The Influence of Climate Variability on the Content, Composition, Pattern of Organic-Carbon Enrichment in Sediments

Abstract

Predictive models of organic carbon (OC) enrichment in sediments are based on process-product approach, whereby the accumulation of OC-rich sediments is correlated with three mechanistic processes: organic productivity, organic oxidation, and organic matter burial rate. These processes are individually complex, highly interlinked, and strongly dependent on certain underlying factors such as water circulation, tectonic setting, and climate condition. Our aim is to generate new insights into the basic processes that drive enhanced OC burial and advance our understanding of how these processes depend on the underlying factors, particularly climate variability. The OC-rich Whitehill Formation in the Karoo Basin of South Africa exhibits a strong imprint of the effects of the complex coupling between the mechanisms of OC enrichment and allows us to illustrate the manner in which climate variability triggers the mechanisms. Our study involves sedimentologic descriptions, organic and inorganic geochemical and isotopic proxies, and high-resolution petrographic observations. The empirical models generated from these data sets are tested against definitions of processes using multivariate statistical methods. Three-fold increases and decreases in TOC values are documented in the shales, corresponding to distinctive shifts in climate from cool to warm conditions and associated rises and fall in sea level, respectively. A strong positive covariation exits between TOC values, carbon chemistry, and the inorganic (Al, P, Ti) contents, reflecting productivity-controlled mechanisms. A less positive correlation exists between TOC and sedimentological and geochemical proxies for redox condition, indicating that the basin was generally depleted in oxygen but only intermittently anoxic. An irregular relationship exists between TOC and laminae thickness and size of pelagic snow, which represent average sediment flux rate. Data from the three closely associated intervals of OC enrichment indicate that the highest TOC at the peak of the cool-warm climatic switch is associated with maximum highstand intervals through a number of mechanisms such as influx of nutrient-rich seawater, trapping of terrigenous clastic materials, salinity to thermally stratified conditions, and remineralization of nutrients from sedimented OM. These processes created eutrophic conditions favorable for higher organic productivity and preservation, with climate variability acting as the master trigger.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018