DO ARCHEAN CHEMICAL SEDIMENTS RECORD ANCIENT SEAWATER RARE EARTH ELEMENT PATTERNS?

A variety of geologic features identified within, and geochemical data collected from, Early Archean rocks exposed in Greenland and Western Australia (3.45 Ga greenstones from the Pilbara Craton, Western Australia; 3.7 Ga Isua Greenstone Belt of southwestern Greenland; 3.8 Ga rocks from Akilia Island off the Greenland coast) have been interpreted by some researchers to be the earliest evidence of life on Earth, whereas others have argued strongly against the presence of any biogenic signatures in these ancient rocks. Although complex and multifaceted, arguments for preservation of biogenic indicators in these Archean rocks are based on sedimentary structures (e.g., stromatolitic bedforms), microscopic features that resemble, to a degree, bacteria, and perhaps most convincing, isotopically light signatures of reduced carbon within these rocks. Recently, investigators have begun to employ the rare earth elements (REE) to help sort out such issues by applying these trace elements to identify protoliths of the Archean rocks in question. In particular, specific features of shale-normalized REE patterns (i.e., positive, shale-normalized La-, Gd-, and Y-anomalies, negative Ce-anomalies, and enrichments in heavy REEs) of Archean rocks that closely resemble shale-normalized REE patterns of modern seawater and reefal microbialites are cited as evidence that these rocks are marine-derived biochemical precipitates, some of which may have formed in sabkha-like settings (i.e., Strelley Pool Chert from Western Australia). Recent studies, however, indicate that solutes are chiefly derived from continental sources in modern sabkhas. Furthermore, we present REE data for 28 groundwater samples from central México that exhibit features of their shale-normalized REE patterns that closely resemble modern seawater and microbialite patterns, as well as the REE patterns of Archean rocks, including the Strelley Pool Chert and greenstones of the Isua Belt in Greenland. The shale-normalized La-, Ce-, Gd-, Y-anomalies, and the heavy REE enrichments of these groundwaters are statistically indistinguishable from modern seawater and Holocene microbialites, and are within the range exhibited by the Archean rocks. Consequently, we suggest that it may be premature, in the absence of Nd isotopic data, to unequivocally conclude that these Archean rocks record REE signatures of an Archean ocean.