Reading
the Darwinist explanations for the Cambrian explosion is like watching a
game of "Who's Got the Button?" The biologists are sure the
paleontologists have the reasons for this unprecedented appearance of
animals in the fossil record. The paleontologists, in turn, point to the
geologists for it. And the geologists rely on the opinion that the
biologists have handfuls of explanatory buttons. When you inspect each
one's hands, though, they come up empty.
It was the geologists' turn to show their empty hands. In the
Geological Society of America Bulletin,
two papers undermine popular assumptions used by the biologists and
paleontologists to account for the explosion. Ask yourself if their work
sounds like "settled science."
No Oxygen Rise Here, Mate
In the first paper by Sperling
et al.,
geologists from Harvard and Queens College undermine the notion
(commonly asserted by the biologists) that it was a rise in oxygen
levels that triggered the explosion. Not that we ever found that
explanation credible (
#1,
#2,
#3,
#4),
but at least it gave the biologists a story with which to entertain the
uninformed. No longer; these geologists found no evidence to support a
global oxygen rise in the Ediacaran period that preceded the Cambrian.
The
causes behind the appearance of abundant macroscopic body and trace
fossils at the end of the Neoproterozoic Era remain debated. Iron
geochemical data from fossiliferous Ediacaran successions in
Newfoundland suggested that the first appearances correlated with an oxygenation event.
A similar relationship was claimed to exist in the Mackenzie Mountains,
Canada, although later stratigraphic studies indicated that the
sections analyzed for geochemistry were incorrectly correlated with
those hosting the fossils. To directly connect fossil occurrences with geochemistry in
the Mackenzie Mountains, we conducted a multiproxy iron, carbon,
sulfur, and trace-element geochemical analysis of stratigraphic sections
hosting both the Cryogenian "Twitya discs" at Bluefish Creek as well
as Ediacaran fossils and simple bilaterian traces at Sekwi Brook. There is no clear oxygenation event correlated with the appearance of macroscopic body fossils or simple bilaterian burrows; however, some change in environment -- a potential partial oxygenation --
is correlated with increasing burrow width higher in the Blueflower
Formation. Data from Sekwi Brook suggest that these organisms were
periodically colonizing apredominantly anoxic and ferruginous basin. This seemingly incongruent observation is accommodated through
accounting for differing time scales between the characteristic
response time of sedimentary redox proxies versus that for ecological
change. Thus,
hypotheses directly connecting ocean oxygenation with the appearance of
macrofossils need not apply to all areas of a heterogeneous Ediacaran
ocean, and stably oxygenated conditions on geological time scales were
not required for the appearanceof these Avalon-assemblage Ediacaran organisms. At least in the Mackenzie Mountains,
the appropriate facies for fossil preservation appears to be the
strongest control on the stratigraphic distribution of macrofossils. [Emphasis added.]
For
non-specialists, this basically means that the evidence connecting
oxygenation (a geochemical condition) with Ediacaran and Cambrian
fossils is equivocal at best. The geologists appear to be trying hard
not to rule it out ("a potential partial oxygenation"), but throughout
the paper, they reveal contradictions with the oxygen hypothesis. Some
of the best fossils appear in non-oxygenated rocks, and other
well-oxygenated rocks have no fossils. Moreover, earlier studies that
seemed to find a correlation were flawed.
The first step in distinguishing coincidence from correlationis determining whether temporal linkages represent a global pattern, regional events, or simply the unrelated appearanceof organisms during a time interval characterized by broadly increasing oxygen levels. Neoproterozoic oxygenation, if present, is increasingly being recognized as regionally heterogeneous (Kah and Bartley, 2011). This is reflected in iron speciation data from the southern Canadian Cordillera showing an increased prevalence of anoxic conditions during the mid- Ediacaran (Canfield et al., 2008), in contrast to the Newfoundland data, and data from the Wernecke Mountains of northwestern Canada which show no change at all(Johnston et al., 2013). Other regions such as Namibia also show heterogeneous but generally anoxic and ferruginous conditions through the late Ediacaran (Wood et al., 2015).Analyzed collectively and statistically, a global database of Proterozoic and Paleozoic iron speciation data shows no overall change to the oxygenation state of marine environments between the Ediacaran and Cambrian (Sperling et al., 2015). These database analyses do not rule out an increase in oxygen through this time period, but they do limit the magnitude of such a change to much less than is normally depicted (e.g., Holland, 2006).
Their
research in Canada also finds interesting geological evidence that
contradicts the picture of slow, gradual deposition. For instance, some
of the fossils are found in turbidites, which represent underwater
landslides. Other fossils appear encased in strata that appear to have
been formed in offshore storm surges. There are unconformities, slumps,
and gaps.
In
sum, the idea that biologists and paleontologists can expect to find a
gradual increase in oxygen below the Cambrian boundary is mistaken.
Figure 7 of the paper correlates outcrops from China, Africa, the U.S.,
and Canada. There's no pattern. "Redox change [i.e., change in reducing
vs oxidizing conditions] may correspond to the appearance of megascopic
fossils in some sections, but stable oxygenation on geologic time scales is not required." So if it's not oxygen, what is it? "The most obvious explanation is simply the distribution of beds capable of preserving fossils." Some places were suitable for preserving fossils, and some weren't. That's all.
Importantly, these data suggest that oxygenation of a basin is not required for the appearance of many Ediacaran taxa. To a first-order approximation, more so than oxygenation, the appearance of fossils of large eukaryotes throughout the entire Cryogenian and Ediacaran succession in NW Canada isdictated almost entirely by the appearance of event beds suitable for their preservation and presentation.
More on the second paper tomorrow.
