Ethan Brand HBSc thesis abstract

Thesis Title: 
Was the Archean Ocean Stratified with Respect to Carbon Isotopes and Rare Earth Elements? A Comparison of Deep and Shallow Water Archean Carbonate

The modern ocean is a complex system that is continually mixed by circulatory ocean currents that keep ocean water relatively homogeneous. This process means that both deep water and shallow water carbonates in today’s ocean have similar isotopic ratios of carbon, as they are direct chemical precipitates of sea water. This however is not true for Archean carbonates.

To explore this idea, deep water chemical sediments were collected from across the Archean Superior province from Red Lake, Schreiber, Beardmore, and Wawa. These samples were compared with shallow water chemical sediment. A total of 28 carbonate samples were gathered from the Uchi, Wabigoon, and Wawa sub-provinces and analyzed for their geochemical properties. The carbonate samples were processed via partial dissolution so as to only dissolve the chemical precipitate, thus assessing the water from which they formed. The samples went through ICP-MS and ICP-AES analysis and through carbon and oxygen isotope analysis in order to better understand their chemistry.

When comparing the REE geochemistry of deep vs shallow water chemical precipitate it was found that Schreiber, Beardmore, and Wawa localities were chemically distinct. These locations all showed a more positive Eu anomaly in deep water. Deep water Schreiber and Wawa had lower Y/Ho ratios than their shallow water counterparts. Heightened Eu values are indicative of greater hydrothermal leaching of basaltic oceanic crust. Lower Y/Ho ratios are due to more hydrothermally sourced Y/Ho ratios with more chondritic values of ~26. Additionally, Schreiber and Beardmore locations showed a greater degree of LREE depletion and heightened HREE enrichment. Red Lake samples however show that both deep and shallow samples had very similar positive Eu anomalies, and super chondritic Y/Ho ratios with some fluctuation.

δ 18O‰ analysis and comparison of deep water samples to shallow water samples found that in Schreiber, Red Lake, and Wawa there was no variation with depth. This proved that δ 18O‰ was unaffected by depth in the water column. δ 13C‰ analysis showed that in Schreiber and Wawa deep water carbonate there is more δ 13C‰ depletion than in shallow water carbonate. The δ 13C‰ of the Red Lake samples did not vary with depth.

Throughout this study the Red Lake samples have been an outlier, with no evidence of stratification, whereas the three other locations did have evidence of Archean ocean stratification. The hydrodynamic system at the Red Lake site may have been governed more by upwelling of deeper water onto the shallow shelf than at the other sites. Due to this unique scenario, it is fair to conclude that the Red Lake locality does not accurately represent typical shallow water in the Archean ocean. This leaves the conclusion that, for the cases of Schreiber, Beardmore, and Wawa, the formative water that precipitated carbonate minerals in these locations varied from depths below storm wave base to the shallow shelf. This is compatible with a greater hydrothermal influence in deep water than in shallow, and more negative δ 13C‰ values in deeper water, suggesting Archean ocean stratification with respect to REE and δ 13C‰.