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MARINE OXYGENATION AND THE EARLY DEVELOPMENT OF PALEOZOIC REEFS

FRIDAY SEPTEMBER 13 – 11 AM

JEONG-HYUN LEE
DEPT. OF GEOLOGICAL SCIENCES, CHUNGNAM NATIONAL UNIVERSITY

ABSTRACT: Microbial carbonates were major components of early Paleozoic reefs until coral-stromatoporoid-bryozoan reefs appeared in the mid-Ordovician. We recognize six Cambrian-Ordovician reef intervals: I, Late Ediacaran (~550–541 Ma) microbial reefs with Cloudina, Namacalathus, and Namapoikia; II, Earliest Cambrian (Fortunian–mid-Age 2) microbial reefs with rare “Ladatheca”; III, Microbial-archaeocyath sponge reefs with radiocyaths, coralomorphs, etc. from mid-Age 2 to Age 4; IV, Microbial-lithistid sponge reefs in the mid-late Cambrian; V, Microbial-lithistid sponge reefs augmented by Calathium, pulchrilaminids, bryozoans, Lichenaria, Amsassia, and Cystostroma in the Early and early Middle Ordovician; VI, Skeletal-dominant reefs mainly constructed by stromatoporoids, corals, and bryozoans together with microbial carbonates in the late Darriwilian and Late Ordovician. This pattern indicates initial increase in skeletal reefs (Intervals I–III) followed by mid-late Cambrian decline (Interval IV) and then Ordovician recovery (Intervals V–VI). We suggest that this largely reflects changes in marine oxygenation mediated by global temperature fluctuations. During Intervals I–III, oxygen increase contributed to diversification of marine animal life. This was followed during Interval IV by a prolonged period of ‘greenhouse’ conditions. As sea-level rose and CO2 increased, the mid–late Cambrian was unusually warm. These elevated temperatures lowered oxygen solubility, promoting widespread thermal stratification that resulted in a tendency to marine dysoxia and hypoxia. Greenhouse conditions also stimulated carbonate platform development, locally further limiting shallow-water circulation. Reduced oxygenation has been linked to episodic extinctions of phytoplankton, trilobites and other metazoans during the mid–late Cambrian. We propose that this tendency to dysoxia-hypoxia in shallow marine environments also limited many metazoan reef-builders, resulting in the rise of a distinctive microbial-lithistid sponge consortium well-adapted to low oxygen conditions that globally dominates mid-late Cambrian reefs. These conditions ameliorated during the Ordovician as CO2 decline lowered temperatures and promoted ocean ventilation. Thus, the prolonged time gap occupied by low diversity reefs between the ‘Cambrian Explosion’ and the ‘Great Ordovician Biodiversification Event (GOBE)’ reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments. This reef perspective could be broadly applicable to marine biotas in general, providing an overarching explanation for the prolonged mid-late Cambrian pause between the Cambrian Explosion and the GOBE.

BIO: Jeong-Hyun Lee is Associate Professor at the Department of Geological Sciences, ​Chungnam National University (South Korea), holds a PhD in Sedimentology from the Seoul National University and has a research interest in the paleoecology of the Cambrian and Ordovician, and the beginning of the Phanerozoic. Jeong-Hyun Lee's work focuses on carbonate sedimentology and invertebrate fossils such as sponges and calcified microbes as well as on chemostratigraphy and provenance of siliciclastic sediments. Understanding how organisms evolved along with changes in environmental condition is of primary interest.
www.cnu.ac.kr/~jeonghyunlee/