Evidence for a lack of biological P-cycling in a Cambrian soil

Abstract:

The earliest fossil land plants are known to exist in the Mid-Ordovician at 472 to 468 Ma and protein sequence analyses suggest that the onset of land colonization may have begun at as early as ca. 700-1000 million years ago (Ma) . However, fully established soil ecosystem may not be in place until after the Devonian (ca. 400 Ma) or even later. Dearth of fossil record on possible fungi- and/or bacteria-dominated early land biota renders it difficult to establish the early history of land colonization on Earth. Here we present a proxy for soil biological P- cycling.

Igneous rock contains typically 0.005-0.4% (wt) phosphate (PO₄^-3). In a biologically active soil weathering profile, phosphorus (P) is cycled by land biota including by those of the most primitive kingdoms. During, for example, pyrophosphate hydrolysis, the P-O bonds in PO₄^-3 breaks and exchange oxygen with ambient water. The biologically processed PO₄^-3 will have typically much higher δ¹⁸Ovalues (15-24‰ VSMOW) than the ones inherited from igneous sources (ca. 6‰). Therefore, an increase in the δ¹⁸O_PO4 from pristine igneous rocks to the upper more weathered ones should be expected if there was an active soil biological P-cycling. An igneous-PO₄ δ¹⁸O value in the more weathered rocks would otherwise indicate a lack of biologically-mediated P-cycling, thus a lack of or very limited land colonization. We examined a weathering profile in the Elk Point Formation (520-503Ma), South Dakota, a paleosol developed on a metagabbro in a subtropical climate of the Mid-Cambrian. Phosphate was extracted from a drill core of this profile and was analyzed for δ¹⁸O_PO4. The δ¹⁸O_PO4 for the weathered and un-weathered igneous rocks are all within a narrow range of 4.8-8.2‰, suggesting that biological P-cycling was insignificant during the weathering of Elk Point metagabbro at ca. 500 Ma. Subaerial, biologically mediated weathering probably did not play a role in geochemical cycling on Earth until much later in Earth history. More paleo-weathering profiles will need to be examined before a full history and impact of the greatest invasion of life on Earth can be constructed.