed samples from two stratigraphic sections sampled during the summer of 2008 for carbon isotope composition.  Combined with data collected previously during the PI’s PhD research, these data indicate the existence of a large carbon isotope gradient (~4‰) between shallow and deep water settings during the Early Triassic.  This gradient does not occur during Middle Triassic time, when both platform interior and basin margin strata exhibit δ¹³C values near 2‰.  These findings are best interpreted to indicate the existence of a large vertical gradient in the carbon isotope composition of dissolved inorganic carbon in the Early Triassic water column.  Such a large gradient has been predicted by Earth systems models investigating the causes of Early Triassic marine anoxia and euxinia, but has not been previously observed in rocks.  The existence of the large carbon isotope gradient from shallow to deep water indicates high rates of primary productivity in the Early Triassic water column, likely associated with an expanded oxygen minimum zone at a few hundred meters depth.  Thus, these results suggest reef recovery may have been delayed during Early Triassic time by the frequent upwelling of anoxic waters from an expanded oxygen minimum zone onto shallow platform-margin settings. 

We have also acquired carbon isotope data from early marine cements in hand samples of reef material from the northern margin of the Great Bank of Guizhou near the town of Bianyang.  We have found that the stratigraphically lowest part of the reef exhibits heavy δ¹³C values relative to younger strata.  This finding indicates that the oldest part of the reef is of earliest Anisian age (Aegean and Bithynian).  Moreover, spatial geostatical analysis (kriging) of the carbon isotope data indicate that the reef was deposited along a slope, prograding and aggrading through Anisian time with one possible episode of step-back of the platform margin.  These observations suggest that a gradient observed in the number of co-occurring framework-building organisms from the lowest and most proximal area of the reef to the stratigaphically highest and most-basinward part of the reef reflects a temporal trend of biotic recovery.  These observations are further supported by lineations observed in a satellite image which may reflect bedding planes in platform-margin clinoforms.  The timing of reef initiation appears to coincide with the collapse of the carbon isotope gradient, presumably reflecting a decrease in the extent of chemical stratification within the water column and associated anoxia.

Financial support from the Petroleum Research Fund has had a substantial positive impact on the research in my lab.  It has allowed PhD student Brian Kelley who participated in fieldwork during 2007 to continue his dissertation research and build a large dataset that will soon be ready for publication.  It also provided the opportunity for post-doctoral fellow Katja Meyer to conduct a new test of the geochemical structure of the Early Triassic marine water column.  Moreover, the large sample suites collected during the first two years of the grant and the field localities identified present the opportunity to continue research on these samples and at these localities far beyond the scope of the currently funded research project.