The Doushantuo Formation of southern China dates from just before the Cambrian Explosion, and has become a source of astonishing information about animals that preceded the appearance of those with hard parts. It contains fossil embryos, algae, achritarchs, and small bilaterians that are purportedly the Earth’s earliest animals. Moreover the formation rests on the cap carbonates of a diamictite reckoned to represent a late Neoproterozoic glacial epoch, and provides a variable trend of carbon-isotope variation that extends up to the base of the Cambrian in southern China. Because the sequence contains a number of volcanic ash beds it is potentially dateable. Using a single-zircon U-Pb method, Daniel Condon of MIT and colleagues from the Chinese Academy of Science have established the ages of both top and base of the Doushantuo Formation with considerable precision (Condon, D. et al. 2005. U-Pb Ages from the Neoproterozoic Doushantuo Formation, China. Science Express, 24 February 2005). Sedimentation is bracketed between 635 and 550 Ma, the oldest age coinciding with that for the Ghaub tillite in Namibia. Time-calibration of the carbon-isotope record allows it to be matched with others in Namibia, Oman and Newoundland. There is one snag; within the sequence is a formation boundary that signifies non-deposition, which the authors correlate with a glacial epoch recognised in Newfoundland (the Gaskiers diamictite), citing sea-level withdrawal as the cause of non-deposition in China. The well-constrained correlation suggests a major, global increase in the burial of 12C that produced a marked negative excursion in d13C that spans around 90% of the Ediacaran Period that saw the rise of large soft-bodied animals shortly before the emergence of shelly faunas. The interpretation placed by the authors on this signature of burial of dead organic matter, which relates to no sign of glaciation, is that it would have elevated oxygen levels in the Late Neoproterozoic oceans. That might have increased productivity by primitive eukaryotes, and possibly opportunities for predation. The uppermost part of the Doushantuo Formation broadly coincides with the first appearance of complex trace fossils and mollusk-like bilaterians, and elsewhere there are signs of the first reef formation by weakly calcified metazoans at around that time. Clearly, it is well-dated sections such as these that may hold the key to what exactly prompted the general secretion of skeletal material; the hallmark of the 10 Ma later explosion in fossil animals.
No graphite in Akilia apatites, no sign of life?
In the first EPN of 2005 evidence was reported that weighed against a sedimentary origin for the ~3.8 Ga ironstones of West Greenland from which isotopically light carbon had been claimed to indicate the earliest signs of life (see Iron isotopes enter the Archaean life debate January 2005 EPN). The original work that claimed a biological signature in carbon from the oldest known metasedimentary rocks focussed on carbon-isotope analyses of apatites in them, in the belief that they would have withstood intense metamorphic alteration because of the resistance of that mineral to chemical reactions. Following close on the heels of that revelation comes one a great deal more worrying for aficionados of biogeochemistry. Geoscientists from Estonia, France, the US and Sweden have systematically made petrographic observations on apatite grains from the rocks of the Akilia Association, including those originally reported as carrying geochemical signs of life existing at that time (Lepland, A. et al. 2005. Questioning the evidence for Earth’s earliest life – Akilia revisited. Geology, v. 33, p. 77-79). Of the 190 individual apatite grains examined in 17 rocks, not one showed the slightest trace of carbonaceous material. It seems that apatite is unlikely to have been the host for the low d13C that caused such a stir in palaeobiological circles when it was first announced, and may well not be a good place to look for biomarkers. It also throws into question what did produce the signal. If it was the bulk rock, then the depletion in 13C could have resulted from temperature induced isotopic fractionation. Another possibility is that the samples were contaminated with modern biological materials, despite the precautions taken to avoid that.