Microbial Carbonate Deposition on Carbonate Platform Slopes can be Favored by Oxygenated Conditions

Session: New Voices in Geobiology

Presenting Author:

Evan G. Ritchey

Authors:

Ritchey, Evan G.¹, Li, Xiaowei², Garber, Joshua Michael³, Lau, Kimberly⁴, ALTINER, Demir⁵, Lehrmann, Daniel James⁶, Yu, Meiyi⁷, Minzoni, Marcello⁸, Schoonover, Erik Jeffery⁹, Sherman, Clark E.¹⁰, Kelley, Brian M.¹¹

(1) Pennsylvania State University, State College, PA, USA, (2) Guizhou University, Guiyang, Guizhou, China, (3) University of St Andrews, St Andrews, Scotland, United Kingdom, (4) Penn State University, University Park, PA, USA, (5) METU (ODTU), Geological Engineering, Ankara, Turkey, (6) Trinity University, San Antonio, TX, USA, (7) Guizhou University, Guiyang, Guizhou 550025, China, (8) University of Alabama, Geological Sciences, Tuscaloosa, AL, USA, (9) Penn State, University Park, PA, USA, (10) University of Puerto Rico at Mayaguez, Department of Marine Sciences, Mayaguez, PR, Puerto Rico, (11) Penn State University, University Park, PA, USA,

Abstract:

Carbonate platform slopes composed of in situ automicrite (fine micrite precipitated by microbial processes or organomineralization) are commonly interpreted to be favored by anoxic settings that limit benthic metazoans. However, automicrite occurs extensively in shallow reef carbonates throughout the Phanerozoic Eon, suggesting that the environmental controls on its occurrence remain poorly constrained. The end-Permian mass extinction is a key interval for studying these controls. Oceans following the extinction are characterized by expanded anoxia and depleted metazoan benthic ecosystems.

The Great Bank of Guizhou (GBG) is an isolated carbonate platform in southern China containing relatively continuous strata from the uppermost Permian to the Upper Triassic. At the Xiliang margin of the GBG, a syncline creates a cross-sectional exposure of the platform, allowing us to sample across paleoenvironmental gradients from slope to basin. Samples were collected along three stratigraphic sections spanning the latest Permian through Middle Triassic and ranging in water depth from approximately 30 to 550 meters. To evaluate the relationship between slope oxygenation and the formation of in situ microbial and skeletal carbonate, we combined petrographic point count data with rare earth element + yttrium (REY) analyses and the cerium (Ce) anomaly proxy on the same stratigraphic samples.

Our results demonstrate that local slope anoxia initiated in the latest Permian and continued through the Early Triassic. By the start of the Middle Triassic, the GBG slope and basin were episodically oxygenated, with lasting oxygenation occurring later in the epoch. Automicrite was absent on the GBG during persistent anoxia but occurred with the onset of episodic oxygenation near the Early to Middle Triassic transition, though it contributed little to the overall rock fabric. It was not until persistent oxygenation that the volumetric abundance, size, and morphological complexity of automicrite increased on the slope. This increased accumulation of automicrite coincided with a parallel rise in the diversity, size, and abundance of skeletal metazoans. Our findings suggest that the poorly oxygenated, metazoan-depleted ecosystems of the Early Triassic slope did not promote microbial carbonate deposition. These results challenge the view that anoxia favors automicrite deposition in slope settings, as well as the broader view that microbial carbonate deposition reflects conditions inhospitable to metazoan life.