Reports: DNI850214-DNI8: Geochemical and Petrographic Investigation of a Novel Calcite-Aragonite Sea Transition in Terminal Proterozoic Time (549 - 548 Ma)

Justin B. Ries, PhD, University of North Carolina (Chapel Hill)

1. Introduction

The objective of this research is to investigate a potential calcite-to-aragonite sea transition recorded in a well preserved terminal Proterozic sequence of marine limestones in Namibia (the ‘Nama Group’).  This putative transition in ocean state was originally identified by the PI by a threshold increase in Sr/Ca ratios within the carbonate sequence, consistent with a change in carbonate polymorph mineralogy from calcite-to-aragonite ca. 549 Ma.  The major challenge to identifying ancient trends in original CaCO3 polymorph mineralogy arises from the conversion of metastable aragonite to low-Mg calcite over 10^5 – 10^6 yr timescales. Thus, original mineralogy must be inferred from secondary indices, such as elemental and/or isotopic geochemistry, petrographic texture, and relic primary crystals.

Research into this problem was focused on three key areas: (1) analyzing the elemental data obtained from the Nama Group samples; (2) creating and analyzing thin-sections of the Nama Group samples in order to identify temporal trends in carbonate polymorph mineralogy; and (3) investigating the boron isotope system (δ11B) as a proxy of seawater pH throughout this apparent terminal Proterozoic calcite-aragonite transition.

2. Research Activity

2.1. Elemental geochemistry

The concentrations of fifty-four elements have been measured in 109 of the Nama Group samples. Elements are partitioned differently into the calcite and aragonite polymorphs of CaCO3. In many cases, these differences in elemental partitioning persist throughout diagenesis, thus serving as a potential indicator of original mineralogy. For instance, Sr/Ca ratios tend to be higher in carbonates that were originally deposited as aragonite and Mg/Ca ratios tend to be higher in carbonates that were originally calcite. Critically, the Nama Group carbonates exhibit a threshold increase in Sr/Ca ratios and a threshold decrease in Mg/Ca ratios at around 549 Ma, consistent with a calcite-to-aragonite sea transition at that time.

2.2. Thin-sections

Thin-sections were made of the 140 Nama Group samples spanning 10 m.y. of terminal Proterozoic time.  Initial analysis of Nama Group thin sections reveals a transition from originally  calcitic petrographic textures (i.e., finer grained) to originally aragonitic textures (recrystallized as calcite, i.e., coarser grained) between 550 and 548 Ma, consistent with the coeval rise in Sr/Ca ratios and fall in Mg/Ca ratios, indicating a transition from calcite-to-aragonite deposition.

2.3. Investigating the boron isotope system (δ11B) as a proxy of seawater pH

Although calcite-aragonite sea transitions are thought to be predominantly driven by seawater Mg/Ca ratios, seawater pH also plays a role in controlling the dominant polymorph mineralogy of carbonates precipitated from seawater.  To this end, the PI has established a collaboration with a boron isotope specialist (Dr. Paolo Montagna), in order to reconstruct seawater pH from the δ11B composition of the Nama Group carbonates.  Initial analysis of the δ11B composition of the Nama Group carbonates suggests that ocean pH ranged from 7.6 to 8.6 over this 9 m.y. interval.  Furthermore, seawater pH consistently oscillated by 0.6 to 1.0 units over relatively short timescales (200 k.y. to 1 m.y.) between 552 to 545 Ma.  These relatively high frequency oscillations in seawater pH are unlikely to have had much bearing on the apparently singular shift from calcite-aragonite seas between 550 and 548 Ma.  However, of potential import is the relative stabilization of ocean pH between 545 and 543 Ma—the 2 m.y. immediately preceding the Cambrian Radiation of animal life.  This was an unexpected finding, yet one that could have major implications for our understanding of this critical interval of Earth history.

3. Future Work (2012-2013)

The PI has received a no-cost extension to continue this research project over the coming year.  Over this time, he will focus on two key areas: (1) performing high-spatial resolution in situ measurement elemental ratios within the Namacalathus and Cloudina fossils present throughout the Nama Group carbonates and (2) investigating additional unconventional isotope systems as proxies of ocean chemistry across this apparent terminal Proterozoic calcite-aragonite transition.

3.1. Measurement of Mg/Ca ratios within the skeletons of Namacalathus and Cloundina fossils

The PI will use either laser ablation ICP-MS or SEM-hosted microprobe to measure elemental ratios within the shells of the Namacalathus and Cloudina fossils. These measurements should confirm the original mineralogy of these early marine calcifiers (calcitic, according to prior reports). Furthermore, since Mg/Ca ratios of calcitic marine organisms are known to reflect ambient seawater Mg/Ca ratios, changes in the Mg/Ca ratios of the Namacalathus and Cloudina fossils throughout the 10 m.y. sequence should monitor seawater Mg/Ca—the putative driver of calcite-aragonite sea transitions—across this critical interval of geologic time.

3.2. Exploration of unconventional isotope systems as proxies of ocean chemistry throughout this apparent calcite-aragonite transition.

The PI has recently established extramural collaborations to analyze the Ca, Sr, and Mg isotope systems, as well as the CO2 isotopologue system, within the Nama Group samples.  Ca- and Mg-isotopic compositions may provide insight into Ca2+ and Mg2+ cycling in the ocean, Sr-isotopic composition may monitor rates of ocean crust production and fluid-rock weathering along the mid-ocean ridge, and the CO2 isotopologue may function as an independent proxy of paleo-seawater temperature, which is also known to influence the polymorph mineralogy of CaCO3 precipitated from seawater.

4. Career benefits

This PRF award was the first extramural grant received by the PI.  The funding enabled the PI to explore a high-risk, high-reward area within his field of expertise.  The primary hypotheses of his proposal were borne out through the empirical observations that were made possible by this award.  Furthermore, the funding has allowed the PI to explore novel isotope systems as proxies of paleo-ocean chemistry (e.g., the boron isotope system as a proxy of seawater pH).  This has allowed him to apply these isotope systems to Precambrian limestones—which, for several of these isotope systems, constitute relatively novel applications.  It has also enabled him to forge beneficial collaborations with leading isotope geochemists throughout the world.  Finally, this award has enabled two undergraduates and two graduate students to pursue related, independent research projects investigating the relationship between seawater chemistry and the formation and preservation of biogenic carbonates.