Reports: ND856757-ND8: What Controls Ooid Grain Size?

Woodward Fischer, PhD, Caltech

Ooids, concentrically coated sand-sized grains composed of CaCO3 phases, represent a mode of carbonate sedimentation found ubiquitously in carbonate successions across Earth history. Ooids have major historical significance as records of climate change and ocean composition and economic significance as a key reservoir lithofacies, yet a lack of quantitative constraints on the mechanisms and pace of ooid genesis has to date limited their utility as paleoenvironmental proxies. Additionally, substantial debate persists concerning the roles of physical, chemical, and microbial processes in their growth, including whether carbonate precipitation on ooid surfaces is driven by seawater chemistry or microbial activity, and what role—if any—sediment transport and abrasion play. To test these ideas, we developed an approach to study ooids in the laboratory.

We designed two sets of experiments to test the ooid evolution models by isolating effects of bed shear velocity (u) and initial grain size (D) on ooid net growth. The first experiment set consisted of 18 experiments, each with a different bed shear stress (u), with all other parameters held constant including initial grain size (D), for two types of natural ooids. The second experiment set consisted of 3 pairs of experiments, each pair with two different initial grain sizes (D), but all other parameters held constant including u. Both sets of experiments employed the same experimental apparatus and used the same seawater carbonate chemistry typical of tropical seawater. For starting materials, we selected two types of natural ooids that represent classic end-members in terms of morphology and environment. And then we used a setup of wet abrasion mills similar to those used to study bedrock erosion processes at Caltech. For each experiment, all variables were held constant and ooid size change was measured after 5 to 17 days.

Ooid populations were characterized before and after each experiment using transmitted and reflected light microscopy, scanning electron microscopy, and grain size via dynamic image analysis using a Particle Size Analyzer measuring a minimum of > 100,000 grains. We estimated volumetric rates of ooid size change by calculating the difference in mean grain volume per unit time, using mean major, intermediate, and minor axis grain dimensions determined from Particle Size Analyzer data.

These experiments produced ooid abrasion and precipitation rates four orders of magnitude faster than radiocarbon net growth rates for natural ooids. These results are important because it carries the corollary expectation that ooids approach a stable size representing a dynamic equilibrium between precipitation and abrasion. These experiments also demonstrated that the mode of sediment transport (e.g., bedload versus suspended load) is significant due to tradeoffs in grain size versus abrasion rate as a function of transport stage: abrasion rates for a given grain size decrease with increasing current energy because the increase in transport by suspension results in fewer grain impacts. We further predicted that there may be no distinct ooid factory, arguing instead that ooids may experience alternating episodes of net abrasion and net precipitation depending on the balance between fluid carbonate chemistry and sediment transport regime, which are expected to vary in both time and space. Ultimately, this new dynamic equilibrium ooid size hypothesis has implications for interpreting ancient oolites in important ways that deviate from ooid factory hypotheses: 1) in combination with other sedimentological observations, ooid size can be used as a quantitative proxy for carbonate saturation state in paleo-seawater, and 2) the mechanism and rate of ooid growth is uniquely set by carbonate chemistry and sediment transport. Notably, this dynamic equilibrium model does not require independent constraints on individual ooid lifetimes or net growth rates; consequently, ooid size provides a direct record of physical and chemical conditions of Earth surface environments, such that sedimentological observations of ooid grainstones can be used to quantify the carbonate chemistry of ancient seawater. The first paper describing this work was published earlier this year in EPSL (Trower et al. 2017), a second manuscript is currently in revision.