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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 trace element geochemistry, petrographic texture, and relic primary crystals.

During the first year of the project (2010-2011), the PI focused his research on three key areas: (1) analyzing the elemental data obtained from the Nama Group samples; (2) creating thin-sections of the Nama Group samples; and (3) characterizing micron scale changes in polymorph mineralogy of the samples via synchrotron micro-X-ray diffraction. During the second year of the project (2011-2012), the PI will focus on: (1) analyzing the petrographic textures of the thin-sections made in year 1 in order to identify temporal trends in carbonate polymorph mineralogy; (2) performing high-spatial resolution elemental analysis of Namacalathus and Cloudina fossils, in order to constrain paleoseawater Mg/Ca ratios across the putative calcite-aragonite sea transition; and (3) developing novel isotopic indicators of original CaCO3 mineralogy—a potentially transformative approach to constraining the evolution of ancient ocean chemistry that was not discussed in the PI’s original proposal.

2. Research Activity (2010-2011)

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 calcitic. 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

Thick ‘thin-sections’ (approximately 60-microns thick) were made of the 140 Nama Group samples spanning 10 m.y. of terminal Proterozoic time.  Thin sections were highly polished and cut 60 microns thick so that they could be analyzed for relic aragonite and  the fine scale Namacalathus and Cloudina fossils could be analyzed for elemental composition via SEM-hosted microprobe and/or laser ablation ICP-MS. Despite being cut thicker than standard petrographic thin-sections—so that they could be analyzed via microprobe and/or LA-ICP-MS—they are still sufficiently thin so as to reveal petrographic textures that are indicative of original mineralogy (e.g., aragonite recrystallized as low-Mg calcite is typically coarser-grained that original low-Mg calcite).

2.3. Synchrotron micro-X-ray diffraction

In 2010, the PI was awarded 21 shifts on the Advance Light Source’s synchrotron radiation micro-X-ray diffraction beamline (granted by Lawrence Berkeley National Lab/US Department of Energy). The PI used a portion of the awarded beam time to scan the Nama Group samples for trace crystals of original aragonite that may have survived the diagenetic transformation of original aragonite to calcite—so called ‘relic aragonite’.  However, despite the micro-XRD’s 5 micron-scale spatial resolution, the PI did not identify any relic aragonite crystals within the Nama Group samples.

3. Future Work (2011-2012)

Over the coming year, the PI will focus on three key areas of the project: (1) analyzing the petrographic textures of the thin-sections in order to identify trends in CaCO3 polymorph mineralogy throughout the Nama Group sequence; (2) performing high-spatial resolution in situ measurement elemental ratios within the Namacalathus and Cloudina fossils present throughout the Nama Group carbonates; and (3) developing novel isotopic indicators of original carbonate mineralogy.

3.1. Analysis of petrographic thin-sections

Petrographic textures can reveal much about the original mineralogy and diagenetic history of marine carbonates.  For example, carbonates that were originally deposited as aragonite and subsequently altered to low-Mg calcite tend to be coarser-grained than carbonates originally deposited as low-Mg calcite. The PI will quantify the petrographic textures of the 140 thin-sections of the Nama Group carbonates using iterative image analysis software. This analysis will seek to identify a threshold increase in average crystal volume (i.e., crystal surface area in section) and/or a threshold decrease in average ratio of crystal surface area (i.e., crystal perimeter in section) to crystal volume (i.e., crystal surface area in section) across the putative calcite-aragonite sea transition.  Textural parameters will be plotted stratigraphically alongside the other independent proxies of original mineralogy.

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

Over the coming year, 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.

3.3. Development of novel isotopic indicators of primary mineralogy and paleo-seawater chemistry

The PI has recently established extramural collaborations to measure the calcium (44/40Ca) and strontium (87Sr/86Sr) isotopic compositions of the Nama Group samples.  These isotopes are thought to be fractionated differently in aragonite vs. calcite and may thus function as additional proxies of original CaCO3 mineralogy should isotopic differences be preserved through diagenesis and neomorphism. Calcium and Sr-isotopic ratios may also provide insight into Ca²⁺-cycling in the ocean and rates of terrestrial and mid-ocean ridge fluid-rock weathering, respectively, which will inform modeling of seawater ionic ratios across this critical interval of geologic time.